[PVC]PVC Technology

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ADMIN: I.W

PVC TECHNOLOGY Fourth Edition

PVC TECHNOLOGY Fourth Edition W. V. TITOW M. Phil., Ph.D., C.Chem., F.R.S.C., F.P.R.I., C. Text., A. T.!. Formerly of the Yarsley Research Laboratories Ltd, Ashtead, Surrey, England

ELSEVIER APPLIED SCIENCE PUBLISHERS LONDON and NEW YORK

ELSEVIER APPLIED SCIENCE PUBLISHERS LTD Ripple Road, Barking, Essex, England Sole Distributor in the USA and Canada ELSEVIER SCIENCE PUBLISHING CO., INC. 52 Vanderbilt Avenue, New York, NY 10017, USA

First edition Second edition Reprinted Third edition Fourth edition Reprinted

1962 1966 1967 1971 1984 1986

British Library Cataloguing in Publication Data PVC technology. -4th ed. 1. Polyvinyl chloride I. Titow, W. V. 668.4'236 TP1180.V48

ISBN-13: 978-94-010-8976-0 e-ISBN-13: 978-94-009-5614-8 DOl: 10.1007/978-94-009-5614-8 WITH 171 TABLES AND 230 ILLUSTRATIONS

©

ELSEVIER APPLIED SCIENCE PUBLISHERS LTD 1984

Softcover reprint of the hardcover 1st edition 1984

Special regulations for readers in the USA This publicatiQ1l .has been registered with the Copyright Clearance Center Inc, (G;~c), Salem, Massachusetts. Information can be obtained from the CCC 'about conditions under which photocopies of parts of this publiCatio~ may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. The selection and presentation of material and the opinions expressed in this publication are the sole responsibility of the authors concerned All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher.

Preface to the Fourth Edition

This book continues the tradition of the first two editions of the late W. S. Penn's original PVC Technology, and the extensively revised third (1971) edition prepared by myself and B. J. Lanham. In the present edition the original general format, and the arrangement of chapters, have been largely preserved, but virtually nothing now remains of Penn's own text: a part of the contents is based on material from the 1971 TitowlLanham version (revised, updated and mainly rewritten): the rest is new, including, inter alia, several chapters specially contributed by experts from the plastics industry in the UK and Europe. The section listing international (ISO) and national (BS, ASTM and DIN) standards relevant to PVC, which was first introduced (as Appendix 1) in the 1971 edition, proved a popular feature: it has now been brought up to date and considerably extended. Two further appendices provide, respectively, comprehensive unit conversion" tables (with additional information on some of the most frequently encountered units, and the SI units), and a list of many properties of interest in PVC materials, with definitions, typical numerical values, and references~to relevant standard test methods. For various reasons, work on this edition involved more than the usual quota of problems: I am truly grateful to the Publisher's Managing Editor, Mr G. B. Olley, for his understanding, patience, unfailing courtesy and friendly encouragement. I am also most appreciative of the helpful attitude of all other members of the Publisher's staff who were concerned with the various aspects of processing the manuscript and bringing the book out. If my own contribution to the book has any merit, then I would like to dedicate it-respectfully and affectionately-to all my friends of the Yarsley Laboratories with whom I was priviledged to share many happy years, participating in the worthwhile work of a good team. W.V.T. v

Acknowledgements

I am much indebted to Messrs W. B. Duncker, F. J. Olivier and D. J. Sieberhagen of Vynide Ltd for their most helpful comments on the draft of Chapter 18 and for the trouble they took-individually and severally-to provide the drawing for Fig. 18.3, data for Table 18.1, and a few items of information on certain practical aspects of calendering. I am also grateful to Mr J. M. Hofmeyr of Union Carbide for the information he kindly supplied on the Ucar range of copolymer resins, and for his permission to use it in Chapter 24. It is a pleasure to record my thanks to Mr R. Coates of AECII Chlor-Alkali and Plastics Ltd for a most useful discussion of the scripts of Chapters 2 and 3, for items of information I have used in Tables 2.5 and 2.6, and for arranging his Company's permission-which I very much appreciate-to reproduce from their technical literature the contents of Tables 3.4-3.7. For the illustrations contained in the Plates my thanks are due to the companies and/or individuals identified in each caption, who kindly provided the original photographs. A small number of graphs and drawings, and one table (Table 14.1), are straight reproductions from other publications: the copyright holders' and authors' permissions to use these items-which are mentioned in each individual case-are much appreciated. A few definitions and sets of numerical data have been directly quoted (with sources clearly identified) from ISO, British and ASTM Standards. Such material from ISO specifications is reproduced by permission of the British Standards Institution granted on behalf of the International Organisation for Standardisation. The extracts from Britvii

viii

Acknowledgements

ish Standards are reproduced by permission of the British Standards Institution, 2 Park Street, London W1A 2BS, from whom complete copies of the standards concerned can be obtained. The material from ASTM Standards is copyright the American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103, and is reprinted with permission. I am particularly grateful to Mrs Rene Chizlett-whose invaluable secretarial contribution to the previous edition was greatly missed with the present one-for timely help with last-minute verification of several items of information on suppliers of commercial PVC materials. Mrs Connie von Gernet typed most of the manuscript-it is a pleasure to acknowledge her professional assistance. I am also most appreciative of Mrs Micky Kruger's secretarial help with two of the chapters and urgent correspondence. No aid on the technical side could be more important than the support and patience of my wife, Margaret Ley-Titow, during the long period, not lacking in stress, when the book was being put together. For all she has done she has my truly appreciative thanks. W.V.T.

Contents

Preface

v

Acknowledgements

vii

List of Contributing Authors

xxix

Chapter 1 Introduction-W. V. TITOW . . . . . . . . . . . . . 1.1 PVC: General Terminology and Relevant Definitions 1.2 Early History and Development of PVC . . . . . 1.3 General Statistics 1.4 Outline of the PVC Sector of the Plastics Industry 1.5 Vinyl CWoride Polymers and Copolymers . . . . 1.5.1 PVC Homopolymers: chemical structure; morphology 1.5.2 Vinyl CWoride Copolymers . . . . . . . 1.5.3 'External' Modification of PVC by Other Polym-

1 1 4 10 12 13 13 19

1.5.4 Properties of PVC Compositions 1.6 CWorinated Polyvinyl Chloride (CPVC) 1.7 Material and Test Standards References . . . . . . . . . . . . . . .

21 24 24 29 30

Chapter 2 Commercial PVC Polymers-W. V. TITOW

37

ers

ix

x

Contents

2.1 Introduction-Production and Main Types . 2.2 Polymer Characteristics Cardinal to Behaviour in Processing and/or Service Performance . 2.2.1 Composition . . . . . . . . . . . . . . 2.2.2 Molecular Weight (Viscosity Number and K Value) . 2.2.3 Polymer Particle Characteristics: particle size and size distribution; particle shape and morphology . 2.2.4 Purity . . . . . . . . . . . . . . . . 2.3 Characterisation and Designation of Commercial PVC Polymers . 2.4 Examples of Basic Properties of Commercial Polymers as Used for Some Major Applications . . 2.5 Commercial Sources of PVC Polymers References. . . . . . . . . . . . . . . . Chapter 3 Commercial PVC Compounds-W. V. TITOW 3.1 Introduction . 3.2 Commercial Sources of PVC Compounds 3.3 Types and Applications of Commercial PVC Compounds . 3.4 Properties and Designation of Commercial PVC Compounds . 3.4.1 Designation . 3.4.2 Properties Used in Characterisation of PVC Compounds ' . 3.4.3 Some Typical Properties of Commercial PVC Compounds References Chapter 4 Elementary Principles of PVC Formnlation-W. V. TITOW 4.1 Introduction . . . . . . . . . . . . . . . . . . . 4.2 The Components, and Basic Types, of a PVC Formulation . 4.3 Formulation Costing-Basic Points . . . . . . . . . . 4.4 Main General Considerations in the Selection of Principal Formulation Components . . . . . . . . . . . . . .

37

41 42 43

46 48 49 55 55 57

59 59 60

61 63 63

65 65

78

79 79

81 83 85

Contents

4.4.1 Nature and Characteristics of Individual Components of a Formulation: PVC polymer; heat stabilisers; plasticisers; lubricants; polymeric modifiers; fillers; colourants 4.4.2 Interactions and Mutual Effects of Formulation Components: compatibility effects; synergism; other mutual effects 4.4.3 Side Effects of Formulation Components: 'secondary functionality' effects; undesirable sideeffects 4.5 Some Special End-use Requirements 4.5.1 Food-contact Applications 4.5.2 Resistance to Weathering 4.5.3 Electrical Insulation 4.6 Examples of Basic Formulations 4.6.1 Film and Sheeting 4.6.2 Calendered Plasticised Vinyl/Asbestos Flooring 4.6.3 4.6.4 4.6.5 4.6.6 4.6.7 4.6.8 4.6.9

(Tile~

Pipe and Tubing Cable Covering and Insulation Gramophone Records Blow-moulded Bottles Injection Mouldings Extruded Profile Paste Formulations

Chapter 5 Theoretical Aspects of PlasticisatiOD'--D. L. BUSZARD

5.1 5.2 5.3 5.4 5.5 5.6

General Introduction . . . . . . . . . Definition of Plasticisers and Plasticisation Chemical Nature of Plasticisers Theories of Plasticisation Stages of Plasticiser Interaction with PVC Polymer Requirements for PVC Plasticisers 5.6.1 Compatibility and Miscibility: the IL value; solubility parameter 8; clear point temperature; Flory-Huggins interaction parameter x; Ap/Po ratio; loop or roll compatibility tests; maximum torque temperature

xi

86 103 105 106 106 107 107 107 107 109 110 111 112 112 113 114 114

117 117 117 119 120 122 124

125

xii

Contents

5.6.2 Effectivity of Plasticisers 5.6.3 Permanence of Plasticisers 5.7 General Relationships between the Structure of Plasticisers and their Behaviour in PVC 5.8 Ageing of Plasticised PVC 5.9 Antiplasticisation References . . . . . . . . .

132 134 136 138 142 143

Chapter 6 Commercial Plasticisers-D. L. BUSZARD 6.1 Introduction . . . . . . . . . . . . 6.2 Classification of Commercially Available Plasticisers 6.3 Group Characteristics of Major Plasticiser Gasses 6.4 Synonyms and Abbreviations 6.5 Group 1 Plasticisers-Phthalates . 6.5.1 Lower Phthalates . . . . 6.5.2 General-purpose Phthalates 6.5.3 Linear Phthalates 6.5.4 Higher Phthalates 6.5.5 Miscellaneous Phthalates 6.6 Group 2 Plasticisers-Phosphates 6.6.1 Triaryl Phosphates . . . 6.6.2 Trialkyl Phosphates 6.6.3 Mixed Alkyl Aryl Phosphates 6.6.4 Halogenated Alkyl Phosphates 6.7 Group 3 Plasticisers-Trimellitates . 6.8 Group 4 Plasticisers-Aliphatic Diesters 6.9 Group 5 Plasticisers-Polymeric Plasticisers 6.10 Group 6 Plasticisers-Miscellaneous Plasticisers 6.10.1 Epoxy Plasticisers 6.10.2 Chlorinated Paraffins 6.10.3 Monoesters . . . . 6.10.4 Glycol Esters 6.10.5 Hydrocarbon Extenders 6.10.6 Other Miscellaneous Plasticisers 6.11 Storage and Handling of Plasticisers 6.12 Plasticiser Manufacturers References . . . . . . . . . . . . . .

147 147 147 148 148 152 152 153 153 156 158 159 159 160 161 163 163 163 165 170 170 171 173 173 174 174 175 180 180

Conren~

Chapter 7 Properties of Plasticised PVC-D. L. BUSZARD 7.1 Introduction . . . . . . . . . . . . . . 7.2 Formulation of a Plasticised PVC Compound 7.2.1 The 'Desirability Function' 7.2.2 Computer-assisted Formulating 7.3 Softness and Tensile Properties ..... 7.3.1 Effect of Plasticiser 7.3.2 Compounding at Equal Efficiency 7.4 Low-temperature Properties 7.5 Permanence Properties 7.5.1 Extraction Resistance 7.5.2 Migration Resistance 7.5.3 Volatile Loss 7.5.4 Automotive Fogging 7.5.5 High-humidity Compatibility 7.6 Flame-retardant Properties 7.7 Electrical Properties 7.8 Weathering and Light Stability . . . 7.9 Resistance to Microbiological Attack 7.10 Resistance to Insect and Rodent Attack 7.11 Stain Resistance . . . . . . . . . . 7.12 Toxicity and Health Aspects of Plasticisers 7.12.1 Plasticisers for Food-contact Application 7.12.2 Health and Safety References . . . . . . . . . . . . . . . . . . . Chapter 8 FiDers in PVC-I. D. HOUNSHAM and W. V. TITOW

8.1 Introduction . . . . . . . . . . . . . . . 8.2 Mineral Fillers . . . . . . . . . . . . . . 8.2.1 Silicates and Silicas: asbestos; talc; clay 8.2.2 Alkaline-earth Metal Sulphates 8.2.3 Calcium Carbonates 8.3 Calcium Carbonate Fillers-Nature, Properties and Applications . . . . . . . . . . . . . . . . . . . . . 8.3.1 General Types: whiting; ground limestone, marble and calcite; ground dolomite; precipitated calcium carbonates

xili

181 181 181 183 183 184 185 185 192 195 196 199 200 202 204 204 206 206 208 209 209 210 210 211 212 215 215 216 216 219 221 224 224

xiv

Contents

8.3.2 Surface Treatments: stearate treatments; organotitanate treatments; proprietary and miscellaneous treatments 8.3.3 Filler Properties and Selection Criteria: maximum particle size; particle size distribution and mean particle size; colour (dry brightness); refractive index; oil (or plasticiser) absorption; dispersion characteristics; cost. 8.3.4 Applications, and Effects of Filler Loading: flooring; plasticised compounds; rigid PVC 8.4 Functional Fillers 8.4.1 Reinforcing Fillers: asbestos (chrysotile) fibres; inorganic microfibres; glass fibres; carbon fibres; glass spheres; fine-particle calcium carbonate 8.4.2 Flame-retardant and Smoke-suppressant Fillers 8.4.3 Miscellaneous Functional Fillers: carbon black; metal powders; wood flour; starch; synthetic silicas 8.5 Some Filler Suppliers and Trade Names References . . . . . . . . . . . . . . .

225

228 232 240 240 247 248 251 253

Chapter 9

Stabilisers: General Aspeds-W. V. TITOW 9.1 Introduction . . . . . . . . 9.2 Degradation of PVC Polymer 9.2.1 Thermal Degradation 9.2.2 Photochemical Degradation 9.3 Ideal Requirements for a Stabiliser, and General Factors Affecting Stabiliser Selection 9.4 Heat Stabilisers 9.4.1 Lead Compounds 9.4.2 Organotin Stabilisers: chemical nature and types; characteristics and applications 9.4.3 Compounds of Other Metals: metal compounds with stabilising effects in PVC; composite metal stabilisers 9.4.4 Organic (Miscellaneous) Stabilisers: esters of aminocrotonic acid; urea derivatives; epoxy compounds; organic phosphites; miscellaneous organic co-stabilisers

255 255 256 256 260 261 263 265 270 275

286

Conren~

9.5 Antioxidants and UV Absorbers 9.5.1 Antioxidants 9.5.2 UV Absorbers 9.6 Main Modes of Stabiliser Action 9.6.1 Lead Stabilisers . . . . 9.6.2 Organotin Stabilisers . . 9.6.3 Other Metal-based Stabilisers 9.6.4 Organic Stabilisers, Antioxidants, UV Stabilisers 9.7 Some General Features and Common Faults of Stabilised Compositions . . . . . 9.7.1 Plate-out . . . . . . . . . . . . 9.7.2 Sulphide Staining 9.8 Testing and Evaluation of Stabiliser Effects 9.8.1 Concept of Stability in Processing, Service and Tests 9.8.2 Heat Stability Testing 9.8.3 Light Stability Testing 9.9 Detection and Analysis of Stabilisers References . . . . . . . . . . . . . . Chapter 10 Commercial Stabillser Practice-P. S. COFFIN

10.1 10.2 10.3 10.4

Introduction Choosing a Commercial Stabiliser . . . The Importance of a Well-balanced Lubricant System One-pack Systems and the Physical Form of Stabiliser Products . . . . . . . . . . . . . . . . . . . . . 10.5 Hygiene and Environmental Considerations . . . . . 10.6 UK Stabiliser Manufacturers-Product Ranges and Applications 10.6.1 Associated Lead Manufacturers Ltd 10.6.2 Ciba-Geigy Ltd . . . . . . . . . ..... 10.6.3 Durham Chemicals Ltd 10.6.4 Diamond Shamrock Polymer Additives Division 10.6.5 Victor Wolf Ltd References . . . . . . . . . . . . . . . . . . . . . "

xv

292 292 294 299 299 300 302 304 305 305 308 311 311 315 328 330 330 335 335 337 339 340 341 342 342 346 348 351 356 356

Chapter 11 Some MisceUaneous Components of PVC Formulations-W.

V. TITOW . . . 11.1 Lubricants . . . . . . . . . . . . . . . . . . . .

359 359

xvi

Contents

11.1.1 Functions, Nature and Effects 11.1.2 Interaction and Co-action of Lubricants with Other PVC Formulation Components: lubricant/stabiliser effects; mutual effects of lubricants and plasticisers; effects of polymeric modifiers; effects of fillers and pigments 11.1.3 Assessment of Lubricant Effects 11.1.4 Sources of Information on Lubricants and their Commercial Suppliers 11.2 Polymeric Modifiers 11.2.1 Processing Aids 11.2.2 Impact Modifiers: impact resistance-its nature, significance and measurement; the impact resistance of PVC; the nature, effects and applications of polymeric impact modifiers for PVC 11.3 Colourants 11.3.1 General Nature and Functioning 11.3.2 General Classification 11.3.3 Forms in which Colourants are Available 11.3.4 Choice of Colourant-Main Considerations: general appearance and colour requirements; processability and stability in processing; stability and permanence in service; health and safety considerations 11.3.5 Some Commercial Pigments 11.4 Antistatic Agents 11.4.1 Static Electricity Charges on PVC: Phenomena and Tests 11.4.2 Nature and Use of Antistatic Agents 11.5 Flame and Smoke Retardants 11.5.1 General Mechanism of Burning of Polymers and Plastics 11.5.2 Flame Retardance and Smoke Suppression in PVC Compositions References

359

364 367 370 371 372

375 401 401 403 405

407 410 419 420 422 424 424 427 435

Chapter 12 MisceUaneous Properties of Special Interest in PVC Materials and Products-W. V. TITOW

12.1 Introduction

439 439

CQnren~

12.2 Low-temperature Properties 12.2.1 Cold Flex Temperature (Clash and Berg) 12.2.2 Cold Bend Temperature 12.2.3 Low Temperature Extensibility of Flexible PVC Sheet 12.3 Heat Resistance . . . . . . . . . . . . 12.4 Permeability 12.5 Environmental Stress Cracking and Crazing 12.6 Weathering Resistance . . . . . . . . . 12.7 Resistance to Biological Attack . . . . . 12.7.1 Microbiological Attack (Biodegradation) 12.7.2 Insect and Animal Depredations: attack by termites; attack by rodents 12.8 Chemical Resistance . . . . . . 12.9 Health Hazards . . . . . . . . 12.9.1 Vinyl Chloride Monomer 12.9.2 PVC Compounds and their Regular Constituents 12.9.3 PVC Decomposition Products 12.9.4 Peripheral Hazards 12.10 Burning Behaviour References . . . . . . . . . .

xvli

439 442 442 442 443 452 466 469 483 483 486 487 495 496 498 499 500 501 509

Chapter 13 Industrial Compounding Technology of Rigid and Plasticised

PVC-W. HENSCHEL and P. FRANZ 13.1 Introduction 13.2 Raw Materials. . . . . . . . . 13.2.1 PVC Polymer and Fillers 13.2.2 Plasticisers 13.2.3 Other Additives . . . . 13.3 Upstream Equipment (Silo Storage to Weighing) 13.3.1 Silo Storage of PVC Polymer and Fillers: silo sizes; materials of silo construction; raw material intake (silo filling); raw material discharge; dust removal system . . . . . . . . . . . . 13.3.2 Conveying of PVC Polymer and Fillers: pneumatic conveying . . . . . . . . . . . . 13.3.3 Storage of Plasticisers: tank sizes; suitable con-

513 513 514 514 519 519 519

519 525

xviii

Contents

struction materials; plasticiser delivery; pointers on pipe laying 13.3.4 Storage of Additives. . . . . . . . . . . 13.3.5 Metering and Weighing: fundamentals of metering and weighing technology; control and monitoring equipment . . . . . . . . 13.4 Mixing . . . . . . . . . . . . . . . . . . . . . . 13.4.1 Blending of Bulk Materials in Overall Solid Phase: introduction; theoretical aspects of mixing, with special reference to dry blending of PVC compositions; mixers for plastics processing; tank-type or intensive mixer . . . . . . 13.4.2 Melt Compounding: compounding and pelletising; compounding of PVC for feeding calenders; extrusion of film, sheet and board; recycling 13.4.3 Preparation of PVC Pastes: silo storage; metering; pasting-up and dispersion; filtering; degassing; ageing; colouring 13.4.4 Machinery: screw-type machines; machine drives; control and instrumentation; interlocks; materials of construction; machines for the production of pastes 13.5 Pellet Cooling and Storage . . . . . . . . . . . . . 13.5.1 Pellet Cooling: nature and outline of the operation; pellet cooler systems 13.5.2 Pellet Mixing and Storage: pellet mixer designs; handling of PVC pellets Chapter 14 Extrusion of PVC-General Aspeds--B. J. LANHAM and W. V. TITOW . . . . 14.1 Introduction 14.2 The Extruder . . . . . . . . . . . . . . . . . . . 14.2.1 Main Components and Their Functions, with Special Reference to Extrusion of PVC: the screw; the barrel; the head and die assembly; heating and cooling; the hopper . . . . . . . 14.2.2 Some General Points Relevant to Extrusion of PVC: machine outputs and energy efficiency in

530 532 532 547

547

577 603

609 660 660 664

673 673 674

674

Contents

modem extrusion practice; some features of, and aids to, modem extrusion; use of computers; some material aspects; some features and common faults of extruded products . . . . . 14.3 PVC Material Flow, Homogenisation and Gelation (Fusion) in the Extrusion Process 14.4 Single-screw Extruders . . . 14.5 Twin-screw Extruders . . . 14.6 Some Commercial Machines 14.7 Ancillary Equipment 14.8 Extrusion of Plasticised PVC 14.8.1 Normal (Relatively Slow) Extrusion 14.8.2 High-speed Extrusion . 14.8.3 Examples of Industrial Extrusion of Plasticised PVC: PVC coating of wire and cable; production of pPVC hose with braid reinforcement References Chapter 15 Injection Moulding of PVC-The late L. W. TURNER 15.1 Introduction . 15.2 Melt Properties of Particular Significance, Melt Behaviour in Relation to Moulding Conditions, and Moulding Compounds . . . . . . . . . . . . . . . . 15.2.1 Moulding compounds . 15.3 Effect of Processing Factors upon Product Properties 15.3.1 Quenching Stresses . 15.3.2 Orientation and Related Features . 15.4 The Moulding Process: Available Equipment; Process Control; Some Features of uPVC Moulding 15.4.1 Rate of Injection and Injection Pressure 15.4.2 Working Surfaces . 15.4.3 Interaction of PVC with Acetal Polymers and Copolymers 15.5 Materials and Applications 15.6 Trouble-shooting . . . . 15.6.1 Machine Selection 15.6.2 Processing Features Specific to PVC 15.6.3 General Considerations. References . . . . . . . . . . . . . . . . . .

xix

682 689 698 699 703

710 713 713 714 717 719 723 723 724 726 728 728 728 729

734 735

735 736 738 738 738

740 740

xx

Contents

Chapter 16 Sheet 'Thermoforming and Related Techniques for PVC-The late L. W. TURNER 16.1 Introduction 16.2 Materials Used . . . . . 16.3 Vacuum Forming of Sheet 16.3.1 Principal Methods: negative forming; plugassisted forming; drape forming; bubble forming; snap-back forming 16.3.2 Details of Methods .... 16.3.3 The Moulds 16.3.4 Finishing . . . . . . 16.4 Matched-mould and Related Methods 16.5 Tolerances in Dimensions and Dimensional Stability of Formed Parts . . . . . . . . . . . . . 16.6 Equipment Suppliers 16.7 Materials Assessment and Design Aspects 16.7.1 Effect on Quality of Draw Ratio and Temperature . . . . . . . . . . . 16.7.2 Thermoformability of CPVC References . . . . . . . . . . . . . . . Chapter 17 Blow Moulding of PVC-W. V. TITOW . . . . . . . . . . 17.1 Basic Features and Historical Development of Blow Moulding 17.2 Blow-moulding Processes and Their Application to PVC 17.2.1 General Characterisation and Main Features of Processes and Systems: main characteristics of extrusion; injection, and dip blow moulding; the role and effects of stretching in stretch-blow moulding; processing and equipment arrangements; cooling methods 17.2.2 Industrial Blow Moulding of PVC: some process and equipment considerations; extrusion blowmoulding equipment; injection blow-moulding equipment; dip blow-moulding equipment; sources of information on blow-moulding

743 743 745 745 745 751 753 754 755 756 757 757 759 761 761

763 763 765

765

Contents

equipment . . . . . . . . 17.3 PVC Compositions for Blow Moulding 17.3.1 The Processing Aspect . . . 17.3.2 The End-use Aspect . . . . 17.3.3 PVC Bottle Formulations: PVC polymer; stabiliser system; impact modifiers; lubrication; other additives . . 17.4 PVC Blow Mouldings 17.4.1 Applications 17.4.2 Properties and Tests References . . . . . . . . . . .

xxi

784 789 789 792 793 795 795 797 800

Chapter 18 Calendering of PVC-W. V. TITOW

18.1 Introduction 18.2 The Calender . . . . . . . . 18.3 The Calendering Operation: General Features and Their Effects on the Structure and Properties of Calendered Sheet 18.4 Calender Lines . . . . . . . . . . . . . . . . . . 18.4.1 General-purpose Line: pre-calender (compounding and feed) section; calendering; the post-calender train 18.4.2 Special Lines and Arrangements: calendered flooring lines; lamination on or at the calander 18.5 The Formulation Aspect . . . . . . . . . . . 18.6 Some Faults and Defects of Calendered Sheeting 18.6.1 Simple Dimensional Faults 18.6.2 Structural Defects . . . . . . . 18.6.3 Faults Manifested in Appearance 18.7 Further Processing of Calendered Sheet 18.7.1 Press Finishing 18.7.2 Press Lamination 18.7.3 Surface Treatments: printing; coating; embossing . . . . . . . . . . . . . . . . . 18.7.4 Continuous Lamination 18.8 Properties and Applications of Calendered Materials References . . . . . . . . . . . . . . . . . . . . .

803 803 804 808 809 809 828 830 833 833 834 835 837 837 837 838 839 840 847

xxii

Contents

Chapter 19

Rigid PVC: Main Products-Production, Properties and Applications-B. J. LANHAM and W. V. TITOW 19.1 Introduction 19.2 Some Material Properties of uPVC 19.3 uPVC Pipes 19.3.1 Types of uPVC Pipe . . . 19.3.2 Production of uPVC Pipe: equipment and process; some formulation aspects . . . . 19.3.3 Pipe Properties and Their Determination 19.3.4 Some Special Pipe Products . 19.4 uPVC Profiles 19.4.1 Main Types and Applications 19.4.2 Production . . . . . . . 19.4.3 Some Formulation Aspects 19.4.4 Testing and Specifications 19.5 uPVC Sheet and Film 19.5.1 Terminology . . . . . 19.5.2 Production . . . . . . 19.5.3 Applications and Properties 19.6 Gramophone Records . . . . . 19.7 Injection-Moulded uPVC Articles References . . . . . . . . . . . . .

849 849 856 866 867 869 878 879 883 883 884 886 889 890 890 891 893 896 897 898

Chapter 20

PVC Sheet and its Fabrication-W. V. TITOW 20.1 Introduction 20.2 Unsupported PVC Sheet Materials 20.3 Main Fabrication Techniques Applicable to PVC Sheet Materials and Parts 20.3.1 Welding: hot-gas welding; extrusion welding; high-frequency welding; heated-tool welding 20.3.2 Bonding: solvent bonding; adhesive bonding 20.3.3 Machining 20.3.4 Conversion and Manipulation of PVC Film and Sheeting for Packaging . . . . . . . . . . , 20.3.5 Surface Decoration, Marking, and Other Surface Processing of PVC Materials and Products: surface decoration; surface marking; surface

901 901 904 910 910 923 930 932

Contents

processing References Chapter 21 PVC Pastes: Properties and Formolation-W. V. TITOW 21.1 Introduction 21.2 PVC Pastes: Rheological Properties and Theory . . .... 21.2.1 Viscosity of a Simple Suspension 21.2.2 Main Compositional Factors Influencing the Apparent Viscosity of PVC Pastes . . . . 21.2.3 Expressions for the Apparent Viscosity of Pastes . . . . . . . . . . . . . . . . . . 21.2.4 Variation of Paste Viscosity with Rate of Shear, or with Time at Constant Shear Rate 21.2.5 Gelation and Fusion of PVC Pastes . . . . 21.2.6 The Measurement of Viscosity of PVC Pastes 21.3 Paste Components and Formulation . . . . . . . . 21.3.1 The Polymer: paste polymers; extender polymers 21.3.2 Plasticisers 21.3.3 Stabilisers 21.3.4 Fillers 21.3.5 Thickening Agents (for Thixotropic Plastisols and Plastigels) 21.3.6 Miscellaneous Paste Components: viscosity depressants; diluents; other minor additives . 21.4 Pastes for Rigid Products: Organosols and Rigisols . 21.4.1 Organosols 21.4.2 Rigisols References

xxiii

932 936

939 939 940 941 942 943 945 951 960 962 %2 965 969 970 973 975 975 975 976 978

Chapter 22 Preparation, Processing and Applications of Pastes-W. V.

TITOW . . . . . . . . 22.1 Introduction 22.1.1 Preparation . . . . . 22.1.2 Conversion to Products 22.2 Applications 22.2.1 Rotational Casting . .

981 981 981 982 986 986

xxiv

Contents

22.2.2 Slush Moulding 22.2.3 Paste Casting 22.2.4 Dip Coating and Moulding: hot-dip coating; hot-dip moulding; cold-dip coating 22.2.5 Spray Coating 22.2.6 Coating of Sheet Materials (Fabrics and Paper): paste coating (spreading) by doctor knife; paste coating by roller; direct-coating process; transfer (reverse) coating process; promotion of adhesion between coating and substrate; surface decoration and finishing of PVC paste coatings; testing of coated materials 22.2.7 Miscellaneous Paste Processing Methods of Minor Significance: low-pressure injection moulding; compression moulding; extrusion References

Chapter 23 PVC Latices-Revised and edited by W. V. TITOW 23.1 Introduction 23.2 Types of PVC Latices 23.2.1 Homopolymer Latices 23.2.2 Unplasticised Copolymer Latices 23.2.3 Plasticised Copolymer Latices . 23.3 Some Properties of Polymeric Products from PVC Latices 23.3.1 Mechanical Properties 23.3.2 Toxicity Considerations 23.4 Compounding 23.4.1 Latex Property Modifiers: latex stability; wetting agents; thickeners; antifoaming agents; pH modifiers and buffers . . . . . . . . 23.4.2 Polymer Property Modifiers: heat stabilisers; plasticisers; fillers; pigments 23.5 Anti-blocking Techniques 23.6 Applications 23.6.1 Textile Applications: as bonding agents in nonwoven fabrics; for coating or impregnation of fabrics . . . . . . . . . . . . . . . . . .

988 991 992 996

998 1010 1012

1013 1013 1016 1017 1017 1018 1018 1018 1019 1019 1020 1029 1039 1040 1040

Contents

xxv

23.6.2 Paper Treatments . . 23.6.3 Leather Finishes 23.6.4 Adhesive Applications References

1042 1044 1044 1045

Chapter 24

PVC Solutions and their AppUcations--W. V. TITOW 24.1 Introduction 24.2 Components of PVC Solutions 24.2.1 The PVC Polymer: homopolymers; copolymers; terpolymers 24.2.2 Solvents and Diluents 24.2.3 Other Solution Constituents 24.3 Preparation of PVC Solutions, and Solution Compositions for Particular Applications 24.4 Applications 24.5 Adhesion of Solution-applied Coatings to Substrates References . . . . . . . . . . . . . . . . . . . . .

1047 1047 1048 1048 1049 1054 1057 1060 1063 1065

Chapter 25

CeUuIar PVC Materials and Products-W. V. TITOW 25.1 Introduction 25.2 Production Methods and Processes 25.2.1 Foams: dispersed-gas blowing: 'chemical' blowing; gas entrainment (mechanical frothing); insitu gas evolution and cross-linking; solvent (monomer) blowing 25.2.2 Other Cellular PVC Materials: the 'lost filler' method; sintering of powder 25.3 Formulation and Process Factors in Foam Production 25.3.1 Effects of Formulation and Processing Variables on Foam Properties . . . . . . . . . . . . 25.3.2 Chemical Blowing Agents-Nature and Operation . . . . . . . . . . . . . . . . . . . 25.4 Some Surface Treatments-Embossing and Lacquer Coating of Flexible Cellular Sheet Materials 25.4.1 Mechankal Embossing . . . . . . . . . 25.4.2 Chemical Emboss . . . . . . . . . . . 25.4.3 Emboss Effects by Screen Printing of Paste

1067 1067 1069

1069 1078 1080 1080 1085 1092 1092 1093 1094

xxvi

Contents

25.4.4 Lacquer Coating 25.5 Examples of Basic Formulations 25.6 Evaluation and Testing References . . . . . . . . . . . . Chapter 26 Applications of PVC-W. V. TITOW . . . . . . . . . . . 26.1 Main Applications of Primary PVC Products . . . . . 26.1.1 Pipes and Tubing: rigid (uPVC) pipes; flexible tubing . . . . . . . . . . . . . . . . . . 26.1.2 Extruded Profiles and Channels . . . . . . . 26.1.3 Unsupported Sheeting and Film: rigid sheet; flexible sheet; foil and film . . . . 26.1.4 Foam: rigid foam; flexible foam . . . . . 26.2 Composite Products (Coated, Laminated, or Filled) 26.2.1 Coated Fabrics 26.2.2 Conveyor Belting 26.2.3 Sheet-type PVC Interior Wall-coverings 26.2.4 PVC Coatings and Coverings on Metal Substrates: wire and cable insulation and coverings; PVC/metal sheet laminates; 26.2.5 Laminates of PVC with Non-metallic Materials: sandwich panels; PVC/polystyrene sheet laminate; PVC/polyacetallaminated sheeting. 26.2.6 Unsupported PVC Flooring and Floor Tiles 26.3 PVC Fibres and Fibre Products . . . . 26.4 Miscellaneous Products and Applications 26.4.1 Gramophone Records 26.4.2 Blown Bottles and Containers 26.4.3 Footwear . . . . . . . . . 26.4.4 Battery Separators . . . . . 26.4.5 Powder-coated Products and Mouldings Produced by Powder-coating Methods 26.4.6 Medical Applications 26.4.7 Applications in Motor Cars . . . . . . . . . 26.4.8 Tubular-frame Furniture and Related Applications 26.5 Some Special, Unusual, or Minor Products and Applications References . . . . . . . . . . . . . . . . . . . . . . .

1094 1095 1095 1101 1103 1104 1104 1106 1107 1110 1111 1111 1112 1113 1114 1115 1116 1117 1117 1117 1118 1118 1118 1118 1120 1121 1121 1122 1125

Contents

xxvii

Appendix 1

Standards Relevant to PVC Materials and Products-Compiled by N. HERBERT and W. V. TITOW 1. Plastics Terminology, Properties and Testing: General 1.1 Terminology: general; common names and abbreviations; equivalent terms in various languages . . . . 1.2 General Test Conditions and Methods: conditioning and testing conditions; some general test methods . 2. Vinyl Polymers and Copolymers . . . . . . . . 2.1 General (Designation, Coding, Characterisation Tests) . . . . . 2.2 Viscosity . . . . . . . . . . 2.3 Chlorine Content . . . . . . . . . . 2.4 Vinyl Acetate Content in VCNA Copolymers 2.5 Ash and/or Sulphated Ash Content . 2.6 Volatile Matter (including Water) 2.7 Impurities and Foreign Matter 2.8 Bulk Density 2.9 Particle Size . . . . 2.10 Bromine Number 2.11 pH of Aqueous Extract 2.12 Miscellaneous Properties Relevant to Processing 2.13 Methanol Extract 2.14 VCM Content 3. Vinyl Compounds 3.1 General (Designation, Coding, Characterisation Tests): rigid compounds; flexible compounds, pastes; miscellaneous 3.2 Properties and Tests: bulk density and pourability; water absorption; temperature effects; mechanical properties; miscellaneous properties and analysis 4. Plasticisers 4.1 Bulk Properties . . . . . . . . . . . . . . . . 4.2 Properties in Association with PVC (Compatibility, Volatility, Migration) 4.3 Effects on PVC . 5. PVC Sheeting and Films 5.1 Rigid 5.2 Flexible 5.3 Sheet and Film Fabrication and Products

1127 1131 1131 1134 1135 1135 1136 1137 1137 1137 1137 1138 1138 1138 1139 1139 1139 1140 1140 1140 1140 1142 1144 1144 1146 1147 1148 1148 1148 1149

xxviii

Contents

6. PVC Pipes, Tubing, and Pipe Fittings 6.1 Rigid Pipes and Fittings, Including Pressure Pipes 6.2 Flexible Tubing 6.3 Miscellaneous Standards R~evant to Pipes 7. PVC-coated Materials and Products 7.1 Coated Fabrics, including Conveyor and Transmission Belting . 7.2 Other Coated Materials and Products 8. Cellular Vinyls 8.1 Rigid Cellular Materials 8.2 Flexible Cellular Materials 8.3 Miscellaneous Standards: definition and classification; physical properties-general; thermal properties-general; flammability and burning; chemical resistance and permeability; insulation materials: cushioning materials, sandwich structures 9. PVC Wire and Cable Insulation, Cable Sheathing and Jacketing 10. PVC Flooring 11. Various Product Standards and Tests 11.1 Colour Bleeding and Staining 11.2 Miscellaneous

1150 1150 1156 1157 1158 1158 1160 1160 1160 1162

1163 1165 1167 1167 1167 1167

Appendix 2 Quantities and Units: The SI System: Unit Conversion Tables--Compiled by W. V. TITOW 1169 Appendix 3 Some Material Properties of PVC Componnds-Compiled by W. V. TITOW Index 1 General

Products

and 1185

. , . . . . . . . . . . . . . . . . . . . . . . 1199

Index 2 Material and Product Trade Names . . . . . . . . . . . . 1223 Index 3 Named Equipment and Processes . . . . . . . . . . . . .

1231

List of Contributing Authors

W. V. Trrow

Formerly Manager (Special Projects), Laboratories Ltd, Ashtead, Surrey, England

Yarsley

Research

D. L. BUSZARD Market Development and Technical Service, Plastics Chemicals, Ciba-Geigy Industrial Chemicals, Tenax Road, Trafford Park, Manchester, MI71WT, England

P. S. COFFIN General Manager-Technical, Roeol Ltd, Rocol House, Swillington, Leeds, LS26 2BS, England P.

FRANZ

Manager of Process R&D Department, Buss Ltd, CH-4133 Pratteln, Switzerland W.

HENSCHEL

Manager of the Design and Construction Department, Buss Ltd, CH-4133 Pratteln, Switzerland

Miss N.

HERBERT

Head, Standards Information Centre, South African Bureau· of Standards, Private Bag X191, Pretoria 0001, Republic of South Africa xxix

xxx

List of Contributing Authors

I. D. HOUNSHAM Sales Manager, PVC Division, Croxton and Garry Ltd, Curtis Road, Dorking, Surrey, RH4 lXA, England

B. J. LANHAM European Marketing Manager, LNP Plastics Nederland BV., PO Box 13, Ottergeerde 24, Raamsdonksveer, The Netherlands The late L. W. TuRNER Formerly Senior Research Associate, Yarsley Technical Centre Ltd, Redhill, Surrey, England

CHAPTER 1

Introduction W. V.

TITOW

1.1 PVC: GENERAL TERMINOLOGY AND RELEVANT DEFINITIONS The letters 'PVC' stand for 'polyvinyl chloride'. Thus the abbreviation, like the full name, should-strictly speaking-specifically denote a homopolymer of vinyl chloride. However, both terms-and in particular the abbreviation-have acquired a different, wider meaning in common usage: to the processor and user, as well as the technologist, 'PVC' is any material or product made of a PVC composition, i.e. of an intimate mixture of a vinyl chloride polymer or copolymer with various additives, some of which (e.g. plasticisers in a flexible PVC composition) may be present in very substantial, occasionally predominant, proportion. It is usual to refer to the polymer constituent of such compositions as PVC resin or PVC polymer. The terms 'compound' and 'formulation' are also sometimes used as if they were synonymous with 'composition', although the purist may claim, with some justification, that 'formulation' is the make-up of a composition (e.g. as recorded on paper), and that the word 'compound' should be reserved for those PVC compositions which are produced by melt compounding (in contradistinction to, say, dry blends or plastisols-see Chapters 13 and 21, respectively). Some of the additives which the formulator includes in a PVC composition are heat stabilisers, necessary in all cases to counteract the inherent thermal instability of PVC resins (especially at the high processing temperatures); others also function as aids in processing (e.g. certain polymeric modifiers, lubricants), whilst still others (e.g. 1

2

W. V. Titow

plasticisers, fillers) modify the material properties to provide the wide applicational versatility that makes PVC so important among the major thermoplastics. In terms of the extent of their effect on the material properties of PVC, plasticisers are the most important group of additives. PVC compositions incorporating plasticisers (and the materials and products made from such compositions) are known as plasticised PVC (sometimes abbreviated to pPVC*); flexible PVC and soft pvc contain plasticisers in quantities high enough to impart these properties to the material. PVC compositions and products which do not incorporate plasticisers are commonly called unplasticised PVC (uPVC*) or sometimes rigid PVC, although the latter term properly extends also to PVC materials which may contain some plasticisers but in a proportion not sufficient to lower the modulus appreciably. Plasticised materials whose plasticiser contents-whilst generally low-do reduce the modulus (and usually the strength and hardness) in comparison with uPVC (but only to values still higher than those normal for flexible or soft PVC) are sometimes referred to as semi-rigid PVc. The term 'vinyl' is also used, as an adjective or noun, in the place of 'PVC' (e.g. in such expressions as 'processing of vinyls', 'vinyl composition', 'vinyl material', 'vinyl upholstery', 'vinyl foam'), especially-and most commonly-where the material concerned is a flexible or soft PVC. This terminology is quite common, and thus sanctioned by usage, but it is worth bearing in mind that it tallies neither with standard definitions in the PVC field nor with systematic chemical nomenclature. Thus the current international standard definition of vinyl resin (ISO 472-1979(E» is a resin made by polymerisation of monomers containing the vinyl group, and hence includes, for example, polystyrene (which is polyvinylbenzene), polyvinyl acetate, polyvinyl alcohol, polyvinyl fluoride and polyvinyl pyrrolidone, along with polyvinyl chloride and all the other polymers of compounds whose main structural component is the vinyl grouping CH z = CH-. Whilst the definition is sound and properly based on the relevant chemical structure, no polymer technologist would refer to, say, expanded polystyrene as 'vinyl foam': in the common parlance of

* These designations (with a space after the first, lower case letter) are prescribed by two international standards: ISO 2898/1 and ISO 1163/1 (but current revision proposals include changes from u PVC and p PVC to PVC-U and PVC-P). The letters iPVC are sometimes used to designate a high-impact compound.

i

introduction

3

the plastics industry the term 'vinyl' is firmly associated with PVC, in the way just mentioned. A few other relevant standard definitions may be noted in passing. Vinyl chloride plastic: 'A plastic based on polymers of vinyl chloride or copolymers of vinyl chloride with other monomers, the vinyl chloride being in the greatest amount by mass'. (ISO 472-1979). Rigid PVC compounds: 'Rigid plastic compounds composed of poly(vinyl chloride), chlorinated poly(vinyl chloride), or vinyl chloride copolymers, and the necessary compounding ingredients. The resin portion of copolymer compounds shall contain at least 80 percent vinyl chloride. The compounding ingredients may consist of lubricants, stabilizers, non-poly(vinyl chloride) resin modifiers, and pigments, essential for processing, property control and colouring.' (ASTM D 1784-81). Unplasticised compounds of polymers of vinyl chloride: 'Compounds based on homopolymers of vinyl chloride, or copolymers with at least 50% of vinyl chloride, or chlorinated poly(vinyl chloride), or mixtures of such polymers with each other or with other polymers, the principal ingredient being a polymer of vinyl chloride. These compounds may also contain fillers, colorants, and such small quantities of other ingredients as are necessary to facilitate fabrication, such as stabilizers and lubricants.' (ISO 1163/1-1980(E)). Non-rigid vinyl chloride polymer and copolymer moulding and extrusion compounds: Compounds based on '... nonrigid vinyl chloride polymer and copolymer classes in which the resin portion of the composition contains at least 90% vinyl chloride. The remaining 10% may include one or more monomers copolymerized with vinyl chloride or consist of other resins mechanically blended with polyvinyl chloride or copolymers thereof. These nonrigid vinyl compounds are defined by a hardness range and include the necessary stabilizers, plasticizers, fillers, dyes, and pigments to meet the designated requirements'. (ASTM D 2287-81). Flexible PVC compounds: 'Compounds ... manufactured from polyvinyl chloride or from a copolymer of vinyl chloride of which the major constituent is vinyl chloride, or from both. Such materials shall be

4

W. V. Titow

suitably compounded with plasticisers and other ingredients.' (BS 2571: 1963). Plasticised compounds of polymers of vinyl chloride* (pPVC): 'Compounds based on homopolymers of vinyl chloride or copolymers with at least 50% of vinyl chloride, or chlorinated poly(vinyl chloride), or mixtures of such polymers with each other or with other polymers, the principal ingredient of the mixtures being a polymer of vinyl chloride. These compounds contain plasticizers and may also contain fillers, colorants, and small quantities of other ingredients such as stabilizers and lubricants'. (ISO 2898/1-1980(E)).

For the sake of convenience, abbreviations (letter symbols) are used for some polymers and copolymers in many places in this book. Such abbreviations are generally in line with the recommendations of the relevant international standard (ISO 1043-1978). However, in a few cases where those recommendations are at variance with common general usage, or in order to avoid inconsistency, the ISO standard has not been followed. Two notable examples are the author's preference for EVA (over 'EN AC' recommended by the ISO standard) as an abbreviation for ethylene/vinyl acetate copolymer, and VCNA for vinyl chloride/vinyl acetate copolymer. The EVA symbol is very widely used (and indeed also recommended by another English-language standard-ASTM D 1600-83), whilst-given its use-IVA' for vinyl acetate is then more consistent than 'VAC' (the abbreviation favoured by all standards), and should be acceptable especially in contexts where there is no chance of confusion with vinyl alcohol (which is in any case usually designated by 'VAL'). It may also be noted that 'A' for acetate is recognised, though not preferred, by ISO 1043-1978.

1.2 EARLY HISTORY AND DEVELOPMENT OF PVC Although Regnault l - 3 prepared some vinyl and/or vinylidene monomers in 1838, and observed the conversion of the latter to a white powder when exposed to sunlight in sealed tubes,2 it is Baumann's polymerisation of vinyl chloride (as well as bromide) in 18724 that is often regarded as the earliest documented preparation of PVC homopolymer: this was certainly among the 'white, solid masses * NB called simply 'plasticised vinyl compounds' in an earlier version of this definition (ISO/DIS 2898, International Draft Standard, 1972).

I

Introduction

5

unaffected by solvents and acids' obtained in that work. The polymer was found to be stable on heating up to Boac, but to decompose rapidly with evolution of acid vapour at higher temperatures. 5 Early manifestations, or at least precursors, of budding practical interest in PVC came in 1912, in the form of British and German patents6 to Ostromislensky (for the production of 'rubber-like masses' from vinyl halides), and in the work in Germany by F. Klatte considered by some to have laid the foundation for the technical production of PVC: 3 Klatte took out a German patent for the production of PVC fibres,3 and Ostromislensky went on to obtain patent cover for 'polyvinyl halides' (in the USA in 1929). 2 In the meantime (c. 1928) patents were also being granted for vinyl chloride/acetate copolymers; in the USA to Du Pont and the Carbide and Carbon Chemicals Corporation, and in Germany to I. G. Farbenindustrie (now BASF).3,7 The first production of the copolymers in America (by the Carbide and Carbon Chemicals Corp.) falls in the period 1928-1930: soon after (1931) B. F. Goodrich introduced their own 'non-rigid vinyl chloride plastics'. 2 In Germany, 1931 saw the first technical-scale production of vinyl chloride polymer and copolymers, and the first preparation (by Hubert and Schonburg) of chlorinated PVC fibres (followed by the first technical production of both the CPVC polymer and the 'Pe-Ce' fibres from it in 19343). Industrial development (with emphasis eventually shifting from the vinyl chloride/acetate copolymer to the homopolymer) thereafter proceeded in both countries, with full commercial production achieved in the late 1930s. Whilst some development work was taking place in the UK in the same period, PVC was first produced there on a commercial scale in 1942-1943 (by ICI and the Distillers Company). It is thus comparatively recently that PVC became a commercial plastics material. The early interest in copolymers (in particular polyvinyl chloride/acetate) was associated with their use as the first practical solution to the problem of thermal decomposition in processing: whilst the general thermal stability of the copolymers is somewhat poorer than that of the homopolymers they can be processed at significantly lower temperatures, at which they are reasonably stable. The effects in this respect of the co-monomer units in the polymer molecules-sometimes referred to as 'internal plasticisation'-are now well understood. Effective 'external' plasticisation of PVC homopolymer by the incorporation of plasticisers first came around 1930, with the finding by several workers5 ,8 that compounding with dibutyl phthalate (DBP) and certain other esters would convert the intractable polymer to a material

6

W. V. Titow

of lower softening point; this could be processed satisfactorily at lower melt temperatures and was-in the solid state at room temperaturerelatively soft and similar to rubber in some respects. Thus, chronologically, external plasticisation came after 'internal' plasticisation by copolymerisation, although it is now the main route to the formulation of flexible and most semi-rigid PVC materials. Among early suggestions of substances for use as plasticisers, now of only historical interest, were tung oil9 and alkyd resins. 1o It was also realised at about the same time (the early 19305) that certain additives, e.g. alkaline-earth metal soaps,11 would act as heat stabilisers. The main present-day applications in which vinyl chloride/acetate copolymers are more suitable than plasticised homopolymer compositions are gramophone records and floor coverings. It is interesting to note that the first of these applications was originally disclosed in the early 1930s,8,12 i.e. around the time when other less practicable and now long defunct proposals were also current, e.g. moulded PVC dentures,13 and an adhesive, consisting of a mixture of PVC with rubber and a cellulose derivative, for sticking patches on worn places in clothing!14 It was World War II that first brought PVC into its own. It was soon realised that plasticised PVC was an effective replacement for rubber in some important applications, notably cable insulation and sheathing. Thus PVC helped to relieve the acute rubber shortage, and at the same time established itself as a material in its own right. From then on it continued growing rapidly in importance, to attain the dominant position which its properties and versatility secure for it today in so many applications. The early processing of PVC, in the pre-war period and to some extent during the war, was largely carried out by methods and on machinery originally developed for rubber or celluloid. The processes involved were mixing, calendering, compression moulding, and extrusion (including wire coating). The paddle-type (Gardner) pre-mixers were in use at an early period, but between about 1942 and 1945 open-mill mixing was widely practised. The use of internal mixers was also adopted when it was found that PVC compounds could be readily mixed in them. The open mills and other machines had to be run at temperatures higher than those appropriate for rubber: as they were normally steam-heated steam pressures had to be increased at the risk of grease melting extensively and draining away from bearings. Electrical heating, particularly for extruders, was a logical development, but one which proposals were also current, e.g. moulded PVC dentures,13 and an adhesive, consisting of a mixture of PVC with rubber and a cellulose

7

1 Introduction

was fully utilised only slowly. The need to modify the rubber extruders employed for the early production work soon became plain, and modifications were made, e.g. to enable the material to be fed-in in granular form, and to provide higher processing temperatures (by electrical heating). A special ram extruder was employed in Germany for a time to produce rigid pipe from a PVC billet. 15 Thanks to the work of Kaufman the early history of PVC polymers, compounds, and processing is well recorded and documented. 15 ,16 The development of modern PVC-processing equipment and of the many specialised processes which form such an important part of present-day PVC technology has paralleled the remarkable expansion of the production of PVC and the scope and number of its applications. The 1970s brought two unforeseen events of major significance both in their initial impact and their lasting effects upon the PVC industry-the oil crisis of 1973/74 (with its aftermath of continuing oil price rises), and the finding that vinyl chloride monomer (VCM) is a carcinogen. The oil crisis-after first causing a serious temporary shortage of the oil-derived principal feedstocks for VCM production (ethylene and acetylene-see also Chapter 2), and hence of PVC polymers (ct. the drop, c. 1974, of the curves of Fig. 1.1)-resulted in large, and continuing, increases in polymer prices. These are the outcome of higher costs of both the feedstocks and the energy (also largely oil-supplied) used to process them into monomers and thence into polymers. It may be noted that one of the developments prompted by this situation has been a refocusing of interest on coal-based raw materials and processes, with special reference to the acetylene route to the production of VCM: HC:=CH + HCI ~ CHr-CHCl

(1)

Albeit normally more energy-intensive than the ethylene route (generally favoured with oil feedstocks also because of the higher cost of acetylene from that source) it can be made completely independent of petrochemicals by producing the acetylene from coke and quicklime via calcium carbide: 3C + CaO~

Ca~

+ CO

(2)

(coke)

Ca~

+ 2H20~ HC:=CH + Ca(OH)2

(3)

i

=:

E c o

"c;l

"t:

u

+'

C

01

.

