Paint and Coating Testing Manual [MNL 17] (14th Ed. of the Gardner-Sward Hbk.) - J. Koleske (ASTM, 1995) WW

Paint and Coating Testing Manual Fourteenth Edition of the Gardner-Sward Handbook

Joseph V. Koleske, Editor

ASTM Manual Series: MNL 17 ASTM Publication Code Number (PCN) 28-017095-14

1916 Race Street, Philadelphia, PA 19103

Library of Congress Cataloging-in-Publication Data Paint and coating testing manual: fourteenth edition of the Gardner-Sward handbook/Joseph V. Koleske, editor. p. cm.--(ASTM manual series; MNL 17) Rev. ed. of: Paint testing manual. 13th ed. 1972. "ASTM publication code number (PCN) 28-017095-14." includes bibliographical references and index. ISBN 0-8031-2060-5 1. Paint materials--Testing. 2. Paint materials--Analysis. I. Koleske, J. V., 1930- . II. Paint testing manual. III. Series. TP936.5.P34 1995 95-10632 667'.6--dc20 CIP

Copyright 9 1995 AMERICAN SOCIETY FOR TESTING AND MATERIALS, Philadelphia, PA. All rights reserved. This material may not be reproduced or copied, in whole or in part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of the publisher.

Photocopy Rights Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by the AMERICAN SOCIETY FOR TESTING AND MATERIALS for users registered with the Copyright Clearance Center (CCC)Transactional Reporting Service, provided that the base fee of $2,50 per copy, plus $0.50 per page is paid directly to CCC, 222 Rosewood Dr., Danvers, MA 01923; Phone: (508) 750-8400; Fax: (508) 750-4744. For those organizations that have been granted a photocopy license by CCC, a separate system of payment has been arranged. The fee code for users of the Transactional Reporting Service is 0-8031-2060-5-95 $2.50 + .50.

NOTE: This manual does not purport to address (all of) the safety problems associated with its use. It is the responsibility of the user of this manual to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

Printed in Ann Arbor, MI June 1995

Foreword THIS PUBLICATION, Paint and Coating Testing Manual: Fourteenth Edition of the Gardner-Sward Handbook, was sponsored by Committee D- 1 on Paint and Related Coatings, Materials, and Applications. The editor was Joseph V. Koleske. This is Manual 17 in ASTM's manual series.

III

Acknowledgments ASTM WOULDLIKE TO EXPRESS its gratitude to the authors of the previous 13 editions of this publication. These publications made significant .contributions to the technology; therefore, ASTM, in its goal to publish books of technical significance, called upon current experts in the field to revise and update this important publication to reflect the changes and advancements that have taken place since the last edition, which was published in 1972.

iv

Contents xi

Preface

xiii

Introduction

PART 1: REGULATIONS Chapter 1--Regulation of Volatile Organic Compound Emissions from Paints and Coatings by John J. Brezinski

PART 2: NATURALLY OCCURRING MATERIALS Chapter 2--Bituminous Coatings by Ben J. Carlozzo

15

Chapter 3mCellulose Esters by L. G. Curtis

23

Chapter 4~Drying Oils by Joseph V. Koleske

26

Chapter 5~Driers and Metallic Soaps by Marvin J. Schnall

30

PART 3: SYNTHETIC MATERIALS Chapter 6~Acrylic Polymers as Coatings Binders by John M. Friel

39

Chapter 7--Alkyd and Polyesters by Al Heitkamp and Don Pellowe

53

Chapter 8--Amino Resins (Reaction Products of Melamine, Urea, etc. with Formaldehyde and Alcohols) by J. Owen Santer Chapter 9mCeramic Coatings by Richard A. Eppler

60

68

CONTENTS Chapter 10mEpoxy Resins in Coatings by Ronald S. Bauer, Edward J. Marx, and Michael J. Watkins

74

Chapter 11 ~Phenolics by John S. Fry

79

Chapter 12~Polyamides by Robert W. Kight

85

Chapter 13~Polyurethane Coatings by Joseph V. Koleske

89

Chapter 14~Silicone Coatings by D. J. Petraitis

95

Chapter 15mVinyl Resins for Coatings by Richard J. Burns

99

Chapter 16--Miscellaneous Materials and Coatings by Joseph V. Koleske

108

PART 4: PLASTICIZERS Chapter 17~Plasticizers by Peter Tan and Leonard G. Krauskopf

115

PART 5: SOLVENTS Chapter 18--Solvents by Stephen A. Yuhas, Jr.

125

PART 6: PIGMENTS Chapter 19--White Pigments by Juergen H. Braun

159

Chapter 20mBlack Pigments by Frank R. SpineUi

179

Chapter 21mColored Organic Pigments by Peter A. Lewis

190

Chapter 22~Inorganic Colored Pigments by Peter A. Lewis

209

Chapter 23~Ceramic Pigments by Richard A. Eppler

214

CONTENTS vii Chapter 24mExtender Pigments by Henry P. Ralston

2t7

Chapter 25--Metallic Pigments by Russell L. Ferguson

223

Chapter 26--Pearlescent Pigments by Carl J. Rieger

229

Chapter 27--Inorganic Anti-Corrosive Pigments by M. Jay Austin

238

Chapter 28mOil Absorption of Pigments by Joseph V. Koleske

252

PART 7: ADDITIVES Chapter 29~Bactericides, Fungicides, and Algicides by Vanja M. King

261

Chapter 30~Thickeners and Rheology Modifiers by Gregory D. Shay

268

PART 8: PHYSICAL CHARACTERISTICS OF LIQUID PAINTS AND COATINGS Chapter 31ADensity and Specific Gravity by Raymond D. Brockhaus

289

Chapter 32nParticle-Size Measurements by George D. Mills

305

Chapter 33ARheology and Viscometry by Richard R. Eley

333

Chapter 34nSurface Energetics by Gordon P. Bierwagen

369

Chapter 35~Solubility Parameters by Charles M. Hansen

383

PART 9: FILMS FOR TESTING Chapter 36--Cure: The Process and Its Measurement by Thomas J. Miranda

407

Chapter 37--Film Preparation for Coating Tests by Robert D. Athey, Jr.

415

viii CONTENTS

Chapter 38--Measurement of Film Thickness by C. M. Wenzler and J. F. Fletcher

424

Chapter 39--Drying Time by Thomas J. Sliva

439

PART 10: OPTICAL PROPERTIES Chapter 40--Color and Light by Fred W. Billmeyer, Jr. and Harry K. Hammond III

447

Chapter 41~Gloss by Harry K. Hammond III and Gabriele KigleBoeckler

470

Chapter 42~Hiding Power by Leonard Schaeffer

481

Chapter 43--Mass Color and Tinting Strength of Pigments by Julio I. Aviles

507

PART 11: PHYSICAL AND MECHANICAL PROPERTIES Chapter 44--Adhesion by Gordon L. Nelson

513

Chapter 45--Abrasion Resistance by Mark P. Morse

525

Chapter 4 6 ~ D y n a m i c Mechanical and Tensile Properties by Loren W. Hill

534

Chapter 47inFlexibility and Toughness by M. P. Morse

547

Chapter 48mHardness by Paul R. Guevin, Jr.

555

Chapter 49mStress Phenomena in Organic Coatings by Dan Y. Perera

585

Chapter 50mSlip Resistance by Paul R. Guevin, Jr.

600

PART 12: ENVIRONMENTAL RESISTANCE Chapter 51--Prevention of Metal Corrosion with Protective Overlays by William H. Smyrl

609

CONTENTS

Chapter 52--Natural Weathering by Lon S. Hicks and Michael J. Crewdson

619

Chapter 53~Accelerated Weathering by Valerie D. Sherbondy

643

Chapter 54~Biological Deterioration of Paint Films by David L. Campbell

654

Chapter 55~Chemical Resistance by Alan H. Brandau

662

Chapter 56~Testing Coatings for Heat Resistance and Flame Retardance by Wayne Ellis Chapter 57--Water-Resistance Testing of Coatings by Wayne Ellis

667 677

PART 13: SPECIFIC PRODUCT TESTING Chapter 58--Aerospace and Aircraft Coatings by Charles R. Hegedus, Stephen J. Spadafora, David F. Pulley, Anthony T. Eng, and Donald J. Hirst

683

Chapter 59~Architectural Coatings by Harry E. Ashton

696

Chapter 60~Artists' Paints by Benjamin Gavett

706

Chapter 61--Automative Product Tests by Rose A. Ryntz

711

Chapter 62--Can Coatings by Martin B. Price

717

Chapter 63--Masonry by Frances Gale and Thomas Sliva

725

Chapter 64--Pipeline Coatings by Loren B. OdeU and AI Siegmund

731

Chapter 65--Sealants by Saul Spindel

735

Chapter 66--Traffic Marking Materials by Larry R. Hacker

741

Chapter 67--Water-Repellent Coatings by Victoria Scarborough and Thomas J. Sliva

748

x CONTENTS

PART 14: ANALYSIS OF PAINTS AND PAINT DEFECTS Chapter 68--Analysis of Paint by Darlene Brezinski

753

Chapter 69--The Analysis of Coatings Failures by George D. Mills

767

PART 15: INSTRUMENTAL ANALYSIS Chapter 70--Atomic Absorption, Emission, and Inductively Coupled Plasma Spectroscopy by Dwight G. Weldon

783

Chapter 71--Chromatography by Rolando C. Domingo

789

Chapter 72~Electron Microscopy by John G. Sheehan

815

Chapter 73~Infrared Spectroscopy by Jack H. Hartshorn

826

Chapter 74--Methods for Polymer Molecular Weight Measurement by Thomas M. Schmitt

835

Chapter 75--Coatings Characterization by Thermal Analysis by C. Michael Neag

841

Chapter 76~UltravioletNisible Spectroscopy by George D. Mills

865

Chapter 77--X-Ray Analysis by A. Monroe Snider, Jr.

871

PART 16: SPECIFICATIONS Chapter 78--Paint and Coatings Specifications and Other Standards by Wayne Ellis

891

Appendix

895

Index

899

Preface AT A JANUARY1967 MEETINGOF ASTM COMMITTEED-1 held in Washington, DC, ASTM (American Society for Testing and Materials) accepted ownership of the Gardner-Sward Handbook from the Gardner Laboratory. It was through this laboratory that Dr. Henry A. Gardner published the previous twelve editions of the manual. Acceptance of this ownership gave ASTM an assumed responsibility for revising, editing, and publishing future editions of this well-known, respected manual. The undertaking was assigned to Committee D-1 on Paint and Related Coatings, Materials, and Applications. This committee established a permanent subcommittee, D01.19 on Gardner-Sward Handbook, chaired by John C. Weaver, to provide technical, editorial, and general policy guidance for preparation of the 13th and subsequent editions of the Gardner-Sward Handbook. The 13th edition was published in 1972 as the Paint Testing Manual (STP 500) with Mr. G. G. Sward as editor. The manual has served the industry well for the past two decades; it contains useful information that cannot be found elsewhere. However, the passage of more than 20 years since its publication is readily apparent in many and perhaps most chapters of the manual. Although updating the manual was discussed through the years, a variety of reasons prevented this task from being accomplished. Feasibility of updating the manual was not realized until mid-1989 when Dr. John J. Brezinski, Union Carbide (retired), and Mrs. Kathleen A. Dernoga, Manager of Acquisitions and Review of ASTM Technical Books and Journals, discussed the matter and the 14th edition was conceived. Between then and the spring of 1990 an outline for the 14th edition was developed and was approved by members of Subcommittee DO1.19. Almost five years later the manual was completed--no wonder such a long period elapsed between editions! The scope of the new edition is in keeping with the stated scope of Subcommittee D01.19: "To provide technical, editorial, and general policy guidance for preparation of the Fourteenth and subsequent editions of the Gardner-Sward Handbook. The handbook is intended for review of both new and experienced paint technologists and the past, present, and foreseeable trends in all kinds of testing within the scope of Committee D-1. It supplements, but does not replace, the pertinent parts of the Society's Book of Standards. It describes briefly and critically all Test Methods believed to have significance in the world of paint technology, whether or not these tests have been adopted officially by the society." In this new edition, ASTM standard methods are described by minimal detail with the various volumes of the ASTM Book of Standards remaining the primary source of such information. An effort was made to include references in the absence of ASTM information concerning industrial, other society, national, and international test methods. For the most part, the manual contains either new chapters or the old topics/chapters in rewritten form. In a few cases, the old manual was merely updated, attesting to either the quality of the earlier writing, the lack of development in the area, or the apparent waning of interest in the topic. A variety of modern topics has been included. Individual authors, experts in their various fields, were given a great deal of freedom in expressing information about their topics. Many things have changed through the years. The chemical emphasis has shifted from natural products to synthetic products, so this edition of the manual contains chapters that deal with a large number of synthetic polymers used in the coating industry. Instrumentation has undergone a marked change with innovative electronics providing the key to many changes. An effort was made to include chapters dealing with a broad variety of instruments.

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PREFACE To the authors, a warm, heart-felt "thank you." You put your talents to work and sacrificed much personal time to make the manual a success. A "thank you" is also due the reviewers, who are a special lot. They must be critical, yet carry out their task in a constructive manner. Because of the customary anonymity accorded reviewers, they should know that some authors made a special effort to express their appreciation for the review comments that they felt strengthened their manuscripts. Those organizations who permitted authors' time, use of support staff, and supplies are truly appreciated. Works such as this manual could not be completed without their generosity--may they prosper. The staff at ASTM is distinctive--they were interested and smilingly helpful to the authors, reviewers, Subcommittee DO1.19, and the editor as they guided us through the maze of the publication assembly process (though they may have gritted their teeth at times). A very special thanks to Monica Siperko of ASTM, who worked closely with the editor in dealing with authors, reviewers, ASTM staff, and manuscripts. Her invaluable, cheerful assistance is appreciated. And last, but certainly not least, the contributions of Maureen Quinn and David Jones of the ASTM editing staff are acknowledged. Their able assistance ensured that the manual was uniform in style and grammar.

Joseph K Koleske Editor

Introduction P A S T TO P R E S E N T More than a score of years has passed since the previous edition of this manual was published, and many changes have taken place in the coating industry and elsewhere since that time. In 1972, the previous publication date, over 90% of all industrial coatings were low-solids, solvent-borne coatings. Total solids ranged from about 5 to 20% by weight. In the early 1970s, solvents were inexpensive, convenient carriers for the binder polymers used in coatings, and there appeared to be little knowledge in the scientific community about the consequences of breathing them, absorbing them through the skin, or placing them either in the atmosphere or in the environment in general. There were exceptions, as when a particular compound was known to be highly toxic. The specific effect of certain solvents as well as other chemicals on certain segments of the population was unknown in the scientific community. Large quantities of solvent were needed to dilute the high-molecular-weight binders to an appropriate application viscosity. High-molecular-weight binders imparted high-quality characteristics to the final coating. In addition, very dilute solutions allowed application of very thin, but continuous, films. These factors coupled with the low cost of energy used to drive the large ovens associated with coating manufacture were major reasons that kept coating systems low in solids and solvent-borne in nature. Even a large percentage of architectual coatings was oil-based, solvent-borne formulations. In an overall sense, products of the coatings industry worked and did a satisfactory job. However, new technologies were being talked about, worked on, and even commercialized, albeit in a small way. Terms such as "powder coatings," "radiation-cure coatings," and "water-borne coatings" were beginning to creep into the language of the coating industry. The technologies promised a great deal, were considered mainly by the innovative, and had many difficulties associated with their introduction. Abbreviations such as EPA, MSDS, OSHA, SARA, TSCA, and similar others that are familiar to us today weren't yet in the industry's jargon. In fact, less than two pages (pp. 418-419) in the previous edition of this manual were dedicated to the topic of atmospheric pollution, and therein basically only Rule 66 was briefly discussed. No criticism is m e a n t - - s u c h was the nature of the topic in the pre-1972 world. As stated previously, "Times have changed," and this new edition devotes a significantly sized chapter to acquaint readers briefly with the topic of regulation of volatile organic compounds emitted from coatings. This topic and the related topics of health and safety are mentioned a number of times in the manual.

POWDER COATINGS Changes other than those of regulation, though related, have taken place in our industry. In 1972, coating journals had discussions about the "powder explosion." Powder coatings were to take over the industry since they were clean, 100% solids systems that could be applied on any substrate that could be either heated for fluid-bed application or made conductive for electrostatic spray application. Although powder coatings had been used in Europe as early as the 1950s, not many powder-coating systems existed in the United States in the 1970s. There was little incentive for largecompany, raw-material suppliers who sold solvents to get into the powder coating business. A prime obstacle was the fact that there was little one could do to alter a Xlll

xiv INTRODUCTION powder coating once it was made. If a fully formulated product such as a powder coating were to be sold by a raw-material supplier, there was a feeling that customers for solvents and other raw materials would be alienated. Also, from the end-user point of view, conversion from in-place, existing application equipment to powder coating equipment required a capital expenditure. This is a factor that always was and still can be a hindrance to conversion from existing to new technology. It did not take long for the fuse of the powder keg to fizzle--but, more importantly, it did not go out. Epoxide powders were in vogue for pipeline coatings and were used on the Alaskan pipeline. In the early days of powder coatings, small amounts of vinyl chloride homopolymer and copolymer, polyester, and nylon powders were used. Fluidbed application methods were first to be commercialized. It was relatively easy to meltmix and grind mixtures of polymers, pigments, plasticizers, and other formulating ingredients to obtain the relatively large particle-size powders used by this method. Electrostatic spray took longer to develop since procedures for manufacture of the fine particle-size powders as well as the sophisticated spray equipment needed for effectively and efficiently handling charged powder particles had to be developed. Powder coatings not only had problems in manufacture and application, but also in other areas such as: changeover from one color, availability and storage of a number of colored powders, flow and leveling, in developing thermoset coatings that would flow and level before cross-linking at an elevated temperature, in blocking during storage and as the powder flowed through the spray-system hoses and gun, in cost coupled with concerns about handling overspray and recovery and disposal. But, something new had been born, and a new industry within the coating industry was going through the throes of growing up in a competitive environment. Today the powder industry segment is strong and is growing. It has developed to the point where it now has its own organization, The Powder Coating Institute, located in Alexandria, VA. Journals such as the Journal of Coatings Technology, Industrial Paint and Powder, etc. now devote entire issues to the topic. Local, national, and international meetings are held to discuss the topic. The biennial trade show Powder Coating '92, held in Cincinnati, attracted over 4000 people, and 163 companies displayed their products. Powder coatings probably will not take over the coating industry, but they now are and continue to be important factors in the industry for the foreseeable future.

RADIATION C U R I N G Another new technology born in the late 1960s was radiation curing. It also showed great early promise and many problems--there were even unrealized problems at the beginning since no one really understood that some of the chemicals used were human sensitizers and strong irritants. Lack of knowledge in the scientific community about the hazards of acrylates resulted in some people becoming sensitized to these compounds. However, the idea of taking a liquid, low-viscosity, coating formulation, applying it to a substrate with conventional equipment, and having the coating essentially instantly converted into a solid, cross-linked film with very little or nil loss to the atmosphere was attractive. Radiation curing involved the use of electron-beam or ultraviolet-light radiation. Free radicals were generated with ultraviolet light (photocure), and electrons were generated with electron beams. Acrylates and maleate polyesters/styrene were very rapidly polymerized in the presence of these active species. Because the reaction took place in thin films, the heat of reaction was readily dissipated and was not a problem to the technology. In the mid-to-late 1970s it was felt that this technology might capture only a percentage or two of the industrial coatings market. Proponents of these essentially 100% solids systems refined the technology. Adaptation of existing equipment to the technology involved relatively simple, low-cost improvement when ultraviolet-light radiation was involved, and many conventional application techniques could be used. Most potential users insisted on the formulated products having low viscosity so that conventional equipment could be used. Such users were the driving force toward low-molecular-weight, reactive products. Innovators developed products that were quite safe to handle. A new branch of the technology that involved cycloaliphatic epoxides and photoinitiators that generated cationic species when photolyzed was begun at the end of decade.

INTRODUCTION

Today, radiation-cure technology is considered a growth technology that is well established. It has developed to the point where it has its own technical society, RadTech International, located in Northbrook, IL, with large numbers of members and attendees at its meetings and exhibitions, which are held in North America, Europe, and Japan. Again, various journals have issues dedicated to the technology, and meetings or segments of meetings are held in various countries throughout the world. Radiation curing is currently a strong force in the market, and it is widely used to provide coatings for flooring, beverage cans (nonfood contact), electronics, plastics, paper, etc. It surely will also be a force in the future for as far ahead as we can see.

HIGH SOLIDS COATINGS A third technology with its inception in the 1970s is high solids. Three factors provided the impetus for this technology and certainly had an effect on powder and radiation-cure coatings. First, the oil embargo during the mid-1970s caused the price of raw materials--including solvents--to increase significantly, and this was coupled with a significant scarcity of both petroleum-derived chemicals and fuel for energy purposes. Second, there was the energy cost involved in operating the huge ovens required to volatilize safely the large amount of solvent removed during the drying of the coatings. Not only were gas and oil costly, they were not readily available. High-solids, low-energy systems were developed in a feverish manner. Popular words in the industry at this time were "high solids" and "low energy cure." Slowly the coating industry was coming to the realization that it might have to change--willingly or otherwise--from low solids and relatively easy to formulate systems to something new, be it powder coatings, radiation-cure coatings, water-borne coatings, or some other new technology. The third factor was related to concern about people and the environment. Everyone was becoming more and more conscious about the environment: in the workplace and the home as well as in a national and global geophysical sense. There was an awareness that solvents were being released into the atmosphere and into other parts of our ecological system, and that those solvents, though certainly not the only nor most important culprits, could have a long-term, deleterious effect on our environment and quality of life. In addition to these factors, and very importantly, the government became strongly involved in regulation of the industry through the Environmental Protection Agency (EPA). The EPA is an agency that administers federal laws concerned with activities that affect the environment (details about the EPA and similar agencies are dealt with elsewhere in the manual). Governmental requirements, naive as some of them may have been at the outset, were established for the coating industry. For example, coatings were to contain no more than 20% volatile organic solvent. An industry that had been using formulations containing about 80 to 90% organic solvent and about 10 to 20% coating polymer was being asked (told) to change in a "quantum leap" manner. The industry was to develop formulations that significantly reduced the amount of organic solvent used and, of course, maintain ease of application, good protection, and aesthetically pleasing appearance. To term such a requirement "naive" may have been an understatement. The difficulty and impracticality of the requirement were realized, and over the years the standard has been modified. Higb(er) solids systems that contain more than 1 pound of polymer per pound of solvent are routinely used. Of course, this results in a markedly reduced volume of solvent that enters the atmosphere. Nonetheless, the change has been made and today almost all coatings are of the higher solids or 100% solids variety. Here, too, the industry discusses advances each year at The Water Borne and Higher Solids Coatings Conference that is cosponsored by the University of Southern Mississippi and the Southern Society and is held in the late winter of each year.

O T H E R N E W COATING T E C H N O L O G I E S Adjuncts to high-solids coating technology are the water-borne systems (though they may have predated organic-based systems) that require minimal cosolvent to achieve good appearance and properties, two-package coating systems that have relatively short pot lives, and water-borne emulsion or latex systems. The latter coatings are important

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INTRODUCTION to the architectural coatings market, and today oil-base paints represent only a small fraction of the huge market for these commodity items. Latex-based paints have increased their solids, decreased volatile organic components, and are formulated with new thickening agents that have excellent flow characteristics. Even when the paint is applied by amateurs, spattering is almost unknown. This industry segment also is included in the above-mentioned symposium as well as at Lehigh University, which is well known for its efforts in the field of water-borne coating technology.

SOLVENTS The area of solvent technology has undergone a number of changes during the past quarter of a century. The changes for the most part are associated with reduction of their health-hazard profiles and characteristics. Knowledge was developed or "rediscovered" about many solvents since the previous edition was published. This knowledge has led either to the demise or to "sharply" curtailed o r restricted usage of what had been many commonly used solvents. This same information was also responsible for innovation and has played an important role in development of new solvents that are less harmful to humans, animals, and the environment. The abbreviation MSDS (Material Safety Data Sheet) became well known for solvents and for other chemicals. Today these information sheets with safety data about materials are not only required for chemicals, they are used by people in laboratories and plants. It goes without saying that they should be required reading for anyone handling chemical compounds.

OTHER INNOVATIONS The advent of new technology to produce functional and decorative coatings involved more than innovative organic chemistry. It also required innovative physical chemistry, material science, polymer science, and engineering. In addition to new chemicals and ways to use them, development of new application equipment and cure equipment was required. Conventional suction-feed spray guns and roll coaters could not be used for many of the new technologies. Powder coatings required development of methods of manufacture, of handling systems that could be quickly cleaned during a color changeover, of methods for placing a charge on the powder, of understanding and elimination of cage effects, of getting good wrap-around on other than flat substrates, of ways to remove fused powder from hangers and conveyors, and so on. Radiation-cure coatings required development of electron beam systems with improved safety features, of efficient ultraviolet light systems that could cure in other than line of sight, of low-viscosity chemicals with improved safety and health characteristics, and more. Water-borne systems had to deal with flash rusting, with minimizing cosolvents, with developing latexes that would quickly dry and fuse while maintaining qualities such as hardness, high gloss, and toughness. High solids required balances between molecular-weight, functional groups and their effects and viscosity, as well as between reactivity and shelf or pot life. It also required coating scientists to obtain high-quality finishes from the small molecules needed to achieve the low viscosity used for reasonable application characteristics at high-solids content. Electronics also played an important role in the changes in coating technology and in the testing of coatings. During the period we are discussing, there has been a "Buck Rogerish" explosion in this industry. Many concepts and products related to such concepts that were considered amazing and with little likelihood of success in 1970 are realities today. Miniaturization technology has made printed circuit assemblies and semiconductors possible. Today, hand-held calculators are almost as powerful as the room-filling computers were in the early 1970s. Personal computers weren't even thought about in 1972. Robots and various forms of robotics have become useful tools in the 1990s. Application of electronics technology to a host of instruments, some of which are described in this manual, has vastly improved our ability to probe and otherwise examine and understand materials that are currently used, new materials as they are being developed, and final coatings in both an as-made and an aged condition. Both reliability and precision of tesing have been improved through new instruments.

INTRODUCTION Yet within this array of new test equipment that has been enabled through electronics and that allows quantitative results to be obtained in a reliable manner, there is still room for and a need for some of the simple, homey tests used for many years. Tests that are easy to apply and that require no elegant or complicated equipment are still desirable. Quickly drawing a nickel over a coating while applying downward pressure to the stroke almost immediately gives one a feeling for how well the coating is adhering to the substrate and to its toughness and formability. Such a test can he performed "on line" and by essentially anyone. Even interpretation of results is not difficult and is largely intuitive. Pencil hardness testing may vary from operator to operator, but one does not need to be a coatings scientist to quickly grasp what the test is measuring and to have a "feel" for a coating's hardness from the test. Solvent double rubs are easy to do. While the exact number of double rubs obtained may vary from individual to individual, the test still gives a quick understanding of the coating's thermoset character as well as the degree of cross-linking. Sharp impacts on the face or reverse side of a coated metal panel can quickly give an understanding about the impact and adhesion characteristics of the coating. These are four simple tests, but they can yield a great deal of understanding about a particular coating in a very short time. Other simple tests also exist. Lest one get the wrong impression from the last few sentences, while these tests are useful, they certainly do not lead to the fundamental understanding that is very important to development of knowledge so necessary for new products. Sophisticated testing puts numbers on test results, probes deep into molecular achitecture, and allows both comparison of competitive products and the development of improved products. Sophisticated analyses also provide the understanding necessary to develop new chemicals and technology that will lead to improvements in existing products and to new products.

SUMMARY Within the changed environment that has been described, the 13th edition of the

Paint Testing Manual has, for the most part, become outdated--as was expected when it was compiled. Many of the methods described have changed, and the needs of the industry have also changed. The 14th edition reflects these changes. Even its title has been changed--to Paint and Coating Testing Manual. The collective effort of the many authors has resulted in a manual that has deemphasized, though certainly not eliminated, natural products, that provides a description of the regulations currently in force for the industry, and that discusses the main polymeric species, colorants, special pigments, extenders, and additives used in the industry today. The manual also deals with the analyses used to dissect and analyze a coating, the instruments used in the industry, and the products of the industry as well as how they are used and tested. Testing procedures for the most part are not detailed in the manual. Rather, the manual is a guide that will lead a coatings scientist to more in-depth treatises about the various topics and to test methods, procedures, and standards of ASTM and other national and international organizations.

Joseph V. Koleske Editor

xvii

Part I: Regulations

MNL17-EB/Jun. 1995

Regulation of Volatile Organic Compound Emissions from Paints and Coatings

1

by J. John Brezinski I

PRIORTOTHE 1960S the coatings industry enjoyed a somewhat predictable regulatory and economic environment. The paint formulator developing a solvent-based coating selected solvents on the basis of evaporation rate, solubility parameter, density, flammability, and, of course, cost. There was no apparent need to consider the relative photochemical reactivity of these materials, nor was there any appreciable incentive to reduce the solvent content of commercially acceptable coatings. It was, of course, recognized that objectionable odors were released from some paints and coatings. Further, air emissions resulting from the evaporation of solvents during hightemperature processing of oils and resins caused occasional complaints from persons living near the coatings plant. The prevailing view of this period was summarized by Francis Scofield in his article in the 13th edition of the Paint Testing Manual entitled "Atmospheric Pollutants" [1]. These "nuisance" types of pollution are a continuing problem but, in general, can be dealt with by dilution and dispersion of the objectionable materials to bring the concentration below a level that can be detected by the neighboring citizenry. Fortunately, most of the materials used by the paint industry are not toxic at concentrations significantly below the range at which they can be detected by the human nose, and sophisticated analytical procedures are rarely needed to deal with these "nuisance" problems. Since the 1960s societal concern about health and the environment has increased appreciably. Actions taken by federal and state legislative bodies have resulted in a steady avalanche of new laws and associated regulations that affect virtually all industry. Among the new federal laws administered by the U.S. Environmental Protection Agency (EPA) that impact significantly on the coatings industry are those shown in Table 1. They are designed to control the emission of pollutants to air, to water, and to soil. In addition, among the new federal standards administered by the Occupational Safety and Health Administration are those that require manufacturers--including those making paints and coatings--to evaluate the hazards of products they make and to provide appropriate safety information to employees and users through the Material Safety Data Sheet (MSDS) and product labels. 9 Hazard Communication Standard (HCS), 1983 9 Occupational Exposure to Hazardous Chemicals in Laboratories, 1990 11046 College Circle, St. Albans, WV 25177. Copyright9 1995 by ASTMInternational

TABLE 1--Federal environmental laws administered by the U.S. Environmental protection agency. Law Clean Air Act, 1970 Amendments of 1977 Amendments of 1990 Clean Water Act of 1972 Amendments of 1977 Safe Drinking Water Act, 1974 Toxic Substances Control Act, 1975 Resource Conservation and Recovery Act, 1980 Comprehensive Environmental Response Compensation and Liability Act, 1980 Superfund Amendments and Reauthorization Act, 1986 Title III, Emergency Planning and Community Right-to-Know, 1986

Abbreviation CAA CAAA-77 CAAA-90 CWA SDWA TSCA RCRA CERCLA (Superfund) SARA SARA, Title III

The discussion in this section will focus on the Clean Air Act and its amendments that, in the author's opinion, have had (and will continue to have) the greatest impact on coatings.

T H E CLEAN AIR ACT AND ITS AMENDMENTS California Smog A precipitating factor influencing the basis for selection of solvents for coatings in the 1960s and early 1970s was the recognition that the emission of solvents from coatings to the atmosphere contributed to the growing "smog" problem in Southern California. The frequency of smog conditions in this area had increased steadily during the 1950s and 1960s as the number of automobiles, trucks, buses, and airplanes increased and as industrial development expanded with the accompanying growth of petroleum and chemical processing and power plant utilization. The smog problem was (and is) most acute in the Los Angeles air basin, an area uniquely situated in a series of plains that originate in the high mountains to the east. The basin enjoys predominantly sunny days with cool moist air flowing with a light westerly wind most of the year. These factors cause a nearly permanent temperature inversion layer, trapping air emissions that combine to produce a persistent eye-irritating smog in the basin.

www.astm.org

4

PAINT AND COATING TESTING MANUAL

In a presentation entitled "Solvent Restriction--Problem or Opportunity," Dr. John Gordon, then of the University of Missouri-Rolla, discussed the major sources of hydrocarbons and nitrogen oxides, which together in the presence of UV radiation react to produce oxidants and ozone, major components of smog [2]. Sunshine HC + NOx UV Radiation Smog (03)

Sources of NOx: Flame of almost any kind, volcanoes, internal combustion engines, forest fires, cigarettes, boilers, space heaters. Processes that Produce Hydrocarbons 9 Petroleum production, refining, transport 9 Internal combustion engines 9 Natural processes--forests and plants (isoprene and terpenes) 9 Surface coatings A 1962 estimate of the contaminants discharged into the Los Angeles air during the summer period revealed that motor vehicles accounted for about 60%, while the use of organic solvents (for all purposes) accounted for about 18% of the organic gases. About one half of the organic solvent emitted was attributed to the coatings industry, chiefly to the use in paint and coatings. Approximately 66% of the NOx released was assigned to gasoline (motor vehicle) combustion, while the combustion of fuels (energy supply) accounted for about 26% [1]. Based on the results of laboratory studies in "smog chambers," in which a mixture of a solvent and nitrogen oxide was exposed for 6 h to light approximately the intensity of noon sunlight, the solvents could be classified as "low" or "high" in photochemical reactivity related to the amount of peroxides and ozone produced. These studies formed the basis for the well-known Rule 66, an air pollution control regulation passed by the Los Angeles Air Pollution Control District. Rule 66 identifies an "approved" solvent as one that contains less than 20% by volume of specific chemicals and is further limited to certain combinations of these chemicals. Thus, approved solvents can contain no more than designated amounts of the combinations shown in Table 2. In effect, Rule 66 promoted the use of specific solvents such as aliphatic and naphthenic hydrocarbons, alcohols, esters, normal ketones, chlorinated hydrocarbons (except trichloroethylene), and nitroparaffins. Rule 66, superseded in 1976 by Rule 442, Usage of Solvents, by the California South Coast Air Quality Management District, was subsequently adopted by various other state jurisdictions. Renewed interest has developed recently in the consideration of solvent photochemical

TABLE 2--Rule 66--Limits of solvent categories in approved mixtures.* 5% Hydrocarbons, alcohols, aldehydes, e s t e r s , ethers or ketones having an olefinic or cycIoolefinic unsaturation

8% Aromatic hydrocarbons (W/8 C atoms)

20% Ethylbenzene, branched ketones, toluene, or trichloroethane

*Calculated as the percent by volumeof the total solvent.

reactivity in state, federal, and international programs related to air quality control.

VOC Definition The United States Environmental Protection Agency (EPA) was created in 1970 by Congress as part of a plan to consolidate several federal environmental activities. Studies directed by the EPA laboratories in Research Triangle Park, NC of the photochemical reactivity of materials in a laboratory smog chamber revealed that when organic materials and nitrogen oxide were irradiated for periods of up to 36 h, even those solvents considered acceptable under Rule 66 reacted to form peroxides and ozone. Only a few materials showed negligible photochemical reactivity, among which were: methane, ethane, methylene chloride, 1,1,1-trichloroethane, and fluorinated compounds. These studies, which were prompted in part by the passage of the Clean Air Act of 1970, led to the conclusion that most organic compounds emitted to the atmosphere contribute to the formation of ozone. On this basis, EPA adopted as a regulatory objective the limit of essentially all volatile organic compounds emitted to the atmosphere from all sources, including paint and coatings applications [3].

Regulatory Definition o f VOC The regulatory definition of volatile organic compounds (VOC) was revised by EPA in 1992. A part of this definition is as follows: Section 51.100 Definitions 2 Volatile organic compounds (VOC) means any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions. (1) This includes any such organic compound other than the following, which have been determined to have negligible photochemical reactivity: methane; ethane; methylene chloride (dichloromethane); 1,1,1-trichloroethane (methyl chloroform); 1,1,1-trichloro-2,2,2-trifluoroethane (CFC-113); trichlorofluoromethane (CFC-11); dichlorodifluoromethane (CFC-12); chlorodifluoromethane (CFC-22); trifluoromethane (FC-23); 1,2-dichloro- 1,1,2,2-tetrafluoroethane (CFC-114); chloropentafluoroethane (CFC-115); 1,1,1-trifluoro 2,2dichloroethane (HCFC-123); 1,1,1,2-tetrafluoroethane (HF-134a); 1,1-dichloro 1-fluoroethane (HCFC-141b); 1chloro 1,1-difluoroethane (HCFC-142b); 2-chloro1,1,1,2-tetrafluoroethane (HCFC- 124); pentafluoroethane (HFC-125); 1,1,2,2-tetrafluoroethane (HFC-134); 1,1,1-trifluoroethane (HFC- 143a); 1,1-difluoroethane (HFC-152a); and perfluorocarbon compounds which fall into these classes: (i) Cyclic, branched, or linear, completely fluorinated alkanes; (ii) Cyclic, branched, or linear, completely fluorinated ethers with no unsaturations; 240 Code of Federal Regulations (CFR) Part 51; Requirements for Preparation, Adoption and Submittal of Implementation Plans; Approval and Promulgation of Implementation Plans. FederalRegister, Vol. 57, No. 22, 22 Feb. 1992, pp. 3941-3946.

CHAPTER 1--REGULATION

(iii) Cyclic, branched, or linear, completely fluorinated tertiary amines with no unsaturations; and (iv) Sulfur containing perfluorocarbons with no unsaturations and with sulfur bonds only to carbon and fluorine. (2) For purposes of determining compliance with emissions limits. VOC will be measured by the test methods in the approved State implementation plan (SIP) or 40 CFR part 60, appendix A, as applicable. Where such a method also measures compounds with negligible photochemical reactivity, these neglibflity-reactive compounds may be excluded as VOC if the amount of such compounds is accurately quantified and such exclusion is approved by the enforcement authority. (3) As a precondition to excluding these compounds as VOC or at any time thereafter, the enforcement authority may require an owner or operator to provide monitoring or testing methods and results demonstrating, to the satisfaction of the enforcement authority, the amount of negligibly-reactive compounds in the source's emissions. (4) For purposes of Federal enforcement for a specific source, the EPA will use the test methods specified in the applicable EPA-approved SIP in a permit issued pursuant to a program approved or promulgated under title V of the Act, or under 40 CFR part 5 I, subpart I or appendix S, or under 40 CFR parts 52 or 60. The EPA will not be bound by an State determination as to appropriate methods for testing or monitoring negligibly-reactive compounds if such determination is not reflected in any of the above provisions.

The Ozone Standard The Clean Air Act of 1970 targeted six criteria pollutants for control: carbon monoxide, lead, nitrogen dioxide, ozone, particulates, and sulfur dioxide. Criteria pollutants are those for which criteria were issued by EPA. These documents include national ambient air quality standards (NAAQS)--levels that protect against adverse effects to health and to plants and materials [4]. Standards for ozone and nitrogen oxides are: Ozone The ozone concentration in the atmosphere cannot exceed 0.12 p p m as a daily m a x i m u m one-hour average more than once per year.

Nitrogen Dioxide The nitrogen dioxide concentration in the atmosphere cannot exceed 0.053

OF VOC EMISSIONS

5

Control Technique Guidelines In 1977, the Agency issued the first of a series of guidance documents for the states related to various industrial coating operations or end-use categories. These documents, called "Control Technique Guidelines (CTG) Series, Control of Volatile Organic Emissions from Stationery Sources," include recommended VOC emission limits, based on EPA's assessment of Reasonably Available Control Technology (RACT): the limits are expressed as pounds of VOC per gallon of coating (minus water), as applied. The Clean Air Act Amendments of 1977 directed that states had to revise their implementation plans for areas out of compliance with the national ozone standard. The revised SIPs were to include sufficient control of VOC emissions from stationery sources, such controls to incorporate the RACT limits for coatings operations for which a CTG was published. The CTG documents relating to surface coatings operations issued through 1992 are shown in Table 3 with recommended limits for VOC content.

NEW SOURCE PERFORMANCE STANDARDS The control of VOC emissions from new coatings plants and from significant modifications of existing plants was addressed by EPA in a series of New Source Performance Standards (NSPS), the first of which issued in 1980. These mandatory standards, which apply uniformly to all parts of the country, define the emission sources more narrowly and impose a tighter level of emission control than that for related existing sources. The VOC limits defined in the NSPS, expressed as kilograms of VOC per liter of applied solids, are based on the best demonstrated technology (BDT) for the specific coating operation. The New Source Performance Standards for surface coatings operations issued through 1992 are shown in Table 4. The emission limits in both the CTG and NSPS documents, in the majority of cases, focus on restricting the VOC content per unit of coating or of coating solids applied in the operation, rather than placing a ceiling on individual plant emissions. The responsibility for establishing emission limits for particular plants, if appropriate, was left to the states [5].

p p m as the annual arithmetic mean concentration.

D E T E R M I N A T I O N OF VOC C O N T E N T CONTROL OF VOC E M I S S I O N S F R O M COATINGS The Clean Air Act addressed air pollution eminating from both existing sources and that from future new plant construction or significant modification of existing sources. States with areas that did not comply with the ozone standard were given primary responsibility to develop appropriate regulations for existing sources to meet the time schedule for compliance specified by Congress. The Federal EPA was assigned oversight responsibility for the state programs that were described in "State Implementation Plans" (SIP).

Federal Reference Method 24 The procedures specified by the federal EPA for testing paint products for compliance with VOC limits are described in Federal Reference Method 24 [6]. This standard employs several ASTM test standards, including those shown in Table 5. Method D 2369 is a key procedure of Federal Method 24. Since 1980, several important revisions have been made in this standard to make it compatible with revisions in Method 24, including the addition in 1990 of instructions for testing multicomponent coatings and the deletion of sections dealing with testing at shorter times. The revised version of Federal

6

PAINT AND COATING TESTING MANUAL TABLE 3--VOC content limits in control technique guidelines (CTG) for surface coating operations. Allowable Limitsb Minus H20

Kg VOC/L Minus H20

2.8 1.2 1.9 2.8 2.8 4.8 2.8 4.2 5.5 3.7 2.8 3.8

0.34 0.14 0.23 0.34 0.34 0.58 0.34 0.51 0.66 0.44 0.34 0.45

1.7 2.6 3.0 3.5

0.20 0.31 0.36 0.42

Clear coat Extreme performance Powder coatings All others

4.3 3.5 0.4 3.0 2.9

0.52 0.42 0.05 0.36 0.35

Vinyl

3.8

0.45

Lb VOC/Gal Coatings Operation

CTG Date"

Appliances, large Auto and light duty trucks

Dec., 1977 May, 1977

Cans

May, 1977

Fabric

May, 1977

Graphic arts--rotogravure and flexography Magnetic tape Magnet wire Metal coil Metal furniture Miscellaneous metal parts and products

Dec., 1978

Paper, film and foil Plastic parts for business machines Polymeric coatings of supporting substrates Pressure sensitive tapes and labels Vinyl and urethane, flexible

Wood paneling, flat

See Paper coating Dec., 1977 May, 1977 Dec., 1977 June, 1978

May, 1977 None None; may be considered fabric coating See Paper coating Fabric: May 1977 and/or Graphic Arts Packaging Rotogravure, Dec. 1978 June, 1978

Primer, electrodeposit Prime coat Guidecoat (surfacer) Topcoat Final repair Sheet basecoat Interior body spray Side seam End seal compound Fabric coating Vinyl coating (Consult CTG or state regulations) (Based on the use of an incinerator) Prime and topcoat or single coat Air dry

Printed interior panels: 6.0 lb/1000 sq. fl of surface coated Natural finish plywood: 12.0 lb/ 1000 sq. ft of surface coated Class II finishes 10.0 lb/1000 sq. ft of surface coated

NOTE:The information presented in this table is not complete. Persons subject to emission control for any of the operations are advisedto consult the state/local regulations for details. aCTG documents are available from the National Technical Information Service, 5285 Port Royal Road, Springfield,VA 22161. bReasonably available control technology (RACT)limits recommended in CTG and, in most cases, adopted in state/local regulations. Reference M e t h o d 24 is also included in the ASTM Manual on Determination o f Volatile Organic Compound (VOC) Content in Paints, Inks, and Related Coating Products, 2nd ed., 1993 [7]. Substantial revisions during 1989-1991 were also m a d e in ASTM D 3960, Practice for D e t e r m i n i n g Volatile Organic C o m p o u n d (VOC) Content of Paints and Related Coatings, a standard developed in ASTM S u b c o m m i t t e e D01.21 to provide a guide for the calculation of VOC and to establish a base for the investigation in ASTM of the precision of VOC co n t en t determination. The definitions a n d symbols used in D 3960 are those ad o p t ed by the EPA and included in the Agency d o c u m e n t "Procedures for Certifying Quantity of Volatile Organic C o m p o u n d s E m i t t e d by Paint, Ink a n d Other Coatings" that was published in 1984 [8]. The general expression for calculating VOC c o n t e n t in gr a m s p er liter of coating less w a t e r and e x e m p t solvent specified in the EPA Control T e c h n i q u e Guidelines issued t h r o u g h 1991 is:

Weight % total volatiles less w a t e r less | (Density of coating) e x e m p t solvent ] VOC = (Volume% 1 _ ( Volume% 1 100% - \ w at er ] \ e x e m p t solvent] or

voc

-

(Wo)(Oc) 100%

-

Vw -

vex

(W~ - W~ - W~x)(Dr 100%-

(1)

(Ww)(Dc/D~)- (W~)(D~/Dr

where VOC = VOC co n t en t in g/L of coating less w a t e r and exe m p t solvent, 141o = weight % of organic volatiles = Wv - Ww - Wex, Wu = weight % of total volatiles = (100% - weight % nonvolatiles), (ASTM D 2369), Ww = weight % of w at er (ASTM D 3792 or ASTM D 4017), 14~x = weight % of e x e m p t solvent (ASTM D 4457),

CHAPTER 1--REGULATION OF VOC EMISSIONS

7

TABLE 4 - - V O C limits in New Source Performance (NSPS) for surface coatings operations. Allowable Limitsb Coatings Operation Appliances, large Auto and light duty trucks

Cans (beverage cans only)

Fabric (coating)

NSPS Date~ Oct.,1982 Dec., 1980

Graphic a r t s - - r o t o g r a v u r e and flexography Magnetic tape Magnet wire Metal coil

None Nov., 1982

Polymeric coatings of supporting substrates Pressure sensitive tapes Vinyl and urethane, flexible Wood paneling, flat

Kg VOC/L Applied Solids

7.5 Prime coat 1.3 Guide coat 11.7 Top coat 12.2 Exterior base 2.4 Clear base coat 3.8 Inside spray 7.4

Aug.,1983

See Polymeric coating of supporting substrate Rotogravure only Nov., 1982 Oct,, 1988

Metal furniture Miscellaneous metal parts and products Plastic parts for business machines

Lb VOC/Gal Applied Solids

0.90 0.16 1.40 1.47 0.29 0.46 0.89

Consult NSPS d o c u m e n t 1.7 Consult NSPS

0.2

. . . . w/o emission control device 2.3 With emission control device 1.2 7.5 . . . .

Oct., 1982; Apr., 1985 None Jan., 1988

Prime a n d color coat 12.52 Texture and touch-up 19,2 90% control from process: Consult NSPS 1.67 8.3 . . .

Sept., 1989 Oct., 1983 June, 1984 None

.

. 0.28 0.15 0.90

.

. 1.5 2.3 0.20 1.0

.

.

.

NOTE: The information presented in this table is not complete. Persons subject to emission control for any of the operations are adv/sed to consult the specific language of the referenced documents and state and local regulations. ~NSPS documents are available from the National Technical Information Service (NTIS), 5285 Port Royal Road, Springfield, VA 22161. bBest demonstrated technology (BDT) emission limits established as NSPS standards. In the NSPS, the limits are expressed as kilograms of VOC per liter of applied solids.

TABLE 5 - - A S T M standards referenced in Federal Reference Method 24. ASTM Method

Test Method f o r -

D 2369-81

Volatile Content of Coatings

D 1475-60

Density of Paint, Varnish, Lacquer, and Related Products

D 3792-79

Water Content of Waterborne Paints by Direct Injection into a Gas Chromatograph

D 4017-81

Water in Paints and Paint Materials by Karl Fischer Method

D 4457-85

Analysis of Dichloromethane a n d 1,1,1Trichloroethane in Paints and Coatings by Direct Injection into a Gas Chromatograph

Vw Vex Dc Dw D,x

= = = = =

volume volume density density density 1475).

% o f w a t e r -- (Ww)(Dc/Dw), % of e x e m p t s o l v e n t = (Wex)(Dc/Dex), o f c o a t i n g a t 25~ i n g/L ( A S T M D 1475), o f w a t e r a t 25~ i n g/L -- 0 . 9 9 7 • 103, a n d of e x e m p t s o l v e n t a t 25~ i n g/L ( A S T M D

T o c o n v e r t f r o m g/L t o lb/gal, m u l t i p l y t h e r e s u l t (VOC c o n t e n t ) b y 8.345 • 10 -3 (lb/gal)/g/L). T o c o n v e r t g/L t o kg/L, d i v i d e t h e r e s u l t b y 10 a. The general expression for VOC content defined in terms of the mass of VOC per unit volume of coating solids applied as s p e c i f i e d i n t h e E P A N e w S o u r c e P e r f o r m a n c e S t a n d a r d s is VOCm =

( W , - Ww - We~)Dc

v.

where V O C m = V O C c o n t e n t i n g/L o f c o a t i n g solids, a n d

(2)

8 PAINT AND COATING TESTING MANUAL V, = Volume % nonvolatile content of the liquid coating, ASTM D 2697. 3 The EPA would have preferred to limit volatile organic compound emissions in the Control Technique Guidelines on the basis of the unit volume of coating solids applied. The adoption in the 1970s of Eq 1, in which VOC content is defined as mass per unit volume of coating less water and less exempt solvents, was necessary as no acceptable consensus procedure was available for determining the volume percent nonvolatile content. In a presentation in Copenhagen in 1990, James C. Berry of U.S. EPA stated: "Though certainly less than ideal, the major attraction is that the expression permits the determination of compliance from the analysis of a coating sample obtained during a plant inspection. In the simplest case, these units require only one volumetric and one gravimetric measurement" [5]. Studies and discussions in ASTM Subcommittee D01.21 that led to the modification and improvements of ASTM standards referenced in Federal Method 24 and in ASTM Practice D 3960 were conducted with the cooperation of EPA personnel of the Office of Air Quality Standards Development at Research Triangle Park, NC.

OTHER VOC-RELATED METHODS STUDIES

AND

ASTM development activity on other VOC-related standards has expanded significantly since 1980. Many of the standards listed in this section have not been approved by the Federal EPA for use in demonstration of compliance with VOC emission control regulations. Use of any of these standards to demonstrate compliance should be coordinated with appropriate regulatory agencies. Among the new standards developed or in process of development are the following:

S t a n d a r d s Specific to t h e A u t o m o b i l e I n d u s t r y 9 D 5087 Test Method for Determining the Amount of Volatile Organic Compounds (VOC) Released from Solvent-Borne Automotive Coatings and Available for Removal in a VOC Control Device (Abatement) 9 D 5066 Practice for the Determination of the Transfer Efficiency Under Production Conditions for Spray Application of Automotive Paints--Weight Basis 9 D 5009 Test Method for Evaluating and Comparing Transfer Efficiency Under Laboratory Conditions These standards were developed with the cooperation of representatives from automotive coating suppliers and the Motor Vehicle Manufacturers Association. Method D 5009 was derived from a study of transfer efficiency conducted for the U.S. Environmental Protection Agency [9].

Masonry Treatments 9 D 5095 Test Method for Determination of the Nonvolatile Content in Silanes, Siloxanesl and Silane-Siloxane 3EPAReference Method 24 does not include an analytical method for determining V~, but states that the value be calculated from the coating manufacturer's formulation.

Blends Used in Masonry Water-Repellent Treatments. 4 In this standard, a catalyst is added prior to the bake cycle to simulate the catalytic effect provided by masonry during actual application of the water-repellent treatment.

Aerosol Spray Paints 9 D 5200 Test Method for Determination of Weight Percent Volatile Content of Solvent-borne Paints in Aerosol Cans 9 D 5325 Test Method for the Determination of Weight Percent Volatile Content of Water-borne Aerosol Paints These standards were developed for potential use related to proposed regulations in California to limit the level of volatile organic material in aerosol paints.

General Application Standards 9 D 5201 Practice for Calculating Formulation Physical Constants of Liquid Paints and Coatings The calculation of various physical constants directly from the paint formulation is a common practice in industry. ASTM D 5201 describes procedures for the calculation of formulation weight solids, volume solids, solvent content, and density of liquid paint based on formulation data (not analytical laboratory determinations). The values obtained may not be acceptable for demonstrating regulatory compliance. 9 D 5286Test Method for Determination of Transfer Efficiency Under General Production Conditions for Spray Application of Paints This standard, a modification of Practice D 5066 developed for use in the automobile industry, describes conditions for determining transfer efficiency under production conditions applicable to spray application of miscellaneous parts. 9 D 5327 Practice for Evaluating and Comparing Transfer Efficiency under General Laboratory Conditions Practice D 5327 provides a useful guide for general research studies related to transfer efficiency. The general approach employed is derived from that developed in Method D 5009 except that D 5327 employs a fixed rather than moving spray station. 9 Revision of D 2697, Test Method for Volume Nonvolatile Matter in Clear or Pigmented Coatings: Use of the Helium Gas Pycnometer (Under Study) The use of the helium gas pycnometer provides a quick, reliable approach to the determination of the dry coating density, a critical parameter in the calculation of volume percent nonvolatile content. 9 Direct Measurement of Volatile Organic Material in WaterReducible Coatings (Under Study) Federal EPA funds supported the preliminary investigation of this novel approach to the "direct" gravimetric determina4This standard has been accepted by the Federal EPA for use in the determination of the VOC of silane- and siloxane-based coatings: Letter, Gary McAllister, EPA, to H. Smith, NJ Dept. of Environmental Protection, 25 March 1992.

CHAPTER 1 - - R E G U L A T I O N OF VOC E M I S S I O N S tion of volatile organic content of waterborne coatings [10]. The method involves collecting, on activated charcoal in weighed tubes, the organic effluent evolved on heating a paint specimen for 1 h at 110~ while purging the reaction vessel with dry nitrogen. Methanol is not captured on the charcoal. 9 D 5403 Test Method for Volatile Content of Radiation Curable Materials

9

tants; reduction of acid rain; and the protection of ozone in the stratosphere. Features of the Act that will impact most on the coatings industry include:

Title I m O z o n e Control in the Atmosphere

The test methods in D 5403 determine the weight percent volatile content of paint, coatings, and inks that are designed to be cured by exposure to ultraviolet light or to a beam of accelerated electrons. After radiation cure, the specimens are baked at 110 + 5~ for 60 min.

Title I specifically directs EPA to develop control technique guidelines and maximum achievable control technology (MACT) standards for aerospace coatings and for shipbuilding coatings and repair. EPA was also directed to prepare new control technique guidelines for additional coatings uses that include:

Inks

9 9 9 9 9

9 D 5328Volatile Organic Compound (VOC40) Content of Non-Heatset Paste Printing Ink Systems at 40~ This standard is patterned, in part, after Method 30 of California's Bay Area Air Pollution Control District in which the specimen is baked for 1 h at 40~ D 5328 is applicable to paste printing inks and vehicles that dry primarily by absorption, polymerization, or related means without the application of heat.

Supplementary Information Further information about the development, significance, and limitations of these VOC-related ASTM standards as well as about the use of ASTM standards for the demonstration of compliance with VOC emission control regulations is available in the ASTM Manual on Determination of Volatile Or-

ganic Compounds in Paints, Inks, and Related Coating Products [7]. Attachments in the second edition of this manual include the 1992 revision of Federal Reference Number 24 and a publication from EPA's Emission Standards Division titled "Procedure for Certifying Quantity of Volatile Organic Compounds Emitted by Paint, Ink and Other Coatings" [8]. Included in the latter publication are "VOC Data Sheets" applicable to coatings "as supplied" by the manufacturer and for coatings "as applied" by the user. The form used for "as supplied" coatings is patterned after a recommendation of the National Paint and Coatings Association.

CLEAN AIR ACT A M E N D M E N T S OF 1 9 9 0 The Clean Air Act Amendments of 1990 defined a comprehensive long-term approach "to achieve and maintain a healthy environment while supporting a strong and sustainable economic growth and sound energy policy.''s A major impetus for these amendments was the continued inability of a number of heavily populated urban areas to meet the requirements of the national ambient air quality standards for ozone and carbon monoxide. Among the specific issues addressed in the Act are: control of ozone in the atmosphere; control of hazardous air pollu5U.S. EPA Office of Air and Radiation, "Implementation Principles for the Clean Air Amendments of 1990."

autobody refinishing plastic parts (business machines) plastic parts (others) offset lithography wood furniture

In addition, EPA plans to promulgate national rules to control VOC emissions from architectural and industrial maintenance coatings and from traffic paints. Solvent emission from consumer and commercial products, including that from aerosols, are currently under study, and regulation of this broad category of products is planned. In the development of these rules, negotiated rulemaking may be employed, a process bringing together representatives of EPA, industry, the states, and environmentalist groups to negotiate the content of a proposed rule. In CAAA-90, ozone nonattainment areas are placed in five classifications based on the mid-1991 ozone level (Table 6), and compliance with the national ozone standard by specific years is mandated in the law. Increasingly strict provisions, including further reduction of VOC emissions, will be imposed on areas, the magnitude to be related to the severity of the ozone problem. Depending on the area classification, several or all of the following will be required: 9 Increased monitoring and more accurate VOC and N O 2 emission inventory 9 Revision of state implementation plans to incorporate RACT limits from previous and future CTGs for all major stationery sources 9 New source review and permits for new or modified stationery sources 9 Reduced emission threshold levels for the definition of major stationery sources, ranging between 10 tons/year for severe classification areas to I00 for marginal or moderate areas TABLE 6--Clean air act amendments--1990 ozone nonattainment area classifications. Classification Marginal Moderate Serious Severe Extreme~

Design Value (Ozone Level)

Years to Achieve Compliance, year

0.121-0.138 0.138-0.160 0.160-0.180 0.180-0.280 0.280 and above

3 (1993) 6 (1996) 9 (1999) 15 (2005) 20 (2010)

~Onlythe Los Angelesarea is in this classification.

10 PAINT AND COATING TESTING MANUAL 9 Higher VOC emission offset r e q u i r e m e n t s for new or modified sources

of 1995 sufficient categories a n d subcategories m u s t be listed to ensure that 90% of the area sources that emit the 30 most hazardous air pollutants are subjected to regulation.

Title III--Air Toxics Program U n d e r Title III, EPA is directed to evaluate a n d control the emission of hazardous air pollutants (HAPS). 189 products are identified in the Act, a n d EPA has the authority to delete or add additional products to this list. I n d u s t r y groups m a y petition EPA to delist products. Among the materials included o n the initial HAP list that are used in paints a n d coatings are those shown in Table 7. The control of emissions of hazardous air pollutants is to be achieved t h r o u g h the p r o m u l g a t i o n of emission standards for source categories a n d subcategories that emit these products. The initial list of categories of sources published by EPA (57FR31576, 16 July 1992) included u n d e r Surface Coating Processes those processes for which a CTG or n a t i o n a l rule has b e e n issued or is planned. Also included is the Manufacture of Paints, Coatings and Adhesives. A draft timetable for regulating the categories of sources of hazardous air pollutants was published by EPA in 1992 (57FR44147, 24 Sept. 1992). By the end of 1994 emission standards were due for the following surface coating processes: magnetic tapes, printing/publishing, shipbuilding a n d ship repair, a n d wood furniture. The Agency was required to p u b l i s h emission limits based o n m a x i m u m achievable control technology (MACT) for 40 of these categories by the end of 1992, with MACT limits to be identified for the r e m a i n i n g categories by 2000. The Act directs that the health impact a n d economic factors he considered in defining appropriate MACT limits. Further, by the end TABLE 7--Clean air act amendments--1990 selected hazardous air pollutants used in paints and coatings. ORGANICMATERIALS Bis (2-ethylhexyl phthalate) Dibutyl phthalate Diethanolamine Dimethyl formamide Dimethyl phthalate Ethylene glycol Formaldehyde Glycol ethers (ethylene oxide-based) Methanol 1,1,1-trichloroethane (methyl chloroform)~ Methylene chloride~ Methyl ethyl ketonea Methyl isobutyl ketone~ 2-nitropropane Styrene Toluenea Xylenes~ INORGANICANDOTHER Ammonia Antimony compounds Cadmium compounds~ Chromium compoundsa Cobalt compounds Lead compounds~ Mercury compounds~ ~Materials included in the EPA/Industry33/50 Project, a voluntaryindustry initiative to reduce the total release and transfer of 17 targeted chemicals by one third by the end of 1992and by one half by the end of 1995 (using 1988as a baseline year).

Title V - - S t a t e Operating Permit Program The state operating p e r m i t p r o g r a m is considered by EPA as a cornerstone of the CAAA-90 a m e n d m e n t s designed to ensure that the ozone n o n a t t a i n m e n t areas meet compliance deadlines. This p r o g r a m will impact o n m a n y previously unregulated coatings m a n u f a c t u r e r s a n d users. The final rule re the operating p e r m i t p r o g r a m was issued in 1992 (57FR32250, 21 July 1992). The operating permit p r o g r a m has been called the "air pollution equivalent" of the NPDES permit p r o g r a m of the Clean Water Act, u n d e r which operating permits are required of sources that discharge pollutants to water. I n the p r o g r a m u n d e r Title V, all federal a n d state air pollution rules a n d regulations will be consolidated u n d e r a single d o c u m e n t wherein the states are given authority to m o n i t o r a n d enforce the regulations. Sufficient funds will be available to the states from a m i n i m u m a n n u a l fee of $25 per ton for each regulated pollutant emitted, the fee to be assessed against all m a j o r sources. Major sources required to have state operating permits include those that emit 10 tons or more per year of a single regulated hazardous air pollutant or 25 tons per year of a c o m b i n a t i o n of hazardous air pollutants. These pollutants include those materials for which a n a t i o n a l emission standard (NESHAP) has been established. Under Title V of CAAA-90, a n d the final rule on operating p e r m i t programs, EPA is to approve (or disapprove) state permit programs w i t h i n one year of receipt; the m a j o r sources m u s t apply for the five-year p e r m i t within one year of the EPA's approval of the state program, a n d all permits m u s t be issued a n d be legally b i n d i n g by the e n d of 1997. U n d e r the rule, states have the option of exempting all n o n m a j o r sources, with some exceptions, from requiring a permit for five years after the state p e r m i t p r o g r a m is approved by EPA. The characterization of a m a j o r source in ozone nona t t a i n m e n t areas is also based o n the a m o u n t of volatile organic c o m p o u n d s emitted annually. The threshold a m o u n t is related to the area classification a n d sources in ozone n o n a t t a i n m e n t areas that emit above the designated a m o u n t of VOC shown in Table 8 are identified as m a j o r sources. These limits vary between 10 tons/year for the "extreme" classification to 100 tons/year for the "marginal or moderate" classification. For ozone transport regions (e.g., one is established in the Northeast), a threshold limit of 50 tons/year of VOC emission applies.

TABLE 8--Clean air act amendments--1990 major source identification based on VOC emissions: limits for area classifications. Ozone Nonattainment Area Classification

VOC Emission Limit, tons/year

Marginal or moderate Serious Severe Extreme

100 50 25 10

CHAPTER 1 - - R E G U L A T I O N OF VOC E M I S S I O N S TABLE 9--Code of federal regulations subchapter topics. Subehapter

Subject

Parts

C

Air Programs New Source Performance Standards Water Programs Solid Waste Superfund/Right-to-Know Effluent Guidelines and Standards Toxic Substances Control Act

50-87 60 104-149 240-281 300- 372 401-471 700-799

D I J N R

TABLE 10--Control technique guidelines and surface coating operations reference documents. EPA Document Reference EPA-450/2-77-008

Coating Operation Vol. II

Auto and light duty trucks Cans Fabric Metal Coil Paper, film, and foil Vol. III Metal furniture Vol. IV Magnet wire Vol. V Appliances, large Vol. VI Miscellaneous metal parts and products Vol. VII Wood paneling, flat Vol. VIII Graphic arts--rotogravure and flexography Vinyl and urethane, flexible

EPA-450/2-77-032 EPA-450/2-77-033 EPA-450/2-77-034 EPA-450/2-78-015 EPA-450/2-78-032 EPA-450/2-78-033

Title VI--Stratospheric

Ozone Protection

The m o s t significant feature of the p r o g r a m to protect ozone in the s t r a t o s p h e r e is the staged p h a s e o u t of 1,1,1trichloroethane, a m a t e r i a l widely used in coatings a n d classified as a "VOC-exempt" solvent by the EPA. P r o d u c t i o n (and use) will be r e d u c e d in i n c r e m e n t s (from the 1989 a m o u n t ) b e g i n n i n g in 1993 to a 50% level for the p e r i o d 1996-1999, t h e n to the 20% level for the p e r i o d 2000-2001, after w h i c h the use of the m a t e r i a l will be prohibited.

Title VII--Enforcement EPA is g r a n t e d b r o a d n e w a u t h o r i t y to i m p o s e penalties a n d substantial fines for various actions including: violations of the State I m p l e m e n t a t i o n Plan; violation of s o m e of the o p e r a t i n g p e r m i t provisions; a n d false s t a t e m e n t s in records, m o n i t o r i n g data, a n d reports. Also i n c l u d e d are provisions for field citations b y inspectors.

Scenario for the 1990s CAAA-90 a n d the m y r i a d of n e w federal a n d state regulations associated with i m p l e m e n t a t i o n of this c o m p r e h e n s i v e law that will issue d u r i n g the 1990s will have a m a j o r i m p a c t on the coatings industry. A m o n g the m a n y u n c e r t a i n t i e s are the n a t u r e a n d level of MACT limits to be defined for essentially all coating operations; the level of new o r stricter VOC

TABLE I 1--Regional offices, U.S. Environmental Protection Agency. Region

States Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, Vermont New Jersey, New York, Puerto Rico, Virgin Islands Delaware, Maryland, Pennsylvania, Virginia, West Virginia, District of Columbia Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, Tennessee Illinois, Indiana, Michigan, Minnesota, Ohio, Wisconsin Arkansas, Louisiana, New Mexico, Oklahoma, Texas Iowa, Kansas, Missouri, Nebraska Colorado, Montana, North Dakota, South Dakota, Utah, Wyoming American Somoa, Arizona, Nevada, Hawaii, Guam, California

10

Alaska, Idaho, Oregon, Washington

11

Address J. F. Kennedy Federal Bldg. Room 2203, Boston, MA 02203 Phone (617) 565-3715 26 Federal Plaza New York. NY 10278 Phone (212) 264-2515 841 Chestnut St, Philadelphia, PA 19107 Phone (800) 438-2474 345 Courtland St. NE Atlanta, GA 30365 Phone (800) 282-0239 in GA (800) 241-1754 in other Region 4 states 230 S. Dearborn St. Chicago, IL 60604 Phone (800) 572-2515 in IL (800) 621-8431 in other Region 5 states 1445 Ross Ave. 12th Floor, Suite 1200 Dallas, TX 75202 Phone (214) 655-2200 726 Minnesota Ave. Kansas City, KS 66101 Phone (913) 236-2803 999 18th St. Suite 500 Denver, CO 80202 Phone (800) 759-4372 215 Fremont St. San Francisco, CA 94105 Phone (415) 974-8076 1200 6th Ave. Seattle, WA 98101 Phone (206) 442-5810

12

PAINT AND COATING TESTING MANUAL

e m i s s i o n limits for coating operations; a n d the time, m a n p o w e r , a n d cost a s s o c i a t e d with c o m p l y i n g with the m a n y new regulations a s s o c i a t e d with the a m e n d m e n t s . As did the decades of the 1970s a n d 1980s, the 1990s will pose a c o n t i n u i n g challenge to r a w m a t e r i a l suppliers to develop a n d provide e n v i r o n m e n t a l l y acceptable m a t e r i a l s as well as to p a i n t f o r m u l a t o r s to develop new o r modified coatings with r e d u c e d VOC content. Additionally, i n c r e a s e d attention to the i m p r o v e m e n t of coating processes a n d to the use of a b a t e m e n t e q u i p m e n t for e m i s s i o n control d u r i n g the a p p l i c a t i o n of coatings is expected.

U.S. EPA regional offices o r to the specific state regulating b o d y responsible for air quality control. The U.S. E n v i r o n m e n t a l Protection Agency has established ten regional offices, each responsible for several states (Table 11). A m o n g the i n d u s t r y o r g a n i z a t i o n s that provide information to their m e m b e r s h i p a b o u t p e n d i n g regulations a n d g u i d a n c e on c o m p l i a n c e with finalized regulations are Chemical M a n u f a c t u r e r s Association (CMA), N a t i o n a l Paint a n d Coatings Association (NPCA), Dry Colour M a n u f a c t u r e r ' s Association (DCMA), a n d Chemical Specialty M a n u f a c t u r e r s Association (CSMA). Several coatings j o u r n a l s p u b l i s h excerpts from regulations a n d s u m m a r y reviews.

REGULATION INFORMATION Published Sources F e d e r a l e n v i r o n m e n t a l regulations, including those prom u l g a t e d u n d e r the Clean Air Act, are p u b l i s h e d in the Code of Federal Regulations (CFR), a series of b o o k s that are generally available in m a j o r libraries a n d law libraries. These regulations as well as those of related state a n d local codes are also o b t a i n a b l e from the associated r e g u l a t o r y offices. Regulations of p a r t i c u l a r interest to the coatings i n d u s t r y can be found in s u b c h a p t e r s of the Code of Federal Regulations (Table 9). The F e d e r a l Control Technique Guidelines for coating operations are not included in the CFR, b u t are available from the National Technical I n f o r m a t i o n Service (NTIS), 5285 Port Royal Road, Springfield, VA 22161. CTG d o c u m e n t s t h r o u g h 1991 are i n c l u d e d in the EPA p u b l i c a t i o n s listed in Table 10. P r o p o s e d regulations are p u b l i s h e d by the EPA in the Federal Register. Typically, a p u b l i c (written) c o m m e n t p e r i o d of 30 to 90 days on the p r o p o s a l s is allowed, a n d often a public h e a r i n g is s c h e d u l e d at w h i c h oral c o m m e n t s can be presented. The c o m m e n t s received are c o n s i d e r e d by the Agency in the d e v e l o p m e n t of a final regulation that is p u b l i s h e d in the Federal Register together with the r e g u l a t i o n c o m p l i a n c e date.

Information S o u r c e s Questions relating to the interpretation, applicability, a n d c o m p l i a n c e to air quality regulations m a y be a d d r e s s e d to the

REFERENCES [1] Scofield, F. in Paint Testing Manual, 13th ed., American Society for Testing and Materials, Philadelphia, 1972, p. 413. [2] Gordon, J., "Solvent Restriction, Problem or Opportunity," presentation to the Chicago Coatings Society, 13 Nov. 1978. [3] EPA Policy Statement, Recommended Policy on Control of Volatile Organic Compounds, FederalRegister, 8 July 1977. [4] "Glossary for Air Pollution Control of Industrial Coating Operations," EPA-450/3-83-013R, Environmental Protection Agency, Washington, DC, December 1983. [5] Berry, J. C., U.S. EPA, "Control of Volatile Organic Compound (VOC) Emissions from Painting Operations in the United States," presentation at the International Symposium on Paint and the Environment, Copenhagen, 12-14 Nov. 1990. [6] Code of Federal Regulations, Vol. 40, Subchapter C., Part 60, Appendix A; Amendments in a Rule published in the Federal Register, Vol. 57, No. 133, 10 July 1992, pp. 30654-30656. [7] Manual on Determination of Volatile Organic Compounds in Paints, Inks, and Related Coating Products, ASTM Manual Series, MNL4, 1989, 2nd ed., 1993. [8] Procedures for Certifying Quantity of Volatile Organic Compounds Emitted by Paint, Ink and Other Coatings, EPA-450/3-84019, Environmental Protection Agency, Washington, DC, December 1984. [9] Development of Proposed Standard Test Method for Spray Painting TransferEfficiency, Vols. I and II, EPA Publication Nos. EPA600/2-88-026a and EPA-600/2-88-026b, Environmental Protection Agency, Research Triangle Park, NC. [10] Method Development for Measuring the VOC Content of WaterBased Coatings, EPA Contract No. 68D90055, Work Assignments No. 28 and 40, Environmental Protection Agency, Research Triangle Park, NC.

Part 2: Naturally Occurring Materials

MNL17-EB/Jun. 1995

Bituminous Coatings by Ben J. Carlozzo 1

INTRODUCTION General Overview IN THE UNITED STATES,the terms "bituminous" and "asphaltic" are often used interchangeably. In Europe, bitumen refers to the mixture of heavy hydrocarbons, free of inorganic impurities. Asphalt is often considered the impure form of the generic material [1]. For our purposes, the ASTM definitions will be used. ASTM Definitions of Terms Relating to Roofing, Waterproofing, and Bituminous Materials (D 1079-87a) [2] defines bitumen as either "...(1) a class of amorphous black or dark colored (solid, semi-solid, or viscous) cementitious substances, natural or manufactured, composed principally of high molecular weight hydrocarbons, soluble in carbon disulfide, and found in asphalts, tars, pitches and asphaltites; or, (2) a generic term used to denote any material composed principally of bitumen." Asphalt is similarly defined as " . . . a dark brown to black cementitious material in which the predominating constituents are bitumens which occur in nature or are obtained in petroleum processing." While the term has historically implied the natural deposits (the Trinidad Lake asphalts on the Island of Trinidad or the Bermudez Lake, Venezuela asphalts), most asphalt used in the United States today for coatings applications i s from petroleum processing [3].

History and Background of Bitumens As one of man's oldest engineering materials, the adhesive and waterproofing properties of bitumen have been known since the earliest days of civilization. The area between the Tigris and Euphrates rivers in Iraq, long believed to be "the cradle of civilization," contains the earliest deposits of asphalt and heavy liquid petroleum. Early historical and biblical accounts tell of the use of asphalt in shipbuilding and foundation mortars. The Egyptians were known to have used asphalt in the mummification process; in fact, the part-Persian word for asphalt, "mumiya," is where our word "mummy" is derived [4]. The first asphalts produced in the United States were derived from California crude oils in the late 19th century. A straight run distillation, often without steam, was able to produce a good-quality material suitable for paving work. 1Mameco International, Inc., Cleveland, OH. Copyright9 1995 by ASTM International

Most of this material was competing with foreign imports from the Lake Trinidad region on the Isle of Trinidad off the north coast of South America. In the early 20th century, Mexican asphalt obtained from Mexican crude oil was used extensively in the eastern United States and gained a reputation as a high-grade standard paving bitumen. Today, asphalts are found throughout the world in several natural deposits of soft bituminous material or as hard, glassy, black bitumen associated with certain rock formations or impregnating various limestone or sandstone-type formations. Additionally, asphalts are derived from colloidally dispersed asphalt hydrocarbons in crude petroleum. This leads to the classification of bitumens into two classes: (1) natural asphalts (bitumens) and (2) artificial or oil asphalts (petroleum asphalts). The purity of bituminous materials is generally related to the degree that they are soluble in certain organic solvents. For years, the degree of solubility in carbon disulfide (CS2) has been a typical method for determining the purity of natural asphalts. ASTM Test Method for Bitumen Content (D 4-86) formalizes this procedure with CS2 solubility as the primary screening test. Most oil asphalts are generally greater than 99% soluble in CS2. The natural asphalts can be further classified by the geographical region of their origin, as well as the extent to which impurities are present, for example, Trinidad, refined, of approximately 50 to 57% bitumen; Cuban, refined, of 80 to 90% purity; Bermudez, refined, of 85 to 92% purity; and various rock asphalts, i.e., limestone, sandstone, tar sands, etc., with varying degrees of bitumen content. A separate class of natural bitumens are the asphaltites. These are also called the solid bitumens and are asphalts without impurities (silts, clays, salts, etc.), although their degree of CS2 solubility varies. Examples of these materials are Gilsonite, grahamite, glance pitch, or manjak, as well as harder materials that show no softening point, such as the pyrobitumens. The most important of these for coatings applications is Gilsonite. Artificial bitumens have been classified into three major groups [5]: 1. Oil or petroleum asphalts are soft to hard asphalts of high solubility in carbon disulfide (more than 99%) and are classed as pure bitumens. They are obtained from the vacu u m or steam distillation of crude oils containing high asphalt content. The distillation concentrates the colloidally dispersed asphalt into the "still bottoms" or "residuum" and is often a solid material.

15 www.astm.org

16 PAINT AND COATING TESTING MANUAL Precipitation methods are also used to recover asphalt from raw lubricating oils. This de-asphalting operation uses propane or other low-boiling hydrocarbons. The materials produced are the so-called asphaltic resins, with the hard, high asphaltene asphalts as the precipitate. Variations are made by controlling the propane stream. Oxidized or "blown" asphalts are obtained by blowing air at high temperatures through soft or liquid petroleum residues. This procedure can take semi-asphaltic materials of low purity and produce considerable amounts of bitumen. The resulting material is harder, with a higher softening point. 2. "Cracked" asphalts are also petroleum derivatives, but are obtained from by-products in oil-cracking processes. Residues are distilled to produce asphalt. They are variable in composition and may contain a certain amount of uncracked paraffinic material. These materials represent asphaltic hydrocarbons approximately intermediate in aromaticity between oil asphalts and the completely aromatic, highly condensed bitumens found in coal tars, water-gas tars, coal carbonization tars, and their pitches. 3. Coal tar, water-gas tars, and their pitches are derived from tars. ASTM D 1079 defines tar as " . . . a brown or black bituminous material, liquid or semi-solid in consistency, in which the predominating constituents are bitumens obtained.., from the processing of coal, petroleum, oil-shale, wood, or other organic materials." The "free carbon" content, or other benzene insoluble matter, distinguishes coal tar from the asphaltites and oil asphalts. The latter are devoid of free carbon. Coal tars and their products are not included in the category of asphalt. In the early 1960s, approximately 70% of all oil asphalts were consumed by the road-paving industries with 20% used in roofing. The solid bitumens and asphaltites of natural origin found their greatest consumption in lacquer, paint, and electrical insulation. Today, the paving industry is still the largest user of these materials, but the scope and area of specialty coatings has broadened considerably. In recent years, asphalts and other bitumens have become increasingly important as the cost of other natural and synthetic binders has continued to escalate. Their ability to act as adhesives with excellent moisture vapor transmission (MVT) properties continues to result in new and varied uses.

Coating Types The types of asphaltic or bituminous coatings available can be classified, in a large part, by the industry of use. Industries considered are: 1. The paints and coatings industry, where bituminous coatings have been used to protect metal from the effects of water and oxygen degradation. 2. The roofing industry, where asphalt coatings are used extensively to weatherproof buildings. 3. The construction industries, where concrete and mortar are waterproofed. 4. The paving industry; where the adhesive properties of asphalt as binder is put to good use in roads.

Specialty Paints and Coatings Asphalt, coal tar, and other bitumens have been used in several specialty areas in the paint and coatings industry. The predominate use has been in the area of pipe coatings and automotive under-body coatings, although containment coatings are fast becoming a sizable market. In pipe coatings, the base bitumen forms an inter-penetrating network with a thermosetting resin to form an impervious barrier to groundwater and the effects of catastrophic rusting. On deep buried pipes or those set in concrete, the cost associated with the use of an expensive binder is offset by the large replacement costs involved. The thermoset resins most frequently used have been the epoxides. The bitumens used in these coatings have generally been the coal tars and pitches. This was primarily due to the compatibility of these highly aromatic materials with epoxy resins, as well as the ease of working with a liquid material. The final film hardness is derived from the cross-linked epoxy network. There has been a growing concern with the toxicity of highly aromatic systems. The result has been that trade sales and light industrial coatings have moved away from coal tar or its pitches. Recently, the asphaltites and oil asphalts have been used in these types of coatings. The trend has been to use softer asphalts. Some form of compatibilizer has also been necessary to make these lower aromatic-content systems stable. In automotive under-body rustproofing, bituminous coatings have found extensive use. These materials are modified with rubbery materials to give flexible coatings with excellent adhesion to metal parts. Many years ago, the predominant bitumen in use had been coal tar. Today, with the move away from highly aromatic products, petroleum asphalts are generally used. To use the harder bitumens such as asphaltites and petroleum asphalts, plasticizers such as di-octyl phthalate or butyl benzyl phthalate are required to soften and liquify the bitumen. Aromatic processing oils have also been used for this purpose. Natural and synthetic waxes are added to prevent chipping from road debris. Given the severe penalties associated with contamination of groundwater, chemical and moisture-resistant coatings for containment dikes are being used more and more in the chemical process industry. Most state and local regulations require the use of a containment wall around every storage tank that may potentially rupture and contaminate the water table. Coatings for this application have included coal tar epoxies and coal tar resinous systems. Gilsonite-based resinous coatings have been widely used and, depending on the chemical nature of the contained material, petroleum asphalt urethanes and epoxides are available. Additional areas where bitumens have shown applicability as specialty coatings have included the areas of sealing soil to minimize water penetration (pond liners, seepage control for levees and dams, and hazardous waste containment) as well as sound deadening on sheet metal and binding other bituminous materials such as coal or lignite for pelletization.

Roof Coatings In roof coatings, bitumens have been important raw materials since the turn of the century. Today, many commercial roofing systems use some form of asphalt or chemically mod-

CHAPTER 2 - - B I T U M I N O U S COATINGS ified asphalt in their construction. The application of an asphalt or polymer-modified hot melt asphaltic material, followed by the application of a reinforcing membrane, is the basic construction of a modern built-up-roof (BUR). In some markets, the current industry trend has been away from the use of hot melt coatings, where a roofing kettle that heats the materials up to 450~ (232~ to reach their application viscosity is required, and toward cold-applied systems. Here, the asphalt is usually modified with solvents, fillers, and thixotropes as well as various additives to result in a formulation that can be applied at ambient temperatures with good flow properties and that which will subsequently dry or cure into a weatherproofing membrane. In these coatings, volatile solvents are varied to control cure times. In general, the solvents are either mineral spirits or naphthas. While asbestos was long a preferred additive for thixotropy and reinforcement, the hazards of working with and removing old installations with asbestos-containing materials have driven the products toward asbestos-free roofing materials. This has led to the use of cellulose, synthetic, and glass fibers as a partial replacement for asbestos. Bentonite and attapulgite clays are then used to obtain the required thixotropy. Today, there are still a significant number of manufacturers that continue to use asbestos in their formulations. The asphalt portion of these coatings usually consists of materials referred to as cutbacks. Various solvents are used to cut (solubilize) the asphalt, depending on the cure times required. The solvent predominantly used today is mineral spirits, with a flash point (tag closed cup) of 104~ (40~ Faster evaporating versions of these cutbacks have been used as primers for better substrate adhesion. These materials generally use faster aromatic solvents, including toluene, xylene, and the aromatic naphthas. The asphalt content varies from 30 to 70% by weight. The preceding materials, while they can, in the strictest sense, be considered coatings, are actually closer to adhesives in performance; that is, these coatings are applied to hold the reinforcing membranes together. Although the last coat applied may be a flood coat of the adhesive coating, the roof is usually not left this way. Weathering characteristics are significantly improved when these roofs are gravel surfaced. This graveled surface blocks harmful ultraviolet (UV) radiation and serves to improve the fire resistance. The most common roof gravels are river-washed gravel, crushed stone, granite, and blast-furnace slag recovered from the iron ore reduction process and composed of silicates and aluminosilicates of lime [6]. Other materials, also available for this purpose, include a variety of small, colored roofing granules, similar to those used on shingles. From 400 to 600 lb (181 to 272 kg) of river-washed gravel per 100 ft2 (9.29 m 2) of roof is used, or, if weight considerations are important, 50 to 60 lb (22.68 to 27.2 kg) of the smaller roofing granules can be used. The bituminous coating is then an adhesive for these gravels. Due to weight limitations on existing roofs and costs associated with roof tear-offs and subsequent reroofing, current philosophy is to maintain the existing roof. When physically possible, restoration instead of replacement is very cost effective. This requires the use of coatings whose purpose is to repair damage to the roof and re-establish or maintain the

17

weather-tight seal. After the repairs are complete, a reflective coating may be applied to act as an ultra-violet (UV) barrier and thermal reflector, or additional gravel added. Several different types of coatings have been available for each of these purposes. Asphalt cutbacks and emulsions are the primary coating used for restoration. They are applied in heavy applications of 40 to 80 rail thick. This allows the coating to cover minor surface defects that are present on the old roof. Splits and cracks can be repaired by using these materials with either fiberglass or polyester reinforcements. A final application gives a reasonably water-tight monolithic appearance. Asphalt emulsions consist of two types. In one, the water is dispersed in the asphalt external phase. In the other, the asphalt is dispersed in a water external phase. The first are called water-in-oil (W/O) emulsions. The later are oil-in-water (O/W) emulsions. Roofing emulsions are predominately water-in-oil emulsions. The oil-in-water emulsions are more widely used in the paving industry and will be discussed in more detail in that section. The water-in-oil emulsions are produced from finely powdered clays, which can act as dispersants for the water. Dispersing agents of this type show some affinity for water or are sufficiently hygroscopic to hold water and bring it into dispersion in the asphalt. The bentonite clays form extremely colloidal gelatinous mixtures and pastes with water and result in asphalt dispersions of very small particle size, These smooth buttery emulsions are very stable and can be fibered for reinforcement and modified with latex resins to obtain a degree of elasticity. Most commercial products are unmodified and yield a final coating possessing all the properties of a gel asphalt after evaporation of the water. Several books are available which offer greater detail in the area of emulsion technology [7-9]. Asphalt emulsions can be left untop-coated, but are frequently coated with reflective topcoats to help control roof top temperatures. For several years, the major type of coatings for this application have been solvent-borne aluminum pigmented bituminous coatings. A wide variety of bitumens have been used, including asphalt, asphaltite, tar, and pitch. Their viscosity has generally been low with moderate levels of volatile solvents present. Most of the solvent-based aluminum bitumen paints in use today are asphalt vehicles made from petroleum asphalt cutbacks. The predominate solvent has been mineral spirits. The pigment used has generally been a leafing grade of aluminum paste. It is reported that some early formulations used cumerone indene resin to improve the leafing characteristics and act as an anti-bronzing agent. A level of 2 lb (0.91 kg) of aluminum paste per gallon of paint is typical in these coatings [IO]. With the recent increase in environmental legislation and an increased awareness of health issues, alternatives to these solvent-borne coatings are beginning to find their place in the market. Specifically, asphalt emulsions of various solids are being used in conjunction with new aluminum pigment technology which allows the manufacture of relatively stable waterborne versions [11,12]. These materials generally consist of petroleum asphalt emulsions that use organophosphatetreated leafing-aluminum pastes. The phosphate passivates the aluminum, giving it more stability on storage. The solids of such coatings vary from 25 to 50% by weight. Additional

18

PAINT AND COATING TESTING MANUAL

modification similar to other emulsion systems is also used in these coatings. Newer technologies to stabilize aluminum pigments in water have recently been introduced. Chemically bound chrome is used to passivate the aluminum [13]. While quite expensive, these products are finding use in the automotive industry. As their cost decreases, perhaps they will be available for the waterborne bituminous aluminum market. Other technologies exist that are nonbituminous in composition. These coatings include elastomeric acrylic latexes, solvent and waterborne urethanes, epoxides, and alkyds. These are usually pigmented with either titanium dioxide (TiO2) or aluminum pastes to give thermally reflective coatings. These types of coatings will be discussed elsewhere in this manual.

Waterproofing Membranes Bitumen-modified waterproofing membranes are used extensively in the construction industry. The most common substrate is poured or cast concrete or mortared "cinder block." Prestressed concrete in the foundations, walls, and roof decks of high-rise buildings is also a suitable candidate for these membranes. In the home construction and repair industries, cinder block foundations and concrete footers are commonly waterproofed with bitumen-modified polymeric coatings. The bitumen of interest in these markets has predominately been coal tar pitch and petroleum asphalts. Waterproofing membranes are generally composed of bitumen in an elastomeric polymer matrix. The aromatic polyurethanes are frequently used for this purpose. In coating structural steel and steel reinforcement bars, coal tar epoxies have been extensively used. Their composition and purpose is similar to that of pipe coatings used for the prevention of underground corrosion. Their composition can be modified to conform to a particular steel coatings application. As in other markets, the use of aromatic coal tars is slowly being replaced by safer soft petroleum asphalts. Environmental issues aside, higher tech systems are beginning to be seen. Other types of coatings for rebar in the last five years have included fused epoxy powder coatings systems and polyethylene dip coated systems. While much more expensive than bitumen-modified systems, their improved performance have made them of interest.

Coatings for Paving The paving industry is probably the oldest using bitumen and its coatings. Asphalt cutbacks have also been known as "road oils." For years these solvent cut materials were used to seal roads as well as coat aggregates for application to the road surface. Today, hot asphalt or cutback is used to prime new paving as well as to repair damaged or worn areas. Today, most road coating uses asphalt emulsions. These are generally chemically stabilized emulsions. The emulsion is prepared beforehand and mixed with aggregate on site and is referred to as chip and seal. Hot asphalt is not required in this application, making it much easier than the use of hot mix paving, where the asphalt is heated to melting before application. In paving, oil-in-water emulsions predominate. The oil-in-water emulsions are formed from the action of a chemical emulsifier, either anionic, cationic, or nonionic in

nature. The anionic and cationic emulsifiers form an emulsion in which the dispersed phase shows a definite charge. These emulsions are said to "break" upon contact with a charged aggregate, yielding the exclusion of one phase from the other. The speed of break can be modified, yielding rapid, medium, or slow setting emulsions. The cationic versions are preferred because the coating formed does not re-emulsify. With anionic emulsifiers, break occurs when emulsions destabilize due to water loss on drying. One disadvantage of this is the possibility of re-emulsification in the early stages of cure. Once the coating has dried, water is no longer a problem. Paving sealers are used to protect new or old asphalt driveways or parking lots. The sealers are generally coal tar in nature due to good resistance to gas and oil. Asphalt sealers can also be used, but they must be latex or polymer modified to improve solvent resistance. Other types of bituminous coatings used in the paving industry include slurry seals and micro surfacing, which uses latex or polymer-modified asphalts with fine aggregate filler as a surface treatment for repair of minor damage to roads. Coal tar is not used in this application because the resulting coating is too slippery. Tack coats consisting of asphalt cutbacks are also used when one layer of asphalt needs to be adhered to another.

I D E N T I F I C A T I O N OF B I T U M I N O U S MATERIALS This section will catalogue several test methods currently available through ASTM for characterization of bituminous paints and coatings. Many of these methods are familiar to the coatings chemist as standard paint-related tests found in Volumes 6.01 through 6.04 of the Annual Book of ASTM Standards. Several others are under the jurisdiction of Committee D 8 on Roofing, Waterproofing, and Bituminous Materials. These methods appear in Volume 4.04 of the Annual Book of

ASTM Standards. Tests on Bituminous

Materials

The following test methods are used to differentiate one type of bitumen from another. They also can distinguish mixtures of bitumens and their purity. As bitumens are considered pseudo-plastic materials, with no true melt point, softening point and penetration are the two major tests routinely performed to identify differences within grades of the different bitumen classes. Viscosities at elevated temperatures are also very important with several instruments and their methods listed. In earlier editions of this manual, several tests were described that were in common use in 1972. Among the tests described were the solubility of bitumens in carbon disulfide (CSa) to identify the purity of a bitumen sample, since by definition only CS2 soluble matter is bitumen. Also listed were tests to determine the presence of asphalt and tar in suspected mixtures (the Oliensis Spot Test and the characteristics of bituminous samples dispersed in solvent). Today, these have been incorporated into the Annual Book of ASTM Standards and will not be described in detail.

CHAPTER 2--BITUMINOUS COATINGS 19 On this note, it is important to point out that each industry that uses bitumens has tended to develop their own series of common pertinent tests over the years. Today most of the pertinent tests have been incorporated as ASTM standards. In addition to ASTM, other organizations have tried to compile these tests for their members' use. The Asphalt Institute, an international, nonprofit organization sponsored by members of the petroleum asphalt industry, also publishes a handbook that has evolved over the past 50 years as the standard reference work in the field of asphalt technology and construction, especially in the paving industry [14]. This reference book cites both ASTM test methods and, where applicable, American Association of State Highway and Transportation Officials (AASHTO) counterparts to these methods. A large part of the manual is devoted to practical how-to information about how to use asphalt, as well as comprehensive data on asphalt technology, and is highly recommended.

D 4799-88 Test Method for Accelerated Weathering Test Conditions and Procedures for Bituminous Materials (Fluorescent UV and Condensation Method) D 4989-90 Test Method for the Apparent Viscosity Flow of Roofing Bitumens Using the Parallel Plate Plastometer E 96-90

Test Methods for Water Vapor Transmission of Materials

E 102-81 Test Method for Saybolt Furol Viscosity of Bituminous Materials at High Temperatures E 108-90 Method for Fire tests of Roof Coverings

Specifications and Test Methods for Asphalt D 71-84

General D 4-86

Test Method for Bitumen Content

D 5-86

Test Method for Penetration of Bituminous Materials

D 36-86

Test Method for Softening Point of Bitumen (Ring and-Ball apparatus)

D 70-82

Test Method for Specific Gravity and Density of Semi-Solid Bituminous Materials

D 88-81

Test Method For Saybolt Viscosity

D 92-90

Test Method For Flash and Fire Points by Cleveland Open Cup

D 95-83

Test Method for Water in Petroleum Products and Bituminous Materials by Distillation

Test Method for Relative Density of Solid Pitch and Asphalt

D 312-89 Specification for Asphalt Used in Roofing D 449-89 Specification for Asphalt Used in Dampproofing and Waterproofing D 1328-86 Test Method for Staining Properties of Asphalt D 1370-84 Test Method for Contact Compatibility Between Asphaltic Materials (Oliensis Test) D 1856-79 Test Method for Recovery of Asphalt from Solution by Abson Method D 2042-81 Test Method for Solubility of Asphalt Materials

in Trichloroethylene D 2521-76 Specification for Asphalt Used in Canal, Ditch, and Pond Lining

D 140-88 Practice for Sampling Bituminous Materials

D 3461-85 Test Method for Softening Point of Asphalts and Pitches (Mettler Cup-and-Ball Method)

D 52%90 Test Method for Accelerated Weathering Test Conditions and Procedures for Bituminous Materials

D 4402-87 Test Method for Viscosity Determinations of Unfilled Asphalts Using the Brookfield Thermosel Apparatus

D 1079-87 Definitions of Terms Relating to Roofing, Waterproofing, and Bituminous Materials D 1669-89 Method for Preparation of Test Panels for Accelerated and Outdoor Weathering of Bituminous Materials D 1670-90 Test Method for Failure End Point in Accelerated and Outdoor Weathering of Bituminous Materials D 4798-88 Test Method for Accelerated Weathering Test Conditions and Procedures for Bituminous Materials (Xenon-Arc Method)

Specification and Test Methods for Coal Tar, Pitches, and Highly Cracked Petroleum Products D 61-75

Test Method for Softening Point of Pitches (Cube-in-Water Method)

D 450-78 Specification for Coal Tar Pitch Used in Roofing, Dampproofing, and Waterproofing D 2318-86 Test Method for Quinoline-Insoluble (QI) Content of Tar and Pitch D 2319-76 Test Method for Softening Point of Pitch (Cubein-Air Method)

20

PAINT AND COATING TESTING MANUAL

D 2320-87 Test Method for Density (Specific Gravity) of Solid Pitch (Pycnometer Method) D 2415-66 Test Method for Ash in Coal Tar and Pitch D 2416-84 Test Method for Coking Value of Tar and Pitch (Modified Conradson)

D 555-89 Guide for Testing Drying Oils D 562-81 Test Method for Consistency of Paints Using the Stormer Viscometer D 609-90 Practice for Preparation of Cold-Rolled Steel Panels for Testing Paint, Varnish, Conversion Coatings, and Related Coating Products

D 2569-89 Test Method for Distillation of Pitch D 2764-81 Test Method for Dimethylformamide-Insoluble (DMF-I) Content of Tar and Pitch D 2962-71 Method for Calculating Volume-Temperature Correction for Coal-Tar Pitches D 3104-87 Test Method for Softening Point of Pitches (Mettler Softening Point Method) D 4072-81 Test Method for Toluene-Insoluble (TI) Content of Tar and Pitch D 4312-89 Test Method for Toluene-Insoluble (TI) Content of Tar and Pitch (Short Method) D 4616-87 Test Method for Microscopical Analysis by Reflected Light and Determination of Mesophase in a Pitch D 4715-87 Test Method for Coking Value of Tar and Pitch (Alcan) D 4746-87 Test Method for Determination of Quinoline Insoluble (QI) Content in Tar and Pitch by Pressure Filtration D 4892-89 Test Method for Density of Solid Pitch (Helium Pycnometer Method) D 4893-89 Test Method for Determination of Pitch Volatility D 5018-89 Test Method for Shear Viscosity of Coal Tar and Petroleum Pitches

D 610o85 Method for Evaluating Degree of Rusting on Painted Steel Surfaces D 662-85 Test Method for Evaluating Degree of Erosion of Exterior Paints D 714-87 Method for Evaluating Degree of Blistering of Paints D 1212-85 Test Methods for Measurement of Wet Film Thickness of Organic Coatings D 1474-85 Test Methods for the Indentation Hardness of Organic Coatings D 1475-90 Test Method for Density of Paint, Varnish, Lacquer, and Related Products D 1540-82 Test Method for Effect of Chemical Agents on Organic Finishes Used in the Transportation Industry D 1542-60 Test Method for Qualitative Detection of Rosin in Varnishes D 1640-83 Test Methods for Drying, Curing, or Film Formation of Organic Coatings at Room Temperature D 1644-88 Test Methods for Nonvolatile Content of Varnishes D 1654-79 Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments D 1849-80 Test Method for Package Stability of Paint

TESTS AND SPECIFICATIONS FOR COATINGS

D 2243-90 Test Method for Freeze-Thaw Resistance of Water-Borne Paints

General Tests for Coatings Several of the test methods familiar to the industrial paints and coatings chemist can also be used to characterize the performance and physical properties of bituminous coatings. The following methods can all be found in Volumes 6.01 through 6.04 of the Annual Book of ASTM Standards.

D 2247-87 Practice for Testing Water Resistance of Coatings in 100% Relative Humidity D 2369-90 Test Methods for Volatile Content of Coatings D 2370-82 Test Method for Tensile Properties of Organic Coatings

Tests and Specifications B 117-90 Method of Salt Spray (Fog) Testing

D 2832-83 Guide for Determining Volatile and Nonvolatile Content of Paint and Related Coatings

D 522-88 Test Methods for Mandrel Bend Test of Attached Organic Coatings

D 3170-87 Test Method for Chip Resistance of Coatings

CHAPTER 2--BITUMINOUS COATINGS 21 D 3359-90 Test Methods for Measuring Adhesion by Tape Test D 3960-90 Practice for Determining Volatile Organic Compound (VOC) Content of Paints and Related Coatings G 6-88

Test Method for Abrasion Resistance of Pipeline Coatings

Solvent-Thinned or Cut-Back Coatings

Paving Sealers D 3320-90 Specification for Emulsified Coal Tar Pitch (Mineral Colloid Type) D 3423-84 Practice for Application of Emulsified Coal Tar Pitch (Mineral Colloid Type) D 4866-88 Performance Specification for Coal Tar Pitch Emulsion Pavement Sealer Mix Formulations Containing Mineral Aggregates and Optional Polymeric Admixtures

General

Specialty Coatings

D 255-70 Method for Steam Distillation of Bituminous Protective Coatings

D 41-85

Specification for Asphalt Primer Used in Roofing and Waterproofing

D 402-76 Test Method for Distillation of Cut-Back Asphaltic (Bituminous) Products

D 43-73

Specification for Creosote Primer Used in Roofing, Dampproofing and Waterproofing

D 529-90 Test Method for Accelerated Weathering Test Conditions and Procedures for Bituminous Materials (Carbon-Arc Method)

D 1187-82 Specification for Asphalt-Base Emulsions for Use as Protective Coatings for Metal

D 3105-90 Index of Methods for Testing Elastomeric and Plastomeric Roofing and Waterproofing Materials

Emulsion Coatings

Roof Coatings D 41-85

Specification for Asphalt Primer Used in Roofing and Waterproofing

D 43-73

Specification for Creosote Primer Used in Roofing, Dampproofing and Waterproofing

D 1227-87 Specification for Emulsified Asphalt Used as a Protective Coating For Built-Up Roofing D 2823-90 Specification for Asphalt Roof Coatings D 2824-85 Specification for Aluminum-Pigmented Asphalt Roof Coatings D 3805-85 Practice for Application of Aluminum-Pigmented Asphalt Roof Coating D 4479-85 Specification for Asphalt Roof Coatings-Asbestos Free D 5076-90 Test Method for Measuring Voids in Roofing and Waterproofing Membranes

Waterproofing Membranes D 41-85

Specification for Asphalt Primer Used in Roofing, Dampproofing and Waterproofing

D 43-73

Specification for Creosote Primer Used in Roofing, Dampproofing and Waterproofing

D 5076-90 Test Method for Measuring Voids in Roofing and Waterproofing Membranes

General D 466-42 Method of Testing Films Deposited from Bituminous Emulsions D 529-90 Practice for Accelerated Weathering Test Conditions and Procedures for Bituminous Materials (Carbon-Arc Method) D 1187-82 Test Method for Asphalt-Base Emulsions for Use as Protective Coatings for Metal D 2939-78 Method for Testing Emulsified Bitumens Used as Protective Coatings

Clay Stabilized Emulsions D 1227-87 Specification for Emulsified Asphalt Used as a Protective Coating For Built-Up Roofing D 2963-78 Test Method for Testing Flow Table Consistency of Clay-Stabilized Asphalt Emulsions D 3320-90 Specification for Emulsified Coal Tar Pitch (Mineral Colloid Type)

Anionic Emulsions D 1227-87 Specification for Emulsified Asphalt Used as a Protective Coating For Built-Up Roofing

Non-Ionic Emulsions D 1227-87 Specification for Emulsified Asphalt Used as a Protective Coating For Built-Up Roofing

Cationic Emulsions D 1227-87 Specification for Emulsified Asphalt Used as a Protective Coating For Built-Up Roofing

22

PAINT AND COATING TESTING MANUAL

Resin Modified Bituminous Coatings Synthetic Resins D 3468-90 Specification for Liquid-Applied N e o p r e n e a n d Chlorosulfonated Polyethylene used in Roofing a n d Waterproofing

CONCLUSION B i t u m i n o u s coatings are still used extensively today. The waterproofing a n d adhesive properties, coupled with the relatively inexpensive costs for m o s t b i t u m i n o u s materials, continue to drive their use in m a n y diverse industrial applications. The p r e c e d i n g i n f o r m a t i o n will give the r e a d e r an u n d e r s t a n d i n g of the c h e m i s t r y a n d uses of b i t u m i n o u s coatings in i n d u s t r y a n d a realization that even several t h o u s a n d years after t h e i r discovery a n d first use the usage of these b i t u m i n o u s r a w m a t e r i a l s as an engineering r a w m a t e r i a l is still growing.

REFERENCES [1] Barth, E.J., Asphalt: Science and Technology, Gordon and Breach Science Publishers, New York, 1962.

[2] Annual Book of ASTM Standards, Section 4, Volume 4, Roofing, Waterproofing, and Bituminous Materials, American Society for Testing and Materials, Philadelphia, 1988, p. 100. [3] Barth, E.J., Asphalt: Science and Technology, Gordon and Breach Science Publishers, New York, 1962, p. 3. [4] Martin, O., Bitumen, Teere, Asphalte, Peche Vol. 11, 1951, p. 285. [5] Barth, E.J., Asphalt: Science and Technology, Gordon and Breach Science Publishers, New York, 1962, pp. 7-9. [6] Griffin, C.W., Manual of Built-Up Roof Systems, 2nd ed., McGraw-Hill Book Co., New York, 1982, pp. 141-151. [7] Barth, E.J., Asphalt: Science and Technology, Gordon and Breach Science Publishers, New York, 1962, pp. 471-558. [8] Bennett, H., Bishop, J.L., and Wulfinghoff, M. F., Practical Emulsions: Materials and Equipment, Vol. 1, Chemical Publishing Company, Inc., New York, 1968. [9] Bennett, H., Bishop, J.L., and Wulfinghoff, M.F., Practical Emulsions: Applications, Vol. 2, Chemical Publishing Company, Inc., New York, 1968. [10] Edwards, J. D. and Wray, R. I., Aluminum Paint and Powder, Reinhold Publishing Corp., New York, 1955, pp. 64-69. [11] Williams, J. E., U.S. Patent No. 4,565,716, 1986. [12] Besold, R., "Aluminum Flake in Waterborne Coatings: Antagonism or Reality," Proceedings, 18th Annual Higher Solids and Waterborne Coatings Symposium, New Orleans, LA, 1991. [13] Chapman, D.P., "Aluminum Pigment Technology for Waterborne and Powder Coatings in the 1990's," Proceedings, 18th Annual Higher Solids and Waterborne Coatings Symposium, New Orleans, LA, 1991. [14] The Asphalt Handbook, Bukowski, J. R., Ed., The Asphalt Institute, Manual Series No. 4 (MS-4), 1989.

MNL17-EB/Jun. 1995

Cellulose Esters by L. G. Curtis 1

CELLULOSE ESTERS ARE THE reaction product of combining organic acids and acid anhydrides with the hydroxyl groups found on the anhydroglucose repeating units of a cellulose molecule. The esterification of the cellulose with the acids and anhydrides occurs quite rapidly and if allowed to proceed to completion, forms a triester with each of the anhydroglucose units, which contain three hydroxyl groups. Because the triester is of little practical use, hydrolysis is necessary to restore desired levels of hydroxyl content, which affects various properties of the cellulose ester such as solubility and compatibility with other coating polymers.

possible. Hydroxyl content and molecular weight possibilities expand this range even further.

FACTORS A F F E C T I N G P E R F O R M A N C E OF CELLULOSE E S T E R S IN COATINGS Performance properties of cellulose acetate buytrate are affected by the chemical composition and the viscosity of the ester. As butyryl increases, solubility, compatibility, flexibility, diluent tolerance, and moisture resistance are increased. Lower butyryl levels are associated with decreased water tolerance, grease resistance, hardness, and increased melting range. As the hydroxyl content of cellulose acetate butyrate varies, several characteristics are also affected. Below 1% hydroxyl, solubility in common coatings type solvents is limited but improves as the hydroxyl increases. At levels around 5%, solubility in lower molecular weight alcohols occurs. At higher hydroxyl levels, reactivity increases, providing crosslinking capability with amino and isocyanate resins. However, in noncross-linking systems, higher levels decrease moisture resistance due to increased hydrophilicity. The viscosity of cellulose esters also influences physical properties of the ester as well as coatings formulated with them. Increasing the viscosity of a particular ester by increasing the molecular weight slightly lowers its solubility and compatibility with other resins, but does not affect hardness and density. Generally, toughness and flexibility are improved with increased molecular weight and viscosity.

P R O D U C T I O N OF CELLULOSE E S T E R S For the production of coating-grade cellulose esters, three organic acids and anhydrides are used, either separately or in combination with each other. Cellulose acetate is the simplest cellulose ester since only acetic acid and acetic anhydride are used in the esterification reaction. If two different organic acids and anhydrides are used simultaneously, the resultant product is referred to as a mixed ester. Examples of mixed cellulose esters are cellulose acetate butyrate and cellulose acetate propionate. In addition to esterification and hydrolysis, several subsequent processing steps are required in the manufacture of cellulose esters including filtration, precipitation, washing, dewatering, drying, and screening. The final product is a dry, free-flowing powder in most instances, although other physical forms can be produced. Unlike cellulose nitrate, organic esters of cellulose are low in flammability and present no handling hazards.

APPLICATIONS F O R CELLULOSE E S T E R S IN COATINGS T Y P E S OF CELLULOSE E S T E R S Protective and decorative coatings for various substrates can be formulated either as air-dry lacquer systems or as converting or curing types often referred to as cross-linked enamels. In many such coatings, cellulose esters are included as either a modifying resin to impart a specific property to the coating or to function as the primary film-forming resin in the formulation. Both types of coatings can be applied over a variety of substrates ranging from paper products to automobiles. Some areas in which cellulose esters are used include automotive OEM and refinish, wood furniture coatings, leather coatings, printing inks, plastic coatings, aircraft coatings, cable lacquers, and various fabric coatings. Cellulose esters are used in coatings to impart such properties as rapid-

Several types of cellulose esters are commercially available, including cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate. Other esters of lesser commercial value are produced, but are not suited for coating applications. Table 1 shows the types of cellulose esters commercially available. Cellulose acetate butyrate is the most commonly used organic cellulose ester for coating applications, and there is an almost infinite number of types that can be produced because of the acetylbutyryl combinations that are ~Principal Technical Representative, Eastman Chemical Company, Building 230, Kingsport, TN 37662.

23 Copyright9 1995 by ASTMInternational

www.astm.org

24

PAINT AND COATING TESTING MANUAL TABLE l - - C o m m e r c i a l cellulose esters (Eastman Chemical Company). Type

Viscosity,~ s

Acetyl,b %

CA-394-608 CA-398-3 CA-398-6 CA-398-10 CA-398-30

60.00 3.00 6.00 10.00 30.00

39.5 39.8 39.8 39.8 39.8

CAP-482-0.5 CAP-482-20 CAP-504-0.2

0.50 20.00 0.20

2.5 2.5 0.6

CAB- 171-158 CAB-321-0.1 CAB-381-0.1 CAB-381-0.5 CAB-381-2 CAB-381-20BP CAB-381-20 CAB-381-20BP CAB-500-5 CAB-531-1 CAB-551-0.01 CAB-551-0.2 CAB-553-0.4

15.00 0.10 0.10 0.50 2.00 2.20 20.00 16.00 5.00 1.90 0.01 0.20 0.40

29.5 18.5 13.5 13.5 13.5 14.5 13.5 15.5 4.0 3.0 2.0 2.0 2.0

Propionyl, %

Butyryl, %

Hydroxyl, %

Melting Range, ~

4.0 3.5 3.5

240-260 230-250 230-250 230-250 230-250

... -.. --.

2.6 1.8 5.0

188-210 188-210 188-210

16.0 31.2 37.0 37.0 37.0 36.0 36.0 36.0 51.0 50.0 52.0 52.0 46.0

1.1 1.3 1.3 1.3 1.3 1.8 1.8 0.8 1.0 1.7 1.8 1.5 4.8

230-240 165-175 155-165 155-165 175-185 175-185 195-205 185-195 165-175 135-150 130-140 127-142 150-160

Celhtlose Acetate . . . . .

. . .

. . . . .

. . . . .

. . . . .

. . . . .

. .

. .

. .

. .

Cellulose Acetate Propionate 45.0 46.0 42.5

Cellulose Acetate Butyrate ... --. .-... ..... ... -.. -.. ... ... .-. .--

~ASTMTest Method for Cellulose Acetate Proprionate and Cellulose Acetate Butyrate (Formula A) (D 817) and Test Methods for Viscosity of Cellulose Derivatives by Ball-Drop Method (D 1343). bASTM D 817. d r y i n g , p i g m e n t c o n t r o l , v i s c o s i t y c o n t r o l , film t o u g h n e s s , and polishability.

TESTING OF CELLULOSE ACETATE C e l l u l o s e a c e t a t e is t e s t e d b y t h e m a n u f a c t u r e r i n a c c o r d a n c e w i t h A S T M T e s t M e t h o d s f o r C e l l u l o s e A c e t a t e s (D 871), w h i c h c o v e r s c o l o r a n d h a z e , c o m b i n e d acetyl, f r e e acidity, h e a t stability, h y d r o x y l c o n t e n t , i n t r i n s i c v i s c o s i t y , m o i s t u r e content, sulfur or sulfate content, a n d solution viscosity. C o a t i n g s m a n u f a c t u r e r s u s u a l l y r e s t r i c t t h e i r t e s t i n g t o viscosity of the ester, solubility a n d a p p e a r a n c e , a n d color a n d haze.

Viscosity V i s c o s i t y m e a s u r e m e n t o f c e l l u l o s e a c e t a t e is c a r r i e d o u t in a c c o r d a n c e w i t h A S T M T e s t M e t h o d s f o r V i s c o s i t y o f Cellul o s e D e r i v a t i v e s b y B a l l - D r o p M e t h o d (D 1343) b a s e d o n t h e ball d r o p o r f a l l i n g ball p r i n c i p l e . A p r e c i s i o n H o e p p l e r visc o m e t e r is u s e d i n m o s t v i s c o s i t y d e t e r m i n a t i o n s w i t h r e s u l t s r e p o r t e d in ASTM seconds. F o r m u l a t i o n s for viscosity determ i n a t i o n a r e s h o w n in T a b l e 2.

Solubility and Appearance T h e s o l u b i l i t y a n d a p p e a r a n c e t e s t is p e r f o r m e d t o d e t e r m i n e t h e p o s s i b l e p r e s e n c e o f i n s o l u b l e gel p a r t i c l e s , fibers, flock, o r o t h e r c o n t a m i n a n t s , u s i n g s o l u t i o n s p r e p a r e d for v i s c o s i t y t e s t i n g . T h e m a t e r i a l t o b e t e s t e d is a d d e d t o 16 oz. (454 g) F r e n c h s q u a r e b o t t l e s a n d v i s u a l l y c o m p a r e d t o a reference standard.

TABLE 2 - - S o l u t i o n s for viscosity measurement of cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate. Cellulose ester Acetoned Acetone/water, 96/94 Ethyl alcohoF Methyl alcoholf Methylene chlorideg

20~ 70 . . . 8 . . . ......

20 ~ 20 b 80 . . . . . . . . . . . -.. 8 . . . . . . . 72

15c 20~ l(F . . . . . . . . . . 80 ... 8.5 ...... . . . . . 9 76.5 .-. 81

Typical Solution Densities, g per mL at 25~ 0.85 0.86 1.25 1.23 0.86

1.24

aFor cellulose acetate having a max of 40.5% acetyl and for most mixed esters having less than about 40% acetyl and more than about 8% propionyl or butyryl. bFor cellulose acetate having 40.5 to 42.7% acetyl and for most of the commercial cellulose acetate propionate and acetate butyrates. CFor cellulose acetate having 42.7 to 44.8% acetyl and for most of the commercial cellulose acetate propionate and acetate butyrates; particularly good for esters containing more than 40% acetyl. aAcetone (99.4 -+ 0.1%) containing 0.3 to 0.5% water and under 0.3% ethyl alcohol. eEthyl alcohol (95% by volume). Formulas 2B, 3A, or 30 denatured ethyl alcohol may be used. fMethyl alcohol (sp gr 20/20~ = 0.785 to 0.795). gMethylene chloride having a boiling range of 39.2 to 40.0~ and less than 0.001% acidity calculated as HCL

Color and Haze The same solutions used for ASTM viscosity and solubility and a p p e a r a n c e testing are normally used for color and haze m e a s u r e m e n t s . T h e s o l u t i o n is t r a n s f e r r e d t o a s p e c i a l 33m m - s q u a r e cell, a n d t h e a n a l y s i s is p e r f o r m e d w i t h a G a r d n e r M o d e l XL-385 c o l o r i m e t e r . L i g h t p a s s e s t h r o u g h t h e s o l u tion, and the p r i m a r y yellowness index and p r i m a r y haze values are s i m u l t a n e o u s l y displayed by t h e i n s t r u m e n t . Seco n d a r y c o l o r , b a s e d o n p l a t i n u m / c o b a l t s t a n d a r d s , a n d seco n d a r y haze, b a s e d on scattered light f r o m a m o n o d i s p e r s e

CHAPTER 3--CELLULOSE ESTERS polystyrene latex suspension, are calculated. APHA color and ASTM haze are predicted from the secondary values.

T E S T I N G OF CELLULOSE ACETATE P R O P I O N A T E A N D CELLULOSE ACETATE BUTYRATE ASTM Test Method for Cellulose Acetate Propionate and Cellulose Acetate Butyrate (D 817) contains the following

25

tests: acetyl and propionyl or butyryl contents; apparent acetyl content; free acidity; ash; color and haze, heat stability, hydroxyl content; moisture content; sulfur content; and viscosity. Usually color and haze and viscosity are the only tests run by the coatings manufacturer. The test methods are the same as those used in the testing of cellulose acetate.

MNL17-EB/Jun. 1995

Drying Oils by Joseph V. K o l e s k e I

D R Y I N G OILS REPRESENT A SMALL PORTION

of the huge fats and

HOCH2--CH(OH)--CH2OH + 3 RxCOOH Glycerol Fatty Acid RxCOOCH2--CH(OOCRx)--CH2OOCRx + 3 H20 Triglyceride or Drying Oil Water

oils industry. 2 In 1987 there were 81 000 metric tons ( - 1 7 8 million pounds) of drying oils consumed in the United States [1]. With an expected annual average growth rate of 2.6%, there should have been 92 000 metric tons consumed in 1992. Such consumption represents about 3% of the total nonfood fats and oils market and about 1% of the combined food and nonfood fats and oils national market. The industry is very mature with relatively little growth expected. Within this industry, whose growth is about the same as the population growth of the United States, drying oil consumption, though relatively very small, has the highest expected growth rate over the above five-year period. The paint and coating industry's need for drying oils is in an overall decline along with other end use markets, such as binders for hardboard, sealants, plasticizers, linoleum, and core oils. Drying oils used in paints and coatings are being replaced with oil-free, synthetic, petroleum-derived oligomeric and polymeric binders carried in a variety of media or in a neat manner. The printing ink market is the only one expected to have small growth in the area of drying oils. Usually in the fats and oils industry, products solid at room temperature are referred to as fats, and products liquid at room temperature are termed oils. Often times the terms "fats" and "oils" are used interchangeably within the industry. Drying oils, except for fish oil, are of vegetable origin. Chemically, drying oils are water-insoluble, unsaturated glycerides of long chain fatty acids with the generalized structure

where R x is anyone of R a, Rb, or R~. This reaction is reversible and when the oils are heated they can transesterify with Ra, Rb, and Rc capable of changing their positions in an inter- and an intramolecular sense. Drying oils also contain saturated glycerides of fatty acids, but these are usually present at relatively low levels and they do not participate in drying or polymerization phenomenon. Hydrolysis of drying oils results in separation into glycerol and the fatty acid. Drying oils can be classified in many ways, but one principle way is to divide them into drying, semidrying, and nondrying (an oxymoron term) oils in accordance with their iodine values, which is a measure of unsaturation content. Although such classification has been rather arbitrary, Rheineck and Austin [2] defined the classes as given in Table 1. The main fatty acids found in drying oils and their chemical composition are given in Table 2. The unsaturated-component content of selected drying oils was given in Table 3. Stearic (18-carbon) and palmitic (16-carbon) acids are the most widely distributed saturated fatty acids found in drying oils. Except for cottonseed oil, which contains 29% palmitic acid, the drying oils listed in Table 3 contain less than about 10% of any particular saturated fatty acid residue. The degree of double bond unsaturation controls the drying rate. The higher the degree of unsaturation or iodine number (see helow), the faster the drying or polymerization of the oil. Double bond position is also important because conjugated bonds, which is the term used to described two carbon-carbon double bonds separated by one carbon-carbon single bond, are more susceptible to autooxidation [4]. Physical characteristics of some typical commercial drying oils are given in Table 4. The oils are particularly characterized by their Iodine Value, which is a measure of the amount of unsaturation present, and Saponification Number, which is an indication of fatty-acid chain molecule weight. Selected property requirement ranges or minimum values for various

CH~--O--OC--Ra

I

CH--O--OC--Rb

I

CH2--O--OC--R c Generalized Triglyceride where R a, Rb, and R~ are the same or different and represent the hydrocarbon chain residues of fatty acids. In simple terms, this means that most oils are mixed triglycerides. The triglycerides are produced by the condensation reaction that occurs between a glycerol molecule and three fatty acid molecules:

TABLE 1--Classification of drying

oils by iodine value. ISenior Consultant, Consolidated Research, Inc., 1513 Brentwood Road, Charleston, WV 25314-2307. 2Information about this industry is developed by the U.S. Department of Commerce. The nature of this market results in information that is usually a few years out of date. 26

Copyright9 1995 by ASTMInternational

www.astm.org

Oil Class

Iodine Value

Drying oil Semidrying Nondrying

> 140 125- 140 < 125

C H A P T E R 4 - - D R Y I N G OILS

27

TABLE 2--Main unsaturated fatty acids found in drying oils. Fatty Acid

No. Carbon Atoms

No. Double Bonds

Structural Formula

Linolenic Linoleic Oleic Eleostearic Licanic Ricinoleic

18 18 18 18 18 18

3 2 1 3 3 1

HOOC(CH2)7CH=CHCHECH~CHCH2CH=CHCH2CH 3 HOOC(CH2)7CH~CHCH2CH~CH(CH2)4CH3 HOOC(CH2)7CH~---CH(CH2)TCH3 HOOC(CH2)7CH~CHCH~--~-CHCH~---CH(CH2)aCH3 HOOC(CH2)ECH(O)(CH2)4CH--~CHCH--~CHCH--~CH(CH2)3CH3 HOOC(CH2)7CH~---CHCH2CH(OH)(CH2)5CH 3

drying oils a n d the ASTM m e t h o d t h a t contains o t h e r specification p r o p e r t i e s are d e l i n e a t e d in Table 5. The ASTM methods cited in Table 5 c o n t a i n references to o t h e r ASTM methods a p p r o p r i a t e for o b t a i n i n g the i n d i c a t e d p r o p e r t i e s a n d for o t h e r p e r t i n e n t properties. The following drying oils are the m o s t i m p o r t a n t m e m b e r s of this class of coating r a w materials. Castor oil is o b t a i n e d from b e a n s of the p l a n t Ricinus communis. The oil differs f r o m the o t h e r oils in t h a t it is m a d e u p of a very high p e r c e n t a g e of the hydroxyl-containing ricinoleic acid residue. Although the fatty a c i d residues in this oil c o n t a i n on the average only a single d o u b l e b o n d a n d for this r e a s o n c a s t o r oil is essentially a n o n d r y i n g oil, it can b e converted into a drying oil b y a d e h y d r a t i o n process in w h i c h its hydroxyl group a n d a n a d j a c e n t h y d r o g e n a t o m are rem o v e d as w a t e r to form a double b o n d conjugate to the previously existing double bond. The resultant p r o d u c t is k n o w n as d e h y d r a t e d castor oil, w h i c h has g o o d drying characteristics. Cottonseed oil is o b t a i n e d from the p l a n t Gossypium malvaceae. Although it is a drying oil, cottonseed oil is s e l d o m u s e d as a n oil in the p a i n t a n d coating industry. Its m a i n use is as a source of fatty acids that are used in the m a n u f a c t u r e of alkyd resins. Linseed oil, w h i c h is o b t a i n e d f r o m seed of the flax p l a n t Linum usitatissimum, is the m o s t c o m m o n a n d widely used oil. It has a high degree of u n s a t u r a t i o n , w h i c h i m p a r t s a short drying time, due to its large percentages of linolenic a n d linoleic triglycerides. It is m a r k e t e d in a n u m b e r of modifications including alkali-refined, acid-refined, boiled, blown, a n d p o l y m e r i z e d linseed oil [2]. P o l y m e r i z e d linseed oils of various acid values a n d viscosities are available. Oiticica oil is o b t a i n e d f r o m the nuts of the tree Licana rigida. It has a very high licanic acid content, a n d the three c o n j u g a t e d d o u b l e b o n d s of this acid result in r a p i d drying

characteristics. It is often u s e d as an alternative or supplem e n t to tung oil. Safflower oil is o b t a i n e d from seed of the p l a n t Carthamus tinctorius. This s e m i d r y i n g oil has drying characteristics bet w e e n those of linseed a n d s o y b e a n oils. Because of its low linolenic acid content, it has low residual u n s a t u r a t i o n after cure a n d very g o o d anti-yellowing characteristics. S o y b e a n oil is o b t a i n e d from the seeds of the widely g r o w n p l a n t Soja hispida. Its m a i n use is in the p r e p a r a t i o n of alkyds. It has a wide variety of uses o t h e r t h a n as a drying oil. I n a n epoxidized form, this oil is widely u s e d as a reactive plasticizer a n d as an acid scavenger. Both epoxidized soyb e a n a n d linseed oil have b e e n r e a c t e d with acrylic acid to form p r o d u c t s with residual acrylate functionality a n d m a r k edly higher viscosity. These acrylated oils have been used as c o m p o n e n t s in r a d i a t i o n - c u r e coating systems that are initia t e d with free radicals. Although tall oil is classified as a drying oil, it is not a triglyceride. The p r o d u c t is o b t a i n e d as the m a j o r b y p r o d u c t of sulfate or Kraft pulping of pine a n d certain o t h e r softw o o d s such as spruce a n d h e m l o c k that are p u l p e d in Scandinavian countries. Crude tall oil is an a p p r o x i m a t e l y 50/40/10 by weight mixture of fatty acids, r o s i n acids, a n d unsaponifiable c o m p o u n d s such as higher alcohols, waxes a n d o t h e r h y d r o c a r b o n s , a n d sterols. Tung oil is o b t a i n e d from seeds of the trees Aleurites fordii a n d Aleurites montana. This relatively high viscosity a n d refractive index oil is r a p i d drying a n d is used in varnishes a n d alkyds w h e r e w a t e r resistance is of p r i n c i p a l i m p o r t a n c e . This oil is also k n o w n as w o o d oil, Chinese w o o d oil, chin a w o o d oil, a n d m u oil. F i s h oils are the only nonvegetable oils in the drying oil class. They are p r i n c i p a l l y o b t a i n e d from m e n h a d e n (Alosa m e n h a d e n ) . These oils are s e m i d r y i n g in n a t u r e a n d c o n t a i n a significant a m o u n t of s a t u r a t e d fatty a c i d residues. I n addition to 16 a n d 18-carbon fatty acid residues, fish oils c o n t a i n

TABLE 3--Weight percentage of major unsaturated fatty acid residues in selected drying oils [2,3] (remainder of oils is essentially all saturated fatty acid residues). Drying Oil Cottonseed Castor Linseed Oiticica Safflower Soybean Sunfloweff Tall Oil Fatty Acids Tung

Linolenic

Linoleic

.-. 40 ... 3 52 16 . . . . . . 1 75 9 51 2 75-52 3 41 3 4

aThere is wide variation in reported values for sunflower oil.

Unsaturated Fatty Acid Oleic Eleostearic 24 7 22 6 13 25 34-14 46 8

. . . . . . . . . . .-. . . . . . . . . . . . 80

Licanic . . . . . . . . . .

. . . .

.

.

Ricinoleic .

.

.

.

. . . .

. . . . . . .

87 . . 78 . . . . . . . . . . .

-..

28

PAINT AND COATING TESTING MANUAL TABLE 4--Physical characteristics of some typical drying oils [5].

Oil

Specific Gravity, 25.5/25.5~

Cottonseed Dehydrated castor Fish Linseed Oiticica Safflower Soybean Sunflower Tall oil Tung

0.919 0.931 0.925 0.926 0.967 0.922 0.920 0.917 .-. 0.915

Iodine Value, Wijs 105 135 158 180 150 145 135 135 133 170

30 to 40% of a r a c h i d o n i c (20-carbon with four double bonds), c l u p a n o d o n i c (22-carbon with five double bonds), a n d nisinic (24-carbon with five d o u b l e bonds) acid residues. Because of the presence of acid residues with high degrees of u n s a t u r a tion, fish oils have a strong t e n d e n c y to yellow after cure due to residual u n s a t u r a t i o n . Fish oils do r e p r e s e n t a source of the very long chain fatty acids that are not p r e s e n t in vegetable oils. Currently, they are not widely used in the coatings' i n d u s t r y with use often d i c t a t e d b y relative price of linseed a n d s o y b e a n oils. Although r a w drying oils are used in coating formulations, the oils are often further p r o c e s s e d before use. S u c h processing includes alkali refining, dehydration, d r i e r addition, h e a t p o l y m e r i z a t i o n that involves heating an oil to selectively advance m o l e c u l a r weight a n d viscosity, a n d oxidation o r blowing w h e r e i n air is b u b b l e d into h e a t e d oil a n d oxygen is t a k e n up with a resultant m o l e c u l a r weight increase. Drying oils are also modified b y r e a c t i o n with maleic anhydride, by copolym e r i z i n g with vinyl m o n o m e r s such as styrene, a n d by epoxidation. Reaction with oxygen is the m o s t i m p o r t a n t r e a c t i o n that drying oils u n d e r g o in the drying or p o l y m e r i z a t i o n process [6, 7]. Oxidation can result in trans i s o m e r formation, cleavage of the c a r b o n - c a r b o n chain along with f o r m a t i o n of volatile byproducts, a n d polymerization. These reactions can be catalyzed with metallic salts such as cobalt n a p t h e n a t e (see next chapter) that p r o m o t e free radical f o r m a t i o n by r e a c t i o n with h y d r o p e r o x i d e s a n d o t h e r peroxides that are f o r m e d in the oxidation process [8]. F a r m e r a n d coworkers [9] were first to describe the m e c h a n i s m of a u t o o x i d a t i o n w h e r e i n they found that four different m o n o h y d r o p e r o x i d e s were f o r m e d w h e n oxygen was r e a c t e d with the methyl ester of oleic acid. A different r e a c t i o n p a t h was involved when linoleic esters were autooxidized since two m o n o h y d r o peroxides a n d one cyclic d i p e r o x i d e were formed. Polymer-

Saponification Value

Acid Value

Refractive Index, 25~

192 190 187 190 190 192 190 192 196 192

1.0 5.0 4.0 3.0 4.0 2.0 2.5 2.0 194.0 0.2

1.465 1.481 1.485 1.478 1.510 1.474 1.473 1.473 -.. 1.517

ization is initiated by r e a c t i o n of oxygen with an u n s a t u r a t e d fatty acid residue a n d free radical f o r m a t i o n followed by chain p r o p a g a t i o n in w h i c h free radicals react with oxygen to form peroxy radicals w h i c h in t u r n react with o t h e r u n s a t u r a tion sites [10]. The p o l y m e r i z a t i o n is t e r m i n a t e d by c o m b i n a tion of various free radicals that exist in the r e a c t i o n mass. Availability of m u l t i p l e d o u b l e b o n d s in s o m e of the molecules results in a crosslinked p o l y m e r i c network. Solidification o r p o l y m e r i z a t i o n o f a d r y i n g oil such as linseed oil can be t h o u g h t of in the following m a n n e r . W h e n the drying oil is exposed to air, there is an i n d u c t i o n p e r i o d d u r i n g w h i c h oxygen is a b s o r b e d a n d it c o n s u m e s antioxid a n t s p r e s e n t in the system. In this step, there is very little a p p a r e n t change in physical or chemical properties. This is followed by a p e r i o d in which there is a m a r k e d oxygen u p t a k e a n d an a p p e a r a n c e of peroxides w h i c h d e c o m p o s e to form free radicals. The free radicals then initate a n a d d i t i o n p o l y m e r i z a t i o n of the u n s a t u r a t i o n and a crosslinked netw o r k results. During the r e a c t i o n scheme, low m o l e c u l a r weight cleavage p r o d u c t s including c a r b o n dioxide a n d w a t e r are formed. ASTM D 1640 S t a n d a r d Test Methods for Drying, Curing, o r F i l m F o r m a t i o n of Organic Coatings at R o o m Temperature has p r o c e d u r e s r e c o m m e n d e d for d e t e r m i n a t i o n of the stages a n d rates of film f o r m a t i o n in the drying o r curing of organic coatings that are to be used at r o o m t e m p e r a t u r e . I n c l u d e d are m e t h o d s for d e t e r m i n i n g tack-free, dry-totouch, dry-hard, dry-through, print-free, a n d dry-to r e c o a t times. In one instance (Section 7.5.1) a p a r t i c u l a r p r o c e d u r e is specified for drying oils. ASTM S t a n d a r d Test M e t h o d for Gel Time of Drying Oils (D 1955), deals with d e t e r m i n a t i o n of the gel t i m e of oiticiica a n d tung oil. This s i m p l e test method, which" involves heating the oil in a test tube a n d observing the t i m e required for the oil to congeal a r o u n d glass r o d relative to a s t a n d a r d of k n o w n behavior, can be used for o t h e r oils

TABLE 5--Selected property requirements for drying oils (indicated ASTM method has other requirements).

Oil

Specific Gravity, 25/25~

Castor, raw Dehydrated castor Linseed, raw Oiticica Safflower Soybean, refined Tung, raw

0.957-0.961 0.926-0.937 0.926-0.931 0.972 (min) 0.922-0.927 0.917-0.924 0.933-0.938

Iodine Value, Wijs 83-88 125-145 177 (min) 135 (min) 140-150 126 (min) 163

Saponification Value

Acid Value, max

ASTM Method

176-184 188-195 189.0-195.0 None 189-195 189-195 I89-195

2.0 6 4.0 8.0 3.0 0.3 5.0

D 960 D 961 D 234 D 601 D 1392 D 1462 D 12

CHAPTER 4 - - D R Y I N G OILS t h a t have c o n j u g a t e d double b o n d or o t h e r gelling characteristics. ASTM S t a n d a r d G u i d e for Testing Drying Oils (D 555) is an overall guide to selection a n d use of p r o c e d u r e s for testing drying oils that are c o m m o n l y u s e d in coatings.

REFERENCES [1] "Fats and Oils Industry Overview," Chemical Economics Handbook, SRI International, Nov. 1990. [2] Rheineck, A. E. and Austin, R. O., Film Forming Compositions, R.R. Myers and J. S. Long, Eds., Marcel Dekker, Inc., New York, Vol. 1, No. 2, 1968. [3] Gunstone, F. D., Chemistry and Biochemistry of Fatty Acids and Their Glycerides, 2nd Ed., Chapman and Hall, Ltd., 1967.

29

[4] Solomon, D.H., The Chemistry of Organic Film Formers, Kreiger, New York, 1977. [5] Gallagher, E. C., "Drying Oils," Paint Testing Manual, G.G. Sward, Ed., 13th ed., The American Society for Testing and Materials, Philadelphia, 1972, p. 53. [6] Harwood, R. J., Chemical Reviews, Vol. 62, 1962, p. 99. [7] Fox, F. L., Unit Three, "Oils for Organic Coatings," Federation Series on Coatings Technology, W. R. Fuller, Ed., Federation of Societies for Paint Technology, Philadelphia, 1965. [8] Russell, G. A., Journal of Chemical Education, Vol. 36, No. 3, 1959, p. 111. [9] Farmer, E. H. and Sutton, D. A., Journal of the Chemical Society, 1946, p. 10. [10] Cowan, J. C., "Drying Oils," Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 8, 3d ed., 1979, pp. 130-150.

MNL17-EB/Jun. 1995

Driers and Metallic Soaps by Marvin J. Schnall I

TABLE l--Coatings applications of metallic soaps.

METALLICSOAPSARE COMPOUNDSof alkaline metals or heavy metals and monobasic carboxylic acids containing from 7 to 22 carbon atoms. The water-insoluble metallic soaps are of particular interest to the coatings industry, although potassium and lithium soaps have limited water solubility. The applications of metallic soaps in coatings include their use as driers, catalysts, stabilizers, biocides, bodying agents, and flatting agents. An overview of metallic soap applications is presented in Table I. This chapter concentrates primarily on metallic soaps used as driers, although a brief review of bodying and flatting applications is included.

Applications

M E T A L L I C S O A P S AS B O D Y I N G A N D FLATTING AGENTS Aluminum stearates, aluminum octoates, and calcium linoleate pulp were at one time widely used as bodying and pigment-suspending agents in solvent-based coatings. Their advantages include fairly high efficiency and ease of incorporation. However, they have a number of deficiencies, including sensitivity to variations in the formulation and in the processing of paints, as well as adverse effects on film hardness and resistance properties. As a result, they have been replaced to a considerable extent by the bentonite clay and organic wax derivatives [2]. Zinc and calcium stearates are used on occasion as flatting additives in clear solvent-based coatings. However, precipitated and fumed silicas are now more commonly used for this purpose. Zinc stearate is also used to facilitate sanding of primers and sealers for wood furniture finishing [2].

Acids

Cobalt, manganese, lead, Tallates, octoates, iron, rare earth, naphthenates, cerium, zirconium, neodecanoates, zinc, calcium, barium, synthetic acids, bismuth, potassium, linoleates,rosinates vanadium, aluminum

Catalysts

Potassium, lithium, cobalt, copper, tin, zinc, manganese

Octoates, naphthenates, laurates, neodecanoates

Stabilizers

Barium, cadmium, tin, zinc, calcium, lead

Proprietary, sulfates, carbonates, stearates

Biocides

Copper, tin, zinc, mercury

Naphthenates, octoates, phenyl acetates, phenyl oleates, laurates, butyl oxides

Bodying agents

Aluminum, calcium

Stearates, octoates, linoleates

Flatting agents Zinc, calcium

Stearates

The induction period occurs because most drying oils contain natural antioxidants, the effects of which must be overcome before the drying process can begin. Oxygen is then absorbed from the air at the unsaturated sites on the oil molecule, and, as the absorption continues, forms peroxides. These peroxides then decompose to form free radicals which act as catalysts to promote cross-linking of the oil or resin molecules at the unsaturated sites, resulting in dried films. These steps will occur in the absence of driers, but driers accelerate the process by means of the following mechanisms [1,5]:

T H E O R Y OF O X I D A T I V E D R Y I N G A N D F U N C T I O N OF D R I E R S [1,5]

1. Removal of natural antioxidants. 2. Acceleration of oxygen absorption. 3. Acceleration of peroxide decomposition.

It is generally accepted that vehicles based on unsaturated oils, including alkyd resins and oleoresinous varnishes, dry by oxidation according to the following steps: 1. Induction period. 2. Oxygen absorption. 3. Peroxide formation. 4. Peroxide decomposition to free radicals. 5. Free radicals promote cross-linking.

Naturally occurring antioxidants may be considered as negative catalysts for oxidation, whereas driers are positive catalysts, counteracting the effects of the antioxidants. The multivalent nature of the drier metal is considered to be a key factor accelerating oxygen absorption in the film. The drier metal is initially in a divalent state and is converted to a trivalent state by the presence of oxygen in the film. The metal then releases the oxygen to the film and is converted back to the divalent form. This action accelerates the process

1Coatings consultant, 620 Airport Rd., Suite 304, Chapel Hill, NC 27514. 30 Copyright9 1995 by ASTMInternational

Metals

Driers

www.astm.org

CHAPTER 5 - - D R I E R S AND METALLIC SOAPS of oxygen absorption, peroxide formation, and peroxide decomposition, which is responsible for oxidative film drying.

DESCRIPTION OF DRIER METALS The metals that act as catalysts to promote oxidation and which may be used in driers are indicated below: Cobalt t Manganese Vanadium

Active

Lead 1 Calcium Zirconium Zinc Iron Rare Earth Cerium Aluminum

31

Water-dispersible driers may be prepared by adding nonionic surfactants to naphthenate or synthetic acid driers. However, proprietary cobalt and manganese drier compounds are available that are purported to be more suitable for water-based coatings. Trade names of the various commercial drier types available are listed in Table 2.

MISCELLANEOUS DRIERS Auxiliary

Cobalt and manganese, particularly cobalt, are the most active drier metals. Cobalt promotes surface drying of films, while manganese affects both surface and through drying. Vanadium has been mentioned in the literature as an active drier but is seldom used in coating formulations. The auxiliary driers are seldom used alone, but rather in combination with cobalt and/or manganese. Their functions are to increase the efficiency of the active drier metals and to increase film hardness. In the past, lead was the most frequently employed auxiliary drier, but it is presently out of favor due to toxicity. Calcium and zirconium driers are most frequently used as lead replacements. Zinc is used primarily for improved film hardness and to prevent wrinkling of thick films. Iron driers are used mainly to improve drying of baking systems when their dark color can be tolerated. Rare earth and cerium driers are recommended occasionally for improved through drying and as oxidation catalysts for baking. Interest in aluminum compounds as auxiliary driers has increased recently with the advent of high-solids alkyd resins. Aluminum compounds are being recommended to improve film hardness with these resins but may at times adversely affect viscosity stability and promote gelation.

D E S C R I P T I O N OF D R I E R ACIDS

Restrictions on solvent emissions have stimulated the development of both higher-solids and water-reducible coatings. In the process of formulating these coatings, chemists are experiencing difficulty obtaining satisfactory drying properties with the conventional metallic soap driers. Alternative drier compounds, including both organics and proprietary metallic complexes, are currently being offered [6]. Some typical examples are shown in Table 3. They are usually recommended in combination with conventional metallic soap driers for improved drying efficiency. Another class of metallic compounds closely related to driers are loss of dry inhibitors or "feeder" driers. These are compounds designed to prevent loss of drying efficiency of paints on aging resulting from the adsorption of driers by pigments, particularly carbon black and organic red pigments. They function by dissolving gradually into the coating vehicle so that the metals are available over a period of time rather than immediately. In this manner, they replace the drier metals that have been adsorbed by the pigments,

TABLE

Type of Drier Synthetic acid

2--Commercial drier types [3]. Trade Name Supplier Cem-All NuXtra Troymax

Octoate

Hex-Cem Octoate

To perform their function, driers should be soluble in the vehicles to which they are added. Solubility is achieved by reacting the drier metals with organic acids to form metallic soaps. The most commonly employed acids are as follows: Linoleates Rosinates Tallates Naphthenates Octoates (2-ethyl hexanoates) Synthetic acids Neodecanoates Chronologically, the linoleates, rosinates, and tallates were the first types developed, followed by the naphthenates and the octoates. A more recent development is the synthetic acid type, which is proprietary but closely related to the octoates. The synthetic acid and neodecanoate driers can be prepared at higher metal concentrations than the other types and are gradually replacing the older acids.

Mooney Chemical, division of OMG Huls America Troy Corp. Mooney Chemical, division of OMG Huls America

Neodecanoate

Ten-Cem

Mooney Chemical, division of OMG

Naphthenate

Nap-All

Mooney Chemical, division of OMG Huls America Troy Corp.

Nuodex Troykyd Tallates

Lin-All

Mooney Chemical, division of OMG

Water dispersible

Hydro-Cem

Mooney Chemical, division of OMG Mooney Chemical, division of OMG Huls America Troy Corp. Ultra adhesives Ultra adhesives Ultra adhesives Ultra adhesives

Hydro-Cure Nuocure Troykyd WD Calcicat Aquacat Magnacat Zircat

32

PAINT AND COATING TESTING MANUAL TABLE 3--Alternate drier compounds.

Trade Name

Company

Activ-8

Drymax Nutra ADR 10%

R.T. Vanderbilt Co. Mooney Chemicals Inc. Huls America Huls America

Nutra LTD 18%

Huls America

Dri-RX

Composition 1,10-phenanthroline 2,2'-dipyridyl 2,2'-dipyridyl proprietary metal complex proprietary metal complex

thereby maintaining satisfactory drying on prolonged storage. Lead compounds, including litharge, were used formerly but have been replaced by lead-free compounds based primarily on less soluble forms of cobalt and other drier metals. Commercially available feeder driers are listed in Table 4. All are lead-free metal complexes except for the last item [3].

D R I E R L E V E L S IN COATINGS Drier requirements for coatings are usually expressed in terms of percent drier metal based on oxidizable vehicle nonvolatile content. A typical calculation is as follows [1]: Assume: 1. In a 1000-g paint formulation, there are 300 g of vehicle nonvolatile. 2. Cobalt drier used is 12% metal by weight. 3. Calcium drier used is 10% metal by weight. 4. Required for optimum drying: 0.05% cobalt plus 0.2% calcium (percent metal based on vehicle nonvolatile). per 1000 g of paint: Cobalt metal required = 0.0005 x 300 g = 0.15 g Calcium metal required = 0.002 x 300 g = 0.6 g (10% calcium drier required) = (0.6 g calcium metal) = (6 g (0.10 g metal/g drier) drier as supplied) (12% cobalt drier required) = (0.15 g cobalt metal) = (1.25 g (0.12 g metal/g drier) drier as supplied) The optimum levels of drier metal required will vary depending on the type of resin system employed and the conditions of drying. Typical metal concentrations for a number of common vehicles are indicated in Table 5.

TABLE 4--Commercial feeder driers.

Company Mooney Chemical, division of OMG Mooney Chemical, division of OMG Troy Corporation Huls America Huls America Huls America

T E S T I N G OF D R Y I N G E F F I C I E N C Y

Trade Name Hex-Cem LFD Hydroxy Ten-Cem Cobalt Troykyd Perma Dry Nuact Cobalt 254 Nuact NOPB Nuact Paste (lead-based)

The procedures used to determine the stages of film formation during the drying of coatings are described in ASTM Test Methods for Drying, Curing, or Film Formation of Organic Coatings at Room Temperature (D 1640) [4]. With coatings containing metallic driers, it is recommended that the paint samples age overnight following the drier additions before drying tests. The drying conditions, shown in Table 6, are usually specified but are subject to agreement between the purchaser and the seller. The methods used to determine the various stages of drying may be summarized as follows:

1. Set-To-Touch-Time--no

transfer of the coating upon lightly touching the film with the finger. 2. Dust-Free-Time--(a) cotton fibers dropped on the film can be removed by blowing lightly; (b) fine calcium carbonate extender dropped on the film can be completely removed by blowing gently and wiping with a cloth or brush. 3. Tack-Free-Times--a specified type of paper or aluminum foil will no longer adhere to the film when applied under specific conditions. 4. Dry-To-Touch-Time--the film no longer adheres to the finger when pressed firmly and does not rub up when rubbed lightly. 5. Dry-Hard-Time--a thumb print applied under specified conditions is completely removed from the film when polished lightly with a soft cloth. 6. Dry-Through-Time--the film is not distorted or detached when the thumb is applied to it in a specified manner and rotated through a 90~ angle. 7. Dry-To-Recoat--a second coat or top coat can be applied without development of lifting or loss of adhesion of the first coat. 8. Print-Free-Time--army duck or cheesecloth applied at a pressure of 3.5 or 6.9 KPa no longer marks the film as determined by photographic standards shown in ASTM Test Method for Print Resistance of Lacquers (D 2091) [4]. In addition to the above subjective tests, a number of mechanical drying time recorders are available. The instrument referred to most frequently in recent literature is the BykGardner Improved Circular Drying Time Recorder [7]. This functions by moving a Teflon stylus over the applied film in a circular path. The pattern left in the film by the stylus after a complete revolution is observed. Recorders are available which make complete revolutions in 1, 6, 12, or 24-h. With the use of a plastic template, set-to-touch, surface dry, and through-dry times may he noted. Development of film hardness is also an important criterion of drier efficiency. Tests [4] used to measure hardness include:

1. Test Method for Film Hardness by Pencil Test (D 3363)--a series of pencils varying in lead hardness from 6B (softest) through 6H (hardest) are pushed into the film, and the hardest pencil that will not penetrate the film is noted.

2. Test Method for Hardness of Organic Coatings by Pendulum Damping Tests (D 4366-87)--either a Konig or a Persoz Pendulum Hardness Tester is employed with the time, in seconds, noted for the swing amplitude of the pendulum to

CHAPTER 5 - - D R I E R S AND METALLIC SOAPS

33

TABLE 5--Typical drier recommendations, percent metal based on vehicle nonvolatile. Cobalt Long oil alkyd-air dry

Medium oil alkyd-air dry Short off alkyd air-dry

Chain-stopped alkyd

Manganese

0.04-0.06 0.04-0.06 0.04-0.06

Zirconium

Calcium

0,1-0.2

0.1-0.3 0.1-0.3 0.1-0.2

0.04-0.06 0.04-0.06 0.04-0.06

0.1-0.3

0.04-0.06 0.04-0.06 0.04-0.06

0.1-0.3

0.1-0.2

0.1-0.2

0.05-0.08 0.05-0.08

Medium oil alkyd-bake

0,0 I-0.03

Oil-modified urethane

0.02-0.04 0.02-0.04

0,1

Zinc

0.02

Alkyd-oil house paint

Epoxy Ester

0.02-0.03 0.02-0.03 0.02-0.04

0.1-0.3 0.1-0.2 0.2 0.1 O.I-0.5

0.1-0.3 0.1-0.3 0.02 0,02-0.04 0.02-0.04 0.02-0.03 0.02-0.03 0.02-0.04

0.03-0.05 0.03-0.05

Acrylic Modified alkyd

0.04-0.08

Oleoresinous varnish

0.02-0.06

Vinyltoluene alkyd

0.02-0.04 0.02-0.04

High solids alkyd

0.2 0.1-0.2 0.1-0.3

0.1-0.2 0.1-0.15 0.1-0.3 0.2-0.3

0.2-0.3 0.1-0.2

0.1-0.2

0.1-0.15

0, I-0.2 0.1-0.2 0.1

0,1 0.2

0.1 0.1

0.04-0.06

0.5-0.9 0.04-0.06

Water-based alkyds (water dispersible driers)

2,2'-Dipyridyl, 30%

0.1-0.3 0.1-0.2

0.02-0.04 Linseed oil

1,10-Phenantbroline

0.5-0.9

0.04-0.06

0.2-0.4

0.04-0.06 0.04-0.06 0.05-0.1

0.1-0.3 0.15-0.3

decrease b y a specified degree w h e n set into oscillation on the d r i e d film.

3. Test Methods for Indentation Hardness of Organic Coatings (D 1474)--either a K n o o p or a Pfund I n d e n t e r is a p p l i e d to a film u n d e r a specified loading, a n d the d e p t h of indentation is m e a s u r e d with the a i d of a microscope. The d e p t h is converted to either a K n o o p or a Pfund H a r d n e s s N u m b e r using the equations in the standard. TABLE 6--Standard drying conditions. Condition

Typical Value

Ambient temperature Relative humidity Film thickness (dry) Substrate Lighting Applicators Coating viscosity

23 + 2~ 50 + 5% 12.5 to 45 ~m Clean glass No direct sunlight Doctor blades Close to normal application

0.2 0.2 0.2-0.4

0.1-0.3 0.15-0.2

0.1-0.3

S P E C I F I C A T I O N S F O R LIQUID P A I N T DRIER Drier specifications as described in ASTM Specification for Liquid Paint Driers (D 600) involve the following classes: Class A 2-ethyl hexanoic acids in p e t r o l e u m spirits. Class B N a p h t h e n i c acids in p e t r o l e u m spirits. Class C N e o d e c a n o i c acids in p e t r o l e u m spirits. Class D Tall oil fatty acids in p e t r o l e u m spirits. Class E Any of the above, plus additives to m a k e the driers w a t e r dispersible. Class F Other unidentified acids a n d acid blends. A c o m p r e h e n s i v e table of liquid p a i n t driers of the above classes is given in ASTM D 600 [4]. The typical p r o p e r t i e s of the driers listed a n d r e p r o d u c e d in Table 7 include p e r c e n t m e t a l concentration, p e r c e n t nonvolatile content, specific gravity, G a r d n e r Color, a n d G a r d n e r - H o l d t viscosity.

34

PAINT AND COATING TESTING MANUAL TABLE

Class

Metal

A A

Calcium Calcium Calcium Calcium Calcium Calcium Calcium Calcium Calcium Calcium Calcium Calcium Calcium Cerium Cerium Cobalt Cobalt Cobalt Cobalt Cobalt Cobalt Cobalt Cobalt Cobalt Iron Iron Iron Iron Iron Lead Lead Lead Lead Lead Lead Lead Manganese Manganese

B B

C D E E F F F F F A B

A A B

C D E E F F A B

F F F A B

C D E F F A B

7--Typical requirements of liquid paint driers, a

Metal Concentration, % Min Max 3.9 4.9 3.9 4.9 4.9 3.9 3.9 5.9 3.9 4.9 5.9 7.9 9.9 5.9 5.9 5.9 11.8 5.9 11.8 5.9 4.9 5.9 5.9 11.8 5.9 5.9 5.9 8.9 11.8 23.8 23.8 23.8 23.8 23.8 23.8 35.8 5.9 5.9

4.1 5.1 4.1 5.1 5.1 4.1 4.1 6.1 4.1 4.1 6.1 8.1 10.1 6.1 6.1 6.1 12.2 6.1 12.2 6.1 5.1 6.1 6.1 12.2 6.1 6.1 6.1 9.1 12.2 24.2 24.2 24.2 24.2 24.2 24.2 36.2 6.1 6.1

Nonvolative Matter, %, Max 50 60 70 85 46 66 63 76 50 60 74 70 65 30 57 45 90 67 65 72 60 71 70 80 50 67 50 78 75 65 67 61 66 71 67 81 50 66

Typical Specific Gravity, 25/25~ Min Max 0.884 C 0.894 0.902 0.932 0.888 0.890 0.905 0.922 0.850 0.900 0.873 0.958 1.000 0.856 0.925 0.875 1.008 0.918 0.984 0.912 0.926 0.945 0.870 1.014 0.900 0.960 0.905 0.950 1.068 1.090 1.125 1.100 1.100 1.125 1.080 1.350 0.888 0.930

.-0.912 0.937 0.970 ..0.918 0.930 0.960 0.884 0.936 0.948 --. 1.030 ... ..0.900 1.060 0.970 ... 0.956 -.. 0.960 0.958 1.040 0.930 0.990 0.930 0.985 ..1.110 1.160 ... 1.125 1.150 1.140 1.393 0.920 0.965

Colorb Gardner (Test Method D 1544)

G-H Viscosity (Test Method D 1545)

3 5 10 11 2 9

A C D T A B

8

G

5 3 4 6 5 7-8

N A B N B K

8

17 Blue/purple Blue/purple Blue/purple Blue Purple Red/purple Blue/purple Blue/violet Blue/violet Dark brown Dark brown Brown Brown Brown 3 11 2 10 7 10 8 Red/brown 17

A

A1 A J B A C A I A J A M A A A A B B A2 A A H A D

Continued

T E S T I N G OF LIQUID P A I N T D R I E R S

6. Drying power--As d e s c r i b e d in t h e s e c t i o n e n t i t l e d "Test-

A S T M Test M e t h o d for L i q u i d P a i n t D r i e r s (D 564) [4] outlines t h e test p r o c e d u r e s e m p l o y e d , i n c l u d i n g b o t h p h y s i c a l a n d c h e m i c a l tests. T h e p h y s i c a l tests i n c l u d e :

7. Viscosity--According to A S T M T e s t M e t h o d for V i s c o s i t y

ing of D r y i n g Efficiency."

1. Appearance--observations for clarity a n d c l e a n n e s s in ac2.

3. 4.

5.

c o r d a n c e w i t h A S T M Test M e t h o d for Clarity a n d Cleanness of P a i n t L i q u i d s (D 2090). Color--according to A S T M D 1544, w h i c h e m p l o y s t h e G a r d n e r n u m e r i c a l c o l o r scale. H o w e v e r , a n u m b e r of driers, i n c l u d i n g cobalt, m a n g a n e s e , nickel, a n d r a r e earth, d o n o t fit i n t o this scale a n d are r e p o r t e d descriptively. Nonvolatile Content--According to A S T M D 1644, M e t h o d A o r B. M e t h o d A involves h e a t i n g s a m p l e s at 105~ for 10 m i n , w h i l e M e t h o d B specifies 150~ for 10 m i n . Miscibility with Oil--One v o l u m e of t h e d r i e r s a m p l e is m i x e d w i t h 19 v o l u m e s of r a w l i n s e e d oil. T h e m i x t u r e is o b s e r v e d for a n y signs of s e p a r a t i o n o r c l o u d i n g o v e r a 24-h period. Stability--The d r i e r s a m p l e is s t o r e d for 7 days at 25~ - 20~ a n d 50~ a n d e x a m i n e d for i n d i c a t i o n s of clotting, gelation, o r p r e c i p i t a t i o n .

o f T r a n s p a r e n t L i q u i d b y B u b b l e T i m e M e t h o d (D 1545). This involves c o m p a r i n g t h e t i m e of travel of b u b b l e s in t u b e s of t h e s a m p l e v e r s u s G a r d n e r - H o l d t s t a n d a r d tubes. T h e s t a n d a r d s w e r e f o r m e r l y d e s i g n a t e d by l e t t e r b u t are n o w i n d i c a t e d d i r e c t l y in stokes. A table in D 1545 indicates t h e c o n v e r s i o n f r o m letters to stokes. C h e m i c a l analysis is u s e d to d e t e r m i n e t h e m e t a l c o n t e n t of l i q u i d p a i n t driers. T h e E D T A m e t h o d is u s e d for m o s t d r i e r m e t a l s (Table 8). T h e l i q u i d d r i e r is d i s s o l v e d o r d i g e s t e d in solvents a n d t h e n t r e a t e d w i t h a n excess of s t a n d a r d E D T A s o l u t i o n ( d i s o d i u m salt of e t h y l e n e d i a m i n e t e t r a c e t i c a c i d dihydrate). T h e excess of E D T A is t h e n t i t r a t e d to a n e n d p o i n t d e t e r m i n e d b y a specified i n d i c a t o r . This m e t h o d is a p p l i c a ble to single m e t a l d r i e r s only, n o t to d r i e r blends. An E D T A m e t h o d is n o t yet a v a i l a b l e for c e r i u m , a n d a n o x i d i m e t r i c d e t e r m i n a t i o n is specified [ASTM Test M e t h o d for C e r i u m in P a i n t D r i e r s by O x i d i m e t r i c D e t e r m i n a t i o n (D 3970)]. T a b l e 8 o u t l i n e s t h e A S T M d e s i g n a t i o n s , i n d i c a t o r s , a n d t i t r a t i n g sol u t i o n s for analysis o f d r i e r m e t a l s by E D T A titration.

CHAPTER 5 - - D R I E R S AND METALLIC SOAPS

35

TABLE 7--Continued

Class

Metal

C D E E F F F A A A B C A A B B D E F F A A A A C C E E F F F F

Manganese Manganese Manganese Manganese Manganese Manganese Manganese Nickel Rare earth a Rare earth Rare earth Rare earth Zinc Zinc Zinc Zinc Zinc Zinc Zinc Zinc Zirconium Zirconium Zirconium Zirconium Zirconium Zirconium Zirconium Zirconmm Zirconmm Zirconium Zirconium Zirconium

Metal Concentration, % Min Max 5.9 5.9 4.9 5.9 5.9 8.9 11.8 9.9 5.9 11.8 3.9 5.9 7.9 17.8 7.9 9.9 7.9 7.9 7.9 15.8 5.9 11.8 17.8 23.8 5.9 11.8 5.9 11.8 5.9 11.8 17.8 23.8

6.1 6.1 5.1 6.1 6.1 9.1 12.2 10.1 6.1 12.2 4.1 6.1 8.1 18.2 8.1 10.1 8.1 8.1 8.1 16.2 6.1 12.2 18.2 24.2 6.1 12.2 6.1 12.2 6.1 12.2 18.2 24.2

Typical Specific Gravity, 25/25~ Min Max

Nonvolative Matter, %, Max 50 72 42 69 55 80 75 70 30 55 35 35 50 90 70 75 70 42 60 80 30 56 55 77 23 46 31 55 28 55 80 93

0,870 0.942 0.911 0.942 0.870 0.950 1.044 1.032 0.630 0.977 0.840 0.876 0.880 1.068 0.915 0.980 1.008 0.946 0.855 1.020 0.860 0.960 1.070 1.240 0.864 0.976 0.864 0.975 0.855 . . 1.090 1.240

.

.-. 0.972 ... 0.965 1.020 1.020 ... .-. 0.880 -.. 0.855 ... 0.906 1.130 0.960 1.044 ... ... 0.963 1.100 0.864 0.992 1.074 -.. .-... ... 1.020 0.870 . . 1.130 1.260

.

Colorb Gardner (Test Method D 1544)

G-H Viscosity (Test Method D 1545)

10 Brown Brown Brown 18 18 16 Green 6 Yellow/green 10 8 6 7 9 8 11 2 7 3 2 4 3 2 2 2 4 4 2 . . . . 4 6

A E E E A H C A E C A A5 G Z A L D A C B A A A J A5 A A A A5 .

. Z J

"Source: National Paint and Coatings Association, Chemical Specialties Section, Raw Material Index, April 1978 edition per Gardner Method D 1544. blf off the scale, as observed by the unaided eye. ~'Only one drier was listed in this category. aThe metal content represents total rare earth metals calculated as cerium, but the drier contains cerium and lanthanum, as well as minor amounts of other rare earth metals. TABLE 8 - - M e t a l analysis by EDTA titration. Metal Cobalt Lead Manganese Calcium or zinc Iron Zirc Vanadium Total rare earth

ASTM Method D D D D D D D D

2373 2374 2375 2613 3804 3969 3988 3989

Solvent

Indicator

Titrating Solution

Glacial acetic acid Glacial acetic acid Toluene-ethanol Toluene-ethanol Isopropanol HSO-HO Isopropanol-HCL Isopropanol-HCL

PANa PAN~ E r i o c h r o m e hlack-T E r i o c h r o m e black-T E r i o c h r o m e black-T Xylenol orange Xylenol orange Xylenol orange

Cupric sulfate Cupric sulfate Zinc chloride Zinc chloride Zinc chloride B i s m u t h nitrate Zinc chloride Zinc chloride

a1(2-Pyridylazo)-2-Naphthenol.

REFERENCES [1] Hurley, R., "Metal Soaps: Drier Stabilizers a n d Related Compounds," Handbook of Coatings Additives, Chap. 13, Marcel Dekker, Inc., New York, 1987, pp. 485-509. [2] Schnall, M., "Thickeners for Solvent-Based Coatings," Handbook of Coatings Additives, Chap. 3, Marcel Dekker, Inc., New York, 1987, pp. 33-34. [3] Raw Materials Index, Chemical Specialties Section, National Paint & Coatings Association, W a s h i n g t o n , DC, March 1989, pp. 2-11 a n d pp. 22-27.

[4] Annual Book of ASTM Standards, Books 6.01 a n d 6.03, American Society for Testing a n d Materials, Philadelphia, 1990. [5] Godbole, V. A., "Use of Metallic Driers in Organic Coatings," Paint India, April 1986, pp. 28-25. [6] Belletiere, S. J. a n d Mahoney, D. M., "Multi-Metallic Complexes: The Next Generation of Driers," Journal of Coatings Technology, Vol. 59, No. 752, S e p t e m b e r 1987, pp. 101-108. [7] Instrument Catalogue, Section 9, "Drying Time," Byk-Gardner, Inc., Silver Springs, MD, 1990.

Part 3: Synthetic Materials

MNL17-EB/Jun. 1995

6

Acrylic Polymers as Coatings Binders by John M. Friel I

ranging from a few hundred to a few thousand centipoise. In later years, 100% solid-grade versions became popular since they reduced the cost and safety concerns associated with shipping resins containing high solvent levels. The coatings manufacturer is then able to dissolve the solid-grade acrylic in a wide range of solvents, thereby having greater control over the formulation ingredients. A wide range of properties can be built into an acrylic coatings binder by careful selection of the type and level of the acrylic monomers used. Some of the important properties for several acrylic homopolymers are shown in Table 1 [3]. Coatings for a wide range of applications can therefore be custom designed. Some of the principal applications for acrylic solution coatings include automotive finishing, factory, and farm implement coatings, general-purpose pigmented coatings, aerosol lacquers, and clear coatings for a multitude of substrates. There are two types of acrylic solution polymers: (1) thermoplastic polymers, which harden simply by loss of solvent through evaporation; and (2) thermosetting polymers, which contain functional groups that react with another functional material (i.e., melamine, epoxy, isocyanate, etc.) to form a cross-linked network. The advantages and disadvantages of each are discussed in detail later in this chapter. Acrylic emulsion polymers (often referred to as acrylic latexes) have become one of the major binder types in use in the coatings industry today. To form an emulsion polymer, the acrylic monomers are emulsified and then polymerized as small droplets in a continuous water phase. The droplets are typically stabilized by surfactants, and usually no solvent is present. While acrylic emulsions are generally associated with quality architectural coatings, they are also used to formulate industrial coatings. In fact, the use of acrylic emulsions in industrial applications is expanding at the expense of

ACRYLICPOLYMERS, WHICHARE USED as coatings binders, are comprised chiefly of esters of acrylic and methacrylic acid that are polyrnerized by additional polymerization, usually using a free radical mechanism: H

CH 3

I (--CH2--C - - ) - -

I --(--CH2--C--)--

I C ~0

I

OR An acrylate

I C=0

t

OR A methacrylate

Interest in acrylic technology dates back to the 1920s when Dr. Otto Rohm developed a practical process for making acrylate esters from ethylene. Shortly afterwards, his associate, Otto Haas, established the first commercial production of methyl and ethyl acrylate in the United States [1]. The first commercial use of an acrylic polymer was as an adhesive-like interlayer for laminated safety glass. Probably the highest profile use of an acrylic began in 1936 with the introduction of thermoplastic, transparent methacrylate sheet. With the advent of World War II, methacrylate sheet became invaluable as a tough, weather-resistant glazing material for military aircraft. Since it could be formed easily and had excellent optical properties, the transparent plastic was used for aircraft canopies, bomber noses, and gun turrets [2]. Acrylic technology soon expanded into the coatings industry in the form of acrylic solution polymers, followed later by acrylic emulsions. The acrylics gained widespread market acceptance as coatings binders due to such outstanding properties as color stability, transparency, and resistance to weathering and aging. The good weathering resistance of acrylic polymers is primarily due to their resistance to hydrolysis and their lack of absorption of ultraviolet (UV) light, the highenergy portion of the light spectrum most responsible for degradation. Acrylic solution polymers (often referred to as acrylic resins) are generally copolymers of acrylate and methacrylate esters prepared by direct solution polymerization in a solvent that has a solubility parameter similar to that of the polymer. Typical solvents include aromatics such as toluene and xylene, as well as ketones and esters. Acrylic resins are typically supplied at about 30 to 50% solids by weight, with viscosities

TABLE 1--Properties of polymethacrylates and polyacrylates [3]. PolymerTypes Tensile Strength, psi Elongation, % Polymethacrylate

Methyl Ethyl Butyl

4 7 230

1000 33 3

750 1800 2000

Polyacrylates

Methyl Ethyl Butyl

1Group leader and research fellow, Architectural Coatings Research, Rohm and Haas Co., Research Laboratories, 727 Norristown Road, Spring House, PA 19477.