2

g3

tI

CIl

4

5

1964

I

Fig. 1.1

¥,----I

1972

x

/',/

/'

,/

I

1976

I

"

,...

./

/'

/"

"",.

/.~

I

1984

__ x

/'/'

/'

---

/'

/'

/'/'

/'

-- -- --

/"

1980

~,.-----Japan

/"

,,'/

/'

,/

Consumption of PVC polymers in the principal consuming areas.

I

1968

/

-----

--~/x--'-~

x

W~st~rn Europ~

./

./

/'

~

~

:0:::::

~

00

1 Introduction

9

The chlorine and hydrogen needed for the HCI used in reaction (1) can be produced by electrolysis of brine (with caustic soda as a saleable by-product). Some industrial plants manufacturing VCM and PVC polymer by this process have been in operation for many years (e.g. the AECI 'Coalplex' plant at Sasolburg, RSA). The discovery, in the early 1970s, that exposure to VCM could cause certain forms of cancer, coupled with the realisation that VCM concentrations in factory atmospheres and its residual contents in PVC polymers were comparatively high, had repercussions on PVC polymer production in several countries. It also caused a serious decline (especially in the USA and Japan) in the use of uPVC films for food packaging, and blow-moulded bottles for beverages and oils. The legal action for 285 million dollars brought in the USA against Borden Chemical and Goodyear Tire and Rubber Co. (two PVC polymer producers) by some supermarkets, in respect of 'damage to health' by PVC film used to wrap meat,17 is an example of the extremes of feeling in some quarters. Soft PVC was comparatively less affected, as the dilution effect of large amounts of plasticiser and greater loss in processing reduced the VCM concentration in the compounds to relatively low proportions. The considerable effort expended on investigating and remedying the situation, together with relevant regulations brought out in the major industrial countries, led to a vast reduction of VCM contents in both the factory air and PVC polymers produced by virtually all main manufacturers. The 'clean-up' brought the content levels down to values now regarded as acceptable on the basis of data obtained in extensive studies. The subject is discussed in more detail in Chapter 12 (Section 12.9.1), and also mentioned in Chapters 2 and 7. A third topic-albeit of comparatively lesser importance in the PVC context than the oil crisis and the VCM problem-which has been receiving increasing attention in recent times is the disposal of plastics waste and re-usable material. Concern with preservation of resources and conservation of the environment provides the main incentive in these two related areas. Dealing with PVC waste involves special considerations. Selective reclamation, i.e. separation from waste mixtures with other plastics (which operation is not a straightforward proposition in itself), and subsequent re-processing are complicated by the wide variety of PVC formulations, and the increased susceptibility to heat degradation in re-processing: the main factors in the latter are the 'heat history' already acquired and the possible presence of

10

W. V. Titow

polymer already partly degraded in the course of past heat treatments and/or service. Re-processing PVC-containing plastics waste without separation will normally entail dealing with mixtures in which large proportions of polyolefins (mainly polyethylene) are present: in view of the poor compatability of polyolefins with PVC this is not a particularly attractive practical proposition either with respect to processing or the resulting product. Disposal of PVC waste also has its special problems, since the polymer is not biodegradable, whilst incineration produces irritant, corrosive and toxic products (see Chapter 12, Section 12.9.3). Claims are made from time to time of successful reclamation of PVC from mixed plastics scrap and waste (e.g. by the 'Mesco' process developed in Japan by Mitsui 18) but the scale of commercial recovery is still relatively small, and the practical limitations of all existing methods are recognised. 18 ,19 The re-use of material from discarded PVC bottles is sometimes cited as a case where a certain measure of success has been achieved. In France such bottles have been processed for some time (on a limited scale) by Societe Dorlyl, to produce reclaimed PVC compounds said to be suitable for the production of certain grades of sewage and drainage pipes, and telephone cable sheathing. * The use of granulated PVC bottles as road-surfacing material in the USA has also been reportedt (as indeed has that of ground glass bottles!). Normal recirculation, in the same process, of the clean PVC scrap generated (e.g. edge trim in calendering-see Chapter 18) is widely practised, in particular with pPVC for non-critical applications. General pPVC scrap, both own and from external sources, is also converted by some processors into such products as cheap garden hose or core composition for cables (see Chapter 13, Section 13.4.2(d».

1.3 GENERAL STATISTICS Today the amount of PVC produced worldwide represents about 30% of the total production of thermoplastics: this is second only to the production of all polyolefins (i.e. low and high density polyethylene and polypropylene together). The consumption of PVC in the principal * Eur. Plast. News, (February 1979), 6(2), 3. t J. Burbidge, Chapter 8, p. 130, of the general source given in Ref. 5.

1 Introduction

11

consuming areas (where most of the production also takes place) is illustrated in Fig. 1.1. A breakdown, by main use, of PVC consumption in 1970 and 1976 is given in Table 1.1 for Western Europe and the USA. Production and consumption statistics for PVC (as well as other plastics) are published each year in the January issue of Modern Plastics International: some relevant information will also be found in the current issue of the Modern Plastics Encyclopedia. Data,

TABLE 1.1 Consumption of PVC Polymers, by Main Outlet, in Western Europe (Including UK) and the USA in 1970 and 1976 Western Europe (1000 metric tonnes)

Outlet

1970 Film and sheet (rigid and flexible) Calendered Extruded Flooring Calendered Coated

} }

430 (21'5%)

1976

} }

USA (1000 metric tonnes)

1970

1976

} }

605 (19·1%)

259} 341 82 (24,8%)

195 (6·1%)

113} 147 34 (10'7%)

550 (27'5%) 230 (11·5%)

785 (24'7%) 335 (10'6%)

215 (15'7%) 186 (13-5%)

682 (31'9%) 150 (7'0%)

Records

40 (2,0%)

78 (2'4%)

64 (4'8%)

68 (3·2%)

Blow-moulded bottles

110 (5'5%)

235 (7-4%)

32 (2·3%)

35 (1·6%)

20 (1·0%) 235 (11'8%)

82 (2'6%) 505 (15·9%)

39 (2'8%) 23 (1·7%)

104 (4'9%) 177 (8'3%)

Misc. coatings (other than flooring)

134 (6'7%)

265 (8'3%)

187 (13'6%)

190 (8'9%)

Others (including plastisol products other than coatings)

72 (3-6%)

90 (2'9%)

138 (10,1%)

283 (13-2%)

2000 (100·0%)

3175 (100,0%)

1372 (100'0%)

2139 (100,0%)

uPVC pipe, conduit and fittings Wire and cable covering

Misc. injection mouldings Misc. extruded products (including rigid profiles and cladding, flexible tubing and profiles)

Total

178 (8'9%)

200 305 105 (14,2%) 75 70

145 (6'8%)

12

W. V. Titow

predominantly for the UK and Europe, usually appear in the January issue of European Plastics News.

1.4 OUTLINE OF THE PVC SECTOR OF THE PLASTICS INDUSTRY Companies operating in the PVC sector of the plastics industry generally fall into one of four main categories, which are as follows: (i) (ii) (iii) (iv)

polymer producers; compounders; processors; companies selling finished goods consisting of or containing PVC.

Polymer importers have not been included in this sequence because they do not normally engage in technical activities. They are however, with the producers, members of the more general category of polymer suppliers. Some companies fall within more than one of the categories listed: for example, the polymer producers all produce compounds; some also produce semi-finished goods. It should be appreciated at this early stage that the number of polymer producers in any country is very small when compared with the large numbers of companies in the other categories. Category (iv) above will contain many companies for which PVC is but a small part of their interests. Nevertheless, such companies, e.g. the automobile producers, can use very large quantities of PVC and are very important to the industry. The principal processes used to convert the PVC to finished and semi-finished goods are extrusion, calendering, injection moulding and spread coating. Although some companies are concerned with more than one of these processes, most tend to specialise in one process. In some cases the processed PVC is marketed directly by the processor (e.g. unplasticised PVC pipes), whilst in other areas the processor passes on the PVC in semi-finished form to another company which employs the material in its products, e.g. vinyl automobile upholstery. In addition to the material producers, converters and users, there are many companies which specialise in the supply of additives for use in PVC compounds, e.g. plasticisers, stabilisers, lubricants, fillers, etc. It is relevant to point out that the value of the total market for some of these materials exceeds that of many other plastics materials.

I

Introduction

13

Also worthy of mention-since without them there would be no PVC industry-are the machinery manufacturers. Many companies have specialised in equipment for PVC processing, and through their development work on plant and equipment new applications for PVC have been made possible.

1.5 VINYL CHLORIDE POLYMERS AND COPOLYMERS 1.5.1 PVC Homopolymers

(a) Chemical Structure The basic repeat unit of the PVC polymer chain is HI HI ]

[

-C-C-

h tl

i

where i is the degree of polymerisation, i.e. the number of repeat units in the molecular chain. The units are linked virtually exclusively 'head-to-tail', i.e. -CHz-CHCI-CHz-CHCI-. In commercial PVC polymers the average values of i range between about 500 and 1500; this corresponds to a theoretical molecular weight range of about 31000-94000.

Note: In practice a given amount of linear, thermoplastic polymer (i.e. a test specimen, processing batch, etc.) will consist of chain molecules made up of the same basic repeat units, but differing in size (chain length). A single chain (polymer molecule) consisting of i repeat units is said to have a degree of polymerisation of i. The molecular weight of such an individual chain (neglecting any small difference due to end groups) may be designated M i , and the species may be referred to as an 'i-mer' or'ith species'. Since, at least in the ideal case, M i is the sum of the weights of all the repeat units in the chain, its value will be different for molecular chains differing in the value of i. The scatter of Mi values (the molecular weight distribution) in a given amount (i.e. sample, batch, etc.) of polymer may be wide or narrow but, as the chains are not all identical, a single molecular weight figure quoted for the whole amount can only be an average value.

14

W. V. Titow

Depending on the method of determination, this value will in practice normally be a number-average molecular weight (M o ) , weight-average molecular weight (Mw ) , or viseosityaverage molecular weight (Mv ).20 For the same batch of polymer these values are numerically in the sequence M w > Mv > Mo, with M v usually closer to Mw than to Mo. In the ideal, theoretical case of all chains being of identical length (the same value of i) M o = M w • Solution viscosity measurement is comparatively straightforward (especially with polymers soluble in convenient solvents) and the data it yields can be used to calculate either M v or-more commonly with PVC in industrial practicesuch related quantities as specific viscosity, * viscosity number, t logarithmic viscosity number or K value§ (ct. also Chapter 2, Table 2.1). The relationships between these various quantities (valid only when the relevant viscosities are determined under the same, standard conditions) are as follows:

,*

Specific viscosity (viscosity increment):

(Tf - Tfo)/Tfo

Viscosity number (formerly known as 'reduced viscosity' or'RV'): (Tf - Tfo)ITfoc Logarithmic viscosity number II (formerly known as 'inherent viscosity' or 'logarithmic reduced viscosity', Le. 'IV' or 'RV'): [In (TfITfo)]le where Tf is the viscosity of a dilute polymer solution (or its time of flow in standard conditions); Tfo is the viscosity of the solvent alone (or its time of flow in standard conditions); c is the concentration of the polymer, in g per ml of solution; and TfITfo is the viscosity ratio (formerly known as 'relative viscosity'). * Still sometimes determined by the (now superseded) Method B of ASTM D 1243-58T. t Method of ISO 174-1974. :j: Method of ASTM D 1243-66 (reapproved 1972). § For PVC normally the Fikentscher K value-ct. DIN 53 726-1983. (see also Chapter 2, Note in Section 2.2.2, and Table 2.1). II Not to be confused with the 'limiting viscosity number' (formerly known as 'intrinsic viscosity'): Iimc-+o [(fl- flo)/f/o C] or limc-+o [(In fl/flo)/c).

15

1 Introduction

The Fikentscher K values corresponding to a range of viscosity ratios of dilute solutions (0·005 g ml- 1 ) of PVC polymer in cyclohexanone at 25°C are listed in DIN 537261983. Most commercial PVC polymers have Fikentscher K values within the range 50-80, equivalent to about 50000-500 000 M w , and 3000090 000 MD. The relationship is illustrated in Fig. 1.2. Polymers of much higher molecular weight have also been made. Like those of other thermoplastics, the properties of PVC polymer are influenced by both the molecular weight and molecular weight distribution: the ratio 100000

5'8

90000

80000

70000

r{ 60000

5-2

~

I~

'"

o

.J

50

5-0+-100000 40000

30000

4·8

--50000 20000 4·6

Fig. 1.2 Relationship between the molecular weight (weight-average and number-average) and the K value of PVC polymers (the Fikentscher K value determined at 25°C on 0·005 g ml- 1 polymer solution in cyclohexanone--d. Chapter 2, Table 2.1 and Note in Section 2.2.2).

w. v.

16

Titow

Mw/M n (known as the 'dispersion of distribution') is a function of the extent of the latter (i.e. the width of the distribution curve, although it is not influenced by its shape). The evaluation of the molecular weight of vinyl chloride homo- and copolymers by gel-permeation chromatography was reviewed recently by Janca and Kolinsky?! The polymerisation temperature influences the molecular weight, and hence the dilute-solution viscosity, of PVC polymers (higher molecular weights obtained at low temperatures). This factor was examined by Ravey and Waterman 22 for the polymerisation temperature range 0-70°C. Intensive heating may be necessary to ensure complete dissolution of PVC polymers produced at sub-zero temperatures, for solution viscosity measurement and determination of M w by light scattering and gel-permeation chromatography.23 The structure of PVC polymer molecules deviates in practice from the theoretical ideal of linear chains of -CHz-CHCl- units terminating in -CHz-CHzCl and -CHCl-CH3 groups. The main differences are listed below: CHAIN BRANCHING

The chains of commercial PVC polymers are branched. Estimates of the extent of branching, based on determinations by various techniques,24,25 range from 0·5 to 20 branches per 1000 carbon atoms, and include the suggestion that 5 out of every 1000 carbons are methyl-branched whilst up to 2 per 1000 carry side-chains of more than 5 carbon atoms. Z6 The two most probable structures for a branch junction are believed to be Z5 ~CH2-CHCI-CH-CHa-CH2-CHCl~

I

R

and ~CH2-CHCI-CH2-TH-CH2-CHCl~

R

where the side-chain R is -CHz-CHCI~, or -CH3 . The presence in PVC polymers of branch junctions involving the

1 Introduction

17

existence of a tertiary chlorine ~CHz-CCl-CHz-eHCI~

I CHz I CHCI

1

has been the subject of much speculation,25 * as their formation (by appropriate radical transfer to polymer in the course of polymerisation) is a valid theoretical possibility, whilst-if present-they would constitute important sites for initiation of thermal or photo degradation of the polymer (owing to the relative ease with which the tertiary CI can be split off-see discussion in Chapter 9). However, no direct evidence of the presence of tertiary CI, or of any functional relationship between the number of chain branches and thermal stability, could be found in a number of studies concerned with these subjects. 25 ,27-30 END-GROUPS

A large variety of end-groups is encountered in commercial PVC polymers. Some are acquired by reactions, in the course of polymerisation, with fragments of initiators, emulsifying or suspending agents, or other 'external' compounds present (to which chain transfer can occur). Others are formed in terminating reactions (chain transfer to monomer or polymer, disproportionation, coupling) involving the monomer and/or any of the species generated during polymerisation. Examples of the end-groups originating in the former way are ~CH2-0C-C6H5; ~CH2-o-S02-0R; and ~CH20H (where R is an alkyl radical): those formed in the 'internal' terminating reactions are ~CHCI-CH=CH2 (an allyl-chloride type of structure in which the CI atom is activated by the unsaturated linkage at the 3,4-position relative to it31 ); ~CH CHCI; ~CCl=CH2; ~CH2-CHCI2; ~CHCI-CH2Cl; and ~CHr-CH2Cl. * cf. also, for example, the discussion by L. I. Nass of the thermal stability of PVC, in the Encyclopedia of pvc (see General Bibliography section at the end of this chapter).

18

w.

V. Titow

OTHER STRUCTURAL PEATURES

The results of several studies32 ,33 confirm the presence of double bonds, distributed randomly in the polymer chain: ordinary PVC polymers may contain up to 15 such bonds per 1000 carbon atoms. 25 Some evidence has also been obtained of the presence of oxygen: 25 apart from oxygen-containing end-groups (see above) some oxygen could be incorporated in the chain during polymerisation, or acquired through oxidation of the polymer; however, the resulting chemical groupings have not been identified.

(b) Morphology Commercial PVC polymers may be regarded as essentially amorphous,34 although crystalline material contents of about 2-10% have been reported on the basis of determinations by X-ray diffraction methods, thermal analysis (DTA, DSC, TMA) , and density measurements. 35-37 The crystallinity is associated with the stereoregular (syndiotactic) polymer fraction. 38-40 The glass transition temperatures (Tg ) of commercial homopolymers lie in the range 80-84°C (as determined by DTA, DSC and TMA).34,35 Annealing above the Tg increases crystallinity and also the crystalline melting temperature (as given by the endothermic peak on DTA curves).35,37 The density of the crystalline fraction has been reported as 1530 kg m-3 (1,530 g cm-3) and that of totally amorphous (quenched) polymer as 1337kgm- 3 (1·337gcm- 3).35 As would be expected, annealing below the Tg has no effect on crystallinity: however it can increase the density-this has been attributed to a reduction of the free volume in the polymer without ordering of its fine structure. 35 Apparently the kind of short-range non-crystalline ('domain') order which can develop in some essentially glassy, but partly crystallisable polymers (bisphenol-A polycarbonate, polyethylene terephthalate)41-44 on heating below the Tg , does not arise in PVc. Relatively highly crystalline PVC polymers (up to about 45% crystallinity) with high syndiotactic material content, have also been prepared (by polymerisation at low temperatures or in certain solvents).24,36,38,45,46 The preparation of fibres and film from such polymers has been reported47 as well as that of plasticised compounds (with about 50 phr DOp).38,47 The melting points of the crystalline polymers can be as high as 265_273°C24 ,46 (cf. commercial PVC polymer-about 210°C in the absence of decomposition). The

i

introduction

19

above-mentioned plasticised compounds of crystalline PVC had a higher modulus and hardness, and lower tensile creep at room temperature than a similar compound of ordinary commercial PVC polymer (but lower tensile strength and extension at break): below -25°C the modulus and hardness were lower (i.e. the flexibility greater) than those of the normal PVC.38 Molecular orientation of the PVC polymer, and the associated structural anisotropy, can be an important factor in the morphology (and the properties) of PVC products (especially uPVC). Thus in PVC mouldings the skin-and-core effects (a well-known, common feature also of other polymer mouldings) can involve, inter alia, a considerable degree of orientation in the skin. 48 Biaxial orientation of the polymer in PVC bottles, films and thermoformed articles increases impact resistance (ct., for example, Chapter 17) and reduces permeability (ct. Chapter 12, Section 12.4). The tensile strength and retraction on heating of extruded products is strongly influenced by longitudinal molecular orientation imparted by stretching in production (ct., for example, Chapter 12, Section 12.3), whilst the high degree of orientation produced in PVC fibres by the drawing process in manufacture is responsible for their very high tensile strength in comparison with other PVC products (cf. Appendix 3). The fracture and yield behaviour of PVC polymers and uPVC compositions is strongly influenced by the extent and nature of molecular orientation. Useful investigational work in these areas has been reported by several authors.49.50--52 1.5.2 Vinyl Chloride Copolymers

Some of these are long-established commercial materials. Others, more recent, have also become of more than academic interest. The oldest, and still most widely used, are vinyl chloride/vinyl acetate copolymers (VCNA). In most of the vinyl chloride copolymers of commercial interest the co-monomer units are in a minor proportion (and randomly distributed) in the polymer chain, i.e. most are internal, random copolymers, with the VC units predominating. Copolymers with vinylidene chloride (VCNDC) are a notable partial exception here, in that whilst those used in certain PVC compositions (e.g. some calendering compounds) for ease of heat-processing contain relatively small amounts of VDC co-monomer, in others (e.g. those for making

20

W. V. Titow

self-supporting, low-permeability films, or barrier layers in composite films) it is the VC co-monomer which is the minor constituent (usually 10-15%). Another exception is constituted by acrylic and (some) modacrylic fibres: by definition,53 the former must contain not less than 85% of acrylonitrile units in the chain (i.e. only up to 15% of a co-monomer, which may be VC), and the latter between 85% and 35%-although the material of Dynel (Union Carbide), a well-known modacrylic fibre, was in fact a 60/40 copolymer of vinyl chloride and acrylonitrile (see also Table 1.2). Some PVC graft copolymers are also noteworthy, e.g. those with ethylene/vinyl acetate copolymers (EVA),62,67 polyolefins,62,68 butadiene/acrylic ester copolymers,62 and acrylic ester polymers. Graft copolymers of the first three kinds were the subjects of early patents by, respectively, Bayer and Dynamit Nobel, Montecatini, and Pechiney-Saint-Gobain. 47 VClEVA graft copolymers are used (typically in blends with PVC homopolymer) in uPVC compositions for outdoor service, notably in window frames, where good weatherability and impact resistance at low temperature are required;3 VC/acrylate grafts are also employed for this application (see Chapter 19, Section 19.4.3). Some graft copolymers with sufficiently high polyvinyl chloride contents (e.g. VC/EVA with 50-70% EVA) can act as plasticisers for PVC homopolymer, or as processing aids. The graft copolymers are, as a rule, more expensive than main-chain copolymers. As has been mentioned in Section 1.2, the chief effects of the presence of a significant proportion of a co-monomer in the vinyl chloride polymer chain are normally similar to (but, in general, more permanent than) those of plasticising a homopolymer with 'external' plasticisers: the processing temperature is reduced (albeit the heat stability also decreases) as is the Tg (and hence the softening temperature and temperature of deflection under load); the hardness also usually decreases, and the extensibility increases.

Note: A partial exception to the general trend, which is of some practical significance, may be noted: some copolymers of vinyl chloride with N-substituted maleinimide derivatives 3,69 have Tg values and Vicat softening points significantly higher than those of vinyl chloride homopolymer: ct., e.g., Hostalit LP HT 5060 (Hoechst)-a copolymer containing 5% of Ncyclohexylmaleinimide. 69 Copolymers are normally more readily soluble than homopolymers (see Chapter 12, Section 12.8, and Chapter 24): when used as surface

1 Introduction

21

coatings they adhere better to many substrates (the adhesion may be further improved by the incorporation of a suitable third co-monomer in the chain-see Chapter 24). Broadly speaking, the morphology of most copolymers is similar to that of PVC homopolymer, except that the reduced regularity of the chain is an extra hindrance to crystallisation. This occurs also in VDCNC copolymers where VC is the minor component: the chain structure of PVDC is favourable to crystallisation and the crystallinity of the homopolymer is normally high;4o.56 the structural regularity and hence ease of crystallisation is progressively reduced as the VC unit content of the polymer chain is increased, until at VC contents;?; 30% the copolymer becomes non-crystalline. It is for this reason that VDCNC copolymer films for barrier applications contain only about 10-15% VC. This content level represents a reasonable combination of easement of processing (the highly crystalline PVDC homopolymer requires high temperatures) and retention of much of the excellent barrier effect of the homopolymer associated with its crystallinity (crystalline regions in polymers are normally impenetrable to diffusant molecules). 1.5.3 'External' Modification of PVC by Other Polymers

PVC polymers can be modified 'externally' by blending with other polymers or copolymers (including vinyl chloride copolymers-see Section 1.5.2 above). On the industrial scale this is widely practised in uPVC compositions, to improve the melt processability, and/or the toughness (impact resistance at normal and low temperatures) as well as-in some cases-the resistance to heat distortion, of the finished product. The polymeric additives incorporated for these purposes are known as processing aids and impact modifiers: they are discussed in Chapter 11. Those of the polymeric additives which are chlorinated (but not vinyl chloride) polymers-e.g. chlorinated polyethylene (see Chapter 11)as well as copolymers of vinyl chloride (e.g. VCNA, VCNDC, or the VC graft copolymers mentioned in Section 1.5.2 above) can have particularly high compatibility with PVC resins: they can sometimes be blended in such large proportions that the composition becomes a plasticised PVC (with high permanence of properties, because the plasticiser is a high polymer-see Chapter 11, Section 11.2). This can also be done with some chlorine-free polymeric additives, e.g. nitrile rubbers (see Chapter 11).

Vinyl chloride/ethylene

Vinyl chloride/propylene (VC/P)

Vinyl chloride/acrylorutrile (VClAN)

Vinyl chloride/vinylidene chloride (VClVDC)

Similar to vinyl chloride/propylene copolymers-S

2. Extruded films (packaging)-S 3. Viscosity-reducing porymer in pastes-S 4. Solution applications (esp. barrier coatingsr-SL 5. Latex applications (esp. paper and textile finishing)-E 6. Fibres, e.g. Saran" (National Plastics Products Co.) Fibres (vinyl chloride is the comonomer in some acrylic or modacrylic fibres, e.g. DyneF (Union Carbide Chemicals Co.) Extruded films (packaging); injection mouldings-S

Refs 57 and 58--polymer production, structure and properties; Ref. 59-polymer composition and density; Ref. ~ackaging film Ref. 61

Commercially available from Air Products and Chemicals Ltd, USA (Airco 400 series) Developed by Union Carbide Chemicals Co.

Refs 40 and 53

Refs 40 and 47-general nature and preparation of the copolymers; Ref. 55-packal;\ing films; Ref. 56-general revIew (with 92 references)

Literature on this copolymer is extensive-see, e.g., relevant titles in the General Bibliography section at the end of this chapter

Literature/References

Chapter 26

6. Chapter 26

5. Chapter23

2. Chapters 19 and 26 3. Chapters 21 and 22 4. Chapters 12, 24 and 26

4. Mentioned, inter alia, in Refs 53 and 54 1. Chapters 2, 3, and 18

2. Chapters 24 and 26. The copolymers used for these applications sometimes contain a third co-monomer 3. Chapters 23 and 26

2. Coatings (solution applications)-SL

3. Adhesives, finishing agents (paper and textiles)-E 4. Fibres, e.g. Vinyon H H (American Viscose Corp.) 1. U nplasticised calendered sheets; mouldings-S

1. Chapters 2, 3 and 26

1. Unplasticised mouldinss (including gramophone records) and sheeting (including sheets for thermoforming and PVC ftooring)-S

Vinyl chloride/vinyl acetate (VCNA)

Relevant chapters/Remarks

Main applications (with indication of the usual method ofproduction of copolymer for use in the applicationa)

Copolymer

TABLE 1.2 Vinyl Chloride Copolymers



;:s

~.

f2-

~

;;-

'-

24

w.

V. Titow

Blends have also been prepared of PVC polymer with a copolymer of vinyl chloride and an unsaturated dimethacrylate compound (DMA), in which the latter component could be cross-linked (via the double bonds in the DMA) to provide a PVC material of improved strength and stability, and better processability when vacuum-formed as a sheet. 7o A further refinement of this concept (and a PVC alloy with an unusual structure) is represented by a blend of PVC polymer with a butadiene/acrylonitrile copolymer in which both components are cross-linked to form two separate but intimately interpenetrating networks. Depending on the cross-link density the properties of the material can range from those of a tough elastomer to those of a soft plastic. 71 Improved ease of processing and higher temperatures of deflection under load are claimed for blends of PVC with styrene/maleic anhydride copolymers (d. for example, Bourland and Wambach in Plastics Engineering, 1983, 39(5), 23-7). Some versions of the blends are available as commercial injection-moulding compounds (e.g. from the Arco Chemical Co., USA). 1.5.4 Properties of PVC Compositions To make their processing possible, and to achieve the required performance in service, PVC polymers are compounded with various additives to make up the compositions which are the substance of the PVC materials and products of industry and commerce. The properties of these compositions form one of the major topics dealt with in this book: almost every chapter features one or more of their aspects, including their durability, their individual and relative importance in particular contexts and applications, their measurement, the ways in which they are influenced by formulation and processing, and others. Many numerical values of properties characteristic of various compositions and products are quoted in Appendix 3, as well as throughout the text. 1.6 CHLORINATED POLYVINYL CHLORIDE (CPVC) This is an old-established material, first produced commercially in the mid-1930s in Germany by chlorination of PVC polymer in solution (in a chlorinated hydrocarbon solvent-typically tetrachloroethane or

1 Introduction

25

chloroform) at elevated temperatures (50 to about 100°C). CPVC made by this process is more soluble in solvents than the parent PVc. The early commercial materials-e.g. Igelit PC (I. G. Farbenindustrie) and Rhenoflex (Dynamit Nobel)-were used in solution-applied surface coatings and adhesives. CPVC fibres were also spun from solvent solutions. These applications still continue to some extent. Around 1960 the dispersion chlorination process came into use. In the originally patented version of this4o ,47 PVC polymer in aqueous dispersion is treated with a large excess of chlorine at relatively low temperatures (up to 60°C) in the presence of a swelling agent (a chlorinated hydrocarbon, e.g. chloroform) under UV light. The early commercial polymers produced in this way are exemplified by Trovidur HT (Dynamit Nobel) and Geon HT (Goodrich). Dispersionchlorinated CPVC polymers are less soluble than those produced by the solution process and their thermal stability is better. In both processes chlorination takes place mainly at the -CHzgroups of the PVC polymer chain (Le. the 1: 2 chlorinated configuration, -CHCI-CHCI-, is preferentially formed) so that the resulting chain structure becomes virtually that of a copolymer of vinyl chloride with 1: 2 dichloroethylene* «a) in Fig. 1.3), rather than that of a vinyl chloride/vinylidene chloride copolymer «b) in Fig. 1.3) which would be given by preferential 1: 1 chlorination. t The large preponderance of the 1: 2 chlorination is shown by the IR spectra of CPVC. It is also evidenced by the products of thermal decomposition: those generated -CHCl-CHCl-CHz-CHCl-CHCl-CHCl-CHCl-CHCl(a) Vinyl chloride/1:2 dichloroethylene copolymer or a chlorinated PVC -CHz-CClz-CHz-CHCl-CHz-CClz-CHz-CClz(b) Vinyl chloride/vinylidene chloride copolymer -CHCl-CHCl-CHCl-CHCl-CHCl-CHCl-CHCl-CHCl(c) Homopolymer of 1: 2 dichloroethylene Fig. 1.3 Simplified representation of polymer segment structures.

* Production by direct polymerisation of the monomers impracticable, although some brittle, low molecular weight products have been obtained in attempts to prepare the homopolymer. 40 t In the dispersion process tendency to 1: 1 chlorination can be increased at high temperatures if the chlorine concentration is allowed to fall. 4o

26

W. V. Titow

by CPVC contain virtually none of the aromatic hydrocarbons produced by the pyrolysis of both PVC homopolymer (see Chapter 12, Section 12.9.3) and VCIVDC copolymers. 4o Complete chlorination would give a polymer very similar to the symmetrical polydichloroethylene* «c) in Fig. 1.3); however, the degree of chlorination of commercial CPVC polymers is considerably lower than this (see Table 1.3). The density and Tg (and hence the Vicat softening point) increase with the chlorine content. TABLE 1.3 Some Properties of Commercial CPVC and PVC Polymers, and FuUy Chlorinated PVC CPVC polymer Chlorine content (weight %) Density (gcm- 3)

Tgeq

Maximum service temperature (for commercial compounds) eq Continuous exposurec Intermittent exposure c

PVC Fully chlorinated homopolymer PVC polymer

65·67 1·52-1·59b 99-123

56·8 1·40 80-84

90

65 80

110

73·2" 1·70 175

"Theoretical figure for polymer of 1 :2 dichloroethylene. b Density of commercial compounds: 1·47-1·62. This coincides almost exactly with the range specified for CPVC pipes and fittings in ISO 3514-1976. C In non-aggressive environments.

Since the same kind of site (the -CH2- group) is preferentially chlorinated in the chains of CPVC polymers prepared by either process, and the total chlorine content ranges are essentially the same, other structural factors must be responsible for the differences in solubility and thermal stability between the products of the two processes. The balance of evidence from investigations carried out since early times47 indicates that these differences are associated with the way in which the chlorine atoms substituted into the -CH2groups are distributed within the polymer chain: the distribution does vary according to the method of preparation in a manner suggesting * Production by direct polymerisation of the monomer impracticable, although some brittle, low molecular weight products have been obtained in attempts to prepare the homopolymer. 40

1 Introduction

27

that the variation is due to different accessibility of the polymer chains to the chlorine in the two processes. Solution chlorination has been reported47 to result in a uniform, random, 'statistical' distribution of the chlorine among the -CHz- groups of the molecular chain, such as would be expected if all the chains (and all segments within an individual chain) were equally accessible to the reagent. The chlorination produced by the dispersior process is believed47 to be hetrogeneous, in two senses: the chlorine contents of different molecular chains are not the same, and each individual chain contains irregularly alternating blocks of polyvinyl chloride and 1: 2 dichloroethylene polymer structures. It has also been found 47 that CPVC produced by the dispersion process from PVC polymer of high steroregularity (high syndiotactic material content) has a higher Vicat softening point than one similarly produced from polymer of low stereoregularity, whilst there is no corresponding difference between analogous solution-chlorinated materials. Chlorination of PVC polymers reduces the forces of attraction between the molecular chains, as evidenced, for example, by the comparatively greater ease and extent of stretching of CPVC films above the Tg . 72 The essentially amorphous morphology of CPVC polymers is probably a factor in this effect, as even the small amount of crystalline material present in commercial PVC polymers would have a constraining effect (with the crystallites acting as quasi cross-links) at temperatures up to the crystalline melting point. In comparison with uPVC, the effect of stretching (especially biaxial stretching) of CPVC sheet upon some of its properties (increases in Young's modulus and yield stress) is greater, although the permeability to COz of CPVC sheet was found to increase with biaxial orientation, in contrast with the reverse effect observed with uPVC: 72 as pointed out by the investigators, the increase in permeability on biaxial stretching is characteristic of essentially amorphous polymers which do not crystallise under tension. 72 ,73 However, the increase in the impact strength of the CPVC sheet, which was also achieved by biaxial stretching in the above investigation, was claimed to be greater than that attainable-in the absence of molecular orientation-through incorporation of impact modifiers. Commercial CPVC compounds are formulated on the same general lines as uPVC compounds (see Chapter 4). However their processing is influenced by the fact that the melt viscosity of the polymer increases sharply with the chlorine content. 74 The compounds are used mainly

28

W. V. Titow

for the production of pipes and pipe fittings for hot-water installations (including, increasingly, domestic central heating systems), where the general similarity of properties to uPVC (including, inter alia, suitability for jointing by solvent welding) combined with the greatly increased temperature resistance in service, are particularly advantageous. Other applications include pipes and fittings for potable water (CPVC is approved for this purpose by several professional and regulatory bodies 75), pipework and associated products (fittings, valves, tanks) for chemical plant (the general chemical resistance of CPVC is comparable with that of uPVC) , extruded profiles, sheets (including co-extruded CPVClpPVC sheets), some electrical appliances, and constructional applications. Some examples of commercial CPVC compounds are: the Lucalor range (Rhone-Poulenc, France), which includes Lucalor RB 1266 specially developed, and recently evaluated, for central heating systems; the Dekadur compounds (Deutsche Kapillar Plastik, West Germany) and the CPVC compounds in the Geon range (B. F. Goodrich, USA). Some of the properties of three Geon compounds are listed, by way of example, in Table 1.4. TABLE 1.4 Some Properties of 'Geon' CPVC Compounds

(Based on manufacturer's published data)

Property

Tensile strength (lbf in- 2 ) (ASTMD 1708) Flexural strength (lbf in- 2) (ASTMD790) Flexural modulus (lbf in- 2) (ASTMD790) Izod (notched) impact strength (ft lbf in -1) Deflection temperature under load eC): at 264lbf in- 2 (ASTMD648) Specific gravity (ASTMD792)

Geon 88933 (high temperature extrusion and injection moulding)

Geon 88934 Geon 88935 (extrusion of (profile co-extrusion pipes and with pPVC-high profiles) ductility compound)

8200

8400

7300

14500

15600

13600

387000

395000

396000

2·3

2·0

3·2

100

102

82

1·52

1·57

1·47

1 Introduction

29

It may be noted in passing that its higher chlorine content reduces the flammability of CPVC in comparison with uPVC.

1.7 MATERIAL AND TEST STANDARDS The properties of PVC materials and products, as well as methods of their characterisation and testing, are the collective subject of a very large number of standard specifications. Whilst some companies (particularly polymer manufacturers) and big user organisations (e.g. government and military procurement departments, motor car manufacturers) operate their own in some cases, the standards of by far the greatest importance to the PVC technologist and user in the Western World are those of the following four groups: (i)

Standards developed by the appropriate technical committees of the International Organisation for Standardization and published by that organisation (ISO standards). The ISO committees dealing with plastics are TC 61: Plastics and TC 138:

Plastic Pipes, Fittings and Valves for the Transport of Fluids. (ii) Standards of the British Standards Institution (BS standards). (iii) Standards of the American Society for Testing and Materials (ASTM standards) (iv) Standards of the German Institute for Standards (DIN Deutsches Institut fUr Normung: DIN standards).

Most other countries also issue their own national standard specifications. Those standards from the four main sources which relate directly to PVC are listed (by number and title) in Appendix 1. The list is divided into sections, grouping the standards by subject and also largely according to their relevance to the chapters dealing with particular topics in this book. * In addition, many 'plastics' standards not specifically or primarily directed to PVC, but nevertheless relevant to particular aspects of PVC materials, products or technology, are mentioned in the introduction to Appendix 1, in Appendix 3, and in various appropriate places in the book. The numerous references to standard specifications throughout this * For example, Section 4 of Appendix 1 lists standards dealing with various aspects of plasticisers, and is thus directly relevant to Chapters 5-7.

30

W. V. Titow

book, and the listings in Appendices 1 and 3, contain in almost every individual case not just the specification number, but also a year of issue, since this can serve as a useful point of reference. Most of the years of issue so quoted should be current at the time of going to press, but it will be appreciated that international and national standards are being periodically amended and revised, with consecutive issues appearing under newer dates. Entirely new standards are also being brought out. The introduction to Appendix 1 provides guidance on keeping up to date with proposed, new, and revised standards. The excellent book by Ives et ai. 76 served for a long time as a valuable source of information on standard tests for plastics (including PVC). An updated version, produced by an editorial team, is now available in a new edition. 77

REFERENCES 1. Regnault, V. (1838). Ann. Chim. Phys., 2,69, 151. 2. Drukker, H. L. (1944). Proc. of Symposium on Plastics, Am. Soc. for Testing Materials, Philadelphia, Pa, USA, pp. 165-77. 3. Domininghaus, H. (1976). Die Kunststoffe und Ihre Eigenschaften, VDI-Verlag GmbH, Diisseldorf, p. 566. 4. Baumann, E. (1872). Ann. Chim. Phys., 163, 308-12. 5. Tester, D. A. (1973). In Developments in PVC Technology, (Eds J. H. L. Henson and A. Whelan), Applied Science Publishers, London, Chapter 1. 6. Ostromislensky, 1. (1912). British Patent No. 6299; German Patent No. 264123. 7. Brydson, J. A. (1975). Plastics Materials, Newnes-Butterworths, London, pp. 248-9. 8. British Patent No. 408969, Carbide and Carbon Chemicals Corp., (1934). 9. US Patent No.1 938662, Du Pont, (1933). 10. British Patent No. 387928, British Thomson-Houston, (1932). 11. Canadian Patent No. 346164 (1934). 12. British Patent No. 388309 (1933); US Patent No. 1932889 (1933). 13. British Patent No. 412442 (1934). 14. German Patent No. 470 149 (1927). 15. Kaufman, M. (1969). Plast. Polym., 37(129),243-51. 16. Kaufman, M. (1969). The History of PVC, Elsevier, London. 17. Anon. (1974). Chern. Engng. News, 52(35),8. 18. Trevitt, E. W. (1976). Polym. Paint Col. J., 166(3918), 193-4. 19. Anon. (1979). Eur. Plast. News, 6(6), 8. 20. Billingham, N. C. and Jenkins, A. D. (1972). In Polymer Science, Vol. 1, (Ed. A. D. Jenkins), North-Holland Publishing Co., AmsterdamLondon, Chapter 2.

i

introduction

31

21. Janca, J. and Kolinsky, M. (1976). Plasty a Kaucuk, 13(5), 138-41. 22. Ravey, M. and Waterman, J. A. (1975). J. Polym. Sci., Polym. Chem. Ed., 13(6), 1475-8. 23. Tavan, M., Palma, G. and Carenza, M. (1975). J. Appl. Polym. ScL, 19(9),2625-7. 24. Pezzin, G. (1969). Plast. Polym., 37(130), 295-301. 25. Braun, D. (1975). In Degradation and Stabilisation of Polymers, (Ed. G. Geuskens), Applied Science Publishers, London, Chapter 2. 26. Schwenk, V., Cavagna, F., Lomker, F., Konig, I. and Streitberger, H. (1979). J. Appl. Polym. Sci., 23, 1589-93. 27. Caraculacu, A. A. (1966). J. Polym. Sci., A-i, 4, 1829, 1839. 28. Caraculacu, A. A., Bezdadea, E. C. and Istrate-Robila, G. (1970). Ibid., 8, 1239. 29. Braun, D. and Weiss, F. (1970). Angew. Makromol. Chem., 13(55), 67-71. 30. Suzuki, T., Nakamura, M., Yasuda, M. and Tatsumi, J. (1971). J. Polym. Sci., C, 33, 281. 31. Fieser, L. F. and Fieser, M. (1944). Organic Chemistry, D. C. Heath & Co., Boston, pp. 152-5. 32. Valko, L. and Tvaroska, I. (1972). Angew. Makromol. Chem., 23, 173. 33. Braun, D. and Quarg, W. (1973). Ibid., 29/30, 163. 34. Haward, R. N. (Ed.) (1973). The Physics of Glassy Polymers, Applied Science Publishers, London, pp. 201-6. 35. Gray, A. and Gilbert M. (1976). Polymer, 17(1), 44-50. 36. D'Amato, R. J. and Strella, S. (1969). Applied Polymer Symposia, No.8, 275-86. 37. Ohta, S., Kajiyama, T. and Takayanagi, M. (1976), Polym. Engng. Sci., 16(7), 465-72. 38. Gugelmetto, P., Pezzin, G., Cerri, E. and Zinelli, G. (1971). Plast. Polym., 39(144), 398-402. 39. Abdel-Alim, A. H. (1975). J. Appl. Polym. Sci., 19(8), 2179-85. 40. Brighton, C. A. (1962). In Advances in PVC Compounding and Processing (Ed. M. Kaufman), Maclaren & Sons Ltd, London, Chapter 1. 41. Titow, W. V., Braden, M., Currell, B. R. and Loneragan, R. J. (1974). J. Appl. Polym. Sci., 18,867-86. 42. Frank, W., Goddar, H. and Stuart, H. A. (1967). Polym. Lett., J. Polym. Sci.. 5,711. 43. Siegmann, A. and Geil, P. H. (1970). 1. Macromol. Sci. (Phys.), 84(2), 239. 44. Kashmiri, M. I. and Sheldon, R. P. (1969). Polym. Lett., J. Polym. Sci., B,7,51. 45. Bockman, O. C. (1965). Brit. Plast., 38(6), 364-5. 46. Gouinlock, E. V. (1975). J. Polym. Sci., Polym. Phys. Ed., 13(5),961-70, and 13(8), 1533-42. 47. Bier, G. (1965). Kunststoffe, 55(9),694-700. 48. Copsey, C. J., Gilbert, M., Marshall, D. E. and Vyvoda, J. C. (1978). 'The dependence of PVC structure and properties on injection moulding variables', paper presented at the PRI International Conference on PVC

32

w. V. Titow

Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 49. Rider, J. G. and Hargreaves, E. (1969). J. Polym. Sci., A-2, 7,829-44. 50. Miller, L. E., Puttick, K. E. and Rider, J. G. (1971). J. Polym. Sci., C, 33, 13-22. 51. Smith, K., Hall, M. G. and Hay, J. N. (1976). Polym. Lett., J. Polym. Sci., 14(12), 751-5. 52. Brady, T. E. (1976). Polym. Engng. Sci., 16(9),638-44. 53. Cook, J. G. (1964). Handbook of Textile Fibres, Merrow Publishing Co., Watford, England. 54. Dux, J. P. (1970). 'Vinyon and related fibres' in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 21, 2nd Edn, John Wiley, New York, pp. 441-51. 55. Oswin, C. R. (1975). Plastic Films and Packaging, Applied Science Publishers, London. 56. Sauntson, B. J. and Brown, G. (1971). Reports on the Progress of Applied Chemistry: Plastics, LVI, 66-76 (Society of Chemical Industry). 57. Cantow, M. J. R, Cline, C. W., Heiberger, C. A., Huibers, D. Th. A. and Phillips, R (1969). Mod. Plast., 46(6), 126-38. 58. Heiberger, C. A., Phillips, Rand Cantow, M. J. R (1969). Polym. Enging. Sci., 9(6), 445-51. 59. Ravey, M. (1975). J. Polym. Sci., Polym. Chem. Ed., 13(11),2635-7. 60. Briston, J. (1976). Packag. Rev., 96(3), 71-2. 61. Sarvetnik, H. A. (1969). Polyvinyl Chloride, Van Nostrand, New York. 62. Goebel, W., Bartl, H., Hardt, D. and Reischl, A. (1965). Kunststoffe, 55, 329-32. 63. Edser, M. H. and Bulezuik, B. W. (1974). Polym. Paint Col. J., (4th December), 1051-6. 64. Ulbricht, J. and Rassler, K. (1976). Plaste u. Kaut., 23(7),487-90. 65. Albert, W. (1963). Kunststoffe, 53(2), 86-93. 66. Bohn, L. (1963). Kunststoffe, 53(2), 93-9. 67. Edser, M. H. and Bulezuik, B. W. (1974). Loc. cit., (18th December), 1090-4. 68. Pegoraro, M., Szilagyi, L., Locati, G., Ballabio, A., Severini, F. and Natta, G. (1968). Chimica e Ind., 50(10), 1075-81. 69. Kiihne, G., Andrascheck, H. J. and Huber, H. (1973). Kunststoffe, 63(3), 139-42. 70. Sasaki, I. and Ide, F. (1975). Polym. Lett., J. Polym. Sci., 13(7), 427-32. 71. Sperling, L. H., Thomas, D. A., Lorenz, J. E. and Nagel, E. J. (1975). J. Appl. Polym. Sci., 19(8), 2225-33. 72. De Vries, A. J. and Bonnebat, C. (1976). Polym. Engng. Sci., 16(2), 93-100. 73. Hopfenberg, H. B. and Stannett, V. (1973). In The Physics of Glassy Polymers, (Ed. R N. Haward), Applied Science Publishers, London, Chapter 9. 74. Arnold, G. H. (1970). Plast. Polym., 38(133),21-6. 75. Anon. (1979). Plast. Technol., 25(9), 31. 76. Ives, G. c., Mead, J. A. and Riley, M. M. (1971). Handbook of Plastics Test Methods, Iliffe Books, London.

I

Introduction

33

77. Brown, R. P. (Ed.) (1981) Handbook of Plastics Test Methods. 2nd Edn, George Godwin Ltd. and the PRI, London.

GENERAL BIBLIOGRAPHY ON PVC SPE Vinyl Professional Activity Group (1964). A Guide to the Literature and Patents Concerning Polyvinyl Chloride Technology, SPE, Stamford, Conn., USA. Sarvetnik, H. A. (1969). Polyvinyl Chloride, Van Nostrand, New York. Canton, M. J. R. (1970). 'Vinyl Polymers (Chloride)', in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 21, 2nd Edn, John Wiley, New York, pp. 369-412. Dux, J. P. (1971). 'Vinyl Chloride Polymers', in Encyclopedia of Polymer Science and Technology, Vol. 14, (Eds H. F. Mark and N. G. Gaylord), Wiley-Interscience, New York, pp. 305-483. Matthews, G. (1971). Vinyl Chloride and Vinyl Acetate Polymers, Plastics Institute Monograph, IIiffe Books, London. Sedlacek, B. (Ed.) (1971). Polyvinyl Chloride: Its Formation and Properties, Proceedings of IUPAC Symposium, Prague 1970. Butterworths, London. Sarvetnik, H. A. (Ed.) (1972). Plastisols and Organosols. Van Nostrand, New York. Brydson, J. A. (1975). Plastics Materials, 3rd Edn, Newnes-Butterworths London, Chapter 12. Yescombe, E. R. (1976). Plastics and Rubber: World Sources of Information, Applied Science Publishers, London, pp. 151, 177-80,359. Nass, L. I. (Ed.) (1978). Encyclopedia of PVC, Marcel Dekker, New York. Burgess, R. H. (Ed.) (1981). Manufacture and Processing of PVC, Applied Science Publishers, London. Owen, E. D. (Ed.) (1984). Degradation and Stabilisation of PVC, Elsevier Applied Science Publishers, London.

~

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.

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Fig. 5.4 Relationships between bulk properties of plasticisers and their plasticising properties. (Graph contributed by Mr T. C. Moorshead.)

room temperature for approximately 7 days before physical properties are determined. In the past, when lower molecular weight plasticisers were used, this effect was often attributed to plasticiser volatility. However, this is impossible since the effect is reversible. Reheating to the processing temperature and subsequent cooling causes the modulus and hardness to revert to their initial values. Several workers83 ,84 have followed the changes in elastic modulus and density with storage time and temperature, and it has been proposed that the stiffening is due to crystallisation of the PVC on storage. This has been confirmed by DSC measurements which show the development of endothermic peaks accompanying the stiffening. It is suggested that the crystallites can be melted on reheating to the processing temperature and re-form again slowly on cooling.

140

D. L. Buszard

130- 60

400

300

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I-

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7

10

11

12

'00

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Fig. 5.5 Relationship between Ap/Po ratio and physical properties of plasticised PVc. 37

5

141

Theoretical Aspects of Plasticisation

90

60

-6050

60

-50-

40

50

40

.

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142

D. L. Buszard

5.9 ANTIPLASTICISATION The addition of small quantities of plasticiser (up to 20%) to a PVC compound leads to an increase in modulus and tensile strength and a reduction in impact strength and elongation at break. This is the opposite behaviour to that which might normally be expected of a plasticiser and has been termed 'antiplasticisation'.85 The phenomenon has been known for a long time. Brous and Semon86 reported the anomalous behaviour of PVC containing up to 18% TCP as early as 1935, and since then it has been examined by many other workers. 87- 99 An illustration may be seen in Fig. 5.6 from the work of Ghersa,87 which shows the effect of low concentrations of DOP on tensile strength, elongation at break, tangent modulus and impact strength. As in plasticisation, the mechanism has not yet been fully elucidated, although the major features have been well researched. Horsley88 demonstrated by X-ray diffraction that systems containing low concentrations of plasticiser possessed increased order on a molecular scale, and attributed it to increasing crystallinity caused by the increased freedom of motion induced by the presence of the plasticiser. However, more recent X-ray and IR data have suggested that only minor changes in crystallinity accompany antiplasticisation. 9o ,91 Bohn,92 using viscoelastic measurements, related the onset of brittleness to the suppression of the viscoelastic f3 relaxation process.

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20

40

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Fig. 7.8 Effect of plasticiser level on 'Clash and Berg' low-temperature flexibility (BS 2782: 1970, Method 104B: NB Current revised version, BS 2782, Method 150B: 1976). 1, DOP; 2, DIDP; 3, DTDP; 4, DOA; 5, Reofos 65; 6, TOTM; 7, ESO; 8, PPA alcohol endstopped; 10, ~9P,

7 Properties of Plasticised

pvc

195

TABLE 7.2

Effect of ,Linevol' Phthalates and Adipates on Low-Temperature Properties of Identical PVC Formulationsa Linevol 79 adipatel Linevol 79 phthalate (Ph,)

Linevol 911 adipatel Linevol 911 phthalate (Ph,)

2-EH-adipatel 2-EH-phthalate (Ph,)

5010 3012020130 0150 5010 3012020130 0150 5010 3012020130 0150

Cold flex temperature

(BS 2782: 1970 M 104 B)("C) -44 -36 -31 -21 -23 -34 -30 -22 -35 -32 -24 -14 Cold flex temperature after 7 days at 100°C

(BS2782:1970MI04B)("C) -16 -15 -16 -13 -23 -30 -29 -22 +13

+9

-8

-5

a Extract

from technical literature of Shell Chemicals International Ltd, reproduced with their permission.

7.5 PERMANENCE PROPERTIES

It is obviously desirable that once a product has been manufactured from a flexible PVC formulation, it should continue to perform with a minimum of change in properties throughout its service life. Failure to perform satisfactorily may be the result of a number of factors: (i)

inaccurate initial specification, e.g. insufficient low-temperature properties; (ii) degradation by heat, light or possibly radiation; (iii) loss of plasticiser resulting in an undesirable change of properties.

The latter may be as a result of extraction, migration or volatile loss of plasticiser. The more practical aspects of these properties together with t\l. 0 important related areas, cable ageing and fogging, will be considered in this section. As was mentioned in Section 5.6.3 in Chapter 5 the rate of loss of plasticiser may be either diffusion- or surface-controlled. That is, the rate-determining step controlling the loss may be either the rate at which the plasticiser molecules can travel through the PVC matrix or the rate of loss of plasticiser from the surface. The dominant step depends on a number of factors but in general, where the overall rate of loss is slow, e.g. the volatile loss of plasticiser during service life, the process is surface-controlled. When the rate of loss is much higher,

D. L. Buszard

196

e.g. in a powerful extraction medium, the rate of plasticiser diffusion is more important. 7.5.1 Extraction Resistance One of the most important reasons for using polymeric plasticisers is their resistance to extraction by solvents. Some figures giving details of the physical properties and extraction resistance of various plasticisers are shown in Table 7.3. The polymeric plasticisers all exhibit better extraction resistance and, in general, somewhat inferior efficiency when compared with monomeric plasticisers. The high molecular weight non-endstopped Dio/pate 150 (Briggs & Townsend) possesses particularly good extraction resistance to nonpolar solvents such as hexane and oils. The endstopped polymerics, Dio/pate 214 and 917, show less resistance to nonpolar solvents, but better resistance to aqueous extractants and also superior compatibility, especially at high humidity, and greater efficiency. The endstopped, mixed adipate/phthalate polymeric Diolpate 171, exhibits reasonable extraction resistance but improved processing behaviour, both with respect to lower plastisol viscosities and faster gelation. It is worth stressing that the good extraction and migration properties of polymeric plasticisers are very dependent on achieving full gelation. The high molecular weight non-endstopped products are particularly difficult in this respect. If it cannot be guaranteed that the TABLE 7.3 Properties and Extraction Resistance of Plasticisers at 60 phr

Viscosity at 25°C (St) BS softness No. Cold flex temperature (0C) Tensile strength (MN m- 2) Elongation at break (%) Volatile loss (%) Extraction loss (%) Hexane Mineral oil Olive oil Water Soap (1%)

DOP

BBP

TXP

ESO Diolpate Diolpate Diolpate Diolpate 150 171 214 917

0·5 45 -20 17-8 335 22·5

0·6 43 -12 11·5 195 16·3

0·9 5 41 41 +2 -12 20·3 16·5 280 365 6·9 9·0

35 15·8 23·5 0·1 12·8

14·0 12·4 15·4 0·5 19·6

15·5 18·6 6·7 10·7 9-6 14·5 +0·3 0·1 21·2 4·1

130

28 -3 18·7 320 2·9 1·2 +0·5 3·1

1-6 7-2

9 42 -6 18·0 315

3-6 7-6 7·3 9·8 0·4 8·2

35 41 -7 17-4 330 H 2·9

z.t

5·8 0·3 2·7

41

44 -4 16·9 350

3-3 2·4 1·9 3·8 0·1 2·9

7 Properties of Plasticised

pvc

197

appropriate compounding/processing equipment for a particular product will achieve full gelation, then it is preferable either to use a lower molecular weight endstopped polymeric or to include a proportion of rapid-gelling plasticiser, such as triaryl phosphate, in the formulation. It can be seen that the replacement of an alkyl by an aryl group in a dialkyl phthalate reduces the nonpolar extraction resistance of the plasticiser. Triaryl phosphates and epoxidised soyabean oil exhibit intermediate extraction resistance between the dialkyl phthalates and the polymeric plasticisers. As mentioned in Section 6.9 of Chapter 6, solid polymeric resins have replaced high viscosity polymeric plasticisers in certain applications. Typical physical and extraction properties of a polyurethanebased solid elastomer are shown in Table 7.4. It will be noticed that whilst such products impart excellent extraction resistance to a PVC compound, they are far less efficient than conventional polymeric plasticisers. The thickness of a sample can also have an effect on its extraction resistance. This is shown schematically in Fig. 7.9. With very thin TABLE 7.4 Physical Properties of a PVC Compound Containing a Solid Polyurethane Elastomer-'Durelast loo,a BS softness No. Cold flex temperature eq 100% modulus (MN m- 2) Tensile strength (MN m- 2) Elongation at break (%) Tear strength (kN m- 1) Volatile loss (%) Extraction loss (%) Hexane Mineral oil Olive oil Water Soap (1%) Formulation:

a Trade

PVC Durelast 100 ESO Cd stearate

45 -22·5 5·6 15 530 59

0·2

+0·3 +0·1 -0·1 +0·7 +0·4 100 100 10 6

name of Briggs & Townsend.

D. L. Buszard

198

B

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Fig. 7.9 Schematic representation of the effect of sample thickness on extraction resistance.

samples the concentration gradient throughout the sample on extraction is very low (region A), whereas thicker samples exhibit a marked concentration gradient with a reservoir of plasticiser in the centre of the sample, thus resulting in the extraction being diffusion-controlled (region B). Extraction resistance is normally quoted as % weight loss of the sample (as in Table 7.3, etc.) or, less commonly, as % loss of plasticiser. However, in certain cases, it has become accepted to quote the results as actual weight loss per unit area of the sample tested. This is so in a number of national and international directives relating to the loss of additives from food-packaging materials into the contained foodstuffs. For example, the EEC draft directiveS on overall migration of plastics additives puts an upper limit of 10 mg per square decimetre of packaging material when tested by a particular method using a range of food simulants (distilled water, citric acid solution, aqueous alcohol and olive oil). Expression of the results in this manner obviously means that, as shown in Fig. 7.9, a particular compound may meet the requirements when tested at one thickness, but may fail when tested in a thicker section. In order to meet the olive oil extraction requirements of the above directives, it has been necessary to reformulate the thin PVC cling film to include a proportion of polymeric plasticiser as shown in Table 7.5.

7 Properties of Plasticised

pvc

199

TABLE 7.5 Reformulation of Stretch Wrap PVC Food Packaging Films to Include a Polymeric Plasticiser Old New formulation formulation

PVC suspension polymer DOA Reoplex 430 (Ciba-Geigy) ESO Ca/Zn stabiliser Antifogging agents Overall migration resistance into olive oil days at 40°C) (mgdm- )

pO

100 25

3 1

100 10 20 5 3 1

32

9

5

7.5.2 Migration Resistance The resistance of a plasticiser to migration from a PVC compound into another material in close contact is often a very important requirement. For example, migration of plasticiser from a PVC refrigerator gasket into a high-impact polystyrene (HIPS) refrigerator liner can lead to cracking of the HIPS in high stress areas or even softening of the HIPS resulting in it adhering to the gasket. Other examples include PVC cables in contact with plastics electrical appliance cases, self-adhesive PVC films, insulation tapes and the adhesion of print and lacquer to PVC films. The degree of migration will depend not only on the type and molecular weight of the plasticiser but also on the nature of the surface with which the PVC compound is in contact, in particular the compatibility of the plasticiser with, and its diffusion coefficient into, that material. This is highlighted in Table 7.6 where it can be seen that plasticisers generally have a greater tendency to migrate into cellulose nitrate than into natural rubber, and even less into polyethylene. It will be noted that the relative tendency of different plasticisers to migrate may be reversed when in contact with different surfaces. For example, the acid endstopping of polypropylene sebacate increases its migration into cellulose nitrate but markedly reduces its migration into natural rubber (Table 7.6).

D. L. Buszard

200

TABLE 7.6 Migration Resistance of Plasticisersa Cellulose Natural Polyethylene nitrate rubber

PPS PPA PPS/acid endstopped PPAlacid endstopped PPA/alcohol endstopped Epoxidised oil

2·7 2·2 9·1

10·7 6·4 9·1 14·1

DOP a Method

2·9 1·0

o

2·5 0·7 1·3 11·0

0·1 0·34 0·3 0·7 0·1 0·25 2·2

according to DIN 53405-1981.

Migration resistance is usually determined by DIN 53 405-1981, in which pre-weighed discs of plasticised PVC are sandwiched between discs of the relevant plastics material and placed between glass plates with a 5 kg weight on top, in an oven at 70°C. Table 7.7 shows the migration (expressed as % weight loss of the PVC disc) of a range of commercially available plasticisers into high impact polystyrene. The resistance to migration of the different plasticisers varies widely and is dependent on structure rather than molecular weight. TABLE 7.7 Migration of Polymeric Plasticisers into Polystyrene % weight loss of PVC disc a Test period (days) Wolftex b 828 Wolflex 848 Wolftex 868 Wolftex PLA 1 3

7 14

0·302 0·623 0·785 0·956

0·019 0·027 0·027 0·037

0·062 0·114 0·132 0·157

0·058 0·127 0·146 0·173

a Modified b

DIN 53 405-1981. Trade name of Victor Wolf.

7.5.3 Volatile Loss The volatile loss of plasticisers from PVC compounds is generally studied in two ways. Compounds are aged under a given set of conditions of temperature, time, airflow and sample size, and the volatilisation assessed either directly by the loss in weight of the test

7 Properties of Plasticised

pvc

201

specimens, or indirectly by the changes in physical properties occurring on ageing. The first method is very common and is the means by which the volatile loss results quoted in Chapter 6 Tables 6.3 and 6.6-6.13 have been obtained. These tests may be carried out either directly in ovens, preferably specially designed to avoid cross-contamination, or with the samples in contact with activated carbon to absorb the plasticiser vapours. Polymeric plasticisers, trimellitates and pentaerythritol esters are the classes of plasticisers exhibiting lowest volatile losses. Figure 7.10 shows the weight loss of PVC plasticised with a number of common plasticisers over an extended period. It is a characteristic of monomeric plasticisers that volatile loss tends to be reasonably linear with time up to fairly high losses. However, the volatile losses from polymeric plasticisers tend to level off at relatively low overall values, as the low molecular weight 'tails' are lost, and then continue unchanged for an extended period. The alternative method of assessing volatile loss is by following the change in physical properties on ageing. It has already been demonstrated by the changes in low-temperature properties on ageing, in Fig. 6.2 (Chapter 6) and Table 7.2. This way of assessing plasticiser volatility is obviously more compound-performance-orientated and is much favoured in the various cable specifications, particularly for high-temperature cables. The change in physical properties does not 40

30

HUG

Hcxaplal PPA

200

400 600 Houri at 100 0

800

1000

Fig. 7.10 Volatilisation of polymeric and other plasticisers.

202

D. L. Buszard

necessarily indicate just loss of plasticiser; particularly when the ageing tests are carried out at high temperature, oxidation of plasticiser and cross-linking and/or chain scission of the PVC molecules can also occur. In these cases, the inclusion of an antioxidant in the formulation can cause an apparent reduction in the plasticiser volatility. In the case of cables, the choice of plasticiser is dependent upon the conditions under which the cable is designed to operate. Generalpurpose insulation and sheathing compounds based on Cs phthalates, often with a chlorinated paraffin, are normally limited to a maximum continuous temperature of 6~5°C. For continuous operation at temperatures higher than this low-volatility plasticisers must be used. Thus for a maximum rating of 75°C, DIDP or perhaps a phosphate plastieiser are suitable, whereas for 90°C cables, DTDP is preferred. Trimellitates, polymeries or pentaerythritol esters are required for 105°C rated cables. 7.5.4 Automotive Fogging One property relating to plasticiser volatility whieh is periodically of interest to PVC technologists is automotive windscreen fogging. This problem of the build-up of an oily condensation or fog on car windscreens causing reduced light transmission, has been recognised since the 1950s and was largely attributed to volatile plasticisers. However, careful analysis of the fog by a number of laboratories suggest that in addition to the PVC plasticisers from crashpads and leathercloth, other additives may also cause problems. These include plasticisers in adhesives, pigment-dispersing media and other components of polyurethanes, antioxidants and even airborne hydrocarbons. Although fogging has been around for many years, the car manufacturing companies do not have a consistent approach to the problem. Some companies have very strict requirements, whilst others are influenced by the cost premium imposed by low-fogging leathercloth and crashpads. There are several different types of fogging tests in use, and a number of different test temperatures, e.g. 60, 75 and 90°C, depending on the part of the car for which the parts are destined. The fogging performance of a compound has been shown to be affected not only by the plasticiser but also by other constituents, e.g. stabilisers and minor impurities such as residual emulsifying agent in the polymer and free alcohol in the plasticiser. A selection of fogging results on various typical formulations is shown in Table 7.8.

7 Properties of Plasticised

pvc

203

TABLE 7.8 Fogging Test Results on Typical PVC Formulations for Automotive Use o CRASH PADS b

Breon S125/12 Blendexc 101 Chemigum d NB B1 A2 1rgastabe CH55 Titanium dioxide Antimony trioxide Reomof LTM 1rgastab 17M Fogging at 90°C

50 50 10 0·5 5 3 25 1·5 95%

CALENDERED SHEETING

Breon S125/12 Reomol LTM Palatino! 911 Reofose95 Titanium dioxide Calcium carbonate Antimony trioxide 1rgastab 17M Irgawax e 372 Fogging at 60°C Fogging at 75°C LEATHERCLOTH

Vinno[8 P70 Solvich 374 NB ReomolLTM Palatinol 911 Reofos 95 Titanium dioxide Antimony trioxide Irgastab 17M Fogging at 60°C Fogging at 75°C

100 50 2 5 5 1 0·3 98% 95%

100 30 20 2 5 1 0·3 97% 98%

33 66 70

33 66

33 66

70

2 5 1 99% 93%

2 5 1 96% 90%

45 25 2 1 98% 91%

Results by Volvo Fogging test method-minimum requirement 90% reflectance. Ii Trade name of BP Chemicals. C Trade name of Borg Warner. d Trade name of Goodyear. e Trade name of Ciba-Geigy. fTrade name of BASF. g Trade name of Wacker Chemie. h Trade name of Solvay. o

204

D. L. Buszard

Obviously the most stringent fogging test is that carried out at 90°C, which is for crashpad and window visor components. To meet the 90% reflectance requirement, it is necessary to formulate with plasticisers such as trimellitates, high molecular weight phthalates or other high molecular weight plasticisers, as well as a carefully chosen polymer and stabiliser system. Normal polymeric plasticisers are generally unsuitable since the low molecular weight 'tails' can fog severely. Trimellitates are technically preferable to high molecular weight phthalates, since it is far easier to strip Cg alcohols from the finished product than C lO-C14 alcohols.

7.5.5 High-humidity Compatibility Many plasticisers which have good compatibility under normal conditions of usage can exhibit severe incompatibility under conditions of high humidity, e.g. in refrigerators or in tropical climates. Non-endstopped polymeric plasticisers such as polypropylene adipate, are particularly bad in this respect. Phosphate plasticisers exhibit very good compatibility at high humidity. 7.6 FLAME-RETARDANT PROPERTIES

Rigid PVC is the most flame-retardant of all the thermoplastic polymers manufactured on a large scale. The addition of plasticisers reduces this flame retardance to a greater or lesser extent, as is shown in Fig. 6.3 of Chapter 6, in terms of the effect of plasticiser concentration on the oxygen index. However, even with high concentrations of non-flame-retardant plasticisers, flexible PVC exhibits a greater flame retardancy than other common polymers such as polyolefins, polystyrene, PMMA, etc., and is often self-extinguishing to small ignition sources. In recent years a greater awareness of the fire hazards associated with the extensive use of polymeric materials has led to increased requirements for more highly flame-retardant flexible PVC compositions. Such applications include cables for use in power stations and high-rise buildings, conveyor belting for coal mining, and wall coverings with low flame-spread characteristics. Flame retardancy in flexible PVC may be achieved either by (a) using a suitable plasticiser, e.g. a phosphate or chlorinated paraffin, or (b) incorporating a flame-retardant additive, such as antimony trioxide

7 Properties of Plasticised

pvc

205

or alumina trihydrate. The latter approach is often used in conjunction with a flame-retardant plasticiser rather than alone. Phosphate plasticisers, particularly triaryl phosphates which are very effective primary plasticisers as well as good flame retardants, offer a convenient means of achieving the necessary properties. They have the additional advantage of being non-pigmenting and hence clear flame-retardant formulations can be produced. Since triaryl phosphates are seldom used as sole plasticiser, their poor low-temperature properties can be offset by blending with other plasticisers. Figure 7.11 demonstrates the oxygen index and the cold flex properties of Reofos 95 (Ciba-Geigy) with di-Linevol 79 (Shell Chemicals) phthalate at overall plasticiser contents of 40, 60 and 80 phr. If the di-Linevol 79 phthalate in the blend is replaced by another non-flame-retardant plasticiser, e.g. an adipate, trimellitate, polymeric or other phthalate plasticiser, then the oxygen index of the blends would be similar, although the cold flex results would be different. The use of Fig. 7.11 35

20

o o

.

0>(

ClI

"0

.!: c ClI

~ >( 025

,/

"0

-40

8

20'+-_ _,...-_ _~_ _r-_----I o 25 50 75 100 100

Fig.7.11

75

0/0 Reofos 95

50

°/oL79P

25

o

Oxygen index and cold flex temperatures of Reofos 95/di-LinevoI79 phthalate blends.

206

D. L. Buszard

in conjunction with the data in Figs 7.2 and 7.8 enables the properties of other blends to be estimated. The use of chlorinated paraffins particularly in conjunction with phosphate plasticisers allows compositions with good flame retardancy to be formulated economically, providing that a high degree of light stability is not required. Alkyl diaryl phosphates can give formulations with reduced smoke evolution. The partial replacement of triaryl phosphates by chlorinated paraffins,6 particularly in the presence of certain fillers, such as magnesium oxide and hydroxide. is also claimed to give a reduction in smoke evolution;? see also Chapter 11, Section 11.5. 7.7 ELECTRICAL PROPERTIES

The electrical properties of greatest interest in flexible PVC are the volume and surface resistivity, and the dielectric properties. Volume resistivity and dielectric strength are naturally very important in wire and cable insulation; high values in these properties enable thinner coatings to be used. The effect of plasticiser type and concentration on the volume resistivity of a PVC compound is summarised in Fig. 7.12. 8 Since, as is apparent from its molecular structure, PVC exhibits a high dipole polarisation, the dielectric constant and power factor of its compounds are very frequency- and temperature-dependent. This limits the use of flexible PVC insulation to lower voltage and low-frequency applications. However, the power losses at high frequencies are successfully utilised in the high-frequency welding of PVC sheet (see Chapter 20). Outside the area of electrical insulation, low values of resistivity, particularly surface resistivity, are often beneficial in reducing problems of static build-up. A good review of the electrical properties of polymers and the effect of plasticisers has been made by Coulson. 8 7.8 WEATHERING AND LIGHT STABILITY

These properties depend primarily on the stabiliser system (Chapters 9 and 12) but they are also affected by the plasticisers used. The weathering and light stability of PVC compounds are usually assessed either by long-term outdoor exposure tests or by more rapid

7 Properties of Plasticised

pvc

207

Eu c:

13

> 10

>-

> >-

.r> .r>

~

~

10'2

....

:J

o >

1010L..J~_ _~:----~:----~:---

40

50

60

70

PlASTICISEIl CONTENT (phr)

Fig. 7.12 Effect of plasticiser type and level on volume resistivity at 23°C. 8

accelerated weathering techniques such as the Atlas Weatherometer* or the Xenotest. t It is very difficult to compare the results of different workers since both the climatic conditions and the accelerated ageing techniques vary so widely, and this often leads to contradictory and anomalous results. Four phenomena are generally associated with the outdoor weathering of PVc. These are: (i)

Discoloration-usually due to degradation of the PVc. This is very dependent on the stabiliser system, but photo-oxidation of plasticiser can accelerate the decomposition.

* Atlas Electrical Device Co.

t Hanau Quarzlampen GmbH.

208

D. L. Buszard

(ii) Loss of ftexibility-due to loss of the plasticiser by volatilisation, extraction Or photo-oxidation. (iii) Light-induced exudation-plasticiser migrating to the surface oxidises, leading to a discoloured, tacky surface layer. This is particularly associated with plasticisers containing carboncarbon double bonds, chlorinated paraffins or high levels of epoxy compounds. (iv) Dirt pick-up-this is often associated with (iii) and is particularly a problem in PVC-coated steel for use in outdoor cladding.

In general, aliphatic diesters impart good light stability, providing they are within their compatibility limits. Straight-chain phthalates are superior to their branched-chain counterparts, although the high linear phthalates exhibit poorer compatibility. Phosphates are poorer than phthalates, except when included in phthalate-plasticised compositions at low concentrations, when they apparently stabilise the formulations. 9 Aromatic phthalates such as BBP have markedly poorer light stability than dialkyl phthalates.

7.9 RESISTANCE TO MICROBIOLOGICAL ATIACK This is particularly important in plasticised PVC products for use outdoors, especially when in contact with soil or in warm humid areas, Examples of products particularly at risk are: buried cables, swimming pool liners and covers, foul-weather clothing, wallcoverings, and shower curtains. Plasticisation increases susceptibility to microbiological attack and no plasticiser is completely immune. Much investigational effort has been devoted to the problemy)"'15 Table 7.9 extracted from the work of Burgess and Darby13 indicates the relative resistance of a range of plasticisers to a mixture of five different fungi. Extensive soil burial tests by Decoste 12 showed that in addition to type, plasticiser concentration is also an important factor. Tests for plasticiser suitability are time-consuming and the advice of manufacturers provides a shorter route to the solution of specific practical problems of formulation. This advice may include recommendations for the use of such suitable chemical additives as fungistats or bacteriostats in the PVC formulation (e.g. Irgasan DP 300/PA-Ciba-Geigy; Estabex ABF-Akzo Chemicals; bioMET additives-M & T Chemicals).

7 Properties of Plasticised

pvc

209

TABLE 7.9 Fungal Resistance of Plasticisers13 ,a

Plasticise,P TXP D79P

DIOP

Polyester B Polyester C Polyester A

DIOA DIOS ESO

% plasticiser

% sample shrinkage

o

o o o

loss

4·19 5·24 6·21 14·58 18·28 53·13 56·43 63·83

3·2 2·2 2·7 8·5

10·7

15·0

Samples sprayed with a mixed spore suspension of: A. flavus; C. herbarum; P. funiculosum; P. pullulans; T. viride; incubated for 14 days at 28°C. b Formulation: PVC, 100; Plasticiser, 54; Epoxy, 1·5; BalCd stabiliser, 3·0. a

7.10 RESISTANCE TO INSECT AND RODENT ATTACK This is also of interest in certain building and outdoor uses of PVC and is particularly important in tropical climates. Plasticisation increases the susceptibility of PVC to attack by insects and rodents. There is some evidence that phosphate plasticisers may be more resistant than others,lO,16 but effective remedies are not primarily a matter of plasticiser selection. Special insecticides or repellants may be used in,17 or applied as coatings to, the PVC compound (e.g. Dieldrin-Shell Chemicals; bioMED·

7.11 STAIN RESISTANCE Flexible PVC is susceptible to staining by many different substances but particularly those which are oil-based (e.g. ball-point pen ink, shoe polish, tar, etc.). It has been shown that the degree of staining increases with the plasticiser content and that at equal levels of softness

D. L. Buszard

210

the type of plasticiser is also important. 18 Plasticisers which give reduced levels of staining are 2,2,4-trimethyl-l,3-pentanediol diisobutyrate (e.g. Kodaflex TXIB-Eastman Chemicals), the monoisobutyrate monobenzoate of the above diol (e.g. Nuoplaz 1046-Tenneco), benzyl butyl phthalate (e.g. Santicizer 16o-Monsanto) and triaryl phosphates (e.g. Reolos So-Ciba-Geigy). The major applications where stain resistance is important are in PVC flooring, and, to a lesser extent, in wall coverings and claddings.

'.n '.n.t

TOXICITY AND HEALTH ASPECTS OF PLASTICISERS

Plasticisers for Food-contact Application

Plasticised PVC is often used in applications in which it comes into direct contact with foodstuffs, e.g. packaging films, bottle seals, can lacquers, conveyor belting used in food preparation, etc. In such applications it is important that only those plasticisers-and indeed all other constituents-which are known to be non-toxic are used. In many parts of the world this is a requirement in law. In the USA for example, only those additives specifically permitted by the PDA 19 may be used. In other countries, such as the UK and Germany, there is currently no specified list of legally approved products, but the recommendations of the BPP20 and BGA,21 respectively, are voluntarily followed. It is likely that in Europe in the near future, the various national lists of approved additives will be replaced by a single Council of Europe or EEC directive. Table 7.10 summarises the more important plasticisers approved in a number of countries and also includes the draft proposals of the Council of Europe. However, this list is not exhaustive and in certain applications additional limitations may apply. For more detailed information a user should contact the plasticiser manufacturer or study the appropriate published national requirements/recommendations directly. Recent studies22 .23 by the US National Cancer Institute (NCI) have indicated that DOP and DOA could cause a statistically significant increase in the observed liver tumours (hepatocellular adenomas and carcinomas) when fed to certain rodents daily over a two-year period. The high levels of dosage do, however, make interpretation of these results difficult and it would not be possible on this evidence alone to

7 Properties of Plasticised

pvc

211

TABLE 7.10 Plasticisers Approved Q for PVC in Contact with Foodstuffs DBP DOP DIOP DIDP DMEP DBS DOS DOA DIDA ESO Reop/ex Reop/ex FG 430 Council of Europe Federal Republic of Germany Franceb HoUand b Italt UK USA

x

x

x x x x x x

x x x x x

x

x x x

x

x x x

x

x

x

x

x

x x

x x x x x x

x x x x x x

x x x

x x

x x

x

x x x x x x x

x x x

x x x x x

X

Limitations not indicated in the table may apply to, for example, maximum permissible concentration, type of food to be packaged, form of finished product, etc. b Subject to an overaU migration limit.

a

evaluate any potential carcinogenic hazard to man. Since it was evident that these findings could have important implications affecting the continued commercial use of these esters, the European Council of Chemical Manufacturers Federation (CEFIC) recommended that further scientific studies specifically designed to investigate the possibility of adverse health affects resulting from human exposure should be carried out. These studies sponsored by the European Plasticiser Manufacturers under the auspices of CEFIC have now been completed and reported. 24 The studies carried out on DOP included DNA binding, dose/time response and a comparative assessment in rodents (rats) and primates (monkeys). These results confirm that there is sound evidence that the NCI study is not relevant to human risk assessment and carcinogenic risk to man has not been demonstrated. Further studies involving a wide range of phthalates and adipates are currently in progress in the USA. This is a voluntary research programme funded by the Plasticiser Manufacturers section of the Chemical Manufacturers Association in conjunction with the FDA.

7.U.2 Health and Safety The majority of plasticisers manufactured today are of a low order of toxicity and constitute little hazard in use either from direct toxic effects or dermatitic effects on handling. However, as with all organic

212

D. L. Buszard

TABLE 7.11 Vapour Pressure of Dialkyl Phthalates Plasticiser Molecular weight

DBP DOP DIDP

278 390 447

Vapour pressure (rnrnHg)

Concentration in saturated air at 160"C (g rn- 3 )

1·2 0·121 0·029

12 1·75 0·48

materials, good working practice should be employed. Adequate ventilation should be ensured in the vicinity of heated equipment where plasticiser fumes may be produced. Table 7.11 indicates the concentrations of plasticiser vapour which could occur in saturated air at 160°C. Laboratory experiments25 and actual measurements on industrial plastisol coating plants indicate that 25-50 kg h- 1 of plasticiser could be lost by volatilisation alone. This indicates the need for good ventilation if the threshold limit value (TLV) of 5 mg m- 2 , generally considered to he the safe maximum concentration by the American Conference of Governmental Industrial Hygienists 26 and adopted by HSE 27 and OSHA, is not to be exceeded. REFERENCES 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Harrington, C. E. (1965). Ind. Quality Control, 21,494-8. Tang, Y. P. and Harris, E. B. (1967). SPE J. 23(11), 91-5. Pugh, D. M. and Wilson, A. S. (1976). Eur. Plast. News, 3(9), 37-42. Combey, M. (1972). Plasticisers, Stabilisers and Fillers, (Ed. P. Ritchie), Iliffe/PRI, London, Ch. 12. EEC Draft Directive on the overall migration limit for the constituents of plastics, materials and articles intended to come into contact with foodstuffs. Ref: R/1444178. Ceasar, H. J. and Davis, P. J. (1975). SPE Tech. Papers, 21, 130-4. Price, R. V. (1979). SPE Tech. Papers, 25,956--63. Coulson, S. H. (1972). Plasticisers, Stabilisers and Fillers, (Ed. P. Ritchie), Iliffe/PRI, London, Ch. 8. Dolozel, B. (1963). Chern. Prurnysl., 13(38), 3, 160-5. Wessel, C. J. (1964). SPE Trans., 4, 193-207. Berk, S., Ebert, H. and Teitell, L. (1957). Ind. Eng. Chern., 49, 1115-24. Decoste, J. B. (1968). Ind. Eng. Chern., 7(4),238-47. Burgess, R. and Darby, A. E. (1964). Brit. Plast., 37(1), 32-7. Burgess, R. and Darby, A. E. (1965). Brit. Plast., 38(2),2-6.

7 Properties of Plasticised

pvc

213

15. Wolkober, Z., Gyarmati, I. and Farkas, P. (1978). Int. Polym. Sci. Technol., 5(4), no. 16. Bultman, J. D., Southwell, C. R. and Beal, R. H. (1972). Naval Res. Lab Report No. 7417, Washington DC. 17. Anon. (1965). Mod. Plast., 42(5), 168. 18. Pinner, S. H. and Massey, B. H. (1963). Brit. Plast., 36(10), 564. 19. USA Food and Drugs Administration (FDA) Code of Federal Practice. 20. 'Plastics for Food Contact Applications-A Code of Practice for Safety in Use', The British Plastics Federation. 21. 'Kunststoffe in Lebensmittelverkehr', Kunststoff-Kommission des Bundesgesundheitsamtes, Berlin, Federal Republic of Germany. 22. NTP Technical Report series 217. 'Carcinogenesis Bioassay of Di(2ethylhexyl)phthalate (CAS No. 117-81-7) in F 344 Rats and B6C 3F1 Mice (Feed Study), NIH Publication No. 82-1773. 23. NTP Technical Report series 212. Carcinogenesis Bioassay of Di(2ethylhexyl)adipate (CAS No. 103-23-1); F 344 Rats and B6C 3F1 Mice (Feed Study), NIH Publication No. 81-1768. 24. CEFIC. (1982). 'Di-(2-ethylhexyl)phthalate (DEHP), CEFIC Plasticiser Toxicological Working Group Report on Developments in DEHP Toxicology, Avenue Louise 250, Bte 71, B-1050 Brussels. 25. Poppe, A. C. (1980). Kunststoffe, 70(1), 38-40. 26. 'Threshold Limit Values for Chemical Substances in Workroom Air for 1978', American Conf. of Govt. Ind. Hygienists, Cincinnati, Ohio. 27. 'Threshold Limit Values 1980', Health and Safety Executive, Guidance Note EH 15/80.

CHAPTER 8

Fillers in PVC I. D. HOUNSHAM and W. V. TlTow

8.1 INTRODUCTION For the purpose of this chapter fillers may be broadly defined as solid particulate or fibrous materials, substantially inert chemically, incorporated in plastics compositions (including PVC) to modify the properties or to reduce material cost. Cost reduction is often the primary reason for the use of a filler, and because of this the term is occasionally treated (incorrectly) as if it was synonymous with 'cheapening extender'. In fact all fillers-when present in significant quantitiesaffect in some measure the material and/or processing properties of the plastic, and some-which may be termed 'functional fillers'-are indeed used, often at increased cost, expressly as property modifiers, e.g. glass fibres as reinforcing filler in uPVC compositions, antimony trioxide or alumina trihydrate as flame retardants in pPVc. It may be noted that the functional aspect is emphasised in the standard definitions of a filler (ct., for example, ISO 472-1979; BS 1755: Part 1: 1967; ASTM D 883-83). In the ideal case the incorporation of a filler might confer the combined benefits of cost reduction with increased output (involving no processing difficulties and no rise in the process/production costs) and some technical advantages in the properties and service performance of the plastic. In practice, usually only one, or some, of these features can be secured, often at the expense of the others, and the selection of a filler (or filler system) will thus be a compromise, dictated by the balance of the technical requirements and cost considerations. For example, whilst such low-cost fillers as ground 215

216

I. D. Hounsham and W. V. Titow

limestone and coarse ground whitings offer the highest material cost savings in many flexible PVC compositions, they can also adversely affect the processing, and some physical properties of the end product; incorporation of glass-fibre reinforcement in uPVC upgrades mechanical properties but increases the cost and affects the processing behaviour of the material. A wide variety of materials has been evaluated as fillers for PVC compositions: in this chapter attention is centred on those which are of current technical interest. Among these, certain kinds of calcium carbonate and chrysotile (white) asbestos have attained particular commercial importance.

8.2 MINERAL FILLERS

Certain minerals, especially some naturally occurring silicates and natural (as well as synthetically produced) carbonates, provide some of the most widely used fillers for PVC. These materials may be considered under three general headings: (i) silicates and silicas; (ii) sulphates of the alkaline-earth metals; (iii) calcium carbonates. 8.2.1 Silicates and Silicas

The silicate minerals of particular interest as filler materials for PVC are asbestos, talc, and clay. Other silicate fillers used in some PVC compositions, but not on a major scale, are wollastonite (a calcium metasilicate 1 ,2 sometimes employed as a filler in floor tiles and plastisol products), nepheline syenite (an anhydrous sodium/potassium/aluminium silicate, useful in some semi-transparent compositions because of its low tinctorial power), mica (a general name for a group of complex potassium/aluminium silicates with plate-like particles, of particular interest in some electrical insulation applications), and slate flour (slate is a complex composed of muscovite mica, chlorite and quartz, i.e. a composite hydrated potassium/aluminium/magnesium silicate combined with silica). The mineral silica fillers (quartz, sand, diatomaceous earth) are of comparatively little interest for PVC compositions, although one

8 Fillers in PVC

217

form-novaculite-has been claimed to be useful in calendered sheet, rigid compositions and foams. 3 (a) Asbestos The only form of asbestos used as a filler in PVC is chrysotile (white asbestos). Chemically this is a hydrated magnesium silicate (3MgO.2SiO.2H zO). It is a fibrous material (fibre length 1-40 mm, fibre diameter 0·01-1 tlm) with a very high fusion point (1500°C) and 100% strength retention at temperatures up to 4000C. 1 Its main applications in PVC are in flooring compositions (where a short-fibre grade is used, improving melt cohesion in processing and providing some reinforcement in the product, including improved hardness and denting resistance), and as reinforcement in pressed sheet (e.g. Duraform-Turner Brothers Asbestos, UK): a longer fibre grade is used in the sheet, which finds application as internal and external cladding material for buildings (especially industrial buildings), corrosion resistant trunking and ducting, and the like. 4 ,5 Despite the increasing stringency of various health and safety regulations, the handling of asbestos and asbestos-filled PVC is still possible, if suitable precautions are observed. 1 ,4 In formulating and processing asbestoscontaining PVC compositions it should be borne in mind that their heat stability and colour may be affected by iron compounds present in chrysotile as minor chemical constituents. (b) Talc

This material also consists of hydrated magnesium and silicon oxides (with admixtures of other minerals and impurities, neither of which are completely removed in preliminary processing). The chemical composition and the particle shape of talc vary somewhat according to the source; the shape may also be influenced by the grinding process: the particles may be granular, plate-like ('platy' or 'scaly'), or needleshaped, with sizes in the range 1-50 tlm.1 Talc is used in some calendered PVC floor tile compounds to increase melt cohesion and the stiffness and hardness of the finished product: 1 its low tinctorial power and its refractive index (which is close to that of PVC resin) also make it of interest as a filler for translucent compositions. (c) Clay There are several varieties of clay, all essentially forms of complex, hydrated alumino-silicates, with varying amounts of other minor

I. D. Hounsham and W. V. Titow

218

constituents (including potassium oxide, titanium dioxide, and iron oxides). Of these only kaolinite ('kaolin', 'china clay') is of significant interest as a filler for PVC. Whilst in some other plastics ground kaolin is used directly, in PVC compositions it is normally employed in the calcined form. Calcination refines the clay (by removing some impurities and some iron compounds, which results in improved whiteness), removes the water of hydration, and improves the processing performance. The main use of china clay in PVC has been in flexible compositions for electrical applications (insulation, cable covering), and also in carpet backing compounds (in those based on PVC latices bentonite clay, as well as kaolinite, may be used-see Chapter 23), latex-based films, and plastisol products (especially coatings on fabrics). However, in all these applications clay has yielded ground to calcium carbonate fillers, which are normally cheaper, grade 90

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General

(b) Rigid: Pipe fittings

Shoe-sole compounds

General

(a) Flexible: 10 } These fine-particle grades give good balance of physical properties, ease of 10 dispersion and processability

Somewhat coarser particles acceptable here

Small particle size and good dispersion are particularly important (for good physical properties and processability, respectively)

Good physical properties and fast output promoted by filler fineness (fastest production with coated grades). Special 'electrical' grades available

Good dispersion and physical properties

Generally as for extruded soil pipe and rain-water goods

Generally similar to those for rigid extrusion compounds, but fine particle size (especially low maximum size) even more important (because of need to ensure good physical properties of compounds based on polymer of lower K value)

Finest particle grades required for good wear properties, resistance to flex cracking, cut growth and other effects of damage in service

2

Ground whiting, s

Ground calcite Ground whiting (s) Precipitated, s

3

Ground calcite

10

2

Ground whiting, s

Soil pipe, rain-water goods

3. Injection-moulding compounds

About 7 5

C). If the composition is initially

314

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W. V. Titow

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100 1

2 3

.... 10

2·3

2·2

2·1

1/& , K- 1 x 103

2·0

Fig.9.5 Arrhenius plot (ct. Section 9.2.1) of log stability time (ts> minutes) as a function of the reciprocal of absolute temperature (1/8) for three PVC compositions of increasing stability (1) 2> 3): schematic representation (but values roughly representative of some uPVC compositions, with HCl emission as the degradation index).

9 Stabilisers: General Aspects

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.S

c

o

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Fig. 9.6 Residual heat life (stability time), tA - tT, tB - tT , tc - tT, of PVC of the same composition, processed under conditions of increasing severity (A> B > C) after an initial heat treatment (T): schematic representation.

subjected to a heat treatment (say melt compounding), under conditions A, or B, or C, or some other conditions, which uses up the amount of its heat life corresponding to the initial portion tT of the stability time, then the residual heat life (all that remains available before significant degradation sets in during subsequent heat treatment) will be represented by t A - tT for conditions A, t B - tT for conditions B, and tc - tT for conditions C. 9.8.2

Heat Stability Testing

Apart from their role in development and research work, heat stability tests on PVC compositions and products are important in the practical contexts of processing and service. Evaluation of the suitability and effectivity of stabilisers, and stabiliser/lubricant systems, in protecting PVC against degradation both under processing conditions and in use, is one of their main applications. Others include direct assessment or prediction of the stability of PVC compositions in various circumstances of treatment and/or exposure, with reference-where relevantto the effects of formulation components and/or heat history in this regard. Investigation of PVC material failures, and 'trouble shooting' generally, are related areas in which stability tests can be helpful. A stability test normally comprises a suitable treatment of the PVC material to induce degradation under controlled conditions, followed by detection, or quantitative determination, of a significant level of the

316

W. V. Titow

manifestation of degradation which is being used as the degradation index in the test. The determination methods employed in such tests can also be applied to PVC materials degraded by means other than the test treatment (e.g. in actual processing or service), but reference to a relevant standard (e.g. results of appropriate calibration tests; specimens exemplifying the effects of actual service in known, relevant conditions) will normally be necessary to characterise the extent of any degradation detected. Stability tests are of two general types: dynamic and static. The methods often used in the evaluation of stabiliser effects are summarised, in a general way, in Table 9.4. Some details of standard tests are given in Table 9.5. In dynamic tests an appropriate weight of the PVC composition is worked at an elevated temperature in sui~able equipment, typically a torque rheometer, internal mixer, extruder, or mill. Stability is assessed either by periodically checking the effect used as the degradation index (commonly colour development in the material), or~specially in a torque rheometer-determining the stability time as the period from the commencement of test processing to the ultimate rise in melt viscosity (and hence in torque) marking the onset of substantial degradation ('decomposition point'). By their nature dynamic tests are primarily relevant to the effects of processing on PVC compositions. Indeed, the test equipment, temperature, and running conditions are often chosen with a view to relating the test results to a particular process. Because PVC cannot be processed without stabilisers, dynamic methods are not suitable for the determination of the heat stability of PVC polymers alone. In a static test, the test treatment essentially consists in heating specimens of PVC material at the test temperature. The specimens are often pieces of sheet of standard size, but they may also be standard weights of powder or pellets of PVC polymer or composition. Note: Where the sheet from which specimens are cut for the test is

specially prepared, e.g. on a mill or by pressing, the preparation should be carefully standardised, so that variations in heat history do not arise to affect any comparison of results. The relevant standard test specifications (cf. Table 9.5) usually include the method and conditions of specimen preparation. In the absence of specific recommendations the following general method may be used 7 for preparing

9 Stabilisers: General Aspects

317

specimen sheet on a laboratory mill (35 x 15 cm), from about 100 g of composition. uPVC: Process for 5 min at 180°C into a sheet about 0·3 mm

thick. pPVC: Process for 5 min at a temperature between 165 and

170°C (depending on nature and amount of plasticiser) into a sheet about 0·5 mm thick. The heating equipment may, typically, be an air-circulation oven, containers for the specimens immersed in a heating bath, or-in some cases-a press with suitable arrangements for heating and cooling the platens. Stability is determined in terms of heating time to reach a certain level of the degradation index used in the test (Le. stability time in those terms): this may be the first appearance, or attainment of a certain degree, of discoloration in the composition (ct. the example below). Colour change of an indicator in continuous contact with volatiles evolved by a specimen, or the other effects mentioned in Tables 9.4 and 9.5, may also be used. Note: A test method which can give rapid results, and in which the

degradative test treatment is combined with measurement of induction time, is differential scanning calorimetry (DSC). This may be applicable where the degradation process is exothermic, so that on the usual DSC plot of heat flow rate versus time the period Of stability (induction time) at the heating temperature used is represented by a flat portion of the curve, and a subsequent drop marks the end of that period. 68 The results of static tests are more relevant to the effects of heat in service than in processing of PVC materials. However, because the equipment required and the test procedures are generally simpler than those of dynamic tests, static tests are sometimes used also to obtain indications of the likely stability in processing, albeit correlations with actual process effects (or with those of dynamic tests) may not be very good. Oven-heating involves free access of air to the PVC specimens, whilst in the heating-bath methods the containers housing the specimens can usually be continuously swept with air (cf. Table 9.5). Thus either of these two methods may be employed where accessibility to air is relevant to the purpose of the test, and their results are sometimes taken as an indication of the PVC material's likely stability

Static

Colour change (to blue) of a Congo Red paper, or universal indicator sensitive at about pH 3, in contact with volatiles evolved during the test. Reduction of colour of Ultramarine Blue incorporated in the test specimens, continuously monitored (by photo cell) during heat treatment in oven. Onset of significant degradation (end of stability time) indicated by sharp drop in blue-light reflectance curve--cf. Fig. 9.8.

Colour change

Colour change

Evolution of HCI

Evolution of HCI

Static

Both static and dynamic

Inspection after set time, or at intervals, and comparison with standards.

Method, and/or stability criteria

Test treatment for which determination method is suitable

Visual

General means

Determination of degradation effects

Development of colour in PVC material

Manifestation of degradation detected or measured

Nelson's Test: Ref. 67

ISO/R 182 (under revision); BS 2782, Method BOA: 1976; DIN 53 381 Part 1

Static: ISO 305; ASTM D 2115; DIN 53 381 Part 2 Dynamic: Refs 7,65,66

References and remarks

TABLE 9.4 Heat Stability Tests Relevant to Practical Assessment of the Effects of Stabilisers in PVC

~

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o

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~

00

Titration

Conductivity determination

Torque-time plot

Evolution of HCI

Evolution of HCI

Melt viscosity rise with onset of degradation of composition in torque rheometer

Volatiles evolved by specimens in test treatment passed through a standard KCI solution. Time for pH (continuously monitored) to drop from about 6 to about 3·9 measured as stability time. a Volatiles evolved by specimens in test treatment absorbed in NaOH solution and HCI determined by back titration. Stability expressed as mg of HCI evolved per g of sample during heating period (30 min). Absorption in water of volatiles evolved by specimens in test treatment. Stability time measured as time for water conductivity to rise by a specified figure. a Stability time determined as time to reach point of torque rise ('decomposition point') on the torque-time graph. Dynamic

Static

Static

Static

Ref. 66

Introduction of test of this kind under consideration for a revised version of ISO/R 182

ASTMD793

ISO/R 182 (under revision); BS 2782, Method l30B: 1976; DIN 53 381 Part 3

a Volatile alkyltin chlorides are formed on heating of PVC compositions containing alkyltin stabilisers (cf. Section 9.6). These can dissociate in aqueous solution, and may thus interfere with HCI determination by conductivity and pH methods.

pH determination

Evolution of HCI

\(:)

'-0

~

" impact modifiers* and processing aidst > stabilisers and lubricants:j: Thus the main operative features of additives incorporated specifically as lubricants are: (i) generally low compatibility with PVC (lowest for external lubricants) ; (ii) chemical nature enabling the lubricant effects to be exerted in significant measure at the low level of addition (between O· 2 and about 3·0 phr for the total lubricant system, usually comprising more than one lubricant). Since the mode of a lubricant's action (internal or external) is closely associated with its compatibility with PVC, several studies have been made of the degree of compatibility of lubricants, measured in terms of some relevant property of the lubricated PVC composition, with a view to classifying the lubricants and/or predicting their suitability in practical applications. 2 The properties measured included the amount of haze introduced by the additive into a clear PVC compound3 (haze being a manifestation of incomplete compatibility), and the reduction of the glass transition temperature (Tg ) caused by its incorporation 4 (lowest Tg with the most highly compatible lubricant). The effect of lubricants on the fusion behaviour of a PVC composition in a *Some impact modifiers (e.g. extensively chlorinated polyethylene; nitrile rubber; VC/EVA graft polymers) are highly compatible with PVC polymersee Section 11.2. t Some processing aids are occasionally used in high proportions to increase the resistance of PVC to deformation at elevated temperatures-see Section

*

11.2.

Lubricants with internal lubricant action are more compatible with PVC polymer than typically external lubricants.

362

W. V. Titow

rheometer or an extruder has also been used as the criterion of the type of lubricant action. In terms of behaviour under suitably standardised mixing conditions in a torque rheometer the most characteristic effects are: Typical external lubrication: Fusion time (i.e. time to reach peak torque-see also Section 11.1.3 below) substantially increased;5--7 torque value may be reduced (because of decrease in external friction); NB in an extruder, increase of fusion time and torque reduction are accompanied by a drop in back pressure. Typical internal lubrication: Torque significantly reduced (in consequence of drop in melt viscosity); little or no effect on fusion time.

However, the effects of an external lubricant on the torque rheometer fusion times of PVC compositions do not always correlate with the corresponding melting rates in an extruder, 7 although limited prediction of extrusion characteristics may be possible from plots of rheometer fusion time against mixing head temperature. 8 An increase of melt flow velocity at the wall face of an extruder die (slip effect) was observed directly by Chauffoureaux and co-workers9 in the case of a PVC compound containing a lubricant with effective external action: this effect was absent when the lubricant used was one with a typically internal behaviour. Various studies directed to characterising and classifying lubricant effects have been briefly reviewed by Gale,? and by Logan and Chung. lO The chemical structure of the lubricant is both the prime factor in its compatibility with PVC and the link between compatibility and the mode of lubricant action. Results of experimental studies in this area, and in particular those of the authoritative work of Illmann l l are consistent with the theoretical expectation that the molecular size (chain length) and polarity (nature and number of polar functional groups) of a lubricant are the main factors determining the compatibility and lubricant behaviour: broadly speaking, short chain length and high polarity make for good compatibility and internal lubrication, whereas relatively long-chain compounds (even with some polar groups) tend to be poorly compatible and act as external lubricants. The types of chemical compound used as lubricants in PVC are shown in Table 11.1: among those, typical examples of low compatibility and external lubrication effects are provided by, say, the polyethylene waxes and calcium stearate, whilst relatively high compatibility and

11

Some Miscellaneous Components of pvc Formulations

363

TABLE 11.1 Compounds Used as Lubricants2,4,9 General type

Class of compound

Examples

Hydrocarbons

Natural hydrocarbons Synthetic hydrocarbons

Paraffins, paraffin oils Synthetic paraffins, low molecular weight polyethylene

Hydrocarbon derivatives

Fatty acids Fatty alcohols

Stearic acid Cetyl, stearyl, octadecyl

Derivatives of organic acids

Metal salts (soaps)

Stearates of barium, calcium, aluminium, and lead

Amides Esters and partial esters

Stearamide Butyl stearate, glycerol monostearate, glycerol monoricinoleate; Stearyl esters, montan acid esters

Wax esters

associated internal lubricant action are exemplified by the partial esters of glycerol. Whereas rigid PVC compositions normally require both internal and external lubricants (unless lubricating stabilisers and/or processing aids are used-see below) only external lubricants will usually be considered for flexible compositions, as in those the plasticiser will provide internal lubrication (some external lubrication may also be supplied by plasticiser extenders if present). However, an external lubricant may be useful, or necessary, even in such heavily plasticised compositions as certain plastisols, especially for intricate mouldings. 2 Because of the need to achieve the right measure of internal and external lubrication, and to balance-in the particular, individual composition-the effects of the lubricants with those of the other additives present, more than one lubricant (i.e. a lubricant system) is normally employed in rigid PVC formulations (where up to four lubricants may be combined in some cases) and sometimes also in pPVC. The mutual effects and interactions of the lubricants with the other formulation components are important considerations, affecting both the choice of the lubricant system and the design of the formulation as a whole.

364

w.

V. Titow

11.1.2 Interaction and Co-action of Lubricants with Other PVC Formulation Components (a) Lubricant/Stabiliser effects STABILISING EFFECfS OF LUBRICANTS

Direct stabilising action: Most lubricants of the metal soap type (see Table 11.1) have some stabilising effect, and some can act as stabilisers in their own right, albeit their action in this role is not as strong as that of the more powerful 'primary' stabilisers. Thus certain metal soap lubricants (e.g. calcium stearate) can be used in either capacity in PVC formulations (but when employed as the sole stabiliser, a relatively large amount will be needed to provide a reasonable measure of long-term stability, and the overall stabilisation will not be as good as that conferred by a smaller proportion of a strong primary stabiliser). Lead stearate and dibasic lead stearate are widely used as lubricating components of lead stabiliser systems. Synergistic action with stabilisers: Some lubricants can enhance the effectivity of some stabilisers by a synergistic effect. The synergistic lubricants are of the internal kind (i.e. comparatively highly compatible with PVC) and usually contain reactive functional groups (in particular hydroxyl) in the molecule. Typical examples are partial esters of glycerol with relatively long-chain saturated or unsaturated aliphatic acids. In those compositions where it occurs, the synergistic effect depends, in a complex way, on several factors, including the PVC resin (type and grade), the nature and amount(s) of stabiliser(s) in the stabiliser system, and the amount of the synergistic lubricant present. The stabiliser systems which benefit from the effects of lubricant synergists are those based on sulphur-containing tin compounds (thiotin or tin mercaptide stabilisers) or on lead compounds. In rigid compositions incorporating calcium stearate as the sole heat stabiliser (e.g. in some pipe compounds for potable water in Europe) most lubricants, whether synergistic or 'neutral' in their effects with other stabiliser systems, can produce some stability improvement. 12 Calcium stearate used as a lubricant synergistically improves heat stabilisation by antimony mercaptide stabilisers. 13 With BalCd, Ba/Cd/Zn, CalZn and Ca/Mg/Zn stabiliser systems synergistic effects are important, but they are provided by epoxy-compound co-

11

Some Miscellaneous Components of pvc Formulations

365

stabilisers, usually in conjunction with an organic phosphite (see Chapters 4, 9 and 10), and the lubricants used do not make any significant contribution in this respect. Improvements in heat stability (mainly long-term) of compositions stabilised with sulphur-containing tin compounds (e.g. rigid films, calendered or extruded) can be promoted by the use, as lubricants, of glycerol partial esters (liquid versions, e.g. glycerol monoricinoleate, are necessary for transparency). Lubricants of this kind also enhance the stability of compositions containing lead stabiliser systems (but discoloration can arise, especially on outdoor exposure, with such inorganic lead stabilisers as lead phosphite or sulphate*) Pentaerythritollfatty acid partial ester lubricants can be particularly effective in synergistically enhancing the long- and short-term stability of lead-stabilised rigid PVC compositions, allowing significant reductions in the amount of stabiliser necessary in many cases. 12 Negative effects can also arise: for example, the presence of glycerol partial ester lubricants can reduce the thermal stability of compositions stabilised with sulphur-free tin stabilisers (whilst very good stability may be maintained if the lubricant is an ester of a monohydric alcohol and long-chain fatty acid-e.g. butyl stearate). Note: On the other hand, the heat stability of uPVC (dry blend) compositions stabilised with a butyl thiotin stabiliser can be impaired if cadmium stearate is included in the lubricant system. 8 Indirect stabilising action: In discharging their primary functions, the lubricants also affect the thermal stability of a PVC composition. By lowering frictional heat build-up (through both internal and external lubrication) and melt viscosity, and hence the effective processing temperature, as well as limiting direct contact between the stock arid hot metal surfaces whilst simultaneously preventing the formation of stagnant deposits (through external lubricant action), lubricants reduce the scope for immediate thermal degradation of the PVC in processing and limit its 'heat history': the first of these two general effects is equivalent to improving short-term thermal stability, and the second a factor enhancing the long-term stability of the composition. 5 ,8 The use

* This is attributable to a reaction of these compounds, in the presence of light, with free glycerol often contained in residual amounts in commercial glycerol esters. 12

366

W. V. Titow

of an effective lubricant system can thus reduce the demands on the stabiliser(s). LUBRICANT ACTION OF STABILISERS

Several stabilisers have some lubricating action (usually mainly of the external kind). This is greatest with some metal stearates: as indicated above, certain compounds in this group (e.g. calcium and lead stearates) may be regarded as lubricants with stabilising properties (see also Chapters 9 and 10). Some tin stabilisers also exert lubricant effects (e.g. dibutyltin dilaurate), as do Ba/Cd soap complex stabilisers. Compositions containing such stabilisers will require less lubricant(s) overall (none in some cases) and/or a different balance of the lubricant system.