NOTE:Psi + 14.22 = k g / c m 2. (Reprinted with permissionof Modem Paint and Coatings. Copyright1973). 39

Copyright9 1995 by ASTM International

9000 5000 1000

www.astm.org

PAINT AND COATING TESTING MANUAL

40

solvent-based systems because of the industry's need to control organic emissions. Over the past 20 years, acrylic emulsion manufacturers have made great strides in improving the properties of acrylic emulsions so that they now offer performance similar to the solvent-based coatings they are replacing. When the first acrylic emulsion designed for use in house paints was introduced in 1953, it had the low-odor, quickdrying, and easy cleanup features of its water-based competitors, styrene-butadiene and poly(vinyl acetate) emulsions; but, in addition, it offered excellent exterior durability that allowed use in exterior paints. During the past 40 years, it has been good exterior durability that enabled acrylic emulsions to replace solvent alkyds as the dominant binder in the exterior house paint market.

ACRYLIC SOLUTION POLYMERS Thermoplastic Resins Thermoplastic acrylic resins are acrylic polymers that are polymerized directly in a suitable solvent and form a film solely by evaporation of the solvent. They do not need to be oxidized or cross-linked to form a hard, resistant finish. They are fast-drying lacquer materials, but they remain permanently soluble. Acrylic resins are usually supplied in strong solvents such as toluene, xylene, or methyl ethyl ketone. They are clear, colorless solutions and, if left unpigmented, will also dry down to clear, colorless films. They are often used in unpigmented form as protective finishes over vacuum metalized plastics and polished metals such as brass. Acrylic resins generally make excellent grind media for dispersing pigments. No external pigment wetting agents are required to make finely dispersed pigment grinds for highgloss lacquers. Also, thermoplastic acrylic polymers are quite unreactive and consequently are stable when mixed with pigments, extenders, and colors. They do not discolor powdered metals, such as aluminum. Acrylics are a uniquely versatile family of polymers since an infinite array of properties can be achieved by carefully selecting combinations of the various acrylic monomers. Each acrylic monomer brings to the polymer its own individual performance characteristics based on its molecular structure. This is particularly true for polymer hardness as determined by the glass transition temperature (Tg) of the monomers that make up the homopolymer (only one monomer) or copolymer (two or more monomers). The Tg of a polymer is a softening point: it is actually a temperature range where the polymer undergoes a second-order transition. At temperatures below the Tg, the polymer is a glass, but above the Tg the polymer is a rubbery material. To approximate the Tg for a copolymer composition, it is useful to utilize the relationship proposed by Fox [4].

1

_

W 1

+

W2

(1)

Tgl and

Tg 2 =

the Tg's of the homopolymers of Monomers 1 and 2 in degrees absolute.

Since thermoplastic acrylics are not cross-linked to achieve a desired level of performance, the concept of Tg and the ability to manipulate Tg as a means to control properties is crucial in designing polymers that meet the needs of the coatings market. The marked difference in Tg's, and consequently polymer characteristics of the acrylics, can phenomenologically be explained by the free-volume theory proposed by Fox and Flory [5] and later refined by several others. The free-volume theory states that the Tg for any given polymer occurs at that temperature where the fractional free volume (i.e., unoccupied space contained within the polymer) reaches some universally constant value that remains unchanged as temperature decreases below Tg. Above this temperature, the free volume increases, permitting sufficient molecular motion so polymer flow can begin. In Fig. 1, Rogers and Mandelkern have plotted specific volume versus temperature for a series of methacrylates as a means of establishing the relationship of Tg to free volume [6]. The arrows ( T ) in Fig. 1 indicate the temperature at which there is an inflection in the specific volume curve indicating a sudden increase in free volume (as temperature increases). This is the Tg. From the graph, it can be calculated that, at Tg, free volume accounts for 15% of the total polymer volume [6]. Simha and Boyer have independently calculated that at Tg, free volume accounts for 11% of a polymer's total volume [7].

1.22 1.21 1.1~ 1.1"

,, -"" ""

11"111"1~."" 09 ,..,"/", . / ' ~ j

,, ""

.,

m

C18

0 -

E

/ " "

1.05

...,. ,.

1.o3

/

1.01 , * '12

0

/

~

./

.,.//

.

f/

/

" -

.-"

0.97

0.05 0.93 0.91 C ~ ~ ~ ~ + ~ 0.89 0.87 0.85 0.83 ~ t t t t t t t f t -80 -60 -40 -20 0 20 40 60 80 100 120 140

Temperature, ~ where W1

and W2 = the weight ratios of Monomers 1 and 2, respectively,

FIG. 1 -Specific volume-temperature relations for the poly-(nalkyl methacrylates). (Reprinted with permission from the American Chemical Society. Copyright 1957,)

CHAPTER 6--ACRYLIC POLYMERS AS COATINGS BINDERS On a molecular level, the Tg differences for the acrylic family of polymers can also be easily explained. The acrylates have an alpha-hydrogen atom next to the carbonyl group, giving them more rotational freedom and hence greater segmental chain motion than the methacrylates. The methacrylates have a bulky methyl group substituted for the alphahydrogen atom, which hinders molecular rotation, thereby increasing chain stiffness. The methacrylates are therefore higher in Tg, harder, higher in tensile strength, and lower in elongation than analogous acrylate polymers. Equally important to Tg and polymer hardness is the length of the ester side chain of the monomer. As the alcohol moiety of the ester side chain becomes larger, the polymer chains are pushed further apart, creating additional free volume, thereby encouraging greater molecular rotation. Consequently, the polymers b e c o m e increasingly soft (as the ester side chain becomes larger) until the effects of side-chain crystallization causes additional hardening effects. Table 2 demonstrates the large range of Tg's that exist for the acrylate and methacrylate family of homopolymers [8]. The second most important parameter governing the film properties of a thermoplastic acrylic polymer is molecular weight (MW). Most dry film properties for thermoplastic acrylics improve with increasing molecular weight up to a MW of about 100 000 and then tend to level off. Tensile strength, elongation, toughness, solvent resistance, and exterior durability are all dependent on molecular weight. This dependence of strength, elongation, and resistance of the acrylic polymer on molecular weight is primarily due to the greater n u m b e r of chain entanglements (which act almost like cross-links), anchoring the polymer chains at higher molecular weight (i.e., longer chain length). However, the viscosity of a solution polymer is proportional to the molecular weight of a polymer according to the Mark-Houwink equation [9]. TABLE

Polymer

n = K(MW) a

(2)

where 91 = solution viscosity K and a = constants derived experimentally for a specific polymer/solvent combination (for polymethyl methacrylate in toluene, K • l0 s = 7.1 and a -- 0.73). The type molecular weight used in determining the constants should be specified. As molecular weight increases, the solution viscosity also increases, thereby posing handling and application problems if the molecular weight becomes too high. For example, high spray solids are desirable for acrylic lacquers because of e c o n o m y and emissions concerns. The lower the molecular weight of the polymer, the lower the viscosity at a given solids content, or conversely, the higher the spray solids at spray viscosity. Consequently, low molecular weight is beneficial to solids and application concerns, whereas high molecular weight is needed for good film properties. The obvious answer to the viscosity versus film property dilemma is to reach an o p t i m u m balance by producing an intermediate molecular weight polymer. For this reason, most thermoplastic acrylic solution polymers have weightaverage molecular weights in the 75 000 to 100 000 range. It is also helpful to narrow the molecular weight distribution, reducing the a m o u n t of low-molecular-weight fractions, which have a deleterious effect on resistance and strength properties, while also minimizing high-molecular-weight portions, which increase viscosity and cause application problems such as cobwebbing of spray-applied acrylic automotive lacquers. Since the application and drying properties of an acrylic resin are largely controlled by the physical characteristics of the solvent contained in the resin, as well as by the interaction of the polymer and solvent, it is essential to carefully select

2--Tg for methacrylate and acrylate homopolymers [8]. Tg, ~ Polymer

poly(methyl methacrylate) poly(ethyl methacrylate) poly(n-propyl methacrylate) poly(isopropyl methacrylate) poly(n-butyl methacrylate) poly(sec-butyl methacrylate) poly(isobutyl methacrylate) poly(t-butyl methacrylate) poly(n-hexyl methacrylate) po]y(2-ethylbutyl methacrylate) poly(n-octyl methacrylate) poly(2-ethylhexyl methacrylate) poly(n-decyl methacrylate) poly(lauryl methacrylate) poly(tetradecyl methacrylate) poly(hexadecyl methacrylate) poly(octadecyl methacrylate) poly(stearyl methacrylate) poly(cyclohexyl methacrylate) poly(isobornyl methacrylate) poly(phenyl methacrylate) poly(benzyl methacrylate) poly(ethylthioethyl methacrylate) poly(3,3,5-trimethylcyclohexylmethacrylate)

41

105 65 35 81 20 60 53 107 - 5 11 - 20 - 10 - 60 - 65 - 72 - 100 104 170(110) 110 54 -20

poly(methyl acrylate) poly(ethyl acrylate) poly(propyl acrylate) poly(isopropyl acrylate) poly(n-butyl acrylate) poly(sec-butyl acrylate) poly(isobutyl acrylate) poly(t-butyl acrylate) poly(hexyl acrylate) (brittle pt) poly(heptyl acrylate) poly(2-heptyl acrylate) poly(2-ethylhexyl acrylate) poly(2-ethylbutyl acrylate) poly(dodecyl acrylate) (brittle pt) poly(hexadecyl acrylate) poly(2-ethoxyethyl acrylate) poly(isobornyl acrylate) poly(cyclohexyl acrylate)

Tg, ~ 6 - 24 -45 - 3 - 55 - 20 - 43 43 -

57 60 38 50 50

- 30 35 - 50 94 16

79

NOTE:The brittle point measured by a fracture test often approximates Tg. (Reprinted by permission of John Wiley & Sons, Inc. From Encylopedia of Polymer Science and Engineering, Vol. l, 2nd ed., New York, Copyright 1985).

42

PAINT AND COATING TESTING MANUAL

the solvent in which the acrylic is dissolved (see Chapter 18 entitled "Solvents"). To ensure good solubility of the polymer, it is important to match the solubility parameter of the solvent to that of the polymer. The solubility parameter is an estimation of the polarity of a solvent or polymer and is related to the intermolecular energy of the molecule (see Chapter 35 entitled "Solubility Parameters"). The solubility parameter concept was defined by Hildebrand [10] and applied to coatings by Burrell. Burrell has published the solubility parameters for an extensive list of solvents [11]. For polymers, the solubility parameter can be calculated by knowing the molecular structure of the repeating unit according to Small's method. Small has published a table of molar attraction constants used to calculate polymer solubility parameters [12]. Besides being helpful in estimating solubility, the solubility parameter concept is helpful in predicting the resistance of polymers to solvents or other organics. In general, the more polar acrylic polymers will have the best resistance to hydrophobic materials, such as gasoline, grease, or oil. More hydrophobic acrylics (with low 8 values) will have better resistance to polar materials, such as water and alcohol. The evaporation rate of the solvent or solvent mixture must also be carefully chosen to accommodate the expected application method and conditions. For spray application, moderately fast evaporating solvents are needed to avoid running and sagging of the low-viscosity paint. For roller coating, a much higher viscosity coating would be used; therefore, slower evaporating solvents are required to avoid skinning on the roller and to allow for flowout of roller pattern created during application of the paint. Since thermoplastic acrylics dry by evaporation of solvent alone, extremely slow-drying solvents, which retard development of properties, should be avoided. The majority of thermoplastic acrylic solution polymers are designed for general-purpose industrial finishing (i.e., metal furniture and product finishing) and have a Tg of approximately 50~ This Tg is generally obtained by copolymerizing combinations of methyl methacrylate (MMA), butyl methacrylate (BMA), ethyl acrylate (EA), butyl acrylate (BA), and ethylhexyl acrylate (EHA). While many other acrylate and methacrylate monomers exist, as indicated in Table 2, these few are the primary acrylic monomers that are commercially available and that are, therefore, the most economically feasible. At a Tg of 50~ these acrylic polymers are intermediate in hardness, having a Tukon hardness of about 11 to 12 [see ASTM Test Methods for Indentation Hardness of Organic Coatings (D 1474)]. They are hard enough to dry rapidly to a tack-free state that allows early handling of the coated product and also hard enough to resist marring, print [see ASTM Test Method for Print Resistance of Lacquers (D 2091)], block [see ASTM Test Method for Blocking Resistance of Architectural Paints (D 4946)], and dirt pickup. Yet, they retain enough flexibility and elongation to have some impact resistance [see ASTM Test Method for Resistance of Organic Coatings to the Effects of Rapid Deformation (Impact) (D 2794)], and some can even be post-fabricated, such as is done with coil coated stock to produce aluminum gutters, trim pieces, etc.

Up to the mid-1950s, nitrocellulose lacquers were the major automotive coating in use. Nitrocellulose finishes gave an excellent original appearance but had poor durability, particularly gloss retention, and required frequent polishing by the car owner for the finish to took good. This problem was eliminated in 1956 when General Motors adopted acrylic lacquers based on poly(methyl methacrylate). The acrylic lacquers gave significantly better durability and allowed for the use of the more modern eye-catching metallic pigments [13]. The acrylic lacquers generally contain external plasticizers, such as the phthalates, which contribute to improved chip resistance and cold crack resistance. No internal plasticizing m o n o m e r (i.e., acrylates) is generally contained in automotive acrylic lacquers, and consequently their Tg is approximately 105~ with a Tukon hardness of about 22. Since they are very hard and fairly high molecular weight (i.e., 100 000), the thermoplastic solution polymers designed for automotive use are not capable of the excellent molecular flow that would be expected of softer/lower-molecular-weight polymers. Consequently, the acrylic lacquers require factory buffing and or baking to obtain the kind of m a x i m u m gloss required for the new car showroom. This disadvantage was shared by the older nitrocellulose lacquers. Also, because of hardness and high molecular weight, the spray solids percent is tow. Since these lacquers are thermoplastic, they are permanently subject to softening by strong solvents, such as toluene or acetone, if for some reason they would contact the automotive finish. Conversely, however, the thermoplastic acrylic lacquers can be easily repaired by an additional coat of paint which "melts" into the original coat, leaving no "two-coat" effects or intercoat adhesion problems.

Thermosetting Acrylic Resins Thermosetting acrylic resins are compositionally very similar to the thermoplastic-type acrylics, with the exception that they contain functional groups, such as carboxyl or hydroxyl, that are capable of reacting with another polymeric or monomeric multifunctional material to produce a three-dimensional network structure. As has already been discussed, the mechanical properties of thermoplastic acrylic polymers for coatings are generally improved by increasing molecular weight, but polymers with overly high molecular weight produce solutions of unworkably high viscosity. An alternate route to improved film properties is to use a thermosetting acrylic polymer, converting linear, moderate-molecularweight polymer chains to an infinite molecular weight structure. This cross-linking reaction takes place after the coating has been applied to the substrate, often by the application of heat, hence the term "thermosetting." To be truly crosslinked, one of the reactive species must have at least two reactive sites, while the other species has at least three reactive sites per molecule or chain. Thermosetting acrylic polymers offer the following advantages over thermoplastic acrylics: (1) improved hardness and toughness, (2) better resistance to softening at elevated temperatures, (3) improved resistance to solvents, stains, and detergents, and (4) lower applied molecular weight, resulting in lower solution viscosity and consequently higher application solids.

CHAPTER 6--ACRYLIC POLYMERS AS COATINGS BINDERS Over the years, numerous chemical reactions utilizing various functional groups have been devised as a means to crosslink acrylic polymers. However, we will elaborate on some of the more commercially significant thermosetting acrylics, namely those cross-linked with nitrogen resins, epoxies, and isocyanates.

Acid-Functional Acrylics Cross-Linked with Epoxy Resins Possibly the most resistant acrylic enamels are based on thermosetting acrylic solution polymers that contain acid functionality and are reacted with an epoxy resin. Typically, the epoxy resin is a condensation product of Bisphenol A and epichlorohydrin (see Chapter 10 entitled, "Epoxy Resins in Coatings").

H I

CH2--C--CH2-\0 /

~----~

~H~

,--, --O--(z

7

&,--, x)--C--(/

\

/ \~/

I

/

&

\ ) - - O - - C H 2 - - C H - - C H2

\

/

\/

O

CH~ \ ~ /

The acrylic solution polymer is made acid functional simply by incorporating acrylic or methacrylic acid into the backbone; when cured under suitable conditions, it reacts with the epoxide to form ester linkages between the two polymers: E P O X Y - - C H - - C H 2 + ACRYLIC--COOH

\/

O EPOXY--CH--CH2

I OH

I OCO--ACRYLIC

An alternative reactant to epoxies based on Bisphenol A/epichlorohydrin is another acrylic polymer wherein epoxide sites have been incorporated by polymerizing glycidyl acrylate, glycidyl methacrylate, or allyl glycidyl ether into the acrylic backbone. This approach is limited in use, however, because both economics and performance favor the Bisphenol A type epoxides. Also, in recent years, epoxy manufacturers have developed many aliphatic epoxides in an attempt to mimic the benefits of the workhorse Bisphenol A based products (i.e., resistance properties) without the accompanying disadvantages (i.e., poor resistance to UV light). The carboxyl-functional acrylic resins typically have a molecular weight in the 10 000 to 50 000 range and a carhoxyl content of 5 to 20%. Some higher solids acrylics are lower in molecular weight with higher acid content. The acid content of the acrylic polymer determines the equivalent weight of

43

epoxy resin required to react with the acrylic to achieve a thoroughly cross-linked system. Styrene or vinyltoluene are often incorporated into the acrylic polymer as "honorary" acrylic monomers because their reaction kinetics with acrylic monomers are fairly good and because they often improve alkali resistance and are low in cost. The cross-linking reaction between the carboxyl group on the acrylic polymer and the epoxy resin is fairly sluggish and requires a base catalyst such as dodecyl trimethyl a m m o n i u m chloride, or tri(dimethylaminomethyl) phenol. In some instance, melamine-formaldehyde resins are sufficiently basic to be used as catalyst and have the additional advantage of entering into the cross-linking reaction [13]. For improved package stability, however, it is preferable to add the base catalyst to the other components just prior to use. Even with catalyst, the baking or stoving requirements for acid/epoxide systems are fairly high, with 15 to 30 min at 150 to 190~ being typical. Of course, the higher the bake temperature, the shorter the bake time required for cure. To determine the optimum conditions for a given system, it is best to cure that system at a variety of baking conditions and then measure properties to determine the temperature and time required for m a x i m u m performance. The primary uses for acid-functional acrylics cross-linked with epoxy resin are as appliance enamels and coatings for interior metal furniture and equipment. Acrylic/epoxy enamels fill the needs of the appliance industry primarily because of their excellent alkali/detergent resistance [see ASTM Practice for Detergent Resistance of Organic Finishes (D 2248-89)], which is critical to the performance of washing machine coatings. Other properties that are important to the appliance industry include: hardness, scratch resistance, grease resistance, stain resistance, as well as flexibility, impact resistance, and adhesion to metal. Acrylic/epoxy enamels, on the whole, offer an excellent balance of these important properties. Typical properties for an enamel based on an acid-functional acrylic resin cross-linked with a Bisphenol A based epoxy are shown in Table 3. The disadvantages associated with acrylic/epoxy coatings are usually concerns brought about by the aromatic nature of the Bisphenol A based epoxy cross-linker. Their most significant limitation is poor resistance to UV light, which restricts their use to interior applications. If used outside, the aromatic Bisphenol A portion of the epoxy would degrade rapidly, and early chalking would occur. In some of the newer high-solids acrylic/epoxies, the epoxy content is very high and can therefore cause discoloration problems even indoors due to UV light. Where this i~ a problem, an aliphatic epoxy can be substituted for at least some of the Bisphenol A based epoxy to reduce sensitivity to UV radiation (see Chapter 10 entitled "Epoxy Resins in Coatings").

Acrylic Polymers Cross-Linked with Amino Resins Acrylic polymers containing acid or hydroxyl functionality can be cross-linked with amino resins such as: urea, melamine, and benzoguanamine formaldehyde condensates (see Chapter 8 entitled "Amino Resins") as follows: AMINO RESIN--NHCH2OR + ACRYLIC--COOH > AMINO RESIN--NHCH20CO--ACRYLIC + ROH

44

PAINT AND COATING TESTING MANUAL

T A B L E 3--Application properties for a white enamel based on an acid functional acrylic resin cross-linked with a Bisphenol A based epoxy [14] (baked 30 rain at approximately 180~

Tukon hardness

16.2

Pencil hardness

2H

Solvent resistance, glass Cellosolve acetate, 15 rain Cellosolve acetate, 60 rain Xylol, 15 min

6B 6B 5B

Stain resistance, cold-rolled steel no stain trace

Mustard, 30 min Ink, 30 min Print resistance, cold-rolled steel 82~

light print

30 min, 2 psi

Detergent resistancea 1% detergent, 74~

200 h

Few--6

Optical properties, CRS (two coats) Original 60~ gloss Gloss after 16 h at 178~ Original color Color after 16 hr at 178~

95.5 95.4 9.2 13.0

Microknife adhesion, CRS "H" Value

22.8

Mandrel Flexibilityb 1/2 in., 1/4 in., 1/8 in. Cold-rolled steel Bonderite 1000

0-0-0 0-0-0

Reverse impact, inch-lbs (Joule) Cold-rolled steel Bonderite 1000

22 (2.48) 15 (1.70)

Direct impact, inch-lbs (Joule) Cold rolled steel Bonderite 1000

35 + (3.96 + ) 50 + (5.65 +)

~ASTMblister rating. A rating of 10 means no blistering, a rating of 0 means very large blisters, with intermediate ratings judged by ASTMphoto standards. ~ = no cracks; 9 = delamination.

AMINO R E S I N - - N H C H 2 O R + A C R Y L I C - - O H ) AMINO R E S I N - - N H C H 2 O--ACRYLIC + ROH Reactions with a m i n o resins containing an -NH-CH2OH group are possible b e c a u s e this group differs from a simple alcohol in that it is far m o r e acidic a n d reactive. Likewise, the methylol e t h e r (when c a p p e d with alcohol) is m o r e reactive t h a n a conventional dialkyl ether. The curing c o n d i t i o n required for acid functional acrylics cross-linked with a m i n o resins is a p p r o x i m a t e l y 30 m i n at 150~ while for an analogous hydroxyl functional acrylic, the r e a c t i o n is m o r e facile, requiring 30 m i n at 125~ with an acid catalyst [15]. Since the acid-methylol r e a c t i o n is relatively slow, it allows significant self-condensation of the a m i n o resin [16]. This detracts from the overall toughness a n d resistance properties. The hydroxyl-functional acrylics are, therefore, favored over acid-functional p o l y m e r s a n d are m o s t often used in

c o m b i n a t i o n with a m e t h y l o l a t e d o r b u t y l a t e d m e l a m i n e f o r m a l d e h y d e or b e n z o g u a n a m i n e - f o r m a l d e h y d e condensate. U r e a - f o r m a l d e h y d e c o n d e n s a t e s are less d u r a b l e a n d have been f o u n d to have lower gloss a n d p o o r e r c h e m i c a l resistance. Hydroxyl functionality is i n c o r p o r a t e d into the acrylic p o l y m e r b y c o p o l y m e r i z i n g m o n o m e r s such as hydroxyethyl acrylate (HEA) o r hydroxyethyl m e t h a c r y l a t e (HEMA). This type of c o m b i n a t i o n p r o d u c e s cross-linked acrylic/amino e n a m e l s with o u t s t a n d i n g exterior durability, g o o d hardness, a n d m a r resistance, as well as excellent resistance to solvent attack. Acrylic/amino t h e r m o s e t t i n g e n a m e l s were, therefore, very successful in replacing the less d u r a b l e a l k y d / m e l a m i n e systems in a u t o m o t i v e t o p c o a t applications, a n d general industrial finishing. Over the years, the a u t o m o tive i n d u s t r y has relied heavily on this type of t h e r m o s e t t i n g acrylic b e c a u s e it offers the o u t s t a n d i n g d u r a b i l i t y of acrylic lacquers b u t with b e t t e r resistance to solvents a n d elevated t e m p e r a t u r e s . It also offers significantly higher a p p l i c a t i o n solids. Also, it does not require factory buffing to achieve high gloss as do the acrylic lacquer coatings. A n o t h e r r e a s o n that acrylic/amino resin t e c h n o l o g y bec a m e so p o p u l a r is b e c a u s e of the versatility of the chemistry, w h e r e b y p r o p e r t i e s can be readily altered b y varying acrylic Tg, acrylic m o n o m e r s , acrylic functionality level, a n d crosslinker type a n d level. This is very i m p o r t a n t in general industrial finishing, where coatings often m u s t be c u s t o m t a i l o r e d to the specific end use. Table 4 briefly d e m o n s t r a t e s the kinds of variation in p e r f o r m a n c e w h i c h can be o b t a i n e d by a few m a n i p u l a t i o n s in c o m p o s i t i o n [17]. An alternate a p p r o a c h to a t h e r m o s e t t i n g acrylic p o l y m e r is to p r e p a r e an acrylic p o l y m e r w h i c h contains functionality a n a l o g o u s to a m e l a m i n e / f o r m a l d e h y d e condensate. Meth31101 or methylol e t h e r groups c a n be a t t a c h e d to an acrylic backbone, and the resulting p o l y m e r can self-condensate, resulting in a cross-linked structure without the need for a n external cross-linking agent. Initially, an acrylic p o l y m e r is m a d e containing a c r y l a m i d e (AM). The p o l y m e r i z a t i o n is usually a conventional free-radical, solution p o l y m e r i z a t i o n c a r r i e d out in alcohol or a c o m b i n a t i o n of alcohol and arom a t i c solvent. As in most t h e r m o s e t t i n g acrylic polymers, m e r c a p t a n is usually included to control m o l e c u l a r weight. After the p o l y m e r i z a t i o n is complete, the p o l y m e r is t r e a t e d with f o r m a l d e h y d e to convert it to the methylol amide. An acid catalyst will b r i n g a b o u t etherification with the alcohol present, usually butanol. The conversion p r o c e e d s as follows

[18]: P O L Y M E R - - C O - - N H 2 + HCHO POLYMER--CO--NH--CH2OH

)

P O L Y M E R - - C O - - N H - - C H a O H + ROH P O L Y M E R - - C O - - N H - - C H 2 O R + H20

)

As an alternative process, the AM m o n o m e r can be methylolated before being polymerized. The finalized methylolated a m i d e acrylic p o l y m e r s condense readily w h e n acid catalyzed at bake conditions of 30 m i n at 150~ The condensation process is a two-stage r e a c t i o n [19]: 2 POLYMER--CO--NH--CHzOH ) POLYMER--CO--NH--CH2--O--CH2-N H - - C O - - P O L Y M E R + H20

CHAPTER 6--ACRYLIC POLYMERS AS COATINGS BINDERS

45

TABLE 4--Compositions, viscosities, and film properties of HEMA-based copolymers containing a variety of other monomers a [17]. Acrylate Styrene HEMA BA BMA St MeSt EtSt Viscosity in 1/1 xylenefoutanol Solids content, % by wt Cross-linking agent, % by wtb

27.5 22.5 .-. 50.0 . . . . . U-V 47.8 30

Methacrylate Styrene

Methacrylate Methyl Styrene

Copolymer Composition, 27.5 . . . . . 22.5 50.0 . .

.

. .

.

. W+ 49.4 30

.

.

% by wt 27.5 . . . 22.5 . . . . 50.0

Methacrylate Ethyl Styrene 27.5 . 22.5 .

.

X+ 49.4 30

... 50.0 X+ 48.3 30

.

Film Properties Color or yellowness factorb Initial After 4 h/219~ Gloss (60~ initial After 4 h/219~ Front impact, in./lb to fail Flexibility, l/s-in, mandrel Knife scratch Resistance to: 20% NaOH, 12-day exposure 50% HAc, 7-h exposure Butyl acetate, 4-h exposure 0.5% Rinso at 74~ blisters after 4 days of immersion

- 2.4 6.2 98 84 2 0 7+

- 4.6 - 0.9 98 92 0 0 7

- 5.2 0.6 94 84 0 0 7

- 3.0 10.1 98 74 0 0 7

10 10 6 9-

10 10 10 9

10 10 10 9

10 10 3 9

~Allcopolymers were POLYMER--CO--NH--CH2--NH--CO-POLYMER + H20 If the m e t h y l o l a t e d a m i d e has b e e n etherified b y b u t a n o l capping, i n s t e a d of w a t e r as a by-product, a mixture of b u t a nol a n d dibutyl ether w o u l d be obtained. These types of selfcondensing acrylic resins will also react with a m i n o resins, b u t usually there is no justification for so doing (i.e., no i m p r o v e m e n t s in performance). The strength of the methylol a m i d e acrylics is that they have no glaring weaknesses a n d m a k e very g o o d g e n e r a l - p u r p o s e resins.

Isocyanate-Reactive Acrylics Acrylic solution p o l y m e r s t h a t are cross-linked with isocyanates (often referred to as acrylic urethanes) are u n i q u e a m o n g the various cross-linking systems b e c a u s e they cure u n d e r a m b i e n t c o n d i t i o n s - - t h e y don't require baking. The isocyanate g r o u p ( - - N = C = O) is extremely reactive a n d will cross-link with any type of functionality having a labile hyd r o g e n atom. This includes amines, alcohols, ureas, urethanes, carboxylic acids, a n d amides. Acrylic p o l y m e r s designed to be cross-linked with isocyanate resins generally contain hydroxyl functionality i n c o r p o r a t e d b y p o l y m e r i z i n g hydroxyethyl acrylate (HEA) o r hydroxyethyl m e t h a c r y l a t e (HEMA) into the acrylic backbone. There are six basic types of curing m e c h a n i s m s for uret h a n e coatings [see ASTM T e r m i n o l o g y Relating to Paint, Varnish, Lacquer, a n d Related Products (D 16-75)]. The two-package polyisocyanate/polyhydroxyl coatings m a k e up "Type V." Of this class, acrylic u r e t h a n e s b a s e d on weatherresistant hydroxyl functional acrylics p r e d o m i n a t e . The hydroxyl functional acrylic reacts with isocyanate resin as follows:

ACRYLIC--OH + R--N = C = O R--NH--COO--ACRYLIC (a urethane)

>

The preferred isocyanates are usually aliphatic, such as the a d d u c t of h e x a m e t h y l e n e d i i s o c y a n a t e (HMDI), b e c a u s e of the p o o r e r color a n d exterior d u r a b i l i t y a s s o c i a t e d with the a r o m a t i c types of isocyanates. While the a r o m a t i c varieties of isocyanate react faster t h a n the aliphatic types, a wide range of catalysts are available w h i c h can be used to speed u p the cure of aliphatic isocyanates. A few such catalysts include triethylene diamine, zinc n a p h t h e n a t e , a n d dibutyl tin-dil a u r a t e [20]. Acrylic solution p o l y m e r s cross-linked with aliphatic isocyanates are ideal for a p p l i c a t i o n s w h e r e a durable, high-perf o r m a n c e coating is r e q u i r e d b u t where b a k i n g is not feasible b e c a u s e of the size or t e m p e r a t u r e sensitivity of the object to be coated. Acrylic u r e t h a n e s are, therefore, invaluable in the t r a n s p o r t a t i o n i n d u s t r y w h e r e high-quality coatings are n e e d e d for aircraft, r a i l r o a d cars, trucks, buses, etc. A u t o m o bile refinishing, heavy e q u i p m e n t coatings, a n d high-perf o r m a n c e m a i n t e n a n c e coatings are also areas w h e r e acrylic u r e t h a n e s are a p p r o p r i a t e . The acrylic u r e t h a n e s c o m b i n e the i n h e r e n t UV resistance a n d exterior d u r a b i l i t y of acrylics with the a m b i e n t cross-linking ability of aliphatic isocyanates to p r o d u c e hard, tough, chemical-resistant, high-perform a n c e coatings. The m a j o r d r a w b a c k of acrylic u r e t h a n e s is that they are a two-package system a n d c a n n o t be m i x e d until r e a d y for a p p l i c a t i o n b e c a u s e they are so reactive a n d have a short "pot life." Over the years, the hydroxyl-functional acrylics have b e e n i m p r o v i n g in p e r f o r m a n c e , a n d n o w the e m p h a s i s is on higher solids content for lower VOCs. To meet m o r e stringent VOC regulations, lower-molecular-weight, higher-solids hydroxyl functional acrylics have b e e n developed. I n c r e a s i n g

PAINT AND COATING TESTING MANUAL

46

the solids of the acrylic r e d u c e s solvent levels in the form u l a t e d coating. To c o m p e n s a t e for lower m o l e c u l a r weight, one w a y to i m p r o v e p e r f o r m a n c e is to increase hydroxyl content, w h i c h in t u r n requires higher levels of isocyanate. An alternative a p p r o a c h to r e d u c e d solvent o r higher solids is to m o d i f y the acrylic u r e t h a n e with a reactive diluent w h i c h is fluid a n d acts like a solvent b u t t h e n reacts to form p a r t of the cross-linked n e t w o r k [21]. One such diluent is a lowmolecular-weight, difunctional oxazolidine w h i c h is nonreactive with isocyanates until a m b i e n t m o i s t u r e opens the ring, releasing b o t h hydroxyl a n d a m i n e functionality [22].

TABLE 5--Copolymerization of ethyl acrylate, methyl methacrylate, and methacrylic acid [23]. Materials: 375.0 g 5.1g 100.0 g 100.0 g 4.0 g 4.0 mL 1.0 g 0.7 g 5 drops

Deionized Water Surfactant Ethyl acrylate (15 ppm MEHQ) Methyl methacrylate (25 ppm MEHQ) Glacial methacrylic acid (100 ppm MEHQ) Ferrous sulfate solution (0.15%) Ammonium persulfate in 5 mL of water Sodium formaldehyde sulfoxylate in 5 mL of water t-butyl hydroperoxide (70%)

Procedure:

R" \

O/C

I

H2C

/

H

H

\

/

~N--R--N/C

I

R"

~O

I

I

CH2 H2C

+ 2H20

>

CH2 HO

I

H2C

HN--R--NH

I

CH 2

I

H2C

OH

I

+ 2R'CHO

CH 2

This type of functionality has the advantage that it is one p a c k a g e stable with isocyanates as long as m o i s t u r e is excluded from the paint. Because it has four reactive sites p e r molecule, it increases cross-link density for m a x i m u m performance, while it decreases solvent emissions.

ACRYLIC EMULSION POLYMERS An acrylic e m u l s i o n is a t w o - p h a s e system in w h i c h acrylic p o l y m e r droplets are d i s p e r s e d in an external w a t e r phase, usually with the a i d of a n emulsifier (i.e., surfactant). Unlike s o m e p o l y m e r emulsions, such as alkyds o r epoxides, w h i c h are emulsified as preexisting resins, acrylic e m u l s i o n s are m a d e b y a n e m u l s i o n p o l y m e r i z a t i o n process w h e r e i n the m o n o m e r droplets are emulsified in w a t e r a n d then p o l y m e r ized. A typical acrylic e m u l s i o n p o l y m e r i z a t i o n recipe is given in Table 5 [23]. The physical c h e m i s t r y of acrylic e m u l s i o n p o l y m e r s is m u c h the s a m e as for their solution p o l y m e r analogs, a n d the film p r o p e r t i e s of the e m u l s i o n s can be controlled by m a n i p u lating p o l y m e r c o m p o s i t i o n a n d m o l e c u l a r weight just as with acrylic solution polymers. However, the viscosity of an e m u l s i o n is unaffected b y p o l y m e r m o l e c u l a r weight since solution principles do not p e r t a i n to e m u l s i o n s (the p o l y m e r is insoluble in the c o n t i n u o u s w a t e r phase). Therefore, for the b e s t possible physical properties, the m o l e c u l a r weight of acrylic emulsions is generally h i g h e r t h a n t h a t of acrylic solution polymers: 100 000 to 1 000 000 for an e m u l s i o n versus 75 000 to 100 000 for a solution polymer. The particle size of an e m u l s i o n is also very i m p o r t a n t in d e t e r m i n i n g p e r f o r m a n c e a n d m u s t be carefully controlled. F o r example, the film-forming ability of a n emulsion, as well as its p i g m e n t b i n d i n g capability, is d e p e n d e n t on particle size, with s m a l l e r particle size being b e t t e r t h a n large particle size. Particle size does affect e m u l s i o n viscosity, with large particle size generally being a s s o c i a t e d with low viscosity.

In a beaker, stir the surfactant with the water until dissolved and adjust the pH to 9.0 by adding 50% sodium hydroxide solution. Transfer this solution into the reaction flask, rinse the beaker with a small amount of deionized water, add the monomers and ferrous sulfate, and stir 15 min with flow of nitrogen before adding the initiators. The maximum temperature of 77~ is attained in 12 to 15 min. Stir 15 min after adding the initiators, then cool to room temperature, adjust to pH 9.5 with 28% aqueous ammonia, and filter; the gums amounted to 0.17%. The free acid (unneutralized) surfactant can also be used as an emulsifier for the above copolymerization. In this case, the period of purging with nitrogen after charging the monomers should not exceed 15 min before the addition of initiators to avoid the formation of polymer emulsion product with excessive viscosity. Filtration of the finished emulsion gave only 0.05% gums. The properties of these emulsions were: Surfactant Form

Sodium Salt

Free Acid

Solids Content, %--Calculated --Found pH at 25~ Viscosity (Brookfield), cP Particle size (light scattering), % Minimum film-forming temperature, ~

35.0 34.3 5.6 7.9~ 22.8 b 22~

36.0 35.6 1.8 10.5a 12.3r 3tY

~Emulsion adjusted to pH 9.5 before measurement. bMeasurement at 2% solids. CMeasurement at 0.4% solids. Acrylic e m u l s i o n p o l y m e r s (also k n o w n as acrylic latexes) have long b e e n a m a i n s t a y of the architectural coatings m a r ket, p a r t i c u l a r l y in exterior p a i n t s w h e r e their o u t s t a n d i n g d u r a b i l i t y is so i m p o r t a n t . However, in recent years, clean air regulations have further s t r e n g t h e n e d the p o s i t i o n of acrylic emulsions, usually at the expense of solvent alkyds. The use of acrylic e m u l s i o n s in industrial coatings a p p l i c a t i o n s has also g r o w n as a result of solvent e m i s s i o n restrictions. At the s a m e time, the p r o p e r t i e s of acrylic e m u l s i o n p o l y m e r s in the industrial coatings m a r k e t has i m p r o v e d so that they n o w offer p e r f o r m a n c e s i m i l a r to their solvent-borne counterparts.

Acrylic E m u l s i o n s for Architectural Coatings Architectural coatings are generally c o n s i d e r e d to be coatings i n t e n d e d for on-site a p p l i c a t i o n to residential, c o m m e r cial, or institutional buildings; they are also k n o w n as t r a d e sales coatings. Over the last 40 years, this m a r k e t has evolved from an entirely oil-based m a r k e t to one d o m i n a t e d b y emulsions. There are three underlying reasons for the takeover of the architectural coatings m a r k e t b y e m u l s i o n polymers. The health, safety, a i r quality, a n d o d o r concerns a s s o c i a t e d with the solvents in oil-based p a i n t s have m o v e d p e o p l e t o w a r d s

CHAPTER 6--ACRYLIC POLYMERS AS COATINGS BINDERS water-based latex paints whenever there is not a large penalty in performance. Also, latex paints are more tolerant of a wide variety of application conditions and can even be applied over damp substrates. Thirdly, emulsion polymers, particularly quality acrylic emulsions, have surpassed oil-based paints for long-term performance and protection in most applications. Although acrylic emulsions generally provide a superior level of performance, their cost is somewhat higher than lower-performance emulsions such as vinyl acetates. Therefore, in segments of the architectural coatings market where performance is not critical, such as for interior flat paints, acrylic emulsions .are not a dominant binder. However, in segments where performance and durability are essential, such as for exterior flat and gloss paints, acrylic emulsions control a very large portion of the market both in the United States and abroad.

Interior Coatings Applications The high-volume, interior-flat market is dominated by vinyl acetate copolymers plasticized with a soft acrylate monomer, usually butyl acrylate at about the 15% weight level. Interior flat paints are aimed primarily at broadwall applications. Performance requirements for this market are fairly modest, with decorative features such as color, sheen level, and hiding being the more infuential factors determining consumer preference. The one resistance property demanded in this m a r k e t is scrub resistance; in this regard, vinyl/acrylics perform satisfactorily. There is a small, premium segment of the interior flat market occupied by all acrylic latexes, and one of the key features which they offer is improved rheology and application characteristics resulting from the better interaction of acrylic latex particles with the new associative thickeners. Associative thickeners (also referred to as rheology modifiers) dramatically enhance flow, brushability, film build, and spatter resistance compared to conventional hydroxyethyl cellulose (HEC) thickeners. Since interior flat paints contain a high pigment loading to increase hiding and reduce cost, the acrylic polymers used in this market tend to be fairly soft with good binding capacity. Typically, they are copolymers of a hard methacrylate monomer such as methyl methacrylate with a soft, commercially available acrylate m o n o m e r such as ethyl acrylate, butyl acrylate, or ethylhexyl acrylate. The Tg is generally around 10~ Interior gloss and semigloss paints have much more demanding requirements than flat paints since they are used for more diverse and challenging substrates such as windows, cabinets, and doors. Acrylic emulsions play a major role in this market, especially at the high-performance end. They are tailored to the specific needs of this market by optimizing the important variables contributing to performance: composition, hardness, molecular weight, and particle size. Acrylic emulsions used in gloss and semigloss paints are copolymers of acrylate and methacrylate monomers and are usually harder than emulsions used in flat paints. They generally have a Tg in the range of 20 to 50~ The harder polymers are necessary to build in block and print resistance, which are needed to keep doors and windows from sticking and to prevent marring and film damage associated with softer polymers. Since gloss and semigloss paints are formulated at low

47

pigment volume concentration (PVC) to obtain gloss, the pigment does not contribute significantly to film hardness; it must all come from the polymer. Since these gloss and semigloss latexes are often used over old oil-based enamels, adhesion to aged oil-based paints is required. This may be accomplished by copolymerizing adhesion promoting functional monomers into the acrylic polymer backbone. A major breakthrough in latex adhesion technology came about with the development of ureide functional acrylic monomers [24]. The adhesion promoter particularly is needed to improve adhesion and blister resistance when the paint film becomes wet, as it might in a bathroom. Since the gloss paints are often used in wet areas such as a bathroom, the water resistance of the dry film is also an important property. To ensure good water resistance, acrylic emulsions used in this market often contain hydrophobic monomers such as styrene. Since interior trim paints are so highly visible, overall appearance properties are critical to the success of the paint job, and features such as flow and levelling, gloss, and film build are expected to be similar to oil-based enamels. This level of outstanding appearance has been possible in recent years with the introduction of associative thickeners. The traditional thickener for latex paints has been hydroxyethyl cellulose (HEC), which thickens by a flocculation mechanism and usually produces poor flow and gloss. Associative thickeners loosely bind to the surface of the latex particles through hydrophobic interactions, forming a network structure which accounts for their thickening action. The degree of interaction between the thickener and the latex particle is largely a result of the surface chemistry of the emulsion particles. Smaller particle-size emulsions have greater surface area and therefore have more interaction with associative thickeners. More hydrophobic latexes have stronger association with the new thickeners. Consequently, small-particlesize, hydrophobic acrylic emulsions have been designed specifically for use with associative thickeners. These newer acrylic emulsions optimize thickener interaction and produce exceptional flow and gloss. In fact, before these new emulsion/thickener systems, truly high-gloss latex paints were out of the question. They also improve brushability and film build, while eliminating the problem of roller spatter. Overall, the appearance properties of the newer small particle-size hydrophobic acrylics, when used in combination with associative thickeners, rivals that of oil-based enamels. In an effort to further improve the performance of acrylic emulsions, the morphology of emulsion particles has become an additional important variable. In the past few years, new composite acrylic emulsions have been introduced, particularly into the interior gloss area, which are made up of two or more nonhomogeneous phases. They are prepared by a twostage polymerization process sometimes referred to as a sequential emulsion polymerization that results in various types of core-shef structures. The goal of this type of polymerization is to incorporate the best characteristics of the different phases. The hard acrylic emulsions typically used in interior gloss paints provide excellent performance but require considerable amounts of coalescent to achieve film formation (i.e., 10 to 20% by weight on polymer solids). This is undesirable both

48

PAINT AND COATING TESTING MANUAL

from a cost and an organic emissions perspective. Using a two-stage polymerization, it is now possible to make hard, block-resistant acrylic emulsions that are also flexible and require lower coalescent levels. This type of polymerization can also be used to achieve a desired surface chemistry while not disturbing the bulk composition of the latex particle. This can be useful in optimizing rheology or improving adhesion characteristics of an acrylic latex.

Exterior Coatings Applications By far the most challenging application for any coating is as an exterior paint required to protect a multitude of substrates in diverse and extreme weather conditions. It is in this demanding role that acrylic emulsions have met virtually all requirements and impressed the industry by their outstanding durability. One primary reason for their success, as mentioned earlier for solution acrylics, is their lack of absorption of ultraviolet light coupled with their inherent hydrolysis resistance. Over the years, acrylic emulsions have evolved from simple polymers troubled by shortcomings, such as poor adhesion or low film build, to sophisticated systems incorporating elements designed to address essentially every major challenge experienced by an exterior paint. One of the toughest demands facing exterior flat house paints is the need to withstand the freeze-thaw type of expansion and contraction of dimensionally unstable substrates such as pine or other soft woods. To avoid the grain cracking that often occurs over this type of substrate, acrylic emulsions designed for flat house paints are fairly soft, with a Tg in the range of 10 to 15~ A coalescing solvent is usually used in the formulation to assist film formation, particularly at lower temperatures. When the coalescent leaves, the acrylic paint film remains pliable and able to withstand substrate swelling and freezing, unlike oil-based house paints which become harder and embrittle on exposure as they continue to crosslink. The primary concern with making the acrylic polymer too soft is that dirt pickup would worsen. Since flat house paints contain a fairly high pigment content (i.e., PVC = 40 to 60%), dirt resistance is enhanced by the pigment loading. Experience over many years indicates that a Tg of 10 to 15~ is the optimum range to balance grain-crack resistance with dirt resistance. For exterior flat house paints, the inclusion of an effective adhesion promoter in the acrylic backbone is crucial for good adhesion. The adhesion promoter greatly improves blister resistance. Furthermore, the improved adhesion enhances crack resistance over dimensionally unstable wood substrates. Painting over a degraded chalky surface is a common practice that can be a potential disaster if sufficient adhesion is not obtained. The chalk acts like a powdery barrier, preventing the emulsion binder from penetrating to the real substrate and establishing an adhesive bond. Studies have shown that smaller particle-size acrylic emulsions are much more effective than larger particle-size emulsions for filtering down through the chalk and obtaining adequate adhesion. For this reason, many exterior grade acrylic emulsions have been designed at a fairly small particle size of about 100 nm

the flocculating mechanism of HEC. Therefore, 100-nm emulsions that were designed to have improved chalk adhesion sacrificed some of the flow and film build of large particle-size (500-nm) emulsions. In an attempt to combine these seemingly mutually exclusive properties, particle-size distributions have been carefully controlled to ensure a tailored mixture of small particles that give good adhesion to chalky surfaces and large particles that help to improve flow in formulations thickened with HEC. An additional benefit of these polymers is their high supplied solids, which may be as high as 60% by weight compared to 40 to 50% for unimodal latexes. Wide particle-size distribution acrylic emulsions do n o t significantly address the low film build associated with smaller particle-size emulsions when thickened with HEC. Film build is particularly important to an exterior paint because the durability of the film is usually proportional to the film thickness, i.e., how much paint is applied to the substrate. This was addressed in the 1980s by the Rohm and Haas Co. with the introduction of a Multilobe | acrylic emulsion, shown in Fig. 2 [26]. This type of polymer has a lobed morphology that is grown out during the polymerization process; it does not result from particle aggregation. The lobes of this polymer are about 350 nm, but it has an effective hydrodynamic volume of about 1000 nm and is, therefore, very effective at imparting high film build in paints thickened with HEC. It also reduces the level of thickener needed to achieve a given viscosity. Since in its commercial form this technology also contains small particles, good adhesion characteristics are retained while film build is optimized. Other important aspects of weatherability are color retention and resistance to chalking. These properties are made worse by the catalytic degradation effects of TiO 2 on the binder, so that high PVC flat paints are generally poorer than low PVC gloss paints. However, the inherent durability of the binder is still a controlling factor, and acrylic polymers have excellent resistance to sunlight and erosion, which contribute to their very good chalk resistance and color retention. Among the common acrylic copolymer compositions in use commercially, MMA/BA polymers are better than MMA/EA polymers, and higher methacrylate containing binders are

[25]. Small-particle-size, large-surface-area emulsions, when thickened with HEC, have poorer flow and film build than larger particle-size emulsions, which are less aggregated by

FIG. 2-Scanning electron micrograph of Multilobe | acrylic particles (courtesy of Rohm and Haas Co,).

CHAPTER 6--ACRYLIC POLYMERS AS COATINGS BINDERS better than acrylics containing higher levels of acrylate monomers. Of course, the methacrylate/acrylate levels are generally determined by the Tg required to achieve the desired balance of crack resistance and dirt pickup. Exterior gloss and semigloss paints are required to withstand similar tortures as their flat paint counterparts and, in addition, must provide equally good dirt resistance at much lower pigment loadings. Acrylic emulsions have been designed that meet all of these challenges and perform very well in environments as diverse as the tropical regions of Asia and the Philippines to the Scandinavian regions of Europe. Since the acrylic emulsion engineered for interior gloss/semigloss paints are intended to be high-performance systems capable of good adhesion even in wet areas, they are often able to be used outside as well. Since exterior gloss paints must have good dirt resistance at low PVC, the acrylic emulsions used in these paints are harder than those used in flat paints and generally have a Tg in the area of 20 to 35~ This Tg range can provide acceptable dirt resistance while still having good grain-crack resistance. The very hardest acrylic emulsions used for interior gloss paints (i.e., above Tg 35~ would not be appropriate outside, at least in areas subject to freezing, because they would be more prone to grain crack. Since brush marks are much more obvious in a gloss paint than in a flat paint, good flow and levelling is much more critical for a gloss paint than for a flat paint. Consequently, older acrylic emulsions intended for semigloss paints (there were no high-gloss latex paints until about 1980) are of large particle size to have the best possible flow with the prevailing thickener of the day, HEC. The flow of these systems could best be described as fair, but overall they have demonstrated an admirable balance of properties and are still popular today. However, newer acrylic emulsions intended for exterior gloss paint applications, particularly those generally referred to as acrylic enamel vehicles, are small in particle size (i.e., 100 nm) to take advantage of the excellent flow, gloss, and rheology available by using associative thickeners. Over the past ten years, the decorative as well as protective capabilities of exterior gloss paints have improved significantly to the point where acrylic emulsions are rapidly replacing alkyd enamels as the preferred coating for exterior trim.

Acrylic Emulsion Maintenance Coatings Maintenance coatings differ substantially from decorative coatings since they are used primarily for their protective features, which prevent substrate deterioration by corrosive elements. Maintenance coatings are generally used to protect metal surfaces such as bridges, storage tanks, and other industrial facilities, often in harsh chemical and corrosive environments. The first acrylic latex binders for corrosionresistant maintenance coatings were introduced commercially in 1964. They are similar in hardness and composition to exterior flat house paint binders with the exception that they are formulated with reactive pigments and additives which help prevent rusting. The surfactants and other "salt and pepper" ingredients used in the polymerization of maintenance acrylic emulsions are carefully selected so as to not aggravate corrosion. These acrylic latex maintenance coatings had the usual advantages in application of water-based paints over solvent

49

alkyd paints along with expected advantages in chalk resistance, color retention, and other decorative qualities. However, to the surprise of some segments of the maintenance industry, acrylic maintenance emulsions often outperformed solvent alkyds for corrosion resistance and overall metal protection. This is partly because the alkyds continue to harden and eventually crack, leaving the substrate exposed and subject to the elements. Acrylic emulsions do not continue to harden once the paint film is dry, and they remain sufficiently pliable to expand and contract with the substrate. The one weakness of the acrylic latexes in the area of maintenance coatings was their low Tg, which reflected a lower hardness than the alkyd paints. This limitation was removed by the development of aqueous gloss enamel binders. These hard latex binders used in interior and exterior gloss paints were fine tuned to maximize corrosion resistance and overall protection. Commercialized in the mid1980s, this new generation of maintenance vehicles has proved very successful in extensive field tests, particularly on bridges in the southeastern United States. The small particle size of these binders fits perfectly with associative thickener technology to give tight water-resistant films, which are an improvement over HEC-thickened paints that can have microscopic defects as a result of the flocculating mechanism of HEC [27].

Acrylic Emulsions for Industrial Coatings Nonreactive Emulsions Industrial coatings users, who have traditionally applied solvent-based polymers, have recently been under pressure to reduce volatile organic emissions. This is particularly true in California, which has historically been at the forefront of clean air legislation. In many instances, these coatings users have complied with the stricter environmental legislation by switching over to water-based systems. Acrylic producers have responded to the needs of these coatings users by developing more sophisticated emulsions capable of meeting the demanding performance requirements of many different end users. Early emulsions aimed at industrial coatings applications were offshoots of architectural coatings technology and were often too soft for industrial coatings uses. Also, high gloss was not possible with these older emulsions. Like the newer gloss enamel emulsions for trade sales use, however, latexes aimed at industrial coatings applications have evolved into hard, resistant binders that match the performance of their solvent-based counterparts. Without this evolution in performance, it is unlikely that industrial coatings users would switch to latex coatings even with the more severe emissions regulations. Thermoplastic acrylic emulsions designed for industrial coatings applications generally have to be harder and faster drying than architectural emulsions and have better corrosion and chemical resistance. The typical Tg range for such acrylics is about 30 to 70~ The film formation problems usually associated with such hard emulsions are somewhat alleviated by the controlled application conditions in the factory, so that low-temperature film formation is generally not required. For general industrial finishing over metal substrates, industrial acrylic emulsions have borrowed technology from maintenance finishes and have optimized sur-

50

PAINT AND COATING TESTING MANUAL

factants, additives, a n d c o m p o s i t i o n s to i m p r o v e r a t h e r t h a n d e t r a c t from c o r r o s i o n resistance. N e w e r heterogeneous acrylic e m u l s i o n s c o m p r i s e d of two o r m o r e phases have recently b e c o m e i m p o r t a n t in the industrial coatings industry. One such type of h e t e r o p o l y m e r , the core-shell polymer, is being used to achieve r a p i d h a r d n e s s d e v e l o p m e n t with i m p r o v e d block a n d p r i n t resistance at low VOC. These p r o p e r t i e s allow the m a n u f a c t u r e r to stack, pack, a n d ship coated parts m o r e quickly [28]. Using core-shell technology, acrylic e m u l s i o n s have been able to rival the p e r f o r m a n c e of traditional, high-solvent-content nitrocellulose lacquers in w o o d coatings a n d furniture finishes. The m a j o r deficiency of acrylic emulsions in these areas is the "warmth" of w a t e r - b a s e d coatings c o m p a r e d to solvent-based materials. " W a r m t h " is a quality w h i c h refers to the feel a n d a p p e a r a n c e of the coated wood.

linked, the infinite m o l e c u l a r weight provides for solvent a n d c h e m i c a l resistance, along with h a r d n e s s a n d toughness. By a d j u s t i n g the level of functionality, the a m o u n t of crosslinker, a n d the Tg of the acrylic emulsion, a system c a n be c u s t o m designed for a specific application. F o r m a n y years, the c o n s t r u c t i o n i n d u s t r y has relied on hydroxyl functional acrylic e m u l s i o n s r e a c t e d with u r e a o r m e l a m i n e to coat p r o d u c t s such as h a r d b o a r d , w o o d panels, shingles, a n d m e t a l coil. In i n t e r i o r applications, such as over w o o d paneling, these e m u l s i o n s offer c o m p a r a b l e cure speed a n d p e r f o r m a n c e to solvent-based alkyd/urea systems. I n coil coating applications, the t h e r m o s e t t i n g acrylics offer high gloss, excellent durability, g o o d c o r r o s i o n protection, as well as g o o d roll coatability. These e m u l s i o n s have b e e n a p p l i e d at line speeds up to 137 m / m i n with g o o d transfer, flow, a n d leveling. Usually these systems are catalyzed with an a c i d catalyst to achieve the fastest/lowest t e m p e r a t u r e cure. A very g o o d p r o p e r t y b a l a n c e is d e m o n s t r a t e d in Table 6 for a n aqueous acrylic m e l a m i n e coil coating e n a m e l [29]. A recent d e v e l o p m e n t in cross-linking acrylic e m u l s i o n technology is an epoxy cross-linking, a m b i e n t cure system w h i c h has m a n y a p p l i c a t i o n s b u t has been f o u n d to be particularly useful in m a i n t e n a n c e coatings. Besides being a m b i e n t curing, a n attractive feature of this system is its excellent early p r o p e r t i e s resulting from the high-molecular-weight acrylic emulsion, w h i c h provides a m p l e resistance characteristics until the epoxy cross-linking is complete. This n e w acrylic/epoxy system is c o m p a r e d to an e p o x y / p o l y a m i d e

Thermosetting Emulsions Just as is the case with solution acrylics, functional groups can be i n c o r p o r a t e d into the p o l y m e r b a c k b o n e of an acrylic e m u l s i o n so that it can react with a n o t h e r functional m a t e r i a l after a p p l i c a t i o n to the substrate, f o r m i n g a cross-linked polymer. Typically hydroxyl o r hydroxyl/acid functional acrylic emulsions are cross-linked with u r e a o r m e l a m i n e resins. Acid functional acrylic e m u l s i o n s can be cross-linked with emulsified epoxy resins. The c h e m i s t r y of these systems is identical to the cross-linking c h e m i s t r y discussed earlier for solvent-based acrylic resins. After the e m u l s i o n is cross-

TABLE 6--Properties for an aqueous acrylic/melamine coil coatings enamel over aluminum and galvanized steel [29]. Substrate

Aluminuma

Primer thickness Topcoat film thickness Gloss 20~ ~ Image clarity Tukon hardness (KHN) Pencil hardness Initial Wet 16 h, 38~ H20 Flexibility--X30 microscope Direct impact, in.-lbs Reverse impact, in.-lbs Metal mark resistance Rheology MEK rubs Cleveland condensing cabinet, 200 h at 60~ After 1000 h Salt Spray Exposure X-scribed area Tape adhesion, % removed Lifting Undercutting Blistering~ Exposed edge Undercutting Blistering~ 1/8-in. mandrel bend Blistering~ White rust Flat Blistering~

0 0.9 to 1.0 65/89 Very good 9

Mini-Spangle Galvanized Steel~ 0.2 0.8

-/80 b

Good 9

H

H

B 2-3T 20 to 25 15 Excellent Excellent 200 Pass

B 3T 35 10 Excellent Excellent 200 Pass

0 None 1/16 in. None

0 None 1/16in. Mod-Dense, No. 6, No. 8

... ...

4/16 in. Mod, No. 2, No. 4

... ...

None None

None

None

~Commercial chromate pretreatrnent. bG1ossdependent on smoothness of substrate. el-9: Higher numbers indicate srnaller blisters. Blister density is rated as few, moderate, or dense. 10 = no blisters.

CHAPTER 6--ACRYLIC POLYMERS AS COATINGS BINDERS

51

TABLE 7--Resistance characteristics of acrylic/epoxy water-borne coatings versus conventional controls [30]. 3 Weeks, Air Dry Acrylic/Epoxy Formulation MEK rubs to remove Spot tests, 15 rain. MEK Toluene Butyl acetate Gasoline Butyl cetlosolve

Solvent Resistance >300

Mod. soft Mod. soft Lt. soft Lt. soft Lt. soft

Epoxy/ Polyamide

Alkyd

>300

120 Lifted

Mod. soft Mod. soft Lt. soft Lt. soft Mod. soft

Lifted Lt. soft Lt, soft Lt. soft Mod. soft

Lt. stain OK Lt. stain OK OK

Med. stain OK Hvy. stain OK OK

No effect No effect

No effect Dissolved

Stain Resistance Spot tests, 24 h Mustard Coffee Red ink Cola Grape juice

OK OK OK OK OK Acid/Base Resistance

24-h immersion, on concrete HCI NaOH

No effect No effect

Accelerated Exposure, 300 h of QUV Exposure Gloss retention, percent 63 4 Fade resistance (green coatings) Good Very poor 800 h of Fade-O-Meter exposure Gloss retention, % 49 2

4 Poor 10

NOTE:Lt. = light; Mod. = moderate; Med. = medium; Hvy. = heavy. (Reprinted with permission of the American Paint and Coatings Journal. Copyright 1992.)

coating and an alkyd coating in Table 7 [30]. The strong points of the acrylic emulsion/epoxy system are its stain, solvent, and c h e m i c a l resistance, along with o ut st an d i n g weathering. No less a key feature is its very good co r r o si o n p e r f o r m a n c e [31].

REFERENCES [1] Chemicals for The Lakeside 1959, p. 20. [2] Chemicals for The Lakeside

Industry, Rohm and Haas Company 1909-1959, Press, R. R. Donnely & Sons Co., Chicago, IL,

Industry, Rohm and Haas Company 1909-1959, Press, R. R. Donnely & Sons Co., Chicago, IL,

1959, p. 21. [3] Brendley, W. H. Jr., "Fundamentals of Acrylic Polymers," Paint and Varnish Production, July 1973. [4] Fox, T. G., Bulletin of the American Physics Society, Vol. 1, 1956, p. 123. [5] Fox, T. G., Jr. and Flory, P. J., Journal of Applied Physics, Vol. 21, 1950, p. 581. [6] Rogers, S. and Mandelkern, L., Journal of Physical Chemistry, Vol. 61, 1957, p. 985. [7] Simha, R. and Boyer, R. F., Journal of Chemical Physics, Vol. 37, No. 5, t 962, p. 1003. [8] Kine, B. B. and Novak, R. W., "Acrylic and Methacrylic Ester Polymers," Encyclopedia of Polymer Science and Engineering, 2nd ed., H. F. Mark, N. M. Bikales, C. G. Overberger, G. Menges, and J. I. Krosckwitz, Eds,, Vol. I, John Wiley and Sons, New York, 1985, pp, 257-258.

[9] Rodriguez, F., Principles of Polymer Systems, McGraw-Hill Book Co., New York, 1970, p. 154. [10] Hildebrand and Scott, The Solubility of Non-Electrolytes, 3rd ed., Rheinhold Publishing Corp., New York, 1949, pp. 129, 301. [11] Burrell, H., Official Digest, Vol. 27, No. 369, 1955, p. 726. [12] Small, P. A., Journal of Applied Chemistry, Vol. 3, 1953, p. 71. [13] Solomon, D. H., The Chemistry of Organic Film Formers, Robert E. Krieger Publishing Co., Huntington, NY, 1977, p. 273. [14] "Acryloid Thermosetting Acrylic Resins," revised October 1966, Rohm and Haas Company promotional literature C-170, Spring House, PA. [15] Solomon, D. H., The Chemistry of Organic Film Formers, Robert E. Krieger Publishing Co., Huntington, NY, 1977, pp. 277-281. [16] Saxon, R. and Lestienne, F. C., Journal of Applied Polymer Science, Vol. 8, 1964, p. 475. [17] Petropoulos, J. C., Frazier, C., and Cadwell, L. E., "Acrylic Coatings Cross-linked with Amino Resins, Symposium on Thermosetting Acrylic Resins," Official Digest, Vol. 33, 1961, p. 729. [18] Solomon, D. H., The Chemistry of Organic Film Formers, Robert E. Krieger Publishing Co., Huntington, NY, 1977, p. 283. [19] Christenson, R. M. and Hart, D. P., Official Digest of the Federation of Societies for Paint Technology, Vol. 33, p. 696. [20] North, A. G., Journal of Paint Technology, Vol. 43, No. 557, 1971, p. 47. [21] Watson, D. M. and Schall, D. C., American Paint and Coatings Journal, 19 Aug. t991, p. 58. [22] Private communication, D. C. Schall, Rohm and Haas Co., Spring House, PA, 1991. [23] "Emulsion Polymerization of Acrylic Monomers," Product Bulletin CM-104 A/cf, Rohm and Haas Co., Spring House, PA. [24] Hankins and Melamed, U.S. Patent 2,881,171, 1959.

52

PAINT AND COATING TESTING MANUAL

[25] Harren, R. E., Organic Coatings: Their Origin and Development, R. B. Seymour and H. F. Mark, Eds., Elsevier Science Publishing Co., Inc., New York, 1990, p. 297. [26] Rohm and Haas Company photograph. [27] Harren, R. E., Organic Coatings: Their Origin and Development, R. B. Seymour and H. F. Mark, Eds., Elsevier Science Publishing Co., Inc., New York, 1990, p. 309. [28] Roman, N., Modern Paint and Coatings, November 1991, p. 38.

[29] Rohm and Haas Co. promotional literature, "82C2," October 1980, p. 2.

[30] Mercurio, A., American Paint & Coatings Journal, 20 Jan. 1992, p. 43.

[31] Klepser, R. J., "Water-based Maintenance Coatings Systems," Maintaining Structures with Coatings, Proceedings of SSPC 91, Steel Structures Painting Council, Pittsburgh, PA, 1991, pp. 9697.

MNL17-EB/Jun. 1995

7

Alkyd and Polyesters by Al Heitkamp ~and Don

Pellowe 2

oxidative polymerization and cross-linking that took place through coreaction of oxygen and the carbon-carbon bond unsaturation part of the fatty acids. The use of vegetable oils and fatty acids as coreactants with the early developed polyesters was the technological breakthrough that led to alkyd resins being the predominate binder for organic coatings. Other developments contributed to the general interest in the products such as new techniques for the production of phthalic anhydride, synthetic glycerin, and other new, novel polyhydric alcohols. Typically, an alkyd could be based on glycerin as the polyol, phthalic anhydride as the polybasic acid, and soya or linseed oil as the vegetable oil. These compounds are coreacted and then reduced with aliphatic or aromatic petroleum-based hydrocarbon solvents. Monofunctional fatty acids such as tall oil fatty acids or special blended fatty acids are commonly found in alkyds as alternatives to vegetable oils.

ALKYD RESINS, COMMONLYKNOWN AS "ALKYDS,"are synthetic polymeric materials that have been used in the coating industry since the 1930s. Today they continue to be the "workhorse" polymers for the paint, coating, and printing ink industries. Alkyd and chemically modified alkyd polymers find use in most types of liquid organic coatings for architectural, air-dry, and baked industrial and maintenance coatings. Alkyds are a special class of polyesters that often have vegetable oil or fatty acids coreacted into the polyester, and these compounds provide the distinctive air-cure feature of many of these compounds. Three major classifications of alkyds are those designed for conventional solids, higher solids, and water-borne coatings. Because there are a large variety of commercially available intermediates and chemical modifiers--i.e., m o n o m e r s - - f o r the preparation of alkyds, they continue to be the most versatile type of polymers for coatings and printing inks. Most alkyds are film-forming polymers with a relatively low glass transition temperature (Tg), i.e., below 0~ They are easily pigmented and readily accept additives to form coatings with a wide range of appearance, performance, and application characteristics. Alkyds are extensively used on wood, metal, plastic, composite, and other substrates such as primers, topcoats, maintenance paints, undercoatings, exterior trim paints, wall paints, and similar end uses. Polyesters for coatings are based on a coreaction of polyhydric alcohols and polybasic acids. Such polyesters may be prepared from one or more polyhydric alcohols and polybasic acids to meet particular coating performance requirements.

ALKYD SYNTHESIS, PROCESSING AND MANUFACTURE Three major categories of chemical intermediates are utilized in the manufacture of alkyd resins: 9 Polybasic organic acid/anhydride--example, phthalic anhydride 9 Polyhydric alcohol--example, glycerin 9 Monobasic fatty acid or triglyceride--example, soya fatty acids or soya oil The stoichiometric proportions and the equivalent weight of these monomers lead to the desired physical properties and molecular weight distribution of the resulting alkyd. The solvent selection and quantities used influence the viscosity, nonvolatile content, and the solvent evaporation rate from coating films. Alkyd processing is mainly a condensation reaction between hydroxyl and carboxyl groups of the chemical intermediates. The main by-product of the reaction is water, and it must be removed during the polymerization process or it will transesterify back into the alkyd and change characteristics. Other chemical reactions are possible during preparation, and these include dimerization of fatty acids or vegetable oils depending on their unsaturation and the alkyd processing temperature. Vegetable oils (triglycerides) are used for economical alkyd manufacture, whereas fatty acid blends are used in high-performance alkyds--particularly in higher solids and water-borne types.

HISTORY Although condensation products of dihydric alcohols and dicarboxylic acids were known at the start of the 20th Century, alkyds modified with drying oils were developed in the late 1920s by Kienle et al. [1-5]. The early condensation products were not soluble in common solvents and did not air dry until monofunctional acid or fatty acids were incorporated into the polymeric material. Kienle coined the term "alkyd" from the alcohols ("al") and acids ("cid") used in their preparation. The early spelling of"alcid" was later changed to the current form, "alkyd." Air-dry films were the result of iMcWhorter Technologies, 1028 South Third Street, Minneapolis, 55415. 2Retired, formerly employed by Frost Paint. 53 Copyright9 1995 by ASTM International

www.astm.org

54

PAINT AND COATING TESTING MANUAL

An alkyd resin can be modified with a number of intermediates. Some of the more common types are: 9 Acrylates 9 Benzoic acid 9 Epoxides 9 Isocyanates 9 Paramethyl styrene 9 Phenolics 9 Polyamides 9 Rosin 9 Silicone 9 Styrene 9 Vinyl toluene

dry nitrogen or carbon dioxide, is introduced to "blanket" the vapor space above at the top of the reaction vessel. The reaction mixture is heated from 350 to 500~ (175 to 260~ The main polymerization occurring is by condensation to form ester groups. Water of condensation exits from the top opening of the reactor. Vigorous mixing and agitation are required throughout the process to insure uniformity of the final resin (Fig. 1). The product provided by this process depends greatly on the procedure conditions followed in temperature and timely removal of water-of-reaction by-product.

Solvent Reflux P r o c e s s

More than one of the above compounds are often used to impart particular characteristics when an alkyd is modified. There are two major methods of preparing or processing alkyds for both laboratory and production scale. These methods are the fusion and solvent reflux processes.

Fusion P r o c e s s In this method of manufacture, the alkyd intermediates are charged into the reaction vessel. Then an inert gas, such as

MOTOR g~ i'~

THERMOMETER

In the solvent reflux process, an azeotropic solvent such as xylene is commonly used in the reaction. The purpose of the azeotropic solvent is to aid in removal of water formed during the condensation reaction. The reflux solvent and water volatilize together and liquefy in a condenser placed above the reaction vessel. A separator or Dean-Stark trap below the condenser collects this liquid mixture, and the azeotrope solvent is returned to the reaction vessel (Fig. 2). The choice of azeotrope solvent affects the temperatures maintained during the reaction. In both fusion and solvent reflux processes, acid number and viscosity are measured until the final desired values are

~'.:"~;'J

GLASS BEARING

MM O.D. TUBING

C

NIUM FOIL RS ALL CORKS

I-1 r- IVll ~ I-" r-II:: I - t l L , ~ L 13 L A ~ - L ; U L

HEATING MANTLE FIG. 1-Apparatus for fusion cooking of alkyds. (Diagram courtesy of ICI Hercules Alkyd Reports.)

CHAPTER 7 - - A L K Y D AND P O L Y E S T E R S

55

ALLEQUIPMENTHAS29/42JOINTS ,EDRICH'S NDENSER MOTOR TRUBORESTIRRER4 THERMOMETER II L E T ~ CO2IN

SOLVENT LAYER WATEF

SEPARATORY TRAP

,. SAMPLE

HEMISPHERICALGLAS-COL HEATINGMANTLE

FIG. 2-Laboratory apparatus for solvent cooking of alkyds. (Diagram courtesy of ICI Hercules Alkyd Reports.) reached. Then the alkyd is thinned with the desired type and amount of organic solvent. Only a small amount, usually less than 3% by typically 1% of the total weight, of the reflux or azeotrope solvent remains in the alkyd. The solvent reflux process advantages are less emission of by-products to the atmosphere and faster processing time. Also, a greater variety of alkyds can be made by this process. The final alkyd solution properties are measured at 25~ Typical tests include color, acid number, hydroxyl number, hardness, viscosity, and percent nonvolatiles.

RAW MATERIALS (INTERMEDIATES) FOR ALKYD RESINS Typical polybasic acids, polyhydric alcohols, and monobasic fatty acids or oils are given in Tables 1, 2, and 3. The numerous possible raw materials available and economic considerations of these lead to versatility of alkyds and to a wide range of commercially available products.

PHYSICAL PROPERTIES The most common physical properties used to identify characteristics of alkyd resins are determined by ASTM methods.

Viscosity The viscosity of alkyds covers a wide range and must be compared to the nonvolatile content and type organic solvent used, ASTM D 1545: Test Method for Viscosity of Transparent Liquids by Bubble Time Method [6]. The bubble tubes and measured times in seconds are easy to run with proper testing equipment and a constant temperature set at 25~ Viscosity is important in reflecting alkyd molecular weight and the final coating application properties, thickness, and minimizing batch-to-batch variation of each specific alkyd. Relatively high-molecular-weight alkyds need to be reduced to application viscosity with a greater amount of solvent or solvent mixture or with solvents that have a particular solvency for the specific alkyd.

NONVOLATILE CONTENT The nonvolatile content of alkyd solutions is determined with ASTM D 1259: Test Method for Nonvolatile Content of Resin Solutions. Alkyd specifications are designed to show a 1 or 2% variation from an agreed on nonvolatile by weight requirement. This method is sometimes varied to a higher oven temperature of 150~ and a shorter dwell time in resin processing use.

56

PAINT AND COATING TESTING MANUAL

TABLE 1--Acids and anhydrides used in alkyd manufacture.

TABLE 3--Vegetable oils used in alkyd manufacture. VEGETABLEOILS

POLYFUNCTIONAL Adipic acid Azelaic acid Chlorendic anhydride Fumaric acid ~Isophthalic acid ~Maleic anhydride aphthalic anhydride Succinic acid Sebacic acid Citric acid aTrimelletic anhydride MONOFUNCTIONAL Abiatic acid ~Benzoic acid Caproic acid Caprylic acid Capric acid Castor oil acids Coconut oil acids Cottonseed fatty acids Lauric fatty acids Linoleic acid Linolenic acid Oleic acid Tallow acids aTall oil fatty acids Tertiary-butyl benzoic acid Special blended fatty acids aMost commonlyused in commercialalkyds. Alkyd resin solutions vary from 30% nonvolatile (flat wall, medium-oil alkyds) to 100% nonvolatile content by weight (very long oil alkyds for exterior paints, stains, latex modifiers, and similar products).

Color The color of alkyd solutions is determined by comparison with a range of color standards referred to as the GardnerHoldt color standards, ASTM D 1544: Test Method for Color of Transparent Liquids (Gardner-Holdt Scale) [8]. The color or degree of yellowness of the alkyd solution may or may not have an effect on the color of the final coating films. TABLE 2--Polyhydric alcohols used in alkyd manufacture. POLYHYDRICALCOHOLS aGlycerin aEthylene glycol Propylene glycol Trimethylol propane aNeopentyl glycol Hexylene glycol Pentanediol 1,3-Butylene glycol Diethylene glycol Triethylene glycol ~Pentaerythritol Methyl glucoside Dipentaerythritol Sorbitol aTrimethylpentanediol Trimethylol ethane ~Mostcommonlyused in commercialalkyds.

Castor oil aCoconut oil Corn oil Cottonseed oil Dehydrated castor oil ~Linseed oil Safflower oil aSoybean oil Tung oil Walnut oil Sunflower oil Menhadden oilb Palm oil

aMostcommonlyused in commercialalkyds. bAnonvegetableoil derived from fish.

Density The density or specific gravity of alkyds is also referred to as the weight per gallon or density and can be determined by following ASTM D 1475: Test Method for Density of Paint, Varnish, Lacquer, and Related Products [9].

Flash Point The flash point of alkyds is mainly of importance as it pertains to shipping the products and formulated paints, i.e., to bill of lading and other regulations. ASTM D 3278: Test Methods for Flash Point of Liquids by Setaflash-Closed-Cup Apparatus [10] is the most common test that will provide conformance with Department of Transportation regulations. However, other ASTM methods are utilized. The method utilized depends on flash cup availability and other specified requirements. Neat alkyds have low vapor pressure. Therefore, the flash point of an alkyd solution reflects the flash point of the solvent used to dissolve the alkyd. It is recommended that flash points on alkyd solutions actually be measured by laboratory determination. The flash point of an alkyd solution is different from that of the actual solvent or solvents incorporated into the solution.

Drying Properties The drying properties of alkyds are of importance when describing the product. Metallic driers are based on cobalt, manganese, iron, lead, calcium, and rare earths reacted with synthetic organic acids, such as vegetable fatty acids, to form soaps. When these driers are added to the alkyd-based coating, they act as catalysts and accelerate the rate of air drying and cross-linking. Driers are formulated in combinations or blends to maximize desired dry film surface and interior characteristics. In recent years, synthetic acid-based metallic driers have gained popularity for two main reasons: (1) higher metal concentration in the drier, and (2) greater uniformity of drier performance. Methods associated with determining drying are given in ASTM D 1640: Test Methods for Drying, Curing, or Film Formation of Organic Coatings at Room Temperature [11].

CHAPTER 7 - - A L K Y D AND P O L Y E S T E R S

Acid Value

H I G H E R S O L I D S ALKYD R E S I N S

The free organic acid groups present in the nonvolatile portion of an alkyd resin is an important property for pigment wetting and performance properties of organic coatings. The acid value of alkyds is typically determined with ASTM D 1639: Test Method for Acid Value of Organic Coating Materials [12]. Reasonably accurate and consistent results can be obtained with this test procedure.

Hydroxyl Value The hydroxyl value or number is a measurement of the free hydroxyl groups remaining in the alkyd that have not been reacted with carboxyl groups during the condensation stage of the alkyd resin preparation process. Hydroxyl value determinations are more difficult to perform than acid number determinations. There are several reasons for this. First, the hydroxyl group can be sterically hindered or less available within the polymer and thus difficult to reach with the reactants. Second, hydroxyl groups on primary carbon atoms are more reactive than those on secondary carbon atoms, and these are more reactive than those located on a tertiary carbon atom. Tertiary-positioned hydroxyl groups are the most difficult to esterify in this determination. Manufacturers can often specify methods that can be used for secondary hydroxyl groups. Hydroxyl numbers are important in determining equivalent weight, which in turn is important to determining the amount of urea formaldehyde, melamine formaldehyde, or urethane prepolymer to react with the alkyd. In the case of oil-modified urethanes, also called uralkyds, the hydroxyl groups coreact with free isocyanate functionality. Although theoretical equivalency based on hydroxyl numbers is a good guideline to establishing performance characteristics, a ladder of co-reactant ratios is important to optimizing particular performance characteristics.

ALKYD R E S I N CLASSIFICATION Unmodified alkyds are classified into four types that depend on oil content--very long-oil, long-oil, medium, and short-oil alkyds. Properties such as speed of drying, ease of brushing, film flexibility, chemical resistance, and exterior gloss retention are all dependent on the oil content. These properties are summarized in Table 4. A summary of alkyd resins comparing types of oil/fatty acids, nonvolatile, solvent, and typical application is given in Table 5.

TABLE 4--Alkyd resin properties related to oil length, Very Long oil LongOil Medium Short Oil Oil content Speed of drying Ease of brushing Film flexibility Chemical resistance Exterior gloss retention

57

Highest Slowest Best Highest Worst Worst

( < < < < (

) ) ) > ) )

Lowest Fastest Worst Lowest Best Best

Higher solids alkyds have been developed to reduce organic solvent emissions in those applications under regulatory restrictive requirements. This is accomplished by the development of polymers with lower viscosities than so-called conventional alkyds. The design and processing of higher solids alkyds result in a lower average molecular weight than conventional alkyds. A narrower molecular weight range of the resin species is necessary to meet air-dry, nonvolatility, and performance properties of the coatings. Another important factor is solvent selection. Organic solvents with greater viscosity reduction of the base or neat alkyd are needed to lower coating hydrocarbon atmospheric emissions. The release of these solvents during coating film formation is an important consideration. The higher solids alkyd resins are available in all classes of "conventional alkyds" such as those shown in Table 6. Higher solids alkyds can replace their conventional solids counterparts in many coating applications, thus affording decreased atmospheric emissions. Such products are used in air-dry architectural enamels and both air-dry and baking industrial primers and topcoats.

W A T E R - B O R N E ALKYD R E S I N S Water-borne alkyds obtain their water reducibility by the use of coupling solvents and amine-neutralized carboxyl groups on the polymer. Typical coupling solvents are ethylene glycol monobutyether, propylene glycol monoethylether, propylene glycol monopropylether, and four-carbon alcohols such as s-butyl alcohol. Water-borne alkyds are available in most classes of "conventional alkyds" such as those shown in Table 7.

SATURATED P O L Y E S T E R S Saturated polyesters are also called oil-free alkyds. The oil or fatty acid modification is zero percent, and this factor results in a polymer that cannot be air dried to a cross-linked coating. Rather, these polymers are formulated with a curing agent or cross-linker and baked. The curing agent can be a urea-formaldehyde or a melamine formaldehyde resin, both of which require baking. Polyurethane prepolymers can be coreacted with polyester resins for air-dry or low-bake coatings in two-component systems. In such systems, the saturated polyester provides the hydroxyl groups for cure with free isocyanate groups on the polyurethane prepolymer. The physical properties of these coatings are outstanding due to the absence of fatty acids, and they afford coatings with excellent color retention, flexibility, exterior durability, and hardness. The type of resins can be adapted to provide higher solids saturated polyesters by redesigning the polymer and using organic solvents with appropriate solvency rather than the customary blends of aromatic hydrocarbons with ketones, alcohols, and glycolethers. Water-borne polyesters are available through design of polymers having acid numbers in the range of 40 to 60. When these products are neutralized with an amine, they become

58 PAINT AND COATING TESTING MANUAL TABLE 5--Description of unmodified alkyd resins. Type Alkyd

Oil or Fatty A c i d

Nonvolatiles

Typical Applications

Solvent

Very long

Linseed Soya Tall oil

85-100%

Aliphatic hydrocarbon

Exterior latex modifier House paint modifier Oil-based stain and ink vehicles and modifiers

Long

Linseed Safflower Soya Sunflower Tall oil acids

60-70%

Aliphatic hydrocarbon

Architectural coatings Maintenance coatings One-coat enamels Exterior enamels Primers Topcoats

Medium

Linseed Safflower Soya Sunflower Tall oil acids Blends

45-50%

Aliphatic hydrocarbon Aromatic hydrocarbon

Farm implements Railway equipment Maintenance

Short

Castor Dehydrated castor Coconut Linseed Soya Tall oil acids Blends

50%

Aromatic hydrocarbon or Rule 66-type solvent blenda

Industrial coatings

~AtypicalRule 66 type solventis isobutanol,VM&Pnaphtha, and xyleneat 8% maximumvolumesolids.Rule 66 is a 1966regulationfrom California'sSouth CoastDistrictthat restricted the amount of aromatichydrocarbon solventin a coatingformulation.In the 1960s,research indicatedthat these types of solventscontributedgreatly to atmospheric ozone formation. Rule 66 legislationwas adopted by many other local and state regulators. soluble in blends of water a n d cosolvents a n d yield systems with low-volatile organic c o m p o u n d content. F o r m u l a t i o n of a coating from these products involves the use of water-borne or water-tolerant ureas a n d melamines. The cured films offer excellent hardness, gloss, a n d flexibility.

TABLE 6--Higher solids alkyd resin types and end uses. Type Long oil Medium oil Short oil Benzoic acid terminated Phenolic modified Silicone modified Copolymer

Typical End Use Architectural enamels Transportation enamels General industrial air-dry and bake enamels Implement enamels Primers Maintenance topcoats Aerosol enamels

TABLE 7--Waterborne alkyd resin types and end uses. Type Long oil Medium oil Short oil Benzoic acid terminated Phenolic modified Silicone modified

Typical End Use Stains and enamels (limited package stability) General industrial air-dry enamels General industrial baking enamels, automotive under the hood parts Implement enamels Primers Maintenance topcoats

SILICONE-MODIFIED POLYESTERS Conventional Types Silicone modification of polyesters is accomplished by use of a silicone intermediate incorporated t h r o u g h reaction at a 30 a n d 50% level. The silicone intermediates are of either hydroxy or methoxy functionality, a n d w h e n they are reacted with the polyester, water or m e t h a n o l is eliminated. This modification improves the weatherability and/or heat resistance of the alkyd a n d resulting organic coating. The siliconemodified polyesters are available in both self-curing a n d baking ( m e l a m i n e formaldehyde resin cross-linked) types. They are used as coil coatings for prefabricated building panels, prefabricated architectural products, metal advertising sign stock, a n d other applications requiring excellent exterior durability and/or good heat resistance.

Higher Solids Types Higher solids silicone-modified polyesters are m a d e by lowering the polyester base molecular weight and/or u s i n g oxygenated solvents such as ketone and ester types as replacements for aromatic hydrocarbons. This s u b s t i t u t i o n yields increased solvency, lower viscosities, lower solvent a m o u n t s , a n d higher nonvolatile c o n t e n t for the polyester solution. The end uses are similar to c o n v e n t i o n a l solventb o r n e silicone polyesters. However, the higher solids, silicone-modified polyester resins do n o t have the self-cross-link-

CHAPTER 7--ALKYD AND POLYESTERS ing option available for conventional types a n d are always c o m b i n e d with a cross-linking agent.

59

[12] ASTM D 1639: Test Method for Acid Value of Organic Coating Materials," Annual Book of ASTM Standards, Section 6, Vol. 6.01, 1992, pp. 192-193.

REFERENCES [1] Kienle, R. H. and Ferguson, C. S., Industrial and Engineering Chemistry, Vol. 21, 1929, p. 349. [2] Kienle, R. H. and Hovey, A. G., Journal of the American Chemical Society, Vol. 51, 1929, p. 509. [3] Kienle, R. H. and Hovey, A. G., Journal of the American Chemical Society, Vol. 52, 1930, p. 3636. [4] Kienle, R. H., Industrial and Engineering Chemistry, Vol. 22, 1930, p. 590. [5] Kienle, R. H., U.S. Patent 1,893,873, 10 Jan. 1933. [6] ASTM D 1545: Test Method for Viscosity of Transparent Liquids by Bubble Time Method, Annual Book of ASTM Standards, Section 6, Vol. 06.03, 1992, pp. 214-215. [7] ASTM D 1259: Test Methods for Nonvolatile Content of Resin Solutions," Annual Book of ASTM Standards, Section 6, Vol. 06.03, 1992, pp. 212-214. [8] ASTM D 1544: Test Method for Color of Transparent Liquids (Gardner-Holdt Scale)," Annual Book of ASTM Standards, Sec-, tion 6, Vol. 06.02, 1992, pp. 267-268. [9] ASTM D 1475: Test Method for Density of Paint, Varnish, Lacquer, and Related Products," Annual Book of ASTM Standards, Section 6, Vol. 06.01, 1992, pp. 178-180. [10] ASTM D 3278: Test Methods for Flash Point of Liquids by Setaflash-Closed-Cup Apparatus," Annual Book of ASTM Standards, Section 6, Vol. 6.03, 1992, pp. 406-412. [11] ASTM D 1640: Test Methods for Drying, Curing, or Film Formation of Organic Coatings at Room Temperature," Annual Book of ASTM Standards, Section 6, Vol. 6.01, 1992, pp. 194-197.

BIBLIOGRAPHY Blegen, J. R. and Fuller, W. P., Alkyd Resins, Unit 5 of the Federation Series of Coatings Technology, Philadelphia, PA, 1967. Holmberg, K., High Solids Alkyd Resins, Marcel Dekker, Inc., New York, 1987. Oldring, P., Resins for Surface Coatings, SITA Technology, London, 1987. Keane, J., et al., Systems and Specifications, Vol. 2, 4th ed., Steel Structures Painting Council, Pittsburgh, PA, 1985. Kask, T. and Lesek, F., Processes and Equipment for Alkyd and Unsat-

urated Polyester Resin Manufacture, Progress in Organic Coatings, Vol. 19, Elsevier Science Publishing Co., New York, 1991, pp. 283-331. Patton, T. C., Alkyd Resin Technology: Fo~7,nulating Techniques and Allied Calculations, Interscience Publishers, Division of John Wiley & Sons, New York-London, 1962. Payne, H., Organic Coating Technology, Vol. 1, Chapter 7, John Wiley and Sons, New York-London, 1965. Singer, E., "Fundamentals of Paint, Varnish, and Lacquer Technology," Chapter IV, American Paint Journal, 1957. "The Technology of Alkyd Resins," Barrett Division of Allied Chemical, Toledo, OH, 1958. Von Fischer, W., Paint and Varnish Technology, Chapter IX, Reinhold Publishing Corporation, New York, 1948. Zacharias, K., "Raw Materials Index, Resin Section," National Paint and Coatings Association, Washington, DC, 1988.

MNL17-EB/Jun. 1995

Amino Resins (Reaction Products of Melamine, Urea, etc. with Formaldehyde and Alcohols) by J. O w e n Santer 1

INTRODUCTION Definition and Description

/

AMINO, OR AMINOPLAST,RESINS for coatings are the p r o d u c t s of the r e a c t i o n of either urea (Fig. 1) o r m e l a m i n e (Fig. 2) with f o r m a l d e h y d e a n d an alcohol. Besides u r e a a n d melamine, o t h e r c o m p o u n d s with s i m i l a r f u n c t i o n a l i t y - - s u c h as b e n z o g u a n a m i n e , glycoluril, e t c . - - a r e also used in specific a p p l i c a t i o n s where certain p r o p e r t y advantages are required. However, use of these m a t e r i a l s is quite limited, a n d sales of a m i n o resins are d o m i n a t e d b y those b a s e d on u r e a a n d melamine, with U.S. c o n s u m p t i o n of a b o u t 100 million lb (45 • 106 kg) p e r year. As p r e p a r e d , a m i n o resins are usually water-white, viscous m a t e r i a l s which m a y c o n t a i n a d d e d solvent to reduce viscosity for ease of handling. W h e r e a solvent is used, it is m o s t often an alcohol such as n-butanol, iso-butanol, o r iso-propanol, all of w h i c h are excellent solvents for a m i n o resins. Mixed solvents, such as n- o r iso-butanol with xylene are also used, especially for the h i g h e r - m o l e c u l a r - w e i g h t resins m a d e with b u t a n o l as a co-reactant. S o m e a m i n o resins are w a t e r soluble or w a t e r reducible with co-solvent. Amino resins for coatings are g r o u p e d s o m e w h a t a r b i t r a r ily into two classes: (1) high solids resins, i.e., resin solutions at ---80% solids (weight/weight), including resins which contain no solvent, a n d (2) conventional resins, i.e., resin solutions at < 8 0 % solids (weight/weight).

FIG. 1-Urea.

NH 2 FIG. 2-Melamine. ways m a d e with m e t h a n o l or c o m b i n a t i o n s of m e t h a n o l a n d butanol, a l t h o u g h a small n u m b e r of high-solids resins are available which are m a d e exclusively with butanol.

Where Used Amino resins are used in coatings to cross-link the p r i m a r y film-former, usually a n acrylic, polyester, o r alkyd resin carrying p r i m a r y o r s e c o n d a r y hydroxyl groups. The crosslinking r e a c t i o n ("cure") is p r i n c i p a l l y one of trans-etherification between hydroxyl groups on the p r i m a r y film-former a n d alkoxymethyl groups on the a m i n o resin. In a d d i t i o n to the trans-etherification reaction, the a m i n o resin a l m o s t always u n d e r g o e s self-condensation reactions. The m a j o r b y - p r o d u c t s of the cure r e a c t i o n include m e t h a nol a n d / o r butanol, formaldehyde, a n d water. Cure t e m p e r a tures are in the range of 180 to 400~ (82 to 204~ for t i m e s w h i c h vary from 20 to 30 m i n at the lower end of the t e m p e r a ture range to p e r h a p s only 30 s at the u p p e r end. An a c i d catalyst m a y be used to accelerate cure, d e p e n d i n g on the cure t e m p e r a t u r e a n d the p a r t i c u l a r a m i n o used. Claims have been m a d e for a m i n o resin f o r m u l a t i o n s w h i c h cure at r o o m t e m p e r a t u r e , but as far as is known, no such f o r m u l a t i o n s are presently c o m m e r c i a l l y available. Urea resins are less expensive t h a n m e l a m i n e resins, w h i c h is u n d e r s t a n d a b l e given that m e l a m i n e is m a d e f r o m urea. Urea resins are also faster curing t h a n m e l a m i n e resins, b u t are m o i s t u r e sensitive a n d therefore not suitable for use outdoors. They are used widely for w o o d finishing, e.g., furniture, kitchen cabinets, a n d in paper, film, a n d foil applica-

History Historically, the first a m i n o resins used in coatings were the r e a c t i o n p r o d u c t s of u r e a o r m e l a m i n e with formaldehyde a n d b u t a n o l (either n- or iso-). They were substantially p o l y m e r i c a n d were f o r m u l a t e d at a b o u t 50 to 60% solids in butanol/xylene mixtures. They have been c o m m e r c i a l l y available for a b o u t 60 years. Parenthetically, it should be noted that resins m a d e by reacting u r e a o r m e l a m i n e with formald e h y d e w i t h o u t subsequent r e a c t i o n with an alcohol have b e e n available for 70 years o r more. These resins are used as m o l d i n g p o w d e r s a n d adhesives a n d are generally unsuitable for coatings applications. High-solids coating resins, usually m a d e with higher ratios of r e a c t e d f o r m a l d e h y d e t h a n the older, conventional resins, have been available for a b o u t 35 years. They are a l m o s t al1Principal technologist, Monsanto Chemical Co., 730 Worcester Street, Springfield, MA 01151. 60 9

Copyright 1995 by ASTM lntcrnational

C=O

www.astm.org

CHAPTER 8 - - A M I N O R E S I N S tions. Wood and paper applications capitalize on the relatively rapid cure of the urea resin since lower temperatures must be used to avoid damage to the substrate. In some wood applications, cure temperature is at or near ambient. Melamine resins, on the other hand, find much b r o a d e r application since they are not nearly as water sensitive as the ureas. Perhaps the largest single use for melamine resins is in automotive OEM (original equipment manufacture), where the finished paint must stand up to extremes of temperature, humidity, and the degradative effects of sunlight, etc, Melamine resins are used also in coil coatings, metal containers, etc. (see E n d U s e s o f A m i n o R e s i n s ) .

SYNTHESIS OF AMINO RESINS Reactions of Synthesis The synthesis of amino resins for coatings is a two-step process. In the first step, the parent compound is reacted with formaldehyde (methylolation reaction); in the second, the methylolated intermediate is reacted with an alcohol (etherification reaction). Equations 1 through 4 exemplify the process, with urea as the parent compound. Reactions with melamine are analogous. H2NCONH2 + CH2O HaNCONHCH20H + CH20

) H2NCONHCH2OH monomethylolurea

(1)

HOCH2NHCONHCH20R + ROH ) ROCH2NHCONHCH2OR + H20 bis(alkoxymethyl)urea

>NCH2N< (methylene) or >NCH2OCH2N< (methylene ether) bridge. The reactions leading to self-condensation may be written as follows: >NCH2OR + HN<

~ >NCH2N< + ROH R = H, alkyl

>NCH2OH + HOCH2N<

(5)

~ >NCH2OCH2N< + H20 (6)

The bridging groups in amino resins manufactured for coatings applications are predominantly methylene ether bridges. When formulated and cured, additional bridges of both types may be formed; how many of each will depend on factors such as the composition of the amino resin, cure temperature, and catalyst level. It can be seen, then, that a variety of amino resins may be prepared, with properties which depend on such factors as the choice of starting material, i.e., urea, melamine, etc., the combining ratios of the various reactants, the choice of alcohol (or alcohols, if more than one is used), and the degree of polymerization of the resin. The principal manufacturers of amino resins for coatings typically offer a product line of 25 or more resins. A generalized composition of a typical melamine resin is shown in Fig. 3.

Structure/Property Variations The difference between conventional solids and high-solids a m i n o resins represents not so much a difference in solids

) HOCH2NHCONHCH20H (2) dimethylolurea

HOCH2NHCONHCH20H + ROH ) HOCH2NHCONHCH2OR + H20

61

(3) (4)

Reactions shown in Eqs 1 and 2 proceed quite rapidly when catalyzed by either acid or base. Reactions shown in Eqs 3 a n d 4 take place only under acid conditions, with the rate of reaction strongly pH dependent; the lower the pH, the faster the reaction. All four reactions are equilibrium reactions. Hence, the extent of the reaction is dependent on the charge ratios of the various reactants and on whether or n o t the reactions are driven by removal of by-products in order to shift the equilibrium. The reactions of melamine are similar to those of urea with one exception. With urea, two of the four available hydrogens are readily reacted with formaldehyde (Eqs 1 and 2), while the remaining two hydrogens react more slowly and require an excess of formaldehyde to force the reaction. With melamine, on the other hand, all six hydrogen atoms may be reacted with relative ease to give hexa(methylol)melamine. The kinetics of the methylolation reactions of urea and melamine have been studied extensively [1-7], but there is nothing in the literature on the kinetics of the etherification reaction. Although both of these reactions are superficially straightforward, a number of other reactions may take place which complicate the kinetics. These reactions are all selfcondensation reactions in which two or more molecules of the parent species are joined together through either a

content as it does a distinction between resin structures. The conventional solids (3 and perhaps somewhat higher for the ureas. Because of the relatively high polymer content, these resins are viscous and must be reduced with solvent to less than 80% to obtain a manageable viscosity. Another, possibly the major, contributor to high viscosity is the residual imino (>NH) and methylo] (>NCH2OH) groups on the amino resin, which can form strong hydrogen bonds with unshared electrons on nitrogen and oxygen. The high-solids amino resins have much higher levels of combined formaldehyde than the conventional solids resins. Typical values for combined formaldehyde are in the range 2.0 to 2.7 for urea resins and 3.5 to 6.0 for melamine resins. The etherifying alcohol is most often methanol, although res-

ROCH2\ /CH2OH ROCH2./N"'~NI~N'H

N. N N

ROCH2/

"CH 2 0 ' ' 9

FIG, 3-Representative structure of a melamine resin,

62

PAINT AND COATING TESTING MANUAL

ins made with both methanol and butanol or even butanol alone are also widely used. These resins are less polymeric, with DP N H and >NCH2OH groups, and (2) the solvent is not low boiling. Apparently, the increase in viscosity as solvent evaporates slows the diffusion rate and effectively prevents complete removal of solvent within the time frame of the test. There may also be a hydrogen-bonding effect between solvent and resin which contributes to the retention of solvent. Other test methods involve much higher temperatures, where resin condensation/degradation does occur. One standard method is the ASTM Test Methods for Volatile Content of Coatings (ASTM D 2369), where a small resin sample (0.3 to 0.5 g) is diluted with xylene and placed in a 110~ oven for 1 h. There are a number of other, similar tests.

Viscosity Measurement Amino resin viscosities are most commonly measured by the Gardner bubble viscometer method. This method is similar to the Test Method for Viscosity of Transparent Liquids by Bubble Time Method (ASTM D 1545). A tube containing the resin under test is placed in a rack containing reference tubes of known viscosity. The tubes are equilibrated to 25~ in a constant temperature bath. The rack is quickly inverted, and the rate of rise of an air bubble in the sample tube is compared against similar bubbles in the reference tubes. The reference tubes are letter graded A through Z and Z1 through Z6.

Solids Content The most common methods used to determine solids content are gravimetric. Solvent is allowed to evaporate from a weighed sample under carefully controlled conditions of time and temperature. The sample is then reweighed. The loss in weight gives a measure of solvent content, and the solids content is obtained by difference. One difficulty with this test is the tendency of amino resins to deformylate and/or selfcondense when heated, with evolution of formaldehyde, alcohol, and water. To the extent that this occurs, the measured solids content will be lower than the "true" value. Frequently, however, the paint formulator is interested in the "contributed solids," i.e., what fraction of the amino resin solution remains in the cured film. In that case, a solids test method which approximates the time and temperature of cure might be more appropriate. For these reasons, solids test methods fall into two groups: (1) methods which reflect the solids content in the absence of self-condensation, etc., and (2) other methods, which reflect varying degrees of self-condensation in addition to loss of formaldehyde and solvent(s). The most common of the first methods is the so-called foil solids test, which is used almost universally for high-solids amino resins. Essentially, a 1-g sample of resin solution is weighed onto a piece of preweighed aluminum foil. The foil is folded over on itself and the sample compressed between the two foil surfaces to provide a thin film about 3 to 4 in. (7 to 10 cm) in diameter. The foil is then opened up to give a thin film on each foil surface. The foil is placed in a 45~ oven for 45 min, at the end of which time it is removed, reweighed, and the solids content calculated. These conditions are known to be sufficiently mild that no resin condensation occurs; nor does the resin lose formaldehyde via demethylolation. Surprisingly, the foil solids test may on occasion overestimate the solids content, particularly when (1) the resin is relatively

Solvent Tolerance There are a number of different solvent tolerance tests. All involve titrating a weighed sample of the amino resin with a standard reagent (solvent). The object of the test is to measure how much of the reagent the amino resin can accept before the solution turns cloudy/milky. Results are typically reported in milliliters of reagent per gram of sample. Typical reagents used include xylene, iso-octane, and the iso-octane/ decahydronaphthalene/toluene mixture described in ASTM Test Method for Solvent Tolerance of Amine Resins (D 1198). While the immediate objective of the solvent tolerance test is to determine the amount of reagent which the amino resin can accept before solution clouding occurs, the real purpose of the test is to gain insight into the structure and composition of the resin and hence have a better understanding of how it will perform in a given coating application. In general, amino resins of high molecular weight, or having high levels of polar functional groups, i.e., >NH, >NCH2OH, will have limited compatibility with the typical hydrocarbons used and hence will give low tolerance test results. Experience shows that a low tolerance value means a faster curing resin and vice versa, especially in the absence of acid catalyst. However, although the tolerance test represents a quick and easy way to measure potential cure response, it does not uniquely define the resin structure. Thus, a low tolerance reading can be caused by either high polarity or high molecular weight or both.

Size Exclusion and High-Performance Liquid Chromatography To obtain more detailed knowledge of resin structure, amino chemists now rely very heavily on gel permeation or size exclusion chromatography (SEC) and on high-performance liquid chromatography (HPLC). The size exclusion

CHAPTER 8--AMINO RESINS chromatograph provides an excellent measure of number and weight-average molecular weight and molecular weight distribution (polydispersity), while HPLC, which fractionates the resin components primarily by functional groups, provides information on resin composition. The more polar species are eluted first, followed by the less polar fractions. Thus, taken together, SEC and HPLC provide detailed information on molecular weight and functionality which cannot be directly obtained or inferred from any of the various solvent tolerance tests. Size exclusion and liquid chromatograms for a representative commercial high-solids methylated melamine resin are shown in Figs. 4 and 5.

Combining Ratios Amino resins may also be characterized by measurement of the amounts of formaldehyde and alcohol which have re-

acted. For example, see hexa(methoxymethyl)melamine (HMMM) (Fig. 6), which has exactly 6 mol each of combined formaldehyde and methanol per mole of melamine. Unlike HMMM, most resins are, of course, mixtures of products which are best described by an average composition. One of the most widely sold commercial high-solids methylated melamine resins has an average combining ratio melamine/formaldehyde/methanol of about 1/5.6/5.1. Because methanol reacts with an already-reacted formaldehyde molecule, a resin can never have combined methanol greater than the combined formaldehyde. The excess formaldehyde, 0.5 tool in the commercial example, represents formaldehyde which has not reacted with methanol and which must therefore be present as methylol (>NCH2OH), bridging groups (>NCHzOCH2NNCH2OCH2OCH3). Acetals are formed when an excess of formaldehyde is used in the

400 -

Monomer

mV

350 -

300 -

I

250"~

200 -

Dimer

150 ~ 100-

50 20

35

30

25

40

45

Minutes FIG. 4-Size exclusion chromatogram of a typical high-solids methylated melamine resin.

600"mV

Hexamethoxy

500-

Pentamethoxy

400 350200 -

Tetra-

I

A

I

1000

20

25

30

63

35

40

45

50

Minutes FIG. 5-High-performance liquid chromatogram of a typical high-solids methylated melamine resin.

64

PAINT AND COATING TESTING MANUAL in Analysis/Analytical Methods. The problem is particularly acute with resins having high methylol functionality.

(C H 30C H2)2N...h/N__,h/N(CH 20C H 3)2 II I N.~N

Viscosity

N(CH2OCH3)2 FIG. 6-Hexa(methoxymethyl)melamine. synthesis. They are therefore present in many high-solids amino resins. Determination of combining ratios may be done most easily by either IH or 13C NMR techniques [8]. Older methods involve complete hydrolysis of the resin to the starting materials, followed by wet-chemical analysis for formaldehyde and gas chromatographic determination of alcohol (methanol or butanol).

Free Formaldehyde Amino resins always contain some unreacted formaldehyde, usually referred to in product specifications as "free" formaldehyde. Free formaldehyde may be analyzed quantitatively by a number of methods. One of the most commonly used is the sodium sutfite method [10]. Formaldehyde reacts rapidly and completely with aqueous sodium sulfite to form a bisulfite addition complex. Sodium hydroxide is liberated quantitatively on a mole-for-mole basis CH20 + Na2SO3 + HaO

~NaOH + CHa(OH)NaSO3 (7)

The NaOH is either titrated directly with a standard HC1 solution, or neutralized with a known excess of standard HC1, which is then back-titrated with NaOH. Care must be taken to ensure that reacted formaldehyde, particularly methylol groups, is not analyzed as free formaldehyde. This can occur because of the following reaction, which can be minimized by performing the titration as rapidly as possible at cool temperatures, e.g., room temperature or lower. >NCH2OH

) >NH + CHaO

(8)

PHYSICAL PROPERTIES General Amino resins are typically viscous liquids, with an aminelike odor. Depending on composition, they may also smell of formaldehyde and/or solvent. They are readily soluble in alcohols, ketones, hydroxy-functional glycol ethers, esters, etc., but have limited solubility in hydrocarbons. Some resins, especially methylol-rich resins with low levels of both combined formaldehyde and combined methanol, are water soluble. Many more are water reducible in the presence of other solvents, e.g., alcohols and glycol ethers. Because of their resinous nature, aminos have neither a well-defined freezing point nor boiling point. Uncured resins typically have glass transition temperatures around -40~ When heated, they undergo decomposition, with release of formaldehyde and alcohol, at temperatures above about 140~ This tendency to decompose causes difficulties in determining the solids content of resin solutions, as described

The viscosity of an amino resin is a function of (1) polymer content (degree of polymerization) and (2) the nature of its functional groups. The latter may be a more important contributor to viscosity than the former. Amino resins are not generally very polymeric, especially in comparison with other coating resins, e.g., polyesters, alkyds, and acrylics. Typically, average degrees of polymerization are in the range of I to 5. High-molecular-weight "tails" increase viscosity significantly. Because of strong hydrogen bonding, resins carrying significant amounts of >NH and >NCH2OH functionality are quite viscous, even though they may not be highly polymerized. There is a marked drop in viscosity when amino resins are diluted with solvent, largely due to breaking of hydrogen bonds. Good solvents (e.g., alcohols) are more effective at reducing viscosity than poor ones [11 ]. Methanol is probably the best, although it is not widely used because of its low boiling point. Isopropanol is almost as effective, and because it is somewhat higher boiling, represents a good compromise.

Surface Tension The surface tension of amino resins is quite strongly related to the nature of the etherifying alcohol and is much less affected by the level of combined formaldehyde and alcohol. In the author's laboratory, surface tension measurements on high-solids, solvent-free resins using a DeNouy tensiometer have given values ranging from about 45 dynes/cm for methylated resins to about 28 dynes/cm for butylated resins. Mixed methyl/butyl resins give intermediate values, depending on the methyl and butyl content. The reduction in surface tension vghen butanol is the etherifying alcohol may be one reason that high-solids butyl and methyl/butyl resins provide improved flow and leveling in high-solids formulations compared to their fully methylated counterparts.

REACTIONS OF AMINOS IN COATINGS Cure Reactions Amino resins in coating formulations cure by reactions which are chemically and mechanistically similar to those which take place during synthesis of the resin. The principal reaction of cure is one of trans-etherification, wherein a hydroxyl group on the primary film-former (acrylic, polyester, or alkyd) reacts with an alkoxymethyl group on the amino resin >NCH2OR + HO--A

~ >NCH20--A + ROH

(9)

where R = alkyl, and A = primary film-former. Additionally, direct etherification may take place, the end result being the same

CHAPTER 8 - - A M I N O R E S I N S >NCH2OH + HO--A

)

>NCH20--A + H20

(10)

where A = primary film-former. These two reactions both result in chemical bond formation between the amino and the primary film-former (cocondensation). Two other reactions may also take place, both of which involve reaction of the amino resin with itself (selfcondensation). These are >NH + ROCHzN<

)

>NCH2N< + ROH

(11)

where R = H, alkyl. >NCH2OH + HOCH2N<

) >NCH2OCHzN< + H20 (12)

Besides the co-condensation and self-condensation reactions, hydrolysis and deformylation reactions may also occur >NCH2OR + H20 >NCH2OCH2OR + H20 >NCHzOH

)

) >NCHzOH + ROH

(13)

>NCH20H + CH20 + ROH

(14)

~ >NH + CH20

(15)

The relative contributions to cure of the co-condensation and self-condensation reactions will depend on a variety of factors. These include: 1. The functionality of the amino resin, i.e., the relative proportions of >NCH2OR, >NCH2OH, and >NH groups present initially, as well as those generated during formulation and/or cure. 2. The functionality (hydroxyl number) of the primary filmformer (coreactant). 3. The amino/coreactant ratio. 4. The level and type of catalyst (weak acid/strong acid). 5. Cure time and temperature. A coreactant resin with a low hydroxyl number is best if formulated with a "polar" amino (i.e., one rich in >NH and/or >NCH2OH) since these groups help build molecular weight during cure via self-condensation, particularly if little or no catalyst is present. Conversely, a high hydroxyl resin is best if matched with an alkoxymethyl-rich amino and cured with a strong acid catalyst. Where high cure temperatures are employed (e.g., can or coil coating operations), the choice of amino resin is less obvious, and, in practice, both polar and nonpolar aminos are used. Acid catalysts are usually used as an aid in curing aminobased formulations. These catalysts include very strong acids such as p-toluenesulfonic acid (PTSA), dodecylbenzenesulfonic acid (DDBSA), dinonylnaphthalenedisulfonic acid (DNNDSA), etc., and weaker acids such as phenyl acid phosphate (PAP), butyl acid phosphate (BAP), etc. Amine blocking agents are sometimes used to help minimize resin advancement prior to cure. Some coatings, particularly those designed for high-bake temperatures, need no catalyst, relying instead on the combination of high temperature and perhaps carboxylic acid functionality on the primary filmformer to bring about cure [12]. While all of the various reactions which take place during cure are accelerated by either acid or heat, it is fair to say that reactions of transetherification are most influenced by catalyst level and type, while reactions of self-condensation are most influ-

65

enced by heat. The trans-etherification reaction takes place very rapidly under strong acid catalysis, even at low temperatures. This is especially true for aminos with a high level of alkoyxmethyl substitution, i.e., a very low NH content, which tends to inhibit catalysis. Thus, most formulations involving resins with high alkoxymethyl ether content and designed for low-temperature cure (250~ or lower) will call for a sulfonic acid catalyst, either blocked or free. Although the individual reactions of cure are reasonably well understood and have been described in numerous papers [13-18], there is still much to be learned about the overall behavior of amino resins during cure, in particular the relative contributions of each of the various reactions. One of the difficulties is, of course, that the coating becomes intractable as cure progresses. Hence, a majority of studies involve analysis of the by-products of cure [13,17,30]. Other methods, such as dynamic mechanical analysis [19], nuclear magnetic resonance [20,21], FTIR [17], ESCA, etc. investigate the structure of the cured film. These techniques are useful not only for analyzing the freshly cured coating, but also as a means of following the coating through its lifetime, either natural or accelerated.

Degradation and Weathering Amino-based cross-linked coatings exposed to the atmosphere are subject to both hydrolysis and UV-degradation. The mechanisms by which melamine resins hydrolyze have been described in detail by Berge [22-24], who was the frst to distinguish between mono- and di-substituted nitrogen with respect to their behavior towards acid or base hydrolysis. Thus, in an alkaline medium, hydrolysis of an alkoxymethyl group on a singly substituted nitrogen is initiated by removal by the base of the proton attached to nitrogen --NHCH2OR + B --NCH2OR

) --I~ICH2OR + BH § ) - - N = CH2 + OR-

- - N = CH2 + H20 OR- + BH +

) --NHCH2OH ) ROH + B

(16) (17) (18) (19)

This mechanism is clearly not applicable to di-substituted nitrogen (N(CH2OR)2), and these groups are in fact extremely resistant to alkaline hydrolysis. On the other hand, acid hydrolysis takes place readily for both mono- and di-substituted nitrogen. Berge proposed two mechanisms (a) specific acid catalysis :, >NCH2OHR +

>NCH2--OR + H + >NCH2OHR + > N C H f + H20

(20)

) >NCH~- + ROH

(21)

) >NCH2OH + H +

(22)

) >NCH2OHR + + A-

(23)

and (b) general acid catalysis >NHCHzOR + HA >NHCH2OHR + + A-

) >N

- - N = CH 2 + H20

= CH 2 +

ROH + HA (24)

) --NHCHEOH

(25)

Berge's work with melamine resins is undoubtedly relevant to acid hydrolysis of paint films, which has been studied by a number of workers.

66

PAINT AND COATING TESTING MANUAL

English et al. [25,26] found that coatings prepared from highly alkylated melamines underwent extensive hydrolysis of residual methoxy groups during two years exposure in Florida, but there was no evidence of hydrolysis of bonds between melamine and the primary film-former. Bauer [2728] used IR to analyze acrylic-melamine coatings exposed to both UV and moisture and found evidence of hydrolysis of both residual methoxy groups and acrylic-melamine bonds, with the rate of hydrolysis being faster in the presence of UV light. The rate of hydrolysis was slowed considerably when a hindered amine light stabilizer was used. In recent years, degradation of melamine-containing automotive coatings has been particularly severe because of etching and spotting due to "acid rain." The problem is compounded because modern high-solids automotive coatings use very high levels of melamine resins (35 to 45% of total binder weight), giving rise to correspondingly high levels of acrylic-melamine bonds and residual alkoxymethyl groups in the cured film, all of which are susceptible to hydrolysis under acid conditions. Suppliers of high-solids coatings for automobiles are presently evaluating and using alternative cross-linkers, such as isocyanates and epoxies, which are more stable under acid rain conditions and which can serve as either a partial or complete replacement for melamines. An interesting aspect of the acid etch problem is that the damage always occurs to relatively new coatings. If a newly painted automobile is protected from the acid environment for the first six to twelve weeks, damage thereafter is much less severe. An obvious conclusion is that the paint is undergoing additional cure (probably melamine self-condensation) as it ages. Automotive paint manufacturers are also actively pursuing waterborne systems, which use higher molecular weight, less functional coreactant resins, and lower levels of melamine cross-linker and which are therefore less severely degraded by acid rain. At the present time, however, these waterborne systems are only used in the base coat, where acid attack is in any case minimized by the protective clear top coat. It is the top coat, with its high melamine content, which is the principal site for acid attack. But it is also the high level of melamine resin which provides the excellent gloss and "distinctness of image" (DOI), characteristic of basecoat/clearcoat technology. The melamine resin also minimizes the amount of solvent required because of its low viscosity at high-formulated solids, behaving in some ways as a reactive diluent and plasticizer.

in exterior applications, despite some of the recent difficulties described earlier in connection with water spotting and acid etch of automobiles. Besides automobiles, they are used in appliance formulations (both coil appliance and conventional post-sprayed), general metal applications, container coatings (beer and beverage cans), etc. In choosing an amino resin for a particular application, consideration must be given not only to interior versus exterior use, but also to possible restrictions on cure conditions and compatibility of the amino resin with its co-reactant resin, both when formulated and as the paint film is formed during solvent flash-off and cure, etc. Compatibility of the amino is especially important in water-borne coatings, which are becoming more widely used. Another factor is the stability of the amino towards advancement (molecular weight buildup) during storage of the formulated paint. Benzoguanamine-based (Fig. 7) amino resins are used where film flexibility and hardness are required, as in some appliance applications (e.g., refrigerator doors made from coil stock, etc.). They also have good corrosion and humidity and detergent resistance. Their use is limited by cost and poor exterior durability due to the pendant phenyl group on the benzoguanamine molecule. Glycoluril (Fig. 8) resins have been available for about a dozen years. They may require a higher cure temperature or a higher catalyst level than melamine-based resins, but show excellent corrosion and humidity resistance and release lower amounts of formaldehyde during cure [29].

ENVIRONMENTAL/TOXICITY The past 20 years have seen increased emphasis on the quality of the environment both in the workplace and beyond. In the coatings industry, this has meant strict controls on exposure of workers to hazardous ingredients in the coating formulation when applied, as well as on the nature and

.2.yN.yN.2

E n d Uses o f A m i n o R e s i n s Amino-based surface coatings protect and decorate the substrate to which they are applied. Their technology and use has developed over many years. As already mentioned, resins based on urea and melamine dominate the field. Urea resins are traditionally used in clear coatings for wood, e.g., furniture, kitchen cabinets, in paper, film, and foil applications, and in some appliance and general industrial coatings. They are also used to some extent in automotive primers. They cannot be used in automotive topcoats because of their sensitivity to hydrolysis. Melamine resins are much more widely used. They give better chemical resistance, as well as resistance to weathering

FIG. 7-Benzoguanamine.

H

H

"N~ ~'N ~ H H FIG. 8-Glycoluril.

CHAPTER 8 - - A M I N O R E S I N S a m o u n t s of volatile organics (the so-called VOCs) released to the e n v i r o n m e n t w h e n the f o r m u l a t i o n is cured. Amino resin suppliers have r e s p o n d e d to these environm e n t a l challenges in a n u m b e r of ways. Chief a m o n g these has been a progressive shift t o w a r d s higher-solids, lowermolecular-weight aminos, w h i c h are n o w the resins of choice of coatings formulators. M a n y a m i n o resins are supplied at 100% nonvolatiles, especially for the a u t o m o t i v e industry. W h e r e solvents are needed, those presenting the least h a z a r d to w o r k e r a n d e n v i r o n m e n t are selected. F o r their part, p a i n t p r o d u c e r s have i n c r e a s e d the functionality of the c o r e a c t a n t resin while lowering its m o l e c u l a r weight to m i n i m i z e solvent use with the object of b u i l d i n g m o l e c u l a r weight to the maxim u m possible extent d u r i n g cure. This has m e a n t using h i g h e r levels of a m i n o resin, as m u c h as 40 to 50% of total b i n d e r weight in s o m e cases. Perhaps the m o s t i n t r a c t a b l e e n v i r o n m e n t a l p r o b l e m with a m i n o resins is the use of f o r m a l d e h y d e in t h e i r m a n u f a c t u r e . F o r m a l d e h y d e is r e c o g n i z e d b y the I n t e r n a t i o n a l Agency for R e s e a r c h on Cancer (IARC) as a carcinogen. The A m e r i c a n Conference of G o v e r n m e n t a l I n d u s t r i a l Hygienists (ACGIH) lists f o r m a l d e h y d e as an "A2" substance, i.e., one suspected of carcinogenic potential for m a n , a n d the O c c u p a t i o n a l Safety a n d Health A d m i n i s t r a t i o n (OSHA) has set w o r k p l a c e exposure limits of 0.75 p p m (8-h t i m e w e i g h t e d average) a n d 2 p p m (15-min s h o r t - t e r m exposure limit). The f o r m a l d e h y d e content of a m i n o resins is p r e d o m i n a n t l y "combined," i.e., chemically reacted, a n d r e p r e s e n t s a b o u t 30 to 50% by weight of the resin. A small a m o u n t , ranging from a b o u t 0.1 to a b o u t 3% is present free, o r unr e a c t e d (see the section entitled A n a l y s i s / A n a l y t i c a l Methods). Amino resin suppliers have m a d e c o n s i d e r a b l e progress over the p a s t several years in lowering the level of free formald e h y d e in their products, w h i c h is i m p o r t a n t b e c a u s e of OSHA labelling requirements. In an ideal situation, all of the c o m b i n e d f o r m a l d e h y d e w o u l d r e m a i n in the coating after cure as p a r t of the p o l y m e r network, In practice, however, some of the c o m b i n e d formaldehyde a n d all of the free f o r m a l d e h y d e is released d u r i n g cure a n d m a y r e a c h the environment, d e p e n d i n g on the mechanics of the coating a n d curing operation. It is the p a r t i a l release of c o m b i n e d f o r m a l d e h y d e d u r i n g cure w h i c h is of m o s t concern, since the a m o u n t released c a n easily be several times t h a t of the free formaldehyde. I n c i n e r a t i o n of off gases, w h e r e possible, is the best solution.

REFERENCES [1] DeJong, J. I. and DeJonge, J., Recueil de Travail Chimie Pay-Bas, Vol. 71, 1952, p. 643.

67

[2] Gordon, M., Halliwell, A., and Wilson, T., Journal of Applied Polymer Science, Vol. 10, 1966, p. 1153. [3] Gordon, M., et al., "The Chemistry of Polymerization Processes," SCI Monograph No. 20, Society of Chemical Industry, London, 1966, p. 187ff. [4] Aldersley, J. W. et al., Polymer, Vol. 9, 1968, p. 345. [5] Okano, M. and Ogata, Y., Journal of the American Chemical Society, Vol. 74, 1952, p. 5728. [6] Braun, D. and Legradic, V., Angewaudte Makromolekular Chemie, Vol. 35, 1974, p. 101. [7] Tomita, B., Journal of Polymer Science, Vol. 15, 1977, p. 2347. [8] Christensen, G., "Analysis of Functional Groups in Amino Resins," Progress in Organic Coatings, Vol. 8, 1980, pp. 211-239. [9] Kambanis, S. M. and Rybicki, J., Journal of Coatings Technology, Vol. 52, No. 667, 1980, p. 61. [10] Walker, J. F., Formaldehyde, 3rd ed., Robert E. Krieger Publishing Co., Huntington, NY, 1975, p. 486. [11] Hill, L. W. and Wicks, Z., Progress in Organic Coatings, Vol. 10, 1982, p. 55. [12] Yamamoto, T., Nakamichi, T., and Ohe, O., Journal of Coatings Technology, Vol. 60, No. 762, 1988, p. 51. [13] Blank, W., Journal of Coatings Technology, Vol. 51, No. 656, 1979, p. 61. [14] Blank, W., Journal of Coatings Technology, Vol. 54, No. 687, 1982, p. 26. [15] Santer, J. O. and Anderson, G. J., Journal of Coatings Technology, Vol. 52, No. 667, 1980, p. 33. [16] Santer, J. O., Progress in Organic Coatings, Vol. 12, 1984, p. 309. [17] Lazzara, M. G., Journal of Coatings Technology, Vol. 56, No. 710, 1984, p. 19. [18] Nakamichi, T., Progress in Organic Coatings, Vol. 14, 1986, p. 23. [19] Hill, L. W. and Kozlowski, K., Journal of Coatings Technology, Vol. 59, No. 751, 1987, p. 63. [20] Bauer, D. R., Progress in Organic Coatings, Vol. 14, 1986, p. 45. [21] Bauer, D. R., Progress in Organic Coatings, Vol. 14, 1986, p. 193. [22] Berge, A., Kvaeven, B., and Ugelstad, J., European Polymer Journal, Vol. 6, 1970, p. 981. [23] Berge, A., Advances in Organic Coatings Science and Technology, Vol. 1, 1979, p. 23. [24] Berge, A., Gudmundsen, S., and Ugelstad, J., European Polymer Journal, Vol. 5, 1969, p. 171. [25] English, A. D., Chase, D. B., and Spinelli, H. J., MacromoIecules, Vol. 16, 1983, p. 1422. [26] English, A. D. and Spinelli, H. J., Journal of Coatings Technology, Vol. 56, No. 711, 1984, p. 43. [27] Bauer, D. R., Journal of Applied Polymer Science, Vol. 27, 1982, p. 3651. [28] Bauer, D. R. and Briggs, L. M., "Characterization of Highly Crosslinked Polymers," American Chemical Society Symposium Series No. 243, Washington, DC, 1984. [29] Parekh, G. G., Journal of Coatings Technology, Vol. 51, No. 658, 1979, p. 101. [30] McGuire, J. M. and Nahm, S. H., Journal of High-Resolution Chromatography, Vol. 14, 1991, p. 241.

MNLI7-EB/Jun.

1995

Ceramic Coatings by Richard A. Eppler ~

GLAZES

organic paints for surface coating applications. When painting with a suitable material will meet all service requirements, it is almost always less expensive to paint. However, organic paints have limitations in several areas where ceramic coatings are a more suitable selection. Vitreous (glassy) ceramic coatings are chosen for application over a substrate for one or more of several reasons [1]. These coatings may be applied to a substrate surface in preference to an organic paint to render the surface chemically more inert, impervious to liquids and gases, more readily cleanable, and more resistant to service temperature, abrasion, and scratching. The chemical durability of ceramic coatings in service substantially exceeds that of organic paints [2]. Vitreous coatings are formulated to be resistant to a variety of reagents, from acids to hot water to alkalies, as well as to essentially all organic media. The only important exception is hydrofluoric acid, which readily attacks all silicate glasses. This outstanding durability, combined with a very smooth surface, renders many ceramic coatings suitable for applications requiring the highest standards of cleanability such as ware that comes in contact with food and drink. These coatings are also suitable for applications requiring true hermeticity, usually to protect sensitive electronic equipment. No organic resins are truly hermetic. Even the most thermally stable organic resins depolymerize at temperatures on the order of 300~ Hence, organic paints are not suitable for applications requiring thermal stability above 300~ For example, stove side panels are painted, but stove tops are porcelain enameled. A similar argument can be made for abrasion resistance. Organic resins are soft (Moh 2 to 3). By contrast, vitreous coatings are harder (Moh 5 to 6), and some plasma coatings are much harder. For example, alumina coatings, plasma sprayed, have Moh = 9. Vitreous coatings are thin layers of glass fused onto the surface of the substrate. When the substrate is a ceramic, the coating is called a glaze. When the substrate is a metal, the coating is called a porcelain enamel. When the substrate is a glass, the coating is called a glass enamel. CERAMIC COATINGS ARE AN ALTERNATIVE t o

A ceramic glaze is a vitreous coating applied to a ceramic substrate, usually a whiteware. Glazes are applied to their substrates by one of several powder-processing techniques: dipping, spraying, and waterfall. The raw materials are both crystalline oxides and frits. In these wet processes the raw materials are dispersed in an aqueous slip for application. After application, the coatings must be dried and fired at high temperatures (up to 1300~ typically 1000 to 1100~ to fuse them onto the substrate.

Applications for Glazes Ceramic glazes find their way into a wide range of applications ranging from coffee mugs to automotive spark plugs. The major markets for ceramic coatings have different requirements, but one common theme is chemical durability and cleanability. The major products that normally use glazes are distributed as follows: 43.9% Sanitaryware 32.9% Wall and floor tile 10.9% Tableware 9.5% Artware 2.8% Electrical porcelain and electronics The total market for these products in the United States was reported to be $3459 billion for 1989 [3], of which the glaze typically consumed 10 to 15% of the total manufacturing cost. Hence, the value of the protective, functional, and decorative properties provided by the coating usually far outweighs the cost.

Leadless Glazes Glazes are essentially mixtures of silica with other oxides added to permit the glaze to form at a readily achievable temperature. In a leadless glaze, the alkali and alkaline earth oxides, together with magnesia (MgO), zinc oxide (ZnO), and boron oxide (B203), are used to provide the fluxing action. Table 1 gives the formulas of a few typical ceramic glazes. Glaze 1 is a feldspathic glaze suitable for use on soft paste porcelains or hard stoneware [4]. This glaze is typical of that used on medieval Chinese porcelains. Glaze 2 is a sanitary-ware glaze [5]. It is derived from the soft paste porcelain glaze by the addition of ZnO in large quantity. Increasing the melting rate by increasing the per-

1Consuhant, Eppler Associates, 400 Cedar Lane, Cheshire, CT 06410. 68 Copyright9 1995 by ASTMInternational

www.astm.org

CHAPTER 9--CERAMIC COATINGS

69

TABLE 1--Typical ceramic glazes in weight percent. Glaze

Na20

KzO

CaO

MgO

ZnO

SrO

BaO

PbO

B203

A1203

SiO2

ZrO2

1 2 3 4 5 6 7 8

2.24 2.05 6.54 1.81 3.06 2,46 0.00 0.85

3.24 3.12 1.47 2.71 1.72 0.00 0.00 1.91

9.71 11.15 7.67 9.16 7.65 3.09 0.00 10.08

0.44 0.00 0.16 0.62 0.00 0,00 0.00 0.00

0.00 5.39 10.18 10.94 0.00 0.00 0.00 0,00

0.00 0.00 0.00 3.07 0.00 0.00 0~00 0.00

0.00 0.00 0.00 2.50 0.00 0.00 0,00 0,00

0.00 0.00 0.00 0.00 16.08 35.30 88.14 28.87

0.00 0.00 1.36 5.47 6.04 8.93 0.00 4.20

14.44 18.58 10.36 7.37 9.57 7.04 0.00 9.17

69.90 59.71 62.25 55.79 55.88 42.45 11.86 35,99

0.00 0.00 0.00 0,57 0,00 0,72 0,00 8.92

cent of fluxes yields a fast-fire, wall-tile glaze such as Glaze 3 [6]. To produce a glaze for dinnerware, the coefficient of t h e r m a l expansion m u s t be reduced to m a t c h that of the ware. Glaze 4 is a n example of a glaze for vitreous hotel c h i n a [7].

TABLE 2--Test methods for ceramic glazes [12]. Number

Title

C 1027

Test Method for Determining Visible Abrasion Resistance of Glazed Ceramic Tile Test Method for Resistance of Ceramic Tile to Chemical Substance Test Method for Measurement of Small Color Differences Between Ceramic Wall or Floor Tile Test Method for Crazing Resistance of Fired Glazed Ceramic Whitewares by a Thermal Shock Method Test Method for Crazing Resistance of Fired Glazed Whitewares by Autoclave Treatment Test Method for Resistance of Overglaze Decorations to Attack by Detergents Test Method for Linear Thermal Expansion of Porcelain Enamel and Glaze Frits and Ceramic Whiteware Materials by the Interferometric Method Test Method for Linear Thermal Expansion of Porcelain Enamel and Glaze Frits and Fired Ceramic Whiteware Products by the Dilatometer Method Test Method for 60-deg Specular Gloss of Glazed Ceramic Whitewares and Related Products Test Method for Lead and Cadmium Extracted from Glazed Ceramic Cookware Test Method for Lead and Cadmium Extracted from Glazed Ceramic Surfaces Test Method for Lead and Cadmium Extracted from Glazed Ceramic Tile

C 650 C 609

Lead-Containing Glazes Litharge (PbO) is used in glazes for several reasons [8], the most i m p o r t a n t of which is the strong fluxing action of PbO, which allows the f o r m u l a t i o n of glazes that m a t u r e at temperatures lower t h a n their leadless counterparts, leading to greater flexibility in the f o r m u l a t i o n of the glaze to o b t a i n desired properties, .Glazes for electronic substrates, artware, a n d some dinnerware, a n d tiles c o n t a i n lead oxide. However, PbO is highly toxic. Therefore, use of lead-containing glazes requires special care in processing a n d in testing the ware produced. Glaze 5 i n Table 1 is a n example of a lead-containing dinnerware glaze [9]. Glaze 6 is a n example of a clear glaze suitable for use o n artware a n d hobbyware bodies [10]. Glaze 7 is an example of a coating used on integrated circuit packages to seal t h e m [11].

C 554 C 424 C 556 C 539 C 372 C 584 C 1034 C 738 C 895

Satin and Matte Glazes Satin a n d matte effects are due to dispersed oxide crystals of appropriate refractive index in the glaze [5]. Calcium aluminosilicate a n d zinc silicate crystals are c o m m o n l y used. The crystals m u s t be very small a n d evenly dispersed if the glaze is to have a smooth, velvet appearance. Glaze 8 in Table 1 is a n example of a matte glaze.

Testing o f Glazes ASTM Committee C-21 on Ceramic Whitewares a n d Related Products has developed several test methods to evaluate the physical properties of ceramic glazes. These are listed in Table 2 [12]. These tests form the basis for most quality control testing programs. There are several m e t h o d s concerned with the fit of the glaze to the substrate. These include: C 5 5 4 - - T e s t Method for Crazing Resistance of Fired Glazed Ceramic Whitewares by a Thermal Shock Method; C 4 2 4 - - T e s t Method for Crazing Resistance of Fired Glazed Whitewares by Autoclave Treatment; C 5 3 9 - - T e s t Method for Linear T h e r m a l E x p a n s i o n of Porcelain E n a m e l a n d Glaze Frits a n d Ceramic Whiteware Materials by the Interferometric Method; and C 3 7 2 - - T e s t Method for Linear T h e r m a l Expansion of Porcelain E n a m e l

a n d Glaze Frits a n d Fired Ceramic Whiteware Products by the Dilatometer Method. Several other ASTM methods are concerned with chemical durability. These include: C 6 5 0 - - T e s t Method for Resistance of Ceramic Tile to Chemical Substances; a n d C 5 5 6 - Test Method for Resistance of Overglaze Decorations to Attack by Detergents. Of particular c o n c e r n are ASTM methods used to control release of lead and c a d m i u m from glazed surfaces, These include: C 1034--Test Method for Lead a n d C a d m i u m Extracted from Glazed Ceramic Cookware; C 7 3 8 - - T e s t Method for Lead a n d C a d m i u m Extracted from Glazed Ceramic Surfaces; a n d C 8 9 5 - - T e s t Method for Lead a n d C a d m i u m Extracted from Glazed Ceramic Tile; as well as C 1035--Specification for Lead and C a d m i u m Extracted from Glazed Ceramic Cookware.

PORCELAIN ENAMELS Porcelain e n a m e l coatings are ceramic coatings designed for application to metals. Conventional porcelain e n a m e l coatings are prepared in a n aqueous system a n d applied to the substrate by spray, dip, or flow coating. The coating is

70

P A I N T A N D COATING T E S T I N G M A N U A L

dried before firing. Newer technology involves dry application of powdered porcelain e n a m e l by electrostatic spray. The total market for porcelain-enameled products was reported to be $5486 billion i n 1989 [13]. About 86% of the products are appliances, such as ranges, water heaters, h o m e laundry, a n d dishwashers. About 6% are cast-iron sanitary ware, a n d 8% are architectural, cookware, a n d miscellaneous items. A porcelain e n a m e l m u s t be formulated such that it will b o n d to the metal substrate. For proper adherance of the e n a m e l to the metal, it is necessary to develop a c o n t i n u o u s electronic structure across the interface [14]. This structure is developed by saturating the e n a m e l coating a n d the substrate metal with a n oxide of the metal [15], which for iron a n d steel substrates is ferrous oxide (FeO). Certain t r a n s i t i o n metal oxides, such as cobalt oxide (COO), nickel oxide (NiO), a n d cupric oxide (CuO), can be added to a n e n a m e l f o r m u l a t i o n to improve the adherence between the metal a n d the substrate. G r o u n d coat enamels c o n t a i n adherance oxides, while cover coat enamels do not.

G r o u n d Coat E n a m e l s A general-purpose g r o u n d coat e n a m e l like E n a m e l 1 in Table 3 is a n alkali borosilicate c o n t a i n i n g small a m o u n t s of adherance oxides to promote the b o n d i n g process. E n a m e l 2 is a h o m e l a u n d r y enamel that has been formulated for outstanding alkali resistance t h r o u g h the addition of large q u a n tities of zirconia (ZrO2) [16]. Hot water t a n k coatings like E n a m e l 3 have very stringent thermal- a n d corrosion-resistance requirements. E n a m e l 4 is a c o n t i n u o u s clean coating. This is a porous coating which provides a m e a n s of volatilizing a n d removing food soils from the i n t e r n a l surfaces of ovens during n o r m a l operation [17].

Cover Coat E n a m e l s Cover coat porcelain enamels are formulated to provide specific color a n d appearance characteristics, a b r a s i o n resistance, surface hardness, a n d resistance to corrosion, heat, a n d t h e r m a l shock. They c a n be clear, semiopaque, or opaque. Opaque enamels such as E n a m e l 5 are used for white a n d pastel coatings [18]. They c o n t a i n high c o n c e n t r a t i o n s of titania (TiO2) to provide the opacification. S e m i o p a q u e enamels like E n a m e l 6 are used for most m e d i u m - s t r e n g t h colors. Clear enamels like E n a m e l 7 are used to produce strong, bright colors. They are similar to g r o u n d coat formulations without the adherance oxides.

Testing o f Porcelain E n a m e l s Test m e t h o d s for porcelain e n a m e l coatings are u n d e r the jurisdiction of ASTM Committee B-8 o n Metallic a n d Inorganic Coatings. The methods are listed in Table 4. Again, they form the basis for most quality control test programs. Several of these test methods are c o n c e r n e d with the chemical durability of porcelain enamels. They include: C 2 8 2 - Test Method for Acid Resistance of Porcelain E n a m e l s (Citric Acid Spot Test); C 6 1 4 - - T e s t Method for Alkali Resistance of Porcelain Enamels; C 7 5 6 - - T e s t Method for Cleanability of Surface Finishes; C 5 3 8 - - T e s t Method for Color Retention of Red, Orange, a n d Yellow Porcelain Enamels; C 8 7 2 - - T e s t Method for Lead a n d C a d m i u m Release from Porcelain E n a m e l Surfaces; a n d C 2 8 3 - - T e s t Method for Resistance of Porcelain E n a m e l e d Utensils to Boiling Acid. A related issue is the possibility of defects providing a pathway from the surface to the substrate, usually called c o n t i n u i t y of coating. Methods in this area include: C 5 3 6 - - T e s t Method for Continuity of Coatings in Glassed Steel E q u i p m e n t by Electrical Testing; C 7 4 3 - - T e s t Method for Continuity of Porcelain

TABLE 3--Typical porcelain enamels in weight percent. Oxide

Enamel1

Enamel2

Enamel3

Enamel4

Enamel5

Enamel6

Enamel7

Li20 Na20 K20 CaO MgO ZnO BaO CoO NiO CuO

0.88 13.15 2.30 6.18 0.00 0.00 7.27 0.47 0.29 0.20

0.81 12.60 1.56 2.80 0.18 0.26 0.73 0.36 0.31 0.00

1.33 13.92 0.00 2.04 0.00 1.27 0.56 0.47 0.00 0.00

0.52 7.30 1.47 0.65 0.00 0.00 0.00 0.03 0.03 13.99

0.89 9.41 6.13 0.00 0.00 0.00 0.00 0.00 0.00 0.00

1.10 8.58 9.15 0.00 0.00 1.04 0.00 0.00 0.00 0.00

1.76 12.23 3.83 0.00 0.00 0.00 0.00 0.00 0.00 0.00

B203 A]203 Cr20 s Sb203

15.37 6.35 0.00 0.00

15.99 11.50 0.00 0.00

7.60 2.02 0.00 0.00

1.18 41.38 1.24 0.30

16.13 2.25 0.00 0.00

16.53 1.34 0.00 0.00

7.11 2.72 0.00 0.00

SiO2 ZrO2 TiO2 MnO2 P205 Nb205 WOs MoOs

44.01 0.00 0.00 0.20 0.70 0.00 0.00 0.00

41.55 6.36 2.55 0.66 0.45 0.00 0.00 0.00

56.05 11.66 0.00 1.81 0.00 0.00 0.00 0.00

24.20 7.24 0.03 0.03 0.00 0.00 0.00 0.00

40.97 0.00 20.97 0.00 1.30 0.06 0.05 0.00

46.74 0.00 13.25 0.00 0.00 0.00 0.00 0.00

59.07 7.86 3.58 0.00 0.00 0.00 0.00 0.47

2.71

2.31

2.19

0.72

3.17

3.93

2.35

F

CHAPTER 9 - - C E R A M I C COATINGS E n a m e l Coatings; a n d C 5 3 7 - - T e s t M e t h o d for Reliability of Glass Coatings on Glassed Steel R e a c t i o n E q u i p m e n t by H i g h Voltage.

TABLE 5--Test methods for glass enamels

ASTM Method C 724

C 735

GLASS ENAMELS C 675 Glass e n a m e l s are vitreous coatings a p p l i e d on glass. They provide a m e a n s of decoration, not an i m p r o v e m e n t in c h e m ical d u r a b i l i t y or in cleanability. These coatings m u s t be mat u r e d at t e m p e r a t u r e s b e l o w the d e f o r m a t i o n p o i n t of glass (1000 to 1200~ o r 538 to 649~ Hence, they require large quantities of fluxing elements so t h a t c h e m i c a l d u r a b i l i t y is difficult to achieve. Glass e n a m e l s are p r o d u c e d in ready-to-use form (paste, t h e r m o p l a s t i c s , s p r a y m e d i u m s , ultraviolet curable med i u m s ) b y a few select m a n u f a c t u r e r s . They r e p r e s e n t a specialty p r o d u c t that is m o r e a k i n to organic p a i n t s t h a n to o t h e r c e r a m i c coatings. The m a r k e t s for this specialty product are c a t e g o r i z e d as tableware, glass containers, architectural, lighting, a n d automotive. As s u p p l i e d to the user, glass e n a m e l s are m e c h a n i c a l mixtures of pigments, fluxes, a n d organic s u s p e n d i n g media. The r e q u i r e m e n t for low maturing t e m p e r a t u r e s necessitates the use of very high lead oxide c o n t a i n i n g borosilicates for the flux. Leadless fluxes are u n d e r development, b u t have n o t yet achieved c o m m e r c i a l l y acceptable properties. The organic s u s p e n d i n g m e d i a are similar to m a t e r i a l s u s e d to m a k e organic paints. TABLE 4--Test methods for porcelain enamels

Number C 448 C 282 C 614 C 756 C 538 C 839 C 536 C 743 C 374 C 346 C 872 C 539 C 537 C 283 C 285 C 703 C 385

[19].

Title Test Methods for Abrasion Resistance of Porcelain Enamels Test Method for Acid Resistance of Porcelain Enamels (Citric Acid Spot Test) Test Method for Alkali Resistance of Porcelain Enamels Test Method for Cleanability of Surface Finishes Test Method for Color Retention of Red, Orange, and Yellow Porcelain Enamels Test Method for Compressive Stress of Porcelain Enamels by Loaded-Beam Method Test Method for Continuity of Coatings in Glassed Steel Equipment by Electrical Testing Test Method for Continuity of Porcelain Enamel Coatings Test Methods for Fusion Flow of Porcelain Enamel Frits (Flow-Button Methods) Test Method for 45-degree Specular Gloss of Ceramic Materials Test Method for Lead and Cadmium Release from Porcelain Enamel Surfaces Test Method for Linear Thermal Expansion of Porcelain Enamel and Glaze Frits and Ceramic Whiteware Materials by the Interferometric Method Test Method for Reliability of Glass Coatings on Glassed Steel Reaction Equipment by High Voltage Test Method for Resistance of Porcelain Enameled Utensils to Boiling Acid Test Method for Sieve Analysis of Wet-Milled and DryMilled Porcelain Enamel Test Methods for Spalling Resistance of PorcelainEnameled Aluminum Test Method for Thermal Shock Resistance of Porcelain-Enameled Utensils

C 676 C 824

C 927 C 978

C 777

71

[12].

Subject Test Methods for Acid Resistance of Ceramic Decorations on Architectural-Type Glass Test Method for Acid Resistance of Ceramic Decorations on Returnable Beer and Beverage Glass Containers Test Methods for Alkali Resistance of Ceramic Decorations on Returnable Beverage Glass Containers Test Method for Detergent Resistance of Ceramic Decorations on Glass Tableware Practice for Specimen Preparation for Determination of Linear Thermal Expansion of Vitreous Glass Enamels and Glass Color Frits by the Dilatometer Method Test Method for Lead and Cadmium Extracted from the Lip and Rim Area of Glass Tumblers Externally Decorated with Ceramic Glass Enamels Test Method for Photoelastic Determination of Residual Stress in a Transparent Glass Matrix Using a Polarizing Microscope and Optical Retardation Compensation Procedures Test Method for Sulfide Resistance of Ceramic Decorations on Glass

Testing of Glass Enamels Test m e t h o d s for glass e n a m e l s are u n d e r the j u r i s d i c t i o n of S u b c o m m i t t e e 14.10 on Glass D e c o r a t i o n of ASTM Committee C-14 on Glass a n d Glass Products. These m e t h o d s [12] are listed in Table 5. Most of these m e t h o d s are c o n c e r n e d with the c h e m i c a l d u r a b i l i t y of glass decorations. They include: C 7 2 4 - - T e s t Methods for Acid Resistance of Ceramic Decorations on Architectural-Type Glass; C 7 3 5 - - T e s t M e t h o d for Acid Resistance of Ceramic Decorations on R e t u r n a b l e Beer a n d Beverage Glass Containers; C 6 7 5 - - T e s t M e t h o d s for Alkali Resistance of Ceramic Decorations on R e t u r n a b l e Beverage Glass Containers; C 6 7 6 - - T e s t M e t h o d for Detergent Resistance of Ceramic Decorations on Glass Tableware; a n d C 9 2 7 - - T e s t M e t h o d for L e a d a n d C a d m i u m E x t r a c t e d f r o m the Lip a n d R i m Area of Glass T u m b l e r s Externally Decor a t e d with C e r a m i c Glass Enamels.

REFRACTORY COATINGS F l a m e s p r a y techniques can be used to a p p l y c e r a m i c coatings in the m o l t e n state to heat-sensitive o r massive substrates that c a n n o t themselves be h e a t e d to high temperatures. Most c e r a m i c coating m a t e r i a l s u s e d currently c a n be a p p l i e d b y flame spraying [20]. Silicates, silicides, carbides, oxides, a n d nitrides have all been d e p o s i t e d by this process. I n these processes, the coating m a t e r i a l is m e l t e d a n d p r o jected as h e a t e d particles onto the suhstrate, w h e r e it instant a n e o u s l y solidifies as a coating. Three m e t h o d s of h e a t i n g a n d propelling the particles in a plastic c o n d i t i o n to the s u b s t r a t e surface include: (1) c o m b u s t i o n flame spraying, (2) p l a s m a arc flame spraying, a n d (3) d e t o n a t i o n gun spraying. C o m b u s t i o n flame spraying is used for c o a t i n g m a t e r i a l s t h a t melt readily. P l a s m a arc flame spraying is used for very re-

72

PAINT AND COATING TESTING MANUAL

fractory materials such as metal carbides. Detonation gun spraying is used for hard, wear-resistant materials such as tungsten carbide. Flame spray coatings generally lack smoothness and are usually porous. They are, therefore, limited to applications such as thermal barrier coatings, where porosity is a virtue, and wear-resistant coatings, where the materials cannot be applied readily by any other technique.

Testing of Refractory Coatings There is only one test method for flame spray coatings in the ASTM standards: C 633--Test Method for Adhesion or Cohesive Strength of Flame-Sprayed Coatings [19].

COATING APPLICATION Ceramic coatings are applied to their substrates by one of several powder-processing techniques. In wet processes the raw materials are dispersed in a slip. Slip preparation involves mixing the ingredients, particle-size reduction, dispersion in water, and the addition of minor amounts of additives to modify the rheological properties of the slip [21-22]. These processes are carried out together in a ball mill comprising a rotating cylinder partly filled with freely moving, impactresistant shapes. The application process for a ceramic coating must be straightforward and foolproof, reproducible, economical, and flexible [21]. Selection of the application technique is one of the most important decisions the coatings engineer makes. Criteria for this selection are type of ware, shape and size of ware, throughput required, energy and labor costs, and space available. All of these factors affect the quality and the cost of a coating process, so that the best solution must be determined on an individual basis. Dipping is a simple, efficient, rapid technique requiring no capital equipment. The ware is immersed in the coating slip, moved around in a controlled way, removed from the slip, shaken to remove excess slip, and set down to drain and dry. Any bare spots are touched up with a finger wet with coating material. Its limitations are extreme sensitivity to operator skill and difficulty in automating volume production. Spraying is a process whereby a coating slip is broken down into a cloud of fine particles that are transferred to the substrate by either pneumatic, mechanical, or electrical forces. The method requires a gun, a container or feed mechanism, an impelling agency, and a properly designed hood or booth maintained under negative pressure [23]. Spraying lends itself to high-volume automated systems [24]. The articles are continuously fed under a battery of angled spray guns. Coating reclaim is an essential part of automated systems. Slip can also be applied mechanically with a rotating atomizer. Slip is passed onto a set of closely spaced rotating disks which throw the coating into a fan of droplets. Costs are similar to spraying. The primary use of this technique is in producing textured coatings on tile. If the substrate is conductive (that is, a metal), the surface quality and uniformity of a ceramic coating can be improved by using the electrostatic spray coating technique [25,26]. In

this system, the slip is broken into droplets either by air atomization or by centrifugal force from a sharp-edged rotating surface. The drops acquire a high negative charge and are dispersed as a fine mist. They are driven forward to the grounded substrate following the lines of force. Hence, coating material can reach the underside of the ware, and fulledge coverage is achieved. There are other techniques for specific applications. Tile require only one face to be glazed, but with a very smooth coating. This suggests the waterfall, or curtain technique [21], where a continuous feed of tiles is carded under a curtain of fluid slip. Painting and brushing are seldom used except for special effects and for applying glaze to inaccessible areas. For substrates which require precisely positioned areas of coating, the silk screen process can be used [20]. Finely powdered dry coating material is dispersed as a smooth paste. Using a squeegee, this paste is pressed through the open areas of a fine mesh screen stretched on a frame. For coating a total piece, costs are excessive. There are a few techniques of application that do not require the preparation of a slip. They include flame spraying, dry powder cast iron enameling, and electrostatic dry powder enameling. Flame spraying can be used to apply ceramic coatings in the molten state to heat-sensitive or massive substrates. Flame spray coatings generally lack smoothness and are usually porous. Equipment and material costs are generally high. In dry powder cast-iron enameling, a casting is heated in a furnace to red heat. It is then withdrawn from the furnace and, while still hot, dusted with dry powdered frit by means of a vibrating sieve placed over the surfaces to be coated. The powdered frit melts and adheres as it falls on the hot surface. This process is also extremely operator sensitive. The most important dry application method, and the one most recently introduced, is dry powder electrostatic application of all-fritted coatings to conductive substrates. This technique involves charging individual coating particles at a high voltage and then spraying them towards the substrate surface. Charging of particles is accomplished by encapsulating the coating material with an organic silane. It is then suspended in clean compressed air in a fluidized bed container [27]. The fluidized powder is siphoned and propelled through powder feed tubes to special electrostatic powder guns for low-pressure application. The powder carries a potential of up to 100 kV, which causes it to seek out and attach itself to the grounded workpiece. Capital costs of this process are substantial, but operating costs are reduced through elimination of slurry preparation and drying of the ware.

REFERENCES [1] Eppler, R. A., "Glazes and Enamels," Chap. 4, Glass Science and Technology, Vol. 1, Academic Press, New York, 1983, pp. 301-337. [2] Eppler, R. A., "Corrosion of Glazes and Enamels," Chap. 12, Corrosion of Glass, Ceramics, and Ceramic Superconductors, D. E. Clark and B. K. Zoitos, Eds., Noyes Publications, Park Ridge, NJ, 1992. [3] Ceramic Industries, August 1990, p. 36.

CHAPTER 9--CERAMIC [4] Tichane, R., "Ching-te-Chen; Views of a Porcelain City," N.Y. State Institute for Glaze Research, Painted Post, NY, 1983. [5] Singer, F. and German, W. L., "Ceramic Glazes," Borax Consolidated, 1964. [6] Orth, W. H., "Effect of Firing Rate on Physical Properties of Wall Tile," American Ceramic Society Bulletin, Vol. 46, No. 9, 1967, pp. 841-844. [7] O'Conor, E. F., Gill, L. D., and Eppler, R. A., "Recent Developments in Leadless Glazes," Ceramic Engineering Society Proceeding& Vol. 5, Nos. 11-17, 1984, pp. 923-932. [8] Eppler, R. A., "Formulation and Processing of Ceramic Glazes for Low Lead Release," Chap. 10, Proceedings, International Conference of Ceramic Foodware Safety, J. F. Smith and M. H. McLaren, Eds., Lead Industries Association, New York, 1976, pp. 74-96. [9] Marquis, J. E., "Lead in Glazes--Benefits and Safety Precautions,"American Ceramic Society Bulletin, Vol. 50, No. 11, 1971, pp. 921-923. [10] Eppler, R. A., "Formulation of Glazes for Low Pb Release," American Ceramic Society Bulletin, Vol. 54, No. 5, 1975, pp. 496-499. [11] Tummala, R. R. and Shaw, R. R., "Glasses in Microelectronics in the Information-ProcessingIndustry," "Commercial Glasses," Advances in Ceramics, Vol. 18, American Ceramic Society, Columbus, OH, 1986, pp. 87-102. [12] ASTM Annual Book of Standards, Part 15.02: Glass, Ceramic Whitewares. [13] Ceramic Industries, August 1990, p. 49. [14] Pask, J. A., "Chemical Reaction and Adherance at Glass-Metal Interfaces," Proceedings, PEI Technical Forum, Vol. 22, 1971, pp. 1-16. [15] King, B. W., Tripp, H. P., and Duckworth, W. H., "Nature of Adherance of Porcelain Enamels to Metals," Journal of the American Ceramic Society, Vol. 42, No. 1t, 1959, pp. 504-525. [16] Eppler, R. A., Hyde, R. L., and Smalley, H. F., "Resistance of Porcelain Enamels to Attack by Aqueous Media: I--Tests for

COATINGS

73

Enamel Resistance and Experimental Results Obtained," American Ceramic Society Bulletin, Vol. 56, No. 12, 1977, pp. 10641067. [17] Monteith, P. G., Linhart, O. C., and Slaga, J. S., "Performance Tests for Properties of Low Temperature Thermal Cleaning Oven Coatings," Proceedings, PEI Technical Forum, Vol. 32, 1970, pp. 73-79. [18] Shannon, R. D. and Friedberg, A. L., "Titania-Opacified Porcelain Enamels," Illinois University Engineering Experimental Station Bulletin, Vol. 456, 1960. [19] ASTM Annual Book of Standards, Part 2.05: Metallic and Inorganic Coatings. [20] Taylor, T.A., Bergeron, C.G., and Eppler, R.A., "Ceramic Coating," Metals Handbook, 9th ed., Vol. V, ASM International, Metals Park, OH, 1982, pp. 532-547. [21] Taylor, J. R. and Bull, A. C., Ceramics Glaze Technology, Pergamon Press, Oxford, England, 1986. [22] Reed, J. S., Introduction to the Principles of Ceramic Processing, John Wiley & Sons, New York, 1988. [23] Bloor, W. A. and Eardley, R. E., "Environmental Conditions in Sanitary Whiteware Shops, II. Glaze Spraying Shops," Transactions, Journal of the British Ceramic Society, Vol. 77, No. 2, 1978, pp. 65-69. [24] Whitmore, M., "Spraying of Earthenware Flatware," Transactions, Journal of the British Ceramic Society, Vol. 73, No. 4, 1974, pp. 125-129. [25] Hebberlein, K., "Electrostatic Glazing of Tableware," Berichte der Deutschen Keramischen Gesellschaft, Vol, 53, No. 2, 1976, pp. 51-55. [26] Lambert, M., "Industrial Application of Electrostatic Enamelling to Parts in Sheet Steel and Cooking Equipment," Vitreous Enameller, Vol. 24, No. 4, 1973, pp. 107-109. [27J ASM Committee on Porcelain Enameling, "Porcelain Enameling," Metals Handbook, 9th ed., Vol. 5, ASM International, Metals Park, OH, 1982.

MNL17-EB/Jun. 1995

Epoxy Resins in Coatings

10

by Ronald S. Bauer, 1 E d w a r d J. Marx, 2 and Michael J. Watkins 2

tain one or more epoxy (oxirane) groups per molecule. The epoxy resins most widely used by far in coatings are the bisphenol A based epoxy resins, the generalized structure of which is given in Fig. 1. In commercial products, the n value ranges from 0 to about 25, although higher-molecular-weight thermoplastic resins having n values of 200 or more are available. As n increases, the epoxy equivalent weight (EEW) increases, as does the number of hydroxyl groups. Thus epoxy resins with low n values are normally cured by reaction of the epoxy group, whereas those resins with higher n values are cured by reaction of the hydroxyl functionality. Resins having n values less than 1 are viscous liquids; they are used mainly in ambient-temperature cure coatings, electrical castings, flooring, electrical laminates, and fiber-reinforced composites. These applications require liquid resins having good flow and are cured through the epoxy ring. The higher n value resins, particularly those above 3000 molecular weight, are normally used in solution and find their greatest application in heat-cured coatings. In these resins the concentration of epoxy groups is low, and so they are cured with materials that react with the hydroxyl groups along the backbone. Table 1 displays a range of typical epoxy resins that are commercially available along with their properties and applications.

EPoxy RESINSHAVEBECOMEtechnologically important materials that find extensive application in high-performance coatings, adhesives, and reinforced plastics. Almost since their commercial introduction in 1950, epoxy resin systems have been used in protective coatings. Historically, protective coatings were the largest single end use for epoxy resins. Although in recent years the noncoating applications of epoxy resin have been growing, coatings still represent about half of the annual epoxy resin usage. Epoxy resin coatings offer a unique combination of adhesion, chemical resistance, and physical properties that provide outstanding protection against severe corrosive environments. They are used extensively in coatings for refineries, chemical plants, and marine equipment such as offshore drilling platforms and merchant ships. Other important applications where epoxy resin coatings are used almost exclusively because of the corrosion protection they afford include automotive, aircraft, and appliance primers as well as to protect both the inside and outside of pipe. Epoxy resins are the predominant thermosetting resin used for the interior linings of beer and beverage cans, cans for hard-to-contain food products such as sauerkraut, tomato juice, and meat products, and for chemical-resistant linings of pails and drums. These coatings are used not only to protect the metal of the container from corrosion, but also to protect the flavor of the contents, which can be affected by direct contact with metal. The principal components of any epoxy resin coating system are the epoxy resin and the curing agent or hardener. Epoxy resins are reactive intermediates that can be liquid or solid, and they are converted into the final coating by reaction with curing agents (hardeners). Curing agents function by reacting with specific groups in the epoxy resin molecule to give a three-dimensional, infusible polymer network. Although the resin and curing agent are common to all epoxy coatings, other materials are incorporated to achieve the desired rheological characteristics, cure speed, and film performance.

CURING AGENTS Epoxy resins are reactive intermediates composed of mixtures of oligomeric materials containing one or more epoxy groups per molecule. To convert epoxy resins into useful products, they must be cross-linked or "cured" into a threedimensional polymer network. Cross-linking agents, or curing agents as they are generally called, function by reaction with or cause the reaction of epoxide or hydroxyl groups in the epoxy resin. The number of curing agents that have been developed over the years for epoxy resins is overwhelming. Selection of the curing agent is as important as that of the base resin; it is dependent on the performance requirements of the film and the constraints dictated by the specific method of application. The most important types of curing agents used in epoxy resin coatings are the amine-functional materials for ambient-cure coatings and the amino- and phenoplast resins for heat-cured systems. The principal amine-functional curing agents used in ambient-cure coatings are aliphatic amine adducts of epoxy resins, polyamides, and ketimines. Aminefunctional materials cure epoxy resins by polyaddition

RESIN TYPES Generically, epoxy resins can be characterized as a group of commercially available oligomeric materials which con1Research adviser and 2senior research chemist, Shell Development Co., Westhollow Research Center, P.O. Box 1380, Houston, TX 77521-1380.

74 Copyright9 1995 by ASTMInternational

www.astm.org

CHAPTER IO--EPOXY RESINS IN COATINGS

o

F

75

--I

CH2- CHCH2 - ' ~ O ~

CH2CHCH2Jn --

O- CH2CH-CH 2 CH3

n is typically 0 to 1 for liquid resins with solid resins having n values as high as 15 or more FIG. I-Idealized structure of a bisphenol epoxy resin.

TABLE 1--Typical properties of bisphenol-based epoxy resins. Average Molecular Wt

Average EEW~

Approximate Average Value of n

Viscosity, P, 25~

Softenin~ Point, ~ v

350

182

...

80

..-

380 600 900 1400 2900

188 310 475 900 1850

0.2 1 2 4 10

140 Semisolid Solid Solid Solid

... 40 70 100 130

3750

3050

13

Solid

150

Applications Solventless and solvent-borne ambient cure coatings, electrical encapsulation, flooring, and filament winding Solvent-borne, ambient cure coatings Powder coatings Heat cure, solvent-borne coatings for cans, drums, primers, etc.

aEEW = epoxideequivalent weight, i.e., grams of resins needed to provide 1 M equivalent of epoxide. Alsoreferred to as WPE (weight per epoxide) and EMM (epoxy molar mass). All three terms are interchangeable. bSoftening point by Durran's mercury method [ASTMSpecification for Epoxy Resins (D 1763)].

t h r o u g h reaction of the amine with the epoxy groups. Aminoand phenoplast cross-linking resins are generally etherified urea-formaldehyde, melamine-formaldehyde, and phenolformaldehyde condensates. The amino- and phenoplast resins cure by condensation with the backbone hydroxyls of the epoxy resin with elimination of water or an alcohol. Aliphatic amines such as ethylene diamine (EDA), diethylene triamine (DETA), and triethylene tetramine (TETA) have always been popular curing agents for epoxy resins because of their ability to readily cure at r o o m temperature. However, aliphatic amines present certain handling hazards because of their high basicity and relatively high vapor pressure. Epoxy/ amine adducts, amine-terminated polyamides, ketimines, and other modified polyamines are less hazardous derivatives of aliphatic amines and often provide performance and handling advantages.

EPOXY RESIN COATINGS There are a n u m b e r of possible ways to classify epoxy resin coatings. Since curing agent types have been divided into ambient-cure and heat-cure, for convenience the types of epoxy coatings can also be classified similarly. The bulk of the ambient-cure or "air-dry" coatings are cured with polyamines or modified polyamines and generally find application as maintenance and marine or architectural coatings. Ambientcure coatings are based on low-molecular-weight epoxy resins having high epoxy group content and are generally cured through reaction of the epoxy group. In addition to the "two-

package" type ambient cure epoxy coatings, air dry epoxy esters, prepared by esterifying epoxy resins with unsaturated fatty acids, dry by the same m e c h a n i s m as alkyd resins. Historically ambient-cure coatings have been applied out of solvent, typically at about 40% weight solids. However, with the regulation of emissions of volatile organics, the trend has been toward coatings with lower volatile organic content, 100% solids, and waterborne systems. Heat-cure coatings are used in industrial finishes, automotive primers, appliance primers, pipe coatings, and coatings for beer and beverage cans, as well as cured coatings for pails and drums. Conventional solvent-borne and waterborne heat-cured coatings are based on the higher-molecularweight epoxy resins, and they are generally cured t h r o u g h reaction of the hydroxyl groups. Powder coatings, however, are generally manufactured from intermediate molecular weight solid resins and are cured t h r o u g h the epoxy group. The usual curing agents for heat-cured epoxy resin coatings are amino- and phenoplast resins, as well as dicyandiamide and polycarboxlic acids, which are used in powder coatings.

TWO-PACKAGE, AMBIENT-CURE COATINGS By far the largest volume of ambient-cure epoxy resin coatings are the "two-package" type, which are typically manufactured from liquid or low-molecular-weight solid epoxy resins cured with a polyamine, amine adduct, or polyamide. A twopackage coating, as the n a m e implies, is a two-part system:

76

PAINT AND COATING TESTING MANUAL

the epoxy component and the curing agent, which are packaged separately and often in volume ratios of 2 to 1 or 4 to 1 of epoxy component to curing agent. Two-package epoxy coatings are mixed just prior to application and are characterized by a limited working life or pot life after the resin and curing agent components are mixed. Commercial systems will have pot lives of a few hours to a couple of days, with typical working times of about 8 to 12 h. Two-package epoxy resin coatings include a broad range of products and properties. Specific film properties depend on selection among many epoxy resins, curing agents, pigments, and modifiers, as well as the ratios of these materials. Resins differ primarily in molecular weight. As the molecular weight of the resin increases, the flexibility, flexibility retention, filmleveling properties, and pot life of the coating increase. However, cross-link density decreases with increasing molecular weight, resulting in reduced solvent and chemical resistance as well as nonvolatile content of the paint at application viscosity. Although aliphatic polyamines are less expensive and have been used extensively as curing agents, polyamine adducts, amine-terminated polyamides, and ketimines are generally preferred. Among the advantages provided by aliphatic amine adducts are: 9 Milder odor. 9 Lower volatility. 9 Less tendency to produce blush when coatings are applied under conditions of high humidity. 9 Greater suitability for application at low temperature. 9 Less tendency to corrode metal containers. Two general types of polyamine adducts are available, i.e., those based on a low-molecular-weight liquid epoxy resin and those based on a high-molecular-weight solid epoxy resin. The performance properties of amine-cured coatings are not significantly different from those of aliphatic polyamine cured systems. Like polyamines, amine adducts offer maxim u m resistance to solvents, acids, and other highly corrosive chemicals. Initial flexibility and impact resistance are excellent, and retention of these properties is adequate for most uses over rigid or semi-rigid substrates. More pounds of polyamide curing agents are consumed annually in the United States than any other type of epoxy resin curing agent. Polyamides are obtained from the condensation of dimerized and/or trimerized fatty acids with aliphatic polyamines such as diethylene triamine and triethylene tetramine to give an amine-terminated polyamide. Polyamide cured epoxy coatings develop superior adhesion to moist and poorly prepared surfaces, and they provide a high degree of corrosion resistance. Like epoxy resins, polyamides are also mixtures of oligomers. Thus, a range of polyamides which vary in viscosity, amine equivalent weight, and reactivity is available. Polyamide cured coatings exhibit somewhat better retention of flexibility and impact resistance on aging than polyamine adducts. Although resistance to solvents and acids is not quite as good as with other types of amine curing agents, polyamides are adequate for most applications where amine cure epoxy coatings are used. Ketones add reversibly to primary amines with the loss of water to give ketimines. The ketimines obtained from the typical polyamine curing agents have rather low volatility compared to the precursor polyamine. Ketimine curing

agents can be considered blocked polyamines, which in the presence of water hydrolyze to produce a ketone and a polyamine. These ketimines react at a practical rate of cure under ambient conditions. Atmospheric moisture, which is absorbed during and following application of the coating, serves as the source of water required to activate the curing agent. Ketimine curing agents are similar in behavior to the aliphatic amine polyamines and amine adducts in rate of cure and performance of cured films, but they provide much longer pot lives. Typical applications of two-package coatings of the above type are in heavy-duty maintenance and marine coatings, tank linings, aircraft primers, internal pipe coatings, for gas transmission lines, coatings for plastic products, and highperformance architectural coatings.