(b) Mutual Effects of Lubricants and Plasticisers PRIMARY PLASTICISERS

The internal lubricating action of primary plasticisers has been mentioned (see Section 11.1.1 above): this makes the addition of internal lubricants to plasticised compositions unnecessary in most cases. However, if the external lubricant used is highly compatible with the plasticiser(s), its lubricating action in the composition will normally be reduced, necessitating an increase in the level of addition. SECONDARY PLASTICISERS AND EXTENDERS

Some of these may exert external as well as internal lubricant effects. However, the extent (or even the occurrence) of external lubrication will depend on the nature (and given that, the amount) of the plasticiser(s) or extender(s) present, and to some extent also on the process. For example, in calendering compositions many polymeric plasticisers, even when used near the compatibility limit, provide no external lubrication so that external lubricants are required to counteract 'stickiness' in processing: on the other hand, in some compositions containing a chlorinated-paraffin extender this additive can provide both internal and external lubrication in sufficient degree. (c) Effects of Polymeric Modifiers (see also Section 11.2) The polymeric additives incorporated in a PVC composition, in relatively minor proportions, as impact modifiers or processing aids, can, in any individual case, affect the total lubricant requirement

11

Some Miscellaneous Components of pvc Formulations

367

and/or that for the internaVexternal lubricant balance in a composite lubricant system, if their own compatibility with the lubricant(s) influences the latter's compatibility with the composition as a whole, or-as, for example, in the case of some processing aids (see below)-because of direct lubricant action. PROCESSING AIDS

Many of these have no lubricant effect, internal or external, in that they do not reduce the melt viscosity or the external friction and 'sticking' tendency of a PVC composition. However, some acrylicbased lubricating processing aids with pronounced external lubricant action are available,14 and poly-a--methylstyrene (of the relatively low molecular weight grade used as a processing aid) lowers the melt viscosity of PVC compositions (i.e. has an internal lubricant effect).15 IMPACf MODIFIERS

Most ABS and MBS modifiers have no lubricant action. With those highly compatible modifiers which may be incorporated in large proportions to act as permanent plasticisers (nitrile rubber, chlorinated polyethylene of high chlorine content, VClEVA graft copolymers) lubricant effects may arise. The presence of some impact modifiers increases the compatibility of external lubricants with the composition, so that the external lubricant has to be carefully selected (and a relatively high amount may have to be used) for optimum results. (d) Effects of Fillers and Pigments Fillers and pigments (especially fine-particle grades) can bind lubricants by absorption (ct. plasticiser demand-Chapters 4 and 8) so that their presence in a PVC (especially uPVC) composition can increase the lubricant requirement. However, this effect may be reversed-at least with regard to external lubrication-if the filler carries a stearate coating (see Chapter 8) as this can not only block absorption of lubricants at the particle surface, but also actually provide additional lubrication. 11.1.3 Assessment of Lubricant Effects

Reference has already been made in Section 11.1.1 to the evaluation of the effects of lubricants in PVC compositions with the aid of a torque rheometer or in an extruder; capillary rheometers are also sometimes

W. V. Titow

368

employed. The torque rheometer is widely used for this purpose (as indeed, in general, for practically oriented studies of melt-processing characteristics of PVC compositions): very popular and well-known commercial equipment of this kind is the Brabender Plasti-Corder. * In essence this consists of a thermostatically heated mixing chamber housing two rotors mounted in a measuring head and driven by a variable speed motor. The equipment is instrumented for continuous measurement of the torque on the rotors (which is a function of the resistance of the PVC composition to the mixing action) and the mix temperature. A plot of torque against mixing time typically shows a rise of the torque as the mix is fluxed, and something of a drop when the fusion point is reached (see Fig. 11.1): the 'fusion time' taken to reach this point is increased by external lubrication; the torque value can also reflect lubricant effects. A standard method for carrying out

CJl~

:J

L.

f-

~

~ ~ - ~-

- - - -

- ----=-=_::"':_=-=_:":_==--=-=-=-=-=_:-

B

A Time

Fig. 11.1 Schematic representation of a 'plastogram' recorded on a Plasti-Corder chart for a uPVC composition. A, Torque (Nm); B, mix temperature (0C); C, mixer temperature (0C).

* Marketed by C. W. Brabender Instruments Inc. (in North America), Brabender OHG (in West Germany), and agents in most countries. The Plasti-Corder is a larger, more sophisticated version of the original Brabender Plastograph. Several models are available, with torque ratings between 100 and 400 Nm and different maximum rotor speeds.

11 Some Miscellaneous Components of pvc Formulations

369

this kind of fusion test is given in ASTM D 2538-79 (see Appendix 1, Section 2.12). For determinations relevant to extrusion characteristics of a PVC composition extrusion heads can be fitted to the Plasti-Corder. 16 The RAPRA torque rheometer* is another wellknown instrument of the internal mixer type. Determinations of melt viscosity (in many cases the value determined will be the apparent viscosity, if the melt behaves as a non-Newtonian fluid) in capillary or, generally, tubular die rheometers may be used to follow the reduction in viscosity brought about by internal lubrication. A standard method, employing a piston plastometer, is given in ASTM D 3364-74 (1979) (see Appendix 1, Section 3.1(d»; this can be useful for direct comparison of internal lubricant effects in the same composition (or two closely similar compositions) but correlation with processing practice may be variable, inter alia because the shear rates imposed by the processing equipment can be different (higher) than those in the plastometer, and the effects of melt elasticity can affect the behaviour differently. Highly sophisticated equipment has been described by Chaufoureaux and co-workers,9 not only capable of demonstrating the overall effects of lubricants (as well as other constituents) on the rheology of a PVC composition, but also providing data indicative of the mechanisms of lubricant action. The concept of 'lubricant value' (LV) has been put forward 17 as a means of comparing-in a general, approximate way-the effectivity of different lubricants and lubricant systems, including the stabiliser/ lubricant combinations formulated for use in particular applications. To calculate the LV, appropriate data from standard determinations in a Brabender Plasti-Corder are used in the formula: LV = (lOOOE)/(T. md) where: E is the total weight of lubricant additive (phr); T is the stock temperature in the mixing compartment (0C); and md is the torque (kgfm). As can be seen, the LV is highest for the most effective lubricants. The LV values of many lead-based stabiliser systems (Biiropan SMS stabilisers-Otto Barlocher GmbH) were found 17 to lie between about 6 and 14. Various laboratory methods of assessment of lubricants have their * Developed in the UK by the Rubber & Plastics Research Association.

370

W. V. Titow

place in more fundamental studies as well as in the preliminary selection and comparison for the purposes of practical formulation of PVC compositions. In the latter case, however, actual processing trials should always be conducted. Much of the development work on lubricant systems for various types of compositions and processes is done by suppliers of lubricants (and stabiliser suppliers, since combined stabiliser/lubricant 'one-pack' systems properly formulated can offer the advantages of optimum compatibility, component balance and synergistic effects). A further extension of the one-pack concept is the inclusion of other additives with the stabiliser/lubricant system, so that the total additive content is tailored for particular requirements. The advantages and limitations of this approach are mentioned in Chapters 9 and 10. Another recent, interesting, line of development has been the introduction of lubricant concentrates in PVC, in the form of PVC particles heavily loaded (about 50% and over) with calcium and barium stearates. The principal advantages of such concentrates are that they are virtually dust-free, can be air-conveyed and have dry-flow properties very similar to those of PVC resins. The effects of incorrect balance, or total amount, of a lubricant system in a PVC composition may include the following, in varying degrees of severity: Processing Overlubrication:

(or wrong balance)

Underlubrication:

(or wrong balance)

Excessive slippage (resulting in lower output or even disruption of production); plate-out High shear resistance (resulting in lower output); degradation of polymer in melt

Product

Surface bloom; haze (in clear compounds); impaired printability Impaired stability (because of excessive heat history) or actual degradation

The presence of a slight excess of external lubricant can have some useful effects. Thus surface gloss may be improved, and surface friction and tendency to blocking reduced.

11.1.4 Sources of Information on Lubricants and Their Commercial Suppliers In addition to the references already quoted in this section, papers published by Jacobson,18 Riethmayer,19 and Stapfer et al. 20 are concerned with the nature, application and effects of lubricants.

11 Some Miscellaneous Components of pvc Formulations

371

Many suppliers of heat stabilisers for PVC (ct. Chapter 10, especially Table 10.1) also supply lubricants. Listings of lubricant suppliers (as well as those of most other additives) in the Western World will be found in the publications mentioned in Section 8.5 of Chapter 8. Many British suppliers are also listed in the Buyers' Guide for Plastics Additives published by the British Plastics Federation. The following may be mentioned by way of a few examples.

UK: Ciba-Geigy Plastics and Additives Co. Industrial Chemicals Division (Irgawax); Diamond Shamrock Ltd (Lankroplast, Lankromark); Croxton and Garry Ltd (Lubriol, Syntewax). Continental Europe: Henkel International GmbH, West Germany (Stenol, Ceroxin, Loxiol); Otto Barlocher GmbH, West Germany (Biiropan); Acima, Switzerland (Metawax, Metaglide). USA: Emery Industries, Inc. (Emerwax); Interstab Chemicals Inc. (Interstab); Nopco Chemical Division of the Diamond Shamrock Chemical Co. (Metasap, Nopcowax); Witco Chemical Corp. (Lubraplus); Petrochemicals Co. Inc. (Monolube). 11.2 POLYMERIC MODIFIERS As has already been mentioned in Chapter 4, the polymeric additives incorporated in PVC compositions may be broadly classified into two groups according to their functions, viz. processing aids and impact modifiers. In general terms, the main differences which form the basis of this classification are in the nature of the polymers used in each of the two capacities (see Sections 11.2.1 and 11.2.2 below), in the usual level of addition (normally substantially lower with the processing aids), and in the type of effect: processing aids-as implied by their name-serve to modify the properties of the PVC stock during heat processing (but have relatively little effect on those of the finished product), whereas the main function of impact modifiers is to improve the impact resistance of the product. However, whilst the above features do typify the group characteristics in a general way, they are not rigidly definitive. Thus there is some overlap in the types of polymer used for the two respective purposes; many impact modifiers have some processing-aid action (albeit this often tends to be manifested at temperatures somewhat higher than those at which typical processing aids exert their effect); processing aids can affect some product properties even at their usual, relatively low level of

372

W. V. Titow

addition. Certain of the polymeric additives are also used in exceptionally high proportions to upgrade the heat-distortion properties of uPVC compositions (see Section 11.2.1), or to combine a toughening effect with plasticisation of virtually ideal permanence (see Section 11.2.2). It is very important for its maximum effectivity (and hence also cost economy) that any additive (and especially one used in relatively minor proportions) should be dispersed as thoroughly as possible in the PVC polymer. This applies to the polymeric modifiers discussed in this section. In the production of PVC pre-mixes for further (melt) compounding, and dry blends for use as feedstocks in melt processing, the order of addition of the formulation components (in conjunction with the temperature at the various mixing stages) plays a significant part in the ultimate degree of dispersion and effectivity of action of the polymeric modifiers. The following guidelines for a particular procedural sequence and conditions in hot high-speed mixing of a powder blend to be used as extrusion feedstock,21 illustrate something of the points important in practice (see also Chapter 13, Section 13.4.1, for a more complete discussion of high-speed mixing): -7

6-7 6-7 7 7 7 7 5 6

7 6-7 7 7 6-7 6-7

5 5 5 5

5 5 4-5 5 5 5 5 5 5

4-5 5 5 4-5 5 5 5 5

5 5 5 5 4-5 5

4B 5 4Y 4Y

4 5 1 3Y 4-5 3Y 4-5 4D 4Y

4Y 4-5 5 4-5 4Y 4-5 3-4 3Y

5 5 4-5 4-5 4Y 4Y

11

(G) Phthalocyanines CPC, ll'form CPC, stable ll' CPC, f3 form Chlorinated CPC Brominated CPC (H) Other Organics Iron nitroso-f3-napthol Aniline black Carbon black (I) Cadmiums Cadmium sulphide

Chemical type

11

P.Yellow 37 Yellow P 3680 Primrose P 500 Lemon Yellow P 3682 Light Orange P 4701K Deep Orange P 4702K

1·0 1·0 1·0 0·75 0·75

0·10 1·0 0·05

P.Green 8 P.Black 1 P.Black 7

Vulcafor Green LS Monolite Fast Black LS Kosmos 70

Cadmium Cadmium Cadmium Cadmium Cadmium

0·06 0·07 0·08 0·16 0·20

Amt for! ISD (%)

P.Blue 15 P.Blue 15 P.Blue 15 P.Green 7 P.Green 41

C.l. Ref. (Pt l)a

Irgalite Blue BLP Vynamon Blue LBS Monastral Fast Blue BGS Vynamon Green BES Vynamon Green 6YS

Brand name

TABLE 11.7-eontd.

5 5 5 5 5

0·5 0·5 0·5

0·5 0·5 0·5 0·5 0·5

7 7 >7 7 7

6 7 >7

0·01 0·01 0·01 0·1 0·1 0·1 0·1 0·1

>7 >7 >7 >7 >7 0·01 0·01 0·01 0·01 0·01

5 5 5 5 5

6-7 6-7 6-7 6-7 6-7

5 5 5 5 5

3 5 5

4-5 5 5 5 7 >7

P

3Y 4Y 3-4Y 4-5 5

F

Heat stabilityd

4-5 5 5 5 5 >7 >7 >7 >7 >7

P F

P

F

Light fastness C

%in patterns b

P.V.Fast Brown G

-

Chrome Green DC 3593

Vynamon Yellow 6GNS Supra Lemon Chrome 4GS Vynamon Yellow CRNS Supra Orange Chrome HYS Supra Scarlet Chrome YS Supra Scarlet Chrome MS

Light Red P 4703K Scarlet P 4704K Red P 4705K Deep Red P 4706K Crimson P 4707K Maroon P 4708K

a

b

2·0 0·75 0·3

1·0 1·0 1·0 1·0 1·0 1·5

1·0 1·0 0·75 0·75 0·75 0·75

= Darkening.

P.Green 17 P.Blue 29 P.Brown 6

P.Red 104

II

P.Yeliow 34

P.Red 108

11

Colour Index (Part I) Ref: N.L. = Not listed. Patterns: F = Full Shade; P = Pastel Shade in white plasticised PVC; D C Light fastness: Daylight, Blue Scale 8-1 ratings BS. dHeat stability: 10 min at 200°C in air oven. Grey Scale 5-1 ratings.

(K) Other Inorganics Chromic oxide Ultramarine Iron oxide

Lead molybdate

II

(J) Chromes Lead chromate

Cadmium selenide

11

Cadmium Cadmium Cadmium Cadmium Cadmium Cadmium

5 5 5

5 5 5 5 5 5

5 5 5 5 5 5

0·1 0·1 0·1

0·1 0·1 0·1 0·1 0·1 0·1

0·1 0·1 0·1 0·1 0·1 0·1

>7 7 >7

7D 7D 7D 7D 7 7

7 7 >7 >7 >7 >7

7 7 7

7D 7D 7D 7 7 7

6-7 6-7 7 7 7 7

5 5 5

5 5 5 5 5 5

5 5 5 5 5 5

5 4Y 5

5 5 5 5 5 5

5 5 5 5 5 5

416

W. V. Titow

Plate A Laboratory-scale equipment for PVC processing (Farrel Bridge Ltd.). (1) Two-roll mill (swing-side, variable friction, rolls 6 in x 13 in).

D. Insolubilised monoazo: These are types in which heavy substitution of the simple monoazo pigment has suppressed solubility to very acceptable levels; usually this is obtained at appreciable financial expense. E. Polycyclic compounds: These are offshoots of the vat dyestuffs used on textiles; this highly selected group of colours give very high strength, fastness and brilliance but at a very high cost.

11

Some Miscellaneous Components of pvc Formulations

417

Plate A-.

a

f}

~

I'>..

§

0;-

~.

~ ~

'1:l

s·

~ ~

S

~

~ ~

~.

~

'1:l ~

..... N

474

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V. Titow

with the specimens vertical, horizontal, or tilted at 45°. In some methods specimens are exposed under glass, so that the effects of certain weathering factors (wind, rain) are excluded (ct. ISO 877-1976, identical with BS 2782: Part 5: Method 540A: 1977; also ASTM G 24-73, and DIN 53388). Outdoor exposure procedures and effects are the subject of several international and national standards, including: ISO 4607: 1978: Plastics-methods of exposure to natural weathering. BS 4618: Section 4.2:1972: Resistance to natural weathering. Section 4.4:1973: The effect of marine exposure.

ASTM D 1435-75: Standard recommended practice for outdoor weathering of plastics. ASTM G 7-77: Atmospheric environmental exposure testing of non-metallic materials. DIN 53386:1974: Testing of plastics; testing of resistance to weathering in nature (outdoor weathering). British Draft Document 76/54334 (eventually to become method 828A of the revised version of BS 2782): Determination of resistance to natural weathering.

Intensified outdoor exposure (2 above) can be effected in various ways. A useful practical method developed by Caryl and Helmick43 involves boosting the amount of natural sunlight incident on the specimen. In their apparatus, commonly known as 'EMMA' (Equatorial Mount with Mirrors for Acceleration), 10 mirrors direct extra sunlight onto specimens, producing up to tenfold intensification. This was found to be capable of accelerating by a factor of nine the degradation of some uPVC materials: 44 the specimens are cooled during exposure to prevent undue temperature rise. A further development of this concept-involving the application of a water spray and air stream to broaden and increase the weathering effects-is embodied in the 'EMMAQUA' apparatus 45 in use in America at the Desert Sunshine Exposure Test Station in Arizona. The rate of onset of 'normal' weathering effects in PVC can also be effectively increased (and hence the time scale of outdoor exposure shortened) if very thin

12

Properties of Special1nterest in PVC Materials and Products

475

films are used as specimens: in this form the surface-to-volume ratio (specific surface) is large, so that most of the material of a specimen is immediately and directly available to the agents instrumental in weathering, and is affected by changes as soon as they begin to occur. A potential disadvantage of this approach is that the exposure period needed to bring about measurable changes may in some cases actually be too short to encompass seasonal variations and sporadic effects characteristic of the 'normal' weathering pattern in the particular locality. In accelerated artificial weathering (3 above) the aim is to reproduce or match in suitable degree the effects of natural weathering by laboratory treatment of comparatively short duration. As in the other two approaches, evaluation of the results should, in the ideal outcome, enable accurate predictions to be made of the useful service life of the material concerned under given climatic conditions. Failing that it is also of interest to be able to: (i) place materials in order of qualitative relative merit with regard to likely weathering resistance in actual service; and if possible (ii) quantify the ranking, even if still on a relative basis. The answers to (i) and (ii) would, of course, follow automatically if the ideal could be achieved of reliably equating a period of standard accelerated exposure in the laboratory to one in actual long-term outdoor exposure (say, for example, 1 h in a Wether-Ometer to 50 h outdoors in Arizona). Unfortunately this is not possible, especially where long-term predictions are concerned. This well-known fact is illustrated, for example, by the data of Kuist and Maxim,46 and Grossman. 47 The former two investigators quote correlation coefficients for results of accelerated laboratory weathering tests and those of outdoor exposure as 0·6--D·9 (i.e. unavoidable variability approximately between 20 and 70%). It is also known, moreover, that differences occur in the rates of failure between long-term outdoor exposures at the same site. However, with good equipment the relative performance of materials in accelerated weathering tests can give a reliable, at least semi-quantitative indication of the relative performance to be expected in the field (i.e. (i) and (ii) above are attainable). Three factors are employed to bring about 'artificial' weathering in accelerated laboratory tests, viz. radiation, heat, and water (as vapour,

476

w. v.

Titow

liquid condensate or spray). * Exposure to radiation (light of wavelengths extending from about 280 nm into the visible region) is the basic feature of all such tests: in most procedures this is combined with the other two factors. Radiation sources commonly used are listed in Table 12.7: the emission characteristics of such equipment are discussed in a paper by Allen et at. 48 It is generally recognised that, as far as radiation effects in the normal weathering of plastics are concerned, it is the UV component of sunlight which is the main operative factor.47-49 This is the basis of the widely held view that the spectral distribution in the UV region of the light used in accelerated laboratory weathering tests should be as similar as possible to that of sunlight, t to reduce the possibility that radiation-induced chemical (and any other) changes in the test speCimens may differ in kind from the corresponding effects of natural weathering. However, a strong case has also been made out for the use of a source (fluorescent UV lamp) emitting intensely and almost exclusively in the 290-340 nm region, where the intensity of sunlight's spectrum is in fact comparatively low. This approach is based on the view that most of the photochemical changes suffered by plastics in natural weathering are attributable to the 290-315 nm UV band,47 and that therefore exposure to a source with strong emission in this region is both sensible and particularly effective as a means of accelerating radiation effects in artificial weathering. Test apparatus employing this type of illumination in conjunction with means of subjecting the specimens to condensed moisture and elevated temperatures (see entry No.4 in Table 12.7) has been claimed47 to give particularly rapid accelerated weathering, and results which correlate well with those of outdoor exposure (within the general limitations mentioned above).

* Resistance to other agencies, of specific interest in connection with weathering in particular environments, is also sometimes assessed, in separate, additional tests. Some examples are: Resistance to salt spray (relevant to marine environments-see, for example, BS 3900:Part F4:1968); to marine exposure generally (see, for example, BS 4618: Section 4.4:1973); to microbiological attack (see Section 12.7 in this chapter); or to exposure to damp heat, water spray, and salt mist (see ISO 4611-1980). t For this purpose sometimes defined in standard terms as 'global radiation', i.e. total radiation--direct, scattered, and reflected, incident upon a horizontal plane in defined conditions (see, for example, Standard D65 of the Commission Internationale de l'Eclairage).

12 Properties of Special1nterest in PVC Materials and Products

477

The following international and national standards are concerned with the methods and apparatus of accelerated weathering of plastics: ISO/R 878-1968: Plastics-Determination of resistance of plastics to colour change upon exposure to light of an enclosed carbon arc. ISO/R 879-1968: Plastics-Determination of resistance of plastics to colour change upon exposure to light of a Xenon lamp. ISO 4892-1981: Plastics-Methods of exposure to laboratory light sources. BS 3900: Methods of test for paints. Part F3:1979: Resistance to artificial weathering (enclosed carbon arc). BS 4618:Section 4.3:1974: Resistance to colour change produced by exposure to light. ASTM D 1920-69 (Re-approved 1976): Determining light dosage in carbon-arc light ageing apparatus. ASTM D 1499-64 (Re-approved 1977): Operating light- and water-exposure apparatus (carbon-arc type) for exposure of plastics. ASTM D 1501-71: Exposure of plastics to fluorescent sunlamp. ASTM D 2565-79: Operating Xenon-arc-type (water-cooled) lightand water-exposure apparatus for exposure of plastics. ASTM G 26-77: Operating light-exposure apparatus (Xenon-arc type) with and without water for exposure of non-metallic materials. ASTM G 53-77: Operating light- and water-exposure apparatus (fluorescent-UV/condensation type) for exposure of non-metallic materials). DIN 53 387:1982: Testing of plastics; accelerated test of weathering resistance (simulation of outdoor exposure by filtered Xenon-arc radiation and artificial rain). DIN 53389:1974: Testing of plastics; short test of the light stability (simulation of global radiation behind glass by filtered Xenon-arc radiation). The results of accelerated weathering are assessed in the same way as they are for natural weathering, i.e. in terms of colour changes,

Atlas Electric Devices Co., Chicago, Ill. 60613 USA Carl Zeiss Inc. New York, USA; Quartz-Lampen GmbH, Hanau, West Germany; John Goodrich, Ludlow, Shropshire, England (ii) Xenotesta

(xenon arc)

Weather-Omete~

(i)

Close

3. Xenon arc with borosilicate glass filter

Atlas Electric Devices Co., Chicago, Ill. 60613 USA

2. 'Sunshine' carbon arc (open carbon arc with 'Corex D' filters)

Weather-Ometera (Sunshine arc)

Poor

Fairly close

Suppliers

Atlas Electric Devices Co .. Chicago, Ill. 60613 USA

Name or designation

Examples of weathering equipment in which used

Fade-Ometera

Approximation of UV spectral distribution to sunlight

1. Enclosed carbon arc

Nature

Radiation sources

TABLE 12.7 Radiation Sources in Common Use in Laboratory Weathering Equipment

ASTM D 2565

ASTM G 23; ASTM G 25.

ASTM G 23; ASTM G 25.

Remarks and References

o~

:::;j

:0::::

~

~

00

Microscal lightfastness tester

Close

Fair (more intense below 320 nm)

Fair

5. Mercury/tungsten lamp

6. Fluorescent UV lamp combined with UV fluorescent 'black-light' source (FSB unit)

7. High-pressure mercury/quartz arc with 'Corex D' filters

a

Several models available. bOriginated by American Cynamid Co., USA. C Ciba-Geigy Technical Service Bulletin PL 9.1, January, 1977. d Originated by National Starch and Chemical Corp., USA.

GP UVA ultra-violet accelerometert

GP-PS/BL b

o-U-V accelerated weathering tester

None, but source effective in producing relevant photochemical degradation (see text)

4. Fluorescent UV lamp (mercury arc lamp phosphor coated)

General Products Manufacturing Co., Morristown, NJ, USA

General Products Manufacturing Co., Morristown, NJ, USA

Microscal Ltd, Ealing, London, England

The O-Panel Co. Cleveland, Ohio 44135, USA

Refs 46 and 48

Refs 48 and 50 One hour FSB exposure sometimes approx. equated to one day outdoors in the UK c

Ref. 48

ASTM G 53; Ref. 47

.$>0 -.J \0

I ;;

"

!}

~

I:l..

;:0

s::>

5' 1:;-

~

~ ~

"tl

s·

~

~

S'

~

~

-t'

~

~

i;.

1"'

tv

......

480

W. V. Titow

deterioration of surface, or changes in other properties. In assessing colour development (yellowing, darkening), colour changes, or resistance to colour fading in artificial weathering, use is often made of standard colour indices or scales, and colour fastness standards. Thus the degree (and hence the development) of yellowness may be measured and described by reference to the Yellowness Index (BS 2782:Part 5: Method 530A; ASTM D 1925·70 (Re-approved 1977)), change in colour by reference to the Grey Scale (ISO 105-A021978(E); BS 2662:1961; BS 4618: Section 4.3:1974; ASTM D 2616-1967 (Re-approved 1979)), and colour fastness in terms of the Blue Wool Standards originally set up for textiles (ISO 105-B011978(E); ISO/R 878-1968; ISO/R 879-1968; BS 1006:1971; BS 4618: Section 4.3:1974). Other relevant standards include: ISO 4582-1980: Plastics-Determination of changes in colour and variations in properties after exposure to daylight under glass, natural weathering or artificial light. BS 2782:Part 5: Method 530B:1976: Determination of the colour of near-white or near-colourless materials. * ASTM G 45-75: Standard recommended practice for specifying limits for fading and discoloration of non-metallic materials.

Thermal and thermomechanical analyses (DTA, DSC, TGA, TMA) have been used as sensitive means of detecting and evaluating the effects of artificial weathering of plastics; they have also been employed as the combined means of both the thermal ageing itself and the evaluation of its effects, useful results being claimed in the prediction of service life of plasticised PVC formulations. 51 The weathering resistance of PVC is cardinally dependent on the formulation, and in particular on the stabiliser system. The individual roles of the major formulation components are mentioned in the relevant chapters. Here the following general points may be made. Much practical experience is now available to provide guidance in the formulation of weathering-resistant PVC compounds for outdoor service. However, by the same token, the limitations of even the best formulations are recognised. Thus, in the extreme conditions encoun* This is also the subject of an ISO draft standard, ISO/DIS 3558.

12 Properties of Special Interest in PVC Materials and Products

481

tered in some hot-climate areas (e.g. in parts of Australia and South Africa) where the intensity of incident sunlight, the proportion of sunlight time, and the ambient temperature are all high, and where other factors (e.g. severe hail or rain storms, large temperature fluctuations) may also operate to aggravate the severity of exposure, the durability-and hence the use-of even the most resistant PVC compositions in long-term exposure situations is limited or precluded altogether. Elsewhere PVC is successfully used in many outdoor applications: e.g. rigid compositions for external wall cladding, 'ranch'-type fencing, road signs, rainwater goods and window frames; and pPVC as coatings on chain-link wire fencing and tarpaulins, as well as (in the form of flexible sheeting) for lining swimming pools and reservoirs (although in this last application the long-term stability is generally inferior to that of some alternative materials, e.g. butyl rubber). Among the formulational factors affecting the outdoor performance of PVC materials, the following are particularly noteworthy. The PVC polymer should have the highest molecular weight consistent with the processing requirements applicable in the particular case. Homopolymers are generally preferable, although in certain compositions (e.g. some PVC window-frame compounds) certain PVC graft copolymers are used as impact modifiers (cf. Chapter 19, Section 19.4.3). The stabiliser system should be carefully selected, and should preferably (with BalCd stabiliser systems invariably) include an epoxy co-stabiliser (typically 2-8 phr): apart from the long practical experience of the beneficial effect of this type of additive, there is investigational evidence for its useful role, inter alia, as an agent facilitating the neutralisation of nascent HCI-evolved in the course of degradation of the PVC polymer-by the main stabiIiser(s) present. 52 ,53 As the main stabilisers, selected tin carboxylates can give very good results, as can BalCd systems supplemented by epoxy co-stabilisers and phosphite chelators. The carboxylates, too, benefit from the presence of epoxy co-stabilisers, although there is some evidence to show that the effect is less pronounced than with BalCd stabilisers. 42 Where lead stabilisers are permissible, dibasic lead phosphite is especially useful because of its UV-screening action and antioxidant effect: combinations of this stabiliser with tribasic or tetrabasic lead sulphate have also proved very effective for weatheringresistant compositions. UV absorbers are commonly included (sometimes in conjunction with antioxidants) in transparent compositions for

482

w.

V. Titow

outdoor use, to provide additional protection against photolytic degradation. Polymeric impact modifiers for weathering-resistant rigid compositions should be carefully selected from among the chlorinated polyethylene, acrylic, and EVA types. Rubbery modifiers (ABS, MBS) are not suitable in this application. Two common pigments are noted for their beneficial effect on the weathering resistance of PVC: carbon black and titanium dioxide. The latter can also apparently enhance the effect of certain colourants which are known to exert a stabilising action of their own: two examples of such colourants are indanthrene blue and carbazole violet. 54 The weathering resistance of plasticised compounds can be improved by keeping the plasticiser content as low as possible, using selected high-permanence plasticisers (some polymerics can be particularly useful), and incorporating epoxy plasticisers as already mentioned. To obtain the best weathering performance possible with a particular formulation, attention should be paid to the processing conditions (in particular excessive heating should be avoided), and preferably no re-worked material should be included. Presence of solvent residues in solvent-cast films can have an effect: residual tetrahydrofuran was found to promote photodegradation of PVC in air. 55 External treatment (surface coating) may sometimes improve weathering resistance: the lacquers applied as thin top coats to flexible PVC sheet products (e.g. coatings on fabrics-see Chapter 22) can have this effect. Such coatings, conventionally applied from solvent solution, normally contain an acrylic polymer as a main component. Co-extruded protective acrylic coatings on rigid PVC products have also made their appearance: for example, improved weatherability is among the advantages claimed for the German 'Vacuplast' PVC window frame system56 produced from acrylic-surfaced extruded profiles (with aluminium reinforcement). Application of UV absorbers to the surface of transparent PVC sheeting by absorption, as well as in surface coatings, has been suggested57 as a way of obtaining more cheaply a degree of weathering protection comparable with that afforded by the conventional incorporation into the compound. The effectivity of internally incorporated stabilisers can be significantly affected by their degree of dispersion, migration through the compound, volatility and extractability. 58

12

Properties of Special Interest in PVC Materials and Products

483

12.7 RESISTANCE TO BIOLOGICAL ATTACK 12.7.1 Microbiological Attack (Biodegradation) In the plastics context this normally means attack by fungi (mould, mildew) or bacteria. Whilst infestation by algae might also be included in the term in its widest connotation, it is not a major problem even with such PVC materials as film linings for canals, reservoirs and swimming pools. The essential mechanism of microbiological attack is enzymatic degradation of the substrate on which the micro-organisms groW. Both bacteria and fungi produce enzymes capable of breaking down many carbon compounds (those containing oxygen-bearing functional groups can be particularly susceptible) to simpler substances utilisable as nutrients. Some of the products of the breakdown process can be coloured, so that the appearance of colour (in PVC often a characteristic pink stain 59) or black spots, as well as deterioration of some properties in consequence of the chemical degradation, are the main outward manifestations of microbiological attack; others include the development of offensive odours, surface tackiness (in soft pvq, or surface cracking. The resistance of PVC materials can vary widely depending on the formulation: some compositions stand up very well to long exposure in the most unfavourable conditions, such as warm, humid environments (indoors or out), soil burial or permanent immersion in water. However, even a very resistant formulation can be affected indirectly, through contact with a material that is prone to attack: thus, for example, mildew may grow on the cotton fabric backing of a PVC-coated protective glove (especially if kept moist with perspiration for long periods) or on the moist paste layer of a PVC-coated wall paper, or micro-organisms may flourish on surface contaminants (grease, dirt) on PVC cladding. In broad terms, the relevant characteristics and effects of the main components of a PVC formulation may be summarised as follows. Like many other synthetic polymers, the polymers and copolymers of vinyl chloride are resistant to attack by micro-organisms. However, some commercial PVC resins may contain residual amounts of emulsifying or suspending agents used in their production, and these may be susceptible. 44 ,60,61 Many plasticisers are vulnerable to microbiological attack, as are some stabilisers and lubricants (especially epoxy

484

W. V. Titow

compounds, some stearates and waxes), and certain antioxidants, although some organotin stabilisers and phosphite co-stabilisers actually tend to inhibit microbiological growth. 6o The general ranking of plasticisers, in order of increasing susceptibility, is: -aryl phosphates and chlorinated paraffin extenders -phthalates and trimellitates -aliphatic esters (with sebacates and ricinoleates tending to be least resistant in this group) -polyesters (with some exceptions) -epoxy esters and epoxidised oils The resistance of plasticised PVC to microbiological attack is also a function of plasticiser content in many cases. 62 Some fillers may be vulnerable, e.g. wood flour in wood-filled PVC compounds used for extruded profiles and trim. 6 Special protective additives are included in PVC formulations at risk of microbiological attack. Those with a positive ability to destroy fungi and bacteria are often referred to as fungicides and bacteriocides, respectively (or, collectively, biocides): the terms 'biostat', 'fungistat' and 'bacteriostat' are applied to substances which deter microorganism growth by whatever mechanism. Kaplan et at. 63 evaluated the action of 32 biocidal compounds in PVC film: they concluded that of those only one, 'copper 8-quinolinolate' (bis(8-quinolinolato)-Cu), provided fully satisfactory protection. N-(trichloromethylthio)phthalimide also gave good results, but was considered less effective and less widely compatible with PVC formulations. This ranking appears to reverse the order that might be inferred from industrial usage: although N-(trichloromethylthio)imides and the copper complex are both used in commercial biostats, compounds of the former group would be regarded as more versatile and possibly more effective at least in some cases. Other compounds of practical interest are phenyl mercury salicylates,6h and organic compounds of arsenic (e.g. Estabex ABF-AKZO Chemie UK Ltd). Some commercial products are claimed to give broad biostatic protection (e.g. TV-2-Sanitized Sales Co. of America Inc.; Mikro-Chek 12-Ferro Chemical Division, * The use of organomercury biocides in flexible PVC has been discontinued on toxicological and environmental grounds despite their usefulness as the only biocides of proven effectivity against Pseudomonas aeruginosa which can cause problems in pPVc.

12

Properties of Special1nterest in PVC Materials and Products

485

USA). A bacteriostat of lower than average toxicity, highly effective in plasticised PVC products (baby pants, curtains, flooring, gloves) against both gram-positive and gram-negative bacteria, is 2,4,4'-trichloro-2'-hydroxydiphenyl ether (Irgasan DP 300--CibaGeigy): it is also active against certain fungi which grow on the skin, e.g. athlete's foot. Its main function is to reduce growth and spread of bacteria and to suppress odour, rather than to protect the PVC itself against bacterial attack. Amounts of biostats used in PVC formulations vary with the nature of the reagent and the formulation itself, within the range of about 0·1-2% by weight of the formulation. The protection they afford is of interest in many applications. In addition to those already referred to in this section, electrical wire and cable coverings, PVC-coated tarpaulins and foul-weather clothing, garden hose, and some pipe formulations may also be mentioned. Testing the resistance of a plastics material to microbiological attack in the laboratory typically involves placing specimens in contact with stock cultures of selected micro-organisms under controlled conditions for a prescribed time, * and determining changes in a selected property or group of properties. The appearance of the specimens before and after the treatment is usually noted, either as part of the evaluation or additionally. Some relevant standards are listed below. Of these BS 4618 gives a short bibliography, and ASTM G 21 lists in an appendix several standard (ASTM) methods for determining changes in the properties which may be monitored in the tests. ISO 846-1978: Plastics-Determination of behaviour under the action of fungi and bacteria-Evaluation by visual examination or measurement of change in mass or physical properties. BS 4618: Section 4.5:1974: The effect on plastics of soil burial and

biological attack. ASTM G 21-70 (Re-approved 1980): Standard recommended practice for determining resistance of synthetic polymeric materials to fungi. ASTM G 22-76: Standard recommended practice for determining

resistance of plastics to bacteria. * Actual soil burial tests are also popular with investigators.

486

W. V. Titow

ASTM G 29-75: Standard recommended practice for determining algal resistance of plastic films.

u.7.2

Insect and Animal Depredations

In practice the only problems of any significance under this heading arise in connection with attack on PVC products by termites and rodents. (a) Attack by Termites Although this only occurs in tropical and sub-tropical countries it can be a problem with PVC products, especially soft PVC (e.g. electrical wire insulation, cable covering, upholstery fabrics and foam). Experience appears to indicate that termites have a preference for soft plastics generally, and hence high loading with hard fillers has been suggested as a possible way to more resistant formulations (see, for example, BS 4618). Other suggestions that have been made from time to time included the use of phosphate plasticisers (regarded as more resistant than other kinds),64 incorporation of lead naphthenate, and incorporation of insecticides in, or their application in coatings to, PVC materials. 65 The effectivity of such measures is by no means established or universal: susceptibility can differ in different localities and with different species of termite, and no PVC material can be guaranteed to be generally immune from attack, even if it has performed satisfactorily in a particular set of conditions. In some countries a metal barrier (tape) is prescribed by regulations to prevent termite and animal attack on the PVC covering of electrical cables, and this is an effective solution in this particular case. (b) Attack by Rodents PVC materials are not, in general, palatable to rodents, and are not a source of food. They are attacked, however, by mice and rats if they form an obstacle on the way to food or water. Some apparently less purposeful gnawing is also experienced from time to time on such PVC products as electrical insulation and conduit, uPVC water pipes, and reservoir linings. Although barriers may be incorporated in some products (e.g. cable coverings, see above) there is no generally applicable way of preventing these depredations. However, they are neither sufficiently frequent nor widespread to constitute a major problem.

12

Properties of Special Interest in PVC Materials and Products

487

12.8 CHEMICAL RESISTANCE At ordinary temperatures PVC homopolymer is resistant to most of the common inorganic reagents (including aqueous salt solutions), oxidising agents (with the partial exception of concentrated nitric acid) reducing agents, aqueous solutions of detergents, oils (mineral, animal and vegetable), fats, aliphatic hydrocarbons and alcohols. Its solvent resistance is, however, limited in certain respects: it can be dissolved by some ketones (tetrahydrofuran, cyclohexanone, isophorone) and swollen to varying degrees by others; some nitroparaffins can swell or even dissolve it; and chlorinated hydrocarbons, aromatic hydrocarbons, aromatic amino compounds, as well as some other reagents (e.g. acetic anhydride) are also swelling agents. Copolymers are somewhat less resistant, especially to organic solvents (cf. vinyl solutions-Chapter 24), save in the exceptional case of copolymers of vinyl chloride with maleic acid imide derivatives (cf. Chapter 1). However, their general resistance characteristics are broadly comparable with those of the homopolymer. As is usual with thermoplastic polymers, the susceptibility of PVC homo and copolymers to chemical attack increases with increasing temperature: the same behaviour is exhibited by uPVC and pPVC compounds. The resistance of compounds can also be lower (in some cases considerably so) than that of the PVC polymer alone, because of the presence of the various additives. However, uPVC compositions are not normally significantly inferior in this respect, although the presence of some impact modifiers may increase solvent susceptibility somewhat, whilst resistance to acids and alkalis may be affected by heavy loading with certain fillers (e.g. whiting and wood flour, respectively). Flexible (pPVC) compositions may be more readily attacked by solvents; the increased susceptibility depends mainly on the nature and amount of plasticiser(s) present. The general chemical resistance characteristics of PVC compositions are summarised in Table 12.8. Additional data for uPVC are given in Tables 12.9 and 12.10. ISO/DATA 7:1979 gives data on the resistance of uPVC pipes to many fluids at up to 60°C. Apart from any direct chemical action, some reagents can affect the properties of PVC materials by leaching or dissolving out important components of the formulation (e.g. plasticisers, stabilisers) even if only from the surface layer. Plasticisers may also be lost by migration into materials in close contact with pPVC (e.g. adhesives, lacquers) whose properties may be affected as a result.

S

S S S

Reducing agents Detergent solutions Inorganic salt solutions

:}

M

M

Remarks

Rigid PVC

No attack up to 60°C, but max. allowable design stress should be lowered

No attack up to 60°C; allowable design stress should be substantially reduced Allowable design stresses should be substantially reduced No attack up to 60°C

Attacked above 20°C; max. allowable design stress should be reduced substantially

:1

Q

General resistance rating

Oxidising agents

Concentrated

Organic acids Alkalis: Dilute

Oxidising (concentrated)

Concentrated

Inorganic acids: Dilute

Reagents

S S S

S

:}

M

U

M

S

Q

General resistance rating

No attack up to 60°C

Some fillers may be affected

Some plasticisers and fillers may be affected

No significant attack up to 20°C; plasticisers and some fillers may be affected at higher temperatures Plasticiser and some fillers may be affected Short-term contact may be acceptable in some cases

Remarks

Plasticised PVC

TABLE 12.8 General Chemical Resistance Characteristics of PVC at Room Temperature

~

:::l 0

~

~

.j>-

oe oe

a

U U U M M M-U U

S U U

U U U M M S-M U

Aliphatic hydrocarbons

Aromatic hydrocarbons Chlorinated hydrocarbons Esters Ethers Ketones Aldehydes Amines Liquid fuels Turpentine Oils: Mineral Vegetable and animal Fats See also Table 12.10

Rating key: S = Satisfactory. M = Moderate (dependent on formulation and conditions). U = Unsatisfactory.

S S S

U U

S

Water

Allowable design stresses should be substantially reduced Some softening possible at elevated temperatures

M-U M-U M-U

M

S

U U U M

U U U S

U

Bromine Fluorine Iodine Aliphatic alcohols

Little attack in the absence of moisture

M

Halogens: Chlorine

Softening, and some effects on certain fillers at elevated temperatures Extraction of plasticisers and some effects on other components possible

.j>.

-a

00

1:;'

'"

l::

~

0

~

~

$::>

;:::

0;-

$::>

::I.

~

~

()

"l:I -.::::

s·

~

~

;:-

'"~'"

~ ~

'"~.

~

.g

...... N

TABLE 12.9

Effects of Chemical Immersion on a High-impact uPVC Compound 30-day immersion at room temperature

Chemical

Tensile strength (lbfin- z)

Weight change

5200 4500 3600 5800 5900 5800 5300 4900 5700 5550 5450

0·81 1·99 4·48 0·50 0·43 0·22 0·32 0·40 0·48 0·04

5500 5600 5400

30-day immersion at 60"C Tensile strength (lbfin- Z)

Weight change 3·29 8·25 15·39 1·64 2·26

0·14 -0·07 -0·01

5000 2450 1450 5800 5700 4900 4700 5450 4200 4950 5250 4800 5900 5800 5700

1·48 0·98 6·59 1·72 -0·19 3·54 0·04 0·58 12·78

4600 4100 5400 5200 5600

1·20 1·21 0·14 0·02 -0·02

2150 2400 5400 5900 5400

5·14 3·17 0·22 -0·05 -0·11

Carbon tetrachloride Trichloroethylene

1850 1550

59·14 103·21

Excessive swelling Excessive swelling

Benzene Castor oil Cotton seed oil Glycerine Hexane Linseed oil Salt solutions (sat.) Barium sulphide Ferric chloride Potassium chloride Sodium dichromate Trisodium phosphate

5000 5300 5150 5450 3000 5150

73-11 0·44 0·18 0·07 4·24 0·07

Excessive swelling 5000 0·07 5400 0·26 5900 0·18

5000 5100 5000 5400 5050

0·61 0·25 0·22 0·23 0·46

5000 4700

0·80 1·53

2850 5200

3·87 0·61

Acetic acid Chromic acid Hydrochloric acid Nitric acid Oxalic acid (sat. soln) Phosphoric acid Stearic acid Sulphuric acid

Butyl alcohol Ethyl alcohol Sodium hydroxide

Formaldehyde Hydrogen peroxide Phenol Turpentine Distilled water

20% 80% (glacial) 10% 30% 40% 30% 30% 60% 75% 100% 20% 50% 80%

10% 30% 50%

(%)

(%)

4800

0·44

5450 4750 4800 5150 4800

3·11 0·32 0·20 0·18 0·78

4200 3·57 4800 1·67 Excessive swelling 1850 35·49 5100 0·94

Acetaldehyde: 100% 40% Acetone Aluminium fluoride Ammonia liquid, 100% Ammonium hydroxide, 0·88 Ammonium fluoride, 20% Ammonium sulphide Amyl acetate, 100% Aniline, 100% Barium chloride Benzaldehyde Benzine Bleach lye, 10% Bromine gas, weak Bromine liquid Bromic acid, 10% Butyl acetate Butyric acid: 20% cone. Butanol: primary secondary Calcium chlorate Calcium hypochlorite, soln Carbon disulphide Chloracetic acid, 100%

Chemical

S S U U S U S S

S S

S

M

S S S S

U

S U S

U

U S M S S S

U

S

S

U S U S S U S

M

U

-

U

-

S

M

M

U

U M U

S

60

S

U

20

(0C)

Temperature

Chloric acid, 1-20% soln Chlorine: gas moist gas liquid Chlorobenzene Chloroform Chlorosulphuric acid Chromic acid Citric acid, sat. Copper fluoride, 2% Copper cyanide Cresol Cresylic acid Cupric fluoride Cyclohexanol Cyciohexanone Dibutyl phthalate Diethylene glycol Diglycolic acid Dioctyl phthalate Ethyl acetate, 100% Ethyl alcohol Ethyl butyrate Ethyl chloride Ethyl ether Ethylene chloride Ethylene dichloride

Chemical

TABLE 12.10 Further Data on Chemical Resistance of Rigid PVCa

S U U U

M

S S U U S S S U U S

M

U U S S S S S

U

S S S

20

(0C)

U

U U U S S S S U S S U U S S S U U S U S U U U

-

S S

60

Temperature

~

l:>..

.;..

\0

I;;

'"

l'::

l:>..

Cl

"'tl

;:,

'"

/;;'"

1:;'

;;; ....

a:: '"

(')

'..



~

~

'"~.

~ ~

tv

......

U U U S U S U S M

-

S

-

S S S S S S S S S U S -

494

W. V. Titow

The good resistance of PVC to many chemicals is utilised in such practical applications as, for example, uPVC wall cladding in chemical plants, or the ducting and fans of fume-extraction systems, and protective pPVC gloves and clothing for workers in the chemical industry, laboratories and stores. The susceptibility of PVC compositions to attack by some solvents is also used to advantage in some processes as well as for certain test purposes: apart from the various applications of PVC polymer and copolymer solutions (see Chapter 24), such uses include, for example, the incorporation of isophorone as a keying agent in printing inks for PVC sheeting and coatings, the use of solvents in tests for the completeness of gelation of PVC paste coatings on fabrics (see Chapter 22), and solvent-swelling tests (in acetone or dichloromethane) for homogeneity and structural integrity of uPVC pipe and other extruded products and mouldings (d., for example, ISO 3472-1975; BS 3506:1969; ASTM D 2152-80; SABS 791-1975; SABS 966-1976). The chemical resistance of plastics materials, including PVC, is normally tested by determining changes in appearance, dimensions, mass, and/or other properties of specimens after a period of contact (usually by immersion) with the chemical(s) concerned. Some of the relevant standards give a list of chemicals for determining the general resistance and specify the properties to be used as assessment criteria (see, for example, ISO 175 and 462; BS 4618: Section 4.1; ASTM D 543). Some basic requirements in respect of the general chemical resistance of uPVC compound are laid down in Table 2 of ASTM D 1784-1981. The international and national specifications of interest in connection with various aspects of chemical resistance of PVC include the following: ISO 175*-1981: Plastics-Determination of the effects of liquid chemicals, including water. IS0/R 462*-1965: (Later incorporated in ISO 175). Recommended practice for the determination of change of mechanical properties after contact with chemical substances. ISO 3473-1977: Unplasticised polyvinyl chloride (PVC) pipesEffect of sulphuric acid-Requirement and test method. * Essentially equivalent to parts of ASTM D 543.

12

Properties of Special1nterest in PVC Materials and Products

495

BS 2782:1970: Method 505A: Resistance to concentrated sulphuric acid of rigid polyvinyl chloride compounds. BS 4618: Section 4.1:1972: Chemical resistance to liquids. ASTM D 543-67. (Re-approved 1978): Resistance of plastics to chemical reagents. ASTM D 1239-55. (Re-approved 1982): Resistance of plastic films to extraction by chemicals. ASTM D 1784-81: Rigid poly(vinyl chloride) (PVC) compounds and chlorinated poly(vinyl chloride) (CPVC) compounds.

DIN 53476:1979: Testing of plastics; Determination of the behaviour against liquids. DIN 53756:1974: Testing of plastics; Storage in contact with chemicals. DIN 53 428:1967: Testing of cellular materials; Determination of the resistance to liquids, vapours, gases and solid materials.

U.9 HEALTH HAZARDS Health hazards arise in the production, processing, use, and disposal of most plastic materials, and PVC is no exception. Some of the hazards are of a general nature, not directly dependent on the composition of the plastic: e.g. risks of injury in operating plastics processing machinery, or the well-publicised danger of suffocation to children using plastics bags as substitute space helmets in play. This section is concerned primarily with those health hazards which are specifically associated with the chemical nature of PVC materials, although some associated 'peripheral' hazards are also briefly mentioned. The main hazard areas may be collectively identified as the risk of harmful effects on contact with the PVC materials themselves, or their individual constituents, or decomposition products, during any of the abovementioned phases of the materials' life history. The principal possible harmful effects are poisoning (in the widest sense of the term), carcinogenic action, irritation and tissue damage, and dermatitis. The forms of contact through which they can arise are ingestion, inhalation, absorption (e.g. through the skin or mucous membranes), or simple

496

w.

V. Titow

'external' contact (which may also lead to some absorption) especially if prolonged or repeated. 12.9.1 Vinyl Chloride Monomer In the case of PVC an important potential health hazard is encountered at the earliest stage of the material's life cycle, in that the vinyl chloride monomer (VCM) is a recognised carcinogen. The hazard continues wherever residual amounts of the monomer are present in PVC resins and compounds, before, during and after their conversion into end products. This situation necessitates precautions against exposure to free VCM in the production of PVC polymers and copolymers, and measures to minimise residual VCM contents of such polymers and the compounds and products based thereon. The general objective is to reduce to an acceptable level the amount of the carcinogen which can be transferred by direct contact, inhaled (or absorbed) as vapour previously volatilised into the atmosphere, or consumed in foods and beverages which can extract it from PVC packaging films or containers. Among the most important problems arising in this connection is the need to know what should be regarded as the maximum permissible concentrations of VCM in PVC materials and the atmosphere, and the associated requirement for suitable methods of determination. Although the carcinogenic activity of VCM (in animals) was first made known only in 1970,66 and links with a form of liver cancer (angiosarcoma) and a rare cancer of the mouth in humans first recognised in the mid-1970s,66,67 much effort has already been devoted to meeting both these needs. Several analytical methods for determining small amounts of VCM in PVC and in air are now available, with sensitivity in many cases better than 1 ppm, and in some down to a few parts per (American) billion. 68 Several commercial detectors and monitors are on the market.69 Gas-chromatography procedures, involving either direct or head-space sampling, can be particularly useful,70-72 although IR spectroscopy and photodetection are also utilised in monitors for VCM in air. 69 Clip-on badges have been developed for the latter purpose.73 Interest continues in possible ways of determining the actual extent of damage caused by VCM in the body: inter alia, a very sensitive method has been reported based on the alkylating action of VCM (as well as of certain other carcinogens) on amino acid constituents of haemoglobin. 74 Ideas on maximum concentrations representing 'acceptable risk'

12

Properties of Specia/1nterest in PVC Materials and Products

497

levels have undergone a considerable change in the past few years with increasing volume and availability of relevant data. The first limits recommended in the UK (in the mid-1970s) for maximum VCM concentration in factory atmospheres started with a time-weighted average figure of 25 ppm (by volume), soon to be brought down to 10 ppm with the further proviso that wherever possible zero concentration should be aimed at. 75 At the same time in West Germany (North Rhine-Westphalia) the maximum concentration limits for factories were being lowered from an initial 50 ppm to 5 ppm,76 whilst in the USA a limit of 1 ppm was being demanded, with the US Food and Drug Administration (FDA) concurrently framing regulations to prohibit the use of rigid and semi-rigid PVC for food-packaging applications (bottles, films) unless it could be shown that no migration of VCM into the contents would occur. Attention was focused on unplasticised PVC, because available evidence indicated that plasticisation reduces residual VCM contents to undetectably low levels. A temporary standard was put out in the USA by the Occupational Safety and Health Administration (OHSA) in 1974, followed by a finalised version in 1978: in the same year relevant rules, limiting VCM emission in industrial plant, were formulated by the US Environmental Protection Agency (EPA), and EEC directives issued in Europe on VCM content in food-packaging materials. These moves made themselves felt in the industry in several ways. PVC resin production, as well as that of packaging films and bottles, was curtailed by some manufacturers unwilling to face the difficulties and expense of reducing VCM concentrations in their plants and products in the face of uncertainty as to what limits might finally be laid down. Prices of some PVC resins and products were affected as production became more expensive where removal of VCM and tighter control over its concentration were being instituted. Some resins, in which the VCM content was reduced by heat treatment ('stripping'), became more glassy and harder to process as a result of this addition to their 'heat history'. On the positive side, R&D work was stimulated towards methods of reducing VCM concentrations in PVC materials and factory atmospheres, methods of determining such concentrations, and the ways in which they were affected by production conditions. Towards the end of the 1970s the practical improvements achieved in production and processing, coupled with the results of the R&D effort, led to a brighter outlook on the VCM risk. Further confirmation has been forthcoming for the relative safety of plasticised PVC

498

W. V. Titow

materials, as has evidence of a substantial drop in residual VCM levels in both PVC materials for food packaging and the foods packaged therein. 67 ,77 It is now practicable to reduce the VCM content of commercial PVC resins to a few parts per 109 (i.e. by a factor of nearly 106 since the early 1970s), and there is strong evidence (from the Ethyl Corporation in the USA) that at, or below, 2 parts per 109 VCM will not migrate into food from PVC materials at a significant rate. 78 ,79 The latest FDA estimates based on this evidence indicate potential maximum VCM levels of less than 5 parts per 1012 in PVC-packaged food. 79 Thus, whilst the fact remains that only complete avoidance of exposure to VCM can entirely eliminate all risk, a high degree of confidence in properly processed PVC as food-packaging material may soon be restored. An excellent review of the VCM problem in all its aspects was published recently by Clayton. 8o It may be noted in passing that exposure to VCM (admittedly in minute quantities) from sources unconnected with PVC may be a real possibility for large numbers of people both in the industry and outside: vinyl chloride has been reportedly found in tobacco smoke (albeit in very small concentrations-up to 0·03 ppm), and the possibility has been mentioned that it may also be formed as a combustion product of other plant materials, including vegetable refuse. 8 ! U.9.2

PVC Compounds and their Regular Constituents

Aside from the effects of VCM, the main health hazard is possible toxicity in food-contact applications involving such PVC products as films and containers: this hazard is usually considered from the point of view of the properties of the individual components of a formulation. It is normal to 'clear' these, before the formulation is finalised and made up, on the basis of experience, and/or information from the manufacturers, and/or the relevant recommendations or rules of the appropriate national authorities and organisations. In the USA the organisations most directly concerned are the ones referred to in the previous section (FDA, OHSA, EPA): the US Department of Health, Education and Welfare (HEW) may also be mentioned in this connection. In the UK and Europe the bodies with related interests and functions (albeit largely different constitutions, and scope and nature of operations) include the UK Health and Safety Executive, British Plastics Federation (BPF) , the UK Chemical Industries

12 Properties of Special1nterest in PVC Materials and Products

499

Association, the West German Federal Health Office, and corresponding organisations in many other countries. Some of these organisations (e.g. FDA, BPF) issue lists of materials (e.g. plasticisers, stabilisers, colourants) approved (or forbidden) for food-contact applications: such applications constitute the area of primary concern in the context of this section. Some aspects of the subject of toxicity of PVC materials are discussed in a brief paper by Estevez. 82 An earlier review, by Phillips and Marks,83 is also still of some interest. In the UK the BPF publishes a code of practice for safety in use of plastics for food-contact applications, based in part on extensive evaluation tests carried out by the British Industrial Biological Research Association (BIBRA). It is normally assumed that PVC homopolymers, vinyl chloride/ acetate and vinyl chloride/vinylidene chloride copolymers are non-toxic in compounds. Several lubricants (in particular stearic acid) are regarded as safe, as are some of the other two principal formulation components, plasticisers and stabilisers, when used in prescribed concentrations: acceptability, especially the concentration limits, may, however, vary according to the conditions. For example, more stringent requirements arise for food-packaging films to be used with fatty foods (e.g. bacon, butter, etc.) capable of leaching out plasticisers, than for non-fatty foods with a high water content (e.g. fruits, vegetables). The packaging of children's toys is also an area of special concern. Detailed, up-to-date information and guidance can be obtained from the organisations mentioned in this section. Some further general information is also given in the chapters on stabilisers and plasticisers.

U.9.3 PVC Decomposition Products If thermal decomposition of PVC is permitted to occur in processing, and when PVC is burned (e.g. in an accidental fire, or as a means of disposal), toxic and irritant fumes are produced. These contain a considerable proportion of hydrogen chloride (usually appearing as an acrid, highly irritant white fume), which is the principal product of thermal breakdown of vinyl chloride homopolymers and copolymers: 84 a sooty, black smoke usually arises from the combustion of plasticisers in flexible PVC compositions. Other pyrolysis products of PVC materials include benzene, toluene, xylene, naphthalene, and certain derivatives of these compounds: 6,84--86 with an adequate supply of

SOO

w.

V. Titow

oxygen, water vapour, CO and CO2 are also formed, as combustion products. 84 U.9.4 Peripheral Hazards

The kinds of hazard that may be mentioned under this heading are relevant to PVC, although not exclusive to it, * as they can arise in the production and processing of other plastics. They are: (i) fire and explosion hazards; (ii) respiratory hazards; (iii) toxic hazards; These occur in the storage and handling of additives and other formulation components (especially in powder form), and in processing operations involving the use of solvents (e.g. making up PVC solutions, printing on PVC materials, preparation and application of solvent-based lacquers for PVC sheet materials). The appropriate precautions are nowadays generally reasonably well known in the industry, but it should also be remembered that many are prescribed by law, and that the statutory requirements vary in different countries. Advice and guidance is available from the organisations mentioned in Section 12.9.2. Relevant information may also be found in the current editions of the following publications: Industrial Hygiene and Toxicology. F. A. Patty (Ed.), Interscience Publishers. Encyclopaedia of Occupational Health and Safety. International Labour Office, Geneva. Health Hazards of the Human Environment. World Health Organisation. Kirk-Othmer Encyclopedia of Chemical Technology. John Wiley & Sons (under 'Industrial Toxicology' and other relevant headings). * However, two specific points may be made regarding PVC: under certain conditions the presence of fine PVC resin dust can lower the explosive limit of VCM/air mixtures; potentially ignitable levels of VCM may arise in high-speed mixing equipment. Guidance on safety in the operation of high-speed mixers is provided in a booklet published jointly by the British Plastics Federation and

Chemical Industries Association Ltd, 'Vinyl CWoride Monomer. Guide to the High Speed Mixing of PVC Resins and Compounds'.

12 Properties of Special Interest in PVC Materials and Products

501

Dangerous Properties of Industrial Materials. N. I. Sax, Van Nostrand Reinhold. Fire Protection Handbook. G. H. Tryon (Ed.), National Fire Protection Association, Boston, Mass., USA. Publications of the American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio, USA, including (i) Documentation for Threshold Limit Values for Substances in Workroom Air; (ii) Industrial Ventilation: A Manual of Recommended Practice. Relevant HMSO Publications, (UK).

12.10 BURNING BERAVIOUR Virtually all plastics are combustible: that is they will-under suitable conditions (e.g. in a sufficiently intense fire)-undergo exothermic oxidative decomposition, accompanied by flame and/or glowing and/or smoke evolution as its main visible manifestations. However, the nature and severity of the conditions required for ignition and sustained combustion are different for different plastics and, conversely, the ignition and combustion behaviour will differ under identical conditions: in both cases the differences are governed by differences in chemical composition and physical state-e.g. a polystyrene film will burn readily in circumstances in which a uPVC one will not; a uPVC bar may have an oxygen index of 40 or only 25 depending on whether it is solid or cellular, and so on. Thus meaningful comparisons can only be made on the basis of tests relevant to the purpose of the comparison and carried out under closely standardised conditions. Moreover, because they are cardinally dependent upon the conditions, the results of laboratory tests are strictly relative (as are any comparisons based upon them) and should not be used as criteria for the prediction of the degree of hazard in actual fire situations. Similarly, such apparently definitive terms as 'self-extinguishing', 'non-flammable', 'flameresistant', 'slow-burning' can only have meaning in relation to a specified set of conditions (e.g. a particular standard test). Even when these principles are observed confusion can still occasionally arise because the terminology of the burning behaviour of plastics is not fully uniform: standardisation, and increasing awareness of the factors and concepts involved, have done much to improve matters, but even standard definitions of the same important term can still differ considerably. For example, two sources of relevant standard defini-

502

W. V. Titow

tions, ISO 3261-1975 * and ASTM E 176-82, t define 'flammable', respectively, as 'capable of undergoing combustion in the gaseous phase with emission of light during or after application of an igniting source' and 'subject to easy ignition and rapid flaming combustion'. Note: Other publications containing relevant terminology are: Addendum 2 (1983) to ISO 472, comprising definitions of terms relating to burning behaviour of plastics; and Compilation of ASTM Standard Definitions published by the American Society for Testing and Materials. Certain terms are also defined in some of the standard specifications listed in Table 12.12.

The burning behaviour of plastics is of great importance in many applications, and hence of interest to the user and technologist alike. The key aspects with which the practically oriented tests are concerned are ignitability, spread of flame, rate of heat release, and amount of smoke generated. The chemical composition of the smoke, whilst not investigated in standard tests, is also important and has been receiving increasing attention as a toxic hazard in fires. In addition, the Fennimore-Martin 'Oxygen Index,88,89 (based on the minimum concentration of oxygen required to support candle-like burning of a standard size specimen in specified conditions) provides a useful means of rating the flammability (in the sense of ease of ignition and burning) of plastics and other materials. Some typical oxygen index values for plastics, including PVC, are shown in Table 12.11. Standard burning tests relevant to (including some specifically devised for) PVC materials and products are listed in Table 12.12. A British standard covering the development, presentation and use of fire tests is now available. 91 The' flammability of PVC (resins and solid uPVC compositions) as determined in standard tests is one of the lowest among those of the common plastics. However, the smoke emission is relatively high, and the smoke is irritant and toxic (see Section 12.9.3). The low flammability is due to the large chlorine content: like the other halogens (cf., for example, PTFE in Table 12.11) chlorine acts as a retardant in the process of combustion (see Chapter 11, Section 11.5). * 'Fire tests-Vocabulary'.

t 'Standard definitions of terms relating to fire tests of building construction

and materials'.

12

Properties of Special Interest in PVC Materials and Products

503

TABLE 12.11 Oxygen Index Values of Some Plastics Materialsa

Material

PVC resin (homopolymer) uPVC compound (medium impact strength) uPVC compound containing 15% glass fibre PVC floor tile (asbestos-filled) pPVC compounds PVDC PTFE Polyamide (nylon 6.6) Polycarbonate Polymethyl methacrylate Polyethylene Polypropylene Polypropylene with flame retardant Polypropylene asbestos-filled Polystyrene uPVC foam pPVC foam Polystyrene foam Polystyrene foam with flame retardant Polyurethane foam Polyurethane foam with flame retardant Polyisocyanurate foam

Oxygen indexb (typical or representative value) 45

40 40 30

21-26

60

95 23 23-27

17-18 17-18 17-18 22 21

18

25 22

18

24 19 22 26

Table based on data from Refs 88, 92, 94 and 95. % Oxygen in the standard gas mixture, required to support candle-like combustion of standard specimen in standard conditions (ASTM 2863). a

b

The performance of PVC compositions in flammability tests falls with decreasing chlorine content (see Fig. 12.7 here, and Fig. 6.3 in Chapter 6). This is the main reason for the well-known fact that plasticisation increases flammability, albeit this effect is reduced where chlorinated extenders or phosphate plasticisers are used, since the former introduce their own chlorine, and the latter act as flame retardants in their own right (see Chapter 11, Section 11.5; and Chapter 7, Section 7.6). An expression relating the halogen content of a polymer to its carbon and hydrogen contents, known as the van Krevelen Composition Parameter, has been found to correlate well with the oxygen index for many polymers, including polyvinyl chloride. 92 The flammability of

Flammability

Plastics: rigid (selfsupporting) sheet or moulding

1. ISO 1210-1982 2. BS 2782: 1970 Method 508D Burning time and/or rate and/ 1. BS 2782:1970: or extent Method 508A 2. BS 2782:1970: Method 508B 3. ASTM D 635-81

1. ISO 871-1980 2. ASTM D 1929-77

Ignition properties

Plastics: pellets; sheet or film

Standard specifications

Incandescence resistance (be- l. ISO 181-1981 haviour during and after con- 2. BS 2782: 1970 tact with incandescent bar at Method 508E 950°C) 3. ASTM D 757-77 4. DIN 53459-1975

Property or characteristic determined

Plastics: rigid sheet or moulding

Material or product

Remarks

1. Bar specimen held horizontally 2. Relates specifically to PVC compounds 3. Bar specimen held horizontally

1. and 2. technically equivalent: self-ignition and flash-ignition temperatures determined (in a hot-air ignition furnace)

1. and 2. Intended for thermosetting plastics 3. Recommended for materials which are self-extinguishing in the test of ASTM D 635 (see below)

All four specifications closely similar technically (employ the 'Schramm! Zebrowski' method)

TABLE 12.12 Standard Burning Tests Relevant to PVC Materials

;;

is

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Flammability and/or burning rate, and/or extent of bum

Plastics: cellular

Smoke generation Horizontal burning characteristics Smoke generation Vertical burning characteristics (flame height, burning time, mass loss)

Plastics: solid or cellular Oxygen index (applicable also to non-plastics materials e.g. wood)

Plastics: film or thin sheet

BS 5111:Part 1:1974 ASTM D 3014-76

BS 4735:1974

1. ISO 4589-1985 2. BS 2782:Part 1: Methods 141 A to C: 1978 3. ASTM D 2863-77 ASTM D 2843-77

4. ASTM D 1433-77

1. ISO/R 1326-1970 2. BS 2782:1970: Method 508Ca 3. ASTM D 568-77

4. UL subject 94 Parts A & B 5. IBM CMH 6-0430102

The 'Butler Chimney' test

2. Restricted to solid (non-cellular) specimens NB Method D for electric cable insulation or sheathing-see below Employing the XP2 smoke chamber

2. Relates specifically to thin flexible PVC sheeting 3. Vertically suspended strip specimen: test results sensitive to thickness 4. Strip specimen supported on 45° incline

4. and 5. Closely similar; vertical bar specimens ignited at lower end; effect of dripping (ignition of cotton by flaming drops) taken into account; tests more severe than 1 and 2

2S lJl

~ ~ 1:;

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~.

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S' "l:I

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~ ~

~.

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Property or characteristic determined

Building materials (including plastics)

Various combustion characteristics (including smoke generation in some cases)

Combustibility

Electrical insulation and Oxygen index cable sheathing (mainly plastics) Ignition and/or spread of flame and/or rate and extent of burning

Material or product

Remarks

1. Agrees with lEC 332 (vertical specimen) 2. Test for rigid sheet and 2. ASTM D 299-82 plate insulation materials 3. Test for non-rigid PVC 3. ASTM D 876-80 tubing used for electrical insulation Test for duration of sustained ISO 1182-1979 flaming 1. ISOrrR 3814-1975 1. Report on tests being developed 2. BS 476 2. A multi-part specification 'Fire tests on building materials and structures' 3. ASTM E 84-81 3. 'Underwriters tunnel furnace test'. 25 ft specimens 4. ASTM E 286-69(1975) 4. The '8 foot tunnel' test 5. DIN 4102 5. A multi-part specification 'Behaviour of building materials and components in fire'

BS 2782:Part 1: Method 141D:1978 1. BS 4066:1969

Standard specifications

TABLE 12. 12-contd.

;e

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:0::::

~

~

Vl

Various combustion characteristics

'Materials' (some relevance to plastics)

a

Now superseded by BS 2782:Part 1: Method 140 D:1980.

Interior materials for Burning rate and extinmotor vehicles (includ- guishing characteristics ing PVC upholstery and mouldings)

Duration of flaming and afterglow, and/or length of char (or melt), and/or flaming drips

Coated fabrics

ISO 3795-1976 (based on US Federal Motor Vehicle Safety Standard 302)

2. DIN 53 438-1977

1. ASTM E 162-81

4. DIN 54332-1975

3. ASTM D 2859-76

1. BS 3424:1973: Method 17 2. BS 5790:1979

Requirements stated in terms of rate of burning; specification much used for PVC upholstery fabrics

1. Test with radiant energy source: 'flame spread index' and smoke evolution measured

2. Specification for upholstery fabrics, including PVCcoated woven and knitted fabrics: flammability tests by the Method of 1. 3. Flammability of textile floor coverings (relevant to PVC-backed carpets) 4. Burning behaviour of textile floor coverings

1. Vertical strip specimen

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508 45

40

25

0·3

004

el,

wt fraction

0-5

0·6

Fig. 12.7 Oxygen index (01) of a PVC composition as a function of its chlorine content (fraction by weight of the total composition). Formulation of PVC composition: 100 PVC resin Plasticiser (DOP) 0-90 phr, as shown White lead 7 phr Ca stearate O· 75 phr A, B, C, D, E, F,

DOP 90 phr; all additives 97·75 phr; CI content 0·287; DOP 60 phr; all additives 67·75 phr; CI content 0·339; DOP 40 phr; all additives 47·75 phr; CI content 0·384; DOP 20 phr; all additives 27·75 phr; CI content 0·445; DOP 0; all additives 7·75 phr; CI content 0·527; PVC resin alone; CI content 0·568.

a PVC composition may be reduced, despite a reduction in the overall chlorine content, through the incorporation of a non-combustible filler (e.g. asbestos fibre), a flame-retardant compound, or a smoke suppressant. The latter two types of additive and their effects are discussed in Section 11.5 of Chapter 11. Much useful information (including an extensive list of literature references) on all aspects of combustion of polymers is contained in a recent book by Cullis and Hirschler. 93 A comprehensive (10-volume)

12

Properties of Special Interest in PVC Materials and Products

509

report * by the National Materials Advisory Board of the USA Academy of Sciences is an important source of reference on subjects falling within the ambit of its title. A list of flammability test methods for plastics (containing national standard tests of 18 countries, as well as some ISO standards and those of the Underwriters Laboratory, NCB) has been published by the Chemical Industries Association Ltd, London. Some data on the evolution of HCI and smoke from PVC (burnt with wooden cribs) are given by Edgerley and Pettett. 84

REFERENCES 1. Eftis, J. and Liebowitz, H. (1975). Engineering Fracture Mechanics, Vol. 7, Pergamon Press, Oxford, pp. 101-35. 2. Plati, E. and Williams, J. G. (1975). Polym. Engng. Sci., 15(6), 470--7. 3. Brown, H. R. (1973). J. Mat. Sci., 8,941-8. 4. Williams, J. G. (1975). 'The determination of fracture toughness from impact tests on polymers', Paper 2 at the PRI Conference on New Developments in Impact Testing, London, 2nd December. 5. Williams, J. G. (1975). 'The fracture mechanics of polymers', Ibid. Paper 14. 6. Titow, W. V. (1977). In Developments in PVC Production and Processing-l (Eds A. Whelan and J. L. Craft), Applied Science Publishers, London, Ch. 4. 7. Reymers, H. (1970). Mod. Plast., September, 78-80. 8. Titow, W. V. Unpublished work. 9. Wilson, A. S., Biggin, I. S. and Pugh, D. M. (1978). 'The influence of volatility on the selection of plasticisers to meet new and developing performance requirements', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 10. Ranney, M. W. (1975). Irradiation in Chemical Processes: Recent Developments, Noyes Data Corp. 11. Scalco, E. and Moore, W. F. (1983). Radiat. Phys. Chern., 21(4),389-96. 12. Yasuda, H. (1975). J. Appl. Polym. Sci., 19(9),2529-36. 13. Titow, W. V. (1978). In Adhesion 2, (Ed. K. W. Allen), Applied Science Publishers, London, Ch. 12. 14. Fujita, H. (1961). Fortschr. Hochpolym.-Forsch., 3, 1-47. 15. Meares, P. (1965). Polymers: Structure and Bulk Properties, Van Nostrand, London, p. 316. 16. Meares, P. (1958). J. Polym. Sci., 27, 391-404. 17. Meares, P. (1966). Eur. Polym. J., 2,95-106.

* 'Fire Safety Aspects of Polymeric Materials' (1979).

510

w.

V. Titow

18. Crank, J. and Park, G. S. (Eds) (1968). Diffusion in Polymers, Academic Press, London. 19. Stafford, G. D. and Braden, M. (1968). J. Dent. Res., 47(2), 341. 20. Carman, P. C. (1956). Flow of Gases Through Porous Media, Butterworths, London. 21. Scheidegger, A. E. (1974). The Physics of Flow Through Porous Media, 3rd Edn, University of Toronto Press, Toronto. 22. Rodebush, W. H. and Langmuir, I. (1942). 'Smokes and filters', US OSRD Report No. 865. 23. Davies, C. N. (1948). 'Fibrous filters for dust and smoke', Proc. of the IX International Congress on Industrial Medicine, London, 13-17 Sept., John Wright and Sons Ltd, Bristol. 24. Iberall, A. S. (1950). J. Res. Nat. Bur. Stds, 45, 398-406. 25. Thomas, D. J. (1952). J. Inst. Heatin!{ Ventilating Engrs. 20(201), 35-70. 26. Hopfenberg, H. B. (Ed.) (1974). Permeability of Plastics Films and Coatings, Plenum Press, New York. 27. Lebovits, A. (1966). Mod. Plast., 43 (March), 139-46, 150, 194-213. 28. Hennessy, B. J., Mead, J. A. and Stenning, T. C. (1966). The Permeability of Plastics Films, Plastics Institute. 29. Pye, D. G., Hoehn, H. H. and Panar, M. (1976). J. Appl. Polym. Sci., 20(7), 1921-31. 30. Salame, M. and Pinsky, J. (1962). Mod. Packag., (September) pp. 153-223. 31. Wilson, G. A. R. (1965). Plastics, (May), pp. 86-115. 32. Brydson, J. A. (1961). Plastics, (December), pp. 107-10. 33. Horsfall, F. and James, D. I. (1973). RAPRA Members J., (September), pp.221-7. 34. ShUT, Y. J. and Ranby, B. (1975). J. Appl. Polym. Sci., 19(7), 1337-46. 35. Kambour, R. P. (1968). Polym. Engng. Sci., 8(4),281-5. 36. Haward, R. N. (Ed.) (1973). The Physics of Glassy Polymers, Applied Science Publishers, London. 37. Ziegler, E. E. (1954). SPE J., 10(4),13-16. 38. Dempsey, L. T. (1967). Polym. Engng. Sci., 7(2),86. 39. Titow, W. V. (1975). Plast. Polym., 43(165),98-101. 40. Faulkner, P. G. and Atkinson, J. R. (1972). Plast. Polym., 40(147), 109-117. 41. Wolf, J. (1967). Gas, 87 (November), 433. 42. Szabo, E. and Lally, R. E. (1975). Polym. Engng. Sci., 15(4),277-84. 43. Caryl, C. R. and Helmick, W. E., US Patent No.2 945 417: Apparatus and Mechanism for Concentration of Solar Rays on Objects to be Tested, 19th July, 1960. 44. Ives, G. c., Mead, J. A. and Riley, M. M. (1971). Handbook of Plastics Test Methods, Iliffe Books, London. 45. Caryl, C. R. (1967). SPE J., 23(1),49. 46. Kuist, C. H. and Maxim, L. D. (1968). SPE J., 24(7), 46-51. 47. Grossman, G. W. (1977). J. Coatings Technol., 49(633),45-54. 48. Allen, N. S., McKellar, J. F. and Wood, D. G. M. (1976). Plast. Rubb.: Mat. Appln., 1(2), 57-61.

12

49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60.

61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82.

Properties of Special Interest in PVC Materials and Products

511

Summers, J. W. (1976). 34th ANTEC SPE Proceedings, pp. 333-5. Kinmonth, R. A., Jr (1964). SPE Trans., 4(3),229-335. Cassel, B. and Gray, A. P. (1977). Plast. Engng, 33(5), 56-8. Wypych, J. (1975). J. Appl. Polym. ScL, 19(12), 3387. Wypych, J. (1976). Ibid, 20(2),557. De Coste, J. B. and Hansen, R. H. (1962). SPE J., 18(4), 431-9. Rabek, J. F., Shur, Y. J. and Ranby, B. (1975), J. Polym. Sci. Polym. Chern. Ed., 13(6), 1285-95. Anon. (1979). Eur. Plast. News, 6(3), 40. Katz, M., Shkolnik, S. and Ron, I. (1976). 34th ANTEC SPE Proceedings, p.511. Allara, D. L., Ibid, p. 245. Scullin, J. P., Girard, T. A. and Koda, C. F. (1965). Rubb. Plast. Age, 46(3), 267-8. Sahajpal, V. K. (1978). 'PVC compounding for low organoleptics and controlled bacteriological growth', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. Mascia, L. (1974). The Role of Additives in Plastics, Edward Arnold, London. De Coste, J. B. (1968). Ind. Eng. Chern., 7(4), 238-47. Kaplan, A. M., Greenberger, M. and Wendt, T. M. (1970). Polym. Engng. Sci., 10(4),241-6. Wessel, C. J. (1964). SPE Trans., 4(3), 193-207. Anon. (1965). Mod. Plast., 42(5), 168. McGinty, L. (1979). New Scientist, 183 (9th Aug.), (1167), 433. Socrates, G. (1979). Plast. Rubb. Wkly, (9th March), p. 10. Anon. (1975). Ibid, (21st February) p. 70. Anon. (1974). Chern. Engng. News, (16th December), pp. 24-5. Draft German Standard DIN 53743-1977. Testing of plastics: Gas-chromatographic determination of vinyl chloride (VC) in polyvinyl chloride (PVC). Anon. (1979). Plast. Technol., 25(9), 13. Berens, A. R., Crider, L. B. and Tomanek, C. J. (1975). J. Appl. Polym. ScL, 19(12), 3169-72. Anon. (1976). Chern. Engng. News, (8th March), p. 6. Anon. (1979). New Scientist, 183 (April 19th), (1151), 185. UK Health and Safety Executive. 'Vinyl Chloride-Code of Practice for Health Precautions', Temporary Format (February 1975). Anon. (1975). Plast. Rubb. Wkly, (18th April), p. 3. Daniels, G. A. and Proctor, D. E. (1975). Mod. Packag., 48(4),45-8. Anon. (1979). Plast. Technol., 25(5), 211. Anon. (1979). Mod. Plast. Int., 9(12),28. Clayton, H. M. (1977). In Developments in PVC Production and Processing-I, (Eds A. Whelan and J. L. Craft), Applied Science Publishers, London, Ch. 3. Anon. (1977). Plast. Rubb. Wkly, (18th March), p. 15. Estevez, J. M. J. (1969). Plast. Polym., 37(129),235-42.

512

W. V. Titow

83. Phillips, I. and Marks, G. C. (1961). Brit. Plast., 34, 319 and 385. 84. Edgerley, P. G. and Pettett, K. (1981). Plast. Rubb. Proc. Appl., 1(2), 133-7. 85. Iida, T., Nakanishi, M. and Goto, K. (1975). J. Polym. Sci. Polym. Chem. Ed., 13(6), 1381-92. 86. Mitera, J. and Michal, J. (1976). Chem. Prum., 26(8),417-20. 87. Clark, C. A. (1972). SPE J., 28(7), 30-5. 88. Fennimore, C. P. and Martin, F. J. (1966). Mod. Plast., 44(3), 141-8. 89. Isaacs, J. L. (1970), J. Fire Flamm., 1(1), 36-47. 90. Oswin, C. R. (1975). Plastic Films and Packaging, Applied Science Publishers, London. 91. BS 6336:1982. Guide to the development and presentation of fire tests and their use in hazard assessment. 92. Grieveson, B. M. (1976). 'The fire hazard of polymers', paper presented at the Polymer Symposium, British Association for the Advancement of Science, Lancaster, England, 3rd September, 1976. 93. Cullis, C. F. and Hirschler, M. M. (1981). The Combustion of Organic Polymers, Clarendon Press, Oxford University Press. 94. Titow, W. V. and Lanham, B. J. (1975). Reinforced Thermoplastics, Applied Science Publishers, London. 95. Ahrens, H. W. and Zahradnik, B. (1973). 'Oxygen index rating of plastics as a guide to their behaviour in fire', CSIR Special Report BOU 29.

CHAPTER 13

Industrial Compounding Technology ofRigid and Plasticised PVC W. HENSCHEL and P. FRANZ

13.1 INTRODUCTION The compounding process represents the link between raw material production and finished-article manufacture. Its function is to combine the PVC resin with the various additives required for processing and for the service properties of the final product, in accordance with the formulation. There are five general types of industrial PVC compounding • operation (Fig. 13.1) -preparation of pre-mixes and dry blends, -melt compounding and pelletising, -compounding for the feeding of film and sheet calenders, ----,production of pastes (plastisols, organosols, plastigels), -recycling. As indicated schematically in Fig. 13.2, the equipment required can be divided into the upstream section ahead of the compounder, the compounder itself, and the downstream equipment. The upstream units are more or less identical for all the five general types of compounding operation, but the compounder and its downstream equipment have to be adapted to the specific requirements of each type. A typical line is shown in Fig. 13.3. The upstream equipment handles the raw materials: it comprises silo storage, conveying, weighing. Included in the compounding section are the PVC pre-mixing operation, the actual compounding and, where pellets are produced, the pelletising operation. 513

514

W. Henschel and P. Franz PROCESSED PVC MATERIALS

COmpounding (mixing)

Compounding

~

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,Extrusion ,Blow moulding

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Rtl-cycling

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Fig. 13.1 Industrial compounding of PVC: general schematic outline. Table 13.1 indicates, for some important PVC products, the proportions produced, respectively, from pre-mix and from pellets as the feedstock. To interpret the table properly, one should bear in mind that the production of film, sheet and board, and of products from plastisols, involves processes with an in-line compounding step between pre-mix and final product. Thus, in these cases final-product processing follows directly on the compounding operation, and there is no need either for pelletising or for the downstream equipment that normally follows that operation. The downstream equipment normally employed for pellets and dry blends handles the cooling, conveying, storage and packaging of the compound.

13.2 RAW MATERIALS 13.2.1 PVC Polymer and Fillers

In terms of the amounts used in PVC compounding, these are the two principal solid raw material components; both are in powder form.

13 Industrial Compounding Technology of Rigid and Plasticised PVC

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Fig. 13.2 PVC compounding: block diagram.

PVC polymer: The characteristics of PVC polymers, and their significance in formulation and processing, are discussed in Chapters 1-4. From the standpoint of the compounding or extrusion operation, it is improtant to emphasise those properties that are crucial to the production of free-flowing, dry powder blends.

2

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D Fig.13.3 Typical PVC compounding line. 1, Storage silo for PVC polymer; 2, production or holding silo for filler; 5, storage tank for plasticiser; 6, station; 8, weighing station for solid components; 9, weighing station for liquid cooling; 13, storage silo for pellets; 14, bagging and palleting; 15, plant control

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518

W. Henschel and P. Franz

TABLE 13.1 Proportions of Important PVC Products Produced, Respectively, from Pre-mix and Pelletised Compounds (World-wide)

Products

Produced from: pre-mix (%) pellets (%)

Extrusions (pipes, profiles, tubes, hoses, siding) Injection mouldings Cables Records Blow mouldings (bottles) Film and sheet Plastisol products

90 25

75

70

100 100

10 75

100 25 30

Table 13.2 lists some of the relevant properties of commercial PVC polymers produced by suspension polymerisation (S-PVC), emulsion polymerisation (E-PVC) and mass polymerisation (M-PVC). TABLE 13.2 Some Properties of PVC Polymers Polymer type

Properties

Kvalue (DIN 53726) Processing Particle shape Particle size (/lIIl)

M-PVC

S-PVC

E-PVC

57-71

55-71

65-75 Sprayed by special process Spherical, whole and broken spheres

Dried with Sprayed rolls Riven, porous up to 1000

Bulk density (g litre -1) 54~30 (DIN 53468) Good Free-flowing property 1·5-5 Plasticiser absorption (ml DOP per 5 g PVC)

Riven, porous

Bead-shaped, Flaky compact, glassy 60-250 60-250 60-500

4~20

600-700

300

~

up to 200 30Q--4()()

Good

Good

Good

Poor

Good

5

2

J-5

0·5-1

J-5

up to 60

13

Industrial Compounding Technology of Rigid and Plasticised PVC

519

Fillers: The use and effects of fillers in PVC are discussed in Chapters 4 and 8. The effect of fillers on the production of hot blends depends on the loading, particle size and plasticiser absorption. High loadings of fine-grained filler make blends flow less freely. Fillers with a porous surface often absorb too much plasticiser, which in turn results in dry mixes. One positive effect worth mentioning is the use of very small amounts of colloidal silica for powdering poorly flowing blends in the cold mixer. 13.2.2 Plasticisers

Plasticisers are the principal liquid components employed in PVC compounding. The nature and classification of plasticisers, their properties, applications, and effects in PVC, are discussed in detail in Chapters 5-7 (also, passim, in Chapters 1, 4 and others). 13.2.3 Other Additives

The other constituents of PVC compositions, which-from the point of view of compounding-may be regarded as additives to the PVC polymer, are discussed in Chapter 4: some are also dealt with in considerable detail in separate chapters or chapter sections, e.g. stabilisers in Chapters 9 and 10; lubricants, colourants, and others in Chapter 11. 13.3 UPSTREAM EQUIPMENT (SILO STORAGE TO WEIGIDNG) 13.3.1 Silo Storage of PVC Polymer and Fillers

Storage of the solid raw material components calls for buildings designed to house bagged or container goods, or for silo installations. For economic reasons, preference is normally given today to batteries of silos capable of holding several thousand tonnes. The lower limit for economical silo storage of raw material components is a percomponent consumption of about 30 tonnes per month. (a) Silo Sizes Silos with capacities of 150 to 250 m3 are generally used for storing PVC in the plastics industry. Whilst smaller silos are also used, those

520

W. Henschel and P. Franz

with volumes of less than 50 m3 are regarded as uneconomical. This minimum size is set by the capacity of the rail tank cars normally employed nowadays for delivering the raw materials from production plants. The ability to discharge the entire contents of a tank car into an empty silo in a single operation is essential. Otherwise, unnecessary waiting time would result for the tank cars. On the other hand, the maximum silo size depends on the transport possibilities from the silo fabrication plant to the erection site. If finished silos have to be transported by road or rail, the acceptable volume is limited to 150 m3 . Typically, the design diameter of the silo tank is 2·4 m, though 3·5 m silos are built occasionally. Larger units-up to a volume of 400 m3 for PVC-can only be transported by water or, if this is impossible, shipped in pieces and welded together on site. In the case of fillers (notably chalk), silos of volume greater than 150 m3 are hardly ever employed because of the relatively high bulk density of the contents. The size of a battery of silos (Plate B) in a plastics plant depends on the procurement possibilities for the raw materials, raw material consumption, the plant's geographic location, and not least the market situation in the raw material sector. (b) Materials of Silo Construction Nearly all the silos erected out-of-doors today are fabricated from an aluminium/magnesium alloy (AIMg 3). It is fair to say that the steel silo with internal coating and external paint finish has been displaced by the standard aluminium alloy silo in the field of PVC compounding. Aluminium alloys are weatherproof, require no maintenance (as no paint peels off and no rust develops) and have a virtually unlimited service life. The plates used have a smooth surface, with a peak-to-valley depth normally less than 20 f.lm. Silo walls of aluminium alloy are much less prone to adhesion of contents than those of other materials. There are no problems with electrostatic charges, because unpainted aluminium is an excellent conductor of electricity. Because the external wall reflects well, there is little product heating as a result of exposure to sunlight. For some years now, silos as large as 150 m3 have also been built of glass-fibre reinforced polyester. This material is superior to aluminium in terms of chemical stability and mechanical abrasion. The disadvantage of static charges causing dust to adhere to the silo wall is countered by using antistatic additives in the material.

13 Industrial Compounding Technology of Rigid and Plasticised PVC

521

Plate B Silo installation for PVC polymer and fillers.

(c) Raw Material Intake (Silo Filling) With increasing use of silo storage facilities, the traditional practice of purchasing solid raw material components in bags or other small containers is being increasingly replaced by bulk purchase with delivery by tanker transport. The advantages are:

-less labour; -no loss of material in transport; -lower raw material prices; -no contamination of the materials and dust-free working conditions. The filling of raw material silos is always accomplished with pneumatic conveying systems. Both road tankers and rail tank cars are used. The vehicle tanks are generally designed to resist conveying

522

w.

Henschel and P. Franz

pressures. Compressed air, and not a suction system, is normally used to empty the vehicle tanks. Screw compressors are used to generate the required flow of oil-free conveying air. Though most road tankers have their own compressors, a stationary compressor installation is required at the plant for emptying rail tank cars. These compressors have a working pressure between 1·5 and 2·5 bar. Air flows lie between 400 and 800Nm3 h- 1 .* With the usual pipe diameters of 80 to 100mm, depending on the material conveyed, this results in conveying capacities of 15 to 30 Mp h -1. t Generally, each silo has its own pipe leading from the connection point for the filling hose. In order to retain flexibility with regard to raw materials, and the ability to handle small batches of different formulations and qualities smoothly, most plants have an additional dumping station for filling the storage silos (or holding bins, or both) with bag goods. Because the amount of bagged goods is usually very small compared with total plant throughput, a manual dumping station is normally sufficient for this purpose. The hourly filling rate achievable by manual opening and dumping of bags is about 3 tonnes. To eliminate bag scrap, it is generally advisable to follow the dumping station with a suitably dimensioned sifting machine before the raw material is conveyed pneumatically to the storage silo through a rotary valve or a pressurised tank. The bag-dumping station must be arranged so that, instead of escaping, the dust raised during dumping is drawn off to a filter by a suitable exhaust system. As a rule these bag-dumping stations are supplied with a built-on filter, so that the filter can be cleaned mechanically after each filling operation to return the dust to the raw material. A proper exhaust system for a bag-dumping station should be laid out for an air flow of about 20 m3 min- 1 at a vacuum of 200 mm w.g. The resulting air withdrawal velocity during dumping is about 0·5ms- 1 . If, in exceptional cases, larger quantities of bagged raw materials are expected, it is advisable to plan for a semi-automatic or fully automatic bag-dumping machine. Such machines are available on the market for dumping rates of about 600 bags per hour. Maximum and minimum level monitors are necessary in storage silos * A German unit = cubic metres per hour at STP (i.e. 20°C, one bar pressure). t Megaponds per hour (i.e. tonnes per hour).

13 Industrial Compounding Technology of Rigid and Plasticised PVC

523

to prevent both overfilling and unplanned emptying. For more sophisticated demands, it is also possible to use continuously operating devices to monitor the filling level at all times. (d) Raw Material Discharge Raw material discharge is a very important factor in the operation of a silo facility. Most conical silo outlets are built with a hopper angle of 60°. Except for plastics pellets, additional discharge aids must be attached to the outlet zone for virtually all fine-grained raw materials. The familiar ability of many pulverulent products to flow freely when fluidised with air is exploited with the aid of aerating devices. The suitability of a product for aeration is determined by its bulk density, angle of repose, grain size distribution and specific surface area. Aeration plates are built based on a number of different systems. The surfaces in contact with the product are made of an air-permeable material. Nylon and polyester are generally used to cover the aeration plates, but air-permeable ceramic materials, sintered metal, and polyethylene board are sometimes used instead. Nozzles are occasionally employed to inject the air into the product, but it is important to design them in such a way that no product can enter the tiny air channels. Aeration plates are laid out to blow in the air successively in different sections, thus achieving a pulsation effect. The air must be completely free of dust or oil. Air pressures as high as 2 bar are required, depending on material depth and bulk density. The assumptions generally employed are a specific surface loading of 2-4 m3 of air per minute and m2 of aeration surface. (See Fig. 13.4). Another important mechanical discharge aid is the vibration plate. It is particularly suitable for products that tend to 'shoot over'. In such cases, it is necessary to hold the products back while metering them to the equipment that follows. From the storage silos, the raw material components are conveyed pneumatically to the weighing station. In small plants, this can be accomplished with a ring pipe and discharge flap above the scale. In larger plants, the raw materials are transported pneumatically from the storage silos to the production (or holding) silos. The latter are located directly above the weighing station in the compounding line. The raw material components are normally metered into the pneumatic conveying lines via blow-through rotary valves. Two-cycle valves are sometimes used, particularly for low-velocity pneumatic conveying or plug conveying.

524

W. Henschel and P. Franz

VI~W

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v~nl

A

A

Qlr conl'lK\lon

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Fig. 13.4 Storage silo for PVC polymer or fillers. (e) Dust Removal System As already noted, virtually all silo storage facilities in the plastics industry are filled by pneumatic conveying systems. The product is usually blown tangentially into the tank; cyclones are very seldom used to separate the product from the air stream. The dust content of the air differs, depending on silo size, filling level and particle size of the product. Suitable filters have to be provided to remove the dust from the air. Most silo installations are laid out with a filter for each tank

13 Industrial Compounding Technology of Rigid and Plasticised PVC

525

unit. The only exceptions are cases where the same product is stored in a number of silos. The filters are provided with fully automatic or semi-automatic purging, depending on the dust loading and throughput. The difference between the two is that fully automatic filters are purged by dust-laden air during service, while semi-automatic filters are cleaned only when the filling operation is complete. Either bag or sheet filters can be used. The advantage of the sheet filter over the bag filter is that it occupies less space. For lower dust loadings and coarse-particle dusts, filters with mechanically actuated purging devices are generally sufficient. For very highly loaded filters and those handling fine dust, jet-type filters with pneumatic purging are generally used. In this case, the dust is purged from the outside of the bags by applying compressed air at about 6 bar pressure to the inside of the filter elements for back-purging. Cotton can be used for the filter fabric, but synthetics such as polyacrylonitrile or polyester are usually favoured. The filter area to be provided is governed by the admissible filter surface loading. Rule-of-thumb figures are: in the case of mechanically purged filter elements, 1 m2 of filter area can handle 1 m3 min- 1 of dust-laden air: in the case of pneumatically purged jet filters, 1 m2 of filter area can be loaded with 3 m3 min- 1 of dust-laden air. 13.3.2 Conveying of PVC Polymer and Fillers

As already indicated, any two operations in a compounding process are generally separated by a transport distance for the solid raw material components, dry blend, finished pellets, or for recycled process or start-up waste. The most suitable conveying system has to be found for each material, depending on flow rate, conveying distance, and special cleanliness requirements. It is also necessary to consider the material temperature (and whether it cools down or heats up), as well as the possibility of its segregation into various fractions. The decision whether pneumatic or mechanical conveying (by means of screws, bucket elevators, etc.) should be given preference will be made in the light of these considerations. (a) Pneumatic Conveying Pneumatic conveying, i.e. the transport of bulk materials in closed pipes with the aid of a stream of air, is standard practice in PVC processing plants just as it is in other industries. The technique has

-1500

b

Piston compressor, radial blower

Radial blower

Short to medium distances

30

150

Charging pellets into bins and machines; removing free-flowing materials from tips or containers Silo filling, suction pick-up from grinding mills

Silo filling

Filling of production or holding silos from storage silos or from bag-dumping stations Filling of holding silos

Filling of storage silos from pressureproof tankers

Application

This system offers special advantages in conjunction with such process steps as drying and cooling.

° 10 000 mm w.g. = 1 kgf cm- 2 .

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Vacuum of about -5000 Piston compressor

Up to 50 (very limited)

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13 Industrial Compounding Technology of Rigid and Plasticised PVC

555

Applications in the processing of PVC: -Pre-mixers for mixing plastic pellets with masterbatch pellets for the colouring of PVC on injection extruders or cable insulation extruders. -Post-mixers for mixing PVC pellets following compounding and prior to packaging for the purpose of batch homogenisation. This makes it possible to equalise production-related quality variations. Large-volume mixers such as double cone, tumbler or vee mixers are used for this purpose. Table 13.6 lists the main technical features of the mixers with rotating containers shown in Fig. 13.14. Mixers with rotating mixing tools:

Operating principle: The different mixer designs can be divided into three groups on the basis of peripheral speed (Vu ): slow-speed 'push' mixers with V u < 2 m S-l slow-speed 'throw' mixers with Vu = 2-12 m S-l high-speed intensive mixers with Vu = 12-50 m S-l In the slow-speed push mixers, the mixing tool displaces the material to the front and the side, so that it flows back into the resulting empty space behind the tool and becomes mixed. The material is treated gently and hardly any size reduction occurs. If the speed of the mixing tool is raised until the centrifugal force exceeds gravity, we have the throw mixer. The material particles are thrown upwards by the mixing tools, follow intersecting trajectories, and are thus mixed together. Mixing times are much shorter than they are in push mixers. Unlike push mixers, which are nearly filled with product, throw mixers are filled to only about 60 or 70% of their volume, to provide sufficient free space into which the mixture components can be thrown. Velocity differences between the various trajectories produce frictional forces that can help break up agglomerates, but friction-related product stressing and heating remains within reasonable limits. Still higher mixing tool speeds do not result in shorter mixing times, but raise the energy absorption, with resultant heating of the material, an effect that is desirable in many cases. Peripheral speeds between 30 and 50 m S-l are common in high-speed intensive mixers. The mixing

Twin shell mixer (vee)

Wobble miX« aod } double cone mixer

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5-10

50-75

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Speed of mixing container (r min-i)

50-75

(%)

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Mixing time

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TABLE 13.6 Technical Data for Rotating-tank Mixers

0·1--40

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0·5-1·0

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Industrial Compounding Technology of Rigid and Plasticised PVC

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tool produces an intensive impact effect, which tends to break up particles besides dispersing the material components. Applications in PVC processing (see Fig. 13.14): -As hot or cold mixers for the blending of raw material components. This function can be performed by helical ribbon mixers, ribbon bar mixers, paddle mixers or ploughshare mixers. It is also often carried out by tank-type mixers, alternatively known as intensive mixers, fluidising mixers or turbo-mixers. -Change-can mixers and orbiting vertical screw mixers are used for the preparation of PVC pastes. -Vertical-screw silo mixers are used for after-mixing of PVC pellets following compounding for batch equalisation. Table 13.7 and Fig. 13.15 contain the main technical data for the mixers with rotating mixing tools shown in Fig. 13.14. Because tank-type or intensive mixers are extremely important in the processing of PVC, a detailed description of this type is provided in the next section.

(d) Tank-type or Intensive Mixer DESIGN AND OPERATING PRINCIPLE

Tank-type or intensive mixers, which are also known as turbo, high-speed, or fluidising mixers, are used mainly as batch mixers for the pre-mixing of the PVC raw material components to form a free-flowing powder blend. Among the best-known designs are those marketed by the following companies: Batagion, Caccia, Covema, Diosna, Fielder, Thyssen, Mixaco, Moritz, MTI, Papenmeier, Spangenberg. Tank-type mixers with high-speed mixing tools are generally built with cylindrical tanks arranged vertically (Fig. 13.16) or horizontally. Inside the tank, the mixing tools are mounted on a vertical or horizontal mixing shaft. The tools, which generally operate at peripheral speeds between 20 and 50 m s -1 differ depending on the mixing job and manufacturer. They may take the shape of radial flights in the form of bars, knives or propellers, or they may be ring-shaped or paddle-shaped. In the vertical design, the tank bottom is either flat or dished, and the ratio of tank diameter to height is approximately 1: 1. The Diosna design employs two cylindrical tanks joined together to form a figure-of-eight shell, which is equipped with two separate drives

Category

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200{}-{)000 100-3500 10-1500 25-4500

5-12

5-10

5-12

15-30

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TABLE 13.7 Technical Data for Rotating-tool Mixers

350-50

1000-250

350-100

50-15

rotational (r min-I)

(10) 8-4

50-20

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Industrial Compounding Technology of Rigid and Plasticised PVC

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13 Industrial Compounding Technology of Rigid and Plasticised PVC

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tools. In this case, of course, the shape and configuration of the tools must be adapted to the lower mixing speeds. The mixing of a rigid PVC composition will normally proceed as follows. The associated changes in power input (motor amperage) are shown in Fig. 13.23. The particulate components are homogenised by the turbulent mixing motion immediately after the high-speed mixer motor is switched on. As this occurs, the mechanical energy of the mixing tool is converted into heat by material friction. The friction, and particle impacts, bring about the particle surface changes previously mentioned and the material flows more freely, causing the energy consumption level to drop until a temperature of about 8SoC is reached. Around this temperature the lubricants melt, and the material becomes sticky andflows less freely, which again raises the energy consumption; this peaks when the liquefied lubricants are being absorbed by the PVC grains: thereafter the material again flows more freely and the energy consumption falls. Another energy peak, this time a small one, is observed at temperatures above 12SoC when the metallic stearates melt. A

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Fig. 13.23 Power input during the blending of rigid PVC in a high-speed mixer. Phase I, mixing and abrading; II, increase in free-flowing property; III, melting of lubricants; IV, dry mixing; V, melting of metallic stearates; VI, hot mixing; VII, cooling of blend in cold mixer.

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0

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13 Industrial Compounding Technology of Rigid and Plasticised PVC

571

When the motor amperage levels off, the mixing operation is over and it is time to discharge the material into the cold mixer. Continued mixing in this temperature range, but below the melting point of the PVC, would merely impair the flow properties as a result of further size reduction, accompanied-perhaps surprisingly-by a drop in bulk density. Thermal degradation would also be initiated. Changes in material temperature in the course of mixing are illustrated, for various PVC compositions, in Figs 13.24-13.27. Figure 13.24(A) shows the energy input and temperature curves of an emulsion PVC blend with 35% plasticiser. In this case the cold plasticiser was fed continuously into the preheated mixture. The start O(OC)

120 110 100 90 80 70 60 50 40 30 20 10 0

0

2

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4

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Fig. 13.25 Temperature change in a high-speed mixer as a function of blending time. 1, S rigid PVC; 2, E rigid PVC; 3, S plasticised PVC 30% pi; 4, S plasticised PVC 40% pi; 5, S plasticised PVC 50% pi; 6, E plasticised PVC 40% pI. (Source: Herfeld Mixaco.)

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W. Henschel and P. Franz

and finish of plasticiser feeding are apparent from the energy input curve. Figure 13.24(B) is a similar plot for suspension polymer, but with the cold plasticiser charged all at once into the mixture. Again, the point at which this took place is reflected in the temperature and energy curves. Both diagrams indicate that the quality of a blend is affected by the method and timing of the plasticiser addition. To obtain homogeneous dispersion of the plasticiser, it is necessary to achieve a mixer temperature of 70 to 80°C. In this temperature range, the PVC particles swell and absorb the continuously fed, and possibly preheated, plasticiser more easily. Figure 13.25 shows the temperature changes as a function of mixing time for rigid and plasticised PVC with various amounts of plasticiser. Figures 13.26(A) and (B) show the temperature curves, respectively, for a rigid PVC pipe blend and a plasticised (60:40) PVC shoe compound blend, both processed in a hot/cold mixer combination. Figure 13.27 illustrates, in a generalised and highly simplified form, the temperature changes of rigid and plasticised PVC compositions as functions of mixing time in hot/cold mixer combinations. Among factors contributing substantially to the achievement of optimal properties in a PVC composition are the timing, sequence and method of additive admixing, as well as the speed programme of the high-speed mixer, mixing time and final temperature. It is therefore a serious mistake to underestimate mixing technology and to regard the mixing step as an unimportant preliminary to subsequent processing. Influence of raw material components and processing conditions on the properties of hot blends: -PVC polymer. Particle structure, shape and size affect the blend flow properties. Suspension and bulk polymers with porous particles absorb the plasticiser easily. Fine-grained emulsion polymers containing emulsifier, and their large-grained clusters which are broken down in the mixer, usually result in pasty stock when the plasticiser is added: this can only be used to produce agglomerate. -Type of plasticiser. Dry mixes are difficult to produce with plasticisers of limited compatibility (which are poorly absorbed by the polymer particles), and with slow-gelling plasticisers. Primary plasticisers with good gelation effect give the best results. -Plasticiser quantity. This is a crucial factor in producing a hot

13 Industrial Compounding Technology of Rigid and Plasticised PVC

575

dry blend. The maximum amount of plasticiser that the hot mixer will tolerate depends on the type and K value of the polymer, as well as the plasticiser type. In some cases high percentages of plasticiser can only be handled by hot mixers to produce agglomerate. The temperature of incipient agglomerate formation, and then that of lump formation, decreases with increasing plasticiser content. In other words: the higher the plasticiser content, the lower the admissible final temperature. Cold mixing at high plasticiser contents will normally result in damp or wet mixtures lacking the ability to flow freely. -Plasticiser viscosity. This mainly influences the diffusion rate into the PVC particles (see Fig. 5.3 in Chapter 5). Medium- to high-viscosity plasticisers therefore require longer mixing times, unless their viscosity is reduced by heating before addition. -Admixing the plasticiser. Understandably, it makes a difference whether the plasticiser is placed in the mixer first and the solid components then added or whether it flows into the cold or hot pre-mix of the other components, and whether its introduction into the running mixer is made in one shot, in portions, or continuously. The best results are obtained by allowing the plasticiser to flow slowly into the pre-mix of the solid components at an elevated temperature (about 7o-S0oq. The feed rate should be set in such a way that the amperage drawn by the drive motor does not rise sharply. After the plasticiser has been added, mixing should be continued for about one minute to make sure all of it has been completely absorbed. If plasticiser is added to suspension or bulk PVC of medium absorption capacity faster than it can be absorbed by the particles, a highly viscous paste will form which will then gel to produce large, tough lumps if mixing is continued. This may cause the drive to cut out because of overloading: in any case, the mixture is unusable and cleaning of the mixer is unpleasant and time-consuming. -Mixing temperature and mixing time. Mixing time depends upon the required final temperature, which is given in the temperature/ time diagrams earlier in this section. For high-speed mixers it ranges between 10 and 20 min, depending on the formulation and whether a hot or hot/cold mix is involved. A reliable rule-ofthumb is that a temperature rise of more than 15°C min -1 should be avoided during mixing: otherwise local overheating can damage the mix. Furthermore, the correct final temperature of the mix is

576

W. Henschel and P. Franz

also affected by the type and quantity of plasticiser employed, as well as by the K value and particle structure of the polymer. It is set correctly when the cooled blend emerging from the cold mixer is dry and free-flowing. -Lubricant: The effects of lubricants in the hot mix depend upon their physical state (solid, liquid, paste), compatibility, melting characteristics and quantity. A liquid lubricant (e.g. paraffin oil) can produce a sticky, non-flowing hot mix if it is poorly compatible with PVC and becomes concentrated on the particle surface. PVC-compatible liquid lubricants are absorbed into the particles with the plasticiser, unless the amount added exceeds the compatibility limit. The melting range of solid lubricants should be chosen in such a way that (1) the lubricant is molten before the final mixture temperature is reached and is dispersed homogeneously throughout all of the material, and (2) the solid physical state has already been regained after the material has cooled down to the cold mixer temperature (about 40°C). In the case of rigid PVC blends, all formulation components except the lubricant are mixed at low speed: then the speed is raised, and the lubricant is added about 20°C below the desired final temperature. For the production of hot mixes, it is therefore advisable to combine PVC-compatible internal lubricants with solid external lubricants in the required melting range. -Fillers: The effect of a filler on the production of hot mixes depends on its amount, particle size and plasticiser absorption. High percentages of very fine-grained fillers impair the freeflowing property. Fillers with porous surfaces usually have undesirably high plasticiser absorptions, but, on the other hand, promote dry mixes. Mixes poorly flowing in the cold mixer can be improved by the addition of very small amounts of colloidal silica. APPLICATION OF INTENSIVE MIXERS

Either high-speed mixers (hot mixers) or hot/cold mixer combinations are used, depending on the production route followed for the final product. Hot mixers (high-speed mixers): Hot mixers are normally used alone ahead of pelletising machines and for direct processing of PVC powder blends. The advantages of hot mixing of PVC powders in high-speed

13 Industrial Compounding Technology of Rigid and Plasticised PVC

577

mixers are: -optimal mix quality and homogeneity; -short cycle times and high output rates; -very free-flowing blends; -pneumatic conveying of dry blends or agglomerates without product segregation; -up to 20-40% increase in bulk density as a result of sintering, thus raising the output rates of processing machines; -low-cost and effective lowering of the residual VC content, partly because of the high mixing temperatures of more than 80°C; -nearly complete elimination of residual moisture in the material.