EPOXY RESIN ESTER, AMBIENT CURE COATINGS Epoxy resin esters are prepared by esterifying the resin with vegetable oil fatty acids. Epoxy esters are usually prepared from solid epoxy resins having EEWs in the range of 900. As in conventional alkyd technology, these coatings are made by esterifying the resin with fatty acid at temperatures of 400 to 450~ (204 to 232~ Initially, the fatty acid reacts with the epoxy ring at lower temperatures, forming hydroxyl esters. Subsequently, these hydroxyl groups and those already present in the resin are esterified at higher temperatures with the aid of esterification catalysts and with azeotropic removal of water. Typically, between 30 and 90% esterification is chosen, depending on the oil length desired. Like alkyd resins, epoxy resin esters may be made in long, medium, and short oil lengths. The oil length refers to the degree of esterification of the epoxy resin with the fatty acid: long indicates 70 to 90% esterification, medium indicates 50 to 70% esterification, and short indicates 30 to 50% esterification. By proper selection of acids and adjustment of reaction ratios, long, medium, or short oil esters may be prepared with drying, semidrying, or nondrying characteristics. The terms "drying" or "air dry"are used instead of ambient-cure since cross-linking of epoxy resin esters does not involve a curing agent. Air-dry epoxy resin ester coatings are "one-package" or one-component systems, since they cross-link or cure only on exposure to air. Air-dry epoxy ester coatings are used in maintenance and marine coatings, especially where mildly corrosive conditions are encountered. They do not, however, possess the outstanding chemical resistance of amine-cured epoxy coatings, but they are superior to alkyd paints. In addition, their toughness and durability make them well suited for longwearing floor finishes.

HEAT-CURED SOLVENT-BORNE EPOXY R E S I N COATINGS Conventional solvent-borne heat-cured or "baked" epoxy resin coatings are based on high-molecular-weight epoxy resins, that is, resins with EEWs around 1750 or greater. The concentration of epoxy groups is low in these resins, and

CHAPTER I O - - E P O X Y R E S I N S IN COATINGS cross-linking occurs principally through the hydroxyl functionality. Thermosetting resins such as urea-formaldehyde, melamine-formaldehyde, and phenol-formaldehyde resins are used as cross-linkers for the coatings of this type. This cross-linking requires heat, and usually a strong acid catalyst is used to accelerate the cure. Thus, these systems are supplied as "one-component" systems, i.e., the resin, curing agent, and accelerator are packaged together. Aminoplast (urea-formaldehyde or melamine-formaldehyde) cross-linking resins are used because of their good color and relatively low cure temperature. They are typically used in linings for beer and beverage containers and as clear coatings for brass and jewelry. Pigmented aminoplast cured coatings are used as coatings for industrial equipment, appliances, and hospital and laboratory furniture. Phenoplast (phenol-formaldehyde) cured coatings are more chemically resistant, and they find application in beer and beverage containers (particularly in Europe), drum and pail linings, internal coatings for pipe, wire coatings, and appliance primers. Phenoplast resins, generally giving coatings of poorer color than arninoplast resins, are used only when maxi m u m resistance to solvents and other chemicals is required.

HEAT-CURED WATERBORNE EPOXY COATINGS The earliest epoxy resin coatings for beer and beverage containers were solvent-borne amino- or phenoplast cured systems, the particular systems used being dependent on the application. Although application technology has changed over the years and is now dominated by waterborne systems, the coatings are still basically amino- or phenoplast cured systems. These coating systems are based on high-molecularweight epoxy resins onto which are grafted terpolymers of, for example, styrene/methacrylic acid/ethyl acrylate. These epoxy/acrylic graft polymers are neutralized with base, such as dimethylethanolamine, to give a resin easily dispersible in water. The dispersed resin can then be cured with an aminoplast resin to give coatings with properties that make them suitable for beer and beverage containers.

ELECTRODEPOSITION COATINGS Epoxy resin electrodeposition coatings are also waterborne coatings formulated from either anionic or cationic epoxy resin polymers. The part to be coated is dipped into the electrodeposition bath, and an appropriate electrical charge is applied, causing the coating to deposit onto the part. The part is then removed from the bath, rinsed, and baked to cure the coating. In the United States, epoxy-based electrodeposition coatings account for over 92% of all electrodeposition coatings. Epoxy-based cathodic electrodeposition (CED) automotive primers dominate this application, accounting for over 82% of all electrodeposition coatings. Over 40 million pounds of epoxy resin are used in the United States in CED automotive primers, making this one of the largest single end uses for epoxy resins in coatings. Virtually every automobile made in the United States, Europe, and Japan is primed with a CED primer. CED primers are used because they afford

77

exceptional corrosion protection and because they are deposited uniformly to all areas of the automobile, even in areas which would be inaccessible to other coating application methods such as spray. Because of their major importance, the remainder of this discussion will deal with CED automotive primers. The preparation of CED coatings generally begins by reacting a bisphenol A based liquid epoxy with bisphenol A to give an epoxy resin with an epoxy equivalent weight in the range of 500 to 1000. This epoxy resin is then reacted with a flexibilizing diol. This diol can be an aliphatic diol or a polyether diol. The principal requirement is that the diol contain primary hydroxyl functionality. These primary hydroxyls are reacted with the epoxy groups in the presence of a suitable catalyst (e.g., a tertiary amine) to form ether linkages between the epoxy and the flexibilizing diol. At this point, the resin will have an epoxy equivalent weight in the range of 1000 to 1500. The remaining epoxy functionality is then reacted with amines. Generally, secondary amines are chosen to minimize further chain extension. One favored method to accomplish this is to use a diketimine of diethylene triamine. During coating preparation, the ketimine groups decompose to give primary amines. These primary amines are fairly basic, resulting in stable dispersions at a relatively high bath pH (pH > 6). At this point, the CED resin preparation is complete. In practice, specialized CED resins are used to make the pigment grind pastes. These are developed to efficiently make stable pigment dispersions, which retain good stability in the CED coating bath. Curing agents used are generally blocked isocyanates. These are chosen to be stable and unreactive in the coatings bath, but to unblock and cure the coating at baking temperature. An example of such a curing agent would be the reaction product of 3 mol of toluene diisocyanate with 1 tool of trimethylolpropane. This is then reacted with 3 mol of a suitable blocking agent, such as 2-ethyl-1-hexanol. Catalysts such as tin or lead salts are generally used to facilitate unblocking and coating cure. The coating is prepared by blending the resin with pigment paste, curing agent, catalysts, additives, and solvents. A low-molecular-weight organic acid, such as lactic or acetic acid, is then added to the mixture to make a m m o n i u m salts with the amine groups in the resin. This mixture is then dispersed in water to make the CED coating. Solvents may be required in the preparation of the CED resin or other components. In order to reduce the volatile organic compound content of the finished coating, it is usually subjected to a vacuum stripping step which can reduce VOC to less than 0.7 lb/gal. When the automobile is dipped into the CED bath, a negative charge is applied to it (making it the cathode) relative to counter electrodes in the bath. Electrolysis of water occurs, forming hydroxide ions in the immediate vicinity of the automobile surface. These hydroxide ions react with the a m m o n i u m ion groups in the resin near the surface, regenerating the neutral amine groups and causing the coating to be deposited onto the surface. In this way, a uniform film is applied to the entire conductive surface of the automobile. The automobile is then removed from the bath, rinsed, and baked.

78

PAINT AND COATING TESTING MANUAL

E P O X Y R E S I N P O W D E R COATINGS Powder coatings are produced by melt blending homogenous dispersions of nonvolatile solid resins, curing agents, pigments, fillers, and various additives. The dispersion is solidified by cooling, ground into a finely divided powder form, and classified for subsequent use. The resultant powder is normally electrostatically deposited onto grounded substrates and, through the application of heat, converted into very high performance thermoset films. The process of applying coating powders allows nearly 100% powder utilization and evolves almost no volatile organic compounds. The 1970's volatiles regulations and energy concerns raised interest in powder coating technology. The real sustaining driving forces for growth, however, have been improvements in powder coating raw materials, formulations, manufacturing technology, and application equipment. The advantages for the use of powder coatings can best be summed up in the "Four E's," used by The Powder Coating Institute: (1) excellence of finish, (2) economy in use, (3) energy efficiency, and (4) environmental acceptability. The Clean Air Act, as amended in 1990, has contributed to even greater interest in the use of powder coatings to meet more stringent volatile organic requirements. Powder coatings is the fastest growing area of coatings technology. Growth rate for powder coatings in the 1990 to 1995 time frame is projected to be at 10 to 12% versus a conventional "wet" coatings rate of about 2%. The unique characteristics of solid epoxy resins account for their choice by formulators for use in powder coatings applications. Bisphenol-A based epoxides with equivalent weights

greater than about 650 are nonsintering and extremely friable. They have relatively low melt viscosity and high reactivity via the terminal oxirane functionality. The addition reaction with amines, phenolics, or carboxylic acid functional curatives allows a wide range of formulations. The primary limitations for bisphenol-A based epoxy resins in powder coatings are yellowing and loss of gloss that occur when these coatings are exposed to exterior weathering conditions. Powder coatings are broadly divided into either "functional" or "decorative" uses. Functional coatings are normally applied at film thicknesses greater than about 3 mi] and are expected to withstand some rather severe service. Examples of functional uses are coatings for exterior and interior pipe, rebar, and various electrical devices. Although decorative powder coatings are functional, these are normally used at a film thickness of 3 mil or less and are not expected to perform significantly better than baked films derived from "wet" coatings. Some examples of decorative uses are coatings for appliances, furniture, and underhood automotive parts.

REFERENCES [1] Lee, H. and Neville, K., Handbook of Epoxy Resins, McGraw-Hill Book Co., New York, 1967. [2] May, C. and Tanaka, Y., Epoxy Resins Chemistry and Technology, Marcel Dekker, Inc., New York, 1973. [3] Bauer, R. S., Epoxy Resin Chemistry, ACS Symposium Series 114, American Chemical Society, Washington, DC, 1979.

MNL17-EB/Jun. 1995 ii

11

Phenolics by John S. Fry 1

CHEMISTRY

DEEINmON: a polymeric, resinous reaction product of a phenol with an aldehyde. Said products may be used alone or in formulations with other polymers to produce useful coatings.

The reaction of phenol with aldehydes to produce resinous products was difficult to understand in the early years because many of said products were insoluble or infusible. After studying the variables, Baekeland at least defined the conditions to produce three stages of products: "A" stage--soluble and fusible; "B" stage--insoluble, but swellable and softenable; and "C" stage--cured to an insoluble and heatresistant material. With the advent of modern analytical tools, the chemistry of the reactions has been more fully defined by various workers [2,3]. A brief description follows.

HISTORY Phenolic resins, initially commercialized in 1909, were the first completely synthetic materials for the burgeoning plastics business. The expansion of several new technologies of the time, namely the electrical, communications, and automotive industries, all required and depended on new materials that had better electrical insulating properties, higher heat resistance, and improved resistance to chemicals, acids, oils, and moisture. The heat-reactive resins, developed by Dr. Leo H. Baekeland [1], were formulated into blends that were convenient for mass production compression moldings and satisfied the above-mentioned requirements. Improved and new items such as coil supports, commutators, distributor heads, telephone sets, vacuum tube bases, radio parts, and electrical switches all blossomed onto the market within a few years.

RAW M A T E R I A L S The commercially important phenols used in coatings resins are shown in Fig. 1. While phenol is the most common, the substituted phenols are also used to vary the solubility and reactivity properties of resins. The cresols, butyl phenol and bisphenol-A, are widely employed while the others have limited or specialty uses. Phenol has three aldehyde reactive ring positions; the 2 and 6 carbon atoms (ortho) and the 4 position (para). Phenols with substituents in the above positions have lower functionalities and are frequently used to modify resin properties. The aldehyde co-reactant of choice for the phenols is formaldehyde, the most reactive of those commercially available. Formaldehyde, a gas, is conveniently handled as an aqueous solution (formalin) or as a solid polymeric form known as paraform. Formaldehyde in aqueous solutions exists as hydrated glycols or low-molecular-weight glycol ethers.

FIRST PHENOLIC COATINGS Concurrent with the above developments, the nonheat-reactive phenolics or "novolak" resins were prepared as a hoped-for substitute for shellac. These resins were not as resilient as shellac and, when used alone, were not successful in coatings. However, combined with the formaldehyde donor, hexamethylene tetramine, the novolaks could be compounded into another set of thermosetting molding materials which found early use in phonograph records. While the above-mentioned novolaks had to wait for success in coatings, the alcohol solutions of the heat-reactive resins were found, by 1911, to form excellent films when baked and cross-linked. These coatings, still in wide use today, are hard and glass-like and have excellent resistance to chemicals, acids, water, and solvents. Early applications included protective coatings for brass beds as well as other hardware items. These solution resins also initiated the manufacture of laminates, which engendered radio circuit boards and, later, printed circuit boards.

CH20 + H20

Both of the above forms of formaldehyde depolymerize on heating to supply reactive formaldehyde for the phenolic reactions. The type of reaction products and resins formed depends on the catalysts and conditions used.

B A S E CATALYST Strong base catalysts (pH above 8) produce initial reaction products such as the methylol phenols shown in Fig. 2a. Phenol can produce five different methylol-related species, while the substituted phenols, with lower functionality, produce fewer methylol derivatives. Further reaction causes the methylol groups to condense with other ring positions (meth-

~Consuhant, 14 Westbrook Ave., S. Somerville, NJ 08876. 79 Copyright9 1995 by ASTM International

) HOCH2OH- HOCH20[CH20]H

www.astm.org

80

PAINT AND COATING TESTING MANUAL

~ ~H3~CH3~ OH

OH

Phenol

Orthocresol

OH

OH

Metacresol

CH3 Paracresol

OH

OH

OH

() 0

0

OH

CH3-C-CH3 I cn3

CH3-C-CH3 I CH 2 I CH 3

OH

CH3

CH3 Xylenols

OH

CH3-C-CH3 I CH2 I CH3=C-CH 3 I

CH3

OH 2,2 Bis (4Hydroxylphenyl) Propane (Bisphenol-A)

p-Phenyl Phenol

p-tert-Butyl Phenol

p-tert Amylphenol

p-tert-Octyl Phenol

FIG. 1-Phenols used to make phenolic resins.

ylene link) or to etherify with other alcohol groups (methylene ether links) (Fig. 2b). Additional reaction raises the molecular weight to branched, heat-reactive resin products which are dehydrated, dissolved in solvents, or isolated as grindable solids for later formulation.

ACID CATALYST At a pH of 0.5 to 1.5, the acid-catalyzed phenol-formaldehyde reaction proceeds through an unstable addition intermediate to condensed, methylene-linked phenolic rings (Fig. 3). When phenol is used, highly branched "novolaks" are obtained. However, when substituted phenols are used, the functionality of the system is reduced to two and mostly linear resins are formed.

INTERMEDIATE

pH

CATALYSIS

When salts of zinc, magnesium, or aluminum are used as catalysts, the pH of the phenol-formaldehyde reaction falls in

the 4 to 7 range. Under such conditions, the formaldehyde addition to the phenolic ring is highly directed to ortho substitution. With excess formaldehyde, hemi-formals and ortho methylol groups are formed. Using lower formaldehyde levels leads to the formation of nonheat-reactive ortho-ortho novolaks. With the highly reactive para ring position still open, these resins have been employed in relatively rapid crosslinking formulations.

TESTING

OF PHENOLIC

RESIN

PRODUCTS

Typical quality control tests for phenolic resin products may include the following. 1. Gel time [ASTM Test Method for Determining Stroke Cure Time of Thermosetting Phenol-Formaldehyde Resins (D 4640-86)] (heat-reactive resins). 2. Volatile content [ASTM Test Method for Volatile Content in Phenolic Resins (D 4639-86)]. 3. Viscosity (solution).

CHAPTER ll--PHENOLICS

OH

OH

OH

+ CH20

~ ~

Base

r

C ' H2OH +

CH2OH OH

<

+

OH

~

H2OH

HOCH2~

H2OH

,'H2OH OH HOCH2~

CH2OH

CH2OH FIG. 2a-Base-catalyzed initial reaction products.

OH

~

OH H2OH

Base y ~

OH

~

CH2

CH2OH+H20

METHYLENELINK Or

OH

0

OH

0

METHYLENE ETHER FIG. 2b-Methylol phenol reaction products.

+ H20

81

82

P A I N T A N D COATING T E S T I N G M A N U A L OH

CH20

H+

~- HO Acid

~

CH 2 ~

O

H

Q CH2

FIG. 3-Acid-catalyzed reaction products. 4. pH [ASTM Test Method for Measuring Apparent pH or Water Insoluble Phenol-Formaldehyde Resins (D 461386)]. 5. Color (Gardner). 6. Specific gravity. Other tests to characterize resins may be used: I. Molecular weight distribution and free phenols--gel permeation chromatography. 2. Structure analysis--NMR, I.R. 3. Thermal analysis (curing curves). 4. Free formaldehyde-hydroxyl amine method.

H E A T REACTIVE R E S I N S IN COATINGS Alcohol-Soluble Resins The thermosetting, alcohol-soluble resins are based on muhifunctional monomers such as phenol, cresols, xylenols, and bisphenol-A. These resins are also soluble in ketones, esters, and glycol ethers, but insoluble in aromatic and aliphatic hydrocarbons. They are compatible with amino resins, epoxies, polyamides, and poly(vinyl butyral) and are, in many cases, formulated with said resins as crosslinking agents. The resins are commercially available as solvent solutions, solids, or waterborne systems. Low-molecular-weight resins with a high methylol content form water solutions, while higher molecular weight resins may be used in dispersion form [4,5]. Once formulated into vehicles, the resins may be applied by spray, dip, brush, or roller coating methods. The films are then baked at typical temperatures of 135 to 250~ for times varying from several minutes to several hours. When used alone, phenolic resins crosslink with the release of water to form 0.2 to 1.0-mil films. Since volatiles are released during cure, bubbles may develop in thicker films. Multiple coats may be applied as long as the intermediate coats are given a short bake. Baked phenolic coatings are hard and glass-like with excellent resistance to organic solvents, boiling water, acids, and acidic or neutral inorganic salts. They are not resistant to alkalies unless combined with epoxides as will be discussed later. These phenolic coatings have excellent electrical resistance and also resist dry heat to 370~ for short periods. As a

result, these products find applications in oil industry drill pipe, production pipe, as drum and pail linings, tank linings, printing plate backing, printed circuit masks, and as corrosion-resistant hardware coatings. Tests performed on such coatings may include: 1. Electrical properties. 2. Physical and mechanical. 3. Environmental resistance. 4. Applications.

P h e n o l i c s as Crosslinking Agents for Other Polymers Coatings based on heat-reactive phenolics alone are not very flexible. To obtain better toughness and flexibility for applications such as can coatings or coil coatings, the phenolic resins are combined with a linear resin, such as an epoxy resin, in ratios of about 15/85 to 50/50 phenolic/epoxy. Upon baking, the phenolic resin's methylol groups react with the secondary hydroxyls of the epoxy resin backbone to crosslink the system (Fig. 4). Depending on the ratio used, some of the phenolic resin may self condense, but to a lesser extent than the pictured cross-linking reaction. In the case of can coatings, thin films (0.2 mil) are applied to tin plate or tin-free steel and typically baked for 10 to 12 rain at 200~ The coated metal is then formed into can bodies and lids, and foods are then packaged. The linings have to withstand a steam sterilization cycle of about 90 min at 250~ (121~ while protecting the metal and the food. For beer and beverage cans, phenolics based on bisphenol-A may be used to minimize critical taste effects. Similar formulations have been used for wire coatings or coil coating primers. Phenolic/epoxy systems are also more resistant to alkalies than phenolics alone and are used for linings when basic substances are packaged.

Heat-Reactive Aromatic Soluble Resins Heat-reactive resins or copolymers based mainly on substituted phenols such as p-tertiary butyl phenol or those with higher alkyl groups form resins which are soluble in aromatic solvents and even tolerate some aliphatic diluents. These resins are blended with blown oils or alkyd resins to form electrical coil and armature impregnation varnishes. Such var-

CHAPTER l l - - P H E N O L I C S

83

CH3 OH CI-I3 CH2

A1

EPOXY

I PHENOLIC

CH 3 OCH2-CH-CH2 I

0

CH3

I

+H20 I

CH2

OH FIG. 4-Crosslink formation.

nishes may be used in both new and reconditioned electrical equipment.

Nonheat-Reactive Resins--Unsubstituted The acid-catalyzed reaction of formaldehyde with an excess of phenol and/or cresols produces the nonheat-reactive, novolak-type resins. These resins do not form useful films by themselves, but when formulated with polyfunctional epoxy resins they act as co-reactants (hardeners) to produce thermoset systems. In this case, the chemistry involves the basecatalyzed reaction of an epoxy group with the phenolic by-

droxyl group to form ether-type crosslinks which are resistant to chemicals, moisture, and heat (Fig. 5). Since this reaction does not produce by-products, thick films may be obtained via powder coatings or high solids solution coatings. Such coatings find application as pipe coatings, reinforcement bar, and electronic encapsulation coatings.

Varnish Resins Substituted phenols containing the ten-butyl group or higher alkyl groups are reacted with formaldehyde under acidic conditions to produce "oil soluble" novolak resins.

B

O.CH2_CH_CH2

+

\o/

FIG. 5-Epoxy phenolic reaction.

84

PAINT AND COATING TESTING MANUAL

Additionally, substituted phenols may be co-reacted with tall oil resin and formaldehyde to form modified resins, which are also "oil soluble." While they are again nonfilm formers, combined with drying oils or alkyds, the above resins produce useful air-dry or baking formulations. Varnishes based on natural oil-soluble resins and drying oils were known long before phenolic resins, but the advent of the synthetic phenolic resins led to more consistent production quality. Varnishes were originally prepared via a "cook" method where the phenolic resin was dissolved in the oils at 230 to 310~ while the oil then polymerized to a specified viscosity. After adding solvents, the varnish was cooled and then finished with additives such as driers, UV absorbers, and anti-skinning agents. In the 1950s, novel phenolic resins of higher molecular weight allowed the cold blend preparation of varnish vehicles [6]. In this process, the resin is dissolved in the solvents and the oils and additives are added at mild temperatures. Such vehicles are still used as baked can coatings, air drying clear wood coatings, porch and deck enamels, maintenance paints, and as alkyd reinforcements. In the 1980s, high solids coatings were mandated by various federal and state regulations. Newer phenolic varnish resins were introduced that allowed vehicle solids of 70 to 80% to be achieved. Applied with airless spray, air drying primers

and aluminum-pigmented topcoats have been successfully used for maintenance painting of major steel structures such as bridges and airport light towers [7]. While phenolic resin technology represents a mature field, the product class is hardly pass6. Newer resins, formulations for waterborne, high solids and powder coatings, and the continued high performance in container and pipe coatings all indicate a promising future for this class of materials.

REFERENCES [1] Baekeland, L. H., Industrial Engineering Chemistry, Vol. 1, No. 3, 1909. [2] Megson, N. J. L., Phenolic Resin Chemistry, Academic, New York, 1958. [3] Martin, R.W., The Chemistry of Phenolic Resins, Wiley, New York, 1956. [4] Harding, J., U.S. Patent 3,823,103, 1974 (to Union Carbide Corporation). [5] Fry, J. S., U.S. Patent 4,124,554, 1978 (to Union Carbide Corporation). [6] Richardson, S. H., Paint and Varnish Production, August 1955. [7] Yee, A. and Fry, J. S., American Paint and Coatings Journal, 23 June 1986, p. 41.

MNL17-EB/Jun. 1995

Polyamides by Robert W. Kight 1

POLYAMIDES ARE POLYCONDENSATIONproducts of dimerized fatty acids and polyamines. Reactive polyamides are oligomers designed primarily for use in the manufacture of twocomponent polyamide/epoxy coatings and adhesives. The two-component coatings are generally labeled Part A and Part B, with the polyamide usually (though not always) contained in Part B. The polyamide may function as the curing agent, coreactant, or hardener for epoxy resin. Polyamides should not be considered as catalysts although they may initiate the reaction; the polyamide reacts with the epoxy resin and becomes part of the polymer. The majority of polyamides used in coatings are viscous liquids that are usually supplied by the coatings manufacturer as a solution in organic solvents. The solution may be a clear amber liquid or may contain pigments in colored systems.

Monocyclic

COOH (I ~H2)8

R'NHCOR + CO2 T MIXEDANHYDRIDE AMIDE

This reaction is important to keep in mind when polyesters prepared from glycols and dicarboxylic acids are used or when acid numbers are unexpectedly high in polyether, polylactone, or other polyols. The end product of this reaction results in stoppage of chain growth and a lower than expected molecular weight. In most cases this is undesirable. Amides will react with isocyanates to form acyl ureas. Isocyanates will dimerize (aromatic) to form uretidine diones and trirnerize (aromatic and aliphatic) to form isocyanurates. These reactions decrease the expected equivalent weight of isocyanates, a cost factor, and will lead to branching, cross-linking, and unexpected molecular weight in-

NR'--

C

~ [--R'NHCOOH] > --R'NH2 + CO2 T CARBAMICACID, AN UNSTABLE INTERMEDIATE

--R'NCO + --R'NH2

R'NCO + RCOOH

--R'N

NR'--

--R'N

I

I

O~-~C C~-~O \/ N

I

R-ISOCYANURATE

Thermoplastic Polyurethanes Thermoplastic polyurethanes are used in Type VI lacquers and in many industrial end uses that require solid products. Their chemistry is similar to that of prepolymers except short-chain extenders are used to connect the prepolymer molecules and build them into long polymeric materials. In a structural sense, they may be described as linear block copolymers of the ABn type. One of the blocks is a relatively long, number-average molecular weight of about 300 to 3000, polyether or polyester that forms the soft or flexible segment. The other block is formed by the reaction of a diisocyanate and a low-molecular-weight diol chain extender. The polar nature of the urethane linkages in the hard segment results in hard segment aggregation and domain segregation from the soft segment. The hard segments act as pseudo cross-links, and as a result tough, strong, elastomeric macromolecules are formed. In a mole sense, these polymers can be viewed as polyol/ diisocyanate/short-chain extender polymers that are formed in an equivalents ratio of 1/X/(X - 1). The n u m b e r X c a n vary from 1 or less to as much as 20 or more, though more typically in coatings X has a value of one or less to about 3 or 4 [10,13]. Because of solubility characteristics, a ratio of about 1/2/1 is often used. A small excess of hydroxyl groups is used to keep final free isocyanate content and storage reactivity at a nil level. When the wide range of values of X, the types of isocyanates, the types and functionalities of polyols, and the range of polyol molecular weight available is considered, it is readily apparent that a myriad of polyurethanes can be prepared and that a broad range of mechanical and chemical properties can be achieved. The chemistry is basically isocyanates reacting with hydroxyl groups to form urethane linkages.

CHAPTER 1 3 - - P O L Y U R E T H A N E COATINGS

Radiation-Curable Urethanes [18-20] Acrylate-terminated polyurethanes are used in a number of ultraviolet light and electron beam curable formulations. The products are termed "urethane acrylates" or "acrylated urethanes." They are prepared by first forming an isocyanateterminated prepolymer from a polyol and then end capping the prepolymer with an hydroxy acrylate such as 2-hydroxyethyl acrylate. The reactions leading to urethane acrylates are almost always carried out in an inert solvent.

boxylic acid-containing diol such as dimethylolpropionic acid (2,2-bis(hydroxymethyt) propionic acid), dihydroxybenzoic acid, sulfonic acids as 2-hydroxymethyl-3-hydroxypropanesulfonic acid, and similar compounds. For example: 2 0 C N - - R ' - - N C O + CH3--C(CH2OH)2--COOH

.

CH3

!

OCN--R'--NHCOOCH2~CHzOOCHN--R'--NCO COOH

20CN--R'--NCO + HO--POLYOL--OH > OCN--W--NHCO--O--POLYOL--O--OCNH--R'--NCO

[ + (n + 1 ) H O - - P O L Y O L - - O H

ISOCYANATE-TERMINATEDPREPOLYMER OCN--R'--NHCO--O--POLYOL--O--OCNH--R'--NCO+ CH2~-CHCOOCH2CH2--OH

93

CH3

I

HO--(POLYOL--OOCHN--R'--NHCOOCH2CCH2OOCHN-/

COOH

2-HYDROXYETHYL ACRYLATy CH2~CHCOOCH2CH2--O--OCNH--R'--NHCO--O--POLYOL-O--OCNH--R'--NHCO--OCH2CH2OOCHC=CH2 A URETHANE ACRYLATE The reactions as depicted above have been idealized. In all commercial and most laboratory preparations there is a significant amount of reaction between the ingredients so that chain extension occurs and molecular weight increases. This causes the final product to have a markedly higher-thanexpected viscosity. Oligomeric compounds such as these are formulated with triacrylates such as trimethylolpropane triacrylate to provide cross-linking, monomeric acrylates, N-vinyl pyrrolidone, or other compounds for viscosity reduction to provide low-viscosity, essentially 100% solids systems that will cure when exposed to actinic radiation. In formulations, the urethane acrylate is considered as the main ingredient contributing to mechanical properties of the cured film. When the actinic radiation source is ultraviolet light, a photoinitiator (for example, 2,2-diethoxyacetophenone or benzophenone in combination with an amine synergist, etc.) is added as a free radical source. Electron beam curable formulations do not require a photoinitiator. Radiation-cured polyurethanes are often used on plastic substrates that require only low or moderate curing temperatures such as clear overprint lacquers on vinyl decals, electronic circuit boards, "no wax" vinyl flooring, and tile. Although radiation-cured colored and pigmented inks and coatings are used in the marketplace, the skill needed in preparing such products, because of difficulty with light penetration or absorption, is readily apparent.

Water-Borne Polyurethanes Water-borne polyurethanes are prepared in bulk or in a solvent by first preparing an ionomer prepolymer that is neutralized and then chain extended to a desired molecular weight. The polymer then is dispersed into water. Both cationic [21,22] and anionic [23,24] systems are known. Cationic systems employ an amine-containing diol such as diethanolamine, methyl diethanolamine, N,N-bis(hydroxyethyl)-a-aminopyridine, lysine, N-hydroxyethylpiperidine, and similar compounds. Anionic systems use a car-

R'--NHCOO),--POLYOL--OH WATER DISPERSIBLE POLYURETHANE Water-borne polyurethane laminating adhesives that are completely free of volatile organic compounds are expected to be the next developments in this area [25]. These adhesives are expected to be for the low-to-medium demand product area such as for snack food and similar packaged products.

Powder Coatings Polyurethane powder coatings are usually urethane-modifled polyesters and polyacrylics that cure at high temperatures. High temperatures are needed for the powdered polymer to flow and level to the extent needed for a particular end use. The key to successful powder coatings is related to a balance between molecular weight and related viscosity and a cross-linking mechanism that is stable under storage conditions and not effected to any significant degree until flow and leveling takes place at the cure temperature. Another requirement is that the glass transition temperature should be sufficiently high that the powder does not block during storage. The main end use for powdered polyurethanes is in the major appliance market--refrigerators, dryer drums, range cabinets, etc.--coatings.

MARKETS Polyurethanes of the various types are used in a number of market areas and end uses. Many of these were mentioned above. Two features of polyurethane coatings that have been often looked on as disadvantages are high cost and special handing of the potentially hazardous isocyanates that are used in manufacture or as curing agents. However, the various industry segments have been able to develop safe handling and use methods that overcome one of the objections. The very- high performance characteristics of polyurethanes, their ability to cure at lower baking temperatures, and the

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PAINT AND COATING TESTING MANUAL T A B L E 1--Polyurethane end uses.

HOME FURNISHINGS Drum dryers Furniture "No wax" flooring and tile Range cabinets Refrigerators Wood floors

PLASTIC SUBSTRATES Fascia Electronic parts and equipment Optical fibers Printed circuit boards Sheet molding compound

INDUSTRIAL MAINTENANCE Bridges Industrial buildings Marine coatings Plant equipment Public utility works Roof coatings Windows

RECREATIONAL PRODUCTS Golf balls Golf clubs Gym floors Toys

MISCELLANEOUS Aerospace coatings Luggage Magnetic tape coatings Mast and spar finishes Medical equipment Safety glass Shoes Vinyl decal overprints Wire coatings

TEXTILES Apparel Leather Tarpaulins Upholstery TRANSPORTATION Aircraft Automotive OEM Automotive refinish Golf carts Motorcycles Railroad cars Trucks and buses Vans

i m p r o v e d total coating solids, i.e., d e c r e a s e d volatile o r g a n i c c o m p o u n d content, that can be o b t a i n e d are factors that offset their high cost. F o r example, p o l y u r e t h a n e s are replac= ing poly(vinyl chloride) plastisols as u n d e r c o a t i n g s a n d sealants in the a u t o m o t i v e a n d o t h e r t r a n s p o r t a t i o n coating m a r ket. Lower coating thickness a n d equivalent o r i m p r o v e d p e r f o r m a n c e m a k e the a p p l i e d cost of the p o l y u r e t h a n e c o m petitive with the plastisol. The textile a r e a is a m o d e r a t e g r o w t h a r e a for t h e r m o p l a s t i c p o l y u r e t h a n e lacquers with the excellent c o m b i n a t i o n of p r o p e r t i e s as the m a i n driving force for use. These include g o o d elasticity at low t e m p e r a tures, a b r a s i o n resistance, solvent a n d w a t e r resistance, d r y cleanability, m a c h i n e washability, a n d a n ability to be prep a r e d in a b r o a d variety of tensile/elongation properties. I n addition, the high p e r f o r m a n c e can be achieved with very thin coatings t h a t do not m a r k e d l y increase fabric weight o r change styling factors such as drape. To decrease volatile organic content, new low-viscosity, aliphatic isocyanates [26] a n d p o l y u r e t h a n e polyols [27] are being developed. Although it is n o t a c o m p l e t e listing, Table 1 is a s u m m a r y of m a n y end uses for p o l y u r e t h a n e coatings. I n the five-year p e r i o d b e t w e e n 1991 a n d 1996, it is estim a t e d t h a t the U.S. p o l y u r e t h a n e coating m a r k e t will g r o w at a c o m p o u n d e d a n n u a l rate of 5% o r f r o m 209 million lb to 265 million lb (95 000 to 123 400 metric tons) [7]. It is exp e c t e d that the two-package (ASTM Type IV a n d V) systems will have a l m o s t d o u b l e the c o m p o u n d e d a n n u a l g r o w t h rate of the overall u r e t h a n e coating market, i.e., a b o u t 10%, with c o n s u m p t i o n rising from 84 million lb in 1991 to 133 million lb in 1996 (38 200 to 60 500 m e t r i c tons). W a t e r b o r n e a n d p o w d e r e d p o l y u r e t h a n e s are also i m p o r t a n t growth areas.

REFERENCES [I] Bayer, 0., Modern Plastics, Vol. 24, 1947, p. 149. [2] Wright, P. and Cumming, A. P. C., Solid Polyurethane Elastomers, Elsevier Publishing Company, Amsterdam, 1969. [3] Bayer, O., Rinke, H., Siefken, W., Orthner, L., and Schild, H., German Patent 728,981 (1942). [4] Bayer, O., Angewandt Chemie, Vol. A59, 1947, p. 275. [5] Schollenberger, C.S., Scott, H., and Moore, G.R., Rubber World, Vol. 137, No. 4, 1948, p. 549. [6] Smith, R. M., "Polyurethanes," Supplement C, Report No. 10C, SRI International, Menlo Park, CA, May 1991. [7] Linak, E., Kalt, F., and Takei, N., "Urethane Surface Coatings," Chemical Economics Handbook, SRI International, Menlo Park, CA, August 1992, p. 592.8000. [8] ASTM D 16: Terminology Relating to Paint, Varnish, Lacquer, and Related Products, Vol. 06.01, ASTM Book of Standards, 1992. [9] "Chemical Products for Resins, Coatings, Sealants, Adhesives, and Elastomers," Hill America Inc., Piscataway, NJ, 1992. [10] Seefried, C. G., Jr., Koleske, J. V., and Critchfield, F. E., "Thermoplastic Urethane Elastomers. I. Effects of Soft Segment Variations," Journal of Applied Polymer Science, Vol. 19, 1975, p. 2493. [11] Bailey, F. E. and Koleske, J. V., Alkylene Oxides and Their Polymers, Marcel Dekker, Inc., New York, 1991, p. 218. [12] Critchfield, F. E., Koleske, J. V., Magnus, G., and Dodd, J. L., "Effect of Short Chain Diol on Properties of Polycaprolactone Based Polyurethanes," Journal ofElastoplastics, Vol. 4, January 1972, p. 22. [13] Seefried, C. G., Jr., Koleske, J. V., and Critchfield, F. E., "Thermoplastic Urethane Elastomers. II. Effects of Hard Segment Variations," Journal of Applied Polymer Science, Vol. 19, 1975, p. 2503. [14] Lee, J. M. and Winfrey, J. C., U.S. Patent No. 3,236,895 (1966). [15] Yeakey, E. L., U.S. Patent No. 3,654,370 (1972). [16] Bishop, T. E., Coady, C.J., Zimmerman, J. M., Noren, G. K., and Fisher, C. E., European Patent Publication 209,641 (1987). [17] Christenson, R. M. and Erikson, J. A., U.S. Patent 4,222,911 (1980). [18] Salim, M. S., Polymer, Paint, Colour Journal, Vol. 177, No. 4203,762 (1987). [19] Martin, B., Radiation Curing, Vol. 13, No. 4, August 1986, p. 8. [20] Hodakowski, L.E. and Carder, C.H., U.S. Patent 4,131,602 (1978). [21] Scriven, R. L. and Chang, W. H., U.S. Patent 4,046,729 (1977). [22] Scriven, R. L. and Chang, W. H., U.S. Patent 4,066,591 (1978). [23] Scriven, R. L. and Chang, W. H., U.S. Patent 4,098,743 (1978). [24] Milligan, C., U.S. Patent 3,412,054 (1968). [25] Long, D. and Barush, J., "VOC-Free Adhesive Use Grows Due to Increased Performance," Adhesive Age, Vol. 36, No. 10, September 1993, p. 42. [26] Wojcik, R. T., "Super-Low-ViscosityAliphatic Isocyanate Crosslinkers for Polyurethane Coatings," Modern Paint and Coatings, Vol. 83, No. 7, July 1993, p. 39. [27] Gardon, J. L., "Polyurethane Polyols: Ester-Bond Free Resins for High Solids Coatings," Journal of Coatings Technology, Vol. 65, No. 819, April 1993, p. 25.

MNL17-EB/Jun. 1995

Silicone Coatings by D. J. Petraitis ~

UNIQUE PROPERTIES OF SILICONES THAT M A K E T H E M U S E F U L AS C O A T I N G S

implants such as heart pacemakers. They have also been used to coat temporary implants such as catheters and surgical drains. Also, thin silicone elastomer coatings are used to provide soft tissue replacements by forming an envelope to encapsulate gels and/or normal saline solutions. Recent applications for biocompatible silicone coatings include drug delivery devices for both transdermal and long-term implantable, controlled-release drug delivery. A final characteristic which makes silicone coatings useful is their inherently low or nonflammability. Typically, silicone elastomer coatings have been rated SE-I when tested via Underwriters' Laboratories Flame Test (UL-94). This property makes silicone coatings ideal for conformal coating of various electrical circuits and devices. In the event of catastrophic thermal degradation, the silicone coatings can and do provide an SiO2 ash coating which may permit the emergency operation of the electrical device on a short-term temporary basis.

Silicone based coatings are among the most useful materials for a wide variety of applications. Because the basic bond energies of Si--C and Si--O bonds are so high, the chemical processes usually associated with aging of coated surfaces are often much slower and in many situations virtually eliminated for silicone coatings. Also, because the Si--O and Si--C bonds are not present in the natural organic world, biocompatibility and resistance to degradation via biochemical and biophysical processes are also significantly reduced. In a similar manner, some silicone resinous coatings and fluorosilicone based coatings, in particular, have excellent solvent resistance. Silicone coatings based on trifluoropropyl methyl polysiloxanes have resistance to swelling from such agents as gasoline, jet fuel, solvents, and various other reagents. Highly branched resinous silicone coatings chemically begin to approach the properties of silica surfaces as the organic pendant content is reduced. As the organic pendant groups are reduced, the SiO4/2 content increases and the chemical resistance increases. Such resinous coatings can provide physical scratch resistance as well as chemical resistance. Silicone elastomeric coatings, however, do not provide good resistance to strong acids and/or bases. Strong acids or bases, in particular at elevated temperatures, can cause depolymerization of the siloxane backbone, resulting in failure, or in the case of silicone elastomeric coatings, dissolution of the coating itself. In a similar manner, silicone coatings are resistant to virtually all frequencies of the electromagnetic spectrum. For complaint coatings, silicones are unsurpassed in resistance to hard radiation, such as that from a cobalt-60 source for doses in excess of 20 Mrd, as well as the ultraviolet and infrared frequencies. When combined with their hydrophobicity and oxygen and ozone resistant properties, silicones provide excellent weatherability characteristics, and when these properties are combined with the resistance to atomic oxygen in low earth orbit, silicone coatings provide protection for organic substrates in varied spacecraft applications. Another family of applications which combine the chemical and biochemical characteristics of silicones are those which are used to provide coatings for medical devices. Silicone coatings are used to encapsulate and seal permanent

F O R M S OF S I L I C O N E C O A T I N G S Silicone coatings are available in various forms ranging from a hard, rigid resin to a compliant elastomer to a soft, almost gel-like character. The rigid resins are typically supplied in a solvent solution and are mixed with curing agents prior to application. Among the most common curing agents are lead and zinc octoate, which require approximately 1 h at 250~ to attain complete cure. The cross-linking mechanism involves the condensation of silanol groups ~-~SiOH + H O S i ~

Specific coating applications include jet engine components, furnace parts, incinerators, high-temperature appliances, and missile coatings. In addition, specific silicone resins have been designed to mix with organic coatings and paints, providing higher performance under moderate heat environments. By varying the R group:Si ratio, the hardness of the final coatings can be varied. As the R: Si ratio is decreased, the cross-link density of the resin effectively increases. Similarly, variation of the R group itself can result in somewhat different flexibility and other properties. Properly designed and condensed resins can be fornmlated to provide hard silicalike abrasion-resistant coatings. Such coatings often involve the condensation of alkoxy groups with silanol groups as well as condensation between silanol groups alone. Technology to minimize shrink and maximize adhesion during the cure

1Vice president, Operations, NuSil Technology, 1040 Cindy Lane, Carpinteria, CA 93013.

95 Copyright9 1995 by ASTMInternational

~ ~ S i O S i F + H20

www.astm.org

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PAINT AND COATING TESTING MANUAL

needs to be incorporated to prevent cracking and subsequent flaking of the coating from the substrate. Aminofunctional alkoxy silanes are often incorporated into the formulation to simultaneously optimize cure rate and adhesion. Silicone elastomeric coatings incorporate the use of polymeric siloxanes with appropriate cross-linkers to provide compliant, flexible coatings. Among the cure mechanisms which result in elastomers are the following 1. ~ S i O H + HSi-~- sn ~ ~SiOSiw~ + H2 2. ~ S i O H + CH3COOSi~ ~ ~ S i O S i ~ + CH3COOH 3. ~--~SiOH + ROSin--- sn ~ ~ O S i - ~ + ROH 4. ~SiCH~---CH2 + HSi I~ Pt ~ ~SiCH2CH2Si~ Sn 5. ~ S i O H + R2NOSi~-s--~2 ~ S i O S i ~ + R2NOH These elastomeric coatings can range from extremely tough, high-strength elastomers to soft gel-like coatings. Typically, the elastomer could have properties within the following ranges: Durometer: Tensile Strength: Elongation: Tear Strength:

Type 00 = 10 Type A = 70 0.34 to 13.8 MPa 50 to 1500% 0.88 to 43.8 kN/m

The properties and the cure systems which are chosen for these elastomeric coatings depend, to a large extent, on the end use and the method of application. For instance, the SiOH + HSi (No. 1) mechanism is often used to provide release coatings for backing paper for pressure sensitive adhesives. The actual coating itself has poor strength but attains its properties by simply impregnating the substrate. The acetoxy cure system (No. 2) is used where one-part convenience is desired, where relatively slow cure is acceptable, and where acetic acid given off during the cure is not a problem. The oxime (No. 5) cure system provides many of the properties of the acetoxy cure system, but results in an oxime leaving group instead of an acetic acid leaving group. Among the applications for the oxime cure systems are coatings for electronic components and protection for organic composites to prevent atomic oxygen degradation, and coating of quartz blankets to provide adequate emissivity and reflectivity characteristics for certain thermal protection surfaces on the space shuttle. The alkoxy 2-part (No. 3) cure system, when combined with certain thermal enhancing fillers such as iron oxide, glass microballoons, and various fibers, is often used to provide ablative and thermally insulating coatings. Various products incorporating the alkoxy two-part cure system are used to protect surfaces and components exposed to plume radiation from various rocket motors and jet engines. The addition cure system (No. 4) has characteristics which permit rapid heat-accelerated cure, tough physical properties, virtually nil shrinkage, and, due to the platinum catalyst, the best overall flame resistance. Applications include solar cell protection, particularly for satellites, and burn-through protection for the liners of solid rocket motors. The only negative characteristic of the addition cure system is its susceptibility to inhibition. Because the system contains partsper-million levels of platinum catalyst, it can be readily "poisoned." Among the most common inhibitors are sulfurcontaining organic rubbers and organo-tin compounds

which are often used as plasticizers in plastics and also as catalysts for other silicone coatings. There are other silicone elastomeric cure systems, and one of the most significant applications is to coat fiberglass blankets for fire resistance. Spark protection welding blankets are a common application for peroxide-cured silicone coatings. Since peroxide-cured silicones require higher temperature cures, their usefulness is constrained by the substrate upper temperature limits. Also, selectivity of the specific peroxide is critical to prevent poor cures due to the oxygen inhibition; characteristic of many peroxides. Another novel silicone elastomer coating which has been developed is a combination cure involving the ultraviolet photoinitiation via free radical formation to provide crosslinking. This ultraviolet mechanism is often combined with a standard cure mechanism to provide a combination cure. This system provides quick surface cure followed by the slower room temperature cure of unexposed, shadowed areas to ultimately provide a fully cured conformal coating. Processes using the combination cure can be used to minimize the time and space required to hold the coated parts until cure is completed before downstream assemblies can take place. Other cure systems have been developed for silicone elastomers, but they find limited use as coating materials and were generally developed for specific applications such as building sealants or glazing compounds. The most common form for silicone coatings is a dispersion of the silicone in solvent. If the coating is based on a tough elastomeric silicone, the uncured elastomer base is most commonly described as a dispersion because it contains insoluble components such as high surface area fumed silica for reinforcement and often other solid components such as titanium dioxide pigments for coloration or reflectivity properties. The carrier solvent for these dispersions may include chlorinated hydrocarbons, fluorochlorohydrocarbons, and both aromatic and aliphatic hydrocarbons. The dispersions also often include blends of solvents to provide the proper combinations of flow, evaporation, and application ease. Among the most common solvents for silicone dispersions are 1,1,1-trichloroethane, VM&P naphthas, and xylene. Lowmolecular-weight alcohols such as ethanol and isopropanol and ketones such as acetone are not suitable because silicones are generally incompatible with these lower-molecular-weight oxygen-containing solvents. Fluorosilicones require the use of such solvents as methyl ethyl ketone and methyl isobutyl ketone for adequate dispersing. Fluorosilicone-dimethyl copolymer-based silicones can be dispersed adequately in 1,1,1-trichloroethane for thin layer application. True solutions can also be made if the silicone contains no insoluble components. For example, true solutions can be made for unfilled silicones or for silicones that are resin reinforced. These coatings have limited use, however, because the final cured elastomeric coating lacks the overall toughness of the filled materials. Recent developments have resulted in silicone coatings which have not involved the use of solvents. Because of environmental concerns, the use of solvent carriers for dispersions and solutions has become less desirable. In particular, fluorochlorocarbons and chlorinated hydrocarbons, despite

CHAPTER 1 4 - - S I L I C O N E COATINGS their low toxicity and nonflammability, are being phased out because of Montreal Protocol Agreements. Similarly, hydrocarbon solvents are undesirable because of their flammability, toxicity, and environmental effects. Silicone-based conformal coatings have been developed without solvent carriers. However, thin layer applications are difficult unless the viscosity is low enough to permit proper coating. Unfortunately, the technology for high-strength, low-viscosity, 100% solids, silicone coating does not exist. The current products, therefore, when cured, are very low strength and do not provide coatings that are resistant to handling. Research is ongoing to develop water-based dispersions, but to date, the demonstrated physical properties, although higher than the 100% solids coatings, are significantly less than the current solvent-based silicone coatings.

Methods of Applications The methods of applications for silicone coatings depend on the device being coated and the specific type of silicone being used. Dipping, spraying, and painting are the most common types of application. The thinnest coatings result from spraying of two solvent dispersion utilizing standard aerosol spray guns. Needless to say, experience involving aerosol spraying is critical for acceptable coatings. Among the variables to consider are the following: viscosity, solvent, percent solids, pot life, and cure system choices. The most securely sealed surface layer is accomplished by dip coating. Again, variables including solvent, bath life, and cure systems must be optimized. Additionally, the evaporation of solvent during the dip processing needs to be compensated for by periodically or continuously adding make-up solvent to maintain optimal bath viscosity. If a one-part humidity-actuated cure system is used, consideration must be given to provide a dry blanket over the bath to prevent a partially cross-linked elastomeric skin from forming. Dry argon is often utilized to prevent moisture in the air from reacting with the silicone base coating. Another consideration for the dip coatings is the possibility of air bubble inclusion. Again, several variables need to be considered. Low viscosity, controlled immersion and withdrawal rates, and vibration of the bath and/or object to be coated can be used to minimize bubble entrapment. Similarly, the use of two distinct solvents with different rates of evaporation are often used to ensure uniform coating with minimal drip regions and minimal bubble formation. Painting or brush coating substrates is yet another method to apply a uniform silicone coating. Painting, however, is usually not applicable for either large areas or mass production coatings. For painting application, virtually all of the variables discussed in the above dipping and spraying also apply. Regardless of the methods of application, the cure parameters demand significant considerations. Vacuum exposure may be used to remove air bubbles and to ensure flow under surface irregularities or impregnation of porous substrates. Vacuum treatment may also be used to enhance removal of the solvents, but care should be taken to prevent evaporation of the reactive volatile components which would prevent cure even after removal from the vacuum. Of course, most commonly, the vacuum removal of solvent is unwarranted and

97

therefore solvent is merely evaporated at ambient pressures. The solvent evaporation can also be enhanced by air circulation and by acceleration with heat. However, the application of heat should be limited or applied in a stepwise manner to prevent solvent entrapment below the surface resulting in solvent bubble formation. Also, for one-part silicone coatings which are cured via moisture activation, it is ineffective to use heat acceleration because humidity is obviously reduced in a normal air circulating oven. If accelerated cure is required for one-part coatings, a steam autoclave may be used, but only

after all of the carrier solvent is removed.

TESTING CONDITIONS The test requirements for silicone coatings include MIL-I46058C for qualifying silicone coatings as insulating compounds for electrical coating applications of printed circuit board assemblies. MIL-I-46058C includes the following tests: Curing Time and Temperature Appearance Coating Thickness Fungus Resistance Insulation Resistance Dielectric Withstanding Voltage Leakage Current Testing Q Resonance Q Resonance after Immersion Thermal Shock Flexibility Thermal Humidity Aging Flammability Materials which are used in applications for spacecraft are tested via ASTM Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment (E 595). This test is used to determine the amount of collected volatile condensable material and total mass loss that eludes from a material when exposed to 125~ for 24 h at vacuum less than 5 • 10 .2 torr. Basically, the maximum CVCM value for coatings intended for space applications is 0.1% and the m a x i m u m TML is 1.0%. The coatings intended for satellite applications require these high levels of purity to prevent the contamination of solar cells, optical surfaces, and other sensitive instrumentation. For most silicone materials, extended devolatilization is required for the polymeric components prior to compounding into the finished product. For silicone elastomeric coatings, the physical properties of the cured elastomer are critical parameters. The tensile strength, elongation, and modulus are defined in ASTM Test Methods for Rubber Properties in Tension (D 412). Durometer and tear strength measurements are defined in ASTM Test Method for Rubber Property--Durometer Hardness (D 2240) and ASTM Test Method for Tear Strength of a Convential Vulcanized Rubber and Thermoplastic Elastomer (D 624) respectively. The viscosity, nonvolatile content, and specific gravity tests are defined in ASTM Test Method for Viscosity of Adhesives (D 1084), ASTM Test Method for Weight Loss of Plasticizers on Heating (D 2288), and ASTM Test Method for Specific Gravity (Relative Density) and Density of Plastics by Displacement (D 792), respectively.

98

PAINT AND COATING TESTING MANUAL

Other tests have been developed for silicone coatings to meet the requirements of specific applications. Included among these are the "blocking" test to determine the propensity of silicone coatings to cause "sticking" to contacted surfaces after application and cure. This test involves contact between the silicone-coated surface and the surface to be tested by subjecting the interface to an applied pressure for a fixed time followed by measurement of the force needed to reseparate the surfaces. A variety of tests have been developed to measure the adhesive force of the coating on the substrate. Again, a number of tests which are oriented toward the specific application have been developed and revised.

SPECIFIC APPLICATIONS FOR SILICONE COATINGS Among the varied applications for silicone coatings is the conformal coating of electronic circuit boards. Because of their previously described stability properties, silicones make ideal conformal coatings. Silicone coatings typically have stiffening points of -65~ and can be formulated with stiffening points as low as - 115~ This makes them ideal for extreme environment electronic device protection. Silicone coatings are used almost exclusively to provide protection from atomic oxygen degradation in low earth orbit (approximately 100 to 500 miles high). Atomic oxygen degradation is significant enough to rapidly erode and degrade organic substrates including epoxies, urethanes, and polyester-based thermosets. Coating protection permits the use of composite materials in space applications where the advantages of high strength and low weight associated with composite materials would be unusable due to their atomic oxygen degradation. The high-temperature stability and excellent dielectric properties of silicone resins make them ideal impregnant coatings for high-energy capacitors used in jet engine ignitions. The inherent stability of silicone coatings when combined with specific fillers including zinc oxide, titanium dioxide, and zinc orthotitanate are often used to provide specific emissivity and reflectance required for thermal control coat-

ings. Similarly, silicone elastomeric coatings are produced by the addition of iron oxide, glass or ceramic microballoons, and graphite fibers, which provide ablation protection. Launch vehicles, launch equipment, and thrust reversers are often coated with specially formulated silicone ablative coatings. The incorporation of phenyl siloxanes into the basic silicone polymeric species provides increased ablative prop~ erties, and various copolymers--including silicone-boranes and silphenylenes--have and are being evaluated to provide protection from impingement of high-energy lasers. As discussed previously, the biocompatibility of silicones makes them ideal for medical applications. Coating permanent implants as well as temporary implants with silicones provides improved safety and efficacy. Foley catheters coated with silicone elastomers result in less patient discomfort and reduced infection rates. For similar reasons, temporary pressure-sensitive silicone adhesive coatings are used to provide adhesion directly to the skin. Combinations of silicone coatings are being investigated for use in various drug delivery devices. Specifically layered coatings of silicones impregnated with drugs can be used for transdermal drug delivery. When combined with a silicone pressure sensitive adhesive, a complete system of controlled drug delivery devices can be fabricated.

NEW REQUIREMENTS FOR SILICONE COATINGS Research and development efforts continue to provide silicone coatings with even more stringent requirements and specifications. Electrical coatings with semiconducting properties for electronic applications and elastomeric coatings with volume resistivities in the 10-4 to 10-5 ohm-centimetre range are being investigated. Silicone coatings with variable electric properties are also being researched. Similarly, silicone coatings which provide specific biological properties are also being developed. Specifically, hydrophilic silicone coatings are being developed for reduced thrombogenicity, and microporous coatings are under development for controlled tissue in-growth response.

MNL17-EB/Jun. 1995

Vinyl Resins for Coatings

15

by Richard J. Burns 1

VINYL RESINS FOR COATINGS

History

Definition

About 1912 Ostromislenski pioneered the industrial investigation of vinyl polymers and made and fractionated poly(vinyl chloride). E. W. Reid invented the copolymers of vinyl chloride and vinyl acetate in 1928. In 1933, Davidson and McClure described applications of vinyl resins including their use as swimming pool coatings [1]. Commercial production of poly(vinyl chloride-vinyl acetate) resins was begun in 1936. Carboxyl-modified copolymers were introduced in 1939 and hydroxyl-modified resins in 1945. The first commercial use of these vinyl resins was in 1936 as a coating for the inside of beer cans. Organosol and plastisol coatings technology that permitted the use of very-high-molecular-weight resins were developed about 1943 [2,3].

THE VINYL RESINS USED IN s o l v e n t - b a s e d c o a t i n g s , i n k s , a n d

adhesives are low-to-medium molecular weight co- and tercopolymers of vinyl chloride, vinyl acetate, or other monomers to improve solubility. Functional monomers contribute specific properties; thus, carboxylic acid-containing monomers provide adhesion, while hydroxyl-containing monomers contribute to reactivity, compatibility with other resins and polymers, or adhesion to specific surfaces. These modified vinyl resins are most often used as thermoplastic, solvent-soluble lacquers, though by formulating with appropriate modifiers, air-dry or baking finishes can be produced having thermoset-like properties. Special techniques have been developed that enable the use of high-molecular-weight vinyl chloride homopolymers as dispersions in organic media called plastisols or organosols that require a heat fusion step to form films or coatings. Vinyl chloride homopolymers and copolymers are also compounded for use as powder coatings for application by either electrostatic spray or fluidized bed techniques. Water-based vinyl chloride polymers and copolymers include high-molecular-weight polymer latexes that require heat to fuse, and also aqueous dispersions of low-molecular-weight polymers that utilize coalescents to form films at room temperature.

Polymerization Vinyl chloride monomer is a gas at standard conditions with a boiling point of - 13.9~ Polymerization is carried out in autoclaves under moderate to high pressure. The reaction is typically initiated by free radical generating compounds such as peroxides. The polymerization is exothermic, and reaction temperature regulation is necessary to control the growth (molecular weight) of the polymer. The use of high pressure and low temperature generally favors the formation of high-molecular-weight resins, and chain transfer agents are commonly used to control molecular growth. The number average molecular weight (M,) of commercially available solvent-soluble vinyl chloride homopolymers and copolymers ranges from a low of a few thousand to about 45 000. The M, of vinyl resins used for plastisol and organosol coatings ranges between about 60 to 110 000 [4].

General Important characteristic features of vinyl resins/coatings are: (1) relatively high glass transition temperature; (2) excellent resistance to water, alcohols, aliphatic hydrocarbons, vegetable oils, dilute acids, and alkali; and (3) inertness in contact with foods (FDA-listed resins only). Vinyl resin films can be degraded by exposure to high temperatures or by long-term exposure to ultraviolet light, with a resultant change in color from clear to amber, red, and eventually black. Suitable heat stabilizers are employed that allow the processing of vinyl coatings at high temperature, while proper pigmentation helps to protect vinyl coatings from attack by UV light. Some stabilizer systems can provide limited protection to clear vinyl films.

Manufacture Vinyl resins for coatings are made by several processes. Polymerization by solution and suspension processes is used to make the solvent-sofuble resins, while emulsion or dispersion polymerization is used to make the much higher molecular weight polymers for plastisols and organosols. Some solvent-soluble grades are also made by the emulsion process. Post-polymerization processes are applied to some resins to achieve special properties.

~Union Carbide Corporation, B o u n d Brook, NJ 08805.

99 Copyright9 1995 by ASTM International

www.astm.org

100 PAINT AND COATING TESTING MANUAL Solution Process

Vinyl Chloride Copolymer Coating Resins

Polymerization is carried out in a solvent in a batch or continuous process. The viscosity of the reaction medium increases as m o n o m e r is converted to polymer, and the extent of polymerization can be monitored and controlled via viscometry. When the appropriate viscosity is attained, the autoclave varnish is stripped of unreacted vinyl chloride monomer, and the polymer is precipitated by the addition of water or water/alcohol mixtures; the slurry is centrifuged to remove most of the liquid, then the resin is dried in fluid-bed dryers. The particle size of the dried resins produced by this process ranges from about 75 to about 200 ~m, and the particle shape is irregular.

Four types of solvent-soluble coating resins offered by Union Carbide are shown in Table 1. These polymers are produced by the solution polymerization process. 1. Vinyl chloride-vinyl acetate copolymers. 2. Carboxyl-modified vinyl chloride-vinyl acetate copolymers. 3. Hydroxyl-modified copolymers of two types: a. Hydroxyalkyl acrylate modified directly polymerized. b. Vinyl-alcohol-modified polymer derived from poly(vinyl chloride-vinyl acetate) in a post-polymerization process. 4. Epoxy-modified vinyl chloride copolymers. Other suppliers of solvent-soluble vinyl resins and their product lines are listed in Tables 2 through 5 for Denka Kagaku, BASF, Wacker Chemie, and Nissan.

Suspension Polymerization Suspension polymerization is generally carried out in a water medium. High-molecular-weight water-soluble colloidal polymers are used in small amounts to stabilize the droplets of suspended monomer(s) to control particle size. The stabilizer used remains with the resin during and after polymerization and resin recovery. Normally the preparation of solutions of suspension resins requires that mild heating be employed to achieve m a x i m u m clarity of solutions at minim u m viscosity. Particles of suspension vinyl resins are characterized as spherical with a size between 100 to 300/~m.

Emulsion Polymerization Like the suspension process, emulsion polymerization is also carried out in water, but in place of water-soluble polymers, surfactants are normally used to stabilize the smaller m o n o m e r droplets during polymerization. A special form of emulsion polymerization called dispersion polymerization uses an oil-soluble rather than water-soluble initiator and produces resin of particles size ranging from about 0.2 to 2 /~m. These high-molecular-weight powdered products are used in plastisol and organosol coatings.

Post-Polymerization Process Some vinyl-alcohol modified resins are prepared in a twostep process. The first step consists of the preparation of a poly(vinyl chloride-acetate) copolymer by either a solution or suspension process. Next, the copolymer resin is dissolved in a suitable solvent and a catalyst is added to partially hydrolyze the pendant acetoxy groups to yield a vinyl alcohol moiety. The modified resin is then precipitated from solution and dried as described for the solution process. The resin thus formed has only secondary hydroxyl groups, which accounts for its unique solubility/compatibility properties. These vinyl-alcohol-containing resins differ from those prepared directly using other hydroxy-containing monomers in their compatibility with alkyds and in the rate of reactivity with coreactants such as isocyanate or amino-formaldehyde cross-linkers.

FDA Status Vinyl copolymer resins are listed by chemical identity in several U.S. Food and Drug Administration regulations such as 21CFR 175.300, 176.170, 176.180, and 177.1210 as components of coatings on metal and paper substrates for use as food contact surfaces of articles used in processing, manufacturing packing, producing, heating, packaging, holding, or transporting food, or as components of closures with sealing gaskets for food containers. Vinyl chloride-acetate copolymers, hydroxyl-modified vinyl chloride-acetate copolymer, and several other vinyl chloride copolymers made with monomers having acid or ester functionality are described.

Vinyl Resins--Analysis There are many references to chemical methods for identifying and characterizing vinyl resins [5,6]. However, the infrared spectra of vinyl resins are very useful for qualitative and quantitative purposes. Spectra of neat vinyl resins can be found in sources such as atlases, encyclopedia of plastics, or specific papers dealing with the subject [7-9]. Also, several ASTM documents deal with the identification and characterization of vinyl resins used in coatings materials. ASTM Guide for Testing Poly(Vinyl Chloride) Resins (D 4368-89) describes methods for homo- and copolymer vinyl resins to determine important characteristics such as total chlorine content for composition, dilute solution viscometry to assess polymer molecular weight, high and low shear viscosity measurements to characterize vinyl dispersion resins for plastisols and organosols [10]. ASTM Test Method for Infrared Identification of Vehicle Solids from Solvent-Reducible Paints (D 2621-87) covers the qualitative characterization of separated paint vehicle solids by infrared spectroscopy. A spectrum for an ortho-phthalic alkyd, vinyl chloride-acetate modified vehicle is presented [11]. ASTM D 2124-70 (Reapproved 1988), Test Method for Analysis of Components in Poly(Vinyl Chloride) Compounds Using an Infrared Spectrophotometric Technique, presents methods whereby vinyl compounds can be separated into components including resins, plasticizers, stabilizers, and fillers. Each component can then be analyzed by infrared technique [12].

CHAPTER 1 5 - - V I N Y L R E S I N S FOR COATINGS

101

TABLE 1--Typical properties of UCAR | solution vinyl resins. Polymer Composition, wt%

Inherent Viscosity, ASTM D 1243

Glass Transition Temperature (Tg), ~

Average Molecular Wt, M.*

Solution Viscosity ~' at 25~ cP

0.74 0.50 0.40

79 72 72

44 000 27 000 22 000

1300 i 600 200

0,50 0.38 0.32

74 72 70

27 000 19 000 15 000

650 100 55

---

67

15 000

--.

2.3 2.3

0.53 0.44

79 77

27 000 22 000

1000 400

Hydroxyl Hydroxyl Hydroxyl

1.~ ~ 1.9 2,0

0.56 0.44 0.30

70 65 65

33 000 24 000 15 000

930 275 70

Hydroxyl

3.0

0.15

54

5 5O0

2O

Reactive

UCAR| Solution Vinyl

Po/y(vinyl chloride)

Poly(vinyl acetate)

VYNS-3 VYHH VYHD

90 86 86

10 14 14

. . . . . . . . . . . . . . . . . . . . . . . . . . .

VMCH VMCC VMCA

86 83 81

13 16 17

1a 1a 2a

Acid Acid Acid

1.0 1.0 2.0

VERR-40

82

9

9 b'c

Epoxy

1.8g~

VAGH VAGD

90 90

4 4

6d 6d

Hydroxyl Hydroxyl

VAGF VAGC VROH

81 81 81

4 4 4

15e 15e 152

VYES-4

67

11

22 e

Functionality Other

Type

Wt%

*Referenced to polystyrene standard. ~Maleic acid. VEpoxy-containing monomer. CSolution--40% resin in MEK/toluene 3 • 2. aPoly(vinylalcohol). ~Hydroxy alkyl acrylate. /Oxirane oxygen. gOn solids. h30%resin in MEK. J20% resin in MEK.

TABLE 2 w S o l u t i o n vinyls from Denki Kagaku. Composition, wt% Grade

Vinyl Chloride

Vinyl Acetate

1000A 1000AS IO00C 1000CS 1000GKT

86 86 86 86 91

14 14 13 13 3

TABLE 3 - - S u s p e n s i o n

vinyls for solution coatings--BASF.

Composition, wt% Grade

Vinyl Chloride

Vinyl Isobutyl Ether

Approximate Solution Viscosity," MPa.s

MP-25 MP-35 MP-45 MP-60

75 75 75 75

25 25 25 25

35 35 45 60

a20% resin solutions in toluene.

Formulation of Solution Coatings A typical vinyl coating usually consists of resin, solvent(s), plasticizer, pigments (required for exterior exposure), and o p t i o n a l i n g r e d i e n t s s u c h as s t a b i l i z e r s , m o d i f y i n g r e s i n ( s ) , and cross-linker(s). T h e r e s i n is n o r m a l l y s e l e c t e d o n t h e b a s i s of its a b i l i t y t o p r o v i d e a d h e s i o n to t h e s u h s t r a t e , W h i l e , i n m o s t cases,

Carboxylic Acid . . . . . . . . . . . . 1 1 ..-

Acrylic Ester

Inherent Viscosity

---.. 6

0.5 0.4 0.5 0.4 0.5

s t r o n g a d h e s i o n is d e s i r e d , t h e r e a r e s p e c i a l c o a t i n g s s u c h as s t r i p p a b l e o r p e e l a b l e c o a t i n g s w h e r e a d h e s i o n is n o t w a n t e d . T a b l e 6 lists t h e r e l a t i v e a d h e s i o n o f s e v e r a l v i n y l r e s i n s t o various polymers and substrates. The resin selection may be made on the need for reactive functionality to produce cross-linked coatings that change the nature of the coating from thermoplastic to thermosetlike properties that are characterized by improved solvent or stain resistance.

Solubility Variation in the ratio of vinyl chloride to vinyl ester and the d e g r e e o f p o l y m e r i z a t i o n p r o d u c e a w i d e r a n g e o f v i n y l polymers with different solubility characteristics. Highest solubility is f a v o r e d b y l o w v i n y l c h l o r i d e c o n t e n t a n d l o w m o l e c u l a r w e i g h t . T h i s r e l a t i o n s h i p i n t e r m s of v i s c o s i t y o f r e s i n s o l u t i o n s is c o m p a r e d i n T a b l e 1.

102

PAINT AND COATING TESTING MANUAL TABLE 4--Suspension vinyls for solution coatings--Wacker Chemie. Composition,wt% Vinyl Acetate Acid

Vinyl Chloride

Grade

Acrylic Ester

Inherent Viscosity

E 15/45 H 15/45 H 15/50

85 85 85

15 15 15

9.. 9.. 9..

0.5 0.5 0.6

H 40/43 H 40/50 H 40/55

60 60 60

40 40 40

9.. 9.. ...

0.46 0.6 0.7

H 40/60 E 15/45M H 15/45M

60 84 82

40 15 17

9-. 9.. 9..

0.88 0.5 0.56

E 15/40A E 20/55A E 22/48A

85 80 78

..... 7

15 20 15

0.4 0.68 0.56

TABLE 5--Suspension vinyls for solution coatings--Nissan.

Grade

Vinyl Chloride

MPR-TA MPR-TS MPR-TM

91 87 86

Composition,wt% Vinyl Vinyl Acetate Alcohol 3 13 13

Acid

6 ... . . . . . . ... 1

... 1 1

aromatic h y d r o c a r b o n s a n d m a k i n g up the difference with esters a n d aliphatic h y d r o c a r b o n s [13]. Though it was necessary to use more oxygenated solvents, the p e r f o r m a n c e of c o m p l i a n t coatings stayed the same. Typical solvent blends used for spray application a n d the reformulated c o m p l i a n t systems are shown i n Table 7.

S o l u t i o n Characteristics Vinyl solution resins are dissolved by ketones, esters, certain chlorinated solvents, a n d some nitroparaffins. As a class, ketones are the best solvents i n terms of the ability to dissolve higher solids at lower solution viscosity. Hydrocarbons are chiefly used as diluents primarily to lower cost. Aromatic a n d aliphatic h y d r o c a r b o n s c a n be usd as diluents. Aromatic hydrocarbons, chiefly toluene a n d xylene a n d higher boiling fractions such as Aromatic 100 or 150, are preferred as they can be used at high levels, in the range of 50 to 65% of the solvent b l e n d depending on the resin composition, molecular weight, a n d desired solids. The Aromatic 100 a n d 150 are usually used only in baking finishes. Aliphatic h y d r o c a r b o n s can be used in limited a m o u n t s , up to a b o u t 30% of the solvent blend. Higher levels can lead to viscosity instability, a n d only low boiling aliphatic hydrocarbons, those with boiling points up to 117~ are suitable. The use of higher boiling aliphatic h y d r o c a r b o n s can cause precipitation of the resin d u r i n g drying. Alcohols are strong precipitants for vinyls a n d are n o t generally used in unmodified vinyl lacquers. However, in some cases vinyls, usually hydroxyl-modified vinyls, are readily formulated with other resins that are carried in alcohol. With these, up to 15 to 20% alcohol m a y be used in the solvent blend. Careful attention m u s t be paid in vinyl coating formulations that c o n t a i n alcohols to be sure that problems do n o t develop d u r i n g application a n d drying of the coatings. Glycol ethers a n d glycol ether esters are sometimes used in vinyl coatings to improve flowout of baked coatings. I n response to regulations restricting the type a n d a m o u n t of solvents used in coatings, such as Los Angeles Rule 66 a n d later versions, vinyl coatings were reformulated as c o m p l i a n t systems by reducing the a m o u n t of b r a n c h e d ketones a n d

F r o m the time a vinyl resin is dissolved, the viscosity of solutions increases with time until a n e q u i l i b r i u m is reached after which the viscosity r e m a i n s constant. This behavior is believed due to the f o r m a t i o n of regions of microcrystallinity between polymer molecules in solution. The extent of the viscosity increase is d e p e n d e n t on: (a) resin molecular weight; (b) solids c o n t e n t of the solution; (c) a n d the strength of the solvent blend. The viscosity increase m a y be small or so large that the solution sets to a gel. Properly formulated vinyl resin solutions usually reach a n e q u i l i b r i u m viscosity i n a b o u t 3 to 5 weeks. Guidelines for the p r e p a r a t i o n of viscosity stable solutions for resins of varying molecular weight are s h o w n in Table 8. Vinyl chloride copolymer solutions also exhibit what is k n o w n as the m e m o r y effect. W h e n a vinyl resin solution is heated to a b o u t 60~ the effect of microcrystallinity is eliminated. If the solution is then cooled to its original temperature, the viscosity will n o t immediately r e t u r n to its original value because of the time lag needed for the effect of the microcrystallinity to redevelop. With time, the viscosity of the solution will r e t u r n to the same value as that of a solution that was not heated. The converse relationship hold w h e n vinyl solutions are cooled. A graphical p r e s e n t a t i o n of the m e m o r y effect is presented in Fig. 1.

Plasticizers Plasticizers are often used with vinyl resin coatings to improve flexibility, formability, a n d impact resistance of the coating. M o n o m e r i c as well as polymeric plasticizers or compatible polymers with low glass t r a n s i t i o n t e m p e r a t u r e (Tg) m a y be used to plasticize a vinyl coating.

CHAPTER 1 5 - - V I N Y L R E S I N S FOR COATINGS

103

TABLE 6RAdhesion of vinyl resins. Product Type

Substrate

Copolymer VYHH

Carboxy Modified VMCH

Hydroxy Modified VAGH VAGF

WB Vinyl AW-875

Steel Galvanized Paper (glassine) Aluminum foil Polyethylene, treated Polypropylene, treated Polysulfone Acrylic PVC plastic ABS plastic Polycarbonate Polyphenylene oxide Polyethylene terephtalate Impact polystryene Inked surface

0 0 10 0 0 0 7 10 I0 10 10 4 0 0 0

10 10 10 10 0 0 10 10 10 I0 10 I0 0 0 5

5 5 10 7 0 0 10 10 10 10 10 0 5 0 8-10

0 0 10 10 0 0 10 10 10 10 10 0 0 0 5

Rating: 10 = Pass Scotch Tape Test, no loss of adhesion. 5 = Some loss of adhesion, not recommended. 0 = No adhesion. TABLE 7--Typical solvent mix for spray-applied coatings, composition, wt%.

Memory

Effect

180

Compliant with Rule

Non-Regulated

Rule 66 Compliant

MIBK 50 MEK Toluene or Xylene _ 50 _ Butyl acetate 100% Cyclohexanone Toluene Xylene VM&P naphtha

66/EPA 33/50 Initiative ~

14 MEK 7 46 Acetone 3 9 MIAK 15 12 Butyl acetate 40 7 Cyclohexanone 9 1___22Toluene 6 100 VM&P naphtha 20

.g

g Cooled

~50% reduction of MEK and toluene, which are EPA 33/50 listed solvents.

Ambient Heated

TABLE 8--Guidelines for viscosity stable solution. Resin Molecular Weight • 10- 3

Resin

Maximum Solids

Typical Solvent Blend, wt%

15 25

MEK/toluene, 67/33 MIBK/toluene, 50/50

28

MIBK/toluene, 50/50

33

MIBK/toluene, 33/67

Pigmentation

60

MEK/toluene, 10/90

Vinyl coatings are p i g m e n t e d (1) to achieve the desired color, a n d (2) to prevent degradation of the vinyl resin from the effect of ultraviolet light for coatings that are i n t e n d e d for use outdoors [14]. Most organic a n d inorganic pigments can be used. However, basic pigments m u s t be avoided with carboxyl-modified resins, as these pigments can react to form irreversible gel. P r i m e or color pigments which absorb UV r a d i a t i o n m u s t be used at a level sufficient to protect the vinyl resin. Extender pigments or fillers do n o t absorb UV r a d i a t i o n a n d can only he used in c o m b i n a t i o n with a UV absorbing pigment. For t i t a n i u m dioxide (TiO2) pigments, a m i n i m u m level of a b o u t 75 p h r is needed to provide resistance to UV light. Other inorganic pigments can be used to replace TiO2 by substituting o n a n equal volume basis. Organic p i g m e n t s that are

I

44 27-34 19-22 15 5.5

VYNS-3 VAGF, VAGH, VMCH, VYHH VAGC, VAGD, VMCC, VYHD VERR, VMCA, VROH VYES-4

Phthalate, phosphate, a n d glycol ester plasticizers are typically used. Plasticizers are selected to meet the r e q u i r e m e n t s of the coating that m a y include low-temperature flexibility, resistance to extraction by solvents, resistance to migration, to humidity, etc. Blends of plasticizers m a y be required to meet specific requirements. Table 9 presents a listing of plasticizers that are c o m m o n l y used with vinyl resins. Care m u s t be exercised in choosing the level of plasticizer as excessive a m o u n t s tend to make the film soft a n d p r o n e to dirt retention. Ordinarily, a level of 25 p h r (parts per h u n d r e d parts resin) of plasticizer is considered a b o u t m a x i m u m for use with coating resins.

0

/

1

I

I

I

2

3

4

5

Weeks FIG. 1 - M e m o r y effect,

104 PAINT AND COATING TESTING MANUAL TABLE 9--Typical plasticizers compatible with solution vinyls. Phthalates Butyl benzyl phthalzate (BBP) Di-2-ethylhexyl phthalate (DOP) Diisooctyl phthalate (DIOP) Diisononyl phthalate (DINP) Diisodecyl phthalate (DIDP)

Linear Dibasic Acid Esters Di-n-butyl sebacate (DBS) Di-2-ethylhexyl adipate (DOA) Diisononyl adipate (DINA) Di-2-ethylhexyl azelate (DOZ) Phosphates

Citrates Acetyl trim-butyl citrate Epoxies Epoxidized soybean oil (ESO) 2-Ethylhexyl epoxytallate

manufactured to smaller particle size are used at a lower concentration, and blends of inorganic and organic pigments are often used to achieve the desired color. Excessive high loading of pigments can lead to early chalking.

Organosols and Plastisols A plastisol is a dispersion of discreet particles of highmolecular-weight vinyl homopolymer resin in plasticizer, with a low level of heat stabilizers sufficient to prevent degradation during baking (fusing). Plastisols normally require a minimum amount of about 55 to 60 parts plasticizer per hundred parts of resin to form a fluid mix. The viscosity of the dispersion is dependent on packing effects, the volume of dispersed resin relative to the volume of liquid plasticizer, the size and shape of the suspended particles, solvating or swelling effect of the plasticizer on the resin particles, and the viscosity of the liquid plasticizer. The relatively high levels of plasticizer needed to produce a flowable liquid mix results in the formation of fused films too soft for use as coatings. Plastisol coatings are usually formulated with the addition of coarser particle-size PVC resins called extenders from suspension or bulk (mass) polymerization that allow the use of less plasticizer and thus harder films. Additionally, small amounts of thinner, usually aliphatic hydrocarbon, are used (up to about 10 wt%) to reduce viscosity and provide better flow and leveling of the plastisol coating. Plastisol coatings do not adhere well to most substrates and most often require the use of a suitable primer. An organosol differs from a plastisol in that much lower levels of plasticizer are used. A combination of weak solvents called dispersants in combination with hydrocarbon solvents called diluents are used to provide sufficient fluid to make a fluid dispersion. Because lower levels of plasticizer are used, films with much greater hardness can be obtained. Commercial organosols are usually modified with a solvent-soluble resin to prevent mud cracking or film splitting during the bake to fuse the film. The modifier resin may contain carboxyl groups to make self-adherent coatings, or it may be a hydroxyl containing resin to provide functionality to react with cross-linkers such as amino or phenol/formaldehyde resins to achieve a degree of thermoset properties. Though vinyl copolymers are usually the modifier of choice for organosols, other polymers such as acrylic polymers may be used.

Tri-2-ethylhexyl phosphate (TOP) Isodecyl diphenyl phosphate Polymerics Adipic acid polyesters Azelaic acid polyesters

Careful consideration must be given to the selection of the solvent/diluent mix for organosols to attain the highest solids coupled with good viscosity stability. Commercial organosols of 50 to 55% nonvolatile by weight are typical. Plastisols and organosols require a high baking temperature of about 350~ (177~ to fuse the films. At elevated temperature, the plasticizer or plasticizer diluent mix exerts a strong solvating or swelling effect on the dispersed PVC resin particles. At fusion, the resin no longer exists as discreet particles, but rather as a continuous, homogeneous film. Films of plastisols or organosols need only to reach fusion temperature and do not have to be held at the fusion temperature for a long time period. Undercuring or baking at temperatures lower than that required for fusion will yield films deficient in tensile strength, elongation, abrasion resistance, and all other properties. Plastisols and organosols also require the use of heat stabilizers to protect the vinyl resin against degradation during the fusion bake. Heat stabilizers are usually combinations of metal salts of organic acids in combination with epoxidized oils or liquid epoxy resins. Special attention must be given to the selection of heat stabilizers for organosols modified with solvent-soluble resin, especially when carboxyl-modified polymers are used. In such cases, the metallic salts must be avoided as these will usually cause gellation; typically, mercapto tin or tin ester compounds are used in combination with an epoxy stabilizer. The type of pigment and level of pigment used in pigmented organosols follow the guidelines given for solution vinyl resins. It is, however, more difficult to prepare pigmented plastisols because there is generally little solvent used to control viscosity. Low oil absorption pigments must be used to avoid excessively high viscosity.

Primers for Plastisols and Organosols Plastisol coatings need a primer to develop good adhesion to metal substrates. An organosol coating may also require a primer if it is not modified with an adhesion-promoting modifier. Suitable primers can be formulated from carboxyl-modified vinyl resins and may require the use of thermoset resins such as amino-formaldehyde or phenolic resins to provide

CHAPTER 15--VINYL RESINS FOR COATINGS resistance to excessive softening from highly plasticized plastisol or organosol coatings.

MAJOR MARKET AREAS FOR VINYL COATINGS R i g i d Packaging

Liners for Interior Surface Coatings, Cans, Can Ends, Closure~Caps and Crowns The first commercial use for vinyl coatings was as the topcoat lacquer for the inside of beer cans. As beverage cans evolved from three- to two-piece construction, the vinyl coating also changed from lacquer to hydroxy vinyl/amino-formaldehyde thermosets to meet the need for higher corrosion resistance. Thermoset coatings of epoxy-modified vinyl resin with carboxyl-modified vinyl resin are used to coat on coil stock. The coated coil stock is then formed into the stay-on tab can ends, an application that requires excellent mechanical properties to withstand the forming steps without cracking. Organosol coatings containing a solution resin component, usually carboxyl-type for adhesion, have also been used on precoated stock for can ends. Vinyl organosols are further modified with amino-formaldehyde or phenolic resins to upgrade chemical resistance and permit the use of such coatings for packaging food that will be autoclaved to sterilize the contents [15]. Vinyl lacquer and vinyl thermoset coatings are used as size coats for metals that are formed in caps and closures for jars or as crowns for beverage bottles. These systems serve as the primer coat for gasketing compounds made with plastisol or vinyl resin dry blends.

Flexible Packaging Solvent-soluble carboxyl-modified vinyl chloride copolymers have good adhesion to most materials used in flexible or semi-rigid packaging including aluminum foil, paper and plastic films such as polyethylene terephthalate, polycarbonate, PVC, and cellophane. This type of resin is used for its adhesion characteristic, ease of heat sealing, and resistance to attack by the packaged product. The vinyl resin may be used alone or modified with plasticizers or other resins and polymers to formulate heat-sealable coatings for applications requiring varying degrees of force needed to open the container. This could range from applications such as blister packaging where the bond needs to be strong enough to cause substrate failure when the package is opened, to items such as jellies or cream containers found in restaurants where a tight but readily peelable bond is required. Vinyl coatings are also used to coat collapsible metal tubes for packaging materials such as pharmaceutical preparations, toothpastes, and the like where the need is for a very flexible coating that will not crack nor be attacked by the contents of the package even though high stresses from collapsing and rolling up the tube are encountered. Other applications include decorative coatings for the aluminum foil paper laminates for cigarette packaging, food wrappers for fast food restaurant items, for butter, marga-

105

rine, soups, and so on. Decorative foil for floral wrappings, decorative labels, and coatings for aluminum foil for the vapor barrier insulation for construction applications are also coated with vinyl resin coatings. Inks The major markets for vinyl inks are on vinyl surface products such as floor and wall coverings, swimming pool liners, vinyl upholstery, and garment fabrics. Ink formulation is quite similar to that used with coatings except solvent choices are somewhat narrowed and higher pigment loadings are needed to achieve hiding in the thin films typical of inks. Vinyl inks are often reverse printed on a clear vinyl film, and the printed film is then laminated to substrates such as wood or metal to make articles having simulated wood finish. Vinyl inks are printed by gravure or screen process because these presses are compatible with the strong solvents needed for vinyls; flexographic printing is not suitable for vinyls because the plates are susceptible to solvent attack. Inks for highly plasticized vinyl surfaces are usually formulated with ester solvents to avoid excessive softening of calendered films and puckering of the films.

Dry Film Printing (Hot Stamp Transfer) In this application, vinyl inks are printed on a carrier sheet such as polyethylene terephthalate, polyethylene, polypropylene, or other suitable surfaces to which the ink will not adhere strongly. The inks are applied and dried usually in web form. When ready for use, the printed carrier film is placed with the inked side on the surface to be decorated. A heated die presses the composite to make intimate contact with the surface, so that when the die is removed, the ink is firmly bonded to the substrate and the carrier is peeled away cleanly.

Maintenance and Marine Finishes Heavy duty marine finishes were developed in the mid1940s. These systems consisted of a poly(vinyl butyral) wash primer, vinyl-red lead anticorrosive intermediate coatings [based on poly(vinyl alcohol)-modified resin needed for adhesion to wash primer], and vinyl copolymer/wood rosin/ cuprous oxide anti-foul top coats. This system has become the subject of numerous specifications; many U.S. Government agencies and agencies of other governments have written specifications with this coating system specified for use below the waterline of ships. Because of their good water resistance, good weathering qualities, flexibility, fast dry and ease of application, and repair, vinyls quickly became established as maintenance finishes. This area includes coatings for locks, dams, appurtenant structures for waterways, interior linings for potable water tanks, steel structures such as bridges, electrical towers, equipment in chemical plants, and the like. Many specifications have been written that require the use of vinyls as maintenance paints [16,17]. The early vinyl maintenance and marine finishes were applied by air atomizing spray guns at low solids. Several coats were needed to attain coverage sufficient for good corrosion

106

PAINT AND COATING TESTING MANUAL

protection. High-build airless spray-applied vinyl coatings were developed in the 1970s to fill the need for coatings systems that could be applied in fewer coats at less expense [18].

Wood Finishes Reactive heavy duty vinyl finishes for wood have been developed consisting of a hydroxyl-modified vinyl resin crosslinked with amino/formaldehyde resins. Alkyd resins were often added to improve film build. Such finishes became established as the standard for kitchen cabinets because of their retention of excellent adhesion and water resistance, particularly when the coated wood becomes wet from high humidity or water splashing. These finishes also have excellent resistance to a variety of household chemicals, solvents, and stains and have been used as fine furniture finishes [19].

Magnetic Recording Media Vinyls, especially hydroxy-modified vinyls, have been used as binders for magnetic iron oxide tapes since the beginning of the development of tape recording. The vinyl resins are used because of their good adhesion, abrasion resistance, and good pigment wetting properties. The early binder formulations used alkyd resin as plasticizers, then polyesters; currently, polyurethane resins are used as the plasticizer as the technology of tapes advanced and placed more stringent requirements on the performance of magnetic tape for audio and video [20].

Powder Coatings Vinyl powder coatings are formulated with vinyl chloride homopolymers and copolymers for application by fluidized bed, powder spray, or electrostatic powder spray. Powder coatings are prepared by dry compounding resins, plasticizer, pigments, and additives in ribbon blenders followed by attrition or dispersion to powder in mixers such as the Henschel mixer. Some powder coatings are prepared by a melt mix technique followed by cryogenic grinding. This latter technique produces powders of smaller particle size [21]. Powder coatings prepared by dry compounding are usually applied by fluidized bed or by spray techniques. The metal parts are heated for fluid bed application so that the powder will adhere to the part and begin to flow to form a film. A bake after the powder application is needed to complete the filmforming process by fusion or melting. Cryogenically ground powder coatings are applied by electrostatic powder spray. With the electrostatic method, it is not necessary to preheat the parts, but a bake is necessary after application to fuse the powder to a film. The finer particle size allows the deposition of smoother and thinner films than is attainable from fluidized bed or powder spray process. However, the high costs of cryogenic grinding made these materials substantially more expensive than dry grinding and as a consequence, the cryogenic ground powders account for only a small share of the PVC powder-coating market. PVC powder coatings are used to coat products such as pipe, fencing, and metal furniture.

PVC Latex Emulsion polymerized vinyl chloride homopolymers and copolymers are used in the latex form not so much to make finished coatings but rather as material coated on a base or support to provide the substrate for items such as wall coverings, backing for carpeting, and the like. In a sense, such use could be considered analogous to a waterborne version of an organosol coating. The vinyl chloride homopolymers need to be modified with a substantial loading of plasticizer, and some grades are sold as preplasticized latexes. These waterbased materials require a high temperature bake to fuse the resin plasticizer mix into a continuous film. By varying the type and amount of comonomer used to make emulsion polymerized copolymer latexes, lower Tg products are available that can use lower temperature bakes to form films.

Waterborne Vinyl Dispersions Waterborne vinyl dispersions made from solution-polymerized vinyl copolymers became available in the 1980s. These waterborne vinyl dispersions are of medium molecular weight and have high Tg, about 80~ Coalescents are needed with these products to form a film. Some dispersions are available with a glycol ether coalescent already present in the product, and a co-solvent free variety is also available. With the latter, the formulator can choose whichever coalescent, glycol-ether, glycol-ether ester, plasticizer, or blend of coalescents that best meets performance requirements. A line of waterborne vinyl dispersions is shown in Table 10. Waterborne vinyl dispersions are used in many ink, coating, and heat-sealable coating applications where solventbased vinyl coatings had been used.

Trends i n V i n y l Coatings To meet the VOC requirements that are either in place or proposed for the future, developments in vinyl coatings have centered on high solids and waterborne systems. For high solids vinyl coatings, substantially increased resin solubility was achieved by reduction in the polymer molecular weight, so that viscosity stable solutions could be prepared at two to three times the level of solids content that was possible with earlier vinyl resins. However, at the low molecular weights needed for high solubility, the performance of coatings made from such resins was greatly reduced in terms of chemical resistance and physical properties. As a result, high solids vinyl resins are modified to contain hydroxyl functionality to allow for reaction with added coreactant materials to build molecular weight. Though the high solids resins may be used alone for less demanding applications, they are

TABLE 10--A line of waterborne vinyl resins dispersions. Grade AW-850 AW-875

Composition, wt% Solids Water~ 38 39

aContains less than 2% amines. bEthylene glycolmonobutylether.

50 61

Cosolventb

pH

12 ..-

7.0 7.0

CHAPTER 1 5 - - V I N Y L R E S I N S FOR COATINGS b e s t used either as a reactive system, with a m i n o - f o r m a l d e h y d e o r isocyanate cross-linker, o r as modifiers for alkyds, polyester-isocyanate, o r e p o x y - a m i n e coatings to i m p r o v e initial drying o r set-to-touch rate, or to improve r e c o a t a b i l i t y [21]. The waterborne vinyl dispersions previously described represent an alternative to high solids vinyls as a way to formulate low VOC coatings. The waterborne vinyls are compatible with a wide variety of other waterborne resins with low VOC content such as acrylics, alkyds, urethanes, and aminoformaldehyde cross-linkers.

REFERENCES [1] Industrial and Engineering Chemistry, Vol. 25, No. 6, June 1933. [2] Myers, R. and Long, J. S., Eds., Treatise on Coatings, Film Forming Compositions, Vol. 1, Part II, Dekker, New York, 1968. [3] Powell, G. M., Federation Series on Coatings Technology, Unit 19, Federation of Societies for Paint Technology, Philadelphia, April 1972. [4] Breziuski, J.J., Koleske, J.V., and Potter, G.H., "Hydrodynamic Properties of Vinyl Chloride-Vinyl Acetate Copolymers in Dilute and Concentrated Solutions," Proceedings of X1 Congress FATIPEC, Florence, Italy, 1972. [5] Paint Testing Manual, ASTM STP 500, 13th ed., G. G. Sward, Ed., ASTM, Philadelphia, 1972. [6] Crompton, T. R., Analysis of Plastics, Pergamon Press, New York, 1984. [7] Infrared Spectra Atlas of Monomers and Polymers, Sadtler Research Labs, Philadelphia, 1980. [8] Burley, R. A. and Bennett, W. J., "Spectroscopic Analysis of Poly(Vinyl Chloride) Compounds," Applied Spectroscopy, APSPA, Vol. 14, 1960, p. 32.

107

[9] An Infrared Spectroscopy Atlas for the Coatings Industry, 4th ed., Vols. I and II, D. R. Brezinski, Ed., Federation of Societies for Coating Technology, Philadelphia, 1991. [10]ASTM Guide for Testing Poly(Vinyl Chloride) Resins (D 4368-89), ASTM Book of Standards, Vol. 06.03, American Society for Testing and Materials, Philadelphia, 1993. [11] ASTM Test Method for Infrared Identification of Vehicle Solids from Solvent-Reducible Paints (D 2621-87), ASTM Book of Standards, Vol. 06.01, American Society for Testing and Materials, Philadelphia, 1993. [12] ASTM Test Method for Analysis of Components in Poly(Vinyl Chloride) Compounds Using an Infrared Spectrophotometric Technique [D 2124-70 (1988)], ASTM Book of Standards, Vol. 08.01, American Society for Testing and Materials, Philadelphia, 1993. [13] Burns, R. J. and McKenna, L. A., Paint and Varnish Production, February 1972. [14] Hardman, D. E. and Brezinski, J. J., "Pigmented Vinyl Copolymer Coatings: A Discussion of Factors Influencing Exterior Durability," Official Digest, Vol. 36, No. 476, 1964, pp. 963-984. [15] Good, R. H., ACS Symposium Series 365, American Chemical Society, Washington, DC, 1988, pp. 203-216. [16] Corps of Engineers CW-099040, U.S. Department of the Army, August 1981. [17] Steel Structures Painting Council, Pittsburgh, PA, Paint No. SSPC-9. [18] Martell, R. J. and Yee, A., Journal of Protective Coatings and Linings, Vol. 5, No. 9, September 1988. [19] Mayer, W. P., Journal of the Oil and Colour Chemists Association, Vol. 73, No. 4, April 1990. [20] Kreiselmaier, K.W., "Pigmentation of Magnetic Tapes," Pigment Handbook, Vol. Ill: Applications and Markets, T. C. Patton, Ed., John Wiley & Sons, New York, 1973. [21] Ginsberg, T., "Vinyl-Modified Epoxy Coatings," Modern Paint and Coatings, November 1988.

MNL17-EB/Jun. 1995

16

Miscellaneous Materials and Coatings by Joseph V. Koleske 1

THIS CHAPTER IS CONCERNED WITH A VARIETY of products that

HO--[CH(CH3)--CH2~L - O - R - O - [ C H 2 - C H ( C H 3 ) ] b - O H

are not discussed elsewhere in the manual. Some topics are mentioned only briefly to indicate that the area has not been forgotten and that the topic is not within the scope of the manual.

Poly(propylene oxide) Polyol H--[O(CH2)40--CO(CH2)4CO].--O(CH2)40-[CO(CH2)4CO--O(CH2)40]v--H

Poly(1,4-butanediol adipate) Polyester Polyol H--[O(CH2)5CO]s--O--R'--O--[CO(CH2)sO]t--H Poly+caprolactone Polyol

POLYOLS Polyols, or polyalcohols as they are sometimes known, are compounds containing one or more, but usually two or more, free hydroxyl groups. Most definitions, and particularly those over ten years old, list typical polyols as compounds such as ethylene glycol, propylene glycol, neopentyl glycol, glycerol or glycerin, trimethylolpropane, pentaerythritol, and sorbitol that were used in the preparation of alkyds and polyesters. Today the word "polyols" is far more encompassing and more often than not refers to alkylene oxide [1 ] and E-caprolactone [2] adducts of the above-mentioned and other monohydric or polyhydric alcohols, low-molecular-weight polyesters prepared from the above mentioned as well as other polyhydroxyl compounds and dicarboxylic acids (particularly adipic acid) [3-5], polytetrahydrofurans prepared by a cationic ring-opening polymerization of tetrahydrofuran [6, 7], and low-molecular-weight polycarbonates [8-10]. There are other compounds that meet the above definition, but they are not usually termed polyols. Compounds such as these are certain vinyl chloride copolymers, hydroxyl-containing glycidyl ether compounds, vinyl alcohol copolymers, and so on. This chapter will not be concerned with these latter compounds since they are dealt with elsewhere in the manual. Polyols are important compounds used in the manufacture of alkyds and polyurethane coatings, of intermediates used in radiation curable formulations, as copolymerizable ingredients in high solids and cationic photocure systems, as well as in a number of other end uses including elastomeric fibers, dentistry, artifact preservation, and pharmaceutical preparations. The two main classes of polyols used in coatings are the polyether polyols, which are typified by the poly(propylene oxide) polyols (PPO), and the polyester polyols, which include both poly(glycol adipates) (PEA) and poly+caprolactone polyols (PCP). Both classes of polyols are available as difunctional and

trifunctional hydroxyl compounds though the adipates are almost always difunctional in nature. Higher functional polyols are known and available, but their usage is less common than that of the di- and trifunctional products. In the above structural formulas, R and R' may be the same or different and - - O - - R - - O - - and - - O - - R ' - - O - - are the residues of the polyhydric alcohol initiators. Difunctional and trifunctional PPOs are usually initiated with 1,2-propylene glycol and glycerol, respectively. The adipate polyols are usually prepared with an excess of diol, so most end groups are hydroxylic rather than carboxylic in nature. Since these polyols are prepared by a condensation reaction, there is no need for an initiator. Caprolactone polyols are initiated with a variety of diols and triols such as diethylene glycol, ethylene glycol, 1,4-butanediol, trimethylol propane, glycerol, etc. The above structure for PPO indicates that the hydroxyl groups are both secondary, which is the usual case. However, from time to time a primary hydroxyl group will be found due to an unexpected opening of the propagating 1,2-epoxide. The subscripts a, b, u, v, s, and t in the above structural formulas can be the same or different, and they can take on a wide variety of values with the number average molecular weight ranging from about 150 to 3000 for polyols usually used in coatings. Details about preparation of urethane coatings based on polycaprolactone polyols for rigid substrates [11] and flexible substrates [12] are available. A variety of other specialty polyols also exist such as poly(butylene oxide) and polybutadiene polyols, which are useful when very high levels of barrier hydrophobicity are needed [13]. Poly(tetramethylene oxide) polyols also have good hydrophobic character. New polyols are also being developed, including polyols based on lactose that have flameretardant characteristics as well as polyols with different end capping, etc. [14]. Although new polyols such as these are often designed, for use in the manufacture of polyurethane foams and elastomers, they can be and are used in coating formulations.

~Senior consultant, Consolidated Research, Inc., 1513 Brentwood

Road, Charleston, WV 25314-2307.

108 Copyright9 1995 by ASTM International

www.astm.org

CHAPTER 1 6 - - M I S C E L L A N E O U S End capping polyols can provide adducts with different properties. For example, poly(propylene oxide) polyols which contain terminal secondary hydroxyl groups can be end capped with ethylene oxide to provide polyols with more reactive primary hydroxyl groups [1, 7]. Ways to apply nuclear magnetic resonance to measure the ethylene oxide content of these and other propylene oxide/ethylene oxide copolymers is detailed in ASTM Test Methods of Polyurethane Raw Materials: Determination of the Polymerized Ethylene Oxide Content of Polyether Polyols (D 4875). Also described in the literature [1, 7] are polyols modified to have amine, allyl, carboxyl, cyano, and vinyl ether end groups. Glycols that are solid and/or that have subliming characteristics, as 2,2'-dimethyl3-hydroxypropyl 2,2'dimethyl-3-hydroxypropionate, can be modified with a few ethylene or propylene oxide groups to yield new polyols that are liquid, have low viscosity, and do not sublime with even a few molecules of ethylene oxide having nil or very little effect on moisture resistance [15]. Polyols can be end capped with an anhydride to form adducts that have free carboxylic acid functionality or a mixture of it and hydroxyl functionality as has been done with the poly-~caprolactone polyols [I 6] or the alkylene oxide capped glycols [17]. In other instances, poly(propylene oxide) polyols have had carboxyl groups grafted to their backbone with acrylic or methacrylic acid. These grafted polyols retain their original hydroxyl end groups and are used in coating formulations

[18]. Polyols can be incorporated into alkyds, made into moisture-curing urethanes, can be cross linked with aminoplasts, and can be cross linked with cycloaliphatic epoxides when terminated with carboxylic acid end groups. In using the polyols, the hydroxyl number [19] is their most important physical characteristic to be measured and used. Five wet chemical methods and two nuclear magnetic resonance methods for determining the hydroxyl number are given in ASTM Method for Testing Polyurethane Polyol Raw Materials: Determination of Hydroxyl Numbers of Polyols (D 4274) and in ASTM Method for Testing Polyurethane Raw Materials: Determination of Primary Hydroxyl Contents of Polyether Polyols (D 4273), respectively. The equivalent weight or combining weight of a polyol is determined from the hydroxyl number by the following relationship Equivalent Weight = 56 100/Hydroxyl Number when potassium hydroxide is used as the titrating agent. Of course, if functionality is known, polyol molecular weight can be calculated by multiplying the equivalent weight by the functionality. Manufacturers provide information about hydroxyl number and usually about methods for analytically determining it. Another important reactivity parameter is the acid number described in ASTM Test Method for Polyurethane Raw Materials: Determination of Acid and Alkalinity Numbers of Polyols (D 4662). Acidity and alkalinity in polyols can affect reactivity, shelf life, color, and hydrolytic stability of coatings prepared from polyols. Polyethers and poly-ecaprolactone polyols usually have very low acid numbers. However, due to the nature of the condensation reaction coupled with transesterification used to produce polyester polyols, these polyols have relatively high acid numbers. Color, which has obvious implications, can be determined with ASTM Test Method for

109

Polyurethane Raw Materials: Determination of Gardner and APHA Color of Polyols (D 4890).

CYCLOALIPHATIC E P O X l D E S Although the topic of epoxides in coatings is the subject of a separate chapter in this manual, that chapter deals with glycidyl or 1,2-epoxides that are not attached to a ring structure. Such epoxides are the largest volume products of all epoxides used, and the main products in this class are the diglycidyl ethers of bisphenol A. However, there is a special class of epoxides, termed "cycloaliphatic epoxides," that are used in specialty coatings and in cationic radiation-cure coatings. These epoxides are characterized by a saturated ring structure that imparts a high degree of weatherability and excellent electrical properties such as dielectric constant, dissipation factor, dielectric breakdown voltage, etc., to coatings and other products made from them. The good weatherability of the cycloaliphatic epoxides is apparent from the fact that they have been used for decades to make the large electrical insulators used in substations [20]. These compounds react well with carboxylic acids, as evidenced by their time-honored use as acid scavengers, and this reactivity often forms the basis for their use in coating formulations. The main commercial cycloaliphatic epoxide is 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate that has the structure H

\/

H O

/c\ H/

II

k/

H

/c\ /H \/c\ / \ H

\/c\ / H

H

H

H

H

This epoxide is well known by the familiar name designation ERL-4221. Table 1 contains the properties of this epoxide and other cycloaliphatic epoxides that are commonly known in the industry. Epoxide equivalent weight can be determined with ASTM Test Methods for Epoxy Content of Epoxy Resins (D 1652). Manufacturers can also be helpful in supplying information about methods of analysis for specific products. Usually these epoxides are reacted with polyols that function as flexibilizing agents for the highly cross-linked polymeric network that results. These epoxides polymerize by nucleophilic attack on the epoxide ring to form an ether linkage and a hydroxyl group on the ring. The hydroxyl group that is formed on the ring is quite acidic in character andwill readily open other cycloaliphatic epoxide groups. In the coatings industry, cycloaliphatic epoxides are used as a major formulating ingredient in cationic, photocurable formulations [22]. Usually they are formulated with polyols, onium-salt photoinitiators, and other ingredients. The onium salts photolyze in the presence of ultraviolet light to form strong protic acids that cause rapid polymerization of the epoxides as well as their copolymerization with active hydrogen compounds such as polyols. The presence of alkalinity including even very weak bases can result in neutralization of the protic acids formed by photolysis. Since the protic acids

110

PAINT AND COATING TESTING MANUAL TABLE l--Commercial cycloaliphatic epoxides and their physical properties [20,21]. Viscosity, cP at 25~

Specific Gravity, 25/25~

Color, 1933 Gardner (max)

Epoxide Equivalent Weight

Boiling Point, ~ (mm Hg)

3,4-Epoxycyclohexylmethyl3,4epoxycyclohexane carboxylate

3"50-450

1.175

1

131-143

Bis(3,4-epoxycyclohexylmethyl)adipate

550-750

1.15

1

2-(3,4-Epoxycyclohexyl-5,5-spiro-3,4epoxy)cyclohexane-m-dioxane

7000-17 000 at 38~

1.18

1-Vinyl-epoxy-3,4-epoxycyclohexane

10 2-74

Passing through minimum opening

Microscope Optical Electron

0.5-500 0.002-15

Martin's, Feret's, or equivalent circle diameter

Zone sensing Resistivity Optical

0.05-500

Volume or area

Gas Permeability Gas Adsorption Brownian Motion Sieving Dry Wet

1-500

Elutriation Laminar flow Cyclone

3-75 8-50

Equivalent spherical diameter (E.S.D.)

0.5-50

E.S.D.

Mercury intrusion

0.01-200

E.S.D.

Centrifugal sedimentation Mass accumulation Photo extinction X-ray

0.05-25 0.5-100 0.01-5

Gravity sedimentation Pipettes & hydrometers Photo extinction X-ray

1-100 0.05-100 0.1-130

Centrifugal classification

E.S.D.

Hydrodynamic chromatography Packed column Capillary column

0.03-2

Cascade impactor

0.05-30

Aerodynamic diameter

It is often useful to use the a p p r o x i m a t i o n of size from surfaces to evaluate o t h e r p a r a m e t e r s derivable f r o m the volu m e function, such as specific surface a r e a a n d density. The b a s i c equations that describe a sphere's surface a r e a a n d volu m e are given by

,t---.-- Martin's Diameter

n ~ 2.'%':5.,~:.~:x-.',::,~

Equivalent "~-Diameter'-~ Feret's Diameter

E.S.D,

0.1-60

Particle Size From Surface Area

DIAMETER DEFINITIONS

|-

E.S.D.

=1

FIG. 5-The relationship of some stated diameters and the physical size of the particle.

Area of a sphere = ~D 2

(4)

Volume of a sphere = ~rD3/6

(5)

These relationships alIow d e t e r m i n a t i o n of the m e a n particle d i a m e t e r if the specific surface (area) is known, Particlesize c h a r a c t e r i z a t i o n s t h a t e m p l o y surface a r e a techniques in the size d e t e r m i n a t i o n yield average diameters. The surface a r e a of a p o w d e r m a y be d e t e r m i n e d from gas a d s o r p t i o n experiments. By a s s u m i n g uniform-size n o n p o r o u s a n d spherical particles, the m e a n particle size m a y b e d e t e r m i n e d from the surface a r e a data. The F i s h e r subsieve sizer is an example of an i n s t r u m e n t of this type (Fig, 6). The "specific surface" o b t a i n e d equals the d e t e r m i n e d surface a r e a divided b y the p r o d u c t of the particle v o l u m e and density. W i t h app r o p r i a t e units conversion, D = 6/[oA]

(6)

312

PAINT AND COATING TESTING MANUAL

1. Perspective of Operating Principles FIG. 6-The Fisher subsieve sizer. (Compliments of Fisher Scientific.) where D = particle diameter, ~m, p = particle density, g/cm 3, and A = specific surface area, m2/g. Any shape other than a sphere, either regular or irregular, has a greater ratio of surface to volume, and consequently the particles will be smaller than calculated. Cracks, crevices, and pores are other factors leading to an increase in specific surface area and to a corresponding decrease in calculated particle size.

Perfectly spherical particles are seldom encountered in mineral pigments and fillers used in paint and coating applications since these particles result from shattering by crushers and high-speed pulverizers. Crystalline materials fracture along crystal planes, while some pigments are acicular (like needles). With the many shapes of particles to be evaluated and characterized, instruments that operate on different principles "see" different values of the size-dependent variable being monitored. As a result, a sample characterized by one machine may yield a somewhat different value when

CHAPTER 32--PARTICLE-SIZE MEASUREMENTS evaluated using a different technique. For this reason, the practitioner must be aware of the size-dependent variable to which his machine is sensitive. Methods [17] for determining specific surface include adsorption of gases by the pigment, adsorption of solutes from solutions by the pigment, and permeability of pressed pigment particles to gases or liquids.

313

Silica ~--Cap Helix~,.~ ~=~., "--': ITo~--~ I'I ] ~Water JaCv::~um

Adsorption of Gases This method is based on determining the number of gas molecules that will adsorb as a monomolecular layer on the surface of the particles. Knowing the area occupied by one molecule and the number of molecules, the specific surface area of a specimen of pigment may be calculated from

s where E = n = N = A --

=

nNA

11 ~

(7)

specific surface area in cm 2, number of moles of gas per gram of pigment, Avogadro's number, 6.02 • 1023, and area covered by each molecule.

Knowing the specific surface, the surface mean diameter may be calculated. Brunauer-Emmett-Teller (BET) Method--The general procedure for this method [18,19] includes introducing the specimen into an enclosed space of known volume followed by degassing with heat and evacuation. A known amount of gas is admitted to the system, and its pressure is determined. The gas is then compressed to a new volume, and the corresponding pressure is determined. The procedure is repeated until enough data for the preparation are obtained. By means of the BET equations, the surface area is found, and from it the average particle size may be calculated. Continuous Flow Method--This method makes use of the BET equation, but the adsorbed gas is determined by concentration measurements in a continuous flow system [20]. Nitrogen gas is adsorbed by the specimen at liquid nitrogen temperature from a stream of nitrogen and helium gas and is later eluted by warming the specimen. The amount of nitrogen liberated is estimated by thermal conductivity. For detecting changes in the amount of adsorbed nitrogen, Beresford et al. [21] used the Perkin-Elmer-Shell sorptometer with conductivity bridge and potentiometric recorder. Gravimetric Method--In addition to the volumetric methods, a gravimetric method using balances patterned after that of McBain and Bahr [22] can be used. The procedure is much simpler but requires care as the apparatus is very fragile and the changes in weight are very small. An essential part (Fig. 7) of the gravimetric method is a fused silica spring balance from which a small glass "bucket" for the specimen is hung. The balance assembly is located in a glass housing connected to a source of gas and to a manometer for determining the pressure. The bucket is hung on the balance, the housing is closed, and the specimen is degassed by evacuation. The zero point of the balance is recorded. Gas is admitted to the housing, and the change in weight of the specimen is determined by noting the change in the elongation of the balance with a traveling microscope. Details are set forth in Ref 17.

Sample

Mercury Manometer

FIG. 7-The Components of a basic gravimetric adsorption apparatus.

Adsorption of Nongaseous Molecules Adsorption of molecular species other than typical gaseous molecules is possible also. These include the adsorption of solvents and surfactants in special cases. Adsorption of Solutes onto Pigments--This method depends on the adsorption of solute molecules onto a particle when it is wet out by a solution containing the dissolved surface active solute [23]. The process requires that the adsorbent be thermodynamically more active than the solvent and the kinetics be such that wet-out times be reasonable. Typical systems used are stearic acid in benzene, dyes in benzene, and wetting agents in water. The amount adsorbed will asymptotically approach a constant value with time. At this value it is assumed that the surface of the particles is covered with a mono-molecular layer of solute. The area occupied by a single molecule of solute may be obtained from tests made with solids of known areas. The amount adsorbed is obtained by analysis of the solution before and after adsorption. A disadvantage of the method is uncertainty about the size and orientation of certain types of molecules. On the other hand, the method is rapid and easy to perform. For this reason it is suited to routine tests where its reliability has been established. Soap Titration of Emulsion Particles--Maron et al. [24] determined surface area and particle size of a synthetic latex by

314

PAINT AND COATING TESTING MANUAL

titrating with a soap until the critical micelle concentration of the soap in the solution was attained. From the effective molecular surface area of the adsorbed soap and the amount adsorbed, surface area and particle size are calculated. The method is calibrated with electron micrographs. Brodnyan and Brown [25] also studied the technique.

Permeation Through Packed Powders In this method, a gas, usually air, is forced through a compacted bed of the pigment. The rate of flow is a function of voids (pore space), pressure drop through the bed, specific surface of the pigment, and viscosity of the gas. A knowledge of all the other factors allows calculation of the specific surface area and, from it, the mean surface diameter. The method is fairly simple and is suitable for routine determinations of spherical or near-spherical particles. Gooden-Smith Method--Using air as the gas, Gooden and Smith [26] designed a permeation method with two special features: (1) the weight of specimen used in grams is numerically equal to its density and (2) every specimen is packed to the same volume (Fig. 8). Together with a graphic calculator, these features greatly simplify the test and make it attractive for routine testing or research in the particle-size range 0.2 to 50/xm.

ter, (3) a capillary specimen tube, and (4) a leveling bulb containing mercury. To provide the constant volume of air, the U-tube is inverted and filled through the stopcock at the bend. Then, with a capillary eyedropper, a weight of mercury equivalent to 1 mL is removed, leaving an equal volume of air after which the stopcock is closed and sealed. The mercury does not pass through the sintered glass disk at the pressure used in the test. To make a test, a plug of pigment is formed inside the specimen tube using the special press. The weight of the specimen is a function of its density and the desired porosity of the plug (usually about 0.5). With the leveling bulb in the lower position, the specimen tube is attached to the side arm of the apparatus, and a vacuum is applied by opening the stopcock at the upper right-hand corner. The vacuum is adjusted, and simultaneously the stopcock is closed, timing is started, and the leveling bulb is raised to its upper position, which forces the 1 mL of air into the system. The timing is stopped when the manometer returns to zero. The stopcock is then opened, and the bulb is returned to its lower position for another run.

Hutto-Davis Method

Separation and Collection: Particle Size by Elutriation

The principle differences between this and the preceding method are: (1) the weight of the powder used to make the compacted pellet is calculated to give a definite porosity, and (2) the volume of air is kept constant for all plugs. Hutto and Davis [27] modified the apparatus of Carman and Malherbe [28] by providing an improved manometer, better control of the volume of air, and an improved press for preparing the specimen. They also designed a nomograph for calculating specific surface and surface area. The apparatus consists of: (1) a U-tube with legs of unequal length and closed at each end with a sintered glass disk, (2) a manome-

This approach is primarily a means for separating or fractionating a quantity of pigment into portions according to particle size rather than providing a value of the sizes separated. Size determinations may then be made on the separated fractions. The technique has utility in evaluating the influence of pigment and filler particle size on the properties of the formulated product. After standardizing operating conditions for a material and measuring sizes by an independent method, approximate size distribution may be estimated for additional material.

air

t nlet

sorew clamp

i

~

ao iurt

oo.I

valve

// /] // / / // / / // // // // // // // // // // // // //

ample-/ plusJ

~

I

t

diffuser

/ / / /

t~bes

back a

J

f r o n t aria

/~

~P---levoliN

l~re samva r e ~ d a t o ~

FIG. 8-Gooden-Smith apparatus for surface area. (Courtesy of Official Digest.)

tube

CHAPTER 32--PARTICLE-SIZE MEASUREMENTS 315 Roller Particle-Size Analyzer This device [29] uses elutriation by air to separate pigments into sizes. The glass U-tube at the bottom of Fig. 9 is charged with the specimen of pigment. Air under pressure is admitted through a nozzle that extends to the bottom of the tube. The other arm of the tube is connected to the metal settling tube. By proper selection of size of nozzle, size of settling tube, and velocity of the air, separation into desired fractions is obtained. The fractions are collected in paper extraction thimbles connected with a gooseneck to the settling tube. During the separating, the U-tube and the settling tube are vibrated vigorously. Using 25-g charges, the average time required for separations ranges from 62 rain for particles up to 5/~m in diameter to 11 min for particles from 40 to 80/~m in diameter.

<

To filter system ~ i :.!:!!)!i ~ to removefine j : ::!i:ili::j: particles from ] ~4k?~": ~ stream J::., :

.•L

Sievieg Finerparticles passthrough

sieve EIMdatlen f Largerparticles tend to settle

Felvation This method [30] combines elutriating and sieving for fractionating fine powders. The basic apparatus (Fig. 10) consists of a column about 4 in. (10 cm) in diameter and 24 in. (61 cm) in height. The specimen is placed in the narrow tube at the right. Fluid is introduced and allowed to pass through at a relatively low rate. The powder soon becomes fluidized in the conical base of the column. The flow of the liquid is increased gradually. The smaller particles reach the sieve and pass through it. The throttling action of the sieve increases the flow through it. Hence, particles that have passed through do not fall back and clog the screen. Eventually, particles just larger than the sieve openings reach the sieve, but they do not block the screen as they reduce the flow around them and then tend to fall from the sieve. The sieve functions only as a gage. It does not support the powder. Highly accurate electroformed sieves are used, but, because of their small size (1 in. (2.5 cm) in diameter), their cost is reasonable. Beside being economical, the method is

Flow from reservoir

Powderto be f " fractionated '% Coarse wire screen

Flu|dizatiee Powderfluidizedby flow FIG. 10-Felvation. Fractionating powders into different sizes by a combination of fluidization, elutriation, and sieving. (Courtesy of Chemical and Engineering News.)

rapid. Complete fractionations can be run in a period as short as 10 min.

Individual Particle S e n s i n g In this technology, individual particles are separated from each other by a diluted suspension in either air (by fluidization followed by entrainment in a flowing stream) or a constantly stirred conductive solution. The particles are then forced to pass through the sensing zone. In one air-fluidized instrument, the time of flight between two precisely placed laser light sources and detectors is used to calculate size. In another, the attenuated light as detected by a photodiode is used to calculate a size. In the liquid suspension instrument, a change in the established electrical resistivity through an electrolytecircuit is measured as a particle passes through a small orifice.

Time of Flight from Light Blockage

FIG. 9 - T h e roller particle-size analyzer.

An instrument based on aerodynamic time-of-fight technology was developed by Dahneke [31]. The measurement range is reported to be from 0.2 to 200/xm, with 150 channels per decade and a count rate of up to 100 000 particles per second. Output may be reported in either geometric or aerodynamic diameter. In this instrument, a gas (usually air for pigment particles) is used to break down agglomerate material and entrain them in a controlled flow; a specially designed "pulse-jet dispenser" is used where a controlled flow of air is directed down onto a sample placed into a small cup (Fig. 11). Depending on the nature of the sample, the operator sets the computer values for particle count rate, flow pressure, and pulse pressure. The disperser flow pressure controlling the carrier gas usually remains constant. The disperser

316

PAINT AND COATING TESTING MANUAL

PULSE JETDISPERSER ~~..~

P

TM

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heath Toroidal Swirl

Pulse Jet

FIG. 11-A diagram of the operation of the dry-powder pulse jet disperser. (Courtesy of Amherst Process Instruments, Inc.) pulse pressure determines the number of particles fed through the analyzer. Measurements are made over 2-s intervals to ensure that the minimum count rate is met. Should the count rate fall below the predetermined count rate, the pressure is increased. Should the count rate exceed the maximum allowed, the pressure is dropped by a factor of 2, thus maintaining the number of counts between the allowed maximum and minimum.

The particles are deaggregated to primary size and fed into the injection tube of the sonic nozzle suspended as single

particles in the air stream moving at about 6 L per minute. There is no operator involvement during the I to 5-min determination. Two laser beams set at a discrete distance apart and directed onto two photomultipliers make up the "sensing zone" (Fig. 12). As the particle passes through the set of laser beams, light that is attenuated and scattered is monitored by the detectors. Signal analysis using cross-correlation techniques give the particle's time-of-flight (TOF) with a resolution of 25 ns. The technique is reported to be able to discriminate between particles with only a 10% size difference. Attachments to the instrument also allow the investigation of sprayed aerosol droplets as well.

Electrical Resistance

FIG. 12-Schematic of the aerosol beam generator and detection mechanism. (Courtesy of Amherst Process Instruments, Inc.)

According to the Coulter principle [32-34], when a particle suspended in an electrically conductive liquid enters a constricted path between electrodes, it changes the pathway electrical resistance between the electrodes. This change produces a fluctuation in the applied voltage that, under proper conditions, is inversely proportional to the volume of the particle. When the dispersion is made to pass through a hole small enough to pass the particles one by one, the resultant series of voltage fluctuations may be electronically amplified, scaled, and counted. A functional diagram is shown in Fig. 13. The beaker and tube contain the suspension. When the stopcock is opened, a controlled vacuum starts flow of the suspension from the beaker into the tube and also unbalances the mercury in the manometer. The stopcock is then closed, and the mercury siphon continues the flow of the suspension. Electrical contacts in the manometer start and stop the counting of the particles. The volume of the manometer tube between the contacts determines the volume of the specimen. This ranges from 0.02 to 5 mL. A fresh volume is drawn for each count. A typical count requires from 3 to 30 s, depending on the size of the hole and the volume between the contacts in the manometer. The Coulter counter was found to be useful by Princen et al. [35] for particle-size determination of emulsified oil systems.

CHAPTER 32--PARTICLE-SIZE MEASUREMENTS

COULTER PRINCIPLE

S

A CURRENT

sieves in the U.S. Series. Especially useful for sizes under 100 /zm is a sieve made by a photoengraving and electroplating process [39]. Such sieves are intended to serve mainly as primary reference standards [see ASTM Specification for Precision Electroformed Sieves (Square-Opening Series) (E161)].

{

PULSEDETECTION, ~

ELECTROLYTE~ WITHDISPERSED PARTICLES

~

~.

317

ION

\\ . ~ ' ~ 1 1 \\ / > - ~ J l l ~ / " J

FIG. 1 3 - T h e Coulter principle: A fluctuation in current flow occurs when a particle paretically blocks the small opening between the electrodes. The size of the particle is related to the amount of change in current flow,

The principle utilized to determine individual particle size is the electrical resistance of the particle as it is forced to pass through a small hole in a glass envelope through which an electrical potential exists.

Particle Size by Sieving Sieving appears to be a simple means for separating pigments into fractions according to size. A significant addition to the physical laws governing the process has been made by Whitby [36], who investigated variables such as size distribution, mesh size, sieve loading, sieve motion, sieve material, and relative humidity. A comprehensive manual on sieving methods has been published by ASTM [37] and the W. S. Tyler Company [38]. For routine testing, sieves conforming to ASTM Specification for Wire-Cloth Sieves for Testing Purposes (E-11) are useful. These sieves are made of woven wire cloth, supported in frames of up to 12 in. (30 cm) in diameter. A skirt protrudes slightly below the sieve, allowing it to nest into the frame of the next size sieve. The openings in successive sieves progress from a base of 1.00 mm in the ratio of the square root of 2 to 1 (i.e., 1.414:1). When selecting a range of sieves from the series, it is recommended that each sieve, each alternate sieve, or each fourth sieve be taken. In this way, the basic ratio between successive sieves remains constant. The U.S. Series of sieves is patterned after the Tyler Series that was introduced in 1910 [38]. In fact., the two are now interchangeable, the only difference between them being the designations of the individual sieves. Those in the U.S. Series are identified preferably by the sizes of the openings in millimeters or micrometers, while an alternate means is by a number approximately equal to the mesh. Tyler sieves are identified by mesh. Equivalent sieves are available in both series. The sieves proposed as standard by the Internationat Standards Organization (ISO) correspond to many of the

Hand Sieving If the sieves are used singly, the following directions that appear in several ASTM test methods, among them D 546, Sieve Analysis of Mineral Filler for Road and Paving Materials, may be used. After transferring the specimen to the sieve, "Hold the sieve, with pan and cover attached, in one hand in a slightly inclined position so that the specimen will be well distributed over the sieve, and at the same time gently strike the side about 150 times per minute against the palm of the hand on the upstroke. Turn the sieve every 25 strokes about one sixth of a revolution in the same direction each time. Continue the operation until not more than 0.05 g passes through the sieve in 1 minute of continuous sieving." Sources of error in hand sieving are operator fatigue and casual attention to directions. While machine sieving eliminates these errors, some specifications still require hand sieving unless it can be shown that machine sieving gives the same results.

Machine Sieving Machine sieving has the advantages of uniformity of treatment and saving time since the operator is free to perform other tasks while the machine is working. Several types of machines are available. The conventional ones impart an oscillating or rotating motion (or both) to the sieve, with regular tapping. None appear to reproduce the motions of hand sieving. Ro-Tap Sieve Shaker--This machine [37] (Fig. 14) manipulates a series of sieves, graduated with respect to mesh size, so as to permit separation of a specimen into sizes. As the name implies, the sieves are given a special rotary motion accompanied at regular intervals by a tap. The nature of the specimen dictates the size of the sieve openings and the timing cycle. This type of machine is recommended in ASTM Test Method for Particle Size or Screen Analysis at No. 4 Sieve (4.75-mm) and Finer for Metal Bearing Ores and Related Materials (E276). Sonic Sifter--In the Allen-Bradley sonic sifter [40] (Fig. 15), the sieves are stationary, and agitation is imparted to the particles by an oscillating column of air. Sieve wear and particle attrition are said to be minimal. The sonic sifter consists of a sieving chamber, a diaphragm at the top vibrating at 60 Hz, and a motor with the necessary controls. The amplitude of vibration is adjustable to the nature of the specimen. A determination may require no more than 60-s operation of the machine. To make an analysis, the sieves, in descending order from top to bottom, are assembled in the holder, and the specimen (not more than 30 g or 10 mL) is transferred to the top sieve. The cone, coupling unit, and the diaphragm are added, and the stack is latched within the chamber. The complete assembly is positioned in the machine, the power level and the time interval are set, and the operation is started. After the sift

318

P A I N T A N D COATING T E S T I N G M A N U A L

FIG. 14-Ro-tap resting sieve shaker. (Courtesy of W. S. Tyler Co.)

interval, the stack is removed and opened, and the fractions are weighed and computed in the usual manner. Air-Jet Sieve--A cross section of the air-jet sieve [41] is shown in Fig. 16. A specimen is seen being processed through

Lock Air C0h Sieve S Light

upport m

6 Standat or 3 Dee

older

M( P~ I [tude flitude ~ot Di'. FIG. 1 5 - D i a g r a m of the Allen-Bradley sonic 'sifter. (Courtesy of Allen-Bradley Co.) .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

the component parts. The complete operational unit includes a vacuum cleaner. The hose of the vacuum cleaner is fastened to the outlet at the lower left. The specimen is placed on the sieve, and the air, which comes from the rotating split nozzle on its way to the vacuum cleaner, agitates the specimen. The finer particles pass the sieve and are collected in a tared filter bag; the coarse particles are retained on the sieve. Sieves with different meshes are used for additional fractionations, and the procedure is repeated. New tared filter bags are used for each fractionation if retention of the fractions is desired. Air velocity must be regulated, and a manometer is used to indicate the vacuum in the housing.

Direct M i c r o s c o p i c M e a s u r e m e n t Microscopes, both optical and electron as well as automated image analyzers, are used to measure particle size directly. Optical techniques suffer somewhat from an inability to focus on the "edge" of a particle at very high magnifications due to a depth of field problem. The lower limit is set by its resolving power and the upper limit by its depth of focus. The microscope is especially useful for measurement of platelike and needle-shape particles that do not obey Stokes' law, on which the sedimentation methods are based. Disadvantages of the method are that it is slow and laborious. Hence, it is used chiefly for the calibration of the more rapid relative methods. ASTM Practice for Particle-Size Analysis of Particu-

CHAPTER 32--PARTICLE-SIZE MEASUREMENTS 3 1 9 The use of the electron microscope for particle characterization is now a routine practice. The resolving power is many times that of the light microscope and may be used in scanning as well as transmission mode. With state-of-the-art surface analysis tools such as energy dispersive X-ray (EDX) built into the scanning electron microscope (SEM) equipment, elemental analysis of a single-pigment particle within the coating matrix can be done conveniently while the coating sample is in the instrument.

Array Method Using Optical Microscope The array method of particle sizing lends itself to systems of monodispersed spheres, as in Fig. 17 (from Duke [48]). These are highly uniform with respect to diameter. When placed on a flat plate, the spheres tend to align themselves into hexagonal arrays. These are characterized by straight rows of the particles. Because of this, the rows can be measured, and the diameter of the particles may be derived by dividing by the number in the row.

Transmission Electron Microscopy (TEM)

FIG. 16-Alpine air-jet sieve. (Courtesy of Gilson, Inc.) late Substances in the Range of 0.2 to 75 p,m by Optical Microscopy (E 20) and a paper by Loveland [42] are good references. Dark field illumination can use the detecting power of the microscope rather than the resolving power for sizes below the resolving power of the optical microscope. Green [43] was the first investigator to systematize the use of the microscope for direct measurement. He dispersed the pigment in a medium on a microscope slide, photographed the dispersion at a known magnification, and projected the image on a screen to increase the magnification for measurement. Dunn [44] bypassed the photomicrograph by projecting the image of the particles directly from the slide on the screen. Ideally, the pigment should be dispersed in the medium in which it is used, but this is rarely done. Green used turpentine, Allen [45] recommended a viscous vehicle, and Eide [46] used fused resins. Gehman and Morris [47] milled the pigment in rubber, dissolved the mix in a solvent, and applied the suspension to the slide. Microscopes equipped with view-through linear scales, circles or ellipses in graduated sizes, may be used as a direct measurement method [42]. The comparison scale may be a micrometer eyepiece, an eyepiece reticle, or a scale engraved on the microscope side.

TEM is useful for the measurement of latex microspheres of 200 nm and larger [49,50]. The procedure usually utilizes the magnification factor of the microscope in the size determination from photomicrographs. Because this factor is not always dependable, photographs are sometimes made against a replica diffraction grating of known spacing. Figure 18 shows 100-nm latex microspheres on a 2160-1ine/mm grating. Sources of error include uncertainty in the accuracy of the line spacing, the roughness of the lines, and the line thickness relative to the size of the particles. Duke [51] has used an internal standard of NIST traceable materials, as shown in Fig. 19, and reports accurate sizing to as small as 50 nm.

Particle Size by Sedimentation The use of the principle that a particle will "fall" through a liquid medium at a rate dependent on its diameter (as well as other variables) is a popular sizing technique. By this

FIG. 17-Microphotograph of 9.87-v.m spheres in arrays. (Courtesy of Stan Duke.)

320

PAINT AND COATING TESTING MANUAL Stokes' L a w For essentially spherical particles, one assumes that Stokes' law will be followed when particles are falling in a fluid under some accelerating potential such as gravity or centrifugal force. This assumption requires that the particles will fall freely under laminar flow conditions [52]. For a perfect sphere, the diameter can be calculated simply from the Stokes' equation.

-~)

FIG. 18-Microphotograph of 100-nm latex spheres against a 2160-1ine/mm grating. (Courtesy of Stan Duke.)

method, the pigment particles are dispersed in a liquid and then allowed to settle under the influence of gravity or centrifugal force. The rate of particle movement through the liquid then gives the particle diameter. Among the methods for measuring the rate of particle movement in a fluid are: (1) collecting the particles on a balance pan suspended in the dispersion, (2) analyzing specimens withdrawn with a pipet from different levels, (3) determining density with hydrometers or "divers," and (4) measuring attenuated light transmission through the dispersion. With low-cost computers and extremely stable rotational control provided by advanced electronic feedback circuits, the sedimentation methods using centrifugal acceleration have become popular. A prerequisite to analysis by the sedimentation process is that the particles must be at their primary size. Vigorous stirring, usually with a dispersant agent or surfactant, both with and without ultrasonics, is often necessary. Agents to prevent flocculation may be required, and adjustments in solution pH may be necessary to stabilize the system. These additives must be evaluated at different levels to determine impact on the derived values.

where D -- diameter of sphere, /z = medium viscosity, v = velocity of fall under influence of gravity, g = gravitational constant, pp -- sphere density, and p = medium density. Most often, the particles are not exactly spherical. Because of this, it is difficult to relate a particle's dimensions directly to the observed settling rate. The usual procedure is to use Stokes' equation to determine the values for d from the sedimentation distance and its corresponding time. The value so determined is then the "Stokes' diameter" of the particle. The definition then becomes "the diameter of a sphere that has the same density and free falling velocity (in a given fluid) as the particle being investigated (within the range of Stokes' law)." This practice avoids the problem of variations in free fall velocities caused by shapes which differ from being a sphere. The assumption holds and permits useful comparisons of the size distributions when similar types of materials are evaluated. The size distribution of particles of different shapes may he compared provided that the phenomenon being studied is dependent on its behavior in the fluid. While the concept of the Stokes' diameter is useful, it is necessary to appreciate its limitations. One should not equate the Stokes' diameter to a sphere of equal volume or other related quantity. Two types of errors may be introduced. In the first case, errors arise when the particle's movements depart from Stokes' law and, in the second case, errors are introduced by the experimental technique itself. Departure from Stokes' law can occur when flow conditions are not met. Stokes' equation is valid under laminar flow only. Consequently, there is an upper limit to the size of a particle that can be evaluated in a given liquid. Large particles can experience too high a velocity in the liquid and create turbulent flow. Vortexes in the liquid in the vicinity of other particles also introduce errors. Laminar flow is determined by the Reynolds number experienced by the particle/fluid system at the fall rate. The Reynolds number, Re, is defined as Re - vdp

FIG. 19-The use of an internal standard of known size. (Courtesy of Stan Duke.)

(8)

(9)

where v is the velocity in the fluid, d is the diameter in centimeters, p is density of the particle in grams per milliliter, and ~ is the viscosity in poise. According to B.S. 3406 [53], the value of Re cannot exceed 0.2 if the introduced error from Stokes' law deviation is to be less than 5%. This would imply

CHAPTER 32--PARTICLE-SIZE MEASUREMENTS that the upper limit of particle size would depend on both the viscosity and density of the fluid. For water, this value would place an upper limit of about 60/~m. However, as might be anticipated from the Reynolds equation, this limit may be stretched somewhat when using either more dense or more viscous liquid media than water. Another problem arises when the concentration of particles is too great. The Stokes' equation assumes single particles having no influence from the wall of the vessel or other particles in close proximity. Wall effects are minimized by employing properly sized vessels. While some investigators have recommended that the volume concentration should be kept less than 0.05%, others recommend concentrations of up to about 0.5 to 1% by volume. Certain investigators feel that this compromise introduces systematic errors and renders the method inappropriate as an absolute measure, although it is suitable as a routine control method. The flocculating nature of the powder in the test fluid will impact the maximum concentration that can be used. B.S. 3406 recommends a concentration of 1% by volume, although it cautions that, under certain circumstances, a small interference may occur. Because of the potential introduction of systematic errors at higher concentrations, efforts to refine the technique so that reduced concentrations are used are underway. Precision with differentials no more than 4% are usually expected. Another problem that must be addressed is the temperature stability of the liquid. Not only is there a change in the liquid viscosity, but thermally produced convection currents grossly impact the sedimentation process. Because of the difficulty in maintaining thermostatic control over the long periods required for gravitational settling, there is usually a lower particle-size limit of about 2/~m when water is used as the suspending liquid.

Gravity Sedimentation The analysis of particles by sedimentation is possible because of Stokes' law, which states that the time of fall of a particle through a viscous medium is proportional to the particle's density (the difference between it and the medium) and its diameter. Of course, if the density of the medium were greater than the particle, the particle would float. Another requirement is that all of the particles must have a common density. This rules out mixtures of different minerals and pigments having different densities since they cannot be determined together. Because of the time required for sedimentation to occur, the simple gravimetric method and the photometric method appear to be limited to pigments of sizes above 0.5 to 1.0/~m. Cadle [53.1] and Orr and Dallavalle [54] have reviewed the many variations of the method. Gravity settling was first proposed by Oden [55,56]. Calbeck and Harner [57] were among the first to use the technique for paint pigments; Jacobsen and Sullivan [58] brought the method to a higher degree of refinement by using an analytical balance with one of the pans replaced with a cup submerged in the suspension. Hydrometer Method--The density of a dispersion is proportional to the concentration of the dispersed material, and methods for particle-size analysis based on the use of the hydrometer have been developed. One such method is ASTM Method for Particle-Size Analysis of Soils (D 422). Another,

321

developed in large part by Bauer [59], is TAPPI Method T649sm, Determination of Particle Size of Clay. Included in the method is a nomograph by Casagrande [60] to aid in the calculations. The same nomograph is found in Test Method for Particle Size Distribution by Hydrometer of the Common White Extender Pigments (D 3360) [61]. Radioactivity Method--Connor and Hardwick [62] report on this method for determining the height of the sedimentation. The radioactivity is either induced in the specimen or is used as an external probe. The technique avoids disturbing the suspension by periodic sampling and can be used with both gravity and centrifugal sedimentation.

Centrifugal Sedimentation The centrifuge is used to provide an accelerating force for sedimentation analysis. Configurations exist using rotating disks with "see-through" ports as well as holders of small seethrough cells. Disk Centrifuge--The technique for the disk centrifuge was developed by Atherton et al. [63]. The apparatus might be considered to be a direct descendant of the single cell centrifuge of Donoghue [64] and the scratch start technique of Marshall [65]. The range is from 0.01 to 0.5/~m. The apparatus consists of a centrifuge unit, a sampling unit, and an electronic control unit [66]. The rotor of the centrifuge is a cell (hollow disk) of plastic or glass that rotates on a horizontal axis (Fig. 20). The back wall is attached to the shaft of the rotor. The front wall has a hole in the middle that serves as an access port. The inside diameter of the standard cell is 10 cm, and the width is 1 cm. In operation, the cell takes liquid up to a minimum radius of 2.3 cm. The speed ranges from 500 to 8000 rpm. The sampling unit (Fig. 21) consists of a probe connected to collection flask and a clock motor. The probe is an L-shape thin-wall steel tube arranged to rotate about the axis of one arm. The other arm terminates in a sharp bevel for scooping up the contents of the cell in a manner analogous to that of an inside cutter in a lathe. The center of rotation of the probe lies below the center of the rotor. To make a test, an appropriate volume of "spin fluid," typically a 4% sucrose solution, is transferred to the rotating cell. After swirling stops (20 to 30 s), the specimen--usually 0.5 mL of a 0.5% dispersion--is expelled from a syringe against the back wall near the center of rotation. The specimen flows outward over the wall unit it reaches the free surface of the spin fluid, where it forms a band about 1 mm thick. The spin fluid and the dispersion do not mix because of the higher density of the former. Thus, zero time for the measurement is defined rather accurately. After the desired time, spin fluid is withdrawn from the cell with the probe, which is driven by a clock motor at 1 rpm. With the pickup end of the probe in the 12 o'clock position, the motor is started automatically. All but 5 mL of the spin fluid is removed for measurement of the undersized fraction of the dispersed powder. The development of detector systems must consider the action on a transmitted light beam. Light-scattering theory dictates that when light of intensity I0 attempts to pass through a dilute suspension (i.e., no multiple scattering) of particles, it is lost or extinguished to an extent that It makes it

322

PAINT AND COATING TESTING MANUAL

FLUID SAMPLE LAYER ENTRY PORT

MOTOR SHAFT

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i~!~ii~~"i

I ~ INJECTION HEAD AND SYRINGE SAMPLE INJECTION NEEDLE

BOSS 9

i 84

9

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~ P E R S P E X

ROTOR

FIG. 20-Diagram illustrating the principle of the disk centrifuge. (Courtesy of Joyce, Loebl & Company, Ltd.)

tBE PIVOT PATH DESC BY PROBE "

LE FINAL COLLECTION POINT FIG. 21-Relationship of disk and probe of disk centrifuge sampling. (Courtesy of Joyce, Loebl & Company, Ltd.)

CHAPTER 32--PARTICLE-SIZE MEASUREMENTS through. Mathematically [67,68], the transmitted light is given by /t = I0 exp( - a~xtL)

(10)

where L is the length through the suspension, and O/ext is the attenuation coefficient (turbidity). The value of O/ext is related to the number concentration of particles, N per unit volume,

by

323

ing of the reference cell, the sample cell, and the rotor speed (Fig. 23). Data are displayed in real time on a built-in CRT as well as printed out on tape after the analysis for a permanent record. Graphs are presented indicating the absorbance versus time, frequency distribution, and cumulative distribution (either percent undersize or percent oversize).

Light A t t e n u a t i o n a n d S c a t t e r i n g T e c h n i q u e s Otext = N C e x t

(11)

where Cext is the extinction cross section of the particles. The extinction efficiency of a particle of cross-sectional Area A is conveniently expressed as

a = Cext/Qext

(12)

This yields the relationship Ctext --

N 1rd2Qext 4

(13)

Photo extinction detectors on modern turbidity instruments essentially determine aext. The internal calculation of Qext requires knowledge of the particle refractive index, size, shape, orientation, state of polarization, and wavelength of the scattered light as well as the refractive index of the suspending liquid. Using Mie theory [67-69], size may be deduced. Oppenheimer [70] has pointed out that a source of potential error is that the refractive index of the suspending liquid and particles must be known for each wavelength. Rotating Cell Holder Centrifuge--Real time monitoring is provided on some instruments and is advantageous when irregular measurements force an abort. One such instrument can perform centrifugal as well as gravity analysis using the same on-board computers. With ten different rotor speeds switchable from 300 to 10 000 rpm, an accelerating potential of up to 9000 g is available. Particles as small as 0.01/zm are reported to be measurable. The instrument uses two small, transparent rectangular cells (Fig. 22) mounted opposite each other on the balanced rotor. While they counterbalance each other, one serves as a reference for the suspension liquid, and the other holds the particle suspension system under analysis. Three photocells in the top monitor three opposite light-emitting diodes (LED) on the back side of the rotor. This allows real-time monitor-

Spectrophotometric Techniques Gamble and Barnett [71] developed a method for measuring particle-size distribution by the scattering of light in the near infrared. The pigment is dispersed in low concentration and placed in a transmission cell for spectrophotometric measurement. From the shape of the spectrophotometric curve, the relative size and size distribution characteristics of an unknown pigment are obtained in terms of calibrated samples (Fig. 24). Bailey [72] also developed a spectral transmission method using infrared wavelengths from 0.4 to 2/~m. The spectrophotometric curve was related to size by graphic comparison with specimens of known sizes. Atherton and Peters [73] measured light scattering by polydispersed system of spherical particles. The size distribution curves for the materials compared favorably with direct measurement with the electron microscope. Leobel [74] used a spectrophotometer to obtain the effect of wavelength on turbidity in the determination of the size of latex particles. The sizes of particles used for calibration were determined from electron micrographs.

Light-Scattering Techniques When solid particles pass through a light beam, the light will interact with the particles in one or more ways. It can either be reflected, refracted, diffracted, or adsorbed and reradiated. This is shown schematically in Fig. 25. The scattered radiation will present a pattern that is dependent on the shape and size distribution of the particles. By evaluating the scattered radiation detected at different locations relative to the beam direction and in different positions, much can be learned about the scattering media. Figure 26 depicts some of the angular intensities of scattered light produced by particles of various size. With low-cost computers and stable de-

FIG. 22-Reference cells used in a centrifuge. (Courtesy of Horbia Instruments, Inc.)

324

PAINT AND COATING TESTING MANUAL

SYSTEM DIAGRAM

PHOTOCELLS

1.SYNCHRO-SIGNAL(REF) 2.ANALOG-SIGNAL 3.SYNCHRO-SIGNAL(SAMPLE)

;;;

,~~2OCELL

(REFERENCE)

(OPTION)

(OPTION)

The CAPA-700 system is comprised of an optical system, a centrifuge, computation circuitry and input/output functions. The rotating disc in the horizontally mounted centrifuge is connected directly to the motor to minimize gravitational effects, and electronic control keeps the rotational speed stable at the set value. The CPU monitors and controls automatic checking for fluctuations in rotational speed, computation formulae and deterioration of the light source and detector, assuring precision data at all times. In the optical system, a reference cell guarantees both optical and mechanical balance for each revolution, which, after CPU processing, gives even higher precision particle size distribution analysis.

FIG. 2 3 - A modern instrument for determining particle size in real time.

LO

z0

B

80

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o

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a.o

3.o

4.0

WAVE LENGTH IN MICRON e. FIG. 2 4 - P a r t i c l e s i z e m a y b e determined from the scattering of radiation in the near infrared region of the s p e c t r u m .

tectors, instruments have been developed that can monitor this scattered radiation, giving fast and reliable data. Angular dependence techniques, which include detection and manipulation of signals from front (reflection) as well as side and rear (diffraction), have been integrated into a single instrument. Angular-dependence, light-scattering techniques have been widely studied for information concerning the sizes of both pigmentary and nonpigmentary particles. In the latter class, polymeric organic materials from molecular to latex sizes have been included. Debye and Bueche [75] described how to characterize the optical inhomogeneities related to size from information concerning the way a system scatters light. Angular-dependence techniques include the work of Brice et al. [76], who designed a photoelectric photometer for measurement of molecular weight by applying the Debye theory. Aughey and Baum [77] note that particles in the size range of large molecules produce variations in the intensity of light scattered at large angles to the illuminating beam. Progressively larger particles produce significant variation in the light scattered at small angles. The light source is a welldefined monochromatic beam. The cell containing the dispersion is stationary, and the photoelectric tube travels around the cell in an arc that covered a useful range from 0.05 to 140 ~ Scattered light reaches the tube through a small aper-

CHAPTER 32--PARTICLE-SIZE MEASUREMENTS

325

Reflected

Absorbed and Reradiated

> ~"

~'-

Refracted

Diffracted FIG. 2 5 - P o s s i b l e interactions w h e n light strikes a particle.

ture in the housing for the tube. The radii of the optical inhomogeneities range from 0.1 to 100/~m. Modern diode array detectors are now available to do similar tasks without the movement. Diffraction of Laser Light--Instruments using diffraction of single-wave-length laser light have been made possible by the development of low-cost, highly stable lasers and diode array detector technology. By fluidizing the particles in a stream of air, they are directed through a cell being illuminated by a laser source. The He-Ne laser, emitting a coherent beam of 632.8-nm wavelength is often used. To increase the crosssectional area of the beam, expanders are often employed (Fig. 27). The diffraction pattern that is seen by the detector is a pattern of rings varying in darkness depending on the size distribution of the passing particles. On-board computers use both the Fraunhofer diffraction and Mie scattering theories to calculate the particle-size distribution of the sample. These methods allow the determination of size distribution based Incident Beam ~

~---=--:..~_~.~_ ~-~---~,,,.~ -'~" ~ ; ,~, . ,~~, .,~-~-

Sphere Smaller than 0.1 the Light Wavelength

Incident

Beam

~'~2

~-

~

.

Sphere about 0.25 the Light Wavelength

on the intensity distribution of the diffracted laser beam. Since the intensity pattern is a function of the actual particle size, mixtures of particles having different densities may be measured together, unlike those relying on Stokes' law. The instrument is reported to address 0.1 to 200-p.m particles. Total Light Scattering--Instruments capable of monitoring light from all sides can effectively measure particles even smaller than the usual limit of 0.1/xm. This limit is imposed because of the physics relating the wavelength of the incident radiation and the interacting particles. By taking advantage of a set of detectors located strategically around the scattering cell, data relating to even very small particles less than 0.1 /xm are determined. As the size of the particle decreases, the intensity of the scattered light obscures critical differences in the angular distribution. In practice, this difficulty in treatment of data leads to the so-called laser-diffraction measurement barrier. Since there is an increase in both the side and rear scattering with very small particles, it is possible to take advantage of the phenomenon. Figure 28 is an instrument diagram with an arrangement of detectors that allow a single instrument to measure a broad range of sizes. Light irradiating the particles is scattered at various angles. For relatively large particles, light is scattered in the forward direction, while, for smaller particles, the light tends to be scattered in all directions. By arranging a lens system and manipulating data from the detectors, the particle-size distribution is calculated from the Mie theory. Since the size of the particles measured is a function of the wavelength of the impinging radiation, the instrument uses a beam of filtered blue light to expand the range.

X-Ray Scattering

Beam

-~---~-.-

'

Sphere

. " - - ~ - ~

larger than t h e

,, L '..2__

Light Wavelength

FIG. 26-Angular intensity of scattered light after striking a particle.

The Debye-Scherrer small angle X-ray scattering technique for observing interference effects related to particle size is well known [78]. Some generalizations on the use of the technique were addressed by Debye [79]. Yudowitch [80] and Danielson et al. [81] both employed the "diffraction peak" procedure for the determination of latex particle size. For control they used a latex whose size had been determined under the electron microscope.

326

PAINT AND COATING TESTING MANUAL

sampleparticle /

diffracted lightpattern

He-Nelaser

beamexpander

condenser lens

I) photo-cell Ycletector

Principle of Operation The He-hie laser emits a beam of 632.8 nm whose flux is enlarged by a beam expander and radiated upon the particles suspended in the liquid. After it has been diffracted and dispersed by the particles, the laser beam passes through the condenser lens and its image is formed at the photo-cell detector located at the focal point of the lens. The diffraction pattern that appears at the detector is a pattern of light and dark concentric rings that corresponds to the particle size distribution of the sample. Now, using both of the Fraunhofer diffraction and Mie scattering theories, the intensity values are used to calculate the particle size distribution of the sample.

tungstenlamp He-Nelaser

~ r -

~

filter

sampleparticle ~ condenser detector I\ lens [~

--- ",~......... - ~ rear

detector

%

scatteredlightpattern

~

~side

detector

detector

Here is how the LA.900 works. Light irradiating the particles is scattered at various angles. If the particles are comparatively large, this scattering tends to be concentrated in a forward direction. As the size of the particles decreases, the light is scattered in all directions. Therefore, to measure larger particles, data from a small angle of scatter is required; to measure smaller particles, a larger angle of scatter is required. On the LA-900, to measure the distribution of low angle scattered light, a condenser lens is used with an array detector at the focal point of the lens. To measure large angle scattered light, detectors are used at the front and rear of the sample chamber. From angular measurement of the scattered light by all of these detectors, the particle distribution is calculated from Mie theory. It is also true that as the wavelength of the light becomes shorter, so does the measurement size of the particles. Therefore the LA-900 uses a He-Ne laser beam and a red light beam and a blue light beam obtained by filtering a tungsten lamp to expand the measurable range.

FIG. 27-Schematic layout for the large-particle system. (Courtesy of Horbia Instruments, Inc.)

Drawdown Techniques for Texture and O v e r s i z e During the manufacturing of paints and coatings, there is a need for rapid, "on-the-floor" testing to determine whether a dispersion has reached its maximum grind. The "grind" gages were developed to provide quick, on-the-spot answers. Some of the tests could be done in the factory at the disperser during the grinding stage of production. Actually, during the production of paints and pigmented coatings, the pigments

are not ground but are only wet out and dispersed to their "primary" size. This primary size is the smallest possible size supplied from the producer.

Thin-Film Drawdown for Oversize Particles This test [82] is suited particularly for detecting oversize particles that adversely affect the gloss of high-gloss industrial enamels. A 2-mL wet film of the enamel is spread on a

CHAPTER 32--PARTICLE-SIZE MEASUREMENTS

327

Flow Schematics reservoir unit (option) ....................... -~ I l i

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layout for a system m e a s u r i n g both large and small p a r t i c l e s (Courtesy of Horbia

glass plate with a Bird applicator blade. Inspection of the film is best made in a dimly lit room with the operator facing a light source such as a window or under a florescent light with a grid that can be reflected from the wet paint sample. With the eye focused on the silhouetted image of the window sash or the reflected pattern from the light or ceiling, extremely fine particles may be detected and compared with a visual standard. They appear as a roughness to the wet surface. A variation of this approach is to illuminate the specimen at grazing incidence with a spotlight and view the film through a magnifier parallel to the specimen. Still another version is the use of wedge-shape or step-wise multithickness films.

Fineness-of-Dispersion Gages Drawdown tests for the detection of oversize particles led to development of fineness-of-dispersion gages, or, as first called, fineness-of-grind gages. The gage is a steel block in which is cut a wedge-shape channel, tapering usually from 4 mils at the deep end to zero at the other end, though other depths and different widths and lengths of the channel are available. Some gages have twin channels. An excess of the sample is placed in the deep end of the channel, and the excess is drawn to the shallow end with a scraper. At some point along the channel, coarse particles or agglomerates become visible. The results are interpreted by reference to standard reference patterns. The addition of slow-evaporating solvents may be required to thin down the system to allow more time for reading the gage without experiencing volume loss by evaporation.

St. Louis Gage--This [83], the first of the dispersion gages, was designed to facilitate the use of the North Standards. The channel is 2 in. (51 cm) long and 0.005 in. (0.13 mm) deep at the deep end. The sample and the selected North Standard are placed side by side in the deep end, drawn by a scraper toward the shallow end, and compared. Hegman Gage--The Hegman gage [84,85] (Fig. 29) made the North standards and the St. Louis gage obsolete. It is essentially a St. Louis gage with the addition of a scale to show the depth of the channel or the distance from the deep end. Instead of evaluating the dispersion in terms of the North standards, the distance from the deep end is reported. The finer the dispersion, the greater the distance. This is the reverse of the North standards, where the finer the dispersion, the lower is the number of the standard. ASTM Gage--The gage specified in ASTM Test Method for Fineness of Dispersion of Pigment-Vehicle Systems (D 1210) is almost identical to the Hegman. The steel block (Fig. 30) is about 180 m m long, 63.35 mm wide, and 12.7 mm thick. The channel is 100/~m deep at the heel. The gage is calibrated according to depth in steps of 10/~m along one edge and to the corresponding nearest 0.1 mil along the other. The dimensions of the scraper are shown in the diagram. Experience has shown that the speed of drawdowns and the angle at which the scraper is held have no important effect on the results. However, the time lapse between the drawdown and reading, as well as operator technique, are important [86]. Readings should be made within 10 s after completion of the drawdown, especially for dispersions with a rating of seven or better due to the volume loss from solvent evaporation. Good practice suggests three drawdowns using

328

PAINT AND COATING TESTING MANUAL

FIG. 29-Hegman fineness-of-dispersion gage. (Compliments of Lynn Shirey.)

fresh material each time. The first drawdown allows accommodation to the light source and a rough estimate of the dispersion pattern and the end point. Subsequent readings may then he made within 3 or 4 s. ASTM directs viewing the drawdown with the line of vision at a right angle to the long dimension of the channel and at an angle between 20 and 30 ~ with the face of the gage in a light that renders the pattern readily visible. Diffuse subdued light is preferred.

Elimination of operator variance is aided by the use of six standard patterns. Figure 31 illustrates reading of the end points, and the scales show the depth of the channel. This scale appears to have some advantage over both the North and the Hegman scales as it is related directly to the dimensions of the oversize particles. Other scales in use include the FSCT (Federation of Societies for Coatings Technology) scale, which divides the distance into ten parts. Constant Depth Gage--The channels in this gage are of constant depth rather than tapered as in the gages previously described. The gage most often used has four channels, 1/2 in. (1.3 cm) wide and 6 in. (15 cm) long, having depths of 0.002 (0.005 cm), 0.0015 (0.0038 cm), 0.0010 (0.0025 cm), and 0.0005 in. (0.004 cm) equivalent to Hegman values of 4, 5, 6, and 7, respectively. Other depths can be supplied. The advantage of this type of gage is the long path available for inspection, a condition that should minimize the influence of bubbles and foreign matter on the ratings. NPIRI Grindometer--Printing ink films are much thinner than those of paints or coatings. Relatively fine particles in the latter would be considered relatively coarse in the ink. Because of this, the printing ink range is addressed with a gage designed by the National Printing Ink Research Institute (NPIRI) called the Grindometer [87,88]. The channel of this gage is only 0.001 in. deep at the deep end, is 1 in. (2.54 cm) wide, and the scale is graduated in steps of 0.0001 in. (0.000 25 cm).

X-Ray Microradiography Technique This technique detects undispersed clumps of pigment (inorganic types only) in paint. It uses low-energy X-rays [89] to resolve oversize material in the range of 1 ~M or less, which is below the range detectable with fineness-of-dispersion gages. A thin film of paint is spread over the surface of photographic-sensitized, fine-grain film. The specimen is exposed

/ MILS

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-20

0

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0

J FIG. 30-ASTM dispersion gage and scraper.

CHAPTER 32--PARTICLE-SIZE

4--

329

where

microns

mils

MEASUREMENTS

= liquid velocity at radius, r, = pressure drop for the distance, L, = inside radius of capillary, = effective radius of liquid at Vr, = viscosity of fluid in flow, and L = length of capillary. Because the larger particles tend to have statistically more volume in the fluid away from the wall, the larger particles are under a greater influence of the faster-moving fluid away from the wall more so than the smaller particles, which can physically fit in the slower-moving layer nearer the wall. Thus, the larger particles advance faster than the smaller ones affecting the separation. Vr (P0 - P/.) R r

--I00

- 80 3--

--60 2 - 40

THE ROLE OF PARTICLE-SIZE REFERENCE TEST MATERIAL

[ 0 ---J

;Piiil --

-

0

4 0 microns 1.6 mils FIG. 31-Typical pattern produced by a dispersion gage.

to X-rays and the film developed after removal of the paint. The film is enlarged optically to reveal silhouettes of the oversize materials on a photographic negative. Determination of size may be made following the methods used for light and electron microscopy.

Hydrodynamic Chromatography: Angstrom Particle Sizing The separation of angstrom-sized particles may be affected by the use of hydrodynamic chromatography. In this technique, the mixture of very small particles is allowed to transgress a long capillary tube in which the liquid medium is flowing in laminar flow. It is welt known that fluids, both sasses and liquids, will experience a pressure drop as a function of distance traveled and of the diameter of the tubing that occurs because the fluid in actual contact with the wall of the tube does not flow and is stationary. For noncompressible fluids, (i.e., liquids), this variation in flow velocity from wall to center of tube may be used to separate particles of differing size. For laminar flow, the velocity of flow through the cross section of the tube increases from the wall to the center, essentially in concentric "layers" [90]. This may be represented by the vectorial diagram in Fig. 32 of the flow velocity profile. The actual velocity of flow of one of the "concentric circular layers" will be a function of its distance from the wall of the tube. The flow velocity gradient is given by

Vr

_ _

(Po - PL)R2 [1 - (r/R) 2] 4~L

(14)

Carefully prepared particles of known size and composition have become an important part of testing for emulsions, powders, film, and processes. When properly used, reference particles are an important factor in demonstrating compliance with standards such as ISO-9000, FDA Good Manufacturing Practices and various military specifications [91]. In the past two decades, the need for instrument calibration has prompted the establishment of several businesses specializing in the production and supply of reference particles, which are available in assortment of materials of various sizes, densities, and grades. A brief discussion of some typical applications will illustrate the usefulness of these test reference particles. They are used for instrument calibration, filter checking, flow tracing, and evaluation of processes such as blending, cleaning, and spraying, to name a few. Instrument calibration and checking covers two broad classes: (1) particle-size analyzers and (2) particle contamination analyzers. Both types of instruments are calibrated or controlled by spherical particle-size standards, primarily polystyrene microspheres, which are normally measured and calibrated by methods traceable to the National Institute of

r f

>

FIG. 32-Vectorial representation of the velocity of a fluid in laminar flow.

330

PAINT AND COATING TESTING MANUAL

S t a n d a r d s a n d Technology (NIST). The spherical c a l i b r a t i o n particles have fairly p r e d i c t a b l e i n s t r u m e n t responses a n d are available as aqueous s u s p e n s i o n s of highly u n i f o r m particles in discrete sizes (diameters) from 0.02 to 2000/zm. I n a d d i t i o n to polystyrene spheres, w h i c h m a y be nonfluorescent o r m a y c o n t a i n various fluorescent dyes, calibration spheres are c o m p o s e d of silica a n d glass. N o n s p h e r i c a l m a t e r i a l s such as alumina, quartz, a n d various milled powders are available as e x p e r i m e n t a l m a t e r i a l s or controls. Particle-size s p e c t r o m e t e r s are used to m e a s u r e the particle-size d i s t r i b u t i o n of powders, suspensions, emulsions, a n d aerosols. Other t h a n c o m p o s i t i o n , particle-size d i s t r i b u t i o n is p r o b a b l y the m o s t i m p o r t a n t variable in p r o d u c t quality and p e r f o r m a n c e . The m a j o r class of instruments, particle counters, are used to m e a s u r e trace a m o u n t s of particle cont a m i n a t i o n in air, water, chemicals, beverages, a n d medicines. A recent variation of the particle c o u n t e r can m e a s u r e particle c o n t a m i n a t i o n on flat surfaces, such as silicon wafers, a n d a variety of optical a n d electronic parts. All these instruments, as well as optical a n d electron microscopes, require calibration a n d reference particles to assure b o t h the quality a n d traceability of m e a s u r e m e n t s . In a d d i t i o n to i n s t r u m e n t evaluation a n d calibration, reference particles a r e used to verify r e t e n t i o n ratings a n d poresize d i s t r i b u t i o n of a i r a n d liquid filter media. Using aerosol particle generators and p r e c i s i o n particle counters up a n d d o w n stream, high-efficiency p a r t i c u l a t e air (HEPA) filters can be certified for use in ultraclean m a n u f a c t u r i n g operations for rigorous c o n t a m i n a t i o n control. Polystyrene spheres with a p p r o x i m a t e l y a 0.25-/zm d i a m e t e r are frequently used for this purpose. Another b r o a d class of reference particle a p p l i c a t i o n is for validating processes, such as for cleaning, blending, dispersing, separation, a n d spraying. F l u o r e s c e n t particles, which have b r i g h t a n d distinctive colors t h a t can be c o n t r a s t e d with o t h e r b a c k g r o u n d materials, are frequently used to follow the flow or direction of a process. Other m a t e r i a l s such as pollens, g r o u n d walnut shells, refractory powders, or other materials of the d e s i r e d particle size a n d specific gravity are available as m o d e l systems for evaluating processes. In conclusion, reference particles are a key ingredient of m o d e r n testing methods, a n d their use should be c o n s i d e r e d at a n early stage in a n y QA/QC p r o g r a m as quality m a n a g e m e n t p r o g r a m s require the r e g u l a r a n d t i m e l y evaluation a n d s t a n d a r d i z a t i o n of particle-sizing equipment.

REFERENCES [1] Thompson, G. W., "The Classification of Fine Particles According to Size," Proceedings, American Society for Testing and Materials, Vol. 10, Part II, 1910, p. 601. [2] Oden, S., "A New Method for Determination of Particle Size in Suspension," Kolloid Zeitschrift, Vol. 18, 1916, p. 33. [3] Oden, S., "Sedimentation Analysis and Its Application to the Physical Chemistry of Clays and Precipitates," Colloid Chemistry, J. Alexander, Ed., Chemical Catalog Co., New York, 1926, Vol. 1, p. 861. [4] Stutz, G. F. A. and Pfund, A. H., "A Relative Method for Determining Particle Size of Pigments," Industrial and Engineering Chemistry, Vol. 19, 1927, p. 51.

[5] Gamble, D. L. and Barnett, C. E., "Scattering in the Near Infrared; A Measure of Particle Size and Size Distribution," Industrial and Engineering Chemistry, Analytical Edition, Vol. 9, 1937, p. 310. [6] Atherton, E. and Peters, R. H., "Light Scattering Measurements on Polydispersed Systems of Spherical Particles," British Journal of Applied Physics, Vol. 4, 1953, p. 344. [7] Bunce, E. H., "Zinc Oxide in Exterior Mixed Paints," Scientific Section Circular, National Paint, Varnish, and Lacquer Association, No. 319, 1927, p. 541. [8] Eide, A. C., "Properties of Zinc Oxide Influencing the Weathering of Paints," Official Digest, Federation of Paint and Varnish Production Clubs, Vol. 7, 1935, p. 164. [9] Morris, H. H., "Titanium Dioxide, Lithopone, and Leaded Zinc," Official Digest, Federation of Paint and Varnish Production Clubs, Vol. 6, 1934, p. 8. [10] Nelson, H. A., "Zinc Sulfide Pigments for Interior Paints," Official Digest, Federation of Paint and Varnish Production Clubs, Vol. 7, 1935, p. 177. [11] Jacobsen, A. E., "Significance of Pigment Particle Size and Shape," Canadian Chemical Process Industries, Vol. 33, 1949, p. 124. [12] Pigment Index, National Paint, Varnish, and Lacquer Association, Washington, DC. [13] Allen, T., Particle-Size Measurement, 4th ed., Chapman and Hall, New York, 1990, p. 5. [14] Allen, T., Particle-Size Measurement, 4th ed., Chapman and Hall, New York, 1990, p. 9. [15] Allen, T. and Khan, A. A., Chemical Engineering, Vol. 238, 1970, pp. 108-112. [16] HORIBA: PARTICLE SIZING SEMINAR, Notes and Workbook, Horiba Instruments, Inc., (714) 250-4811, Irvine, CA, 1992, p. 5. [17] Fries, R., "The Determination of Particle Size by Adsorption Methods," Particle-Size Measurement, STP 234, American Society for Testing and Materials, Philadelphia, 1958, p. 259. [18] Brunauer, S., Emmett, P. H., and Teller, E., "Absorption of Gases in Multimolecular Layers," Journal, American Chemical Society, Vol. 60, 1938, p. 309. [19] Emmett, P. H., "A New Method for Measuring the Surface Areas of Finely Divided Materials and for Determining the Size of Particles," Particle Size Determinations in the Subsieve Range, STP 51, American Society for Testing and Materials, Philadelphia, 1941, p. 95. [20] Nelsen, F. M. and Eggersten, F. T., "Determination of Surface Area. Adsorption Measurement by Continuous Flow Method," Analytical Chemistry, Vol. 30, 1958, p. 1387. [21] Beresford, J., Carr, W., and Lombard, G. J., "Surface Area of Pigments," Journal of the Oil and Colour Chemists' Association, Vol. 48, 1965, p. 293. [22] McBain, J. W. and Bahr, A.M., "A New Sorption Balance," Journal of the American Chemical Society, Vol. 48, 1926, p. 690. [23] Ewing, W. W., "Specific Surface of Pigments by Adsorption from Solution," Particle Size Deternzination in the Subsieve Range, STP 51, American Society for Testing and Materials, Philadelphia, 1958, p. 259. [24] Maron, S. H., Elder, M. E., and Ulevitch, I. W., "Surface Area and Particle Size of Synthetic Latex Containing Fatty Acid Soap," Journal of Colloid Science, Vol. 9, 1954, p. 89. [25] Brodnyan, J. G. and Brown, G. L., "The Soap Titration of Acrylic Emulsions," Journal of Colloid Science, Vol. 15, 1960, p. 75. [26] Gooden, E. L. and Smith, C. M., "Measuring Average Particle Diameter of Powders," Industrial and Engineering Chemistry, Analytical Edition, Vol. 12, 1940, p. 479. [27] Hutto, F. B., Jr. and Davis, D. W., "An Improved Air Permeability Apparatus," Official Digest, Federation of Paint and Varnish Production Clubs, Vol. 31, 1959, p. 429.

CHAPTER 32--PARTICLE-SIZE MEASUREMENTS [28] Carman, P. C. and Malherbe, P. leR., "Routine Measurement of Surface of Paint Pigments and Fine Powders," Journal, Society of Chemical Industry, London, Vol. 69, 1950, p. 134. [29] Roller, P. S., "Measurement of Particle Size with an Accurate Air Analyzer," Proceedings, American Society for Testing and Materials, Philadelphia, Vol. 32, Part II, 1932, p. 607.

[30] Anonymous, "Felvation Speeds Powder Fractionation," Chemical and Engineering News, 6 March 1967, p. 50. [31] Dahneke, B. E. and Cheng, Y. S., "Properties of Continuum Source Particle Beams. L Calculation Methods and Results,"

Journal of Aerosol Science, Vol. 10, 1979, pp. 257-274. [32] Berg, R. H., "Electronic Size Analysis of Subsieve Particles by Flowing Through a Small Liquid Resister," Particle-Size Measurement, STP 234, American Society for Testing and Materials, Philadelphia, 1958, p. 245. [33] "Theory of the Coulter Counter," Coulter Electronics Industrial Division, 2601 N. Mannheim Road, Franklin Park, IL 60131. [34] Valentine, L., "Measurement of Particle Size with the Coulter Counter," Peintures, Pigments, Vernis, Vol. 39, 1963, p. 214. [35] Princen, L. H., Stolp, J. A., and Kwolek, W. F., "Emulsificationof Linseed Oil. I. Effects of Oil Viscosity, Temperature, Time of Agitation, and Age of Emulsions on Particle Size Distribution," Journal of Paint Technology, Vol. 39, 1967, p. 183. [36] Whitby, T., "The Mechanism of Fine Sieving," Particle-size Measurement, STP 234, American Society for Testing and Materials, Philadelphia, 1958, p. 3. [37] Manual on Test Sieving Methods, STP 447, American Society for Testing and Materials, Philadelphia, 1969. [38] "Testing Sieves and Their Uses," Handbook 53, W. S. Tyler Co., Cleveland, 1991. [39] Daescher, H. W., Siebert, E. E., and Peters, E. D., "Application of Preformed Micromesh Sieves to the Determination of Particle Size Distribution," Particle-Size Measurement, STP 234, American Society for Testing and Materials, Philadelphia, 1958, p. 26. [40] "Allen-Bradley Sonic Sifter," Publication 6801, Allen-Bradley Co., Milwaukee, WI, 1965. [41] "Air-Jet Sieve," Bulletin l-A, Alpine American Corp., 3 Michigan Drive, Natick, MA 01760. [42] Loveland, R. P., "Methods of Particle Size Analysis," Particle-Size Measurement, STP 234, American Society for Testing and Materials, Philadelphia, 1958, p. 57. [43] Green H., "A Photographic Method for Determination of Particle Size of Paint and Rubber Pigments," Journal, Franklin Institute, Vol. 192, 1921, p. 637. [44] Dunn, E. J., Jr., "Microscopic Measurement for the Determination of Particle Size of Pigments and Powders," Industrial and Engineering Chemistry, Analytical Edition, Vol. 2, 1930, p. 59. [45] Allen, R. P., "Preparation of Pigment Mounts for Microscopy," Industrial and Engineering Chemistry, Analytical Edition, Vol. 14, 1942, p. 92. [46] Eide, A. C., personal communication to G. G. Sward, 1945. [47] Gehman, S. D. and Morris, J. C., "Measurement of Average Particle Size of Fine Pigments," Industrial and Engineering Chemistry, Analytical Edition, Vol. 4, 1932, p. 157. [48] Duke, S. D. and Layendecker, E. B., "Improved Array Method for Size Calibration of Monodispersed Spherical Particles by Optical Microscope," Particle Science and Technology, Vol. 7, 1989, pp. 209-216. [49] Maron, S. H., Moore, C., and Powell, A. S., "Electron Microscopy of Synthetic Lattices," Journal of Applied Physics, Vol. 23, 1955, pp. 900-905. [50] Bradford, E. B. and Vanderhoff, J. W., "Electron Microscopy of Monodispersed Latexes," Journal of Applied Physics, Vol. 26, 1955, pp. 864-870. [51] Duke, S. D. and Layendecker, E. B., "Internal Standard Method for Size Calibration of Sub-Micrometer Spherical Particles by

331

Electron Microscope," Proceedings of the Fine Particle Society, 1988. [52] Kaye, B. H., Direct Characterization of Particles of Fine Pigments, John Wiley & Sons, New York, 1981. [53] British Standard 3406. [53.1] Cadle, R. D., Particle Size Theory and Industrial Application, Reinhold, New York, 1965. [54] Orr, C. and Dallavalle, J. M., Fine Particle Measurement, Macmillan, New York, 1959. [55] Oden, S., "A New Method for Determination of Particle Size in Suspension," KoUoid Zeitschrift, Vol. 18, 1916, p. 33. [56] Oden, S., "Sedimentation Analysis and Its Application to the Physical Chemistry of Clays and Precipitates," Colloid Chemistry, J. Alexander, Ed., Chemical Catalog Co., New York, Vol. 1, 1926, p. 861. [57] Calbeck, J. H. and Harner, H. E., "Particle Size and Distribution by Sedimentation Method," Industrial and Engineering Chemistry, Vol. 19, 1927, p. 58. [58] Jacobsen, A. E. and Sullivan, W. F., "Method of Particle Size Distribution for the Entire Subsieve Size Range," Industrial and Engineering Chemistry, Analytical Edition, Vol. 19, 1947, p. 855. [59] Bauer, E. E., "Recent Developments in the Hydrometer Method as Applied to Soils," Particle-Size Measurement, STP 234, American Society for Testing and Materials, Philadelphia, 1958, p. 89. [60] Casagrande, A., Die Araometer Method zur Bestimmung der Kornverteilung yon Baden and anderen Materialen, Julius Springer, Germany, 1934. [61] Annual Book of ASTM Standards, Vol. 02.05, ASTM, Philadelphia, 1993. [62] Connor, P. and Hardwick, W. H., "The Use of Radioactivity in Particle Size Determination," Industrial Chemistry, Vol. 36, 1960, p. 427. [63] Atherton, E., Copper, A. C., and Fox, M. R., "The Measurement of Particle Size and Its Practical Significance in Vat-Dye Quality," Journal of the Society of Dyers and CoIourists, Vol. 80, 1964, p. 521. [64] Donogue, J. K. and Bostock, W., "New Technique for Particle Size Analysis by Centrifugal Sedimentation," Transactions, Institute of Chemical Engineering, Vol. 33, 1953, p. 72. [65] Marshall, C. E., "The Degree of Dispersion of the Clays, I. The Technique and Accuracy of Mechanical Analysis Using the Centrifuge," Journal, Society of Chemical Industry, London, Vol. 50, 1931, p. 444. [66] Beresford, J., "Size Analysis of Organic Pigment Using the ICIJoyce Loebl Disc Centrifuge," Journal, Oil and Colour Chemists' Association, Vol. 50, 1967, p. 594. [67] Kerker, M., The Scattering of Light and Other Electromagnetic Radiation, Academic Press, New York, 1969. [68] Bohren, C. F. and Huffman, D. R., Absorption and Scattering of Light by Small Particles, Wiley-Interscience, New York, 1983. [69] Mie, G., "Beitrage zur Optik truber Modien, spezeill Kollaidaler Metallosungen," Annalen der Physik 25, Vol. 3, 1908, pp. 377445. [70] Oppenheimer, L. E., "Interpretation of Disk Centrifuge Data," Journal of CoUoid Interface Science, Vol. 92, 1983, p. 350. [71] Gamble, D. L. and Baranett, C. E., "Scattering in the Near Infrared; A Measure of Particle Size and Size Distribution," Industrial and Engineering Chemistry, Analytical Edition, Vol. 9, 1937, p. 310. [72] Bailey, E. D., "Particle Size by Spectral Transmission," Industrial and Engineering Chemistry, Analytical Edition, Vol. 18, 1946, p. 365. [73] Atherton, E. and Peters, R. H., "Light Scattering Measurements on Polydispersed Systems of Spherical Particles," British Journal of Applied Physics, Vol. 4, 1953, p. 344.

332

PAINT AND COATING TESTING MANUAL

[74] Loebel, A. B., "Determination of Average Particle Size Synthetic Lattices by Turbidity Measurements," Official Digest, Federation of Paint and Varnish Production Clubs, Vol. 31, 1959, p. 200. [75] Debye, P. and Bueche, A. M., "Scattering by an Inhomogeneous Solid," Journal of Applied Physics, Vol. 20, 1949, p. 518. [76] Brice, B. A., Halwer, M., and Speiser, R., "Photoelectric Light-

[82] Pigments Division, duPont Co., White Pigments for Paint, Sec-

Scattering Photometer for Determining High Molecular Weights," Journal, Optical Society of America, Vol. 40, 1950, p. 768. [77] Aughey, W. H. and Baum, F. J., "Angular Dependence Light Scattering--A High Resolution Recording Instrument for the Angular Range 0.05-140," Journal, Optical Society of America, Vol. 44, 1954, p. 833. [78] Marculaitis, W. J., "Particle Size and Particle Size Distribution of Pigments by Small Angle X-Ray Scattering," Journal of Colloid Science, Vol. 12, 1957, p. 581. [79] Debye, P., "Light Scattering as a Tool," OfficialDigest, Federation of Paint and Varnish Production Clubs, Vol. 36, 1964, p. 518. [80] Yudowitch, K. L., "Latex Particle Size from X-ray Diffraction Peaks," Journal of Applied Physics, Vol. 22, 1951, p. 214. [81] Danielson, W. E., Shenfil, and Du Mond, J. W. M., "Latex Particle Size Determination Using Diffraction Peaks Obtained With the Point Focusing X-Ray Monochromator," Journal of Applied Physics, Vol. 23, 1952, p. 860.

[84] Fasig, E. W., "The Hegman (Sherwin-Williams)Fineness Gage," Drugs, Oils, and Paints, Vol. 54, 1938, p. 438. [85] Purdy, J. M., "The Hegmen Fineness Gage," Paint, Oil, and Chemical Review, Vol. 109, 1946, p. 14. [86] Doubleday, D. and Barkman, A., "Reading the Hegman Grind Gage," Official Digest, Federation of Paint and Varnish Production Clubs, Vol. 22, 1950, p. 598. [87] National Printing Ink Research Institute, "The NPIRI Produc-

tion II, 1956, p. 11.

[83] St. Louis Paint and Varnish Production Club, "Effects of Wetting Agents upon Pigment Dispersion," Scientific Section Circular, National Paint, Varnish, and Lacquer Association, No. 471, 1934, p. 429.

tion Grindometer," Project Report 12, 1949.

[88] Walker, W. C. and Zettlemoyuer, A. C., "Fundamentals of Grindometers," American Ink Maker, Vol. 28, No. 7, 1950, p. 31. [89] Venuto, L. J. and Hess, W. M., "A New Look at Carbon Black," American Ink Maker, Vol. 45, No. 10, 1967, p. 42. [90] "Flow of Fluids Through Valves, Fittings, and Pipe," Technical Paper No. 410, Crane Co., New York, 1985.

[91] Private communication with Stan Duke, Duke Scientific Corp., Palo Alto, CA 94303, 1993.

MNL17-EB/Jun. 1995

Rheology and Viscometry

33

by Richard R. Eley I

G~ Plateau m o d u l u s G* Complex s h e a r m o d u l u s G' Storage m o d u l u s G" Loss m o d u l u s K Consistency in p o w e r law model; c o n s t a n t in viscosity-molecular weight equation L Capillary tube length M w Weight-average p o l y m e r m o l e c u l a r weight M c Critical (entanglement) p o l y m e r m o l e c u l a r weight P Pressure Q Volumetric flow rate R Capillary tube radius; particle radius; air b u b b l e radius T Absolute t e m p e r a t u r e Tg Glass t r a n s i t i o n t e m p e r a t u r e a Curvature p a r a m e t e r in Eq 15; a m p l i t u d e of coating surface striations a0 Time-zero a m p l i t u d e of coating surface striations e Base of the n a t u r a l l o g a r i t h m i c scale = 2.718 28 f Frequency, Hertz g Gravitational acceleration h Film thickness k B o l t z m a n n ' s constant; general rate c o n s t a n t l Length m Kinetic energy correction t e r m for capillary tube flow, Eqs 72 a n d 73 n Power law exponent t Time v Velocity x Coordinate parallel to substrate y Coordinate n o r m a l to substrate

Nomenclature

ap a /3 7 ~w 8

T0 ~/| ~d Tie ~l "Or ~' [~] 0 )t v p tT (r0 ~r ~% Zs %

the thin tO A

De F

Fg G

Fluidity integral, Eq 64 Rate of decay of r o u g h n e s s amplitude, s- l Time c o n s t a n t characteristic of a critical shear rate, s Dimensionless shear strain S h e a r strain rate, s-1 Wall s h e a r rate in t u b e flow, s 1 Phase shift; thickness of a d s o r b e d p o l y m e r layer Extensional (Hencky) strain, dimensionless Extensional (Hencky) strain rate, s-1 Coefficient of viscosity Zero-shear viscosity H i g h - s h e a r limiting N e w t o n i a n viscosity Dispersion viscosity Extensional viscosity Liquid-phase viscosity Relative viscosity, ~d/~t D y n a m i c viscosity Intrinsic viscosity (limiting viscosity n u m b e r ) Angle of inclination with respect to the vertical Wavelength of coating surface striations; elastic stress relaxation t i m e c o n s t a n t K i n e m a t i c viscosity Ratio of c i r c u m f e r e n c e of a circle to its d i a m e t e r Density Surface tension; generalized stress Initial i m p o s e d stress Tensile stress Shear stress; viscosity-kinetic time constant for drying, wicking, or t h i x o t r o p y Yield stress Tangential surface s h e a r stress Wall s h e a r stress Volume fraction of internal (dispersed) p h a s e of a dispersion Effective volume fraction for a d i s p e r s i o n M a x i m u m packing fraction of a dispersion, where ~r ---~ ~ Angular frequency ( = 2wf)(rad/s) Area of s h e a r face D e b o r a h n u m b e r ( )t/t ) Force Force of gravity Shear modulus

INTRODUCTION THE VALUEOF RHEOLOGICALSCIENCEis increasingly being realized in the coatings laboratory. One r e a s o n is economic: As m u c h as half the cost of new p r o d u c t d e v e l o p m e n t c a n be c o n s u m e d in solving rheology-related p r o b l e m s of m a n u f a c ture or p e r f o r m a n c e . Moreover, the rheology of an established p r o d u c t can go a w r y due to r e f o r m u l a t i o n or a r a w m a t e r i a l or process change, a n d such p r o b l e m s are generally in urgent need of solution. Rheological analysis can be a costeffective aid to the coatings f o r m u l a t o r in u n d e r s t a n d i n g a n d solving p r o d u c t a n d process difficulties. Traditionally, m e a s u r e m e n t of the viscosity or "consistency" of p a i n t s a n d coatings h a d b e e n b a s e d on a n u m b e r of test m e t h o d s having certain perceived virtues: They were

lThe Glidden Company (member ICI Paints), Glidden Research Center, 16651 Sprague Road, Strongsville, OH 44136. 333 Copyright9 1995 by ASTM International

www.astm.org

334

PAINT AND COATING TESTING MANUAL

undemanding in operator skill, required inexpensive equipment, and the result was usually a single number requiring no interpretation. These tests proved useful enough, especially as tools of the experienced formulator or bench chemist. This was partly because, in the times when many industrial coatings were solvent-borne and relatively low in solids, the product rheology was seldom far from Newtonian. In the latter case, any test serves as well as any other. The advent of environmentally compliant technologies, such as waterborne, higher solids, and powder coatings, changed this situation. With the new formulations, non-Newtonian, time-dependent, and viscoelastic behaviors were encountered. Here, the traditional methods were inadequate for the reasons that many of the latter tests yield only a single-point measurement, use arbitrary (rather than absolute) viscosity units, or use a flow field which is "nonviscometric. "2 Any of these shortcomings render a method inappropriate for non-Newtonian materials. A recurrent failing in product viscosity testing is the employment of methods not relevant to the performance property in question. This deprives the test of any analytical or predictive power relative to paint performance. Thus, for example, a low-shear viscosity measured using a spindle viscometer, paddle viscometer, or orifice cup bears no relation to the performance of a paint during application, generally a high-shear process. Part of the aim, therefore, is the development of tests which are related specifically to the critical processes paints must undergo. Rheological factors play a key role in all stages of a coating's lifetime, from manufacture to the final career of a protective, decorative film. The many processes embodied in that history involve broad aspects of the science of rheology. Table 1 lists some coatings processes, each process having associated with it several distinct deformation characteristics. For each deformation type, a specific rheological test is required to measure its effect on the process. Many of these measurement quantities are independent, i.e., they cannot be inferred from another rheological property. Obviously, if one were to measure only steady-shear viscosity, and over a limited range of shear rate at that, the real cause of a performance problem may be missed. Thus, the characterization necessary for complete understanding of rheology-controlled behavior may require the use of more than one instrument and technique. Notwithstanding, many problems can be solved from shear viscosity and viscoelastic data alone provided the experiments performed are well designed and the results properly interpreted. In order to take full advantage of the capabilities built into the sophisticated rheological instruments now being widely used in the coatings industry, more than a passing knowledge of the subject of rheology is required. This chapter is therefore somewhat tutorial in style, endeavoring to provide the novice an entry point to the discipline. Certain concepts are discussed at some length because of their importance or complexity. One of these is viscoelasticity. It is worth the time because (a) it is established that viscoelasticity exerts control over coating processes when present, and (b) a number of 2"Viscometric" shear flow requires that the flow be "everywhere indistinguishable from steady simple shear" [1].

commercial instruments are capable of viscoelastic characterization of materials. Finally, in the words of Professor Ken Walters, "Rheology is a difficult subject." This is certainly true, combining as it does several disciplines under one banner: mathematics, physics, physical chemistry, colloid and polymer science, continuum mechanics, etc. This is not to mention the special complexities of the experimental methods used. Furthermore, rheology requires mastery of concepts peculiar to the science with which most persons have had limited opportunity to become familiar in the normal course of a technical education. Nevertheless, it is quite possible for the nonspecialist to acquire a useful working knowledge of the principles and practice of rheology and to use this profitably in linking formulation and performance for coatings.

DEFINITIONS OF BASIC TERMS Rheology The derivation of the term theology is from the Greek rhein, "to flow." The classical definition of the science of rheology is "the study of the deformation and flow of matter." Some have pointed out a redundancy in this definition since flow is a subset of deformation, as we shall see. An operational definition of rheology would be "the study of the response of certain materials to the stresses imposed on them." In order to quantify the deformation and flow behavior of materials, three basic terms must be defined. The first two of these relate to the measurement of the deformation (strain and strain rate) and the third to the measurement of the force required to deform the material (stress).

Deformation (Strain) A deformation is a change in shape and/or volume of a material in response to an applied stress. The equivalent engineering term is strain. During the lifetime of a coating, the deformation history may be complex (see Table 1), with the most important components being simple shear and extension. For purposes of definition, we will limit our discussion to simple shear. Simple shear deformation is exactly analogous to the spreading of a deck of playing cards, each card representing a thin volume element (or shear plane) displaced relative to its nearest neighbor (Fig. 1). If a force F is applied to the uppermost volume element (thickness dy), the material will deform by the displacement of adjacent volume elements by a distance dx. The total thickness is Ay, and the total displacement is Ax. The shear strain, % is the ratio of the net displacement, Ax, to the distance of separation of the confining surfaces, Ay. V-

Ax

ay

(1)

Strain Rate In order to measure the viscosity, or resistance to flow, of a fluid, we must know not only the extent, of deformation (strain), but also the rate of deformation (strain rate). The strain rate is the change in strain per unit time, or the time

CHAPTER

33--RHEOLOGY

AND

VISCOMETRY

335

TABLE 1--Rheological components of coatings processes. Deformation Type or Attribute

Rheological Property

Roll Coating

Squeezing flow Stretching flow Shearing flow High strain rate Large accelerations Large decelerations Surface area transients

Biaxial extensional viscosity Uniaxial extensional viscosity Shear viscosity High shear viscosity Elasticity (G') G' recovery Dynamic surface tension

Drying

Spray

High shear rate Large accelerations High-strain-rate extensional flow Surface area transients

High shear viscosity Elasticity (G') Extensional viscosity/elasticity

Flash-off

Brushing/rolling

Medium shear rate Stretching flow Surface area transients

Shear viscosity Extensional viscosity/elasticity Dynamic surface tension

Leveling/sagging

Slow shear flow Surface-stress driven Transient

Low shear stress viscosity Dynamic surface tension Structure recovery kinetics

Curtain coating

Extensional flow Surface area transients Shear (pumping, extrusion)

Extensional viscosity/elasticity Dynamic surface tension Shear viscosity

Process

Shear Stress ('r) -

F A

Shear Rate (5') -

Shear Strain (3') -

Ax Ay

Viscosity (~/) =

Dynamic surface tension

Wicking Drying

dv

Stress

dy

Force a p p l i e d to a m a t e r i a l creates a state of stress w i t h i n the material. Stress can be expressed in units of force p e r unit a r e a (e.g., dyne/cm2), or, equivalently, energy p e r unit v o l u m e (e.g., erg/cma). In t e r m s of Fig. 1, the s h e a r stress, T, is the force, F, necessary to m a i n t a i n steady shearing m o t i o n against the resistance of the confined fluid divided b y the a r e a of the s h e a r face, A.

7 "it

Y

r -

dv = dx/dt

F

Non-Rheological Effects

F A

(3)

A ~y

~

EXx

"

X

FIG. 1-Basic term definitions for simple shear. derivative of the strain, s y m b o l i z e d ~, w h e r e the "dot" signifies "time derivative of." Therefore, from Fig. 1 _ A~, _ A ( ~ / A y )

At

At

_ A(~/At______) _ A v

Ay

(2)

Ay

If the d e f o r m a t i o n is simple shear, the strain rate is t h e n called the s h e a r rate. The d i m e n s i o n a l i t y of y is L T - 1 L - 1 (e.g., cm/s p e r cm). Unit cancellation leaves reciprocal t i m e (s 1)as the unit of s h e a r rate. It is conceptually helpful, however, to r e m e m b e r that, as s h o w n b y Eq 2, the s h e a r rate is actually a velocity g r a d i e n t (change in velocity p e r unit gap b e t w e e n shearing surfaces, A v / A y ) .

Viscosity The viscosity of a fluid characterizes its resistance to flow. The resistance to flow is, in turn, a m e a s u r e of the friction b e t w e e n the flow units of the fluid (e.g., molecules) o r m a y also be a m e a s u r e of the attractive forces b e t w e e n the flow units. Thus, a "viscous" fluid (one reluctant to flow) m a y be so b e c a u s e of high m o l e c u l a r weight (as in m o t o r oil) o r be of relatively low m o l e c u l a r weight, b u t having strong i n t e r m o lecular interactions (e.g., h y d r o g e n bonds, as b e t w e e n sugar molecules in honey). The s e p a r a t i o n of molecules in flow dissipates energy, chiefly as frictional heat. Flow, therefore, is a process w h i c h costs energy, of w h i c h the viscosity is a measure. F o r the case of s h e a r d e f o r m a t i o n , the viscosity, a?, is calculated as the ratio of s h e a r stress to s h e a r rate. The viscosity, therefore, is the energy p e r unit volume dissipated to a t t a i n a unit velocity gradient. T rl = -7 Y

(4)

336

PAINT AND COATING TESTING MANUAL

Modulus Materials c o m p l y with an a p p l i e d stress b y deforming, o r u n d e r g o i n g strain. F o r ideal H o o k e a n materials, the strain will be p r o p o r t i o n a l to the a p p l i e d stress. The m o d u l u s is the p r o p o r t i o n a l i t y constant b e t w e e n the stress a n d strain. F o r example C -

T

(s)

Y

w h e r e the s h e a r modulus, G, is equal to the ratio of the s h e a r stress a n d s h e a r strain. Most p o l y m e r i c m a t e r i a l s a n d m o s t coatings systems as well are non-Hookean, i.e., the m o d u l u s is not a m a t e r i a l constant, b u t will d e p e n d on b o t h rate a n d extent of deformation.

Units Various systems of units for rheological variables are in use a n d m a y be e n c o u n t e r e d in the literature. Until recently, the m o s t c o m m o n system of units for rheological t e r m s was the cgs (centimeter-gram-second), o r "smaLl metric" system. However, m a n y technical p u b l i c a t i o n s a n d scientific j o u r n a l s n o w specify that Systeme Internationale (SI) units be a d h e r e d to. The SI system is b a s e d on the "large metric," o r MKS (meter-kilogram-second) units, with s o m e a d d i t i o n a l n a m e d units. The units associated with the above variables, according to the various systems, are given in Table 2.

CLASSES OF RHEOLOGICAL B E H A V I O R Newtonian

Fluids

I s a a c Newton p o s t u l a t e d that the force resisting m o t i o n of liquids is p r o p o r t i o n a l to the rate at which one a t t e m p t s to "separate the parts" of the liquid. In t e r m s of o u r defined quantities, this w o u l d be expressed as r ~ v

(6)

r = n~/

(7)

or

w h e r e "0 is a c o n s t a n t of proportionality, called the coefficient of viscosity o r simply the viscosity. E q u a t i o n 7 is the simplest example of a flow model, a n expression w h i c h allows one to predict the flow p r o p e r t i e s of a m a t e r i a l in response to an a p p l i e d stress. Fluids w h i c h obey Eq 7 over a range of shear rate are said to be N e w t o n i a n over t h a t range. The viscosity of N e w t o n i a n fluids is a m a t e r i a l c o n s t a n t a n d d e p e n d s only on the t h e r m o d y n a m i c variables

of t e m p e r a t u r e , pressure, a n d concentration. If the viscosity of such a m a t e r i a l is m e a s u r e d at any s h e a r rate o r s h e a r stress, the viscosity u n d e r all conditions of d e f o r m a t i o n is known.

Non-Newtonian Fluids Of m a t e r i a l s e n c o u n t e r e d in the coatings industry, only dilute o r low-molecular-weight p o l y m e r solutions or stable dispersions of low c o n c e n t r a t i o n are likely to be Newtonian. In general, p o l y m e r solutions, colloids, dispersions, a n d suspensions of particulate solids will be non-Newtonian. F o r n o n - N e w t o n i a n materials, the viscosity is no longer a material constant, b u t is called a m a t e r i a l f u n c t i o n - - i n this case, a "function" of the s h e a r rate (or s h e a r stress). F o r non-Newt o n i a n fluids, a viscosity m e a s u r e d at a single s h e a r rate is not an a d e q u a t e r e p r e s e n t a t i o n of the rheology of the system.

C L A S S E S OF N O N - N E W T O N I A N B E H A V I O R In the following sections, the various types of non-Newt o n i a n flow behavior will be outlined. At the s a m e time, several m a t h e m a t i c a l expressions which can describe nonN e w t o n i a n flow will be introduced. M a t h e m a t i c a l m o d e l s are useful for s u m m a r i z i n g flow b e h a v i o r quantitatively. To be sure, m a t e r i a l s m a y be evaluated by qualitative c o m p a r i s o n of flow curves (e.g., b y visual inspection), b u t the m a t h e m a t i cal analysis of a flow curve has significant value. F o r one thing, the m o d e l constants m a y have physical significance. F o r example, s o m e of the m o d e l s contain a yield stress t e r m (see P l a s t i c (Yield) B e h a v i o r ) as a fitted p a r a m e t e r . The m a g n i t u d e of this p a r a m e t e r m a y be coupled to sag o r leveling p e r f o r m a n c e [2]. A later section of this chapter, LEVELING, gives examples of h o w sagging can be p r e d i c t e d for n o n - N e w t o n i a n fluids using constants from m o d e l s discussed below. F u r t h e r m o r e , the values of m o d e l p a r a m e t e r s m a y be associated with f o r m u l a t i o n variables, allowing one, in principle, to o p t i m i z e rheology b y adjusting c o m p o s i t i o n in a rational way. It should be u n d e r s t o o d t h a t the models a b o u t to be discussed are actually idealizations a n d therefore limited in their ability to r e p r e s e n t the b e h a v i o r of real materials. The models can describe real b e h a v i o r at least over a limited range of stress or strain rate. Thus, a second use of m a t h e matical m o d e l s of flow is to m a k e p r e d i c t i o n s of flow behavior, b e a r i n g in m i n d that it is d a n g e r o u s to extrapolate the m o d e l s b e y o n d their range of validity. As m e n t i o n e d above, the simplest flow m o d e l is the Newt o n i a n expression, which has only one constant, the coeffi-

TABLE 2--Units. Variable

CGS

Strain Strain rate Stress Viscosity

Dimensionless s- ~ Dyne/cm2 Poise (P)(= 1 dyne 9 s / c m 2 ) or centipoise (cP) (1 cP = 0.01 P = 1 mPa 9 s) dyne/cm 2

Modulus

MKS

SI

. . . . . . s- 1 sN/M 2 Pascal (Pa)( = 1 N/M z) ... Pa - s (= 10 P) or millipascal-second (mPa 9s)(= 1 cP) N/M2 Pa

CHAPTER 3 3 - - R H E O L O G Y A N D V I S C O M E T R Y

337

rl

~p

cient of viscosity, 4. To describe m o r e c o m p l i c a t e d behavior, we wilt have to a d d a d d i t i o n a l coefficients a n d terms, the physical significance of w h i c h will be given w h e n possible.

Shear-Dependent Viscosity Materials in w h i c h the viscosity falls with increasing shear rate 3 are d e s i g n a t e d shear thinning. S i m p l e shear t h i n n i n g b e h a v i o r w i t h o u t either t i m e d e p e n d e n c e (see u n d e r T i m e D e p e n d e n t Fluids) o r a yield stress (see u n d e r Plastic (Yield) B e h a v i o r ) is termedpseudoplastic, a c o m m o n type of n o n - N e w t o n i a n b e h a v i o r in coatings systems. Viscosity rising with increasing rate of s h e a r is called shear thickening. The t e r m dilatancy (see u n d e r Shear Thickening Fluids) is often a p p l i e d to s h e a r thickening behavior, although this refers strictly to s h e a r thickening a c c o m p a n i e d b y a v o l u m e increase, as the t e r m implies. Figure 2 shows curves illustrating viscosity-shear rate relationships for N e w t o n i a n a n d nonN e w t o n i a n fluids. As stated, N e w t o n i a n b e h a v i o r is the simplest of all a n d is described b y Eq 4. The viscosity, 4, is a m a t e r i a l c o n s t a n t i n d e p e n d e n t of s h e a r rate (Contour N, Fig. 2). More complicated viscosity-shear rate b e h a v i o r requires a m o r e c o m p l e x expression to m o d e l it. The first level of complexity is to a d d yield b e h a v i o r (see u n d e r P l a s t i c (Yield) B e h a v i o r ) to the N e w t o n i a n model, resulting in the B i n g h a m equation [3] ,r-

r0

= 4//

(8)

E q u a t i o n 8 says that, above the yield stress (%), the shear stress (minus the yield stress) is directly p r o p o r t i o n a l to s h e a r rate. 4 It is a c o m m o n m i s c o n c e p t i o n that this c o r r e s p o n d s to N e w t o n i a n b e h a v i o r above the yield stress. Figure 2, Curve B shows that the B i n g h a m m o d e l displays power-law-like behavior, b u t deviates from the p o w e r law (Curve PL) at higher s h e a r rate, a p p r o a c h i n g the plastic viscosity, ~/e, as a limit. Casson [4] modified B i n g h a m ' s equation by taking the square r o o t of all t e r m s 'T 1/2 - -

"1"1/2 =

41~:,/2' 1/2

(9)

The Casson m o d e l is n o t empirical, b u t was theoretically derived specifically for systems whose p r i m a r y flow units are rigid rods. Casson's equation is r e p u t e d to hold for a variety of p a i n t systems, particularly as modified b y Asbeck [5] ,~1/2 _ ,01/2 = ,/1/2,-1/2

(10)

which is somewhat surprising since few paints utilize rodshaped particles as fillersor pigments. In fact, this author's experience is that frequently the Casson model does not represent paint flow as well as certain other models (see below). In Eqs 9 a n d 10, q~, s o m e t i m e s called the Casson viscosity, is not truly an "infinite-shear viscosity," b u t is a limit t h a t is a p p r o a c h e d , c o r r e s p o n d i n g to s o m e u n k n o w n high shear rate. The value o b t a i n e d for the Casson viscosity will d e p e n d on the m a x i m u m e x p e r i m e n t a l s h e a r rate. As with all such models, the u s e r m u s t be a w a r e that the constants resulting 3Any definitions or descriptions of shear rate-dependent behavior may likewise be stated in terms of shear stress dependence. 41n the Bingham expression and in other models incorporating a yield stress term, it is important to note that the equations describe flow behavior only when r > %. When r -< %, 5' = 0 (i.e., "q = co),and there is no flow.

~HB N

~

~

PL (n> 1) PL (n< 1)

FIG. 2-Viscosity-shear rate curves for simple flow models. Symbols represent the following fluid models: N = Newtonian; B = Bingham (~b, = plastic viscosity); HB = HerscheI-Bulkley; PL = power law (n = exponent).

from a Casson analysis are not necessarily true m a t e r i a l constants. F o r example, b e c a u s e the analysis of flow d a t a is simply curve-fitting, a finite yield stress will generally be o b t a i n e d w h e t h e r o r not the m a t e r i a l really possesses yield behavior. The next c o m p l i c a t i o n we will i n t r o d u c e is to let the exponent of the s h e a r rate in the N e w t o n i a n law be other t h a n unity. In o t h e r words, the s h e a r stress now will d e p e n d on s o m e p o w e r of the s h e a r rate ' r = K * '~

(11)

E q u a t i o n 11 is k n o w n as the p o w e r law or OstwalddeWaele model. Here, K is a constant, s o m e t i m e s called the "consistency, "s w h i c h has r e p l a c e d the coefficient of viscosity, 4. This is necessary b e c a u s e the exponent n can be o t h e r t h a n unity, in w h i c h case K will not have p r o p e r viscosity units associated with it. The p o w e r law exponent, n, has b e e n t e r m e d the "flow index." Its value is characteristic of the d e p e n d e n c e of viscosity on s h e a r rate, i.e., w h e t h e r the viscosity rises or falls with increasing shear. Dividing Eq 11 t h r o u g h b y , yields a form of the p o w e r law which relates the viscosity to the s h e a r rate 4 = K'* n-I

(12)

w h e r e K' = (l/K) ira. Obviously, i f n = 1 in Eqs 11 a n d 12, the p o w e r law reduces to the N e w t o n i a n law. A value of n < 1 c o r r e s p o n d s to s h e a r thinning b e h a v i o r a n d n > 1 to s h e a r thickening (Fig. 2 curves PL). The next c o m p l i c a t i o n we shall consider is to a d d a yield stress t e r m to the p o w e r law expression r - ro = K , "

(13)

5Mathematically, K corresponds (in numerical magnitude but not dimensionally) to the viscosity at unit shear rate (1 reciprocal second).

338

PAINT AND COATING

TESTING

MANUAL

which is known as the Herschel-Bulkley equation. This model describes power law behavior above the yield point (Curve HB, Fig. 2). Figure 3 shows a generalized equilibrium flow curve [6, 7]. This figure represents the general features of the shear rate dependence of viscosity for non-Newtonian fluids with any time-dependent or relaxation behavior removed. It consists of a low shear rate Newtonian regime, Region I, an exponential shear thinning regime, Region II, a high-shear Newtonian regime, Region III, and m a y include a shear-thickening regime, Region IV. The figure is explained in detail (see S h e a r Thinning Fluids). The chief limitation of the power law models is that they are valid only over a limited range, namely, the linear portion of Region II of Fig. 3. They cannot account for the upper or lower Newtonian regions and, in fact, predict infinite viscosity at zero shear rate and zero viscosity at infinite shear rate, both unrealistic limiting behaviors. Nevertheless, the power law models are found to be quite useful within their limitations, particularly for engineering-type calculations. The Herschel-Bulkley equation has been found to be superior to a n u m b e r of other models in describing the flow behavior of a wide variety of coatings materials over a useful range of deformation conditions [8]. Extending the range of validity beyond Region II in Fig. 3 requires more elaborate models. A simple extension of the power law model is to add an upper Newtonian limiting viscosity, 7]~ (14)

= ~ + K'~, "-1

This expression is known as the Sisko model [9] and includes Regions II and III of Fig. 3. Of several proposed models encompassing Regions I, II, and III inclusively, two in particular have perhaps found wider acceptance and utility in the literature. These are the Cross and Carreau models. Hieber et al. [10] recently wrote a general form of which the Cross and Carreau models are special cases (here modified to include Region III)

~/0

log "17

I[I

log "Y FIG. 3-Generalized equilibrium flow curve: rio is the zeroshear viscosity (random structure, maximum disorder); ~q| is the high-shear limiting viscosity (maximum order). Region I is the first Newtonian plateau; Region II the power law regime; Region III the second Newtonian plateau; Region IV the shearthickening regime. (Adapted from Ref 6.)

"qo -- "r/~ = ~ -~ (1 + [/37],)(,-,)/a

(15)

Here, T0 is the first Newtonian plateau viscosity and a is a constant that determines the curvature of the transition region between the lower Newtonian regime and the power law regime. The value of a can be a measure of the breadth of the molecular weight distribution of a polymer [10] or perhaps the particle-size distribution of a colloidal dispersion. Setting a = 1 - n in the above expression yields the Cross equation; setting a = 2 gives the Carreau-B model. The parameter n has the identical meaning as in the simple power law model (Eqs 11 and 12), i.e., it is the slope of the power law region in a loglog plot of shear stress versus shear rate. The constant/3 has the dimension of time and is actually a time constant representing a characteristic time of the system. This time constant m a y be related to, for example, the diffusional or rotational relaxation time of the flow units (be they colloidal particles or polymer chains) or to the time for rupture of particle flocs or aggregates under shear. The location of the transition from the initial Newtonian plateau (Region I) to the shear thinning regime (Region II) in Fig. 3 is governed by the value of/3 in an inverse sense: increasing/3 decreases the shear rate of the onset of shear thinning and vice versa. In other words, /3 defines a characteristic shear rate of transition, "]/,r [11-13]. ~ltr -

1

/3

(16)

It is tempting to postulate that /3 corresponds to the time constant for Brownian diffusion of the primary flow units of a fluid. Stokes-Smoluchowski-Einstein theory gives us the value of/3 for a particulate dispersion from 48w~R 3 /3 - - kT

(17)

where r/ is the viscosity of the continuous phase, R is the particle radius, k the Boltzmann constant, and T absolute temperature. For a typical aqueous latex dispersion, R = 125 nm, ~1 = 0.05 P, for which (at 25~ = 0.36 s, corresponding to "Ytr = 2.8 s - l . When the experimental shear rate equals 1//3, the shearordering effect begins to dominate the randomizing effects of Brownian motion, and onset of shear thinning is seen [14] (see S h e a r - T h i n n i n g Fluids). This event corresponds to Point c in Fig. 3. Equations 16 and 17 show how the transition from Newtonian to shear thinning behavior m a y be controlled. Any variation which increases the value of/3 (such as increasing the effective particle size, the continuous-phase viscosity, or lowering the temperature) will move the shear thinning transition to lower shear rates. Decreasing/3 extends Newtonian behavior to higher shear rates. While Eq 17 is strictly valid only for very dilute dispersions, it still provides qualitative guidelines for manipulating the rheology of dispersions. For concentrated dispersions, "Oshould be taken as the viscosity of the dispersion [15-17]. It follows from the above discussion that the broader the size distribution of the flow units, the wider the spectrum of relaxation times and the more gradual the transition from Region I to Region II (corresponding to a smaller value of a).

CHAPTER 33--RHEOLOGY AND VISCOMETRY The shear rate of transition is fixed by/3 (a mean relaxation time), while a represents the distribution of relaxation times.

Shear-Thinning Fluids The term pseudoplastic has been applied t o fluids which decrease in viscosity with increasing shear rate (or shear stress) and implies shear-thinning behavior without yield stress. However, the term is passing out of use in favor of the more general description shear thinning. Particulate dispersions, polymer colloids, and polymer solutions can display this behavior above a certain concentration threshold. Viscosity is a measure of the dissipation of energy or the "energy cost" to flow. Shear thinning behavior, therefore, implies that an increase of shear rate causes a structural change in the fluid that allows it to flow more efficiently, consequently with less energy loss. The mechanism 6 involves a shear-induced increase in order, or anisotropy, within the system. Thermal (Brownian) motion tends to keep systems disordered (of random order). Shear forces work against this, tending to impose orderliness. If shear rates are low, the randomizing forces win out and the viscosity does not change for small increases of shear rate (Point a to Point b in Fig. 3). Since the structure is no less random anywhere in Region I than at zero shear rate, the viscosity equals ~?o,the zero-shear value. As the shear rate approaches a critical magnitude (see Shear-Dependent Viscosity), the competition of thermal randomizing and shear ordering starts to favor ordering (Point c in Fig. 3). In the case of polymers in solution, randomly coiled polymer chains tend to stretch in the direction of shear, partially uncoil, and align in more or less parallel fashion depending on the strength of the shear field. The particles of a dispersion tend to line up like "strings of pearls" (Fig. 4) and eventually in ordered planes perpendicular to the shear gradient [18-20]. The result is a steadily decreasing viscosity with increasing shear rate as the degree of order increases. Ultimately, if the shear rate is high enough, the maximum amount of shear ordering possible is attained and the viscosity again becomes independent of shear rate (Newtonian). Figure 3 shows the overall way in which viscosity varies with shear rate for systems such as those described above where Region I is the low-shear Newtonian regime (where Brownian motion controls structure). Region II is the shearthinning segment of the flow curve (where shear forces control the structure). It is found that the viscosity decreases exponentially with shear rate here; hence, it is often referred to as the "power law" regime. Region III is the high-shear Newtonian regime. Here, the maximum degree of shear ordering has been attained; thus, the viscosity is once again independent of shear rate. The high-shear limiting Newtonian viscosity is usually given the symbol ~ . Region IV is a shear-thickening region which is occasionally seen, especially with concentrated dispersions. In actuality, shear thickening in dispersions may occur at virtually any magnitude of 6The following discussion applies strictly to "stable" systems, i.e., those in which the net force between flow units is repulsive and therefore which do not flocculate. The shear-thinning mechanism for unstable systems (net interparticle force attractive) is discussed in the subsection entitled Mechanism of Thixotropy.

339

shear rate depending on dispersion concentration [21], so that one or more of the other regimes are obliterated. That is, the equilibrium flow curve may consist of Regions I to IV, I, II, and IV only, I and IV only, or IV only. Note, once again, that structural order and viscous dissipation are inversely related. An increase in order means decreasing viscosity (Region II), while a decrease in order results in an increase in viscosity (Region IV) [22]. It follows that, for Newtonian behavior, no change must occur in structural ordering with shear. If the disperse system is unstable, i.e., tending to flocculate, the dotted curve may be followed (Fig. 5) instead of displaying a low-shear Newtonian regime [7]. Some systems may possess an apparent yield stress (see

Plastic (Yield) Behavior). (See DISPERSION RHEOLOGY for additional discussion of particulate system rheology.) As a general statement, the range of accurate measurement of most laboratory viscometers (for typical coatings fluids) is in the power law region. It may require extraordinary methods or special instrumentation to characterize fluid behavior in Regions I or III.

Shear-Thickening Fluids We have seen above that shear thinning involves a shearinduced increase in order of a system. This allows the elements of a fluid to move or flow with minimum expenditure of energy. Conversely, shear thickening evidences that shear has caused a decrease in order of a system. The resulting disordered system dissipates more energy during flow and hence is more viscous. An example of this is provided by the catastrophic increase in viscosity observed by Hoffrnan [19], resulting from the "buckling" or "log-jamming" of ordered, layered arrays of particles. One frequently encountered type of shear thickening behavior is dilatancy. Properly, dflatant behavior is shear thickening accompanied by an increase in volume of the fluid. It is most commonly observed in relatively concentrated disperse systems. In a dilatant system, the disperse phase particles are "wetted" with the minimum amount of liquid continuous phase. Furthermore, at rest, the particles of the disperse phase are in a random close-packed structure for which the interstitial volume is relatively minimal (Fig. 6). If the dispersion moves only slowly, adequate time exists for the meager liquid phase to flow sufficiently to maintain the dispersed phase in a "wetted" state, and the system is able to maintain its close-packed structure. Faster or more forceful motion causes a liquid-starved condition because the interstitial volume increases when the system is deformed or made to flow (Fig. 6). There is no longer enough liquid to lubricate the system. The particles are, therefore, incompletely wetted, and forced flow would ultimately create microscopic voids, leading to fracture of the material. The surface of a dilatant material may appear dry when stressed due to the withdrawal of surface liquid into the increased interstitial volume. This is seen when walking on wet sand on the beach. The resistance to deformation of the material can increase tremendously with increased deformation rate due to these effects. During the course of a pigment grind operation, a fairly sudden maximum in viscosity is often seen and is an indication that a good grind (i.e., to primary particles) has been achieved. In fact, the surge in viscosity and power draw result

340 PAINT AND COATING TESTING MANUAL o

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9 Q o

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FIG. 4-Dilute suspension of glass spheres in a polymer solution confined between glass plates with small plate separation: (a) just after loading, particles are randomly distributed; (b)-(d) after being sheared in a side-to-side direction at both increasing duration and shear rate. (By permission from J. Michel, R. P~tzold, and R. Donis, R h e o l o g i c a Acre, Vol. 16, 1977, p. 317. Cited in Ref 20.)

permit flow under even very low stresses. For this reason, paints in a dilatant state may suffer from rapid settling and be difficult to redisperse. For most coatings operations, dilatancy is, in fact, undesirable. Pumping of dilatant dispersions may result in high back-pressure in lines, excessive

03

~X \\ \\

log "~

X\ \n \

Dilatant Behavior

\\

log FIG. 5-Flow curve seen for unstable (flocculating) dispersions is indicated by the ascending curve going toward infinite viscosity at low shear rate.

from the grind having become dilatant. The dilatancy is desirable here because it facilitates energy transfer throughout the grind. Thus, dilatancy is frequently an indicator of achievement of a stable dispersion to primary particles. Dilatant systems, in general, are not also thixotropic and therefore

random close-packed

under shear

Volume Expansion Under Shear FIG. 6-Dilatant behavior-volume expansion under shear. Random close-packed structure gives way to less-efficient packing with necessary volume dilation.

CHAPTER 33--RHEOLOGY AND VISCOMETRY 341 wear of system components, high power consumption, and loss of metering. In industrial rollcoating, dilatancy shortens coating lifetime on the roll, causing "dry edges" and loss of film thickness control. Dilatancy is very sensitive to dilution and can be dramatically reduced or eliminated by a small reduction in paint solids. Also, addition of flocculants, electrolyte, a particulate phase of different particle size, or warming can alleviate undesirable dilatancy. Flow curve measurement provides an excellent means of quantifying the degree of reduction of dilatancy achieved by these measures.

Time-Dependent Fluids Time-dependent fluids are those whose viscosity is a function of both shear rate (or shear stress) and time. The most common such behavior encountered in coatings is time-dependent shear thinning or thixotropy [23]. At constant shear rate and temperature, the viscosity of thixotropic fluids will fall, eventually reaching a constant value. If the shear rate is changed, the viscosity will approach a new equilibrium value at a characteristic rate. This behavior is illustrated in Fig. 7, showing the way viscosity changes for a time-dependent system when the imposed shear rate (or shear stress) is changed in steps. Initially, the shear rate is zero, and the viscosity is very high or infinite for a thixotropic system. As a shear rate greater than zero is imposed (%), the viscosity drops exponentially, reaching a constant value. Increasing the shear rate to ~/2 decreases the viscosity further to a new equilibrium value. A decrease of the shear rate to % results in an exponential rise of viscosity to a higher equilibrium value. If, instead of a multiple steady-shear experiment, just described, we were to carry out a shear rate (or shear stress) ramp experiment (Fig. 8a), the time dependence would result in a loop (Fig. 8b). The explanation of the loop is given below, but has to do with the fact that in a ramp experiment the equilibrium structure is never attained. The circuit in Fig. 8b is called a thixotropic loop. Roughly bisecting the loop is the equilibrium-structure curve generated from the equilibrium viscosity data of Fig. 7. The analogous time-dependent behavior for

"Y--o

>4,1

"YI>0

"Y3 < "ir 1

r/

Time FIG. 7-Time-dependent fluid behavior. At ~, = 0, viscosity is high (or infinite), Imposing ~'1 > 0 causes viscosity to fall exponentially, reaching new equilibrium value. Subsequent changes in ~, result in re-equilibration of structure, hence viscosity.

shear thickening materials has been called rheopexy (a rather rare phenomenon). The preferred term now for the latter is

antithixotropy. "Thixotropic Index" Test We emphasize that the most important characteristic of thixotropic behavior is not shear thinning alone, but time dependence. The relatively slow change of Viscosity provides a means of control of flow behavior and is the reason thixotropy is formulated into coatings. Hence, it is important to correctly characterize the time-dependent aspect of the behavior. For this reason, the so-called "Thixotropic Index" test is of limited benefit for characterization of such systems. We will digress for a moment to discuss the Thixotropic Index test since it is very widespread in practice. This misnamed test determines the ratio of viscosities measured at arbitrary high- and low-shear conditions. ASTM Test Method D 2196 specifies taking the ratio of viscosities measured on the Brookfield Synchro-Lectric viscometer at two speeds, representing a ten-fold speed ratio. Since only equilibrium viscosities are measured, the test yields no information about time dependence and should be called the "Shear-Thinning Index" instead (it is so termed in ASTM D 2196). Its value, like many quality-control tests, is in its simplicity and "quickand-easy" character, requiting no interpretation. As long as a given coating formulation changes little, so that the hidden kinetic factor would be expected to change little, the "Thix Index" can reveal when something has gone awry (i.e., can be useful as a gross indicator whether a formulation error has occurred). However, it is always possible that the recovery kinetics of the system have changed, which would go completely unnoticed if the "Thix Index" were the only test used to evaluate the rheology. Below, we will present test methods which can be virtually as simple as the "Thix Index," but which give more useful, relevant, and complete information about thixotropic systems.

Mechanism of Thixotropy Thixotropy is formally defined as an isothermal, reversible sol-gel (fluid-solid) transformation [24]. As stated, it is experienced as a viscosity which is both time- and shear-dependent. Its origin is the breakdown, under shear, of internal fluid structure to smaller flow units or the reassembly of structure from smaller units when shear is relaxed. The mechanism of shear thinning in thixotropic (time-dependent) systems is different from pseudoplastic (time-independent) systems. In pseudoplastic systems, shear thinning is the consequence of order externally imposed by shear [14]. In thixotropic systems, an internally imposed, viscosifying structure exists at rest, and the viscosity falls because of the collapse of that structure under shear. (Flocculated systems possess extra mechanisms of energy dissipation [25]; therefore, their viscosity will be higher than that of a deflocculated dispersion of the same composition.) A further important difference between the two is in the amount of time the structure requires to respond to changes in shear rate. In the case of time-independent systems, rapid structural equilibration quickly accommodates to changes in shear rate. The accommodation process essentially is particle diffusion, which is quite rapid for submicron particles (see Shear-Dependent Viscosity). The viscosity (a measure of structure)

342

PAINT AND COATING TESTING MANUAL

~

A

T T OT

=

o,T

gRLr~ C~'Vr

[

Time

~1 ~p

time

FIG. 8-(A) Shear rate or shear stress ramp experiment. Shear rate or stress increases linearly (sometimes logarithmically, optionally) to preselected maximum over a selected sweep time period, resulting in a given acceleration gradient (~) or stress rate (~-); (B) Thixotropic loop. Equilibrium structure curve (approximately bisecting loop) corresponds to data from an experiment like that in Fig. 7.

thus always "keeps up" with changes in shear rate for such systems. For thixotropic systems, the rate of structural reorganization is slower than the experimental rate of change of shear rate (~) or slow with respect to the time of observation at constant shear rate. In a "shear rate ramp" experiment (see below), the structural breakdown always lags behind the ever-increasing shear rate, so that the viscosity on the "up" r a m p will be higher than the equilibrium viscosity at a given shear rate. On the "down" side of the ramp, the rebuilding structure lags behind the rate of reduction of shear rate, so that the measured viscosity is always lower than the equilibrium value. Thus, a "thixotropic loop" is seen, the size of which should be proportional to the structural time constant for a given r a m p time. Holding the shear rate constant, as in the description above, allows the structure breakdown process to "catch up." The viscosity then continues to change until an equilibrium between structural breakdown and recovery rates is achieved (see Fig. 7).

Thixotropy Test Methods Two c o m m o n test methods for experimental characterization of thixotropy are the "Thixotropy Index" test, and the "Thixotropic Loop" test, described above. In the latter, a p r o g r a m m e d increase and decrease of shear rate (or shear stress) is applied to a material (Fig. 8A), resulting in the generation of a loop (Fig. 8B). The size of the loop can be obtained by numerical integration and is taken as an indicator of the rate of structural breakdown and recovery for a given r a m p time (the larger the loop, the slower the breakdown-recovery rates)9 This m e t h o d is a difficult one to do correctly, however, and is more complex than first appears 9 The result (loop size) depends on the shear history of the s a m p l e - - t h e time interval since its last shear experience-and the magnitude of the shear undergone 9 It also depends on the shear rate (or shear stress) m a x i m u m in the loop experim e n t and the r a m p rate (# or ~). A better m e t h o d is one which puts the thixotropic material into a k n o w n state, i.e., having had a controlled shear experience, followed by the test9 Such a method is the step-shear

+--o

')'1> 0

~t2< ~1

(

f f

--~/00

7 Time FIG. 9-Step-shear method for thixotropic recovery. High shear rate ~'1 simulates application process, followed by low shear rate ~'2, emulating film after application. Viscosity recovers from zero-time value ~1(o)with a characteristic time constant 7, eventually reaching final level ~(| test (Fig. 9). In it, a high shear rate (order of 104 s -1 desirable) is applied to the material, allowing time for the viscosity (hence, structure) to reach equilibrium; then the shear rate is suddenly reduced to a very low value (order of 1 s-1). The equilibrium a m o u n t of structure which can exist at the lower shear rate is greater than at the previous high shear rate, so reassembly of fluid structure occurs, accompanied by an approximately exponential rise of viscosity. Meaningful physical constants can be extracted from the data by fitting the following equation to the recovery curve -q(t) = ~/(0) + [~(o0) - n(0)] [1 - e -t/v]

(18)

where ~(t) is the viscosity as a function of time, t, "O(0) the sheared-out viscosity (at time zero), ~(~) the infinite-time recovered viscosity level, and 9 the time constant describing the recovery rate9 The ratio ~(~)/~, which we have termed the Recovery Parameter, has been found in our laboratories to correlate well to thixotropy-related properties such as sag

CHAPTER 3 3 - - R H E O L O G Y AND V I S C O M E T R Y

TABLE 3--Gel coat thixotropy test data comparison: z is the recovery time constant and ~(~) the recovered viscosity level from a step-shear experiment; ~(m)/r is the recovery parameter. Composition

~, s

r/(00),p

~(~)/~, P/s

Thix Index

Sag?

With additive Without additive

8.9 18.2

116 97.3

13 5.4

4.04 4.24

No Yes

resistance a n d air e n t r a i n m e n t [26]. Table 3 shows d a t a for two gel coat formulas, o n e sagging a n d one nonsagging. The conventional "Thix Index" results do not predict the sag behavior and, in fact, are o p p o s i t e w h a t w o u l d be expected from the observed behavior. The s t e p - s h e a r e x p e r i m e n t a l p a r a m e ters T a n d ~(~) are given, along with their ratio. The nonsagging f o r m u l a ("with additive") has b o t h a s h o r t e r recovery t i m e a n d a higher final recovered viscosity, a n d the recovery p a r a m e t e r takes b o t h of these into account to p r e d i c t significantly b e t t e r sag resistance t h a n for the "without additive" material. The step-shear test puts useful t h i x o t r o p y c h a r a c t e r i z a t i o n within r e a c h of a n y d e v e l o p m e n t lab, for it can be p e r f o r m e d on a n inexpensive v i s c o m e t e r (such as the Brookfield Synchro-Lectric o r Wells-Brookfield Cone/Plate Viscometer) as well as on m o r e s o p h i s t i c a t e d i n s t r u m e n t a t i o n . The latter offer advantages, of course, p a r t i c u l a r l y if possessing viscoelastic c h a r a c t e r i z a t i o n capability. The ideal way to characterize thixotropic recovery is to b r e a k d o w n the structure u n d e r high, steady shear (as in the step-shear method), t h e n step d o w n to a s m a l l - a m p l i t u d e (small strain) oscillatorys h e a r test, as described by Dodge [27] (Fig. 10a). The r e b u i l d i n g of structure can then be followed b y m e a n s of the viscoelastic p a r a m e t e r s , w h i c h are sensitive p r o b e s of the fluid structure responsible for thixotropy. This m e t h o d m o r e faithfully m i m i c s processes o c c u r r i n g in the relatively quiescent film after application. Figure 10b shows the recovery curves of the viscoelastic m o d u l i G' a n d G" (see The Viscoelastic Parameters and Their Measurement) from such a step-shear experiment.

Plastic (Yield) Behavior An ideal plastic m a t e r i a l behaves as an elastic solid until a critical stress is applied, w h e r e u p o n it will "yield" a n d bec o m e fluid. This critical stress is the yield stress, the minim u m stress necessary to initiate flow. Ideally, the p r o p e r t i e s of a m a t e r i a l exhibiting yield b e h a v i o r w o u l d be those of an elastic, or "Hookean," solid, b e l o w the yield stress. F o r such materials, t h e steady-shear viscosity w o u l d be infinite (or undefined), the d e f o r m a t i o n l i n e a r with stress, a n d the yield value a m a t e r i a l constant. F o r m o s t real materials, however, d e f o r m a t i o n b e l o w the yield p o i n t is a c o m b i n a t i o n of elastic strain a n d viscous flow. This is b e c a u s e interparticle forces ("secondary bonds") are of a range of types with a corres p o n d i n g range of r e l a x a t i o n times [28]. As a consequence, the m e a s u r e d yield value will d e p e n d on the rate at w h i c h the stress is i n c r e a s e d up to the p o i n t w h e r e flow occurs (the faster the rate of stress increase, the h i g h e r the m e a s u r e d yield value). 7 7For this reason, yield stress values cited in the literature are meaningless unless the exact experimental methodology is provided.

343

High Steady-Shear Rate

Small-Amplitude Viscoelastic Measurement

Time FIG. lOa-Viscoelastic characterization of structural recovery by step-shear method using small amplitude oscillatory strain for recovery phase.

ts r--t

E q S

v 2

o O O~ O3

b

7

cY CL < I

~-3 o O

0.0

I 2.5

I 5.0

7.5

TIME (minutes) FIG. lOb-Experimental curves of G' (elastic modulus, []) and G" (viscous modulus, O) versus time for a viscoelastic stepshear experiment.

Barnes a n d Walters [29] have c l a i m e d that m o s t m a t e r i a l s with an a p p a r e n t yield stress will be found, in reality, to have a high b u t finite viscosity if m e a s u r e d at sufficiently low stresses. In practical terms, yield stress b e h a v i o r can have i m p o r tant c o n s e q u e n c e s for the processing, stability, a n d application of coatings. There is quite obviously s o m e sort of flow d i s c o n t i n u i t y that occurs at low stresses, the m a g n i t u d e of w h i c h is a p p a r e n t l y related to the n u m b e r a n d strength of interparticle attractive forces [28,30]. The yield stress is therefore a n "engineering reality" [31 ] that m u s t be taken into

344

PAINT AND COATING TESTING MANUAL

account when formulating paints or dispersions. A yield stress m a y be desirable or undesirable depending on the process in question. Materials with a yield stress will often exhibit thixotropy and viscoelasticity as well.

Static Versus D y n a m i c Yield Stress There are two types of yield stress which m a y be measured. The first is the static yield stress, measured by the startup of flow from nonflowing conditions. The second is the dynamic yield stress, which is the shear stress at which a presently fluid material suddenly turns solid (or exhibits a flow discontinuity). Experimental methods exist for measuring both [32], and the type which is relevant to the particular process in question should be the one measured. For example, for long-term suspension of solids or for start-up flow in pumps, the static yield stress is relevant. Postapplication leveling and sag behavior would be governed by dynamic yield stress characteristics, including the kinetics of structural recovery (see thixotropy in T i m e - D e p e n d e n t Fluids). Some have used curve-fitting techniques to obtain a value for the yield stress by fitting a mathematical model for flow to a rheological flow curve. The Bingham, Herschel-Bulkley, and Casson models contain a yield stress term and have been c o m m o n l y used to obtain a yield stress from measured flow behavior. Such methods are questionable, however, since one is attempting to infer a property of the solid state from behavior of the fluid

[25,33]. Yield Stress Test M e t h o d s Startup-of-flow methods include the use of penetrometertype instruments such as a thermomechanical analyzer, where increasing force is applied to a standard probe. The yield stress is determined as the break point of a forcepenetration curve [34]. In recent years, a new type of instrument known as a controlled-stress rheometer has become commercially available that is well-suited to measurement of yield stress and other properties of structured materials. For this instrument, the shear stress is the independent (or controlled) variable, while the dependent (or measured) variable is the shear rate. This allows an experiment to be done whereby the stress is gradually increased from zero or a very low value, registering zero shear rate (i.e., infinite viscosity) until the applied stress reaches the yield value, whereupon the viscosity becomes finite. The instrument reports this value as the measured yield stress. Another recent method employs a special vaned rotor to remedy problems of slip with yield-stress materials [35-37]. The vaned rotor consists of rectangular blades or vanes fixed to a rotating shaft. Advantages of this geometry are: (1) there is little disturbance of the sample when this type of probe is inserted, and (2) when the rotor turns, the material moves as a solid "cylinder." Thus, the shear surface is within the material itself, avoiding problems of wall slip.

Practical Aspects o f Yield B e h a v i o r Some of the practical consequences of yield behavior occur in the processing of, for example, pigment dispersions. Dispersions with high yield stresses m a y be difficult to dispense accurately or reproducibly via automatic metering systems. Pumps m a y actually refuse to move or m a y cavitate while attempting to p u m p such materials; solenoid valves m a y

"freeze." Coatings with significant yield stresses m a y exhibit poor leveling [2] since, as the shear stress decreases during the course of the leveling process, the material effectively becomes immobile when the dynamic yield stress is reached. On the other hand, a small yield stress can be of great value in inhibiting settling of particulate suspensions. One can easily calculate the shear stress exerted on the surrounding medium by a spherical particle falling under the influence of gravity

Fg _ Force of gravity on particle T--

A

Surface area of particle

Fg = 4 7rR3(pp _ Pl)g;A

4~rR2

~- = -~ (pp - p~)g where R is the particle radius, pp the particle density, and Pl the liquid density. For a titanium dioxide particle of radius 0.2 m m and density 4 g/cm 3, the shear stress due to gravity acting on the particle is about 0.2 dyne/cm z. To prevent settling of such a particle, the surrounding m e d i u m need only resist with an opposing stress of greater than 0.2 dyne/cm z. Even this, alas, is impossible for a Newtonian liquid, be it water or honey, since it will flow under all stresses no matter h o w slight (it is only a question of how slowly). A pigment particle will inevitably settle out of such a fluid, especially for viscosities of practical magnitudes. However, if the suspending m e d i u m possesses a yield stress equal to or greater than the particle shear stress, the particle "thinks" it is suspended in a solid and will be suspended indefinitely. This a r g u m e n t assumes more or less ideal yield behavior, which, as described above, m a y rarely be encountered. A m e d i u m with a measured yield stress of apparently sufficient magnitude m a y or m a y not permit the particle to settle over long periods of time due to the possibility of viscous flow below the apparent yield point. Therefore, to ensure adequate practical stability, one should build in a higher yield value than that calculated from the above.

Elastic Liquids (Viscoelasticity) In ideal viscous flow, all energy input is converted either to heat or energy of motion. None is stored (i.e., none is converted into potential energy). Therefore, viscous flow results in irreversible deformation. Newtonian liquids show essentially ideal viscous (also called "inelastic") behavior over a wide range of deformation rate. For ideal elastic CHookean") substances, all the energy of deformation is stored, similar to a stretched rubber band. Consequently, elastic deformation is not permanent, but is in fact completely reversible. Real fluids can display elasticity, but mixed with viscous character, in varying degrees. Hence, the term viscoelastic is applied to such materials. Not all non-Newtonian fluids have significant elastic properties, but m a n y do. The presence of significant elasticity in colloidal systems generally means there is a microscopic three-dimensional structure, or association network (rnicrostructure), within the fluid. Elementary flow units are linked together in some fashion such that the struc-

CHAPTER 33--RHEOLOGY tural relaxation time (see below) is measurably long. This structure, and particularly the destruction and rebuilding process that occurs during and after coating application, can have great consequences for application and film formation processes. An example of this is the influence of elasticity on leveling of an applied coating [38] (see also Massouda [39] and Glass [40]). As the term implies, viscoelasticity refers to a material response which is a combination of viscous and elastic behavior. Viscous flow superimposed on elastic strain results iia the "relaxation," or gradual disappearance, of stress within the strained object. This is manifested as an imperfect, or fading, stress memory. Figure 11 illustrates the material response to an applied elongational stress for a material with permanent stress m e m o r y (e.g., rubber band) and one with fading stress m e m o r y (e.g., "bouncing putty"). The Viscoelastic P a r a m e t e r s a n d Their M e a s u r e m e n t A convenient way of experimental characterization of viscoelastic materials is by alternating the direction of strain (or stress). Most often, a sinusoidal deformation is employed (Fig. 12). The strain amplitude must be kept small so that the material response remains in the linear viscoelastic region where stress and strain are linearly related. 8 In perfectly elastic behavior (Hookean spring), the stress and strain are "in phase" with each other, that is, u p o n deformation, the maxim u m stress and m a x i m u m strain occur at the same instant in time. If there is any viscous (energy-loss) component in the material response, the stress and strain maxima will not be coincident, but will be "out of phase." This happens because viscous flow relieves the stress within the material, causing the stress m a x i m u m to occur before the strain maximum. That is, as the rate of strain decreases near the m a x i m u m (or m i n i m u m ) of the strain sine wave, stress relaxation "catches up" and overtakes the stress-building effect of the strain. The separation in time of the stress and strain maxima is called the phase shift (orphase angle, the fraction of a complete cycle in degrees or radians that the phase shift represents). The phase shift is often given the symbol g. The faster the viscous stress-relaxation process, the earlier in the cycle the stress m a x i m u m will occur, i.e., the larger the phase shift will be. The limiting value of the phase angle for purely viscous liquids is 90 ~ (see Fig. 12). The reason is that, for sinusoidal deformation, the m a x i m u m strain rate (maximum slope of the strain sine wave) occurs 90 ~ ahead of the m a x i m u m strain, and that, for Newtonian liquids, the stress is proportional to strain rate. Therefore, the m a x i m u m stress must occur at the m a x i m u m strain rate, which corresponds to a phase shift of 90 ~. The derivation of viscoelastic parameters from a sinusoidal shear experiment begins with the calculation of the complex shear modulus, G* (refer to Fig. 13). This is simply the ratio of the m a x i m u m stress, ~o, to the m a x i m u m strain, 70 (G* = %/70). F r o m G* m a y be separated G', the modulus of elasticity (storage modulus), and G", the viscous modulus (loss modulus). Figure 13 demonstrates the geometric relationship of G* to its in-phase and out-of-phase components, G' and G", governed by the phase angle, 6. It is obvious from geometry 8This is a requirement because the equations used are valid only in the linear viscoelastic region.

AND VISCOMETRY

345

rubber band

oB viscoelastic material

time FIG. 11-Viscoelastic stress-memory loss: stretch-force experiment. Rubber band is cross-linked polymer, does not relax stress ((rE) when stretched. Viscoelastic material can accommodate to strain by molecular motion (viscous flow), allowing elastic stress to decay. 6=o ~

Stress response, elastic solid 6=90 c

Stress response, viscous liquid

Time FIG. 12-Phase shift/~ for ideal viscous and elastic bodies.

that when 8 is zero, G* = G', and when 6 = 90 ~ G* = G". The physical meaning is that, when 8 = 0, all of the measured modulus is due to elastic effects, and when 6 = 90 ~ the modulus consists entirely of viscous effects. This follows from the discussion of the phase shift, above. By trigonometry, G' = G* cos 6 and G" = G* sin 6. Alternately stated, G' is the c o m p o n e n t of the complex shear modulus which is in phase with the strain. G' therefore represents the elastic part of viscoelastic behavior. The viscous c o m p o n e n t (G"), on the other hand, is derived from the part of the modulus which is out of phase with the strain, but in phase with the strain rate. Consider a dynamic test in which a sinusoidally varying strain is applied to a viscoelastic material. The angular frequency of the deformation is given by to -- 2~'f, where f i s the frequency of oscillation (s i) and to is in rad/s. Now, the amplitude of the strain with time is given by 7 = 7oCOS tot

(19)

which describes a cosine wave of m a x i m u m amplitude 3'o and period 1/to. The strain rate experienced by the material is then j, = - my o sin 0Jt

(20)

346

PAINT AND COATING TESTING MANUAL

out-of- T phase axisl

G'

in-phaseaxis

I

"

Complex

G* = s h e a r stress

modulus

shear strain

Storage modulus

G' = G ' c o s

Loss modulus

G"

G'sin

=

G

Dynamic viscosity

O0 -

'YO

(~

I!

'J~'--

FIG. 13-Viscoelastic relationships in the complex plane. The oscillating strain p r o d u c e s a stress response o- = o-oCOS (cot + 8)

(21)

w h e r e 8 is the p h a s e shift, advancing the p h a s e of the stress relative to the strain. E q u a t i o n 21 m a y be equivalently w r i t t e n

[96] o-* = % cos 8 cos cot - o-0sin 8 sin cot

(22)

w h i c h describes a complex stress, where, by complex plane relationships analogous to Fig. 13, o-0 cos 8 is the stress comp o n e n t in-phase with strain a n d o-osin 8 the stress c o m p o n e n t w h i c h is out-of-phase with the strain (but in-phase with the strain rate). We therefore can define a d y n a m i c viscosity, "o', as the quotient of the stress in-phase with the strain rate divided by the strain rate 7'

.

o-osin 8 . . ~o

o-o sin 8 . . Y0co

.

G* sin 8

G"

co

co

(23)

The above expression is o b t a i n e d using the relationships % (strain-rate a m p l i t u d e ) = Y0coa n d G* = o-0/Y0. The m a g n i t u d e s of G" a n d G' reveal the relative i m p o r t a n c e of viscous a n d elastic b e h a v i o r in the m e c h a n i c a l response of a material. In Fig. 13, again by trigonometry, the t a n g e n t of the p h a s e angle equals the ratio G'TG'. Thus, t a n 8 quantifies the b a l a n c e of energy loss to energy storage m e a s u r e d u n d e r certain conditions of t e m p e r a t u r e , pressure, a n d frequency o r rate of d e f o r m a t i o n . F o r solids, t a n 8 can be useful to p r e d i c t the likelihood of brittle o r ductile failure of a p o l y m e r or the s o u n d a b s o r p t i o n o r v i b r a t i o n d a m p i n g properties. F o r liquids, t a n 8 can m o n i t o r the progress of fluid r e s t r u c t u r i n g in thixotropic recovery. The latter m a y be useful in c o m p u t i n g sag resistance for w h e n t a n 8 b e c o m e s less t h a n unity, the system has essentially b e c o m e i m m o b o l i z e d (i.e., r e a c h e d a " d y n a m i c yield point"). W h e n t a n 8 > 1, viscous flow (i.e., a steady-state viscosity) is possible. W h e n t a n 8 < 1, however, the m a t e r i a l is m o r e elastic t h a n viscous (i.e., a p e r c o l a t i n g n e t w o r k extends t h r o u g h o u t the bulk, a n d it is essentially an elastic solid).

Viscoelastic Models It turns out that viscoelasticity can be r a t h e r realistically m o d e l e d b y simple m e c h a n i c a l analogues. These are useful not only as an aid to c o n c e p t u a l i z a t i o n of viscoelastic behavior, b u t also in helping to u n d e r s t a n d the e l e m e n t a r y m a t h e matics of viscoelasticity. As we have said, viscoelasticity is a

c o m b i n a t i o n of two idealized behaviors: H o o k e a n elasticity and N e w t o n i a n viscosity. The m e c h a n i c a l a n a l o g u e of H o o k e a n b e h a v i o r is a spring of force c o n s t a n t G, a n d t h a t of N e w t o n i a n flow is a "dashpot" (a piston-in-cylinder filled with a viscous fluid of viscosity "O).These elements are c o m b i n e d in various ways to m o d e l viscoelastic m e c h a n i c a l response. To build o u r first model, we will connect a spring and d a s h p o t in series (Fig. 14), an a r r a n g e m e n t k n o w n as the Maxwell model. To i m a g i n e w h a t response the m o d e l has, let's a p p l y a c o n s t a n t stress, o-0, to one end, the o t h e r being fixed. By the way, o u r d a s h p o t is c o n s i d e r e d to be infinitely long so that the piston never r u n s out of travel. This being the case, one can see that an e q u i l i b r i u m strain w o u l d never be reached, b u t the d a s h p o t w o u l d continually move as long as the stress is applied. Since the m o d e l can "flow" w i t h o u t limit, this is obviously a m o d e l for viscoelastic liquids ( s o m e t i m e s called "elasticoviscous"). We have just d e s c r i b e d w h a t is k n o w n as a creep experiment, in w h i c h a s u d d e n stress o-0 is applied, a n d the evolution of strain, or d e f o r m a t i o n , is followed with time. The creep of a Maxwell liquid is not very interesting, however. The strain-time curve consists m e r e l y of a straight line with intercept equal to o-0/G a n d slope of o-0/~ (Eq 26). We will use the Maxwell m o d e l to illustrate h o w equations describing viscoelastic b e h a v i o r m a y be derived. Just as the stress is the s a m e in every p a r t of a stretched string, so is the stress the s a m e on b o t h elements for the Maxwell model, a n d also, therefore, the rate of change of stress is identical as well. O'Total =

o-Elastic =

O-Vi . . . . .

( = O-0)

(24)

and

o"T = 6-e = ou

\ \ \ \ \

G

FIG. 14-Maxwell model. G is force constant of spring; ~ the viscosity of dashpot fluid; ~ro is the applied stress.

(25)

CHAPTER 33--RHEOLOGYAND

F u r t h e r m o r e , the total strain is clearly the s u m of the strains u n d e r g o n e by the two elements 7r = 3`E + 3`v = 0-E + 0-_~# G B

VISCOMETRY

347

Stress

a0

(26)

The expression on the right follows f r o m the definition of the m o d u l u s (G = 0-/3,), Eq 4, a n d the fact t h a t 3` = ~/t. It is also true that ~T = 7E + ~v = 0-E + 0-v G B

e

An alternative to the creep m e t h o d that is p a r t i c u l a r l y useful for viscoelastic solids is to apply a s u d d e n d e f o r m a t i o n a n d follow the decay of stress w i t h time. This is similar to the e x p e r i m e n t d e p i c t e d in Fig. 11 a n d is k n o w n as stress relaxation. S u d d e n i m p o s i t i o n of a strain 3`o results in instantaneous lengthening of the spring. The d a s h p o t experiences, in turn, an initial stress 0-0 from the extended spring, causing a g r a d u a l m o v e m e n t of the d a s h p o t ' s piston, resulting in relaxa t i o n of the stress. By m a n i p u l a t i o n of the above equations, we can arrive at a quantitative w a y of describing this relaxation process. Since the strain in a stress-relaxation e x p e r i m e n t is constant (3`0), the total strain rate, 7T = 0. Therefore, f r o m Eq 27 0-v _

oE _

B

G

d0-/dt

G

a0

(27)

(28)

X (='r//e)

TEe

relaxation time constant

FIG. 15-Stress relaxation (constant strain) experiment. initial elastic stress r o relaxes viscously with a time constant ,1 ( = ~q/G). A corresponds to the point where stress has fallen to a value ~role.

m e n t s - - s e e below.) E q u a t i o n s derived for the Maxwell m o d e l [42,43] show how the viscoelastic t i m e constant, X, m a y b e o b t a i n e d from the oscillatory d a t a G~0X B'co = G . . . .

rearranging

1 + 602)i 2

do"_

G dt

0-v

B

(29)

and G~2,~. 2

Integrating Eq 29 from t i m e 0 to t i m e t results in the Maxwell stress relaxation expression or = 0-oe - ~ " "

(30)

Figure 15 illustrates the decay of stress in a c o n s t a n t strain or stress relaxation e x p e r i m e n t for the Maxwell model, described b y Eq 30. Note that the quantity B/G has the d i m e n sion of time. We will assign to this q u a n t i t y the symbol A a n d refer to A as the stress-relaxation time constant. It can be seen from Eq 30 t h a t w h e n t = B/G, or = 0-o/e.~Therefore, A represents the time r e q u i r e d for the stress to fall to 1/e of its initial value (Fig. 15) a n d gives a convenient w a y of quantifying the rate of d e c a y of stress in viscoelastic materials. The t i m e constant, h, could also be called the stress m e m o131 t i m e c o n s t a n t since it is related to the t i m e it takes for a viscoelastic m a t e r i a l to "forget" its initial elastic stress level w h e n subjected to a c o n s t a n t strain. As such, X d e t e r m i n e s the role t h a t viscoelasticity plays in a n industrial process. That is, elastic m a t e r i a l s generate a n "extra (elastic) stress" w h e n deformed, w h i c h m a y result, for example, in the stabilization of fluid structures w h i c h w o u l d o r d i n a r i l y collapse t o o quickly to be i m p o r t a n t . These include, for example, liquid fibers a n d webs, w h i c h result in roller s p a t t e r [41]. The magn i t u d e of the u n d e s i r a b l e effect will d e p e n d on the rate at w h i c h the elastic extra stress decays or "relaxes," as we'll see below. In m o d e r n rheometers, sinusoidal oscillation is the m o s t c o m m o n m e t h o d of viscoelastic characterization. (Creep is also available as a viscoelastic test m o d e for controlled-stress i n s t r u m e n t s a n d stress relaxation for controlled-rate instru-

(31)

G' - -

-

1 + ~o2h 2

(32)

from w h i c h tan 6 -

G"

G'

-

1

~0h

(33)

w h e r e G~ is the p l a t e a u m o d u l u s (limiting value of G ' r e a c h e d at high frequency = Maxwell spring constant). E q u a t i o n 33 says that w h e n G' a n d G" cross over (tan 6 = 1), X = l/c0c (~0C is the crossover frequency). The p l a t e a u m o d u l u s can be obt a i n e d from Eq 32, as well, since at the crossover p o i n t G~ = 2G'. Similarly, at crossover, the zero-frequency p l a t e a u viscosity, B0 = 2B' (from Eq 31). If the m e c h a n i c a l elements are c o n n e c t e d in parallel r a t h e r t h a n series, they each experience identical strain, b u t the stresses are n o w additive. This a r r a n g e m e n t is called the Kelvin-Voigt m o d e l (Fig~ 16). Because the elements are n o w in parallel, the Kelvin-Voigt m o d e l can only u n d e r g o finite strain, limited b y the extensibility of the spring. Therefore, this is a m o d e l for a viscoelastic solid b e l o w its yield point. E q u a t i o n s for creep, stress relaxation, a n d t a n 8 for the Kelvin-Voigt m o d e l are Kelvin-Voigt creep 3' = G (1 - e -t/a)

(34)

Kelvin-Voigt stress relaxation or = ToG

(35)

348

PAINT AND COATING TESTING MANUAL

G

\ \ \ \ \

/

0

71 72

II

y

G1 FIG. 18-Burgers model. Symbol definitions same as Fig. 13. Subscript 1 refers to the Voigt element, 2 to the Maxwell element.

FIG. 16-Kelvin-Voigt same as Fig. 14.

then be extracted from the data by analysis according to one or more of the above models.

model. Symbol definitions

Viscoelasticity and Industrial Processes

tan 6 = toh

(36)

These equations describe an exponentially increasing strain at constant stress (see Fig. 17 and Eq 34) and a nonrelaxing stress at constant strain (Eq 35), respectively. The Maxwell and Voigt models, by themselves, are too simple to describe accurately most real viscoelastic materials. However, a Maxwell element connected in series with a Kelvin-Voigt element turns out to model the linear viscoelasticity of m a n y real systems rather well. Figure 18 shows such an arrangement, known as the Burgers model. For completeness (and also because an error in the Burgers stress-relaxation expression has crept into the literature), equations describing creep and stress relaxation behavior for the Burgers model are given Burgers creep % + ~r~ + ~11(1 - e -t/A') = G--2

(37)

~2

Burgers stress relaxation tr = ~/oG2e-t/'x2 + ~0G1

(38)

where hi = ~1/GI and h2 = ~2/G2. The creep and stress relaxation behavior for the Burgers model are shown in Figs. 19 and 20. Modern rheometers of the type known as "controlled stress" (see Rotational Instruments) are capable of performing creep measurements. Viscoelastic constants can Strain,

3'

/

retarded spring motion: "Y(t)= ~0 (l_e-t/X)

Time FIG. 17-Kelvin-Voigt creep (constant stress) experiment. At constant applied stress ~ro,strain increases exponentially with time constant A ( = -q/G).

Now, since industrial processes are of m a n y types, how do we assess the effect of elasticity on a given process? That depends in part on the length of time the process stress is applied to the material. That is, what's most important is not how rapidly a stress is applied nor even the magnitude of the stress, but for how long the stress is applied relative to the time required for any elastic "extra" stress to decay. This suggests taking a ratio of the stress relaxation time, A, to the time (duration) of the process stress, t De -

A t

(39)

This ratio is a defined theological term known as the Deborah number, De. It is n a m e d for the Biblical prophetess Deborah, who prophesied that the "mountains flow before the Lord" [44]. This is a perfectly accurate statement, made long before being verified by the science of geology, of the fact that, on God's time scale, rock formations can be observed to undergo permanent deformation, or flow. In other words, if the stress time scale, t, greatly exceeds the relaxation time, h (De ~ 1), the material will respond as a viscous fluid (because elastic stress has time to decay). Conversely, if De ~ 1, the stress duration is too brief to provide an opportunity for viscous relaxation, and the material behaves as if an elastic solid. Thus, the Deborah n u m b e r quantifies the proportion of elastic to viscous control of a process. This is one reason why a determination of the viscoelastic properties of paints and coatings is important. The results of an analysis by Keunings [38] of the effect of viscoelasticity on leveling can be adapted to the situation of a typical paint, and the influence of the elastic-stress relaxation time constant on leveling rate is shown in Table 4. To be sure, real materials m a y not exhibit simple exponential stress decay (i.e., a single relaxation time), but rather m a y possess a spectrum of relaxation times. However, the mechanical response will be dominated by a "mean" relaxation time (or sometimes the longest relaxation time [38]), obtained from experiments such as described in foregoing sections. Simple viscoelastic dispersions can show Maxwellian behavior with a single relaxation time [45,46]. One of the reasons that associative thickeners (ATs) have been so successful in being able to thicken paints without at the same time adversely affecting flow and leveling is no doubt due in part to their low elasticity. Even though ATs

CHAPTER

33--RHEOLOGY

AND VISCOMETRY

349

slope = strain rate, dT/dt

Strain,

3'

t viscous flow: O~QO

-

.y

t/x)

instantaneous O0 { strain:

77 =

or,

"Y(t)= ~t

G22

Time FIG. 19-Burgers creep (constant stress) experiment. Under applied stress ~0, the Burgers model undergoes instantaneous deformation equal in magnitude to (rolG. In next segment of curve ("retarded spring motion") strain increases exponentially with time. Finally, spring is fully extended and viscous flow occurs with a constant strain rate.

O

viscous relaxation: "Y0G2e-t/x2 _~ unrelaxed stress:

oGi time FIG. 20-Burgers model stress relaxation (constant strain). Stress generated by initial strain ~'o decays exponentially governed by the relaxation time ~ ( = ~/~G2). Voigt element contributes unrelaxed stress ~r| = ~'oG1.

TABLE 4--Leveling rate dependence on Maxwell relaxation time, h. c~ = da/dt, where a = roughness amplitude, and tl/2 = time for a ~ 0.5ao (initial amplitude). ;t, s 0.0 0.1 1.0 10.0 50.0 100.0 c~, s -t tl/2

34.5 0,020

7.75 0.09

0.972 0.67

0.0997 6.95

0.02 34.7

0.01 69.3

generate a three-dimensional network structure within the fluid, 9 they typically would have negligible G' values, in contrast to typical cellulosic thickeners, for example. This is believed to be due to the extreme lability of the micellar junctions of the associative network, resulting in very short network relaxation times [46,47]. Thus, the decay of elastic stress in ATs is so rapid that such stresses are virtually unobserved (De ~ 1). 9See Chapter 30--Thickeners and rheology modifiers.

As stated before, the process consequences of viscoelasticity stem partly from the stabilization of otherwise unstable liquid structures by the elastic "extra stress." Thus, liquid fibers and "webs" which would ordinarily collapse by surface tension are stabilized, producing, for example, excessive rollcoat spatter (or "misting") and ribbing, and inhibiting atomization of sprayed materials. The importance of viscoelasticity for a particular process is gauged, using the Deborah number, by the ratio of the stress-relaxation time constant to the time duration of the process stress. Of course, both the magnitude and lifetime of the elastic stress will be important, for together they will govern the degree of stabilization. The analysis of the problem is complicated by the fact that m a n y processes, particularly of coatings application, involve strain magnitudes outside the range of linear viscoelasticity. Approaches to nonlinear viscoelasticity exist, but are beyond the scope of this chapter. Quantitative prediction of viscoelas-

350

PAINT AND COATING TESTING MANUAL

tic effects would require the use of a model for the fluid behavior, or viscoelastic constitutive equation, also outside the scope of this presentation. However, it is often possible to build experimental correlations between coating elasticity and performance problems, so that guidance may be provided for formulation efforts to solve them. For most modern coatings, in particular, the origin of elasticity is likely to be an associated structure built up from a dispersed phase rather than polymeric entanglement. Such particulate flocs are generally shear-sensitive and are reduced or destroyed by high-shear application processes. The elastic modulus G' will be seen to decrease with increasing strain and strain rate outside the linear regime. Thus, in most cases, G' is a measure of structure that has its greatest effect at low strain and strain rate (unless the elastic character is highpolymeric in origin). Therefore, under most conditions of application, structure (therefore elasticity) is destroyed and must rebuild in the applied film. Herein, it is seen that thixotropy and viscoelasticity are kindred phenomena.

strain rate thickening. For example, Lu [48] observed that polyacrylamide thickeners in latex paint systems showed extensional thickening behavior, whereas hydroxyethylcellulose-type thickeners did not. Coatings application processes are generally high strain rate, so it is clear that ~/e can dominate the mechanical response, generally leading to detrimental consequences. Extensional stresses can stabilize liquid "webs" and fibers, such as form in direct rollcoating, allowing them to grow large instead of dissipating. This can result in heavy ribbing and "misting" (roll spatter) [49]. On the other hand, the breakup of a liquid jet to form atomized droplets in spray application is suppressed by a high extensional viscosity because the liquid fibers formed intermediary to droplet formation are inhibited from disintegration. J. E. Glass has been the chief proponent of the study of extensional viscosity in relation to paint performance, correlating it to roller spatter [41] and the performance of sprayed coatings [50]. Massouda did a clever and, unfortunately overlooked study of the relationship of extensional stress measurements to viscoelastic relaxation kinetics and thence to spattering of paints [39].

EXTENSIONAL RHEOLOGY The other important deformation occurring in coatings besides simple shear is extensional (or elongational) deformation. Extensional or "stretching" deformation causes an increase in length and decrease in cross section of an object. In a simple shear field, particles or polymer coils (i.e., the flow units) rotate with a velocity ~/2. The rotational motion lessens the friction between the solvent and solute particles. In an extensional flow, this rotational accommodation to the flow field is not possible because there is no velocity gradient normal to the flow direction. The separation of flow units is thus more costly in terms of energy dissipation due to friction. Thus, the viscosity of a Newtonian fluid in extension turns out to be three times greater than its viscosity measured in shear (Trouton rule). The extensional viscosity (~e) is calculated as the ratio of tensile stress to extensional deformation rate

The strain in extension is usually defined as a Hencky strain, 9 = Al/lo, where l0 is the original length and Al the increase in length under tensile stress tre. The Hencky strain rate is then the time derivative of the strain or 1 dl

l dt

There are, unfortunately, few commercial instruments suitable for extensional measurement on coatings. Carri-Med (division of TA Instruments) markets the Spin-Line Rheometer (SLR) utilizing the fiber-spinning geometry. Rheometrics' RFX rheometer is probably better suited to lowviscosity fluids, using opposed-nozzle flow to measure extensional forces (the latter instrument offers dual high-shear and extensional measurement capability). With the necessary electromechanical design and fabrication resources, one might attempt to build a simple extensional rheometer, perhaps patterned after that of Gupta [51]. The relatively simple technique of convergent flow analysis was used by Lu for measuring extensional properties of latex paints [48]. An overview of extensional rheometry has recently been published by James and Walters [52].

(40)

~e -

-

Extensional Viscosity Measurement

-

d(lnl) dt

(41)

E x t e n s i o n a l V i s c o s i t y in C o a t i n g s P r o c e s s e s

Many coatings processes involve stretching (elongational) flows (Table 1), and when coatings can support large extensional stresses (i.e., high extensional viscosity) performance can be dominated by such flows. As mentioned, the viscosity of a Newtonian liquid in extension is three times that measured in shear. For non-Newtonian fluids, the Trouton ratio [~(~)/~(~)] can be as much as 10 4. Furthermore, whereas the shear viscosity is usually a decreasing function of shear rate, extensional viscosity frequently displays strong extensional

POLYMER MELT AND SOLUTION RHEOLOGY High-polymer solution rheology is a subject of relatively little interest with respect to coatings due to the shift from solvent-borne to environmentally compliant technologies, such as water-borne, higher solids, and powder coatings. The polymers used for high solids and powder coatings are little more than oligomers in order to achieve necessary flow for satisfactory processing and film formation. Even the modest molecular weight polymers used in solvent-borne varnishes and paints are below the entanglement molecular weight, Me, hence, both their neat and solution rheology are Newtonian. Similarly, powder coating melts are Newtonian until near gelation [53]. The viscosity of polymer solutions below Mc is proportional to the weight-average molecular weight, Mw,

"qo = KMw(Mw < Mc)

(42)

where K is a constant dependent on chain flexibility (Tg), polymer-solvent interaction, temperature, etc. If Mw > Mc,

CHAPTER 3 3 - - R H E O L O G Y AND VISCOMETRY the motion of entangling polymers becomes much more complex, and the viscosity now depends on M~ raised to the power of approximately 3.4 [54]. Trl0-----KMw34(Mw > Mc)

DISPERSION

RHEOLOGY

A dispersion (or suspension) consists of a suspended or dispersed discontinuous phase contained in a continuous phase. As an example, for coatings in particular, this might be a system in which a fine particle size solid is wetted by and thoroughly mixed in a liquid. However, the dispersed phase may be a liquid or semisolid (or gaseous) as well. The dividing line between dispersions and suspensions is essentially one of particle size. Dispersions are generally of colloidal dimension, about 10 nm to 1/.~m. Due to their small size and mass, colloidal particles are very slow settling or nonsettling because Brownian motion effectively keeps them randomly suspended. Suspensions range in size above 1/~m and generally exhibit rapid setting because Brownian forces are ineffective with such massive particles, and also because Van der Waals attractive forces increase in proportion to particle size. Addition of a particulate phase to a liquid modifies its viscosity and sometimes its rheology. The modification to the continuous-phase viscosity made by an added particulate phase depends on characteristics of the particle and on the particle concentration. The contribution of a single particle to the viscosity of a dispersion is characterized by its intrinsic viscosity (or limiting viscosity number), [~]. The primary factors governing [~] are particle shape and deformability. The influence of particle concentration on viscosity is expressed by the volume fraction (or internal phase volume) 4, the fraction of the total volume of the suspension occupied by the suspended material and which is a dimensionless number. A convenient way of expressing the effect of a dispersed phase on the viscosity of a liquid is by normalizing the dispersion viscosity to the pure-liquid viscosity. This ratio is termed the relative viscosity ~r and is also dimensionless. ~r

-

a~d

It is reasonable that the viscosity of a liquid will be augmented by a factor equal to the product of the particle intrinsic viscosity and the concentration of particles.

(43)

The viscosity of polymer melts follows a similar relationship. It should be noted that polymers having highly polar or hydrogen-bonding functionalities (such as commonly used in high-solids formulations) can show nonlinear dependence of viscosity on molecular weight even below M~ due to transient intermolecular associations [55]. The sole use of high-polymer binders in coatings today is where they exist as a separate phase dispersed in a liquid carrier medium, m Such materials are known as polymer latexes, or latex dispersions. Because the latex polymer is segregated from, and therefore noninteracting with, the solvent, the rheology of latexes is much simpler than that of the same high polymer in the solution state. The rheology of polymer dispersions will be discussed below.

(44)

~z

~~ high-molecular-weight, water-soluble polymers are used as thickeners. However, they are used at low levels, and their effects on rheology are mainly colloidal/osmotic rather than as solution polymers.

351

n,

=

I

+ [n]~'

(45)

Einstein [56] was the first to calculate the intrinsic viscosity for noninteracting rigid spheres in a Newtonian liquid and obtained the number 2.5 ~]r

=

1 + 2.5~b

(46)

The intrinsic viscosities of other particle shapes (e.g., prolate and oblate spheroids--discs and rods) have been calculated [57] and are always greater than 2.5. This means that any deviation from spherical particle shape will increase dispersion viscosity. Equation 46 is valid only in the very dilute regime (d~ < 0.05). Batchelor [58] extended the rigorous treatment to somewhat higher volume fractions by using a second-order expression in ~]r = 1 + 2.5q6 + k~ 2

(47)

The value of k ranges from 5.2 to 6.2. Figure 21 illustrates the typical dependence of 7It on 4~ for actual dispersions (using data of Eilers [59]) and shows curves corresponding to the predictions of Einstein's and Batchelor's equations. It can be seen that Batchelor's equation predicts a finite viscosity at 4~ = 1, which is not realistic. Also in Fig. 21, it is seen that ~], rises toward infinity at a volume fraction considerably smaller than unity. The volume fraction corresponding to ~r oo is denoted thin, the m a x i m u m volume fraction, or maxim u m packing fraction. At ~ = thin, the density of particle packing is such that the dispersion can no longer flow. The value of ~,, will be system-dependent and will be determined by particle shape, particle-size distribution, the ionic strength of the medium, the degree of particle flocculation, and the exact manner in which the particles arrange themselves (pack) in three-dimensional space. Numerous models have been proposed to take account of the limiting concentration parameter q~,,, [15,59-61]. Probably the model which has been most successful in fitting a variety of data is the Krieger-Dougherty equation [62] ,, =

(

1 - g-~!

(48)

Figure 21 shows the fit of the Krieger-Dougherty model to some data of Eilers [59] with a value of [~] close to the Einstein value. The m a x i m u m packing fraction will, in fact, have different, unique values at low and high shear rates because the strength of the shear field determines the way particles pack together. Recent reviews of the rheology of polymer colloids are recommended for further reading

[6,14]. It should be noted here that, although the dispersion rheology is controlled by the disperse-phase volume fraction, (b, the effect of the particle phase on the rheology may be greater than expected on the basis of the volume of material added to make up the dispersion. This is because the disperse-phase volume may be augmented by various effects tending to increase the effective particle radius. It is the "hydrodynamically effective" particle volume that determines the rheology. Thus, the rheological behavior will be found to scale with the "effective" volume fraction, ~e, rather than with

352

PAINT AND COATING

TESTING

Eile'rs' relative viscosity data ....... Einstein model --! Batchelor model Krieger-Dougherty fit

[]

MANUAL

] / 1

~-

~. o

Lt~ ~

~[ 0.00

: 0.15

I 0.30

Volume

I 0.45

I 0.60

0.75

Fraction, ~o

FIG. 21 -Relative dispersion viscosity data of Eilers, together with best-fit curves for the Einstein model, Batchelor model, and the Krieger-Dougherty model.

the formulated volume fraction, 4~. The effective particle radius can be increased in a number of ways, often as a consequence of employing various methods of achieving stabilization of dispersions. Adsorption of a polymeric stabilizer onto the surface of a particle adds the thickness of the stabilizer layer to the particle radius. This layer prevents the close approach and flocculation of particles by steric interactions between stabilizer layers, resulting in steric stabilization. If the thickness of the steric stabilizing layer on the particle is 6, the effective volume fraction is [63] ~e= q~[1-t- ( ~ ) 3 1

(49)

where R is the original particle radius. Quantitative viscomettic methods have been developed for inferring the adsorbedlayer thickness [63,64]. Aqueous dispersions are often stabilized by association of repulsive electrical charge with the particle, known as electrostatic stabilization. The charge may be due to adsorption of ions, anionic or cationic surfactants or polyelectrolytes, or, in the case of polymer colloids, to the presence of ionizable groups which are part of the polymer molecules. (In the case of functionalized polymer latexes, such groups tend to migrate to the particle surface.) The surface ionic charge propagates an electrical potential field into the aqueous phase, depending on the ionic strength of the medium. In the presence of dissolved counterions, an ionic "atmosphere" develops around the charged particle. These phenomena are the origin of the so-called electroviscous effects, which have to do with the way the electrical field surrounding the particle affects the effective volume fraction or collision cross section, and the nature of the hydrodynamic interaction between particle and surrounding liquid [57,65]. Dispersion rheology can be greatly altered as a consequence of these effects, as the solution ion concentration or pH are varied. Krieger and Eguiluz showed the ~/r of a dispersion of uniform polystyrene

latex spheres (4 = 0.4) decreased from 106 to 10 for a twoorders-of-magnitude increase in concentration of electrolyte [66]. In that study, the low-electrolyte latexes exhibited apparent yield stress behavior, a consequence of the increase of the effective volume fraction of the latex particles due to the expansion of the repulsive electrostatic field. The effect is so great that the particles are actually "locked" into crystalline arrays that can diffract light, producing striking iridescent colors. As the electrolyte concentration is increased, the counterion "cloud" both shrinks the electrostatic field and shields particle fields from each other. Consequently, the viscosity drops dramatically (remember that the volume change depends on the cube of the radius change). Rheology is particularly useful for dispersion characterization because of its sensitivity to microstructure. Dispersion rheology has two broad aspects: (1) the dependence of viscosity on the concentration of the dispersed phase, and (2) the dependence of viscosity on shear stress and shear rate, discussed in CLASSES OF NON-NEWTONIAN BEHAVIOR. To the foregoing are added effects of particle shape, rigidity, particle size and particle size distribution, and interparticle forces, both attractive and repulsive. All these factors combine to determine the microstructure of the dispersion and hence its rheology. Dispersions can exhibit the full range of rheological behavior mentioned previously, including Newtonian, shear-dependent, time-dependent, plastic, and elastic behavior. A dispersion of noninteracting spherical particles will be Newtonian up to about q~ = 0.2 [15]. Above this point, onset of non-Newtonian character begins due to particle interactions and hydrodynamic factors. Mechanisms for these effects are discussed in the section entitled Shear-Thinning Fluids.

SAGGING A coating layer on any but a horizontal surface will experience a tangential shear stress due to gravity of magnitude "r = pgh cos 0

(50)

where p is the liquid density, g the gravitational acceleration, and h the uniform layer depth. 0 is the angle of inclination of the substrate to the vertical. For a vertical substrate, ~- = pgh (Fig. 22). The shear stress on any layer within a coating will be equal to the load from the outer layers. For a vertical surface r = pg(h - y)

(51)

is the shear stress acting on a layer a distance y from the substrate due to the weight of the outer layer of thickness h - y (Fig. 22). Coating layers having a yield stress % (see Plastic (Yield) Behavior) will not sag unless the gravitational shear stress exceeds the yield value. The shear plane where yield occurs will therefore be at y' (for ideal yield behavior) when the following expression is satisfied % +

Distance Perpendicular to Surface FIG. 2-Schematic of surface excess concentration as calculated as difference for bulk concentration.

372

PAINT AND COATING TESTING MANUAL

classic example being soaps, with an alkyl nonpolar group and a [carboxylate + metal ion] polar group. In concentrated systems of surface-active materials, especially those of low inherent solubility in the liquid phase, at a certain concentration the addition of further surfactant no longer depresses the surface tension, as the surface is saturated, and the molecules of the surfactant begin to form selfordered structures in the bulk liquid called micelles [12]. Micelles are aggregates of surfactant molecules that form in solution to minimize the total system energy. In water they orient with the hydrophobic portion of the molecule directed toward the interior of the micelle, and in nonaqueous systems they have the polar end in the interior. At the concentration of surfactant that micelles just begin to form, m a n y of the solution properties also undergo rapid change, and this breakpoint in surfactant solution properties is k n o w n as the critical micelle concentration (CMC) (Fig. 3). The CMC is a unique concentration for a surfactant/solvent combination at a specified temperature. The book by Rosen [13] gives an expanded discussion of surfactant behavior, and Tanford's book [14] gives an expanded description of micellar behavior in aqueous systems. These texts also describe the complex aggregates that form in surfactant/solvent mixtures, including membrane-like structures and liquid crystals. As Gibbs first noted, the presence of a material adsorbing preferentially at an interface imparts an elasticity to the surface, as it will tend to resist expansion and contraction. This surface elasticity is given by %_

dv _ dlnA

dv dlnF

(7)

where A is the surface area. This elasticity is the resistance to disturbance of a film by the presence of a surfactant. The effects of this surface elasticity have been reviewed with respect to its effects in liquid films and liquid threads [15]. Surface elasticity measurements have been used to character-

CMC

ize the properties of m o n o m o l e c u l a r layers at surfaces [16]. If there are diffusional effects due to soluble surfactants (see below, "Dynamic Properties of Surfaces"), they will reduce the surface elasticity, effectively short circuiting the surface tension changes by surface replenishment [17-19]. In these cases, Eq 7 should be modified to read

%=

- (d~,/d In F) [1 + (h/2)(dc/dF)]

where c is the surfactant concentration and h is given by the relationship h = 2 ~/D

Liquid/Solid Interfaces: Wetting and Contact

Angles

W h e n a liquid solution is in contact with a bulk solid, the Young equation holds true as %l - %v + ~v cos O = 0

~

3_SurfaceTension ~N~4-Equlval.entvlty ~

C C Surfaotant Concentration FIG. 3-Schematic of physical property changes in surtactant solution at critical micelle concentration (CMC).

(10)

where %t, %v, and 7zv are the solid/liquid, solid/vapor, and liquid/vapor surface tensions, respectively, and O is the contact angle formed between the liquid and solid [9,22]. This is shown in Fig. 4 and discussed extensively in the references given above. The contact angle between a liquid and a solid is in some ways a measure of the difference in surface energies of the solid and liquid. If a liquid "wets" a solid, the contact angle is zero, and the liquid spreads spontaneously over the surface of the solid, displacing air in contact with the solid surface. If the contact angle is greater than zero, a liquid will not wet and spread on a solid. An extreme example of this is found with Teflon TM, which is of such a low surface energy that it is wet by few pure liquids or solutions. In general, pure

2-OsmoticPressure

Property

(9)

where D is the surfactant diffusion coefficient, and to is the frequency of the surface motion [18]. Surface elasticity effects are a major consideration in foam stability [20] and in coating application processes [21], where there are rapid surface area changes and creation of new surfaces.

J Physioal

(8)

FIG. 4-Schematic of contact angle in G/L/S system.

CHAPTER 3 4 - - S U R F A C E E N E R G E T I C S hydrocarbon and fluorocarbon interfaces that are nonpolar in nature have very low surface energies and are difficult to wet, while polar materials such as metals and metal oxides have high surface energies and are easily wet. Oils or other impurities on the surface of a polar substrate will cause the material to act as a low surface energy solid, making it difficult to wet. There is extensive qualitative discussion in the literature concerning polar and nonpolar contributions to surface energetics, much of it of value only in a qualitative sense as a guide to interpreting the wetting behavior of pure liquids. Good and Fowkes have both published extensively in this area (see Refs 1 and 23 for a more complete listing of these citations), and the literature is full of equations of arguable theoretical value invoking empirical relations between composition and surface energetics, including acidbase interactions [23]. In coatings application, it is very crucial to have the liquid coating wet the substrate. Substrate cleaning and the use of surfactants to depress the liquid coating surface tension and reduce the contact angle to zero are two of the most common steps to achieve this. It is essential to have this wetting of the substrate to obtain good adhesion of the coating to the substrate, hence the importance of proper surface preparation for coatings use. Rough surfaces tend to have lower contact angles than smooth surfaces of the same composition [24]. This is why sanding or other surface roughening will give better wetting of a material by a coating. There is also often a difference in the contact angle measured by an advancing drop and a receding drop, the advancing angle being the larger. This is often due to surface roughness, causing a hysteresis in the contact angle [25]. Surface compositional differences and their scale have also been shown to give hysteresis, both by themselves and together with surface roughness [26]. These literature references, plus many others (see Ref I for a more complete discussion), indicate the difficulties in obtaining completely unambiguous contact angle measurements, especially when considering both advancing and receding drops on solid surfaces. Zisman and coworkers have also shown that there is a "critical surface tension" characteristic of a given surface [27]. This is determined by plotting the cosine of the contact angle O versus surface tension and extrapolating to cos O = 1 (zero contact angle) and determining the surface tension at this value. This is often used to determine the relative surface energy of a solid and the surface tension required of a wetting liquid for that surface. Pure liquids can be used as test liquids for the measurement of critical surface tension, or surfactant solutions can be utilized [13,28]. Some precautions need to be taken on the choice of test liquids as the polarity of the test liquid may influence measurements and thus the extrapolation procedure used to estimate the critical surface tension (see Ref 1 for further details). Exposure to exterior weathering has been noted to decrease contact angles of organic coatings and other polymeric films [29]. Similar effects are noted when these surfaces are modified by corona or flame treatment, and these treatments are often used to give wetting and adhesion of coatings to hard-to-wet plastic substrates such as polyethylene and polypropylene. In addition to these surface treatments, it has been found that certain chlorinated olefinic polymers improve the wetting and adhesion of coatings to polyolefin plastics [30].

373

D Y N A M I C P R O P E R T I E S OF LIQUID SURFACES When fresh surfaces are formed in a liquid solution, such as in coatings application processes, manufacture, etc., the surface tension and composition of the fresh surface of a solution will be different from the equilibrium value. At the time of new surface creation, the surface tension of the liquid will be nearly equal to the surface tension of the pure solvent at the instant of the surface formation. The surface tension then decreases as the surfactant in the solution diffuses to the interface. The dynamics of the diffusion from bulk to interface, or vice versa in the case of surface compression, has been well studied, initially by Ward and Tordai [31] and later by Sutherland [32], Hansen [33], Joos [34], and others. There are also possible complicating effects if the adsorbing material is polymeric, for which case additional time-dependent changes in the surface tension may occur as the polymer molecules relax to their equilibrium conformation at the surface, or if there is an energy barrier to adsorption of the material [35,36]. Special methods have been developed to measure dynamic surface tension, and these are discussed below, along with static surface tension measurement methods. It is especially important to consider and eliminate dynamic surface tension effects if one is trying to measure the equilibrium value of ~7(t) for t --~ o0],meaning for example, a Wilhelmy plate rather than a Du Nuoy ring is the appropriate measurement for systems that are other than pure liquids. These dynamic effects are especially important in coating application processes and all coating operations that generate fresh surfaces [37]. There are also dynamic effects in the wetting of surfaces that must be considered for coating application operations. The first is the dynamics of wetting in thin film spreading as discussed by de Gennes [38]; the other is the dynamics of contact angles on moving substrates [39].

M E A S U R E M E N T OF T H E S U R F A C E T E N S I O N OF L I Q U I D S Introduction There are many ways of measuring surface tension, but in essence, they are all related to two effects of capillarity. The first effect is the excess pressure due to surface tension at a curved interface. This is described by the Young-Laplace equation as Ap -= T(1/R, + 1/R2)

(11)

where Ap is the excess pressure due to the curved interface, and R1 and R2 are the two principal radii of curvature of the interface. In the case of a sphere of radius r, where the radii of curvature are both equal to r, this equation reduces to

Ap = 2T/r

(12)

The other capillary effect is that the surface tension of a liquid exerts a force on a solid body immersed in it equal to the surface tension times the perimeter of the body times the contact angle the liquid makes with the solid. If one is using a balance, one can write AW = ~ , c o s O

(13)

374

PAINT AND COATING TESTING MANUAL

where ~ is the perimeter and O is the contact angle, and AW is the extra force on the solid body due to surface tension. These two effects yield, together with various modifications of geometry, etc., m a n y methods to measure static and dynamic surface tension. The most widely used methods are described below. In all measurements of surface tension, the cleanliness of all apparatus and the purity of all materials is of utmost importance. Organic impurities in aqueous systems will have drastic effects in reducing the surface tension values measured. The concentration levels necessary to alter surface tension measurements are as little as i0 -s M. Trace a m o u n t s of impurities on solid apparatus surfaces can alter contact angles and, as will be shown, the measured surface tension values. All water used in surface tension measurements should be at least double distilled, and often the presence of a strong oxidizing agent in one step of the distillation ensures that trace surfactants are removed. The water should be used fresh, as surface active impurities can be leached from glass and plastic containers. The same holds true for all solvents used in surface characterization studies.

CO ketone

~ I 3 triplet d

--O--ether

66.6 111.0

=I 2 twind

31.5

81.9

~ C I 3 triplet d

I

52.0

~C12 twin d

24,0

18.0

--F

CI

16

C-6 r i n g

(saturated)

C-5 r i n g

16

-5.5

> C = olefin

.

13.5

- - C H = olefin

Phenyl .

28.5

- 19.2

C H 2 = olefin

>C<

- 1.0

16.1

CH<

33.5

CH2<

Aliphatic

CH~-

Group

Functional

Molar Volumef AV (cm3/mol)

TABLE

4300 e

5 400

(7 000)

(5 850)

2 000

3 000

7 000

4 150

10 440

7 120

6 600

4 300

(4 050)

4 150

800

11 700

Same

Same

Same

Same

(4400)

Same

Same

Same

Same

Same

Same

Same

(1650 +- 150)

9350 e

6400 e

6100 e 3600 e

7 650 4 550 8 000

4700 e

5 900

2960 ~

3670 e

4 600 3 700

1315 e 2200 e

1 650 2 760

800 e

1360 e

250

-..

7630

1 700

250

250

Same

Same

Same

Same

Same

Same

Same

Aromatic

1 000

...

1 030

1 030

1 030

350

820

1 180

1 125

Aliphatic

Total Parameter AV,52 ( c a l / m o l ) (.~

00 O0

CHAPTER 3S--SOLUBILITY PARAMETERS

389

nED kJ/mol

i/

50

40

ss-

-

,_~ I 0;,;~~

~.~"

i'"

,,,s

.-

~.s "'~

30

~

L...~

20

io

///,/~

0

50 100 150 200 250 V, ~mS/mo l FIG. 1-Energy of vaporization for straight chain hydrocarbons as a function of molar volume and reduced temperature.

85o

~

~

. . . . . . -I o.~ . . . . . . .

600 600 -

~

,. . . . . . .

~

- ~ o. o 0.60 0.65

0.70 80

70

80

90 V,

100

110

120

0.65

~

~

~50-

50

4 0.50

130

em~/mol

FIG. 2-Cohesive energy density for cycloalkanes as a function of molar volume and reduced temperature.

earlier data to SI units. All three of these figures have been straight line extrapolated into a higher range of molar volumes than that reported by Barton. Energies found with these extrapolations have also provided consistent results. The solubility parameters in SI units, MPa 1/2, are 2.0455 times larger than those in the older centimeter, gram, second system, (ca]/cc)1/2, which still finds extensive use in the United States, for example. The figure for the aliphatic liquids gives the dispersion cohesive energy, AEd, whereas the other two figures directly

i 80

90

, 100

110

j 120

, - - ~ o.,o

~ 130

140

150

160

170

V, c m ~ / m o l

FIG. 3-Cohesive energy density for aromatic hydrocarbons as a function of molar volume and reduced temperature.

report the dispersion cohesive energy density, c. The latter is much simpler to use since one need only take the square root of the value found from the figure to find the respective partial solubility parameter. Barton also presented a similar figure for the aliphatic solvents, but it is inconsistent with the energy figure and in error. Its use is not recommended. When substituted cycloaliphatics or substituted aromatics are considered, simultaneous consideration of the two separate parts of the molecules is required. The dispersion energies are evaluated for each of the types of molecules involved, and a weighted average for the molecule of interest based on numbers of significant atoms is taken. For example, hexyl benzene

390

P A I N T A N D COATING T E S T I N G M A N U A L

would be the arithmetic average of the dispersion energies for an aliphatic and an aromatic liquid, each with the given molar volume of hexyl benzene. Liquids such as chlorobenzene, toluene, and ring compounds with alkyl substitutions with only two or three carbon atoms have been considered as cyclic compounds only. Such weighting has been found necessary to satisfy Eq 6. The critical temperature, To, is required to use the dispersion energy figures. If the critical temperature cannot be found, it must be estimated. A table of the Lydersen group contributions, AT, [32] as given by Hoy [31] for calculation of the critical temperature, is included here as Table 2. In some cases the desired groups may not be in the table, which means some educated guessing is required. The end result does not appear too sensitive to these situations. The normal boiling temperature, Tb, is also required in this calculation. This is not always available, either, and must be estimated by similarity, group contribution, or other technique. The Lydersen group contribution method involves the use of Eqs 8 and 9.

Tb/Tr = 0.567 + s

-

(EAT)2

(8)

and

T~ = T/T~

(9)

where T has been taken as 298.15 K. The dispersion parameter is an atomic force parameter. The size of the atom is important. It has been found that corrections are required for atoms significantly larger than carbon, such as chlorine, sulfur, bromine, etc., but not for oxygen or nitrogen, which have a similar size. The carbon atom in hydrocarbons is the basis of the dispersion parameter in its present form. These corrections are applied by first finding the dispersion cohesive energy from the appropriate figure. This requires multiplication by the molar volume for the cyclic compounds using data from the figures here, since these figures give the cohesive energy densities. The dispersion cohesive energy is then increased by adding on the correction factor. This correction factor for chlorine, bromine, and sulfur has been taken as 1650 J/tool for each of these atoms in the molecule. Dividing by the molar volume and then taking the square root gives the (large atom corrected) dispersion solubility parameter. The need for these corrections has been confirmed many times, both for interpretation of experimental data and to allow Eq 6 to balance. Research is definitely needed in this area. The impact of these corrections is, of course, larger for the smaller molecular species. The taking of square roots of the larger numbers involved with the larger molecular species reduces the errors involved in these cases since the corrections themselves are relatively small. It can be seen from the dispersion parameters of the cyclic compounds that the ring also has an effect similar to increasing the effective size of the interacting species. The dispersion energies are larger for cycloaliphatic compounds than for their aliphatic counterparts, and they are higher for aromatic compounds than for the corresponding cycloaliphatics. Similar effects also appear with the ester group. This group appears to act as if it were, in effect, an entity which is larger than the corresponding compound containing only carbon

(i.e., its homomorph), and it has a higher dispersion solubility parameter without any special need for corrections. The careful evaluation of the dispersion cohesive energy may not have major impact on the value of the dispersion solubility parameter itself because of the taking of square roots. Larger problems arise because of Eq 4. Energy assigned to the dispersion portion can not be reused when finding the other partial parameters using Eq 4 (or Eq 6). This is one reason group contributions are recommended in some cases below.

CALCULATION OF T H E POLAR SOLUBILITY PARAMETER, ~ The earliest assignments of a "polar" solubility parameter were given by Blanks and Prausnitz [6]. These parameters were in fact the combined polar and hydrogen bonding parameters as used by Hansen and cannot be considered polar in the current context. The first Hansen polar parameters [7] were reassigned new values by Skaarup according to the B6ttcher equation [9]. This equation requires the molar volume, the dipole moment, DM, the refractive index, and the dielectric constant. These are not available for many compounds, and the calculation is somewhat more difficult than using the much simpler equation developed by Beerbower [11] 37.4

8p - ( ~ ) ~ d (DM)

(10)

The constant 37.4 gives this parameter in SI units. Equation 10 has been consistently used by the author over the past few years, particularly in view of its reported reliability [26]. This reported reliability appears to be correct. The molar volume must be known or estimated in one way or another. This leaves only the dipole moment to be found or estimated. Standard reference works have tables of dipole moments, with the most extensive listing still being McClellan [33]. Other data sources also have this parameter as well as other relevant parameters and data such as latent heats and critical temperatures. The so-called DIPPR database has been found useful for many compounds of reasonably common usage, but many interesting compounds are not included in DIPPR. This abbreviation is for Design Institute for Physical Property Research, 2 Project 801 of the American Institute of Chemical Engineers at the Pennsylvania State University [34]. When no dipole moment is available, similarity with other compounds, group contributions, or experimental data can be used to estimate the polar solubility parameter. It must be noted that the fact of zero dipole moment in symmetrical molecules is not basis enough to assign a zero polar solubility parameter. An outstanding example of variations of this kind can be found with carbon disulfide. The reported dipole moments are mostly 0 for gas phase measurements, supplemented by 0.08 in hexane, 0.4 in carbon tetrachloride, 0.49 in chlorobenzene, and 1.21 in nitrobenzene. 2Design Institute for Physical Property Research, Department of Chemical Engineering, 167 Fenske Laboratory, The Pennsylvania State University, University Park, PA 16802.

CHAPTER TABLE 2 - - L y d e r s e n

group

Aliphatic, AT

Cyclic, AT

>C< ~CH2

0.020 0.020 0.012 0.000 0.018

... 0.013 0.012 - 0.007 ---

=CH--

0.018

0.011

=C< ~CH =CH

0.000 . . . .

Group CH3 CH 2

>CH--

aromatic aromatic

--O->O epoxide --COO->C~O --CHO --COzO

0.021 . . 0.047 0.040 0.048 . .

--OH--~ --H-o --OH primary --OH sec . . --OH tert. --OH phenolic

.

. .

. .

. .

.

.

.

. ... 0.033 ...

.

.

.

. . . . . . . . . . 0.082 . . . 9. . . . . 0.035

.

--NH 2 --NH->N---C~-~N

0.031 0.031 0.014 0.060

--NCO HCON< --CONH . --CON< --CONH2 --OCONH .

. . .

.

.

. . .

.

.

.

.

.

. ...

...

. . . . . .

. .

. .

. .

. .

0.015 0.015

- - C I 1~ --C1 2 ~ C12 t w i n C1 a r o m a t i c

0.017 9. . . . . . . . . . . . . . . . .

...

--Br --Br

0.010 . .

..-

--F --I

.

.

. . .

.

4 5 6 7

. . . .

. . .

Ortho Meta Para

. . .

Bicycloheptyl Tricyclodecane

. .

ring ring ring ring

... . . .

. . .

.

. .

.

. . . .

.

. . .

. . .

. . .

. .

. .

. .

. . . . .

.

0.0175 0.0267 0.0497 0.0400 0.0445 0.0863

0.16 . . 0.47 0.29 0.33 . .

0.0343 0.0077 0.0493 0.0440 0.0593 0.0060

0.06 . . . . . . . . - 0.02

0.0318 . .

.

.

0.018 0.012

Conjugation c is double bond trans double bond Member Member Member Member

.

0.227 0.227 0.210 0.210 0.198 0.198 0.198 . . . . . .

0.0345 0.0274 0.0093 0.0539

0.008 . .

.

.

.

.

Cyclic, b5'

0.0226 0.0200 0.0131 0.0040 0.0192 0.0184 0.0129 0.0178 0.0149

... 0.184 0.192 0.154 --0.154 0.154 . .

. .

. .

.

.

.

0.12 .

... 0.02 ... .

.

.

.

. . . .

. . . .

. . . .

. . . .

...

0.095 0.135 0.17 0.36

0.0539 0.0546 0.0843 0.0729 0.0897 0.0938

--S---SH

aromatic

.

.-. 0.024 0.007 ...

. . . . . . . . . .

Aliphatic, AP

Aer

0.014

.

PARAMETERS

constants.

0.011 . .

35--SOLUBILITY

. . . . . .

.

.

.

. . . . . .

... 0.09 0,13 ... . . . . . .

0.27 .

. . . . . .

. . . . . .

. . . . . . 0.24

.

.

0.0311 0.0317 0.0521 0.0245

0.320 . . . . . . . . .

. . .

. . .

. . .

0.0392 0.0313

0.50 . .

.

.

.

0.006

0.224 0.83

.

-..

... .

... ...

0.0035 0.0010 0.0020

. . .

. . .

. . .

. . .

. . .

. . .

.

0.0118 0.003 0.0035 0.0069

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . .

. . .

0.0015 0.0010 0.0060

. . .

. . .

. . .

. . .

. . .

. . .

. .

. .

0.0034 0.0095

. .

. .

. .

. .

. .

. .

. .

. .

. .

. .

. . .

.

.

.

391

392

PAINT AND COATING TESTING MANUAL

There is a clear increase with increasing solubility parameter of the media. The latter and highest value has been found experimentally most fitting for correlating permeation through a fluoropolymer film used for chemical protective clothing [35]. Many fluoropolymers have considerable polarity. The lower dipole moments seem to fit in other instances. Diethylether has also presented problems as an outlier in terms of dissolving or not, or rapid permeation or not. Here the reported dipole moments [33] vary from 0.74 to 2.0 with a preferred value of 1.17, and with 1.79 in chloroform. Choosing a given value seems rather arbitrary. The chameleonic cyclic forms of the linear glycol ethers would also seem to provide for a basis of altered dipole moments in various media [29]. When Eq 10 can not be used, the polar solubility parameter has been found using the Beerbower table of group contributions, by similarity to related compounds, and/or by subtraction of the dispersion and hydrogen bonding cohesive energies from the total cohesive energy. The question in each case is: "Which data are available and judged most reliable?" New group contributions have also been developed from related compounds where their dipole moments are available. These new polar group contributions then become supplementary to the Beerbower table. For large molecules, especially those with long hydrocarbon chains, the accurate calculation of the relatively small polar (and hydrogen bonding) contributions present special difficulties. The latent heats are not generally available with sufficient accuracy to allow subtraction of two large numbers from each other to find a very small one. In such cases the similarity and group contribution methods are thought best. Unfortunately, latent heats found in a widely used handbook [36] are not clearly reported as to the reference temperature. There is an indication that these are 25~ data, but checking indicated many of the data were identical with boiling point data reported elsewhere in the literature. A more recent edition of this handbook has a completely different and less voluminous section for the latent heat of evaporation [37]. Again, even moderate variations in reported heats of vaporization can cause severe problems in calculating the polar (or hydrogen bonding) parameter when Eqs 4 or 6 are strictly adhered to.

CALCULATION OF THE HYDROGEN BONDING SOLUBILITY PARAMETER, 8h In the earliest work, the hydrogen bonding parameter was almost always found from the subtraction of the polar and dispersion energies of vaporization from the total energy of vaporization. This is still widely used where the required data are available and reliable. At this stage, however, the group contribution techniques are considered reasonably reliable for most of the required calculations and, in fact, more reliable than estimating several of the other parameters to ultimately arrive at the subtraction step just mentioned. Therefore, in the absence of reliable latent heat and dipole moment data, group contributions are judged to be the best alternative. Similarity to related compounds can also be used, of course, and the result of such a procedure should be essentially the same as for using group contributions.

SUPPLEMENTARY CALCULATIONS AND PROCEDURES The procedures listed above are those most frequently used by the author in calculating the three partial solubility parameters for liquids where some data are available. There are a number of other calculations and procedures which are also helpful. Latent heat data at 25~ have consistently been found from latent heats at another temperature using the relation given by Fishtine [38].

1

~

This is done even if the melting point of the compound being considered is higher than 25~ The result is consistent with all the other parameters, and to date no problems with particularly faulty predictions have been noted in this respect, i.e., it appears as if the predictions are not significantly in error. When the latent heat is given in cal/mole, the above equation is used to estimate the latent heat at 25~ RT equal to 592 cal/mole is then subtracted from this according to Eq 12 to find the total cohesion energy, ~ v , in cgs units at this temperature: Mq.,, = z~E, = AH~, - a T

(12)

Only very limited attempts have been made to calculate solubility parameters at a higher temperature. Solubility parameter correlations of phenomena at higher temperatures have generally been found satisfactory when the established 25~ parameters have been used. Recalculation to higher temperatures is possible, but has not been found necessary. In this direct but approximate approach it is assumed that the parameters all demonstrate the same temperature dependence, which, of course, is not the case. It might be noted in this connection that the hydrogen bonding parameter, in particular, is the most sensitive to temperature. As the temperature is increased, more and more hydrogen bonds are progressively broken, and this parameter will decrease more rapidly than the others. The gas phase dipole moment is not temperature dependent, although the volume of the fluid does change with temperature, which will change its cohesive energy density. Beerbower has suggested relations to predict the changes of the partial solubility parameters with temperature [11]. The coefficient of thermal expansion, ~, appears in all of these relations. These are

d(8,,) _ dT

d(~a) _ dT

1.25 a8 d

(13)

d(Sp) _ dT

0.5 asp

(14)

~h (1.22 x 10 -3 + 0.5 ~)

(15)

A computer program has been developed by the author to assign the three Hansen parameters for solvents based on experimental data alone. This has been used in several cases

CHAPTER 3 5 - - S O L U B I L I T Y PARAMETERS where the parameters for the given liquids were desired with a high degree of accuracy. The procedure is to enter solvent quality, good or bad, into the program for a reasonably large number of polymers where the solubility parameters and appropriate radius of interaction for the polymers are known. The program then locates that set of 6d, 8p, and ~h parameters for the solvent which best satisfies the requirements of a location within the spheres of the appropriate polymers where solvent quality is good and outside of the appropriate spheres where it is bad. An additional aid in estimating the Hansen parameters for many compounds is that these parameters can be found by interpolation or extrapolation, especially for homologous seties. The first member may not necessarily be a straight line extrapolation, but comparisons with related compounds should always be made where possible to confirm assignments. Plotting the parameters reported in Table 3 for homologous series among the esters, nitroparaffins, ketones, alcohols, and glycol ethers will aid in finding the parameters for related compounds. Table 3 contains Hansen solubility parameters for a large number of liquids and plasticizers. These are given in SI units.

SOLUBILITY PARAMETERS FOR POLYMERS The solubility parameters for numerous polymers and film formers are given in Table 4. Suppliers and trademarks are given in Table 5. These data are based on solubility determinations unless otherwise noted. There are four parameters, the three describing the nonpolar, polar, and hydrogen bonding interactions as for the liquids, and the fourth, R0, a radius of interaction for the type of interaction described. Most of these are taken from a report [39] from the Scandinavian Paint and Printing Ink Research Institute. (This institute unfortunately no longer exists.) Additional values have been contributed according to the notes in the table to indicate the types of data which have been correlated with these techniques. Barton [40] has also provided solubility parameters for many polymers in a recent handbook. Experimental determination of polymer solubility parameters involves trying to dissolve the polymer at a given concentration, usually 10% by weight, in a selection of solvents intended to maximize information regarding all types of interaction. Whenever possible the author uses a set of parameters indicated with an "*" in Table 3. The "yes" or "no" solubility data can be plotted by hand or processed by computer to yield a "spherical" characterization as described above. Teas [41] has developed a triangular plotting technique which helps visualization of three parameters on a plane sheet of paper. Examples are found in Ref 3 and Ref 4 as well. Swelling, weight gain, solvent resistance, and surface attack have also been used as a primary data to characterize polymers.

393

APPLICATIONS There are many applications documented in the literature where solubility parameters have aided in selection of solvents, understanding and controlling processes, and, in general, offered guidance where affinities among materials are of prime importance. To find the optimum solvent for a polymer using solubility parameters, it is most desirable to have the solubility parameters for the polymer. Matching the parameters of an already existing solvent or combination of solvents can be done, but does not necessarily optimize the new situation. The optimum depends on what is desired of the system. A solvent with highest possible affinity for the polymer is both expensive and probably not necessary. Most coatings applications involve solvents safely within the solubility limit with a maximum of cheaper hydrocarbon solvent. Some safety is advised because temperature changes, potential variations in production, etc. can lead to a situation where solvent quality changes in an adverse manner. Balance of solvent quality on evaporation of mixed solvents is also necessary. Here again computer approaches are possible. An oxygenated solvent frequently added to hydrocarbon solvent and which has been cost effective in increasing the very important hydrogen bonding solubility parameters has been n-butanol. The mixture of equal parts xylene and n-butanol can be used in conjunction with many polymers, but a third solvent, such as a ketone or ester, is often included in small amounts to increase the polar parameter/solvency of the mixture. Glycol ethers can also be added to hydrocarbon solvents with advantage, and the polar and hydrogen bonding parameters are higher than had n-butanol been added to the same concentration. There are many possibilities, and a solubility parameter approach is particularly valuable in quickly limiting the number of candidates. Coalescing solvents in water-reducible coatings are often those with somewhat higher hydrogen bonding parameters than the polymer, which also means they are water soluble or have considerable water solubility. The distribution between the water phase and the dispersed polymer phase depends on the relative affinities for water and the polymer. Solvents which are not particularly water soluble will preferentially be found in the polymer phase. Such coalescing solvents may be preferred for applications to porous substrates, making certain they are where they are needed. Otherwise a water-soluble coalescing solvent would tend to follow the aqueous phase penetrating the substrate and not be available to do its job in the film itself. When water evaporates the solvent must dissolve to some extent in the polymer to promote coalescence. This can be determined and adjusted by either increasing or decreasing the affinity for the polymer. Amines are frequently added in water-reducible coatings to neutralize acid groups in polymers, thus providing a watersolubilizing amine salt. Amine in excess of that required for total neutralization acts like a solvent. Such amine salts have been characterized separately to demonstrate that they have higher solubility parameters than either (acetic) acid or organic bases [42]. These salts are hydrophilic and have very little affinity for coatings polymers, which means they are to be found in a stabilizing role in interfaces in the aqueous

394

PAINT AND COATING TESTING MANUAL TABLE 3 - - H a n s e n solubility parameters for selected liquids. The solvents in alphabetical order.

No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

Solvent Acetaldehyde Acetic acid Acetic anhydride *Acetone Acetonitrile *Acetophenone Acrylonitrile Allyl alcohol Amyl acetate Aniline Anisole Benzaldehyde *Benzene 1.3-Benzenediol Benzoic acid Benzonitrile Benzyl alcohol Benzyl butyl phthalate Benzyl chloride Biphenyl Bromobenzene Bromochloromethane Bromoform 1-Bromonaphtalene Bromotrifluoromethane Butane 1.3-Butanediol *l-Butanol 2-Butanol *Butyl acetate Sec-butyl acetate Butyl acrylate Butylamine Butyl lactate Butyraldehyde Butyric acid *Gamma butyrolactone Butyronitrile Carbon disulfide *Carbon tetrachloride *Chlorobenzene 1-Chlorobutane Chlorodifluoromethane *Chloroform 3-Chloro- 1-propanol m-cresol Cyclohexane *Cyclohexanol Cyclohexanone Cyclohexylamine Cyclohexylchloride Cis-decahydronaphthalene Trans-decahydronaphthalene Decane 1-Decanol *Diacetone alcohol Dibenzyl ether Dibutyl phthalate Dibutyl sebacate Dibutyl stearate *o-dichlorobenzene 2.2-Dichlorodiethyl ether Dichlorodifluoromethane 1.1-Dichloroethane 1.1-Dichloroethylene Di-(2-chloro-isopropyl) ether Dichloromonofluoromethane 1.2-Dichlorotetrafluoroethane Di-iso-butyl carbino] Diethanolamine Diethylamine

Dispersion

Polar

Hydrogen Bonding

Molar Volume

14.7 14.5 16.0 15.5 15.3 19.6 16.4 16.2 15.8 19.4 17.8 19.4 18.4 18.0 18.2 17.4 18.4 19.0 18.8 21.4 20.5 17.3 21.4 20.3 9.6 14.1 16.6 16.0 15.8 15.8 15.0 15.6 16.2 15.8 14.7 14.9 19.0 15.3 20.5 17.8 19.0 16.2 12.3 17.8 17.5 18.0 16.8 17.4 17.8 17.2 17.3 18.8 18.0 15.7 17.5 15.8 17.3 17.8 13.9 14.5 19.2 18.8 12.3 16.5 17.0 19.0 15.8 12.6 14.9 17.2 14.9

8.0 8.0 11.7 10.4 18.0 8.6 17.4 10.8 3.3 5.1 4.1 7.4 0.0 8.4 6.9 9.0 6.3 11.2 7.1 1.0 5.5 5.7 4.1 3.1 2.4 0.0 10.0 5.7 5.7 3.7 3.7 6.2 4.5 6.5 5.3 4.1 16.6 12.4 0.0 0.0 4.3 5.5 6.3 3.1 5.7 5.1 0.0 4.1 6.3 3.1 5.5 0.0 0.0 0.0 2.6 8.2 3.7 8.6 4.5 3.7 6.3 9.0 2.0 8.2 6.8 8.2 3.1 1.8 3.1 10.8 2.3

11.3 13.5 10.2 7.0 6.1 3.7 6.8 16.8 6.1 10.2 6.7 5.3 2.0 21.0 9.8 3.3 13.7 3.1 2.6 2.0 4.1 3.5 6.1 4.1 0.0 0.0 21.5 15.8 14.5 6.3 7.6 4.9 8.0 10.2 7.0 10.6 7.4 5.1 0.6 0.6 2.0 2.0 5.7 5.7 14.7 12.9 0.2 13.5 5.1 6.5 2.0 0.0 0.0 0.0 10.0 10.8 7.3 4.1 4.1 3.5 3.3 5.7 0.0 0.4 4.5 5.1 5.7 0.0 10.8 21.2 6.1

57.1 57.1 94.5 74.0 52.6 117.4 67.1 68.4 148.0 91.5 119.1 101.5 89.4 87.5 100.0 102.6 103.6 306.0 115.0 154.1 105.3 65.0 87.5 140.0 97.0 101.4 89.9 91.5 92.0 132.5 133.6 143.8 99.0 149.0 88.5 110.0 76.8 87.3 60.0 97.1 102.1 104.5 72.9 80.7 84.2 104.7 108.7 106.0 104.0 113.8 118.6 156.9 156.9 195.9 191.8 124.2 192.7 266.0 339.0 382.0 112.8 117.2 92.3 84.8 79.0 146.0 75.4 117.6 177.8 95.9 103.2

CHAPTER 3 5 - - S O L U B I L I T Y P A R A M E T E R S

395

TABLE 3 - - H a n s e n solubility parameters for selected liquids. The solvents in alphabetical order.

No.

Solvent

Dispersion

Polar

Hydrogen Bonding

Molar Volume

72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142

2-(Diethylamino)ethanol Para-diethylbenzene Diethyl carbonate *Diethylene glycol Diethylene glycol butyl ether acetate Diethylene glycol hexyl ether Diethylene glycol monobutyl ether Diethylene glycol monoethyl ether Diethylene glycol monomethyl ether Diethylenetriamine *Diethyl ether Diethyl ketone Diethyl phthalate Diethyl sulfate Diethyl sulfide Di(isobutyl) ketone Di(2-methoxyethyl) ether N.N-dimethylacetamide *Dimethylforrnamide 1.1-Dimethylhydrazine Dimethyl phthalate Dimethyl sulfone *Dimethyl sulfoxide Dioctyl phthalate * 1,4-Dioxane Dipropylamine *Dipropylene glycol Dipropylene glycol methyl ether Dodecane Eicosane Epichlorohydrin Ethanethiol *Ethanol *Ethanolamine *Ethyl acetate Ethyl acrylate Ethyl amyl ketone Ethylbenzene Ethyl bromide 2-Ethyl- 1-butanol Ethyl butyl ketone Ethyl chloride Ethyl chloroformate Ethyl cinnamate Ethylene carbonate Ethylene cyanohydrin Ethylenediamine Ethylene dibromide *Ethylene dichloride *Ethylene glycol Ethylene glycol butyl ether acetate *Ethylene glycol monobutyl ether *Ethylene glycol monoethyl ether Ethylene glycol monoethyl ether acetate *Ethylene glycol monomethyl ether Ethylene glycol monomethyl ether acetate Ethyl formate 2-Ethyl hexanol Ethyl lactate *Formamide Formic acid Furan Furfural Furfuryl alcohol Glycerol Heptane Hexadecane Hexamethylphosphoramide *Hexane Hexylene glycol Isoamyl acetate

14.9 18.0 16.6 16.6 16.0 16.0 16.0 16.1 16.2 16.7 14.5 15.8 17.6 15.7 16.8 16.0 15.7 16.8 17.4 15.3 18.6 19.0 18.4 16.6 19.0 15.3 16.5 15.5 16.0 16.5 19.0 15.7 15.8 17.0 15.8 15.5 16.2 17.8 16,5 15.8 16.2 15,7 15.5 18.4 19.4 17.2 16.6 19.2 19.0 17.0 15.3 16.0 16.2 15.9 16.2 15.9 15.5 15.9 16.0 17.2 14.3 17.8 18.6 17.4 17.4 15.3 16.3 18.5 14.9 15.7 15.3

5.8 0.0 3.1 12.0 4.1 6.0 7.0 9.2 7.8 13.3 2.9 7.6 9.6 14.7 3.1 3.7 6.1 11.5 13.7 5.9 10.8 19.4 16.4 7.0 1.8 1.4 10.6 5.7 0,0 0,0 10.2 6.5 8.8 15.5 5.3 7.1 4.5 0.6 8.0 4.3 5.0 6.1 10.0 8.2 21.7 18.8 8.8 3.5 7,4 11.0 4.5 5.1 9.2 4.7 9.2 5.5 8.4 3.3 7.6 26.2 11.9 1.8 14.9 7,6 12.1 0.0 0.0 8.6 0.0 8.4 3,1

12.0 0.6 6.1 20.7 8.2 10.0 10.6 12.2 12.6 14.3 5.1 4.7, 4.5 7.1 2.0 4.1 6.5 10.2 11.3 11.0 4.9 12.3 10.2 3.1 7.4 4.1 17.7 11.2 0.0 0,0 3,7 7.1 19.4 21.2 7.2 5.5 4.1 1.4 5.1 13.5 4,1 2.9 6.7 4.1 5.1 17.6 17.0 8.6 4.1 26.0 8.8 12.3 14.3 10.6 16.4 11.6 8.4 11.8 12.5 19.0 16.6 5.3 5.1 15.1 29.3 0.0 0.0 11.3 0.0 17.8 7.0

133.2 156.9 121.0 94.9 208.2 204.3 170.6 130.9 118.0 108.0 104,8 106.4 198.0 131.5 107,4 177.1 142.0 92.5 77.0 76.0 163.0 75.0 71.3 377.0 85.7 136,9 130.9 157.4 228.6 359.8 79.9 74.3 58.5 59.8 98.5 108.8 156.0 123.1 76.9 123.2 139.0 70,0 95.6 166.8 66.0 68.3 67.3 87.0 79.4 55.8 171.2 131.6 97.8 136.1 79.1 121.6 80.2 156.6 115.0 39.8 37.8 72.5 83.2 86.5 73.3 147.4 294.1 175,7 131.6 123,0 148,8

396

PAINT AND COATING TESTING MANUAL TABLE 3 C o n t i n u e d - - H a n s e n solubility parameters for selected liquids. The solvents in alphabetical order.

No. 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213

Solvent Isobutyl acetate Isobutyl alcohol Isobutyl isobutyrate Isooctyl alcohol Isopentane *Isophorone Isopropyl palmitate Mesitylene Mesityl oxide Methacrylonitrile *Methanol o-Methoxyphenol Methyl acetate Methyl acrylate Methylal Methyl amyl acetate Methyl butyl ketone Methyl chloride Methylcyclohexane *Methylene dichloride Methylene diiodide Methyl ethyl ketone Methyl isoamyl ketone Methyl isobutyl carbinol *Methyl isobutyl ketone Methyl methacrylate 1-Methylnaphthalene Methyl oleate 2-Methyl- l-propanol *Methyl-2-pyrrolidone Methyl salicylate Morpholine Naphtha.high-flash Naphthalene *Nitrobenzene *Nitroethane *Nitromethane 1-Nitropropane *2-Nitropropane Nonane Nonyl phenol Nonyl phenoxy ethanol Octane Octanoic acid 1-Octanol 2-Octanol Oleic acid Oleyl alcohol Pentane 2.4-Pentanedione 1-Pentanol Perfluoro(dimethylcyclohexane) Perfluoroheptane Peiaquoromethylcyclohexane Phenol Bis-(m-phenoxyphenyl) ether 1-Propanol ~ 2-Propanol Propionitrile Propylamine Propyl chloride *Propylene carbonate *Propylene glycol Propylene glycol monobutyl ether Propylene glycol monoethyl ether Propylene glycol monoisobutyl ether Propylene glycol monomethyl ether Propylene glycol monophenyl ether Propylene glycol monopropyl ether Pyridine 2-Pyrolidone

Dispersion

Polar

Hydrogen Bonding

Molar Volume

15.1 15.1 15.1 14.4 13.7 16.6 14.3 18.0 16.4 15.3 15.1 18.0 15.5 15.3 15.0 15.2 15.3 15.3 16.0 18.2 17.8 16.0 16.0 15.4 15.3 17.5 20.6 14.5 15.1 18.0 16.0 18.8 17.9 19.2 20.0 16.0 15.8 16.6 16.2 15.7 16.5 16.7 15.5 15.1 17.0 16.1 14.3 14.3 14.5 17.1 15.9 12.4 12.0 12.4 18.0 19.6 16.0 15.8 15.3 16.9 16.0 20.0 16.8 15.3 15.7 15. l 15.6 17.4 15.8 19.0 19.4

3.7 5.7 2.9 7.3 O.0 8.2 3.9 0.0 6.1 10.8 12.3 8.2 7.2 9.3 1.8 3.1 6.1 6.1 0.0 6.3 3.9 9.0 5.7 3.3 6.1 5.5 0.8 3.9 5.7 12.3 8.0 4.9 0.7 2.0 8.6 15.5 l 8.8 12.3 12.1 0.0 4.1 10.2 0.0 3.3 3.3 4.9 3.1 2.6 0.0 9.0 4.5 0.0 0.0 0.0 5.9 3.1 6.8 6.1 14.3 4.9 7.8 18.0 9.4 4.5 6.5 4.7 6.3 5.3 7.0 8.8 17.4

6.3 15.9 5.9 12.9 O.0 7.4 3.7 0.6 6.1 3.6 22.3 13.3 7.6 5.9 8.6 6.8 4.1 3.9 1.0 6.1 5.5 5.1 4.1 12.3 4.1 4.3 4.7 3.7 15.9 7.2 12.3 9.2 1.8 5.9 4.1 4.5 5.1 5.5 4.1 0.0 9.2 8.4 0.0 8.2 11.9 11.0 5.5 8.0 0.0 4.1 13.9 0.0 0.0 0.0 14.9 5.1 17.4 16.4 5.5 8.6 2.0 4.1 23.3 9.2 10.5 9.8 11.6 11.5 9.2 5.9 11.3

133.5 92.8 163.0 156.6 i 17.4 150.5 330.0 139.8 115.6 83.9 40.7 109.5 79.7 89.7 169.4 167.4 123.6 55.4 128.3 63.9 80.5 90.1 142.8 127.2 125.8 106.5 138.8 340.0 92.8 96.5 129.0 87.1 181.8 111.5 102.7 71.5 54.3 88.4 86.9 179.7 231.0 275.0 163.5 159.0 157.7 159.1 320.0 316.0 116.2 103.1 108.6 217.4 227.3 196.0 87.5 373.0 75.2 76.8 70.9 83.0 88.1 85.0 73.6 132.0 115.6 132.2 93.8 143.2 130.3 80.9 76.4

CHAPTER 3 5 - - S O L U B I L I T Y P A R A M E T E R S

397

TABLE 3 Continued--Hansen solubility parameters for selected liquids. The solvents in alphabetical order.

No. 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246

Solvent Quinoline Stearic acid Styrene Succinic anhydride 1.1.2.2-Tetrabromoethane 1.1.2.2-Tetrachloroethane Tetrachloroethylene Tetraethylorthosilicate *Tetrahydrofuran Tetrahydronaphthalene Tetramethylurea *Toluene Tributyl phosphate Trichlorobiphenyl 1.1.1-Trichloroethane *Trichloroethylene Trichlorofluoromethane 1.1.2-Trichlorotrifluoroethane Tricresyl phosphate Tridecyl alcohol Triethanolamine Triethylamine Triethyleneglycol Triethylene glycol monooleyl ether Triethylphosphate Trifluoroacetic acid Trirnethylhenzene 2.2.2.4-Trimethylpentane 2.2.4-Trimethyl- 1.3-pentanediol M.I. butyral Trimethylphosphate Water Xylene o-xylene

Dispersion

Polar

Hydrogen Bonding

Molar Volume

19.4 16.3 18.6 18.6 22.6 18.8 19.0 13.9 16.8 19.6 16.7 18.0 16.3 19.2 16.8 18.0 15.3 14.7 19.0 14.3 17.3 17.8 16.0 13.3 16.7 15.6 17.8 14.1 15.1 16.7 15.5 17.6 17.8

7.0 3.3 1.0 19.2 5.1 5.1 6.5 0.4 5.7 2.0 8.2 1.4 6.3 5.3 4.3 3.1 2.0 1.6 12.3 3.1 22.4 0.4 12.5 3.1 11.4 9.9 0.4 0.0 6.1 15.9 16.0 1.0 1.0

7.6 5.5 4.1 16.6 8.2 9.4 2.9 0.6 8.0 2.9 11.0 2.0 4.3 4.1 2.0 5.3 0.0 0.0 4.5 9.0 23.3 1.0 18.6 8.4 9.2 11.6 1.0 0.0 9.8 10.2 42.3 3.1 3.1

118.0 326.0 115.6 66.8 116.8 105.2 101.1 224.0 81.7 136.0 120.4 106.8 345.0 187.0 99.3 90.2 92.8 119.2 316.0 242.0 133.2 138.6 114.0 418.5 171.0 74.2 133.6 166.1 227.4 115.8 18.0 123.3 121.2

*Indicates use in author's standard set of test solvents. phase, still being a t t a c h e d to the polymer. Electrostatic repulsion contributes to stability as well. Surface active agents, w h e t h e r n o n i o n i c or ionic, are also to b e f o u n d w h e r e the affinities of the respective parts of their molecules dictate their placement. The h y d r o p h i l i c end with a high h y d r o g e n b o n d i n g p a r a m e t e r will be in the aqueous phase, a n d the h y d r o p h o b i c end will seek o u t an e n v i r o n m e n t where energy differences are lowest. Increases in t e m p e r a t u r e lead to lower h y d r o g e n b o n d i n g p a r a m e t e r s , especially. F o r this r e a s o n solvents with high h y d r o g e n b o n d i n g p a r a m e t e r s , such as glycols, glycol ethers, a n d alcohols, b e c o m e b e t t e r solvents for m o s t p o l y m e r s at higher t e m p e r a t u r e s . This can m a r k e d l y affect h o t - r o o m stability in water-reducible coatings, for example, since m o r e of the solvent will p a r t i t i o n to the p o l y m e r phase, w h i c h swells, b e c o m e s m o r e fluid, a n d has altered affinities for stabilizing surface active agents, for example. They m a y dissolve too readily in the d i s p e r s e d polymer. Carefully controlled, these t e m p e r a t u r e effects are a n advantage in water-reducible, oven-cured coatings, leading to higher film integrity. A simple a p p r o a c h to m a n y practical p r o b l e m s is to m a k e a t w o - d i m e n s i o n a l plot of p o l a r versus h y d r o g e n b o n d i n g par a m e t e r s with a circle (or e s t i m a t e d circle) for the p o l y m e r in question. One can plot the points for potential solvents a n d quickly arrive at a starting c o m p o s i t i o n for an experiment. This can subsequently be a d j u s t e d if necessary. It should also be kept in m i n d t h a t cyclic solvents generally have higher n o n p o l a r p a r a m e t e r s t h a n aliphatic solvents.

A long list of applications, in coatings a n d elsewhere, was p r o v i d e d as early as 1975 in a review article b y B a r t o n [43]. Many a p p l i c a t i o n s are given in Table 6 a n d in the p r e s e n t discussion to give a n idea of w h a t c a n b e studied systematically using this concept. More recent references a n d varied a p p l i c a t i o n s can be found in Refs 3, 4, a n d 44. B e e r b o w e r [44] has given m a n y of the m o r e theoretical a p p l i c a t i o n s for solubility p a r a m e t e r s including correlations of the R e h b i n d e r effect of crushing strength of a l u m i n u m oxide u n d e r various liquids, the w o r k of a d h e s i o n for liquids on mercury, the Joffe effect of the consequences of i m m e r sion in various liquids on the fracture strength of soda-lime glass, a n d correlations of friction on polyethylene t r e a t e d with fuming sulfuric acid H2SO4 + SO3 [44]. Direct a n d practical a p p l i c a t i o n s of solubility p a r a m e t e r s in coatings have i n c l u d e d their use as an aid in the selection of solvents a n d solvent b l e n d s for m a n y years. Most solvent suppliers a n d frequent solvent users have c o m p u t e r prog r a m s for this purpose, although as n o t e d above, such prog r a m s are not an absolute necessity. R e f o r m u l a t i o n to meet e n v i r o n m e n t a l r e q u i r e m e n t s is especially i m p o r t a n t in this respect since one can quickly evaluate w h i c h of the alternatives is m o s t likely to m e e t the given requirements. This includes reducing a m o u n t s of volatile organic solvent (VOC) a n d often involves mixtures. The m a i n thing to r e m e m b e r in solvent selection using solubility p a r a m e t e r s is that the resultant values for mixtures c a n be e s t i m a t e d from volume fraction averages for each

398

P A I N T A N D COATING T E S T I N G M A N U A L TABLE 4 C o n t i n u e d - - S o l u b i l i t y parameters for polymers.

TABLE 4--Solubility parameters for polymers. D

P

H

D

R

Cellulose acetobutyrate Cellit BP-300"

12.0

18.2

12.4

6.7 10.2

10.8

4.3 6.9

3.9 5.9

7.4 14.2 11.4 10.5 11.1 9.6 7.2

9.4 6.1 9.0 9.0 13.4 10.7 14.8

13.7 17.7 9.1 7.9 11.7 7.8 14.9

Epoxy curing agents 100 115 125 140

23.8 20.3 24.9 26.9

5.3 6.6 3.1 2.4

16.2 16.1 14.1 9.6 18.7 20.3 18.5 24.0

17.7 19.1 21.1 16.0 20.6 19.4 17.9 18.7 19.9

10.6 12.2 14.6 13.1 7.8 7.4 9.6 9.6 8.1

11.6 9.5 9.9 8.0 12.0 16.2 9.2 11.4 11.6 13.1 6.0 9.8 5.9 8.2 9.9 8.2 6.0 9.8

22.7 19.3

6.4 11.4

8.2 19.4 14.3 12.4

Polyurethane Desrnophen 651 Desmophen 800 Desmophen 850" Desmophen 1100 Desmophen 1150 Desmophen 1200 Desmophen 1700 Desmolac 4200 Macrynal SM 510N

Phenolic resins Super Beckacite 1001~ Phenodur 373 U"

Hydrocarbon resins Piccolyte S-I00" Piccopale 110" Piccoumarone 450 La

16.1 17.2 19.0

0.4 1.2 5.4

2.8 3.5 5.6

8.4 6.4 9.4

Styrene-butadiene elastomer (SBR) Polysar 5630"

17.2

3.3

2.6

6.4

8.6

4.1

9.4

Acrylonitrile-butadiene elastomer Hycar 1052 a

18.2

Polybutadiene Buna H~ls B-10"

2.2

3.3

6.2

5.3 10.4

18.1 18.2

3.4 4.7

4.9 2.0

3.6 5.0

19.9

0.0

0.0

9.4

Nitrocellulose 15.1 1/2-sec-Nitrocellulose H 23 a

14.4

17.4 17.4

Chlorinated polypropylene 19.8

Chlorosulfonated polyethylene Hypalon 20 b Hypalon 30 b

Cyclized rubber Alpexb

16.2

1.4

-0.8

14.2 16.9 17.4 19.7 16.1

2.5 2.5 4.3 14.3 3.7

17.2

7.4

Polyvinylchloride 8.2

3.4

21.2 17.2 19.6 19.2

0.9 9.2 5.7 4.6

8.3 14.0 10.7 7.6

15.4 10.4 11.4 10.4

17.0 18.9 20.4

-1.9 9.6 0.4

14.6 7.4 11.1 6.2 14.0 12.9

17.5 17.6 19.7

11.3 10.0 12.9

5.9 8.5 3.7 9.3 12.8 11.4

18.6 20.2 18.2

12.9 11.2 4.3

10.3 8.3 13.3 11.2 12.7 10.4

17.2 16.2 18.6 18.4 18.2

9.4 6.8 10.8 9.4 10.3

3.9 10.4 5.7 9.1 4.1 11.5 6.5 10.7 7.7 8.4

20.5

11.0

9.4 13.4

20.8

5.6

4.2 12.4

Polyamide Versamid 930" Versamid 961 Versamid 965

Isocyanate Desmodur L Demodur N ~ Suprasec F-5100"

Polyvinylbutyral Mowithal B 30 H Mowithal B 60 H Butvar B 76 a

Polyacrylate Lucite 2042" Lucite 2044 Plexigum MB 319 Plexigum M 527 PMMAa

Polyvinylacetate Polystyrene

9.4

4.6 12.4 4.0 7.2 8.4 7.4 14.7 11.4 7.9 8.9

8.6 11.2

Rosin derivatives Cellolyn 102a Pentatyn 255 ~ Pentalyn 830" Ester Gum 8L~

Polystyren LG~

Polyisobutylene

Vipla KR a

3.8 10.0 3.9 6.1

6.4

Polyisoprene

Lutonal IC/1203 ~ Lutonal I60 Polyvinylbutyl ether Lignin p o w d e r ~ Modaflow Multiflow

9.5 4.3

Chlorinated rubber

Mowilith 50" 17.1

Cariflex IR 305 a

6.8 9.8 7.9 11.9

Parlon P 10" 14.0 21.3 18.1 17.4 21.0 18.9 23.4

R

8.3 7.6

5.9 9.9

Epoxy

Versamid Versamid Versamid Versamid

20.0 17.0

Pergut S 5 Allopren R 10 17.9 20.1

Araldite DY 025 Epikote 828 Epikote 1001 Epikote 1004 Epikote 1007 Epikote 1009 Phenoxy PKHH

Cereclor 70 Chlorparaffin 40

7.4

Ethyl cellulose Ethocel HE 10 ind b Ethocel Std 20 ind b

H

Chlorparaffin

16.6

Cellulose acetate Cellidora Aa

P

Vinylchloride/copolymers Laroflex MP 45 Vilit MB 30 Vilit MC 31 Vilit MC 39 Vinylite VAGD Vinylite VAGH Vinylite VMCA Vinylite VMCC Vinylite VMCH Vinylite VYHH Vinylite VYLF

18.4 20.0 20.0 18.4 17.1 16.5 17.7 17.6 17.6 17.4 18.1

8.4 8.3 8.3 7.6 10.4 10.9 11.1 11.1 11.1 10.2 10.3

5.8 6.7 6.7 6.7 6.5 6.4 6.9 6.8 6.4 5.9 4.2

9.0 9.4 9.4 6.8 7.5 7.7 8.7 8.8 8.6 7.8 8.3

CHAPTER 3 5 - - S O L U B I L I T Y P A R A M E T E R S T A B L E 4 Continued--Solubilityparameters for polymers. D

P

H

TABLE 5--List of suppliers and trademarks for paint binders and

Alkyds and polyesters 18.6 23.0 22.9 22.6 20.5 19.2 23.6 20.6 17.3 22.6 20.0 18.1 18.0 18.8 17.7

10.0 2.2 15.2 13.8 9.3 5.3 1.0 4.6 4.2 I3.1 6.2 9.0 11.6 12.0 13.0

5.0 4.2 7.6 8.1 9.1 6.3 7.6 5.5 7.9 5.8 7.0 4.8 8.5 6.0 7.6

10.4 16.9 18.1 17.1 12.4 11.9 19.0 12.6 9.3 16.8 9.5 10.4 9.0 11.5 11.5

Amino resins BE 370 Beetle 681 Cymel 300~ Cymel 325 Dynomin MM 9 DynominUM 15 Soamin M 60 Synresen A 560 Plastopal H~ Uformite MX-61

20.7 22.2 19.9 25.5 18.8 19.9 15.9 22.1 20.3 22.7

6.1 -0.4 8.3 15.2 14.0 15.8 8.1 5.0 8.1 2.8

12.7 10.1 10.4 9.5 12.3 13.4 6.5 11.3 14.6 5.4

14.8 18.4 14.4 22.2 10.5 11.7 10.6 15.5 12.4 16.2

Acrylate resins Uracron 15 Paraloid P 400 Paraloid P 410 Paraloid experimental resin QR 954

19.2 19.2 19.6 18.4

7.7 9.6 9.1 9.8

5.7 9.3 6.8 10.0

10.6 12.2 12.2 12.4

19.4 16.6

9.9 1.9

10.1 8.0

6.9 8.0

Silicone resins Baysilon UD 125 Wacker 190 F

A d d i t i o n a l S p e c i a l Data

DEN 438 (Dow epoxy novolak) DEN 444 (Dow epoxy novolak) Zink silicate (CR)--Chemical resistance 2-Comp epoxy (CR)--Chemical resistance Polyvinylidinefluoride Coal tar pitch PA6 polyamide chemical resistance PA66 polyamide solubility PAll polyamide chemical resistance Cellophane swelling EVOH--solubility (ethylenevinyl alcohol)

polymers.

R

B i n d e r s in s o l u t i o n

Alftalat AC 366 Alftalat AM 756 Alftalat AN 896 Alftalat AN 950 Alftalat AT 316 Alflalat AT 576 Mkydal F 261 HS Alkydal F 41 Durofta] T 354 Dynapol L 812 Dynapol L 850 Plexal C-34a Soalkyd 1935-EGAX Vesturit BL 908 Vesturit BL 915

399

20.3 19.5 23.5 18.4

15.4 11.6 17.5 9.4

5.3 15.1 9.3 10.0 16.8 15.6 10.1 7.0

17.0 18.7 17.0 17.4 17.0 16.1 20.5

12.1 7.5 3.4 9.8 4.4 18.5 10.5

10.2 8.9 10.6 14.6 10.6 14.5 12.3

4.1 5.8 5.1 5.1 5.1 9.3 7.3

Note: D = dispersion;P = permanent dipoles;H = hydrogenbonding;R = interaction radius. ~Takenfrom Hansen, C. M., "Solubilityin the CoatingsIndustry,"Fi~rgoch Lack, VoL 17, No. 4, 1971, pp. 69-77. bCalculatedfrom solubilitydata in PolymerHandbook, 2nd ed., John Wiley & Sons, Inc., New York, 1975.

solubility p a r a m e t e r c o m p o n e n t . Solvent quality can be adjusted by the RED n u m b e r concept or graphically as described above. A c o m p u t e r search for nearest neighbors for a given single solvent has b e e n used m a n y times to locate alternates. A similar application is to predict which other solvents will

Suppliers

Trademarks

Bayer (D)

Cellit, Desmophen, Desmolac, Pergut, Cellidora, Desmodur, Baysilon Alkydal Piccolyte, Cellolyn, Pentalyn, Ester Gum, Parlon Araldite Epikote, Cariflex Vinylite, Phenoxy Macrynal, Phenodur, Alpex, Mowithal, Alftalat, Mowilith Super Beckasite, Uformite Polysar Hycar Vilit, Vesturit, Buna Hills Lutonal, Laroflex, Plastopal, Polystyren Modaflow, Multiflow, Butvar Vipla Cereclor, Allopren, Suprasec Lucite l/2-sec, nitrocellulose H 23 Plexigum Paraloid Dynapol Soamin Beetle Dynomin Uracron Super Beckasite, Uformite Wacker Ethocel Versamid Chlorparaffin Synresen Cymel Plexal Piccopal, Piccoumarone

Hercules (USA) Ciba-Geigy Shell (D) Union Carbide (USA) Hoechst (D) Reichhold (CH) Polymer Corp. (CAN) Goodrich (USA) Hills (D) BASF (O) Monsanto (USA) Montecatini Edison (I) ICI (GB) Du Pont (USA) Hagedom (D) R6hm (D) Rohm and Haas (USA) Dynamit Nobel (D) SOAB (S) BIP Chemicals (GB) Dyno Cyanamid (N) DSM Resins (S) Reichhold Chemie (CH) Wacker (D) Dow Chemical (CH) Cray Valley Prod. (GB) W. Biesterfeld (D) Synres (NL) American Cyanamide (USA) Polyplex (DK) Pennsylvania Industrial Chemical Corp. (USA)

probably be aggressive to a chemically resistant coating where very limited data have indicated a single solvent or two are s o m e w h a t aggressive. A nearest n e i g h b o r search involves calculation of the q u a n t i t y Rs for a whole database, for example, a n d then arranging the p r i n t o u t i n RED n u m b e r order with the potentially most aggressive at the top of the list. Solvents with RED less t h a n 1.0 are "good" a n d easily recognized. I n m a n y cases a m a r g i n a l solvent is desired, in which case RED n u m b e r s just u n d e r 1.0 will be sought. Marginal solvent quality will ensure that p o l y m e r adsorbed onto p i g m e n t surfaces has little reason to dissolve away from that surface where it is desired as a stabilizing factor i n the product. The solvent in this case should have a RED n u m b e r for the pigm e n t surface greater t h a n 1.0 to aid i n the p l a n n e d affinity approach to p i g m e n t dispersion stability. A sketch of the optim u m relations is given in Fig. 4, where the m a r g i n a l solvent is N u m b e r 1. Solvent 2 would p r o b a b l y be too expensive and, i n addition, will probably dissolve the polymer too well. I n special applications this extended polymer c h a i n configuration is desirable, b u t a solid a n c h o r to the p i g m e n t surface is required. A good a n c h o r has high affinity for the p i g m e n t surface a n d m a r g i n a l or n o affinity for the solvent. Solvent 3

400

PAINT AND COATING TESTING MANUAL

TABLE 6--Examples of the use of the solubility parameter.

Activity coefficients Aerosol formulation Biological materials and compatibility Chromatography Coal solvent extraction Compressed gases Cosmetics Cryogenic solvents Dispersion Dyes Emulsions Gas-Liquid solubility Grease removal Membrane permeability and swelling Paint film appearance Pharmaceutical Pigments Plasticizers, polymers, resins Plasticization Polymer and plasticizer compatibility Printing ink Reaction rate of radical polymerization Resistance of plastics to solvents Rubber blends Solid surface characterization--organic and inorganic Solid surface modification Solvent extraction Solvent formulation, environmental aspects Surface tension Urea-water solutions Vaporization of plasticizers Viscosity of polymer systems Water-based polymer systems, coalescents

would adsorb onto the pigment surface preferentially, and pigment dispersion stability will be poor. Pigments have been characterized by long-time suspension studies where some solvents will suspend the fines for days, months, or years, while others of similar viscosity yield rapid settling. The suspending solvents are the "good" ones and can be used to define a sphere for surface wetting/adsorption. In other cases better matches between solvent and polymer solubility parameters are required. This is true when two polymers are mixed and one of them precipitates. This is most likely the polymer with larger molecular weight, and it must be dissolved better. Lower RED numbers with respect

to this polymer are desired, while still maintaining affinity for the other polymer. Miscible blends of two polymers have been found using a solvent mixture composed exclusively of nonsolvents. This is demonstrated schematically in Fig. 5, where it can be seen that different percentage blends of Solvents 1 and 2 will have different relative affinities for the polymers. No other alternative theory of polymer solution thermodynamics can duplicate this predictive ability. Polymer miscibility is enhanced by larger overlapping solubility regions for the polymers as sketched in Fig. 6. Polymers A and B should be compatible, while C will not be. Such a systematic analysis allows modification of a given polymer to provide more overlap and enhanced compatibility. The advantages of a copolymer containing the monomers of A or B and C should also be evident. Such a copolymer will essentially couple the system together. Van Dyk et al. [45] have correlated the inherent viscosity of polymer solutions with the solubility parameter. This is interesting in that the solubility parameter is a thermodynamic consideration, while the viscosity is a kinetic phenomena. Solvents with higher affinities give greater polymer chain extension in solution, and the inherent viscosity--the solution viscosity divided by the solvent viscosity at polymer concentrations approaching zero--is an expression reflecting polymer chain extension in solution. Higher intrinsic viscosities were found for solvents with solubility parameters nearest the polymer solubility parameters. The use of supercritical gases as solvents has become more common in recent years. Space limitations prevent going into the details of these developments. It should be noted, however, that when a gas is compressed its cohesive energy density increases. This means that nonpolar gases with their low nonpolar solubility parameters can begin to dissolve given organic materials which otherwise have solubility parameters which are too high. Increasing the nonpolar solubility parameter of the gas by increasing the pressure causes a

Pigment

~p ~p

~h FIG. 4-Solubility parameter relations for optimum pigment dispersion stability.