Hot/cold mixer combinations: Hot/cold mixer combinations have won wide acceptance among PVC compounders in the past 15 years. As already mentioned, they enable the hot mix from the high-speed mixer to be transferred directly into the cold mixer and cooled down to about 4Q-50°C, whereby unnecessary 'heat history' (and hence thermal degradation) of the material is avoided, tendency to agglomeration during storage is counteracted, and the free-flowing property improved by the cold shock. Hot/cold mixer combinations are used both for feeding pelletising extruders and for direct processing of the PVC powder blend for use in injection moulding and blow moulding. Virtually all rigid PVC blends are produced on hot/cold mixer combinations, even if pelletising follows. During mixing of suspension and bulk polymers, high friction sometimes produces an electrostatic charge in the material, which impairs flow properties, especially in batches with relatively low bulk densities. One way to eliminate the charge is to cool the mix rapidly to about 40°C in the cold mixer. For certain purposes, e.g. the production of large pipes, crystal-clear blown films and similar sensitive products, which have to be absolutely free of moisture, the PVC batch can also be de-gassed during mixing. This can be done by using vacuum mixers or by blowing or drawing dry, filtered air under the hot-mixer cover during the final phase of the mixing process. 13.4.2 Melt Compounding In some production processes, such as the manufacture of records or the extrusion of pipe and profiles, it is sometimes possible to use

W. Henschel and P. Franz

578

pre-mixes, produced as described in Section 13.4.1, directly as feedstock for the processing machines. However, where only limited homogenisation is achievable in the processing equipment (such as for example, simple extruders, light-duty and heavy-duty calenders), or if a simple pre-mix is not acceptable as feedstock (by reason of particular material-handling arrangements or feed requirements), or where stringent quality specifications for the final product necessitate the highest degree of homogenisation of the composition, the material must be melt-compounded. Table 13.1 indicates the percentage of melt-compounded PVC used in the main application areas. While dry blending interdisperses the formulation components uniformly on a macroscopic, and down to the microscopic scale, the level of interdispersion produced by melt compounding may be described as microscopic to sub-microscopic, and largely intermolecular for those components which are sufficiently compatible. This degree of dispersion optimises the effectivity of additives combined with the PVC resin in the composition. It can only be achieved when the resin is in an elasto-viscous (melt) state during the compounding: this is brought about by thermomechanical means. PVC is a heat-sensitive material. The extent of thermal decomposition in processing depends on stock temperature and residence time, as illustrated by Fig. 13.28; therefore both must be carefully controlled in the compounding process. In this, thermomechanical stressing of the PVC polymer changes the structure of the original particles and hence

104.11 T.mp.,otur.

UNACCEPTABLE FOR EXTRUSION

(OC)

EXTRUDABLE AND WEATHERABLE 2S

50

75

Uillmot. Proc.sslng TI_ (1041.....1••)

100

AREA OF HEAT HISTORY IN BUSS COMPOUNDING AND PELLETISING LINES TYPE IIG

Fig. 13.28 Influence of processing conditions ('heat history') on the processability and performance of melt-compounded PVc.

13 Industrial Compounding Technology of Rigid and Plasticised PVC

579

the rheological behaviour of the material. Raising the stock temperatures and residence times results in more severe structural disruption, and unravelling of the macromolecules, ultimately leading to a higher degree of gelation. Where pellets are being fed to an extruder, for instance, a higher degree of gelation requires higher melting energies during extrusion. Because modern final-processing extruders are pushed to the performance limit, raising the melting power may make certain extrusion operations unprofitable as a result of reduced output. Thus the compounding process has to be matched to the operating conditions in the final processing (production) step, whilst achieving its fundamental objective, viz. the maximum homogenisation and degree of dispersion of the additives in the PVC resin, with the lowest possible input of thermomechanical energy and minimum residence time, so that the 'heat history' (and hence degradation) of the resin is kept to a minimum. Besides homogenising the composition, compounding converts it into a physical form suitable for subsequent processing. In recent years, too, sharply increased processing speeds on calenders and extruders have added effective de-gassing of the PVC compoundoriginally effected on mixing rolls and in internal mixers-to the list of the functions of compounding. Whilst they are still in use in laboratories and in some small-scale operations, mixing rolls and internal mixers find only limited application in present-day industrial melt-compounding of PVC, which is now largely the domain of screw-type compounding machines. Continuous compounding on such equipment was introduced about 30 years ago with a view to better control of residence times, improved efficacy of mixing, reduction of compound quality fluctuation (and hence better reproducibility), process rationalisation, and elimination of the effects of chance in batch and intermittent operation. In view of this, the discussion in this section is confined to compounding processes involving continuously operating screw-type machines. In such machines, 80-90% of the energy required for fluxing the polymers and homogenising the mixture is obtained by the conversion of mechanical shearing energy. Only 10-20% of the total energy requirement is provided by heating the barrels and screws. The main job of the heating system is to ensure that the screw and barrel surfaces in contact· with the material are kept at a desired set-point temperature; inter alia this prevents overheating and scorching of the PVC stock as a result of wall slippage: thus the heating system must be

580

W. Henschel and P. Franz

capable of both supplying and dissipating heat. For this reason systems based on the circulation of liquid heat-transfer media, such as water or HT oil, in screws and separate barrel zones have gained wide acceptance in industrial practice. (a) Compounding and pelletising Both rigid and plasticised PVC compositions are melt-compounded. The general types of compound produced are shown in Fig. 13.29. The characteristic curves of various rigid and plasticised PVC compositions, obtained on a high-pressure capillary rheometer (Fig. 13.30), demonstrate the wide differences in rheological behaviour that have to be coped with during the compounding process to obtain the necessary degree of gelation. The graph shows that with plasticised PVC compositions high outputs are obtained within a relatively narrow (and generally low) pressure range. This applies in melt compounding, so that-in the context of this process-plasticised PVC may be regarded as relatively insensitive to pressure and shear. For rigid PVC the graph shows a pronounced influence of pressure on output, i.e. in this sense rigid PVC compositions are sensitive to pressure and shear. Because energy dissipation in screw-type machines is essentially a function of shear rate and system pressure, adjustment of the degree of gelation of rigid PVC calls for special elements to control these parameters in this kind of equipment (see also further discussion below).

MELTCOMPOUNDED PVC COMPOSITIONS

FLEXIBLE

BOTTLE EXTRUSION CABLE COMPOUNDS COMPOUNDS COMPOUNDS

INJECTION MOLDING COMPOUNDS

EXTRUSION COMPOUNDS

Fig. 13.29 Melt-compounded PVC compositions.

13

Industrial Compounding Technology of Rigid and Plasticised PVC

plasticised PVC [k~] Oulpul

2

581

rigid PVC 6

3

21)

1,5

1,0

0,5

o

10

20

30

40

so

60

70

Fig. 13.30 Characteristic flow curves of some PVC compositions in a capillary rheometer. 1, Shoe compounds; 2, water hose compounds; 3, general-purpose extrusion grades; 4, cable compounds (insulation, sheathing); 5, 6, 7, blow-moulding compounds; 8, general-purpose extrusion grades; 9, 10, profile compounds; 11, window profile and siding compounds. Both rigid and plasticised PVC are compounded for pelletising by essentially the same process (Fig. 13.31). The free-flowing PVC blend from the pre-mixer passes through a holding bin before being charged into the feed hopper which serves as a product surge bin and a volumetric metering element. In some cases the hopper is designed additionally for powder de-aeration, but this does not eliminate the need for de-gassing the fluxed stock. An agitator with suitably shaped arms is employed in the upper part to prevent bridging (Fig. 13.32). A separately driven twin-screw metering device is sometimes used below the hopper (as, for example, in the feed section of the Plastifikator machines). An alternative is to add a vertical metering screw to the bottom of the agitator, a concept employed, for instance, in the design of the Buss-Kneader and the Kombiplast machine. In either case, the speed of the metering screws determines the volumetric flow of product fed into the screw-type machine. In the intake zone of the machine the metered blend is picked up and carried along to the plasticating and homogenising zone. In this zone, part of the total shear energy is converted into heat and the PVC is fluxed (plasticated).

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Fig. 13.31 PVC pelletising: flow sheet. 1, Silos for solid components (resin, filler); 2, tanks for plasticisers; 3, discharge and conveying devices; 4, plasticiser supply pumps; 5, batch weighing station for solid components; 6, batch weighing station for plasticiser; 7, pre-mixer; 8, compounding and pelletising unit; 9, vacuum pump; 10, pellet cooler.

FEED HOPPER WITH HORIZONTAL TWIN SCREW

FEED HOPPER WITH VERTICAL 51 GLE SCREW

Fig. 13.32 Typical feed hopper designs.

13 Industrial Compounding Technology of Rigid and Plasticised PVC

583

Another portion of the shear energy is used for breaking down and dispersing the additives. The ways and means of imparting the energy to the material are discussed in Section 13.4.4(a) ('screw-type machines'). Homogenisation is one of the three objectives of the compounding process, the other two being de-gassing and conversion of the compound into a form suitable for further processing. In practice, cylindrical pellets with diameters of 2·5-4 mm and lengths from 1 to 4 mm have proved optimal for feeding final processing machines. To produce the pellets, it is necessary to force the plastic PVC stock through a multi-hole die plate. One way to do this is to incorporate in the screw-type machine a suitable metering and pumping zone, and to mount the pelletising die plate directly on the end of such a machine. This approach represents a rigid combination of compounding and forming (i.e. pelletising). Research and development work started at the end of the 1960s demonstrated, however, that the optimal design and operating parameters for compounding and forming, respectively, are often diametrically opposed, so that it makes more sense to separate the two stages clearly from one another in high-production compounding lines. This approach resulted in the development of two-stage compounding lines, with one screw-type machine for compounding and a second one for pelletising. Usually the pelletising screw is set up in cascade fashion following the compounding screw. Pelletising machines are normally single-screw designs, but the compounding step may be handled by either single-screw or multi-screw machines. In some cascade compounding units, the homogeneously fluxed PVC stock drops in lump form through a connecting tube from the outlet end of the compounding stage into the feed opening of the pelletising screw. Vacuum can be applied to the connecting tube to de-gas the PVC stock. The pelletising screw proceeds to build up the pressure required for extruding the compounded stock through the pelletiser's die plate. With this separate arrangement, design and process parameters such as speed, shear and temperature profile can be selected in such a way that a minimum amount of energy is dissipated during the forming phase, which is probably the most critical phase of the entire compounding process. As already mentioned, when compounding rigid PVC compositions it is necessary to control effectively the system pressure in the

584

W. Henschel and P. Franz BUSS-KNEADER

REGULA TlNG SCREW

PELLETISER

Fig. 13.33 Back-pressure control by regulating screw.

compounding stage in order to maintain the desired degree of gelation in the PVC pellets. This control may be achieved in various ways. Buss-Kneader compounding lines of the KG type employ a regulating screw in place of the connecting tube between the compounding and pelletising stages (Fig. 13.33). The speed of this 2·5 LID single screw is separately and infinitely variable: its adjustment provides accurate control over back-pressure in the kneader, and hence over energy dissipation in the homogenisation zone and therefore over the stock temperature and degree of gelation. On the twin-screw ZSK Kombiplast machines both screws can be shifted axially with a gear motor and adjusting spindle. This moves the kneading elements at the discharge end (Fig. 13.34, shaded area) into the outlet orifice to a greater or lesser extent, so that the back-pressure can be varied infinitely without stopping production. Another approach to controlling pressure conditions is adopted in the twin-shaft MPC/v system. Two 'barrel valves' are employed (Fig. 13.35), one in the kneading disk area and the other at the discharge

Fig. 13.34 Back-pressure control by adjustable outlet orifice.

13 Industrial Compounding Technology of Rigid and Plasticised PVC

585

Centreline olugilulol shults - -

Fig. 13.35 MPCN barrel valve.

end. This kind of valve constitutes a movable saddle-shaped cut-out in the barrel. When the valve body is turned manually, the saddle is set at right angles to the direction of product flow, which closes the gusset space between the two shafts and causes the pressure to build up. In the open position the saddle piece in the valve body is lined up with the barrel saddle, thus offering the least resistance to stock flow. All three arrangements allow the composition to be de-gassed in transit to the pelletising screw. Two basic cutting arrangements are available for pelletising~entral cutting and side cutting (Fig. 13.36). Side cutting has gained wide

KNIFE SHAFT At-() DIE PL ATE CONCENTRIC

KNIFE SHAFT NEXT TO DIE PLATE

Fig. 13.36 Basic cutting arrangements in pelletising equipment.

586

W. Henschel and P. Franz

acceptance among compounders because of the favourable cutting conditions it provides. In both methods the plasticated PVC strands forced through the multi-hole die plate by the pelletising screw are cut into pellets by rotating knives on the outer surface of the die plate. Pellet length can be changed by altering the speed of knife rotation. The die head is enclosed in a collector casing that catches the pellets chopped off on the die plate, pre-cools them, and feeds them to the pneumatic conveying system. Because of their relatively high melt viscosity and low stickiness, most rigid and plasticised PVC formulations can be pelletised 'dry' by the hot die-face cutting method, i.e. room air is sufficient for cooling and conveying. In special cases, such as heavily plasticised formulations for the shoe industry or for medical purposes, record compounds with high vinyl acetate content, or bottle compounds with low K value polymer, it may be necessary to counteract pellet sticking by supplementing the pre-cooling in the collector casing and the conveying line with low levels of water mist. In this case, water and compressed air are mixed in a mixing valve (Fig. 13.37) and injected into the collector casing. The spontaneous evaporation of the water affords an additional cooling effect, and prevents any direct contact between the PVC pellets and free water. A flow meter is used to control the amount of water atomised. Particularly in the cable industry, impurities such as fibres, wood chips, paper cuttings, etc., which enter the compounding process in the raw materials, can seriously impair product quality and affect profitability. It is therefore becoming common practice in the cable

COMPRESSED AIR

~+-"'T'"-DC::.t---

WATER

PNEUMATIC PELLET CONVEYING

Fig. 13.37 Water-mist cooling of cut pellets.

13 Industrial Compounding Technology of Rigid and Plasticised PVC

587

DIE PLATE

PACK

Fig. 13.38 Die plate with strainer. industry to filter such impurities out of the composition during compounding. Because the contamination level is generally relatively low, it does not always pay to use an automatic screen changer. An economical solution is to insert behind a pelletising die plate a sandwich screen pack consisting, for instance, of two 20 mesh support screens flanking a 60 mesh strainer screen (Fig. 13.38). The die plate must be designed to accept this screen pack. Experience has shown that the screen pack has to be changed once per working shift. The line is shut down, the die head swung out, and the contaminated screen removed. Wire and cable insulation is normally coloured all the way through. The cable industry prefers to colour the cable with pigment concentrates or masterbatches, which are normally prepared from a complete pre-mix with all components. A compounding unit must therefore be capable, in addition to its normal compounding duties, of dispersing pigments in high concentrations uniformly without streaking. One aspect of masterbatch production is the preparation of semiconductor PVC compounds, in which carbon black loadings as high as 30% are dispersed in the PVC mixture. In this case, too, a complete pre-mix of all components is supplied to the compounding unit. Apart from cable applications, these compounds are also used for producing antistatically finished PVC film and sheet. Another special demand placed on compounding technology by the cable industry arises from the trend towards increased use of

588

W. Henschel and P. Franz

crosslinkable polyethylenes, either the traditional, peroxidically crosslinkable polyethylene or Sioplas (developed by Dow Corning, UK). A modern compounding line must therefore be sufficiently versatile to compound and pelletise these crosslinkable polyethylene compounds as well as a broad range of PVC formulations. The specific energy normally dissipated in the compounding step is about 0·06-0·08 kWh kg- 1 for rigid PVC formulations and 0·040·06 kWh kg- 1 for plasticised PVC formulations. In the forming step, i.e. in the pelletising screw, additional specific energy is expended for pressure build-up and extrusion of the PVC stock through the die plate. A rule-of-thumb from actual practice indicates that the specific energy requirement for extrusion through the die plate equals about 1/10 to 1/5 of the energy dissipated during compounding. The introduction of energy causes the stock temperature to rise as it moves through the kneader. Figure 13.39 shows typical stock temperature profiles for various PVC formulations. Compounding lines now available on the market cover a production band from 100 kg h -1 to 6 t h -1. Besides these production units, scaled-down laboratory units are offered for formulation development and colour matching as a basis for production. Such units must employ

T (·e) 200

4

180

3

160

2

140

1

120 100

eo 60 40

20

~

~

~

~

I.ENGlH

('tttot )

Fig. 13.39 PVC pelletisiing: temperature profiles. 1, Record compounds; 2, plasticised PVC; 3, rigid PVC; 4, rigid PVC, US grades.

13

Industrial Compounding Technology of Rigid and Plasticised PVC

589

the same mixing and homogenisation process as their production counterparts if the results are to be convertible. (b) Compounding of pvc for Feeding Calenders Calendering is a major process for the production of continuous film and sheet. In this process the compounding step is arranged in-line, i.e. the process runs from raw material through compounding to film or sheet forming on a continuous basis. The operation stages of a calender line as outlined in Fig. 13.40 must therefore be well matched to form an integral, interconnected system. These stages are common to regular calender lines, coating lines and light-duty calender lines, so the following remarks on the calendering process are intended to cover those processes as well. Because the calender is essentially a forming machine with no substantial mixing or homogenising effect, the compounding section must fulfil the following functions in the interconnected system: -fluxing of the polymers and homogenisation of the stock; -achievement of an optimal degree of gelation for the calendering process; -de-gassing of the stock and straining (where necessary); -conversion of the fluxed stock into a form suitable for feeding the calender. The provision of feedstock by the compounding section must, at any given time, match the calender's throughput rate.

Fig. 13.40 Block diagram of a calender line.

590

W. Henschel and P. Franz

These requirements apply to the entire range of PVC film and sheet formulations, which can be classified into three main groups according to their plasticiser content: -rigid (substantially without plasticiser); -semi-rigid (10-25% plasticiser-Shore A hardness 95-85); -plasticised (25-40% plasticiser-Shore A hardness 85-60). For plasticised PVC film and sheet, a standardised calendering practice has emerged in which a 'rolling bank' is set up in the calender nip. In the case of rigid PVC, processed at temperatures higher than those in plasticised PVC lines, one of two calendering methodsrespectively known as the high- and low-temperature processes-may be used, depending on the type of sheet or film being produced. In the high-temperature process, the calender bowls are heated to progressively higher temperatures with-typically-the intake bowls at about 160°C, and the last nip at about 210°C. The film or sheet is then passed through the usual cooling roll train. In the low-temperature process the reverse heating order is used. The temperature drops from, say, 175°C at the intake to 145°C at the last bowls, i.e. the fluxed stock is formed in a relatively cool state. Normally, the films produced by this method are fed directly to an orienting calender, where they are heated rapidly on melting rolls heated to about 240°C, and then cooled sharply down to 110°C. This provides optimal film orientation. Known under the name of the 'Luvitherm process', this low-temperature process has gained acceptance in Europe for the production of tearproof films. The calendering method also determines how the temperature will be controlled in the compounding step. For the high-temperature process, the PVC stock must be delivered to the calender fully plasticated; for the low-temperature process, it should be barely plasticated (crumbly), i.e. it has to be fed to the calender at a lower temperature. The calender design is determined essentially by the type of film and sheet to be produced on it. The design determines, in turn, the overall layout, position of the compounding unit relative to the calender, and the necessary conveying and feeding elements. Figure 13.41 shows the bowl arrangements in various four-roll calender types. At the present time, the 'L' and inverted 'L' (also called 'F') types are dominant.

13

(i)

(iv)

Industrial Compounding Technology of Rigid and Plasticised PVC

(iI)

(v)

591

(Iii)

(vi)

Fig. 13.41 Bowl arrangements in 4-bowl calenders. (i) Inverted 'L' type (sometimes referred to as 'F' type); (ii) 'z' type; (iii) inclined '5' (or 'Z') type, down-stack; (iv) inclined '5' (or 'Z') type, up-stack; (v) 'L' type; (vi) vertical stack. Arrangements (i), (ii) and (iii) are widely used in PVC sheet production.

From a development history rich in tradition, two basic methods for feeding calenders have emerged: -direct feeding; -feeding through a set of mixing rolls (two-roll mill) set up between compounding unit and calender. The mixing rolls are still justified where sport runs and a broad range of formulations are handled on a given calender line. In this case the mixing rolls serve as a material-surge unit, keeping the stock hot. This makes it possible to change formulations without any interruption of the calender operation in some cases. By passing through the roll nip repeatedly, the stock can also be de-gassed to a limited extent. However, it is necessary to keep the stock temperature in the

592

W. Henschel and P. Franz

compounding step lower because of the additional work done on the mixing rolls and the longer retention time of the PVC stock at elevated temperature. The endless ribbon of PVC emerging from the mixing rolls is slit and fed to the calender in endless strips. With direct calender feeding, which is undoubtedly the most economical solution in terms of capital investment, labour and space requirements, no mixing rolls are used. This type of feeding is appropriate mainly for the processing of large runs and relatively limited formulation ranges. The job of the compounding unit in this case is to gel the PVC stock to the degree required for the calender and to de-gas it effectively. Whether the calender is fed directly or via mixing rolls, the stock is delivered to the intake nip by conveyor belts. A separate wig-wag belt is sometimes used just before the calender to achieve better distribution of the stock acrosss the intake. These conveyor belts have to be selected carefully with regard to thermal and abrasion resistance; otherwise the stock can be contaminated and the quality of the PVC film or sheet impaired. Another point to watch is that bits of metal can sometimes enter the calender line with the raw materials. The consequences of even the smallest metal particles entering the rolls of the mixing bank or the calender can be much more serious for the machinery than for film or sheet quality. To avoid such damage, it is essential to set up a metal detector, capable of picking up non-magnetic as well as magnetic particles, between the compounding unit and the calender. Normally, the metal detector is located at the beginning of the conveyor belt. Whenever it responds to a bit of metal in the PVC stock, the belt is thrown into reverse for a preset period of time to eject the foreign object. It is a feature of the calendering process that the throughput varies widely. The variation may be deliberate or random (sporadic). The deliberate variation is a consequence of changes in (range of) formulations and qualities handled on the calender line. These determine calendering speeds and film or sheet gauges: since the full width of the calender is normally used, the throughput (and hence feed supply) required can be calculated roughly from calendering speed and film or sheet gauge. The random, momentary throughput variations stem from the nature of the calendering operation. To ensure trouble-free running, a certain amount of stock should always be maintained on the mixing rolls or at the calender intake. Because degradation is a function of the intensity and duration of heating (retention time) this buffer should be kept to a

13 Industrial Compounding Technology of Rigid and Plasticised PVC

593

minimum. If it starts increasing in size, the throughput of the compounding line must be cut back, and vice versa. Despite various attempts, no satisfactory method has been found as yet to measure the buffer stock volume on the mixing rolls or at the calender intake automatically, and to employ the resulting signal to control the throughput of the compounding unit. Such monitoring and throughput regulation must, therefore, still remain among the operator's duties. The compounding unit must be capable of delivering stock of unvarying degree of gelation over the entire output range. The overall variation can cover a ratio of 1: 10, i.e. the minimum throughput equals one tenth of the maximum throughput. As already described for PVC compounding generally, in compounding for the calender a free-flowing blend is first produced from the individual raw material components, and then discharged into a holding silo (Fig. 13.42). The blend leaves the holding silo-normally

Fig. 13.42 A typical compounding train for PVC calendering: flow sheet. 1, Truck unloading station for solid components; 2, debagging station for solid components; 3, pneumatic conveying system; 4, silos for solid components (resin, filler); 5, day bins for solid components; 6, day bins for plasticisers; 7, batch weighing station for solid components; 8, batch weighing station for liquid components; 9, minor ingredients proportioning station; 10, pre-mixer; 11, buffer silo; 12, compounder; 13, conveying belt with metal separator; 14, two-roll mill; 15, conveying belt; 16, calender; 17, edge trim cutter; 18, silo for cut edge trimmings.

594

W. Henschel and P. Franz

under gravity-to enter a feed hopper serving as buffer bin and volumetric metering element (see Section 13.4.2(a)). The metered blend flows into the screw-type kneader, where it is densified, plasticated and homogenised. To simplify operation and make it possible to maintain optimal compounding conditions when the throughput changes, systems have been developed for synchronising the feeds of the feed hopper shaft and the kneading screw. The speed ratio characteristic of the particular formulation is set on a potentiometer. When it is necessary to alter the throughput the operator needs only to raise or reduce the kneading screw speed by pressing a single pushbutton. The synchronisation system adjusts the metering rate automatically, so that the specific plastication rate remains constant regardless of the momentary throughput. For effective de-gassing, where necessary, the screw-type kneader is equipped with a de-gassing zone (Fig. 13.43), to which vacuum (70-200 mbar) is applied. If allowed to remain, the air, moisture and other volatiles introduced with the pre-mix can cause blistering of the calendered film or sheet. Their withdrawal from the stock is promoted and enhanced by the continuous creation and exposure of new surface in the course of compounding. Figure 13.44 demonstrates the effect of de-gassing on PVC stock. A die on the end of the kneader forms the stock into one or more strands. These can be fed either to mixing rolls or the calender. For

-~ ~

BUSS-KNEADER

Fig. 13.43 De-gassing arrangements in screw-type compounders.

13

Industrial Compounding Technology of Rigid and Plasticised PVC

595

Fig. 13.44 Effect of de-gassing of PVC film/sheet stock (crystal-clear, rigid composition). Samples taken at the outlet of a screw-type kneader. Left, material processed without de-gassing (high bubble content); right, de-gassed, bubble-free material.

simpler and more uniform nip feeding, the strands can be sliced into chips with a cutting device working in conjunction with the die (Fig. 13.45). Straining of the stock may be desirable, particularly for the production of plasticised PVC sheet and film, to remove foreign objects such as fibres, paper cuttings, wood chips, etc., which may have entered the mixture from the raw material packages.

Fig. 13.45 Kneader die with cutting device.

596

w.

Henschel and P. Franz

This is done by setting up a 'strainer extruder' following the compounding unit. Normally a single-screw machine, it is equipped with screen pack and die. Three basic alternatives exist for straining, depending on the average degree of raw material contamination expected: -Use of automatic screen changer for high contamination levels requiring an hourly screen change. -Use of a tandem strainer head, i.e. a twin strainer head arrangement permitting the contaminated strainer to be swung out and the clean head swung in in a matter of minutes. This solution is practical in cases where medium to low contamination of the raw materials calls for just one screen change per shift. -Insertion of a screen pack in the extruder head, a possibility that is justified only in cases of minimal contamination, i.e. where a screen change is required only at the end of a production run. During the production of PVC film and sheet, the product is trimmed twice: a hot trim on the calender, and a cold trim to the desired width. The resulting trimmings are recycled. Mixing rolls and internal mixers were widely used for compounding in the early years of calendering. They are still sometimes included in a compounding section which may be used intermittently, for particular runs or formulations, as an alternative to the main compounding set-up (see Chapter 18). The edge trim can be fed back to these machines just as it is. However, attempts to feed it directly into continuously operating screw-type machines have usually failed because of various problems, including difficulties in metering. The usual current practice is to grind the trim in grinding mills together with unsaleable starting film and sheet to obtain 'regrind' particles with edge lengths of 5-10 mm. As a rule, this regrind is added to the virgin blend in the pre-mixing operation at rates of about 10-20%. In isolated cases-particularly where semi-rigid or plasticised film or sheet is being produced-the regrind can be charged directly into the kind of feed hopper shown in Fig. 13.32. The mixing effect of the agitator arms designed to prevent bridging in the hopper is sufficient to intermix the regrind with the powder blend adequately. The specific energy required for compounding is in the 0·060·10 kWh kg- I range. Typical stock temperature profiles resulting from this energy input are shown in Fig. 13.46. The compounding units available on the market cover a throughput range from 100 kg h -1 to 5th- I .

13

Industrial Compounding Technology of Rigid and Plasticised PVC

T (OC)

597

2

ZXl 180

1iO

v.o lZ)

m

eo 60

IIJ Z)

}1.

Fig. 13.46 Temperature profiles, PVC sheet and film stock. 1, Plasticised PVC; 2, rigid PVc.

In the early 1970s some leading companies attempted to set up the entire calender line, from pre-mixing to finished film and sheet, in the form of an integral control loop employing a central process computer, with the idea of saving labour and reducing out-of-tolerance film and sheet (with consequent raw material and other savings). These projects demonstrated, however, that the entire system is so complex and the mutual interaction of the parameters involved so varied, that it makes more sense to divide the calender line into three subloops, pre-mixing, compounding, and calendering. Automation and control can be applied in goal-oriented land comprehensible fashion in these subloops. Automation in the pre-mixing operation is aimed at producing the desired blend with the desired temperature, bulk density and flow properties for delivery to the compounding line. The compounding control system is responsible for maintaining the throughput called for at any given moment in conjunction with the prescribed outlet temperature and fluidity of the stock. The control loops on the calender are essentially responsible for· guaranteeing film and sheet thickness and thickness tolerances in the lengthwise and transverse directions.

Fig. 13.47

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PVC cascade extruder with sheeting line. 1, Extruder; 2, flat sheet die; 3, triple-roll smoothing system; 4, roller conveyor; 5, edge trimmer; 6, twin-roll haul-off unit; 7, length cutter; 8, sheet deposit.

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13 Industrial Compounding Technology of Rigid and Plasticised PVC

599

(c) Extrusion of Film, Sheet and Board Considerable progress has recently been made in work on production of rigid and plasticised PVC film, sheet and board by extrusion. Film as thin as 0·1 mm and board as thick as 10 mm have been produced on a commercial scale. Maximum finished width at present is 1·2 m. As in the case of calendering, these systems employ compounding equipment in line with the extrusion set-up. In the example shown in Fig. 13.47, a cascade extruder (here equipped with a slit die for film formation) is used as the combined melt compounding and extrusion unit (see also Plate Cl). The operation of this machine is discussed in Section 13.4.4(a)). Production systems of the type illustrated by Fig. 13.47 and Plate C2 are designed for outputs up to 500 kg h- 1 for film and 600 kg h- 1 for sheet and board.

(d) Recycling At the time of writing, no general strategy for recovery of plastics scrap has yet gained wide acceptance. Undoubtedly this is attributable

-.. -•• ••

Plate C.l Cascade extruder (Barmag).

600

W. Henschel and P. Franz

Plate C.2 Sheeting line employing a cascade extruder.

to questions such as the possible applications and quality profile of recycled plastics scrap, price and profitability, technological potential and limits, the consumer mentality, and not least the problem of communication among various industries. Nevertheless, the current ecological situation is making it necessary to assess technological progress not only according to increased sales and profitability, but also according to recycling possibilities. We shall probably wait a long time for a satisfactory solution to the problem of reclaiming valuable substances from municipal refuse. From the standpoint of PVC compounding technology, however, several interesting examples can be cited of recovery of scrap produced during a manufacturing process. The simplest and easiest problem to solve is the handling of in-line scrap, so it is not surprising that recovery of the edge trim and starting scrap produced in the manufacture of PVC film and sheet has been practised (as described in Section 13.4.2(b» virtually since the beginning of calendering technology. Recycling practice is similar in PVC pelletising operations when products with the wrong composition or colour shade are produced. Recently, two possibilities have been investigated for the recycling of die-cutting scrap in the processing of PVC sheet, viz. production of

13 Industrial Compounding Technology of Rigid and Plasticised PVC

601

secondary sheet and production of extruded profiles or pipes. First the scrap is ground up in a grinding mill. The stabiliser used in the original process is added again in a pre-mixer, and pigments are often added to achieve a uniform, desired colour. This pre-mix is fed into the compounding unit. If the regrind includes rigid, semi-rigid and plasticised PVC, the rigid and semi-rigid fractions can be charged into the compounding unit through a first inlet opening and plasticated in an initial kneading zone. The plasticised portion is then fed into this fluxed stock, which ensures the gentlest and most homogeneous processing (Fig. 13.48). Because the plasticiser cannot diffuse in the PVC regrind within a reasonable time, as it can in the case of virgin PVC, any additional plasticiser required is injected directly into the kneading zone of the compounding unit by a pump. It is advisable to use a strainer in order to remove any contamination from the stock. Afterwards, the calendering process is carried out as usual. For the production of profiles and pipes, the homogeneous stock is pelletised (as described in Section 13.4.2(a» following compounding. Then the pellets are fed to an extrusion line. In the cable sector, compounders were confronted with the problem of recycling copperless insulation and sheathing scrap. The approach taken in this case was to use this scrap for producing filling core mixtures. The purpose of the filling cores is to fill out the cavities between a cable's conductors (Fig. 13.49). Since their composition is not subject to any special electrical or mechanical specifications, it is normally made as inexpensive as possible, usually receiving up to 700 RIGID PVC SCRAP

PLASTICISED PVC SCRAP

PLASTlC~ ~

~~Wl STRAINER

Fig. 13.48 Recycling of film scrap.

602

w.

Henschel and P. Franz

INNER SHEATHING REINFORCEMENT Fig. 13.49 Cable design.

parts of chalk filler. At this level of loading the PVC acts mainly only as binder for the filler. The reground PVC scrap, with a particle size of 5-10 mm, is fed into the first inlet of a compounding unit designed specifically for this application (Fig. 13.50). The regrind is plasticated homogeneously in the first kneader zone, enabling it to absorb the high filler loading metered in at the second inlet opening without any problems. To plasticise the filler cores further (for increased flexibility) plasticiser is injected into the kneading chamber by a pump. The homogeneous stock is pelletised as described in Section 13.4.2(a). PVC SCRAP

FILLER

PlASTIC~ ~ Fig. 13.50 Compounding of cable filler cores.

13 Industrial Compounding Technology of Rigid and Plasticised PVC

603

Another feature of the equipment shown is that virgin blends can be compounded on the same line. Cork flour-obtained as a waste product in the manufacture of bottle corks~an be admixed to either a PVC virgin blend or a PVC regrind blend and compounded as described in Section 13.4.2(b) for producing flooring materials on a calender. The presence of the cork is believed to increase the resilience of such flooring.

13.4.3 Preparation of PVC Pastes The preparation of PVC pastes consists in dispersing suitable PVC polymer grades in plasticisers, or sometimes plasticisers and solvents, together with fillers, colourants and stabilisers. Paste viscosities of the order of 103-1·5x104 cP are required for ultimate processing. The nature, properties and applications of PVC pastes are discussed in detail in Chapters 21 and 22. Because the pastes are viscous liquids suitable for further processing and giving rise to homogeneous products when fused (gelled) by heating, melt compounding is not relevant or appropriate. Indeed care must be taken during paste preparation to keep any heat development or temperature rise to a minimum in order to prevent premature gelation. Thus screw-type machines of the type used for melt compounding are not applicable. For quite a long time, PVC pastes were prepared exclusively in slow-speed mixers (see Table 13.8), as it was felt that the compositions would gel if handled in high-speed mixers. In the mid-1950s, however, TABLE 13.8 Mixing Equipment for PVC Pastes Continuous

Batch Vertical Low speed

Horizontal

Horseshoe mixer Ribbon blender Planetary mixer Paddle mixer Perl mill High speed

Dissolver

Roll mills

Vertical

3-Roll mill

Dissolver Buss mixing turbine MT

604

W. Henschel and P. Franz

rationalisation efforts led to the first trials with such mixers: the use of dissolvers with infinitely variable speed control eliminated such recognised shortcomings as limitations on mixing tool speed and inadequate coverage of the entire mixture volume, and marked a real improvement in the efficiency of producing pastes on a batch basis. Today, changing economic conditions have again evoked demands for improved efficiency and automation, for better working conditions and anti-pollution measures, for improved space utilisation and higher per capita outputs. To meet these demands, continuously operating units such as continuous dissolvers and the Buss Mixing Turbine were developed. Regardless of whether the continuous or batch approach is used, industrial preparation of stock pastes can be broken down into the following stages (Fig. 13.51): -silo storage of components; -metering of components; -pasting-up and dispersion; -filtering; -de-gassing (de-aeration); -ageing. The preparation of pigment pastes, and stock paste colouring, run in parallel with the preparation of stock pastes. (a) Silo Storage This is practised as described in Section 13.3. (b) Metering Because of the special nature and mixing requirements of PVC pastes it is necessary to charge the components into the mixer in fractions. In the simplest case, the components are batch-weighed and the portions charged into the mixer manually. In automated processes this manual operation has been replaced by gravimetric metering devices for the bulk materials and metering pumps for the plasticisers. For the bulk materials, either belt weighers or differential weighers (loss-in-weight feeders) can be used. While the metering devices used for batch operation operate intermittently, i.e. are switched off after the desired amount of a component has been fed, metering devices for continuous operation supply an uninterrupted flow of material that remains constant per unit time.

TANK)

pASTE BUFFER

AGEING

DE-GASSING

FIL TERING

DISPERSING)

(WETTING,

PREPARATIONI

METERING

Fig. 13.51

_g _PL_A_STlCISER

I

I I I

--©--,-

FILLER PLASTICISER

~

STABILISER

9-~9~-c-Q -.~T~r~ PVC

CONTINUOUS

Flow diagram for industrial preparation of PVC paste.

I-M_:_~_Y_~_:_IG_AE_L_~_¥_~_~__s_TA9_~E_R

BATCH

~

§

~

~

~

0;'

~.

:::!:!

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~

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0~ ~ ::>;, oQ.

5

~ ;;;' s.

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~

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::;

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For batch operation, the gravimetric metering devices can be program-controlled to deliver the desired metering sequence without any difficulty. Metering accuracy is approximately ±0·5%, which is better than the accuracy normally obtained in manual weighing. (c) Pasting-up and Dispersion Although a distinction is made between these two operations one follows the other in continuous sequence. In batch operation, the liquid components are placed in the dissolver first, and the portionwise addition of the solid components is then commenced, with the mixing tool rotating. This practice largely avoids the formation of lumps, and promotes wetting-out of the solid particles. When the last solids fraction has been charged and mixed into the plasticiser, the pasting-up step is over. Dispersion is initiated by speeding up the mixing tool to step up the stock's turbulence and hence the shearing effect. The upper limit on speed is determined by the drive motor rating and depends on the viscosity of paste. In the case of the continuous-preparation system shown in Fig. 13.51, the bulk components are not charged in portions, but are comminuted in the inlet head of the Buss Mixing Turbine by suitable rotor elements. The plasticiser is sprayed in, so that the solid and liquid phases intermix in finely dispersed form. Thorough wetting-out of the surfaces of the solid particles is the result. When low-viscosity plastisols are being produced by the batch process, only part of the total amount of plasticiser is placed in the mixer at the start: otherwise the shearing forces for the dispersion operation would not be sufficiently high. The remainder is added, to effect final dilution, only after the dispersion operation has been completed. In the continuous-preparation equipment of Fig. 13.51, the same result is obtained by spraying-in the plasticiser at various points along the mixing turbine axis. In the batch process, the pasting-up operation in the dissolver takes 8-10 min depending on batch size, and the dispersion operation about 5-8 min (i.e. 13-18 min total mixing time). The peripheral speeds of the mixing tool during dispersion are between 20 and 30 m s-1. In the continuous process employing the Buss Mixing Turbine, the average residence times for the entire mixing operation lie between 15 and 30 s at rotor peripheral speeds between 10 and 25 m S-I, depending on the throughput and formulation. The very large time savings, at generally comparable peripheral speeds, offered by the continuous process arise

13 Industrial Compounding Technology of Rigid and Plasticised PVC

607

because in the Buss Mixing Turbine the mix components come into contact at individual particle (or plasticiser droplet) level. This accelerates both the pasting-up and dispersion operations (particularly the latter). Batch mixing in the dissolver and continuous mixing in a turbine both produce pastes so smooth that they are ready for use and normally require no additional grinding step on mixing rolls (as practised in some traditional, or small-scale, paste preparation methods). (d) Filtering Paste quality can be impaired by the kind of contaminants mentioned in the discussion of melt compounding, or by the presence of isolated undispersed lumps. In cases where the paste is passed through mixing rolls for grinding, partly gelled particles (small lumps) of paste can cling to the ends of the rolls and get into the product at the end of the grinding operation. All such impurities must be filtered out. Although a number of filter devices are available, the vacuum filter described below is the one most frequently used. It can either be moved around the workshop on a portable frame and fitted onto the vacuum-tight mixing and transport tanks as required, or set up for stationary use between mixer and de-gassing tank. The PVC paste fed into the hopper of the device (Fig. 13.52) passes through a screen pack mounted in a quick-change frame: the screen packs are much the same as those employed in automatic screen changers for thermoplastics

Fig. 13.52 Vacuum filter for PVC paste.

608

W. Henschel and P. Franz

processing. One pack is in operation while the second waits its turn. The paste is drawn through the screen by vacuum in the tank below. When the screens become clogged, the two packs are reversed and the contaminated one replaced by a new pack. Clogging of a screen is indicated by an increase in tank vacuum. The screen pack mesh must be fine enough to retain particles larger than 200 tlm. The filtered paste passes directly on to the de-gassing step. (e) De-gassing Air enters the mixing unit with the bulk materials. In addition, the PVC particles contain air and moisture as a result of their structure. Steadily rising quality requirements have been forcing producers of imitation leather, floorings, toys, etc., to de-gas their PVC pastes effectively, and this has how become standard practice. Low-viscosity pastes are de-gassed by the application of a 5-50 mbar vacuum for about 10 min (about 20 min for high-viscosity pastes). Effective de-gassing requires that the surface of the stock be continuously renewed, thus shortening the diffusion paths and the evacuation time. That is why vacuum dissolvers or vacuum planetary mixers are employed for the de-gassing of PVC pastes. For continuous operation, appropriate continuous degassing units are available on the market. (f) Ageing The procedure just described is typical of the preparation of medium to large batches of stock (uncoloured) PVC pastes. Directly after preparation the stock paste is pumped into a storage tank where it is kept-under slow stirring-to serve as buffer stock between paste production and subsequent processing. The storage may also serve as an ageing period (of up to about 24 h) during which the paste viscosity attains a desired value (see also Chapter 21). (g) Colouring PVC pastes are coloured by the addition of colour pastes, produced by dispersing colourants in plasticisers (batchwise in planetary mixers or dissolvers, or by the continuous method in mixing turbines), and homogenising by milling (in ball or roll mills). The method of preparation is thus similar to that used in the paint and varnish industry. Colour pastes are added to PVC stock pastes in high-speed mixers or dissolvers with average mixing tool peripheral speeds of 10-12 m S-l,

13 Industrial Compounding Technology of Rigid and Plasticised PVC

609

peaking as high as 22-25ms- 1 . The stock paste is first stirred briefly, and then the batch of colourant, appropriate to the paste formulation, is added slowly into the vortex forming around the mixing tool. In cases where smoothness is of cardinal importance (e.g. coating pastes for the production of artificial leather) the paste may be milled after colouring. If this is done on 3-roll mills, it is advisable to pass the milled paste through a vacuum filter to preserve the high quality of the ultimate coating by removing any gelled particles that may have formed on the rolls. 13.4.4

~achinery

(a) Screw-type Machines The compounding of PVC is a particularly difficult operation because of the need to minimise the material's 'heat history', which is a function of the time and temperature of treatment. PVC compositions must not be compounded at high stock temperatures for long periods. Consequently, the energy required for plasticisation and homogenisation must be supplied to the system by the gentlest possible shearing within the shortest practicable stock residence time. Another complicating factor is the tendency of PVC compositions to slip on processing surfaces. For all of these reasons, PVC compounding is normally carried out by screw-type machines that operate with relatively little wall slippage and have a sufficiently effective mixing action to plasticate and homogenise the stock at the lowest possible level of energy conversion. Some machines typical of equipment embodying these features are listed in Table 13.9 and briefly discussed below. The list is given by way of illustration only, and is not exhaustive. THE PLASTIFIKATOR

Applications: Compounding and pelletising of plasticised PVC compositions (cable, shoe, profile, hose, flooring compounds). Equipment and operational sequence (Fig. 13.53): The pre-mix passes through a holding tank to reach the stirred feed hopper, 1. The variable-speed twin feed screws, 2, (intermeshing and co-rotating) determine the rate of material feed to the shear cone in the plastication section, 3. In smaller Plastifikator models, the homogeneous stock is picked up by a discharge screw, 4, arranged coaxially with the plastication cone and driven by the same shaft; this

Werner u. Pfleiderer, Stuttgart West Germany Buss Ltd, Basle, Switzerland Barmag, Remscheid-Lennep, West Germany

Manufacturer

Werner u. Pfleiderer, Stuttgart, West Germany FCM continuous mixer {Farrel Corp., Ansonia, CT, USA David Bridge Co. Ltd, Rochdale, UK Pomini-Farrel, Castellanza, Italy Baker Perkins Chern. Machinery MPCN Ltd, Hanley, Stoke-an-Trent, UK Bitruder BT Reifenhauser KG, Troisdorfsiglar, West Germany Planetary EKK Kleinewefers Kunstoff PWE and ZSE Maschinen GmbH, Bochum, West Germany Hermann Berstorff Maschinenbau WE GmbH, Hannover, West Germany

Twin screw Kombiplast

Buss-Kneader Cascade extruder

Single screw Plastifikator

Machine type and designation

X X X X X

X X X X

X X

Rigid PVC

Process

X

X

X

Extrusion

X

feeding

+ pelletising Calender-

X

X X

X

Plasticised PVC

Compounding

TABLE 13.9 Typical Screw-type Compounding Machines

.....

'"

l::l ;3

~

:-0

l::l..

l::l ;3

~

~ ~ g.

~

0

0\

13

Industrial Compounding Technology

at Rigid and Plasticised PVC

611

Fig. 13.53 Plastifikator PK400. (See text for details.) screw forces the stock through the pelletiser die plate. In larger models, the discharge screw constitutes a separate unit with its own drive, gear box, bearings and supports. Cutting knives, 5, offset from the pelletiser die plate, chop the emerging strands into pellets by the dry cutting process. The pellets are gathered in a collector casing, 6, and conveyed pneumatically to the pellet cooler. Working principle (Fig. 13.54): The plastication and homogenisation comprises a shear cone, 1, rotating inside the conical housing, 2. The tapered part of the shear cone is fitted with spiral fins, 3. A closed ring of powder formed, in the first part of the shearing gap between the shear cone and housing, becomes sintered under the influence of shear forces, and gels in the next section. The fins and increasing shear cone diameter serve to divide the product into individual strands; these are squeezed in the constricting gaps between fins and housing wall to form thin layers, only to be re-formed again. Axial displacement of the shear cone in relation to the housing varies the shear gap and therefore the amount of shear energy dissipated. s~tion

612

W. Henschel and P. Franz

3

2 1

flow

Fig. 13.54 Working principle of the Plastifikator. (See text for details.) Temperature control system: The cone housing is equipped with an electric heater. The barrel of the discharge screw is also electrically heated, but is fitted with an air cooling system in addition. If the machines are separate, the discharge screw can be cooled with a separate heat transfer medium. De-gassing: Where the plastication machine and discharge screw are separate, a vacuum of 400 to 550 mbar can be applied to the transition section between the two to de-gas the stock. Operation and cleaning: During starting, a diverter can be placed between the plastification section and discharge screw to remove any PVC powder emerging; this prevents clogging of the die plate or screen pack (if any). For cleaning, the plastification section and discharge screw are run empty and purged of residual powder by passing warm pellets through. The pelletiser head can be swung out to permit easy removal of any product remaining on the screw tip or back of the die plate. For extreme colour or formulation changes that call for a thorough cleaning, the discharge screw with pelletising head of the smaller models can be rolled away on an assembly dolly. Then the shear cone can be pulled out in front of the barrel on a bar sliding in the main shaft. On the larger models, the cone is rolled away together with the transfer tube on guides, making the parts in contact with the product freely accessible. Energy input:

-cold pre-mix: 0·1-0·12 kWh kg-I; -hot/cold blend (50°C): 0·08-0·1 kWh kg-I; -hot blend (85-90°C): 0·06-0·08 kWh kg-I. Output rates and technical data: See Table 13.10.

Production rate: Plasticised PVC (kg h -i) Drive rating: Feed screw (kW) Main drive (kW) Pelletising (kW) Fan (kW) Separate pelletising screw (kW) Speed range: Feed screw (r min-i) Shearing (r min-i) Pelletising (r min-i) Fan (r min-i) Separate pelletising screw (r min-i) Heating: Cone barrel (kW) Extruder barrel (kW) Pelletising (kW) Cooling: Fan (m 3 h- i) Water cooling, bearing (litre h- 1) Water cooling, cone barrel (optional) (litre h- 1) Water cooling, gear-box (optional) (litre h- 1) Weight: Plastifikator (kg) Control cabinet (kg)

4 2·5 1·25 800

-

4 2·5 1·25 800

-

3200 270

-

3500 270

-

60

20-120 52 240-900 2800

20-120 42 204-900 2800

60

4 30 0·75 0·37

4 22 0·75 0·37

-

250-350

-

Model

PKlOOIV11

150-250

PK lOOIVI

TABLE 13.10 Technical Data for the Plastifikator

10 4 2

4500 500

-

800 100 -

-

30-150 88 230-1150 2800

-

4 55 1·5 0·37

450-900

PK 400/111

13100 700

2 x 800 200 500 500

20 16 4·8

29-117 75 312-1560 2800 35

5·5 160 3 2 x 0·37 37

1200-2400

PK lOoom

..

Vol

a......

(")

-..

...........

614

W. Henschel and P. Franz

THE BUSS·KNEADER

Applications: -compounding and pelletising of (i) plasticised PVC blends (cable, shoe, profile, hose, flooring compounds), and (ii) rigid PVC formulations (bottle, profile, record compounds); -compounding of rigid, semi-rigid and plasticised PVC blends for calender feeding (packaging, deep-drawing, decoration, upholstery and engineering film and sheet).