~h FIG. 5-Solubility relations for polymer mixtures can be quickly evaluated to ensure solution stability. Even mixtures of nonsolvents can be systematically used to regulate solution behavior,

CHAPTER 35--SOLUBILITY PARAMETERS

401

14 3: Dewetting

12

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closer match with the corresponding parameter for potential solutes. Similar behavior is found for polar gases such as carbon dioxide. The prevailing pressure and temperature conditions determine its cohesive energy density and changes in pressure or temperature change solubility relations for this reason. Whereas nonpolar gases are most suitably used for relatively nonpolar solutes, carbon dioxide--and in principle other polar gases--are most suitably used in connection with more polar solutes. The solubility parameters for carbon dioxide have been reported [46] based on the room temperature solubility of the gas in different liquids (Sd, 8,, ~h equal to 15.3, 6.9, 4.1). These parameters resemble those of a higher ketone. Pigment wetting/suspension characteristics have been presented earlier [2,4,10]. Mixtures of nonsuspending solvents could also be found which, when admixed, provided predictably higher affinity and suspension of pigment particles for prolonged periods of time. An exceptionally clear demonstration of pigment adsorption properties is given in Ref 4 where a triangular plot of the three partial parameters shows the clearly different adsorption properties of untreated zinc oxide powder and organic phosphate surface-treated zinc oxide powder. This triangular approach to plotting was developed by Teas [41]. Other surface characterizations have also been given for surfaces such as coatings and metal substrates [ l 7,18]. These characterizations have been of the type sketched in Fig. 7. Such cohesive energy plots can lead to systematic modifications of systems to improve adhesion. It might be added parenthetically that equal information can be obtained from plots of the cosine of the contact angle versus the solubility parameter as for plots of these same data versus liquid surface tension [17]. The energy information obtained in these types of studies, the critical surface tension, corresponds to the condition of RED equal to 1.0, with a solubility parameter approach, that is, marginal affinity. The list of applications has recently been expanded to include correlations of the breakthrough times for common

FIG. 7-Schematic diagrams showing surface energy/contact angle characterizations using the Hansen solubility parameters (cohesion energy parameters).

types of chemical protective clothing such as butyl rubber, nitrile rubber, plasticized polyvinyl chloride, neoprene, Viton rubber, polyvinyl alcohol, etc. [47]. Figure 8 shows the importance of combined use of the RED number and molecular volume in correlating 3-h breakthrough times for a fluoropolymer chemical protective product [48]. Monomers with terminal double bonds diffuse more rapidly than comparisons with non-double-bonded molecules of similar size and solubility parameter would have predicted. The smaller cross section at the end of the molecule reminds one of a nail, and the preferred direction and relative rapidity of transport become easily understandable. In addition it should be noted that even biological materials, some of which have interest for coatings applications and use, have been assigned Hansen solubility parameters [49]. These include keratin (which relates to skin permeation), fat, sucrose, blood serum and zein (which are proteins), urea, lignin (wood penetration), and chlorophyll (which closely resembles lignin) [46,49]. The characterization of other biological materials is also possible, of course. Even inorganic salts have been characterized by solubility parameters [50]. The practice of dissolving polymers in solutions of organic liquids and inorganic salts can thus be explained by the solubility parameter. A solubility parameter correlation of the chemical resistance of an inorganic zinc silicate coating is reported in Table 4. Finally it might be noted that Hildebrand presented a chapter on the solubility parameters of metals [1]. Unfortunately we do not often coat metal, but rather metal oxides, for which no solubility parameter work has been reported.

CONCLUSION The background and many uses of the solubility parameter concept have been described in detail. Tables of solubility parameters for many liquids and polymers have been presented. Systematic use of solubility, swelling, or permeation

402

PAINT AND COATING TESTING MANUAL

D, P, H, R = 16.6, 5.4, 4.0, 3.8 FIT = 0 . 9 9 7 FOR 68 < MV < 98

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d a t a for p o l y m e r s allows their c h a r a c t e r i z a t i o n by these s a m e cohesive energy p a r a m e t e r s . Surfaces can also be characterized using these s a m e p a r a m e t e r s . This provides d a t a for opt i m i z i n g solvent selection, i m p r o v i n g compatibility, a n d e n h a n c i n g p i g m e n t d i s p e r s i o n a n d adhesion. W h e n all of the m a t e r i a l s involved in a given p r o d u c t a n d a p p l i c a t i o n can be c h a r a c t e r i z e d with the s a m e energy p a r a m e t e r s , the possibility exists to predict interactions a m o n g them, even in complic a t e d situations. M e t h o d s for e s t i m a t i n g the three H a n s e n solubility p a r a m e t e r s have b e e n given with as m u c h detail as has b e e n possible to ensure their m o r e u n i f o r m use in the future. It has n o t b e e n possible to deal with all the p r i m a r i l y theoretical p r o b l e m s with the solubility p a r a m e t e r concept. S o m e of these are dealt with in the literature cited, m o s t n o t a b l y in Refs 3, 4, 12, a n d 20. A simple a p p r o a c h is d e s c r i b e d to u n d e r s t a n d affinities in such varied m a t e r i a l s as gases, liquids, polymers, biological materials, surfaces, organic a n d inorganic coatings, inorganic salts, a n d metals. An a p p e a l is m a d e to the scientific c o m m u n i t y to e x p a n d r e s e a r c h on this seemingly universal approach. REFERENCES [1] Hildebrand, J. and Scott, R. L., The Solubility of Nonelectrolytes, 3rd ed., Reinhold, New York, 1950.

[48].

[2] Hildebrand, J. and Scott, R. L., Regular Solutions, Prentice-Hall Inc., Englewood Cliffs, NJ, 1962. [3] Barton, A. F. M., Handbook of Solubility Parameters and Other Cohesion Parameters, CRC Press Inc., Boca Raton, FL, 1983. [4] Gardon, J. L. and Teas, J. P., "Solubility Parameters," Treatise on Coatings, Vol. 2, Characterization of Coatings: Physical Techniques, Part II, R. R. Myers and J. S. Long, Eds., Marcel Dekker, New York, 1976, Chapter 8. [5] Burrell, H., "Solubility Parameters for Film Formers," Official Digest, Vol. 27, No. 369, 1972, pp. 726-758; Burrell, H., "A Solvent Formulating Chart," Official Digest, Vol. 29, No. 394, 1957, pp. 1159-1173; Burrell, H., "The Use of the Solubility Parameter Concept in the United States," VI Federation d'Associations de Techniciens des Industries des Peintures, Vernis, Emaux et Encres d'Imprimerie de l'Europe Continentale, Congress Book, 1962, pp. 21-30. [6] Blanks, R. F. and Prausnitz, J. M., "Thermodynamics of Polymer Solubility in Polar and Nonpolar Systems," Industrial and Engineering Chemistry, Fundamentals, Vo]. 3, No. 1, 1964, pp. 1-8. [7] Hansen, C. M., "The Three Dimensional Solubility Parameter-Key to Paint Component Affinities I," Journal of Paint Technology, Vol. 39, No. 505, 1967, pp. 104-117. [8] Hansen, C. M., "The Three Dimensional Solubility Parameter-Key to Paint Component Affinities II," Journal of Paint Technology, Vol. 39, No. 511, 1967, pp. 505-510. [9] Hansen, C. M. and Skaarup, K., "The Three Dimensional Solubility Parameter--Key to Paint Component Affinities III," Journal of Paint Technology, Vol. 39, No. 511, 1967, pp. 511-514.

CHAPTER 35--SOLUBILITY PARAMETERS [10] Hansen, C. M., "The Three Dimensional Solubility Parameter and Solvent Diffusion Coefficient," Danish Technical Press, Copenhagen, 1967 (doctoral dissertation). [I 1] Hansen, C. M. and Beerbower, A., "SolubilityParameters," Kirk-

Othmer Encyclopedia of Chemical Technology, Supplement Volume, 2nd ed., A. Standen, Ed., Interscience, New York, 1971, pp. 889-910.

[12] Barton, A. F. M., "Applications of Solubility Parameters and Other Cohesion Energy Parameters in Polymer Science and Technology," Pure and Applied Chemistry, Vol. 57, No. 7, 1985, pp. 905-912. [13] S~rensen, P., "Application of the Acid/Base Concept Describing the Interaction between Pigments, Binders, and Solvents," Journal of Paint Technology, Vol. 47, No. 602, 1975, pp, 31-39. [14] Van Dyk, J. W,, paper presented at the Fourth Chemical Congress of America, New York, 25-30 Aug. 1991. [15] Anonymous (Note: This was in fact Van Dyk, J. W. but this does not appear on the bulletin), "Using Dimethyl Sulfoxide (DMSO) in Industrial Formulations," Bulletin No, 102, Gaylord Chemical Corp., Slidell, LA, 1992. [16] Karger, B. L., Snyder, L. R., and Eon, C., "Expanded Solubility Parameter Treatment for Classification and Use of Chromatographic Solvents and Adsorbents," Analytical Chemistry, Vol. 50, No. 14, 1978, pp. 2126-2136. [17] Hansen, C. M. and Wallstr6m, E., "On the Use of Cohesion Parameters to Characterize Surfaces," Journal of Adhesion, Vol. 15, 1983, pp. 275-286. [18] Hansen, C. M., "Characterization of Surfaces by Spreading Liquids," Journal of Paint Technology, Vol. 42, No. 550, 1970, pp. 660-664; Hansen, C. M., "Surface Dewetting and Coatings Performance,"Journal of Paint Technology, Vol. 44, No. 570, 1972, pp. 57-60. [19] Hansen, C. M. and Pierce, P. E., "Surface Effects in Coatings Processes," Industrial and Engineering Chemistry, Product Research and Development, Vol. 13, No. 4, 1974, pp. 218-225. [20] Gardon, J. L., "Critical Review of Concepts Common to Cohesive Energy Density, Surface Tension, Tensile Strength, Heat of Mixing, Interracial Tension and Butt Joint Strength," Journal of Colloid and Interface Science, Vol. 59, No. 3, 1977, pp. 582-596. [21] Patterson, D., "Role of Free Volume Changes in Polymer Solution Thermodynamics," Journal of Polymer Science: Part C, No. 16, 1966, pp. 3379-3389. [22] Flory, P. J., Principles of Polymer Chemistry, Cornell University Press, New York, 1953. [23] van Krevelen, D. W. and Hoftyzer, P. J., Properties of Polymers: Their Estimation and Correlation with Chemical Structure, 2nd ed., Elsevier, Amsterdam, 1976. [24] Beerbower, A., "Environmental Capability of Liquids," Interdisciplinary Approach to Liquid Lubricant Technology, NASAPublication SP-318, 1973, pp. 365-431. [25] Fedors, R. F., "A Method for Estimating both the Solubility Parameters and Molar Volumes of Liquids," Polymer Engineering and Science, Vol. 14, No. 2, 1974, pp. 147-154 and 472. [26] Koenhen, D, N. and Smolders, C.A., "The Determination of Solubility Parameters of Solvents and Polymers by Means of Correlation with Other Physical Quantities," Journal of Applied Polymer Science, Vol. 19, 1975, pp. 1163-1179. [27] Anonymous, "Co-Act--A Dynamic Program for Solvent Selection," brochure, Exxon Chemical International, Inc., 1989. [28] Dante, M. F., Bittar, A. D., and Caillault, J. J., "Program Calculates Solvent Properties and Solubility Parameters," Modern Paint and Coatings, Vol. 79; No. 9, 1989, pp. 46-51. [29] Hoy, K. L., "New Values of the Solubility Parameters from Vapor Pressure Data," Journal of Paint Technology, Vol. 42, No. 541, 1970, pp. 76-118. [30] Myers, M. M. and Abu-Isa, I. A., "Elastomer Solvent Interactions III-Effects of Methanol Mixtures on Fluorocarbon Elasto-

403

mers," Journal of Applied Polymer Science, Vol. 32, 1986, pp. 3515-3539. [31] Hoy, K. L., "Tables of Solubility Parameters," Union Carbide Corp., Research and Development Dept., South Charleston, WV, 1985 (1st ed., 1969). [32] Reid, R. C. and Sherwood, T. K., Propertiesof Gases and Liquids, McGraw-Hill, New York, 1958 (Lydersen Method, see also Ref

31). [33] McLellan, A. L, Tables of Experimental Dipole Moments, W. H. Freeman, San Francisco, 1963.

[34] "Tables of Physical and Thermodynamic Properties of Pure Compounds," American Institute of Chemical Engineers Design Institute for Physical Property Research, Project 801, Data Compilation, R. P. Danner and T. E. Daubert, Project Supervisors, DIPPR Data Compilation Project, Department of Chemical Engineering, Pennsylvania State University, University Park, PA. [35] Hansen, C. M., "Selection of Chemicals for Permeation Testing Based on New Solubility Parameter Models for Challenge 5100 and Challenge 5200," under contract DTCG50-89-P-0333 for the U.S. Coast Guard, June 1989, Danish Isotope Centre, Copenhagen. [36] CRC Handbook of Chemistry and Physics, 65th ed., R. C. Weast, Editor-in-Chief, Boca Raton, FL, CRC Press Inc., 1988-1989, pp. C-672-C-683, [37] Majer, V., "Enthalpy of Vaporization of Organic Compounds," Handbook of Chemistry and Physics, 72nd ed., D. R. Lide, Editor-in-Chief, Boca Raton, CRC Press Inc., 1991-1992, pp. 6-1006-107. [38] Fishtine, S. H., "Reliable Latent Heats of Vaporization," Industrial and Engineering Chemistry, Vol. 55, No. 4, 1963, pp. 20-28; also, Vol. 55, No. 5, pp. 55-60 and Vol. 55, No. 6, pp. 47-56. [39] Saarnak, A., Hansen, C. M., and Wallstr6m, E., "Solubility Parameters, Characterization of Paints and Polymers," report from Scandinavian Paint and Printing Ink Research Institute, January 1990. [40] Barton, A. F. M., Handbook of Polymer-Liquid Interaction Parameters and Solubility Parameters, CRC Press, Inc., Boca Raton, FL, 1990. [41] Teas, J. P., "Graphic Analysis of Resin Solubilities," Journal of Paint Technology, Vol. 40, No. 516, 2968, pp. 19-25. [42] Hansen, C.M., "Some Aspects of Acid/Base Interactions" (Einige Aspekte der S/iure/Base-Wechselwirkung) (in German), Farbe und Lack, Vol. 83, No. 7, 1977, pp. 595-598. [43] Barton, A, F. M., "Solubility Parameters," Chemical Reviews, Vol. 75, No. 6, 1975, pp. 731-753. [44] Beerbower, A., "Boundary Lubrication--Scientificand Technical Applications Forecast, AD747336," Office of the Chief of Research and Development, Department of the Army, Washington, DC, 1972. [45] Van Dyk, J. W., Frisch, H. L., and Wu, D. T., "Solubility, Solvency, Solubility Parameters," Industrial and Engineering Chemistry Product Research and Development, Vol. 24, No. 3, 1985, pp. 473-478. [46] Hansen, C. M., "25 Years with Solubility Parameters," (25 reed Opl~selighedsparametrene) (in Danish), Dansk Kemi, Vol. 73, No. 8, 1992, pp. 18-22. [47] Hansen, C. M. and Hansen, K. M., "SolubilityParameter Prediction of the Barrier Properties of Chemical Protective Clothing,"

Performance of Protective Clothing: Second Symposium, ASTM STP 989, S.Z. Mansdorf, R. Sager, and A. P. Nielsen, Eds., American Society for Testing and Materials, Philadelphia, 1988, pp. 197-208. [48] Hansen, C. M., Billing, C. B., and Bentz, A. P., "Selection and Use of Molecular Parameters to Predict Permeation Through Fluoropolymer-Based Protective Clothing Materials," The Per-

formance of Protective Clothing; Fourth Volume, ASTM STP 1133,

404

PAINT AND COATING TESTING MANUAL

J. P. McBriarty and N. W. Henry, Eds., American Society for Testing and Materials, Philadelphia, 1992, pp. 894-907. [49] Hansen, C. M., "The Affinities of Organic Solvents in Biological Systems," Journal of the American Industrial Hygiene Association, Vol. 49, No. 6, 1988, pp. 301-308.

[50] Hansen, C. M., "The Universality of the Solubility Parameter," Industrial and Engineering Chemistry Product Research and Development, Vol. 8, No. 1, 1969, pp. 2-11.

Part 9: Films for Testing

MNL17-EB/Jun. 1995 i

Cure: The Process and Its Measurement

36

by Thomas J. Miranda 1

INTRODUC~ON One of the most critical, and often misunderstood properties of a coating is cure and its measurement. The optimum physical and chemical properties of a coating depend largely on proper curing conditions. For example, a thermosetting acrylic polymer when applied to a panel and dried without curing will be brittle and have poor solvent resistance and low hardness. When properly cured, the film properties will change dramatically and the properties which were designed into the coating become apparent. Similarly, an oil-based paint will be soft and tacky and slowly become hard and scratch resistant as its cure develops. To appreciate the scope of the material in this chapter, it may be well to review a few film-forming mechanisms, polymer fundamentals, and some background into the development of tests and what they mean.

Background and Film Formation

Polymers A polymer is a high-molecular-weight interreaction product of related polyfunctional molecules [1 ]. The term polymer is derived from the Greek poly (many) and met (part). Polymers are formed from monomers (mono -- one) (met -- part) by a process of polymerization. Polymers consist of long chains of macromolecules with molecular weights of a few thousand up to many millions. For example, ultrahigh-molecular-weight polyethylene may have a molecular weight of 4 to 6 million, while alkyds have molecular weights of a few thousand. The concept of macromolecules was developed by Herman Staudinger in 1920 [2]. Staudinger's theory was supported by the work of Wallace Carothers, who introduced the concept of functionality and the distinction between thermoplastic and thermosetting polymers [3]. There are two types of polymers: addition and condensation. Addition polymers have high molecular weight and are characterized by fast chain reaction syntheses that are exothermic and initiated by free radical, coordination polymerization, cationic, group transfer, or anionic mechanisms. Free radical-initiated polymerization is initiated by incorporation of a free radical producer such as benzoyl peroxide which decomposes with heat to form free radicals, which in turn initiate polymerization. Coordination polymerization is Senior Consultant, Consolidated Research, Inc., 16731 Brick Rd., Granger, IN 46530.

initiated by complex initiator systems such as those used to polymerize ethylene or propylene and sometimes referred to as Ziegler-Natta catalysts made from an organometal and an inorganic compound like titanium tetrachloride. Cationic initiation can be done using Lewis acids such as aluminum chloride or boron trifluoride and are usually carried out at low temperatures. Butyl rubber is made by such a process. Group transfer is a relatively new type of polymer initiation, while the anionic processes use organometals (butyl lithium) to produce polymers. Block polymers are made by anionic processes. Condensation polymers have lower molecular weight than addition types and are produced by stepwise addition with elimination of water as a byproduct and require a longer time to synthesize using an endothermic process [4].

Cure Concept and Illustrations We can simplistically illustrate the concept by considering a long chain polymer (thermoplastic) as a wire fence with a number of strands (Fig. I) such as used on a farm. An individual can pass through the fence by spreading the strands apart, and standing on the fence will cause the strand to sag or elongate. If, however, we take that fence and lace it with wire such that a hogwire fence is produced, then we see the changes in the physical properties of this (thermoset) fence. First, it would be difficult to pass through the fence; second, one can stand on the fence since it is stronger and will not yield as would the single strand fence. If one considers the ability to pass through the fence as solvent action, it is easy to visualize how solvent molecules can permeate and dissolve a thermoplastic polymer, but difficult or impossible to dissolve a thermoset polymer. In fact, a thermoset polymer will tend to swell, such as a rubber band in a hydrocarbon solvent. This is because the solvent molecules can solvate the linear portions of the rubber, but are constrained by the sulfur crosslinks from totally dissolving the polymer. Another aspect of cross-linking is that the molecular weight increases enormously, which explains why solvent resistance increases and certain physical properties such as hardness, scratch resistance, and toughness are enhanced. The thermoset polymer is a three-dimensional network polymer. Cure must not be confused with dry time such as that measured in oil-containing polymers which dry by interaction with atmospheric oxygen. Here such terms as set-to-touch, print free, time to lint free, and time for sand pickup relate to the speed at which solvent evaporates and not to the ultimate cure or cross-linking of a coating system. Cure, therefore, refers to the cross-linking of a polymer to produce a three-dimensional network.

407 Copyright9 1995 by ASTMInternational

www.astm.org

408 PAINT AND COATING TESTING MANUAL THERMOPLASTIC S

S

S

S

S

s

I

Lacquers Lacquers consist of a thermoplastic polymer which is dissolved in a solvent. When the coating is applied, the solvent evaporates and a thermoplastic film is formed. This is the type of coating used in certain furniture and automotive finishes.

s

I

S

[

S

(crosslinking)

s

S

Coating films can be made by a number of methods. The principal mechanisms of film formation are represented by lacquers, emulsions, and thermosetting (via oxidative crosslinking or by reactive cross-linking) coatings. For a review of film formation, the reader is referred to the Federation of Societies for Coatings Technology Monograph on Film Formation [5].

S

S

THERMOSET

CURE MECHANISMS

(no c r o s s l i n k i n g )

I

l S

S

Emulsions

= solvent molecules = linear polymer = crosslinked segments

Another means for forming a thermoplastic film is film formation from an emulsion. In this case, a polymer, such as a vinyl acetate-acrylic copolymer emulsion (or latex), is cast onto a substrate and then the water carrier leaves the film by evaporation. In the latex, water is not a solvent but a dispersing medium. Microscopically, the polymer molecules look like tiny ball bearings suspended in water (Fig. 2). Surface active agents keep the polymer from settling. As the water evaporates, hydrodynamic pressure causes the small spheres to crowd closer and closer together until capillary action forces the particles to fuse and form a continuous film. If the polymer is too hard, a poorly adherent film forms. This is why paint manufacturers recommend that water-based, latextype paints not be applied at low temperatures. At low temperatures, the particles cannot fuse properly, and the paint will fail prematurely. To facilitate fusion, small amounts of coalescing solvent such as diethylene glycol monobutyl ether (butyl Carbitol TM Union Carbide Corp.) are added. These solvents lower the glass transition temperature of the copolymer and facilitates fusion. These are examples of physical drying as compared to chemical drying discussed next (see Fig. 2).

FIG. I-Polymers.

TYPES OF FILMS As described above, polymers can be classified as thermoplastic and thermoset in nature when derived from addition and condensation polymerization as well as by their physical response to heat and solvent action.

Thermoplastic A long-chain polymer such as polyethylene is considered thermoplastic since it can be dissolved in certain solvents and can be repeatedly softened or melted when heated and hardened when cooled.

Thermoset On the other hand, a polymer prepared, for example, from a phenol-formaldehyde condensate cannot be liquified and solidified repeatedly after it is formed. These polymers are termed thermosetting and tend to degrade if heated repeatedly. The reason for this is that they are cross-linked.

Oxidative Cross-Linking In oxidative cross-linking, a polymer is prepared from an unsaturated fatty acid or oil such as castor, tall, soya, linseed, or tung oil, and a small amount of a siccative (drier) such as

OWOWOWOWOWOWOWOWOWOWOW W

W

W

W

W

W

W

0000000000OO000000OOOO

O=Polymer

particles

W=Water

FIG. 2-Drying of a latex film.

Latex

on

surface

Water

evaporates

Fused

film

W

CHAPTER 36--CURE: THE PROCESS AND ITS M E A S U R E M E N T lead, cobalt, manganese, or zirconium salts, for example, naphthenates, octoates, etc., is added. The film is cast by brushing, spraying, or other means and allowed to dry. In the first stages, the solvent evaporates, leaving a tacky film. The film can be easily deformed or thumb printed. But, as the film ages, it slowly becomes less tacky and finally print free and cured. What has happened? The film slowly absorbed oxygen from the atmosphere and formed hydroperoxides on the carbon alpha to the double bond. The drier decomposed the hydroperoxide and formed free radicals that initiated crosslinks between the chains derived from the fatty acid portion of the alkyd (which imparts improved properties to the film), resulting in a cured or cross-linked film. A simplified reaction scheme is shown in the following [6]: R--CH=CH--CH2--CH~-~-CH--R' 02 ) R--CH--CH=CH--CH~-~CH--R'

I

OOH R--CH--CH=CH--CH~---CH--R' + .OOH where R is CH3(CH2) 4 and R' is (CH2)TCO2CH 3. The initial products are converted to polymers according to: Initiation: Propagation

RH

02

+

) R" + "OOH

R" + 0 2

)

409

Melamine can be condensed with up to 6 mol of formaldehyde and 6 mol of methanol to form hexamethoxymethyl melamine (HMMM). The reaction product has the following structure and is often referred to as MF resin: N(CH2OCH3)2

/A N

N

t

II

(CH3OCH2)2N--C

C--N(CH2OCH3)2 N

Hexamethoxymethyl melamine Hexamethoxymethyl melamine can be reacted to contain from one to six methoxymethyl groups. Methanol is used to cap the hydroxyl group to prevent premature reaction. Other alcohols are commonly used, including butanol and isobutanol. In applying a melamine formaldehyde resin, the resin usually is reacted with a functional polymer such as a thermosetting acrylic or alkyd polymer containing an acid or hydroxyl group that can react with the MF resin. This can be depicted in the following manner: 2 Polymer--[- + M--N(CH2OCH3)2

)

1

CH2OH

RO0"

Polymer-----[-RO0" + RH

Termination

) ROOH

RO0" + R"

)

RO0" + ROO"

+ R"

CH2(~

ROOR

/ CH2 M--N + 2CH3OH \

> ROOR + 02

R'+R"

>R--R

Termination by coupling doubles the size of the molecule since two fragments are now joined (assuming that the chain length is about equal). In this way oil-bearing oxidative curing leads to networks having the proper chemical and physical properties.

Reactive Cross-linking In this type of cross-linking, a reactive intermediate is added to a polymer and a further chemical reaction is initiated either by heating alone, heating in the presence of a catalyst, or by some other form of initiation.

CH~

I

CH20

Polymer

I

where M denotes a methoxy-substituted melamine functional unit. In this illustration, only two of the melamine methoxymethyl functions are shown reacting with the hydroxyl groups of an alkyd or other hydroxyl functional polymer. Note also the formation of a byproduct, methanol. Formaldehyde can also form as a byproduct.

Urea

Melamine Melamine formaldehyde condensates are used to cure a reactive polymer. Melamine is 1,3,5 triamino-s-triazine and is produced from the condensation of 3 moles of dicyandiamide: NH2

A scheme similar to that described above for melamine is also applicable to urea formaldehyde (UF) resins. In this case, urea is reacted with formaldehyde and endcapped with either methyl, butyl, or isopropyl alcohols. A fully reacted urea intermediate would have a structure (CHaOCH2)2--N--C --N--(CH2OCH3) 2

II 0

I

iL

H2N--C

C--NH 2 N

Melamine

N,N' bishydroxymethylurea and is capable of reacting with hydroxyl groups to produce a cross-link with the elimination of byproducts as water, alcohol, and formaldehyde.

410 PAINT AND COATING TESTING MANUAL Epoxy Epoxy resins are derivatives of cyclic ethers. Aromatic epoxy resins are produced from the reaction of epichlorohydrin and bisphenol A, and the latter is a condensation product of acetone and phenol. Aliphatic epoxy resins are prepared by the peroxidation of unsaturated linear or cyclic olefins. A simple epoxy, the diglycidyl ether of bisphenol A, is shown below (Z = phenylene group) O

O

CH3

/\

/\

i

CH2--CHz--O--Z--C--Z--O--CHz--CH--CH2

[

CH3 These epoxy functional groups can react with active hydrogen groups such as amines, hydroxyls, and acids. For example, they can react with acid groups in a thermosetting acrylic, alkyd, or other polymer and produce a cross-linked polymer. It is interesting to note that the reaction product of the epoxy and acid function produce additional hydroxyl groups which can be reacted with a melamine formaldehyde or urea formaldehyde adduct to produce improved properties, such as hardness, chemical resistance, or solvent resistance in the cured films. The reaction of two acrylic polymer chains with an epoxy resin is shown as follows: [ COOH 0

/\

i

I

CH3 COOH I

Acrylic polymer

l Acrylic polymer H

O

O

I

CH~

t

T

HN

I

CH--CH2--O--Z--C--Z--O--CH2--CH CH3

OH

CH2

t I

O C-~O

I

Acrylic polymer where Z represents a phenylene group. Cross-linked acrylic polymer

Isocyanates

I

O=C--O

I

(CH2)2 Acrylic polymer

T

= CH3--C6H

3-

(from toluene diisocyanate)

Urethane cross-linking is also used in polyesters where the hydroxyl groups are obtained from polyfunctional alcohols, such as trimethylolpropane or pentaerythritol. Similarly, epoxy resins can be cross-linked through the hydroxyl groups formed during epoxy reactions.

Catalyzed Cross-linking

Phenolics Phenolic resins are condensation reaction products of phenol or substituted phenols and formaldehyde. Two types of phenolics are obtained depending upon the catalyst and reaction conditions. Using an acid in its preparation, a thermoplastic, soluble novolac resin is obtained. Under basic conditions, the thermoset product formed is a resole resin which is cross-linked at the final stages of the reaction. A phenolformaldehyde condensate may have a structure as shown:

The term urethane applies to the reaction product of an hydroxyl group and an isocyanate. For example, an alcohol and isocyanate react by rearrangement to produce a urethane CH3--CH2--OH + R - - N ~ C = O

~R--N--CqO

L I

H Alcohol

+

Isocyanate

NH

This type of cross-linking is used in phenolic and silicone reactions wherein a catalyst is added to facilitate the crosslinking reaction.

CH3

I

O--C~-~O

Urethane cross-linked acrylic

CH2--CH--CH2--O--Z--C--Z--O--CH2--CH--CH2Epoxy resin

I C=O I

Acrylic polymer

I I

(CH2)2

0

CH3

/\

Acrylic polymer

ings, and high-performance coatings such as for aircraft. In this case the isocyanate is a di- or triisocyanate, which leads to cross-linking. Toluene diisocyanate is widely used, hut other isocyanates such as 1,4-diphenyl methane diisocyanate, isophorone diisocyanate, and trimers of hexamethylene diisocyanate are also used. The higher-molecular-weight isocyanates are preferred for their lower vapor pressure, thus providing for improved working conditions. Isocyanates are used to cross-link a variety of polymers containing hydroxyl or amine functionality. In the case of amine functionality, a urea cross-link is formed. For example, an acrylic polymer can be prepared from methytmethacrylate, butyl acrylate, and hydroxyethyl methacrylate. This polymer can be cross-linked through the pendant hydroxyl groups of the hydroxyethyl methacrylate portion by polyfunctional isocyanates:

O--CHz--CH3 Urethane

This reaction is used in applying isocyanates to the production of insulation and seating foams, reaction injection mold-

OH

I

C

C

If I HOH2C--C C--CH2OH \// C CH2OH Phenol-formaldehyde (P--F) condensate

CHAPTER 36--CURE: THE PROCESS AND ITS M E A S U R E M E N T Alkyd resins can be cross-linked by reaction of the hydroxymethyl function and an hydroxyl group on the alkyd with the elimination of water: OH

411

source for initiation. Electron beam (EB) and ultraviolet radiation is used to cure coating films. These processes induce either free radical or carbonium ions to initiate polymerization, which occurs rapidly at ambient conditions.

I /C\~

2

+ HOH2C--C

I

C--CH2OH

HEAT

I II C C \//

CH2OH

C CH2OH Alk, 'd

Phenol-Formaldehyde Condensate

f --CH20

H2C--C

C--CH2

II I C C \// C I ALKYD

CH2OH

OH2C-- + 2 H20

ALKYD

Cross-linked alkyd

Silicones These useful polymers are prepared from silicon by chlorination, then subsequent reaction with alkyl or aryl halides to form alkyl/aryl chlorosilanes. Upon hydrolysis, these produce silicones. The alkyl silicones, such as dimethyl silicone, CH3

I

[

Si

J

O--]n--

CH3 are low-molecular-weight or cyclic materials used in antifoam applications. The linear forms are specialty elastomers. Blends of methyl and phenyl silicones have higher-temperature resistance and find use in heat-resistant coatings or polymers. Silicones can be cross-linked when prepared with multi functional silane monomers such as trichloromethyl silane. Upon hydrolysis, the silicone chains can be cross-linked through siloxane bridges: CH3

f

[

Si

I

O--].--

O [

I

Si

[

O--In-

CH3 Cross-linked silicone Silicones can also be cross-linked by peroxide initiators. In this case a silicone monomer containing a vinyl or allyl group is copolymerized to provide an active site along the chain. When used in a gasket or caulk, a small amount of a peroxide is added which initiates polymerization and cross-links the resin system. Silicone reactions are usually catalyzed by 01"gano zinc or tin compounds. Curing can also be accomplished using radiation as a

CURE MEASUREMENT It is very important for the user of coatings to know whether a coating is adequately cured. This can be done quickly in qualitative testing or in more sophisticated test methods requiring instrumentation. The rapid methods are desirable because of time and cost, but in some cases do not really tell the true story. In some cases, even instrumented testing only tests a certain range of cross-linking and could lead to erroneous conclusions. We shall discuss the qualitative methods first, then proceed to the more quantitative. What should be borne in mind is that we are attempting to ascertain a molecular process, cross-linking, and its relationship to coating performance. Many thermoplastic coatings "dry" by simple evaporation of solvent, and though cure was defined above as the crosslinking of a polymeric system, such drying of lacquers can be considered a form of curing even though no cross-linking occurs. This is true for nitrocellulose lacquers, cellulose acetate, and solvent soluble acrylics where no cross-linking takes place. To measure cure for such coating, the time for either print-free or tack-free is measured. At some time during evaporation, the coating goes from tacky, to set-to-touch, to tackfree. Upon further drying, the coating becomes print-free, whereby a thumb print will not be visible on the coating. This can also be measured by dropping cotton linters onto the surface and noting the time after which they do not adhere to the coating. Also see ASTM D 2091, which describes a standard method for print resistance of lacquers. These tests also apply to coatings which dry by oxidative cross-linking, for example, alkyds. Here, times are noted for solvent evaporation, and the coating is monitored for set-totouch, then print-free.

Solvent Rubs (ASTM D 4752 Test Method for Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub) A convenient, often-used method for determining crosslinking involves solvent rubs. A cloth soaked with methyl ethyl ketone (MEK) is rubbed vigorously on the film, and then the film is examined after a specific number of rubs. Acetone is also used for this test. If the film is removed, softened, or loses gloss, then the film is considered not adequately crosslinked. Films which are unaffected are considered cured. For example, certain acrylic, epoxy, or polyester coatings can survive this test. The problem with this test is that the individual testing the coating is not standardized. One person may exert much more pressure than another, and this can affect the final result.

Hardness Measurements Another way of obtaining information concerning crosslinking is to measure some physical property such as, hard-

412

PAINT AND COATING TESTING MANUAL

ness, scratch resistance, or impact resistance. There are a number of tests which provide this information.

Knoop and Pfund Hardness (ASTM D 1474 Test Methods for Indentation Hardness or Organic Coatings) A useful test of cure is to measure the Knoop hardness in which a pyramidal diamond indenter is pressed into the film to cause indentation. The method is described in ASTM D 1474. A weight is applied to the indenter for a specific time, and the length of the indentation line is measured with a microscope. The Knoop hardness number (KHN) is calculated from [8]:

KHN = L/Ap = L/lZCp where L = applied load (kg), Ap = projected indentation area (mm2), l = length in mm of the long diagonal of the indentation mark, and Cp is an instrument constant. Because of the thinness of coating films, this method is more useful in plastics since it requires almost a 75% indentation into the film and can be influenced by substrate considerations. The Pfund Hardness Number (PHN) is obtained using a hemispherical quartz or sapphire indenter, which is pressed into the film. The Pfund Hardness Number is calculated

from: PFN = L/A = 4L/wd2 = 1.27 L/d 2 where L = applied load in kilograms, A = area of projected indentation in mm 2, and d = diameter of the projected indentation in mm.

Sward (See ASTM D 2134, Sward Rocker, and ASTM D 4366, Pendulum Damping) Sward hardness depends upon the change in surface properties and viscoelastic properties of a film. This method, which was discontinued in 1990, is used for automotive finishes and consists of a rocker containing two spirit bubble indicators. The pendulum is rocked, and the number of swings are recorded and a reading taken at the point where the swings are equal to half the original value. Several types of pendulum units are used including Sward, Perzoz, and Koenig. As cure increases, the Sward number increases. The value of a Sward test is that a n u m b e r is obtained which can be compared to other coatings. This test must also be conducted under very clean conditions, as lint and surface imperfections can interfere. Temperature control is important in this test as in other tests involving determining physical properties of coatings. The problem with this test is that it only measures cross-linking density to a point; then cannot detect over cure since a plot of Sward numbers versus cure tends to reach a limiting value. A comparison of values obtained by different test methods is shown in Table 1 [9]. Note how the values differ, especially between different brands of pencils and the limiting value of Sward Hardness.

Pencil (ASTM D 3363 Test Method for Film Hardness by Pencil Test) Pencil hardness is a test which was developed by the manufacturers of pencils who tried to develop a means for checking

the quality of pencils. Someone suggested scratching a paint film, and it was noticed that the different hardnesses of pencils were able to penetrate through the coating. This test was later used by coating technologists, and it is widely used in the industry today. In this technique a number of pencils with known hardnesses are employed. The pencil is t r i m m e d so that 5 mm of lead is exposed. The lead diameter is 1.8 m m and is sharpened by rubbing it perpendicular to the surface of No. 400 carbide abrasive paper. The pencil is held at a 45 ~ angle to the coating and pushed along the surface to peel away the coating. The pencil which fails to scratch the coating is the value used as pencil hardness. This method is simple, the equipment low in cost, and the results can be quickly obtained. However, values can differ depending upon the operator, the method of sharpening the lead, and the variances of lead hardness from different pencil lots and manufacturers.

Gardner Impact (ASTM D 2794 Test Method for Resistance of Organic Coatings to the Effects o f Rapid Deformation (Impact)) This is a valuable test which measures the impact resistance of a coating and can be used to correlate with cure. An undercured coating may exhibit a lower Gardner Impact value, but as cross-linking density increases, the impact values improve. The test consists of placing a coated flat panel under a weighted spherical ball assembly and then dropping the weighted ball onto the panel from different heights. The cylinder in which the ball assembly is mounted is calibrated such that an operator can measure the impact directly in inch pounds. Impact measurements are done by dropping the ball directly on the coating surface or on the reverse side. The results are reported as inch-pounds direct or reverse. Direct impact is less severe than reverse impact. A dimple is formed in the test panel which can be examined visually or with a X10 glass to determine the extent of cracking which occurs. An appliance acrylic may have a direct impact of only 10 in.-lb, while a polyester urethane powder coating may have a reverse impact of 160 in.-lb. The value of the Gardner Impact test is that the test can be done quickly, it is widely recognized in the coatings industry, and it gives some correlation with cross-link density in that the optimum physical properties of a coating develop as the molecular weight increases.

Thermal Analysis Thermal analysis is an important analytical tool for determining the response of material to changes in temperatures. This method of analysis can be used to monitor the glass transition temperature, Tg, of a coating, and this can be related to the cross-linking density. For example, a ladder of paint panels cured at various times or temperatures can be prepared and then studied by thermal analysis to determine the change in Tg as a function of bake schedule. From this, the optimum cure cycle can be determined to insure a quality finish. Thermal analysis units have several modes for determining Te: differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA) are useful tools for obtaining quantitative mea-

CHAPTER 36--CURE: THE PROCESS AND ITS MEASUREMENT 413 TABLE l mHardness test correlation. Panel No.

KIlN

Sward

A

B

Pencil (Brands) C

D

E

3.09 4.33 2.77 2.61 5.81

24 28 24 22 38

5B 4B 5B 3B 2B

6B 6B 6B 4B 2B

5B 6B 5B 5B 2B

6B 6B 4B 4B 2B

4B 4B 3B 3B HB

1 2 3 4 5 10

25.7

54

H

H

H

H

2H

12

39.1

40

3H

2H

2H

4H

3H

40

8H

9H

7H

7H

9H

14

sures of cross-linking b y m e a s u r i n g Tg. The glass t r a n s i t i o n t e m p e r a t u r e is a s e c o n d o r d e r t r a n s i t i o n t h a t r e p r e s e n t s a t e m p e r a t u r e w h e r e segmental m o t i o n occurs along a p o l y m e r c h a i n backbone, a n d it is a c c o m p a n i e d by a volume change. A c o m p a r a t i v e s t u d y of changes of cross-linking m e a s u r e d b y S w a r d hardness, evaporative rate analysis, a n d DSC s h o w e d t h a t Tgchanges c a n be followed to high levels of cross-linking, w h e r e a s S w a r d m e a s u r e m e n t s r e a c h e d a limiting value while Tg c o n t i n u e d to rise [10]. W i t h DSC, a plot of e n d o t h e r m / e x o t h e r m is m a d e as a function of t e m p e r a t u r e at a set rate of t e m p e r a t u r e rise. As the Tg is reached, there is an a b r u p t change in the e n d o t h e r m w h i c h a p p e a r s on a plot. Similarly, Tg can be m e a s u r e d using a plot of c h a n g e in e x p a n s i o n with t e m p e r a t u r e with the TMA mode. Here a s a m p l e is p l a c e d u n d e r a quartz p r o b e a n d t h e n heated. As the film expands a plot is o b t a i n e d in w h i c h a change in slope occurs. The intersection of the tangents of the e x p a n s i o n curve yields the Tgvalue. I n the p e n e t r a t i o n m o d e a d e p r e s s i o n occurs indicating the Tg. Plots can be m a d e of Tg versus b a k e cycle to note the o p t i m u m Tgof a coating. F u r t h e r details on t h e r m a l analysis a p p l i c a t i o n s can be f o u n d in the literature [11 - 13 ].

Dynamic Mechanical Analysis Another w a y of o b t a i n i n g Tg is to use d y n a m i c m e c h a n i c a l analysis in which a coating film is fixed to the ends of a tuning fork a n d the fork driven over a frequency range a n d at different t e m p e r a t u r e s o r at a fixed frequency w i t h varying t e m p e r ature to provide a plot of d a m p i n g versus t e m p e r a t u r e . This i n f o r m a t i o n yields Tg as well as m e c h a n i c a l d a m p i n g inform a t i o n a b o u t the film [14].

Torsion Pendulum [14,15] A n o t h e r m e t h o d for m e a s u r i n g d y n a m i c m e c h a n i c a l p r o p erties of coatings is to s u s p e n d the film with a weight, then twist the weight so that it will oscillate as a p e n d u l u m . This is not entirely acceptable since coating films m a y n o t have the strength to s u p p o r t the weight of the a p p a r a t u s . One way to o v e r c o m e this is to invert the a p p a r a t u s so that the p e n d u l u m weight is s u s p e n d e d w i t h counterweights. A m o r e practical m e t h o d is to use the t o r s i o n b r a i d p e n d u l u m in w h i c h a fiberglass b r a i d is s a t u r a t e d w i t h a coating a n d the p e n d u l u m oscillated. As the coating cures, changes in oscillation can be c o r r e l a t e d with d a m p i n g a n d Tg.

Impedance Measurements Myers [16] s t u d i e d the drying behavior of latex systems using ultrasonic i m p e d a n c e m e a s u r e m e n t s , I n this work, a latex coating was cast on a quartz crystal a n d ultrasonic energy was b e a m e d at an 11 ~ angle at the u n d e r s i d e of the coating a n d reflected to a d e t e c t o r w h i c h m e a s u r e d the attenu a t i o n of the initial b e a m as a b s o r b e d by the drying coatings. As w a t e r evaporated, there were changes in i m p e d a n c e t h a t could be c o r r e l a t e d to drying.

Evaporative Rate Analysis A novel m e t h o d for studying cure is evaporative rate analysis (ERA) developed by John Anderson [17]. This w o r k was carried out to d e t e r m i n e the cleanliness of the surface of spacecraft. Anderson r e a s o n e d that if a radioactive C 14 liquid w o u l d be p l a c e d on a clean surface, the e v a p o r a t i o n rate would be r e t a r d e d due to cleanliness. In fact, however, the opposite was true. Nevertheless, this m e t h o d was developed to m e a s u r e degree of cleanliness. If a small a m o u n t of diethyl succinate-C -14 is d e p o s i t e d on a surface a n d a controlled sweep of n i t r o g e n is applied, the rate of e v a p o r a t i o n can be m o n i t o r e d using a Geiger counter. The principle is that on a clean surface there is no i n t e r a c t i o n between the solvent a n d the clean substrate, a n d n o r m a l e v a p o r a t i o n occurs. If, however, the surface contains a c o n t a m i n a n t , then the radioactive liquid will interact with the surface c o n t a m i n a n t a n d r e t a r d the rate of evaporation of the r a d i o c a r b o n . Plots c a n be o b t a i n e d relating cleanliness to e v a p o r a t i o n rate. Anderson a p p l i e d this technique to the cure of organic coatings. H e s h o w e d t h a t in an u n d e r c u r e d surface solvent r e t e n t i o n increased, leading to longer residence of the rad i o c a r b o n on an u n d e r c u r e d surface. This is u n d e r s t a n d a b l e in view of the example given in the beginning of this c h a p t e r of the ability of a solvent to pass t h r o u g h a wire fence analogy. By p r e p a r i n g c o a t e d panels having different cure times o r t e m p e r a t u r e s , t h e n m e a s u r i n g the ERA of each panel, a plot of r e t e n t i o n versus bake is obtained. This m e t h o d was a p p l i e d to the m e a s u r e m e n t of cure of a variety of coatings by m a n y coatings technologists [10,18]. The only p r o b l e m was t h a t there was no quantitative m e a n s for m e a s u r i n g the relationship of ERA to cure until it was s h o w n that DSC of Tg

414

PAINT AND COATING TESTING MANUAL

could be correlated to the values obtained using ERA meas u r e m e n t s [10].

REFERENCES [1] D'Alelio, G. F., Fundamental Principles of Polymerization, John Wiley & Sons, Inc., New York, 1952, pp. 5-22. [2] Staudinger, H., Uber Polymerization, Vol. 53, 1920, pp. 10731085. [3] Mark, H. and Whitby, G. S., Eds., Collected Papers of Wallace Hume Carothers on High Polymeric Substances, Interscience, New York, 1940. [4] Paul, S., Surface Coatings, Chapter 1, John Wiley & Sons, New York, 1985. [5] Wicks, Z. W., Jr., Film Formation, Federation Series on Coatings Technology, Blue Bell, PA, June 1986. [6] Paul, S., Surface Coatings, John Wiley & Sons, New York, 1985, pp. 452-453. [7] Brown, W. H. and Miranda, T. J., Official Digest, Vol. 36, No. 475, 1964, p. 92.

[8] Paul, S., Surface Coatings, John Wiley & Sons, New York, 1985, p. 485. [9] Sato, K., Progress in Organic Coatings, Vol. 8, No. 1, 1980. [10] Miranda, T. J., Journal of Paint Technology, Vol. 43, No. 553, 1971, p. 51. [11] Seymour, R. B. and Carraher, C. E., Jr., Polymer Chemistry, An Introduction, 3rd ed., Marcel Dekker, New York, 1992, p. 139. [12] Stevens, M. P., Polymer Chemistry, An Introduction, 2nd ed., Oxford University Press, 1990, p. 167. [13] Miranda, T. J., Mechanical Behavior of Materials, Vol. III, The Society of Materials Science, Japan 1972, p. 392. [14] Lambourne, R., Ed., Paint and Surface Coatings, Ellis Horwood Ltd., 1987, p. 607. [15] Allcock, H. R. and Lampe, F. W., Contemporary Polymer Chem/stry, 2nd ed., Prentice Hall, Englewood Cliffs, NJ, 1990, p. 427. [16] Myers, R. R., Journal of Polymer Science, C, Vol. 35, No. 3, 1971. [17] Anderson, J. L., Root, D. E., and Green, G., Journal of Paint Technology, Vol. 40, No. 320, 1968. [18] Rossi, A. G. and Paolini, A., Journal of Paint Technology, Vol. 40, No. 328, 1968.

MNL17-EB/Jun. 1995

Film Preparation for Coating Tests

37

by Robert D. Athey, Jr. 1

THE PERFORMANCEOF A COATINGFILMin a test is likely to be dependent on the physical form of the film. For instance, film thickness is an important factor in physical and appearance measurements (until the coating gets too thick), so there must be some control of the film thickness. Appearance is also related to how smooth the film surface is, and care to make the film appropriately will ensure that the appearance measurements are germane to the end-use. The substrate used as the carrier for the test film, even temporarily, may affect the property measurements, as well. The primary concern in making films for tests is that the film prepared be homogeneous and consistent with previous or future films for the same test. The jargon of the trade calls the art of making films "casting," but many film formation methods are used to form the film, and each method has its own set of advantages and drawbacks.

T E S T R E Q U I R E M E N T S OF F I L M S Test requirements come in two classes, appearance and physical properties. Choice of application method for the film may affect the appearance in some cases. For instance, the "multicolor" paints must be applied by dip or spray techniques for laboratory testing, or the appearance does not give the desired mottle of nearly circular spots. Roller, brush, or even drawdown bar will cause these spots to become streaks. Choice of application method for appearance includes drying technique, as some films require special drying conditions to attain their desired special appearance. Examples include leafing aluminum flake "metallic" look paints, hammertones, and wrinkle finishes. Choice of application method driven by physical property testing is also necessary. Think carefully of the three kinds of stresses (tensile, compression, shear) and recognize that all are blended in a hardness or adhesion evaluation by indenter, pencil, mandrel bend, or dart impact test on a coating. Recognizing these stress combinations can make one aware of film preparation needs. In cases where adhesion is stronger than cohesion of the film, one should be able to distinguish between adhesive failure and cohesive failure. Film preparation should not conflict with these objectives. The physical properties to be tested in specific ASTM methods have a "significance and use" section in the method to ascribe the relation of the test value to some "in use" perfor~Athey Technologies, P.O. Drawer 7, E1 Cerrito, CA 94530-0007.

mance criterion. The preparation of the test film should thus correspond to the standard field application of the material, a s well.

Another portion of film influence on the test is film thickness. The presentation by the Technical Committee from Toronto in the 1990 National FSCT meeting and Paint Show [1] dealt with hardness measurements in films of varying thickness. However, the control of film thickness was not as straightforward as might have been thought. Hardness values turned out to be dependent on film thickness along with other equally important variables [2]. Fluid rheology, concentration, and other factors govern the amount of coating left after the application process. Knowing the process variables associated with a film-casting technique are essential to getting what is needed for the ultimate test and results therefrom. Certain other properties of the film (opacity, permeability, erosion rate by some attacking mode, etc.) will depend on the thickness of the film. So, understanding thickness control in the casting process is crucial to obtaining meaningful results in terms of meeting specifications in reproducibility. In instances where fluid or gas permeation resistance or corrosion barrier properties are to be tested, a "pinhole," "holiday," or "mudcrack" in the film is a fatal flaw. The film preparation technique must eliminate (as much as possible) any such fatal flaw, or the test method should prescribe what is to be done (for example, replications) in cases where an unseen fatal flaw is detected by the test.

FILM C A S T I N G T E C H N I Q U E S Free Films Free films, that is, films not applied permanently to a substrate, are used for a wide variety of tests and accordingly vary in thickness and size. The most common castings of free films are used for permeation or strength and elongation testing. They can also be used for cold flex tests or moisture/ solvent absorption studies related to permeation. Free film thickness can be measured as described in ASTM D 1005: Test Method for Measurement of Dry-Film Thickness of Organic Coatings Using Micrometers. Probably the most common ~ee film castings are laid down on a "nonstick" surface, such as the silicone-coated papers (available through Leneta and other suppliers) or TEFLON TM or polyethylene sheets. Release substrates are described in ASTM D 823: Test Methods for Producing Films of Uniform

415 Copyright9 1995 by ASTMInternational

www.astm.org

416

P A I N T A N D COATING T E S T I N G M A N U A L

Thickness of Paint, Varnish, and Related Products on Test Panels. Lack of release-substrate wetting is the most common problem encountered when trying to obtain a continuous film. Water-borne coatings are especially difficult to formulate for good wetting of these substrates. One trick to use with such formulations is to prepare the sample at higher than normal solids content, which results in increased viscosity, and resistance to beading is improved. An alternative is to apply several coats until the dried "beaded-up" portions merge to form a continuous film. A vacuum plate makes it easier to hold down the paper or polymer film for film casting and is an aid to good results. Figure I is a diagram of the vacuum plate formed from aluminum. Older versions had eyes at the bottom surface through which screws could affix the plate to a table top. If this system is to be vacuum supplied from a sink faucet aspirator, a liquid trap (vacuum side-fitted Erlenmeyer flask) should be placed in line between the vacuum plate and the aspirator. This will ensure that any liquid coming from the aspirator will not get to the vacuum plate or coating substrate. Grenko [3] suggested several coated paper techniques for obtaining free films, such as decal paper coated on the face side or outdated matte or semimatte photopaper. The decal paper could be floated on water and the paper peeled off. Grenko did not describe the removal of photopapers from the film. He also cited Sager [4] for a technique using an embroidery hoop for holding a nonmoisture proof cellophane. After the coating film had been applied, one simply poured water on the inverted cellophane and the coating film dropped off. These techniques are likely to be most appropriate for solvent borne or 100% reactive formulations. Water-borne coatings may be inappropriately leached or weakened by the water used to loosen and isolate the films. Grenko [3] also mentioned the use of metal substrates for free film isolation. Brightwell [5] used glass onto which a thin silver film had been vacuum deposited. Bayor and Kempf [6] used thin aluminum foil that was later dissolved from most of the back with hydrochloric acid doped with a minute amount of platinum chloride. The latter dissolved off only enough aluminum foil to leave a foil "frame" at the edges of the film to support it and to facilitate handling. This technique with aluminum foil dissolution in an acidic, aqueous medium presumes no sensitivity of the coating components to acid or water.

FIG. 1 - V a c u u m plate---pulling a vacuum through the side nozzle holds paper or paperboard substrates tight for drawdown film casting. (Courtesy P. N. Gardner Co.)

For thicker films (1 cm or more), a PYREX TM or TEFLON TM coated baking pan can be used with some success. For instance, about 300 mL of a latex formulation in a 9 by 12 in. (23 to 30 cm) Pyrex TM baking dish will yield plenty of thick film for testing. Occasionally, there is a "mudcracking" problem, but it can be reduced periodically by releasing the edges of the film from the vertical sides of the dish with a spatula to allow uninhibited shrinkage. This way usually results in increased size pieces, and 1 infl (6.25 cm 2) is all you need for the moisture absorption or solvent swell test. Using this technique, it is possible to obtain large enough pieces for moisture vapor transmission tests at application thicknesses that are common for roof coatings. The "mudcracking" problem is serious for any coating. The cause is the lack of "wet gel strength" to resist the tensile stresses within the shrinking membrane. Strictly speaking, unless the coating cross-links, it is not a gel, but simply a high-viscosity fluid or "pseudogel." The problem may be alleviated by assuring that the upper surface does not "skin over" through controlling the humidity or solvent-vapor environment over the drying surface. The literature describes mercury pool casting [7,8] or tin foil casting [9] with mercury amalgamation used to remove the tin without stressing the coating film. ASTM D 4708 Practice for Preparation of Free Films of Organic Coatings also describes the tin foil amalgamation technique. These techniques are no longer in common use, as the mercury vapor is deemed a health hazard. Rarely, but occasionally, a free film of a polymer or binder material is needed for testing water vapor permeation or absorption, for instance. Some of these tend to be tacky, and powder applied to the surface will make them easier to handle. Although pearl cornstarch is preferred over the powdered talcs, baby powders and similar talcum powders are often used. Rotational casting is a means of obtaining nonporous, nonmudcracked free films. The standard jar-mill roller (see Fig. 2) may be used with a variety of open containers (quart or gallon jars, earless paint cans with a 4-in. (10-cm) circle cut out of the bottom . . . . ) to centrifugally cast the film with good thickness control. An alternative device, the Caframo REAX 2

FIG. 2-Lab roller mill--Although this device is most often used for pebble milling, an empty earless paint can on the rollers may be used to cast films. (Courtesy of Indco Inc.)

CHAPTER 37--FILM PREPARATION FOR COATING TESTS rotating mixer [10], may be used to hold the paint can with its main axis along the rotation axis of the clamp (see Fig. 3). This rotational casting technique was used successfully in a study by the Los Angeles Society for Coating Technology Technical Committee to obtain tensile and moisture vapor permeation film samples from acrylic latex compounded with silane modified talcs [II]. It is derived from an older practice at the Thiokol Rubber Co. to form films of castable elastomers in a rolling pipe [12]. Equipment for this technique is easy to set up and use. Introduce 100 mL of coating into a 1-gal can and let it roll overnight. This will result in a film approximately 6 by 18 in. (15 by 46 cm). That is a sufficient amount for 6-in. (15-cm) tensile bars and 2-in. (5-cm) permeation circles. Close control of film thickness is achieved by knowing the area of the cylinder and concentration. From these, the weight of coating needed can be calculated. Films of tacky coatings or pressuresensitive adhesive formulations can be made by this technique. After the film is formed, the can is placed in a refrigerator or freezer to facilitate film removal. To obtain a centrifugally cast powder coating film, a sample of the powder is introduced into the can. Then, a hot air gun is held at the outside of the rolling can, and the powder coating will melt to coat the inside. Care is needed to eliminate scorching, and practice is needed to determine how close the flame should be to the can to get a good film. Other techniques to obtain a film include melting on a Teflon TM sheet on a hot plate or in between Teflon sheets in a heated (Carver, PHI . . . . ) press. Spraying is occasionally used to make free films (e.g., onto a silicone-coated paper). There is no problem with this technique when it is done by a skilled practitioner. However, films made with this technique may not give as good permeation test results as from one made by a film from a fluid-flow technique. A note of caution should be added about handling free films made for physical testing. Any undue stresses may chip or crack the film. This is especially important for permeation or physical property tests. In the case of the latter, some materi-

FIG. 3-Caframo "REAX 2"---This device was designed as a rotational mixer for fluids, but a gallon paint can will fit in it for rotational film casting, as well. (Courtesy of Caframo Ltd.)

417

als are "notch sensitive," that is, the results of a tensile or other physical test may be reduced by the presence of a notch, scratch, or crack in the film [13].

Dry Coatings on Substrates Test coatings on a substrate are generally applied for a wide variety of appearance and physical property tests. Color and opacity are commonly evaluated on coated paper charts, though unsealed paper charts with a porous surface are available, as well. Coating on substrates to simulate actual use is common so the paint lab has many kinds of steel, aluminum, wood, or other substrates in stock as needed by the customer or the paint company. Plastic panels are rarer, but can be made available when needed. Several companies now offer such products. Plastic panels may need special surface preparation techniques to assure wetting and adhesion (solvent washes, chemical etchs with alkali or acid, low-temperature plasma, or corona discharge or chemical oxidation, etc.). As painting plastics become more and more important for automotive, computer, sign, and other industries, the manufacturers will make panels available to potential paint suppliers. Panels of metal are a special case for the preparation of test films for paint. One may have to do a precoating surface preparation on the metal, and consultation with appropriate specifying agencies may be necessary. The Steel Structures Painting Council has many grades of hot-rolled steel preparation techniques that vary from wire brushing through sand blasting to expose "white metal" [14]. Other preparations such as phosphatizing, galvanizing, anodizing, etc., are unique to specific industries or industry segments and will require lab preparation of panels or freshly treated panels from the customer or a commercial source. An annual handbook contains descriptions of some of these techniques for lab usage [15]. Preordered steel panels that are subsequently stored in special corrosion-inhibitor-treated packaging may skew exposure tests if they are not rigorously cleaned of corrosion inhibitor just prior to the coating steps. Cleaning procedures may require rinsing with hydrocarbon solvent or acetone and hot-air drying just before paints are applied. Exposure panels made of metal must also have special edge protection to ensure that only the coated face has the exposure test applied. The casting technique on a paper or paperboard substrate is a skill one learns through apprenticeship. First, the panel needs to be clean and dust free. For horizontal castings, which is a way to obtain good flat films, it is necessary to ensure that the surface is level during casting and drying. Tape the chart or panel to the level surface so it doesn't curl as the film loses solvent or dispersing media and tries to shrink. Unsealed paper charts will curl in a convex manner since any water in the coating swells the paper fibers, and the substrate may try to curl the sheet in a concave manner when the film tries to shrink across the top surface from which evaporation is taking place. Vacuum plates are available, as noted earlier, to hold down the paper, foil, or cardboard chart during the film-casting process. The paper board chart should be stored in the same humidity environment that will be used in drying/ curing the coating as the wood-pulp fibers are hygroscopic.

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PAINT AND COATING TESTING MANUAL

Concrete, plaster, a n d asphalt/gravel test blocks o r panels m a y be p r e p a r e d in the lab o r m a y be p u r c h a s e d . However, w h e n e v e r possible such blocks should be o b t a i n e d from the c u s t o m e r since paint-lab results m a y n o t c o m p a r e well w i t h the c u s t o m e r ' s results if details such as porosity, surface salinity, surface pH, soluble c a l c i u m o r s o d i u m ion content, etc., are not equivalent. These m a t e r i a l s m a y be hygroscopic a n d will need equilibration in the drying/curing e n v i r o n m e n t p r i o r to coating. I n addition, s o m e concrete castings, t h a t is, tilt-up wall constructions, often have surface residues of f o r m coatings that h a d been u s e d to a i d release from the forms. Surface c h a r a c t e r i z a t i o n for such release aids o r rigorous cleaning m a y be needed. There is a n ASTM s t a n d a r d p r a c t i c e for p r e p a r a t i o n of m o r t a r panels for testing paints: ASTM M e t h o d of Making a n d P r e p a r i n g Concrete a n d M a s o n r y Panels for Testing Paint Finishes (D 1734). Grenko [3] has d e s c r i b e d press coating of b i t u m e n s on a l u m i n u m o r felts, with shims p l a c e d in the press to a t t a i n a p p r o p r i a t e thickness. His w o r k was b a s e d on earlier w o r k by Greenfield [16].

EQUIPMENT FOR FILM PREPARATION D r a w d o w n Bars The d r a w d o w n b a r s are the simplest of the film casting devices to use. One s i m p l y m a k e s a p u d d l e of coating on the desired s u b s t r a t e a n d moves the d r a w d o w n b a r over the puddle to m a k e the d e s i r e d wet film thickness. Application of the film with a d r a w d o w n b a r n o m i n a l l y gives a g o o d c o n t i n u o u s film, b u t the spacing b e t w e e n the b a r a n d the s u b s t r a t e m a y have little to do with the film thickness obtained. S p e e d o f pulling the d r a w d o w n b a r over the fluid will affect the film thickness since fluid rheology influences film thickness. Highmolecular-weight p o l y m e r s m a y cause the edges of the film to d r a w in a n d increase the wet-film thickness. The f o r m u l a t i o n m a y not wet the d r a w d o w n bar, a n d this a d d s a n o t h e r factor to the control of film thickness. Wetz a n d coworkers [17] s h o w e d a l m o s t 100% d e p o s i t i o n of the wet film a n d f o u n d a r e l a t i o n s h i p b e t w e e n wet-film a n d dry-film thickness. Tel = 0.057 + 0.588GS(0~/Or), o r Tdf = 0.057 + 0.955T~(og#r), where:

Wet Films for Testing There are a variety of reasons one m a y w a n t to have a close look at the wet film. S i m p l y w a t c h i n g the drying process m a y m a k e a n y p r o b l e m s with m o i s t u r e c o n d e n s a t i o n on the film, p i g m e n t float, etc., apparent. There are test films m a d e to assess wet hiding or sag resistance (ASTM D 4400: Test Methods for Sag Resistance of Paints Using a M u l t i n o t c h Applicator). G o o d film casting practice, for example, a level surface for casting, consistent technique, etc., is a r e q u i r e m e n t for r e p r o d u c i b i l i t y of results. E q u i p m e n t n e e d e d for special tests will be included in the following section.

Tar = Twf = Ot = 0f = G = S =

dry film thickness, mils wet film thickness, mils density of liquid density of dry film clearance of d r a w d o w n bar(mils), a n d weight fraction of solids in liquid.

Grenko [3] m a k e s the p o i n t t h a t these relationships are only true for d r a w d o w n blades having substantial flat surface between the leading edge of the b a r and the trailing edge. The B y k - G a r d n e r i n f o r m a t i o n [18] suggests the following expectations for varying film thicknesses cast:

TABLE l--Drawdown bars for film casting. Type "Bird" knife Two-path applicator (Wasag [11]) Eight-path applicator

Gardner "Microm" applicator (also Hercules-Gardner adjustable [3]) "Universal" blade applicator

Dow latex film applicator

Design Details

Suppliex~

Reference

Permanent fixed gap Bar has two cuts of different depth machined into top and bottom Stainless steel square tube with eight different depth cuts in each of top and bottom edges Micrometers on each side lower or raise blade of "U" shaped deviced Blade forms "U" shape with sides having slots and thumb screws to raise or lower "U" shaped blade with wider cut for larger gap so second cast film covers first

P.N. Gardner P.N. Gardner

[20]

P.N. Gardner

[20]

P.N. Gardner

[20]

P.N. Gardner

[20]

Byk-Gardner

[3]

aThere may be many other suppliers, but only one is cited herein as a space savings technique.

[3]

CHAPTER 37--FILM PREPARATION FOR COATING TESTS 15 to 100/xm 100 to 300 txm 300 to 500 txm over 500/~m

419

50% 60% 80% 90%

The hand-held drawdown bars are covered in an ASTM specification: Method E in ASTM D 832, Practice for Rubber Conditioning for Low-Temperature Testing. A wide variety of drawdown bars are commercially available, though your local machine shop can make special orders of any design, if needed. Table 1 lists a few of the available types of drawdown bars, with commercial sources reports on them. Some rectangular design devices have differing gap depths on the sides so one may choose the film thickness needed for casting (see Figs. 4 and 5). Stainless steel or aluminum are the preferred materials of construction, as corrosion can damage the region of the drawbar controlling thickness of applied film. Good laboratory practice dictates immediate cleaning of the paint contact surfaces after every usage to minimize the threat of corrosion or other damage. A caution on marking the drawdown bars, as some manufacturers label them not with the gap spacing, but a number half the gap spacing since that is the expected wet film thickness to be obtained. Some drawdown bars have film thickness adjustment choices by micrometers or other techniques (see Table 1 and Fig. 6). When these are used, the settings should be checked with feeler gages to make certain user wear has not made them inaccurate. These also require dismantling and cleaning after every use. If oil is used to inhibit corrosion and make the mechanics move smoothly, make sure they are rigorously cleaned of oil prior to usage.

FIG. 6-Film casting knife--Micrometers adjust the blade clearance. (Courtesy of Byk-Gardner Inc.)

Several drawdown bars have gradations of cuts into the wet-film thickness controlling surfaces. For instance, the Erichsen suppliers offer the Kruse Multi Clearance applicator, having six to ten adjacent film strips of 10 to 200/zm in thickness for assessment of color yield, opacity . . . . (see Fig. 7) [19]. A similar device, the Leneta TG19 Logicator, is intended for hiding power and spreading rate measurements. It is available from P. N. Gardner [20] and has eight "gates" ranging from 2.65 to 10.4 rail in depth. There are motorized film application devices specified in ASTM D 823, Test Methods for Producing Films of Uniform Thickness of Paint, Varnish, and Related Products on Test Panels, as Method C. The Model 509/1 Film Applicator from Erichsen [19] may be fitted with any sort of drawdown bar or blade (see Fig. 8). Speed of the motion of the applicator may be preset, so the variation among several samples may be minimized. Two devices Grenko [3] included with drawdown bars are more like bulk coating devices as they do not require putting down a puddle of coating before applying the blade. Indeed, these devices contain the coating and apply it to a stack of

FIG. 4-Applicator frame~step gap applicator-Two film thickness choices are available with this device. (Courtesy of Byk-Gardner Inc.)

FIG. 5-Multiple clearance applicator-Eight film thickness choices are available on one device. (Courtesy of Byk-Gardner Inc.)

FIG. 7-Multiple gap drawdown bar--Six or eight gaps are machined along the same edge for casting side-by-side films for comparison. (Courtesy of Erichsen GMBH & Co.)

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PAINT AND COATING TESTING MANUAL

FIG. 9-Pfund crytometernThe white style shown is for black/dark paints, while a similar black style is used for white/light paints. (Courtesy of Byk-Gardner Inc.)

FIG. 8-Electrically driven drawdown device--Four speeds may be chosen to control drawdown rate. (Courtesy of Erichsen GMBH & Co.)

sheets, one sheet at a time as the sheets are pulled out from underneath the device. One such device is the Parks Film-OGraph cylinder. One puts the cylinder on a stack of Leneta charts or metal panels, fills the cylinder with the coating, and pulls each sheet from underneath the cylinder. A portion of the rim edge of the cylinder is milled out to act as the gate to allow coating at a desired thickness. One may even use it for very viscous coatings (even putty) by forcing the fluid through the tube with a loose-fitting plug from the top and wiping the bottom of the tube over the substrate to be coated. The second of the devices discussed was the Parks Rapid coater. Paper sheets were stacked in the bottom of a box with their ends out of a side slot at the bottom. The box was then filled with coating fluid, and each sheet was withdrawn individually. Grenko [3] noted this was not a precision device, but it was adequate for some purposes. Grenko [3] also described the flat Parks Film-O-Graph, which used a flat plate with spring clips to hold shims along the sides of the sheet of substrate. One poured the coating onto the substrate between the shims and used a bar such as a ruler or other straight edge to doctor off the excess and make the coating as thick as the shims. Bending of the scraper bar by the very slightest amount would make very thin films almost impossible, but for thick films (for example, roof coatings), this technique worked well. Some applicators are meant only for the wet film tests. Two especially useful applicators for these are the sag test devices and the wet hiding test devices. The latter, most frequently the Pfund Cryptometers (available from Erichsen [19] and Byk-Gardner [20]), have black or white glass (ceramic) beds over which a transparent wedged cover levels the coating film (see Fig. 9). Although these devices are quite hard, care in cleaning will assure they do not get scratched. They must be stored carefully, wrapped, and covered. The sag test film casters essentially drawdown several narrow films in gradations of film thickness. Many are simply slotted U-shaped drawdown bars with variations in slot depth from 1 to 6 mil, 3 to 12 mil, and 14 to 60 mil. However, the New York Paint Club Technical Committee designed a special "leveling test blade" that had double slots for each depth

with a wide space between each set of double slots [21]. Their slots are 1, 2, 4, 8 and 16 rail in depth (see Fig. 10). As paint film drip or sag depends on the rheological characteristics, the thickness of the coating that first sags may be related to the gravity-applied shear stress by the outer elements furthest away from the substrate on that film. Other rheological factors (high shear viscosity, ca. 10 000 s-1, for instance) are also important in sag and leveling. Sag test devices complying with ASTM D 3730, Guide for Testing High-Performance Interior Architectural Wall Coatings, and several federal specifications and the New York leveling test blades are available from P.N. Gardner [20] and BykGardner [18].

Wire-Wound R o d s For very thin films, the wire-wound rods are quite useful for quickly and easily applying films. The wire-wound rods are simple rods of 12 or 16-in. (30 to 40-cm) length, of I/4, 3/8, or 1/2 in. (0.63, 0.45, or 1.27 cm) diameter, with a sprial wind of wire tight about 75 to 80% of the rod. Table 2 shows a selection of wet film thicknesses obtained from the rods with different wire diameter. The film laid down with a wire-wound rod is almost exactly a tenth the thickness of the wire winding. Strictly speaking, as the film has the rod depart, there are ridges in the film in the direction of travel of the rod. However, these collapse to make a quite smooth film unless there is rheological inhibition in the formulation or the coating is very fast drying. This technique is quite effective to simulate paper or can coating end products. There are industrial production systems that era-

FIG. l O - N e w York paint club leveling test blade--The slotted blades are for assessment of sag and leveling. (Courtesy of Byk-Gardner Inc.)

CHAPTER 37--FILM PREPARATION FOR COATING TESTS TABLE 2--Selected coating thicknesses from wire wound rods. Size 2.5 3 3.5 4 4.5 5 5.5 6 10 20 30 40 50 60 70 80 90

Wire Diameter (in.) 0.0025 0.003 0.0035 0.004 0.0045 0.005 0.0055 0.006 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

Wet Film Thickness Mils Micrometers 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

6.4 7.6 8.9 10.2 11.4 12.7 14.0 15.2 25.4 50.8 76.2 101.6 127.0 152.4 177.8 203.2 228.6

ploy w i r e - w o u n d rods to m e t e r coatings onto roll substrates, as well. A s i m i l a r device called the "spiral-film applicator" is available from E r i c h s e n [19]. It is a w i r e - w o u n d r o d with a perpendicular h a n d l e (see Fig. 11). It is available to apply film thicknesses from 10 to 2 0 0 / ~ m in widths of 80, 150, or 220 ram. B y k - G a r d n e r offers a handle that attaches to the ends of the w i r e - w o u n d r o d for lab film p r e p a r a t i o n as well [18]. The Accu-Lab TM Lab D r a w d o w n Machine has a r o d h o l d e r a n d s u b s t r a t e h o l d e r to assure precision in l a b o r a t o r y m a n u a l film p r e p a r a t i o n [22]. There are also two-wire rods in w h i c h a second, smaller wire is w o u n d in the grooves m a d e b y the first w i n d i n g wire. These yield h i g h e r a p p l i c a t i o n rates t h a n do the single-wound wire rods. There is potentially an advantage to the two-wire rods sincc they leave a different p a t t e r n in the coating. The ridges left are larger a n d m a y level faster a n d uniformly.

421

Spray Outs S p r a y application, including b y weight on a panel h a n g i n g on a b a l a n c e in a spray booth, can be done with good precision. Good s p r a y out a p p l i c a t i o n on panels can be done by those skilled in the art. To control the weight a p p l i e d in a s p r a y booth, a triple b e a m b a l a n c e m a y be a t t a c h e d to a b e a m 5 o r 6 ft. (1.5 o r 1.8 m) above the floor, a n d a hanger-wire panel h o l d e r is h u n g from it as the s p r a y target. A c a r d b o a r d shield over the b a l a n c e will keep off overspray. Tare the balance a n d p r e s e t weight n e e d e d to apply. Take off a few tenths of a g r a m to give a p o i n t e r i n d i c a t o r w a r n i n g w h e n the s p r a y has a l m o s t r e a c h e d the desired weight. S p r a y quickly to minimize evaporation. There are a u t o m a t e d s p r a y devices. Grenko [3] related the a u t o m a t e d spray device design from Bell Labs [23] to M e t h o d A of ASTM D 823. The Bell Labs design has the gun travel over s t a t i o n a r y panels, while an alternative design has the panels on a moving belt u n d e r a s t a t i o n a r y s p r a y gun, s i m i l a r to a design r e p o r t e d from Battelle [24]. I n either case, the a m o u n t of coating a p p l i e d to the panel is controlled by the speed of the moving belt a n d by nonvolatile content of the s p r a y e d fluid. G r e n k o [3] notes the s a m p l e panels on the moving belt m a y be held to the belt with m a g n e t s or suction cups. Erichsen [19], G a r d n e r [20], a n d others offer autom a t e d spray a p p l i c a t o r s for lab s a m p l e p r e p a r a t i o n (see Figs. 12 a n d 13). The s p r a y technique is p a r t i c u l a r l y i m p o r t a n t , b u t p r e c a u tions are necessary to assure evenness of coating on the substrate. The spray p a t t e r n should extend b e y o n d the edges of the substrate w h e n the nozzle is a i m e d at the center of the target. The s p r a y p a t t e r n - - b e it flat, fan, o r c i r c u l a r - - h a s less p a i n t at the edges t h a n at the center. Moving the s p r a y nozzle across the target or the target u n d e r the center of the nozzle m a k e s the d e p o s i t i o n m o r e likely to be consistent in thickness.

Dip Coating

FIG. 11-Spiral wire drawdown applicator--The handle shown grips the end of the wire-wound rod for application with one hand. (Courtesy of Erichsen GMBH & Co.)

Dip coating is p a r t i c u l a r l y i m p o r t a n t in s o m e special cases. F o r instance, the edge p r o t e c t i o n of c o r r o d i b l e panels in p r e p a r a t i o n for the a p p l i c a t i o n of a test coating is a c o m m o n practice. The Golden Gate Society for Coatings Technology Technical C o m m i t t e e use a red lead vinyl 1/2-in. (1.27-cm) dip for edge p r o t e c t i o n of their exposure panels quite successfully, as a t t a c k c o r r o d e d only the desired exposure surfaces [25]. The p o i n t is that d i p p i n g assures film thickness w i t h o u t holidays t h a t o t h e r techniques do not. This is p a r t i c u l a r l y true for o d d - s h a p e d test specimens such as fragments of a p r e s s u r i z e d gas cylinder in an u n p u b l i s h e d Mellon Institute c o r r o s i o n study. Grenko [3] briefly reviewed two-dip coaters. Bruins [26] first designed one using a tire p u m p a n d needle valve to control the rate of panel w i t h d r a w a l from the coating, while Payne designed an electric m o t o r - d r i v e n device a d o p t e d for ASTM D 823 M e t h o d B. A c o m m e r c i a l lab device is available from P. N. G a r d n e r [20], with variable w i t h d r a w a l speeds at 2 to 20 in. (5 to 50 cm) p e r minute. It c a n d i p a p a n e l of up to 2 lb (0.9 ks) a n d 1 ft 2 (0.3 m) in a r e a (12 by 12 in. (3.65 by 3.65 cm).

422

PAINT AND COATING TESTING MANUAL Again, care must be taken while dip coating. Film thickness control is a battle between wetting surface forces and the shear forces of drainage through the thickness of the film. Drip edges on the bottom edge may be avoided to some degree by having the panel holder inverted to hold the bottom edge upward for a portion (or intermittent portions) of the drying period.

Spin Coating Grenko [3] described the work of Walker and Thompson [27] attaching a panel to a turntable and rotating for I min at 300 rpm to obtain a 25-/zm varnish film thickness. He also described the Sward-Gardner [28] relation of film thickness (F) to viscosity (V, in poise) and nonvolatiles, % N, as

F = O . 4 N + V4 + 3 where spin rate was 290 rpm for 60 s. Their work had a precision of 5 to 10%, and corrections for time or rpm variations were offered. Parker and Siddle [29] suggested modifying the method by adjusting viscosity to equivalence for all fluids to be compared and using volume solids rather than weight percent nonvolatiles. Plots of film thickness versus volume percent nonvolatile were straight lines for nonthixotropic fluids, but curvature existed for thixotropic fluids. A commercial lab spin coating device is currently available from Erichsen [19]. There are two versions, one with 600 rpm speed set, and another adjustable from 50 to 2000 rpm (see Fig. 14). FIG. 12-Single setting lab spray applicator, (Courtesy of Erichsen GMBH & Co.)

O T H E R TIPS ON PRACTICE OF THE ART Dust is always a problem, especially in formulation labs that have pigment dusting in the lab and plant. Cover cast

FIG. 13-Programmable lab spray applicator. (Courtesy of Erichsen GMBH & Co,)

FIG. 14-Lab spin coating device. (Courtesy of Erichsen GMBH & Co.)

CHAPTER 37--FILM PREPARATION FOR COATING TESTS 423 films immediately to keep the dust off. The easiest cover is the top of a box from a typewriter paper or file folder s h i p m e n t container. Simply cut out 1/2 in. (or 1 cm) strip from two, three, or four sides for free flow of air. Of course, a similar cover can be made from this plywood. Keep the cover o n top of the lab refrigerator or book shelf so it's always at hand. Make sure the film is drying u n d e r appropriate temperature a n d h u m i d i t y conditions. S o m e t h i n g in the f o r m u l a t i o n m a y respond i n a n adverse m a n n e r to c o n d e n s i n g m o i s t u r e that can form droplets o n the surface as evaporating solvent rapidly cools the system. I n moisture-cure a n d reactive twopackage urethanes, the c o n d e n s i n g moisture m a y react with the isocyanates to modify degree of cure, which c a n reduce strength, solvent resistance, etc. In systems that do not react with the c o n d e n s i n g water, one m a y still get pits, pinholes, or haziness from the c o n d e n s i n g water droplets o n the surface. Paint applicators are learning to pay a t t e n t i o n to h u m i d i t y variation a n d its effects o n the end p r o d u c t surface.

CONCLUSION How the film is prepared for testing can have a d r a m a t i c effect o n the test results. The thought p u t into film preparation prior to p r e p a r a t i o n a n d the care used in casting can he crucial factors in o b t a i n i n g m e a n i n g f u l a n d reproducible results.

REFERENCES [1] Toronto Society for Coatings Technology Technical Committee presentation, 1990 National Paint Show Voss/APJ Competition presentation, available from the Federation of Societies for Coatings Technology, 492 Norristown Rd., Blue Bell, PA 194222350. [2] Athey, R. D., Jr., "Coating Tests--Hardness of the Film," European Coatings Journal, Vol. 92, No. 10, December 1992, p. 461. [3] Grenko, C., "Preparation of Films for Test," ASTM STP 500, Paint Testing Manual, G. G. Sward, Ed., American Society for Testing and Materials, Philadelphia, 1972. [4] Sager, T. P., "The Preparation of Thin Films," Industrial and Engineering Chemistry, Analytical Edition, IENAA, Vol. 9, 1937, p. 156. [5] Brightwell, E.P., "An Optical Method for Measuring Film Thickness of Paint Films," Official Digest, Federation of Paint and Varnish Clubs, ODPFA, Vol. 28, 1956, p. 412. [6] Bayor, E. H. and Kempf, L., "Preparing Fragile Paint and Varnish Films," Industrial and Engineering Chemistry, Analytical Edition, Vol. 9, 1937, p. 49. [7] Clarke, G. L. and Tschentke, H. L., "Physicochemical Studies on the Mechanism of Drying of Linseed Oil. 1. Changes in Density of Films," Industrial and Engineering Chemistry, IECHA, Vol. 21, 1929, p. 621.

[8] Gloor, W. E., "Effect of Heat and Light on Nitrocellulose Films," Industrial and Engineering Chemistry, IECHA, Vol. 23, 1931, p. 980. [9] Long, J. S., Egge, W. S., and Wetterau, P. C., "Action of Heat and Blowing on Linseed and Perilla Oils and Glycerides Derived from Them," Official Digest, Federation of Paint and Varnish Clubs, ODFPA, Vol. 19, 1927, p. 30. [10] Caframo Lab Products, P.O. Box 70, Wiarton, Ontario, Canada NOH 2TO (519-534-1080). [11] Athey, R. D., Jr. et al., "Latex Coating Formulation Evaluation of Organosilane Treated Talcs: A Statistically Designed Study-Part II. Experiment Design and Test Results," Journal of Waterborne Coatings, Vol. 8, No. 2, May 1985, p. 10. [12] DePugh, C. C., private communication. [13] Takano, M. and Nielsen, L. E., "The Notch Sensitivity of Sensitive Materials," Journal of Applied Polymer Science, Vol. 21, 1976, p. 2193. [14] Steel Structures Painting Manual, Systems and Specifications, Vol. 2, Steel Structures Painting Council, 4400 Fifth Ave., Pittsburgh, PA 15213. [15] Metal Finishing Guidebook and Directory Issue, M. Murphy, Ed., Elsevier Publishing, 3 University Plaza, Hackensack, NJ. [16] Greenfield, S. H., "A Method of Preparing Uniform Films of Bituminous Materials," ASTM Bulletin, American Society for Testing and Materials, No. 193, October 1953, p. 30. [17] Wetz, J. M,, Golding, B,, and Case, L.C., "Film Thickness Relationships of Organic Coatings," Official Digest, Federation of Paint and Varnish Production Clubs, ODFPA, Vol. 31, 1959, p. 419. [18] Instruments Catalog '91, Section 8, Byk-Gardner Inc., 2435 Linden Lane, Silver Spring, MD 20910 (301-589-9044). [19] Erichsen, T. J. Bell, 1340 Home Ave., Akron, OH 44310 (216633-3644). [20] P. N. Gardner Co. Inc., 316 N. E. 1st St., Pompano Beach, FL 33061-6688 (800-762-2478). [21] Industry Tech, 188 Scarlet Boulevard, Oldsmar, FL 34677 (813855-5054). [22] New York Paint Club Technical Committee, Official Digest, Vol. 32, No. 430, 1960, p. 1435. [23] Arlt, H. G., "Paint Films of Controlled Thickness," Bell Laboratories Record, Vol. XIV, 1936, p. 216. [24] Mueller, E. R., "A Simple Semi-automatic Laboratory Spraying Device," Products Finishing, PRFIA, Vol. 15, No. 2, 1950, pp. 36, 38. [25] Golden Gate Society for Coatings Technology, "Corrosion Inhibitive Performance of Some Commercial Water-Reducible NonToxic Primers," Journal of Coatings Technology, Vol. 53, No. 682, November 1981, p. 29. [26] Bruins, P. F., "Production of Uniform Test Films of Shellac and Other Finishes," Industrial and Engineering Chemistry, Analytical Edition, IENAA, Vol. 9, 1937, p. 376. [27] Walker, P. H. and Thompson, J. G., "Some Physical Properties of Paints," Proceedings, American Society for Testing and Materials, ASTEA, Vol. 22, Part II, 1922, p. 465. [28] Sward, G. G. and Gradner, H. A., "Uniform Varnish Films for Exposure Tests," Industrial and Engineering Chemistry, INCHA, Vol. 19, 1927, p. 363. [29] Parker, R. C. and Siddle, F. J., "The Hardness of Paint, Varnish and Lacquer Films," Journal, Oil and Colour Chemists Association, JOCCA, Vol. 21, 1938, p. 363.

MNL17-EB/Jun. 1995

38

Measurement of Film Thickness by C. M. Wenzler 1 and J. F. Fletcher I

the i n n e r wheel is referenced to the scale on the o u t e r wheel

O N E OF T H E MAJOR R E Q U I R E M E N T S OF PAINT a n d

coating testing is the m e a s u r e m e n t a n d control of film thickness. E n s u r i n g that a specification is achieved is i m p o r t a n t in l a b o r a t o r y tests, p a i n t application, a u t o m a t i c spraying, a n d o t h e r coating a p p l i c a t i o n methods. A n u m b e r of test m e t h o d s are available, a n d the choice is d e p e n d e n t on: (1) the l o c a t i o n - - l a b o r a t o r y o r site; (2) the m a t e r i a l c o a t e d - - m e t a l (ferrous o r nonferrous), wood, plaster, b r i c k a n d plastic; (3) the c o n d i t i o n of the c o a t i n g - - w e t o r dry; a n d (4) the c o n d i t i o n of the s u r f a c e - - r o u g h or smooth, flat o r shaped, thick or thin, etc.

a n d the value noted, Starting from m a x i m u m value avoids the possibility of p u s h i n g p a i n t a h e a d of the i n n e r disk, creating an e r r o r c o n d i t i o n w h e n the gage indicates a value h i g h e r t h a n the true wet film thickness. A n u m b e r of ranges are available.

Pfund Wet Film Gage As s h o w n in Fig. 2, this gage consists of a convex lens, L, whose r a d i u s of curvature is 250 m m , at the l o w e r e n d of t u b e T1 t h a t slides in the o u t e r t u b e T2. C o m p r e s s i o n springs, S, keep the lens out of contact with the p a i n t film until p r e s s u r e is a p p l i e d on tube T1. In m a k i n g a m e a s u r e m e n t , the gage is p l a c e d on the p a i n t e d surface a n d the lens is p u s h e d slowly t h r o u g h the film until s t o p p e d by the substrate. The p r e s s u r e is released, a n d the d i a m e t e r of the spot of p a i n t t r a n s f e r r e d to the lens is m e a s u r e d . A 1 to 1 ratio for the thickness a d d e d by the p a i n t d i s p l a c e d b y the lens to the actual thickness has been a s s u m e d a n d is a c c o u n t e d for in the e q u a t i o n

WET FILM THICKNESS The m e a s u r e m e n t of wet film thickness provides the first o p p o r t u n i t y to check the coating a n d its a p p l i c a t i o n process. It also offers an a s s e s s m e n t of the s p r e a d i n g rate w h e n a p a i n t is applied. It is very i m p o r t a n t that wet film m e a s u r e m e n t s are m a d e as soon as the coating is a p p l i e d to avoid e r r o r due to solvent loss d u r i n g the curing process. Reference to the technical d a t a for v o l u m e solids in the coating is r e q u i r e d to establish the wet a n d d r y ratio so that wet film thickness values can be converted to d r y film equivalents. I n m o s t cases wet film thickness gages can be cleaned with solvents a n d reused. ASTM Test Methods for M e a s u r e m e n t of W e t Film Thickness of Organic Coatings (D 1212) s t a n d a r d i z e s the I n m o n t wet film gage (formally k n o w n as I n t e r c h e m i c a l a n d comm o n l y k n o w n as the wet film wheel) a n d the Pfund wet film gage. These two gages are detailed in the following sections, a n d two n o n s t a n d a r d i z e d m e t h o d s are also described.

t = D2/16R

where D = diameter, m m , of spot, a n d R = r a d i u s of curvature, m m , of the lens. Table 1 gives film thickness a n d the c o r r e s p o n d i n g spreading rate in square feet p e r gallon for spots from 3 to 38 m m in diameter. It has been observed that a substantial p r o p o r t i o n of p a i n t s do n o t obey the 1 to 1 relationship. The actual thickness, o b t a i n e d by i n d e p e n d e n t methods, m a y be several times, o r only a fraction of, the thickness calculated b y the equation. A small a m o u n t of t h i n n e r a d d e d to a p a i n t m a y increase t h e d i a m e t e r of the spot on the lens a n d give a c o r r e s p o n d i n g increase in the calculated thickness. This p h e n o m e n o n has been a s c r i b e d to the effects of surface tension. Hence, for b e s t results, a correction factor should be established for each type of p a i n t b a s e d on the k n o w n thickness of a freshly prep a r e d film m e a s u r e d b y the I n m o n t gage. R e p r o d u c i b i l i t y is within a b o u t 2% for films 2 mils ( 5 0 / z m ) thick, decreasing to a b o u t 10% for films 5 mils (125/zm) thick, then b e c o m i n g better as thickness increases.

Inmont Wet Film Gage (Wet Film Wheel) The I n m o n t wet film gage consists of two concentric o u t e r disks with an inner eccentric disk with a s m a l l e r d i a m e t e r p o s i t i o n e d b e t w e e n t h e m as s h o w n in Fig. 1. The o u t e r disks are scaled, with the clearance between the i n n e r disk and the o u t e r disk from zero to a m a x i m u m value as shown. The gate is used as follows. The disks are p l a c e d with the m a x i m u m clearance on the s p e c i m e n of coating a n d rolled t o w a r d the m i n i m u m clearance in either direction. The p a i n t will coat the i n n e r disk until the clearance is greater t h a n the wet film thickness. The p o i n t at w h i c h the coating stops on 1Elcometer Instruments, Ltd., Manchester, England. 424 Copyright9 1995 by ASTM International

(1)

www.astm.org

CHAPTER 3 8 - - M E A S U R E M E N T OF FILM THICKNESS

425

ECCENTRICINNERWHEEL

WETF~M I

.

.

.

.

FIG. 1-1nmont gage (Interchemieall wet film wheel(left) Notch Gages (Wet Film Comb) Notch gages are formed on the edge of a piece of material so that each notch has a different clearance from the reference shoulders to its neighbors (Fig. 3). Many different materials are used, such as stainless steel, aluminum, and plastic in a variety of shapes: square, rectangle, triangle, hexagon, etc. It should be noted that combs made of aluminum are known to wear on rough surfaces. It is a simple low-cost device which is useful when approximate values are satisfactory as the notches have discreet values and are not continuous. The gage is dipped vertically into the film until the reference shoulders are resting firmly on the substrate. The thickness of the film is between the highest coated notch face and the next highest notch value. Several different methods of manufacture of these gages exist from spark-eroded stainless steel precision combs, through punched aluminum sheet, to plastic flow molded combs. The stainless steel combs can be certified with measurements of the tooth displacements, which are traceable to national standards. As the stainless steel is hard wearing, this certificate can be valid over a period of up to one year. On the other hand, plastic combs, although manufactured from sol-

A

r

and 120 photo(right).

vent-resistant ABS plastic, should only be used once as the solvent in the coating may soften the plastic. Plastic combs can be tagged and kept as a permanent record of wet film measurement. Aluminum combs are prone to wear, and the condition of the gage should be monitored before use. Gages are available in many ranges, from 0.5 rail (2.5/xm) to 160 mils (4 mm/4000 /xm) (for high-build, low-solventcontent coatings). The teeth are normally square ended, but for thicker coatings pointed teeth are often used. Notch gages are supplied by various coating equipment suppliers, e.g., Nordson, Elcometer Inc. and are described in ASTM Practice for Measurement of Wet Film Thickness of Organic Coatings by Notched Gages (D 4414).

Needle Micrometer This method was used to study the relationship between the clearance of a doctor blade and the thickness of the wet film left by the blade [1]. A needle is attached vertically to the objective holder of a microscope. The barrel is lowered until the needle just touches the film that has been spread on a plain metal panel. The contact is observed through a horizon-

8

r,

FIG. 2-Pfundgage(left) and photo(right). (Figuretakenfrompreviousedition of this manual,)

426

PAINT AND COATING TESTING MANUAL TABLE 1--Spreading rate by Pfund film gage.

Diameter of Spot, mm

mm

Mils

Ft/Gal

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 32 34 36 38

0.002 25 0.004 00 0.006 25 0.009 00 0.012 25 0.016 00 0.020 25 0.025 00 0.030 25 0.036 00 0.042 25 0.049 00 0.056 25 0.064 00 0.072 25 0.081 00 0.090 25 0.100 00 0.110 25 0.121 00 0.132 25 0.144 00 0.156 25 0.169 00 0.182 25 0.196 00 0.210 25 0.225 00 0.256 00 0.289 00 0.324 00 0.361 00

0.089 0.157 0.246 0.354 0.482 0.630 0.805 0.985 1.19 1.42 1.66 1.93 2.21 2.52 2.84 3.19 3.55 3.94 4.34 4.76 5.20 5.67 6.15 6.65 7.18 7.72 8.28 8.96 10.08 11.38 12.76 14.21

18 088 10 175 6 512 4 522 3 321 2 543 2 009 1 628 1 345 1 130 963 830 723 636 563 502 450 407 369 336 307 282 260 241 223 207 193 180 158 141 125 113

Thickness

tal microscope (Fig. 4). W h e n contact is made, the needle a n d its image reflected by the film just meet. The needle is t h e n lowered into the film until it just touches the metal panel. This contact is noted by the deflection of a galvanometer in series with the panel, a dry cell, a n d the needle. The thickness of the film is calculated from the n u m b e r of t u r n s made by the focusing screw of the vertical microscope between the two points of contact. It should be noted that this technique is only applicable to m e a s u r e m e n t s m a d e in a laboratory as it is impractical for work on site.

DRY FILM THICKNESS (DESTRUCTIVE METHODS) As there are m a n y circumstances u n d e r which coatings a n d paints are used, n o single m e t h o d of dry film thickness meas u r e m e n t is universal. Some methods are destructive, a n d these are most often used w h e n nondestructive methods are n o t applicable. The nondestructive methods are limited to coatings on metals.

Micrometers and Dial Gages W h e n a chip or flake of coating is freed from the surface of the coated object, its thickness can be m e a s u r e d directly using a micrometer. Alternatively, the total thickness of the substrate a n d coating can be measured; the substrate can

then be r e m e a s u r e d after removing the coating with a scraper or solvent. The coating thickness is then the difference between the two m e a s u r e m e n t s . ASTM Test Method for M e a s u r e m e n t of Dry-Film Thickness of Organic Coatings (D 1005) details a preferred procedure using a dial m i c r o m e t e r m o u n t e d o n a support with a clamp to hold the specimen (see Fig. 4). This m e t h o d is r e c o m m e n d e d only for thicknesses over 0.5 mil a n d is accurate _+0. I mil. A hand-held dial gage, the Elcometer Model 126 (Fig. 5), m a y be used. I n this case the dial records the difference i n position between the foot that sits o n the surface of the coating a n d the stylus with its ball end, which passes through a hole prepared in the coating to the surface of the substrate. This is particularly useful for site work. However, for best accuracy a n d precision, the firmly m o u n t e d dial gage specified in ASTM Method D 1005 is preferred.

Gardner Needle Thickness Gage This i n s t r u m e n t is designed to m e a s u r e the thickness of electrically n o n c o n d u c t i n g films o n metal (conducting) substrates. It is small e n o u g h to be used i n the field where the substrate can be m a d e part of the electric circuit. The needle makes only a m i n u t e p u n c t u r e in the film. I n m a n y instances, particularly in a go no-go determination, the damage is so slight that the method m a y be considered nondestructive for m a n y e n d users. The case a l u m i n u m h o u s i n g contains the needle screws for forcing the needle through the film a n d a lamp to signal w h e n the needle contacts the substrate. For use in the field a n d for occasional use in the laboratory, the electric circuit comprises the needle, the substrate, a dry cell, the lamp, a n d a cord that connects with the substrate. I n the laboratory, if m a n y m e a s u r e m e n t s are to be made, it is advisable to use a step-down t r a n s f o r m e r a n d to c o n n e c t to a 110-V source. The thickness is read o n a dial attached to the screw. One t u r n of the dial raises or lowers the needle by 2 mils. The dial is graduated in steps of 0.05 mil. Range is 0 to 15 mils. The zero setting of the needle is obtained by retracting the needle within the housing, placing the gage o n a plane metal block, a n d lowering the needle until the lamp signals contact. The block is replaced by the specimen, a n d the needle is lowered u n t i l the lamp again signals contact. The difference between the two readings is the thickness of the film.

Gardner Carboloy Drill Thickness Gage In m a n y instances films are so hard that they successfully resist p e n e t r a t i o n by the needle p e n e t r o m e t e r described above. This difficulty has been overcome by substituting a Carboloy drill for the needle. The drill is a needle t e r m i n a t i n g in a p y r a m i d having three faces. The drill is secured in a chuck that can be rotated a n d advanced independently. The rotation is controlled by finger action o n a k n o b at the u p p e r end of the chuck shaft. I n all other respects, the operation is the same as that of the needle gage. This m e t h o d has been standardized by ASTM as Procedure B in Test Method for M e a s u r e m e n t of Dry Film Thickness of Non-metallic Coatings of Paint, Varnish, Lacquer, a n d Related Products Applied on a N o n - m a g n e t i c Base (D 1400).

CHAPTER 3 8 - - M E A S U ~ M E N T

FIG. 3-Wet film comb 115 and photo.

FIG. 4-Dial micrometer.

OF FILM THICKNESS

427

428

PAINT AND COATING TESTING MANUAL ple, specimens with poor adhesion may be torn off, exposing the base, even if the chisel does not penetrate the film.

Microscope for F i l m T h i c k n e s s

Classical Method To use a microscope to assess film thickness, a section is prepared and the width of the coating is measured using a graticule in the eyepiece of the microscope. For an approximate assessment, a flake of the coating can be used, but for best results from this method the specimen should be prepared as follows. The specimen is mounted in a block of wax. The face of the mount is cut or ground to a smooth surface. The prepared specimen is then inspected under the microscope. ASTM Method for Microscopic Measurement of Dry Film Thickness of Coatings on Wood Products (D 2691-88) is based on this method; however, as paints are often hard and brittle, a grinding and polishing method is preferred to the blade method indicated in D 2691.

Brightwell Method [2]

FIG. 5-Elcometer dial gage 126.

Gardner Gage Stand Although the Gardner needle gage and the Carboloy drill gage may be operated by manually holding the gage against the specimen, using the Gardner gage stand is less tiring and more accurate, especially when many measurements are to be made. The stand provides constant known pressure [up to 10 lb (5 kg)] on the specimen and ensures that the needle or drill are always perpendicular to the specimen.

This method does not require removal of a chip and elaborate mounting and preparation. A tiny furrow is made in the film or a small chip is removed. A prism or ribbon of light is projected on the selected area at an angle of 45 ~. The distortion of the beam is examined with a microscope equipped with a micrometer eyepiece. Apparatus for this is available in the Schmaltz optical surface analyzer (Carl Zeiss). The apparatus is calibrated by measuring known depths milled in a smooth metal block. The ribbon of light is focused on line A, and the filar micrometer reading is recorded. The procedure is repeated on line BC. The difference multiplied by the calibration factor equals the thickness of the film. Table 2 compares results by this method with results with a micrometer.

Stopped Method [3] Gardner Micro-Depth Gage Although in outward appearance this gage resembles the gages described in the last two subsections, only the establishing of the zero setting is the same. Measurement is not restricted to nonmetallic films on metal--any type of film on any type of substrate may be measured, and the film is always damaged. In this gage, a chisel replaces the needle of the gage. The zero setting having been established, the chisel is advanced by an amount estimated to be less than the thickness of the film. The gage is placed on the specimen and drawn toward the operator through a distance of a few millimetres. If the scratch made by the chisel does not penetrate the film, the chisel is advanced by a small increment, and another scratch is made. The procedure is repeated until the substrate is reached and exposed. Inspection is best made with the aid of a low-power magnifier. The range is 0 to 40 mils (1000/~m). Repeatability depends on the magnitudes of the increments and the compressibility of the film and substrate. For exam-

A cut is made in the film with a sharp knife. The microscope is focused, in turn, on the upper and lower edges of the cut. The thickness of the film is computed from the vertical adjustments of the microscope. If the value is not known, it may be found as follows: Put a piece of plate glass on the stage. Lower the tube until it just touches the plate. Record the reading of the fine adjustment. Now raise the tube as far as possible and again record the fine adjustment. Now raise the tube as far as possible and again record the fine adjustment.

TABLE 2--Film thickness (mils) by optical surface analyzer and dial micrometer (Brightwell). Surface Analyzer

Dial Micrometer

0.39 0.46 0.71 0.98

0.4 0.5 0.8 1.0

1.51 2.28 3.58

1.4 2.1 3.5

1.30

1.3

CHAPTER 38--MEASU~MENT OF FILM THICKNESS 4 2 9 The distance of the tube from the plate divided by the number of turns of the adjusting screw gives the value for each turn.

Tooke Inspection Gage (Paint Inspection Gage P.I.G.)

[4] This gage (shown in Fig. 6) provides for estimating the thickness of a film from the geometry of a V-groove cut in the film by a special tool. With the aid of a x 50 illuminated magnifier equipped with a reticle in the eyepiece, the operator measures the lateral distance from the top edge of the cut and the projection of the intersection of the cut and the substrate. To make a measurement, a "bench mark" of ink applied to the surface of the film serves to make the top edge of the cut readily visible. A short cut is then made at a right angle to the bench mark with the selected cutting tip. Film thickness is then obtained by counting the scale divisions as described previously. The relationship among the tips is summarized in Table 3.

FIG. 6-Groove in paint film. TABLE 3--Tooke inspection gage tip specifications.

Tip

Maximum Coating Thickness,mils

Precision of Thickness Determination Represents,mils

One Division on Reticle Scale, mils

• 1 x2 x 10

50 20 3

_+0.25 _+0.13 -+0.025

1.0 0.5 0.1

Saberg Drill This method is similar to the Tooke inspection gage described in the previous subsection; however, a circular drill is used to penetrate the film. The hole can then be inspected using the x 50 magnifier with graticule, and the width of the cut from the outer edge to the print where the drill penetrates to the substrate is a measure of the coating thickness. For the instrument kit supplied by Elcometer Inc., Rochester Hills, M148309 (Model 195), the calculation of coating thickness is as follows: 1. For measurement in mils, multiply graduations by 0.79. 2. For measurement in micrometers, multiply graduations by 20.0. 3. This method is now described in ASTM Test Method for Measurement of Dry Film Thickness of Protective Coating Systems by Destructive Means (D 4138).

D R Y FILM T H I C K N E S S (NONDESTRUCTIVE METHODS) Permanent Magnet Thickness Gages Permanent magnet coating thickness gages can be used to determine the thickness of films applied to magnetic substrates such as steel, iron, magnetic stainless steel, etc., providing that the coating is nonmagnetic. Materials such as nickel and cobalt, that are naturally magnetic have to be treated with care, while paints containing magnetic particles, such as some ferrous micaceous iron oxide (MIO), can cause errors when using magnetic gages. Simple magnetic coating thickness gages or mechanical gages use the principle that the attractive force between a permanent magnet and the magnetic metal substrate is inversely proportional to the distance between them. The principal limitations are (1) the film must be sufficiently hard to prevent indentation, and (2) the film must not be tacky causing the magnet to be held by the surface of the coating. Electronic magnetic coating thickness gages are also available, but these will be described in a separate section entitled Electromagnetic Thickness Gages; however, this section

does include the electronic gage based on the magneto-resistor probe.

Magne-Gage [5] This instrument (Fig. 7) consists of a small permanent bar magnet, 2 mm in diameter, suspended from a horizontal lever arm. The arm is actuated through a spiral spring by turning a dial. The tip of the magnet is brought into contact with the paint film (on iron or steel), and the dial is then turned until the magnet is detached. The attractive force between the magnet and the film support is indicated on the dial, and the thickness of the nonmagnetic paint film is obtained from a calibration curve relating thickness to dial reading. The gage can be used to measure coatings on convex and concave surfaces as well as on flat ones provided the radius of the curvature is not too small. Unless special calibrations are made, cylinders should not be less than 1/2 in. (1.27 cm) in diameter, spheres not less than 3/4 in. (1.9 cm), and fiat pieces should be at least 3/4 in. (1.9 cm) square. Magnets for thicknesses in the following ranges are available: 0.0 to 0.002, 0.002 to 0.007, and 0.007 to 0.025 in. ASTM Method for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Organic Coatings Applied to a Ferrous Base (D 1186) has standardized the operation on the magne gage.

Hand-HeM Magnetic Gages These gages are being superseded by electronic gages, and some--such as the Tinsley gage and the chemigage--are no longer manufactured. However, many thousands of these types are still in use, so it is appropriate to describe them. The simplest form of these gages contains a magnet suspended from a coil spring housed in a pen-style body manufactured from aluminum or plastic. A scale is drawn on the body and a marker used to indicate the extension of the spring on the scale. The reading on the scale when the magnet lifts off the surface corresponds to the thickness of the coating. The scale on these instruments is nonlinear, leading to poor resolution at the maximum range, usually 20 mils (500/~m),

430

PAINT AND COATING TESTING MANUAL

FIG. 7-Magne gage (courtesy of American Instrument Co.).

FIG. 8-Elcometer pull off gage 157.

a n d great care m u s t be t a k e n to ensure that they are u s e d vertically to avoid the influence of gravity on the springm a g n e t c o m b i n a t i o n . S o m e of these types of gages have design features to o v e r c o m e the gravity effect w h e n the gage is u s e d horizontally. An example of this type of gage is the E l c o m e t e r Model 157 (Fig. 8). A c o m m o n form of the h a n d - h e l d m a g n e t i c gage is s h o w n in Fig 9, a n d its principle is illustrated in Fig. 10. A b a l a n c e d b e a m w i t h a m a g n e t fitted to one e n d a n d c o u n t e r b a l a n c e d b y a b r a s s weight at the o t h e r is a t t a c h e d at the pivot to a helical spring. The o t h e r end of the spring is a t t a c h e d to a ring

FIG. 9-Elcometer magnetic coating thickness gage 211.

holding the scale. R o t a t i o n of the ring raises or lowers the magnet. The gage is placed o n the surface to b e tested in a n y o r i e n t a t i o n as the b a l a n c e weight ensures that gravity effects are neutralized. The ring or scale wheel is p u s h e d f o r w a r d

CHAPTER 38--MEASUREMENT OF FILM THICKNESS 431 Elcometer (Model I01)

ELCOMETER 111 INSPECTOR MAGNETIC ATTRACTION

OUTER CASE , BALANCEWEIGHT ~'--'J~ \ TENSION SPRING ! ' ~ --~\ ~ SCALE _..~ "--"-'-'~, ~ BALANCEARM ~. ( . . ~ ,....~"k% " \ / MAGNET

--

,/COATING

!

,! FIG. lO-Magnetic coating thickness gage principle.

(anticlockwise rotation) to bring the magnet in contact with the coating and to set the scale to a maximum reading. The ring or scale wheel is then rotated clockwise until the magnet breaks free, the thickness of the coating being indicated by a pointer. This type of gage is calibrated against plated standards available from NIST, and accuracy against these standards of - 5% of reading are achieved. It should be noted that the steel used to make NIST standards is not always representative of engineering steels, and therefore a practical operating accuracy of _+10% is obtained in the field.

The Elcometer is one of the first magnetic thickness gages to be commercially available, being patented in 1948. Based on magnetic permeability, the magnetic flux acts on an armature suspended in between the two magnetic arms of the unit (Fig. 11) north and south poles. The turning moment of the magnetic flux is countered by a helical spring, and the magnitude of the magnetic flux changes with the distance between the ends of the magnetic arms (ball feet) and the substrate beneath the coating. A pointer attached to the armature indicates the thickness of the coating. Instruments covering ranges from 0 to 3 mil (0 to 80/~m) to 0 to 0.75 in. (0 to 18 mm) are available.

Magneto Resistor (Electronic) This instrument combines a permanent magnet with a magneto-resistor in a probe to provide a signal which varies with the intensity of the magnetic field, which in turn is influenced by the distance of the magnetic substrate from the tip of the probe (Fig. 12). The scale of the instrument is nonlinear and uses an analogue meter movement to indicate the thickness. In operation it is necessary to set zero on the uncoated metal and calibrate to a thickness of known value to obtain the best accuracy. These instruments have been superseded by the electromagnetic induction types and by digital electronics, but again many instruments are in use and the principle embodied in the probe is still used for ferrite detection in stainless steel

ELCOMETER FERRITE MEASUREMENT FERRITECTOR TYPE 1581

ELCOMETER MAGNETIC RELUCTANCE PRINCIPLE

SCALE

PERMANENT MAGNET

NOgEA#'ggus \

--

~;~

t:.;iil

~

j

MAGNETIC LINES

OFFORCE

SENSING ELEk

WELD SEAM

FIG. 11-Elcometer 101 principle.

,STAINLESS STEEL

I:ERRITE

FIG. 12-Magneto resistor principle.

432

PAINT AND COATING TESTING MANUAL

weld materials. In this application the magnetic properties of the substrate change with the ferrite content and the gage is calibrated in reverse, that is to say, zero, with the probe away from the influence of the magnetic material (free air) and maximum (Ferrite Number 28) on a 20 mil/thou in. (500-/~m)-thick coating on a mild steel base. For ferrite measurement, the probe is applied to an uncoated weld.

E l e c t r o m a g n e t i c T h i c k n e s s Gages The electromagnetic induction method for measuring film thickness is based on the effect of a magnetic metal substrate on the balance of the alternating magnetic field in the probe tip, generated in the probe (Fig. 13) by a signal applied to the central coil. When the probe is away from the influence of the substrate (free air condition), the net voltage output from the two outer coils tends to zero. As the substrate is brought towards the tip, the field is increasingly out of balance between the two outer coils until, with the uncoated substrate in contact with the tip, the net voltage output from the coils tends toward V max.

ELCOMETER ELECTROMAGNETIC INDUCTION PRINCIPLE

ELECTROMAGNETIC FIELD

S

i

DETECTIONCOILS

:!: i: mlm[:!:|!:|l

Analogue Electromagnetic Thickness Gages The simplest form of instrument uses a nonlinear scale on a meter movement to display the voltage output from the probe in thickness terms. The Elcometer Model 145 (Fig. 14) is an example of this type of unit. In common with most electronic coating thickness gages, the instrument must be calibrated by setting the needle to zero with the probe on the uncoated magnetic metal substrate and then setting the upper scale point using the calibration control with the probe on a known thickness of coating. Two forms of thickness standard are in common use: (1) the plated type, as illustrated by the standards available from NIST and other sources, and (2) the measured and unmeasured plastic shims available commercially from gage manufacturers. In many applications the plastic shims are preferred as calibration on the work to be measured reduces the errors of calibration due to surface finish, curvature, and substrate composition. More details of this are given later in this chapter under EFFECTS OF SURFACE FINISH, CURVATURE AND SUBSTRATE COMPOSITION ON

ELECTROMAGNETIC SUREMENTS.

= m l i:!t:! ==ml t tl atoll ili|ll ~i

The voltage output from the probe can be amplified and calibrated and then used to display a thickness value. Three types of electronic coating thickness gages have been developed on the basis of this probe technology using analogue, digital, and microprocessor electronics. These types of gages have been standardized by ASTM as Test Method B of D 1186.

m] mum ~

:i: i:

~ I

1 ~

ENERGISECOIL

PROBE BALANCED ~l:f :1:l:ir~i"

/PROBE

TIP

AND EDDY CURRENT MEA-

Digital Electromagnetic Thickness Gages Advances in electronic components and instruments design techniques have made it possible to significantly reduce the size of coating thickness gages while returning and enhancing the features users find necessary. The use of digital electronics means that the voltage output from the probe can be converted to a numerical value early in the processing of the signal, thus reducing the effects of instrument temperature changes and component drift on the accuracy of the result. It is also possible to use a linear scaling, making it possible to have a fixed resolution over the full range of the instrument scale although resolutions are often enhanced in the 0 to 10 rail range (0 to 100/~m when scaled in metric units). An illustration of this type of instrument is shown in Fig. 15, but it should be noted that this type of product is already being superseded by microprocessor-based designs. The same principles of calibration apply to digital electromagnetic coating thickness gages as apply to the analogue types, and accuracy capabilities of _+5% of readings are achieved in the field.

Microprocessor Electromagnetic Thickness Gages

I

TO FERROUS SUBSTRATE

FIG. 13-Electromagnetic induction principle.

The development of microprocessor electronics and their application to portable coating thickness gages has made it possible to improve the accuracy and reproducibility of these instruments as well as allowing developments in range, special calibration techniques for rough surfaces, memory of readings, statistical calculation, and printouts. In a microprocessor design, the characteristic of the probe voltage output against coating thickness value is stored in the

CHAPTER 3 8 - - M E A S U R E M E N T OF FILM THICKNESS

FIG. 14-Analogue electromagnetic thickness gage 145.

FIG. 15-Digital electromagnetic thickness gage 245.

433

434

PAINT AND COATING TESTING MANUAL

memory of the instrument for many values over the range of the probe. The actual voltage output from the probe is digitized and then compared with the stored values. A thickness value is then calculated from these data and displayed. This is achieved in typically 0.3 s. Using this technique, accuracies of _+1% of reading are possible. As a microprocessor instrument is in effect a dedicated computer, many calculations can be performed on the data, and features such as correction for temperature changes, storage of calibration conditions and corrections to these calibrations, averages, and other statistical values can be included in the instrument's software. Figure 16 illustrates one of the microprocessor-based electromagnetic thickness gages Model 256, available from Elcometer Inc., Rochester Hills, Michigan. This instrument is available in three levels of software: (1) basic~ measurements only; (2) statistical--measurement, memory, and statistical calculation; and (3) top--measurement, memory, statistics, and printout. Figure 17 illustrates the trends in microprocessor designs with the smallest electronic coating thickness gage available, Model 345, Launched in October 1991, the unit supersedes analogue and digital designs at a lower cost.

Eddy Current Thickness Gages The eddy current method for measuring film thickness is applied to coatings on nonferrous metals. It is based on the

effect that a high-frequency alternating field (3 000 000 Hz or 3 MHz) has an electrically conductive surface causing highly localized current flow or eddy currents. These currents generate their own magnetic impedance of the coil, generating a high-frequency field. The magnitude of these changes is proportional to the distance from the probe coil to the substrate, that is, to the thickness of the coating (Fig. 18). Calibration by adjustment to zero on a piece of metal of the same type, shape, and thickness as the samples to be measured is vital to ensure accuracy. Instruments are available using analogue, digital, or microprocessor designs, but many microprocessor instruments offer the facility for a dual ferrous (F) electromagnetic induction and nonferrous (N) eddy current principle instrument using the two different probe designs such as the eddy current instruments illustrated in Fig. 19. This type of gage is also described as ASTM D 1400.

E F F E C T S OF S U R F A C E F I N I S H , CURVATURE, AND SUBSTRATE COMPOSITION ON ELECTROMAGNETIC AND EDDY CURRENT MEASUREMENTS The accuracy of coating thickness measurements carried out using the methods described in the last two subsections depends on the technique used in calibrating the instruments. The three major influences on the calibration are

FIG. 16-Microprocessor electromagnetic thickness gage 256.

CHAPTER 3 8 - - M E A S U R E M E N T OF FILM THICKNESS

435

profile. This is achieved b y using a thin foil 1.0 mil (25 p.m) over the profile to set the lower c a l i b r a t i o n p o i n t a n d a thicker foil 5.0 mils (125/zm) o r 10.0 mils (250/zm) to set the u p p e r value over the profile. The i n s t r u m e n t will t h e n indicate the thickness over the peaks for the coating b e t w e e n the values of foil chosen. This m e t h o d is m o s t a c c u r a t e a n d r e p r o d u c i b l e w h e n 15 to 20 readings are t a k e n on each c a l i b r a t i o n foil to establish a m e a n value a n d the m e a n value is t h e n reset to the correct value of the foil. Trials have shown that the m e a n of 15 to 20 readings taken over a n a r e a of coating give a m e a n value within a few p e r c e n t of the actual value over p e a k s determ i n e d by sectioning. The m e t h o d does not, however, take into a c c o u n t situations w h e r e access to the s u b s t r a t e is not possible for c a l i b r a t i o n purposes. In this case, the m e t h o d d e s c r i b e d in SSPC PA2 w h e r e a c o r r e c t i o n factor is a p p l i e d to r e a d i n g s taken using a s m o o t h surface c a l i b r a t i o n in the i n s t r u m e n t is m o r e a p p r o p r i a t e . I n either case it is i m p o r t a n t to agree w i t h the m e t h o d before m e a s u r e m e n t s start to avoid discrepancies in reporting.

Curvature

FIG. 17-Smallest microprocessor electromagnetic thickness gage 345.

surface finish, curvature, a n d shape of the substrate c o m p o s i tion.

Surface Finish A variety of surface finishes are to be f o u n d on m e t a l to w h i c h a protective or decorative coating is to be applied. In s o m e cases the coatings u s e d require an a n c h o r p a t t e r n of profile d e p t h w h i c h forms a p a r t of the specification. C o m p a rators such as the K e a n e - T a t o r Surface Profile C o m p a r a t o r or the I n t e r n a t i o n a l S t a n d a r d s O r g a n i z a t i o n ISO 8503 are u s e d to d e t e r m i n e the surface finish after shot o r grit blasting. Also Testex Tape a n d the E l c o m e t e r Surface Profile Gage can be used to m e a s u r e p e a k to valley heights of profiles. These i n s t r u m e n t s are shown in Fig. 20 along with the p h o t o g r a p h i c s t a n d a r d s for cleanliness ASTM Pictorial Surface P r e p a r a tion S t a n d a r d s for Painting Steel Surfaces D 2200-1989 a n d SSPC-VIS1. Surface finish also influences c a l i b r a t i o n as the q u a n t i t y of m e t a l directly b e n e a t h the p r o b e is r e d u c e d by the effects of shot a n d grit blasting as the p r o b e tip sits on the highest peaks. This has the effect of increasing the value of thickness i n d i c a t e d using a gage c a l i b r a t e d on a s m o o t h surface by as m u c h as 1.5 mils (35/~m) at 4 mils (100/~m) coating thickness for the highest values of profile. It is possible to use a r o u g h surface c a l i b r a t i o n t e c h n i q u e to eliminate this e r r o r a n d m a k e the i n s t r u m e n t r e a d the correct value of coating thickness over the peaks by using the statistical p o w e r of the m i c r o p r o c e s s o r type gages to calibrate on the

The shape a n d metal wall thickness can also influence the a c c u r a c y of the calibration. The degree to w h i c h a p a r t i c u l a r i n s t r u m e n t is affected d e p e n d s on the design of the probe. M a n y m o d e r n i n s t r u m e n t s exceed the limits identified in SSPC PA2 of 1 Nov. 1982. The effect of shape is m o s t evident w h e n taking readings on an u n c o a t e d sample. W i t h an i n s t r u m e n t c a l i b r a t e d on a s m o o t h piece of m e t a l 0.125 in. (3.175 m m ) thick, changes of m o r e t h a n 0.2 rail ( 5 / z m ) in the r e a d i n g at zero will be seen on curves with a r a d i u s below 0.12 in. (3 m m ) convex o r 0.96 in. (25 m m ) concave on a typical e l e c t r o m a g n e t i c induction probe. Values will vary b e t w e e n m a n u f a c t u r e r s a n d f r o m different p r o b e types. This e r r o r can also be e l i m i n a t e d by c a l i b r a t i o n on a shape closely r e p r e s e n t i n g the s p e c i m e n to be tested. However, it should be n o t e d that once below the values of curvature i n d i c a t e d in the m a n u f a c t u r e r ' s literature, changes in curvature have a significant effect on calibration, i.e., the calibration on a shape will not be applicable to a n o t h e r shape.

Substrate Composition In the case of e l e c t r o m a g n e t i c i n d u c t i o n probes, m o s t are insensitive to the m a j o r i t y of steel specifications in general engineering use. However, w h e n h i g h - c a r b o n steels are coated, the c a r b o n content sufficiently alters the m a g n e t i c p r o p e r t i e s of the steel to cause the n o r m a l c a l i b r a t i o n curve a p p l i e d within the i n s t r u m e n t s to be in e r r o r with respect to linearity. Thus, a n i n s t r u m e n t c a l i b r a t e d at zero a n d say 5 mils (125/~m) m a y have an e r r o r at 2 mils (50 ~ m ) of m o r e t h a n 0.2 rail (50 ~m) or 10%. A similar effect can be seen with s o m e cast irons. This e r r o r can be o v e r c o m e b y calibrating as for the r o u g h surface d e s c r i b e d in the section earlier in this c h a p t e r entitled Surface Finish. F o r best accuracy, choose a foil just b e l o w the expected coating thickness value for the lower c a l i b r a t i o n

436

PAINT AND COATING TESTING MANUAL

FIG. 18-Eddy current principle.

FIG. 19-Microprocessor eddy current gage model 300.

CHAPTER 38--MEASUREMENT OF FILM THICKNESS

437

FIG. 20-Surface profile instruments (group) with model numbers as brochure. p o i n t s a n d a value well above the expected coating thickness value for the u p p e r c a l i b r a t i o n point. W h e n coatings a p p l i e d to n o n f e r r o u s metals are being m e a s u r e d using eddy c u r r e n t techniques, the c o m p o s i t i o n of the s u b s t r a t e a n d its effect on electrical conductivity are the i m p o r t a n t factors with respect to calibration. Materials such as a l u m i n u m a n d c o p p e r have very s i m i l a r characteristics a n d similar c a l i b r a t i o n values. However, zinc, brass, a n d o t h e r n o n f e r r o u s metals a n d alloys have different characteristics, a n d c a l i b r a t i o n on an u n c o a t e d s a m p l e is essential. Differences of up to 2 mils (50 ~m) c a n be seen b e t w e e n TABLE 4--X-ray fluorescence can be used for these applications.

Coating Chromium Cadmium Copper Nickel Nickel-phosphorous Zinc Zinc-nickel Gold Rhodium Palladium Silver Tin Titanium nitride Ferrous oxide

Substrate Steel, copper Steel, copper Steel, zinc, brass Steel, copper, Kovar, aluminum, Alloy 42, inconel Steel, copper, Kovar, aluminum, Alloy 42 Steel, copper, brass Steel Nickel, aluminum, Kovar Nickel, gold Nickel Steel, copper, Kovar Steel, copper, Kovar, aluminum Steel Aluminum, plastic

"zero" with a n a l u m i n u m calibration a n d zero on a b r a s s component.

S T A T I S T I C S IN FILM T H I C K N E S S MEASUREMENT As m a n y r a n d o m variations c a n be expected in a coating process, it is a p p r o p r i a t e to classify the thickness of the coating using a statistical analysis. In fact, m a n y n a t i o n a l specifications utilize a statistical a p p r o a c h in r e c o g n i t i o n of these variations, e.g., SSPC PA2. The sources of these variations are many, a n d only a few examples can be cited h e r e - - o p e r a t o r e r r o r in taking the m e a s u r e m e n t , recording error, variation due to surface o r curvature or composition, local variation in substrate due to local heat t r e a t m e n t o r due to f o r m i n g o r working the metal, inclusions in the metal or in the coating, etc. The influence of these factors can be greatly r e d u c e d by taking a statistically significant n u m b e r of r e a d i n g s for each a r e a of the coating to be tested. This g r o u p of readings can then be s u m m a r i z e d using m e a n a n d either s t a n d a r d deviation or range to show the average a n d the s p r e a d of r e a d i n g s a b o u t the average. A statistically significant n u m b e r of readings w o u l d be 20 to 50; however, if the process is u n d e r statistical control as defined, five readings in each g r o u p o r s u b g r o u p is sufficient. Many of the m i c r o p r o c e s s o r - b a s e d coating thickness ins t r u m e n t s are capable of calculating a n d r e c o r d i n g m e a n

438 PAINT AND COATING TESTING MANUAL values (~), standard deviation (or), and highest and lowest values (range) within a batch of readings. It is important to establish the method of evaluating the information before embarking on an evaluation of a coating system so that the correct disciplines are applied to collecting the data and evaluating it for further decisions.

X-RAY FLUORESCENCE (XRF) Over recent years, developments in performance and reductions in cost have pushed X-ray fluorescence to center stage, particularly for metal-on-metal applications and ever smaller parts. Beta-ray backscatter (BBS) techniques had been widely used to measure plated coatings; however, limitations in performance--e.g., a m i n i m u m of 20% difference in atomic n u m b e r between the coating and the substrate is r e q u i r e d - - m e a n that while gold over nickel, copper, or Kovar can be measured, nickel over copper or Kovar cannot be measured using BBS techniques. Other disadvantages exist, such as limits in the aperture/component geometry, and measurement times have led to the further development of XRF techniques and technology.

Principle of XRF Measurement [6] If sufficient light energy collides with an electron, it is possible for the electron to be driven out of its atomic orbit, a process known as the photoelectric effect. An atom with an electron removed from its orbit is unstable, so to restore equilibrium, an electron from a higher shell must drop into the vacant orbit. This transition causes an emission of energy in the form of a light wave or photon. When the inner shell electrons are ejected from an atom, the emitted p h o t o n has high energy, and they fall into the region of the electromatic spectrum called X-rays. X-rays have characteristic energy levels determined by the element which is emitting and can therefore be used to identify the elements in a sample.

In XRF instruments an X-ray source or tube is used to produce p h o t o n emissions as they have an energy distribution capable of fluorescing all elements c o m m o n l y used in plating. The X-ray beam can be accurately illuminated to provide a small focal spot and high-intensity energy suitable for noncontact measurement of complex layers on small components. The characteristic X-rays emitted by the target materials are detected using a gas-filled "proportional counter" in which the passage of the X-ray ionizes the gas and produces a pulse of electrical charge proportional to the energy of the X-ray. The XRF instruments' electronics convert the charge pulse into a digital signal that can be interpreted as thickness or analyzed for composition and produce the measurement information by comparison with standards of k n o w n thickness. XRF instruments have developed with optical alignment systems and motor-driven sample stages to position the sample and computerized analytical equipment to store calibration data to calculate and present data to the user in a suitable format. Table 4 shows some of the applications which can be successfully measured using XRF.

REFERENCES [1] New Jersey Zinc Co., "Leaves from a Paint Research Note Book," No. 1, 1937, p. 33. [2] Brightwell, E. P., "An Optical Method for Measuring Film Thickness of Paint Films," Official Digest, Federation of Paint and Varnish Production Clubs, ODFPA, Vol. 28, 1956, p. 412. [3] Stoppel, E. A., "Measurement of Thickness of Varnish Films," Proceedings, American Society for Testing and Materials, ASTEA, Vol. 23, Part 1, 1923, p. 286. [4] Tooke, R., Jr., "A Paint Inspection Gage," Official Digest, Federation of Societies for Paint Technology, ODFPA, Vol. 35, 1963, pp. 691-698. [5] Brenner, A., "Magnetic Method for Measuring the Thickness of Non-magnetic Coatings on Iron and Steel," Journal of Research, National Bureau of Standards, JRNBA, Vol. 2, 1938, p. 357. [6] Stebel, M. D. and Silvermann, W. M., "XRF Programmable Plating Thickness Measurement Instrumentation," Proceedings of the International Coil Winding Association, November 1984.

MNL17-EB/Jun. 1995

39

Drying Time by Thomas J. Sliva 1

THE PROCESS OF DRYING INVOLVESseveral physical and/or chemical changes, such as solvent evaporation, oxidation, and polymerization, all of which are time dependent. The various stages of drying that occur in organic films may be subjective, difficult to measure reproducibly, and are influenced by many factors such as film thickness, substrate, temperature, humidity, light, and air circulation. Therefore, it is essential that most of these variables must be minimized in order to make drying time determinations more quantitative.

PREPARATION OF S P E C I M E N S Substrate Preparation It is essential that the substrate to be used and the applied wet film thickness be agreed upon in advance, preferably to conform to the intended use of the coating. Flat glass panels are typically the substrate of choice. Ground and polished glass plates are more suitable for low-viscosity coatings that may tend to crawl. All panels must be thoroughly cleaned, dried, and placed in a horizontal position on a level surface.

Application The test coating should be filtered to remove any dirt or contamination. Test films are typically prepared, in duplicate, using a drawdown or doctor blade adjusted to obtain a uniform film thickness. Films should be drawn down at a uniform rate of application to avoid drag on the coating. It is recommended that all test films should be prepared and tested by one operator properly skilled in the method to be used and that a control (known) coating be run alongside the test coating. All testing should be done within an area, any point of which is not less than 1/2 in. (15 mm) from the edge of the test film. Table I can be used as a general guide for film application when nothing more specific is agreed upon between the purchaser and seller. The dry film thicknesses shown in Table 1 are suggested. Other methods of application, such as spraying, dipping, or flood coat, may be used provided the film thickness obtained is consistent with that recommended under actual usage. Other substrates, such as metal, may be used provided they are smooth and flat.

ENVIRONMENT When determining drying time, a controlled environment is essential. Variations in temperature, relative humidity, and circulation of air and light will have an effect on the drying time of a coating. The typical standard environment used for determining the drying time of air dry coatings is a temperature of 73.4 ~ +_ 3.6~ (23 ~ _ 2~ and a relative humidity of 50 +__ 5% under diffuse daylight (about 25 fc). Relative humidity should be strictly controlled for moisture-cure and two-package urethane coatings since their cure is greatly affected by the existing relative humidity. The effect of variation in temperature was discussed by Algeo and Jones [1], who observed a difference of 4 h for a particular paint dried at 73 and 77~ (22.7 and 25~ both at 50% relative humidity. All testing should be conducted in a well-ventilated room free from direct drafts and dust. Airflow is important in determining drying time. For films that dry by oxidation, the rate of drying is a function of the concentration of oxygen at the interface. Since oxygen can reach the surface only by diffusion, the rate of drying is a function of the thickness of the stationary air layer. For films that dry by solvent evaporation, the continuous removal of solvent-laden air hastens drying.

TEST M E T H O D S ASTM D 1640: Test Methods for Drying, Curing, or Film Formation o f Organic Coatings at Room Temperature Method D 1640 is the most commonly used method to determine the various stages and rates of film formation in the drying of organic coatings normally used under conditions of ambient room temperature. The method describes eight stages of the drying process: I. Set-To-Touch Time

The test film is lightly touched with the tip of a clean finger, and the fingertip is immediately placed against a piece of clean, clear glass to determine when the film does not adhere to the finger or transfer to the glass. 2. Dust-Free Time

1Assistant technical director, DL Laboratories, 116 East 16th St., New York, NY 10003.

This test is generally performed by either of two methods that determine when dust or cotton fibers lightly dropped

439 Copyright9 1995 by ASTMInternational

www.astm.org

440 P A I N T A N D C O A T I N G T E S T I N G M A N U A L TABLE 1--Recommended film thickness of materials to be tested. Material Oil paints Enamels Drying oils Water-based paints Varnishes Lacquers, resins solutions

Dry Film Thickness, mils 1.8 1.0 1.0 1.0 0.85 0.5

+ 0.2 + 0.1 _+ 0.1 _+ 0.1 _+ 0.1 _+ 0.1

I I

I

,

!

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on the test film can be r e m o v e d by blowing over the test film. a. Cotton Fiber Test Method Individual a b s o r b e n t cotton fibers are d r o p p e d from a height of 1 in. (25 ram). The film is c o n s i d e r e d to be dust free w h e n the cotton fibers can be lightly b l o w n off the test film.

3. Tack-Free Time The test film is c o n s i d e r e d to be tack free when no stickiness is observed u n d e r m o d e r a t e pressure. This can be m e a s u r e d by either of two methods: a. Paper Test Method A special p a p e r (K-4 Power Cable Paper) [3] is placed on the test film u n d e r a weight of 2 psi (13.8 kPa). After 5 s, the weight is r e m o v e d a n d the test film inverted. If the p a p e r d r o p s off within 10 s, the film is c o n s i d e r e d tackfree. A variation of the above m e t h o d is used to test the tackfree time of insulating varnishes. The varnish is considered tack free w h e n the p a p e r is placed on the test film u n d e r a weight of 1 lb (450 g) for 1 m i n and tested as above. b. Tack Tester This is a m e c h a n i c a l device w h i c h consists of a strip of metal 1 in. (25 ram) wide, 3 in. (75 m m ) long, a n d 0.016 to 0.018 in. (0.41 to 0.46 m m ) in thickness. It is b e n t to form a base 1 in. (25 m m ) square a n d a vertical length 1 by 2 in. (25 b y 50 ram) angled at 135 ~ The b o t t o m of the base of the tester is covered with a l u m i n u m foil [4] (Fig. 1). A 300-g weight is placed on the center of the base a n d allowed to set for 5 s. The test film is tack free w h e n the tester tips over i m m e d i a t e l y after the weight is removed. Occasionally, tack-free t i m e m a y be longer t h a n dryh a r d or d r y - t h r o u g h t i m e due to the inclusion of external plasticizers in the coating. 4. Dry-To-Touch Time The test film is c o n s i d e r e d dry-to-touch w h e n no m a r k is left w h e n the film is t o u c h e d by a finger. The following variations are used: a. Drying Oils--The film is c o n s i d e r e d dry-to-touch w h e n it does not r u b u p a p p r e c i a b l y w h e n a finger is r u b b e d lightly across the surface. b. Lacquers (and S e a l e r s ) - - T h e film is c o n s i d e r e d dry-totouch w h e n no p r o n o u n c e d m a r k s are left by a finger touching the film. Sealers are generally tested on w o o d o r o t h e r p o r o u s substrates. 5. Dry-Hard Time

I

i

'

I

I

FIG. 1-Zapon Tack Tester. The base of the tester is padded and wrapped with aluminum foil. The weight, at right, is set on the base for a definite interval. After the weight is removed, the time required for the tester to tip over is the measure of tack.

The test film is c o n s i d e r e d d r y - h a r d after m a x i m u m downw a r d t h u m b p r e s s u r e (without twisting) a p p l i e d to the test film leaves no m a r k w h e n the contacted a r e a is lightly p o l i s h e d with a soft cloth.

6. Dry-Through (Dry-To-Handle) Time The test panel is p l a c e d in a h o r i z o n t a l p o s i t i o n at such a height t h a n w h e n a t h u m b is p l a c e d on the film, the a r m of the o p e r a t o r is in a vertical line from the wrist to the shoulder. The o p e r a t o r b e a r s d o w n on the film with the t h u m b , exerting m o d e r a t e p r e s s u r e a n d at the s a m e t i m e twisting the t h u m b t h r o u g h an angle of 90 ~. The test film is considered d r y t h r o u g h w h e n the film is not distorted b y b e a r i n g d o w n with m o d e r a t e t h u m b p r e s s u r e a n d twisting 90 ~. 7. Dry-To-Recoat Time The test film meets this r e q u i r e m e n t w h e n a second coat can be a p p l i e d w i t h o u t causing a n y film irregularities, e.g., lifting, wrinkling. 8. Print-Free Time The test film meets this r e q u i r e m e n t w h e n i m p r i n t i n g fabric u n d e r a p r e s s u r e of 1/2 o r 1 lb/in. 2 (3.5 or 7.0 kPa) shows the coating to be p r i n t free. This p r o c e d u r e is similar to ASTM D 2091: Test M e t h o d for Print Resistance of Lacquers. An i n d i c a t i o n of the a c c u r a c y of these m e t h o d s is the precision s t a t e m e n t developed in ASTM D 1640 in w h i c h d u p l i c a t e d e t e r m i n a t i o n s within a l a b o r a t o r y should agree within __+10% [5].

Federal Test Method Standard 141C, Method 4061.2: Drying Time This m e t h o d is similar to ASTM D 1640. It includes essentially the above stages of drying with the exception of dry-to-

CHAPTER 3 9 - - D R Y I N G TIME touch. However, it includes a test for free-from-after-tack. This test is applicable to coatings w h e r e tackiness persists beyond, o r r e a p p e a r s at, the t h r o u g h - d r y stage. It is similar to the P a p e r Test Method, discussed earlier in this chapter, except that a 2.8-kg (6.2-1b) weight is used. ISO Standard

9117: Paints and Varnishesm

Determination o f Through-Dry State and Through-Dry T i m e - - M e t h o d o f Test This s t a n d a r d describes a m e t h o d for d e t e r m i n i n g u n d e r s t a n d a r d conditions w h e t h e r a single coat or a m u l t i - c o a t system of p a i n t or related m a t e r i a l has, after a specified drying period, r e a c h e d the t h r o u g h - d r y state, i.e., a pass/fail test. The test p r o c e d u r e m a y also be u s e d to d e t e r m i n e the t i m e taken to achieve that state.

1. Through-Dry State This state defines the c o n d i t i o n of a film in w h i c h it is d r y t h r o u g h o u t its thickness as o p p o s e d to that c o n d i t i o n in w h i c h the surface of the film is dry b u t the b u l k of the

| iii

coating is still mobile. A single coating o r a m u l t i - c o a t syst e m of p a i n t o r varnish is c o n s i d e r e d to be t h r o u g h - d r y w h e n a specified gauze a t t a c h e d to a p l u n g e r is placed on the test film u n d e r specified pressm-e (1500 g) for 10 s, after w h i c h time the p l u n g e r h e a d is t u r n e d t h r o u g h an angle of 90 ~ over a p e r i o d of 2 s a n d r e m o v e d (Fig. 2). If no d a m a g e or m a r k i n g s are n o t e d on the test panel, the film is said to have achieved "through-dry state." 2. Through-Dry Time This is the p e r i o d of t i m e b e t w e e n a p p l i c a t i o n of a coating to a p r e p a r e d test p a n e l a n d the time to achieve the "through-dry state" as outlined above.

British Standard B.S. 3900: Methods o f Test for Paints Parts C-1 t h r n C-4 of British S t a n d a r d B.S. 3900 describe drying tests for d e t e r m i n i n g the wet edge time, surface drying, hard-drying, a n d f r e e d o m f r o m residual tack tests. Part C-8 describes a test for d e t e r m i n i n g print-free state or time.

I Plunger

~

Ball-joint ..-- Plunger head ~-. Baseplate

Baseplate and plunger assembly - Plunger

\ IIIIIIIII

Clip Rod for rotating - plunger head ~L~auze

Rubber disc

441

Double-faced adhessive tape

Detail showing assembled plunger head FIG. 2-Through-Dry Tester.

442

PAINT AND COATING TESTING MANUAL

1. Wet Edge Time (BS3900, C1) This procedure is used for determining whether the edge of a film of paint remains "alive" after a specified period of drying. Following a touch-up coat over the film after the specified drying period, the area is evaluated for lack of film continuity, absence of leveling, or variation in color or sheen.

2. Surface Drying (BS3900, C2) This procedure is used to determine the time after which a coating is applied and when approximately 0.5 g of the ballotini (small transparent solid glass spheres) can be poured onto the surface of the film from a height of between 50 and 150 m m and lightly brushed away without damaging the surface.

3. Hard-Drying Time (BS3900, C3) A rubber-faced plunger is covered with cotton twill, rough side outwards, and then loaded to a toal weight of 1.8 kg (4 lb). The rotating plunger drops into the panel and makes a three-quarter turn while in contact. The paint film is dryhard when no damage is observed (Fig. 3).

4. Freedom from Residual Tack (BS3900, C4) After a specified drying period, a paper-backed gold leaf is placed on the test panel and covered with a microscope slide and an 800-g weight. After I0 s, the weight and slide are removed and the panel is held vertically and lightly tapped to detach the gold leaf. The surface of the paint film is examined for adhesion of gold leaf.

5. Print-Free (BS3900, C8)

The state of a coating or varnish when gauze of a specified grade, under specified force and after a specified time, does not leave an imprint on the surface of a coating.

DIN 53 150: D e t e r m i n a t i o n o f D r y i n g T i m e o f Paints Drying time is determined in this method by the adherence or nonadherence of sand or paper to the film under various loadings. Stage I is determined with sand (0.16 to 0.315 mm) or glass beads (ballotini). The sand is allowed to remain on the film for 10 s. The remaining stages are determined using disks of typewriter paper (22 mm in diameter and weighing about 60 g/m 2) and various loads ranging from 5 to 5000 g/cm 2. Interposed between the load and the test disk is a soft rubber cushion. The load remains on the disks for 60 s. The criteria for the seven stages are as follows: 1. 2. 3. 4.

Sand easily removed with a soft brush. Disk under load of 5 g/cm 2 does not adhere. Disk under load of 50 g/cm 2 does not adhere. Disk under load of 500 g/cm 2 does not adhere, temporarily marred. 5. Disk under load of 500 g/cm 2 does not adhere, not marred. 6. Disk under load of 5000 g/cm2 does not adhere, temporarily marred. 7. Disk under load of 5000 g/cm 2 does not adhere, not marred.

FIG. 3-Hard-drying time apparatus: Assembly.

but film is but film is but film is hut film is

CHAPTER 3 9 - - D R Y I N G TIME

443

FIG. 4-Circular drying time recorder. (Courtesy of Byk-Gardner.)

FIG. 5-Straight line recorder. (Courtesy of Byk-Gardner.)

MECHANICAL DEVICES In an attempt to improve the accuracy and reproducibility of the drying time test procedure, various mechanical devices have been developed. The following sections outline these devices and the procedures used in determining drying characteristics.

Circular Drying-Time Recorder The device consists of a synchronous motor in a metal case resting on a rubber-tipped tripod and rotating a vertical shaft. A pivotal arm assembly is attached to the shaft, operating a vertical stylus with a Teflon sphere that does not stick to the drying film [6]. Under a 12-g load, the stylus scribes an arc in

the drying film. Motor speeds are available to cover drying times of 1, 6, 12 and 24 h (Fig. 4). A transparent template with time increments can be placed over the dried coating at the end of the test. The appropriate time circle can then be used to determine the dry time. During the early stages of drying, the coating tends to flow back into the wake of the stylus. When the tendency of the flow has ceased, the film may be considered set. As the drying process continues, a skin will form. Visually, this part of the film formation is seen when the stylus begins to tear the surface of the film. The film may be considered surface dry or dust free when the skin is no longer ruptured by the stylus. It is considered through dry when the stylus rides above the film. Circular drying time devices have been developed for use when determining the drying time of bake finishes that cure

444

PAINT AND COATING TESTING MANUAL Five-gram brass weights m a y be a d d e d to apply greater pressure on the needles a n d thus r e c o r d t h r o u g h drying. The i n s t r u m e n t has also been found useful in evaluating gel t i m e of m a n y t w o - c o m p o n e n t surface coatings.

I . C . I . D r y i n g T i m e Recorder This i n s t r u m e n t consists of a m e t a l box p l a t f o r m w h i c h will a c c o m m o d a t e flat panels, usually m a d e of glass. A gantry moves between a n d parallel to the long d i m e n s i o n of the flat panels [8]. This g a n t r y will c a r r y up to three d e t a c h a b l e devices for each panel. These include a flock dispenser, a s a n d hopper, a b a n d a g e roller, a n d a ball-pointed needle. These can be used in any c o m b i n a t i o n to test up to six coatings, three on each panel. The different stages in drying w h i c h can be m e a s u r e d , dep e n d i n g on the device used, are dust free, surface dry, a n d dry through.

N O PICK-UP T I M E TRAFFIC P A I N T R O L L E R This device is d e s c r i b e d in ASTM S t a n d a r d D 711: Test M e t h o d for No-Pick-Up Time of Traffic Paint. The a p p a r a t u s consists of a steel cylinder weighing 11 lb, 14 oz (5385 g) with two O-rings [6]. It is rolled along a drying film of traffic p a i n t w h i c h has b e e n a p p l i e d on a glass plate. The p a i n t is d r y w h e n no p a i n t adheres to the O-rings (Fig. 6).

REFERENCES FIG. 6-No pick-up time traffic paint roller. (Courtesy of BykoGardner.) at elevated t e m p e r a t u r e s [up to 500~ (260~ The compactness of the i n s t r u m e n t allows the u s e r to place it in an oven at a specified t e m p e r a t u r e .

Straight Line Drying Time Recorder This device consists of multiple needles being d r a w n over m u l t i p l e (up to six) parallel coated glass strips [7]. Its speed can be varied to cover drying p e r i o d s of 6, 12 a n d 24 h (Fig. 5). It defines the following stages in the drying process: 1. The first stage is a p e a r - s h a p e d d e p r e s s i o n c o r r e s p o n d i n g to the t i m e it takes for the solvent to evaporate. 2. The second stage is the cutting of a c o n t i n u o u s track corres p o n d i n g to a sol-gel transition. 3. The third stage is an i n t e r r u p t e d t r a c k c o r r e s p o n d i n g to the surface d r y time. 4. In the fourth stage, the needle no longer penetrates the film, indicating the final drying time.

[1] Algeo, W. J. and Jones, P. A., "Factors Influencing the Accurate Measurement of Drying Rates of Protective Coatings," Journal of Paint Technology, JPTYA, Vol. 41, 1969, p. 235. [2] The dust-free tester was designed and built by Technical Subcommittee 37 of the New York Paint and Varnish Production Club and is described in "Investigation of Methods for Measuring Drying Time," Official Digest, Federation of Paint and Varnish Production Clubs, ODFPA, Vol. 20, November 1948, pp. 836-843. This paper also includes a study of the Zapon Tack Tester. [3] Paper meeting this requirement may be obtained from Crocker Technical Papers, Inc., 431 Westminster St., Fitchburg, MA 01420, their Grade R 20-34. [4] U.S. Patent 2,406,989, 3 Sept. 1946. It is known as the Zapon Tack Tester. [5] See Prane, J. W., "A Latin Square Drying Time Study," Paint Industry Magazine. August 1961, for a study of precision of drying time measurements. [6] Available from Byk-Gardner, Inc, Gardner Laboratory 2435 Linden Lane, Silver Spring, MD 20910 or Paul N. Gardner Co., Inc., 316 N. E. First Street, Pompano Beach, FL 33060. [7] Available from T. J. Bell Inc., 1340 Home Avenue, Akron, Ohio 44310, as well as manufacturers listed in Ref 6. [8] Available from Erickson GMBH & Co., KG, D-5870 Herner, Germany and Paul N. Gardner Co., Inc., 316 N.E. First Street, Pompano Beach, FL 33060.

Part 10: Optical Properties

MNL17-EB/Jun. 1995 ii

Color and Light by Fred W. Bitlmeyer, Jr. 1 and Harry K. Hammond, 1112

BECAUSE COLOR IS A SIGNIFICANT FACTOR i n t h e a p p e a r a n c e o f

an object, it is an important characteristic of any paint. Appearance, of which color is a part, is one quality of a product that every customer can judge for himself. No matter how good the physical properties of a paint, if its color does not meet the expectation of the customer, the finished product will be rated as unsatisfactory. Color, often thought to be a property of the paint itself, depends on three objective aspects: (1) the spectral composition of the light in which the paint is viewed, (2) the spectral reflectance of the paint, and (3) the spectral response of the eye of the observer. The subjective interpretation of the response to these aspects by the brain is also an essential part of color. Describing the color of a paint or other material requires consideration of all of these and not merely the spectral character of the material. The three objective aspects of color are considered in sections entitled LIGHT SOURCE, REFLECTION AND TRANSMISSION, and THE EYE. The sciences involved include chemistry, physics, physiology, and psychology. These are broad subjects, and only enough discussion is included to provide a background for understanding the development of test methods. Readers desiring to pursue these subjects in detail should consult an appropriate text [1-6].

International Electrotechnical Commission (IEC). However, this is structured from the viewpoint of illuminating engineering. It is less readily available and a much more costly document than ASTM E 284.

LIGHT SOURCES Light is electromagnetic radiation weighted by the response of the normal h u m a n eye. It occupies a small portion of the electromagnetic spectrum between ultraviolet and infrared radiation. Its wavelength range is approximately 380 to 780 nm (Fig. 1).

Natural and Artificial Daylight Despite modern dependence on interior illumination, daylight is still an important light source since most objects are at some time viewed in it. The spectral composition of daylight, however, is quite variable, depending upon the hour of day, the season of year, and the amount of cloud cover. One way of dealing with this variability is to use standard light sources and their spectral power distributions when making visual or instrumental color measurements and calculations (see later in this chapter under CIE Standard Sources and

Illuminants). TERMINOLOGY Incandescent Sources To understand this chapter and to make the best use of it, the reader should be familiar with the terminology of appearance. The precise definition of terms is becoming increasingly important in today's world community. The paint terminology standard, ASTM Definitions of Terms Relating to Paint, Varnish, Lacquer, and Related Products (D 16), is the primary source of terms and definitions relating to paint, but it contains very few appearance terms. The reader should refer to ASTM Terminology of Appearance (E 284) for terms and definitions relating to color and other appearance attributes. All significant terms used in this section are defined in ASTM E 284. An important international source of appearance terms is the International Lighting Vocabulary [7], published jointly by the International Commission on Illumination (CIE) and the ~Color consultant, 1294 Garner Avenue, Schenectady, NY 123095746. 2Consulting scientist, BYK-Gardner, Inc., 2435 Linden Lane, Silver Spring, MD 20910.

Other light sources must replace daylight when appropriate. For use in homes, incandescent lamplight is generally preferred because it imparts a soft, mellow effect similar to that of candlelight.

Fluorescent Sources In stores and offices, fluorescent lamps can provide high levels of illumination with low power consumption and heat generation. The most commonly used fluorescent lamp, known as cool white, has a spectral distribution consisting of a relatively smooth curve throughout the visible spectrum. This arises from the fluorescent emission from a phosphor coated on the inside of the lamp tube. The fluorescence is excited by ultraviolet radiation from mercury vapor inside the tube. This lamp is, however, deficient in power in the red end of the visible spectrum. Modifications of it, known as deluxe and super-deluxe versions, have been designed to overcome this deficiency. Fluorescent lamps have also been

447 Copyright9 1995 by ASTM International

www.astm.org

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PAINT AND COATING TESTING MANUAL

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Other Sources Other light sources have been developed for special purposes. They include arc lamps (sodium, mercury, neon, xenon), metal halide lamps, and high-intensity discharge (HID) lamps. None of these lamps has been adopted as standard for use in color measurement.

Color-Matching Booths Because of the variation in spectral composition of different natural and artificial sources, it is essential that visual color matching be done under standardized illumination, such as that provided by a color-matching booth. This device allows the colorist to compare the colors of specimens under controlled and standardized illumination. Carefully manufactured and maintained light booths permit a colorist to make a visual match with confidence that the illumination duplicates that used at another time or place. However, the spectral power distribution of daylight illumination in colormatching booths is not the same as that of natural daylight.

REFLECTION AND TRANSMISSION Opaque, Transparent, and Translucent Films When light strikes an object, some of it may be reflected, some may be absorbed, and if the object is not opaque, some may be transmitted. The reflected light may be concentrated in a glossy, mirror-like reflection, scattered uniformly in all directions, or distributed between these two extremes, which are known as specular reflection and diffuse reflection, respectively. A highly polished metal can reflect as much as 99% of the incident light in the specular direction. A white powder, such as barium sulfate, scatters light uniformly in all directions, and it, too, can reflect as much as 99% of the incident light. Specular reflection is related to the visual perception of gloss; diffuse reflection is related to the visual perception of lightness and, when it is wavelength dependent, to that of color. Transmission can also be diffuse or regular, depending on whether or not light is scattered in passing through a material. Specimens that both transmit and reflect light are called translucent. A spectrophotometer is used to provide information on the spectrally selective character of a material. Figure 2 shows typical spectral reflectance curves of some paints. A trained colorist can obtain valuable information from such curves, but spectral data alone are unsatisfactory as a means for color identification. Among the ASTM standards on reflectance and transmittance measurement [8], the most useful include ASTM Practice for Obtaining Spectrophotometric Data for Object-Color Evaluation (E 1164), ASTM Test Method for Reflectance Factor and Color by Spectrophotometry Using Hemispherical

CHAPTER 100%

WHITE

40--COLOR

AND LIGHT

449

Infrared radiation, with wavelengths longer than 780 nm, is associated with heat transfer. It is widely used for the identification and analysis of chemical compounds. The nearinfrared region, with wavelengths from 780 to about '10 000 nm, is important for camouflage detection. Most paint pigments do not absorb radiation in this region, but some inorganic pigments reflect visible light and absorb radiation in the near-infrared.

Fluorescence PERCENT REFLECTANCE

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700 WAVELENGTH, NANOMETERS FIG. 2-Spectrophotometric curves typical of those measured on paint films. Note the sharp drop in the curve for rutile titanium dioxide (white) as the violet end of the spectrum is approached. The drop continues in the ultraviolet, where this pigment absorbs light strongly. (Based on Ref. 2.)

Geometry (E 1331), ASTM Test Method for Color and ColorDifference Measurement by Tristimulus (Filter) Colorimetry (E 1347), ASTM Test Method for Transmittance and Color by Spectrophotometry Using Hemispherical Geometry (E 1348), and ASTM Test Method for Reflectance Factor and Color by Spectrophotometry Using Bidirectional Geometry (E 1349). 3

Ultraviolet a n d I n f r a r e d Spectral Regions Ultraviolet and infrared radiation can have important effects on paint. Ultraviolet radiation, with wavelengths shorter than 380 nm, is the principal stimulus of fluorescence of certain pigments, is an aid to identification and analytical determination of certain ingredients of paint, and may promote decomposition of pigments or binders. Colorless pigments absorbing in the ultraviolet region can impart protection against such decomposition. Ruffle titanium dioxide absorbs in the ultraviolet, as its spectral curve shows (see Fig. 2). 3Asnoted in ASTM E 284, in the Discussion under reflectance, "The term reflectance is often used in a general sense or as an abbreviation for reflectance factor . . . . " This simplifying convention is used in this chapter, as it is in many textbooks. The reader should refer to ASTM E 284 for the definitions of reflectance, transmittance, and radiance, and the corresponding factors. Note that commercial instruments measure reflectance factor, not reflectance.

Some materials have the property of fluorescing when irradiated by ultraviolet or short-wavelength visible radiation. They emit radiation at longer wavelengths in the visible range or even in the near-infrared. The effect of fluorescence is to increase the apparent reflectance since the eye responds to the sum of the fluoresced and the reflected energy. This sum may even exceed the amount of light reflected by an ideal white material at the wavelengths of maximum fluorescent emission. Many fluorescent pigments have relatively poor lightfastness in outdoor applications. Most modern colorimeters and spectrophotometers are designed to evaluate properly the colors of fluorescent materials, although many do not have light sources adequately simulating the ultraviolet content of natural daylight. In such a case the instrument will not produce the same amount of fluorescence as would daylight. Two ASTM standards apply to the measurement of fluorescence. ASTM Test Method for Identifying Fluorescence in Object-Color Specimens by Spectrophotometry (E 1247) provides two instrumental methods to supplement simple visual examination of the specimen under ultraviolet light to detect the presence of fluorescence. ASTM Practice for Color Measurement of Fluorescent Materials (E 991) specifies the instrument geometry required for the measurement and shows how to assess the performance of daylight-simulating instrument light sources.

Retroreflection Retroreflection is defined in ASTM E 284 as "reflection in which the reflected rays are preferentially returned in directions close to the opposite of the direction of the incident rays . . . . "It is important in paints and coatings used for signs viewed at night, pavement and pedestrian markings, and other safety devices. The measurement of retroreflection requires special instrumentation and special test methods for the determination of daytime and nighttime colors of retroreflecting materials. Among the ASTM standards dealing with this subject are: ASTM Practice for Describing Retroreflection (E 808), ASTM Practice for Measuring Photometric Characteristics of Retroreflectors (E 809), ASTM Practice for Measuring Colorimetric Characteristics of Retroreflectors under Nighttime Conditions (E 811), ASTM Test Method for Coefficient of Retroreflection of Retroreflective Sheeting (E810), ASTM Test Method for Retroreflectance of Horizontal Coatings (D 4061), ASTM Guide to Properties of High Visibility Materials Used to Improve Individual Safety (F 923), and ASTM Specification for Nightime Photometric Performance of Retroreflective Pedestrian Markings for Visibility Enhancement (E 1501).

450

PAINT AND COATING TESTING MANUAL

THE EYE

Perception

The Visual System

Perception is defined as the translation of retinal images by the observer into meaningful information about the environment. The perception of objects and their colors thus represents the overall response of the visual system, including both the eye and the brain. Vision is called a psychophysical phen o m e n o n - p h y s i c a l in the way light reaches the eye, psychological in how the brain interprets the neural signals. The psychological factor determines, for example, whether a given color combination is interpreted as pleasing or displeasing. The mechanism of seeing is physical; the interpretation of what is seen is psychological. Objective color measurement is, however, confined to physical aspects. For example, the perceived color of a specimen may be changed by changing the color of the area surrounding it. This phenomenon, called simultaneous contrast, cannot yet be evaluated instrumentally. Another example of a perceptual phenomenon is chromatic adaptation, defined as the changes in the visual system's sensitivities due to changes in the spectral quality of the illuminating and viewing conditions. These changes tend to compensate, for example, for the effect of the change in illumination from distinctly bluish daylight to distinctly yellowish incandescent lamplight. The colors of familiar objects tend to appear the same (they tend to exhibit color constancy) when the observer goes between environments illuminated by the two kinds of light. Yet the actual colors have all been shifted because of the change in spectral composition of the incident light.

The human eye functions in a manner similar to a camera. It has a lens to focus images of objects and an iris to control the amount of light that enters (Fig. 3). A complex lightsensitive layer, called the retina, plays a role analogous to that of the film in a camera. Neither the structure of the retina nor its function are fully understood. It contains two different types of light receptors that send information along neural pathways to the visual cortex of the brain. They are called rods and cones because of their shapes. The rods are responsible for black-and-white vision at low light levels; they are not considered further in this section. At usual daylight levels, the rods are overwhelmed and do not contribute to vision. The cones are responsible for color vision. There are three types of cones, each with a different spectral sensitivity. The exact spectral response of each type of cone is not known, although it is assumed that each cone response function is related to the absorption curve of its pigment. The absorption curves are broad and overlapping (Fig. 4). They peak in the short, middle, and long wavelength regions of the visible spectrum; thus the designations blue, green, and red (sensitive) cones are sometimes used. Detailed models of color vision have been proposed [10,11], but they are presently based on incomplete information. What happens to the neural signals from the retina on the way to and in the brain is not well understood, but for most work related to color and appearance, it does not need to be.

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CHAPTER 40--COLOR AND LIGHT 451 LIGHTNESS 0

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The V a r i a b l e s o f Perceived Color The fact that the eye perceives color because it has three types of cones with differing spectral sensitivities implies, and experience confirms, that perceived color should have three variables. Several sets of these variables are of interest because of their wide use.

Object Colors: Cylindrical Systems Of great interest to the paint colorist are the variables applying to the perception of object colors. Hue is always one of the variables; the Munsell system (see later in this chapter under Munsell System) is an example. Hue is defined as the attribute of color described by common names such as red, yellow, green, blue, etc. The hues are commonly arranged in a circle in the order of their appearance in the spectrum, with the circle closed by the purples, mixtures of the red and blue at the ends of the visible spectrum (Fig. 5). A second important variable of object colors is lightness, the attribute by which an object is judged to reflect more or less light. It is often represented graphically by a line through the center of the hue circle and perpendicular to its plane (also shown in Fig. 5). The upper and lower ends of this line, often called the neutral or achromatic axis, are white and black, respectively. The third variable in this set has several different names, referring to variations among what we can perceive: Chroma, saturation, and colorfulness are examples. The common fac-

tor among these names is a sense of the amount, in contrast to the kind, of hue in the color. In this section we use chroma as the name for this third variable and show it also in Fig. 5. This quantity is exemplified by the distance between the point representing the color and the neutral axis.

Colored Lights When we deal with colored lights instead of objects, two changes need to be made in the above system: Lightness is replaced by brightness and chroma by saturation. Brightness in this sense is defined as the attribute by which an area appears to emit more or less light.

Object Colors: Opponent Systems A widely used alternative to the hue-lightness-chroma system described above is an opponent-color system that mimics the behavior of the neural signals transmitted from the retina to the brain. The lightness axis, often labeled L, is retained, but the hue circle is replaced by two opponent-type axes at right angles and perpendicular to the lightness axis (Fig. 6). Commonly they are a redness-greenness and a yellowness-blueness axis labeled a and b, respectively, as in the figure. Scales of this type are displayed in many color-measuring systems; examples are given later in this chapter under

COLOR ORDER SYSTEMS. Color Constancy a n d M e t a m e r i s m

Color Constancy As previously noted under Perception, color constancy is the general tendency of colors to remain constant in appearance when the color of the illumination is changed. Note that this term refers to what happens to the color of a single specimen when the illumination is changed.

Metamerism Of greater concern to the colorist, because it is of industrial importance and largely under his control, is what happens to the relationship of two colors when the illumination is changed. Suppose, for example, that two colors, matching in

452

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daylight, are formulated by using different sets of pigments. The two colors may not match under another type of illumination, such as incandescent lamplight, since the two specimens may exhibit different types and degrees of color constancy. This phenomenon is known as illuminant metamerism, and the colors are said to be metameric. Illuminant metamerism is defined as the property of two specimens having different spectral characteristics (resulting in the example from the use of different pigments; see Fig. 7) and having the same color when viewed under a given source, but different colors when viewed under a different source. Observer metamerism also exists in which two colors match to some observers but not to others. Only when colors have identical spectral curves can they be expected to match under all types of light and to all observers;

this is why the same pigment formulation should be used when remaking the color. Whenever pigments used for the match have different spectral characteristics from those used in the sample, the resultant color match should be tested for the absence of metamerism by several observers and under several different types of illumination, for example, daylight, incandescent lamp light, and fluorescent lamp light, preferably using a color-matching booth. If the match is not satisfactory, spectrophotometric analysis of the two formulations should be carried out to determine their spectral differences, and the new formulation should be adjusted to minimize these differences. ASTM Practice for Visual Evaluation of Metamerism (D 4086) specifies procedures for identifying the presence of metamerism and evaluating it semiquantitatively. Means of minimizing metamerism in both visual and instrumentally aided color matching are described later in this chapter in the section entitled COLOR MATCHING.

C O L O R I M E T R Y A N D T H E CIE S Y S T E M Colorimetry is defined as the science of color measurement. Its modern development began in 1931, when, in the interest of standardization and to focus attention on the properties of material objects such as paint films, international standards and recommendations were established by the International Commission on Illumination (Commission Internationale de l'l~clairage, CIE). These recommendations [12] define standard lights and observers and a methodology for combining their properties with those of the objects to describe color and related appearance parameters. CIE Standard Sources and Illuminants Here it is necessary to note two conventions of CIE terminology [7] reflected in ASTM E 284. A source is defined as a real emitter of light, whereas an illuminant is defined as a

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CHAPTER 40--COLOR AND LIGHT 453

In 1931, the CIE defined a tungsten-filament incandescent lamp of 2856 K color temperature (see later in this section under Other Features of the CIE System) as Source A. Later, when measurement of spectral power distributions became easier, the fundamental definition was changed to the lamp's spectral power distribution, known as Illuminant A. The spectral power distribution of Illuminant A is given in Ref 12, in a CIE/ISO standard [13], and in abbreviated form in ASTM Practice for Computing the Colors of Objects by Using the CIE System (E 308).

spond to the use of imaginary primary lights designated X, Y, and Z. The Standard Observer is defined by the amounts, s y(4), and 2(4), of these primaries required to match the spectrum colors; these are plotted in Fig. 9. The symbol (X) indicates that the quantity depends on the wavelength, 4. The quantities ~(4), y(4), and s are known as the color-matching functions of the 2931 CIE Standard Observer. The transformation from real primaries to X, Y, and Z was made so that the color-matching function ~(4) is equal to the spectral luminous efficiency function V(4), that is, the effectiveness of radiation to stimulate the perception of light. This choice means that the tristimulus value Y of a given color, called its luminance, contains all the information about the lightness of the color.

Daylight Source and Illuminants

1964 Supplementary Standard Observer

The CIE also recommended in 1931 standard Source C, consisting of liquid filters used in combination with Source A, representing north-sky daylight. Later, the fundamental definition was changed to that of Illuminant C [12]. Source and Illuminant C do not duplicate the ultraviolet content of natural daylight and thus do not provide correct daylight color rendition of fluorescent materials. For this reason, the CIE adopted in 1968 a series of standard illuminants duplicating the spectral power distributions of various phases of natural daylight, called the D series. They are designated by their correlated color temperatures (see later in this section under Other Features of the CIE System). The most important of these daylight illuminants [12,13] is D65, with a correlated color temperature of 6500 K. The spectral power distributions of Illuminant C and several of the D series are tabulated in ASTM E 308. Unfortunately, very few real sources, whether for visual or instrumental use, simulated any of the D illuminants satisfactorily. The CIE has recommended procedures for assessing the quality of daylight simulators [14]. The relative spectral power distributions of CIE Standard Illuminants A, C, and D65 are shown in Fig. 8.