Equipment and operational sequence (Plates D.l and D.2): The pre-mix moves through a holding silo to the feed hopper, which is equipped with an agitator. The bottom section of the agitator is designed in the form of a vertical screw, so that speed variation of the agitator shaft feeds product continuously at the desired rate into the kneading section. If pellets are being produced (Plate D.l), the stock homogenised in the kneader is transferred cascade-fashion to the pelletising screw, which forces the product through the pelletising die plate. Cutting knives supported next to the die plate chop the emerging strands into pellets by the dry cutting process. The pellets are caught in a collector casing and conveyed pneumatically to the pellet cooler. In the case of calender feeding (Plate D.2), the homogeneous stock is sliced into chips in a die mounted on the kneader; the chips are then fed to the calender. Working principle (Figs 13.55 and 13.56): The Buss-Kneader is a continuously operating single-screw machine of special screw design. Conventional single-screw machines have an uninterrupted Archimedes' screw, which merely rotates around its longitudinal axis (Fig. 13.55(a)). In the Buss-Kneader, each turn of the screw helix is interrupted by three gaps to form the screw kneading tools, or 'screw flights'. Their counterparts in the barrel, the kneading pins or teeth, are arranged in three rows (Fig. 13.55(b)). An axial oscillation is superimposed on the rotation of the screw. A special gear box generates this characteristic Buss-Kneader motion. The mechanism ensures that each turn of the screw is accompanied by one back-and-forth movement. The operating principle can be explained by reference to a projection of the screw profile onto a flat plane (Figure 13.56(A)). The

13

Industrial Compounding Technology of Rigid and Plasticised PVC

615

616

W. Henschel and P. Franz

Plate D.2 Buss-Kneader PRKJE.

screw flight shaded in the figure is considered for this purpose, together with the four marked kneading teeth which work in conjunction with it. The sequence of drawings in Fig. 13.56(A)-(F) shows the tracks of the kneading teeth relative to the screw flights. Each consecutive drawing represents a quarter-turn of the screw shaft, i.e. the rotation positions of 0, 90, 180, 270 and 360 angular degrees. In the drawings, the screw rotation corresponds to movement of the screw flights from bottom to top; the product is conveyed from right to left. The starting position is shown in the first drawing. In the course of a turn through 90 (Part (B», the dark kneading tooth, which started at the top flight tip, has wiped past the long left-hand flank of the screw flight. As it did so, the product was sheared in the gap between the kneading tooth and shaded screw flight, and the flank of this flight was cleaned at the same time. The other marked teeth have not yet 'moved into action with the shaded screw flight, but they have been working with other flights. 0

13 Industrial Compounding Technology of Rigid and Plasticised PVC

617

Fig. 13.55 The screw and barrel of: a, conventional single-screw machine; b, the Buss-Kneader.

As the screw turns through another 90° (Part (e», so that it is describing a half turn in relation to the starting position, the right-hand kneading tooth of the two shown at the top of the projection in the starting position wipes past the short right-hand flank of the shaded flight, thus shearing the product and cleaning this flank. During this rotary motion, the middle and the two upper kneading teeth pass through their respective gaps between the screw flights and push the product back into the next turn of the helix. Note: Thus, superimposed on the conveying effect of the screw pitch and right-to-Ieft movement, there is a backward movement of material. This combined motion has been referred to in the literature as 'pilgrim's step' (two steps forward and one back).

618

W. Henschel and P. l,ranz D

c::::::::::::

u

619

13 Industrial Compounding Technology of Rigid and Plasticised PVC

c:::::::

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c

620

W. Henschel and P. Franz

Naturally, the forward conveying effect produced by the pitch and stroke of the screw is greater than the backward movement of material actuated by the passage of the kneading teeth through the gaps. The net material movement is thus in the direction of the outlet of the machine. The backward movement of material adds the crucial benefit of axial mixing to the radial mixing effect. This is why the homogenisation process in the Buss-Kneader takes less time than in conventional screw-type machines. In completing the 180° turn the screw has sheared the material and performed a cleaning operation on both the left-hand (long) flank of the screw-flight and the right-hand (short) flank. In the course of the next 90° turn (Part (D» the shearing of the material proceeds as the left-hand (short) flank of the left-hand tooth of the pair shown at the top in the starting position is being cleaned. Axial product exchange takes place again as the kneading tooth passes through the gap. The result of the last 90° turn (Part (E», which completes one full revolution of the screw, is tha,t the kneading tooth shown at the lower tip of the shaded flight in the starting position (Part (A» wipes past the right-hand (long) flank of the flight, again shearing the material and cleaning the flank. The interaction of the four kneading teeth with the shaded screw flight during one full turn of the screw has thus sheared the material on all four flanks of the flight, mixed it both axially and radially, and mutually cleaned the screw flight and teeth. As can be seen from the diagrams the same events-though with the roles of the collaborating flanks and kneading teeth exchanged-take place on all of the other screw flights. This is demonstrated by the enlarged detail of the relevant projection: Part (F) shows that the coverage of the kneading tooth tracks in relation to the screw surface is complete, with no surfaces or spaces left unwiped. This is the basis of the thorough mixing, kneading and self-cleaning action of the Buss-Kneader. On the KG models, the regulating screw can be used to set the back-pressure at the kneader outlet steplessly by remote control. This provides finger-tip control of the energy input. On PRK models for feeding calenders, the same back-pressure control is achieved with the aid of a die with infinitely variable cross-section.

Temperature control system: The barrels of both the kneader and the

13 Industrial Compounding Technology of Rigid and Plasticised PVC

621

pelletising screws are fitted with jackets divided into several zones. The screws are hollow right up to the tip. This design permits accurate control of screw and barrel temperatures by means of liquid heat-transfer media, usually HT oil. Pelletising die plates are heated either electrically or with liquid. De-gassing: On the KG and WKG models, which are used for compounding and pelletising, de-gassing of the stock takes place at the transition point between Buss-Kneader and pelletising screw. On PRKlE models for feeding calenders, the Buss-Kneader is equipped with its own de-gassing zone at the outlet end. The de-gassing systems provide for vacuum as high as 50-60 mbar. Operation and cleaning: When the KG and WKG pelletisers are started up, the initial material can be eliminated with a diverter between the Buss-Kneader and pelletising screw. In calender installations, the starting material can be eliminated at the kneader outlet. Buss-Kneaders and pelletising screws are cleaned by running them empty and purging residual powder with cleaning pellets. The pelletising head can be swung out to allow easy removal of material remaining between screw tip and die plate. For extreme product changes requiring a thorough cleaning, the Buss-Kneader barrel flaps open (Plate E), so that both screw and barrel are freely accessible. The barrel of the pelletising screw can be drawn off the screw on an integral roll-away chassis. Because all parts are fastened to the machine base and therefore remain properly positioned, this cleaning procedure requires no special tools, hoists or specially trained personnel. Energy input:

-plasticised PVC: 0.04-0·06 kWh kg-I; -rigid PVC: 0·05--0·08 kWh kg-I. Output rates and technical data: See Tables 13.11 and 13.12, and Fig. 13.57. THE CASCADE EXTRUDER

Applications:

-rigid and plasticised PVC film, sheet and board extrusion; -pelletising.

622

W. Henschel and P. Franz

Plate E

The split barrel of a Buss-Kneader.

13 Industrial Compounding Technology of Rigid and Plasticised PVC

623

Equipment and operational sequence (Fig. 13.58 and Plate Cl): The cascade extruder is made up of two single-screw extruders, the fluxing extruder and the discharge extruder. PVC pre-mix is fed into the former through a simple inlet hopper without any agitator or metering device. The fluxing screw mixes the blend and partly agglomerates the stock into particles of approximately uniform size. The production rate depends on the momentary bulk density of the pre-mix and the screw speed. When the agglomerated stock emerges from the fluxing extruder, it drops through a de-gassing tube (to which vacuum as high as 80 mbar can be applied)' into the intake section of the discharge extruder. Retention time in the de-gassing tube can be regulated by altering the agglomerate level in the tube. The discharge extruder's job is to complete plastication of the partially fluxed material, homogenise it, and force it through the outlet die. Working principle (Fig. 13.59): The energy required for agglomeration and plastication of the PVC stock is supplied by heating the barrel and by shearing the stock between screw core and barrel wall. The screws are provided with a 'heat pipe' to prevent overheating and scorching. To improve its intake characteristics, the fluxing extruder is fitted with a cooled, grooved bushing in the inlet area. The intake section of the discharge extruder is designed in the form of a tapered slot. Temperature control: The barrels are heated electrically. Screw temperature is controlled by the heat pipe system. De-gassing: In the transition tube between fluxing and discharge extruders. Output and technical data: See Table 13.13 and Fig. 13.60. THE KOMBIPLAST

Applications: Compounding and pelletising of plasticised and rigid PVC. Equipment and operational sequence (Fig. 13.61): The pre-mixed material is fed into the feed zone of the twin-screw unit (ZSK or ZDS-K) by a vertical force-feed screw. The compounded stock drops cascade-fashion into the ES-A pelletising screw which forces it through

Rigid PVC· (kg b- I) Plasticised PVC (kg h- I) (at elevated PR screw speed)

Production rate:

Total weigbt' (kg) No. of beating units Heating capacity (approx. kW)

Dtive rating (kW) Screw speed (r min-I) Screw diameter (mm) Screw lengtb (LID) Heating zones PeUetising: Drive rating (kW) Knife speed (r min-I) No. of knives Size of die boles (mm dia.) Die beater (kW)

15 40-300 46 7 4

0·6 16-80 46 2

40 75

1000 3 30

PR

ET

GS

1·1 120-600 2 3/3·6 0·5

3 20-80 70 6 3

WKG4·6-7

Technical data for KGIWKG (rigid and plasticised PVC)

1·8 7-47 80 2

ET

400 750

4500 3 30

70 40-240 100 7 4

PR

GS

1·5 120-600 2 3/3·6 1·7

11 10-47 140 4 3

WKG1Q-14

3 4-30 140 2·5

ET

1500

BOO

1‫סס‬oo

3 60

107 40-120 140 7 4

PR 5·8 10-40 140 2·5 2

RS

KG 14-18

2·2 12(}..6()() 2 3/3·6 3

24 8-40 180 6 3

GS 4·5 4-30 215 2

ET

1600 3000

17500 3 60

210 40-120 200 7 4

PR

12·5 5-39 200 2·5 2

RS

KG2Q-25

TABLE 13.11 Technical Data For Buss-Kneader Models KG and WKG

3 120-600 2 3/3·6 4

52 8-40 250 6 3

GS

7·5 4-25 300 2

ET

3200 S-7ooo

38500 6 120

510 40-120 300 7 4

PR

23 5-35 300 2·5 2

RS

GS

7·5 100-1000 2 3/3·6 7

95 6-30 320 6 3

KG3Q-32

N

;:,

'"

~

~

;:, !'>..

'"

";~,-

1;;

~

~

~

0"-

a b

GS

ET PR 70 40-240 100 7 4

750

1·8 7-47 80 2

75

1·1 12lHiOO 2 3 0·5

3 20-80 70 6 3

4500 3 30

IS 40-300 46 7 4 1·5 12Q-{;()(J 2 3 1·7

11 10-47 140 4 3

GS

WKGIO-14

1000 3 30

PR

ET

0·6 16-80 46 2

WKG4·6-7 ET 3 4-30 140 2·5

1500

8700 3 30

107 40-240 140 7 4

PR

2·2 12lHiOO 2 3 3

24 8-40 180 4 3

GS

WKGl4-18

Approximate figures, not including heating/cooling units, pellet cooling installation, control cabinet or main drive motor. Approximate figures; may vary depending on formulation and operating conditions.

Total weight" (kg) No. of heating units Heating capacity (approx. kW) Production rate: b Plasticised PVC (kg h- I)

Drive tating (kW) Sctew speed (r min-I) Screw diameter (mm) Screw length (LID) Heating zones Pelletising: Drive rating (kW) Knife speed (r min-I) No. of knives Size of die holes (mm dia.) Die heater (kW)

Technical data for WKG (plastic/sed PVC)

ET 4·5 4-30 215 2

3000

60

15800 3

210 40-240 200 7 4

PR

3 12lHiOO 2 3 4

52 8-40 250 4 3

GS

WKG20-25 ET 7·5 4-25 300 2

6-7000

35000 4 120

510 40-120 300 7 4

PR

7·5 100-1000 2 3 7

95 6-30 320 4 3

GS

WKG30-32

Ul

~

~

'"tl

It

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[

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~

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TABLE 13.12 Technical Data for Buss-Kneader Models PRK and PRK/E PRK, PRKIE

PRK, PRKIE

PRK, PRKIE

100

140

200

Feed hopper: Capacity (litre) Screw diameter (mm) Screw drive (kW) Screw speed range (r min-I)

150 100 1·8 6·7-47

280 or 430 140 4·4 2·2-44

430 or 830 215 5·5 1·4-28

Buss-Kneader: Screw diameter (mm) Screw length PRK (LID) Screw length PRKlE (LID) Screw speed range (r min-I) Gear box model Kneader drive (kW)

100 7 12 120 G20 60

140 7 11 120 G45 120

200 7 11 120 G130 200

Heating/cooling equipment: No. of units Installed heating capacity (kW)

2 20

2 36

2 36

3800

6300

11700

Approx. total weight (kg) (not including heating/cooling units, control station or kneader drive equipment)

the pelletising die. Cutting knives mounted next to the die plate chop the emerging strands into pellets, which are conveyed pneumatically to the pellet cooler. Working principle: The ZSK is a twin-screw machine with intermeshing, co-rotating screws equipped with kneading discs (Fig. 13.62) which shear and knead the product and change the local direction of stock flow as it passes the barrel wall. The shearing occurs, and energy is imparted to the stock, as a result of the velocity drop between the kneading discs and the barrel wall and in the saddle area between the two screws. Shearing intensity is determined both by the speed of the screws and the inherent resistance of the kneading discs. The configuaration and arrangement of the discs shown schematically in Fig. 13.62 give rise to squeezing forces which pass the material forward along the screws while it is subjected to shearing and mixing by the consecutive disc elements. The design ensures that the root of each screw is wiped by the flight tip of the other. This self-cleaning

13

Industrial Compounding Technology of Rigid and Plasticised PVC

Output in kg h- 1 Model PRK/E 2000

Output in kg h-1 Model PRK 3000

/

1670 PRK/E 200 1350

1000

/~

670

/

330

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/

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~

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2500

2000

1500 PRK/E 140 ~

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20 40 60 80 Speed of Buss-Kneader screw

PRK/~ 100 I

1000

500

PRK100

100 r min- 1

120

0

Fig. 13.57 Output diagram for Buss-Kneader models PRK and PRKlE.

profile eliminates dead spots and normalises the residence time of the material. Temperature control system: The ZDS-K and ES-A barrels are electrically heated. The ES-A unit is cooled by an air blower, whilst the barrel of the ZDS-K unit has channels for cooling with liquid heat-transfer media. De-gassing: De-gassing is carried out at the transition point between the ZDS-K and ES-A units. Operation and maintenance: The starting product can be diverted between the ZDS-K and ES-A units. For cleaning, both units are run empty, and residual powder is purged with a cleaning material. The pelletising head can be rolled away, which facilitates the removal of product between screw tip and die plate. The barrel sections of the ZDS-K and the barrel of the ES-A can be dismantled for more thorough cleaning work.

628

W. Henschel and P. Franz

1

u > p.. ...

sa...

,

-... 0

'0

;:I

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'0 ~

0

u en ~

....0 U

l=: .~ en 0

0

00

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~

13 Industrial Compounding Technology of Rigid and Plasticised PVC

629

Standard Extruder n

Process Steps

IVonabieland Fixed Operating Conditions

Fig. 13.59 Process steps and operating conditions of a standard cascade extruder.

Energy input: 0·04-0·08 kWh kg-I. Output and technical data: See Table 13.14. THE FCM CONTINUOUS MIXER

Applications:

-- dry blend> pellets (which will

14 Extrusion of PVC-General Aspects

687

have been melt-compounded). The preparation of these types of PVC compound is described in Chapter 13, Sections 13.4.1 (pre-mix and dry blend powders) and 13.4.2 (pellets). Their use as extruder feeds is further discussed in the introduction to Chapter 19. PVC COMPOSITIONS FOR EXTRUSION As in injection moulding of PVC, processing by extrusion is easier with plasticised compositions, so that, in formulating, the processing considerations (as against those associated with service requirements) are less critical than in the case of uPVc. With the latter, the type of extruder used (i.e. whether single- or twin-screw) also influences the formulation. This, and other aspects of formulating uPVC compositions for use in the manufacture of major extruded products, are discussed in Sections 19.3-19.5 of Chapter 19, in Chapter 4, and Chapters 9 and 11 (in connection with the roles of stabilisers, lubricants and other formulation constituents): they are also mentioned at various other points in the book.

FORMULATION OF

PLATE-OUT

As mentioned in Section 9.7 of Chapter 9, plate-out-i.e. persistent, sticky deposits on working surfaces of processing machinery--. is

:::1

~

~

;:-

;:

r-

~

!J:l

Fig. 14.5 A typical building-wire insulation and sheathing line. 1, Dual reel flyer pay-off for insulation or earth wire ;;; when sheathing; 2, wire straightener; 3, rotating pay-offs for insulated core to make flat twin-sheathed cable; 4, cable guides; 5, extruder; 6, hopper loader and colour masterbatch feeder; 7, control panel; 8, embossing unit; 9, multi-pass capstan/cooling trough; 10, diameter control; 11, spark tester; 12, accumulator to control take-up speed; 13, high-speed dual reel take-up; 14, stripping extruder. (Reproduced, with permission, from Ref. 9.)

~! ~-

, .."

8

;::J 00

14 Extrusion of PVC-General Aspects

719

Good accounts of extrusion-coating of wire and cable with PVC have been published by Barnett,9 and Burton and Clarke. 7o (b) Production of pPVC Hose with Braid Reinforcement An example of a modern, continuous production operation is the Maillefer line (Meillefer SA, Ecublens, Switzerland).?l A primary extruder (operating on either pellet or powder feed) extrudes a tube (the ultimate inner part of the hose): this is vacuum-sized, cooled and dried, and thereupon enters an in-line winding section where filament reinforcement is applied-in a braid configuration-by a counterrotating, multi-strand helical winder. The tube next passes through an adhesive-coating station and a pre-heater, and then enters the cross-head die of a second extruder, where it is coated with an outer PVC layer. The composite tube emerging from the die is passed through a cooling bath and caterpillar haul-off (which provides traction for the whole operation), to a winding and cutting station. A typical production speed (with 60-mm screw extruders) for 20-mm hose is quoted as 15-20 m min- l depending on the PVC compound and the reinforcement winding density.

REFERENCES 1. Anders, D. (1978). 36th ANTEC SPE Proceedings, pp. 726-31. 2. Fisher, E. G. (1964). Extrusion of Plastics, Illiffe Books Ltd, and The Plastics Institute, London. 3. Titow, W. V. and Lanham, B. J. (1975). Reinforced Thermoplastics, Applied Science Publishers, London. 4. Burbridge, J. (1973). In Developments in PVC Technology, (Eds J. H. L. Henson and A. Whelan), Applied Science Publishers, London, Ch. 8. 5. Anon. (1982). Eur. Plast. News, 9(11), 3~. 6. Anon. (1982). Mod. Plast. Int., U(lO), 24. 7. McCandless, W. W. and Maddy, W. D. (1981). Plast. Technol., 27(2), 89-93. 8. Avery, D. H. and Csongor, D. (1978). 36th ANTEC SPE Proceedings, pp. 446-8. 9. Barnett, G. P. (1977). In Developments in PVC Production and Processing, (Eds A. Whelan and J. L. Craft), Applied Science Publishers, London, Ch. 8. 10. Grant, D. and Wilkinson, P. (1968). Plast. Polym., 36(124), 33~1. 11. Anon. (1979). Mod. Plast. Int., 9(1), 30-1. 12. Anon. (1982). Mod. Plast. Int.. 12(3), 20. 13. Anon. (1980). Plast. Technol.. 26(13). 62-4.

720

B. J. Lanham and W. V. Titow

14. Carley, J. F. (1968). SPE J., 24(2),36-41. 15. Yarsley, V. E. and Flavell, W. (1956). Cellulosic Plastics: Part 1: Cellulose Acetate, Cellulose Esters, and Regenerated Cellulose, Plastics Monograph No. C. 6., The Plastics Institute, London. 16. Rice, W. T. (1980). Plast. Technol., 26(2), 87-91. 17. Anon. (1982). Mod. Plast. Int., 12(11), 24-5. 18. Anon. (1981). Mod. Plast. Int., 11(11), 35. 19. Anon. (1981). Eur. Plast. News, 8(11), 37. 20. Kautz, G. and Schumacher, F. (1977). Kunststoffe, 67(10),585. 21. Brockschmidt, A. (1982). Plast. Techno!., 28(5), 35-7. 22. Anon. (1982). Eur. Plast. News, 9(11), 37. 23. Brockschmidt, A. (1982). Plast. Techno!., 28(3), 73-6. 24. Fenner, R. T. (1979). Plast. Rubb. Int., 4(5), 219-22. 25. Fischer, P. (1981). Plast. Rubb. News, January, 39-45. 26. Parnaby, J., Kochhar, A. K. and Wood, B. (1975). Polym. Engng. Sci., 15(8), 594-605. 27. Lippoldt, R. F. (1978). 36th ANTEC SPE Proceedings, pp. 737-9. 28. Lippoldt, R. F. (1978). Plast. Engng, 34(9), 37-9. 29. Marks, G. C. (1973). Developments in PVC Technology (Eds J. H. L. Henson and A. Whelan), Applied Science Publishers, London, Ch. 2. 30. Chartoff, R. P. (1976). 34th ANTEC SPE Proceedings, pp. 347-9. 31. Summers, J. W., Isner, J. D. and Rabinovitch, E. B. (1978), 36th ANTEC SPE Proceedings, pp. 757-9. 32. Parey, J. and Menges, G. (1981). J. Vinyl Technol., 3(3), l52-{). 33. Menges, G., Berndtsen, N. and Opfermann, J. (1979). Kunststoffe, 69(9), 562-9. 34. Benjamin, P. (1978). 'The influence of processing on the properties of PVC pipe,' paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978; and (1980) Plast. Rubb.: Mat. Appln, November, 151-{)0. 35. Daniels, C. A. and Longeway, G. D. (1979). Polym. Engng. Sci., 19(3), 181-9. 36. Hattori, T., Tanaka, K. and Matsuo, M. (1972). Polym. Engng. Sci., 12, 199. 37. Faulkner, P. G. (1975). J. Macromol. Sci. (phys.) , B11,251. 38. Khanna, R. (1977). Pigment Resin Techno!., 6(7), 11-14. 39. Krzewki, R. J. and Sieglaff, C. L. (1978). Polym. Engng. Sci., 18, 1174. 40. Kulas, F. R. and Thorshaug, N. P. (1979). J. App!. Polym. Sci, 23, 1781-94. 41. Summers, J. W. and Rabinovitch, E. B. (1981). J. Macromo!. Sci. (Phys.) , B20(2), 219. 42. Krzewki, R. J. and Collins, E. A. (1981). J. Macromo!. Sci. (phys.) , B20(4), 443. 43. Huxtable, J., Cogswell, F. N. and Wriggles, J. D. (1981). Plast. Rubb. Process. Appln, 1(1), 87-93. 44. Isherwood, D. P. and Katwiremu, J. B. (1982). Plast. Rubb. Process. Appln, 2(3), 253-63. 45. Riley, D. W. and Klein, I. (1978). 36th ANTEC SPE Proceedings, po. 525-8.

14 Extrusion of PVC-General Aspects

721

46. Press, J. B. (1978). 'The selection of PVC polymers, additives and fillers and the choice of equipment to maximise output and quality and to minimise cost,' paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, fr-7 April, 1978. 47. Collins, E. A. and Metzger, A. P. (1970). Polym. Engng. Sci., 10, 57. 48. Logan, M. S. and Chung, C. I. (1979). Polym. Engng. Sci., 19(15), 1110-16. 49. Summers, J. W. (1981). J. Vinyl Technol., 3, 107-9. 50. Marshall, G. P. and Birch, M. W. (1982). Plast. Rubb. Process. Appln, 2(4), 369-79. 51. Parey, J. and Zajchowski, S. (1981). Plastverarbeiter, 32(6), 724-6. 52. Gonze, A. (1971). Chim. Ind., 104(4/5),422-7. 53. Lamberty, M. (1974). Plast. Mod. Elast., December, 82-9. 54. Lanham, B. J. (1969). Plast. Rubb. Wkly, (286), 10-11. 55. Martin, B. (1970). Plast. Polym., 38(134), 113-19. 56. Klein, I. and Tadmor, Z. (1969). Mod. Plast., 48(9), 16fr-70. 57. Sweetapple, L. (1968). Plast. Technol., 14(11), 75-83. 58. Anon. (1981). Plast. Techno!., 27(7), 15-17. 59. Wyman, C. E. (1975). Polym. Engng. Sci., 15(8), 60fr-11. 60. Prause, J. J. (1967). Plast. Techno!., 13(11), 41-5. 61. Prause, J. J. (1968). Plast. Technol., 14(2),29-33. 62. Prause, J. J. (1968). Plast. Technol., 14(3), 52-7. 63. Jewmenow, S. D. and Kim, W. S. (1973). Plaste u. Kaut. 20, 356. 64. Kim. W. S.,Statschkow, W. W. and Jewmenow, S. D. (1973). Plasteu. Kaut., 20,696. 65. Korney, A. F. Jr. (1969). SPE J., 25(7),27-9; 25(9), 28-31. 66. Kramer, A. (1969). Kunststoffe, 59(7),409-16. 67. Anon. (1967). Brit. Plast., 40(12), 57-61. 68. Carley, J. F., Endo, T. and Krantz, W. B. (1978). 36th ANTEC SPE, Proceedings, pp. 453-61. 69. Fenner, R. T. and Nadiri, F. (1979). Polym. Engng. Sci., 19(3),203-10. 70. Burton, V. A. C. and Clarke, J. J. (1978). 'Cable extrusion machinery for PVC', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, fr-7 April, 1978. 71. Anon. (1979). Mod. Plast. Int., 9(1),20.

CHAPTER 15

Injection Moulding of pvc The late L. W. TURNER

15.1 INTRODUCTION Injection moulding is a commonly operated melt process, more extensively applied to thermoplastics other than PVC than to PVC itself, chiefly because of two inherent features especially appertaining to the unplasticised PVC compounds. These features are thermal instability and high melt viscosity, both of which create situations which those responsible for the process must anticipate and control carefully. There is little doubt that as moulding technology improves and as the position with regard to VCM becomes less hazardous the injection moulding of PVC will extend. Injection moulding is a cyclic process in which precision in repetition yields significant applicational, economic and quality returns. The procedure by which an injection machine is used to produce adequate components can best be envisaged in distinct stages: feeding the melting cylinder with compound; preparing plastic melt by thermal and mechanical effects; forcing the melt from the melting cylinder into the mould; cooling the contents of the mould; finally emptying the mould. While post-ejection processes may need to be applied-jigging, trimming, decorating, assembling, cementing-these will not be dealt with here. The two features that have been emphasised above as being particularly significant with PVC, are especially so during melt preparation and mould filling. For this reason attention will be largely devoted to the behaviour of PVC in these two stages of the overall cycle of operations. It is here that the plastic melt reaches its maximum temperature and is forced to flow through restricting features such as cylinder nozzle, sprue, runners and gates. 723

724

L. W. Turner

Concerning the other stages of the cyclic process of injection moulding, it is generally satisfactory to consider PVC as one would other thermoplastics. Thus, in the section below on trouble-shooting, this approach is taken specifically, and the account emphasises the effects which might be met with due to instability under melt conditions.

15.2 MELT PROPERTIES OF PARTICULAR SIGNIFICANCE, MELT BEHAVIOUR IN RELATION TO MOULDING CONDITIONS, AND MOULDING COMPOUNDS While, in many respects, PVC responds to the process of injection moulding as do other thermoplastics, it is notable for two specific reactions to moulding conditions which tend rather to set it apart. These are: development of decomposition, in times of the order of moulding cycles, at those levels of temperature found to be necessary; and a tendency for the melt to have a higher viscosity than many other materials at permissible temperature levels, when the polymer is unaided by the presence of plasticisers or other flow promoters. It is often stated that PVC shows a marked tendency to decompose by reason of loss of HCI from the molecular chain accompanied by darkening-at temperatures from, say, I80 a C upwards. But such a description of its behaviour during moulding must be regarded as oversimplistic. In fact degradative effects can become apparent over a range of temperatures, including levels below I80 a C, in relation to the time for which the melt is held at these levels. This leads to the concept of residence time (often applied by moulders when setting up a trouble-shooting programme) which also tends to be employed in too simple a fashion in considering moulding performance. What has to be taken into account is the integrated thermal input to the material (from any input source) during both melt preparation and mould-filling stages. The shear-work being done on a highly viscous polymer melt can result in reduction of the molecular entanglements, thus aiding flow; but this effect will not normally give rise to difficulties in production control or in product quality. Rather more serious is the molecular damage which high shear levels can cause, especially to longer chains present in the melt. This latter degradative effect is separate from, and additional to, the thermal degradative effect and is

15 Injection Moulding of pvc

725

almost always much less prominent and camouflaged by thermal effects. Firstly, to illustrate the straightforward effect of material temperature, the graph in Fig. 15.1 has been prepared from data given in the literature! for three types of moulding compound (referred to again in Section 15.6). The graph defines reasonable limits for residence times: i.e. to the right of a given curve the residence time is too long for safe operation with the compound concerned at a selected temperature; to the left conditions are safe (no thermal/time decomposition). The graph gives data for specific time-temperature pairs, but from the discussion above it will be realised that to deal adequately with degradative effects occurring during moulding, the broader picture of integrated thermal input has to be considered. Secondly, it is necessary to realise that the degradation which might occur during moulding cannot be attributed to one specific timetemperature value pair. The melt reaches the mould cavity by what might be regarded as an

230

220

V.

210

''"-

::J

,"200 '-

'"

a. E

~190 180

lima,s

Fig. 15.1 Time-temperature behaviour of typical PVC compounds. A, Pipe-fitting compound; B, rigid general-purpose compound; C, flexible compound. Generalised representation, based on data from Ref. 1.

726

L. W. Turner

integration of a series of temperature-time steps which vary continuously as the plastic material proceeds from hopper to mould entry. Shear-work, dissipated as thermal energy, raising the melt temperature, takes place in the melt in the forward zones of the screw and barrel, in the nozzle, and finally in the gate. Thermal conductivity is poor, and non-homogeneous build-up of temperature will occur. Poor streamlining in the nozzle-to-gate region can exacerbate the situation by causing local stagnation of flow, which adds seriously to the time component of the thermal input. The rheological behaviour of PVC2 is important in relation to injection moulding because melt preparation usually involves an extrusion screw and barrel arrangement. Melting and flow behaviour during this heating process and flow during the moulding operation are dependent on the rheology, which in turn depends on the composition, i.e. the PVC polymer and the additives employed. Shear-work is another feature dependent upon composition via slip and flow behaviour. 15.2.1 Moulding Compounds

In the case of plasticised compositions, the general difficulties associated with the high melt viscosity and susceptibility to thermal degradation of PVC compounds are, to some extent, relieved by the presence of the plasticiser(s) which makes for easier melt flow (lowers melt viscosity) and hence for processability at somewhat lower heat inputs and temperatures. It is thus the uPVC compounds that present the greatest problems-a circumstance responsible for their general unpopularity with moulders, and long operative as a restricting factor on the scope and volume of the injection moulding of 'rigids'. However, the problems can be considerably eased by suitable formulation, and in particular by the use of special PVC homopolymer resins of relatively low molecular weight (cf. also Chapter 4, Section 4.4.1(a)). 'Easy flow' rigid injection-moulding compounds* available from most big suppliers are commonly based on such resins (in which the lower molecular weight is also usually combined with fine particle size and medium porosity). With good equipment and moulding practice easy flow compounds permit production of relatively thinwalled mouldings of large surface area; deep draws, as well as normally

* e.g. Geon 110 x 346 (B. F. Goodrich); Ethyl 7042 (Ethyl Corp.).

15 1njection Moulding of pvc

727

difficult surface detail, can also be achieved in many cases. Although it is sometimes claimed that the compounds can be successfully run-with only a few modifications-on equipment set up for the processing of materials less difficult than uPVC (e.g. ABS) it is important for effective operation with any uPVC composition to pay attention to the equipment and process considerations summarised in Section 15.4. Compounds based on vinyl chloride copolymers, or on combinations of homopolymer with flow-promoting resins, may be used for ease of processing, where the ultimate properties of the resulting products are consonant with the end-use requirements. Note: A standard flow test3 in a piston plastometer (melt index apparatus with a modified die) provides a useful method of comparison of PVC compounds with particular reference to the effects of compositional factors and thermal and rheological stability. Whilst some correlation of the test results with actual processing behaviour is also claimed3 (especially for compounds of generally similar composition) the validity of this claim in particular cases must be subject to such factors as the differences in shear rates between test and processing conditions (shear generally higher in the latter) and the effects of the elastic nature of the melt (not normally brought out in a plastometer test).

The mould shrinkage of rigid PVC moulding compounds is relatively low: typically 0·1-0·7%. That of plasticised compositions is generally more variable and much higher. Note: Shrinkage of plastics mouldings can vary with the rate of operation (cycle time). Useful general comments on the practical aspects of mould shrinkage calculations for optimum processing rates have been published by W. B. Glenn. 4

uPVC moulding compounds are not very sensitive to moisture and do not require drying before use, except where significant moisture pick-up may have resulted from prolonged explosure to humid atmospheres. Even in such cases a hopper drier on the injection machine may be sufficiently effective. Any oven pre-drying should normally be done on shallow layers (up to about 4 cm), with the temperature and time of drying preferably not exceeding 65°C and 3 h, respectively.

728

L. W. Turner

15.3 EFFECT OF PROCESS FACTORS UPON PRODUCT PROPERTIES Some reference to process factors affecting product properties, aside from the effects attributable to thermal decomposition, is necessary. PVC polymer, in common with other thermoplastics, by reason of its characteristic long chain-like structure, yields melts of high viscosity and poor thermal conductivity. These physical attributes give rise to mouldings in which quenching stresses and orientation both exist. These can have important effects upon product properties, though it is not uncommon for moulders to be aware of and concerned about orientation while neglecting to consider the effects of quenching stresses. Other morphological features of mouldings, e.g. skin and core effects, also arise, and influence the properties. A useful summary of methods of characterisation of injection mouldings was published recently by Haworth et al. 5

15.3.1 Quenching Stresses Injection into a mould which is much cooler than the melt results in rapid chilling of surface layers of the moulding. Inner layers, by reason of poor conductivity in the melt and the solid, cool more slowly and stresses are set up within the moulding cross-section. Commonly, compressive stresses of considerable magnitude arise in the surface layers and balancing tensile stresses in the central zone; together these are sufficient to affect mechanical behaviour and environmental resistance of the moulding.

15.3.2 Orientation and Related Features The flow pattern and the effect of the shear involved in melt flow give rise to anisotropy of mechanical and other properties. Mouldings are most resistant to fracture when deformed across the line of orientation and less so along this direction. This situation can be exaggerated if fillers are present in the compound, especially if they are fibrous. Orientation is not uniform either along a flow path, or, at any given point, through the moulding thickness. Development of orientation relates to the shear stress required to fill the mould cavity and through this physical parameter becomes related

15 1njection Moulding of pvc

729

to gate size, mould temperature, wall thickness, injection rate, melt temperature, and so on. Thus the effective level of applied shear stress can often be employed as an overriding control parameter and guide in troubleshooting, enabling some of the complex inter-relationships between various machine settings to be more simply recognised in so far as moulding properties are being affected. A useful reference to the effects of process conditions on injection-moulded product properties is due to Gilbert et al., 6 who give a description of the behaviour of a selected rigid formulation involving typical ingredients. A low level of crystallinity in mouldings made from this formulation is observed, as are differences between the skin and core, layered structure developing during mould filling and cooling. The distinct skin layer is highly oriented as shown by its strong birefringence (when a thin cut section is viewed through the thickness), whereas the core is largely unoriented. The skin layer decreases in thickness from, say, 0·4 mm down to around 0·1 mm as moulding thickness increases from 1·6 to 6 mm. Additionally, the thickness of this oriented skin is governed by each of the commonly varied process factors: filling rate, mould temperature, material temperature. Unfortunately no reference is made to the quenching stresses which must have developed in parallel with the oriented skin layer and for this reason the consideration of the effects on product properties is not too helpful to the reader. In referring to this paper it has to be recognised that mould shrinkage data are not given though the word 'shrinkage' is used. Such 'shrinkage' data as are given relate to recovery at an elevated temperature (130°C).

15.4 THE MOULDING PROCESS: AVAILABLE EQUIPMENT; PROCESS CONTROL; SOME FEATURES OF uPVC MOULDING

The elementary principles of injection moulding (including machine features, mould design and processing conditions) are outlined in an leI publication. 7 Useful practical guidance on the equipment, the basic process, process operation and control, and diagnosis and cure of moulding faults is provided in a recent book by Brown. s A brief general discussion of the main considerations in injection machine selection has been published by Ireland and Smith. 9

730

L. W. Turner

Plate J Injection unit for the Peco-Loewy 20/90R injection-moulding machine: shot weight 30 lb; 2500 ton locking unit. (Courtesy of the designer Mr T. Seklecki, formerly of Peco, now with Gay's (Hampton) Ltd, Hampton, Middlesex, England.) The modern injection-moulding machine is a sophisticated piece of equipment but, at the present moment, its design details are in a transitional stage. Quite extensive changes are taking place, not so much in the actual mechanics of the separate process stages which it provides, but almost entirely in regard to the detailed control of these process steps, their balance and integration. This is due to a background already developed in monitoring and feed-back control during the last decade, plus the well-nigh revolutionary possibilities inherent in the use of microprocessors. By the use of these, inexpensive integrated control can be achieved. Moulders involved in large runs of closely related products (e.g. pipe fittings) can set up processing units which are unique to the product in satisfying, at optimum level, specific processing requirements. Having in mind the nature of PVC and the specific nature of some of its applications, the development in dedicated process control is most welcome to moulders. Others, wishing to make a wide range of products, can gain advantage from the greater ease of setting up and re-establishing process conditions, thus achieving greater consistency of operation, less rejects and less down-time.

15 Injection Moulding of pvc

731

Extensive surveys of injection-moulding machines are published from time to time in the technical literature. Note: Two recent surveys appeared, respectively, in the March 1983 issues of Plastics and Rubber Weekly and in European Plastics News for June 1983.

The data provided for the various manufacturers' machines normally relate to machine characteristics and construction, with the wide range of equipment available divided into four groups according to the mould-clamping force. Whilst manufacturers will advise how particular machines can best be set up for PVC working, the following general points may be mentioned by way of a few broad guidelines. For moulding uPVC, the machine should preferably have a clamping pressure capacity of up to 3·5 tons (UK) per square inch of proj~cted area of moulding (i.e. about 55 MN m- 2 , in round figures), although anything from about 2 tons upwards (i.e. about 30 MN m- 2 plus) should normally be sufficient with most easy-flow compounds. In many cases a shot size between about two-thirds and three-quarters of barrel capacity will represent the practical optimum for melt residence times sufficiently short to avoid undesirable heat effects or excessive heat history when operating near the top limit of the relatively narrow melt temperature range (see further on) desirable for ease of flow. With screws specially designed for uPVC processing ('PVC screws') shot sizes up to full capacity may be practicable. With decreasing shot sizes (preferably not below about one-fifth of barrel capacity) the balance of the effects of melt temperature, residence time, and cycle timealways important with PVC-becomes progressively more critical. Several features of equipment and operation desirable or essential for the best results in PVC moulding reflect the need to cater for the two special factors which have already been emphasised, viz. the susceptibility of the material to heat degradation, and the relatively high melt viscosity (of uPVC). Thus streamlining right up to the nozzle exit is important to avoid creation of points of increased friction in the flowing melt, and pockets of stagnant material where thermal decomposition may occur with consequent contamination of, and destabilising effect on, the melt. For analogous reasons it is usually considered essential for rigid PVC to have a conical extension (smear-head tip) on the screw which-with a suitably shaped barrel head-virtually empties the barrel at the finish of mould-fill, as it attains its most forward position. Some relevant comments about screw

732

L. W. Turner

tip patterns for different circumstances have been published by Tulley and Harris,I and about the moulding of rigid PVC generally by Huber. lO The screw should preferably be of a design intended for uPVC processing: LID ratios in the range 14-24: 1, and compression ratios of 1·5-2·0: 1, may be regarded as optimal, although higher compression ratios (up to about 3: 1) can be used if the effects of the associated greater shear-heat generation are properly monitored and controlled (by lowering the rotational speed and back-pressure of the screw). Conversely, lower compression ratios allow a wider range, and higher values, of back-pressures and rotational speeds. As an example, the following values have been recommended for an easy-flow high-impact uPVC compound (Ethyl 7042).11 Screw compression ratio Back-pressure range (Ibf in -2) Rotational speed range (r min-I)

l'S:l 2·0:1 2·S: 1 G-1000 0-400 G-100 2G-100 2G-80 2G-SO

3·0: 1 G-SO 2G-30

It can be worthwhile to set the machine to operate under zero cushion mode: otherwise a minimum cushion (say 2~ mm) is desirable. The flow routes (nozzle, runners) should be as short and generous as possible. The nozzle should preferably be reverse-tapered, to reduce friction-heating possibilities in this area. Gates should also be generous (and round wherever possible), with small lands and edges radiused towards the component, for ease of flow, rapid mould filling and reduction of pressure losses. Pin-gating is undesirable, although it may be used in some cases (in particular for small parts moulded with easy flow compounds). Adequate venting of moulds is important. Maintenance of optimum stock temperatures at the various stages of the material's processing in the machine is particularly important with uPVC, for ease of flow, avoidance of decomposition, and minimum heat history. Close attention should be paid to the proper setting and control of all heater temperatures, as well as-and in conjunction with-the factors influencing shear heating in the barrel (the back-pressure and rotational speed of the screw-see above), and frictional heating thereafter (size and configuration of nozzle, flow channels and gates; rate of injection). The melt temperature should be monitored directly, either through intermittent checks (e.g. with a thermocouple inserted into melt flushing out of the nozzle while the barrel is retracted from the mould) or continuously by a sensor (thermocouple) so positioned that it does not cause additional friction

15

733

Injection Moulding of PVC

or stagnant spot in the flowing melt. It is generally advantageous to set the barrel and nozzle heaters for a temperature profile rising fairly sharply from the rear (feed) zone of the barrel to the nozzle. The set temperatures should preferably lag somewhat behind the desired stock temperatures, the heat needed to make up the difference coming from the right amount of mechanical working of the material in the barrel secured by appropriate speed and back-pressure of the screw. For a given machine and mould set-up (and with a particular screw type, speed and back-pressure) the heater temperature settings will vary somewhat according to the composition processed (see examples in Table 15.1), but an arrangement of this general kind offers the following advantages. The final fusion of the stock takes place well forward in the barrel, so that the amount and dwell time of the hottest material are minimised, whilst any volatiles have an escape route through the interstices between incompletely fused compound particles in the rear of the barrel, with consequent reduction of the possibility of porosity in moulding arising from this source. During a mechanical stoppage, the material in contact with the cylinder walls (especially the fused melt in the front part of the cylinder) will be hotter, at least initially, than the wall surface, so that the immediate effect should be its cooling-and not continued heating-with consequent delay of the onset of decomposition. TABLE 15.1

Examples of Recommendations for Temperature Regimes in the Processing of Some uPVC Compounds Type of compound

Temperature settings

eC)

Barrel zone heaters

General-purpose easy flow, high-impact Pipe-fitting compound (Type 1°) Universal pipe-fitting compound (Type 1/2°) ° See Section 15.5.

Nozzle heater

Melt temperature (0C)

Rear (feed section)

Middle

Front

135

160

170

170-175

200-210

140

155

170

170-175

190-200

145

160

175

175-180

200-205

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L. W. Turner

Note: In the case of stoppages of appreciable length, the heater temperature settings should be reduced, and the barrel should be pulled back from the mould and purged out slowly. If the stoppage is prolonged, or when shutting down, the barrel should be completely emptied and purged with a purging compound (polystyrene, ABS, acrylic, or HDPE) at a relatively low temperature (about 160°C): uPVC should never be allowed to solidify in the barrel. If the temperature settings on the rear and centre zone heaters are too low, the torque on the screw trying to work the relatively cold material may be excessive: to prevent overloading the motor, an upward adjustment should be made (say in steps of 5°C) until the screw is running normally. The melt temperature should preferably not exceed 205°C, and should never be higher than 215°C: the range 18Q-205°C may be regarded as typical for most processing. The mould (inlet water) temperatures used are normally within the general range 2Q-70°C, with 2Q--40°C usual for many easy-flow compounds (the actual optimum depending on the composition, and the wall thickness, flow length and moulding size), and the higher temperatures for the older types of uPVc. Temperature control techniques for injection-moulding machines are summarised in a recent paper by Ingham. 12 The following further points may be mentioned.

15.4.1

Rate of Injection and Injection Pressure

The rate of injection should be the optimum for rapid, complete filling of the mould, but consistent with avoidance of decomposition (first manifested in the appearance of discoloration at and near the gate point-d. Section 15.6) through excessive frictional heating. Note: If the injection rate is reduced too far, weak weld lines, sink marks, or even short shots may result. When the characteristic signs of decomposition appear at a generally reasonable injection rate, consideration should be given to enlarging (and streamlining) at points where melt flow may be unduly restricted (nozzle orifice, sprue, runners, gate). As a general principle, automatic adjustment of injection rate (via a closed-loop control arrangement) is highly desirable.

15 1njection Moulding of pvc

735

Injection pressure is interdependent with injection rate, and also influenced by other factors, notably melt fluidity and the geometry and temperature of the mould. As a general guide, for uPVC the first-stage injection pressure can advantageously be within one-half to threequarters of the total pressure available on the equipment, and the holding (dwell) pressure will usually be between one-half and two-thirds of the first-stage pressure. It is desirable that the equipment should be able to provide pressure up to 25000 lbf in -2 (about 172MNm- 2). 15.4.2 Working Surfaces

The working surfaces of machine and moulds must be resistant to acid corrosion. Stainless steel moulds are desirable wherever the type of work and run lengths can justify the initial expense.

Note: Galling can be a serious problem where stainless steel surfaces in contact undergo repeated relative movement. For this reason any moving parts in a stainless steel mould (ejector pins, sliding cores) are usually of chromium-plated hardened steel. The mould surfaces, as well as those of the screw, may also be of chromium-plated hardened steel, or of hardened steel nickel-plated (by the electroless process) over-plated with chromium (the electroless nickel finish alone may not provide sufficient corrosion protection, especially where the mould contains deep recesses). Deep nitriding may be adequate as the sole protective finish in some cases. It is strongly preferable that all surfaces (those of the platens, etc.) accessible to any gaseous products of decomposition of the PVC material should also be suitably protected.

Note: For out-of-use periods (following the end of a run, before a shut-down) the surfaces of the mould, including those of the moving parts, should be treated to neutralise acidity and inhibit corrosion. Suitable treatments (neutralisers and inhibitors, usually in sprayable form) are available on the market. 15.4.3 Interaction of PVC with Acetal Polymers and Copolymers

PVC and the acetal resins (e.g. Delrin-Du Pont; HostaformHoechst; Kematal-Amcel UK; Celcon-Amcel USA) interact strong-

736

L. W. Turner

ly at elevated temperatures, with resultant vigorous mutual degradation. The two materials should on no account be allowed to come together in plastics processing equipment (injection-moulding machines, extruders, melt-compounding mixers, etc.). It is highly preferable that the same machine should not be used for the processing-especially consecutive processing-of these materials: at the very least extremely thorough purging must be carried out (with a suitable purging compound-see above) before a change-over. Two brief general summaries of considerations important in the moulding of uPVC have been published recently by Whelan,13 and Murrey and Dito. 14 15.5 MATERIALS AND APPLICATIONS It should be obvious from what has been said concerning the rather

unique aspects of PVC moulding that the moulder, prospective or current, is greatly aided if he has available to him property details of the compounds he is intending to use. By and large, manufacturers can help but it is not too wise to rely entirely on such sources and the moulder would do well to look at some of the more fundamental aspects of his activity. The purposes for which PVC moulding compounds are used are numerous and the compounds will range in type. It is, however, possible to classify and thus to understand better what the main types are and how they will mould. An excellent account of such a classification is given in Ref. 1. The classification recognises two basic types of unplasticised material, and a general category of plasticised compounds: Rigid (unplasticised) Flexible (plasticised)

1. Typical for pipes and pipe fittings 2. Typical general-purpose General: used for flexible demands

The differences. between the two types of rigid composition arise in consequence of the requirements to be met. Type 1: Mouldings usually thick-sectioned; high strength and stiffness, high rupture strength, good corrosion resistance. Minimum cost because of large number of items used, runner stabilised. Type 2: Designed for thinner sections and a wide range of

15 1njection Moulding of pvc

737

conditions. Higher level of stabilisation and better flow properties for thinner, larger-area products. The flexible (plasticised) grades, while considered as one broad type, will vary widely in respect of types and amounts of plasticisers used. In the moulding of PVC-and especially uPVC-pellets are used as the feedstock in the majority of cases. This is mainly because they consist of melt-compounded material which is already well homogenised and uniformly fused (d. Chapter 13), and thus-unlike powder (dry-blend) feeds-does not have to rely for final homogenisation on the treatment received in the barrel of the injection machine which is not primarily designed as a mixing unit. This consideration normally outweighs that of the more extensive heat history the pellets acquire in consequence of the melt-compounding operation. The most important area for the extension of injection moulding of PVC undoubtedly lies in rigid mouldings of thinner sections. The technical problems will be clear from the text above: the successful development of new applications has depended upon improving processing characteristics by increasing thermal stability and promoting easier flow through the use of suitable PVC polymers and additives of the appropriate types. Availability of screw/ram machines has created better processing conditions in making for better thermal homogeneity and reducing residence times. It is not likely that piston machines will be employed in the future in any significant way. Where really thin disposable items are required (as in the packaging field where PVC is regaining acceptance now that VCM concentration hazards are controlled) these will not be made by injection moulding but by forming of extruded sheet. A process sometimes referred to as 'flow moulding'* enables large mouldings to be produced on relatively small injection machines, by utilising pressure developed by the. rotation of the screw (instead of that produced by the injection stroke) as mould-filling pressure. Inter alia, large (30 lb) PVC pipe fittings have been produced on a 250 ton injection machine 15 by this process, which may be considered as a combination of extrusion (as the means of supplying melt to the mould) and injection moulding (in that a conventional-though modified-injection-moulding equipment is used). The modifications include adjustment of the hydraulics to keep the screw in the forward * Developed by Durapipe Ltd, Cannock, UK.

L. W. Turner

738

position (feeding the stock through a gap between the screw tip and the rear face of the nozzle); provision for adjustment of the size of this gap (where the main working of the material to produce plastication is effected through the shear imparted) to regulate the amount of shear; special provisions for screw and barrel cooling; and overall control of temperatures and pressures. The process is claimed to be particularly suited to heat-sensitive materials such as PVc.

15.6 TROUBLE-SHOOTING Many processing and material factors have to be balanced in order to achieve high outputs of good quality mouldings. Achieving an acceptable balance can sometimes be a difficult task requiring experience and skill. Two principles are here emphasised: good knowledge of the material concerned (this is perhaps more important with PVC compounds than with any other thermoplastic) and use of such control aids as the equipment can provide. Three groups of considerations which arise in aiming at continuous in-balance quality mouldings are distinguished below. They relate to machine selection; to those processing features pertaining more specifically to PVC; and to those features which are of general significance in injection moulding of thermoplastics.

15.6.1 Machine Selection The moulder with a limited range of machines faces problems not necessarily met with by the one who can match more closely machine size to moulding shot-weight and cooling time. Here the essential feature is 'residence time', or the 'temperature-time integral' to be more accurate. During operation it is important to maintain observation of heater control indications (watching out, inter alia, for persistent and increasing override), filling rate (to spot any tendency to increase with running time), and changes in thickness of cushion layer.

15.6.2 Processing Features Specific to PVC These relate almost entirely to the behaviour of the material, there being an inevitable interaction between these features and those mentioned in Section 15.6.1 above.

i5 injection Moulding of pvc

739

TABLE 15.2

Common Faults in PVC Mouldings

Observation 1. Splay marks or

silvery streaks emanating from the gate position

2. Dark centre to sprue or internally in gate area

Significance May be due to condensed moisture but, more important, could be initial signs of decomposition due to thermal input increased above acceptable level Progressive overheating of material (sometimes associated with hot spot at screw tip)

3. Dark, streaky areas on surface of sprue

Increasing overheating of material

4. More prominent areas of darkened material on sprue surface often extending from sprue into the moulding

Some decomposition of material due to overheating

5. Dark streaks spreading from gate point

High shear or stagnation in this region

6. Dark decomposition

Local 'burning'

spot at the same point in each moulding (sometimes in weld line)

Adjustment Choice of reduction of forward band heater settings (especially nozzle temperature), screw speed, back-pressure, filling rate, singly or in various combinations

Adjust barrel temperature and/or reduce backpressure, and/or screw speed; if fault persists attend to shape or smooth runs of screw tip; perhaps change to distorted tip to give an increase of local shear Reduce band-heater settings, and/or backpressure and speed of the screw Retract barrel and make a few air shots to remove overheated stock. Return barrel, reduce heater settings; increase cooling, lower screw speed and/or backpressure; consider residence time Better streamlining required: slow down input rate, attend to streamlining if necessary; consider enlarging nozzle and/or gate(s) First try reducing injection rate; if fault persists attend to proper mould venting and radiusing of corners

740

L. W. Turner

Observation of the product provides the clues from which process adjustments can be decided upon. Table 15.2 lists the most frequent manifestations of problems in the moulding of uPVC (1-4 may be regarded as a sequence in order of increasing severity). 15.6.3 General Considerations

General moulding faults such as warping, sink marks, short shots, flashing, weld lines and others can be treated as they would be for other thermoplastics, so long as it is consistently borne in mind that adjustments covering an increase in the temperature-time input must be undertaken with caution by careful observation of signs of incipient decomposition. Mould filling rate should also be capable of being adjusted and maintained at a specific level. Pressure control via feedback of data obtained from pressure sensors in the mould cavity(ies) is a useful way of controlling mould filling, since rates and pressures are interrelated. Some control experts favour sensors mounted elsewhere. Mould temperatures should be accurately maintained. This calls for correct provision of cooling channels in the mould and control of the fluid from circulating units of adequate performance. Regarding the improvement of older equipment (again neglecting piston machinery), in addition to improvements to the screw by removal of unstreamlined non-return valves and the introduction of conical extensions to the screw tip, it is often a simple matter to add proprietary control features. The simplest and the best control improvement which can be introduced by the addition of a single item is cavity pressure control, though more specifically for PVC, screw-advance rate control has some advantages especially where forward-zone streamlining has proved difficult.