The data for the CIE 1931 Standard Observer were obtained with a visual colorimeter in which the field of view subtended an angle of only 2~ at the eye of the observer. This was selected to correspond to the size of the fovea, that part of the retina containing only cones used in color vision. Later, the CIE studied color vision in a 10~ field, with the central 2 ~ portion disregarded. This corresponds to sample sizes more like those used in commerce, but for which the spectral sensitivity of the eye is somewhat different from that for the 2~ field. With the newer data, the CIE established the 1964 Supplementary Standard Observer [12,15]; see also ASTM E 308. Where confusion might result, quantities referring to the 1964 Supplementary Standard Observer are given the subscript 10; for example, its color-matching functions are :~jo(4), 3~1o(4), and 21o(4).

table or figure giving the spectral power distribution of the corresponding source.

Incandescent Source and Illuminant

Fluorescent Illuminants In 1986, the CIE defined [12], but did not recommend as standard illuminants, a series of twelve spectral power distributions representative of various types of fluorescent lamps, including cool white, lamps simulating daylight well, and three-band lamps. These data should be used when calculations involving fluorescent lamps are required.

CIE Standard Observers

1931 CIE Standard Observer In order to evaluate colors consistently, a standard observer was defined by the CIE in 1931 [12,15] by evaluating the spectral responses of a small group of well-trained individuals. The spectral responses of the CIE 1931 Standard Observer were determined by means of experiments, like those described later in this chapter under Additive Mixing of Lights, in which the observer determined the amounts of three primary colors (red, green, and blue) required to match the colors of all wavelengths of the visible spectrum. These sets of three values are called tristimulus values. For convenience, the data were transformed mathematically to corre-

Calculation of Tristimulus Values The tristimulus values X, I1, and Z of a color can in principle be obtained by direct matching, as were the tristimulus values of the spectrum colors defining the standard observers. But this is impractical, and one of two other methods is always used. One of these involves the design and use of tristimulus (filter) colorimeters and is discussed later in this chapter under Tristimus (Filter) Colorimeters. The other requires knowledge of the spectral reflectance curve of the specimen, obtained by spectrophotometry, and the following procedure. At any wavelength, the contribution to a tristimulus value is given by the product of the relative spectral power of the illuminant, S(4), the reflectance of the specimen, R(A), and one of the color-matching functions of the observer, for example, ~(4). These products are summed over the visible wavelengths, then normalized by multiplication by a normalization factor k; for example, X = k X S(4) R(A):~(A) (and similar equations for Y and Z), where E is the sign for summation over the visible wavelength region. The quantity k is chosen to make Y for perfect white equal to 100: k = 100/E S(4)2~(h) The CIE had defined "perfect white" as the perfect reflecting diffuser, the ideal reflecting surface that neither absorbs or transmits light, but reflects all of it. Most textbooks,

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FIG. 8-Relative spectral power distributions of CIE standard Illuminants A (incandescent lamp light), C (north-sky daylight), and D65 (actual daylight) [6].

ASTM E 308, and Ref 12 provide examples of the use of these equations. Because the fundamental process is integration, for which s u m m a t i o n is an approximation, the process is usually referred to as (tristimulus) integration. If a large n u m b e r of sets of reflectance data are to be integrated for the same illuminant and observer, it is convenient to combine quantities such as S()0 g(,~) by multiplying them together and normalizing the products so that k -- 1. The resulting tristimulus weighting factors can then be stored and used by multiplying them by whatever function R(X) is desired. The CIE has not tabulated or r e c o m m e n d e d specific sets of tristimulus weighting functions, but a substantial n u m b e r can be found in ASTM E 308. They m a y be used for wavelength intervals of 10 or 20 nm. For closer intervals, such as 5 or I nm, the tables in E 308, Ref 12, or Refs 13 and 15 should be used.

Chromaticity Coordinates and Diagram An important use of the CIE tristimulus values X, Y, and Z is the calculation of coordinates describing the chromaticity of a color, that is, its hue and chroma, ignoring its luminance or lightness. The CIE chromaticity coordinates x, y, and z are c o m p u t e d from the tristimulus values X, Y, and Z by dividing each of these by the s u m X + Y + Z. Thus x = X/(X + Y + Z), etc. Since x + y + z -- 1, only two chromaticity coordinates need be given; usually they are x and y. These chromaticity coordinates can be plotted to yield the 1931 CIE x, y chromaticity diagram, shown in Fig. 10, or the 1964 CIE xl0, Yl0 diagram for the Supplementary Standard Observer; the two are quite similar. Features of the chromaticity diagrams include: (1) the locations of the spectrum colors around the horseshoe-shaped

CHAPTER 40--COLOR AND LIGHT 455 2.00

1.50

u

"~ 1.00

0,50

0 400

~ 500

600 700 Wavelength, nm FIG. 9-The color-matching functions of the CIE 1931 standard observer; they are the tristimulus values of the colors of the spectrum [1].

spectrum locus, from 400 nm (violet) at the lower left to 700 nm (red) at the right; (2) the straight line along which purples lie joining these two ends of the spectrum locus; and (3) the location of whites (illuminant points) near the middle of the diagram. As an alternative to the three tristimulus values, colors can be specified by the chromaticity coordinates x and y together with the luminance Y (or their equivalents in the 1964 system). These can be arranged in a threedimensional color space (Fig. 11).

spectrum locus. For purple colors where the line would end at the purple locus, it is extended back through the white point to the spectrum locus, and the wavelength at that point is designated as the complementary wavelength of the sample. The fractional distance from the white point to the sample point relative to the distance to the spectrum or purple locus is called the (excitation) purity of the sample. Dominant wavelength correlates well with the hue of the sample, but purity does not correlate well with any perceived quantity and is little used today.

Other Features of the CIE System

Blackbody Locus, Color Temperature, and Correlated Color Temperature

Dominant Wavelength, Complementary Wavelength, and Purity Dominant wavelength is defined as the wavelength along the spectrum locus at the end of a line drawn from the white point (usually Illuminant C) through the sample point to the

When a metal, such as a lamp filament, is heated, it first radiates heat in the infrared region, then light with a chromaticity at the red c o m e r of the diagram. As it gets hotter, the chromaticity shifts through the oranges and yellows. The line

456

P A I N T A N D COATING T E S T I N G M A N U A L

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,

1

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Z2:~

0

0

0.2

0.4

0.8

0.6 Z

FIG. 10-The CIE 1931 x,y chromaticity diagram showing the various features described in the text [I].

along which these chromaticities lie for perfect absorbers and emitters of radiation is called the blackbody locus and is shown in Fig. 10. If the metal did not melt, its color would continue along this locus through white to light blue at infinitely high temperature. The temperature of a perfect blackbody can be correlated with its chromaticity and is called the color temperature of the body. It is measured in kelvins, K; the color temperature of Illuminant A is, for example, 2856 K. Many light sources, including the phases of daylight and fluorescent lamps, have chromaticities that are close to, but not on, the blackbody locus. In that case, the color temperature closest to the chromaticity of the source is used and called its correlated color temperature. An example is Standard Illuminant D65, with a correlated color temperature of 6500 K.

U n i f o r m Color S p a c e s From almost the beginning of the CIE system, it was recognized that distances in the CIE space did not correlate well with visual estimates of the magnitudes of color differences. Many proposals have been made for deriving quantities that are more uniformly visually spaced. In 1976, the CIE recom-

mended two more nearly uniform color spaces that, although not perfect, have been widely used. Here we describe the one that is most widely used in the paint and related industries and mention briefly the second, more useful when colored lights are considered. The equations for these two spaces, known by official acronyms CIELAB and CIELUV, are found in Refs 1 to 6 and 12 and in ASTM E 308.

CIE 1976 L*, a*, b* (CIELAB) Space CIELAB is an opponent-type color space, with a lightness axis L*, a redness (positive values)-greenness (negative) axis a*, and a yellowness (positive)-blueness (negative) axis b*, all mutually perpendicular, as illustrated in Fig. 6. The transformations from Yto L*, from X and Y to a*, and from Z and Y to b* are all nonlinear, using cube-root functions. The equations for these transformations are:

L*= 116 (Y/Y.) 1/3- 16 a • =

500

[(S/Xn)

1/3 -

(y/y.)l,3]

b* = 200 [(Y/Yn) 1/3 -- ( Z / Z n ) 1/3] where X,~, Y., and Z. are the tristimulus values of the illuminant or white point, and there are some restrictions on the

CHAPTER 40--COLOR AND LIGHT

457

- 100

-80

-60 -

Ir,~

-40

0.8 0.6 0.4 Y

0.6

u0 FIG. 11 - A three-dimensional representation of CIE color space with the luminance (lightness) axis Y rising above the chromaticity diagram [1].

use of the equations for very small values of X, Y, or Z described in the references cited above. Because CIELAB does not have tristimulus values or chromaticity coordinates as defined for the 1931 and 1964 CIE systems in Chromaticity Coordinates and Diagram, it does not have a chromaticity diagram. An alternative set of CIELAB coordinates retains L* but combines a* and b* to give chroma C* and hue angle h (measured in degrees): C* -= (a .2 + b*2) 1/2,

h = tan -1 (b*/a*)

These correlate well with visual judgments of lightness, chroma, and hue, respectively. Perhaps the widest use of CIELAB is in the calculation of color differences (see later in this chapter under Color Difference Calculations).

CIE 1976 L*, u*, v* (CIELUV) Space The CIELUV space has a chromaticity diagram with coordinates u' and v', which are linear transformations ofx and y, respectively. The linearity is important for the additive mixing of colored lights (see later in this chapter under Additive Mixing of Lights). For the three-dimensional CIELUV space, these are combined with the (nonlinear) L* transformation to give opponent-type axes u* and v*, whose meanings are the same as those of CIELAB a* and b*, respectively. An alternative set of hue angle and chroma coordinates, like those in CIELAB, and a color-difference equation, much less widely used than the CIELAB equation, are also part of this system.

COLOR

ORDER

SYSTEMS

In this section are discussed briefly the major color order systems, consisting of physical exemplifications or atlases illustrating underlying systems. References 16 and 17 provide useful general coverage. Munsell System

Dating from the early 1900s, the Munsell system is accepted by most users as the standard for equal visual spacing. It is described in ASTM Test Method for Specifying Color by the Munsell System (D 1535). Its color solid is like that of Fig. 5 with the sole exception that lightness is called value in the Munsell system. Munsell Hue is designated by position around the hue circle in a notation combining letters designating five major hues (red, yellow, green, blue, purple) and their pairs (R, YR, Y, GY, G, BG, B, PB, P, RP) with numbers from 1 to 10. Munsell Value, to which CIE lightness L* is a good approximation, runs from zero for black to 10 for white. Munsell Chroma, which expresses the degree of departure of the color from the gray of the same lightness, starts at zero and is open-ended. To describe a color in the Munsell system, the hue, value, and chroma are noted in a prescribed sequence, as for example, 8R 4/10. This designation indicates that the hue is red (toward yellow-red), the value is 4, and the chroma is 10. The Munsel[ Book of Color is available in two collections of painted color chips. The glossy finish collection contains approximately 1600 removable chips, the matte collection ap-

458

PAINT AND COATING TESTING MANUAL

v0

0.2

0.4

0.6

0.8

X

FIG. 12-Lines of constant Munsell Hue and closed curves of constant Munsell Chroma, at Munsell Value 5 plotted on the ClE 1931 chromaticity diagram [19].

p r o x i m a t e l y 1200 p e r m a n e n t l y m o u n t e d chips. The chips are a r r a n g e d on pages of c o n s t a n t Munsell Hue; in any one row they are perceived as having the s a m e Munsell Value and in a n y one c o l u m n as having the s a m e Munsell C h r o m a w h e n viewed u n d e r daylight i l l u m i n a t i o n on a m e d i u m - g r a y background. The colors progress from very light at the top to very d a r k at the b o t t o m a n d from n e u t r a l at the left to high c h r o m a at the right (or the reverse on facing pages). In 1943, the underlying Munsell system was defined in colorimetric t e r m s [18], a n d since 1968 the chips in the glossy editions of the Munsell Book of Color have b e e n p r o d u c e d to m a t c h these specifications. Figure 12 shows contours of equal Munsell H u e a n d C h r o m a at Value 5 on the 1931 CIE x, y c h r o m a t i c i t y diagram.

ISCC-NBS System In the 1950s the Inter-Society Color Council (ISCC) a n d the N a t i o n a l B u r e a u of S t a n d a r d s (NBS) developed the ISCCNBS M e t h o d of Designating Colors [20] b a s e d on the Munsell system b u t g r o u p i n g s i m i l a r colors to p r o d u c e a s m a l l e r n u m b e r (267) of categories. These are d e s i g n a t e d by descriptive color names, for example, d a r k r e d d i s h orange for the group c o n t a i n i n g 8R 4/10.

Universal Color Language The Munselt system a n d the ISCC-NBS system are b o t h parts of a Universal Color Language [20], a six-level system for describing color to a n y d e s i r e d degree of accuracy. Level 1 consists of the use of 13 hue a n d neutral names. At this level, the color with Munsell n o t a t i o n 8R 4/10 w o u l d be d e s c r i b e d as orange. I n Level 2, w h i c h has 29 categories, i n t e r m e d i a t e hue n a m e s are added. Here the color w o u l d be d e s c r i b e d as r e d d i s h orange. Level 3 is the ISCC-NBS system, a n d Level 4 is the Munsell system as used in the Munsell Book of Color; designations at these levels are given above. Level 5 uses i n t e r p o l a t e d Munsell notations b a s e d on visual c o m p a r i s o n of the color with Munsell Book chips. W i t h practice it is possible to interpolate accurately to 1/10 value step, 1/4 c h r o m a step, a n d from 1 hue step at c h r o m a / 2 to 1/4 hue step at c h r o m a n e a r / 1 0 a n d above. Thus, at this level the color might be designated very a c c u r a t e l y as 8.25 R 4.1/9.75. The final stage, Level 6, of the Universal Color L a n g u a g e is b a s e d on the results of color m e a s u r e m e n t , expressed as CIE chrom a t i c i t y c o o r d i n a t e s x, y, a n d L u m i n a n c e Y. N o w the color might be specified with greatest a c c u r a c y as x = 0.527, y = 0.343, Y = 12.5.

CHAPTER 4 0 - - C O L O R AND LIGHT 459

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B FIG. 13-Two opposing constant-hue pages in the Ostwald system [1]. Ostwald S y s t e m This system [21], which is not illustrated by any set of samples now commercially available, consists of 24 charts of approximately constant hue containing sets of chips having nearly constant chroma but different lightnesses. The chips are arranged (Fig. 13) on each chart in a triangular array, as shown. The most important atlas based on the Ostwald system in the United States was the Color Harmony Manual, available in the 1940s and 1950s [22].

DIN S y s t e m The DIN Color System is the official German Standard Color System [23]. Its coordinates are hue (for which dominant wavelength is used), saturation (like chroma except representing difference from black instead of from gray of the same lightness), and relative darkness. Contours of constant DIN hue and saturation, applicable to all lightness levels, are shown on the 1931 CIE x, y diagram in Fig. 14. From time to time, atlases of DIN samples have been available in glossy and matte finish and in transparent films. In the former two, chips are arranged on pages of constant hue, similar to the Munsel] arrangement.

colors. For example, an orange might be said to have 80% resemblance to red and 20% to yellow; its hue notation would be Y80R. Chromaticness is the resemblance of the color to the (imaginary) color of the same hue having the maximum possible chromatic content. Lightness is not an explicit variable in the system; the third resemblance can be to either white or black. Scales of resemblances to white, black, and the aforementioned maximally chromatic color are chosen so that these three add to 100%, so only one of the resemblances to white or black need be specified. For the notation, resemblance to black has been selected, so that the three variables of the system are blackness, chromaticness, and hue. The complete NCS notation for the orange sample considered above in the Munse]l system, for example, is approximately 3070 YBOR. In the NCS atlas, pages of constant hue notation contain chips arranged on a grid in the form of an equilateral triangle, as shown in Fig. 15. Note that there is a similarity to the Ostwald triangle, the difference being that in the Ostwald system the corner points are real samples; in the NCS, they are imaginary elementary colors. The NCS is subsidized by the Swedish government and aggressively marketed, and thus is available at lower cost than most other atlases.

OSA-UCS System Natural Color S y s t e m Developed in the late 1960s and 1970s, the Natural Color System (NCS) [24] is the national standard color order system in Sweden and several other European countries. It is based on an entirely different principle from that of the other systems discussed, namely the resemblances of colors to six imaginary elementary colors, unique red, yellow, green, and blue, and black and white. The four hues are placed 90 ~apart on the hue circle. This does not lead to visually equal hue spacing. Two opponent-type axes result, perpendicular to the black-white lightness axis. The third variable of the system is chromaticness. In terms of resemblances, hue is defined as the resemblance of a color to the two nearest chromatic elementary

The Optical Society of American Uniform Color Scales system [25,26] was developed by a committee of the OSA between 1947 and 1976. Although the committee concluded that no perfectly equally visually spaced system can exist because color space itself is not Euclidean, it attempted to achieve the best possible compromise and is generally thought to have succeeded. The 558 samples of the OSA atlas are arranged in a rhombohedral lattice in which each sample, except those at the edges, has twelve nearest neighbors. The axes of the color solid are lightness, L (with both positive and negative values around zero at middle gray) and two opponent axes, j (from the French ]aune for yellow), with positive values toward yellow and negative values toward purple-blue, and g (for greenness), with blue-greens at the positive end and

460

PAINT AND

COATING

TESTING

MANUAL

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[16,

pinks at the negative end. One of the features of the lattice is the existence of many planes of closely related samples, sometimes varying along all three axes, that appear strikingly beautiful. The OSA-UCS system is described in ASTM Practice for Specifying Color by Using the Optical Society of America Uniform Color Scales System (E 1360). A portion of the OSA-UCS lattice illustrating the arrangement of samples is shown in Fig. 16, and Fig. 17 shows the spacing of samples of the L = 0 plane on the 1964 CIE xl0, Yl0 diagram.

Colorcurve System This new color order system was introduced in the United States in 1989. It is based on a color space similar to CIE 1964 tristimulus space [27]. Aim points are laid out according to additive mixing (see subsection later in this chapter entitled Additive Mixing of Lights) of CIE 1964 tristimulus values Xlo, Yl0, Zlo for Illuminant D65, starting from eight specified CIELAB points. The CIELAB lightness axis L* (designated only as L) is retained, but coordinates along the CIELAB-type opponent axes are replaced by a simple numbering system giving the sample location in terms of the number of lattice steps between it and the neutral axis in the directions of two

adjacent major axes directed toward reds, yellows, greens, and blues. A typical Colorcurve notation would be L40 R1Y3, representing at lightness level 40 the lattice point one step away from neutral toward the reds and three steps toward the yellows. This is an orange only slightly different from the sample discussed above. The Colorcurve atlas contains about 2200 painted samples. Another feature of the Colorcurve system is that spectral reflectances are furnished for the aim points. These can be additively mixed like tristimulus values to obtain the spectral curve corresponding to any Colorcurve notation; this can be the basis for computer colorant formulation (see subsection later in this chapter entitled I n s t r u m e n t a l and ComputerAided Color Matching) to provide a match with minimum metamerism to the surrounding Colorcurve samples. ASTM Practice for Specifying and Matching Color Using the Colorcurve System (E 1541) describes the system in more detail, including a tabulation of spectral reflectances, tristimulus values, and CIELAB coordinates for the sample aim points.

Printed Systems For each of the color order systems described above, atlas samples consist of painted chips individually matched to the

CHAPTER 40--COLOR 0.6

W

'1

AND LIGHT

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I

0.3

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0.4

I

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Xlo FIG. 17-Section of the CIE 1964 Xlo, Ylo diagram showing the locations of points on the L = 0 plane of the OSA system (ASTM E 1360, from Ref 3).

and it is especially difficult to reproduce the samples accurately in subsequent printings or editions. Their cost is, however, relatively low.

+L

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Single-Number Color Scales Occasions arise in which the colors of samples vary along a single direction in color space. In such a case the color can be described adequately by a single scale value. Three cases are of interest: whiteness, yellowness, and series ranging from colorless to highly colored as the concentration of a colored component increases. Whiteness Indices

.g"

I~

-I1-1

-L FIG. 16-Cuboctahedron around the center point of the OSA system showing its three axes, _+L (lightness), +_j(yellownessblueness), and _ g(greenness-redness). There are twelve nearest neighbors around the center point, with the L,j,g coordihates shown. The entire OSA space is formed by extending this figure in all directions [16].

specified aim points with a mean accuracy of about one CIELAB unit. Many other collections, produced by printing, exist. Of necessity, they all have the characteristic that large groups of samples are produced at the same time; it is not possible to adjust the color of each one individually for closest conformance to the aim point. In many instances average accuracy is much poorer than that for the painted atlases,

Whiteness scales start at the point corresponding to an ideal white, which may be the perfect reflecting diffuser (see subsection earlier in this chapter entitled Calculation of Tristimulus Values) or some other industry standard, often assigned a whiteness of 100. From this number are subtracted amounts corresponding to departures toward lower lightness and the addition of chromaticness. For example, in ASTM Test Method for Indexes of Whiteness and Yellowness of Near-White, Opaque Materials (E 313), the recommended equation for whiteness index W I can be expressed in terms of CIE 1931 tristimulus values for Illuminant C as WI = Y-

4(0.847Z - Y)

The first term is the luminance, Y, which approximates perceived lightness, and the second term is a measure of departure from white toward yellow. The CIE has defined a whiteness index [12] as W = Y + 800(x. - x) + 1700(y. - y) where x and y are the 1931 chromaticity coordinates of the sample with lightness Y, and x . and y. are the values for the

462

PAINT AND COATING TESTING MANUAL

illuminant point. CIE 1964 values can also be used. This equation allows for departures from white in any hue direction, weighted by the factors shown. In each case the value for the perfect white is 100.

Yellowness Indices Indices of departure from achromatic toward yellow are formulated similarly to the second term in the ASTM E 313 WI equation given above. The indices start at zero and increase as yellowness increases. In ASTM E 313, the yellowness index YI is given by

YI = 100 (1 - 0.847Z/Y) A similar equation, given in ASTM Test Method for Yellowness Index of Plastics (D 1925), includes the effect of change in tristimulus value X as well as Y and Z

YI-- 100[(1.28X- 1.06Z)/Y] This equation is equally applicable to paint films.

Scales for Liquids When only a limited range of color is involved, for example, in the testing of the color of oil, clear varnish, lacquer, or solvents used in the paint industry, simple methods are used consisting of comparison of the specimen with standard colored solutions or glasses ranging from colorless to highly colored. A standardized series of these colors is used to provide a specialized color scale. The color is often a measure of concentration of ingredient. One difficulty in the use of these special color scales is that the color of the specimen may not match that of the standard; this can make rating on a singlenumber scale difficult. Nevertheless, their simplicity, low cost, and adaptability to special situations has resulted in wide use of single-number scales for certain applications. Among such scales useful to the paint industry are those described in ASTM Test Method for Saybolt Color of Petroleum Products (Saybolt Chromometer Method) (D 156), ASTM Test Methods for Soluble Nitrocellulose Base Solutions (D 365), ASTM Test Method for Color of Clear Liquids (Platinum-Cobalt Scale) (D 1209), ASTM Test Method for ASTM Color of Petroleum Products (ASTM Color Scale) (D 1500), ASTM Test Method for Color of Transparent Liquids (Gardner Color Scale) (D 1544), ASTM Test Method for Color of Solid Aromatic Hydrocarbons and Related Materials in the Molten State (Platinum Cobalt Scale) (D 1686), and ASTM Test Method for Measurement of Color of Low-Colored Clear Liquids Using the Hunterlab Color Difference Meter (E 450). These and other single-number color scales were intercompared [28].

I N D U S T R I A L COLOR M E A S U R E M E N T Instruments Using the Eye as Detector The eye is the ultimate arbiter in color evaluation because, of course, it passes the final judgment on the acceptability of a color. The first color-measuring instruments used the eye as detector, and this practice is still followed when use of a simple color comparator suffices, as in many of the test methods for single-number color scales described in the pre-

ceding paragraph. But there are many factors, such as fatigue, poor color memory, and subjectivity, as well as the perceptual phenomena described earlier under Perception, that make the eye at least suspect for close color evaluation work. Therefore the use of the eye as detector has been almost entirely replaced by the use of two types of photoelectric instruments developed since the 1940s for color measurement.

Spectrophotometers Spectrophotometers provide detailed information on the color properties of specimens, in the form of spectral reflectances (or transmittances), required for the calculation of CIE tristimulus values and derived color coordinates, as described earlier under Colorimetry a n d the CIE System and in ASTM E 308. Spectrophotometers are described here and their abridgments in the form of tristimulus (filter) colorimeters in the next section. When radiation strikes a sample, it may be reflected, absorbed, or transmitted. Each of these can be measured by most spectrophotometers. All spectrophotometers consist of a light source, a monochromator, arrangements for illuminating and viewing the sample, a photodetector, and an output device. In modern instruments, the latter consists of a computer that processes the signals from the detector and provides for the calculation of tristimulus values and a wide variety of other color-related quantities. Each of these components is described briefly.

Light Source In most cases the exact nature of the light source in a spectrophotometer is of little importance as long as it has adequate power and stability at all wavelengths in the visible spectrum. Incandescent lamps or xenon flash tubes are widely used. When fluorescent samples are measured (see earlier under Fluorescence), the source must illuminate the sample directly and may be filtered to simulate a standard source, such as CIE daylight D65. Most color measuring spectrophotometers are designed with these features. ASTM E 991 provides more information on the measurement of fluorescent samples.

Geometry of Illumination and View In most spectrophotometers, the geometry of sample illumination and viewing follows CIE recommendations [12]. Two standard geometries are widely used. In hemispherical geometry, light from the source usually illuminates the white interior of a hollow, approximately spherical cavity called an integrating sphere, and the diffused light from the sphere illuminates the sample from all angles in the hemisphere bounded by its surface. The sample is viewed at an angle near the normal or perpendicular to its surface. If the sample is glossy, the specular or mirror reflection from its surface will result in a portion of the wall of the integrating sphere also being viewed. The user is given the option of making this part of the sphere white, thus including the specular component, or black, excluding it. In bidirectional geometry, illuminating and viewing are at angles along the normal to the sample surface (designated 0 ~ and 45 ~ from the normal, or the reverse. The use of

CHAPTER 40--COLOR AND LIGHT 4 6 3 bidirectional geometry with the specimen illuminated by white light is required when fluorescent samples are measured. Hemispherical and bidirectional geometries are described in ASTM Guide for Selection of Geometric Conditions for Measurement of Reflection and Transmission Properties of Materials (E 179) and in ASTM E 1164 as well as by the CIE [12]. In typical 45/0 or 0/45 geometry, several illuminating or viewing beams, distributed around the azimuth at 45 ~ to the normal, may be used. When a specimen exhibits directionality, that is, its reflectance changes when it is rotated in its own plane, the use of an instrument with multiple beams provides data that average over the directionality, giving a single number characteristic of the average properties of the specimen. If it is desired to measure the directionality, an instrument with one illuminating (or viewing) beam, or two 180~ apart in azimuth, should be used and measurements made at several different specimen orientations. For the measurement of specimens exhibiting goniochromatism, in which the reflectance changes when the illuminating or viewing angles are changed, the use of special instruments capable of measuring at different combinations of these angles is required. Several such multiangle instruments, known as goniospectrophotometers, are commercially available. To date, no firm recommendations have been made on the number of combinations or the optimum angles required to characterize such goniochromatic specimens. Such specimens of interest in the paint industry include the so-called metallic and pearlescent coatings for the automotive industry.

Monochromator, Detector, and Output The light reflected or transmitted by the specimen is directed to the monochromator, in which a narrow (typically 10 to 20 nm) band of wavelengths is selected from the full spectrum of the incident light. The monochromators in modern instruments usually use holographic diffraction gratings or interference filters to isolate the narrow wavelength range. Spectral light is received by the detector, usually a silicon photodiode. Detection may be accomplished by an array of diodes, each positioned permanently to receive light of a given wavelength, thus eliminating the need for a spectrum scanning device. The electrical signal from the detector is usually amplified, digitized, and entered in an interfaced computer. In addition to ASTM E 1164, the following test methods cover the operation of modern spectrophotometers" ASTM E 1331, E 1348, and E 1349.

Spectroradiometers Instruments closely related to spectrophotometers but made to measure light incident from external sources are known as spectroradiometers. They could be used in the paint industry when, for example, the specimen must be sensed remotely after illumination external to the instrument. Methods for making such measurements are described in ASTM Practice for Obtaining Spectroradiometric Data from Radiant Sources for Colorimetry (E 1341) and ASTM Practice for Obtaining Colorimetric Data from a Visual Display Unit Using Colorimeters (E 1455).

Obtaining Tristimulus Values from Spectral Data The first step in utilizing spectral data is the calculation of CIE tristimulus values, as described earlier under Calculation of Tristimulus Values. This step is performed automatically as part of the measurement sequence in all modern spectrophotometers designed for color measurement. It is usual that the spectral bandpass of the monochromator and the measurement interval are selected to be the same. For highest accuracy, the correct set of tristimulus weighting factors, corresponding to the instrument bandpass, selected from among those added to ASTM E 308 in 1994, must be used in calculating tristimulus values. In addition, the user must select the standard illuminant and the standard observer used, and (when hemispherical measuring geometry is utilized) select the inclusion or exclusion of the specular component in reflectance measurement.

Spectrocolorimeters Some spectrophotometers are designed so that the measured spectral reflectances or transmittances cannot be accessed for examination; only the resulting tristimulus values and other color coordinates can be printed out. Such instruments are designated spectrocolorimeters.

Tristimulus (Filter) Colorimeters Among the earliest photoelectric color-measuring instruments were those in which the source-filter-photodetector combinations duplicate the tristimulus functions of the Standard Observer and a CIE Standard Illuminant, usually Illuminant C [29]. How well their filters are designed and matched to the spectral characteristics of the source and of the detector determines how accurately the instrument performs. Today a high degree of accuracy is attained in the resulting tristimulus values. Because of their ease of operation, good precision, and relatively low cost, tristimulus (filter) colorimeters have found wide application for industrial control. They are used primarily as color-difference meters to evaluate the difference in color between a production specimen and a standard of similar spectral character. This last limitation is important for most colorimeters. Because of their design, most colorimeters provide colorimetric data for only one combination of illuminant and observer and therefore cannot detect metamerism. When specimens are metameric, a colorimeter can give incorrect data on the differences among them. Filter colorimeters should not be used to evaluate pairs of specimens that may be metameric. This limitation should be clearly understood. The use of colorimeters is described in ASTM E 1347. This standard replaces ASTM Test Method for Directional Reflectance Factor, 45-deg 0-deg, of Opaque Specimens by BroadBand Filter Reflectometry (E 97) (discontinued in 1992).

Selection and Calibration o f Instruments The potential user of color-measuring instrumentation must consider with care the selection of the proper instrument(s), depending on his samples and measurement needs. ASTM E 179 and ASTM Practice for Identification of Instrumental Methods of Color or Color-Difference Measurement

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PAINT AND COATING TESTING MANUAL

of Materials (E 805) address this topic. They should be consulted, if possible, before the purchase of a new instrument. The user of any color-measuring instrument must give regular attention to its calibration. Appropriate calibration standards, available from instrument manufacturers and, in some cases, national standardizing laboratories, should be obtained, maintained in good condition, and used at frequent intervals in an established routine [30]. A collaborative reference program [31] is available by means of which a user's instrument performance can be compared to that obtained for the same samples by the color community as a whole and by national standardizing laboratories. With proper calibration and maintenance, modern colormeasuring instruments are capable of a repeatability and reproducibility far greater than the repeatability with which the samples themselves can normally be prepared for industrial measurement. Thus, special care must be taken to establish a highly reproducible method of paint sample preparation and to establish by careful measurement and periodic verification the uncertainties associated with this step in the measurement process. ASTM standards that deal with these matters are E 1164, E 1331, ASTM Practice for Reducing the Effect of Variability of Color Measurement by Use of Multiple Measurements (E 1345), E 1348, and E 1349. See also ASTM Practice for Selection of Coating Specimens and Their Preparation for Appearance Measurements (D 3964) and Standard Guide for the Preparation, Maintenance, and Distribution of Physical Product Standards for Color and Geometric Appearance of Coatings (D 5531). The following two subsections apply mainly to spectrophotometers.

Precision The modern age of computer-interfaced color measuring spectrophotometers began in the mid-1970s. By then it had been clearly demonstrated that such instruments could surpass the eye in the precision of color measurement. Repeatability of instrumental color measurement is about 0.1 CIELAB unit of color difference [32] (see later in this chapter under Color Difference Calculations). The average reproducibility within a group of similar instruments is about 0.2 CIELAB units [33]. Note that 0.5 CIELAB unit is approximately the smallest difference that can be detected visually. Repeatability and reproducibility values are published in the Precision and Bias section of ASTM E 1164.

Bias This quantity, the difference between a measured result for a standard sample and the result when it is measured in a reference laboratory, is difficult to quantify in the color field where there are no "absolute" values. It is usual to accept the results obtained in a national standardizing laboratory as the reference. On this basis, when instruments are selected to minimize differences in measuring geometry and are properly calibrated and used, the average bias for a group of standard samples can be as low as 0.5 CIELAB unit [34]. In addition, the differences between the two sets of measurements can be utilized effectively to model and correct the systematic spectrophotometric errors leading to the bias [35].

C o m m e r c i a l Instruments Instrument design changes have been so frequent in recent years that published information in a manual such as this would almost certainly become obsolete by the time the manuscript is printed. For this reason, detailed information on suppliers and their instrumentation is not included. Such information, including types of instruments manufactured and their design features and performance characteristics, can best be found by first consulting buyers' guides [36,37] that are revised annually. Another useful source of information is the lists of exhibitors at paint shows, such as the descriptions published annually in the Journal of Coatings Technology. In contacting suppliers of instrumentation, the user might request that they send literature, measure samples sent them, or have a representative visit the user to demonstrate the instrument. Another procedure is to negotiate a rental-purchase contract. While some suppliers are reluctant to negotiate such contracts, the procedure provides users the opportunity to evaluate the instrument with their own operators at their own facilities. Most major manufacturers currently in the field supply both spectrophotometers and colorimeters meeting the descriptions given earlier under

Spectrophotometers and Tristimulus (Filter) Colorimeters. Several recent trends provide insight into possible future developments in color-measuring equipment and its uses. These trends can be classified in three groups: (1) miniaturization, (2) multi-angle evaluation, and (3) location and type of use.

1. Miniaturization. Many manufacturers now supply portable spectrophotometers and colorimeters that provide small size and light weight. Many are battery operated for full portability; others may use fiber-optics probes to separate the measuring head from the remainder of the instrument. In many, the precision is good, but limitations related to measurement geometry may increase bias. If very small illuminated and viewed areas are used, there may be an advantage in that smaller portions of samples can be measured, but averaging of multiple measurements (see ASTM E 1345) may be required to obtain representative values for nonuniform samples. 2. Multi-Angle Evaluation. Several manufacturers now supply instruments with multi-angle geometry for the measurement of goniochromatic samples, such as those containing metal flake or pearlescent pigments. For research in this field, a goniospectrophotometer is required. For quality control, however, use of one angle of illumination with several angles of view may suffice. As noted earlier under Spectrophotometers, the selection of illuminating and viewing angles has not yet been standardized. 3. Location and Type of Use. In earlier times, production samples were taken to a quality-control laboratory for evaluation. This procedure is still desirable in some instances to permit control of such variables as ambient light, temperature and humidity, and atmospheric contamination, but today time is being saved by making measurements on the production floor. For continuous processes, on-line instrumentation is finding greater use. In addition, color measurement and computer-aided color matching (see later in

C H A P T E R 4 0 - - C O L O R AND L I G H T this chapter under Instrumental and Computer-Aided Color Matching) are today finding extensive use at pointof-purchase locations such as retail paint stores.

COLOR DIFFERENCE EVALUATION FOR COLOR CONTROL The control of the color of industrial paint products may be aided by use of the results of color measurement, as described in the last section. The instruments provide color coordinates of sample and standard from which accurate measures of color difference may be derived. These data can also be used to establish color tolerances. Of course, color differences may also be judged visually, and several ASTM standards address this topic: ASTM Practice for Visual Estimation of Color Differences (D 1729), ASTM Test Method for Evaluation of Visual Color Difference With a Gray Scale (D 2616), and ASTM Guide to the Selection, Evaluation, and Training of Observers (E 1499). This guide also describes materials and methods for the testing and evaluation of the observer's color vision.

465

Alternative equations for determining AH* and its sign have been described [41]. The unit of CIELAB color difference is two to three times the just perceptible color difference. In the use of color differences, it is important to examine the components of the color difference, such as AL*, Aa*, and Ab*, or AL*, AC*, and AH*, as well as AE* itself. The magnitudes and signs of these components can provide valuable information on remedial action needed to bring a production batch on shade. The interpretation of measured color differences becomes more complex when the specimens are goniochromatic and measurements are made at several combinations of illuminating and viewing angles. The quantitative meaning of color differences as the angles change has not yet been thoroughly studied. Since the adoption of the CIELAB equations and their inclusion in ASTM D 2244, a number of new color-difference equations have been developed. One of these, known as the CMC (1 :c) equation [42,43], is a modification of CIELAB that has improved uniformity of visual perception of its color differences. It will be included in an "ASTM Guide to the Selection and Use of Color-Difference Equations" (under development).

Color-Difference Calculations

Color Tolerances

In the 1931 and 1964 CIE systems, equal distances in different parts of color space do not represent equally perceived color differences. Although many proposals for more uniform color spaces have been made, an ideally uniform space has not been and may not ever be devised. However, modern color-measuring instruments compute sets of color coordinates that correlate better with perceived color differences than do CIE tristimulus values. Among the many modifications of CIE tristimulus values that have been proposed for more uniform color spaces, those proposed by Hunter [29], MacAdam [38-40], and the CIE [12] are preferred for use in the paint industry and are given in ASTM Test Method for Calculation of Color Differences from Instrumentally Measured Color Coordinates (D 2244). The most widely used of these is the CIE 1976 L*a*b* (CIELAB) space discussed earlier in the subsection entitled Uniform Color Spaces. Omitting subscripts ab throughout for convenience, the total CIELAB color difference, AE*, is given by

A valuable means of recording the experience gained in the production of a given color is through the use of color tolerances, expressed as numerical limits to AE* and its components or as color-tolerance charts. Because no color space is entirely uniform and industrial color tolerances are often based on acceptability considerations between buyer and seller, not perceptibility, color differences should be used only as a guide until enough experience has been accumulated for each color to allow specification of a tolerance limit,

AE* = [(AL*)2 + (Aa*)2 + (Ab*)2] 1'z where, for example, AL* = LTRIA L -- LSTANDARD. It is convenient to express AE* in terms of differences in the Munselltype coordinates hue, chroma, and lightness, but the hue angle h defined earlier in the subsection entitled Uniform Color Spaces does not have the same dimensions and scaling as chroma C* and lightness L*. Instead of using h, it is necessary to define CIELAB hue difference AH* to combine with AC* and AL* AH* = [(AE*) 2 - (AC*)2 - (AL*)2]'/2 Then ~*

= [ ( A H * ) 2 -~ (AC~") 2 -}c (z~kn*)2] I/2

Tolerance Charts Color-tolerance charts are usually enlarged sections of a nearly visually equally spaced diagram, such as the CIELAB a*, b* diagram. A color-tolerance chart is set up with the coordinates of the standard at the center, and the differences Aa* and Ab* of production batches are entered as they are made (Fig. 18a). The process is described in ASTM Practice for Establishing Color and Gloss Tolerance (D 3134). As experience is gained, it should be possible to draw a tolerancelimit curve enclosing most, if not all, of the acceptable batches and excluding most, if not all, of the unacceptable batches. Because no known diagram is perfectly visually uniform, the tolerance limit figure may not be a circle or an ellipse and may not be centered on the location of the standard. Lightness differences between the standard and batch must also be taken into account. This can be done by use of a separate tolerance for AL*, but better practice is to use a second chart (Fig. 18b), in which AL* is plotted against either Aa* or Ab*, depending on the data. The batch readings should fall within the tolerance figure on both charts. It must be emphasized that the final criterion for acceptance of a color is always its visual appearance, to which instrumental measurements, color differences, and tolerance

466

PAINT AND COATING TESTING MANUAL 0 0 0

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(b) ~ Tolerances FIG. 18-Color tolerance charts plotted in the CIELAB system: (a) chromaticity tolerance figure on a Aa*, Ab* diagram; (b) lightness tolerance figure on a AL*, Aa* diagram. In this case, a plot of AL* versus Ab* would have served as well. (Based on ASTM D 3134.)

charts provide only clues. Instrument malfunctions, miscalibrations, miscalculations, and operator errors can be revealed only by use of confirming visual observations. Indices

of Metamerism

Another important use of color-difference equations is to quantify indices of metamerism as an aid in reducing this defect between standard and batch. The CIE [12,44] has recommended that indices of metamerism be calculated as the color differences between two samples that match under a standard illuminant or to a standard observer when measured for a test illuminant or a test observer. For illuminant metamerism, the standard illuminant is usually daylight, and the second can be selected by the user. For observer metamerism, the standard observer is either the CIE 193l or 1964 observer, and a corresponding second "standard deviate" observer is provided [44] based on the range of normal color vision found in the human population.

COLOR MIXING

area of the card, produce white again (Fig. 19). In this area, the reflected lights from the three sources add together. These three lights are called the additive primaries. By adjusting the intensities of each of the three lights, a wide range of colors can be produced. The procedure just described can provide an analytical tool for color measurement. If an adjacent spot on the white card is illuminated by light of an unknown color, it could be matched visually, in principle, with the three-light combination by adjusting their intensities. This is the basis for color matching by addition of lights. The additive principle is used to produce the colors in television [45]. The analytical device described above is also a simple version of the visual colorimeter used to generate the tristimulus values of the spectrum colors that form the basis for the CIE Standard Observers (see earlier in this chapter under CIE S t a n d a r d Observers). At each wavelength, the total light reflected is the sum of the power reflected from each of the three primary lights. Since tristimulus values are obtained by adding such sums across the spectrum, they too are produced by additive mixing. It is not necessary to know the spectral nature of the additive primaries to predict the resulting colors.

Additive Mixing of Lights

When a white card is illuminated, its apparent color is the color of the incident light; a red light, for example, makes it appear red. Three colored lights (red, green, and blue), if carefully chosen, would, when placed to overlap on the same

Subtractive

Mixing in T r a n s p a r e n t Films

Now consider the light falling on a white card after passing through a film that contains three transparent colorants, yel-

CHAPTER 40--COLOR AND LIGHT 4 6 7

RED

/

~

GREEN

YELLOW

MAGENTA A

result would be black (Fig. 20). These three colorants are called the subtractive primaries. If varying quantities of them were combined in subtractive mixing, a wide variety of colors could be produced. However, these mixture colors cannot be predicted from the colors of the primaries alone, as in the additive mixing described above. One must also know their spectral character and compute, wavelength by wavelength, how much light is removed by each primary through absorption, using the well-known Beer's law relationship. From what is not absorbed, tristimulus values can be calculated in the usual way.

CYAN

Pigment M i x i n g

BLUE

FIG. 19-Representation of the additive mixing of colored lights showing the additive primaries red, green, and blue forming the mixture colors yellow, cyan, and magenta where the primaries overlap in pairs and white where all three overlap.

low, cyan (blue-green), and magenta (red-purple). This might be the situation with a free-standing transparent plastic film, for example. If the three colorants were ideally chosen, all the light would be absorbed when all three were present, and the

CYAN

YELLOW

Most paint films contain, in addition to three colored pigments (one of which may be black), a white pigment used as an opacifier. The mixing of colors produced in this way is more complex than simple subtractive mixing in transparent films because of the scattering of light caused by the white pigment (and by opaque colored pigments as well). Again, it is necessary to know the spectral character of all the colorants present. The Beer's law calculation of the transparent case is replaced by the Kubelka-Munk relation: For opaque films, as is usually the case in the paint industry, this is a simple equation relating the reflectance R(h) of the film at each wavelength to the~ratio of two constants describing what happens to the light in the film

K(A)/S(A) = [1 - R(A)]2/2R(A) where K(A) is the Kubelka-Munk absorption coefficient, and S(~t) is the Kubelka-Munk scattering coefficient. Here R(X) must be expressed as a fraction, not the usual percentage. The quantities K(X) and S(X) refer to the mixture of pigments in the paint film. These are calculated by adding the separate contributions of each pigment, using its K(X) and S()t) weighted by its concentration C in the mixture K(A)MIXTUR E --

C~KI(A) + C2K2(A) + 9 9 9

S(X)MIXTURE

C l S l ( X ) -b C282()k) -{- 9 . .

where there are as many terms as there are pigments used. These equations form the basis for computer color matching described in the next section.

GREEN

COLOR MATCHING RED

X

BLUE

MAGENTA

FIG, 20-Representation of the subtractive color mixing of transparent colorants showing the subtractive primaries yellow, cyan, and magenta forming the mixture colors green, blue, and red where the primaries are mixed in pairs and black where all three are mixed together.

One of the major objectives of industrial coloring is to match the color a customer wants. Whether this is done visually or with the aid of instruments and computers is often a matter of work load and economics. Therefore we address both visual and instrumental matching. In either case, a major objective of color matching in paint systems is the formulation of a nonmetameric match to a given paint sample. The difference in spectral character of the samples of a metameric pair is the property determining their metamerism, and achieving a nonmetameric match places several requirements on the formulation. First, the same pigments must be used. This requires identification of the pigments in the sample to be matched. Second, the pigments must be used in the same resin system since the spectral properties of pigments can depend upon

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PAINT AND COATING TESTING MANUAL

the choice of binder. Finally, the s a m e degree of d i s p e r s i o n m u s t be achieved since the a b s o r p t i o n a n d scattering p r o p e r ties of p i g m e n t s that d e t e r m i n e their spectral c h a r a c t e r change with degree of dispersion. This usually m e a n s that the s a m e m e t h o d of d i s p e r s i o n m u s t be used.

Visual Color Matching The selection of visual color m a t c h e r s should be m a d e with great care, a n d reference should be m a d e to ASTM E 1499 for details of h o w to m a k e the selection, test the color vision of the candidates, a n d t r a i n t h e m in m a k i n g visual judgments. Beyond this there is no substitute for the experience of the visual color matcher. He or she m u s t learn the skill by practice. This should include b e c o m i n g familiar with one of the better color o r d e r systems a n d atlases a n d learning h o w to use it, coupled with knowledge of the b e h a v i o r of the p a i n t system a n d p i g m e n t s available, to predict the colors that result from mixing t h e m u n d e r carefully controlled conditions. Careful r e c o r d keeping to d o c u m e n t the results is essential. Visual m e t h o d s of recognizing a n d controlling m e t a m e r i s m have been d e s c r i b e d [46,47]. These require no m o r e than knowledge of the b e h a v i o r in mixtures of the p i g m e n t s used a n d a good color m a t c h i n g b o o t h (see earlier in this c h a p t e r u n d e r Color Matching Booths).

Instrumental and Computer-Aided Color Matching The i m p o r t a n c e of eliminating m e t a m e r i s m dictates that, for a color m a t c h i n g aid, the i n s t r u m e n t of choice is clearly the s p e c t r o p h o t o m e t e r . Display of the spectral curves of sampies, and the c o m p u t e r - a i d e d c o l o r - m a t c h i n g o p e r a t i o n s that lead to spectral curve shapes m i n i m i z i n g m e t a m e r i s m , cannot be achieved by the use of o t h e r types of instruments. The s p e c t r o p h o t o m e t e r can also be used in a simple but powerful m e t h o d of organic p i g m e n t identification [48] b a s e d on ext r a c t i o n of the p i g m e n t s into solution followed by spectrop h o t o m e t r y . This can be an i m p o r t a n t aid to selecting the s a m e p i g m e n t a t i o n for the m a t c h as was used in the standard. M a n y m o d e r n c o l o r - m e a s u r i n g s p e c t r o p h o t o m e t e r s can be o b t a i n e d with c o m p u t e r software for color matching. Most of these systems w o r k very well, b u t it m u s t be e m p h a s i z e d that the i n v e s t m e n t in b u i l d i n g up the d a t a b a s e that is essential for their use, a n d in taking all the steps necessary to bring the coloring process u n d e r precise control, is not a small one; only when large n u m b e r s of colors m u s t be m a t c h e d on a daily basis can the cost of setting up a n d m a i n t a i n i n g such a system be justified. The details of h o w c o m p u t e r color m a t c h i n g works are b e y o n d the scope of this chapter. In s u m m a r y , most systems call for m e a s u r e m e n t of the s a m p l e to put its spectral d a t a into the computer. These d a t a are m a t c h e d at each of (usually) 16 wavelengths across the s p e c t r u m by Kubelka-Munktype calculations (see earlier u n d e r Pigment Mixing), using stored values of K(A) and S(h) for the useful p i g m e n t s in the p r o d u c t line. The results a r e the p i g m e n t concentrations in the formulation. Several f o r m u l a t i o n s arising from m i x i n g different p i g m e n t s are usually produced. Additional calcula-

tions of m e t a m e r i s m indices a n d p i g m e n t costs allow the selection of the m o s t suitable results. C o m p u t e r color m a t c h i n g has been discussed in a b o o k [49], a n d some textbooks [1,2,5,50,51] provide useful summaries. The underlying K u b e l k a - M u n k theory, applied to p a i n t systems, has been the subject of several articles [52-55] directed to p a i n t colorists. More complex theory a p p e a r s to be n e e d e d only in special cases, such as m a t c h i n g a u t o m o t i v e p a i n t s containing metal flake or pearlescent pigments. The d e v e l o p m e n t of such theories is still in its early stages.

REFERENCES [1] Billmeyer, F. W., Jr. and Saltzman, M., Principles of Color Technology, 2nd ed., Wiley, New York, 1981. [2] Hunter, R. S. and Harold, R. W., The Measurement of Appearance, 2nd ed., Wiley, New York, 1987. [3] Wyszecki, G. and Stiles, W.S., Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed., Wiley, New York, 1982. [4] Hunt, R. W. G., Measuring Colour, 2nd ed., Ellis Horwood, Chichester, England, 1991. [5] McDonald, R., Ed., Colour Physics for Industry, Society of Dyers and Colourists, Bradford, England, 1987. [6] McLaren, K., The Colour Science of Dyes and Pigments, 2nd ed., Adam Hilger, Bristol, England, 1986. [7] CIE-IEC International Lighting Vocabulary, Publication CIE No. 17.4, Central Bureau of the CIE, Vienna, 1987. [8] ASTM Standards on Color and Appearance Measurement, 4th ed., ASTM, Philadelphia, 1994. [9] Burnham, R. W., Hanes, R. M., and Bartleson, C. J., Color: A Guide to Basic Facts and Concepts, Wiley, New York, 1963. [10] Boynton, R.M., Human Color Vision, Holt, Rinehart and Winston, New York, 1979. [1t] Hurvich, L. M., Color Vision, Sinauer, Sunderland, MA, 1981. [12] Colorimetry, 2nd ed., Publication CIE No. 15.2, Central Bureau of the CIE, Vienna, 1986. [13] ClE Standard on Colorimetric Illuminants, Publication CIE No. $001 (ISO IS 10526), Central Bureau of the CIE, Vienna, 1986. [14] A Method for Assessing the Quality of Daylight Simulators for Colorimetry, Publication CIE No. 51, Central Bureau of the CIE, Vienna, 1981. [15] CIE Standard on Colorimetric Observers, Publication CIE No. $002 (ISO IS 10527), Central Bureau of the CIE, Vienna, 1986. [16] Billmeyer, F. W., Jr., "A Survey of Color Order Systems," Color Research and Application, Vol. 12, 1987, pp. 173-186. [17] Agoston, G. A., Color Theory and Its Application in Art and Design, 2nd ed., Springer, New York, 1987, Chaps. 8-10. [18] Newhall, S. M., Nickerson, D., and Judd, D. B., "Final Report of the O. S. A. Subcomnqittee on the Spacing of the Munsell Colors," Journal, Optical Society of America, Vol. 33, 1943, pp. 385-418. [19] Judd, D. B., Color in Business, Science and Industry, 1st ed., Wiley, New York, 1952. [20] Kelly, K. L. and Judd, D. B., "Color: Universal Language and Dictionary of Names," NBS Special Publication 440, U.S. Government Printing Office, Washington, 1976. [21] Jacobson, E., Basic Color." An Interpretation of the Ostwald System, Paul Theobald, Chicago, 1948. [22] Granville, W. C., "Color Harmony Manual, a Color Atlas Based on the Ostwald Color System," Color Research and Application, Vol. 19, 1994, pp. 77-98. [23] Richter, M. and Witt, K., "The Story of the DIN System," Color Research and Application, Vol. 11, 1986, pp. 138-145.

CHAPTER 40--COLOR AND LIGHT [24] HS.rd, A. and Sivik, L., "NCS--Natural Color System: a Swedish Standard for Color Notation," Color Research and Application, Vol. 6, 1981, pp. 129-138.

[25] MacAdam, D. L., "Uniform Color Scales," Journal, Optical Society of America, Vol. 64, 1974, pp. 1691-1702. [26] Nickerson, D., "Uniform Color Scales Samples: A Unique Set," Color Research and Application, Vol. 6, 1981, pp. 7-33. [27] Stanziola, R., "The Colorcurve System| '' Color Research and Application, Vol. 17, 1992, pp. 263-272. [28] Johnston, R.M., "Colorimetry of Transparent Materials," Journal of Paint Technology, Vol. 43, No. 553, 1971, pp. 42-50. [29] Hunter, R.S., "Photoelectric Tristimulus Colorimetry with Three Filters," Journal, Optical Society of America, Vol. 32, 1942, pp. 509-538. [30] Carter, E. C., Billmeyer, F. W., Jr., and Rich, D. C., "Guide to Material Standards and Their Use in Color Measurement," ISCC Technical Report 89-1, Inter-Society Color Council, Princeton, NJ, 1989; see also Carter, E. C. and Billmeyer, F, W., Jr., "Material Standards and Their Use in Color Measurement," Color Research and Application, Vol. 4, 1979, pp. 96-100. [31] Color & Appearance Interlaboratory Testing, Collaborative Testing Services, Inc., Herndon, VA 22070. [32] Billmeyer, F. W., Jr. and Alessi, P. J., "Assessment of Color-measuring Instruments," Color Research and Application, Vol. 6, 1981, pp. 195-202. [33] Stanziola, R., Momiroff, B., and Hemmendinger, H., "The Spectro Sensor--A New Generation Spectrophotometer," Color Research and Application, Vol. 4, 1979, pp. 157-163. [34] Billmeyer, F. W., Jr. and Hemmendinger, H., "Instrumentation for Color Measurement and its Performance," Golden Jubilee of Colour in the C1E, Society of Dyers and Colourists, Bradford, England, 1981, pp. 98-112. [35] Berns, R. S. and Petersen, K. H., "Empirical Modeling of Systematic Spectrophotometric Errors," Color Research and Application, Vol. 13, 1988, pp. 243-256. [36] Optical Industry and Systems Purchasing Directory, Optical Pubfishing Co., Pittsfield, MA, annually. [37] Modern Paint and Coatings Paint Red Book, Communications Channels, Inc., Atlanta, annually. [38] MacAdam, D. L., "Visual Sensitivities to Color Differences in Daylight," Journal, Optical Society of America, Vol. 32, 1942, pp. 247-274. [39] Chickering, K.D., "Optimization of the MacAdam-Modified 1965 Friele Color-Difference Formula," Journal, Optical Society of America, Vol. 57, 1967, pp. 537-541. [40] Chickering, K. D., "FMC Color-Difference Formulas: Clarification Concerning Usage," Journal, Optical Society of America, Vol. 61, 1971, pp. 118-122.

469

[41] S~ve, R., "New Formula for the Computation of CIE 1976 Hue Difference," Color Research and Application, Vol. 16, 1991, pp. 217-218.

[42] McDonald, R., "Industrial Pass/Fail Colour Matching," Journal, Society of Dyers and Colourists, Vol. 96, 1980; Part I, pp. 372-376; Part II, pp. 418-433; Part III, pp. 486-495.

[43] News: "CMC Colour-Difference Formula," Color Research and Application, Vol. 9, 1984, p. 250. [44] Special Metamerism Index: Change in Observer, Publication CIE No. 80, Central Bureau of the CIE, Vienna, 1989.

[45] Hunt, R. W. G., The Reproduction of Colour in Photography, Printing and Television, 4th ed., Fountain Press, Tolworth, England, 1988, distributed by Van Nostrand Reinhold, New York.

[46] Longley, W. V., "A Visual Approach to Controlling Metamerism," Color Research and Application, Vol. 1, 1976, pp. 43-49.

[47] Winey, R. K., "Computer Color Matching with the Aid of Visual Techniques," Color Research and Application, Vol. 3, 1987, pp. 165-167.

[48] Kumar, R., Billmeyer, F. W., Jr., and Saltzman, M., "Identification of Organic Pigments in Paints," Journal of Coatings Technology, Vol. 57, No. 720, 1985, pp. 49-54; see also Billmeyer, F. W., Jr., Saltzman, M., and Kumar, R., "Identification of Organic Pigments by Solution Spectrophotometry," Color Research and Application, Vol. 7, 1982, pp. 327-337. [49] Kuehni, R. G., Computer Colorant Formulation, D.C. Heath, Lexington, MA, 1975. [50] Judd, D. B. and Wyszecki, G., Color in Business, Science, and Industry, 3rd ed., Wiley, New York, 1975. [51] Allen, E., "Colorant Formulation and Shading," Chap. 7 in Color Measurement, F. Grum and C. J. Bartleson, Eds. Academic, New York, 1980, pp. 290-336. [52] Billmeyer, F. W., Jr. and Abrams, R. L., "Predicting Reflectance and Color of Paint Films by Kubelka-MunkAnalysis," Journal of Paint Technology, Vol. 45, No. 579, 1973; Part I, pp. 23-30; Part II, pp. 31-38. [53] Mudgett, P. S. and Richards, L. W., "Kubelka-Munk Scattering and Absorption Coefficients for Use with Glossy, Opaque Objects," Journal of Paint Technology, Vol. 45, No. 586, 1973, pp. 43-53. [54] Phillips, D. G. and Billmeyer, F. W., Jr., "Predicting Reflectance and Color of Paint Films by Kubelka-Munk Analysis. IV. Kubelka-MunkScattering Coefficient," Journal of Coatings Technology, Vol. 48, No. 616, 1976, pp. 30-36. [55] Rich, D. C., "Computer-Aided Design and Manufacturing of the Color of Decorative and Protective Coatings," Journal of Coatings Technology, Vol. 67, No. 840, 1995, pp. 53-60.

MNL17-EB/Jun. 1995

Gloss by Harry K. Hammond 1111 and Gabriele Kigle-Boeckler2

THE APPEARANCEOFANOBJECTor material can be described by its color and gloss characteristics. Like color, gloss can be subdivided into several aspects depending on viewing conditions. In 1937, Richard Hunter identified five aspects of gloss [1] and the functions of reflectance by which they could be evaluated. Latest studies by K. Lex [2] expanded Hunter's gloss terms and divided them into two groups. One group is based on visual observation with the eye focused on the surface of the material (Fig. 1). For the other group, the eye is focused on the image of the object reflected by the material (Fig. 2). However, experience has shown that no single objective measurement of gloss will provide perfect correlation with the integrated subjective appraisal of glossiness that the eye so quickly renders. For this reason, the gloss evaluation requirement of an object or material should first be examined and the most useful gloss measurement aspects then selected.

Specular Gloss Specular gloss is defined in ASTM E 284 [3] as the "ratio of flux reflected in specular direction to incident flux for a specified angle of incidence and source and receptor angular apertures" (Fig. 4). This aspect of gloss has been measured most frequently because it is the one for which an instrument is most easily constructed. In practice the divergence angles of source and receptor are precisely specified in ASTM Test Method for Specular Gloss (D 523) [4] as are the directional angles of incidence and reflection. Tolerances are specified for all angles. For simplicity, glossmeter geometries are identified by reference to the incidence angles, most frequently 20, 60, and 85~. However, the associated source and receptor aperture angles and their tolerances play a vital role in determining the reproducibility of instrument readings.

Sheen A S P E C T S OF GLOSS A N D T H E I R DEFINITION The simple term "gloss" is defined in an ASTM standard, Terminology of Appearance (E 284) [3] as "angular selectivity of reflectance, involving surface-reflected light, responsible for the degree to which reflected highlights or images of objects may be seen as superimposed on a surface." To indicate specific types of "angular selectivity," such as those involving specular gloss, sheen, or haze and to illustrate the difference between an evaluation where the focus is on the surface and one where the focus is on the reflected image, the complexity of the phenomenon "gloss" is illustrated in Fig. 3. By focusing on the reflected image of an object, an observer obtains information on how distinctly the object is reflected by the surface. A reflected light source may appear brilliant or diffuse depending on the specular gloss of the surface. The outline of a reflected object may appear distinct or blurred depending on image clarity. A halo surrounding the image of the reflected object is an indication of haze. Focusing on the surface of an object provides information on the size, depth, and shape of surface structures contributing to such things as waviness or directionality of brush marks. ~Consulting scientist, BYK-Gardner USA,2435 Linden Lane, Silver Spring, MD 20910. 2Technical marketing manager, gloss, BYK-Gardener USA, 2435 Linden Lane, Silver Spring, MD 20910.

Sheen is defined in ASTM E 284 [3] as "the specular gloss at a large angle of incidence for an otherwise matte specimen." The usual angle for sheen measurement is 85 ~ from the perpendicular to the specimen. This is about the maximum angle that can be used without encountering difficulty in positioning the optics to illuminate and view the specimen at neargrazing angles.

Haze Haze in coating films is often designated "reflection haze" because in plastics there is encountered a near-forward scattering in transmission that is designated transmission haze. ASTM E 284 [3] defines haze in reflection as "percent of reflected light scattered by a specimen having a glossy surface so that its direction deviates more than a specified angle from the direction of specular reflection."

Image C l a r i t y This aspect of gloss has often been referred to as "distinctness of image." It is essentially independent of haze or change in specular gloss. ASTM E 284 [3] defines distinctness-ofimage gloss as "the aspect of gloss characterized by the sharpness of images of objects produced by reflection at a surface." During visual observation, the sharpness of the light-dark edge of a reflected object can be observed. Image

470 Copyright9 1995 by ASTMInternational

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CHAPTER 4 1 - - G L O S S

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clarity is a critical parameter of glossy surfaces having only small amounts of waviness.

has m a n y small indentations that are perceived as a pattern of both highlighted and non-highlighted areas. This pattern is interpreted by an observer as a three-dimensional structure of hills and valleys.

Waviness One obvious type of waviness is designated "orange peel." ASTM E 284 [3] defines orange peel as "the appearance of irregularity of a surface resembling the skin of an orange." A surface m a y be described as exhibiting orange peel when it

Directionality ASTM E 284 [3] defines directionality, perceived, as "the degree to which the appearance of a surface changes as the

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surface is rotated in its own plane, under fixed conditions of illumination and viewing." A surface exhibits directionality when specular gloss measurements are a function of the direction for which measurements are made. When paint is applied by brushing in one direction, the brush marks can cause the surface to have a directional characteristic. Metallic materials frequently exhibit directionality when the surface is polished in one direction.

VISUAL G L O S S EVALUATION Early investigations of gloss were carried out by observing differences in the characteristics of images reflected in the surfaces of specimens. In 1932, the Detroit Paint and Varnish Production Club [5] reported on investigations carried out with their distinctness-of-image gloss comparator. In 1936, Hunter [6] reported an investigation where reflected images of a target pattern were observed.

FIG. 4-Reflected light flux distribution from a semigloss surface is depicted by a broken curve. Three lines are shown proceeding from the specimen surface. The center one depicts the specular direction, the image of a ray reflected from a mirror-like surface. The two other rays represent the range of angles passing through the aperture (AP) to the detector (shown as a rectangle).

CHAPTER 4 1 - - G L O S S

473

FIG. 5-"Landolt Rings." Broken rings of bvarious sizes are used to provide a scale for visual evaluation of the distinctness of surface-reflected images?

Only in comparatively recent times has there been a major effort to investigate visual scaling of gloss and to endeavor to develop correlation between visual and instrumental measurements. O'Donnell did a doctoral thesis on visual gloss scaling at Rensselear Polytechnic Institute, Troy, New York. Results were first presented, in part, at an ASTM Symposium in 1984 [7] and more fully in a journal article in 1987 [8].

Development o f a Documentary Standard In 1990, ASTM Committee D-1 on Paint and Related Coatings and Materials published ASTM Method for Visual Evaluation of Gloss Differences Between Surfaces of Similar Appearance (D 4449) [9] for making visual evaluations of gloss between surfaces of similar appearance. It uses two types of

light sources. One source consists of a tubular fluorescent desk lamp modified by placing a matte-black reflecting material behind the tubes and a coarse wire-mesh screen in front. The directions of illumination and view can be adjusted to be 20, 60, or 85 ~as desired for comparing specimens having high gloss, intermediate gloss, or sheen. The other light source is a clear-bulb incandescent lamp. Light from the selected source illuminates the specimens at the chosen angle. The sharpness of reflected images permits a subjective comparison of the relative gloss of similar surfaces.

Use o f Landolt Rings Landolt rings have been used by ophthamologists to evaluate visual acuity for nearly a hundred years [10]. The test

474

PAINT AND COATING TESTING MANUAL

FIG. 6-Schematic diagram of a gloss meter. Source is on left, detector on right. Lenses are used to provide beam control. Source aperture is designated AP1, receptor aperture AP2.

consists of locating the gaps in a graduated series of sizes of incomplete rings whose radial thickness and gap are equal to one fifth the diameter of the ring. For gloss evaluation of trans-illuminated rings, reflections are viewed on mylar. Rings have different sizes and different gap orientations. An image-gloss scale is associated with the different sizes of rings. An image-gloss scale ranging from 10 to 100 in steps of ten was established for eleven sizes of rings from the largest to the smallest. The development of the scale is not documented, but it ostensibility took place in the General Motors Automotive Division about January 1977. Visual observers select the smallest size of ring for which they can call the gap orientation correctly. The numerical size of the rings is used as an inverse index of distinctness-of-image gloss (Fig. 5). ASTM has not published a method for visually evaluating distinctness-of-image gloss by using Landolt rings, but equipment for this purpose is available?

Visual Evaluation of Orange Peel The automotive industry established a physical standard for orange peel consisting of ten high-gloss panels with various degrees of orange peel structure. 4 The panels are visually ranked from 1 to 10 with Panel Number 1 depicting very pronounced orange peel and Panel Number 10 illustrating no orange peel. The visual observer can use these panels as a supportive tool to evaluate degree of orange peel. 3Apparatus for evaluating distinctness of image using Landolt rings, available from Paul N. Gardner Co., Pompano Beach, FL. 4Set of orange-peel panels can be obtained from Advanced Coatings Technology, 273 Industrial Dr., Hillsdale, MI 49242.

INSTRUMENTAL MEASUREMENT TECHNIQUES Specular Gloss Measurement The design of many gloss meters is based on the precise measurement of the specular component of reflected light. A light source, usually a small filament incandescent lamp, is placed at the focal point of a collimating lens. The axis of the collimated beam is set to the desired angle of illumination. A receptor lens with an aperture in the focal plane followed by an illumination detector completes the basic optical design (Fig. 6 and Fig. 7). The size of the receptor aperture and the size of the source image in that aperture are the elements that complete the optics and that determine the high, low, and intermediate gloss scale readings. The specular angle largely determines the magnitude of the reflected light. However, the tolerances assigned to the source and receptor apertures are what determine the accuracy and reproducibility of measurements made with instruments having the same angles of illumination and view. Periodic calibration or verification of instrument performance requires the use of calibrated gloss standards. For permanence they should be made of glass or ceramic material. ASTM D 523 recommends the use of a primary standard of polished black glass of known refractive index for which the Fresnel (specular) reflectance [11] has been computed for the angle of incidence of the geometry for which the instrument is designed. Since about 1990, national standardizing laboratories have preferred a primary reference stan-

CHAPTER 41--GLOSS

FIG. 7-Photograph of a modern miniature glossmeter. Note size of the hand holding the instrument?

FIG. 8-Diagram depicting the positions of source and receptor for the three geometries of ASTM D 523.

SApparatus designated "micro-TRLgloss," available from BYKGardner USA, Silver Spring, MD.

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PAINT AND COATING T E S T I N G M A N U A L

dard made of a wedge of clear quartz. Polished black glass working standards can be calibrated by direct comparison of their reflectances with that of a quartz wedge. Experience has shown that a single measurement geometry, such as 60 ~, m a y not provide instrument readings of gloss that correlate well with visual observations when comparing different gloss levels. This is why ASTM D 523 [4] provides for measurement at three different angles of incidence, namely 20, 60, and 85 ~ (Fig. 8). Each of the three geometries uses the same source aperture, but a different receptor aperture. The choice of geometry depends on whether one is: (1) making a general evaluation of gloss, (2) comparing high-gloss finishes or (3) evaluating low-gloss specimens for sheen. ASTM D 523 [4] states that the 60 ~ geometry is used for intercomparing most specimens and for determining when the 20 or 85 ~ geometry may be more applicable. The 20 ~geometry is advantageous for comparing specimens having 60 ~ gloss values higher than 70. The 85 ~geometry is used for comparing specimens for sheen or near-grazing shininess. It is most frequently applied when specimens have 60 ~ gloss values lower than 10. The ASTM d o c u m e n t a r y gloss standard originally published in 1939 contained only the 60 ~ geometry [12]. The desirability of using an auxiliary geometry of 85 ~ for sheen evaluation was recognized shortly afterward. However, the use of another geometry with smaller angles of incidence and view, such as 20 ~ and a smaller receptor aperture to provide improved differentiation of high-gloss finishes was not published until 1947 [13]. The three geometries, 20, 60, and 85 ~, were originally published as separate ASTM standards. In 1953, ASTM D 523 was revised to incorporate all three geometries, and it still does. Meanwhile the Paint Committee of the International Organization for Standardization, ISO TC-35, was investigating gloss measurements with various commercial instruments prior to drafting an international standard. A paper documenting what the committee had been doing was published in 1976 by the Chairman of the Gloss Task Group, Dr. Ulrich Zorll [14]. In 1978, the ISO Paint Committee published ISO 2813, essentially an international version of ASTM D 523 [15]. In keeping with the usual ISO procedure, the standard was made available in English, French, and German. Instrument manufacturers report that measurement precision, reproducibility, and data-handling capabilities of gloss meters have been improved markedly in recent years. New instruments have been designed that are smaller, more portable, and more convenient to use. Data storage and analysis are frequently included as well as the capability of electronically transferring data to a personal computer [16,17].

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Goniophotometry Gonio means angle, and photometer means light; so a goniophotometer is an instrument for measuring the angular distribution of reflected or transmitted light. This type of instrument is used regularly in the research laboratory to investigate the distribution of light flux (Fig. 9). A goniophotometer with appropriate apertures can also be used to provide gloss data for a wide variety of angles and apertures. When goniophotometric measurements are desired,

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reference should be m a d e to ASTM Practice for Gon i o p h o t o m e t r y of Objects a n d Materials (E 167) [18], Analysis of g o n i o p h o t o m e t r i c curves was t r e a t e d by Nimeroff [19].

Measurement of Reflection Haze Haze is a gloss p a r a m e t e r that causes a high-gloss surface to a p p e a r milky and lighter, thereby losing contrast. W h e n visually evaluating the reflected image of a hazy object, one observes halos a r o u n d distinct reflection outlines caused by scattered light. Haze can result from various m a t e r i a l or process p a r a m e t e r s such as degree of dispersion, flocculation, i n c o m p a t i b i l i t y of raw m a t e r i a l s (pigment-additiveresin) o r from p o o r a p p l i c a t i o n procedures. Microscopic irregularities cause small a m o u n t s of reflected light to be scattered adjacent to the direction of specular reflection or to modify the refractive index of the m a t e r i a l just b e n e a t h the

each side of the s p e c u l a r r e c e p t o r a p e r t u r e (Fig. 10). ASTM Test Methods for M e a s u r e m e n t of Gloss of High Gloss Surfaces by G o n i o p h o t o m e t r y (E 430) [20] describes two methods for evaluating reflection haze, one at 20 ~, a n o t h e r at 30 ~. An i n s t r u m e n t using a 30 ~ angle has been c o m m e r c i a l l y available for m o r e than.15 y e a r s ] Since a b o u t 1992, a 20 ~ laboratory gloss m e t e r has been e q u i p p e d with auxiliary a p e r t u r e s for haze evaluation, thus p e r m i t t i n g m e a s u r e m e n t of 20 ~ specular gloss and haze with the s a m e i n s t r u m e n t (Fig. 11).

Measurement of Image Clarity

Haze is m o s t often associated with high-gloss surfaces when small a m o u n t s of reflected light are scattered in a region 1 to 4 ~ from the direction of specular reflection. Therefore, it is useful to place apertures several degrees wide on

A variety of different technologies is c o m m e r c i a l l y available to m e a s u r e i m a g e clarity [21-24]. The two m o s t often used principles are: (1) m e a s u r e m e n t of the c o n t r a s t in dep e n d e n c e of the wavelength of the transferred sinus wave (fourier-transformation) resulting in a m o d u l a t i o n transfer function o r (2) evaluation of the steepness of the reflection indicatrix. ASTM E 430 [20] describes the design of a commercially available i n s t r u m e n t b a s e d on the evaluation of the reflection indicatrix (Fig. 12). The i n s t r u m e n t illuminates the s p e c i m e n at a 30 ~ angle a n d m e a s u r e s the light reflected at 0.3 ~ from the s p e c u l a r angle with an a p e r t u r e of 0.3 ~ width. An a t t e m p t was m a d e to correlate the surface waviness of coatings with i m a g e clarity m e a s u r e m e n t s [23,25]. However, different surface profiles provided nearly the s a m e i m a g e clarity readings [21].

6Apparatus designated "haze-gloss," available from BYK-Gardner USA, Silver Spring, MD.

7Apparatus designated "Dorigon," available from Hunter Associates Laboratory, Inc., Reston, VA.

surface.

478

PAINT AND COATING TESTING MANUAL

FIG. 13-Photograph of an orange-peel-measuring instrument?

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8Apparatus designated "wave-scan," available from BYK-Gardner USA, Silver Spring, MD.

CHAPTER 4 1 - - G L O S S

Measurement of Waviness (Orange Peel) The p h e n o m e n a of waviness is most observable on a glossy surface, a critical appearance p h e n o m e n a in the automotive industry. Waviness has been evaluated by visual m e a n s a n d by use of a profilometer. The correlation between profilometry m e a s u r e m e n t s a n d visual perception is satisfactory for surfaces with similar optical properties. The operation of a profilometer, however, is very time c o n s u m i n g a n d limited to laboratory use. W h e n the eye of an observer is focused o n a painted surface, various types of waviness can be identified that involve size, structure, a n d shape. Variations in process or material parameters c a n cause differences in surface structure. For example, poor flow or levelling properties of a coating will usually cause a long wave structure often called orange peel. Changes i n substrate roughness, o n the other hand, will exhibit a shortwave structure of higher frequency. Because waviness is often caused o n the p r o d u c t i o n line, it is i m p o r t a n t to control it there. After considerable research, a n i n s t r u m e n t was produced in 1992 that will provide a n objective evaluation of waviness [26] (Fig. 13). A diode laser source is used to i l l u m i n a t e the specimen at 60 ~. The reflected light intensity is evaluated at the specular angle. During the m e a s u r e m e n t the i n s t r u m e n t is moved along the surface for a distance of a b o u t 10 cm. The intensity of the reflected light is a m a x i m u m w h e n c o m i n g from a valley or peak of a n orange peel element. The detector receives less light from the slopes (Fig. 14). The h u m a n eye c a n n o t resolve the actual heights of the structural elements of a p a i n t e d surface (2 to 4/xm), b u t the contrast between light a n d dark areas provides a n i m p r e s s i o n of depth. The contrast of a surface structure can be expressed by use of the statistical p a r a m e t e r "variance." The final m e a s u r e m e n t results are divided into a short wave c o m p o n e n t (structure size between 0.06 a n d 1 ram) a n d a long wave c o m p o n e n t (structure size between 1 a n d 10 ram) using a n electronic filtering procedure. This evaluation reflects the s i m u l a t i o n of the visual perception at different distances a n d permits categorizing structure sizes with their causes.

REFERENCES [1] Hunter, R. S., "Methods of Determining Gloss," NBS Research Paper RP 958, Journal of Research, National Bureau of Standards, Vol. 18, No. 77, January 1937, p. 281. [2] Lex, K., "Die erweiterte Glanzmessung und die Messung von Oberflaechenstrukturen," Pruftechnik bei Lackherstellung und Lackverarbeitung, Vincentz Verlag, Hannover, Germany 1992, pp. 70-74. [3] ASTM Standard E 284, Terminology of Appearance, Annual Book of ASTM Standards, Vol. 06,01, American Society for Testing and Materials, Philadelphia, PA, 1994. [4] ASTM Standard D 523, Test Method for Specular Gloss, Annual Book of ASTM Standards, Vol. 06.01, American Society for Testing and Materials, Philadelphia, PA, 1994.

479

[5] The Detroit Club, "Accurate Gloss Measurement by a Practical Means," Scientific Section Circular, No. 423, National Paint, Varnish, and Lacquer Association (NAPVA), 1932. [6] Hunter, R. S., "Gloss Investigations Using Reflected Images of a Target Pattern," Journal of Research, National Bureau of Standards, JRNBS, Vol. 16, 1936, pp. 359-366. [7] O'Donnell, F. X. D. and Billmeyer, F. W., Jr., "Psychometric Scaling of Gloss," Review and Evaluation of Appearance: Measurements and Techniques, ASTM STP 914, American Society for Testing and Materials, Philadelphia, PA, 1986, pp. 14-32. [8] Billmeyer, F. W., Jr. and O'Donnell, F. X. D., "Visual Gloss Scaling and MultidimensionalScaling Analysis of Painted Specimens," Color Research and Application, Vol. 12, No. 6, December 1987, pp. 315-326. [9] ASTM Standard D 4449, Test Method for Visual Evaluation of Gloss Differences Between Surfaces of Similar Appearance, Annual Book of ASTM Standards, Vol. 06.01, American Society for Testing and Materials, Philadelphia, PA, 1994. [10] Landolt, E., "Nouveaux Opto-types pour la determination de l'acuite visuelle," Archives d'Ophthalmologie, Vol. 19, 1899, pp. 465-471. [11] Fresnel, A., "Calcul des Tientes que Polarisation Developpe dan Lames Cristallesees," Annal Chemie et Physic, Vol. 17, 1821, p. 312. [12] Hunter, R. S. and Judd, D. B., "Development of a Method of Classifying Paints According to Gloss," ASTM Bulletin, No. 97, 1939, p. 11. [13] Homing, S. C. and Morse, M. P., "The Measurement of Gloss of Paint Panels," Official Digest, Federation of Paint and Varnish Production Clubs, ODFPA, Vol. 19, 1947, pp. 153-160. [14] Zorll, U., "Progress Towards International Agreement on Gloss Measurement of Paint Films," Journal of the Oil and Color Chemists Association, Vol. 59, 1976, pp. 439-442. [15] ISO 2813, Paints and Varnishes--Measurement of Specular Gloss of Non-Metallic Paint Films at 20, 60 and 85, International Organization for Standardization. [16] New Product, "The Micro-Gloss Family," Color Research and Application, Vol. 15, No. 4, August 1990, p. 242. [17] Paint Red Book, Communication Channels, Inc., 6255 Barfield Road, Atlanta, GA 30328. [18] ASTM Standard E 167, Practice for Goniophotometry of Objects and Materials, Annual Book of ASTM Standards, Vol. 06.01, American Society for Testing and Materials, Philadelphia, PA, 1994. [19] Nimeroff, I., "Analysis of Goniophotometric Reflection Curves," Journal of Research, National Bureau of Standards, JRNBA, Vol. 48, No. 5, pp. 441-448; also Journal, Optical Society of America, JOSA, Vol. 42, 1952, pp. 579-583. [20] ASTM Standard E 430, Test Methods for Measurement of Gloss of High-Gloss Surfaces by Goniophotometry, Annual Book of ASTM Standards, Vol. 06.01, American Society for Testing and Materials, Philadelphia, PA, 1994. [21] Czepluch, W., "Visuelle und messtechnische Oberflaechencharakterisierung dnrch Glanz," Industrie-Lack, Vol. 58, No. 4, 1990, pp. 149-153. [22] International Standard ISO 10 215, Anodized aluminum and its alloys--Visual method of image clarity of anodic oxidation coatings. [23] Loof, H., "Goniophotometry with the Zeiss GP 2," Journal of Paint Technology, Vol. 38, No. 501, 1966, pp. 632-639.

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PAINT AND COATING TESTING MANUAL

[24] Matsuta, M., Kito, K., and Kubota, T., "New Portable Orange Peel Meter for Paint Coatings," Proceedings, Williamsburg Conference, 8-11 Feb. 1987, pp. 25-28. [25] Ladstaedter, E. and Gessner, N., "Die quantitative Erfassung yon Reflexionsvermoegen, Verlaufsqualitaet und Glanzschleier

mit dem Gonioreflektometer GR-COMP," Farbe und Lack, 1985, Nr. 1 l, 1979, pp. 920-924. [26] New Product, "Wave-Scan for the Measurement of Surface Structure," Color Research & Application, Vol. 18, No. 1, February 1993, p. 69.

MNL17-EB/Jun. 1995

42

Hiding Power by Leonard Schaeffer 1

1. CONCEPTS, RELATIONSHIPS, TERMINOLOGY

e. Standard Test Substrates

a. Opacity When light enters a paint film, some or all of it is absorbed or reflected by the film before reaching the substrate, thereby hiding the substrate to a lesser or greater degree. The light that reaches the substrate is partly absorbed by it and partly reflected back in conformance with the substrate's visual pattern. Reflection from the substrate eventually emerges from the film carrying the substrate reflectivity information perceived as visibility or lack thereof and referred to as hiding. Opacity m a y he qualitatively defined as the property of a paint film that enables it to prevent the passage of light and thereby to hide the substrate on which it has been applied. Note that opacity is a film property, whereas hiding power is a property of the whole paint. Hiding is a more general term used frequently to refer to either opacity or hiding power.

An opacity test substrate generally has an ordered pattern of contrasting colors or shades, usually black and white, although black and grey and grey and white are also used. Juxtaposition of contrasting areas permits both visual observation and photometric measurement of film opacity. For photometric measurements only, individual black glass and white glass panels are sometimes employed to take advantage of their excellent planarity. Clear plastic can also be used as an opacity test substrate by placing it over black-and-white backgrounds. Standard black-and-white opacity test substrates are defined in paint technology as having CIE-Y reflectances of 0.01 (1%) m a x i m u m and 0.80 (80%), respectively. 2 White test areas are seldom exactly 80%, but equations are available for correction of photometric values to that standard (see Eqs 1 and 2).

L Contrast Ratio The extent to which a paint film obscures or hides the contrasting features of the test substrate on which it is uniformly applied is the measure of its opacity. This is expressed photometrically as the ratio of the luminous (CIE-Y) reflectance over the darker to that over the lighter area of the substrate, which is referred to as the contrast ratio (CR). The Y-reflectance is employed because this CIE parameter is designed to match the sensitivity of the h u m a n eye. The CR and the reflectance are expressed as a percentage or as a decimal fraction, the latter to be assumed unless otherwise indicated. A CR value of unity indicates that too little light has reached the substrate for the substrate reflectance characteristics to have a measurable effect on the emergent light flux; thus, there is complete absence of contrast, or complete hiding. Lesser CR values define intermediate levels of contrast, or incomplete hiding. The contrast ratio of a given paint film varies with substrate reflectances and therefore has significance only with respect to a known substrate and primarily to a standard black-and-white substrate as defined in 1.e. In practice, the white area of a commercially available blackand-white substrate normally deviates somewhat from the ideal reflectance of 8 0 % whereas the black area is normally 1% or less, which has no measurable effect on test results and is therefore treated mathematically as having zero reflec-

b. Light Absorption If most of the light is absorbed by the film before reaching the substrate, the film is dark in color and hides the substrate well, in which case hiding has been produced by light absorption.

c. Light Scattering If most of the light entering the film is reflected back and reemerges without having reached the substrate, the film is white or light in color and hides the substrate well. The reflection mechanism of the film involves multiple internal refractions and reflections that scatter the light to produce a net reversal in its direction. Hiding in this case is produced by light scattering.

d. Incomplete Hiding In all cases, the light-absorbing and light-scattering properties of the film act together to produce its opacity. If the film is low in both light-absorption and light-scattering ability, m u c h of the light reaches the suhstrate. Such a film therefore hides poorly and is characterized as being low in opacity.

2CIE = Commission Internationale d'Eclairage. Reflectances are

measured with specular (mirror) reflection excluded.

1The Leneta Company, 15 Whitney Road, Mahwah, NJ 07430.

481 Copyright9 1995 by ASTM International

www.astm.org

482

PAINT AND COATING TESTING MANUAL

tance. Conventional symbols used in this c o n n e c t i o n are as follows: W = the reflectance of the white a r e a of the test substrate, R~ = the reflectance of the p a i n t film over a white a r e a of reflectance W, R0 = the reflectance of the p a i n t film over the black area, C~ = Ro/Rw, the CR of the a p p l i e d p a i n t film, C0.80 = Ro/Ro.8o, the CR w h e n W = 0.80, a n d C = C0.80, the a b b r e v i a t i o n s o m e t i m e s used in equations. In careful hiding p o w e r m e a s u r e m e n t s , if the white s u b s t r a t e reflectance deviates m o r e t h a n 0.01 from the s t a n d a r d value of 0.80, one of the following c o r r e c t i o n equations 3 is employed C0.80 = f(Ro, Rw, W) =

WR0(1 - 0.80R0) (1) R o ( W - 0.80) + 0.80Rw(1 - WRo)

Co.so = f(Cw, Ro, W) -

WCw(1 - 0.80R0) (2) Cw(W - 0.80) + 0.80(1 - WRo)

g. Visual Observations of Contrast Although i n t e r m e d i a t e levels of c o n t r a s t c a n n o t be directly quantified by visual means, the eye is qualitatively very sensitive to c o n t r a s t variations. It can identify equalities o r n e a r l y c o m p l e t e a b s e n c e of c o n t r a s t with c o n s i d e r a b l e precision, w h i c h is the basis for the original as well as several c u r r e n t hiding p o w e r m e t h o d s to be described. Indeed, such visual observations are the b a s i c criteria of w h a t constitutes hiding a n d h i d i n g power, to w h i c h all i n s t r u m e n t a l hiding m e a s u r e m e n t s trace t h e i r validity.

h. Film Thickness This is usually expressed in t h o u s a n d t h s of an inch (mils) o r in m i c r o m e t e r s (~m). A liquid p a i n t usually contains a s u b s t a n t i a l q u a n t i t y of volatiles, so that its d r y film thickness (DFT) is substantially less t h a n the original wet film thickness (WFT). The W F T of a r c h i t e c t u r a l p a i n t s a p p l i e d in the field are typically in the n e i g h b o r h o o d of 3 to 4 mils (75 to 100/~m). W i t h o t h e r coating types, it m i g h t b e as low as 1 mil (25/~m) or as high as 60 mils (1500/zm). W i t h volatile-free liquid coatings, the W F T a n d DFT are the s a m e except for a possible small increase in density d u r i n g curing. W i t h powd e r coatings, for w h i c h film f o r m a t i o n a n d curing are concurrent, the t e r m W F T is i n a p p l i c a b l e a n d DFT r e d u n d a n t , so t h a t it is a p p r o p r i a t e to refer simply to film thickness.

SR is usually expressed in square feet p e r p o u n d (ft2/lb) o r square metres p e r k i l o g r a m (m2/kg). SR is inversely related to the film thickness; thus, for a given paint, the lower the SR, the higher the film thickness a n d film opacity.

j. Spreading Rate and Film Thickness Relationships Let H= T= D= t=

s p r e a d i n g rate of the coating (equivalent to SR), 4 wet film thickness (equivalent to WFT), 4 coating density (prior to loss of volatiles), d r y film thickness (exclusive of air) s (equivalent to DFT), d r y film density ( d i s p l a c e m e n t density), 5 d N = nonvolatile fraction by weight (equivalent to NVW), and N v = nonvolatile fraction by volume (equivalent to NVV). (1) U.S. U n i t s H(flZ/gal) • T(mil) = 1604.2

(3)

H (fl2/gal) • t (mil) = 1604.2ND + d

(4)

H(ftZ/lb) • T(mil) = 1604.2 + D (lb/gal)

(5)

H (flZ/lb) x t (mil) = 1604.2N + d(lb/gal)

(6)

(2) M e t r i c U n i t s 6 H(mZ/L) x T(p~m) = 1000

(7)

H(m2/L) x t (p~m) = 1000ND + d

(8)

H(m2/kg) x T(/~m) = 1000 -: D(kg/L) H(mZ/kg) • t (/zm) = 1000N § d (kg/L)

(9) (10)

(3) U . S . - - M e t r i c Unit C o n v e r s i o n s n (ft2/gal) = H (mZ/L) • 40.746 H (ft2/lb) = H (m2/kg) • 4.8882

(11) (12)

T(mil) = T(txm) + 25.4

(13)

D (Ib/gal) = D (kg/L) X 8.3454

(14)

(4) D r y V e r s u s W e t F i l m R e l a t i o n s h i p s 7 ND = N~cl

(15)

t = N~T

(16)

T = td + ND

(17)

2. D E F I N I T I O N OF H I D I N G P O W E R ( H P ) i. Spreading Rate W h e n p a i n t is applied, w h e t h e r for test p u r p o s e s o r in actual usage, the a r e a covered p e r unit q u a n t i t y of p a i n t is called the s p r e a d i n g rate (SR) for that p a r t i c u l a r application. W h e n the q u a n t i t y of coating is expressed volumetrically, as is usual with liquid paints, the SR is usually expressed in square feet p e r gallon (ft2/gal) o r square metres p e r litre (m2/L). W h e n the quantity is expressed gravimetrically, the 3Derived from Eq 39 by equating to W = 0.80.

Qualitatively, h i d i n g p o w e r is the p r o p e r t y of a p a i n t t h a t is m a n i f e s t e d as o p a c i t y in its films. Quantitatively, it is the 4Note that WFT and SR, when the latter is expressed by volume, are inverse ways of stating the same information. 5Refers to films containing no air or hypothetically compressed to exclude air. 6The following metric notations are identities: kg/L = g/mL = g/cm 3 = g/cc. 7These are applicable to both common and metric units since the units all cancel.

CHAPTER 42--HIDING POWER 483 spreading rate at which the film opacity is just sufficient to give complete hiding over the specified standard black-andwhite substrate (see 1.e). The "complete hiding" point is determined visually in some test procedures and photometrically in others.

a. Visual Hiding P o w e r End-Point In the visual determination of hiding power, the operator increases the film thickness gradually and records the amount of paint applied at the supposedly exact point of complete hiding. In practice, instead of perceiving such a point, a range of uncertainty is reached beyond which, when hiding seems unquestionably complete, it also seems that the true end-point has been exceeded. To resolve this dilemma and to obtain repeatable results, the operator chooses an endpoint at which it seems that, only a negligible increase in film thickness is required to completely obscure the contrasting features of the substrate. This so-called complete hiding endpoint is therefore more accurately described as just short of complete hiding.

b. Photometric Hiding Power End-Point Uncertainty as to the end-point also exists when measuring hiding power photometrically. A curve of film thickness versus contrast ratio approaches CR = 1 asymptotically, so in theory there is no point at which the contrast is completely obscured. Thus in practice the CR end-point for hiding power measurements must be less than unity. A CR value of 0.98 is generally accepted in paint technology as representing the point of photometric complete hiding because it is in fact very close to being visually complete, and a higher CR end-point could not be identified with as much precision. The concept of 98% as the contrast ratio for complete hiding was originally based on the Weber-Fechner law, which states that differences of 2% in reflectance (with moderate illumination) are imperceptible to the h u m a n eye [1]. Actually, this level of contrast, though slight, is definitely visible.

3. T H E R O L E OF P I G M E N T S I N H I D I N G POWER a. Binders and Pigments A typical paint binder, by itself, forms a transparent and virtually colorless film that neither absorbs nor scatters light to any appreciable degree and therefore makes no contribution to the hiding power of the coating of which it is a part. This task resides entirely in the pigment constituent of the paint. Pigments are fine-particle-size, insoluble, and usually crystalline solids that when dispersed in paint vehicles contribute to the various properties of the mixture, among which are the optical properties of color and hiding power. Pigments that absorb light strongly over the entire visible spectrum are black; those that are optically selective, absorbing strongly in parts of the visible spectrum and poorly in other parts, are colored, viz. blue, red, yellow, etc., corresponding to the spectral region of nonabsorption. Those that absorb poorly over the entire visible spectrum are white.

b. White Pigments When dispersed in a paint binder, some white pigments scatter light strongly and thereby contribute to hiding, while others scatter very poorly and make little if any contribution. On that basis white pigments are classified as hiding pigments or as extenders. White hiding pigments in a paint formulation are sometimes called "prime pigments" as distinguished from the nonhiding "extender" types. The latter are also referred to as "inerts" in view of their apparent passivity with regard to both light absorption and scattering. The difference in scattering behavior between hiding and extender pigments is a function of their refractive indices.

c. Refractive Index Most pigments are crystalline in nature. If a single crystal of white pigment were grown sufficiently large, it would be perceived as shiny and transparent like glass, and objects observed through it would look bent and distorted as when observed through a glass prism. This is due to the change in direction, referred to as refraction, that occurs when light passes between media in which it has different velocities, as illustrated in Fig. 1. The relationship between the angles in this figure is expressed by Snell's law of refraction n = sin//sin r

(18)

in which i and r are the angles of incidence and refraction, respectively, and n is a constant referred to as the refractive index, which is the ratio of the velocity of light in the incident to that in the refraction medium. If the large pigment crystal postulated previously is pulverized and dispersed in a paint film, each small particle will refract incident light in the same way as described for the large one. Light will also be partially reflected at the surface, and both refractions and reflections will occur within the pigment particle itself. This activity, endlessly repeated with a multitude of pigment particles as illustrated in Fig. 2 (Ref 2, p. 1), results in the scattering of the original incident light with concomitant film opacity and paint hiding power. The greater the difference between the refractive indices of the pigment and the surrounding medium, the greater the amount of light scattering. Refractive

I

of

!

\

I FIG. 1-Bending of a light ray by refraction towards the normal as it enters a medium of lower light velocity (higher refractive index).

484

P A I N T A N D COATING T E S T I N G M A N U A L

d. White Hiding P i g m e n t s A r o u g h i n d i c a t i o n of the relative hiding p o w e r of a white p i g m e n t can be calculated from its refractive index using the Fresnell equation of reflectivity (Ref 2, p. 1), F%-

( n - nm) 2 • 100 (n + rim) 2

(19)

w h e r e F is the Fresnell reflectivity, n is the refractive index of the pigment, a n d nm is the refractive index of the m e d i u m in which it is dispersed. Since the refractive index of a p a i n t b i n d e r is in general very close to 1.5, Eq 19 can be rewritten as

['~

(n + 1.5) 2

• 100

(20)

Tables 1 a n d 2 illustrate the use of this equation a n d the general principle that the h i g h e r the refractive index of a p i g m e n t the greater its hiding power. The relative hiding p o w e r values shown therein indicate the m a g n i t u d e of variation related to index of refraction. Other factors can also affect hiding p o w e r substantially, as discussed in Section 9.

FIG. 2-Light-scattering behavior of a pigmented film.

indices are r e p o r t e d in Tables 1 a n d 2 with respect to a v a c u u m as the m e d i u m of incidence. Values with respect to air are practically the same. Since white p i g m e n t s are crystalline in nature, they usually possess different refractive indices along the different crystal axes. Their indices also vary somew h a t with the wavelength of the light, generally being higher at the blue (short wave length) end of the s p e c t r u m t h a n at the red (long wavelength) end. Tables 1 a n d 2 give average values [1,3].

TABLE 1--Refractive index and relative hiding power of some white hiding pigments. Pigment

Refractive Index

Relative HP, F%a

Titanium dioxide (ruffle) Titanium dioxide (anatase) Zirconium oxide Zinc sulfide Antimony oxide Zinc oxide White lead carbonate White lead sulfate Lithopone

2.76 2.55 2.40 2.37 2.19 2.02 2.01 1.93 1.84

8.8 6.7 5.3 5.0 3.5 2.2 2.1 1.6 1.0

~Calculatedfrom Eq 20.

TABLE 2--Refractive index and relative hiding power of some extender pigments. Pigment

Refractive Index

Relative HP, F%a

Barium sulfate Calcium sulfate Calcium carbonate Magnesium silicate Aluminum silicate Silica

1.64 1.59 1.57 1.57 1.55 1.55

0.20 0.08 0.05 0.05 0.03 0.03

aCalculated from Eq 20.

(n -- 1.5) 2

e. E x t e n d e r P i g m e n t s Pigments in this category have low refractive indices in the n e i g h b o r h o o d of 1.5. In the form of a powder, with the surr o u n d i n g m e d i u m being air with a refractive index of 1.0, the difference in the two indices p r o d u c e s substantial light scattering, so that extender p i g m e n t s look white. But d i s p e r s e d in p a i n t binders, w h i c h like themselves typically have a value of a b o u t 1.5, they scatter light very p o o r l y and are virtually transparent. This is i n d i c a t e d by the low HP values listed for t h e m in Table 2 as c o m p a r e d with the white hiding p i g m e n t s in Table 1. Although extender p i g m e n t s are also referred to as "inerts," the latter t e r m is s o m e w h a t of a m i s n o m e r . They have an indirect b u t strong influence on light scattering a n d hiding p o w e r t h r o u g h p h e n o m e n a referred to as "crowding" and "dry hiding." They also have i m p o r t a n t effects on o t h e r physical p r o p e r t i e s of paints such as consistency a n d gloss.

f. Colored P i g m e n t s If a p i g m e n t a b s o r b s s o m e wavelengths of light m o r e strongly t h a n others, it reflects b a c k a higher p r o p o r t i o n of the weakly a b s o r b e d wavelengths a n d is perceived as having the color of the latter (e.g., red, blue, yellow, etc.). Light a b s o r p t i o n of this n a t u r e is referred to as "selective." Colored p i g m e n t s can vary greatly in hiding p o w e r d e p e n d i n g on their light a b s o r p t i o n a n d light-scattering abilities. W i t h r e g a r d to light scattering, as with white p i g m e n t s this is a function of the refractive index or m o r e specifically the difference in refractive index b e t w e e n the p i g m e n t a n d s u r r o u n d i n g medium. Refractive indices of colored p i g m e n t s vary widely with wavelength, ranging from 1.3 to 2.7. These variations cause such p h e n o m e n a as bronzing, dichroism, color change with film thickness, a n d differences in u n d e r t o n e u p o n dilution with white p i g m e n t s (Ref 4, p. 22).

CHAPTER 42--HIDING POWER

485

4. EARLY VISUAL H I D I N G P O W E R METHODS a. Brushouts The earliest methods for determining hiding power employed the practical procedure of brushing the paint uniformly onto combination black-and-white test substrates, increasing the amount of paint in small increments until reaching the point of essentially complete hiding at which the amount of visual contrast was considered negligible. The quantity of paint was determined by weighing the container and brush in grams before and after painting. The corresponding spreading rate (SR) is the hiding power (HP) by definition and was calculated from the equation SR (ft2/gal) = Test Area (ft2) • Paint Density (lb/gal) x 454 Weight of Paint (g)

(21)

For single-pigment paints, the value calculated from Eq 21 can be converted to pigment hiding power using the equation SRpigment (ftZ/lb) -

SRpaint (ftZ/ga]) (22) Pigment Concentration (lb/gal)

FIG. 3-Gardner contrast hiding power board.

Variants of these equations provide for the use of metric instead of U.S. units.

black to white. After printing, a coat of nitrocellulose lacquer or other suitable clear sealer was applied. Many of those chart types became and continue to be commercially available.

b. Early Test Substrates

c. Contrast Design and Visual Sensitivity

Originally in the study of hiding power, test surfaces were prepared in individual laboratories by painting black stripes on white-painted panels. In response to the need for standardized test surfaces, studies were made on oil cloth and linoleum having printed checkerboard-type designs [5]. The Gardner Contrast Hiding Power Board was a two-square-foot area glass checkerboard with black and white squares painted on the underside of a thin piece of glass (Fig. 3). The first formalized ASTM method used a linoleum checkerboard surface in the brushout test procedure described in 4.a. The Gardner glass board was used in the same way. Since the "complete hiding" end point in those early methods was determined when the paint was freshly applied and still wet, the resultant hiding power value pertained only to the wet hiding power of the paint, not to the dry. This was not a problem in the earliest days of hiding power measurement when typical paints contained relatively little volatile constituent and the opacity of the film therefore did not change markedly while drying. But, with the advent of modern paint formulations containing substantial amounts of volatiles, the composition and with it the opacity of the dry paint film could be substantially different than that of the initially applied film. The need to measure dry hiding power therefore became of paramount importance. As a practical problem in this connection, expensive linoleum and glass test surfaces had to be cleaned for reuse after each test, which made it very difficult to use them for the study of dry hiding power. This problem was partly overcome with the introduction of paper test charts circa 1931 that were printed in various designs such as checkerboard, concentric diamond-shaped bands, spirals, crescents, etc., and with various degrees of contrast such as black-white, black-grey, grey-white, and a graded series of stripes from

Kraemer and Schupp [6] evaluated contrast surfaces in a variety of designs prepared on glossy photographic paper. These included the customary checkerboard design, a design of narrow 15-ram-wide bands, another with much broader bands, and one with dark half circles on a light background. The results seemed to favor a narrow band design subsequently employed by the Krebs Pigment Co. in preparing the diamond stripe grey and white contrast charts illustrated in Fig. 4. The test area of that chart was 1 ftz (0.0929 mE). The use of a grey-and-white contrast combination was based on the idea that this is more representative than black and white of the degree of contrast encountered by paints in actual use.

d. Relative Dry Hiding Power--Krebs Method Although the introduction of paper test charts as replacements for linoleum or glass made dry hiding power measurements easier, they were still not easy enough. The problem was that it required the preparation of a considerable number of paint-outs at various spreading rates to obtain one that after drying could be identified with confidence as representing the "complete hiding" end-point. One solution to this problem was to determine comparative or relative dry hiding power. In the Krebs Pigment Co. method, their square-foot grey-and-white diamond stripe chart was used for that purpose in the following manner: A partial hiding ladder of six to eight brushout standards is made by applying a standard paint at spreading rates ranging from 400 to 800 ftE/gal (10 to 20 m2/L) and allowing the brushouts to dry. The spreading rates are precontrolled approximately by syringing specified volumes of paint onto each chart and then determined accurately by weight measurements and calculation as described

486

PAINT AND COATING TESTING MANUAL

!

!

II

II Ii i

II

504540353025201510

5

B

C

i

' i j

I

!

!

l

I

I

Di

A

FIG. 5-Diagram of early model of Pfund cryptometer. causes the line to disappear. The wet film thickness at the point of complete hiding is determined from the scale reading at the toe of the wedge and the thickness of the shim at the heel, from which the HP in ftZ/gal or mZ/L can be calculated using Eq 3 or Eq 7. Dark-colored paints cannot be measured using this instrument because of the lack of contrast with the black glass background. Nelson and Norris of the New Jersey Zinc Co. used this cryptometer to determine HP of colored pigments with results as shown in Table 3. The pastes were prepared by rubbing the colors in No. 0000 lithograph varnish. The rubbing was regulated to represent the maximum development usually obtained in practice. In addition to the regular black glass instrument, they used one specially made with white glass for several measurements.

(2) Black and White FIG. 4-Krebs diamond-stripe hiding power chart. in 4.a. A single test paint panel is likewise prepared at an intermediate spreading rate. After drying, that panel is compared with the standard panels to determine the two that bracket it in contrast. Then by visual interpolation a fairly precise estimate is made of the spreading rate of the standard paint required to match the contrast of the test paint panel. The relative dry hiding power of the test paint is the percent ratio of its spreading rate to that of the standard paint at equal visual contrast, thus % Relative Hiding Power

-

SRTestPaint X 100

(23)

SRstd. Paint

Pfund [8] introduced the black-and-white cryptometer in 1930. It eliminated the well, making cleaning easier, and worked for use with paints of any color because of the blackand-white instead of all-black background. Referring to Fig. 6, the black glass B and the white glass W are fused along line LM. Longitudinal grooves catch the overflow of paint. The wedge is moved to the right to make the line disappear, then to the left to make it reappear. The position of the plate is reversed and the observations repeated. From the mean of all readings, the hiding is calculated as with the all-black cryptometer (4.e.(1)). Comparison of results for white paints shows that the two cryptometer types (all-black and blackand-white) yield the same values within experimental error (Ref 4, p. 22).

(3) Rotary Type e. P f u n d C r y p t o m e t e r s

(1) All Black Introduced in 1919, this was one of the first laboratory instruments made for determining HP [7]. Referring to Fig. 5, A is a plate of black glass whose upper surface is optically flat; B is a transverse groove 10 mm wide and about 2 mm deep. Beginning at the left edge of the groove is a millimetre scale etched in the upper surface of Plate A. C is a plate of clear glass whose lower surface is optically flat. D is a steel shim cemented to C so that a wedge of paint may be formed between the plates. This wedge abruptly becomes "infinitely thick" at B, and so long as hiding is not complete, the line of demarcation is visible. Sliding the wedge to the left eventually

The rotary cryptometer was a short-lived device designed to overcome the jerky movement of the top plate of the regular cryptometer [9]. The wedge of the cryptometers of Figs. 5 and 6 was replaced with a circular glass plate mounted in a metal frame (Fig. 7). The thickness of the film was read on a scale located on the bottom plate. While the movement of the plate was much smoother with this instrument, it was found that bubbles often obscured the end-point.

(4) Assessment of Cryptometers (Ref 4, p. 25) The cryptometer is a simple instrument requiring only small quantities of paint, and determinations are quickly made. However, reading the end-point is difficult, and the mean of a number of determinations is therefore advisable.

CHAPTER 4 2 - - H I D I N G P O W E R TABLE 3--Hiding power (m2/kg) of some colored pigments

487

f. Hallet H i d i m e t e r

measured with a Pfund cryptometer~ (Nelson and Norris). Black Glass

White Glass

... ..29 72 106 51 129 187 188 101 167 181 62 98 154 202 150 91 130 23 31 44 17 29 66 75 65 36 137 224 160 41 35

105 41 -.. ... --. -.. ... ... ... ..... ..--. ... ... ... ... ..-.. 27 34 56 20 .-. -.. ... ... ... ... ... ... ... ...

Lampblack Carbon Black Chromic oxide Prussian blue Chinese blue Blue toner Light green Medium green Deep green Light green Medium green Deep green Light green Medium green Light green Medium green Deep green Green toner Green toner dark Chrome yellow Hansa yellow Lt. chrome orange Med. chrome orange Dk. chrome orange Lithol toner Lithol toner Maroon toner Madder lake Toluidine toner Light para toner Deep para toner Light para toner Deep para toner

Along with the Pfund cryptometer, the Hallet hidimeter

[12] was one of the very early devices for evaluating HP. The objective of a regular microscope is replaced by a long tube fitted with a plain g r o u n d glass objective; the eyepiece is replaced with a small hole. The principle of the device is the light-diffusing property of g r o u n d glass. If a contrast substrate is viewed t h r o u g h a plate of g r o u n d glass, the contrast b o u n d a r i e s b e c o m e more b l u r r e d as the distance b e t w e e n plate a n d substrate increases. If a liquid p a i n t sample is sandwiched b e t w e e n them, it blurs the b o u n d a r y further, a n d the distance required to make the b o u n d a r y disappear decreases. Since that distance is the thickness of the intervening p a i n t film, it is a m e a s u r e of the hiding power of the paint. This m e a s u r e m e n t is essentially comparative because it cannot be translated into regular hiding power units.

5. E A R L Y P H O T O M E T R I C H I D I N G P O W E R METHODS a. Pfund P r e c i s i o n Cryptometer I n this device (Fig. 8) a photoelectric cell is used to m e a s u r e the reflectance of p a i n t c o n t a i n e d in a wedge-shaped layer [13]. The base plate consists of black-and-white areas B a n d W, whose b o u n d a r y is parallel to the length of the plate instead of perpendicular as with the visual cryptometer. The photoelectric device is shifted until a position is f o u n d where the reflectance of the p a i n t over the black area is 98% of that over the white area. The film thickness a n d HP calculations are the same as with the visual cryptometers described in section 4.e. This cryptometer eliminates the uncertainties of the visual type, as there is n o sliding of the top plate over the base plate a n d no need to estimate visually the appearance a n d disappearance of a n indistinct line. However, it retains the disadvantage of p e r m i t t i n g only wet hiding m e a s u r e m e n t s a n d has therefore b e e n superseded by other photometric devices a n d methods that p e r m i t the m e a s u r e m e n t of dry hiding power.

~Multiplyby 4.9 to obtain HP in fta/lb.

Most users c a n repeat their o w n results, b u t agreement a m o n g different users is n o t satisfactory although it is improved by the use of a s t a n d a r d p a i n t [10]. Another m a j o r disadvantage of cryptometers is that they m e a s u r e only wet HP. One study [11 ] reported that cryptometers were satisfactory with low-opacity b u t not high-opacity paints. Consideration of its advantages a n d disadvantages suggests that the cryptometer is better suited for control work t h a n for specification requirements. The cryptometers s h o w n in Figs. 5 a n d 6 c o n t i n u e to be commercially available.

b. H a n s t o c k M e t h o d

! ! I'

L

~%\\\ ~\\\"/////////,~,~:~v////////////,,; r ,,x~\x~

,:;:.:/"...(///,/~",;,k4/;

i

1,4

FIG. 6-Pfund black-and-white r

Hanstock [14] studied the relationship of light transmission through free p a i n t films to opacity a n d HP o n a blackand-white substrate. For his t r a n s m i s s i o n work, he employed a flicker p h o t o m e t e r a n d f o u n d that p a i n t films were perfectly diffusing a n d that films having the same degree of light transm i s s i o n had approximately equal opacity. He further showed the correspondence b e t w e e n refractive index, the Fresnel relationship, a n d HP. The p r o b l e m with the t r a n s m i s s i o n concept is that m o d e m paints have so m u c h opacity it is difficult to m e a s u r e accurately the t r a n s m i s s i o n of films of c o m m e r c i a l thickness. Moreover, HP is concerned in practice with p a i n t in i n t i m a t e contact with opaque surfaces a n d not as a free film. Consequently, m e a s u r e m e n t of light t r a n s m i s s i o n t h r o u g h p a i n t films is done today only for very specialized research.

1

488

P A I N T A N D COATING T E S T I N G M A N U A L

"1 I

I

FIG. 7-Rotary cryptometer.

N

i f "

....-;,,/K

t ~, i ," i7.) L~ W

',,ii/4.-

B

,,, I le , / . ,I;.'.&

W P

\

..z'~'~"'v

bY'z G

9

i,.j..'i;.L' ................... 1 F

Detail of Plate.

~"x"

"

P~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .r~ . I Wl

FIG. 8-Pfund precision cryptometer. Uses a photoelectric cell instead of the eye to measure reflectance.

c. Use o f t h e Fell E q u a t i o n An e m p i r i c a l relationship b e t w e e n spreading rate (SR) a n d c o n t r a s t ratio (CR) was f o u n d b y Fell a n d r e p o r t e d by S a w y e r [15] in the following form l o g ( C R x 10) = m • SR + b

(24)

w h e r e m a n d b are e x p e r i m e n t a l constants. Since the g r a p h for this equation is a straight line, it is a simple m a t t e r after d e t e r m i n i n g m a n d b from m e a s u r e m e n t s at two CR levels to find the SR required for any desired CR.

This p r o c e d u r e was a d a p t e d b y M a r c h e s e a n d Z i m m e r m a n to d e t e r m i n e the hiding p o w e r of p a i n t s at a c o n t r a s t ratio of 0.98, a n d the m e t h o d was used for m a n y years by the Titan i u m Pigment Co. (Ref 4, p. 24). Experience has s h o w n that r e a s o n a b l y satisfactory results can be o b t a i n e d if the equation 'is used for i n t e r p o l a t i o n b e t w e e n points close to the desired c o n t r a s t ratio. But, as p o i n t e d out by Switzer [16], e x t r a p o l a t i o n of results can lead to serious errors. He further p o i n t e d out that the Fell equation m e t h o d allows only a single estimate of HP from at least two test applications, thus re-

CHAPTER 42--HIDING POWER quiring a considerable effort to obtain an estimate of intralaboratory precision.

d. New York Paint Club (NYPC) Method This method employed doctor blades to apply films at several thicknesses on black-and-white cardboard hiding power charts. After the films had dried, reflectance, weight, and area measurements were made from which contrast ratios and corresponding wet film thicknesses were calculated. CR values (rather than log 10 CR as in the Fell equation) were plotted against reciprocal film thickness and the HP calculated from the wet film thickness at 0.98 CR. If the white area of the chart deviated appreciably from the standard reflectance of 0.80, the contrast ratio was corrected using Eq 1 or Eq 2. The Club reported that most of the effort in this method was to determine film thicknesses. To minimize that effort they modified the method by casting films on black and white glass plates and determined thicknesses with an Interchemical wet-film thickness gage in accordance with ASTM Test Methods for Measurement of Wet Film Thickness of Organic Coatings (D 1212). Any error in film thickness, of course, carries over to the HP value. According to Mitton, the revised method sacrifices accuracy and precision for speed [17]. In addition, graphical averaging makes it burdensome to estimate the precision with which the HP has been determined for the same reason pointed out in 5.c in connection with the Fell equation method.

e. Van Eyken-Anderson Method The method proposed by Van Eyken and Anderson [18] uses CRs and film thicknesses in the same way as the NYPC method described in 5.d, except that films of different thicknesses are applied in a single operation by using a doctor blade having seven clearances. A die is used to prepare uniform area punch-outs of the paper charts to determine spreading rate by the basic weight-area-density-NVW calculation (see Eq 26). The defects of this method are that the small areas used for reflectance and weighing make the achievement of good precision difficult (Ref 4, p. 31), and there is no provision for correcting CR if the white substrate reflectance differs from 0.80. f. Federal Test for Dry Opacity This is Method 4121 of U.S. Federal Test Method Standard No. 141. It is a pass-fail test calling for a minimum dry film contrast ratio at a specified WFT. Black-and-white hiding power charts are used as the test suhstrate. For routine testing, the paint may be applied either by brush or doctor blade. For referee tests, application is by doctor blade only. The density and the nonvolatile content of the paint are also required. Several drawdowns are made to bracket the specified WFT. The weight of dry paint film is determined for a measured area on each drawdown and the WFT then calculated from the equation 61 M(g) WFT (mils) = A (in. 2) • N • D (g/mL) where

(25)

489

M = the dry film weight, N = the fractional nonvolatile content of the paint by weight, A -- the film area, and D -= the density of the paint. The CR of each chart is measured and plotted against the corresponding WFT. From a smooth curve drawn through the points, the CR at the specified WFT is obtained. If this is equal to or greater than the specified CR, then the requirement for dry opacity has been met.

6. G E N E R A L H I D I N G P O W E R METHODOLOGY a. Film Application The objective is to determine the spreading rate at a specified level of dry film opacity, which is usually full hiding as perceived visually or corresponding to the contrast ratio: C = R0/R0,0 = 0.98. The basic experimental procedure is to apply a uniform film on a suitable test substrate, to observe its opacity either visually or photometrically, and to determine its spreading rate. Since it is not possible to apply a film with precision at a predetermined dry opacity, several such applications need to be made over a range of spreading rates and their results plotted graphically or otherwise interpolated to the desired hiding power end-point. This laborious procedure is exemplified in the visual methods discussed in 4.a and 4.d and in the contrast ratio (CR) methods discussed in 5.e and 5.f. The Fell equation and NYPC methods (5.c and 5.d) attempt to reduce the workload to only two spreading rate determinations by plotting SR against CR or log CR and finding the hiding power end-point graphically on the basis of perceived empirical straight-line relationships. Kubelka-Munk theory (see Section 8) shows how the end-point can be calculated with just one spreading rate determination.

b. Spreading Rate (or Film Thickness) Determination In both visual and photometric hiding power methods, the procedures for observing film opacity are well-defined and can be performed with dispatch. The experimental task most demanding on the operator's time and ingenuity is to determine the spreading rate or film thickness of the applied coating with good precision. Although gages are available to measure wet and dry film thickness directly and quickly, the most accurate procedure, by far, is to determine the weight of applied paint film on a measured test area and then to calculate the spreading rate or film thickness as described in 4.a and 5.f. The equations in both of those methods contain mixed metric and common units. When the units are all metric, the equations are simpler. Letting M = dry film weight and A = the film area and using the symbolism in 1.j: H (m2/L) = A (cm 2),N.D (kg/L) 10M (g) r(/zm) -

104M (g) A (cm2)-N.D (kg/L)

(26)

(27)

490

PAINT AND COATING TESTING MANUAL

See 1.j.2 for equations interrelating spreading rate, wet film thickness, and dry film thickness. See 1.j.3 for conversions between metric and U.S. units. If the volatiles have a relatively low evaporation rate as with most architectural coatings, the film might be weighed rapidly before appreciable loss of volatiles, in which case Eqs 26 and 27 would still apply but with M as the wet film weight and N as unity. The disadvantage of this procedure is that it demands very skillful and speedy manipulation to minimize loss of volatiles before weighing. For that same reason, it is not applicable at all to coatings containing fast evaporating solvents. With powder coatings, for which the spreading rate is normally expressed on a weight basis, Eq 26 becomes n (m2/kg) -

A (cm2).N 10M (g)

(28)

Assuming negligible volatile content, the value of N in this equation can be taken as unity.

c. P h o t o m e t r i c M e a s u r e m e n t s The CIE-Y reflectance is measured because this function defines the human eye's quantitative response to the luminous character of light across the visual spectrum. This is valid for chromatic as well as nonchromatic colors, as reported by Tough [19], who found good correlation in a large series of colored paints between visual hiding power measurements and contrast ratio values based on CIE-Y measurements with a spectrophotometer. The end-point of 0.98 CR is effective with colors, although it appears that other endpoints, for example CIELAB color difference: AE = 1.5, would make some difference in the relative HP of various colored paints [20]. However, the simplicity of the 0.98 endpoint and its history of validity and general agreement among various workers make it the best choice regardless of color (Ref 4, p. 31). CIE-Y measurements can be made with the green filter of a tri-stimulus colorimeter or with a spectrophotometer. When properly standardized, results with the two instrument types should be the same. As a precaution, there should be coordination between correspondent laboratories with regard to instrumentation. In all cases, reflectance measurements must be made excluding surface reflection, which is implicit for instruments designed with 00/45~geometry and optional with most other instrument types.

discoloration at high temperatures, but they are used widely with air-dried coatings for general hiding power observations. Black-and-white charts can be used for precision photometric hiding power measurements by taking appropriate steps to allow for weight variations in the substrate due to humidity and inherent random variations in the area weight of paper. These steps include the use of unpainted control charts and the averaging of multiple test results. Charts with combinations of gray and black, gray and white, and gradations of gray on a white background are used in visual hiding power tests to obtain what are considered to be more practical hiding power measurements.

b. Clear Plastic Film Polyester is the preferred chemical type. Because of heat distortion, its use is generally confined to air-dried coatings. After the film has dried, a square of convenient size is cut and the area measured. Values of R0 and Rw are read by placing the painted plastic film alternately on a black and a white background with the underside moistened with a suitable liquid (e.g., mineral spirits or dibutyl phthalate) to remove the air interface and establish good optical contact. The dry film weight is determined as the difference in the weight of the painted and unpainted substrate by stripping off the paint film with a strong solvent.

c. Glass Panels Individual black and white glass panels are used to take advantage of the superior levelness of glass for casting of uniform films and because the hard, smooth surface permits rapid wet film thickness measurements with an ASTM-type of wet-film thickness gage (5.d). The same information is obtained less rapidly but with much better precision by scraping off and weighing a defined area of dry film and calculating as described in Section 6.b. In some tests, contrast ratios are calculated on the questionable assumption that separate film applications on black and white glass panels are identical in film thickness.

d. Painted Metal Panels 7. C U R R E N T L Y U S E D T E S T S U B S T R A T E S The substrate is generally the major factor affecting the specific experimental details of a test procedure. It is selected or specified on the basis of its adaptability to the type of coating being tested and for its perceived advantages in the required or preferred test procedure.

a. Paperboard Charts Substrates of this type are described in 4.d. Their employment with baking finishes is limited because of distortion and

Panels of this type are generally used with coatings that are applied by spraying and cured by baking. The weight of the applied film is determined by weighing the panel before the coating is applied and again after drying. The spreading rate or film thickness is then calculated as described in 6.b. If desired, the dry film thickness can be determined without weighing, though with considerably less precision, by direct measurement with a magnetic or an eddy current thickness gage. Black-and-white panels are used for contrast ratio measurements or for visual observation of opacity. The use of allblack panels is described by Mitton for measuring the hiding power of baking enamels [21].

CHAPTER 42--HIDING

8. KUBELKA-MUNK (K-M) TWO-CONSTANT THEORY a.

Subscripts x indicates an experimentally d e t e r m i n e d value, e.g., Tx, Hx,

Px. c indicates a value calculated for a specified c o n t r a s t r a t i o

C,e.g.,Pc, Hc. 0.98 indicates a value calculated for C = 0.98, e.g., H0.98, T0.98. H indicates a value p e r t a i n i n g to a s p r e a d i n g rate, e.g., Cn,

PH, s.. T indicates a value p e r t a i n i n g to a film thickness, e.g., C~,

S T. a a n d b are simplifying functions of R=, defined by

a = I/2 (1/R~ + R~)

(29)

b -= V2 (1/R= - R=)

(30)

F r o m these definitions are derived the a d d i t i o n a l relationships b = (a z - 1) 1/2 R==a-b=a-(a

b. Equation

Symbols

The symbols u s e d here are b a s e d on ASTM Test M e t h o d for Hiding Power of Paints by Reflectometry (D 2805-88) as follows: G = the s u b s t r a t e reflectance F o r a white s u b s t r a t e G = W. F o r a s t a n d a r d white substrate G = W = 0.80. F o r a b l a c k s u b s t r a t e G -- B. F o r a s t a n d a r d b l a c k s u b s t r a t e G = B 130%. Results for a flexible polyurethane primer are a tensile strength of 3500 psi, a yield strength of 3200 psi, an elongation at break of 90%, and an elongation at yield of 40%. The plot shows that a polyurethane topcoat yields and breaks at about the same point: 4000 psi and 22% elongation. The plot for an epoxy primer shows brittle behavior (no yield point), a tensile strength of 2300 psi, and an elongation at break of 7%. The authors [33] report that replacing the epoxy primer by the flexible polyurethane primer eliminated the need for the sealant coat. SSA has been used extensively to characterize cationic UVcured cycloaliphatic epoxy/polyol coatings [34-36]. The mechanism of introduction of polyol is a chain transfer step [34], which permits use of a wide range of epoxy:polyol ratio without need for stoichiometric balance. Selection of flexible polyols with this wide formulating latitude permits preparation of cross-linked films with an extremely wide range of mechanical properties. Numerous stress-strain curves have been presented for this type of system with oligomeric propylene oxide polyols [35] and with oligomeric caprolactone polyols [36]. Examples of stress-strain curves for a cycloaliphatic diepoxide (CYRACURE | UVR-6110, Union Carbide) and a propylene oxide triol (MW = 702) are shown in Fig. 7 [35]. The corresponding tensile properties are given in Table 3. Films from Compositions 1 through 4 all give brittle failure (compare Fig. 6A) and only moderate changes in tensile properties (Table 3) despite a large change in composition. At Composition 5 (70/30 epoxy:polyol, see Table 3) a yield point is first noted, and thereafter very large property changes occur despite relatively small changes in composition. Compositions in the 4 to 6 range (see Table 3) are reported to give excellent post forrnability, as required for coatings on beverage can ends, while maintaining adequate hardness and solvent resistance [34-36].

Relationship to Other Mechanical Properties Stress-strain analysis (SSA) is used in a general way to assess suitability of a binder for various coating end uses. Most coating chemists associate modulus with coating hardness and percentage elongation at break with coating flexibility. Quantitative correlations of these properties are not published very often, however. The lack of published correlations may result from the fact, noted above, that paint tests of adherent coatings depend on coating-substrate interactions, whereas SSA is carried out on free films. ASTM Test Method

Fief. No. 1

62.0

48.2 6

:E 34.5

5 4

20.7

3

6.9 ~

0

4

8

0

12 16 20 24 28 PercentElongation FIG. 7-Stress-strain curves for UV-cured cycloaliphatic epoxide films flexibilized with oligomeric propylene oxide triol. Strain rate is 40% per minute. See Table 3 for tensile properties.

for Elongation of Attached Organic Coatings with Conical Mandrel Apparatus (D 522-88) describes how to calculate percent elongation from the crack length in a conical mandrel bend test. Comparison of elongation of adherent coatings by the conical mandrel method and elongation of the same coating as a free film from SSA would certainly be of interest, but such comparisons were not found in the literature. The logic of associating yield behavior in SSA with post formability of coil coated metal was noted in 1977 [37]. In 1987, Koleske [35,36] confirmed that compositions that exhibited a yield point performed well in the demanding postforming operations carried out on beverage can ends. Evans and Fogel [38] provided convincing evidence that gloss retention during abrasion of floor coatings is related to the area under stress-strain curves. This area, divided by sample volume, is called "work-to-break" or "toughness" as noted in the subsection of this chapter entitled Definitions of Tensile Properties. The authors provide a clear example of the need to match strain rates when attempting to correlate SSA results with paint test results. Failure of attempts to correlate pencil hardness with penetration hardness, e.g., Tukon Hardness, probably result because the former has a strong requirement for toughness, whereas the latter is more dependent on the modulus value at room temperature (see chapter subsection entitled

Relationship to Other Mechanical Properties). DMA is much more generally applicable to determination of cross-link density (see chapter subsection entitled Determination of Cross-Link Density) than is SSA. If the Tg of a coating binder is well below the temperature at which

CHAPTER 46--DYNAMIC

MECHANICAL AND TENSILE PROPERTIES

545

TABLE 3--Tensile propertiesa of UV-cured cycloaliphatic epoxide films flexibilized with oligomeric propylene oxide triol. Film N o. b

1 2 3 4 5 6 7 8

C~176176162 Epoxide, Triol, wt% wt% 90.0 85.0 80.0 75.0 70.0 66.7 63.4 60.0

10.0 15.0 20.0 25.0 30.0 33.3 36.6 40.0

Tensile Modulus,d psi 3.88 3.72 3.33 2.95 2.05 1.48 0.70 0.26

x X • x x • • X

10s 10s 10s 10s 105 105 10s 105

Tensile Strength, psi 9.5 8.9 8.4 7.0 4.3 3.7 2.5 2.0

x X x X x • x x

10 3 10 3 10 3 10 3

103 103 103 103

Elongation, % 6.6 6.6 7.4 8.1 16.2 24.3 54.0 88.4

~Strain rate, 40% per minute. bKeyedto the plots in Fig. 7. CWeight% of polymericbinder. (Filmsalso contain 2.9 wt% photoinitiator and 0.5 wt% flow agent.) a1% secant modulus. (The modulus range expressed in pascals is 2.68 • 109,No. 1. to 1.79 • l0s, No. 8.)

SSA is carried out, t h e n the m o d u l u s from the initial slope of the stress-strain curve is a r u b b e r y plateau m o d u l u s a n d Eq 16 is valid at least in principle. I n practice, curvature in stressstrain curves a n d p e r m a n e n t d e f o r m a t i o n usually result in inappropriate m o d u l u s values. An innovative approach to avoiding the p e r m a n e n t deformation p r o b l e m consists of reversing the extension mode of SSA so that a retraction plot is also obtained. Hergenrother [39] has applied this tensile retraction m e t h o d for d e t e r m i n a t i o n of cross-link density of elastomeric polyurethanes.

CONCLUSIONS A wide range of a u t o m a t e d a n d computer-controlled ins t r u m e n t s is available for d e t e r m i n a t i o n of d y n a m i c m e c h a n ical a n d tensile properties. Careful review of variable features is necessary to insure suitability for property d e t e r m i n a t i o n s o n coating samples of n o r m a l thickness. D e t e r m i n a t i o n of basic physical properties makes it possible to integrate structure/property knowledge from m a n y polymer fields with coatings research a n d development. Free film coating data are m u c h more useful w h e n thoughtfully interpreted in relation to results from tests carried out with films intact o n their enduse substrates. This review includes m a n y examples that illustrate the benefits of c o m b i n i n g DMA or SSA data with results from well controlled a n d d o c u m e n t e d tests as provided by the ASTM. The goal of m u c h of the discussion provided here is better u n d e r s t a n d i n g of hardness, flexibility, post-formability, solvent resistance, a n d abrasion resistance. DMA a n d SSA are often c o m p l e m e n t a r y because strains imposed on test samples are very different. SSA provides i n f o r m a t i o n on yield behavior a n d failure at high strains. DMA provides low strain properties a n d reveals the viscoelastic n a t u r e of coatings very directly a n d quantitatively. For u n p i g m e n t e d thermoset coatings, values of storage modulus, E', in the r u b b e r y plateau can be used to calculate cross-link density (XLD). D e t e r m i n a t i o n of XLD usually makes it possible to confirm or deny that purposeful ref o r m u l a t i o n or changes in resin structure have had the desired effects.

REFERENCES [1] Aklonis, J. J. and MacKnight, W. J., Chapter 2 in Introduction to Polymer Viscoelasticity, 2nd ed., Wiley Interscience, New York, 1983. [2] Nielsen, L. E., Mechanical Properties of Polymers and Composites, Vol. I, Marcel Dekker, New York, 1974. [3] Murayama, T., Dynamic Mechanical Analysis of Polymeric Material, Elsevier, New York, 1978. [4] Sperling, L. H., Chapter 1 in Sound and Vibration Damping in Polymers, R. D. Corsaro and L. H. Sperling, Eds., ACS Symposium Series 424, American Chemical Society, Washington, 1990. [5] Hill, L. W., "Mechanical Properties of Coatings," Federation Series on Coatings Technology, D. Brezinski and T. J. Miranda, Eds., Federation of Societies for Coatings Technology, Philadelphia, 1987. [6] Schurr, G. G., Section 5.5, "Tensile Strength and Elongation," Paint Testing Manual, 13th ed., G. G. Sward, Ed., American Society for Testing and Materials, Philadelphia, 1972. [7] Takano, M. and Nielsen, L. E., "The Notch Sensitivity of Polymeric Materials," Journal of Applied Polymer Science, Vol. 20, 1976, p. 2193. [8] Wicks, Jr., Z. W., Jones, F. N., and Pappas, S. P., Organic Coatings Science and Technology, Vol. 1, Film Formation, Components and Appearance, Wiley, New York, 1992; Vol. 2, Applications, Properties, and Performance, 1994. [9] Hartman, B., Chapter 2 in Sound and Vibration Damping in Polymers, R. D. Corsaro and L. H. Sperling, Eds., ACS Symposium Series 424, American Chemical Society, Washington, 1990. [10] Manson, J. A. and Sperling, L. H., Polymer Blends and Composites, Plenum Press, New York, 1976, Chapters 3, 8, and 13. [11] Cooper, S. L. and Estes, G. M., "Multiphase Polymers," ACS Advances in Chemistry Series 176, American Chemical Society, Washington, 1979. [12] Roller, M. B., "The Glass Transition: What's the Point?," Journal of Coatings Technology, Vol. 54, No. 691, 1982, p. 33. [13] Hill, L. W. and Kozlowski, K., "The Relationship Between Dynamic Mechanical Measurements and Coatings Properties," Advances in Organic Coatings Science and Technology, Vol. 10, Proceedings of the Twelfth International Conference in Organic Coatings Science and Technology, A. V. Patsis, Ed., Technomic, Inc., Lancaster, PA, 1986, p. 31. [14] Provder, T., Holsworth, R. M., and Grentzer, T. H., "Dynamic Mechanical Analyzer for Thermal Mechanical Characterization of Organic Coatings," Chapter 4 in Polymer Characterization, C.

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PAINT AND COATING TESTING MANUAL

D. Craver, Ed., ACS Advances in Chemistry Series 203, American Chemical Society, Washington, 1983. [15] Zosel, A., "Mechanical Behavior of Coating Films," Progress in Organic Coatings, Vol. 8, 1980, p. 47. [16] Skrovanek, D. J., "The Assessment of Cure by Dynamic Thermal Analysis," Progress in Organic Coatings, Vol. 18, 1990, p. 89. [17] Hill, L. W. and Kozlowski, K., "Crosslink Density of High Solids MF-Cured Coatings," Journal of Coatings Technology, Vol. 59, No. 751, 1987, p. 63. [I8] Hill, L. W., "Structure/Property Relationships of Thermoset Coatings," Journal of Coatings Technology, Vol. 64, No. 808, 1992, p. 29. [19] Grillet, A. C., Galy, J., Gerard, J-F., and Pascault, J-P., "Mechanical and Viscoelastic Properties of Epoxy Networks Cured with Aromatic Diamines," Polymer, Vol. 32, No. 10, 1991, p. 1885. [20] Yeo, J. K., Sperling, L. H., and Thomas, D. A., "Rubber Elasticity of Poly (n-butyl Acrylate) Networks Formed with Multifunctional Crosslinkers," Journal of Applied Polymer Science, Vol. 26, 1981, p. 3977. [21] Scholtens, B. J. R., Tiemersma-Thoone, G. P. J. M., and van der Linde, R., "Thermoviscoelastic and Thermoanalytic Characterization of Some Reactive Polyester Powder Coatings Systems," Verfkroniek, Vol. 62, 1989, p. 238. [22] Higginbottom, H. P., Bowers, G. R., Grande, J. S., and Hill, L. W., "Structure/Property Studies of MF-eured Powder Coatings," Progress in Organic Coatings, Vol. 20, 1992, p. 301. [23] Oshikubo, T., Yoshida, T., and Tanaka, S., "Studies on Acrylic Resins and Melamine Formaldehyde Resins for High Solids Coatings," Proceedings, Tenth International Conference in Organic Coatings Science and Technology, 9-13 July 1984, Athens, Greece, p. 317. [24] Scanlon, J., "The Effect of Flaws on the Elastic Properties of Vulcanizates," Journal of Polymer Science, Vol. 43, 1960, p. 501. [25] Flory, P. J., "Molecular Theory of Rubber Elasticity," Polymer Journal, Vol. 17, No. 1, 1985, p. 1. [26] Flory, P. J. and Erman, B., "Theory of Elasticity of Polymer Networks," Macromolecules, Vol. 15, 1982, p. 800. [27] Erman, B. and Flory, P. J., "Relationships Between Stress, Strain and Molecular Constitution of Polymer Networks. Comparison of Theory and Experiments," Macromolecules, Vol. 15, 1982, p. 806.

[28] Graessley, W. W., "The Entanglement Concept in Polymer Rheology," Advances in Polymer Science, Vol. 16, Springer-Verlag, NY, 1974, p. 1.

[29] Sato, K., "The Hardness of Coating Films," Progress in Organic Coatings, Vol. 8, 1980, p. I. [30] Heijboer, J., "Dynamic Mechanical Properties and Impact Resistance," Journal of Polymer Science, Vol. C16, 1968, p. 3755. [31] Roller, M. B. and Gillham, J. K., "Application of Dynamic Mechanical Testing to Thermoset Coatings Research and Development," Journal of Coatings Technology, Vol. 50, No, 636, 1978, p. 57. [32] Ryntz, R. A., Gunn, V. E., Zou, H., Duan, Y. L., Xiao, H. X., and Frisch, K. C., "Effect of Siloxane Modification on the Physical Attributes of an Automotive Coating," Journal of Coatings Technology, Vol. 64, No. 813, 1992, p. 83. [33] Hegedus, C. R., Pulley, D. F., Spadafora, S. J., Eng, A. T., and Hirst, D. J., "A Review of Organic Coating Technology for U.S. Naval Aircraft," Journal of Coatings Technology, Vol. 61, No. 778, 1989, p. 31. [34] Koleske, J. V., "Cationic Radiation Curing," Federation Series on Coatings Technology, D. Brezinski and T, J. Miranda, Eds., Federation of Societies for Coatings Technology, Philadelphia, 1991. [35] Koleske, J. V., "Mechanical Properties of Cationic Ultraviolet Light-Cured Cycloaliphatic Epoxide Systems," Proceedings, Radcure Europe '87, 4-7 May 1987, Munich, W. Germany. [36] Koleske, J. V., "Copolymerization and Properties of Cationic, Ultraviolet Light-Cured Cycloaliphatic Epoxide Systems," Proceedings, RadTech '88--N. America, 24-28 April 1988, New Orleans, p. 353. [37] Hill, L. W., "Stress Analysis: A Tool for Understanding Coating Performance," Progress in Organic Coatings, Vol. 5, 1977, p. 277. [38] Evans, R. M. and Fogel, J., "Comparison of Tensile and Morphological Properties with Abrasion Resistance of Urethane Films," Journal of Coatings Technology, Vol. 49, No. 634, 1977, p. 50. [39] Hergenrother, W. L., "Determination of the Molecular Weight Between Cross-links of Elastomeric Stocks by Tensile Retraction Measurements. II Polyurethanes," Journal of Applied Polymer Science, Vol. 32, 1986, p. 3683.

MNL17-EB/Jun. 1995

Flexibility and Toughness* by Mark P. Morse I

DEFINITIONS

Coatings, as the polymers from which they are prepared, are viscoelastic in nature, that is, they behave both as viscous liquids and as elastic solids. The coatings have elastic recovery and yet will flow with time when placed under a stress. In general, viscoelastic behavior and mechanical properties are markedly affected when a coating enters the glass transition, softening point, or other relaxation. To be certain that the properties of a coating will fulfill the needs of its intended use, the viscoelastic behavior of the coating should be measured, controlled, and designed to meet the particular end use. The softening point of a coating can be used as an index of flexibility. The softening point is between the temperature where the coating changes from being hard and glassy and the temperature where it is leathery or rubbery. For example, if a coating has a softening temperature region near the temperature of the forming operation, the coating is less susceptible to failure by cracking or a similar mechanism than if the softening region was above the forming temperature. Measurement of energy storage (related to elasticity) and energy loss (related to viscous losses) as a function of temperature is a means of predicting impact resistance. Impact resistance of a paint film can be considered as energy dissipation by vibration or rotation of various molecular segments so that at no time will sufficient energy be focused to cause fracture. Since the impact tests performed on paint films often produce deformations beyond the elastic limit of the films, flow within the films must take place or fracture will occur [3]. To obtain good impact resistance, the paint film must consist of a polymer that has a sufficiently high molecular weight to have strong intermolecular entanglement (and therefore, high tensile strength), but sufficiently low viscosity (by choice of proper molecular constituents and limiting molecular weight) that flow and accompanying energy dissipation will take place. Polymer viscosity increases as molecular weight increases so that polymers with very high molecular weights will have greater flexibility than those polymers with intermediate or low molecular weights. At the same time, molecular weights below the critical molecular weight for entanglement lead to very low tensile strengths and the mechanical behavior observed is brittleness. It has been found that modulus is the dominant factor in the relationship between the tensile properties of a coating and its impact resistance [4]. In addition to dynamic mechanical behavior, the relaxation behavior as measured by dissipation or damping of coatings has been determined by application of dynamic electrical tests [5]. In a dielectric relaxation test, a periodic electrical

To PERFORM PROPERLYIN USE, a coating must possess the proper amount of flexibility and toughness to withstand cracking when subjected to stresses produced by shrinking or swelling, forming, mechanical abuse, and weathering. Flexibility is the ability of a material to be bent or flexed without cracking or undergoing other failure. Toughness is the strength and resilience of a material; it is the material's ability to withstand great strain imposed in a short time period without tearing, breaking, or rupture.

INTERPRETATION The flexibility of a coating applied to a substrate depends not only on its distensibility, but also on the coating thickness and on the adhesion between coating and substrate. Good adhesion tends to give better apparent flexibility than does poor adhesion. The toughness of a coating is dependent on its hardness, stiffness, resiliency, distensibility, and the existence of an energy dissipation mechanism that operates at temperatures far below room temperature and is discernable by dynamic mechanical measurements made over a broad temperature or frequency range. Generally, the bend and impact tests used to evaluate flexibility and toughness are much more severe than actual service conditions. This is because the tests are usually performed on relatively fresh, unaged coating films. Since coating films tend to lose flexibility during use due to volatilization of free plasticizing components and chemical changes such as degradation, cross-linking, and the like, these severe tests that exceed normal expectations are useful in predicting long-term serviceability [1].

BASIC P R O P E R T I E S A F F E C T I N G COATING PERFORMANCE Both flexibility and toughness depend on very basic properties: the viscoelastic behavior of the coating and its physical transitions and relaxations. The following is a discussion of these properties taken from a paper by Skrovanek and Schoff [2]. *This chpater is an abridged and modified version of the chapter entitled "Flexibility," written by G a r m o n d G. Schurr, found in the previous edition of this manual. fConsultant, 71 S. Shelburne Rd., Springfield, PA 19064.

54"/ Copyright9 1995 by ASTM International

www.astm.org

548

PAINT AND COATING TESTING MANUAL

potential is a p p l i e d to the s a m p l e coating situated b e t w e e n a p a i r of electrodes. The dielectric c o n s t a n t a n d dissipation fact o r are m e a s u r e d as a function of frequency a n d t e m p e r a t u r e .

TECHNIQUES FOR MEASURING BASIC VISCOELASTIC PROPERTIES Thermal Mechanical Analyzer (TMA) This i n s t r u m e n t employs t r a n s d u c e r s to sense the p o s i t i o n of a vertical r o d that rests on the surface of a coating sample. The i n s t r u m e n t is usually e q u i p p e d with a furnace a n d prog r a m p l a n n e r so that beating, cooling, a n d isothermal temp e r a t u r e o p e r a t i o n s can be employed. W i t h its use, softening points a n d glass transitions can be d e t e r m i n e d from plots of coating i n d e n t a t i o n as a function of t e m p e r a t u r e . Also, changes in stresses within a coating at a c o n s t a n t t e m p e r a t u r e (creep) can be d e t e r m i n e d from plots of i n d e n t a t i o n versus time [2].

Dynamic Mechanical Thermal Analyzer (DMTA) This i n s t r u m e n t p r o d u c e s vibrations in a coating film over a wide frequency range a n d / o r t e m p e r a t u r e range. It can scan a wide range of s a m p l e t e m p e r a t u r e s at different rates. The resulting d e f o r m a t i o n s from the sinusoidally applied stresses are analyzed to c o m p u t e values related to energy storage a n d energy loss [2].

EXTERNAL FACTORS AFFECTING FLEXIBILITY AND TOUGHNESS Flexibility a n d toughness are not c o n s t a n t characteristics of a specific coating. A n u m b e r of external factors affect these properties.

Humidity W a t e r is a g o o d plasticizer for a l m o s t all p a i n t films. A change in relative h u m i d i t y of as little as 2% can be detected in flexibility m e a s u r e m e n t s . S o m e p a i n t films, such as those b a s e d on latexes, i m b i b e m o i s t u r e very rapidly, whereas others r e a c h e q u i l i b r i u m with the a t m o s p h e r e very slowly. It is imperative that tests be c o n d u c t e d in an a t m o s p h e r e of controlled relative h u m i d i t y a n d that the s p e c i m e n s are cond i t i o n e d in that a t m o s p h e r e for a d a y or m o r e before the tests are p e r f o r m e d . Generally, flexibility a n d toughness tests are c a r r i e d out at a relative h u m i d i t y of 50 _+ 5%. The 10% tolerance is needed b e c a u s e of the difficulty in m o r e accurately controlling relative h u m i d i t y in m o s t laboratories. If the e n v i r o n m e n t c a n n o t be controlled at this r e c o m m e n d e d level, then the relative h u m i d i t y should be m e a s u r e d a n d r e p o r t e d along with the m e c h a n i c a l properties.

Temperature The flexibility a n d toughness of coatings are d e p e n d e n t on t e m p e r a t u r e . This is p a r t i c u l a r l y true of t h e r m o p l a s t i c coatings, b u t it also is a factor for t h e r m o s e t coatings. These

coatings have a definite second o r d e r t r a n s i t i o n t e m p e r a t u r e k n o w n as the glass t r a n s i t i o n t e m p e r a t u r e , Tg. Coatings at a t e m p e r a t u r e b e l o w Tg are h a r d a n d brittle with p o o r flexibility a n d i m p a c t resistance unless there is a n o t h e r relaxation at low t e m p e r a t u r e s as exists in p o l y c a r b o n a t e s that have a high Tg of a b o u t 160~ (at 1 Hz) a n d yet have excellent i m p a c t resistance b e c a u s e of a relaxation that occurs at a b o u t - 90~ (at 1 Hz). If coatings do not have this type loss m e c h a n i s m , at t e m p e r a t u r e s just above Tg they are flexible, a n d at t e m p e r a tures substantially above Tg they t e n d to develop viscous r a t h e r t h a n elastic properties. There is a t e n d e n c y for all t h e r m o p l a s t i c coatings to have identical flexibility p r o p e r t i e s if these p r o p e r t i e s are m e a s u r e d at the s a m e t e m p e r a t u r e relative to Tg, for example, at 10~ above Tg [1,6]. Flexibility a n d toughness m e a s u r e m e n t s are usually m a d e at a t e m p e r a t u r e of 25 +_ I~ after the coatings are equilib r a t e d at that t e m p e r a t u r e . However, there are m a n y instances w h e n test are p e r f o r m e d at lower t e m p e r a t u r e s as m i g h t be e n c o u n t e r e d in cold climates.

Strain Rate Strain rate is the rate at w h i c h a coating s p e c i m e n is elongated a n d is usually expressed in p e r c e n t per minute, in./in./min o r cm/cm/min. This is the rate of extension relative to s p e c i m e n size. That is, if a s p e c i m e n 10 c m long is elongated at rate of 1 cm/min, it is the s a m e as a s p e c i m e n 1 c m long being elongated at a rate of O. 1 c m / m i n (1 m m / m i n ) . In b o t h cases, the strain rate is 10% p e r minute. Strain rate has a great influence on the flexibility a n d toughness of a coating. In general, the effect of increasing the strain rate is similar to decreasing the coating t e m p e r a t u r e , that is, as the strain rate is increased, flexibility a n d toughness decrease. There can be critical strain rates where flexibility has s h a r p changes w h i c h are very similar to the effects p r o d u c e d at the glass transition t e m p e r a t u r e [7]. This m e a n s that the strain rate used in a test m u s t be closely controlled. In s o m e tests, such as b e n d test, this is difficult to do. This also m e a n s that tests p e r f o r m e d at a low strain rate (cupping test) are likely to p r o d u c e different flexibility ratings t h a n those p r o d u c e d by a high strain rate (conical m a n d r e l test) [1, 7].

FLEXIBILITY AND TOUGHNESS MEASUREMENTS Mandrel Bend Tests Both conical a n d cylindrical m a n d r e l s are often used for evaluating the flexibility of coatings. Even t h o u g h it is difficult to control the strain rate in these m a n u a l l y o p e r a t e d tests, they can provide very useful flexibility ratings.

Conical Mandrel Tests A conical m a n d r e l test consists of m a n u a l l y b e n d i n g a coated metal panel over a cone. As described in ASTM Test M e t h o d for E l o n g a t i o n of Attached Organic Coatings with Conical M a n d r e l A p p a r a t u s (D 522), a conical m a n d r e l tester consists of a metal cone, a rotating panel b e n d i n g arm, a n d panel clamps. These items are all m o u n t e d on a m e t a l base as

CHAPTER 4 7 - - F L E X I B I L I T Y AND TOUGHNESS

549

FIG. 1-Bending a specimen over a conical mandrel (courtesy of Gardner Laboratory) [I]. illustrated in Fig. 1. The cone is smooth steel 8 in. (203 mm) in length with a diameter of 1/8 in. (3 mm) at one end and a diameter of 1.5 in. (38 mm) at the other end. When a coating is applied on a V32-in. (0.8 mm)-thick coldrolled steel panel, as specified in ASTM D 522, a bend over the mandrel produces an elongation of 3% at the large end of the cone and of 30% at the small end of the cone. The coated panel is bent 135 ~ around the cone in approximately 1 s to obtain a crack resistance rating under simulated abuse conditions. In some instances, longer bend times have been found to be useful. For example, if the percent elongation of the coating at the point of cracking is to be determined, the method specifies a bend time of 15 s. Since variations in temperature and humidity can affect mandrel bend tests, it is imperative that the coated panels be conditioned at a standard temperature and relative humidity before performing the test, which is conducted under the same conditions. The crack resistance value of a coating is obtained by measuring the distance from the furthest end of the crack to the small end of the mandrel. This distance is converted to cone diameter by means of a plot given in ASTM D 522. The mandrel diameter at which cracking occurs is taken as the crack resistance value. If the elongation of the coating at the onset of cracking is to be reported, a bend time of 15 s is used and the diameter at which the onset of cracking occurred is converted to percent elongation from a plot given in ASTM Test Methods for Mandrel Bend Test of Attached Organic Coatings (D 522).

Cylindrical Mandrel Bend Tests When executing cylindrical mandrel flexibility tests, a coated panel is bent manually over one or more cylindrical rods or surfaces of different diameters. ASTM D 522 states that the testing device should include mandrels with I-in.

FIG. 2-Bending a specimen over a cylindrical mandrel (courtesy of Gardner Laboratory) [ 1]. (25 mm), 3/4-in. (19 mm), 1/2-in. (12.7 mm), 3/s-in. (9.5 mm), 1/4-in. (6.4 mm), and 1/8-in. (3.2 mm) diameters. Examples of cylindrical mandrel testers are given in Figs. 2 and 3. The panel should be bent over a mandrel with the uncoated side of the panel in contact with the mandrel surface. The panel should be bent approximately 180 ~around the mandrel at a uniform velocity in a time of i s. If cracking has not occurred, the procedure is repeated using successively

550

PAINT AND COATING

TESTING

MANUAL where t is the thickness of the coated panel a n d r is the radius of the mandrel. Actually, observed elongations are greater t h a n values calculated from the above expression a n d vary with different types of metal substrates. Table 1 contains i n f o r m a t i o n about the influence of panel thickness a n d type metal on percent elongation of a coating. Crack resistance of a coating is d e p e n d e n t o n its thickness, that is, the thicker the film, the lower the crack resistance. Values of crack resistance obtained by the m a n d r e l b e n d tests should be corrected for film thickness w h e n c o m p a r i s o n s are made between different coatings. ASTM D 522 contains corrections to be added to elongation values o b t a i n e d with coatings having thickness greater t h a n 1 rail (0.03 m m ) w h e n applied to 1/8-in. (0.8-mm)-thick steel panels (Table 1). Conical m a n d r e l b e n d test procedures similar to those given in ASTM D 522 are found in ISO Method 6860 a n d BS 3900. Cylindrical m a n d r e l b e n d test procedures similar to those given in ASTM D 522 are found in ISO 1519, DIN 35 152, and BS 3900 E l .

T-Bend Tests T-bend tests are a m e a n s of evaluating the flexibility of coated strip metal that is to be formed during a fabrication process (Fig. 4). Multiple 180 ~ bends of the coated metal are made, a n d the a m o u n t of cracking produced at each b e n d is visually determined. Ratings are classified as 0T, 1T, 2T, 3T, a n d so on. The 0T ( p r o n o u n c e d zero T) b e n d consists of m a k i n g a 180 ~ b e n d with the p a i n t o n the outside of the b e n d a n d pressing the b e n d flat so there is no space between the metal surfaces. This operation is repeated successively to produce a 1T ( p r o n o u n c e d one T), 2T, 3T, etc. b e n d s (Fig. 5). These successive bends result in two, three, etc. thickness o f the metal a r o u n d the first bend. It should be a p p a r e n t that the greater the n u m b e r of thicknesses a r o u n d which the coated metal is bent, the less severe the test. The DiAcro Brake F o r m m a c h i n e is suitable for this test.

FIG. 3 - A n illustration of a cylindrical mandrel test apparatus.

smaller and smaller diameter m a n d r e l s until cracking is apparent. The cracking-resistance value of a coated panel is the m i n i m u m diameter at which cracking does not appear. This testing procedure can be applied as a "pass~fail" test by d e t e r m i n i n g whether cracking is produced by b e n d i n g over a specified m a n d r e l diameter. A table for converting m a n d r e l diameter to percent elongation is given in ASTM D 522. The relationship between diameter of a m a n d r e l and the elongation of a coating has b e e n derived by Schuh a n d Theuerer [8] to be: Percent Elongation = lO0(t/(2r + t))

(1)

TABLE 1--Factors affecting elongation measurements of coated panels by mandrel bend tests. Panel Thickness

I/8

CORRECTIONSTOBE ADDEDFORTHICKNESSOFPANELSUBSTRATE 1/64in. Correction, % 1.5 2.0 3.0 4.0 5.9 1/32 in. Correction, % 3.0 4.0 5.0 7.7 11.1 1/18in. Correction, % 5.9 7.7 11.1 14.3 20.0

11.1 20.0 33.3

1/8

CORRECTIONSTOBE ADDEDFORTYPEOFMETALPANELSUBSTRATE 3/4 hard brass Correction, % 3.4 4.6 6.9 9.6 14.2 Annealed brass Correction, % 3.6 4.9 7.5 10.3 15.9 Cold-rolled steel Correction, % 3.3 4.4 6.7 9.0 13.8

29.1 33.5 28.0

1

3/4

Mandrel Diameter, in. 1/2 3/8

1/4

Metal Type

1

3/4

Mandrel Diameter, in. J/2 3/8

1/4

Metal Type

I

3/4

Mandrel Diameter, in. 1/2 3/8

1/4

CORRECTIONSFORFILMTHICKNESSTOBE ADDEDPERMIL OFCOATING 3/4 hard brass Correction, % 0.21 0.26 0.38 0.50 0.73 Annealed brass Correction, % 0.21 0.26 0.38 0.50 0.74 Cold-rolled steel Correction, % 0.21 0.26 0.38 0.50 0.73

1/8

1.38 1.43 1.37

CHAPTER 4 7 - - F L E X I B I L I T Y AND TOUGHNESS

551

SPECIMEN DINGDIE

FIG. 4-T-Bend test using a die around which the specimen is bent (ASTM D 4145: Test Method for Coating Flexibility of Prepainted Sheet).

iNSERTTHISENDINV I S E ~ \ ~ 1/2in. TO3/4 in.

///

I

fl

COATEDSURFACE ~ /

A

2 in. MINIMUM

WIDTH

0T BEND

(.,

/COATED SURFACE/ 1TBEND FIG. 6-Erichsen Cupping Tester (early model),

SCOATED 9 URFACE. /

2TBEND

9

3TBEND FIG. 5-T-Bend test in which the coated specimen is bent around itself (ASTM D 4145: Test Method for Coating Flexibility of Prepainted Sheet).

Test results are reported as passing the smallest T-bend on which cracks are observed. In some cases, cracking can be detected by removal of a pressure-sensitive tape placed on the bend edges and observing the degree of removed coating particles. ASTM Test Method for Coating Flexibility of Prepainted Sheet (D 4145) describes this test procedure.

Cupping Tests A relatively slow rate of forming test can be conducted with a cupping tester that pushes a punch into the unpainted side of a coated panel until the increasing deformation produces cracks in the coating. Test procedures are given in ISO TC 35, BS 3900 E4, NFT 30-019, SIS 18 41 77, DIN 50 101, and DIN 50 102. There are six models of Erichsen Cupping Testers; they provide different test conditions to simulate different forming operations. Two of these models are shown in Figs. 6 and 7. BYK-Gardner Cupping Testers are a]so suitable for conducting these test procedures (Fig. 8). The BYK-Gardner devices use a spherical punch and provide a range of cupping speeds. The maximum cupping depth is approximately 18 m m (0.7 in.). The cupping action is stopped when cracking in the coating is visually detected. The depth of cupping at that point is indicated on a digital display and is considered to be the flexibility rating. The cupping tester can be equipped with a stereo microscope for observing the onset of cracking.

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PAINT AND COATING TESTING MANUAL

FIG. 8-BYK-Gardner Cupping Tester.

Forming Tests

9

FIG. 7-Erichsen machine for testing stamping lacquers (early model). (Figure from previous edition of this manual.)

In many industrial operations, metal is coated flat and then formed into various shapes by drawing the coated metal. This can be simulated directly or by elongating a coated metal sheet. A testing machine that has provided useful formability evaluations is the Erichsen Stamping Testing Machine (Fig. 7). It provides deformations in a few seconds by application of high pressure to a stamping tool. The stamped coated metal is examined for cracking as a "pass/fail" test. Any tension testing instrument capable of rapidly elongating a metal strip can be used for determining drawability. A coated metal strip would be elongated at a high rate of strain until cracking occurs. The elongation would be measured with an extensiometer [1]. Drawability would be reported as the percent elongation obtained just before cracking is observed. Since elongation is rate dependent, the rate of elongation used should be reported. ASTM Test Method for Formability of Attached Organic Coatings with Impact-Wedge Bend Apparatus (D 3281) describes a procedure for determining the formability of coated metal strips using an impact wedge bend apparatus termed the Coverall Bend Tester (Fig. 9). At the start of the test, the coated panel is bent 170 to 180 ~ over a 1/s-in. (3.2 mm) cylindrical mandrel attached to an impact platform. The platform is adjusted to provide a taper of 0 to 1/s-in. This allows the platform to create a wedge that provides stress angles between 170 and 180~ The end of the coated panel with the 180~ stress angle is defined as having 0T bend. The height of the impacter is adjusted until the load to produce a 0T bend is determined. The distance of the cracking produced in the coating by the impact is measured from the edge of the most severe bend outward to the edge of the least severe bend. The amount of film removed from the coating is indicative of its lack of formability. ASTM Test Method for Formability of Zinc-Rich Primer/ Chromate Complex Coatings on Steel (D 4146) also provides a procedure for determining the formability of coated strip metal. An outline of a testing machine that can produce a sufficiently high pressure for pressing a l%-in. (41-mm)diameter indenter ball into the coated metal is provided. The rate of forming can be adjusted over a range of 0.2 to 1.0 in./min (4.8 to 25 mm/min). A dial gage monitors the movement of the indenting ball. Adhesive tape is applied over the dome formed in the metal, and the tape is rapidly removed. The amount of coating removed is given a rating by comparing it with a set of photographic standards.

CHAPTER 4 7 - - F L E X I B I L I T Y AND TOUGHNESS

FIG. 9-Coverail bend test, After specimen is bent over a 1/e-in. mandrel, the bent portion is shaped into a wedge when a more severe test is needed (ASTM D 3281: Test Method for Formability of Attached Organic Coatings with impact-Wedge Bend Apparatus).

553

cover a range of 0.5 to 60% elongation. See Federal Test Method Standard 141C, Method 6226. ASTM Test Method for Impact Resistance of Pipeline Coatings (Falling Weight Test) (G 14) describes a test procedure for determining the impact resistance of pipe coatings. A fixed weight of 3.0 lb (1.36 kg) and having a s/8-in, nose diameter is dropped through a guide tube onto a coated pipe specimen. The height of the weight is adjusted until the minimum height at which cracking appears is attained. A pin hold detector is used to determine the presence of cracks in the impacted pipe. An equation is given for calculating the impact resistance from the weight and its height of drop required to just produce cracking. A different type of impact tester was developed and is being used at the Bell Laboratories of AT&T (Fig. 11). A coated panel is subjected to repeated glancing blows by a case-hardened steel ball at the end of a short arm that is pivoted to another arm connected to a rotating shaft. During the test, the coated panel is mounted on a platform that moves so that successive blows do not strike the same spot. The energy level of the blows may be held constant, as in a "pass/fail" test, or it

Impact Resistance Tests The most commonly used impact testers drop a weight onto an indenter resting on the surface of a coated panel that is resting on a platform (Fig. 10). A die in an opening in the platform allows the panel to be pushed down by the indenter to form a dimple in the panel. The weight is dropped through a guiding tube whose height is marked in increments. There are a number of possible combinations of weights, indenter sizes, die sizes, and weight heights that can be used in performing impact tests. The tests can be performed by impacting either the coating directly (coating facing upward) or indirectly (coating facing downward). Cracking observed on or around the impact-produced dimple is considered failure, and the force to produce the cracking is given in inch-pounds (killigrams-meters), that is, weight times height. The test can be performed either to determine the inch-pounds required to produce cracking or to determine whether a coating passes or fails at a specified inch-pound value. ASTM Test Method for Resistance of Organic Coatings to the Effects of Rapid Deformation (Impact) (D 2794) describes such a test procedure and offers three procedures for determining the degree of cracking produced in an impact deformation: (a) visual inspection with a magnifier, (b) visual inspection after application of an acidified copper sulfate solution, and (c) use of a pin hole detector. The General Electric Impact Flexibility Tool is used for simultaneously making several indentations of different sizes. From these indentations, conclusions can be made regarding crack resistance and the amount of draw that a coating applied to sheet metal can tolerate. This tester consists of a steel cylinder that has knobs (segments of spheres) of different radii machined on each end. The cylinder is dropped onto a coated panel that is supported coating side down by a rubber pad. The height of drop is adjusted so that the boundary of the cylinder is just discernible. This procedure assures that each knob is used to its limit. Eight knobs

FIG. 10-Gardner impact tester, a falling weight impact tester.

554 PAINT AND COATING TESTING MANUAL visually r a t e d for cracking. In s o m e cases, an exposure of 2 h in a n oven at 150~ (65~ is i n t r o d u c e d into the above cycle conditions. A r a p i d cold crack t e s t that has been developed is b a s e d on the use of cooled air entering a transparent, i n s u l a t e d box. Cold air t h a t has b e e n cooled at a r a p i d controlled rate is i n t r o d u c e d into the box, a n d the coatings are observed for cracking. The coatings are then r a t e d by d e t e r m i n i n g the t e m p e r a t u r e decrease from r o o m t e m p e r a t u r e that is required to p r o d u c e visual cracks in the coatings [4].

Effects of Aging and Weathering

FIG. 11-Bell Laboratories impact tester and specimen. Ball on end of the rotating arm repeatedly strikes specimen, which is moving from left to right (courtesy of Bell Laboratories) [1].

c a n be a d j u s t e d by c h a n g i n g the speed of the rotating shaft to d e t e r m i n e the onset of cracking. If the h a m m e r energy level r e q u i r e d to d e s t r o y the coating is desired, a t r a n s p a r e n t , calib r a t e d scale of shaft speed in revolutions is s u p e r i m p o s e d over the i m p a c t pattern. This tester is not c o m m e r c i a l l y available. There are a n u m b e r of o t h e r i m p a c t testers t h a t have been developed over the years a n d u s e d to s o m e extent. These include the P a r l i n - d u P o n t Tester, C a m p I m p a c t Test, H a r t I m p a c t Tester, Ball Punch, General Electric Ball Drop, a n d Navy Falling Ball test. None of these testers are c o m m e r c i a l l y available.

The u l t i m a t e m e a s u r e of satisfactory flexibility a n d toughness of a coating a p p l i e d to a s u b s t r a t e is p e r f o r m a n c e u n d e r service conditions. Most flexibility a n d toughness tests are p e r f o r m e d on relatively fresh-coated panels, that is, tests are usually p e r f o r m e d after the panels have been c o n d i t i o n e d in a specified a t m o s p h e r e for a specified p e r i o d b e t w e e n 24 h a n d seven days. The results o b t a i n e d are applicable to service conditions if these are c o n c e r n e d with post f o r m i n g o r service i n d o o r s w i t h o u t a d e g r a d i n g a t m o s p h e r e , since m o s t coatings do not c h a n g e a p p r e c i a b l y in their physical service p r o p e r t i e s u n d e r such conditions. However, if the service c o n d i t i o n s include exposure to weathering, this factor can cause appreciable changes to o c c u r in the coatings properties. The effects of moisture, t e m p e r a t u r e changes, a n d exposure to sunlight (ultraviolet wave lengths) e n c o u n t e r e d in o u t d o o r exposure generally reduce the flexibility a n d toughness of organic coatings. Therefore, it often is desirable to c o n d u c t tests for flexibility a n d toughness after p e r i o d s of w e a t h e r i n g to d e t e r m i n e h o w a coating will p e r f o r m u n d e r actual w e a t h e r conditions [1].

Cold Crack Resistance Tests

REFERENCES

Tests in w h i c h coatings on substrates are cycled t h r o u g h elevated t e m p e r a t u r e , low t e m p e r a t u r e , a n d r o o m t e m p e r a ture e n v i r o n m e n t s are called cold c r a c k tests. They have b e e n used in the coatings i n d u s t r y for m a n y years as an i n d i c a t i o n of the ability of a coating to resist cracking in service a n d therefore are c o n s i d e r e d to be tests of coating flexibility. ASTM Test M e t h o d for T e m p e r a t u r e - C h a n g e Resistance of Clear Nitrocellulose L a c q u e r Films Applied to W o o d (D 1211) is a n e x a m p l e of such a cold c r a c k test. It describes a proced u r e for testing l a c q u e r coatings a p p l i e d on wood. The testing cycles consists of 1 h at 120~ (49~ 1 h at - 5 ~ ( - 2 1 ~ a n d 1/2 h at r o o m t e m p e r a t u r e . Results are r e p o r t e d as the n u m b e r of cycles r e q u i r e d to p r o d u c e visible cracking in the coating. Automotive coatings are subjected to cold crack cycle tests. A typical test for exterior coatings on m e t a l panels consists of (1) equilibration at r o o m t e m p e r a t u r e , (2) exposure in a hum i d i t y c a b i n e t at 100~ (38~ a n d 100% relative h u m i d i t y for 20 h, a n d (3) exposure in a freezer at - 22~ ( - 30~ for 4 h. After r e m o v a l from the freezer, the coated panels are allowed to s t a n d at r o o m t e m p e r a t u r e for 2 h. Then the coatings are

[1] Schurr, G. G., "Flexibility,"Paint Testing Manual, ASTM STP 500, 13th ed., H. A. Gardner and G. G. Sward, Eds., American Society for Testing and Materials, Philadelphia, 1972, pp. 333-337. [2] Skrovanek, D. J. and Schoff, C. K., "Mechanical Analysis of Organic Coatings," Progress in Organic Coatings, Vol. 16, 1988, pp. 135-163. [3] Moore, R. J., "Molecular Basis for Impact Resistance of Epoxy Paint Films," Journal of Paint Technology, VoL 43, No. 554, March 1971, pp. 39-46. [4] Morse, M. P., "Physical Properties of Paint Films Relating to Service," presented at Gordon Research Conferences, Organic Coatings Section, 15-19 Aug. 1955. [5] Varadarajan, K., "Review of Dielectric and Dynamic Mechanical Relaxation Techniques for the Characterization of Organic Coatings," Journal of Coatings Technology, Vol. 55, No. 704, September 1983, pp. 95-104. [6] Tordella, J. P., "Mechanical Properties of Amorphous Polymers," Official Digest, Vol. 37, 1965, p. 349. [7] Supnik, R. H., "Rate Sensitivity: Its Measurement and Significance," Materials Research Standards, Vol. 2, 1962, p. 498. [8] Schuh, A. E. and Theuerer, H. C., "Measurement of Distensibility of Organic Finishes," Industrial and Engineering Chemistry, Vol. 9, 1937, p. 9.

MNL17-EB/Jun. 1995

Hardness by Paul R. Gudvin, Jr. 1

P H Y S I C A L C O N C E P T S OF H A R D N E S S TESTING HARDNESS IS A TERM HAVING a d i f f e r e n t m e a n i n g to d i f f e r e n t

people. It is resistance to penetration to a metallurgist, resistance to wear to a lubrication engineer, a measure of flow stress to a design engineer, resistance to scratching to a mineralogist, and resistance to cutting to a machinist. While these actions appear to differ greatly in character, they are all related to the plastic flow stress of the material, i.e., Young's Modulus, Y [1]. K. Sato wrote an overview paper on the hardness of coating films [2] which merits mentioning and reviewing. Hardness is not a fundamental property of materials but a composite one dependent on the elastic moduli, elastic limit, the hardening produced by "working" a metal, etc. Empirical relationships are used to determine other properties from the easily measured hardness, but all such schemes are of doubtfill or limited validity. Hardness testing can be a very useful tool for studying modern materials, but it is plagued by well-known experimental difficulties. Reasons for the unusual behavior of hardness data at very low loads are explored by Monte Carlo simulation, which will be discussed later. These simulations bear remarkable resemblance to the results of actual hardness experiments. The limit of hardness as load or indentation depth tends to zero, which is shown to depend on experimental error rather than upon intrinsic material properties. The large scatter of hardness data at very low loads is ensured by the accepted definition of hardness. A new definition of hardness is suggested which eliminates much of this scatter and possesses a limit as indentation depth approaches zero. Some simple calculations are used to show the utility of this new approach to hardness testing. Over the years, many methods and devices have been employed to measure the hardness of organic finishes. P. C. Wheeler, chairman of a technical committee within the Dallas Paint and Varnish Production Club, as it was called then, reported [3] the results of a survey of findings concerning how coating hardness is measured. Following this survey, the first meeting of what is now called ASTM Task Group DO1.23.14, Hardness, Mar and Abrasion Resistance, was held in June 1947. The Hardness Group of Subcommittee XVIII, as they were called at the time, of Committee D-1 was organized in Atlantic City to provide an opportunity for expres~President, P. R. Gu6vin Associates, P.O. Box 811, Westerville, OH 43086-0811.

sion of opinions by those present concerning their understandings of concepts that were connoted by hardness as applied to organiccoating films. The consensus was that the subject of hardness is very complex. Several of the characteristics of an organic film are simultaneously judgmentally weighed in order of relative importance to obtain the usual expression of hardness judgment. The same physical characteristics of films were employed, and the weighing importance of the various characteristics chosen is not carried out in the same manner; choices are operator dependent and are based on experience with results in practical use of the material. At the time of this first meeting in 1947, the purpose of the group was to (1) study the subject of hardness and attempt to define some of the physical properties or attributes of an organic coating which affect hardness, (2) limit the study to the development of procedures for measuring the attributes of film hardness, and (3) further evaluate limitations to smooth films of organic coatings as they are normally applied on a substrate. Switzer [4] conducted a survey to determine which methods were being used by the coatings industry to measure hardness. They found that some form of scratching or abrasion was used 84% of the time, pendulum or damping hardness (also referred to as entropy hardness) 56% of the time, and indentation hardness 20% of the time. The percentages total more than 100% because some companies surveyed use more than one procedure to evaluate hardness. In 1991, ASTM Task Group D01.23.14 conducted a similar survey. The study showed pencil hardness to be the most commonly used test method with the Sward-type rocker method the next most widely used [5]. The m o d e m trend in industries as a whole has been towards an increasing use of indentation methods.

Scratch Hardness Scratch hardness is the oldest form of hardness measurement and was probably first developed by mineralogists. Back in 1822, F. Mohs [6] evaluated comparative scratch hardness of many materials. Assuming the liquid state to be equivalent to "zero" hardness, he arranged solid materials into ten hardness groups, rating them as follows: talc, 1; gypsum, 2; calcite, 3; fluorite, 4; apatite, 5; orthoclase, 6; quartz, 7; topaz, 8; corundum, 9; diamond, 10. As the system was set up, any material of a given Mohs hardness number could scratch any other material with a lower Mohs hardness, and about 99% of all known materials have hardness ranging from Mohs 1 to 9. However, the Mohs scale, though conve-

555 Copyright9 1995 by ASTM International

www.astm.org

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P A I N T AND COATING T E S T I N G M A N U A L

nient to apply, is essentially qualitative in nature. The wide variety of hardness test procedures that have been used may be described by the following sections. As user demands for improved resistance to scratch and hardness increased for applications such as automotive finishes, high-performance coatings were developed and became more widely used. As the primary objective was to improve the surface hardness rather than resistance to deformation of the coating, scratch resistance tests were examined [7]. The coatings industry developed, adapted, or adopted various instruments and test methods, which are described below.

Bierbaum Microcharacter--This instrument, designed by C. H. Bierbaum [8-12], has a polished prismatic diamond cube (diamond pyramid) held in an elastic support as a scratching tool. It is a rather elaborate device consisting of a microscope, stage, and diamond tool on a balanced arm. The sharp point is the corner of a cube, one edge of which acts as a leading edge, being inclined to the horizontal surface of the specimen at 35.25 ~. The lubricated specimen is slowly moved under the point, and the standard load is 3 g. The scratch width w in micrometre is measured under a microscope according to recommendations made by the inventor, and several readings are averaged to give: Bierbaum Microhardness -

104 w2

(1)

Bierbaum scratch hardness is the ratio of the load on the diamond, in kilograms, to the square of the scratch width (w), in millimetres. It was marketed in the United States by the AmeriCan Optical Company primarily for testing the hardness of metals. It had been used on plastics [13] but never gained general acceptance for use on organic finishes. In 1958, ASTM Committee D-20 on Plastics adopted D 1526-58T, Tentative Method of Test for Bierbaum Scratch

Hardness of Plastic Materials. Their published [14] statement said: This method fills a need for a test to determine the relative resistance of a plastic surface to defacement by a sharp abrasive particle as occurs in tableware, optical elements, and similar applications. When the Bierbaum Microcharacter Hardness Tester wasn't useful any more for ASTM purposes, it was withdrawn in 1964.

Clemen Scratch Hardness Tester--The current instrument, shown in Fig. 1, is marketed by Erichsen GMBH & Co. This device is available in two versions: hand-operated and motor driven. Both determine the scratch resistance of protective surface coatings, such as paint and lacquer finishes, plastic coatings, etc. It consists of a sliding test panel carrier mounted on a base frame. A scratching tool is fixed at the end and a sliding weight in the middle of a counterpoised lever that is supported by two pillars. Operated by means of a dropping and lifting mechanism, the scratching stylus or needle is moved along the test surface during the working stroke, but is lifted for the return traverse. The weight-loaded blade or ball-shaped carbide tool is applied with a defined force (0 to 20 N) to the specimen, which is moved with constant speed. The scratch hardness is measured by the force necessary to cut through the coating to the substrate. It is operated manually or uses a motorized drive. Suitable scratching tools are: the Clemen scratching stylus, a chisel-shaped tool with a tungsten carbide edge, or a scratching needle according to Danske Elv~erkers Forening (DEF) 1053, Method 14, which is an inexpensive, ball-shaped, hardened-steel tool easily replaced when worn. Dantuma Scratch Tester--This device was developed in 1940 by H. Dantuma in the physical laboratory of Sikkens (now Akzo-Sikkens) [15,16]. It employs a novel means of increasing the load during travel of the scratching tool across

FIG. 1-Clemen Scratch Hardness Tester. (Courtesy of Erichsen GMBH & Co.)

CHAPTER 48--HARDNESS

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559

pearance. It has three tungsten carbide hemispheres or ball points 0.5 mm (Opel), 0.75 mm (Bosch), and 1.0 m m (to relate to International Standardization Organization [ISO] 1518) in diameter that are spring loaded. Holding the instrument upright and placing its point on the test surface, one draws a 5 to 10-ram-long line at a rate of approximately 10 mm/s. The stylus should produce a scratch that is just visible. By locking the slider each time, one can control the applied pressure, which is marked in newtons. Thus, one can gradually approach the correct setting. Three scales are engraved into the pencil for the three pressure ranges: 1.0 to 3 N (accuracy 0.1 N), 2.0 to 10 N (accuracy 0.5 N), and 3.0 to 20 N (accuracy 1.0 N). The hardness is defined as the minimum load or force, in g, on the ball point, that leaves a mark in the surface just visible to the unaided eye [I8].

"% !. ".... ~

FIG. 2-Dantuma Scratch Tester. (Figure from previous edition of this manual.)

the film. Referring to Fig. 2, as Arm B, with Tool G resting on Panel H, is lowered, Arm A follows. The scratching tool travels from H to L, a distance of 5 cm. The load varies from 0 to 5000 g. Operation may be by hand crank or motor. Four types of hardened-steel scratching tools are provided: a ball 1 rnm in diameter, a simulated finger nail, and two wedges. The Dantuma Scratch Tester was never produced for distribution.

du Pont Scratch Testing Machine--This type of instrument was once used at the du Pont lacquer plant at Parlin, New Jersey, for determining the hardness of lacquers and their resistance to scratching. It was one of the first to electrically signal the end point of the test. It is shown in Fig. 3. The device consists of a wooden base on which is mounted: (1) a fulcrum holding a graduated level equipped with weights and a needle point, (2) a transformer, (3) a 6-V lamp, and (4) a metal plate. These parts are connected in series, the transformer being used to step down ordinary light voltage to that of the small lamp. The metal panel, coated with lacquer, is placed coated side up on the metal plate under the needle. The weight is adjusted, and the panel is drawn along the plate in the direction of the long axis of the instrument. This operation is repeated, each time using an increased weight, until the needle penetrates the film. When this happens, the electric circuit is closed, and the lamp lights. Erichsen Hardness Tester--The Erichsen Company [17] markets a pocket-size hardness tester, Model 318, shown in Fig. 4, that somewhat resembles a mechanical pencil in ap-

Graham-Linton Hardness Tester--This device, illustrated in Fig. 5, might also be considered an adhesion tester. As shown in the figure, it is essentially a small, circular blade upon which pressure is exerted by a coil spring. A scale, graduated in 100-g increments from 0 to 2000 g, indicates the load on the blade. Hoffman Scratch Tester--The Hoffman Tester is one of the "old line" instruments in the paint industry and comes as close as any instrument to date of being "the paint chemist's educated knife." Figure 6 shows the original instrument that was developed and patented [19]. There is a low carriage with a weighted level on one end. The scratching tool is a sharpedged, hardened-steel cylinder with its axis at an angle of 45 ~ to the plane of the film. This cylinder is attached to the lever arm, and the load is varied by varying the position of the weight on the lever. This instrument has been used for adhesion and mar resistance tests. A General Electric Company test method [20] and a federal test method [21] specify how the instrument is to be used. Figure 7 shows the refined instrurnent. The edge of the hardened-steel tool is positioned at 45 ~ with respect to the test surface and can be loaded at any value between 0 and 250 g or 0 and 2500 g. The Hoffman Tester, when used in the lower range of up to 250 g loading, has been used for determining scratch resistance of a surface coating. It has had its greatest application, however, in the high range, up to 2500 g, for cutting completely through the coating to the support surface for measuring such properties such as degree of cure and adhesion. In use, the desired loading is set on the cutter dial, and the lower edge of the instrument is held against the test surface to

FIG, 3-du Pont Scratch Testing Machine.

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FIG. 6-Original Hoffman Scratch Tester.

FIG. 4-Erichsen Model 318 Hardness Tester. (Courtesy of Erichsen GMBH & Co.)

FIG. 7-Current Hoffman Scratch Tester. (Courtesy of The Paul N. Gardner Company, Inc.)

Inspector's Dur-O-Test Pocket Size Hardness Tester--The

FIG. 5-Graham-Linton Hardness Tester.

ensure uniform contact. While held in this position, the instrument is drawn in a direction away from the cutter. The nature of the mark left by the cutter, if any, is observed, and the test may then be repeated at a different cutter loading.

Inspector's Dur-O-Test Pocket Size Hardness Tester, shown in Fig. 8, is a simple pocket instrument used to evaluate the vulnerability (scratch hardness) of surfaces such as coatings, varnishes, plastic coverings, etc. Especially valuable for quick "on line" tests in plants, paint shops, building sites, etc., it determines the force required to scratch or scar a surface with a defined spherical tool. It consists of an engraving needle with spherical tip of tungsten-carbide of 0.75-ram (0.03-in.) diameter. The spring tension of the tip can be altered and set with a fixing device. The instrument has three interchangeable springs. The limitation is an inability to test elastic coatings. In the operation, the scratch needle extends slightly out of the jacket. A line is drawn on the surface to be tested in 1 s while maintaining sufficient pressure to keep the needle against the rod stop. If the tension is high, the surface will be clearly marked. If low, there will be no mark. The correct tension will result in a barely visible mark.

Laurie-Baily Hardness Tester--This apparatus [22], shown in Fig. 9, was among the first to be developed for films and was invented by A. P. Laurie and F. G. Baily of Heriot-Watt College, Edinburgh. The apparatus consists essentially of a hardened, blunt steel point upon which pressure is exerted by

CHAPTER 48--HARDNESS

559

by hand, the speed recommended by Parker and Siddle being 30 cm/min. Appreciably greater speeds give inconsistent results.

FIG. 8-Inspector's Dur-O-Test Pocket Size Hardness Tester. (Courtesy of The Paul N. Gardner Company, Inc.)

Pencil Hardness Tester--Rating the hardness of an organic finish according to the hardness of a lead (graphite) pencil that will just scratch it was described by Wilkinson [23]. Gardner [24] studied the method using pencils sharpened to different shapes: sharp cones, rounded cones, and chisels. He found the principal source of error lay in the character of the point because it was difficult to reproduce points. Other sources of error were the pressure on the pencil and the angle at which the pencil was held while it was moved over the organic finish. Gardner built a device to hold eight pencils at one time at an angle of 45 ~ to the panel, but found that it was impossible to align all pencils uniformly. Modern production has overcome this problem, and several companies offer a pencil hardness gage composed of eight mechanical drawing lead holders (pencils) permanently mounted in a circular array on a plastic cylinder. A small metal tube through the center of the cylinder provides storage for spare leads and a guide for positioning pencils for a test. ASTM Test Method for Film Hardness Test (D 3363) is practical for laboratory use, for use on a production line, or in the field to assess quantitatively the rigidity or firmness (elastic modulus) of organic coatings applied to rigid substrates such as metal or plastic. Hardness values may define requirements for particular coating applications or may be used to evaluate state of cure or aging of a coating. In this test, pencil leads of increasing hardness values are forced against a coated surface in a precisely defined manner until one lead mars (marks) the surface. Surface hardness is defined by the hardest pencil grade which fails to mar the organic coating surface. Today, pencils are available in about 14 different grades of hardness, ranging from the softest, 6B, to the hardest, 6H, although hardnesses greater than 6H have been available. Pencil leads are blends of graphite, clay, and binders. They range in hardness from softest to hardest as follows: 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, and 6H. Over the years certain features of the method have been standardized. They are: 1. The lead is "squared" against fine abrasive paper (as opposed to a sharp, fine point). 2. The pencil is held at an angle of 45 ~to the surface of the test panel.

FIG. 9-Laurie-Bailey Hardness Tester.

a vertical coil spring. The spring tension is controlled by an adjusting screw and can vary from 0 to 2000 g. The finish to be tested is placed under the point and slowly moved horizontally by hand as the pressure is increased until a scratch is made. Then the scale readings are recorded.

Parker-Siddle Scratch Tester--This tester is a very simple form of the Schopper type in which the toad on a needle is increased as the scratch is made. The point of the needle is a hemisphere 0.2 mm in diameter. The panel carrier is moved

Variations in the method have occurred with respect to how the test is actually carried out. Smith [25] made a study in 1956 and used the following method: 1. Strip the wood from the lead for a distance of approximately 1/4in. (6 ram) using care not to nick the lead. Square the exposed lead by a gentle rotary motion against No. 400 carbide abrasive paper. 2. Hold the pencil in a writing position, that is, at approximately 45 ~and push forward against the film. Use pressure short of breaking the lead. By turning the pencil after a test, a new edge is available for use, and three or four trials may be made with one dressing of the lead. 3. Clean the marks with a soap or "artgum" eraser. Any marring of the surface, visible at an oblique angle in strong

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PAINT AND COATING TESTING MANUAL

light, indicates that the pencil is h a r d e r t h a n the film. The h a r d n e s s is expressed as the grade of the next softer pencil.

that c a n n o t be r e m o v e d by r u b b i n g with a soft r u b b e r eraser.

In developing this m e t h o d , S m i t h d e t e r m i n e d the pencil h a r d n e s s of 14 different organic finishes that varied widely in hardness. Five different b r a n d s o f pencils were used, a n d the results are shown in Table 1. The results i n d i c a t e d that there were variations in h a r d n e s s between different b r a n d s of pencils a n d that it was necessary to use only one b r a n d to o b t a i n reproducibility. S m i t h also c o m p a r e d the K n o o p h a r d n e s s a n d S w a r d r o c k e r h a r d n e s s versus pencil h a r d n e s s of the 14 organic finishes. These results, also in Table 1, showed that pencil hardness r a t e d the various organic finishes in the s a m e o r d e r of h a r d n e s s as the two other m o r e e l a b o r a t e methods. This is interesting because, according to Smith, three widely different m e c h a n i s m s were involved. They are:

In 1972, ASTM Task G r o u p D01.53.02 Cure reviewed the results of the H o u s t o n Society for Paint Technology a n d und e r t o o k p r e p a r a t i o n of a pencil h a r d n e s s test m e t h o d . By 1973, r o u n d - r o b i n testing had been completed, a n d D 3363 received final approval on 25 Oct. 1974.

S w a r d r o c k e r = d e f o r m a t i o n within the elastic limit. K n o o p h a r d n e s s = d e f o r m a t i o n beyond the elastic limit. Pencil = d e f o r m a t i o n b e y o n d the elastic limit a n d tearing a w a y of material. Smith's w o r k p r o v i d e d a b r o a d f o u n d a t i o n for the pencil h a r d n e s s test, a n d m a n y features of his m e t h o d are used today. It was the exact m a n n e r in which the pencil was applied to the test that was never adopted. Those test m e t h o d s which have been a d o p t e d and are used a l m o s t universally were outlined by the H o u s t o n Society for Coatings Technology [26] in 1966. The H o u s t o n Society described two b a s i c methods, which were: 1. The Disbonding Method in which the pencil, at an angle of 45 ~ is p u s h e d into the organic finish. The organic finish is considered to have failed w h e n a pencil removes chips, flakes, scales, or shears the finish from the substrate without breaking the lead of the pencil. This m e t h o d has also been an a d h e s i o n test. 2. The Indentation Method consists of using the pencil as an i n d e n t a t i o n i n s t r u m e n t by d r a w i n g the p o i n t (at an angle of 45 ~ across the film to p r o d u c e a c o n t i n u o u s indentation. The H o u s t o n Society developed a special carriage to hold the pencil a n d apply a l o a d of 200 g to it while the carriage was being d r a w n across the test panel. The organic finish is considered to have failed w h e n the pencil leaves an indentation in the film (visible u n d e r a • 15 magnifying glass)

Rondeau Scratch Tester--This device, shown in Fig. 1O, was developed a n d p a t e n t e d [27] by H e r b e r t F. Rondeau, also belongs to the type where the load on the scratching tool a u t o m a t i c a l l y increases as the test is being m a d e . The tool, p a r a b o l i c in shape, is m o u n t e d on the free end of a cantilever spring, one end being moveable in a slot in the frame. At the start of a test, the tool rests on the test surface u n d e r zero load. The finish end of the slot is 0.100 in. (2.5 m m ) n e a r e r to the test surface t h a n it is at the start. At the finish end, the l o a d is the rated value of the spring. At i n t e r m e d i a t e distances, the load is p r o p o r t i o n a l to the distance. Three springs are provided, giving loads of 300, 600, a n d 1200 g at the finish end. Scheppard-Schmitt Scratch Dynamometer--The principle of this device was e m p l o y e d by E a s t m a n Kodak's S. E. Schepp a r d a n d J. J. S c h m i t t [28] in the d e v e l o p m e n t of a n e w scratch hardness instrument. The scratching tool is a hardened steel, 45 ~ tetrahedron. M e a s u r e m e n t of scratch resistance is expressed as the threshold load p r o d u c i n g a scratch o r by a curve expressing the relation between the load a n d size (width) of the scratch. Schopper Hardness Tester--This device [29], shown in Fig. 11, was one of the first to provide for a u t o m a t i c a l l y increasing the load on the scratching tool while the scratch is being made. Arms extending u p w a r d from the panel c a r r i e r end in slots above the b e a m carrying the scratching tool. A roller resting on the b e a m is guided by the slots. As the panel carrier is d r a w n along, the roller travels with it, t h e r e b y increasing the load on the scratching tool. Provision is m a d e for automatically lifting the load from the s p e c i m e n at the end of each trip a n d also for a sidewise d i s p l a c e m e n t of the s p e c i m e n to provide a new p a t h for r e p e a t tests. The i n t e r p r e t a t i o n of results is the s a m e as with other types of scratching devices, that is, according to the c h a r a c t e r of the m a r k u n d e r a partic-

T A B L E 1--Hardness test correlation (Smith).

Panel No.

Knoop Hardness

Sward Hardness

Pencil Brand A

Pencil Brand B

Pencil Brand C

Pencil Brand D

Pencil Brand E

1 2 3 4 5 6 7 8 9 10

3.09 4.33 2.77 2.61 5.81 9.23 11.2 21.1 17.4 25.7

11

21.0

12 13 14

39.1 34.9 ...

24 28 24 22 38 50 25 58 54 54 60 40 30 40

5B 4B 5B 3B 2B HB HB F F H 2H 3H 6H 8H

6B 6B 6B 4B 2B F F H F H 2H 2H 5H 9H

5B 6B 5B 5B 2B HB HB H F H 2H 3H 5H 7H

6B 6B 4B 4B 2B HB HB H H H 4H 4H 5H 7H

4B 4B 4B 3B 3B HB H 2H 2H 2H 3H 4H 6H 9H

CHAPTER 48--HARDNESS AT START OF TEST AT END OF TRAVEL -,; " ~ l

.~'--

J 4

INGREASING STYLUS FORGE --

/

'~:':':::!-V~,':

suP~T A2-.2~'~ '.ULL YLUS XGURS,O..,O0 .. \

\~,,,~, \,,, oa~:_ i x\\\~\ xx\\\\x\~, \\',,,,x\\\\\\\\x\\,, \\xk\\\~~, ,xx\

.u ,oRT

|

TEST

SURFAGE

ZERO FORGE AT STARTIN6 POSITION

FIG. 10-Rondeau Scratch Tester.

561

under controlled conditions that enable quantitative evaluation of the ability of coatings to withstand repeated horizontal and vertical abrasions. There are three scratching tools: a 1-mm cutting carbide sphere, a Clemen designed scratch cutting tool, and a VW designed scratch cutting tool; two speeds: 35 and 1 mm/s, and two load ranges: 0 to 20 and 0 to 90 N. The test is performed by selecting a load which is applied to the cutter and can remain either constant in value or automatically increased at a linear rate. The weighted cutter is brought into contact with the coated surface at a constant speed, and it penetrates the coating when a critical force is attained. The force required to be applied to the tool to penetrate the coating and just touch the substrate is the measure at the scratch hardness. Testing using the automatically increased force shortens the test and quickly evaluates the scratch resistance at the coating.

Simmons Scratch Tester--This coating hardness measuring instrument [31] is another tester of the increasing load type and is suitable only for films on metal. When the stylus breaks through the film, a relay stops the machine. Hardness is reported as the weight necessary to penetrate the film.

FIG. 11-Schopper Hardness Tester. (Photo from previous edition of this manual.)

ular load or tool, or the load at which a particular tool makes a mark.

Sheen Scratch Tester--This instrument determines the scratch resistance (i.e., scratch hardness) of paint coatings and is designed to meet the requirements of the British Standards (BS) Method of Test for Paints (BS3900: Part E2). Its usefulness, however, extends beyond the rigid limits for operating conditions set by this authority as the test provides data outside the specification. Performance is related to many factors that include the hardness of the coating with other physical properties such as adhesion, lubricity, resilience, etc., as well as the influence of coating thickness and curing conditions. It is a quantitative indication of the extent to which serious damage is resisted when a loaded needle is raked across a relatively smooth, flat surface. The instrument is shown in Fig. 12. The needle arm is counterpoised and rigid to prevent whip or chatter at the ball point. Weights totaling 2000 g and providing increments of 100 g from 100 g are supplied and additional weights are available. A total load of 6 kg can be used for very hard coatings. A 2000-g weight (or two) is useful for baked coatings. The l-ram tungsten carbide-tipped needles are held in a chuck and can readily be removed for inspection and replacement. Sikkens Scratch Hardness Tester--Sikkens Scratch Hardness Tester, Model 601, is marketed by Erichsen Company [30] and conforms to ISO 1518, BS 3900:E2 and Stichting Nederlands Normalisatie-Instituut (NEN) 5336. This device simulates a scratching or scouring action and creates stresses

Steel Wool Scratch Tester--The Panelgraphic Rotary Steel Wool Scratch Tester (Fig. 13) is constructed so scratch resistance may be measured using loads of 13 and 24 lb/in. 2 (0.9 and 1.7 kg/cm 2) on the steel wool pad attached to a square testing foot of 1.25 in. 2 (8.065 cm2). The 0000 steel wool on the testing foot is then rotated for five revolutions, after which the sample is visually inspected for scratches in the coating and rated. The test then may be repeated in other locations to determine uniformity of the coating. This instrument is being removed from production and will not be available for purchase. While this test may be acceptable for materials that are homogeneous or essentially homogeneous, when attempting to measure the scratch resistance of relatively thin coatings, other complicating factors arise. The measured scratch resistance of the coating is dependent upon factors independent of the coating itself, including the coating thickness and the substrate over which the coating is applied. Stated another way, scratch resistance is not an intrinsic property of a coating, and it may mean different things depending on how the property is measured. When attempting to measure only the resistance of a material to surface scratching, the concept of "mar resistance" comes into play. Teledyne Taber Shear~Scratch Tester--Shear and scratch tests are significant because rigorous controls are exercised over these parameters affecting materials' resistance to shear and scratch. The unique Model 502 Teledyne Taber Shear/ Scratch Tester [32] features three cutting tools: the S-20 tungsten carbide tool for shear testing and 139-55 and 139-58 diamond tools for scratch testing. The instrument is shown in Fig. 14. A tool is fixed to the underside of a beam pivoted on ball bearings. Riders provide for adjusting the load on the tool between 0 and 1000 g. The test films for this tester are prepared on panels containing a hole in the middle for locating on a turntable. In making a test, the panel is rotated counterclockwise. Three tools are provided: "thumb-nail" contour shear tool (S-20) lapped to a

562

PAINT AND COATING TESTING MANUAL

FIG. 12-Sheen Scratch Tester. (Courtesy of The Paul N. Gardner Company, Inc.)

FIG. 13-Steel Wool Scratch Tester. Courtesy of The Paul N. Gardner Company, Inc.) 25-mm radius with a 30 ~ clearance, a diamond cut to the shape (diamond pyramid) of a corner of a cube, and a diamond cut to the shape of a cone.

Wolff-Wilborn Scratch-Hardness Tester--This test apparatus, shown in Fig. 15, is another pencil method that belongs to the group of scratch-hardness testing instruments that is a

FIG. 14-Teledyne Taber Shear/Scratch Tester. (Courtesy of The Paul N. Gardner Company, Inc.)

simple and quick method for testing the surface hardness of coatings with regard to stresses inflicted by scratching with sharp edges or other rough surfaces. The speed of measuring even permits testing during the production, e.g., coil coating. The test instrument, Model 291, enables the test to be carried out in accordance with Wolff-Wilborn and ensures that the specified force and angle remain constant throughout. This

CHAPTER 48--HARDNESS

563

V. (DIN 53 153) [33]. Like its n a m e implies, it is a universal h a r d n e s s tester.

Indentation Hardness

FIG, 15-Wolff-Wilborn Scratch Hardness Tester. (Courtesy of Erichsen GMBH & Co.)

i n s t r u m e n t is specified in MIL C 27227, w h i c h has b e e n disc o n t i n u e d as a specification. In the test, pencils of various grades of h a r d n e s s are moved over the p a i n t e d surface at an angle of 45 ~ to the h o r i z o n t a l with a 7.5-N (735-dyne) force. The softest pencil h a r d n e s s that p r o d u c e s injury to the film is the Wolff-Wilborn h a r d n e s s of the coating.

Universal Hardness and Adhesion Test Instrument--This Erichsen Model 413 h a r d n e s s tester, s h o w n in Fig. 16, will allow a n o p e r a t o r to d e t e r m i n e Clemen S c r a t c h Hardness, Micro S c r a t c h H a r d n e s s ( m a r resistance), a n d the Buchholz I n d e n t a t i o n H a r d n e s s test Deutsches Institut ftir N o r m u n g e.

M i c r o h a r d n e s s testing has proved to be very p o p u l a r in m a n y industries b e c a u s e of its simplicity a n d nondestructive nature. It has b e e n p a r t i c u l a r l y successful for quality control w o r k w h e r e it can be used as an i n d i c a t o r of surface durability and, in s o m e cases, of strength. Static i n d e n t a t i o n hardness tests are "nondestructive" physical tests that e m p l o y either a ball, cone, or p y r a m i d t h a t is forced into a surface. The load p e r unit a r e a of i m p r e s s i o n is t a k e n as the m e a s u r e of hardness. The testers use indentors classified by the following names: Brinell, Rockwell, Vickers, a n d Knoop. A sketch of these indenters is s h o w n in Fig. 17. The h a r d n e s s of a m a t e r i a l can be defined as a m e a s u r e of its resistance to indentation. Basically, a n i n d e n t a t i o n hardness test can be classified into two categories [34]: 1. Those b a s e d on a m e a s u r e of the residual d e f o r m a t i o n after the i n d e n t e r was removed. 2. Those b a s e d on the l o a d - i n d e n t a t i o n characteristics. I n d e n t i o n h a r d n e s s values have been r e p o r t e d in a n u m b e r of different ways, such as: d e p t h of the indentation, the w i d t h of the indent, the l o a d necessary to p r o d u c e a specific d e p t h o r length of indent, the l o a d divided by the projected, p l a n a r a r e a of indent, etc. The l o a d divided by the projected, p l a n a r a r e a of indent is really an expression of p r e s s u r e a n d has b e c o m e the widely accepted m e t h o d of r e p o r t i n g i n d e n t a t i o n hardness. Consider the force on an a n n u l u s of r a d i u s x a n d the w i d t h dS. The load L is d i s t r i b u t e d over the c o n t a c t a r e a as a pressure P. The a r e a of the a n n u l u s lying on the curved surface of the i n d e n t a t i o n is 2wxdS, a n d the force on it is P2wxdS a n d can be resolved into two c o m p o n e n t vectors, dH a n d dV. By conditions of symmetry, the h o r i z o n t a l c o m p o n e n t dH is an-

FIG. 16-Universal Hardness and Adhesion Test Instrument. (Courtesy of Erichsen GMBH & Co.)

564

PAINT AND COATING TESTING MANUAL

FIG. 17-Hardness indenters' geometries and indentation shapes. (Courtesy of Wilson Instruments, Inc.)

E n g l a n d in 1925 by R. S m i t h a n d G. S a n d l a n d [36]. Its early a c c e p t a n c e by i n d u s t r y was limited to the largest l a b o r a t o ries, a n d its use was chiefly for research purposes. I n d e n t a t i o n h a r d n e s s testing using the Brinell a n d Scleroscope m e t h o d s grew in i m p o r t a n c e , a n d d u r i n g W o r l d W a r I practically all h a r d n e s s testing was d o n e o n either one or the o t h e r of these instruments. During this time, Stanley P. Rockwell, a metallurgist in a large ball-bearing m a n u f a c t u r ing plant, was p a r t i c u l a r l y c o n c e r n e d with h a r d n e s s control of ball races. As a result, he invented the tester w h i c h has b e c o m e k n o w n as the Rockwell H a r d n e s s Tester. The i n d e n t a t i o n h a r d n e s s of a m a t e r i a l is related to its modulus. The t h e o r y of the i n d e n t a t i o n h a r d n e s s of an elastic m a t e r i a l test has b e e n derived for a spherical indenter. Young's m o d u l u s E is related to the indenting force F, the r a d i u s of the i n d e n t e r r, a n d the d e p t h of i n d e n t a t i o n h. If the m o d u l u s of the i n d e n t e r is m u c h greater t h a n the m o d u l u s of the test surface, the r e l a t i o n s h i p is E = (3/4)(1 - vZ)r - 1/2h -2/3F

nihilated by an equal a n d opposite dH on the opposite side of the annulus. The vertical c o m p o n e n t dV is therefore P2zrxdS and

d2

L = P-n--4

P -

4L 7rd2

-

L Ap

(2) (3)

where P is the m e a n pressure a n d Ap is the projected, p l a n a r a r e a of indent. Therefore, the m e a n pressure on the surface of the i n d e n t e r is equal to the ratio of the load L to Ap. The i n d e n t a t i o n hardness tests are p e r f o r m e d by pressing an i n d e n t e r of p r e s c r i b e d g e o m e t r y against the test surface. The load is controlled at s o m e c o n s t a n t value, a n d the duration of the i n d e n t a t i o n process is usually specified for a viscoelastic material. The size of the i n d e n t a t i o n m a y be m e a s u r e d with a m i c r o s c o p e after the removal of the load. An alternate p r o c e d u r e is to m e a s u r e the d e p t h of i n d e n t a t i o n after a given time interval. The latter p r o c e d u r e is preferred for viscoelastic bodies. The h a r d n e s s n u m b e r is generally calculated by dividing the l o a d by the area of the indentation. The h a r d n e s s values o b t a i n e d are i n d e p e n d e n t of the s p e c i m e n thickness if the i n d e n t a t i o n d e p t h is less t h a n one tenth the s a m p l e thickness. Since coating films are very thin, the i n d e n t a t i o n a p p a r a t u s m u s t be capable of m e a s u r i n g precisely very small indentations. Because it is difficult to set the zero position, a small p r e l o a d m a y be applied before the a p p l i c a t i o n of the m a i n load. A n u m b e r of different instruments, d e s c r i b e d later, have been devised for m a k i n g i n d e n t a t i o n hardness m e a s u r e m e n t s on organic coatings. The beginning of the twentieth century m a r k e d a milestone in the history of h a r d n e s s testing. In t 900, Dr. J. Brinell, chief engineer at Fagersta I r o n Works in Sweden, p r e s e n t e d a pap e r to the Swedish Society of Technologists in w h i c h he described his ball test. In the s a m e year, he showed his hardness tester at the Paris Exposition. Following the Brinell innovation was the d e v e l o p m e n t of the scleroscope (1906) [35]. The 136 ~ d i a m o n d p y r a m i d hardness indenter, comm o n l y referred to as the Vickers indenter, was i n t r o d u c e d in

(4)

w h e r e v is Poisson's ratio (lateral c o n t r a c t i o n versus longitudinal extension). In the case of viscoelastic materials, a similar relationship holds, b u t the variation with i n d e n t e r r a d i u s a n d p e n e t r a t i o n are s o m e w h a t modified. M e r c u r i o [37] has discussed the relationship of Tukon h a r d n e s s to modulus. The theory of i n d e n t a t i o n h a r d n e s s tests on h o m o g e n e o u s m a t e r i a l s has received m u c h interest in the last few decades. D. T a b o r has b e e n w o r k i n g intensely in this area. In his recent p a p e r [38] he said The hardness of a solid is usually u n d e r s t o o d to m e a n its resistance to local d e f o r m a t i o n . The simplest m e t h o d of quantifying it is to press a h a r d i n d e n t e r of specific geometry into the body, divide the load by the a r e a of the i n d e n t i o n formed, a n d express the a n s w e r in units of k i l o g r a m s p e r square millimeters o r pascals (1 kg m m -2 10 7 Pa) . . . . F o r elastic solids such as rubber, the i n d e n t a t i o n p r e s s u r e is a direct m e a s u r e of the elastic p r o p e r t i e s of the material. A n o t h e r example of this type study is that of Lebouvier et al.

[39]. W. W. W a l k e r [40] evaluated the K n o o p h a r d n e s s of three organic coatings using a Model LR Tukon M i c r o h a r d n e s s Tester in a c c o r d a n c e with ASTM Test Methods for Indentation H a r d n e s s of Organic Coatings (D 1474) except he calib r a t e d the i n s t r u m e n t at 100 g l o a d a n d ran the tests at 200 g load. In addition, he tested the pencil hardness of the s a m e coatings in a c c o r d a n c e with ASTM Test M e t h o d D 3363. C o m p a r a t i v e d a t a are shown in Table 2. W. W. W a l k e r c o n c l u d e d that a useful correlation existed between the 200-g K n o o p i n d e n t a t i o n h a r d n e s s a n d pencil TABLE 2--Comparison of pencil and Knoop hardness of selected coatings.

Paint

Epoxy powder Polyurethane Solvent Epoxy Metal Panel

Pencil Lead No.

Lead Hardness, KI-IN

5H 51.5 3H 45.3 H 31.7 . . . . . .

Paint Hardness, KHN

Difference

30.2 22.7 8.9 195 _+ 1

21.3 22.6 22.8 ...

HARDNESS hardness of thick paint films but that further work needs to be done. Krautkrgmer Branson conducted a similar test using their MicroDur Portable Hardness Tester fitted with a Vickers indenter. The preliminary results, shown in Fig. 18, represent an evaluation of eight organic coatings. The results are promising. However, additional work is needed.

Bell Telephone Laboratories Indenting Rheometer--The BTL Indenting Rheometer, shown in Fig. 19, was designed and developed by Eugene M. Corcoran of the Bell Telephone Laboratories specifically for use as an indenting rheometer, sensitive enough for use with organic coatings but with sufficient load-deflection capacity to make it useful for relatively thick materials such as molded plastics and casting resins. Vicker8

To achieve this, two separate head assemblies were required. Basically the rheometer consists of a specimen stage or platform, indenter-LVDT transducer head assembly, weights, transducer amplifier indicator, and a 10-in. (25.4 cm) strip chart recorder. In normal use, the specimen or test panel is clamped on the platform, and the instrument is zeroed in with the indent or tip just touching the specimen. This is done by using the knurled rings on the heads and the platform ring (the rings on the sensitive head have 80 threads per inch or 80 threads/25.4 mm) to obtain a coarse adjustment follow by a fine adjustment on the transducer amplifierindicator. A load (weight) is applied to the weight tray (by means of an overhead pulley), and the depth of indentation is recorded as a function of time. After a specified period of time, the load is removed and the recovery is recorded.

Hardness

800

700

-

9

=

'

~

600

L-. . . . . . . . . . . . ~

-

~

....

500

......................................................................................................................................................

400

.........................................................................................................................................................................................

300

.........................................................................................................................................................................

200

I 2H

I 2H

L 2H

I H

565

J H

, H

i H

Pencil Hardness FIG. 18-Comparison of Vickers hardness versus pencil hardness.

FIG. 19-Bell Telephone Laboratories Indenting Rheometer.

i H

566

PAINT AND COATING TESTING MANUAL

Brinell Indentation Hardness Tester--In 1900, J. A. Brinell p u b l i s h e d the results of his tests that involved pressing steel balls into m a t e r i a l s [41]. The Brinell test is b a s e d on the following f o r m u l a H -

P

(5)

1rD (D - X/-D ~ - d2 2 The r e l a t i o n s h i p b e t w e e n the h a r d n e s s H a n d the d i a m e t e r of the d e p r e s s i o n d has b e e n w o r k e d out on a form for a given size steel ball whose d i a m e t e r is D.

Buchholz Indention Hardness Tester--This device, s h o w n in Fig. 20, is m a d e by the E r i c h s e n C o m p a n y [42] a n d has b e e n s t a n d a r d i z e d in G e r m a n y [33]. It is a simple, p o r t a b l e i n s t r u m e n t and, like the Knoop, m e a s u r e s the length of the recovered i n d e n t i m p r e s s i o n after the toad has b e e n removed. In fact, the resultant impression, t h o u g h m u c h larger, is quite similar in a p p e a r a n c e to the K n o o p impression. A d i a g r a m of the Buchholz I n d e n t i o n H a r d n e s s Tester in use is shown in Fig. 21. Basically a weighted (500-g) wheel, with an included angle of 60 ~ from center to each edge (total angle of 120~ is p l a c e d on a c o a t e d test panel. The test panel is m a r k e d "5" in the diagram, a n d the coating is m a r k e d "4." The wheel is removed, a n d the length of the i m p r e s s i o n m a d e b y the indenter, m a r k e d "3" in the diagram, that r e m a i n s is m e a s u r e d by m e a n s of a small, • 20 microscope, m a r k e d "2" in the diagram, a n d an a t t a c h e d light source, m a r k e d "1" in the diagram. To facilitate m e a s u r e m e n t , the i m p r e s s i o n is ill u m i n a t e d from the side, t h e r e b y creating a s h a d o w in that side of the impression. The i m p r e s s i o n m a y be m e a s u r e d to within 0.05 m m ( c o m p a r e d to 0.001 m m for the Knoop). CDIC Hardness Penetrometer--Still a n o t h e r variation applied to artists' colors is the CDIC (the old Cincinnati-DaytonI n d i a n a p o l i s - C o l u m b u s Paint a n d Varnish P r o d u c t i o n Club) H a r d n e s s P e n e t r o m e t e r [43]. A d i a g r a m of it is s h o w n in Fig. 22. By this device, a 1/2-in. chisel is caused to p e n e t r a t e the film that has been a p p l i e d to metal. Chisel a n d m e t a l are wired in series with an electric bulb, w h i c h lights up w h e n the chisel contacts the panel. The i n s t r u m e n t was never c o m m e r cialized. Fischerscope | Microhardness Tester2--The F i s c h e r s c o p e | H100V, shown in Fig. 23, is a d y n a m i c m i c r o h a r d n e s s tester w h i c h can be used on a variety of materials, including coatings, to m e a s u r e h a r d n e s s u n d e r load. It works with very small test loads up to 256 mN. D e t e r m i n a t i o n of h a r d n e s s b a s e d on the plastic and elastic d e f o r m a t i o n of a m a t e r i a l is the direct result of m e a s u r e m e n t s u n d e r load. H a r d n e s s m e a s u r e m e n t is expressed in N / m m 2, c o r r e s p o n d i n g to the quotient of l o a d P over area of i m p r e s s i o n A (whereby A can be derived directly from the d e p t h of indentation). This definition of h a r d n e s s is physically meaningful b y providing a meas u r e m e n t of hardness to an u n c e r t a i n t y of _+ 1%. This requires exact m e a s u r e m e n t of l o a d a n d i n d e n t a t i o n depth, w h i c h is possible with a F i s c h e r s c o p e | H100V M i c r o h a r d ness Tester. 2Available from Fischer Technology, Inc., 750 Marshall Phelps Rd., Windsor, CT 06095.

FIG. 20-Buchholz Indention Hardness Tester. (Courtesy of Erichsen GMBH & Co.)

FIG. 21-Buchholz Indention Hardness Tester in use. (Courtesy of Erichsen GMBH & Co.)

Using a F i s c h e r s c o p e | H100V M i c r o h a r d n e s s Tester, W. W. Weiler developed a dynamic, nondestructive test m e t h o d to m e a s u r e the m i c r o h a r d n e s s of surface layers, coatings, a n d h o m o g e n e o u s m a t e r i a l s in the ultra-low l o a d range of 0.025 to 1 N [44]. The m e t h o d was b a s e d on using a conventional Vickers i n d e n t e r c o u p l e d to a d i s p l a c e m e n t m e a s u r i n g device.

General Electric Indention Tester3--This device, developed b y C. Dantsizen, consists of a dial m i c r o m e t e r , the foot of w h i c h t e r m i n a t e s in a metal sphere 0.20 in. (5 m m ) in d i a m e ter, a n d with m e a n s for applying a load. The d e p t h of indentation is r e a d on the dial. The General Electric I n d e n t i o n Tester was not widely used within the General Electric Co. n o r did it gain i n d u s t r y acceptance. ICI Pneumatic Microindenter--This c o m m e r c i a l l y available device, 4 shown in Fig. 24, was developed b y M o n k a n d 3Christian Dantsizen, personal communication to G. C. Sward, 1938. 4Available from Research Equipment (London), Ltd., 64 Wellington Road, Hampton Hill, Middlesex, England.

CHAPTER 4 8 - - H A R D N E S S

the apparatus and went further than Gardner et al. [46] in the interpretation and meaning of the curves. Included in the data were curves showing how an alkyd finish changed properties as a function of temperature and accelerated weathering.

R o

C.,SEL--H

a

567

J ;

,

30 ~ BEVEL-'--

EO

._t

FIG. 2 2 - C D I C H a r d n e s s P e n e t r o m e t e r .

Wright [45], As the name implies, it is a pneumatic type (air pressure) instrument that measures and records the depth of indentation or penetration of a ball-ended needle under the application of a constant load and the recovery subsequent to removal of the load. The needles have steel or sapphire ends and vary from 0.0025 to 0.063 in. (1.6 mm) in radius. A preload of 0.1 g must be applied. A 5-in. (12.7-cm), pneumatically operated, strip-chart recorder provides curves of the indentation and recovery thereof. An indenter movement of 6 /xm causes a full-scale deflection on the recorder. A calibration knob is divided into 0.5-/xm divisions, thereby giving an accuracy of about 0.2/xm (0.008 rail). However, the chart can be read to within 0.1 /xm (0.004 mil). The specimen stage or table is a Frigister unit which can raise or lower the specimen temperature. Disadvantages are that it is not a sturdy instrument and must be handled with some care. The panel or specimen must be small enough to fit on the rather small Frigister stage. Also, the use of relatively sharp indenters (referred to by the authors as needles) means that, in many cases, the organic coating will be cut or penetrated, yielding spurious results. The load limit appears to be less than 100 g, and the design does not appear to be conducive to the use of commercially made Knoop or Vickers pyramid indenters. Finally, results obtained using the Frigister to heat or cool the specimen can be misleading. If the indenter is at room temperature, then the specimen temperature where the indenter contacts will not be the same as the temperature of the Frigister. However, the I e I pneumatic microindentation apparatus was still the first commercially available instrument that appears to have the sensitivity necessary to be seriously considered as an indenting rheometer suitable for use with organic coatings. Monk and Wright gave some results obtained from the use of

Knoop Indenter--At the National Bureau of Standards (now known as the National Institute of Standards and Technology), Frederick Knoop and his associates [47] developed a diamond-based pyramid indentation tool as an improvement over the Vickers indenter. This indenter gave well-defined indentations and reproducibility of results when testing glass and crystals of the Mohs scale, and dental plastics and enamels. The Knoop indenter, illustrated in Fig. 25, is a pyramidal diamond with included longitudinal angles of 172~ 30' and an included traverse angle of 130~ 0'. It produces a diamondshaped (rhomb) indentation having long and short diagonals of an approximate ratio of 7 to 1. The depth of indentation is about 1/30th of its length. In essence, the Knoop indenter is a shallow double wedge. The Knoop indenter is subsequently mounted in a machine which applies a load, without impact, at a constant rate and has a microscope equipped with a filar eyepiece for measuring the size of the indentation within _ 1%. Although the Knoop indentation hardness method was developed originally for measuring the hardness of metals, shortly thereafter Lysaght [48] suggested its use for organic coatings. Gusman [49] reported on its use for organic coatings. The instrument was specified in ASTM D 1474, adopted in 1957. In its use for organic coatings, a load of 25 g is applied for 18 s, after which time the indenter is removed from the coating, and the length of the long diagonal of the impression remaining in the coating is measured as quickly as possible. This dimension is then used to obtain the Knoop Hardness Number (KHN), Which is the ratio of the load, in kg/mm 2, to the projected planar area. KHN-

L

L

Ap

ICp

(6)

where L = load in kilograms applied to the indenter, Ap -- projected area of indention in m m 2, l = measured length of the long diagonal of the indentation in ram, and Cp = indentor constant relating l t o Ap, usually 7.028 • 10-10. Most writers refer to I and Ap as the unrecovered length and area, but as we shall see later, this is not true. Elastic recovery of the indentation impression takes place the instant the indentor is removed, and substantial viscoelastic recovery takes place before a measurement can be made. The real difference between the P fund and Knoop methods is that with the Pfund, the hardness measurement is made when the indenter is under load, while with the Knoop the measurement is made of the indentation impression remaining after both the load and indenter have been removed. This remaining Knoop impression is smaller than the original made while under the loaded indentor because all of the elastic and substantially all of the viscoelastic (creep) recovery occur in the indentation impression once the load is

568

PAINT AND COATING TESTING MANUAL

removed. This will be explained and actually shown in the subsection on theory. Therefore, the operation of the Pfund and Knoop methods rests on two entirely different principles. Each measurement represents a completely and substantially different point on a viscoelastic creep-creep recovery curve for any given material or organic coating (see Fig. 26). Yet, the amazing part is that,

quite coincidentally, the numerical results can be quite similar. This similarity in numerical results (PHN ~ KHN) with organic coatings probably could never have been achieved deliberately. How fortunate for the paint industry that the equivalence exists. Of the two test methods, the Modified Pfund and the Tukon gage with the Knoop indenter, the former is a dynamic hard-

FIG, 23-Fischerscope | H100V Microhardness Tester. (Courtesy Fischer Technology, Inc.)

FIG. 24-1CI Pneumatic Microindenter. (Courtesy Research Equipment (London), Ltd.)

CHAPTER 48--HARDNESS ~

RATIOOF DIAGONALS

172~30' INCLUDEDANGLES 130~ 7.11 TO 1 FIG, 25-Diagram of Knoop Diamond Indenter.

ness test and the latter is a static hardness test. Those in the automotive industry know how difficult it can be to obtain repeatable Knoop hardness numbers on metallic finishes. Secondly, with some types of finishes, the ends of the long diagonal of the Knoop impression recover, yielding rounded ends. In others, the organic finish sometimes recovers in such a manner as to partially close the indent near the ends of the long diagonal. However, the main reason for this preference is that the Pfund measures the indent under load. That is when the coating is resisting the indentation.

Pfund Hardness Tester--After numerous, unsuccessful attempts to grade the hardness of varnishes by means of the scratch test in which graded pencils, crystals, etc. were used, A. H. P fund, an associate professor of physics from Johns Hopkins University, Baltimore, MD, modified the Brinell Indention Test [50]. In this method the measurements are made on the organic coating while the loaded indentor is in contact with the coating. At first a 1/16-in.-diameter steel ball was forced under load into the varnish, and the diameter of the resultant circular impression was measured under the microscope. This was soon changed to a quartz cylinder terminating in a hemisphere 1/4-in. (6 ram) in diameter. The results are expressed as the load on the indentor, in grams, necessary to achieve a specified diameter of indent. The device, shown in

Fig. 27, consists of a counterbalanced brass beam containing the Indenter C. Illuminating light is reflected into the indenter by the clear Glass G and reflected back up to the Microscope 0, where the planar diameter of indent is measured by means of a filar eyepiece (shown in the upper right hand corner of Fig. 27). The results are expressed as the load on the indentor, in grams, necessary to achieve a specified diameter of indent. Table 3 shows typical results at an indent of 3 divisions (each division is approximately 0.1 ram). Additional data can be found in the work by Pfund, and Schuh and Theuerer [51,52]. In making a hardness determination, instead of attempting to find the exact load necessary to produce the specified diameter of indent, it was preferable to apply loads producing diameters both greater and less than the value sought and then interpolate to the specified diameter. However, this method is not precise because the relationship between load and diameter is not linear. Although this method of always achieving the same indent results in geometrically similar indents in all cases, this theoretical consideration of geometrically similar indents is significant only when the material being measured is thick enough for the hardness measurement to be uninfluenced by the thickness of the material. Such is not the case here. The theoretical consideration of geometrically similar indents will be discussed in the subsection on theory. As this method was rather tedious and time consuming, the instrument and method were modified in the early 1950s by the Bell Telephone Laboratories. The modified Pfund and test method were incorporated into ASTM D 1474 [53]. The Modified Pfund device is shown in Fig. 28. The instrument develS

--[

:

E.EO

S

=

Y=)I +Y2 . )'2 =

L

569

S .

.

S

.

S

i-e

S

st)

YU= f ( ~ l

TIME LOAD LOAD APPLIED REMOVED FIG. 26-Creep and creep recovery curves of viscoelastic material.

t

-t/k1

J

570

PAINT

AND

COATING

TESTING

MANUAL

C)

J

,L

M2

Ai

W

V, /

FIG. 27-Diagram of Pfund Hardness Indenter.

oped at the Bell Telephone L a b o r a t o r i e s a n d initially a d o p t e d by the ASTM as M e t h o d D 1474 is shown in Fig. 29. The i n d e n t e r is a t r a n s p a r e n t , colorless synthetic quartz o r s a p p h i r e h e m i s p h e r e whose spherical r a d i u s is 0.125-in. (1/4in.) (6 m m ) d i a m e t e r with a m a x i m u m spherical eccentricity of 0.002 in. (0.05 ram). The i n d e n t e r is m o u n t e d in a h o l d e r weighing 1000 g so that the i n d e n t e r is always u n d e r a l o a d of 1000 g w h e n m a k i n g m e a s u r e m e n t s . Hence, we see that in this m e t h o d the l o a d is kept c o n s t a n t a n d the resultant diameter of i n d e n t is recorded. This is exactly opposite to the original Pfund m e t h o d . In operation, the test panel is b r o u g h t into contact with the l o a d e d indenter, a n d after 60 s (while still u n d e r load) the d i a m e t e r of the circular i m p r e s s i o n is m e a s u r e d by m e a n s of a filar m i c r o m e t e r m o u n t e d in the eyepiece of the microscope. E a c h filar division r e p r e s e n t s 0.1 m m , a n d the diameter of the i m p r e s s i o n is converted into a Pfund h a r d n e s s n u m b e r (PHN), expressed in k g / m m 2 units, as follows PHN -

L L 1.27 - - - - - A 'rrd 2 d2

(7)

4 FIG. 28-BTL designed modified Pfund Hardness Gage. TABLE 3--Pfund hardness.

Thickness, rail

Hardness at Three Divisions

A

0.7 1.3 3.0

730 380 47

B

0.6 1.I 3.5

435 130 ,, TIME

FIG. 2-Schematic description of stress (S) dependence on time for latex coatings [15]: (a) and (b) PVC < CPVC; (c) and (d) PVC > CPVC; (e) PVC < CPVC in the presence of a poor coalescent; (f) PVC > CPVC in the presence of a poor coalescent.

CHAPTER

49--STRESS

PHENOMENA

IN ORGANIC

COATINGS

587

vs

Since in accordance with the molecular theory of rubber elasticity in its simplified form [22,23]

I

E,. = 3veRTr

(6)

Sr • 2Er r

(7)

and

giving

I

ve .

Er

Sr

. . . 3R T~ 6R Trr

(8)

where R, S,, Er, and Tr are, respectively, the gas constant, the stress, the elastic modulus, and the temperature at the beginning of the rubbery region. If Sr is measured and e~ is known or determined from separate measurements (e.g., by thermomechanical analysis, TMA), ve can be calculated. Others have used the evaluation of Tg by stress measurements to investigate the effect of a pretreatment on certain pigments [24], the state of cure of baking enamels [21], and the modification of epoxy coatings [19a,25].

TIME FIG. 3-Schematic representation of the dependence of V s / V E and stress (S) on time [16]. V s = volume of solvent present in the film; Vr = volume of the dry film.

[', I

Variation of Relative Humidity Dimensional changes induced by absorption and desorption of water as a result of variation in RH is another cause of stress development in a coating [11,26,27]. As in the case of temperature, if a mismatch between the expansion coefficients of the coating and the substrate exists, a hygroscopic stress (S H) will arise in the coating. Since the hygroscopic strain g~ is given by eH_ ~(ct~ - asH) A RH

(9)

one can write that S H =

fRH2 JRHI

E I

(a~ - aH)dRH

(10)

-- V

where ~ and ~ are, respectively, the hygroscopic expansion coefficients of the coating and the substrate. Some examples of the S H dependence on RH is given in Fig. 6.

INTERDEPENDENCE

T~-%-T FIG. 4-Schematic re ~resentation of dependences of S, E, ~ r and v on temperature

(D US]. the Tg can be determined with fair accuracy just by carrying out a few measurements in the glassy region and then extrapolating the straight line to S T = O. For coatings with a significant stress in the rubbery region (e.g., highly cross-linked thermosets), the measurement of stress in this region might provide a way to approximately determine the cross-link density, Ve [21].

OF STRESSES

While in previous subchapters the various stresses (S F, S r, S~) were discussed separately, in practice they can act together in such a way that the total stress (S,ot) is small or, as in many cases, very important [11] S,ot = S r +-- S T + S H

(11)

The positive and negative signs are arbitrarily chosen. The positive sign denotes a coating tendency to contract (tensile stress) and the negative sign a coating tendency to expand (compressive stress). S e is practically always positive. Equation 11 indicates the existance of two climatic conditions which might provoke a high stress in a coating: 1. Low temperatures and RH's induce high tensile stresses (e.g., a dry, cold winter). 2. High temperatures and RH's induce high compressive stresses (e.g., a humid, warm summer).

588

PAINT

AND

COATING

TESTING

MANUAL

10" S

1

2

~

k

3

'"~ 6 1'0 2b 3'o 4'o 5'o 6'0 -fo T " ~

6 lb 2'o 3'0 4'o 5'o 6 lb 2'o 3'0 FIG. 5-Stress (S, MPa) dependenceon temperature (T, ~ for three coatings at RH = 0% [an epoxy (1), a polyurethane (2), and an epoxy/melamine system (3)] [18]. According to Eq 11 a n d d e p e n d i n g on the type of coating a n d the way it was cured, a n u m b e r of situations can arise. F o r example, for a t h e r m o s e t t i n g coating cured at a high t e m p e r a t u r e (i.e., at T > Tg, RH = 0%) a n d then exposed to different RH's, since S e ~ 0, the Sto t is given b y Stot m S T -

SH

10-

(12)

In Fig. 6, for one coating, the stress is positive regardless of RH, m e a n i n g that S T is always higher t h a n S"; b u t cases w h e r e negative stress values are o b t a i n e d are not rare. It could be d e m o n s t r a t e d [11] that the s a m e coating conditioned in an identical e n v i r o n m e n t b u t with different previous histories can develop different values of stress. Thus, a coating c u r e d u n d e r i s o t h e r m a l and c o n s t a n t RH conditions (e.g., 21~ a n d 50%) will develop a total stress given by Stot = S e - S "

S

86 4

2

"gQ~ I

(13)

Now, if the c o a t e d s u b s t r a t e is first h e a t e d at T > Tg for sufficient t i m e to enable a m a x i m u m stress relaxation a n d t h e n b r o u g h t b a c k to the initial conditions (i.e., 21~ a n d 50% RH) Stot = S r - S H

(14)

The results o b t a i n e d [ 1 1 ] show that in certain cases (e.g., for a n epoxy coating), the Stot can be very different [Sr (Eq 13) = 5 MPa; Sto t (Eq 14) = 0.4 MPa]. Interesting cases are those where the Tg of a coating is close to or below the e x p e r i m e n t a l t e m p e r a t u r e once they are i m m e r s e d in w a t e r or exposed to a high RH [28]. Under such conditions S r or S r can relax a n d therefore S t o t = S H. Such a situation is illustrated schematically in Fig. 7 a n d for a p a r t i c u l a r epoxy coating in Fig. 8. Once a coated s u b s t r a t e is i m m e r s e d in w a t e r one can observe first the d e v e l o p m e n t of a hygroscopic compressive stress followed by its decrease. The time necessary to r e a c h zero stress is m a i n l y d e p e n d e n t on the type of coating. The w i t h d r a w a l of the coated substrate from w a t e r provokes first the d e v e l o p m e n t of a relatively high tensile hygroscopic stress followed by its decrease. The decrease in stress (Curves 2 a n d 4 in Fig. 7) is due to relaxation processes facilitated by the low Tg of the coating. The m u c h higher stress values a t t a i n e d

1

I

100 FIG. 6-Stress (S, MPa) dependence on relative humidity (RH, %) for a polyester powder coating (1); an epoxy (2); a polyurethane (3); and a latex coating (4) [27].

after the i m m e d i a t e w i t h d r a w a l of the coated substrate f r o m w a t e r (Curve 3 in Fig. 7), in c o m p a r i s o n with the initial stress (Sto ~ = S ~ - S " or Stot = S r - S ~I) are due to the high hygroscopic tensile stress (S,o, = S ~ ) . The above findings are i m p o r t a n t not only for u n d e r s t a n d ing the m e c h a n i s m of the stress d e v e l o p m e n t in organic coatings b u t also for practical reasons such as choosing the experimental conditions of n a t u r a l o r accelerated w e a t h e r i n g tests, i.e., the m a g n i t u d e of stress developed in a coating m i g h t d e p e n d as m u c h on n a t u r a l climatic conditions as on the type a n d o r d e r of cycle selected in the laboratory.

CHAPTER 4 9 - - S T R E S S P H E N O M E N A IN ORGANIC COATINGS

589

S

HeO (I) TINE

0 I

I I t I I FIG. 7-Schematic description of the stress (S) dependence on time at two experimental conditions (water and 50% RH). X = the initial stress [28].

4-% 3' 0

2'

C

0

~_

1:

J

0-

--2'

1

27d H20 (1)

I

I

I

76d 50 (X)

TIIE (d)

0 20 40 60 80 100 FIG. 8-Epoxy coating. Stress (S, MPa) dependence on time (d, day) at two experimental conditions (water and 50% RH); ~ = initial stress [28].

STRESS

MEASUREMENT

E q u a t i o n s 1, 3, 5, and 10 show that, if one knows the values of E, e, a n d ~, of a coating, then in principle the various stresses can be calculated. However, except for relatively simple cases, this is difficult b e c a u s e the above coating characteristics can be time, t e m p e r a t u r e , a n d relative h u m i d i t y dependent.

Therefore, efforts were m a d e to directly d e t e r m i n e the stress arising in a coating. Among the m e t h o d s one can find in the literature are: optical [29-32], strain gages [19a,33,34], brittle l a c q u e r m a t e r i a l s [35], X-ray diffraction [36], a n d cantilever (beam) [2,4, 8,11, 26, 37-43 ]. The cantilever (beam) m e t h o d a p p e a r s to be the m o s t widely used a n d is suitable for d e t e r m i n i n g the stress in a n organic coating. This m e t h o d m a k e s use of the fact t h a t for a

590

PAINT AND COATING TESTING MANUAL

FIG. 9-Schematic description of the vertically fixed at one end cantilever (beam) method [67].

FIG. 10-Schematic description of the freely supported beam method [67]. coating under stress, applied on a substrate, the coated substrate will deflect in the direction which relieves the stress. Since the deflection can be measured and the elastic properties of the substrate are k n o w n from separate determinations, the stress can be calculated. Two variations of the cantilever (beam) method are described in the literature: a one-sided coated substrate either (1) fixed vertically at one end [4,8,26,37] (Fig. 9) or (2) freely supported on two knife edges [2,11,41,43] (Fig. 10). The stress analysis of (1) is more complicated and shows that to eliminate the effect of clamping on the coated sub-

strate deflection, its measurement should be made at a distance higher than 80 m m from the clamping point [8]. Variation (2) is m u c h simpler to analyze and can be designed to eliminate the effect of weight loss on the coated substrate deflection by choosing the right distance between the two knife edges [43]. Each variation has its advantages, but if correctly used they should give identical results. For example (1) is more suited to evaluating stress in water [28] and (2) to determine the effect of temperature [2,11]. Among the techniques used to measure the deflection of coated substrates, one can mention: capacitive transducers [42,43], laser [44], travelling microscope [4, 8,26], automatic micrometer [11]. A commercial apparatus, the CoRI stressmeter (Fig. 11), is based on Variation (2) and the mathematical analysis described in Ref 43. This apparatus is almost completely automatic and enables one to measure the stress from about - 5 to 100~ under a variety of RH's. A n u m b e r of mathematical equations are proposed in the literature to calculate the stress, but in the author's opinion those proposed by Corcoran [40] are the closest to the real situation (e.g., considers the fact the stress develops in two directions) S = S --

dEst3 dE(t + c) + 312c(t + c)(1 - Vs) 12(1 - v)

4d'Est3 + 4d'E(t + c) 3Fc(t + c)(1 - Vs) 12(1 - v)

(15)

(16)

where d = deflection of the substrate (Fig. 9), d' = deflection in the middle of substrate (Fig. 10), E s = elastic modulus of the substrate,

FIG. 11-CoRI stressmeter apparatus (Courtesy of Braive Instruments, Liege, Belgium).

CHAPTER 4 9 - - S T R E S S P H E N O M E N A I N ORGANIC COATINGS Vs t c /(Eq 15)

= = = =

Poisson's ratio of the substrate, thickness of the substrate, thickness of the coating, length of the coated substrate between the point at which it is clamped and the point at which the deflection is measured (Fig. 9), and l(Eq 16) = distance between the two knife edges (Fig. 10). Equations 15 and 16 assume, among other things, good adhesion between the coating and the substrate, isotropic elastic properties of the coating and the substrate, the elastic limit of the substrate is not exceeded, and the stress is constant throughout the coating thickness. The second term in Eqs 15 and 16, which contains a number of coating properties difficult to determine, can be neglected if Es "> E and t -> c. Most commonly, stainless steel or cold laminated steel [4,8,11,26,42,43] shims are used. Other substrates such as aluminum can also be used. The elastic modulus, Es, of each shim can be determined prior to use with the CoRI stressmeter by applying Eq 17

p/3 Es

-

(17)

4d't3b

where P = weight placed in the middle of the substrate, b width of the substrate, and d', l, t = as in Eq 16. =

It is important to add that, although it is not difficult to make the measurements, nevertheless great care is necessary. One should always use the correct substrate thickness, adequately condition it, and precisely calibrate the apparatus.

J

EFFECT OF COATING COMPONENTS Since E, E, and v (Eqs 1, 3, 5, and 10) are known to be affected by the coating components, one should expect the same to hold for stress development. This section will briefly review the influence of pigmentation, solvents, and binder.

Pigmentation It has been shown that pigmentation, both the pigment volume concentration (PVC) and the type of filler (i.e., pigments and extenders), affects the development of internal stress [7,41,45-50]. To illustrate this, examples are presented in Figs. 12 to 17. Figures 12 and 13 show, respectively, the stress dependence on time for a thermoplastic binder in solution and a latex, filled with a titanium dioxide (TiO2). Some PVCs are above the CPVC, and some are below the CPVC. The different stages occurring during the film formation, discussed previously, can be recognized. In Stage 1 stress increases relatively rapidly. For latex coatings, this stage corresponds to the transition phase of the evaporation kinetics when the greatest part of the coalescence occurs. In Stage 2 (which corresponds to Phase 2 of evaporation kinetics), depending on the PVC and the type of filler, the stress can decrease or first decrease and then increase. For PVC < CPVC, this decrease is mostly due to the relaxation process, but for PVC > CPVC is mostly due to relief processes such as filler/binder dislocations and/or formation of microfissures.

Determination o f the CPVC The plot of the maximum internal stress (Sin) as a function of PVC enables one to determine the CPVC of a coating. Some examples are presented in Figs. 14 and 15.

t3: S

so

"IJ I

....o..---o z ~ r'"

l&_ I ~ ~,s5 I

591

_

I

1 3 5 7 5 25 105 5 FIG. 1 2 - S t r e s s (S, MPa) as a function of time [hour (h) and day (d)] for a polyisobutyl methacrylate filled with Ti02. The numbers in the figure indicate the different PVCs (%) investigated [48]. CPVC = 51%.

592

PAINT AND COATING TESTING MANUAL

1.1t S 0"91 0.7

0.1]~ I"

HOUR I I I I l l I I I I I 1 3 5 7 1 5 FIG. 1 3 - S t r e s s (S, MPa) as a function of time for a TiO~) [50]: 45% (O); 5 0 % ([]); 55% (X); 60(A). C P V C

These figures, as well as o t h e r results p r e s e n t e d in Refs 48 a n d 49, clearly indicate that S m is a function of PVC. S m increases with PVC up to a certain PVC a n d then decreases. This PVC c o r r e s p o n d s to the CPVC, indicating the possibility of accurately d e t e r m i n i n g this characteristic [51,52] from stress m e a s u r e m e n t s , a n d agrees well with the CPVCs calculated or d e t e r m i n e d by o t h e r m e t h o d s (density, various mechanical properties). The d e p e n d e n c e of S m on PVC can be u n d e r s t o o d if one considers Eq 1 a n d the w a y E, e, a n d v are affected by the PVC. F o r PVC < CPVC, E increases with increasing PVC b e c a u s e the E of an inorganic filler is in general higher t h a n that of an organic b i n d e r [53]. F o r PVC > CPVC, E decreases as a result of the increasing film d i s c o n t i n u i t y [22, 54]. Since in general a n d v are d e c r e a s e d (or are little affected) by the PVC, it follows that the increase of S m with PVC is m a i n l y due to the effect of PVC on E. Figures 14 a n d 15 also show that the m a g n i t u d e of Srn is d e p e n d e n t on the type of filler. There are fillers w h i c h induce a higher stress (e.g., TiO z, r e d iron oxide) t h a n others (e.g., CaCO 3, talc). This is due to the filler/binder interaction (reinforcing effect), w h i c h is d e t e r m i n e d by the n a t u r e and, in particular, by the surface area a n d the acid/base c h a r a c t e r of the b i n d e r [55]. E x a m p l e s of S m = f(PVC) for coatings containing a mixture of fillers are p r e s e n t e d in Figs. 16 a n d 17. An e x a m i n a t i o n of the results o b t a i n e d with b i n a r y a n d t e r n a r y filler coating systems indicates that the stress S m t can be calculated b y m e a n s of an additive rule x=i

Smt = n l S m l + rlzSm2 + ... + rliSrrl i = ~ x=l

n~Smx

(18)

I ' l I l I I I I I I l l I r 9 13 17 21 25 29 33 latex (vinyl acetate/vinyl versatate copolymer filled with = 52%.

~s]SB

XI II

t3

It 9

A

3

PVC t0 20 30 40 50 60 70 FIG. 1 4 - M a x i m u m internal stress (Sm, MPa) as a function of PVC (%) for a solvent-based thermoplastic binder filled with a TiO2 (X), a red iron oxide (O), a yellow iron oxide (A), and a talc (G) [48].

CHAPTER 4 9 - - S T R E S S P H E N O M E N A I N ORGANIC COATINGS

_

Sm

!

I

593

7•Sm

:

o

i

5 ./

/

I

,',

3,2-

3-

2.4--

_

1.6-

.?.-

28-

OA-

FIG. 15-Sm (MPa) as a function of PVC (%) for a styrene acrylic copolymer filled with a Ti02 (Q), a calcium carbonate (X), and talc

(9 [49].

where

nl, n2, n~ = volume fraction of different fillers present in the mixtures, and

Srnl, Srn2, Srni = maximum stress of different single filler systems at a given A below or equal to I (A -- reduced PVC, PVC/CPVC). In the literature one can atso find simplified methods to determine the CPVC of latex coatings based on the same principle. They simply compare the force [56] or the deflection induced by the internal stress for flexible plastic substrates [57] (Fig. 18) coated with paints of different PVCs. These methods can be useful, but one has to be aware that they are valid only if both the thickness of the various paints and the time necessary to reach the maximum stress are the same.

The presence of solvents in a coating can affect the magnitude and especially the rate of development of internal stress. This is illustrated in Fig. 19. For thermoplastic binders in solution, the slower the evaporation of a solvent from a coating, the slower the development of internal stress and vice versa. One should note that the coating cast from fast-evaporating solvents (Curve 1, Fig. 19) produces slightly higher stress values than those cast from more slowly evaporating ones (Curves 3 and 4, Fig. 19). The results obtained were explained by using Eq I, the principle of plasticizing effectiveness of solvents and the stress relaxation favored by the presence in the coating of the slower evaporating solvent [16]. For coatings containing a mixture of solvents (Fig. 20), both the development rate and the stress magnitude are mainly determined by the presence in the film of the less volatile solvent(s). The situation may be different if the film formation is a result of solvent evaporation and cross-linking processes (e.g., epoxy and polyurethane coatings). Under such circumstances the coating containing faster evaporating solvents can develop smaller stress values [58]. For such coatings, the volume of solvent present in the film after most of the crosslinking has occurred (Eq 2) and which determines the magnitude of eF will increase the slower the solvent evaporates. If the stress relaxation process is negligible, it follows that for

594

PAINT AND COATING TESTING MANUAL cizing effectiveness [17, 60], the molar volume, and the steric hindrance of solvents. The Plasticizing effectiveness affects the internal stress magnitude, while the molecular dimensions affect the evaporation kinetics and consequently the rate of the stress development.

3,6"

Sm 3.2-

Binder 2.8 I I

2.4-

I I I I I II

2.

1.6

Il I I

1.20.8 0.4

10

30

50

70

FIG. 17-Sm (MPa) as a function of PVC (%) for a latex binder containing a TiO2 (O), calcium carbonate (X), and their mixture: TiO2 (n = 0.4)/CaCl 3 (n = 0.6) (17) [49].

such coating systems the faster the solvent evaporates the smaller the internal stress. The influence of solvents on internal stress is also evident for latex coatings where certain solvents, for example the coalescents, play an important role in the film formation process [1,5,50,59]. Examples of how the level and the type of coalescent affect internal stress are presented in Figs. 21 and 22, respectively. Figure 21 shows that: (1) the coalescent level affects the time necessary to reach the maximum stress, and (2) for each formulation, there is an optimal coalescent level to obtain a tight continuous coating developing the lowest internal stress. In Fig. 22, the results obtained with three solvents used in latex coatings are given. One can see that the type of solvent influences both the value and the development rate of the internal stress. As in the case of thermoplastic coatings, the influence of solvents on internal stress development in latex coatings was explained [15,50,59] by taking into consideration the plasti-

To understand the role of the binder, the essential component of an organic coating on stress development, one can consider once again the general Eq 1 (see also Eqs 3, 5, and 10) and/or the Tg of a coating with respect to the experimental temperature. Equation 1 indicates that the stress is directly affected by the magnitude of E, e, and u of the binder. The smaller the values of E, e, and v, the smaller the magnitude of stress. With respect to the Tg, it should be remembered that the binders having their Tg below or close to the film formation temperature (T) develop a negligible internal stress, while those having their Tg > T develop an important one. This is due to the fact that at T > Tg~ the mobility of the binder molecular segments is high and the stress arising during film formation can partially or totally relax. Moreover, it can be shown that for a thermoplastic binder in solution having a Tg > T, the lower the Tg of the binder, the less will be the solvent in the film after its formation (see Eqs 2 and 3) and therefore the smaller the internal strain and stress in the dry coating [16]. In brief, any change occurring in the molecular structure of a binder (e.g., crosslinking, crystallinity, molecular weight, steric hindrance) might induce a change in E, E, v, and Tg and thus affect stress development.

STRESS VERSUS ADHESION AND COHESION It is accepted that the stress arising in a coating can reduce the adhesion and cohesion, two crucial properties of an organic coating for obtaining durable coatings [1-10,61-65]. The way the stress affects adhesion is described in detail in Refs 61 and 62. It is shown that the application of an energy balance analysis [66] leads to the factor /3 acting against adhesion Ee 2 /3 = cU,.~--c 1-v

(19)

where Ur is the recoverable strain energy. Because it is the energy that expresses the effect of e, tensile or compressive strains are identical in their effect [61]. To accommodate the effect of stress [18,27,67], Eqs 1 and 19 are combined /3 ~ cSE

(20)

Equation 20 indicates that adhesion is directly affected by c, S, and 9 and the larger their values the higher the tendency of the coating to detach from its substrate. Thus, if a high stress arises in a coating, in order to prevent its detachment, it should be applied in a thinner layer.

CHAPTER 4 9 - - S T R E S S PHENOMENA IN ORGANIC COATINGS

FIG. 18-Determination of the CPVC by comparing the curvature of painted plastic substrates [57].

4-5

1 2-

1TIME

'

2'0

'

,~

'

~o

'

do

'

16o

'

do

'

~

'

'

FIG. 19-Stress (S, MPa) as a function of time (day) in a thermoplastic varnish cast

from methyl ethyl ketone and ethyl acetate (1); toluene (2); xylene (3); methyl isobutyl ketone and isobutyl acetate (4--), at 52% RH and 21~ [.16"].

3.2-

2.4~ e 1.60.8-

O~

~o ~ ~

A0 A

~o

0

TIN(

J 2'o ' ~

' 6'0'

~o

' ~o'

1~o'

!~o'

1~o'

~o'26o

FIG. 20-Stress (S, MPa) as a function of time (day) for a thermoplastic varnish

cast from toluene (X); isobutyl acetate (9 (W/W) (A)] [16].

and their mixtures [1/1 (W/W) (0) and 113

595

596

PAINT

AND

COATING

TESTING

MANUAL

2.o-_S 1.6-

1.2-

0.8-

percent Texanol by weight of binder solids [501.

71+'~

1.4

1. 0.6

O

1 3 5 7 10 FIG. 22-Stress (S, MPa) as a function of time (h, hour; d, day) for a styrene acrylic latex paint containing Texanol (9 Dalpad A (X); Dalpad A + propylene glycol (0)

[501. Since stress develops in most organic coatings, the product c S e should also be considered in adhesion tests performed in

laboratories. The corrected mathematical equations for pull off and peeling at 90 ~ are given by Eqs 21 and 22 [61,67], respectively

(21) F b

- ~- "1, - cSE

where o- = K = 3' = F = b =

(22)

stress applied to pull the coating from the substrate, bulk modulus of the coating, interracial work of adhesion, force applied to peel the coating, and width of the coating.

Equations 20, 21, and 22 also indicate the possibility of determining the adhesion of a coating (i.e., the factor y) with-

out applying any external force. This is due to the fact that at a particular film thickness a spontaneous detachment should occur [61]. Unfortunately, this method can only be used for badly adherent coatings. For all other coatings, extremely high film thicknesses, difficult to apply and cure, would be necessary. If the adhesion forces exceed the cohesive strength of a coating and the stress developed is high, damage will preferentially occur in the coating (cracking, fissuring) rather than at the coating/substrate interface. The verification of this principle can be realized by determining the stress and the ultimate properties of a coating. Since most of organic coatings are viscoelastic, it is essential that these properties be evaluated under the conditions (strain rate, temperature, RH) corresponding to the stress development in the coating. The presence of stress concentrations (e.g., existence of heterogeneities) in the coating and of a fatigue process are factors decreasing the overall stress at which the coating will crack.

sis

CHAPTER 4 9 - - S T R E S S PHENOMENA IN ORGANIC COATINGS

16s.

RH = 5%

597

4 9 Initial 9 168h 9 552 h x 1008 h

12

3

60 ~ 5% RH

2~

8 -I 4

-2 -3

0

21 ~ 90% RH

-4

, 0

20

40

60

80

FIG, 23-Stress (S, MPa) as a function of temperature (T, ~ for a primer/clearcoat system after different periods of weathering [9].

0

400

800

1200

tauv Screen FIG, 25-Stress ($, MPa) as a function of time of weathering (tour, hour) for a primer/ ciearcoat system at two climatic conditions

12

[g].

T=21 ~ ~. Initial 9

W E A T H E R I N G AND S T R E S S DEVELOPMENT

168h

9 552 h x 1008 h

8

4

0

-4 I

0

1

20

I

i

40

I

i

60

i

I

80

I

'l

I O0

RH FIG. 24-Stress (S, MPa) as a function of RH (%) for a primer/clearcoat system after different periods of weathering [9].

In most cases when organic coatings are exposed to weathering (accelerated or natural), they undergo chemical and physical modifications which are expressed in the change of Tg, E, a, v, and cross-link density. Under such circumstances and according to Eqs 1, 3, 5, and 10, one can also expect to see changes in stress development. Confirmation of this and the role played by stress in the coating deterioration is given in Ref 9. In this study a clearcoat/basecoat system exposed in a QUV apparatus to alternating dry and wet cycles cracked after about 1000 h. By measuring the stress as a function of temperature and RH, it has been shown that weathering provoked: (1) an increase of Tg, which in turn induced higher tensile stresses (Fig. 23), and (2) an increase of coating sensitivity to moisture, which induced higher compressive stresses (Fig. 24). The representation of the stress as a function of time of weathering for two experimental conditions similar to those present in the accelerating apparatus (Fig. 25) indicates that every time the experimental conditions changed (every 4 h) the coating ~was exposed to an increasing stress. The processes thought to lead to fissuring of the clearcoat are described in Fig. 26. Briefly, the chemical degradations induced by UV, water, and oxygen, which decrease the coating cohesion, combined

598

PAINT AND COATING TESTING MANUAL

Chemical

Weathering

Degradation

Physical Effects

Light/Oxygen/Water

1

1

1

- Breaking of bonds Formation of new crosslinks

Clearcoat

- Formation of hydrophUic groups Basecoat

- Weight loss (volatilisation, material rinsed by water) - Increase of Tr - Water absorption

Embdttlement + Stress development [ Cracking

=-_.---

FIG. 2 6 - S c h e m a t i c representation of processes causing crack formation in a clearcoat [9].

w i t h t h e fatigue p r o c e s s at steadily i n c r e a s i n g h y g r o t h e r m a l stress levels are t h e c a u s e s of t h e c o a t i n g d e g r a d a t i o n .

REFERENCES [I] K6ning, W., Proceedings, VIth FATIPEC Congress, Wiesbaden, Germany, 1962, p. 424. [2] Dannenberg, H., Society of Plastic Engineering Journal, Vol. 21, 1965, p. 669. [3] Prosser, J. L., Modern Paint and Coatings, July I977, p. 47. [4] Saarnak, A., Nilsson, E., and Kornum, L. O., Journal of the Oil and Colour Chemists" Association, Vo]. 59, 1976, p. 427. [5] Hamburg, H. R. and Morgans, W. M., Hess's Paint Film Defects: Their Causes and Cure, 3rd ed., Chapman and Hall, London, 1979. [6] Croll, S. G., Journal of Applied Polymer Science, Vol. 23, 1979, p. 847. [7] Sato, K., Progress in Organic Coatings, Vol. 8, No. 2, 1980, p. 143. [8] Perera, D. Y. and Vanden Eynde, D., Journal of Coatings Technology, Vol. 53, No. 677, 1981, p. 39. [9] Oosterbroek, M., Lammers, R. J., van der Ven, L. G. T., and Perera, D. Y., Journal of Coatings Technology, Vol. 63, No. 797, 1991, p. 55. [10] Kamarchik, P., Jr. and Jurezak, E. A., Proceedings of Radtech, Edinburgh, Scotland, Great Britain, 1991. [11] Perera, D. Y. and Vanden Eynde, D., Journal of Coatings Technology, Vol. 59, No. 748, 1987, p. 55. [12] Crackin, F. L. and Bersch, C. F., Society of Plastic Engineers Journal, Vol. 15, 1959, p. 791. [13] Bierwagen, G. P., Journal of Coatings Technology, Vol. 51, No. 658, 1979, p. 117. [14] Bauer, C. L., Farris, R. J., and Vratsanos, M. S., Journal of Coatings Technology, Vol. 60, No. 760, 1988, p. 51. [15] Perera, D. Y. and Vanden Eynde, D., Journal of Coatings Technology, Vol. 56, No. 716, 1984, p. 111. [16] Perera, D.Y. and Vanden Eynde, D., Journal of Coatings Technology, Vol. 55, No. 699, 1983, p. 37. [17] Hansen, Ch. M., Industrial and Engineering Chemistry Research, Vol. 9, 1970, p. 282. [18] Perera, D. Y., Proceedings, XVlth International Conference in Organic Coatings Science and Technology, Athens, Greece, 1990, p. 309. [19] Shimbo, M., Ochi, M., and Arai, K., Journal of Coatings Technology: (a) Vol. 56, No. 713, 1984, p. 45, and (b) Vol. 57, No. 728, 1985, p. 93.

[20] Geldermanns, P., Goldsmith, C., and Bedetti, F., Proceedings, First International Technical Conference on Polyamides, Ellenville, NY, November 1982, Society of Plastic Engineers. [21] Perera, D.Y.: (a) Proceedings, XIXth FATIPEC Congress, Aachen, Vol. I, 1988, p. 1; and (b) Material Priifung, Vol. 31, 1989, p. 57. [22] Zosel, A., Progress in Organic Coatings, Vol. 8, 1980, p. 47. [23] Hill, L. W. and Kozlowski, K., Journal of Coatings Technology, Vol. 59, No. 751, 1987, p. 63. [24] Yamabe, H. and Funke, W., Farbe Und Lack, Vol. 96, No. 7, 1990, p. 497. [25] Shimbo, M., Ochi, M., Inamura, T., and Inoue, M., Journal of Materials Science, Vol. 20, 1985, p. 2965. [26] Nilsson, E., Fdrg och Lack, Vol. 2, 1975, p. 318; Vol. 23, 1977, p. 179; Vol. 23, 1977, p. 199. [27] Perera, D.Y. and Vanden Eynde, D., Proceedings, XVIth FATIPEC Congress, Liege, Belgium, Vol. 1, 1982, p. 129. [28] Perera, D.Y. and Vanden Eynde, D., Proceedings, XXth FATIPEC Congress, Nice, 1990, p. 125. [29] De Waard, R., Stock, Ch. R., and Alefrey, T. Jr., ASTM Bulletin, TP56, 1952, p. 53. [30] Zubov, P. J., Lepilkina, L. A., Gilman, T. P., and Leites, A. Z., Colloi'd Journal, Vol. 23, 1961, p. 23. [31] Imamura, H., Mokuzai Gakkaishi, Vol. 16, 1970, p. 168; Vol. 19, 1973, pp. 89 and 393; Vol. 22, 1976, pp. 325 and 331. [32] Theocaris, P. S. and Paipetis, S. A., Journal of Strain Analysis, Vol. 8, 1973, p. 286. [33] Shimbo, M., Ochi, M., and Shigeto, Y., Journal of Applied Polymer Science, Vol. 26, 1981, p. 2265. [34] Association Belge pour l'Etude, l'Essai et l'Emploi des Mat6riaux (ABEM), "Cours d'initiation ~tl'analyse des contraintes," Bruxelles, 1973. [35] Kanno, A. and Murato, Y., Proceedings, 15th Japanese Congress on Materials Research, Japan, 1972, p. 177. [36] Nakamura, K., Nishino, T., and Airu, X., Proceedings, XXth Congress AFTPV, Nice, France, 1991, p. 73. [37] Sanzharovskii, A. T., Vysokomolekularnie Soedinenia, Vol. 2, No. 11, 1960, pp. I698-1702, 1703-1708, 1709-1714. [38] Gusman, S., Paint Technology, January 1963, p. 17. [39] Simpson, W. and Boyle, D. A., Journal of the Oil and Colour Chemists' Association, Vol. 46, 1963, p. 331. [40] Corcoran, E. M., Journal of Paint Technology, Vol. 41, No. 538, 1969, p. 635. [41] Aronson, P. D., Journal of the Oil and Colour Chemists' Association, Vol. 57, 1974, p. 66. [42] Crolt, S. G., Journal of Coatings Technology: (a)Vol. 50, No. 638, 1978, p. 33; (b) Vo]. 51, No. 659, 1979, p. 49.

CHAPTER 49--STRESS PHENOMENA IN ORGANIC COATINGS

599

[43] Croll, S. G., Journal of the Oil and Colour Chemists' Association,

[57] D6rr, H. and Holzinger, F., "Le dioxyde de titane KRONOS

Vol. 63, 1980, p. 271. [44] O'Brien, R. N. and Michalik, W., Journal of Coatings Technology, Vol. 58, No. 735, 1986, p. 25. [45] Kris, G. J. and Sanzharovskii, A. T., Lakokrasochnye MateriaIy i Primenenie, Vol. 3, 1970, p. 27. [46] Haagen, H., Farbe und Lack, Vol. 85, No. 2, 1979, p. 94. [47] Croll, S. G., Polymer, Vol. 20, No. 11, 1979, p. 14. [48] Perera, D. Y. and Vanden Eynde, D., Journal of Coatings Technology, Vol. 53, No. 678, 1981, p. 40. [49] Perera, D. Y. and Vanden Eynde, D., Journal of Coatings Technology, Vol. 56, No. 717, 1984, p. 47. [50] Perera, D.Y. and Vanden Eynde, D., Journal of the Oil and Colour Chemists' Association, Vol. 11, 1985, p. 275. [51] Bierwagen, G. P., Journal ofiPaint Technology, Vol. 44, No. 574, 1972, p. 45. [52] Bierwagen, G. P. and Mallinger, R. G., Journal of Coatings Technology, Vol. 54, No. 690, 1982, p. 73. [53] Sato, K., Progress in Organic Coatings, Vol. 4, 1976, p. 271. [54] Bierwagen, G. P. and Hay, T. K., Progress in Organic Coatings, Vol. 3, 1975, p. 281. [55] Toussaint, A. and D'Hont, L., Journal of the Oil and Colour Chemists' Association, Vol. 64, 1981, p. 302. [56] Helmen, T. and Strauch, D., Farbe und Lack, Vol. 96, No. 10, 1990, p. 769.

darts les peintures-6mulsion, '~ Kronos International, Inc., Leverkusen, Germany, 1990. [58] Croll, S. G., (a) Journal of the Oil and CoIour Chemists' Association, Vol. 63, 1980, p. 230, and (b)Journal of Coatings Technology, Vol. 53, No. 672, 1981, p. 85. [59] Perera, D.Y. and Vanden Eynde, D., Journal of Coatings Technology, Vol. 56, No. 718, 1984, p. 69. [60] Hansen, Ch. M., Official Digest, Vol. 37, No. 480, 1965, p. 57. [61] Croll, S. G., Journal of Coatings Technology, Vol. 52, No. 665, 1980, p. 35. [62] Croll, S. G., Journal of the Oil and Colour Chemists' Association, Vol. 63, 1980, p. 200. [63] Pierce, P. E. and Schoff, C. K., "Coating Film Defects," Federation of Societies for Coatings Technology, Philadelphia, PA, 1988. [64] Schmid, E.V., Polymers Paint Colour Journal, VoL 180, No. 4258, 1990, p. 212. [65] Farris, R. 3., Maden, M. A., and Goldfarb, J., Proceedings, The Adhesion Society, 14th Annual Meeting, Clearwater, FL, 1991, p. 138. [66] Kendall, K., Journal of Physics D: Applied Physics, Vol. 4, 1971, p. 1186; Vol. 6, 1973, p. 1782. [67] Perera, D. Y., Proceedings, XVIlth FATIPEC Congress, Lugano, Switzerland, Vol. 1, 1984, p. 13.

MNL17-EB/Jun. 1995

Slip Resistance by Paul R. Gudvin, Jr. 1

tional Safety a n d Health f u n d e d a project that dealt with slipperiness a n d safety of workers engaged in structural steel erection [11].

AN OVERVIEW OF THIS SUBJECT as it relates to the coating i n d u s t r y was p u b l i s h e d in 1978 [1]. This c h a p t e r is an expansion a n d u p d a t e of that paper. Although a m a j o r i t y of the i n s t r u m e n t s a n d test m e t h o d s for slip resistance were develo p e d in ASTM c o m m i t t e e s with responsibilities o t h e r t h a n for paint, they all can be used to m e a s u r e the slip resistance of paints a n d coatings. Specific s t a n d a r d s for slip resistance and its testing m a i n l y refer to flooring a n d o t h e r p e d e s t r i a n w a l k w a y areas. At this t i m e there are no coefficient of friction (COF) s t a n d a r d s in the coating industry. One industry, the floor polish industry, established a COF value of less t h a n 0.5 as a slippery surface a n d a value of greater t h a n 0.5 as a not slippery surface [2]. OSHA has p r o p o s e d that the COF for working surfaces, such as walkways a n d a r o u n d machinery, in a p l a n t be not less t h a n 0.5 [3]. The U.S. D e p a r t m e n t of Justice has p r o p o s e d a m i n i m u m COF of 0.6 for level walking surfaces and of 0.8 for r a m p s for its Americans with Disabilities Act [4]. If these p r o p o s a l s b e c o m e law, p a i n t e d walking surfaces m u s t meet or exceed these standards.

Definitions The following are useful, specific definitions that have b e e n developed by ASTM C o m m i t t e e D-21 on Polishes a n d f o u n d in ASTM Test M e t h o d for Static Coefficient of F r i c t i o n of Polish-Coated F l o o r Surfaces as M e a s u r e d by the J a m e s Machine (D 2047) [12]. Coefficient of Friction--The ratio of the h o r i z o n t a l c o m p o n e n t of force r e q u i r e d to overcome friction, to the vertical c o m p o n e n t of the object weight or n o r m a l force a p p l i e d t h r o u g h the object, which tends to cause the friction. Dynamic Coefficient of Friction--The ratio of the horizontal c o m p o n e n t of force required to cause a b o d y to slide at a relatively constant velocity to the vertical comp o n e n t of the weight of the object or force a p p l i e d to it. The relatively constant velocity used to cause the b o d y to slide over the surface is to be not less t h a n 0.125 ft/s n o r m o r e t h a n 0.5 ft/s (38 to 152 mm/s). The vertical c o m p o nent shall result in a contact pressure of not less t h a n 1 psi (6.9 kPa) n o r m o r e t h a n 13 psi (90 kPa) applied u n i f o r m l y over the area in m u t u a l contact. Friction--The resistance developed b e t w e e n the physical contacting surface of two bodies when there is movem e n t or t e n d e n c y for m o v e m e n t of one b o d y relative to a n o t h e r parallel to the plane of contact. Slip Resistance--That p r o p e r t y of a floor surface that is designed to prevent slipping. A surface having a static coefficient of friction of 0.5 o r greater as m e a s u r e d in a c c o r d a n c e with ASTM D 2047 is considered to be a slipresistant surface. Static Coefficient of Friction--The ratio of the horizontal c o m p o n e n t of force a p p l i e d to a b o d y that just overcomes the friction or resistance to slipping to the vertical c o m p o n e n t of the weight of the object or force a p p l i e d to it. The vertical c o m p o n e n t shall result in a contact pressure of not less t h a n 1 psi (6.9 kPa) n o r m o r e t h a n 13 psi (90 kPa) applied u n i f o r m l y over the area in m u t u a l contact.

ASTM ACTIVITY M a n y studies of COF m e a s u r e m e n t by various ASTM committees o c c u r r e d over the p a s t 20 years. F o r the most part, these studies have been divided into specific c o m m i t t e e s with a task group of C o m m i t t e e F-13 on Safety a n d Traction for F o o t w e a r u n d e r t a k i n g the task to i m p r o v e c o m m u n i c a t i o n s between the various groups studying COF a n d thus prevent d u p l i c a t i o n of effort. Task G r o u p D01.23.15 on Slip Resistance is involved with m e a s u r e m e n t of p a i n t and coatings COF. The group has developed a test m e t h o d that describes a slip-angle test a p p a r a tus a n d a horizontal-pull slip m e t e r that functions u n d e r wet a n d d r y conditions a n d t h a t can be used to m e a s u r e b o t h static a n d d y n a m i c COF. Other areas of activity dealing with the COF include cer a m i c s [5], plastics [6], polishes [7], skid resistance [8], a n d footwear [9]. A study b y the Consumers' Union [10] i n d i c a t e d a need for good m e a s u r e m e n t of slipperiness. Evidence in 1976 i n d i c a t e d that available i n s t r u m e n t a t i o n was inadequate a n d that results o b t a i n e d with it d i d not correlate with practical observations. The N a t i o n a l Institute of Occupa~Consultant, P. R. GuOvin Associates, P.O. Box 811, Westerville, OH 43086-0811.

In a general sense, slipperiness can be defined as the tendency or liability to cause s o m e t h i n g to slide s u d d e n l y or involuntarily. In m a n y cases, an organic p o l y m e r surface (i.e., a coating) is involved. In terms of the flooring m a r k e t area, for which these definitions were written, the flooring surface

600 Copyright9 1995 by ASTM International

www.astm.org

CHAPTER

could be an alkyd e n a m e l on a p o r c h or deck, a vinyl- or p o l y u r e t h a n e - c o a t e d athletic g y m n a s i u m floor, a waxed vin y l - c o m p o s i t i o n floor tile, an epoxy-coated concrete factor floor, a n d so on. Slipperiness can be c o n s i d e r e d as being m a d e up of two factors, skid resistance a n d slip resistance. Skid can be defined as an act of sliding w i t h o u t r o t a t i o n a n d slip as a slide t h a t occurs suddenly o r involuntarily.

C O N C E P T OF T H E C O E F F I C I E N T OF FRICTION Slip is a w o r d that has m a n y m e a n i n g s [13,14]. In certain areas of the coating industry, slip is c o n s i d e r e d to be the ability of an object to move in a relatively free or u n e n c u m b e r e d b u t controlled m a n n e r w h e n one sheet of metal passes over a n o t h e r sheet as in a coating-line feeding operation, w h e n it moves along a conveyor system in coating, printing, fabrication, or p a c k a g i n g operations, and the like. The coating m u s t have p r o p e r "slip" or "lubricity" to allow the coated object to pass t h r o u g h the coating system to a further fabrication or p a c k a g i n g section of the line. In fact, slip is very i m p o r t a n t to fabrication operations w h e r e i n the c o a t e d object is in contact with a tooling system as in fabrication of bottle caps, or with other m e t a l surfaces in b e n d i n g operations, etc. Yet, o t h e r coatings such as floor coatings are designed to decrease this ability to move easily u n d e r an applied force. In either case, the frictional resistance b e t w e e n objects is being modified a n d controlled with such control being att a i n e d b y the inherent p r o p e r t i e s of the m a t e r i a l s used for the coating or use of additives in the coating. It is i m p o r t a n t to u n d e r s t a n d that at times low friction is desired a n d that at other times r e a s o n a b l y high friction is desired. Friction is the p r o p e r t y t h a t d e t e r m i n e s the degree of slip or resistance to slip t h a t exists. Both static a n d d y n a m i c o r kinetic friction are i m p o r t a n t p r o p e r t i e s of coatings. Static friction is the a m o u n t of friction b e t w e e n two surfaces at the precise instant w h e n one c o m m e n c e s to move over the other. D y n a m i c friction is the friction b e t w e e n two surfaces w h e n they are moving p a s t one a n o t h e r w i t h o u t interruption. The coefficient of static friction is usually equal to or greater t h a n t h a t of d y n a m i c friction [15]. The coefficient of friction, ix, is a m e a s u r e of slip with a high coefficient of friction denoting p o o r slip a n d a low coefficient of friction denoting g o o d slip [16]. This p a r a m e t e r is the ratio of the force F (frictional force) r e q u i r e d to move one surface over a n o t h e r surface to the total force W pressing the two surfaces together ix = F/W A c u r r e n t theory of the m e c h a n i s m of dry friction describes the force of friction arising from the s h e a r strength of m i n u t e interaction areas or areas of true c o n t a c t b e t w e e n the contacting surfaces. These i n t e r a c t i o n areas are d i s t r i b u t e d in a m o r e or less r a n d o m m a n n e r over the total a p p a r e n t contact area. This can be expressed b y the following relation for the friction force F = s.A

where s is the average s h e a r strength of the interaction areas, a n d A is a r e a of actual contact. Since it is the interaction areas

50--SLIP

RESISTANCE

601

t h a t actually c a r r y the n o r m a l l o a d W b e t w e e n the two surfaces, the following relation also exists W = p,~.A

w h e r e Pm is defined as the flow pressure of the softer m a t e r i a l in the vicinity of these local areas of true contact. W h e n A is e l i m i n a t e d b e t w e e n these equations, the friction coefficient is obtained ix -- F / W = s/p,~

F r i c t i o n is the universal force b e t w e e n surfaces t h a t opposes sliding motion. W h e n surfaces of bodies are in contact, the interactive force at the surface m a y have c o m p o n e n t s b o t h p e r p e n d i c u l a r a n d tangential to the surface. The perpendicular c o m p o n e n t is called the " n o r m a l force," a n d the tangential force is called the "friction force." W h e n there is relative sliding b e t w e e n the bodies, the frictional force always acts in the opposite direction of this motion. Most dry surfaces behave a p p r o x i m a t e l y a n d within limits according to Coulomb's frictional law. C o u l o m b f o u n d that just before motion, the friction b e t w e e n two surfaces is slightly greater t h a n w h e n the surfaces are in steady m o t i o n relative to each other, that the frictional force is p r o p o r t i o n a l to the n o r m a l force pressing the surfaces together, and t h a t this force is i n d e p e n d e n t of both the contact a r e a and, except at the start, the speed of relative m o t i o n of the bodies. The constant ratio of the tangential force to the n o r m a l force is k n o w n as the "coefficient of friction" (COF) a n d d e p e n d s on the n a t u r e of the two surfaces. To initiate sliding against friction, it is necessary to a p p l y a tangential force at least as great as the COF a n d the n o r m a l force before the onset of m o t i o n take place. The a p p l i e d tangential force is resisted by the equal a n d opposite force of static friction, a n d the force r e q u i r e d to overcome static friction is usually greater t h a n the force n e e d e d to sustain u n i f o r m sliding motion.

D E T E R M I N A T I O N OF T H E C O E F F I C I E N T OF FRICTION Three types of i n s t r u m e n t s are used to m e a s u r e the COF, a n d these are illustrated in Fig. 1 [17]. These are drag-type meters that are b a s e d on fx = F/W, p e n d u l u m - t y p e m e t e r s that m e a s u r e the energy loss of the p e n d u l u m as an indirect i n d i c a t i o n of the d y n a m i c friction, a n d articulated-strut devices that are b a s e d on the direct a n d f u n d a m e n t a l principle of the resolution of forces that take place w h e n an object slides d o w n an incline as described below. The angle at which a flat or plane surface m u s t be inclined for a solid object to slide with a steady speed d o w n the incline is the "angle of friction." The tangent of this angle is the COF b e t w e e n the solid block of m a t e r i a l a n d the inclined plane /x = tan4~ The principle involved in this equation is used in the slipangle a p p a r a t u s d e s c r i b e d in ASTM Test Methods for Measuring Static F r i c t i o n of Coatings Surfaces (D 4518) [18]. The state of the art of slip-resistance studies t h r o u g h 1975 has b e e n s u m m a r i z e d by B r u n g r a b e r [19].

602

PAINT AND COATING TESTING MANUAL a. DRAG TYPE METER

~

l:Ol~lM*=ilal

I

FLOORSURFACE b. ARTICULATED STRUT DEVICE

' ~ S I T I O N AT

INITIAL POSITION

FLOORSURFACE / i C. PENDULUM DEVICE

f

\\

q•

i FLOORSURFACE

/

FIG. 1-Schematics of different friction measurement devices (courtesy of the National Institute of Standards and Technology).

S E N S O R MATERIALS It is obvious that two surfaces are required for COF measurements of any surface, and the sensor material, or surface against which the specified compound is tested, should be defined and specified. It is essential to obtaining meaningful, reproducible results that the sensor material be selected to represent use conditions and be well defined. Properties such as uniformity (surface character including flatness, roughness, chemical composition, resilience, and shear modulus), permanence in that chemical and physical characteristics should not change with time, and availability in a usable form that does not require excessive preparation should all be considered when selecting a sensor. Both sensor material and test material should be reported when the coefficient of friction is given.

The sensor material may be composed of the same compound as the test compound or different. In most ASTM studies of flooring, leather is used as the primary sensor material. It is commonly used in the manufacture of shoes, and, probably more important, it has the lowest COF of any shoe-sole material. Although the rationale for its selection is not well documented, leather conforming to Federal Specification KK-L-165C is specified as the sensor material. Where a nonleather sensor material is to be used, rubber conforming to ASTM Test Method for Rubber Property--Abrasion Resistance (NBS Abrader) (D 1630) [20] is used. Neoprene has been used in certain round-robin studies. In other instances, three leathers with three levels of oil content, two Kraton | thermoplastic elastomers, and l 5 different rubbers were used to generate statistical data [21] for ASTM Test Method for

CHAPTER 50--SLIP RESISTANCE Static Slip Resistance of F o o t w e a r Sole, Heel, or Related Materials by H o r i z o n t a l Pull S l i p m e t e r (HPS) (F 609) [22].

LUBRICANTS In those instances w h e r e slip is to be i n c r e a s e d (friction decreased), a variety of slip agents or lubricants are available. These include m i c r o n i z e d polyethylene p o w d e r s a n d silicones. See Table 1 for m o r e slip agents. M a n y slip agents also function as a b r a s i o n - r e s i s t a n c e a n d m a r - r e d u c t i o n agents. In certain instances, they can be u s e d as a n t i b l o c k agents. Lubricants are often used in the plastics processing industry where they function as melt viscosity reducers, flow agents to improve flow onto metal surfaces, and, at times, costabilizers. Factors t h a t s h o u l d be c o n s i d e r e d in selecting a l u b r i c a n t include melting point, polarity, a n d solubility. Of course, the l u b r i c a n t should not interfere with a d h e s i o n or any crosslinking m e c h a n i s m that is used. C o m m o n l u b r i c a n t families as well as selected specific lubricants are listed in Table 1. Polyethylene a n d polytetrafluoroethylene are available in a p o w d e r o r m i c r o n i z e d form in a variety of particle sizes [23], a n d silicones are available as f o r m u l a t e d p r o d u c t s designed for use in the coating i n d u s t r y [24,25]. The m i c r o n i z e d polymers are i n c o m p a t i b l e a n d act as a filler that rises to the coating surface where they function as tiny "ball bearings" that decrease friction a n d often i m p r o v e a b r a s i o n a n d m a r resistance. TABLE 1--Lubricants.

Esters Wax esters Fatty alcohol esters Fatty esters Glycerol esters Fatty Acid Amides Alkanolamides Monoamides Bisamides Metallic Compounds Aluminum stearate Barium stearate Calcium stearate Molybdenum sulfide Zinc stearate

METHODS FOR DETERMINING C O E F F I C I E N T OF FRICTION The test m e t h o d s a n d e q u i p m e n t d e s c r i b e d b e l o w were developed for flooring materials. However, they can be applied to testing the slip resistance a n d COF of p a i n t a n d coatings on various substrates against themselves as well as against a variety of substrates. ASTM Test M e t h o d for Determining the Static Coefficient of Friction of Ceramic Tile a n d Other Like Surfaces by the H o r i z o n t a l D y n a m o m e t e r Pull-Meter M e t h o d (C 1028-84) [26] utilizes a pull m e t e r - a n d - h e e l a s s e m b l y to m e a s u r e the COF of tile a n d like materials. The test m e t h o d specifies a s t a n d a r d c e r a m i c tile with an average COF b e t w e e n 0.45 a n d 0.55 and a neolite sensor material. Currently c o n s i d e r a t i o n is being given in an ASTM s u b c o m m i t t e e to modifying this device for i m p r o v e d r e p r o d u c i b i l i t y o r discontinuing the method. ASTM Test M e t h o d for Static a n d Kinetic Coefficients of Friction of Plastic F i l m a n d Sheeting (D 1894) [27] is conc e r n e d with d e t e r m i n a t i o n of static a n d d y n a m i c COFs. The test m e t h o d specifies only the force-measuring i n s t r u m e n t since the test m e t h o d uses a plastic film o r sheet sliding over itself. One c o m p a n y that m a n u f a c t u r e s slip-resistance coatings i n t e n d e d for c o m p l i a n c e with OSHA a n d ADA requirem e n t s specifies this test m e t h o d for d e t e r m i n a t i o n of p r o d u c t COF. ASTM Test M e t h o d for Static Coefficient o f F r i c t i o n of Polish-Coated F l o o r Surfaces as M e a s u r e d by the J a m e s Machine (D 2047) [12] is the only slip-resistance test m e t h o d recognized b y the floor-polish industry. Unfortunately, its use is limited to the l a b o r a t o r y since it is n o t portable. Sensor m a t e r i a l s of leather a n d r u b b e r are specified. M e t h o d A of ASTM Test Methods for Measuring Static F r i c t i o n of Coating Surfaces (D 4518) [18] involves a platform containing a sled that is slowly raised until the angle of m o v e m e n t is reached. M e t h o d B involves a horizontal-pull tester. S o m e l a b o r a t o r i e s have modified their I n s t r o n tensile testers to p e r f o r m similar operations. This a p p r o a c h has the advantage of providing a graphic r e c o r d of the forces involved. Other ASTM m e t h o d s for m e a s u r i n g friction include ASTM Test Method of Measuring Surface Frictional Properties Using the British P e n d u l u m Tester (E 303) [28], ASTM Cons u m e r Safety Specification for Slip-Resistant Bathing Facili-

TABLE 2--Canadian government standards for coefficient of

friction.

Waxes and Other Hydrocarbons Fluoropolymers Micronized polyethylene Micronized polytetrafluoroethylene Montan waxes Oxidized polyethylene waxes Paraffins, low melting Polyethylene waxes Silicones

603

Dry

Static COF Wet Oily

Dry

Sliding COF Wet Oily

For leather with: Epoxy coating Polyurethane coating

0.75 0.85

0.75 0.85

-.. -..

0.50 0.50

0.50 0.50

--. .--

1.00 1.00

0.90 0.85

0.70 1.00

0,80 0.85

0.80 0.85

0.40 0.70

For rubber with: Epoxy coating Polyurethane coating

604

P A I N T A N D COATING T E S T I N G M A N U A L TABLE 3--Devices for measuring the coefficient of friction.

Device Dynamometer Pull Meter

Manufacturer

Comment

Reference

DRAG-TYPEFRICTIONMEASURINGDEVICES Chatillion Inc. Tests are being discontinued by ASTM C21.06

Gardco Washability Wear Friction Tester, Model D 12VF

Paul N. Gardner Co. Inc.

Instrument is being evaluated at several companies

[27]

Coefficient of Friction Tester, Model FM-1055 and -1055F

Paul N. Gardner Co. Inc.

Meets ASTM D-1894-78 requirements. Model F has a force transducer for strip chart readout.

[27]

Instrumentors Slip/Peel Tester Model SP-101B

Instrumentors, Inc., Cleveland, OH; available from IMASS, Accord, MA

Designed to meet ASTM D 1894

Floor Friction Tester, Model 80

Technical Products Co.

Portable device

TMI Slip & Friction Tester, Models 32-06

Test Machine Inc., Amity, NY

Flat bed plate laboratory horizontal pull slipmeter

TOPAKA| Horizontal Slip Tester

Pioneer Eclipse Co.

Described in ASTM D-21 Proposal P 128

Universal Slip-Resistance Tester

William English Ltd.

Device has been removed from market

Whiteley Model HPS III Slip Master

Whiteley Industries, Inc.

Can be used to test in accordance with ASTM F 609

British Portable Skid Resistance Tester

DYNAMICPENDULUM-TYPESKIDRESISTANCETESTER Road Research ASTM E 303 utilizes this device Laboratory, Crowthorne, Berkshire, England

[7]

[29-32]

[28]

Sigler Coefficient of Friction Machine

Frazier Precision Instrument Co., Hagerstown, MD

Specified in Federal Test Method Standard No. 501a

[33-34]

Tortus Floor Friction Tester

Ceramic Research, Penkhall Stake-onTrout, England

Very good for microinvestigations of floor surfaces

[35-38]

Ergodyne Slip-Resistance Tester

ARTICULATEDSTRUTTESTERS William English Ltd., Small and lightweight in nature Alva, FL

James Machine

AIDE, Inc., Racine, WI

Used in ASTM D 2047. Considered to be a comparison standard testing device.

[12,39]

NBS/Brungraber Slip Tester, Model Mark I and Model Mark II

Slip-Test Inc., Lewisburg, PA

Mark I useful on dry surfaces and Mark II useful on wet surfaces

[5,32,40-41]

Model 9505 Mobility/Lubricity Tester

Altek Co., Torrington, CT

Used to measure slip resistance such as beverage exterior can coatings

ties (F 462) [22], ASTM Test M e t h o d for Static Coefficient of Friction of Shoe Sole and Heel Materials as Measured by the J a m e s M ach i n e (F 489) [22], and ASTM Test M e t h o d for Static Slip Resistance of Footwear, Sole, Heel, or Related Materials by Horizontal Pull S li p m e te r (HPS) (F 609) [22].

Few slip-resistance standards exist in m o s t industries, including the coating industry, even though there are m a n y painted surfaces in pedestrian walkways. The United States Navy has two specifications--MIL-D-23003A Military Specification-Deck Covering Co m p o u n d , Nonslip, Rollable and

CHAPTER 50--SLIP ~ S I S T A N C E MIL-D-24483A Military Specification-Deck Covering, SprayOn, N o n s l i p - - t h a t specify COF values. Both specifications use h o r i z o n t a l slip testers. The C a n a d i a n G o v e r n m e n t Specification B o a r d a d o p t e d two s t a n d a r d s for deck coatings based on the above U.S. Navy specifications. One involves nonslip epoxy coatings, I-GP192, and the o t h e r nonslip involves p o l y u r e t h a n e coatings, 1GP-200. The COF specifications for these materials are given in Table 2. This i n f o r m a t i o n points out s o m e i m p o r t a n t aspects of slip resistance m e a s u r e m e n t s . First, value differences in slipperiness of various shoe-sole m a t e r i a l s u n d e r different conditions are used. Second, it points out the importance of specifying the n a t u r e of the shoe-sole material. Overall, leather has lower COFs t h a n rubber.

TEST D E V I C E S F O R M E A S U R I N G T H E COF As m e n t i o n e d earlier, there are three types of devices used to m e a s u r e the COF, n a m e l y drag, p e n d u l u m , and articulated strut-based devices, The drag-type meters can be subdivided into two classes: (1) horizontal-pull slip meters that are portable, inexpensive, a n d u s e d directly on a floor or o t h e r surface u n d e r test a n d (2) b e n c h - t o p slip meters that are used p r i m a r ily in the laboratory. Both of the devices in these subclasses are s o m e t i m e s referred to as "fish scale-type testers." The devices are simple, m o t o r i z e d p o w e r units with force-measuring devices such as d y n a m o m e t e r s . One such device, developed at an i n s u r a n c e c o m p a n y [42], has been used by an ASTM s u b c o m m i t t e e in a r o u n d - r o b i n study [21] to evaluate w a l k w a y slipperiness [43]. Use of such devices has b e e n valid a t e d in a n o t h e r study [44]. General results from s o m e ASTM m e m b e r s indicates that p e n d u l u m - t y p e devices are not applicable for further c o n s i d e r a t i o n in the m e a s u r e m e n t of the COF. P e n d u l u m - t y p e COF devices [12, 33-34] consist of a p e n d u l u m that is faced with a certain shoe-sole or heel material. The p e n d u l u m can be adjusted to sweep a p a t h across a flooring surface so that the contact pressure between the facing a n d the floor follows a p r e d e t e r m i n e d , t i m e - d e p e n d e n t pattern. The p e n d u l u m ' s resultant loss of energy is p u r p o r t e d to be a m e a s u r e of the d y n a m i c friction. Articulated-strut meters [12,39-41] involve a p p l i c a t i o n of a known, constant vertical force to a shoe that is faced with a p a r t i c u l a r sole o r heel m a t e r i a l along with a p p l i c a t i o n of an increasing lateral (forward) force until slip occurs. The ratio of the lateral force at slip to the k n o w n vertical force is the static COF. The vertical force is a p p l i e d to the top so t h a t the article tested is only subjected to a vertical load. As the test progresses, the articulated strut is slowly inclined so the test article continues to be u n d e r a constant vertical l o a d a n d in a d d i t i o n u n d e r an increasing h o r i z o n t a l or tangential l o a d until slip occurs. The tangent of the angle that the articulated strut m a k e s with respect to the vertical at the instant of slip is taken to be the ratio of the horizontal a n d vertical c o m p o nents of the force a p p l i e d to the show a n d thus is the COF. Devices of the three types are s u m m a r i z e d in Table 3.

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REFERENCES [1] Guevin, P. R., "Review of Skid and Slip Resistance Standards Relatable to Coatings," Journal of Coatings Technology, Vol. 50, No. 643, August 1978, pp. 33-38. [2] Federal Register, Vol. 24, (Tuesday, 17 March 1955), pp. 15131524. [3] Federal Register, Vol. 55, No. 69 (Tuesday, 10 April 1990), pp. 13360-13441. [4] Federal Register, Vol. 56, No. 14 (Tuesday, 22 Jan. 1991), pp. 2296-2395. [5] Ceramic Engineering and Science Proceedings, Vol. 13, Nos. 1-2, 1992, pp. 1-91. [6] ASTM Research Report D20-1131, 9 Sept. 1986. [7] Annual Book of ASTM Standards, Vol. 15.04 (1984, 1985, 1986). [8] "Walkway Surfaces: Measurement of Slip Resistance," Walkway Surfaces: Measurement of Slip Resistance, ASTM STP 649, C. Anderson and J. Senne, Eds., ASTM, Philadelphia, 1978. [9] "Slips, Stumbles, and Falls: Pedestrian Footwear and Surfaces," Slips, Stumbles, and Falls: Pedestrian Footwear and Surfaces, ASTM STP 1103, B. E. Gray, Ed., ASTM, Philadelphia, 1990. [10] Consumer Reports, Vol. 42, No. 7, July 1976, pp. 417-419. [11] Stanevich, R., "Correlation of Subjective Slipperiness Judgments with Quantitative COF Measurements for Structural Steel," CDC Contract No. 200-86-2929, NIOSH, Morgantown, WV, 31 Oct. 1987. [12] Book of ASTM Standards, Vol. 15.04. [13] Paint~Coatings Dictionary, S. LeSota, Ed., Federation of Societies for Coatings Technology, Philadelphia, 1978. [14] Additives for Plastics, Vol. 1, R.B. Seymour, Ed., Academic Press, New York, 1978. [15] Cramp, A. P. and Masters, L.W., "Preliminary Study of the Slipperiness of Flooring," National Bureau of Standards, NBSIR 74-613 (July 1974). [16] Burwell, J. T. and Rabinowicz, E., "The Nature of the Coefficient of Friction," Journal of Applied Physics, Vol. 24, 1953, pp. 136-139. [17] Adler, S. C. and Pierman, B. C., "A History of Walkway SlipResistance Research at the National Bureau of Standards," NBS Special Publication 565, National Bureau of Standards, Washington, DC, December 1979. [18] Annual Book of ASTM Standards, Vol. 06.01. [19] Brungraber, R. J., "An Overview of Floor Slip-Resistance Research With Annotated Bibliography," Report NBSTN 895, National Bureau of Standards, Washington, DC, January 1976. [20] Annual Book of ASTM Standards, Vol. 09.01. [21] ASTM Research Report F13-I001, 27 July 1979. [22] Annual Book of ASTM Standards, Vol. 15.07. [23] "Innovation in Powder Technology," Technical Data Brochure, Shamrock Chemicals Corporation, Newark, NJ. [24] "Byk-Mallinckrodt Paint-Additives," Technical Data Notebook, Byk-Mallinckrodt USA, Inc., Wallingford, CT. [25] "Dow Coming | Additives," Technical Data Brochure 24-391 E-93, Dow Coming Corporation, Midland, MI. [26] Annual Book of ASTM Standards, Vol. 15.02. [27] Annual Book of ASTM Standards, Vol. 08.02. [28] Annual Book of ASTM Standards, Vol. 04.03. [29] English, W., "Horizontal Pull Slipmeter," U.S. Patent 4,895,015 (1990). [30] English, W., "Improved Tribometry on Walking Surfaces," Slips, Stumbles, and Falls: Pedestrian Footwear and Surfaces, ASTM STP 1103, B. E. Gray, Ed., ASTM, Philadelphia, 1990, pp. 73-81. [31] English, W., "Improved Static Coefficient of Traction Meter," Ceramic Engineering & Science Proceedings, Vol. 13, Nos. 1-2, 1992, pp. 22-28. [32] Kohr, R. L., "A Comparative Analysis of the Slipperiness of Floor Cleaning Chemicals Using Three Slip Meters," Ceramic

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14-21. [33] Sigler, P. A., Geib, M. N., and Boone, T. H., "Measurement of Slipperiness of Walkway Surfaces," Research Report, 1897, National Bureau of Standards, Washington, DC, Journal of Research, Vol. 40, 1948, pp. 339-346. [34] Jablonsky, R. D., "Standardization of Test Methods for Measurement of Floor Slipperiness," Walking Surfaces: Measurement of Slip Resistance, ASTM STP 649, C. Anderson and J. Senne, Eds., ASTM, Philadelphia, 1978. [35] Harrison, R. and Malkin, F., "A Small Mobile Apparatus for Measuring the Coefficient of Friction of Floors," Journal of Physics D: Applied Physics, Vol. 13, 1980, pp. L77-L79. [36] Brough, R., Malkin, F., and Harrison, R., "Measurement of the Coefficient of Friction of Floors," Journal of Physics D: Applied Physics, Vol. 12, 1979, pp. 517-528. [37] Proctor, T. D. and Coleman, V., "Slipping, Tripping and Falling Accidents in Great Britain--Present and Future," Journal of Occupational Accidents, Vol. 9, 1988, pp. 269-285.

[38] Andres, R. O. and Chaffin, D. B., "Ergonomic Analysis of SlipResistance Measurement Devices," Ergonomics, Vol. 28, No. 7, 1985, pp. 1065-1080.

[39] James, S. V., "What is a Safe Floor Finish?" Soap and Sanitary Chemicals, Vol. 20, October 1944, pp. 111-115. [40] Brungraber, R. J., "Portable Tester for Measuring the Static Coefficient of Friction between a Floor Surface or the Like and a Shoe Sole or Heel Material or the Like," U.S. Patent 3,975,940 (1976). [41] Brungraber, R. J., "Portable Tester for Measuring Slip Resistance," U.S. Patent 4,759,209 (1988). [42] Irvine, C.H., "A New Slipmeter for Evaluating Walkway Slipperiness," Materials Research & Standards, Vol. 7, No. 12, December 1967, pp. 535-542. [43] Irvine, C. H., "Evaluation of Some Factors Affecting Measurement of Slip Resistance of Shoe Sole Materials on Floor Surfaces," Journal of Testing and Materials, Vol. 4, No. 2, March 1976, pp. 133-138. [44] Irvine, C. H., "Evaluation of the Effect of Contact-Time When Measuring Floor Slip Resistance," Journal of Testing and Evaluation, Vol. 14, No. 1, January 1986, p. 19-22.

Part 12: Environmental Resistance

MNL17-EB/Jun. 1995

51

Prevention of Metal Corrosion with Protective Overlayers by William H. Smyrl I

enjoys significant economic leverage, and, as evidence, one may cite the widespread use of coatings, films, and inhibitors for metals and semiconductors in many service environments. All engineering metals used in modern technological societies are unstable with respect to corrosion, and the result is a loss of properties. Natural oxide films provide protection against continued attack for some metals, and alloying extends the life of other metals by developing highly stable passive films. Where metals may not be protected by oxide films, other modification methods have been developed to reduce corrosive attack. In reality, the improvement of corrosion resistance of metals by modification of the surface has been practiced since the invention of metal tools. Some of the earliest techniques to prevent corrosion involved coating with greases or natural oils. More modern methods were developed in the 19th and 20th centuries and include multiple coatings, zinc galvanizing, electroplating of other pure metals, and vacuum physical vapor deposition of mostly pure metal coatings by electron beam and sputtering techniques. The metal coatings are better barriers than organic films because of the lower permeability of the former to moisture, oxygen, and ions. Inhibitors or conversion coatings and primers for paints are cheaper than metal coatings and are used widely by paint manufacturers even though they remain highly proprietary in nature. The use of organic coatings to protect metal surfaces is practiced widely. Much of the use is for atmospheric exposure of motor vehicles as well as for structural units such as bridges and buildings. The successful implementation of existing technologies has greatly reduced the effects of corrosion of automobiles, for example, in the past decade in response to consumer demand. Despite many recent advances, coating technologists and scientists acknowledge that much is unknown and that new processes and understanding are the keys for further progress [1]. Defects in the metal substrate and in the overlayers are among the primary concerns because they are the source of localized corrosion phenomena. Defects may occur on length scales from atomic-level lattice vacancies to arrays of defects at grain boundaries (for crystalline materials) or to random pores or cracks (for example, in noncrystalline films). Avoiding such defects by proper quality control is a major concern in coatings science and technology. THE PREVENTION OF CORROSION BY SURFACE PROCESSING

~Professor, Corrosion Research Center, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455.

In the discussion that follows, aspects of corrosion that involve thermodynamics and kinetics will be developed as a basis for the description of the specific nature of corrosion of metals under protective films and overlayers. Some emphasis will be given to protection of thin metal films and microstructures that are particularly sensitive to corrosion and whose successful protection provides a basis for advancing protection technology in general.

CORROSION IN AQUEOUS SOLUTIONS The driving force for a reaction is the change in Gibbs free energy, AG, for reactants to products. Mathematically, this may be expressed by

Aa=ECproducts

The summation signs are used as a general notation to indicate that all reactants and products are included in the calculation. From the nature of the free energy function, this calculation applies to initial (reactants) and final (product) states and is independent of intervening states. The reaction may be investigated under controlled reversible conditions such as in an electrochemical cell or under irreversible conditions such as in corrosion, and the same total free energy change (AG) will be appropriate. A quite general predictive capability may be applied to specific corrosion reactions since all the available thermodynamic data may be used for corrosion calculations directly. This enables the position of final equilibrium of the corrosion system to be established. The thermodynamic calculations have the limitation that no information concerning the rate of the reaction is provided, only what the final state will be for the process. The value of AG for reactions of elements to form a compound, all in their standard states at a particular temperature, is the standard free energy of formation of the compound, AG~r. Here, the subscript T denotes the temperature. Description of the detailed calculations are beyond the scope of this discussion, but several excellent textbooks are available [2,3]. The most extensive tabulations of thermochemical data for chemical compounds in their standard state at 25~ are in a series of National Bureau of Standards publications [4]. These are NBS Technical Notes 270-3, 270-4, and 270-5, which supersede the older NBS Circular 500 for the elements they cover. These tabulations also update the older data of

609 Copyright9 1995 by ASTMInternational

EC reactants

www.astm.org

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L a t i m e r [5]. The b o o k by L a t i m e r [5] r e m a i n s a valuable reference b e c a u s e of the d e s c r i p t i o n of techniques to e s t i m a t e t h e r m o d y n a m i c quantities w h e r e reliable d a t a are sparse. Lewis a n d Randall [3] t a b u l a t e d t h e r m o d y n a m i c data, including d a t a for aqueous solutions of a n u m b e r of electrolytic solutions that are valuable for c o r r o s i o n calculations. There is overlap b e t w e e n this t a b u l a t i o n a n d the JANAF t h e r m o c h e m i c a l tables [6], b u t the latter also tabulate the s t a n d a r d enthalpy a n d free energy of f o r m a t i o n of chemical comp o u n d s at several t e m p e r a t u r e s along with - (G O - I~298)/T. Oxide a n d hydroxide solubility are strongly influenced by the p H of the a q u e o u s phase. P o u r b a i x recognized this a n d s u m m a r i z e d the t h e r m o d y n a m i c stability of m e t a l - a q u e o u s systems by the use of p o t e n t i a l - p H diagrams. The t h e r m o d y n a m i c d a t a t a b u l a t i o n s a l r e a d y quoted [3-6] should be utilized for detailed calculations, however. Usually w h e n studying corrosion, one is n o t c o n c e r n e d with the conditions for t h e r m o d y n a m i c stability, b u t r a t h e r with the rate of attack a n d h o w it m a y be altered in basically unstable conditions b e c a u s e only a limited n u m b e r of systems have absolute or thermodynamic stability. As a practical m a t t e r it is necessary to accept s o m e rate of c o r r o s i o n a n d / o r to control or mitigate the rate of attack. Thus kinetic stability is always relative a n d subject to i n t e r r u p t i o n if control is not maintained. Controlling the rate of d e g r a d a t i o n m a y be accomplished, for example, by the use of cathodic or a n o d i c protection, the use of inhibitors, the m a i n t e n a n c e of protective surface films, or buffering the c o m p o s i t i o n of an otherwise aggressive solution. All these techniques are used widely to extend the life of metallic structures with continuing improvement. Corrosion reactions are electrochemical in nature. The kinetics of the c o r r o s i o n reactions are t h e n related to the kinetics of the electrochemical reactions that occur during the c o r r o s i o n process. The reactions involve not one b u t at least two electron transfer reactions, and the reactions are not in series but are in parallel. Coupling of parallel o r s i m u l t a n e o u s reactions is a f u n d a m e n t a l feature of the c o r r o s i o n process. E a c h of the s i m u l t a n e o u s reactions m a y consist of multiple steps, respectively, as described above, b u t the simultaneous, i n d e p e n d e n t reactions are coupled electrically. The independent reactions occur on the s a m e surface at the s a m e time, b u t also at the s a m e potential. The reactions m a y be c o u p l e d chemically as well, e.g., t h r o u g h p H effects, but this is not essential. The specific relation that defines the coupling of s i m u l t a n e o u s c o r r o s i o n reactions on an isolated metal surface is

EIo=anodic

E cathodic

There will then be zero net c u r r e n t to the c o r r o d i n g metal electrode. The relationship is w r i t t e n in terms of currents (Ia a n d Ic) r a t h e r t h a n c u r r e n t densities for r e a s o n s w h i c h will be discussed. The potential at w h i c h the b a l a n c e is satisfied is the m i x e d or corrosion potential. It is d e t e r m i n e d by the rates of the s i m u l t a n e o u s reactions a n d is not defined by the state of the system in a t h e r m o d y n a m i c sense. The corrosion potential always lies b e t w e e n the e q u i l i b r i u m potentials of the a n o d i c a n d c a t h o d i c processes, respectively.

As a s u m m a r y , the general c o r r o s i o n of metallic m a t e r i a l s in aqueous solutions is well understood. The a n o d i c or oxidation r e a c t i o n of the metal is s u p e r i m p o s e d on a c a t h o d i c reaction, a n d the two are b a l a n c e d locally on a h o m o g e n e o u s surface. The rate of the reaction is a function of b o t h the rate of m e t a l dissolution a n d the rate of the cathodic (reduction) reaction. E a c h r e a c t i o n m a y be influenced in general b y the c o m p o s i t i o n of the solution, especially the pH a n d the electrolyte anion, a n d by the n a t u r e of the (oxide) films, if any, w h i c h m a y be f o r m e d at the metal/electrolyte interface. If several oxidizing species are present in the solution, each m a y act in parallel so that the total rate of metal dissolution is increased. F o r example, m o s t metals will react directly to displace h y d r o g e n from w a t e r a n d to p r o d u c e an oxide of the metal or s o m e o t h e r c o r r o s i o n process. The a d d i t i o n of oxygen will increase the rate of c o r r o s i o n of the metal, usually in direct p r o p o r t i o n to the c o n c e n t r a t i o n of the oxygen added. The specific details will vary with each metal to reflect the t h e r m o d y n a m i c , kinetic, a n d m a s s transfer driving forces that are acting [7]. H e t e r o g e n e o u s surfaces are c o m m o n l y observed in corrosion situations a n d are of the three general classes: (1) the inclusion of foreign metal i m p u r i t i e s on the metal surface, (2) the n o n u n i f o r m coverage of the surface by a film, either an oxide film o r a n artificial coating in a q u e o u s solutions, a n d (3) n o n u n i f o r m conditions in the electrolyte environment. All these are of great i m p o r t a n c e b e c a u s e localized, or nonuniform, corrosion of metals m a y be caused by any of the three. A form of galvanic corrosion a n d pitting c o r r o s i o n is caused by the first type of heterogeneity, while crevice a n d pitting c o r r o s i o n are p r o d u c e d by b o t h (2) a n d (3). Restrictions of geometry, e.g., in crevices a n d corners, prevent mixing of solutions everywhere, a n d local b u i l d u p of r e a c t i o n p r o d u c t s o r the e x h a u s t i o n of an oxidant m a y occur. The local kinetics will be relatively i n d e p e n d e n t of t h a t in o t h e r regions except t h a t there m a y be coupling t h r o u g h the electric field a n d electrical c u r r e n t m a y flow b e t w e e n a localized c o r r o s i o n site a n d the s u r r o u n d i n g surface. This m a y lead to n o n u n i f o r m corrosion, p a r t i c u l a r l y where the b u i l d u p of p r o d u c t s increases the aggressiveness of the local solution. In this case, c o r r o s i o n will be m o s t severe, not where the c o n c e n t r a t i o n or flux of the bulk solution o x i d a n t is highest b u t where it is lowest. Crevice c o r r o s i o n is c o n s i d e r e d to be an e x a m p l e of this type of attack, a n d the aggressive solution within a crevice or pit is one w h i c h is m o r e acidic t h a n the external solution. Anodic dissolution, plus hydrolysis of the p r o d u c t m e t a l ion, causes an increase of h y d r o g e n ion concentration. On the o t h e r hand, r e d u c t i o n of either h y d r o g e n ions o r dissolved oxygen reduces the h y d r o g e n ion concentration. If the net c o r r o s i o n r e a c t i o n plus hydrolysis w o u l d lead to a n increase of h y d r o g e n ion concentration, the process m a y occur indep e n d e n t l y of any o t h e r process a n d w o u l d accelerate with time to s o m e steady state w h e r e diffusion out of the occluded region w o u l d limit the buildup. If the c o r r o s i o n r e a c t i o n plus hydrolysis leads to no net change in H + concentration, a n acid solution in a crevice o r pit could only be c r e a t e d b y s e p a r a t i o n of the a n o d i c a n d cathodic regions. Concentrating the c a t h o d i c r e a c t i o n on the o u t e r surface w o u l d occur naturally if dissolved oxygen, for example, were the p r i m a r y b u l k oxidant. Coupling this with a net anodic r e a c t i o n (plus hy-

CHAPTER 5 1 - - P R E V E N T I O N OF METAL CORROSION drolysis) in the inner region for an overall current balance would lead to a steady state crevice or pit. For separation to occur as described above, a quite general condition imposed on the corrosion kinetics must be obeyed. The outside surface must support a cathodic reaction, and it must be supported at a potential that is positive of the potential of the anodic reaction in the crevice. The direction of current flow through the solution establishes this criterion. A qualitative laboratory test may be used to identify metal solution combinations that could cause localized attack by the mechanism described above. The test involves the corrosion kinetics on the metal of interest. Cathodic currents must be observed on the metal in the exterior solution at potentials that are positive of the anodic region for the crevice conditions or the separated reactions will not support increased anodic dissolution in the isolated region. This is a very definitive test, and very few metal-environment combinations match the criterion. Ohmic drop restricts the penetration of current into a small-gap, occluded region [8]. This causes the anodic reaction to be distributed over a relatively small region, which concentrates the attack. At greater depths in the gap, the metal is isolated from the external surface reactions. Newman [9] calculated the limited depth to which a reaction may penetrate inside a circular geometry, in this case a cathodic protection reaction. The reaction is concentrated near the opening. Composition gradients are considered to be important for pitting and differential aeration corrosion as well. For pitting corrosion, similar conditions to those for crevice corrosion are considered important. Pits may be initiated in ways that are different from crevice corrosion, e.g., at foreign metal inclusions. However, the propagation of pits depends largely on a locally aggressive solution. Stirring to eliminate concentration effects will stop the growth of pits. Differential aeration could also drive corrosion at locally variable rates under an electrolyte film of nonuniform thickness. The diffusion-limited flux of oxygen through the film would be directly proportional to the film thickness. If the local corrosion rate is limited by the oxygen flux, the attack will be most severe at low film thicknesses. For active/passive metals, increase of the oxygen flux may exceed the peak current for active dissolution and cause the metal to adopt the passive state. In this case, then, thin films of electrolyte will reduce the corrosion rate.

A T M O S P H E R I C C O R R O S I O N OF M E T A L S Most atmospheric corrosion tests have been conducted in environments such as indoor atmosphere, outdoor atmosphere, and laboratory tests under simulated conditions. Indoor corrosion studies have been performed for the electronics, computer, and communication industries for the development of more durable materials with desirable structural, magnetic, and electrical properties. On the other hand, outdoor studies aimed at understanding corrosion behavior are highly dependent on atmospheric weather factors, especially in marine and urban areas. The latter studies have been performed in the automobile, marine, and aircraft industries. Laboratory tests attempt to use accelerated methods under

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simulated atmosphere or aqueous conditions. Electrochemical methods have been used extensively in such tests to analyze and monitor the corrosion behavior of metals. Several weather factors are known to influence outdoor corrosion [10-13]. Precipitation, ambient and dew-point temperatures, atmospheric pollutants, wind direction and wind velocity, and solar radiation can be considered as weather factors in outdoor and/or urban corrosion tests. Among these factors, moisture or relative humidity, temperature, and pollutants such as sulfur dioxide and chlorides are the most important variables. Relative humidity is known to be the most important factor in determining the atmospheric corrosion rate. It has been reported that rapid acceleration of corrosion occurs beyond a certain value of relative humidity, defined as the critical humidity [14-15]. The period in which the relative humidity exceeds the critical humidity is called the time-of-wetness, and this factor is quite significant in determining atmospheric corrosion rate of metals [16]. In addition, in the presence of a pollutant such as sulfur dioxide, the critical humidity at which corrosion is enhanced to a significant extent will decrease with increasing pollutant concentration [17-I8]. It has been reported that comparatively large aggregates of water are present on oxyhydroxide surfaces at humidities below 40% [19]. Even on clean metal surfaces obtained under ultrahigh vacuum or reducing conditions, significant quantities of water are adsorbed on air-formed films when exposed to the environments containing only oxygen and water vapor for more than a microsecond [13]. As a result, monolayers of adsorbed water may provide the medium for electrochemical microcells that may drive a heterogeneous corrosion process. Water may also exist in the form of complex mixtures with oxides, hydroxides, and mixed oxyhydroxides [19-20]. The corrosion rate of metals is accelerated by the presence of air pollutants such as sulfur dioxide, nitrite, nitrate, hydrogen sulfide, chloride, and some kinds of salts [10,15]. These species may derive from gas-borne particles or from reactions at the surface. Reaction with adsorbed water monolayers yield electrolyte films that facilitate further corrosion processes. Among these pollutants, sulfur dioxide, chlorine gases, sulfur gases, and ozone are important species that promote corrosion in the presence of water. The corrosion-accelerating effect of sulfur dioxide with humidity has been reported by many investigators [I0,13,15]. Vernon [15] suggested that sulfur dioxide may change the pH in electrolyte films present on metal surfaces and enhance the corrosion rate. Rice et al. [13] also suggested that sulfur dioxide is readily soluble in water to form sulfurous acid; these local acidic regions accelerate oxide formation, and the corrosion rate is also enhanced by other electrochemical effects. It has been reported that wetting of the metal surface is promoted in the presence of ammonia, and the water droplets contain higher concentrations of sulfates than for the same concentration of sulfur dioxide with no ammonia [10,22]. The effect of chlorine gas or chloride on atmospheric corrosion has been reported [10,13]. In aqueous electrochemical corrosion studies, the chloride ions usually enhance pitting corrosion of many metals and also degrade the oxide surfaces. Rice et al. [13] reported that chlorine gases reduce the surface pH and yield hygroscopic corrosion products that influence the amount of adsorbed water. A direct relationship

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PAINT AND COATING TESTING MANUAL

between the amount of chlorides in corrosion products and atmospheric corrosion rates was reported by Sereda [10]. The effect of ozone gas on copper and silver corrosion has been known to be significant, while cobalt, nickel, and iron are insensitive to ozone [13]. It has been reported that ozone may enhance the corrosion rate of metals sensitive to H2S. The atmospheric corrosion rate can be measured either in field tests in different atmospheres or with accelerated tests in the laboratory. The field tests require long exposure times and yield complicated data that prevent detailed analysis. Accelerated tests are performed under simulated atmospheric conditions, and they are the easiest way to acquire more information with various setup conditions. However, it may not be possible to simulate practical service conditions. There are several methods to monitor and control the corrosion rates by means of either field or laboratory tests. The conventional method is the weight loss determination, which requires long-term exposure unless a continuous method is used that involves the quartz crystal microbalance [23-24]. Another method is the electrical resistance method and measurement using electrochemical cells. Electrical resistance methods use the changes in the electric resistance of thin wires or foils to monitor failure, but they cannot be used for determination of the instantaneous corrosion behavior

[25-27]. Electrochemical methods have been developed to take advantage of the electrochemical basis for atmospheric corrosion [28-29]. Corrosion currents can be monitored electrochemically, and the instantaneous value of current can be detected. One way to monitor atmospheric corrosion with an electrochemical method is to design a cell that will work under thin electrolyte layers (less than 500 ~xm) with consideration of the effects of corrosion products and dilute pollutants [30]. Electrochemical methods for monitoring atmospheric corrosion have been well reviewed by Mansfeld [30-31]. Most of the studies have been aimed at macroscopic measurement of time-of-wetness that is associated with electrochemical corrosion [10,12,16,17]. Galvanic cells with electrodes of different metals have been commonly used [16, 32]. Sereda [10] has developed galvanic cells of platinum-iron and platinum-zinc couples to determine the time-of-wetness. Time-of-wetness was arbitrarily defined as the interval during which the external potential exceeded 0.2 V. This figure was the period during which the relative humidity was greater than 85% [12,16]. Tomashov [33] has used sandwich-type galvanic cells of ironcopper, iron-zinc, iron-aluminum, and copper-aluminum. They concluded that the method was suitable for fast determination of the corrosivity of the atmosphere and that the direct measurement of corrosion rate for testing metals was possible. Several investigators [17,34-36] have used galvanic cells consisting of steel-copper and electrolytic cells consisting of individual metals (steel, zinc, or copper) to which an external potential was applied. They concluded that the cell current gave qualitative agreement with the weight-loss data. Recently, extensive studies have been performed by Mansfeld and his coworkers [30-31,37-39]. They used galvanic cells and electrolytic cells which consisted of two electrodes and three electrodes. Galvanic cells such as copper-steel, copperzinc, steel-zinc, steel-aluminum, and aluminum-zinc couples

were used to study the effects of pollutants, relative humidity, and so on. They used the electrolytic cells such as two- and three-electrode cells for studies of the corrosion kinetics and for the measurement of corrosion currents. The polarization resistance method was used to determine atmospheric corrosion kinetics under thin electrolytic layers. Mazza et al. [40] have used a galvanic cell that consisted of a sandwich formed of bronze covered by its artificial corrosion products on which a high-porosity gold film was applied. They monitored the corrosion current with a zero resistance ammeter and obtained instantaneous and continuous information on the corrosion rate of the bronze. Tosto and Bruco [41] used galvanic cells of copper-steel to obtain the relation between the corrosion content and relative humidity. They found that the corrosion current depended on relative humidity (RH). As a rapid electrochemical method for monitoring atmospheric corrosion, measurements of electrode po*ential using a suitable reference electrode have been developed by several investigators [42-43]. Although the method gives no absolute data on corrosion rates, it is a fast and easy method for on-site investigations. Thin film methods to measure corrosion rates were discussed by Howard [44]. Pourbaix and his coworkers [42, 45] developed an accelerated electrochemical wet and dry method that was designed to use alternate immersion cycles in an electrolyte bath. The electrode potential was monitored when the steel electrode was in the wet part of the cycle. They concluded that their method was selective and yielded reproducible data. Electrochemical cells designed to simulate thin electrolytic films formed during atmospheric corrosion have been developed by several investigators [46-47]. Fishman and Crowe [47] have studied the thin film of electrolyte with a potentiostatic polarization technique. The corrosion current increased with an increase of relative humidity. They concluded that the resultant corrosion rates were consistent with those reported from long-term weight loss measurements. Fiaud [46] created a thin electrolytic film (80-/xm thickness) using the device similar to one developed in the field of thin layer electrochemistry [48]. Platinum and nickel were used as electrodes and sodium sulfate (Na2SO4) solution was used as the electrolyte with change of pH by addition of sulfur dioxide (H2SO4). SO2 gas was introduced into the electrolyte through a membrane. They observed the depolarization effect of SO2, oxidation of SO2, and reduction of SO 2 with use of cyclic voltammetry and linear polarization techniques.

C O R R O S I O N OF T H I N METAL FILMS A N D MICROSTRUCTURES Corrosion of a metal occurs by the same fundamental reactions whether it is used in a large structure like an automobile, a bridge, or a heat exchanger, or in a small structure characteristic of magnetic, optical, or microelectronic devices, or under a protective layer [49]. The uniqueness of each application is tied up in the definition of the environment to which the metal is exposed or which develops with time, as well as the definition of a characteristic size of the corroding material. Since the time to failure of a material (i.e., its lifetime) is normally inversely proportional to the corrosion rate and directly proportional to the thickness of the corroding

CHAPTER 5 I - - P R E V E N T I O N OF METAL C O R R O S I O N material (its characteristic size), small dimensions are more susceptible to corrosion failure and loss of properties. For example (see Fig. 1), a 50-nm cobalt magnetic film may be corroded completely in about 38 h at a corrosion rate of 1 /~A/cm2. The desired lifetime is about five years, so a protective film (e.g., diamond-like carbon) must be used to moderate the rate of attack. The protective layer must be thin to read or write to the cobalt film with the magnetic head, and defects in the protective layer will lead to localized corrosion attac. Wear and friction are mechanical processes that result from the relative movement of the disk and head. The head is designed to fly very close to the disk to take advantage of the magnetic properties [50], but it comes to rest on the disk surface when the system is idle. Humidity and other factors affect wear and friction, and layers or films may be added to lubricate the magnetic film. Of more interest here, however, are the chemical effects that cause corrosion. Accelerated tests have been used to determine disk reliability [51], tests that measure the affects of wear, friction, atmospheric contaminants, humidity, oxygen diffusion, and galvanic corrosion. Also described by Antler and Dunbar et al. [51] is the comparison between field test experience and laboratorysimulated corrosion test results. Earlier results on microelectronics failures are reviewed by Schnable et al. [52], Comizzoli et al. [53], Wood [54], and Stojadinovic et al. [55]. Whatever the mode, the result is a loss of information at the site of degradation and the loss of properties. Better preparation and processing, or better design, may reduce flaws and defects that cause mechanical failure, but they may not re-

THIN FILM MATERIALS CONTINUOUS FILM MAGNETIC MEDIA CONTINUOUS FILM OPTICAL MEDIA

IMULTIPLELAYERSANDWICHARRAYS 5oo ~,

METAL CONDUCTOR LINES METAL CONTACTS

I

DL- CARBON

~I'

COBALTALLOY

500 ,~

Ni P

J PARALLELLINES- SPOTS

Fq

FI

CVD - SiO 2

F AI

TYPICAL DIMENSIONS

TYPICAL DIMENSIONS

50 nm (500~,) THICKNESS

1 grn

EXAMPLE:

Feo,74Tb0,26FOR MAGNETO-OPTIC STORAGE MEDIA - HIGH DENSITY MEMORY DEVICES. MAGNETIC PROPERTIES ARE DE6RADED BY OXIDATION DURING LOCAL HEATING (2200 "C) BY LASER DURING DATA STORAGE.

FIG. 1-Thin film materials for magnetic, optical, metal conductor lines and microelect~onic contacts make them highly susceptible to have small dimensions are highly susceptible to corrosion.

J

613

duce corrosion that is the result of the natural instability of the metal in an aggressive environment. Rarely are the thin film metals stable in the environment, so techniques must be found to stabilize the structures and extend the lifetimes. In the other example of Fig. 1, aluminum interconnects in microelectronic devices have characteristic widths of 1 ~m or smaller. Ionic conduction along adsorbed water layers at the silicon dioxide (SiO2) surface can lead to electrochemical corrosion and "breaks" in the A1 conductor. If the corrosion rate were I/~A/cm2, the lifetime of the A1 interconnects would be about 48 days, rather than the 10 to 20-year lifetime desired. A protective layer is required for this application as well. The corrosion phenomena of thin films chosen for magnetic, optical, or electrical applications have unique characteristics, but they are often similar to those observed for bulk materials [35, 50, 56]. Thin films have bulk metallurgical properties in thicknesses larger than 1 to 3 nm and have the same chemical reactions as well. Both observations lead to the general conclusion that both bulk metals and films 30 nm or thicker will have similar corrosion behavior. On the other hand, thin film materials have small grain size and are prepared for magnetic disk applications in "tracked" or grooved geometries. The small grain size causes films to have more homogeneous properties, with fewer inclusions and smaller chemical segregation effects than ordinary bulk materials. The tracks have sharp edges and dimensions to generate unique morphologies in the films. The homogeneous properties would make the films less susceptible to corrosion, but the defects generated at edges could be sites for enhanced attack. The dimensions and geometry of the tracks may lead to nonuniform chemical composition in the recesses, which would produce localized corrosion effects as well. Atmospheric corrosion has been studied under simulated conditions for thin magnetic films [35], and, as in other cases, it was found that the affect of humidity and atmospheric pollutants was synergistic. The level of humidity may influence the condensation of thin moisture films on surfaces, which will facilitate transport across surfaces and may cause the accumulation of water in microscopic domains. In the latter, the concentrations of dissolved contaminants may approach saturation conditions. The contamination may come from dust or inclusions of other layers [49]. The conditions are difficult to simulate in the laboratory because of the lack of knowledge of local conditions in the microscopic regions that are relevant to the problem. In addition, it has been difficult to make in situ measurements for conditions that simulate atmospheric-corrosion measurements, which would give a direct indication of the processes responsible for corrosive attack. Several standard tests have been developed to assess atmospheric corrosion damage [ASTM Test Method for Assessing Galvanic Corrosion Caused by the Atmosphere (G 104-89); ASTM Practice for Conducting Atmospheric Corrosion Tests on Metals (G 50-76 (1990)); ASTM Practice for Rating of Electroplated Panels Subjected to Atmospheric Exposure (B 537-70 (1992)] without addressing the mechanism of the attack. The second topic relates to protective layers and encapsulants. Pore-free conventional protective layers over magnetic films are too thick to be compatible with the magnetic properties of thin film disk materials. In addition, poly-

614

PAINT AND COATING TESTING MANUAL

mer films can change the adhesion properties of the surface and interfere with the operation of the magnetic head. Highly resistive but electroactive overlayers could lead to galvanic attack of the substrate through holes in the thin film. Sputtered diamond-like carbon films [51,57-60] could fall into this category (see Fig. 1). A protective layer plated or sputtered over an active metal may have pores and defects that will permit the corrosive medium to contact the active substrate metal and thereby promote galvanic corrosion. The holes or defects may be present on a heterogeneous surface in the geometry of either regular arrays or random arrays. The mathematical modeling of galvanic behavior in plating corrosion systems has been discussed by Smyrl and Newman [21], where earlier work was also reviewed. They determined the current and potential distributions of galvanic corrosion system, which consisted of anodic disks in a cathodic plane as shown in Fig. 2. They solved the Laplace equation for potential with nonlinear (Butler-Volmer) boundary conditions with the use of finite difference method. The numerical modeling of galvanic corrosion in which the geometry consists of various array forms has been performed by Morris and Smyrl [61-63] in this laboratory. Either regular or random arrays of disks in the cathodic plane were used for the simulation of a heterogeneous surface. Most treatments of the regular array use the symmetry element derived from symmetrical geometry of the system. For mathematical simplicity, a particular hexagonal symmetry element can be approximated by a circular geometry, thus eliminating any angular dependence. For random arrays, the arrays simulated using a Voroni tessellation of the plane into random polygons as shown in Fig. 3 were used for the disk-cathodic plane geometry. The Voroni tessellation has proven to be useful for modelling the transport and mechanical properties of disordered or composite media. The geometrical properties of the Voroni tessellation and algorithms for generating the tessellation have been described by Winterfeld [64]. The model established by Morris and Smyrl included the Laplace equation for potential distribution with nonlinear boundary conditions, and it was solved by a finite element method. The potential distribution of the system was obtained from numerical simulation of a regular array of disks over a cathodic plane. The disks were of alternating sizes (disks with two different diameters) distributed on the surrounding plane. The models for the tertiary current distribution, which includes both potential distribution and mass conservation with use of a geometry of the random array, are in progress in our laboratory. Since the total anodic current must be equal to the total cathodic current, the area ratio between anodic and cathodic components of the total area is an important parameter in galvanic corrosion. If the currents to each area were uniform, the area ratio is the only parameter that would affect the galvanic interaction for a particular combination of metals. On the other hand, Smyrl and Newman [2I] found that ohmic effects in the electrolyte may cause a nonuniform galvanic current distribution on the component areas, and this leads to the conclusion that under such conditions some parts of the cathodic area are not important in determining the total galvanic current. The effect is even more pronounced under circumstances where the electrolyte phase is very thin, that is, galvanic current from cathode to anode flows only near

FIG. 2-Multilayer "sandwich" arrays may have underlayers exposed through holes or holidays in the overlayer, and galvanic interactions may enhance the corrosion rate in such systems.

where they join, and more remote areas of each are relatively unaffected by the galvanic coupling. It has recently been found that the active perimeter measure of the interactions is more relevant than the area ratio, and the former may be used to correlate the behavior of several geometries [61-63]. The nonuniform current distribution is also obtained if the cathodic surrounding plane is highly resistive but electroactive. For example, resistive sputtered carbon films would cause the cathodic galvanic current to flow only to areas at the periphery of holes and defects [65], and the total area would not be important in determining the galvanic current. However, smaller holes would increase the galvanic current at a constant area ratio because the active perimeter would increase. In summary, investigations in bulk solution provide a basis on which to begin to analyze atmospheric corrosion behavior. As the electrolyte phase decreases in thickness, ohmic and diffusion effects become more dominant and galvanic

OR

FIG. 3-Simulations of galvanic interactions in multilayer arrays have been carried out with regular patterns or with (more realistic) Voronoi tessellation representations.

CHAPTER 51--PREVENTION OF METAL CORROSION coupling is strongly affected. The more remote areas will show the behavior expected for uniform exposure to an aggressive environment. Effects of local composition and local physical geometry then will become more dominant. Behavior in the local areas would be expected to be very similar to the behavior in bulk electrolyte at the same composition conditions. Further general comparisons must be developed as further research is conducted.

COATINGS AND OVERLAYERS FOR CORROSION CONTROL In the past 15 to 20 years, an explosion of interest in surface modification techniques has mostly involved the deposition of thin films, the application of coatings, and the formation of surface alloys. The development of many of the techniques has been driven by the need for the semiconductor electronics industry to create improved processing procedures. As a spin off of the advancing technology, other fields, such as corrosion protection, have benefited from the new processes. A recent panel report [66] has summarized the general surface modification techniques that are used. The techniques are divided into three broad categories: 9 Low-energy inorganic coating techniques. For the most part these are mature technologies that have been used for many years. 9 Polymer coatings include traditional paints, thermoplastics, poly(vinyl chloride)s, epoxies, urethanes, and poly(methylmethacrylate). 9 Techniques involving the use of energetic ions. The techniques have developed rapidly in the last 10 to 15 years; several have neither reached maturity nor found use for corrosion protection. Only those designated as low cost and scalable for widespread use are viable for corrosion protection, except in critical applications. In addition, most techniques that require vacuum processing are too costly for most applications. Inorganic sol-gel films are formed from a sol through continuous stages of increasing concentration of a solid precursor. Typically, the sol is a solution of polymeric species or a suspension of"oligomers" including particles in the colloidalsize range. During deposition through states of increasing solids concentration, the sol might gel, but the gel state is often a fleeting transient that quickly empties of liquid. Nevertheless, the structures formed during this stage have varying amounts of porosity and influence the structure of the final film. This processing offers good control of composition and homogeneity at low temperatures. It is not directional nor equipment intensive. Complex shapes of arbitrary size can be coated with good uniformity. The cost of high-purity liquid precursors may be high, but for thin-film applications the materials cost would be acceptable. Films deposited using energetic deposition techniques are dense, highly adherent, have few pinholes, and can be laid down at low temperatures. They are attractive for corrosion protection. Ion-beam-assisted deposition and ion implantation have the best adhesion properties, while RF sputtering has the best throwing power. Three important factors affecting the performance of films are porosity, adhesion, and stress. Although there are compressive stresses, in ion-im-

615

planted surfaces, for example, delamination by buckling is practically unknown. Effective porosity in the treated layer could exist due to shadowing of the surface from the beam by contaminating particles. The problem has not been observed, but the exact reason is not known. With the exception of ion implantation [67], only a few studies on corrosion have been done on films deposited using energetic deposition methods. Ion-beam-assisted deposition coatings are adherent and more ductile than bulk materials due to the microcrystalline or amorphous structures. The adhesion is better for the films deposited by energetic beam techniques as compared to films derived from physical vapor deposition. More details may be found in the cited report [66] or in the original literature. Polymeric materials are widely used as protective coatings because they are transport barriers which limit access of reactive species (i.e., water, oxygen, ions) to the substrate surface. Leidheiser and Granata [68] have discussed the roles that each of these species may play in degradation processes on metal surfaces, and, in particular, the role of ion transport through bulk films and "ion channels" in films. Several techniques are discussed in this paper for characterizing ion transport: d-c measurements, electrochemical impedance analysis, under-the-coating sensing, and radio tracer measurements. Characteristic d-c resistances of 1011 ohm.cm 2 are observed for films without continuous aqueous pathways through the coating, as first described by Asbeck and Van Loo [69]. The resistance drops to the order of 108 ohm-cm 2 if continuous aqueous pathways exist where such pathways have high rates of transport. It is also clear that films and coatings are heterogeneous and the aqueous pathways are surrounded by regions of lower transport rates. The resistance of films may also decrease with time as the ion channels or pathways equilibrate with an external aqueous environment. For films with high resistance and no ion channels, the impedance of the film is dominated by its geometric capacitance. For films of lower resistance, the low-frequency impedance is dominated by the sum of the resistance of the film and the resistance of the electrolyte. If corrosion proceeds under the coating because of ingress of the aqueous environment, the low-frequency impedance decreases in value. It has been argued that there is a strong correlation between the sites for corrosion under the film and the intersection of the ion channels with the underlying surface, but it has been difficult to confirm the correlation with direct observations. The nature of the easy pathways for transport appears to be related to several factors. One of the factors is the presence of pigment and filler particles, which could facilitate the formation of aqueous pathways adjacent to the pigment or filler and would be influenced by the interaction of the particles with the polymer matrix [68]. The channels could also form by coalescence of voids or pores in the polymer matrix, and this would be influenced by the formation processes of the films. Aggregation of solvent in the film could be influenced by the prior history of the film, by the presence of impurities, and by retained solvent. The presence of channels has been demonstrated to be a function of the glass transition temperature (Tg) of films as well. That is, below T~, the polymer will be brittle unless a secondary, low-temperature relaxation exists, and this will favor the formation of microcracks and defects. Above Tg, the

616

PAINT AND COATING TESTING MANUAL

film will be m o r e flexible a n d less susceptible to f o r m a t i o n of fracture channels. A r m s t r o n g et al. [70] have investigated the influence of Tg on ion t r a n s p o r t a n d p e r m e a b i l i t y in chlorin a t e d r u b b e r films. Pigment a n d filler particles can have a beneficial influence b e c a u s e of the r e d u c e d t r a n s p o r t of water, oxygen, a n d ions. The effect will d e p e n d on the p i g m e n t volume fraction, the c h e m i c a l composition, the geometry, a n d the d i s p e r s i o n as noted by Burns a n d Bradley [71]. E q u i l i b r i u m w a t e r u p t a k e m a y cause plasticization a n d subsequent d e p r e s s i o n of Tg, as well as swelling, w h i c h counteracts the effects of r e d u c e d t r a n s p o r t rate c a u s e d by the solid particles [72]. Pigments that have oxidizing c h a r a c t e r can induce passivation of the underlying metal, as observed for c h r o m a t e o r v a n a d a t e additives [73]. Other p i g m e n t s m a y inhibit the cathodic r e a c t i o n a n d thus suppress c o r r o s i o n as well [74]. De-adhesion of organic coatings is responsible for enh a n c e d c o r r o s i o n rates on one h a n d a n d is the result of c o r r o s i o n on the o t h e r hand. Leidheiser [75] has discussed de-adhesion processes which include: loss of a d h e s i o n w h e n wet, cathodic delamination, c a t h o d i c blistering, swelling of the polymer, gas blistering b y corrosion, o s m o t i c blistering, t h e r m a l cycling, a n d anodic u n d e r m i n i n g . W i t h few exceptions, the loss of a d h e s i o n processes requires that reactive species such as water, oxygen, a n d ions p e n e t r a t e t h r o u g h the coating. Bonds of the coating with the surface of a steel substrate m a y be a t t a c k e d b y high p H conditions, which are the result of c o r r o s i o n reactions o r i m p o s e d c a t h o d i c p r o t e c t i o n conditions. In either case, OH ions are p a r t i c u l a r l y aggressive a n d cause d i s b o n d i n g on steel. In a recent investigation b y S t r a t m a n [76], d i s b o n d i n g was followed by m o n i t o r i n g the surface potential of a p o l y m e r - c o a t e d steel surface with a s c a n n i n g Kelvin p r o b e technique A recent international m e e t i n g [1] reviewed the unsolved p r o b l e m s of c o r r o s i o n p r o t e c t i o n by organic coatings, described the c u r r e n t u n d e r s t a n d i n g of the technology, a n d outlined s o m e focus for further progress. In a d d i t i o n to the principles of b a r r i e r layer t r a n s p o r t that have been d e s c r i b e d above, there was discussion on the effects of: (1) p r e t r e a t m e n t of surfaces, (2) the c o n t r i b u t i o n m a d e by surface inhomogeneities of the substrate, (3) the critical size of a w a t e r p h a s e w h i c h m a y be responsible for corrosive attack, (4) stress in the film a n d stress in the substrate, a n d (5) incorpor a t i o n of c o r r o s i o n inhibitors in protective films. F u n k e [77] later s u m m a r i z e d the continuing uncertainties that exist in studying corrosion p r o t e c t i o n p r o p e r t i e s of organic coatings. S o m e scatter of b e h a v i o r is caused by the age a n d history of the c o a t i n g - - f r e s h coatings are m o r e susceptible to swelling a n d changes in composition. Disbonding m a y initiate at defects, b u t it m a y also occur in the absence of holidays o r defects. The w a t e r that is a s s o c i a t e d with d i s b o n d i n g could be t r a n s p o r t e d along the surface a n d not by p e r m e a t i o n t h r o u g h the film. Ions m a y also move along the interface. All these c o n s i d e r a t i o n s have considerable implications for electroc h e m i c a l c h a r a c t e r i z a t i o n techniques. A review of various types of organic coatings a n d their applications in various service conditions is p r o v i d e d by Tator [78].

SUMMARY The a t m o s p h e r i c c o r r o s i o n of metals is one of the m o s t i m p o r t a n t single p r o b l e m s facing c o r r o s i o n science a n d technology. F r o m small n a n o s t r u c t u r e s to large buildings a n d bridges, coating techniques are being developed to m o d e r a t e the rate of d e g r a d a t i o n with s o m e success. The use of lowcost coatings continues to increase as the coatings are m a d e m o r e i m p e r m e a b l e a n d m o r e a d h e r e n t to the p r o t e c t e d substrate. Higher-cost films applied b y high-energy m e t h o d s are finding wider use in critical a p p l i c a t i o n s w h e r e conventional coatings are inadequate. In all systems w h e r e p r o t e c t i o n is necessary, the early detection of c o r r o s i o n is desirable in o r d e r to p l a n r e p l a c e m e n t a n d m a i n t e n a n c e m e a s u r e s a n d to avoid c a t a s t r o p h i c failures. Detection of the presence of corr o s i o n can be a c c o m p l i s h e d in two ways: detection of the agent that causes c o r r o s i o n or detection of the results of the c o r r o s i o n process either on the m a t e r i a l of interest or on a s p e c i m e n of the material. Sensors a n d m o n i t o r s are receiving greater attention in accelerated life testing of materials, a n d eventually they will be developed m o r e widely for o p e r a t i n g systems or in p o r t a b l e m o n i t o r i n g systems. The savings to i n d u s t r y a n d the public at large w o u l d be in the billions of dollars if the onset of failure processes could be detected p r i o r to their c u l m i n a t i o n in a c a t a s t r o p h i c event [79].

REFERENCES [1] Funke, W., Leidheiser, H., Jr., Dickie, R. A., Dinger, H., Fischer, W., Haagen, H., Herrmann, K., Mosle, H. G., Oechsner, W. P., Ruf, J., Scantlebury, J. S., Vogoda, M., and Sykes, J. M., "Unsolved Problems of Corrosion Protection by Organic Coatings: A Discussion," Journal of Coatings Technology, Vol. 58, 1986, p. 79. [2] Guggenheim, E. A., Thermodynamics, North-Holland, Amsterdam, 1959. [3] Lewis, G. N. and Randall, M., Thermodynamics, revised by K. S. Pitzer and L. Brewer, McGraw-Hill, New York, 1961. [4] NBS Technical Notes 2710-3, 270-4, 270-5, U.S. Government Printing Office, Washington, DC, 1968-1971. [5] Latimer, W. M., Oxidation Potentials, Prentice-Hall, Englewood Cliffs, NJ, 1952. [6] JANAF Thermochemical Tables, NSRDS-NBS 37, U.S. Government Printing Office, Washington, DC, 1968-1971. [7] Smyrl, W. H., "Electrochemistry and Corrosion on Homogeneous and Heterogeneous Metal Surfaces," Comprehensive Treatise on Electrochemistry, Vol. 4, Bockris, Conway, Yeager, and White, Eds., Plenum Press, New York, 1981. [8] Newman, J. and Tiedeman, W., "Flow Through Porous Electrodes," Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, H. Gerischer and C. W. Tobias, Eds., Wiley Interscience, New York, 1978, p. 353. [9] Newman, J., "Mass Transport and Potential Distributions in the Geometries of Localized Corrosion," Localized Corrosion, R. Staehle, Ed., NACE, Houston, 1974. [10] Sereda, P. J., "Weather Factors Affecting Corrosion of Metals," Corrosion in Natural Environments, ASTM STP 558, ASTM, Philadelphia, 1974, p. 7. [11] Perez, F.C., "Atmospheric Corrosion of Steel in a Humid Climate-Influence of Pollution, Humidity, Temperature, Solar Radiation and Rainfall," Corrosion, Vol. 40, 1984, p. 170.

C H A P T E R 5 1 - - P R E V E N T I O N OF M E T A L C O R R O S I O N [12] Guttman, H., "Effects of Atmospheric Factors on the Corrosion of Rolled Zinc," Metal Corrosion in the Atmosphere, ASTM STP 435, 1968, p. 223. [13] Rice, D. W., Cappell, R. J., Phipps, P. B. P., and Peterson, P., "Indoor Atmospheric Corrosion of Copper, Silver, Nickel, Cobalt, and Iron," Atmospheric Corrosion, W. H. Ailor, Ed., John Wiley & Sons, New York, 1982, p. 651. [14] Freitag, W. O., "Testing for Indoor Corrosion," Atmospheric Corrosion, W. H. Ailor, Ed., John Wiley and Sons, New York, 1982. [15] Vernon, W. H. J., "A Laboratory Study of the Atmospheric Corrosion of Metals. Part 1. The Corrosion of Copper, with Particular Reference to the Influence of Sulfur Dioxide in Air of Various Relative Humidities," Transactions of the Faraday Society, Vol. 27, 1931, p. 255. [16] Chawla, S. K., Anguish, T., and Payer, J. E., "Microsensors for Corrosion Control," Materials Performance, May 1980, p. 68. [17] Kucera, V. and Mattson, E., "Electrochemical Technique for Determination of the Instantaneous Rate of Atmospheric Corrosion," Corrosion in Natural Environments, ASTM STP 558, 1974, p. 239. [18] Phipps, P. B. P. and Rice, D. W., "The Role of Water in Atmospheric Corrosion," Corrosion Chemistry, Vol. 235, G. Brubaker and P. B. P. Phipps, Eds., ACS Symposium Series 89, 1979. [19] Rice, D. W., Phipps, P. B. P., and Tremoureux, R., "Atmospheric Corrosion of Cobalt," Journal of the Electrochemical Society, Vol. 126, 1979, p. 1459. [20] Klier, K., Shen, J. H., and Zettlemoyer, A., "Water on Silica and Silicate Surface. Partially Hydrophobic Silicas," Journal of Physical Chemistry, Vol. 77, 1973, p. 1458. [21] Smyrl, W. H. and Newman, J., "Current and Potential Distributions in Plating Corrosion Systems," Journal of the Electrochemical Society, Vol. 123, 1976, p. 1423. [22] Scott, W. D. and Hobbs, P. V., "The Formation of Sulfate in Water Droplets," Journal of the Atmospheric Sciences, Vol. 24, 1967, p. 54. [23] Smyrl, W. H. and Lien, M., "The Electrochemical OCM (Quartz Crystal Microbalance) Method," New Methods and Experimental Approaches in Electrochemistry, T. Osaka et al., Eds., Kodansha, Tokyo, 1993. [24] Smyrl, W. H. and Naoi, K., "Corrosion Studies with the Quartz Crystal Microbalance," Perspectives on Corrosion, G. Prentice and W. H. Smyrl, Eds., AIChE Symposium Series 278, Vol. 6, 1990. [25] Burns, R.M. and Campbell, W.E., "Electrical Resistance Method of Measuring Corrosion of Lead by Acid Vapors," Transactions of the Electrochemical Society, Vol. 55, 1929, p. 271. [26] Sereda, P. J., "Atmospheric Factors Affecting the Corrosion of Steel," Industrial and Engineering Chemistry, Vol. 52, 1960, p. 157. [27] Enrico, F., Riccio, V., and Martini, B., "An Electrical Resistance Method for Measuring Rates of Corrosion of Electrodeposited Metals in Laboratory Tests," Transactions of the Institute of Metal Finishing, Vol. 41, 1964, p. 74. [28] Evans, U. R., "Mechanism of Atmospheric Rusting," Corrosion Science, Vol. 12, 1972, p. 227. [29] White, H. S., "Corrosion Principles in Microelectronics," Electronic Packaging and Corrosion in Microelectronics, M. E. Nicholson, Ed., ASM International, Metals Park, OH, 1987. [30] Mansfeld, F., "Electrochemical Methods for Atmospheric Corrosion Studies," Atmospheric Corrosion, Vol. 139, W. H. Ailor, Ed., John Wiley & Sons, New York, 1982. [31] Mansfeld, F., "Evaluation of Electrochemical Techniques for Monitoring of Atmospheric Corrosion Phenomena," Electrochemical Corrosion Testing, ASTM STP 727, 1981, p. 215.

617

[32] Guttman, H. and Sereda, P. J., "Measurement of Atmospheric Factors Affecting the Corrosion of Metals," Metal Corrosion in the Atmosphere, ASTM STP 435, 1968, p. 326. [33] Tomashov, N. D., Theory of Corrosion and Protection of Metals, MacMillan, New York, 1966.

[34] Agarwala, V. S., "A Probe for Monitoring Corrosion in Marine Environments," Atmospheric Corrosion, Vol. 183, W. H. Allot, Ed., John Wiley & Sons, New York, 1982.

[35] McKenzie, M. and Vassie, P. R., "Use of Weight Loss Coupons and Electrical Resistance Probes in Atmospheric Corrosion Tests," British Corrosion Journal, Vol. 20, 1985, p. 117. [36] Kucera, V. and Gullman, J., "Practical Experience with an Electrochemical Technique for Atmospheric Corrosion Monitoring," Electrochemical Corrosion Testing, ASTM STP 727, 1981, p. 238. [37] Mansfeld, F. and Tsai, S., "Laboratory Studies of Atmospheric Corrosion. I. Weight Loss and Electrochemical Measurements," Corrosion Science, Vol. 20, 1980, p. 853. [38] Mansfeld, F. and Kenkel, J. V., "Electrochemical Monitoring of Atmospheric Corrosion Phenomena," Corrosion Science, Vol. 16, 1976, p. 111. [39] Mansfeld, F. and Kenkel, J. V., "Electrochemical Measurements of Time-of-Wetness and Atmospheric Corrosion Rates," Corrosion, Vol. 33, 1977, p. 13. [40] Mazza, B., Pedeferri, P., Re, G., and Sinigaggla, D., "Behaviour of a Galvanic Cell Simulating Atmospheric Corrosion Conditions of Gold Plated Bronzes," Corrosion Science, Vol. 17, 1977, p. 535. [41] Tosto, S. and Brusco, G., "Effect of Relative Humidity on the Corrosion Kinetics of HSLA and Low Carbon Steel," Corrosion, Vol. 40, 1984, p. 507. [42] Pourbaix, M., "Applications of Electrochemistry in Corrosion Science and in Practice," Corrosion Science, Vol. 14, 1974a, p. 25. [43] Vassie, P. R. and McKenzie, M., "Electrode Potentials for onSite Monitoring of Atmospheric Corrosion of Steel," Corrosion Science, Vol. 25, 1985, p. 1. [44] Howard, R. T., "Environmentally Related Reliability in Microelectronic Packaging," Electronic Packaging and Corrosion in Microelectronics, M. E. Nicholson, Ed., ASM International, Metals Park, OH, 1987. [45] Pourbaix, M., Muylder, J. V., Pourbaix, A., and Kessel, J., "An Electrochemical Wet and Dry Method for Atmospheric Corrosion Testing," Atmospheric Corrosion, Vol. 167, W. H. Ailor, Ed., John Wiley & Sons, New York, 1982. [46] Fiaud, C., "Electrochemical Behavior of Atmospheric Pollutants in Thin Liquid Layers Related to Atmospheric Corrosion," Atmospheric Corrosion, Vol. 161, W. H. Ailor, Ed., John Wiley & Sons, New York, 1982. [47] Fishman, S. G. and Crowe, C. R., "The Application of Potentiostatic Polarization Techniques to Corrosion Under Thin Condensed Moisture Layers," Corrosion Science, Vol. 17, 1977, p. 27. [48] Hubbard, A. T. and Anson, F. C., "The Theory and Practice of Electrochemistry with Thin Layer Cells," Electroanalytical Chemistry, Vol. 5, A. J. Bard, Ed., Marcel Dekker, New York, 1971. [49] Comizzoli, R. B., Frankenthal, R. P., Lobnig, R. E., Peins, G. A., Psota-kelt, L. A., Siconolfi, D. J., and Sinclair, J. D., "Corrosion of Electronic Materials and Devices by Submicron Atmospheric Particles," Interface, Vol. 2, No. 3, 1993, p. 26. [50] Lee, W., "Thin Films for Optical Data Storage," Journal of Vacuum Science and Technology, Vol. A3, 1985, p. 640. [51] Antler, M. and Dunbar, J. J., "Environmental Testing of Materials for Indoor Exposure," IEEE Transactions, Vol. CHMT-1, 1978, p. 17.

618

PAINT AND COATING TESTING MANUAL

[52] Schnable, G. L., Comizzoli, R. B., White, L. K., and Kern, W., "A Survey of Corrosion Failure Mechanisms in Microelectronic Devices," RCA Review, Vol. 40, 1979, p. 416. [53] Comizzoli, R. B., White, L. K., Kern, W., and Schnable, G. L., Report RADC-TR-80-236, July 1980, Final Technical Report, Contract F30602-78-C-0276, 1 Sept. 1978 to 31 Aug. 1979, Rome Air Development Center (RBRP), Griffiss AFB, NY. [54] Wood, J., "Reliability and Degradation of Silicon Devices and Integrated Circuits," Reliability and Degradation: Semiconductor Devices and Circuits, M. J. Howes and D. V. Morgan, Eds., John Wiley, New York, 1981. [55] Stojadinovic, N. D. and Ristic, S. D., "Failure Physics of Integrated Circuits and Relationship to Reliability," Physica Status Solidi, Vol. A75, 1983, p. 11. [56] Howard, J. K., "Thin Films for Magnetic Recording Technology: A Review," Journal of Vacuum Science and Technology, Vol. A4, 1986, p. 1. [57] Jansen, F., Machonkin, M., Kaplan, S., and Hark, S., "The Effects of Hydrogeneration on the Properties of Ion Beam Sputter Deposited Amorphous Carbon," Journal of Vacuum Science and Technology, Vol. A3, 1985, p. 605. [58] Nyaiesh, A. R., Kirby, R.E., King, R. K., and Garwin, E. L., "New Radio Frequency Technique for Deposition of Hard Carbon Films," Journal of Vacuum Science and Technology, Vol. A3, 1985, p. 610. [59] Koeppe, P. W., Kapoor, V. J., Mirtich, H. J., Banks, B. A., and Bulino, D.A., "Summary Abstract: Characterization of IonBeam Deposited Diamond-like Carbon Coating on Semiconductors," Journal of Vacuum Science and Technology, Vol. A3, 1985, p. 2327. [60] Savvides, N. and Window, B., "Diamond-like Amorphous Carbon Films Prepared by Magnetron Sputtering of Graphite," Journal of Vacuum Science and Technology, Vol. A3, 1985, p. 2386. [61] Morris, R. G. and Smyrl, W. H., "Galvanic Interactions on Random Heterogeneous Surfaces," Journal of the Electrochemical Society, Vol. 136, 1989, p. 3237. [62] Morris, R. G. and Smyrl, W. H., "Current and Potential Distributions in Thin Electrolyte Layer Galvanic Cells," Journal of the Electrochemical Society, Vol. 136, 1989, p. 3229. [63] Morris, R. B. and Smyrl, W. H., "Electrode Processes on Heterogeneous Surfaces. I. Galvanic Interactions on Regular Geometry," American Institute of Chemical Engineers Journal, Vol. 34, 1988, p. 723. [64] Winterfeld, P. H., Ph.D. thesis, University of Minnesota (1980). [65] Kassimati, A. and Smyrl, W. H., "Galvanic Corrosion of Sandwich Structures," Journal of the Electrochemical Society, Vol. 136, 1989, p. 2158.

[66] Smyrl, W. H., Halley, J. W., Hubler, G., Hurd, A., MacDonald, D., Snyder, D., and Williams, J., "Corrosion Protection," Critical Reviews in Surface Chemistry, Vol. 3, 1993, p. 271. [67] McCafferty, E., Natishan, P. M., and Hubler, G. K., "Surface Modification of Aluminum by High Energy Ion Beams," Interface, Vol. 2, No. 3, 1993, p. 45. [68] Leidheiser, H., Jr. and Granata, R. D., "Ion Transport Through Protective Polymeric Coatings Exposed to an Aqueous Phase,"

IBM Journal of Research Development, Vol. 32, 1988, p. 582. [69] Asbeck, W. K. and Van Loo, M., "Critical Pigment Volume and Permeation of Paint Films," Industrial and Engineering Chemistry, Vol. 41, 1949, p. 1470.

[70] Armstrong, R. D., Handyside, T. M., and Johnson, B. W., "Factors Determining Ionic Currents in PVC Protective Coatings,"

Corrosion Science, Vol. 30, 1990, p. 569. [71 ] Burns, R. M. and Bradley, W. W., Protective Coatings for Metals, 2nd ed., Reinhold, New York, 1955.

[72] Mastronardi, P., Carfagna, C., and Nicolais, L., "The Effect of the Transport Properties of Epoxy Based Coatings on Metallic Substrate Corrosion,"Journal of Materials Science, Vol. 18, 1983, p. 197. [73] Yamamoto, T., Okai, T., Oda, M., and Okumura, Y., "A Novel Anti-Corrosive Pigment Containing Vanadate/Phosphate," Ad-

vances in Corrosion

Protection by Organic Coatings,

D.

Scantlebury and M. Kendig, Eds., The Electrochemical Society, Pennington, 1989. [74] Guest, N., Scantlebury, J. D., John, G. R., and Thomas, N. L., "Metal Complex Agents as Possible Film Forming Anti Corrosives on Mild Steel," Advances in Corrosion Protection by Organic Coatings, D. Scantlebury and M. Kendig, Eds., The Electrochemical Society, Pennington, NJ, 1989. [75] Leidheiser, H., Jr., "Mechanisms of De-Adhesion of Organic Coatings from Metal Surfaces," Polymeric Materials for Corrosion Control, R. A. Dickie and F. L. Floyd, Eds., ACS Symposium Series 322, American Chemical Society, Washington, DC, 1986. [76] Stratmann, M. and Streckel, H., "Monitoring the Disbonding of Organic Films by the Kelvin Probe Method," Berichte Bunsengesetlschaft Physical Chemistry, Vol. 92, 1988, p. 1244. [77] Funke, W., "Electrochemical Measurements for Characterizing Corrosion Protective Properties of Organic Coatings," Advances in Corrosion Protection by Organic Coatings, D. Scantlebury and M. Kendig, Eds., The Electrochemical Society, Pennington, NJ, 1989. [78] Tator, K. B., "Organic Coatings and Linings," Metals Handbook Ninth Edition, Vol. 13, 1987, p. 399. [79] Smyrl, W. H. and Butler, M. A., "Corrosion Sensors," Interface, Vol. 2, No. 4, 1993, p. 35.

MNL17-EB/Jun. 1995

Natural Weathering by Lon S. Hicks I and Michael J. Crewdson 2

NATURALWEATHERINGDESCRIBESTHE PROCESSof exposing materials to the effects of the outdoor environment. Deterioration caused by the atmosphere occurs to all materials placed in an outdoor environment. Natural weathering tests are an extremely important part of the process of determining the aging characteristics of a material. Weathering tests are used to improve the durability of exposed materials. All materials exposed to the elements deteriorate; the rate and extent of deterioration is dependent upon the material and the severity of the exposure conditions. Weathering tests provide the means to improve the resistance of a material to those factors. Paint is currently the single most important material tested for its weathering-resistance properties. Paint is used as a decorative item, but equally as important, paint is used as a protective layer. Painted products are found in many varied environments with equally varied effects upon exposed surfaces. Weathering tests offer an initial view of the expected performance of the paint to the environment. Natural weathering tests are not, however, simply a case of setting specimens out in the sun and watching what happens; a meaningful test involves a more thorough understanding. A number of important factors must be carefully considered when planning and conducting a weathering test: the cause and effect relationship between the weather and the material, the subtle differences in exposure techniques, and the reporting and inspection methods employed. To achieve the most reliable test result, these fundamentals must be appreciated.

HISTORICAL The origins of natural weathering testing of paints goes back as far as early caveman drawings in the Ice Age. We do not know if these cave dwellers painted on the outside walls, for if they did, the elements would have long since eroded away the evidence. Inside, where conditions are less severe, the paintings have survived tens of thousands of years. The modern paint era was the catalyst for current weathering tests. Consumer awareness of deterioration propelled the search for more durable products. At the same time, paint manufacturing companies became cognizant of the fact that durable products sell better; thus they have increased their efforts to improve paint durability. At this time, the only ~Vice president and technical director, respectively, Sub-Tropical Testing Service, 8290 S.W. 120th St., Miami, FL 33156. 2General manager/technical director, Q-Lab, 13131 SW 122nd Ave., Miami, FL 33186.

testing was "real world," with results being returned from product surveys and consumer complaints. The idea for exposure fences and racks to expose paint specimens moved the testing to the research and development stage and away from the after market survey. Research into improved coatings were made on specimens before the product went to market, thus reducing the possibility for failures in the marketplace. In the early 1920s certain climates were noticed to be more severe on exposed materials than others. At the same time, Florida experienced a land boom with increased development; as the number of consumer items increases, it became apparent that life expectancy for these products was much less in the "Sunshine State" than in the Northern Temperate Zone. Prior to this time, natural weathering tests were conducted mostly in the Northeast and Midwest, where the majority of the large paint companies were situated, but the discovery of the faster weathering led several of these manufacturers (along with car companies) to set up their own exposure stations in South Florida. Also at this time, several independent weathering stations were founded (see Fig. 1) to provide this important service to all companies. The cooperation between independent test stations and paint manufacturing companies over the last 60 years has led to a g~eater understanding of the cause and effect relationship betkveen the weather and material and also to refining and improving techniques used to conduct weathering tests. Early weathering tests were almost always at a 45 ~ angle facing south as this was considered the optimum position (see Fig. 2). The fact that most exposure stations in the early years were all in the Northern United States made this even more so. As the focus of outdoor weathering shifted to Florida, realization set in concerning the importance of the angle of exposure and other variables affecting the outcome.

F A C T O R S OF I N F L U E N C E in 1. 2. 3. 4. 5.

The major influencing factors in the atmosphere involved the process of weathering are: Sunlight. Temperature. Moisture Pollution Biodeterioration.

619 Copyright9 1995 by ASTMInternational

www.astm.org

620

PAINT AND COATING TESTING MANUAL

FIG. 1-Aerial view of exposure site.

FIG. 2-Overview of exposure racks.

Paint will degrade w h e n exposed to the elements in the atmosphere due to the action of these influencing factors, w h i c h cause the basic structure to b r e a k down. These factors interact synergistically to p r o d u c e the d e t e r i o r a t i o n of the material we see as a w e a t h e r i n g effect. The relative p r o p o r t i o n of each of these factors is i m p o r tant in d e t e r m i n i n g h o w the overall d e g r a d a t i o n process will occur. W h e n the factors of influence are changed, so is the

d e g r a d a t i o n produced. Any test m e t h o d e m p l o y e d to examine w e a t h e r i n g m u s t be designed so t h a t the exposure c o n d i t i o n is as s i m i l a r as possible to that of the i n t e n d e d end use of the material. This is the only way a c c u r a t e p r e d i c t i o n s m a y be m a d e concerning the expected life of the material. An o u t d o o r w e a t h e r i n g test is c o n d u c t e d using the n a t u r a l elements in an u n c o n t r o l l e d environment. Test fixtures a n d m o u n t i n g techniques can be chosen to create a s i m i l a r expo-

CHAPTER 52--NATURAL WEATHERING sure position as the end use. These techniques include changing angle and orientation, backing type, and the use of specially designed frames to produce a specific microclimate. The material under test is itself a major factor in the test. The simultaneous exposure of a reference specimen with a known long-term weathering history will ensure that the testing is realistic. If the expected results are given by the reference specimen, there is a greater degree of confidence that the influencing parameters will produce the correct type of changes in the specimen, which is exposed simultaneously.

Sunlight The effect of exposure to sunlight is the fundamental cause of the weathering deterioration of most materials. The primary component of paint weathering is photodegradation.

Spectral Power Distribution The wavelength distribution of sunlight that reaches the Earth's surface is important because of the relative effect on the material caused by each wavelength region. Sunlight can be divided into three major regions: ultraviolet (UV), visible, and infrared (IR). Each region has its own distinct wavelength range [1] (see Fig. 3).

1. Ultraviolet--wavelengths less than 400 nm. 2. Visible--wavelengths between 400 to 700 nm. 3. Infrared--wavelengths above 700 nm.

Range

Wavelengths, nm

UVC UVB UVA

less than 280 280-320 320-400

There is no UVC at the Earth's surface as wavelengths below 295 to 300 nm are filtered out by the atmosphere [1]. The percent composition of the UV regions is shown in Fig. 6. Generally, the shorter the UV wavelength, the more damaging its effect on materials. The UVA and the shorter wavelength UVB are responsible for most photodegradation. Therefore the range of sunlight that comprises the smallest percentage of the solar spectrum is the primary cause for material degradation. This becomes clear when we consider the quantum theory, which describes light as discrete packages of energy called photons. The lower the wavelength, the greater the energy contained in these packets (Fig. 7). Photodegradation occurs as a result of light energy breaking a chemical bond in the exposed material, causing a deterioration of the physical structure [2]. As the wavelengths become shorter, the energy of each individual packet becomes greater, allowing that photon to break progressively stronger molecular bonds. Thus, chemical structures able to withstand irradiation at 350 nm may not be able to endure radi-

TABLE 1--Relative proportion of sunlight wavelength ranges.

The relative proportion of each of these wavelength regions is shown in Table 1 and described in Fig. 4. The visible region constitutes the largest portion of the overall solar energy; however, it is the UV portion of the sun's energy that is the most destructive element. The ultraviolet can itself he divided into three distinct wavelength ranges [1]; only the UVA and the UVB reach the Earth's surface (Fig. 5).

Range

Wavelength, nm

UVC UV UVB UVA

.. I.co z uJ Iz

600 400

i

200 0

6

7

8

9

10

11

12

1

2

3

4

5

6

7

8

T I M E O F DAY

TOTAL

TUV

(xlO)[

FIG. 8-Daily intensity levels of sunlight, Miami, Florida: Total and ultraviolet, 26~ tilt angle. (TUV is shown by the lower line.)

will receive the most radiation, as shown in Table 2. Variable angle exposures where the tilt angle of the rack is changed seasonably to follow the zenith of the sun can be used to increase the radiant exposure by up to 10% depending on location and local climate variability. Short-term exposures, those less than one year, will be affected by seasonal differences in solar energy due to angle of exposure. This important factor must be considered when equating exposure intervals. Timing exposures by amount of radiant exposure will reduce some of these inequities. The use of total ultraviolet

energy as a means of timing exposures will further help to even out the seasonal differences in the weathering effect produced. The radiant energy for one year for Miami and Arizona for each of the commonly used exposure angles is shown in Table 2.

Temperature The temperature of a material on exposure is a primary factor whose influence must be understood in order to relate

624

PAINT AND COATING TESTING MANUAL

,u. I

NOV.-MAR.

SOUTH

IL

ANGLE AT S O L A R NOON FIG. 9-Angle subtended by sunlight on exposed panel, the cause and effect relationship of weathering. For most materials, the moderate high temperatures found in service do not cause deterioration per se. However, when the temperature rises in conjunction with solar radiation, the rate and type of deterioration can vary greatly. In a normal first-order chemical reaction, raising the temperature of the environment by 10~ leads to a doubling of the speed of that forward reaction. However, because material degradation is not a simple one-step chemical reaction, weathering deterioration does not proceed twice as fast when the temperature is raised by 10~ There is a definite indica-

tion, though, that once the breakdown mechanism has been initiated, an increase in thermal load will accelerate the rate of change. A general rule of thumb that has been accepted recently is that an increase of 20~ in the temperature of an exposed specimen results in a doubling of the deterioration rate. Many chemical reactions have a threshold activation energy, i.e., the temperature level at which a sequence of events will be initiated. Therefore, as thermal input increases, higher threshold levels are surpassed and new mechanisms are initiated. This may cause a change in the overall deterioration produced if the new pathway is significantly different from that at the lower temperature. It is therefore necessary to conduct the weathering test at the correct temperature. The comparison being made here is that of the actual end use exposure condition to that found in the test method. By having the temperature too low, the rate of change is slowed and deterioration will not be as severe. Conversely, if the temperature is too high, the rate may proceed too quickly and the effect will be more severe. Paint specimens on exposure derive their temperature from two sources: the ambient air and the radiated infrared from sunlight. The intensity of the sunlight will determine how much higher the temperature of an exposed specimen will be above ambient. Wind speed will assist in reducing the temperatures slightly [7]. The average specimen temperature will show the same seasonal range as that for air temperature (Fig. 11), but will be determined on any given day by the level of solar irradiance (Fig. 12). The angle of exposure, paint color, and season also contribute to determine the actual temperature at any given time. Thus the specimens are generally hotter during the day than at night, in the summer than in the winter, and on a sunny day rather than on a cloudy one. On any given day, the tem-

SOLAR RADIANT ENERGY MIAMI, FLORIDA 7001

_

~

600

400 300 200

J/,N F~S M/,R ACR UkY JdN J6L AU'GSEPT OCT NOV D~;C MONTHS I'-'~'- 45 DEGREES

I

5 DEGREES

~

26 DEGREES

I J D

FIG. 10-Seasonal variations in solar energy measured at different fixed angles of exposure.

CHAPTER 52--NATURAL WEATHERING TABLE 2--Solar radiant energy, average 1990-1991.

Month

45~

Solar Radiation, MJ m 2, Angleof Exposure 5~ 26~

VA~

January Florida Arizona

556.79 612.50

420.48 425.49

514.34

556.79

February Florida Arizona

587,50 607.94

483.37 476.40

618.09

587.50

601.51 631.54

569.90 601.35

661.61

661.61

545.76 724.23

573.12 784.77

623.35

573.35

468.30 735.51

600.77 897.85

577.21

600.77

441.17 648.94

589.38 856.13

529.25

589.38

434.21 649.66

566.95 830.43

531.66

566.95

475.86 684.23

572.30 774.86

557.65

572.30

475.00 658.02

502.78 647.56

526.62

526.62

March Florida Arizona

April Florida Arizona

June Florida Arizona

July Florida Arizona

August Florida Arizona

September Florida Arizona

545.50 712.30

500.14 596.29

561.06

545.60

522.87 669.54

425.90 471.38

509.88

522.87

524.10 601.06

394.72 400.22

492.33

524.10

6178.55 7935.44

6199.76 7762.70

6703.02

6827.51

November Florida Arizona

December Florida Arizona

Total Florida Arizona

Moisture Water is a p r i m a r y factor affecting the deterioration of exposed materials. I n c o n j u n c t i o n with solar radiation a n d high temperatures, the moisture content in a n d s u r r o u n d i n g a n exposed sample is very i m p o r t a n t in d e t e r m i n i n g the weathering response of that material [8]. The presence of water falls into two categories: 1. Gaseous. 2. Aqueous.

NOTE:VA = variableangle,

The gaseous phase is that which describes the moisture c o n t e n t of the air. The a m o u n t of water vapor contained in the a m b i e n t air is the absolute humidity. W a r m air is able to hold more water i n the vapor phase t h a n cold air. The relationship between the actual moisture c o n t e n t of air a n d the m a x i m u m c o n t e n t at any particular t e m p e r a t u r e is the relative humidity. W h e n the air is fully saturated, the relative h u m i d i t y is 100%. Any material placed on exposure will endeavor to m a i n t a i n a moisture c o n t e n t e q u i l i b r i u m with its s u r r o u n d i n g s [9]. Physical stresses are created as the material loses or gains water c o n t e n t in order to equilibrate, as can be seen in Fig. 13. The greater the range of h u m i d i t y in the enveloping atmosphere, the greater the overall stress on the material. Because moisture is a m a j o r factor in the synergistic effect of weather o n exposed materials, a higher moisture c o n t e n t will contribute to increased degradation more t h a n will lower moisture levels. A relative h u m i d i t y value of 70% is considered the critical threshold for corrosion [10]. This indicates that a constant cycling of h u m i d i t y levels at high values increases the rate of deterioration.

Rainfall

October Florida Arizona

perature of a n exposed p a i n t sample will follow closely the irradiance level of the sun. The temperature of the exposed material can be m a n i p u l a t e d even in an o u t d o o r exposure test; these techniques are discussed later in "Accelerated Natural Weathering."

Relative Humidity

May Florida Arizona

625

The effective source of visible surface moisture has two origins, a n d they each have a different effect on the exposed material. The distinction between the two types m u s t be clearly understood. Rain is a n external source of water that is applied to the material via the surface layers. As most rainfall is of short duration, this effect is primarily at the surface a n d does not play a direct role in the deterioration of the bulk of the material. Rainfall has the greatest influence at the surface a n d is responsible for washing away surface layers d u r i n g periods of heavy rain. For example, rain assists in increasing the rate at which a specimen m a y chalk, b u t m a y also help to remove surface attachments such as dirt a n d mildew. Rainfall will cause a t h e r m a l shock on exposed specimens, which can be severe in certain circumstances. W h e n the rain occurs at a time when the material is heated from r a d i a n t exposure, the cooling effect causes a rapid drop in the temperature of the specimen. This thermal shock can cause mechanical stress as the specimen contracts. I n the s u m m e r in

626

PAINT AND COATING TESTING MANUAL

TEMPERATURE MIAMI 1990

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FIG. l 1-Daily maximum black panel versus ambient temperature. (Lower line equals ambient temperature.)

RADIANT

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JAN FEB MAR APR

MAY JUN JUL AUG SEPT OCT NOV DEC MONTHS

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FIG. 12-Daily maximum black box temperature versus daily solar radiant energy. (Upper line equals black box temperature,)

Florida, the temperature of a black panel can drop from 70 to 25~ in about 2 rain.

Condensation Condensation arises when the sample temperature drops below the dew point temperature of the surrounding air. This causes water vapor in the air to condense on the material. The physical and chemical responses of the material caused by condensation have a far greater effect on the material than rainfall. The condensation effect can also be apparent at

much deeper levels within the material. Condensation moisture has high levels of dissolved oxygen, which accentuates photodegradation by taking part in the chemical reaction. The total amount of time a sample is wet is an extremely important factor in the determination of the degree and rate of degradation. Prolonged exposure to surface condensation will allow the absorption or permeation of a relatively high level of water and oxygen, and as the day progresses a strong pressure will be exerted by the atmosphere for water desorption. The longer the intervals of wetting and drying, the

CHAPTER 5 2 - - N A T U R A L W E A T H E R I N G

MOISTURE EQUILIBRIUM +

MATERIAL V E R S U S ATMOSPHERE

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FIG. 13-Ambient versus sample moisture cycling. deeper into the material will be the cycling effect. Water absorption and desorption rates are also a function of the diffusion coefficient of the material. The faster the rate of cycling, the more the effect remains at the surface and the less penetration is made into the bulk of the material. The photochemical degradation mechanism is also accentuated by the presence of moisture. Rates of deterioration are greatest when the solar radiation is present in conjunction with high moisture levels. It follows, therefore, that surface breakdown effects such as chalking will occur due to rainfall, condensation, and sunlight interaction, whereas bulk substrate deterioration such as cracking is due in large part to the synergistic effects of humidity, condensation, and temperature, causing mechanical stress and release of strain in the paint film. As surface layers break down, more of the interior bulk of the material is exposed as a new surface layer. The role of moisture in exposed material breakdown is very important, but it is also important to know the source of the moisture. The different physical (and chemical) effects produced will differ with each source. A climate such as Miami's (Fig. 14), with a range of high humidity and a high percentage of total wet time (approximately 50%), will allow moisture to play a full part in the weathering process [11].

Pollution Atmospheric pollution is a significant, but lesser part of the overall weathering effect. Industrial emissions consisting of compounds that mix with water to form acid rain cause chemical reactions to occur on exposed materials. The severe long-term effect of pollution on even durable materials can be seen by the effect on ancient monuments and statues, which have withstood centuries of exposure only to succumb in recent years to atmospheric contaminants. Short-term exposure to pollution can be just as severe if the material is susceptible. Pollution is a powerful influence on paint durability, but its effect is difficult to control and monitor. Analysis of the atmosphere will reveal the constituents of the pollution, and comparison to exposure in a clean pollution-free atmosphere

627

will show the additional effect of the pollution on overall weathering. Although pollutants may directly affect the paint independently of other weathering factors, most often the changes do not occur until the other influences are present. Changes in paints on exposure in industrial areas, which are seen in a relatively short time but which do not show changes in rural areas, are a clear indicator of the effect of the pollution. Solar energy can act to change the chemical structure of the emitted waste product; for example, atmospheric SO2 in water does not become sulfuric acid until radiated. The sulfuric acid then causes a color change due to reaction with the pigment. Large-scale pollution such as the effect of acid rain is felt throughout most of the northeastern United States, with the greatest effect in Pennsylvania, New Jersey, and New York. Acid rain resistance can be included as part of the routine testing procedure by exposure at known polluted sites. The effect is known and is widespread. Local pollution problems such as the smog in Los Angeles create a problem for the testing industry. Because the chemical effect of each emission is different, the weathering result of a single material may vary from city to city depending upon the industrial base.

Biodeterioration The phenomenon of material degradation due to attack by biological organisms can take many forms. The most common form of biodeterioration of specimens exposed outdoors in Florida is mildew. The organisms that constitute mildew are fungi, which are individually microscopic but can be seen with the unaided eye when grouped in colonies or having threaded structures called hypha. The spores of the fungi attach to the surface of the material, using the constituent chemicals at the surface as a food source. All fungi are characterized by their inability to synthesize their own foods from carbon dioxide and water. They must therefore utilize an external supply of carbohydrate as their energy source. The exact chemical nature of that source will vary for each fungal species, but the more opportunistic species can use the paint directly. As the mildew grows, surface conditions are altered. Breakdown products are formed that can act as a food source for a second fungal species, which will find the surface of the paint a favorable place to live. This succession of different organisms constitutes the microecology of the painted material's surface [12]. The complicated natural order that occurs in mildew attack on exposed products makes it very difficult to ascertain which species is causing the most damage. Real-time exposure testing outdoors in conditions favorable to growth is the only way to truly test the mildewresistance properties of any product. Single culture testing of mildewcides and mildewstats are useful as a preliminary test o n l y .

The subtropical region of Florida is used extensively for the evaluation of fungus growth. The warm, wet climate promotes rapid growth on unprotected materials, with visible hyphae present in just a few days. A number of different exposure methods are employed to study mildew growth, the most common being a vertical north orientation. This gives a

628

PAINT AND COATING TESTING MANUAL

MOISTURE LEVELS MIAMI 1990 _ _ .

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40 30

20 10 0 JAN FEB MAR APR MAY JUN JUL AUG SEPT OCT NOV DEC MONTHS

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RELATIVE HUMIDITY %

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TOTAL WET TIME (HR)

FIG. 14-Daily maximum humidity and total wet time.

shaded, damp location without the natural mildewstatic effect of solar UV radiation. A vertical south position is also used to study mildew growth as part of the total deterioration mechanism. Exposure under a roof overhang is commonly used (Fig. 15), and a tropical forest exposure is the most rigorous example of natural exposure conditions. Mildew organisms thrive in the damp conditions found in subtropical-type climates, and growth is greatest during the summer months when rainfall and ambient relative humidities are at their highest level and temperatures are 20 to 30~ Growth is slowed when average daily humidity drops significantly below 70%; however, during the winter, steady growth will still occur on northern exposures and in tropical forests, where moisture levels are higher and temperatures are in the optimum range. Algae as a biodeterioration factor is different from mildew. Algae is a green plant and able to synthesize food from sunlight, carbon dioxide, and water. Its attachment to paint surfaces is not an actively attacking one as' is the case with mildew. The paint film is merely supplying a substrate for the algae. By its presence on the surface, however, algae affects the microclimate and resulting deterioration may occur.

CLIMATOLOGY Natural weather exposure testing of paints and coatings is conducted at many different sites, each with its own distinct set of influencing factors. It is the combination of the weathering factors that causes the deterioration particular to that location. As indicated earlier, paint exposure testing facilities are located in many areas, but the subtropical area of Florida is regarded by the United States and by much of the rest of the world as the primary benchmark testing climate. This is be-

cause the exposure consists of all of the major environmental parameters in high quantity except pollution. Each climate type has its own distinct characteristic pattern, which derives from the range of influencing parameters at that location. These can be measured and examined to determine the relative importance of each. Several important climatological types used in weathering are described below. Climatological Types

Subtropical The subtropical climate of southern Florida is recognized worldwide as the primary benchmark testing location. It is the climate to which all other locations are inevitably compared. It is no accident that this area is as important as it is, for it is the only true subtropical region within the United States. Due to the historical considerations discussed earlier, Florida is accepted worldwide as the standard in natural weathering. The great majority of materials will fail faster in Florida than anywhere else because the climate contains all of the major influencing parameters except pollution and in relatively high amounts (Fig. 16). The factors act together synergistically, each one increasing the effect of the other. The high temperatures and humidity complement the photodegradation from solar radiation. The sun's energy is more potent as there is a greater relative proportion of UV to total energy. The time of total wetness is close to 50% of the total time, thereby ensuring a constant cycle of wetting and drying.

Desert The desert of the Southwest United States (especially the Phoenix area) has also come to be regarded as a primary weathering testing site. Exposure conditions in the desert are very different than in Florida. The desert is widely used because the climate consists of clear skies and prolonged un-

CHAPTER 5 2 - - N A T U R A L WEATHERING

629

be examined during natural exposure testing in southern Florida.

Temperate with Pollution Exposure in an industrial environment will indicate whether a paint is susceptible to atmospheric pollution. There are many sources of pollution emission into the atmosphere, and the method of administration to the paint is usually by washout during rainfall. In the United States the geographic area most affected by acid rain is the Northeast, with the next most severe area covering most of the rest of the East and the Midwest [13]. It is no coincidence that this is the industrial heart of the country. The U.S. paint industry has long used Louisville, Kentucky as a typical temperate industrial exposure (Fig. 19). As a large paint manufacturing center, this area has become accepted as a location for exposure to acid rain. The temperature continental climate can be as severe as Florida, but only for a short period during the summer when the temperatures are high, humidity increases, and sunlight levels are similar to the subtropics. Rainfall pH values below 4.5 have been recorded in some areas of Pennsylvania and New Jersey, as shown in Fig. 20. Paints and other exposed materials used in this area must be made to withstand this acid rain effect. This can be tested in Florida by artificially adding the acidic solution to the outdoor exposure test; however, outdoor exposures should be conducted in the actual polluted environment to best determine resistance.

Marine Marine Atmosphere--Exposure to a marine atmosphere FIG. 15-Vertical exposure under roof.

interrupted solar radiation. High temperatures are produced on exposed materials, and there is a higher absolute dosage of solar UV than in Florida (Fig. 17). For most materials, however, especially paints, the greater solar UV does not produce faster deterioration than in Florida. This is due to the lack of moisture in the exposure. Without moisture cycling, the effect of temperature and UV is much reduced. There is also no biodeterioration and no corrosion associated with desert exposure.

Extreme Cold At the opposite end of the exposure scale from the desert is exposure testing in extreme cold weather. Low temperatures are accompanied typically by lower sunlight. International Falls, Minnesota is used as an exposure site for cold weather testing because this area frequently records the lowest temperature in the continental United States (Fig. 18). Cold weather exposure is used to test for specific failure types: cold cracking and freeze thaw deterioration. As the temperature passes through the freeze threshold, any water on the surface or in the bulk of the material forms into ice crystals. As the water freezes, its volume increases by approximately 10%, which is enough to create a substantial physical force on the paint film. This is the one environmental factor that cannot

adds the corrosive influence of salt spray. Predominant test sites are located on the coasts of Florida, New Jersey, North Carolina, Texas, and California. The salt aerosols in the atmosphere settle on the exposed panel and work to enhance rusting. The closer to the surf, the greater the percentage of salt in the atmosphere. A marine atmosphere test combines the natural weathering of the site's climate with the additional natural corrosivity of the sea salt. Marine Immersion--Exposure in seawater is used to test antifouling paints and rust-protective agents. Two exposure possibilities exist: floating immersion and fixed exposure. The floating rack positions the panels either completely or partially immersed; the level remains the same through each tide cycle. The fixed exposure in the tidal zone causes the panels to be alternately submerged and dry.

Climate Affects Exposure Weathering deterioration is a function of the material's reaction to the influencing parameters. The two primary testing locations are both located in hot, sunny climates that produce accelerated deterioration compared to most of the rest of the United States. If different climates produce different exposure results, there is a possibility that exposure to only one set of conditions will not reveal the deterioration that may occur in another set [14]. Paint deterioration is determined by a number of mechanisms, each with its own threshold level for initiation. There is a level for each of the influencing factors at which the paint

630

PAINT AND COATING TESTING MANUAL

MIAMI, FLORIDA CLIMATE RANGE 100 90 80 70 60 50 40 30

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PHOENIX, ARIZONA CLIMATE RANGE 100 90 80 70 60 50 40 30

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FIG. 17-Climate summary, desert (Phoenix, Arizona).

will become susceptible. When the factors interact, the overall influence is a complicated matrix that is constantly changing. Degradation is the result of the material's response to its environment. When the environment changes, so do the changes that occur in the material. This does not always manifest as a slower or faster response, but also perhaps as a different one. Generally, however, it is practical to consider Florida as the most severe climate because it has all the influencing parameters in high abundance all year round.

Other climates are missing one of the factors or are reduced in effectiveness due to the winter season. Instrumentation The atmospheric data required for weathering testing go above and beyond that given by the National Weather Service. The factors affecting material durability must be monitored to understand the cause and effect relationship between paint and the atmosphere. An exposure station weather re-

CHAPTER 52--NATURAL WEATHERING

631

INTERNATIONAL FALLS, MINNESOTA CLIMATE RANGE lOO 90 80

31

70 60 50 40 30 20 10 0 -10 -20

,

I

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MAY JUN JUL AUG SEPT OCT MONTHS .

.

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.

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LOUISVILLE, KENTUCKY CLIMATE RANGE 100 90 80 +35

70 60 50 40 30 20 10 0

-I-

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JAN FEB M,~R APR MAY JUN JUL AUG SEPT OCT NOV DEC MONTHS

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FIG. 19-Climate summary, temperate (Louisville, Kentucky). port comprises information relevant to the exposed specimen. The major additional items reported are the solar energy, black panel (or specimen) temperature, and total time of wetness. These are the most relevant atmospheric measurements concerning material exposure. Table 3 lists several parameters and equipment used to collect the information. As discussed earlier, for weathering requirements, the total solar radiation is measured using a pyranometer, such as the Eppley Precision Spectral Pyranometer (PSP) (Fig. 21),

which is sensitive in the range of 300 to 3000 nm. The ultraviolet is measured using a sensor that is sensitive in the range of 300 to 400 nm. The widely used Eppley Total UV Radiometer (TUVR) (Fig. 22) has a sensing range of 295 to 385 nm. These "total" sensors collect incoming radiation from all angles, having a 180 ~ sensitive diffusion cover. This is termed hemispherical radiation. Total radiation measurements include both the direct beam from the sun and the diffuse energy from the atmosphere [15]. Direct beam radiation, excluding the diffuse sky, is measured with a pyrheliometer such as the

632

PAINT AND COATING TESTING MANUAL

,~..

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FIG. 21-Precision spectral pyranometer (PSP).

FIG. 20-Precipitation pH in northeastern United States.

TABLE 3--Weathering instruments and sensors. Measured Parameter

Equipment Used

Total hemispherical Total ultraviolet Normal incidence

Solar Energy Pyranometer (hemispherical) PSP UV Radiometer (hemispherical) TUVR Pyrheliometer (6 ~ solid angle) NIP

Ambient Black panel White panel Sample temperature

Temperature Thermometer Black panel sensor RTD probe White panel sensor RTD probe Thermocouple attached to panel

Rainfall amount Rainfall duration Total wet time Relative humidity Wet bulb temperature Wind speed Salt aerosols Sea water temperature Air quality

Moisture Rain gage Heated wetness sensor (impedance) TOW sensor (impedance) Psychrometer or hygrograph Thermometer (in wick) Miscellaneous Anemometer and wind vane Wet candle Floating thermometer EPA measurements

NOTE: All sensors should be connected to continuously monitoring and recording devices, e.g., computer-controlleddata acquisition system.

FIG. 22-Total ultraviolet radiometer (TUVR).

Eppley N o r m a l Incidence P y r a n o m e t e r (NIP). This has a 6 ~ solid angle field of view and m e a s u r e s f r o m 300 to 3000 nm. To w o r k COlTectly, it must be m o u n t e d on a solar tracking device.

The intensity of the radiation is integrated with respect to time, thus allowing the a m o u n t of radiant exposure to be calculated. Values for radiant exposure are given in megajoules per square metre (MJ m-2). This is the r e c o m -

CHAPTER 52--NATURAL WEATHERING

FIG, 23-Black panel thermometer.

633

m e n d e d SI unit; an older t e r m used for these values is the langley, w h i c h is 1 calorie p e r square centimetre (1 cal cm-2). To directly convert langleys to megajoules, m u l t i p l y the langley a m o u n t by 0.041 84. This accounts for b o t h the conversion from calories to joules a n d the change in unit area. Black panel t e m p e r a t u r e is a m e a s u r e m e n t widely used in w e a t h e r i n g exposures, b o t h n a t u r a l and accelerated. It is the surface t e m p e r a t u r e of a b l a c k - p a i n t e d metal panel (Fig. 23) a n d is used to o b t a i n a reference t e m p e r a t u r e for a p a i n t panel exposed to a r a d i a n t energy source. E x p o s e d m a t e r i a l s a b s o r b infrared energy from a light source, w h i c h causes their t e m p e r a t u r e to rise above that of the a m b i e n t air temperature. A b l a c k panel m e a s u r e m e n t is the expected maxim u m t e m p e r a t u r e a colored panel might achieve w h e n exp o s e d to a p a r t i c u l a r energy source since b l a c k a b s o r b s all light energy. This t e m p e r a t u r e value is thus an i n d i c a t i o n of the actual exposure t e m p e r a t u r e , which is m o r e relevant t h a n air t e m p e r a t u r e . The black panel t e m p e r a t u r e will follow the light intensity o u t p u t level of the sun. Total t i m e of wetness is the a c c u m u l a t e d t i m e an exposed p a i n t panel has visible surface m o i s t u r e [16]. The two sources for this are rainfall and condensation. Surface m o i s t u r e can be m e a s u r e d b y using a sensing e l e m e n t that requires w a t e r to complete an electrical circuit, thus o p e r a t i n g a r e c o r d i n g device. This will r e c o r d the a c c u m u l a t e d time for b o t h rainfall a n d condensation; the time for rainfall can be removed to o b t a i n the time due to c o n d e n s a t i o n only. The sensor is described in m o r e detail in ASTM Practice for Meas u r e m e n t of Time-of-Wetness on Surfaces E x p o s e d to Wetting Conditions as in A t m o s p h e r i c Corrosion Testing (G 84). A rainfall d u r a t i o n sensor (Fig. 24) m e a s u r e s only the wetness time due to precipitation. The sensor is essentially the s a m e as for total wetness m e a s u r e m e n t except that a c o n s t a n t heating e l e m e n t is i n c o r p o r a t e d so that any surface m o i s t u r e will evaporate quickly, switching off the sensor very shortly after the rainfall has stopped. Keeping the t e m p e r a -

FIG. 24-Rainfall duration sensor.

634

PAINT AND COATING TESTING MANUAL

ture of the sensor above the d e w p o i n t will also prevent cond e n s a t i o n from forming. In a d d i t i o n to the specialized i n s t r u m e n t s detailed above, m a n y m o r e recognizable p a r a m e t e r s m u s t be m e a s u r e d . Ambient t e m p e r a t u r e a n d relative h u m i d i t y can yield general i n f o r m a t i o n c o n c e r n i n g the climate at the test site a n d the c o n d i t i o n of the a t m o s p h e r e s u r r o u n d i n g the exposed specimen. These m e a s u r e m e n t s are m a d e following W o r l d Meteorological O r g a n i z a t i o n (WMO) practices with m e r c u r y therm o m e t e r s , psychrometers, o r - - m o r e c o m m o n l y - - r e c o r d i n g h y g r o t h e r m o g r a p h s in a W M O shelter (Fig. 25). W i n d speed is a factor in the cooling of p a i n t panels a n d is m e a s u r e d with an a n e m o m e t e r . A r a i n gage (Fig. 26) collects rainfall to determ i n e the a m o u n t of precipitation. The rate of rainfall can be important; 25 m m in 30 m i n can be quite different from the effect of 5 m m in 2 h. M1 of these d a t a can be collected a n d displayed in such a m a n n e r as to c h a r a c t e r i z e a w e a t h e r i n g climate. If the influencing p a r a m e t e r s are c o m p a r e d , it is possible to convey the severity of the w e a t h e r with respect to the exposed m a t e r i a l s (Fig. 27).

FIG. 26-Rain gage.

EXPOSURE TYPES Exposure Frames

FIG. 2 5 - W M O instrument house.

The m o s t c o m m o n type of exposure frame in use t o d a y is the a l u m i n u m rack designed for open (unbacked) exposures (Fig. 28). This type of rack a n d associated h a r d w a r e should be constructed with n o n c o r r o d i b l e m a t e r i a l to prevent any contact c o r r o s i o n effect with a specimen. Typical racks consist of a r e c t a n g u l a r frame, adjustable hinged flaps with vinyl strips, a n d a c l a m p i n g device to keep specimens securely fastened (Fig. 29). This type of exposure frame is very versatile in that it can be tilted to any specified angle or direction a n d can be adjusted to a c c o m m o d a t e different-size specimens. S o m e advantages in using this type of frame are: (1) any size s p e c i m e n can easily be m o u n t e d and removed, and (2) the hinged flap with vinyl strip can be adjusted to protect the top p o r t i o n of the s p e c i m e n so that any changes in a p p e a r a n c e resulting from w e a t h e r i n g can be detected. A n o t h e r type of exposure frame is k n o w n as the "black box" (Fig. 30). This insulated exposure is used m a i n l y for recreating the conditions found on the flat surfaces of an a u t o m o bile. It consists of an a l u m i n u m box p a i n t e d black a n d open on top. Panels to be tested are p l a c e d over the open t o p side. If there are insufficient panels to cover the open area, black

CHAPTER 5 2 - - N A T U R A L W E A T H E R I N G

635

MIAMI CLIMATIC DATA JULY 1990

50 40 LU ,_J

30 r

20 10 0 2 3 4 5 6 7 8 9 10 11 12 13 14 1516171819202122

232425262728 293031

DAYS I --I-- BLACK PANEL (C) -'t-SOLAR RADIATIONl ~

WET TIME (HR) -1

FIG. 27-Morlthly weather summary, Miami, Florida.

FIG. 28-Exposure rack with panels.

metal "dummy" panels must be used to completely cover the top side to maintain a high temperature during the daytime. A third type of exposure frame in use but to a lesser degree is the solid wood rack. Like the black box, this frame also elevates sample temperature, but not to the degree of a black box. Generally, 3/4-in. (19-mm) untreated plywood is used when constructing this type of rack. Depending on the condition to be simulated, the plywood can be left unpainted or painted black to further increase sample temperature. Using

this type of frame is commonly referred to as "backed exposure. Exposure Angles The choice of exposure angle that a paint manufacturer uses for outdoor weathering is a critical decision. The angle of the specimens on exposure have a definite impact on the amount and type of degradation that can be generated. The exposure angle will affect the amount of solar radiation de-

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PAINT AND COATING TESTING MANUAL

FIG. 2 9 - C l o s e u p of panels on exposure.

FIG. 3 0 - B l a c k box at 5 ~ south.

posited on the sample, the a m o u n t of time the sample remains wet from rainfall or dew, and the t e m p e r a t u r e of the sample (Table 4). To generate meaningful test results, it is i m p o r t a n t to expose specimens at an angle similar to end use. The basic angles of exposure that are used m o s t c o m m o n l y are as follows:

1.45 Degrees South--Specimens are exposed at an angle of 45 ~ f r o m horizontal and facing the e q u a t o r (see Fig. 28). This is the m o s t c o m m o n angle used for architectural house paints and industrial coatings. This angle is also

TABLE 4--Panel exposure temperatures. Temperature, C Exposure Type Open back, 45 ~ Solid back, 45 ~ Open back, 5~ Solid back, 5~ Black box, 5~ Ambient air

Maximum White Black 45 50 48 52 55

60 70 67 73 76 (32)

Minimum White Black 20 19 20 19 17

18 17 18 17 15 (20)

CHAPTER 5 2 - - N A T U R A L WEATHERING

637

FIG. 3 1 - P a n e l s on e x p o s u r e at 5 ~.

used as a c o m p r o m i s e w h e n end use p o s i t i o n is u n k n o w n or variable. 2.5 Degrees South--Specimens are exposed at an angle of 5 ~ from horizontal a n d facing the e q u a t o r (Fig. 31). This angle is p r e f e r r e d over horizontal to avoid w a t e r p o n d i n g on the sample. W h e n exposing in a s u b t r o p i c a l environment, the 5 ~ angle will increase the yearly a m o u n t of solar radiation, especially during the summer, a n d also the total time of wetness w h e n c o m p a r e d to 45 ~ south. This angle is also used for black box testing and for a u t o m o t i v e finishes. 3.90-Degree Racks--Specimens are exposed at a right angle to the g r o u n d (Fig. 32). E x p o s u r e racks can be facing n o r t h or south d e p e n d i n g on the end use of the coating. Generally, w h e n testing for m i l d e w resistance, architectural paints are exposed vertically (90 ~) n o r t h to m a x i m i z e moisture retention. 4. Station Latitude--Exposure frames are tilted so that the specimens are exposed at an angle from horizontal equal to the geographic latitude of the exposure station. F o r the two b e n c h m a r k w e a t h e r i n g locations, Miami, F l o r i d a is at 26 ~ latitude a n d Phoenix, Arizona is at 34 ~ latitude. 5. Variable Angle--The angle of exposure of the specimens is c h a n g e d on a seasonal basis to ensure m a x i m u m solar r a d i a n t exposure. F o r the Miami, F l o r i d a w e a t h e r i n g location, the schedule calls for 5 ~ exposure in the s u m m e r , 45 ~ exposure in the winter, a n d 26 ~ in the m o n t h s of the equinox (see Table 5 for angle schedule).

Orientation

FIG. 3 2 - W o o d siding panels on e x p o s u r e at 90 ~

S t a n d a r d exposure is for the s p e c i m e n to face the equator. In the N o r t h e r n H e m i s p h e r e this m e a n s t h a t south is the p r e d o m i n a n t direction for the exposure of specimens. This is the m o s t severe direction as it enables the sun to shine directly on the specimens for a greater p o r t i o n of the day. Exposures in any other direction would only be used if any

638

P A I N T A N D COATING T E S T I N G M A N U A L

TABLE 5--Variable angle schedule per ASTM E 782. Month March April-August September October-February

Tilt Angle Latitude Latitude Latitude Latitude

angle angle - 20 ~ angle angle + 20 ~

NOTE: Latitude angle = tilt angle equal to the latitude at the exposure location. o t h e r factor of influence was c o n c e n t r a t e d so heavily in a different direction that it took p r e c e d e n c e over the solar influence. This m a y be the case w h e r e m a r i n e a t m o s p h e r e exposures are used a n d the s a m p l e faces the shoreline r a t h e r t h a n south. The m a j o r exception to the south exposures is found with the testing of p a i n t s for residential use. Paint applied to w o o d e n sidings is extremely susceptible to the effect of m i l d e w attack, especially on the s h a d e d side of the house, w h i c h is p r e d o m i n a t e l y m o r e conducive to fungus growth. A vertical (90 ~) n o r t h exposure is c o m m o n for p a i n t e d w o o d e n sidings. However, at times, equal m i l d e w g r o w t h on replicate specimens has been observed by the a u t h o r on vertical (90 ~ south exposures.

Mounting There are several different m e t h o d s for m o u n t i n g specim e n s onto a test frame. The type of backing chosen d e p e n d s m o r e or less on the m a t e r i a l being tested a n d the end use application. Generally, since m o s t paints are applied to a wood, a l u m i n u m , or steel substrate, m o u n t i n g of specimens is done on an o p e n - b a c k e d rack. However, if a s o m e w h a t accelerated test is necessary or a s p e c i m e n m u s t be tested to a p a r t i c u l a r standard, then m o u n t i n g specimens in a black box m i g h t be necessary. A different m o u n t i n g technique w o u l d apply to residential siding material. In this case, m o u n t i n g against a w o o d e n backing w o u l d m o r e closely simulate end use conditions. A n o t h e r i m p o r t a n t criterion that can have an effect on the o u t c o m e of an exposure test is the location a n d s u r r o u n d i n g obstructions in the i m m e d i a t e a r e a of the test site. In accordance with ASTM Practice for A t m o s p h e r i c E n v i r o n m e n t a l E x p o s u r e Testing of N o n m e t a l l i c Materials (G 7), the test r a c k is to be p l a c e d in a l o c a t i o n so that no s h a d o w from a n e i g h b o r i n g o b s t r u c t i o n shall fall on a n y s p e c i m e n w h e n the sun's angle of elevation is greater t h a n 20 ~. Also according to ASTM G 7, the a r e a b e n e a t h a n d in the vicinity of the weathering racks should be c h a r a c t e r i z e d by low reflectance a n d by g r o u n d cover typical of that climatological area. In desert areas this will be gravel, w h e r e a s in m o s t t e m p e r a t e areas the g r o u n d cover will be low-cut grass.

ACCELERATED NATURAL WEATHERING The possibility of deriving an accelerated test from o u t d o o r exposure seems like a m i s n o m e r . There are, however, m a n y test m e t h o d s that a i m to do just that. The basic idea is very simple: to artificially increase the severity of the o u t d o o r exposure to b o o s t d e g r a d a t i o n rates. The s e c o n d a r y a i m of

the changes is to provide either a m o r e realistic exposure o r to p r o d u c e faster d e t e r i o r a t i o n w i t h o u t the loss of a n y agreem e n t in test results c o m p a r e d to the actual end use environm e n t [14]. O u t d o o r exposure in M i a m i can be considered an accelera t e d test in its own right if the i n t e n d e d c o m p a r i s o n is to exposure in a t e m p e r a t e o r n o r t h e r n climate. All the necessary p a r a m e t e r s are p r e s e n t in greater a m o u n t s . The corollary is simple: increase the severity of the influencing p a r a m e ters a n d the rate o r degree of d e t e r i o r a t i o n will increase. A s t r a i g h t f o r w a r d o u t d o o r test involves p o s i t i o n i n g the s a m p l e on a frame w h e r e the elements of the w e a t h e r can w o r k t o g e t h e r to p r o d u c e the deterioration. To accelerate the overall w e a t h e r i n g process, one or m o r e of the influencing p a r a m e t e r s can be increased. The p a r a m e t e r s that can be i n c r e a s e d are t e m p e r a t u r e , wetness, and solar radiation, a n d there are several s t a n d a r d test m e t h o d s designed to do just that.

Black Box The first p a r a m e t e r n o r m a l l y modified w h e n there is a need for acceleration is the test for s p e c i m e n t e m p e r a t u r e b e c a u s e it is the easiest to control. There will be an increase in the rate of the f o r w a r d d e g r a d a t i o n r e a c t i o n as the t h e r m a l l o a d is increased. The simplest w a y to increase the t e m p e r a t u r e is to p o s i t i o n the s p e c i m e n on a solid backing of d a r k color with a high insulation factor (R). This m e t h o d is widely used for residential sidings, roofing materials, a n d others. A c o m p a r i s o n of the t e m p e r a t u r e s of black a n d white panels on various exposure frames on one s u m m e r day in F l o r i d a is shown in Table 4. The black box exposure is used to test a u t o m o t i v e finishes, a n d its specific p u r p o s e is to increase the t e m p e r a t u r e of the test panels to the s a m e as the nearly h o r i z o n t a l surfaces of a car. This test m e t h o d is d e s c r i b e d in ASTM Practice for Conducting Accelerated O u t d o o r E x p o s u r e Tests of Coatings (D 4141). C o m m i t t e e DO 1.27 on Accelerated Testing of Paints has c o n d u c t e d several studies r e g a r d i n g the black box [17], w h i c h was originally developed by General Motors. The box is m a d e of a l u m i n u m , p a i n t e d black, with the p a i n t panels forming the top surface. The black box exposure also serves to lower the panel t e m p e r a t u r e overnight b e l o w that of the s u r r o u n d i n g air, creating a longer c o n d e n s a t i o n period. The conditions as they affect the p a i n t panels are a good s i m u l a t i o n of the actual end use condition, so this test m a y be considered by s o m e n o t as an accelerated test b u t as one that is realistic of the end use for that climate. If the definition of an accelerated o u t d o o r test is one w h e r e the influencing conditions are altered over the o p e n - b a c k e d direct weathering test, the Black Box Test is indeed a n accelerated test. A further acceleration is i n t r o d u c e d using the H e a t e d Black Box Test. This is a black box i n c o r p o r a t i n g heating elements inside the air space. The air t e m p e r a t u r e inside the box is m a i n t a i n e d at an artificially high level even w h e n there is no incident solar energy.

CHAPTER 5 2 - - N A T U R A L W E A T H E R I N G

Salt Spray In Florida, corrosion tests are accentuated by spraying with salt water in the Scab Test. This uses the natural effects of the weather to break down the coating, then increases the corrosion at a scribe mark. The paint panels are exposed to direct weathering at either 45 or 5~ and are sprayed at regular intervals (normally twice weekly) with a 5% sodium chloride solution (Fig. 33). The spray is scheduled for times when the paint panel will be dry, thus maximizing the absorption effect. This test has proven to be an excellent method for determining the corrosion resistance of automotive coatings. The advantage is that the results are very similar to the results of actual car corrosion determined from field studies. The simultaneous effect of sunlight and saltwater produces an overall deterioration effect rather than a single failure mode as is usually given by salt fog tests such as ASTM Test Method of Salt Spray (Fog) Testing (B 117).

Spray Rack The stresses due to moisture vapor content into and out of the specimen are important for the overall breakdown of most materials. Moisture vapor acts as a catalyst in the photochemical breakdown process. In Arizona's desert environment with high temperature and solar radiation, exposure tests are accelerated using the spray rack, which wets the specimens on a regular basis during the day and overnight. The result produces an effect similar to Florida exposure, plus a slight speedup in the deterioration rate. This rate of increase has been seen as ranging from one to two times the rate for direct weathering in Florida. The exposure is conducted as a solid-backed direct weathering, with the specimens sprayed daily on a regular basis during the night and/or day with distilled or deionized water.

639

The day-time spray is normally of very short duration, intended as a harsh thermal and moisture shock to the paint surface. The nighttime spray cycles are of longer duration, intended to soak the paint thoroughly in a simulation of the overnight condensation effect. This occurs naturally very rarely in the desert of Arizona. Both of these accelerated outdoor tests recognize the importance that wetness plays in deterioration. One important note here is that the timing of the water application is vital to produce the correct type of failure.

Follow the Sun Increasing the level of solar radiation for an outdoor test can be done in two ways. The exposure frame can be designed to track the sun across the sky so that the specimens remain at normal incidence, which will yield greater radiant exposure. The limit for this test method is that the intensity level cannot exceed 1 Sun. A simpler method simply adjusts the tilt angle of the rack in response to the height of the sun in the sky. This "variable angle" exposure uses the schedule outlined in ASTM Practice for Exposure of Cover Materials for Solar Collectors to Natural Weathering Under Conditions Simulating Operational Mode (E 782), which is summarized in Table 5 and is based on the latitude of the exposure. The effect is to increase the solar radiation dosages up to 10% over the fixed latitude angle rack.

Fresnel Reflector Concentration An equatorial mount using an array of mirrors to reflect sunlight back onto an exposure frame will greatly increase the incident solar enegy (Fig. 34). This is the Fresnel Reflector Concentrated Sunlight device, as specified in ASTM Practice for Performing Accelerated Outdoor Weathering of Nonme-

FIG. 33-Salt spray testing (Scab Test) in Florida.

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PAINT AND COATING TESTING MANUAL

FIG. 34-Fresnel reflector concentration device.

tallic Materials Using Concentrated Natural Sunlight (G 90) a n d w h e n used for testing coatings p e r ASTM D 4141. This type of exposure is m o r e closely related to that p r o d u c e d by a n o t h e r accelerated device, the xenon arc, t h a n to o u t d o o r test methods. The c o n c e n t r a t i o n is a factor of one for each m i r r o r (less the efficiency of the reflector, w h i c h is a b o u t 80%), so in effect the solar i r r a d i a n c e on the s p e c i m e n is a b o u t eight times the n o r m a l incidence tracking frame. The high intensity of the light exposure causes the s a m p l e t e m p e r ature to rise also; if a w a t e r s p r a y cycle is added, all weathering factors are increased. The high intensity of the solar energy on the p a i n t causes d e t e r i o r a t i o n rates of 4 to 16 times the rate for nonaccelerated exposure. There is no direct relation b e t w e e n a m o u n t of r a d i a n t exposure a n d extent of d e g r a d a t i o n w h e n the i r r a d i a n c e level is altered. Caution m u s t therefore be used w h e n quoting acceleration a n d correlation factors from testing on this machine. W i t h the sun as the energy source, m o s t of the p r o b l e m of m a t c h i n g light sources is removed. This can o c c u r when using s o m e l a b o r a t o r y - a c c e l e r a t e d w e a t h e r i n g devices. The paint panels are m o u n t e d on a target b o a r d that is p o s i t i o n e d upside d o w n on the b o t t o m of the air tunnel. The m i r r o r s are a r r a n g e d in a Fresnel array, whose focal p o i n t is the target board. The panels are cooled by forced air from the air tunnel, w h i c h is directed across the front a n d b a c k of the panel. A set of nozzles is p o s i t i o n e d so that w a t e r s p r a y m a y be d i r e c t e d onto the face of the panels. The device works o p t i m a l l y w h e n there is u n o b s t r u c t e d direct b e a m r a d i a t i o n as the m a c h i n e is designed to follow the sun a n d reflects only the i n c o m i n g energy from a small solid angle. It is important, therefore, for the m a c h i n e to be aligned with the sun at all times during the day. Clouds will d i s r u p t the o p t i m u m condition, a n d overcast days will prevent the Fresnel C o n c e n t r a t o r from o p e r a t i n g at its m o s t

efficient. As a result, the m a c h i n e is n o r m a l l y e m p l o y e d in the desert, where clear skies a n d strong sunlight are f o u n d a l m o s t every day. It w o u l d be incorrect, however, to say that this device will not w o r k correctly out of the desert environment; the time for a c c u m u l a t i o n of energy, however, w o u l d be greater. The test d u r a t i o n for the Fresnel Concentration is determ i n e d by solar r a d i a n t exposure, n o r m a l l y the total ultraviolet. As the energy levels at the target b o a r d are too high for direct m e a s u r e m e n t s on a p e r m a n e n t basis, the dosage m u s t be calculated from direct m e a s u r e m e n t s using a c o r r e c t i o n factor for the reflectance of the mirrors. The m i r r o r m a t e r i a l is chosen for its flat reflectance properties, t h e r e b y reflecting all wavelengths equally. If the reflectance value of the m i r r o r m a t e r i a l is m o n i t o r e d closely, it is possible to m a k e a c c u r a t e calculation of the intensity level at the s p e c i m e n position. W h e n the i r r a d i a n c e is integrated, dosage levels for the Fresnel C o n c e n t r a t i o n Device are eight times greater t h a n n o r m a l incidence. One y e a r of r a d i a n t exposure from a fixed rack can be r e a c h e d in a b o u t six weeks exposure on this device. The c o m m o n t h r e a d to all o u t d o o r accelerated (or accelerated outdoor) tests is that one or m o r e factors have been increased to speed up the rate of the deterioration. This is usually i n t e n d e d to give the test a m o r e realistic c o m p a r i s o n to an i n t e n d e d end-use environment. The exception to this w o u l d be tests that increase solar intensity above naturally found levels.

INSPECTION AND REPORTING Visual I n t e r i m inspections of a m a t e r i a l to c h a r t the d e g r a d a t i o n progression t h r o u g h o u t a n exposure p e r i o d will p r o d u c e ira-

CHAPTER 52--NATURAL WEATHERING 6 4 1 p o r t a n t i n f o r m a t i o n on the rate at w h i c h a s a m p l e degrades a n d at w h a t t i m e interval certain p r o p e r t i e s b e c o m e noticeable. To a c c o m p l i s h this, visual or i n s t r u m e n t a l techniques are used to assess the severity of degradation. W h e n visual inspections are p e r f o r m e d at p e r i o d i c intervals, a n u m b e r of variables are i n t r o d u c e d t h a t m a k e it difficult to get consistent a n d r e p e a t a b l e assessments. To minimize these variables, inspection p r o c e d u r e s m u s t be standardized. W h e n viewing specimens for color change or for w e a t h e r i n g effects, it is very i m p o r t a n t to use the s a m e light source a n d viewing conditions. F o r this reason, evaluations are c o n d u c t e d a c c o r d i n g to ASTM Practice for Visual Evaluation of Color Differences of Opaque Materials (D 1729). In this test m e t h o d , spectral qualities are outlined for different light sources, w h i c h are to b e used for various evaluations. A viewing b o o t h will allow the m o s t consistent readings over an extended p e r i o d of t i m e as o p p o s e d t o examining changes outdoors where daylight is not always the same.

Reporting Scales W h e n evaluating the ability of a coating to w i t h s t a n d w e a t h e r i n g effects, the changes over a p e r i o d of t i m e m u s t be charted. To do this with consistency, r e p o r t i n g scales are e m p l o y e d to r e c o r d the a m o u n t of change. In m a n y instances it is not practical for the p e r s o n m o s t interested in the durability of a coating to see h o w a p a r t i c u l a r coating p e r f o r m s over a p e r i o d of time. This is quite often the case as s p e c i m e n s are exposed in a different c o u n t r y or region of the United States in o r d e r to achieve answers in a s h o r t e r p e r i o d of time. This m a k e s it necessary to leave the inspection process up to a n o t h e r individual. A p r o b l e m is the fact that no two people perceive a degree of change in exactly the s a m e way. In o r d e r to m i n i m i z e this p r o b l e m , certain p r o c e d u r e s are followed so that all interested parties will "'see" the defect(s) the same way. This is done by using generally accepted e x a m i n a t i o n p r o c e d u r e s a n d the use of p h o t o g r a p h i c reference standards. The A m e r i c a n Society for Testing a n d Materials (ASTM), the F e d e r a t i o n of Societies for Coatings Technology (FSCT), a n d the I n t e r n a t i o n a l S t a n d a r d s Organization (ISO) each have e x a m i n a t i o n p r o c e d u r e s similar in quantitative descriptions. The m a i n difference lies in the quantitative a n d qualitative n u m b e r i n g scales used. FSCT uses a scale from 10 to 0, with 10 being "perfect" o r "absent of failures." ISO uses a scale from 0 to 10, with 0 representing a n " u n c h a n g e d " condition. I n d e p e n d e n t test labs in the United States use the ASTM/FSCT scale in their ratings unless i n s t r u c t e d otherwise by their client. It should be n o t e d that "perfect" is not a p r e f e r r e d term, a n d the testing labs will use the t e r m "as received."

Failure Modes Evaluating a n d recording type a n d degree of failure m o d e s in a p a r t i c u l a r coating is not a difficult task if visual inspections are p e r f o r m e d properly. Although m o s t ratings are subjective in t e r m s of degree of failure, an experienced a n d wellt r a i n e d i n s p e c t o r consistent in his or h e r ratings is able to p o r t r a y a good image of any d e g r a d a t i o n present. In o r d e r to do this, all relevant s t a n d a r d s including the FSCT pictorial s t a n d a r d s m u s t be used w h e n applicable. Table 6 lists the m o r e c o m m o n l y u s e d evaluation s t a n d a r d s for p a i n t failure

TABLE 6--List of surface appearance standards. Property

ASTM Method

Color change (v) Chalking Manual Tape Dirt retention (p) Mildew growth (p) Checking (p) Cracking (p) Flaking (p) Blistering (p) Surface rust (p) Scribe rust (p) Erosion (p) Color (i) Gloss (i) D of I (i) NOTE:( v )

=

ASTM D 1729 ASTM D 4214 ASTM D 4214, TNO ASTM D 3274 ASTM D 660 ASTM D 661 ASTM D 772 ASTM D 714 ASTM D 610 ASTM D 1654 ASTM D 662 ASTM D 2244 ASTM D 523 ASTM E 413

visual; (i) = instrumental; (p) = pictorial.

reporting. More c o m p l e t e descriptions of these effects can be found in o t h e r chapters of this manual.

Nondestructive Nondestructive testing is a i m e d at inspection w i t h o u t the d i s t u r b a n c e of the integrity of the specimen. This will, by the above definition, include mostly all the visual evaluations a n d all the i n s t r u m e n t a l l y assisted optical m e a s u r e m e n t s . True nondestructive testing, however, is a i m e d at investigating defects that c a n n o t be seen easily with the eyes or surface s c a n n i n g equipment. Nondestructive tests aim to d e t e r m i n e physical changes in the structure of a m a t e r i a l that c a n n o t be d e t e r m i n e d visually. It is particularly useful in seeing changes at the molecular level, in the b u l k p a r t of the material, or in underlying layers. Many of the c o m m o n physical tests e m p l o y e d to assist in evaluating the effect of w e a t h e r i n g are in fact "destructive," The p a i n t s a m p l e is d i s t u r b e d during the m e a s u r e m e n t process, as in testing a d h e s i o n or chalking. W h e n a destructive test is used, several disadvantages are evident: the s p e c i m e n c a n n o t continue exposure b e c a u s e the place w h e r e the test was c o n d u c t e d will affect the continuity, a n d a p e r m a n e n t r e c o r d of the c o n d i t i o n at that time is lost. These p r o b l e m s can be overcome by using a large n u m b e r of replicates so t h a t there will be enough virgin m a t e r i a l for testing. W h e n the s a m e s a m p l e can be studied t h r o u g h o u t the test, it m a y be easier to get a m u c h clearer picture of the sequence of events of the p r o g r e s s i o n of failure m e c h a n i s m . This is especially true at the earliest sign of changes in the material. If it is possible to d e t e r m i n e the changes that o c c u r at the m o l e c u l a r level during the initial stages of d e t e r i o r a t i o n a n d follow the p r o g r e s s i o n t h r o u g h to failure, it is possible to develop m o d e l s which will assist in p r e d i c t i n g long-term durability from s h o r t - t e r m testing. The range of m e t h o d s i n c l u d e d in nondestructive testing is varied, a n d Table 7 lists those which have b e e n d e m o n s t r a t e d to have an a p p l i c a t i o n in weathering.

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PAINT AND COATING TESTING MANUAL TABLE 7--List of nondestructive testing methods. Property

ASTMMethod

Electromagnetic Strain gage Infrared thermography Electron microscopy X-ray radiography Ultrasonic holography Video imaging systems

ASTM D 1186 ...~ ...a ...a ...a ...a ...~

aStandard for applicationto coatings being developed.

TABLE 8--List of physical testing standards. Property

ASTMMethod

Impact Elasticity Elongation Bend Hardness Adhesion Abrasion Mar Chip resistance

ASTM D 2704 ASTM D 522 ASTM D 522 ASTM D 3363 ASTM D 3359 ASTM D 4060 ASTM D 3170

Mechanical Properties There is a m a j o r distinction that m u s t be made between surface appearance changes in a material a n d intrinsic physical properties. Changes can occur on the surface without affecting the bulk of the material. A m a j o r portion of a complete weathering p r o g r a m is the m e a s u r e m e n t of the physical changes produced during exposure. A result of UV degradation a n d associated failure modes can have a direct impact on the relationship between stress a n d strain of a coating and the elastic a n d inelastic reaction w h e n force is applied. To have a better u n d e r s t a n d i n g of the true performance of a weathered specimen, it is necessary to perform tests to determine certain mechanical properties. Table 8 lists the more c o m m o n l y performed m e c h a n i c a l tests o n weathered p a i n t panels. A more complete description of their relevance is given in other chapters of this m a n u a l .

REFERENCES [1] Luckiesh, M., Artificial Sunlight, D. Van Nostrand Co., Inc., New York, 1930. [2] Searle, N.D., "Weathering," Encyclopedia of Polymer Science and Engineering, Wiley, New York, 1989. [3] Mathew, W. R., "Predicting the Effects of Weathering on Color," Plastics Engineering, May 1986. [4] Bennett, I., "Monthly Maps of Mean Daily Insolance for the United States," Solar Energy, Vol. IX, No. 3, July-September 1965. [5] Juriaanse, A. and Zahradnik, B., "Natural Weathering of Polypropylene: Counteracting the Influence of Season and Exposure Orientation," Proceedings, ANTEC, 1986. [6] Auld, W. E., natural weathering engineering drawings, personal copy, 1961. [7] Fischer, R.M., Murray, W. P., and Ketola, W. D., "Thermal Variability in Outdoor Exposure Tests," Progress in Organic Chemistry, Fall, 1990. [8] Kreese, P., "Influence of Inert Pigments on the Anti-Corrosive Properties of Paint Films," Polymers Paint and Colour Journal, 1978. [9] Lindberg, B., "Moisture and Painted Wood," Journal of the Oil and Colour Chemists Association, June 1986. [10] Evans, U. R., The Corrosion and Oxidation of Metals: Scientific Principles and Practical Application, Arnold Publishing, London, 1960. [11] Crewdson, M. J., "Corrosion Test Methods: A Further Review," National Coil Coaters Association Proceedings, April 1986. [12] O'Neill, T.B., "Succession and Interrelationships of Microorganisms on Painted Surfaces," Journal of Paint and Coatings Technology, Vol. 58, No. 734, 1986. [13] Likens, G. E., "Acid Precipitation," Chemical and Engineering News, November 1976. [14] Crewdson, M.J., "The Present Status of Weathering in the United States," Proceedings, Suga International Weathering Symposium, October 1988. [15] Zerlaut, G. A., "Solar Radiation Measurements: Calibration and Standardization Efforts," Advances in Solar Energy, American Solar Energy Society, 1982, pp. 43-66. [16] Sereda et al., "Measurement of the Time of Wetness by Moisture Sensors and their Calibration," Atmospheric Corrosion of Metals, ASTM STP 767, 1982. [17] Morse, M. P. in Permanence of Organic Materials, ASTM STP 781, ASTM, Philadelphia, 1982.

MNLI7-EB/Jun.

Accelerated Weatherin9

1995

53

by Valerie D. Sherbondy I

PAINTS AND COATINGS ARE USED both to protect substrates and to provide an aesthetically pleasing appearance. In an outdoor environment, both of these functions can be affected by weathering. The four major factors involved in weathering are sunlight, moisture, oxygen, and heat. Light, especially in the ultraviolet (UV) region, may lead to discoloration, premature loss of gloss, scaling, embrittlement, and chalking. Moisture may cause blistering, flaking, mildew, and early 10ss of adhesion. Heat may cause embrittlement, cracking, peeling, and checking. Oxygen in the atmosphere promotes oxidation of the surface of the coating, which may eventually lead to oxidation of internal layers, causing embrittlement, softening, cracking, or crazing. These elements contribute individually as well as in combination to cause coating failures. Naturally occurring and man-made chemicals in the environment also contribute to coating degradation and could be considered a fifth element of weathering. However, the type and levels of chemicals can vary dramatically, even over short distances. Therefore, they cannot be considered as universal in character as the four factors mentioned previously. Perhaps as a consequence of this, and also partly due to tradition, chemical resistance testing is usually considered to be separate from artificial weathering. Therefore, this chapter will consider only those devices that incorporate an ultraviolet light source. Although the effects of chemicals cannot be ignored, they are discussed elsewhere in the manual. Accelerated or artificial weathering involves the use of laboratory equipment to simulate the degradation that occurs during actual outdoor exposure [1-3]. Accelerated weathering is the term most often applied to artificial weathering because the elements of light, heat, and moisture are either longer in duration or more intense than the actual time and conditions encountered in outdoor exposure. This causes the coating to weather or degrade more rapidly, in a time sense, than when placed in a natural environment. However, in an attempt to accelerate the effects of natural weathering, the laboratory conditions may be overly aggressive and thus cause results that are not attained during natural weathering. Artificial weathering devices should be designed to produce test conditions that are controllable and reproducible, so that data are reproducible on a day-to-day basis and comparable on a laboratory-to-laboratory basis. This differs from outdoor or natural weathering where there are no controls over environmental factors. Due to this variability of conditions, the results of natural weathering often are not reproducible. For

~Assistant laboratory director, KTA-Tator, Inc., 115 Technology Drive, Pittsburgh, PA 15275.

example, weathering factors are different for various parts of the world and for different countries. Some locations receive more sunlight and heat, while others are cloudy and cool. There are also seasonal variations. Even for long-term testing, the data for a specimen placed outside during January would not necessarily concur with data for a duplicate specimen placed outside in July because of initial exposure effects. Even if the same location and time of year are used, the natural weathering factors change from year to year. In contrast to natural weathering, data produced under controlled laboratory conditions can be used as comparative data, assuming the same equipment, equally aged lamps, etc., even if testing was conducted during different times of the year or over several years. In addition to reproducibility, a great advantage of accelerated weathering is that the results are available more quickly than with natural weathering, which is a phenomenon that may take years. The time saved on testing can translate into cost savings when developing new products or when choosing a new coating system for buildings, tanks, or equipment where historic data about coating performance are not available. Even though there are many advantages to accelerated weathering, it is important to recognize that these data should be used for comparative purposes only since at this time a direct correlation to natural weathering has not been proven for any weathering device currently being used. One reason a correlation has not been developed is because data obtained through artificial weathering have been produced by subjecting the sample to unnatural conditions, and the results may or may not duplicate those found during natural weathering. If similar failure modes are experienced in an accelerated weathering device and at an outdoor test site, a direct correlation may be developed for that product. The calculated correlation would be valid only for the tested material under the stated, specific conditions, and the actual field conditions usually differ from the conditions found in a test fence area. It should be noted that not all results obtained by accelerated weathering devices are reproducible [3]. Studies have concluded that even though the settings may be the same, the maintenance of the device--including light source, filters, and gauges--affect the exposure levels and thus the results of the testing. The cleaning and disposal of light filters is often subjective and a major source of variation that changes the amount of light at critical wavelengths. The intensity of the light is also dependent on the changing of, rotation of, and power supplied to the light source. These factors may speed

643 Copyright9 1995 by ASTMInternational

www.astm.org

644

PAINT AND COATING TESTING MANUAL

up or slow down the degradation process due to changes in both temperature and UV light exposure. Since this is known to occur, testing specifications are sometimes written to include a standard material with the test specimens or to include an actual measurement of the light during the testing. This measurement is discussed later in this chapter. Even if the light source was not changing with time, each device has areas of more intense exposure. For this reason, most devices have recommended manual rotation procedures for the exposed specimens. These procedures should be performed at regularly spaced intervals to decrease the effects of uneven exposure. These effects would be greater on short-term tests, where the specimens would not be moved through the different stages. Aside from the conditions of the device, the conditions surrounding the device also affect performance. Most devices require a ventilated area to maintain ambient conditions so temperature changes within the device can be maintained or obtained. For example, a relatively high ambient temperature may reduce the amount of condensation on some devices, while high amounts of ventilation may slow the temperature recovery after a water spray in other devices. It should be noted that ambient conditions are not specified by ASTM standards, although the manufacturers recommend general guidelines. Ambient conditions will vary between laboratories, adding an unknown variable that may affect the results.

and the UV-C region is 200 to 280 nm. Although the UV-C is the most damaging region, these wavelengths are filtered out by the atmosphere. Therefore, if light sources produced output in this region, they would cause abnormal degradation and their use should be avoided when performing accelerated weathering unless there is a possibility of UV-C exposure. Both the UV-A and the UV-B regions cause degradation of coatings. The energy of the shorter wavelengths present in the UV-B region, - 9 1 to 102 kcal/mol (3.8 to 4.3 J/tool), cause more severe and rapid degradation of coatings than the wavelengths in the UV-A region. In the UV-B region, the energy levels are high enough to break carbon-nitrogen, carbon-carbon, nitrogen-hydrogen, carbon-oxygen, and carbon-hydrogen bonds in the polymeric portion of the coating. In the UV-A region, the longer wavelengths do not have sufficient energy, - 7 1 to 91 kcal/mol (3.0 to 3.8 J/tool), to break certain bonds, namely carbon-hydrogen. Thus, it is apparent that UV-A radiation differs from UV-B radiation in both wavelength and severity of damage. The breaking of chemical bonds first leads to a degradation of the coating surface layers and is manifested by chalking, fading, and loss of gloss. Once the outer layers of polymer are lost, pigments are exposed. Without the protective polymeric binder, the pigments can fade and erode, causing a change in color and/or appearance. Light has been simulated in accelerated weathering devices by mercury arcs, open and enclosed carbon arcs, fluorescent lamps, and reflection of collected sunlight. However, as the results produced by accelerated testing were compared to the results obtained from natural weathering, the light sources were modified to attempt to achieve similar results. Depending on the exact coating type and the service environment, several different specifications have been developed that indicate which light source should be used. Most of these test specifications require the use of open or "sunshine" carbon arcs, xenon arcs, and fluorescent lamps. These light sources were chosen since they were found to more closely simulate the degrading ultraviolet light range of sunlight or to rapidly produce dramatic changes in the coating.

E L E M E N T S OF W E A T H E R I N G There are many component factors that contribute to the weathering of a coating. The general components are light, moisture, heat, and oxygen, which are always present in various amounts. This section concentrates on these general components and on how they are simulated and intensified. L i g h t [4]

Sunlight is composed of light from the visible, ultraviolet, and infrared regions of the electromagnetic spectrum (Fig. 1). The damaging region of sunlight has been determined to be the ultraviolet light (UV) region, especially the shorter wavelengths ranging down to 295 nm. The portion of sunlight in the UV region is relatively small, only 5 to 7%, due to the filtering effects of the atmosphere. The UV region has been divided into three domains: UV-A, UV-B, and UV-C. The UV-A region is 315 to 400 nm, the UV-B region is 280 to 315 nm,

Enclosed Carbon Arc Originally, these lamps were used to test the light fastness of textiles. They were then combined with a water spray to be an industry-wide artificial testing device. Although mercury and carbon arcs have been in use for over 75 years, when they were used to test paint, it was discovered that they produce light that accelerates chalking and fading more than crack-

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CHAPTER 53--ACCELERATED WEATHERING ing. The c a r b o n arc consists of neutral solid a n d cored c a r b o n electrodes. The flame p o r t i o n of the l a m p is enclosed in a borosilicate globe. The globe creates a semisealed atmosphere that sustains the arc a n d filters out UV light b e l o w 275 nm. The l a m p s p r o d u c e two large spectral peaks t h a t center n e a r 350 a n d 380 nm, b u t they deliver essentially no emission b e l o w 340 nm, w h e r e the light is m o r e severe in its d e g r a d a t i o n capabilities a n d m o r e d a m a g i n g to p o l y m e r i c m a t e r i a l s (Fig. 2). I n addition, even the light in the visible region, 400 to 800 nm, was also low c o m p a r e d to sunlight, m e a n i n g that any visible color change due to the change of the p i g m e n t w o u l d require a lengthy exposure. As a result, the i n d u s t r y d e m a n d e d a light source that p r o v i d e d a b e t t e r simulation of n a t u r a l sunlight, especially since m o r e d u r a b l e materials were being developed.

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I n response to i n d u s t r y d e m a n d s , the o p e n flame c a r b o n arc or sunshine c a r b o n arc was developed. This d e s c e n d e n t of the c a r b o n arc p r o d u c e d a s p e c t r u m closer to that of sunlight. The l a m p operates in a free flow of air instead of in a globe. The l a m p is c o m p o s e d of c o p p e r - c o a t e d electrodes a n d a central core of rare earth. The l a m p is s u r r o u n d e d by a stainless-steel filter frame that also acts as an air duct. The f r a m e holds flat, optical, heat-resistant, borosilicate glass panels that filter p o r t i o n s of the lower wavelength light from the spectrum. The s p e c t r u m p r o d u c e d by this light source is m o r e similar to that of sunlight in the region b e t w e e n 310 a n d 370 n m (Fig. 3). However, there was still a spectral i m b a l a n c e due to a large b a n d b e t w e e n 370 a n d 450 nm, b u t the region b e t w e e n 450 a n d 800 n m was m u c h closer to n a t u r a l sunlight t h a n the enclosed c a r b o n arc l a m p s h o w n in Fig. 3. The disadvantage of this light source is the emission b e t w e e n 260 a n d 310 nm, which includes a p o r t i o n of the u n d e s i r a b l e UV-C region. Although this light source was an i m p r o v e m e n t , the i n d u s t r y requested the spectral d i s t r i b u t i o n of the light be even closer to that of sunlight.

FIG. 3 - S p e c t r u m of light produced by a sunshine carbon arc. Used with permission of the Atlas Electric Devices Co.

Xenon Arc The a d o p t i o n of the xenon arc l a m p for accelerated weathering devices was the next i m p r o v e m e n t in artificial weathering equipment. The xenon l a m p consists of a b u r n e r tube a n d a light filter system. There are two types of xenon arc lamps. One type of xenon arc l a m p is cooled by w a t e r circulated t h r o u g h the l a m p housing. The cooling w a t e r also filters out the long-wavelength infrared light. The o t h e r type of xenon l a m p is air cooled. Both l a m p types p r o d u c e a s p e c t r u m closer to sunlight w h e n filtered a n d set at the correct irr a d i a n c e setting. There are several filters a n d c o m b i n a t i o n s of filters that can be used. The three c o m m o n filter c o m b i n a tions used for artificial w e a t h e r i n g of p a i n t are quartz/borosilicate, borosilicate/borosilicate, a n d quartz/quartz. The first c o m b i n a t i o n allows the UV region to extend d o w n to 270 n m (Fig. 4). The borosilicate/borosilicate c o m b i n a t i o n has a cutoff at 280 nm, w h i c h makes the s p e c t r u m closer to t h a t of

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646

PAINT AND COATING TESTING MANUAL

natural sunlight, which cuts off at 295 nm. The quartz/quartz combination produces a spectrum that extends to below 250 nm, i.e., into the undesirable UV-C region, and although used to test coatings, it is the least popular of the three filter combinations 9 Since the xenon arc source decays as the lamp ages, the irradiance of the light source should be monitored and can be controlled by adjusting the lamp wattage. Most new devices have a monitoring system to compensate for the decay. The most c o m m o n settings are 0.35 or 0.55 W/m 2at 340 nm. Both settings produce a spectrum with approximately the same cutoff wavelength (determined by the filter). The settings are both within the range of natural sunlight. The 0.55 W/m 2 setting is closer to s u m m e r sunlight, while the 0.35 W/m z setting is closer to winter sunlight. If the wattage settings are varied, the degradation rate will change.

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solvent is poured over the film, and the length of time required to puncture the film is noted. This test would be considered a crude screening test. More precise data can be derived from the distensibility test where a tensile strength strip (see ASTM D 2370.8) is clamped with either a tensile tester jaw or an alligator clip on the upper end, and the bottom end is clamped with an alligator clip with a 12-g weight and immersed in a clear beaker containing the solvent reagent (Fig. 4). The beaker is immediately marked with the initial length, and the time required to elongate the strip one inch is recorded.

Solvent Rub Resistance

FIG. 5-Apparatus for alkali resistance test.

provides a quick relative test without having to wait for exposure results. The MEK resistance of some two-component ethyl silicate zinc-rich primers has been shown to correlate well with the cure of the primer as determined by diffuse reflectance infrared spectroscopy.2

ACID R E S I S T A N C E Acid resistance is determined by exposing a coated panel to freshly prepared mortar as well as a hydrochloric acid solution and is fully described in ASTM D 3260, Test Method for Resistance to Acid and Mortar of Factory-Applied Clear Coatings on Extruded Aluminum Products. The acid resistance test is performed by first sealing the edges of a specially coated panel with a paraffin and beeswax mixture and then

Although solvent resistance can be evaluated using ASTM D 1308, many use or adapt a solvent rubbing technique'with a gauze cloth soaked in a solvent (MEK is common) and rubbing with the thumb back and forth in 2-in. (5.08-cm) strokes. This procedure, ASTM D 4752, Test Method for Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub, is imprecise because the person's strength and the size of the thumb is variable. Nevertheless, it 2Starr, T. L., Henton, L. E., Lewis, W. S., and Rideout, F. A., "Improved Field Reliability of High Performance Coatings Systems: Phase II--Develop Procedures and Criteria in Critical Performance Areas," available from Steel Structures Painting Council, 4400 Fifth Ave., Pittsburgh, PA 15213.

FIG. 6 - Q panel is scored with an "x" to expose bare substrate.

CHAPTER 55--CHEMICAL RESISTANCE

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0--Angle of lid, 90 to 125 deg. l--Thermometer and thermostat for controlling heater in base. 2--Automatic water levelling device. 3--Hurnidifying tower. 4--Automatic temperature regulator for controlling heater. 5--Immersion heater, nonrusting. 6---Air inlet, multiple openings. 7--Air tube to spray nozzle. 8--Strip heater in base. 9--Hinged top, hydraulically operated, or counterbalanced. 10---Brackets for rods supporting specimens, or test table. 1 l--Internal reservoir. 12--Spray nozzle above reservoir, suitably designed, located, and bamed. 12A--Spray nozzle housed in dispersion tower located preferably in center of cabinet. 13--Water seal. 14--Combination drain and exhaust+ Exhaust at opposite side of test space from spray nozzle, but preferably in combination with drain, waste trap, and forced draft waste pipe. 16--Complete separation between forced draft waste pipe and combination drain and exhaust to avoid undesirable suction or back pressure. 17--Forced draft waste pipe. 18--Automatic levelling device for reservoir. 19--Waste trap. 20---Air space or water jacket. 21--Test table or rack, well below roof area. FIG. 7 - D i a g r a m of s a l t - s p r a y (fog) c a b i n e t .

immersing the panel in a 10 vol% solution of 31.6% solution of HC1 at ambient temperature for 6 h, followed by rinsing, drying, and examination for blistering, peeling, lifting, crazing, flaking, or discoloration. Mortar resistance is performed by applying a fresh mortar patty, prepared to a specified formula, to both sides of the specially coated panel and then placing in a high relative humidity cabinet for seven days. The mortar is then carefully removed and the panel wiped off with a damp cloth followed by examination as with the acid test.

ALKALI A N D D E T E R G E N T R E S I S T A N C E ASTM D 1308, Test Method for Effect of Household Chemicals on Clear and Pigmented Organic Finishes, is also recommended for evaluating alkali and detergent resistance. It is a simple common sense technique that can be used for many materials or chemicals that can stain or discolor a coating. The choice of testing materials should be related to the coatings end use. As previously mentioned, ASTM D 1308 describes techniques for either immersing the coated substrate in the reagent, putting a small amount of the material on the

coated surface and covering with a small watch glass, or just leaving the material on the surface uncovered. The coating is then checked for staining after a period of time or at 1, 2, 4, 6, 24, and 48-h intervals. For detergent resistance of appliance finishes, a solution of a specified detergent containing a high percentage of sodium phosphate is applied and the temperature maintained at 165~ (73.8~ for the duration of exposure, usually 250 to 500 h. The test panel is submerged at least six inches into the solution. This is a more severe test than a spot test, but it is more representative of actual service conditions. Examination is done after rinsing and blotting the panel and looking for any manifestation of coating failure. ASTM D 1647, Test Method for Resistance of Dried Films of Varnishes to Water and Alkali, describes an alkali resistance immersion test using coated test tubes. The use of test tubes coated by dipping into the coating prevents the reagent from creeping under the edge of a flow-on film. As many as 20 tubes are prepared for this test, allowing examination after 1 to 8, 16, and 24 h (Fig. 5). The exposed specimens are rinsed with water and allowed to dry for 30 min before examining for whitening, blistering, or removal of the film.

666

PAINT AND COATING TESTING MANUAL

WATER A N D M O I S T U R E R E S I S T A N C E There are a n u m b e r of types of chambers such as moisture chambers, salt fog cabinets, and other accelerated weathering equipment that tests coating moisture resistance as well as other resistance criteria. This type c h a m b e r is useful for spot checking accelerated long-term environmental durability. But to be practical, all that is needed to test moisture or water resistance is to partially immerse a coated sample in a glass beaker containing water. The water is maintained at 100~ (37.7~ for an extended period of time, and the panels are periodically checked for discoloration, whitening, or blistering of the film. The test results are compared to a specification or a standard sample run concurrently. The procedure for this test is ASTM D 870, Standard Practice for Testing Water Resistance of Coatings Using Water Immersion.

time. The metal panels are exposed to the settling fog of an atomized neutral (pH 6.5 to 7.2) sodium chloride solution consisting of 5 parts by weight sodium chloride and 95 parts distilled or deionized water. The sample is then periodically checked to see if the rusted exposed metal has propagated under the coating causing coating failure. As with water resistance, the results are compared to a standard or a specification. A typical salt spray cabinet as shown in Fig. 7 incorporates a basic chamber, an air saturator tower, a salt solution reservoir, atomizing nozzles, specimen supports, a heater, and controls for maintaining specified temperature. Such chambers are available commercially from several suppliers. The testing procedure, ASTM B 117, Test Method of Salt Spray (Fog) Testing, describes this method in more detail.

REFERENCES SALT FOG T E S T Salt fog resistance is important for marine, automobile, and aircraft coatings and any other exterior coating exposed to salt spray by being near the ocean or exposed to salted road conditions. The severe corrosion caused by salt is well known. The test requires a salt fog cabinet and coated panels. The coating is scored to the bare substrate with a X shape (Fig. 6). The edges are sealed with a weather-proof tape, and the panel is placed in the cabinet for a specified period of

[1] Lambourne, R., Ed., "Paint and Surface Coatings: Theory and Practice," John Wiley and Sons, New York, 1987, pp. 664-671. [2] Nowacki, L. J., "Protective Linings for Steel Shipping Containers," Corrosion, CORRA, Vol. 14, 1958, p. 100. [3] Hough, R. W., Chairman, "The Bratt Conductivity Cell for Measuring Chemical Resistance," Official Digest, Federation of Paint and Varnish Production Clubs, ODFPA, Vol. 31, 1959, p. 1460. [4] Gearhart, W. M. and Ball, F. M., "Half-Second Cellulose AcetateButyrate: IV," Official Digest, Federation of Societies for Coatings Technology, Vol. 31, 1959, p. 1460.

MNL17-EB/Jun. 1995

Testing Coatings for Heat Resistance and Flame Retardance by Wayne Ellis 1

HEAT RESISTANCE

degradation including rust formation, blistering, loss of adhesion, dulling, and ~chalking.

WHEN ORGANICCOATINGSare exposed to elevated tempera-

tures, the initial effect is usually softening, followed by hardening, embrittlement, and degradation. The rate of response and the extent of degradation depend on coating composition, temperature, and length of exposure. There is a distinction in the terminology used to define the resistance of coatings to such changes. Below 400~ (200~ the property is called "thermal resistance," while above that temperature the property is identified as "heat resistance." Both thermal and heat resistance relate not to occasional heating, but to resistance to change from exposure to a constant heat influence over months or years. Special organic coatings can be formulated to provide thermal resistance, such as those designed to protect steel surfaces exposed to elevated temperature during service life. The upper temperature limit for most organic coatings is in the range of 400~ (200~ In general, inorganic coatings exhibit better heat resistance than organic coatings. A special group of products are thermal protection coatings, including ablative coatings to protect space vehicles during atmospheric re-entry and intumescent coatings that protect wood or other cellulosic surfaces exposed to fire. Evaluation of heat resistance is carried out on coated specimens exposed to selected temperatures representative of service conditions and later subjected to other exposure conditions to determine susceptibility to loss of protective function.

ASTM M e t h o d D 1211 Another test method involving temperature exposure is ASTM Test Method for Temperature-Change Resistance of Clear Nitrocellulose Lacquer Films Applied to Wood (D 1211). A common failure of clear films applied to wood is cracking or checking that may occur over a period of time, either with the grain or at an angle to the grain. This "cold check" test is designed to accelerate the occurrence of checks or cracks by cycling at the temperatures of exposure. In this procedure, which constitutes a single cycle, lacquer-coated wood panels are first subjected to a temperature of 120 • 5~ (48.9 + 2.8~ for 1 h, followed by exposure at - 5 - 2~ ( - 21 • l~ for 1 h, and then a return to room temperature for 30 min. The number of cycles used is a decision by the testing agency.

FLAME RETARDANCE Terminology The technical terminology relating to flame retardance must be understood. The general concept concerned is burning characteristics. (With one exception [1], these standard definitions are found in ASTM Terminology of Fire Standards (E 176). Several interrelated terms are: 9 fire performance test--A procedure that measures the response of a material, product, or assembly to heat or flame under controlled conditions. A fire performance test allows the quantitative description of one or more fire performance characteristics for the specific fire and other parameters of the test. 9 fire resistance--The property of a material or an assemblage to withstand fire or to provide protection from it. 9 fire retardancy--In paint, this is the ability of a paint to retard the spread of flame over a coated suhstrate, usually at the sacrifice of the paint film. 9 fire-retardant coating--A fluid-applied surface covering on a combustible material that delays ignition and combustion of the material when the coating is exposed to fire. o flame resistance--The ability to withstand flame impingement or give protection from it. 9 flame-retardant coating--A fluid-applied surface covering on a combustible material which delays ignition and re-

ASTM M e t h o d D 2 4 8 5 Typical of this kind of testing is ASTM Test Methods for Evaluating Coatings for High-Temperature Service (D 2485). This method provides an accelerated means of determining performance when coatings are exposed to high temperatures. In Method A, for interior service coatings, coated steel panels are heated for 24 h in a muffle furnace at a selected temperature. One panel is plunged immediately into water for thermal shock, while another is cooled and then subjected to a bend test. In Method B, for exterior service coatings, coated steel panels are heated in a muffle furnace at increasing steps of temperature from 400 to 800~ (205 to 425~ One panel is subjected to salt spray for 24 h, while another is exposed outdoors for twelve months. When test exposures are completedelhe panels are examined and evaluated for film ~Deceased, formerly of Harleysville, PA.

667 Copyright9 1995 by ASTMInternational

www.astm.org

668

P A I N T A N D COATING T E S T I N G M A N U A L

duces flame spread when the covering is exposed to flame impingement. 9 flame-spread index--A n u m b e r or classification indicating a comparative measure derived from observations made during the progress of the boundary of a zone of flame under defined test conditions. 9 surface flame spread--The propagation of a flame away from the source of ignition across the surface of a liquid or a solid.

Rationale for Test Selection Note the conceptual difference in the definitions between fire-retardant and flame-retardant coating. Testing coatings for burning characteristics may be carried out to compare the response to flame exposure of different specimens or to evaluate the performance under fire conditions. In the former mode, small-scale testing is appropriate. But, while smallscale testing may be useful in preliminary evaluations in the latter mode, large-scale testing is essential if test results are to be used as a measure of performance in fires in buildings or other structures. Small-scale tests are categorized as flametest standards. Large-scale tests are categorized as fire-testresponse standards. Only the latter are appropriate in fire risk assessments. The scale of test is indicated in each of the following test method descriptions.

Tests for Combustibility A S T M Method E 136 ASTM Test Method for Behavior of Materials in a Vertical Tube Furnace at 750~ (E 136) is a small-scale method designed to identify materials that do not aid combustion or add appreciable heat to an ambient fire. This method is not intended to apply to laminated or coated materials; however, it is useful in evaluating material believed not to be combustible. Figure 1 shows a cross section of the vertical tube furnace. The specimen, nominally 1.5 in. (38 mm) wide by 1.5 in. (38 mm) thick in cross section with thermocouples attached, is inserted in a furnace controlled at 1382 + 10~ (750 +_ 5.5~ and held there until temperatures at the specimen thermocouples are at furnace temperature. The standard contains an instructive commentary section describing the rationale for the test, its limitations, and usefulness of the test results. This test method presently does not contain a precision and bias statement.

A S T M Method D 2863 ASTM Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-like Combustion of Plastics (Oxygen Index) (D 2863) is a small-scale method that measures the minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support flaming combustion. Figure 2 shows the typical equipment layout. Film or thin-sheet test specimens are nominally 2.0 in (52

T H E R M O C O U P L E LEADS T3AND T, (THERMOCOUPLE T= PLACED AT I t~P.ATI~hl r T 1=r IMITIAI

THERMOCOUPL (204ram DOWN) TERMINALS FOR 5( NICHROME WIRE ~ES ~,L ER

HIGH TE INSULA1

l CYLINDER

YLINDER :~COVER SPACING AND SU (3) FOR INNER C~t METAL RING

Inch-Pound Equivalents mm in,

76 3

102 4

152 6 FIG. 1 - C r o s s

254 10 section of furnace assembly.

273 10-V4

6.4 cm 2 1 in.2

1.29 No, 16 Awg

CHAPTER 5 6 - - T E S T I N G COATINGS FOR H E A T R E SISTA N C E

GLASS COLUMN (MINIMUM DIMENSION 450 MM H x 75 MM ID)

METAL SHROU

A~

(OPENING DIAM. = 50 MM)

GLASS COLUMN SECTION A - A OPTIONAL DEVICE FOR RESTRICTING COLUMN OPENING (SEE NOTE 3)

E NOTE 5)

|

1. 2. 3. 4. 5.

BurningSpecimen Clampwith Rod Support Igniter Wire Screen Ring Stand

6. 7. 8. 9. 10,

Glass Beads in a Bed Brass Base Tee Cut-Off Valve Orificein Holder

FIG. 2-Typical equipment layout.

11. 12. 13. 14. 15.

PressureGage PrecisionPressureRegulator Filter NeedleValve Rotameter

669

670

PAINT AND COATING TESTING MANUAL

mm) wide and 5.5 in. (140 mm) long. The minimum oxygen concentration is measured under equilibrium conditions of candle-like burning. The equilibrium is established by the relation between the heat generated from the combustion of the specimen and the heat lost to the surroundings as measured by one or the other of two arbitrary criteria, namely, a time of burning or a length of specimen burned. This point is approached from both sides of the critical oxygen concentration to establish the oxygen index. While this method is useful for comparing resistance to combustion of organic materials, correlation with burning characteristics under actual use conditions is not implied. Precision: In a testing program in which 18 laboratories checked the same five materials, the standard deviation was 0.4 for materials with an oxygen index below 21% and 0.7 to 1.4 for materials with an oxygen index above 21%. Table 1 contains results from another test series. Bias: There are no recognized standards on which to base an estimate of bias. A S T M Method D 1360 ASTM Test Method for Fire Retardancy of Paints (Cabinet Method) D 1360 is a small-scale test method. This procedure measures the relative fire-retardant properties of coatings by determining the weight loss and char index of coated wood panels exposed to flaming ethyl alcohol. It is useful for comparing the burning behavior of coating materials under specified conditions; however, the test results do not always correlate with large-scale spread-of-flame tests, such as ASTM Test Method for Surface Burning Characteristics of Building Materials (E 84). D 1360 is a measure of combustibility, through weight loss, rather than a measure of flame-spread index value. The precision of this method is yet to be determined.

Tests for Flame Spread A S T M Method D 3806 ASTM Test Method for Small-Scale Evaluation of FireRetardant Paints (2-ft Tunnel Method) (D 3806) determines the protection afforded to a substrate by a coating by evaluating the flame spread over the coated surface when the specimen is ignited under controlled conditions in a small tunnel. Figure 3 shows the layout of the 2-ft (610 mm) tunnel apparatus. The test panels are wood, 1/4 by 37/8 by 237/8 in. (6 by 100 by 605 mm). The tunnel apparatus is calibrated with a zeroflame-spread panel and with another reference panel coated with a paint rated for flame spread "by the method of ASTM E 84. A standard gas flame is applied to the lower end of the inclined test panel, and the progress of the flame front upward is observed, measured, and recorded at 15-s intervals. The test results establish a basis for comparing surface-burning characteristics of different coatings without specific consideration of all the end-use parameters that might affect surface-burning characteristics under actual fire conditions.

TABLE I--Precision results. Type

Laboratory-To-Laboratory Standard Deviation

B C D

0.5 to 1,1 0.4 to 1.5 0.5 to 1.4

Within Laboratory Standard Deviation Below 0,2 0.1 to 0.3 (est.) Below 0.6

The method is useful in laboratory comparison of intumescent paint formulations, but large-scale testing by Method E 84 is desirable for correlation with real fire conditions. Precision: Table 2 indicates the repeatability of this method. The standard deviation shown relates to the maximum difference that would be expected between duplicate specimens. The degree of repeatability is dependent on the level of flame-spread ratings. A S T M Method E 162 ASTM Test Method for Surface Flammability of Materials Using a Radiant Heat Energy Source (E 162) is a small-scale procedure, intended for research and development purposes, for measuring the surface spread of flame under exposure to a radiant heat source. Figure 4 shows details of the test equipment. The specimen, which is representative of the material or assembly being evaluated, has dimensions of 6 by 18 in. (150 by 460 mm) by the sheet thickness. It is placed at a downward angle of 30~ from the vertical, facing a gas-fired porous refractory heat-radiating surface, 12 by 18 in. (300 by 460 mm), operating at 1238~ (670~ The rate at which flames travel along the surface depends on the physical and thermal properties of the specimen. The orientation of the specimen is such that ignition is forced near its upper edge and the flame front progresses downward. The time of arrival of the flame front at each of 3-in. (76-mm) marks on the specimen holder is recorded, and observations are made for flashing, dripping, and any other behavior characteristics that appear to be of interest. The maximum temperature rise as indicated by thermocouples in the stack is recorded. The test is completed when the flame front has progressed to the 15-in. (380-mm) mark or after an exposure of 15 rain, whichever occurs first, provided the maximum temperature of the stack thermocouples is reached. A flame-spread index, I~, is calculated as the product of the flame-spread factor, Fs, and the heat evolution factor, Q. F~ is calculated from a plot of I s = FsQ with vertical distance as a function of flame-arrival time at each horizontal mark. The Q factor is calculated from the equation Q ~ CT//3, where C is an arbitrary constant, T is the observed maximum stack-thermocouple temperature difference between the temperature-time curve for the specimen and that for a similar curve of a calibration specimen, and/3 is the mean stack thermocouple rise for unit heat input of a calibration burner. The flame-spread indices determined by this method may be compared with those determined in ASTM Method E 84, although correlation is not necessarily obtained. A precision and bias statement for this method is being developed by ASTM Committee E-5 on Fire Standards. A S T M Method E 84 ASTM Test Method for Surface Burning Characteristics of Building Materials (E 84) is also called the 25-ft (7.6-m) tunnel test, or the Steiner tunnel test, or the NFPA 255 test. A large-scale test, E 84 is applicable to exposed finishes on ceilings or walls. The test is conducted with the material in a horizontal position with the exposed surface facing downward. The purpose of the test is to determine the relative

CHAPTER 5 6 - - T E S T I N G COATINGS FOR H E A T R E S I S T A N C E

671

Materials of Construction and Equipment List Item No

Item

A B C D

Base Plate Framework Side and Front Panels Side Glass

E

Burner Gas Control Valve (not shown) Gage, Gas (not shown)

Item No

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1/2 x 10 x 24 Steel plate 1/g x 1 x Angle iron throughout 1/4Transite cut to specifications 1/s x 2 x 24 Polished Vycor Cut as ShOWn Artificial Gas Model 1/a Orifice, 1" Pipe size, outlet 3 oz/in 2

F G H I

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REAR VIEW

672

PAINT AND COATING TESTING MANUAL TABLE 2~Repeatability. ~

Flame-Spread Ratings

StandardDeviation

Coefficientof Variation

0 to 25 25 to 75 75 to 135 0 to 135

1.2 3.1 3.65 2.85

9.6 6.2 3.5 4.6

aThisinformationwas derivedfromJournalofPaint Technology,Vol.39, No. 511, 1967,p. 495.

burning behavior of the specimen by observing flame spread. The test exposes a nominal 24-ft (7.32-m)-long by 20-in. (508mm)-wide specimen to a controlled air flow and flaming fire exposure adjusted to spread the flame along the entire length of a reference material (select-grade red oak) in 5.5 min. A photoelectric cell is placed at the vent end to detect and

record smoke emission. Figure 5 shows details of the test furnace and the specimen placement. The specimen is exposed to the standard flame conditions for 10 rain or less if the specimen is completely consumed in the fire area and no further progressive burning is evident and if the photoelectric cell reading has returned to the baseline. The flame-spread index (FSI) is determined by calculation from the plotted time-distance curve in relationship to the area under a similar curve for the reference material, select grade red oak. Arbitrarily, red oak is assigned a flame-spread index of 100 and the inorganic reinforced-cement board an FSI rating of zero. The precision and bias of this method has yet to be determined. Several studies have been conducted to examine the relationship of the flame-spread index (FSI) test results obtained for materials with their behavior in large-scale fire-growth

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CHAPTER 56--TESTING COATINGS FOR HEAT RESISTANCE

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FIG. 5 - ( t o p ) T e s t f u r n a c e , s h o w i n g s o m e critical d i m e n s i o n s ( n o t a c o n s t r u c t i o n d r a w i n g ) ; ( b o t t o m ) t e s t f u r n a c e s h o w i n g s o m e critical d i m e n s i o n s ( S e c t i o n " B - B " is n o t a c o n s t r u c t i o n d r a w i n g ) .

673

674

PAINT AND COATING

TESTING

MANUAL

affles

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e x p e r i m e n t s a n d with the fire-test response tory methods. Reports of o t h e r c o m p a r i s o n s e x p e r i m e n t s can be found in the references C o m p a r i s o n s have also been m a d e b e t w e e n 84, E 162, a n d D 3894 [1]. ASTM

of o t h e r laborawith large-scale to this chapter. Test Methods E

Method D 3894

ASTM Test M e t h o d for Evaluation of Fire Response of Rigid Cellular Plastics Using a Small Corner Configuration (D 3894) is i n t e n d e d for evaluation of cellular plastics; however, this large-scale test is useful for c h a r a c t e r i z a t i o n of the b u r n ing b e h a v i o r of coatings a p p l i e d to representative substrates. It simulates configurations of two a d j a c e n t walls o r two adjacent walls a n d ceiling. It is useful in p r e d i c t i n g p e r f o r m a n c e of such m a t e r i a l s in a F a c t o r y M u t u a l L a b o r a t o r i e s full-scale c o r n e r wall test. 2 Figure 6 shows the c o r n e r c o n s t r u c t i o n of the D 3894 setup. The test configuration provides a critical surface g e o m e t r y affording a c o m b i n e d h e a t flux that includes the conductive, convective, a n d radiative responses of a n y specific b u r n i n g material. The s p e c i m e n consists of a corner, s u p p o r t e d b y an outside frame, m a d e from panels each m e a s u r i n g 610 by 1220 by T m m (2 by 4 ft by T in.), T being the thickness of the test panel, a n d a ceiling section of s i m i l a r c o m p o s i t i o n m e a s u r i n g 1220 by 1220 by T m m (4 by 4 ft b y T in.). A s t a n d a r d b u r n e r is used for ignition. Visual observations are made, a n d a r e c o r d of flame-travel distance from the test c o r n e r j u n c t i o n o u t w a r d on the h o r i z o n t a l p l a n e is m a d e at the start of the test a n d at least every 15 s for the test duration. The test is t e r m i n a t e d 2For details contact Factory Mutual Research Corp., P.O. Box 09102, Norwood, MA 02062.

after 20 m i n o r s o o n e r if the flame b e c o m e s uncontrollable. Precision: Tables 3 a n d 4 are b a s e d on a r o u n d r o b i n involving five m a t e r i a l s tested by five laboratories. Bias: There are no

recognized s t a n d a r d s by w h i c h to estimate bias of this test method.

Full-Scale Compartment Fire Tests ASTM

Method E 603

At p r e s e n t there is no ASTM s t a n d a r d test m e t h o d for c o n d u c t i n g full-scale r o o m fire tests, although there is considerable e x p e r i m e n t a l activity. ASTM Guide for R o o m Fire Exp e r i m e n t s (E 603) describes full-scale c o m p a r t m e n t fire exp e r i m e n t s designed to evaluate the fire characteristics of materials, products, o r systems u n d e r actual fire conditions. E 603 is i n t e n d e d as a guide for design of e x p e r i m e n t s a n d for the i n t e r p r e t a t i o n of the results. The guide m a y be used to establish l a b o r a t o r y conditions that simulate a given set of fire c o n d i t i o n s as realistically as possible. TABLE 3--Precision for maximum flame spread for polyisocyanurate cellular plastic, glass-fiber-filled, 1-in. thick, covered with aluminum foil.

Walls and ceiling Walls only

Values Expressed as Percent of the Average

Average, in.

l.*ra

VRb

lrc

IR d

34 27

5 21

13 32

14 59

37 91

~vr is the within-laboratory coefficient of variation of the average. bur is the between-laboratories coefficientof variation of the average. clr = 2.83 v~. dlR = 2.83 Vn.

CHAPTER 5 6 - - T E S T I N G COATINGS FOR H E A T R E SISTAN C E

TABLE 4--Precision for time to maximum flame travel. Average, min

Values Expressed as Percent of the Average uf

vR b

lrc

ll~d

Walls and Ceiling: Material: 4 0.46 12 26 34 74 5 0.56 26 35 74 99 3 1.02 26 58 74 164 1 4.76 18 19 51 54 2 8.51 8 12 23 34 Walls Only: Material: 4 1.34 12 49 34 139 5 2.28 91 95 257 269 3 6.88 10 50 28 141 1 7.39 15 28 42 79 2 9.14 12 15 34 42 Materials: 4 = Rigid polyurethane cellular plastic, 25.4 mm (I in.), w/FR > 25 FS. 5 = Rigid polyurethane cellular plastic, 25.4 mm (1 in.), 2 pcf. 3 = Rigid isocyanurate cellular plastic, 25:4 mm (1 in.), 2 pcf. 1 = Plywood, 1/2 in. AD. 2 = Same as in Table 3. ovr is the within-laboratory coefficient of variation of the average. byR is the between-laboratories coefficient of variation of the average. Cl,. = 2.83 ur. d l R = 2.83 v R .

Analysis of d a t a from r o o m fire experiments can serve two purposes: (1) to relate the severity of the r o o m fire e x p e r i m e n t to the l a b o r a t o r y - m e a s u r e d fire properties, which establishes e m p i r i c a l relationships that validate the strengths a n d the weaknesses of the various fire test m e t h o d s n o w being used o r p r o p o s e d to control the m a t e r i a l s to be used in the rooms; a n d (2) to evaluate m a t e r i a l s a n d p r o d u c t s for acceptability, p a r t i c u l a r l y those m a t e r i a l s for w h i c h t h e r e are no a d e q u a t e m a t e r i a l p r o p e r t y tests. This s t a n d a r d contains a detailed rationale for full-scale c o m p a r t m e n t fire tests. It should be consulted in detail.

Testing Intumescent, Fire-Retardant, and Ablative

structural units t h a t constitute p e r m a n e n t a n d integral parts of a finished building. E 119 is the p r i n c i p a l m e t h o d for evaluating structural integrity of b u i l d i n g assemblies u n d e r actual fire conditions. It is the m e t h o d d e s i g n a t e d in b u i l d i n g codes a n d fire codes to rate structural integrity in fire exposure. The test exposes a full-scale s p e c i m e n within a s t a n d a r d furnace to a s t a n d a r d fire that is controlled to achieve specified t e m p e r a t u r e s t h r o u g h o u t a p r e s c r i b e d t i m e period. The test m e a s u r e s h e a t t r a n s m i s s i o n a n d m e a s u r e m e n t of load-carrying ability of the s p e c i m e n d u r i n g the test exposure. Figure 7 is the time-temp e r a t u r e relationship used in controlling the test. Precision a n d bias of the m e t h o d are not stated. M e t h o d E 119 has b e e n in wide a n d c o n t i n u o u s use since 1917 for fire rating b u i l d i n g assemblies. It has wide acceptance. There are no s t a n d a r d test m e t h o d s for ablative coatings. NASA SP 5014 [2] describes the general topic. Thermogravim e t r i c analysis is useful in u n d e r s t a n d i n g the effect of high t e m p e r a t u r e on inorganic as well as organic coatings.

ASTM Method E 1131 ASTM Test M e t h o d for C o m p o s i t i o n a l Analysis by Thermogravimetry (E 1131) provides a general t e c h n i q u e i n c o r p o r a t ing t h e r m o g r a v i m e t r y to d e t e r m i n e a m o u n t s of volatile matter, c o m b u s t i b l e material, a n d ash content of c o m p o u n d s . It is applicable, using either an inert or a reactive gas environment, to solids a n d liquids in the t e m p e r a t u r e range f r o m r o o m t e m p e r a t u r e to 1830~ (1000~ The test m e t h o d is an e m p i r i c a l technique using t h e r m o g r a v i m e t r y in w h i c h the m a s s of a substance, h e a t e d at a controlled rate in an a p p r o priate environment, is r e c o r d e d as a function of t i m e or temperature.

Historical M e t h o d s The heat a n d fire test m e t h o d s d e s c r i b e d are those m o s t used t o d a y in c h a r a c t e r i z i n g coatings. Historically, a variety of o t h e r bench-scale screening tests have b e e n used, b u t they are no longer in vogue as they do not relate to actual service

Coatings These special-purpose coatings are i n t e n d e d to protect substrates from d a m a g e by fire exposure. I n t u m e s c e n t coatings are designed for cellulosic substrates, as fire-retardant coatings for structural steel substrates, a n d as ablative coatings for the exterior of re-entry space vehicles. See the reference section for m o r e specific references. I n t u m e s c e n t coatings a p p l i e d to w o o d panels are tested for flame-spread index by ASTM E 84. Evaluation of fire-retard a n t coatings for the p r o t e c t i o n of structural steel is cond u c t e d a c c o r d i n g to ASTM E 119.

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ASTM Method E 119 ASTM M e t h o d for Fire Tests of Building Construction a n d Materials (E 119) is a large-scale fire e n d u r a n c e test applicable to assemblies of m a s o n r y units a n d to c o m p o s i t e assemblies of structural m a t e r i a l s for buildings, including loadbearing walls a n d o t h e r walls, partitions, columns, girders, beams, slabs, a n d c o m p o s i t e b e a m a n d slab assemblies for floors a n d roofs. It is also applicable to o t h e r assemblies a n d

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PAINT AND COATING TESTING MANUAL

conditions. Details of the following historical tests m a y be f o u n d in ASTM STP 500 [3]: Schulz fire-retardant tester, NJZ box test, stick a n d wick test, vertical m a t c h test, crib test, firetube test, SS-A-118 acoustical tile test, Schlyter test, a n d the 8-ft t u n n e l test.

Recommendations for Further Reading Additional b a c k g r o u n d material can be f o u n d i n Refs 4-7.

REFERENCES [1] McGuire, J. H., "The Spread of Fire in Corridors," Fire Technology, Vol. 4, No. 2, May 1968, pp. 103-108.

[2] Plunkett, J. D., "NASA Contributions to the Technology of Inorganic Coatings," NASA SP-5014, 1964. [3] Van Heuckeroth, A. W., "Fire Retardance and Heat Resistance," Chap. 6.3, Paint Testing Manual, 13th ed., American Society for Testing and Materials, Philadelphia,1972, pp. 355-365. [4] Babrauskas, V. and Williamson, R. B., "Historical Basis of Fire Resistance Testing," Fire Technology, Vol. 14, Nos. 3 and 4, 1978, pp. 184-198, 304-316. [5] Magee, R. S. and McAlevy, R. F., III, "The Mechanism of Flame Spread," Journal of Fire and Flammability, Vol. 2, 1971, pp. 271297. [6] Shorter, G. W., Ed., Fire Test Performance, ASTM STP 464, p. 1070. [7] Williamson, R. B. and Baron, F. M., "A Corner Test to Simulate Residential Fires," Journal of Fire and Flammability, Vol. 4, April 1973, pp. 91-105.

MNL17-EB/Jun. 1995

Water-Resistance Testing of Coatings by Wayne Ellis* hand, is evaluated on free films.) W a t e r resistance m a y be evaluated as p a r t of o t h e r test regimes, such as exposure testing, w a t e r repellency, corrosion, c h e m i c a l resistance, saltfog resistance, a n d cycling tests such as humidity-cycling a n d light-and-water-exposure testing.

OTHER CHAPTERS IN THIS MANUALcover the influence of w a t e r on the f o r m u l a t i o n a n d d u r a b i l i t y of organic coatings. Recent ASTM p u b l i c a t i o n s a b o u t the m o i s t u r e effects in b u i l d i n g m a t e r i a l s are listed in the B i b l i o g r a p h y at the end of this chapter. These p u b l i c a t i o n s describe m o i s t u r e p r o b l e m s a n d solutions involving coatings a n d their uses. Although this c h a p t e r is i n t e n d e d to describe only the p r i n c i p a l testing a n d evaluation of w a t e r resistance, s o m e general r e m a r k s m a y be helpful in u n d e r s t a n d i n g the test conditions.

TRADITIONAL TEST METHODS

E F F E C T S O N C O A T I N G S OF E X P O S U R E TO WATER AND WATER VAPOR The a d h e s i o n of coatings to substrates is strongly influenced b y the a b s o r p t i o n of w a t e r a n d b y the p e r m e a b i l i t y of the coating to w a t e r vapor. The m e c h a n i s m of this influence proceeds as follows: 1 1. A b s o r p t i o n of w a t e r molecules in the coating film. 2. Inclusion of w a t e r in the interface b e t w e e n film a n d substrate. 3. Blister formation. 4. Corrosion/erosion of the substrate. 5. Flaking o r peeling of the film.

Spot tests a n d i m m e r s i o n tests of coatings a p p l i e d to substrates t r a d i t i o n a l l y have b e e n used as "quick a n d dirty" techniques to c o m p a r e specimens. Criteria for evaluation include softening, blistering, solvation, color change, loss of adhesion, a n d rusting o r o t h e r d e t e r i o r a t i o n of the substrate. These observations should lead to further a n d m o r e c o m p r e hensive testing related to i n t e n d e d conditions of p r o d u c t use. Such testing m a y include exposure to controlled condensation, 100% relative humidity, or w a t e r fog to evaluate moisture blistering resistance. Even m o r e intensive testing m a y involve w a s h a b i l i t y a n d scrub resistance.

SPECIMEN PREPARATION

Although there is no fixed relationship b e t w e e n w a t e r absorption a n d w a t e r v a p o r permeability, generally the h i g h e r the w a t e r a b s o r p t i o n the m o r e p e r m e a b l e the film is to w a t e r vapor. N o r m a l l y the p e r m e a b i l i t y m e a s u r e m e n t s are m a d e on freshly a p p l i e d films. It should be n o t e d t h a t with progressive aging a n d weathering, films b e c o m e m o r e cross-linked, a n d in the case of water-sensitive binders, the water-soluble additives are w a s h e d out b y exposure to r a i n a n d dew. Hence such films will show decreasing p e r m e a b i l i t y with time. Water resistance is defined b y ASTM as " m e a s u r e d ability to r e t a r d b o t h p e n e t r a t i o n a n d wetting by w a t e r in liquid form". 2 Nearly always it is a r e q u i r e d p r o p e r t y for coatings. W a t e r resistance generally is m e a s u r e d on specimens of coatings a p p l i e d to n o m i n a l l y i m p e r m e a b l e substrates such as metal, wood, o r masonry. (Water v a p o r permeability, on the other *Wayne Ellis is deceased. He was a former Chairman of the Board of ASTM and served as a standards consultant and First Chairman of the Board of the Building Environment and Thermal Envelope Council. ~Eric V. Schmid, Exterior Durability of Organic Coatings, Redhill, Surrey: FMJ International Publications, 1988. 2ASTM D 996, Terminology of Packaging and Distribution Environment.

Careful p r e p a r a t i o n of coated specimens is essential to assure a p r o p e r a n d meaningful relationship to field exposure a n d to avoid false test results. The s u b s t r a t e c o m p o s i t i o n a n d surface p r e p a r a t i o n , s p e c i m e n p r e p a r a t i o n , a n d the n u m b e r of specimens should be agreed u p o n b e t w e e n involved parties p r i o r to testing. Applicable m e t h o d s for the p r e p a r a t i o n of test panels are given in ASTM Methods D 609, 3 D 1734, 4 a n d Practices D 1730. 5 Test Methods D 8236 cover a p p l i c a t i o n techniques for the p r o d u c t i o n of u n i f o r m films.

IMMERSION TESTING ASTM Practice for Testing W a t e r Resistance of Coatings Using W a t e r I m m e r s i o n (D 870) describes basic principles aD 609, Practice for Preparation of Cold-Rolled Steel Panels for Testing Paint, Varnish, Conversion Coatings, and Related Coating Products. 4D 1734, Method of Making and Preparing Concrete and Masonry Panels for Testing Paint Finishes. 5D 1730, Practices for Preparation of Aluminum and AluminumAlloy Surfaces for Painting, 6D 823, Test Methods for Producing Films of Uniform Thickness of Paint, Varnish, and Related Products on Test Panels.

677 Copyright9 1995 by ASTM International

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PAINT AND COATING TESTING MANUAL 9

~:~12 " ~

0--Angle of lid, 90 to 125 ~ l~Thermometer and thermostat for controlling heater (Item No. 8) in base 2--Automatic water levelling device 3~Humidifying tOWer C--Automatic temperature regulator for corztrolling heater (Item No. 5) 5--Immersion heater, nonrusting 6.--Air inlet, multiple openings 7--Air tube to spray nozzle 8~Strip heater in ba.se 9--Hinged top, hydraulically operated, or counterbalanced 10--Brackets for rods supporting specimens, or test table 11 ~lnternal reservoir 12--Spray nozzle alcove reservoir, suitably designed, located, and baffled 12AmSprey nozzle housed in dispersion tower located preferably in center of cabinet 13--Water seal 14---Combination drain and exhaust. Exhaust at opposite side of test space from spray nozzle (Item 12), but preferably in coml0ination with drain, waste trap, anti forced draft waste pipe (Items 16, 17, and 19). 1 6 ~ o m p l e t e separation between forced draft waste pipe (Item 17) and combination drain and exhaust (Items 14 and 19) to avoid undesirable suction or back pressure. 17--Forced draft waste pipe 18~Automatir levelling device for reservoir 19--Waste trap 20~Air space or water jacket 21--Test table or rack, well below roof area FIG. l - T y p i c a l

salt s p r a y c a b i n e t (Fig. X1.1 of A S T M B 117).

and operating procedures for testing water resistance of coat-

ings by the partial or complete immersion of coated specimens in distilled or demineralized water at ambient or elevated temperatures. Coated specimens are partly or wholly immersed in water in a container that is resistant to corrosion. The exposure conditions are varied by selecting the temperature of the water and the duration of the test. Failure may be caused by a number of factors, including a deficiency in the coating itself, contamination of the substrate, or inadequate surface preparation. Any effects such as color change, blistering, loss of adhesion, softening, or embrittlement are observed and reported. These test results typically are a pass or fail determination, but the degree of failure also may be measured.

(Fig. l), a water reservoir, a supply of suitably conditioned compressed air, one or more atomizing nozzles, specimen supports, provision for heating the chamber, and necessary means of control. Coated specimens are placed in an enclosed chamber where a water fog surrounds them. The temperature of the chamber is usually maintained at 100~ (38~ The exposure condition is varied by selecting the duration of the test. Failure in water fog tests may be caused by a number of factors, including a deficiency in the coating itself, contamination of the substrate, or inadequate surface preparation. Any effects such as softening (measured by pencil hardness), color change, gloss change, blisters, loss of adhesion, embrittlement, and rusting or corrosion of substrate are observed and reported.

WATER FOG T E S T I N G ASTM Practice for Testing Water Resistance of Coatings Using Water Fog Apparatus (D 1735) covers the basic principles and operating procedures for testing water resistance of coatings in an apparatus similar to that used for salt spray testing. 7 The apparatus required consists of a fog chamber 7ASTM B 117, Test Method of Salt Spray (Fog) Testing.

100% RELATIVE H U M I D I T Y T E S T I N G Practice for Testing Water Resistance of Coatings in 100% Relative Humidity (D 2247) covers the basic principles and operating procedures for testing water resistance of coatings by exposing coated specimens in an atmosphere maintained at 100% relative humidity so that condensation forms on the

CHAPTER 5 7 - - W A T E R - R E S I S T A N C E TESTING OF COATINGS

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specimens. The apparatus (Fig. 2) consists of a test chamber, a heated water tank, and suitable controls. Heated water vapor is generated at the bottom of the chamber, causing saturation of the air immediately above the water tank. As the saturated mixture rises, it cools below the dew point temperature, causing condensation on the specimens suspended above. Condensation is uncontrolled. (For testing at 100% RH with controlled condensation, see ASTM D 4585, described below.) Effects of exposure at 100% relative humidity may be color change, blistering, loss of adhesion, softening, or embrittlement. Any such effects are observed and reported. They may be caused by a number of factors including a deficiency in the coating itself, contamination of the substrate, or inadequate surface preparation. CONTROLLED

CONDENSATION

TESTING

Practice for Testing Water Resistance of Coatings Using Controlled Condensation (D 4585) differs from D 2247 in that the coated specimens are mounted with the uncoated face exposed to room temperature air. The apparatus (Figs. 3 and 4) consists of a test chamber in which the specimens form the roof of a chamber that is fitted with suitable water supply and controls. Water vapor is generated by heating a pan of water at the bottom of the test chamber. The specimens form the roof or walls of the test chamber so that the back sides of the specimens are exposed to the cooling effects of room temperature air. The resulting heat transfer causes water vapor to condense on the coated specimens as liquid water saturated with

m

FIG. 3 - C o n t r o l l e d condensation apparatus (Fig. 1 of A S T M D 4585).

air. The temperature and amount of condensate forming on the specimens are controlled by the test temperature and room temperature. The specimens are inclined so that the condensate runs off the test surface by gravity and is replaced by fresh condensate in a continuous process during the condensate cycle. Exposure conditions are varied by selecting the temperature of the test, the duration of the test, and periodic drying of the specimens. Failure may be caused by a number of factors including a deficiency in the coating itself, contamination of the substrate, or inadequate surface preparation. Any effects such as color change, blistering, loss of adhesion, softening, or embrittlement are observed and reported. CYCLE TESTING Test Method for Finishes on Primed Metallic Substrates for Humidity-Thermal Cycle Cracking (D 2246) covers an accel-

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PAINT AND COATING TESTING MANUAL

which one or m o r e cracks is visible. Evaluation before a n d after exposure m a y be done for checking (D 6608), for cracking (D 6619), for erosion (D 6621~ a n d s o m e t i m e s for w a t e r b e a d i n g (D 292111). ASTM Test M e t h o d for H u m i d - D r y Cycling for Coatings on W o o d a n d W o o d Products (D 3459) is a p r o c e d u r e for evaluation of coatings designed for use on interior w o o d and w o o d p r o d u c t s by exposure alternately to low a n d high h u m i d i t y at an elevated t e m p e r a t u r e . Test panels having a m i n i m u m a r e a of 12 by 12 in. (300 b y 300 ram) are exposed in 48-h cycles in exposure c h a m b e r s m a i n t a i n e d at 97 _+ 2% relative h u m i d i t y a n d 122 ___ 3.5~ (50 -+ 2~ the o t h e r c h a m b e r m a i n t a i n e d at 50 _+ 5% relative h u m i d i t y a n d 73.5 _ 3.5~ (23 _ 2~ At each change of conditions during cycling, the panels are inspected u n d e r strong light for possible d a m a g e or change, w h i c h m a y be in the base m a t e r i a l o r in the coating. Testing for the effect of w a t e r exposure c o m b i n e d with o t h e r exposures is not d e s c r i b e d in this chapter. Such testing includes light-and-water exposure a p p a r a t u s using carbonarc ultraviolet, 12 Xenon-arc/3 a n d fluorescent U V . 14 FIG. 4-Apparatus and cross section (Fig. 2 of ASTM D 4585).

e r a t e d m e a n s for d e t e r m i n i n g the t e n d e n c y of an organic coating to fail by cracking w h e n exposed to h u m i d i t y - t h e r m a l cycling. It is evaluated by alternate exposure of p r e p a r e d s p e c i m e n s in a c a b i n e t m a i n t a i n e d at 100~ (38~ a n d 100% relative h u m i d i t y (with c o n t i n u o u s c o n d e n s a t i o n on the specimens), then in a cold box at - 10~ ( - 23~ allowing only a m a x i m u m of 30 s for transfer. S p e c i m e n s are r a t e d (using a grid overlay) by counting the n u m b e r of grid squares w i t h i n

BIBLIOGRAPHY Lieff, M. and Trechsel, H. R., Eds,, Moisture Migration in Buildings, ASTM STP 779, 1982. Schwartz, T. A., Ed., Water in Exterior Building Walls: Problems and Solutions, ASTM STP 1107, 1991. Trechsel, H. R. and Bomberg, M., Eds., Water Vapor Transmission Through Building Walls and Systems: Mechanisms and Measurement, ASTM STP 1039, 1989.

8D 660, Method for Evaluating Degree of Checking of Exterior Paints. 9D 661, Method for Evaluating Degree of Cracking of Exterior Paints. 1~ 662, Method for Evaluating Erosion of Exterior Paints. HD 2921, Method for Qualitative Tests for the Presence of Water Repellents and Preservatives in Wood Products. 12ASTM D 5031, Practice for Testing Paints, Varnishes, Laquers, and Related Products Using Enclosed Carbon-Arc Light and Water Exposure Apparatus. ~3ASTM G 26, Practice for Operating Light-Exposure Apparatus (Xenon-Arc Type) With and Without Water for Exposure of Nonmetallic Materials. 14ASTM D 4587, Practice for Conducting Tests on Paints and Related Coatings and Materials Using a Fluorescent UV-Condensation Light- and Water-Exposure Apparatus.

Part 13: Specific Product Testing

MNL17-EB/Jun. 1995

Aerospace and Aircraft Coatings by Charles R. Hegedus, 1 Stephen J. Spadafora, 2 David F. Pulley, 2 Anthony T. Eng, 2 and Donald J. Hirst 2

ORGANICCOATINGSARE PRIMARILYapplied to aircraft for environmental protection and appearance. Reference 1 condudes, "The rate controlling parameter for the corrosion of aircraft alloys, excluding the mechanical damage factor, is the degradation time of the protective coating system." This dearly indicates the importance of the coating system's durability and its ability to control corrosion and erosion. Relative to appearance, commercial aircraft benefit from the aesthetic characteristics of the coating system, while military aircraft rely on camouflage properties to minimize enemy detection and tracking during mission operations. To meet operational requirements, aircraft coating systems traditionally consist of a primer and a topcoat. Primers inhibit corrosion of the substrate and enhance adhesion of subsequent topcoats, while topcoats are applied for appearance and to enhance overall durability of the coating system. Self-priming topcoats, which perform as both primer and topcoat in a single coating, have recently been introduced [2,3]. In addition, specialty coatings are strategically applied to perform various functions such as protection against rain erosion, chafing, immersion in fuel, and high temperature. References 4 through 6 provide more detail on the formulation and properties of aircraft coatings. A number of factors affect the performance of aircraft coatings, including the substrate material, the aircraft's operational environment, and flight conditions. Aircraft structures and skins are manufactured from numerous metallic alloys and polymeric composites with a variety of pre-paint treatments, thus complicating the adhesion and corrosion inhibition characteristics of the coating system. Environmental conditions also can vary dramatically (arctic, tropical, mafine, industrial, desert, etc.). Skin temperatures during flight can range from - 5 4 to 177~ ( - 6 5 to 350~ while ground conditions may be relatively benign or highly corrosive. Aircraft type and mission also play important roles in coating system performance. A commuter aircraft that hops from island to island in the tropics sees frequent pressurization and depressurization along with high temperature, humidity, and salt water. In contrast, a military tactical aircraft may fly far fewer hours but will experience extreme structural loads during flight conditions. These flight conditions place environmental and mechanical stresses on the aircraft coating ~Lead Applications Chemist, Air Products and Chemicals, 7201 Hamilton Blvd.. Allentown, PA 18195-1501. 2Chemical Engineers, Materials Engineer, and Materials Engineering Technician, respectively, Naval Air Warfare Center, Aircraft Division, Warminster, PA 18974-0591.

system. Therefore, selection of appropriate test and evaluation procedures is an essential component for determining acceptable coatings for aircraft application.

VISCOSITY The viscosity of aircraft coating components, component mixtures, and raw materials is valuable to the formulator, manufacturer, and applicator for assessment of rheological characteristics. These characteristics affect paint application properties such as atomization, leveling, sagging, and brushability.

Cup Methods (Cup Viscometers) In aircraft coating specifications and at application sites, Zahn and Ford cup viscometers are used for admixed paints because they are inexpensive, easy to use and maintain, and produce practical quantitative data. Although the standmounted, standard Ford cup is more accurate due to its stability, deeper capillary orifice, and larger volume, the diptype Zahn cup is preferred since it is easiest to use and maintain. Zahn and Ford viscometers are described in ASTM Test Method for Viscosity by Dip-Type Viscosity Cups (D 4212) and ASTM Test Method for Viscosity of Paints, Varnishes, and Lacquers by Ford Viscosity Cups (D 1200), respectively.

Brookfield and Stormer Methods Brookfield and Stormer viscometers are rotation-type viscometers described in ASTM Test Method for Rheological Properties of Non-Newtonian Materials by Rotational (Brookfield) Viscometer (D 2196) and ASTM Test Method for Consistency of Paints Using the Stormer Viscometer (D 562), respectively. The Brookfield viscometer, which measures absolute viscosities in centipoises (cP) using a rotating spindle, is particularly effective in determining viscosities of nonNewtonian fluids due to its ability to measure shear stress (that is, torque of rotating spindle) at various speeds and shear rates. Since the cup viscometers described above offer relative simplicity and ease of use, the Brookfield viscometer is not used at application sites. However, it is frequently used as a research tool to characterize the viscosities of polymeric resin materials and dispersions. The Stormer or Krebs-Stormer viscometer uses a rotating paddle to measure relative viscosity expressed in Krebs units.

683 Copyright9 1995 by ASTM International

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PAINT AND COATING TESTING MANUAL

It is designed to provide controlled, uniform, and relative data based on a paddle-stirring motion. This type of viscometer is rarely used in the laboratory or required in current specifications due to the ease of use and maintenance of cup methods and to the accuracy of the Brookfield method.

DENSITY Wet coating density measurements provide a check on the theoretical density value and on the uniformity of the manufactured product. Determination of density by any convenient or suitable method in this Manual is acceptable; however, a weight-per-gallon cup is normally used due to the ease of use of this instrument.

F I N E N E S S OF G R I N D A N D COARSE PARTICLES Fineness of grind and presence of coarse particles are determined to assess the quality and uniformity of the pigment dispersion and coating finish. In order to produce a high gloss coating of good appearance, a paint should be free of coarse particles. However, the extremely low gloss requirements of some aircraft camouflage paints require relatively large particle sizes. Fineness of grind is determined by ASTM Test Method for Fineness of Dispersion of Pigment-Vehicle Systems (D 1210), commonly referred to as the Hegman scale. The coarse particle content is determined by the weight retained on a 325-mesh sieve as specified in ASTM Test Methods for Coarse Particles in Pigments, Pastes, and Paints (D 185).

S O L I D S A N D VOLATILE C O N C E N T R A T I O N / CONTENT General Several analysis techniques are used to determine the total solids, pigment concentration, and volatile concentration of aircraft coatings. This information can be used as a check on the coating composition when compared to the theoretical value as determined from the formulation. It can also be used to determine the quality of an as-received product and its potential surface coverage per volume of paint. In addition, restrictions on the volatile organic compounds (VOC) content of coatings increase the importance of determining the volatile concentration, and methods to determine this value are continuously being developed and refined. The following methods are currently used to determine these compositional properties for aircraft coatings.

Total Solids Content The total solids content of a coating, often referred to as its nonvolatile content, is a measure of the combined polymer and pigment content in a paint. It is typically represented as the weight fraction or percentage of these "solid" components relative to the as-received "wet" coating. For aircraft coatings,

this is determined by subtracting the volatile fraction of the coating from the total to determine the nonvolatile content. The method specified in ASTM Test Method for Volatile Content of Coatings (D 2369) is used. A method that provides a volumetric assessment is ASTM Test Method for Volume Nonvolatile Matter in Clear or Pigmented Coatings (D 2697).

Pigment Concentration Three methods are available to determine the pigment weight concentration within a coating: ASTM Test Method for Determination of Pigment Content of Solvent-Type Paints by High Speed Centrifuging (D 2698); ASTM Practice for Separation of Vehicle from Solvent-Reducible Paints (D 2372); and Federal Standard 141 Method 4021, Pigment Content (Ordinary Centrifuge). These methods use the fact that pigment particles, generally being more dense than the vehicle, will settle under centrifugal force. One distinction between these methods is the variation in rinsing solvent(s) used to separate the polymer from the pigment. Although each method utilizes a different solvent blend, it is common to deviate from the specified method by using solvents which are appropriate for the specific paint under analysis. All three methods result in a quantitative determination of pigment weight concentration in the paint; however, the latter two methods lend themselves to chemical analysis of the pigment sample after this determination.

Volatile Concentration ASTM D 2369 is a standard experimental method to determine the total volatile content of a coating. In contrast, ASTM Practice for Volatile Organic Compound (VOC) Content of Paints and Related Coatings (D 3960), offers a method of calculating the VOC using the nonvolatile content, the water content (if any), and the density of the coating. These latter values are predetermined using referenced ASTM methods, and they are subsequently used in the calculations to determine VOC of the paint. It should be noted that in solventborne coatings the VOC is the volatile content; however, water-borne coatings obviously have a nonorganic volatile component which must he taken into account when performing these calculations. The VOC is typically recorded in units of grams of organic volatiles per liter of paint with pounds per gallon used as an alternate.

Chemical Analysis A variety of methods are used to analyze the chemical composition of aircraft coatings and their components. The specific technique which is selected is determined by the material being analyzed and the level of quantitative or qualitative data required. Some of the more common techniques to analyze polymer and solvent systems are gas chromatography, infrared and ultraviolet spectroscopy, and nuclear magnetic resonance (NMR). Atomic absorption and X-ray spectroscopy are common techniques for determining the chemistry of inorganic pigments. Table 1 provides some of the common ASTM methods used to analyze aircraft coatings.

CHAPTER 5 8 - - A E R O S P A C E AND AIRCRAFT COATINGS

TABLE 1--Analytical methods for aircraft paint components. Compound Ketones Isocyanate

Water Lead, Cadmium, and Cobalt Chromium

ASTMMethod D 2804: Test Method for Purity of Methyl Ethyl Ketone by Gas Chromatography D 3432: Test Method for Unreacted Toluene Diisocyanates in Urethane Prepolymers and Coating Solutions by Gas Chromatography D 4017: Test Method for Water in Paints and Paint Material by Karl Fischer Method D 3335: Test Method for Low Concentrations of Lead, Cadmium, and Cobalt in Paint by Atomic Absorption Spectroscopy D 3718: Test Method for Low Concentrations of Chromium in Paint by Atomic Absorption Spectroscopy

Because of growing concerns over potentially toxic materials, restrictions are being placed on the lead and chromium content of many aircraft paints. These restrictions are in direct conflict with corrosion control requirements since chromate salts, such as strontium chromate, barium chromate, and zinc chromate, have been shown to be excellent corrosion inhibitors for many metals. Therefore, many specifications require analysis for these pigments to ensure either their presence or their absence. Specific methods to determine lead and chromate content are listed in Table 1.

S T O R A G E STABILITY

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used to evaluate specific aspects of the material at that time, such as FTMS 141 method 3011.2, Condition in Container, method 3021.1, Skinning, or method 4208, Evaluating Degree of Settling of Paint. In most cases, it is essential that: (1) the paint be free from skinning, (2) the pigment has not reagglomerated or formed a compacted cake at the bottom of the container and it can be easily redispersed to form a consistent mixture, and (3) the applied coating has properties similar to when it was manufactured.

Accelerated Conditions To speed the evaluation of a coating's behavior under storage conditions, methods have been devised to accelerate this behavior by subjecting the coating to extreme conditions. One common example is specified in FTMS 141 method 3019.1, Storage Stability at Thermal Extremes, which subjects the coating to 49~ (120~ or - 12~ (10~ for 168 h, depending on the type of storage suspected. Methods involving cyclic exposure to high and low temperatures have also been used. With the development of high-performance waterborne coatings for aerospace applications, one major concern is the freeze-thaw stability of these coatings. ASTM Test Method for Freeze-Thaw Resistance of Water-Borne Paints (D 2243) is used to evaluate coating consistency and performance after 17 h at - 18~ (0~ Another technique for predicting storage stability of coatings is by evaluating the settling properties of pigments under accelerated conditions. Aircraft coating specifications rarely contain this type of evaluation; however, centrifugation is often used as a research tool to evaluate the tendency of various pigments to settle and compact in specific vehicles. The centrifugal force and the duration are selected on a case by case basis to ascertain differences between systems.

General The effects of long-term storage on aircraft coating performance are a major concern. Long durations of time and extreme temperatures can have drastic effects on the chemical and physical nature of paints, causing them to have different properties than when they were originally manufactured. Since many aircraft manufacturing, rework, and maintenance activities have paint storage facilities which have only moderate environmental controls, determining the effects of these conditions is necessary. Of primary importance are chemical stability and pigment dispersion. These properties are assessed in the laboratory following long-term and accelerated storage conditions.

FLASH POINT General The flash point of a coating is the minimum fluid temperature at which the solvent vapors are ignited by a spark or flame. It can be predicted roughly as the weighted average of the individual flash points for each of the solvents in a coating formulation. The closed-cup technique is generally preferred since container breakage during shipment and storage often leads to flammable vapors being trapped in a confined space. Naturally, these flash points tend to be lower than the corresponding open-cup values.

Long-Term Evaluation Methods typically used for determination of stability of aircraft coatings are specified in ASTM Test Method for Package Stability of Paint (D 1849), and Federal Test Method Standard (FTMS) 141 method 3022.1, Storage Stability (Filled Container). In these methods, the packaged coating is allowed to sit undisturbed at ambient conditions for an appropriate period of time. (One year is typical for aircraft coatings.) At that time, the coating is reevaluated to compare its physical and optical properties with those originally found for the as-manufactured material. Other methods may be

Pensky-Martens The Pensky-Martens test is a closed-cup method that can be conducted on an admixed coating or on one of the separate components in the liquid (uncured) state. The material may be refrigerated to bring it to a temperature below the expected flash point. It is then placed in a closed metal cup, heated slowly while stirring, and periodically exposed to a pilot flame through a shutter mechanism. A thermometer immersed in the fluid measures the coating temperature. The flash point is the minimum temperature at which the solvent vapors ignite,

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yielding a large flame that propagates over the surface of the fluid. The procedure is covered in ASTM Test Methods for Flash Point by Pensky-Martens Closed Tester (D 93), Method A for clear coatings and Method B for pigmented coatings.

The Setaflash test uses an enclosed apparatus with automatic controls to determine the flash point. It is easier to operate than the Pensky-Martens tester and utilizes an electric heater for efficient heat transfer and greater accuracy. ASTM Test Methods for Flash Point of Liquids by SetaflashClosed-Cup Apparatus (D 3278), Method B describes the test procedure.

drying times of paint films. As many as eight stages of the paint drying process are recognized by coatings authorities. Determination of a particular set of drying properties is important for aerospace coatings due to the strict time and processing constraints placed on production and maintenance painting facilities. No particular drying time parameters are universal throughout the aircraft industry. Each paint facility has acceptable limits for these drying properties depending on their function, schedule, and climatic conditions. For example, set-to-touch time may be more important to a small parts shop where components are handled shortly after painting. In contrast, dry-hard times may be paramount at a production facility for painting entire aircraft that must be flown shortly after painting.

Tag

Set-to-Touch

The Tag tester is similar to Pensky-Martens, except that the cup is placed in a bath containing a mixture of water and ethylene glycol. Only coatings with no suspended solids (such as, fillers) can be evaluated. ASTM Test Method for Flash Point by Tag Closed Tester (D 56) is the applicable test method.

A film is set-to-touch when it clings weakly to the finger under gentle pressure but none of the film transfers to the finger. This property indicates that the painted piece can be handled gently, but excessive contact will diminish the quality of the coating. This property may be considered important to shops painting small aircraft components which must be moved from the application area.

POT LIFE

Tack-Free

Pot life is the length of time in which the flow properties (such as, viscosity) of catalyzed paints will not change within an acceptable range for application. Acceptable coating conditions can vary from no change up to gellation. Pot life requirements in the aerospace industry tend to be controlled by production limitations. Since a normal production work shift is 8 h, many paints have been required to have a pot life spanning this period. More recently, restrictions on the volatile organic content compounds (VOC) of paints have resulted in high solids coatings which tend to inherently possess higher viscosity and shorter pot life (2 to 6 h). Pot life of aircraft coatings is usually determined by measuring viscosity as a function of time after mixing the paint for application. Substantial increases in viscosity are an indication that the pot life is nearly expended. It should be noted that the issue of pot life can be circumvented via the use of plural-component spray equipment, which is becoming more common in the production painting of aircraft. This equipment stores the various components of a multi-component paint separately and then mixes those components at the desired ratio immediately before or at the spray gun. This equipment not only negates pot life as a factor but also minimizes the amount of catalyzed multi-component paint that is wasted due to unused material. The only drawback is that this equipment is most effectively and efficiently used when large volumes of paint (5 gal or more) are used as opposed to small quantities (1 gal or less), which are typical of touchup maintenance operations.

Basically, tack is the tenacity of the film to cling to foreign objects. This component of drying is not considered of major importance in production painting; however, it may be used to ascertain the overall drying characteristics of a coating.

Setaflash

Dry-to-Recoat This stage of drying is considered to be the most important for painting of aircraft because it is one of the major factors controlling the production rate. Dry-to-recoat is the time at which a second coat, or specified overcoat (such as, topcoat), can be applied without developing irregularities in the coating system, such as lifting, blistering, or loss of adhesion. If overcoated prior to the recoat time, these defects can be caused by a number of factors, one of which is the trapping of solvent in the original coat.

Dry-Hard The most common technique is to squeeze or pinch the coated surface with the thumb and forefinger with maximum pressure. The dry-hard time is when this procedure can be performed without leaving a permanent mark on the coating surface. This drying stage is also important in production painting of aircraft since painting is usually the last stage of maintenance and the aircraft can be flown after the coating is dry-hard.

FILM THICKNESS DRYING TIME ASTM Test Method for Drying, Curing, or Film Formation of Organic Coatings at Room Temperature (D 1640) defines

Aircraft coatings are developed and manufactured to be applied within acceptable thickness limits. Coatings which are applied outside of these limits will not exhibit their opti-

CHAPTER 5 8 - - A E R O S P A C E AND AIRCRAFT COATINGS mum performance properties. For example, thin coatings may not have the required tensile and shear strength, making them susceptible to cracking and chipping, especially around fastener heads and at panel seams. In contrast, coatings which are too thick may lack flexibility and can exhibit excessive internal stresses. Another important consideration is the excessive weight that coatings add to an aircraft, causing additional fuel consumption. Therefore, coating thickness is diligently controlled and measurements are performed in both the wet and dry states. Wet coating films are checked for thickness during aircraft application processes using various wet film thickness gages according to ASTM Test Method for Measurement of Wet Film Thickness of Organic Coatings (D 1212). A tooth gage is commonly used in the aerospace industry since it is simple to use, inexpensive, and can be used to obtain good approximations of dry film thickness. These approximations are made by multiplying the wet film thickness by the coating volume percent solids. The thickness of cured aircraft coatings can be determined either destructively or nondestructively. Destructive methods involve the chipping or cutting of dry films from a substrate and subsequent measurement of the coating thickness with a gage. This approach is typically not taken for aircraft coatings since the coating-substrate bond must be destroyed. Nondestructive techniques include micrometer, eddy current, magnetic induction, and a magneto-resistor/thermistor system. These are the most desirable methods of dry film thickness determination used in the aerospace industry, both in the field and in the laboratory due to their accuracy and ease of use. One problem encountered is that aircraft are constructed of a number of types of materials. Therefore, the method used to determine coating thickness may depend on the substrate material, especially if a nondestructive method is used.

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tant. In addition, color change is often used to assess the effects of environmental exposure (weathering, immersion in fluids, etc.) on aircraft coatings. Means of characterizing aircraft coating colors range from qualitative visual assessment to quantitative measurements with instruments, the latter providing the obvious advantage of consistent, numerical results. For the latter, ASTM Test Method for Calculation of Color Differences from Instrumentally Measured Color Coordinates (D 2244) is followed, resulting in a color description in either the XYZ or Lab color coordinates. Color differences, as qualified by a delta E value, are determined by using these color coordinates as described elsewhere in this manual to determine delta E. One additional note must be made with respect to color measurement of aircraft coatings. Colorimeters and spectrophotometers can both be used to quantitatively measure color. Colorimeters use a specified light source with defined radiation wavelengths and intensities, typically resulting in a direct output of XYZ and/or Lab color coordinates. Spectrophotometers measure light reflected from the coating surface over an entire wavelength region. (In the case of color characterization, this is the visible region of the spectrum.) Of the two, colorimeters are less expensive and easier to use. However, colorimeters do not account for metamerism. Metamerism is a phenomenon that occurs when an object appears to be different colors under different light sources. This effect is observed with some coatings for military aircraft, which use unconventional pigments to obtain camouflage properties. In order to confirm that two colors are the same, their total reflectance over the entire visible spectrum must match. These data are obtained using a spectrophotometer. In addition, spectrophotometers are frequently used for military aircraft coatings to determine their reflectance characteristics outside of the visible region, most commonly the infrared and ultraviolet regions.

OPTICAL PROPERTIES General The appearance of an applied coating is determined by a number of optical properties (color, gloss, opacity, etc.). In the commercial aerospace industry, where public opinion is a consideration, these optical properties can strongly affect the initial impression of the aircraft and are considered to be important with respect to customer recognition. The optical properties of coating systems for military aircraft affect the detectability of these aircraft and thus effect their survivability in potentially threatening scenarios. In both the commercial and military sectors, optical properties are specified with strict tolerances and are closely monitored with instrumentation as well as with the naked eye.

Color The colors used on commercial aircraft tend to be strong and vibrant for recognition and attraction. The colors of military aircraft coatings are empirically selected in order to achieve either theater-specific (desert, forest, arctic, etc.) or multi-theater (world-wide) camouflage paint scheme requirements. In either case, paint color is considered to be impor-

Opacity (Contrast Ratio) Opacity or hiding power is the ability of a coating to mask the underlying substrate. Opacity of aircraft coatings is usually quantified by its contrast ratio according to ASTM Test Method for Hiding Power of Paints by Reflectometry (D 2805). In this method, the coating is applied over white and black substrates. (Contrast ratio or Leneta charts are typically used.) A colorimeter is used to measure the coating's luminous reflectance (Y) on the black and white surfaces. Contrast ratio is then determined by: Contrast ratio = Y(hlack)/Y(white) Contrast ratio is a function of coating dry film thickness. The method must define the coating thickness at which the contrast ratio is determined; 2.0 mils is common for aircraft topcoats. Applying a coating to a precise thickness is difficult and may require several attempts. An alternative approach is to determine the contrast ratio at several coating thicknesses and subsequently fit the data to a quadratic equation. This equation can then be used to determine the contrast ratio at the desired coating thickness.

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PAINT AND COATING TESTING MANUAL

Reflectance and Gloss Incident light which is reflected from a surface has two components, specular and diffuse. Specular reflectance or gloss represents light that has been reflected off a surface when the angle of incidence equals the angle of viewing. Diffuse reflectance is light reflected in all directions other than specular. Specular reflectance data are usually obtained via glossmeters whereas diffuse reflectance data are obtained by the use of goniophotometers. Gloss of aircraft coatings is measured by instruments that have been standardized to either 60 ~ or 85 ~ incident angles according to ASTM Test Method for Specular Gloss (D 523). Commercial aircraft coatings have high gloss and are usually characterized at 60 ~. In contrast, many military aircraft are painted with low gloss coatings for camouflage purposes. In addition to gloss analysis at 60 ~, military coatings are also analyzed at 85 ~ in order to minimize glare at grazing angles. Directional reflectance represents the optical data that are obtained from analyzing reflected diffuse light. This type of data are normally obtained via goniophotometric equipment. Several methods describe procedures for these measurements, including ASTM Test Method for Directional Reflectance Factor, 45 deg, 0 deg, of Opaque Specimens by BroadBand Filter Reflectometry (E 1347) and Practice for Goniophotometry of Objects and Materials (E 167). The use of diffuse reflectance of aircraft coatings is typically limited to those for tactical military functions which require camouflage properties.

ADHESION General For aircraft coatings to provide maximum protection against degradation, they must firmly adhere to their substrate. Because of the complex nature of adhesion, various techniques have been devised to determine the adhesive characteristics of coatings. Since a coating's adhesion to its substrate can be significantly affected by environmental conditions, these tests are often performed after exposure to accelerated conditions such as immersion in water at elevated temperature. Common adhesion tests used to evaluate aircraft coatings are tape and scrape tests. Other techniques, such as mechanical peel, tensile, and shear tests, are used less frequently.

Tape Tests Adhesion tape tests are described in ASTM Test Methods for Measuring Adhesion by Tape Test (D 3359). These are the easiest and most versatile of the adhesion tests because they can be conducted in both laboratory and field environments. Tape tests are performed by cutting through the applied coating, into the substrate. A variety of scribe patterns can be used. One common pattern (described in the "A" method of D 3359), which is used to evaluate aircraft coatings, is two scribes forming an "X." An alternative pattern (described in the "B" method of D 3359), is formed by scribing eleven parallel lines through the coating, followed by a second set of eleven lines which are perpendicular to the first set. These

scribe lines create a matrix of 100 squares. Adhesive tape is firmly applied over the scribe area and is subsequently removed with a quick upward motion. Coating adhesion is then characterized using standard ratings for the amount of coating removed according to the ASTM method. As mentioned above, the coated specimens can be subjected to a specific environment prior to testing adhesion characteristics. This practice is most commonly performed with the tape tests. A typical exposure for aerospace coatings is a 24-h immersion in water at 21~ (70~ More severe exposures are becoming common as coating technology advances. One example is immersion in water at 65~ (150~ for seven days. Immediately upon removal from the water, the coating is scribed and tested.

Scrape Adhesion Test The scrape adhesion test is described in ASTM Test Method for Adhesion of Organic Coatings by Scrape Adhesion (D 2197). Scrape adhesion is used to characterize the adhesive and shear strength properties at the primer-substrate interface. The instrument used for this test is the Gardner Labs Scrape Adhesion Test Apparatus (Model SG- 1605). The specimen for this test has an area which is uncoated with the substrate exposed. The test is performed by guiding a weighted stylus at a 45 ~ angle to the specimen along the exposed substrate into the coating system. The scrape adhesion value is recorded as the heaviest weight used without shearing the coating from the substrate. Typical scrape adhesion values for aerospace primers fall into the 3 to 5-kS range. The scrape adhesion test is also used to determine the intercoat adhesion between a topcoat and a primer. When testing this property, a specimen with a section of primer exposed is used and the stylus is guided across the primer into the topcoat. Typical scrape adhesion values for aerospace topcoats also fall into the range of 3 to 5 ks. This test can also be performed after exposure to severe environmental conditions.

Peel and Tensile Tests Peel test methods are rarely used for aircraft coatings. Instead, a modified version of an adhesive peel test is occasionally used. In this test, two thin metal strips are bonded together using the coating as the adhesive. One end of the specimen is left unbonded, and the metal strips at this end are separated to form a "T." They are subsequently pulled apart in a tensile machine, and the force needed to pull the two strips apart is recorded as the adhesive peel strength of the coating. The type of failure (adhesive or cohesive) is also recorded. An adhesive failure indicates that the coating strength exceeds that of the coating-substrate adhesive strength. The most common tensile adhesion test used on aerospace coatings is a tensile pull-off test, also referred to as a button test. In this test, a flat head paten is bonded to the surface of the coating with an adhesive. A tensile force is then applied to the paten perpendicular to the coating surface until the paten is removed. The location at which the paten is removed from the surface must be carefully examined. In many cases, the adhesive which bonds the paten to the coating will fail or the coating may fail cohesively. In these cases, it can only be

CHAPTER 58--AEROSPACE AND AIRCRAFT COATINGS stated that the adhesive strength of the coating exceeds the tensile strength recorded. Only if the coating fails adhesively is the recorded tensile strength that of the coating-substrate adhesion.

689

heights. The weight of the impact cylinder and the highest height that causes no cracking or disbondment of the coating are used to calculate an impact strength, usually in inchpounds. This test can be performed directly on the coated side of the panel or, like the GE impact test, on the reverse, uncoated side.

FLEXIBILITY General Flexibility is an important property for aerospace coatings, particularly at low temperatures ( - 51~ which is common for aircraft cruising at high altitudes. Cracking of coatings at skin joints and around fastener heads can lead to corrosion of exposed areas. Corrosion inhibiting epoxy primers used on aircraft tend to be brittle and exhibit poor flexibility, whereas the urethane topcoats are generally more flexible, especially at low temperatures. Sealant materials and elastomeric primers occasionally are used to improve the overall flexibility of the paint system.

Mandrel Bend The flexibility of high-performance coatings is commonly characterized by the mandrel bend test method outlined in ASTM Test Methods for Mandrel Bend Test of Attached Organic Coatings (D 522). This test is normally conducted at low temperatures such as - 51~ and is performed by bending a painted specimen 180~ around mandrels of various diameters with the coating aligned away from the mandrel. After allowing the specimen to return to room temperature, the coating is examined for cracking along the bend. The smallest (most severe) mandrel diameter which the coating can withstand without cracking is recorded. One standard military specification requirement for this test on low gloss coatings is a 2-in. mandrel bend while more flexible coatings can withstand mandrel bends of 88 to 88 in. at low temperatures without cracking.

Tensile (Elongation) and Fatigue Tests For aerospace coatings, free films of the coating generally 5 mils thick or greater are prepared and tested for tensile characteristics. Epoxy primers have relatively high tensile strengths (>2500 psi) but very poor elongations (

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