REFERENCES 1. Tulley, F. T. and Harris, B. C. (1979). In Encyclopedia of PVC, (Ed. L. I.

Nass), Marcel Dekker, New York, Ch. 24, p. 1313. 2. Shah, P. L. (1979). Ibid, Ch. 22, p. 1153. 3. ASTM D 3364-74 (Reapproved 1979). Flow rates for poly (vinyl chloride) and rheologically unstable thermoplastics. 4. Glenn, W. B. (1980). Plast. Technol., 26(10), 73-5. 5. Haworth, B., Sandilands, G. J. and White, J. R. (1980). Plast. Rubb. Int., 5(3), 109-13.

15 1njection Moulding of pvc

741

6. Gilbert, M., Marshall, D. E., Voyvoda, J. C. and Copsey, C. J. (1979). Plast. Rub.: Process., 3,96. 7. 'The Principles of Injection Moulding,' ICI Technical Service Note G 103 (2nd edn), ICI Plastics Division, Welwyn Garden City, Herts, England. 8. Brown, J. (1979). Injection Moulding of Plastic Components-A Guide to Efficiency, Fault Diagnosis and Cure. McGraw-Hill Book Co. (UK) Ltd, London. 9. Ireland, R. A. and Smith, J. B. (1977). Plast. Rubb. Int., 2(5),225-7. 10. Huber, M. (1977). Plast. Rubb. Wkly, 18th February, pp. 2~23. 11. Technical Bulletins 8/73, Ethyl Corporation, Polymer Division, Baton Rouge, Louisiana, USA. 12. Ingham, P. H. J. (1979). Plast. Rubb. Int., 4(5), 211-15. 13. Whelan, A. (1981). Brit. Plast. Rubb., April, p. 25. 14. Murrey, J. L. and Dito, A. J. (1981). Plast. Techno/., 27(12),79-82. 15. Anon. (1981). Mod. Plast. Int., 11(11), 36.

CHAPTER 16

Sheet Thermoforming and Related Techniques for pvc

The late L. W. TURNER

16.1 INTRODUCTION Sheet thermoforming has been described as the one method whose development sprang uniquely from plastics technological history and was not picked up from metallurgy. This contrasts with the case of such solid-phase forming processes as forging, stamping, and certain others, originally developed for metals and only relatively recently explored in the plastics context. 1 Now super-plastic metals technology has taken up the sheet-forming idea from the plastics industry. Plastic sheet thermoforming is a secondary process, depending on, as primary process, sheet extrusion to supply its starting material. In fact it is well to bear in mind that extruded forms other than sheet (e.g. tubes), or sheet produced by methods other than extrusion (e.g. cast as is PMMA sheet, or laminated) can be thermoformed. It was in the early Celluloid or Xylonite days of the plastics industry that methods of shaping various sheet, rod or tube forms first came into industrial use. For PVC it is largely the forming of extruded and calendered sheet that must be considered. Additionally, though, there are combination processes which start by converting granules to a dimensioned pre-form of planar solid sheet geometry and then form this into a 'drawn' article (the 'Topformer,2,3 process for example). . There are three basic methods of sheet thermoforming: (i) vacuum forming; (ii) pressure forming; (iii) matched-mould forming. 743

744

L. W. Turner

In each the sheet is softened by raising its temperature to an optimum point in the 'rubbery' region to enable large deformations to be achieved. A persistent technological goal for forming operations with various sheet materials is to save energy and promote rapid-cycle operations by forming below the softening point. This is still not a widely established technique and material formulation is especially important. The methods of traditional sheet forming are quite distinct as a group from injection moulding and blow moulding, although in certain respects sometimes rival both. The advantages are that plant of lighter construction can be used, which reduces capital investment in moulds and equipment. Production can be very rapid indeed and it is comparatively simple and cheap to change designs. Apart from any financial considerations, there are a number of technical advantages. It is possible to make quite large articles cheaply and simply from thin sheets covering large areas. Design details not involving sharp changes in thickness can be reproduced and preprinting of sheet is possible; this latter has no equivalent in injection moulding and blow moulding, though in these processes in-the-mould decoration is possible,4,5 but not too common. It should not be thought that thermoforming is free from difficulties. One of the major problems is that of the inevitable development of considerable orientation as the 'rubbery' material is stretched, and cooled in the stretched form. With deep forming, in particular, orientation can be substantial and the objects produced will distort excessively above certain temperatures. Extensive thinning of the material can occur and control of distribution of thickness to maintain stiffness and strength is a persistent difficulty. Some quenching stresses may also be developed due to rapid cooling of the warm sheet. This aspect is discussed in some detail later in this section. Sheet forming was perhaps at one stage envisaged as being primarily suitable for comparatively short runs of large objects. The scope has gradually widened, however, and today fully automatic machines are available which will produce small objects, e.g. beakers, at a high rate, and such forming may be integrated with the sheet extrusion stage. Sheet-forming equipment until recently has tended to be rather poorly controlled, but this situation has now begun to change rapidly due to the employment of improved engineering and, very recently, microprocessor-based process monitoring and control. Good process control is important in order to achieve economic operations with high yields of good quality products.

16 Sheet Thermoforming and Related Techniques for

pvc

745

16.2 MATERIALS USED Almost any thermoplastic which can be made into a satisfactory sheet may be used for thermoforming provided that: (i) it does not decompose at the processing temperatures, and (ii) it has properties which make it suitable for the finished application. Materials which are used do differ, however, in their response to the process conditions. Many materials are used: commercially, the commoner ones are ABS, HIPS, CA, PVC, PE, PP, and PMMA. In general, the thermoforming performance of vinyl chloride homopolymer sheet tends to be only moderate compared to ABS and HIPS, although suitable formulation (and especially inclusion of appropriate polymeric modifiers) can make a substantial difference. Whilst a copolymer-based sheet (again without special compositional modification) will normally be much easier to form than an otherwise comparable but homopolymer-based one, the lower softening point (and related ease of distortion after processing) tends to make it less acceptable. In general, chlorinated PVC is a better thermoforming material than unmodified uPVC. It also offers better dimensional stability (at room and elevated temperatures) of the finished product (see also Section 16.7). Rigid, homopolymer-based PVC compositions incorporating polymeric modifiers (including some regular high-impact compositions) are widely used for thermoforming. It is this kind of composition that is the main material considered here. One commercial example is Kydex sheeting (Rohm and Haas-ef. Chapter 20), an acrylic-modified uPVC material specially formulated for thermoforming. Some thermoformable composite sheet materials, incorporating PVC as one of the components, may also be mentioned. These are exemplified by ABS/PVC solid-sheet laminates (represented, inter alia, by some grades of Raya/ite sheeting3-Uniroyal Plastics Division), and foam sandwich laminates with a PVC foam core and ABS sheet facings, used for the production-by thermoforming-of interior fittings for aircraft (side panels, window and door surrounds, side-liners). 6 16.3 VACUUM FORMING OF SHEET 16.3.1 Principal Methods A useful brief summary of the general types of thermoforming processes and the main advantages and limitations of their industrial

746

L. W. Turner

embodiments was published recently by O'Neill,? and one of thermoforming techniques and set-ups by Lubin et al. 8 Here forming methods relating to sheet are described in some detail since sheet is by far the commonest prepared form processed in this way; however-as has been mentioned-other forms, also made by extrusion, or cast, can be shaped by adapted thermoforming techniques. There are five basic methods of vacuum forming. These are: (a) (b) (c) (d) (e)

negative forming (or straight vacuum forming); plug-assist forming; drape forming; bubble or blister forming; and snap-back forming.

The essentials of each method are given below. (a) Negative Forming (Fig. 16.1) In this method the sheet is first fixed in a clamp frame over the mould. It is heated from above and once it has reached the required temperature is dropped onto the mould and sealed around the edges; vacuum is applied from underneath and this draws the sheet down into the shape of the female mould. When the sheet has cooled it may either be removed by hand, or ejected by air pressure from underneath. Method (b) is used for objects where drawing is greater than 2 in. in depth.

(b) Plug-assist Forming (Fig. 16.2) This technique is really an extension of method (a). A female mould is again used but the drawing process is assisted by a plug very roughly conforming to the shape of the moulding. The plug is forced down onto the sheet, thus starting the forming process which is completed by vacuum as in method (a). The plug has to be heated as well as the sheet and the temperature of both has to be controlled fairly closely. The plug also has to descend quite quickly. (c) Drape Forming (Fig. 16.3) This method is used for deeper drawing and thicker wall sections and involves the use of a male mould. The male former can either be pushed up into the heated sheet or alternatively the latter may be pulled onto it. Vacuum is applied to complete the process once a seal

16 Sheet Thermoforming and Related Techniques for

pvc

747

Stoge I

Key

Heeter B Clomping frome C Plostics moteriol o Former E Gosket F Vocuum box G Pressure line H Vocuum line A

Stoge I-Heeter ·pleced over sheet Stege 2-Vocuum opplied Stoge 3-Heotor removed from sheet. which is eliowed to cool before vocuum is releosed

Fig. 16.1 Vacuum forming-negative forming. (BX Plastics Ltd.)

has been made. As in method (b) the temperature of the mould and speed of ascent or descent is important. Speed is also essential in the rate of air evacuation. (d) Bubble Forming (Fig. 16.4) In this method the sheet is heated, clamped over the vacuum box and a specific air pressure applied to blow it into a bubble. A shaped male plug is then forced into the bubble and vacuum is applied in the usual way.

748

L. W. Turner

DT-.....IIIIIIIil_~iLE

'r-rSloge I

Stege 2

C-

c:..

Sloge 3

A B C D E F G

Heater Plug Plastics material Clamping frame Gasket Former Vacuum box

Stoge 4

Key H Air inlet I Air outlet J Vacuum line Stage 1 - Heater placed over sheet Stage 2 - Plug partly down Stage 3 - Plug fully down - no vacuum Stage 4 - Vacuum applied

Fig. 16.2 Vacuum forming-plug assisted. (BX Plastics Ltd.)

16 Sheet Thermoforming and Related Techniques for

pvc

749

Stage 2

Key A Heater B Clamping frame C

o E F

G H I

Plastics material Gaskets Former Vacuum box Air inlet Air outlet Vacuum pipe

Stage I-Heater over sheet Stage 2-Drope up, no vacuum Stage 3-Vocuum applied

Fig. 16.3 Vacuum forming-drape forming. (BX Plastics Ltd.)

Fairly deep draws with reasonably uniform wall thicknesses can be achieved by this method.

(e) Snap-back Forming This method is really the reverse of method (d). As before, the heated sheet is clamped over a vacuum box and is sucked into the box by vacuum, thus forming an 'upside down' blister or bubble. A male plug

750

L. W. Turner

A

Steg. 2

A B C D E F G

Heater Clamping frame Plastics material Gaskets Former Vacuum box Air inlet

Key H Air outlet I Vacuum pipe Stage 1 - Heater over sheet Stage 2 - Bubble blown Stage 3 - Drape up, no vacuum Stage 4 - Vacuum applied

Fig. 16.4 Vacuum forming-bubble forming. (BX Plastics Ltd.)

16 Sheet Thermoforming and Related Techniques for

pvc

751

or mould then enters this from above until it seals onto the sheet. The vacuum is then released on the box side and applied to the male plug side and the sheet 'snaps back' onto the mould.

16.3.2 Details of Methods One of the important practical points is to apply vacuum as rapidly as possible. If, however, a pump of sufficient self-capacity were used this would be an extremely costly operation. The standard method everywhere is to use an accumulator tank with a smaller pump than would otherwise be required, as the vacuum is required intermittently. In the case of some of the methods, particularly plug-assist forming, a slip ring is used for clamping. This enables the material to slip through the ring during the forming process, which has several advantages. It makes more material available, which prevents edge-tearing, buckling or undue thinning. When clamps are used there must be a good seal between the mould edge and the material. The drape and plug-assisted methods are ideally suitable for producing sections with sharply radiused corners and intricate designs. In the snap-back forming method the material has to be sucked down to a predetermined length which nowadays is controlled by a photoelectric eye. The various aspects of heat control are most important in vacuum-forming methods. Pre-heating may be effected by means of radiant heaters or convection ovens. The former are almost invariably used for thin sheets, particularly in continuous production, and a bank of heaters placed about 2 in from the sheet has been found to be very satisfactory. Convection ovens are usually used for thicker sheets in view of the poor thermal conductivity of rigid PVC. It has been generally established that the temperature at which the sheet should be heated for optimum results is that temperature which gives maximum elongation at break. Figure 16.5 illustrates the principle for a calendered high-impact PVC sheet. In this particular case the optimum temperature is about B2°C, to which temperature the sheet should therefore be uniformly heated. It may be noted that although the material does not have a peak the curve is fairly sharp, and some rigid vinyls give curves sharper than this. These would, therefore, rapidly deteriorate if heated more than a degree or two above the optimum elongation temperature. Once the material has been brought to the correct temperature it

752

L. W. Turner 700

.. o c

o

i.,. 400 c o

iii

20'Cl!:--=--~~-----:-'----""""o:---~""""-_--:-:! ioo /10 120 130 140 150

Temperature,OC Fig. 16.5 Variation of elongation at break with temperature.

should be formed as quickly as possible. A maximum forming time of 60 s should be imposed but it should preferably be below this, particularly with sheets 0·050 in. thick or less, when 30 s or shorter is preferable. Although moulds can be heated (and often are for some materials) it is not general practice with rigid PVC, the relatively low softening point being significant in this respect; whatever the choice it should be consistent. Another point of detail is a method of assisting the flow into the required positions. Interference bars or plates can be placed at strategic points to guide the material in the desired directions. Early descriptions of some methods of producing vacuum formings of uniform wall thickness have been published by Lee and Welham9 and Bretton and Welham. lo

16 Sheet Thermoforming and Related Techniques for

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753

16.3.3 The Moulds The moulds are made from many types of materials; obviously the cheapest consistent with adequate strength and inertness for service is used. Any metal except copper or brass may be used and other materials include phenolic resins and laminates, laminated wood, hardwood, or even plaster for short runs. Light alloy castings are frequently used for long runs. As already explained, both male and female moulds are used, although only one or the other is normally employed for a given method. This keeps mould costs down to a minimum. The choice depends on the method and other factors. For example, deeper forming can be obtained better over a male than a female mould. However, a female mould is best used if a design is incorporated on the outside of the moulding, as this gives much sharper impressions. It is also used for low depth of draw and where a number of cavities need to be placed close together for large-scale production. On the other hand a male mould is used if internal dimensions are of importance. Some guidance can be given regarding the dimensions of the mould compared with the depth of draw. In the case of a male mould the height can be equal to the narrowest dimension and often greater. In the case of a female mould the depth of draw should not exceed half the minimum moulding width. Wooden moulds, etc., are made by machining. Other types, which are cast from metal for long production runs, are made by the use of a wooden pattern. Here shrinkage considerations are of the utmost importance; not only does the shrinkage of the metal used for the mould itself have to be taken into account but also the shrinkage of the formed article coming off the mould. Such moulds as these sustain no damage, except by carelessness, and they are likely to last for a long time. Those made from thermosetting plastics and the like are likely to sustain much more damage even with metal spraying, metal fillers or electroforming. All the moulds must be vented to apply vacuum at various points. The closer the detail, or wherever really close conformity to the mould is required, the greater the number of vents needed. At most the holes will not exceed 0·030 in. in diameter, although half the thickness of the material may sometimes be used as a guide. On the side away from the material, the holes can be opened much wider and tapered to give more rapid evacuation. Where moulds are made by any sort of casting

754

L. W. Turner

process the vent holes can be made by inserting wires into the wooden pattern. With large male moulds, particularly where large production runs are involved, it may be necessary to have some method of cooling. The mould is then cored for water circulation. Walls can never be exactly upright; all must have a draft with a minimum of 3°, a larger one giving a better product. The radii should be as large as possible, certainly at least the thickness of the material being moulded. As with many moulding processes undercuts should preferably be avoided. However, the comparatively simple moulds allow removable parts to be used so that undercuts can be included. Inserts can easily be incorporated in the mould surface and so can embossed lettering on the surface. In the case of large runs it is clearly preferable to trim the article in the mould to avoid a subsequent deflashing process. For this purpose a cut-off is required and the mould must be metal, preferably hardened at the cutting edges.

16.3.4 Finishing The finishing and decoration of vacuum-formed parts is frequently necessary. In particular, it is necessary to remove flash, which can be done by saw, trimmers, guillotine, or male and female dies in toggle or clicking presses. Precisely which method is chosen will depend on the size of the object and the rate of production. Apart from deflashing, the cutting of various shapes in the component is frequently necessary. It is advisable to employ .jigs for such purposes to guarantee accuracy and speed up production; low- or high-speed routers can be used for the cutting. Other finishing operations are often necessary. Various parts have often to be joined together; if high-frequency welding is not possible (see Chapter 20) then parts have to be joined by adhesives. These may simply consist of polymer solutions (in, say, cyclohexanone) or some of the many proprietary adhesives available today (see Chapter 20). Some impact adhesives may well be necessary if dissimilar materials are used. Another type of finishing operation is the folding of edges, e.g. in containers which have to receive a sliding top; for this purpose it is possible to obtain heated folding or bending machines. Decoration sometimes presents problems, but there is no problem in decorating the finished object by masking and spraying or vacuum-

16 Sheet Thermoforming and Related Techniques for

pvc

755

metallising by conventional techniques. The problem arises when the sheets are decorated before forming. This means that inks and paints must be able to stretch, and stretch without much loss of colour. Far more important, however, is the problem of allowing for the distortion which takes place during forming. One way of overcoming this is to form an object in the usual way without decoration. It is then decorated or printed and heated, when, because of the residual strain or elastic memory of the material, it returns to its flat state. This gives a pattern of the distorted decoration which can be converted into the printing block.

16.4 MATCHED·MOULD AND RELATED METHODS An early review of these methods has been made by Mottram and LeverY It is, of course, possible to use female and matching male dies employing either conventional pressures or relatively low pressures of say 200-100Ibfin- 2 . The actual pressure will depend on the flow characteristics and thickness of the sheet. In some instances it is possible to use a female die only. Shallow mouldings can be made in this manner with thin sheet, examples being 0·030-in. thick material and maximum draws of about 0·250 in. Thicker sheets could be used, but this would reduce the possible deformation. A good example of the possibilities here is the production of printing plates, illustrating that extremely accurate reproductions can be made. The pressures used are only 100 Ibfin- 2 and the opposite side of the material is backed with a rubber blanket. A considerable number of small containers are made from rigid PVC by the use of matched metal dies. Such methods are usually continuous; the stock of sheet being fed from a roll, heated continuously by radiant heat, stamped and removed by means of a take-up roll. Because the dies are continually subjected to heat it is necessary to core them for water cooling. In the design of the dies for this purpose the corners and edges should be rounded if possible to assist flow. The clearances between the surfaces of the male and female dies should be at least equal to the thickness of the material, as this avoids undue wear. Finally, it is preferable to introduce cut-offs at the edges of the die to eliminate the necessity for a finishing operation. It is necessary to employ clamping frames, particularly with deep drawn objects. If the depth of draw is not great. then the edges of the

756

L. W. Turner

material may be clamped at once. If the draw is deep, some of the drawing must be allowed to occur before the edges are clamped. If this is not done, excessive thinning, undue strain or even tearing may occur. With both vacuum- and pressure-forming attention to detail is of the utmost importance. Uniformity of heating is essential and the material must reach the correct temperature just as it is removed from the oven for pressing. The forming stroke should be quite rapid, but controlled, and at the bottom of its stroke it should stay there long enough to cool the material below its heat distortion point, usually a few seconds only. Conditions will have to be varied according to the material and thickness, and should be established at the start of a production run. If the sheet used is too hot it will cause loss of embossing on an embossed sheet; if it is too cold or the clamping is insufficient then wrinkled mouldings will result. Even the cut-off must not take place too soon, since, if there is insufficient cooling, warped mouldings will result, or alternatively the edges will pull away. The cutting edges must also be kept in good condition or rough trimming will result. The dies must be properly aligned and be close together, otherwise air trapping may occur with consequent shiny spots which look most unpleasant. It is quite clear from all these methods that great care must be exercised in establishing correct details of production. It is for this reason that fully automatic production is so valuable, since conditions can be set and maintained. 16.5 TOLERANCES IN DIMENSION AND DIMENSIONAL STABILITY OF FORMED PARTS It has already been indicated that if a thermoformed article is heated it

tends to revert to a flat sheet. This is not important for many applications, where the components produced are unlikely to be subjected to elevated temperatures. However, there are certain applications where the problem can be serious. Some of these are car parts, such as fascia panels, which are liable to be affected by strong sunlight, illuminated displays and lighting fittings, etc. A careful study of the subject is therefore required. It has been established by experiment that the two factors which most closely control the degree of distortion are the softening point of the material used and the material temperature during the forming

16 Sheet Thermoforming and Related Techniques for

pvc

757

process. The amount of distortion which occurs (during a test, say, of 2 h at 70°C) depends on the amount of distortion put into the component in the first place. Thus shallow-formed articles will distort much less than deep-drawn ones. Having established the two most important factors influencing distortion the next problem is what to do about it. There is no complete answer but certain precautions can be taken. First, the higher the softening point of the material used, the less the distortion; this is a very important factor. Thus in choosing a vinyl compound, the one with the highest softening point must be used. The second important point is to use as high a forming temperature as possible, for as long as possible. This allows more random movement of molecules, i.e. relaxation to reduce excessive orientation. Unfortunately there are several limits to the temperature which can be used, not least of which is rate of production which has to be extended when temperatures are high. High temperatures will destroy surface finish and even partially destroy embossing, although in some cases embossed sheet that can resist forming temperatures can be supplied. The optimum temperature from the forming point of view is that at which the elongation at break reaches a maximum. Obviously the forming temperature cannot be allowed to exceed this. 16.6 EQUIPMENT SUPPLIERS A number of thermoforming machines are made3,12-many automatic-as well as some completely integrated, computerised thermoforming lines. * A list of some suppliers is given in Table 16.1. 16.7 MATERIALS ASSESSMENT AND DESIGN ASPECTS The term 'optimum temperature' was used earlier when the processes of sheet thermoforming were being introduced and commented upon. This relates to the fact that in uniaxial creep-type tests carried through to rupture and in similar tests carried out in a biaxial-stretching mode at different temperatures, the materials under discussion show

* e.g. the Shelley 'Linear Series' (M. L. Shelley and Partners Ltd, Huntingdon in the UK, and Deacon North America, Bristol, cr in the USA).

TABLE 16.1

Some British and European Manufacturers of Sheet Thermoforming Equipment Anchor Plastics Machinery, Weirvale Industrial Estate, Denham Way, Maple Cross, Herts, UK

Stewart Machinery Sales, The Old George, Lavendon, Olney, Bucks, UK

Leesona Plastics Machinery, Falkland Close, Coventry, CV48AU, UK

Engineering Developments (Farnborough), Belmore Road, Farnborough, Hants, GUI47NW, UK

Ridat Engineering, Fishponds Road, Wokingham, Berks, UK

Mortimer Plastics Machinery, Coronation Road, London, NWlO 7PT, UK

Ataroth Plastics Machinery, Unit A3, Wem, Salop, UK

Technoimpex, Hungarian Foreign Machine Ind. Trade Co., PO Box 183, Dorottya u. 6 H, 1390 Budapest 62, Hungary

Maschinenbau Gabler, Niels-Bohr-Ring 5a, PO Box 1690, D-2400 Lubeck 1, West Germany

Hanwood Engineering Services, Unit B3, Stafford Park 2, Telford, Salop, UK

M. L. Shelley and Partners, St. Peters Road, Huntingdon, Cambridgeshire, UK

OMV Officine Meccaniche, Veronesi Spa, Lungadige Attiraglio 34, Parona, Verona, Italy

Bone Craven Daniels, Bath Road, Stroud, Gloucestershire, GL5 3TL, UK

VTM, Ing Rudolf Wybranietz, 435 Recklinghausen, Hubertusstrasse 41, Postfach 1268, West Germany

Meaf, Groeninx van Zoelenstraat 33 (industrieterrein) , Yerseke 3622, Netherlands

16 Sheet Thermoforming and Related Techniques for pvc

759

maximum elongation (before rupture) within particular 'optimum' temperature regions depending on the material formulation. A broader peak is preferable to a narrow one as the latter obviously allows less processing tolerance. Various relatively simple techniques can be used to compare materials and to a degree assess forming characteristics. In Ref. 13 rapid creep tests are described comparing the forming behaviour of two PVC sheet formulations. The tests consist of applying a predetermined dead load to strips of sheet material at different temperatures, the load being such that extension to high strains in the range 4-5 or up to break-point can be achieved in about 1·4 s. As described the creep curve is observed optically, but simpler means of recording deformation can be employed to provide a useful evaluatory procedure. In such a test a peak elongation at a certain temperature can be observed (Fig. 16.6). The manner in which the data curve falls away above and below this peak (operating region) gives useful guidance as to the behaviour of the test material. The peak position on the temperature scale, its sharpness and the shape of the overall curve can be correlated with actual forming experiments. A similar rupture test has been described,14 demonstrating the effect of temperature on strain development. In this it is shown that rapid uniaxial creep can be factorised into stress and time factors, and a fracture envelope can be readily obtained. Correlation between such creep data and actual thermoforming can usefully be made as in Refs 13 and 15. For this purpose the draw ratio becomes the significant process variable to observe at various temperatures to correlate with laboratory creep data. 16.7.1 Effect on Quality of Draw Ratio and Temperature

While it can be seen that acceptable forming can be conducted over a reasonable range of temperatures, other factors can obtrude (process rate, energy consumed, handling difficulties, discoloration and embritdement) so that, in practice, a processor would aim to conduct his operation close to the shaded area in Fig. 16.7 which typifies the product quality/forming temperature relationship. Overall the most significant comparison between materials illustrated in this sketch is the heavy-line boundary between good and bad forming; this can closely be observed from uniaxial creep data, with due regard to the differences between stretching one way and two.

L. W. Turner

760

optimum Strain l~v~1

c .~

[Crl'Kld

Ar~a of ruptur~

tim~

iii Incr~sing

straining load

rat~ und~r fix~d T~mperatur~

Fig. 16.6 Effect of temperature on rapid strain (based on Ref. 13).

Biaxial creep data, such as might be obtained by blowing up a clamped disc of sheet material, can be employed for similar correlations, but uniaxial tests have the advantage of being the simplest conceivable test pattern. It must be added that care should be taken if the intention is to use creep data to indicate the effect of the pressure employed in forming.

(Cold) rupture

Holes

o

~

~ o

Discoloration degradation Details not reproduced

Temperature

Fig. 16.7 Quality aspects of correlation between draw ratio and temperature (based on Ref. 15).

16 Sheet Thermoforming and Related Techniques for

pvc

761

16.7.2 Thermoformability of CPVC

The extensibility, uniaxial and biaxial, of chlorinated PVC (a simple composition without polymeric modifiers) is good over a relatively wide range of temperatures above the Tg • This behaviour, and the associated improvement in thermoformability over uPVC, must be largely due to the reduction of inter-chain attraction brought about by the presence of additional chlorine atoms in CPVC (ct. Chapter 1, Section 1.6). These points are supported by the results of an experimental study by De Vries and Bonnebat,16 which also demonstrated the molecular orientation imparted by stretching, and the considerable improvements in some properties, notably impact resistance and barrier effects (i.e. permeability reduction), in biaxially stretched CPVC sheet. Note: Similar effects of biaxial orientation have been reported by Brady17 for uPVC, where biaxial stretching under optimum conditions to about 2 x 2 final extension could substantially improve the impact resistance in the absence of polymeric modifiers.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Kulkarni, K. M. (1979). Polym. Engng. Sci., 19(7), 474--81. 'Topformer', Literature from Bone Craven Daniels, Stroud, Glos, UK. Anon. (1979). Plast. Rubb. Wkly, 23rd November, pp. 17-26. Titow, W. V. and Turner, L. W. (1966). 21st ANTEC SPE Proceedings, 'Plastics in Europe', Paper No.2, pp. 1-5. Titow, W. V. (1966). Plast. Technol., U(8), 3~0. Anon. (1979). Mod. Plast. Int., 9(1), 35. O'Neill, W. A. (1980). Plast. Rubb. Int., 5(5), 185-7. Lubin, G., Mele, J. J. and Sheehan, M. J. (1978). 36th ANTEC SPE Proceedings, pp. 50-6. Lee, D. J. A. and Welham, F. A. (1959). Brit. Plast., 32(6), 265. Bretton, R. H. and Welham, F. A. (1961). Ibid, 34(5), 244. Mottram, S. and Lever, D. A. (1957). The Industrial Chemist, May. Anon. (1981). Eur. Plast. News, 8(9), 66--74. Harris, B. L. and Bruins, P. F. (1973). In Basic Principles of Thermoforming, (Ed. P. F. Bruins), Gordon and Breach, New York, p. 81. Turner, L. W. (1958). Society of Chemical Industry, Monograph No. 17.

762

L. W. Turner

15. Malpass, V. E. and White, C. H. (1973). In Basic Principles of Thermoforming, (Ed. P. F. Bruins), Gordon and Breach, New York, p. 103. 16. De Vries, A. J. and Bonnebat, C. (1976). Polym. Engng Sci., 16(2), 93-100. 17. Brady, T. E. (1976). Ibid, 16(9),638-44.

CHAPTER 17

Blow Moulding of pvc W. V.

TITOW

17.1 BASIC FEATURES AND HISTORICAL DEVELOPMENT OF BLOW MOULDING

The general principle of blow moulding may be stated as follows: a gas under pressure is introduced into the interior of an envelope of heat-softened material, which is thereby expanded into conformity with the cavity of a containing mould, to form a hollow article when solidified on cooling. In the blow moulding of plastics the envelope is normally tubular. It is self-evident that to be useful for blow moulding, a material must-when suitably softened by heating-have the right rheological properties and sufficient cohesion at the processing temperature. It is equally obvious that if the moulding is to be fit for use the material must also possess the appropriate service properties. Other characteristics are also needed, e.g. sufficient thermal stability under processing conditions. The thermoplastics first available on a large scale (cellulose acetate, polystyrene) did not adequately meet the combination of requirements relevant in the production and use of blow moulded containers (a natural application for blow mouldings, first embodied in glass bottles). Moreover, the earliest attempts at blow moulding these materials, which were closely modelled on the blowing of glass bottles (from a 'gob' of hot melt), were not very successful: as frequently happens, direct transplantation of a technique from one technology into another did not provide the best route. A combination of two factors ultimately made possible, and initiated, the development of blow moulding of plastics and its rapid growth. The factors were: 763

764

W. V. Titow

(i) the advent of polyethylene-a thermoplastic with processing characteristics well suited to blow moulding, and end-use properties acceptable in containers for many purposes; (ii) the introduction of the extrusion-blow method, pioneered after the Second World War by the Plax Corporation and others in the USA, l and by Kautex in Europe. * Blow-moulded uPVC bottles (produced by the extrusion-based process) first made their commercial-scale appearance around 1960. In the UK, for example, a prestigious chain store began to market fruit squashes in such containers in 1962. 2 As practised today, all the numerous variants of blow moulding include the following essential elements: The production of a tube of the thermoplastic material used. If made by extrusion, the tube (which is initially open-ended) is called a parison. If injection or dip moulded (in which case it is always produced with one end closed) the tube is correctly termed a preform. (ii) Expansion of the hot tube (at this stage always sealed at one end; sometimes at both ends) with compressed air inside a mould. (iii) Cooling the article so formed and removal from the mould. (i)

Individual processes differ in the ways in which these basic operations are performed, and in the number and nature of associated process steps (see Section 17.2). Some methods in which no parison or preform is used were explored early in the development of blow moulding, but none has attained industrial importance. In one example,3 two hot thermoplastic sheets are clamped in a mould and thereby edge-sealed in the outline of the shape required in the ultimate moulding. The envelope so formed is expanded in the mould with compressed air, and the resulting moulding cooled in the mould, removed and trimmed. uPVC is one of the group of plastics important as blow-moulding materials. The other members of this group are polyethylene * Until the appearance of Kautex machines in the late 1950s blow-moulding equipment in industrial use was custom-made for patented versions of the process worked by the patent holders and their licensees. For example, in the UK the patented Plax process (British Patent No. 516262) was for many years operated, under an exclusive licence, by British Xylonite.

17 Blow Moulding of pvc

765

(especially the high density polymer), polyethylene terephthalate, certain modified acrylonitrile polymers, polypropylene, and polycarbonate. Some of these-notably HDPE and PET-eompete with uPVC in many applications (see Section 17.4.1). A few examples of their non-competitive uses are: HDPE petrol tanks for certain makes of motor car, large cans and drums; PP header tanks for truck radiators; PC baby-feed bottles. Blow-moulded containers with composite walls made up of layers of different materials are produced from coextruded parisons or-more recently-eo-injection-moulded preforms. This is done to secure sets of properties not obtainable with a single material: often the combination of properties achieved in this way is low permeability with good mechanical strength and stiffness. Typical examples of such composites are HDPE with polyvinyl alcohol or with an acrylonitrile polymer (in both cases an intermediate adhesive layer is normally also present). PVC is not used as a component of such products.

17.2 BLOW-MOULDING PROCESSES AND THEIR APPLICATION TO PVC 17.2.1 General Characterisation and Main Features of Process and Systems A blow-moulding process can be characterised by reference to three features: the method of production of the first tubular precursor of the ultimate moulding, the presence or absence of a stretching step in the operation(s) whereby the tube is transformed into the moulding, and the nature of the processing and equipment arrangements. A broad classification on the basis of the first two features may be illustrated by the simple schematic flow diagrams of Fig. 17.1. It is this kind of classification that is implicit in such widely used terms as 'extrusion blow moulding', 'injection stretch-blow moulding', 'dip blow moulding', and the like. The processing and equipment arrangements comprise a number of elements which may be combined in various ways in a particular blow-moulding set-up. The nature of these elements and the way in which they are combined may collectively be regarded as the third main distinguishing feature of a blow-moulding process (cf. (c) below).

PLASTICS

extrusion

Fig. 17.1

FEEDSTOCK

PLASTICS

FEEDSTOCK

PLASTICS

by piston push on melt

in cavity: ) complete silJlpmg of pretonn round mandrel

dip mandrel into melt

mould, stretch and blow, 0001

)

MOULDED ARTICLE

PREFORM (CLOSE-ENDED TUBEWrrn FULLY FORMED NECK)

MOULDED (ORIENTED) ARTICLE

blow,oool

transfer to second (article) mould,

transfer to second (article)

HOT MELT IN CAVrrY

I

PREFORM (CLOSE-ENDED TUBE WITH FULLY FORMED NECK)

MOULDED ARTICLE

(article) mould,) blOW, 0001

transfer to second

MOULDED ARTICLE

PREFORM transfer to second (CLOSE-ENDED (article) mould, ) MOULDED TUBE WITH (ORIENTED) stretch and blow, FULLY FORMED ARTICLE 0001 NECK)

enclose in mould. blow, 0001

enclose in first (pretonn) mould, blow (possibly 0001)

OPEN-ENDED TUBE (PARISON)

DIP (DISPLACEMEN1) BLOW MOULDING

INJECTION STRETCH-BLOW MOULDING

INJECTION BLOW MOULDING

EXTRUSION STRETCH-BLOW MOULDING

EXTRUSION BLOW MOULDING

Characteristic operational elements in the main types of blow moulding process. Schematic outline.

or injection

~

moulding

injection

FEEDSTOCK - - +

~

o

~

:-::: ::l

~

17 Blow Moulding of PVC

767

(a) Main Characteristics of Extrusion, Injection, and Dip Blow Moulding EXTRUSION BLOW MOULDING

The typical sequence of operations in simple extrusion blow moulding of a bottle is shown in Fig. 17.2A. As can be seen, a tube is extruded, and immediately enclosed-while still hot-in a split mould. In the action of closing upon it, the mould seals one end of the tube, leaving outside a 'tail' of waste material (and in some cases also pinched-off pieces or flash in other areas). Note: In the arrangement shown in Fig. 17.2A the tube is closed at the bottom, the neck being formed at the top, and the bottle subsequently blown 'right-side-up'. The alternative arrangement is to have the bottom end of the parison tube descending over a blowing mandrel: the tube is then closed at the top and the bottle blown in the 'upside-down' position. See also Section (c) below.

The parison tube is severed (before, during, or after the closing of the mould) and expanded inside the mould by air pressure; the resulting moulding is cooled in the mould and ejected. In some versions of the process the flash (always formed in extrusion blow moulding) is trimmed off in the mould; in others trimming is a separate, post-moulding operation. In some cases such operations as printing or labelling, filling, and closure application, are carried out in-line. In a typical extrusion stretch-blow moulding process, schematically illustrated in Figs 17.2B and 17.3, the extruded parison is first blow-moulded in one mould into a complete preform (undersize in relation to the ultimate moulding), and this is then stretched and blown to final shape in a second mould. The stretching (longitudinal extension) of the preform is commonly effected mechanically, by an extending telescopic pin, and the radial extension by the final blow (cf. Fig. 17.3). The object of stretch-blowing is to produce biaxial molecular orientation in the moulding-the effects and advantages of this are discussed in Section (b) below. For the best results the orientation should be imparted at a temperature somewhat below the extrusion temperature. In most industrial equipment provision is made for temperature conditioning of the preform during and/or after its cooling in the course of formation in the pre-blowing mould: the

W. V. Titow

768 A - Feed·hopper B - Extruder C - ElI1ru ,on head

D Mould buring d'stributor E - Blower head.

Stege I - EXTRUSION The parilon is ell1ruded between two half·mouldl.

Stage 3 • BLOW·MOULDING The blown perison upands end Ilanans out on tha mould ~vity wall•.

Stege 2 • CLOSING·IN The parison Is clamped into the mould.

Stege 4 • EJECTION Once cooled. tha bonia i. immediately ajacted from the mould.

Fig. 17.2A Extrusion blow moulding of PVC: schematic representation. Direct blow moulding: a machine, and operational sequence. (SideI equipment c. 1970: Courtesy of Sidel through their UK agents Engelmann and Buckham Ltd.)

17 Blow Moulding of pvc

769

Fig. 17.2B A modern Sidel continuous-extrusion machine for stretch-blow moulding of bottles. 1, Parison; 2, preform mould; 3, temperature conditioning of preforms; 4, conveyor; 5, bottle mould; 6, demoulding.

conditioning may take place at a separate station (as, for example, on the Sidel MSF machines 4). Extrusion blow moulding was chronologically the first to come into industrial use: stretch-blowing is a relatively recent development in this method as well as in injection blow moulding. Apart from this refinement, which is now available as an integral part of most leading manufacturers' equipment (in some cases also as a retrofit modification-e.g. for the older versions of Kautex KEB machines 4 ), a modern extrusion blow-moulding set-up usually includes parison programming. This term is widely used for programmable adjustment of the wall thickness of a parison in a number of zones along its length. However, in its broadest sense, the expression may also be considered to include certain other ways of parison modification and control now available. In variable-thickness dies, both the central mandrel ('pin') and the die ring interior are tapered, so that a relatively slight vertical movement of the former (actuated by a suitable control mechanism) alters significantly the annular gap between them at the die face. The number of points at which parison thickness can be varied depends on the particular programming system used. Many are available, from various manufacturers: some of these systems are incorporated, as standard components, in particular makes of blow-moulding equipment. The

W. V. Titow

770

1

2

5

3

6

Fig. 17.3 Operational sequence in a typical extrusion stretch-blow moulding process. Schematic representation. 1, Extrusion of parison; 2, blow moulding of preform; 3, transfer of preform to article mould; 4, mechanical stretching (by extensible pin) and blowing of preform; 5, completion of formation of article; 6, removal of article from mould.

17 Blow Moulding of pvc

771

systems range from relatively inexpensive, simple thickness programmers offering control at a limited number of points, adequate for long-run production of simple blow mouldings (e.g. the Moog* system for to-point thickness programming), to sophisticated computerised, fully automatic systems for control at up to 50 points (e.g. the 49-point parison programming facility of the MACa Vlt integrated microprocessor control system for extrusion blow-moulding equipment5). Other examples are the 30-point parison programmer used on some Bekum+ machines,6 and the Hunkar§ programmers (some with 32 set points and higher). In the modern thickness programming systems operated by microprocessors, the required thickness values at the number of points available are pre-set on the control unit, which then continuously automatically computes and executes the necessary adjustments of the movable die elements. Programming capabilities other than parison thickness control offered by some of the modern systems include parison length control, parison stretch compensation, parison ovalisation, and what is sometimes called 'deformable die control'-provision for programming (through automatic control of mutual movement of the die mandrel and bushing) for minimising the amount of material in the pinch-off area(s) of the moulding. It is largely self-evident that the main direct object of parison programming is optimisation of the wall thickness and thickness distribution in the finished moulding. The advantages this brings are two-fold: better structure (with the attendant improvement in strength properties and resistance to damage), and cost savings. The savings are in two areas-reduction of the amount of material, and more effective processing (especially easier and quicker cooling of mouldings which contain no unnecessarily thick areas, and generally only the necessary minimum amount of material). In an informative article (published in 1976), Hunkar7 gives, inter alia, the following figures (based on p~rformance studies of over 800 blow-moulding machines) for the economic gains resulting from parison thickness programming in the * Moog Inc., Electronics and Systems Div., USA, and Moog GmbH, West Germany: the group also supplies more advanced systems with full microprocessor control, e.g. the Moog 40-point analog parison programmer. t Barber-Colman Co. Industrial Instruments Div., Rockford, Ill., USA. :\: Bekum Maschinenfabriken GmbH, Berlin, West Germany. § Hunkar Laboratories Inc., Cincinnati, OH, USA.

772

W. V. Titow

production of PVC bottles: Nominal size Average weight (g) Material savings (%) Production increase (%)

1 quart 54·6 15·8 13·3

1 litre 43·5 11·5

9·6

16-220z 22·3 19·3 29·8

Another source! quotes estimated savings of 20-30% in part weight and cycle time with electronic parison-programming systems. The above advantages of parison programming must be considered in relation to the greater equipment cost and die complexity (undesirable in principle with a heat-sensitive material like PVC). Thus, whilst parison programmers are operated, and useful, with PVC, possibilities sometimes exist-and should not be overlooked-of saving material and securing good thickness distribution simply by suitable design of the moulding. INJECTION BLOW MOULDING

In this type of process a preform is injection-moulded around a metal mandrel (core rod) in a closed mould. It is then transferred to a second mould where it is blown to final shape, cooled and removed. In some versions of the process the sequence is continuous, the temperature of the preform being maintained for the blowing step which follows directly after moulding. In certain others the preforms are cooled, and may be stored, to be reheated and blown in a separate operation. In either of these two general variants of the process the hot preform may be stretched in the blowing mould to produce orientation in the ultimate article in essentially the same way as in extrusion stretch-blow moulding. In comparison with extrusion blow moulding, the advantages of the injection-based process may be summarised as follows. Exact, pre-designed shaping of the preform with the attendant improvement in product quality and material and processing cost savings, analogous to the advantages of accurate parison programming in extrusion blow moulding; lack of waste; virtually constant weight of mouldings (but less easy to alter than in the extrusion-based process); a moulded neck finish in bottles, giving accurate dimensions (and particularly useful for crown closures); better quality finish, with no nip closure line (pinch weld). The main disadvantages are: ratio of neck diameter of bottles to the length limited in practice to about 1 : 12 (due to core rod deflection

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problems); containers with handles not readily produced; highly oval cross-sectional shapes not as successful as with extrusion blow moulding; normally higher tooling costs. Injection blow moulding originated roughly at the same time as extrusion blow moulding, and some containers were being made-in polystyrene and polyethylene-by early versions of this process (involving manual transfer of a single preform on its core from the injection to the blowing mould) from about 1948 onwards. 8 However, large-scale production of injection blow-moulded PVC containers came considerably later than industrial extrusion blow moulding of this material, largely because of heat stability problems. DIP BLOW MOULDING

Commercial equipment for this process (also called 'displacement blow moulding') first appeared in the 1970s. The operational features of the available machines differ in some respects, but the general essential elements of the method are common to all. These may be outlined as follows. A predetermined quantity of hot plastics melt is introduced into a heated cavity, kept at a controlled, optimum temperature. This may be done by horizontal extrusion (as in the original SchloemannSiemag equipment9 ), or vertical (downward) extrusion or injection (as in the early Saum Systems design lO and its adapted version represented by the Leesona displacement injection blow-moulding machine ll ), but in all cases the cavity is not closed, so that the melt enters at relatively low pressure. A metal mandrel (core rod), kept at the required temperature by circulating heat-exchange liquid, is pushed---eentrally-into the melt in the cavity: this is done either by lowering the mandrel or by raising the cavity. As it moves into the cavity, the mandrel displaces the melt upwards, shaping it into an annulus defined by the mandrel body and the cavity walls. The annulus being moulded in this way is the body of the ultimate preform. The shaping of the latter is finally completed-when full mandrel penetration has been reached-by the upward movement of a piston positioned at the bottom of the cavity, which ensures that the melt fills all available space, thereby positively moulding the top (neck portion) of the preform (inside a split mould, sometimes referred to as 'thread mould', enclosing the top of the mandrel) and finalising the material distribution. The mandrel carrying the preform just produced is withdrawn (or the cavity lowered away from it), and then enclosed in the article

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(blowing) mould. Air at normal blowing pressure is admitted through a channel in the mandrel, blow-moulding the body of the preform to the shape of the cavity. The resulting article is cooled and ejected when the blowing mould and the neck mould round the top of the mandrel have opened. The advantages claimed for dip blow moulding are: moderate cost of machine and moulds (because they do not have to withstand high pressures during the moulding of the preform); precision moulding of the neck; no nip closure or gate marks on the article (which are characteristic of, respectively, extrusion and injection blow-moulded containers); virtual absence of scrap; ease of production of wide necks in containers; close control over wall thickness by virtue of the method of producing the preform and the fact that the preform diameter can be close to that of the ultimate moulding. (b) The Role and Effects of Stretching in Stretch-blow Moulding The combination of longitudinal stretching with transverse expansion effected in stretch-blow moulding imparts to the product a considerable degree of biaxial orientation. This improves several of the properties particularly important in containers, which constitute the vast majority of commercially produced PVC blow mouldings. The properties concerned are: impact resistance (as measured in drop impact tests, this property can be better by factors of 2-4 in stretch-blow-moulded PVC bottles as compared with similar bottles produced by simple blowing); bursting pressure; resistance to compression and to deformation by top loading (important in stacking); rigidity; clarity of transparent containers; and permeability (which is reduced by biaxial orientation). Some properties of biaxially oriented PVC are discussed, in relation to those of CPVC, in a paper by De Vries and Bonnebat. 12 Brady's study of the effect of biaxial stretching upon the mechanical properties of uPVC13 (O·015-in thick calendered sheet) indicated, inter alia, that optimum property improvements are obtainable at a stretch factor of x2 in each direction, and that lOO°C was the optimum stretching temperature. The improvements in properties obtained by stretch-blowing make possible significant savings in the amount of material used (because container wall thickness can be reduced without sacrificing performance) and in processing costs (mainly because thinner walls make for faster cooling). Furthermore, the material cost can be lowered in many cases by 'reducing or eliminating impact modifiers. Savings-in

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comparison with otherwise similar but unstretched blow mouldingsquoted as typical include, for example, weight reduction of 20-25% on 1·5litre mineral water bottles produced in rigid PVC on a Bekum BMO-4D machine,14 an average material cost saving of about 10% with typical bottle formulations through elimination of impact modifiers,15 and possible overall production cost savings of up to 20%, despite the somewhat higher equipment cost of stretch-blow moulding. 16 At the same time it should not be overlooked that stretch-blow moulding is applicable primarily to simple, symmetrical shapes (e.g. not to containers with handles), and that stretch-blown PVC bottles can still be no cheaper than bottles conventionally blown in, say, high-density polyethylene which-despite their inferiority in some properties-may be acceptable for many applications where clarity is not a requirement.

(c) Processing and Equipment Arrangements These arrangements comprise a number of elements which may be combined in various ways in a particular blow-moulding set-up. The elements and their particular combination collectively constitute another general distinguishing feature of a blow-moulding process. SINGLE- AND TWO-STAGE OPERATIONS

A blow-moulding process-whether based on extrusion or Injection moulding-may be of the single-stage or two-stage type. That is, the complete production may be carried out in the same machine, or on two separate machines: in the latter case it is normally the blowing (of a separately made parison or preform) that constitutes the second stage. Industrial dip-moulding processes are normally all single-stage. The version of simple extrusion blow moulding where the parison is blown directly under the extruder head is an example of a single-stage, single-station process. It may be noted that in this kind of operation extrusion must be intermittent, to allow for completion of blowing, cooling and ejection of the moulding, before the mould receives the next parison. In the widely used multi-station extrusion blow-moulding systems, a mould receives the parison (which is severed at the appropriate point in the sequence) and is moved out of the way while another is presented to the extruder die. Here extrusion can be continuous, as parison blowing and subsequent cooling of the resulting moulding in a

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direct blowing process, or-in a stretch-blowing process-the blowing of a preform, its transfer to a blowing mould, and the stretch-blowing, cooling and ejection of the final article, take place at stations away from the extruder head. Continuous extrusion systems are preferable for PVC (and other heat-sensitive materials) because they do not entail stationary residence of the hot melt in the extruder, whilst optimum melt temperature and uniform temperature distribution are easier to maintain under constant, controlled flow conditions, so that the risk of overheating is reduced. The simplest multi-station system consists of two reciprocating moulds which are presented alternately to the extruder die. More complex systems employ a number of moulds mounted on a rotary support which may revolve in a horizontal or vertical plane. Blow moulding on a multi-station system (even one with many stations and process steps) will still be a single-stage process if complete production takes place on the same machine, in a continuous sequence of operations. If, however, the sequence is interrupted and the moulding finished on separate equipment, then the process becomes a two-stage one. Note: In the context of blow-moulding processes, the terms 'step' and 'stage' are sometimes used as if they were synonymous. However, the usage adopted in this section is the most common; it is also logical and avoids ambiguity if consistently followed.

Examples of a two-stage process are the Marrick process and the Corpoplast process. The former, developed in the 1960s,17 is now mainly of historical interest. In its first stage PVC tubes were extruded, cooled, and cut into parison lengths. The prefabricated parisons could then be stored and transported to different locations as required. For blow moulding, which took place on separate, special equipment, the parisons were heated (on mandrels) to the required processing temperature. The heating was carried out in ovens: as can be seen, very close temperature control was essential for successful operation, as well as accurate allowance for dimensional changes on heating. The plant and 'know-how' for this stage of the process were provided, under licence, by the Marrick Manufacturing Company Ltd (UK), from whom parison tubes were also available. The advantages claimed for the process included a low scrap percentage, and a potentially high output (for the 1960s) made possible by the use of multi-cavity blowing units: in a single-stage process these would have to be fed by

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multi-head extrusion with its attendant increased risk of heat degradation of the material. Thus the heat stability problem was eased whilst the production rate at the moulding stage was not limited by the rate of extrusion. However, the Marrick process, apart from lacking the optimisation of wall thickness and economy of material use available through parison programming in modern extrusion blowmoulding systems, was also generally less competitive in present-day terms. None the less, it had played its part in the industrial production of PVC bottles. It has also paved the way for the Corpoplast process in which similar ideas have been given a sophisticated, modern format. The Corpoplast system for the production of stretch-blown bottles in PVC, polyacrylonitrile, and polyethylene terephthalate, originated in West Germany. * In the first stage, preforms are produced either by injection moulding, or from lengths of extruded tube (by heating the ends and moulding a neck on one side and a rounded, closed bottom on the other: 18 machines are available for either kind of operation. The preforms are cooled for storage or transport. In the second stage each preform is heated (on a mandrel telescopically extensible and channelled for subsequent stretching and blowing operations) by IR radiation. The heaters are specially designed for quick, effective heating of the body of the preform to the optimum temperature for orientation by stretch-blowing: one of the operational features is the matching of the peak emission wavelengths to those at which the plastics material absorbs most strongly.t The heating is uniform, as it does not depend on conduction through the material. During the heating step the pre-moulded neck is screened from the radiation. The heated preform is stretch-blown in the way already mentioned. Preform production and blowing may also be run in direct sequence. The advantages claimed for the Corpoplast process are: high output rates; versatility of equipment, permitting different arrangements with good control over process variables; relative ease of setting up and running bottle production at the filling plant (with preforms produced at a different site transported more conveniently and cheaply than

* Initially marketed by Gildermeister Corpoplast GmbH: now KruppCorpoplast Maschinenbau GmbH, Hamburg. UK agent: Ritter Plastics Machinery, Bracknell, Berks. In the USA exclusive rights held by Owens Illinois, Toledo, Ohio. t An early discussion of this way of increasing the efficiency of radiant heating of polymeric materials was published by Grant and Foster. 19

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finished bottles); waste re-processing minimised (with injectionmoulded preforms); advantages associated with stretch-blowing (improved container properties, material and process economies). However, the costs of equipment and tooling are relatively high, and licence fees and royalties are a factor. Apart from the version of the Corpoplast process in which the preforms are injection-moulded, injection blow moulding as commonly practised is a single-stage operation. It must also, of necessity, involve at least two stations, because the preform has to be transferred to another mould for blowing. Indeed, the first machine designs to come into industrial use operated on a two-station system. 1 Currently the most popular configuration is a rotary three- or four-station arrangement: in a typical version, core rods are mounted on a turret which indexes to the stations in turn. With a three-station turret these will be the injection station, the blowing station, and the ejection station. A four-station system may be set up to operate in various ways; some of the common ones are given below. Station 4 Station 1 Station 2 Station 3 Injection ~ Pre-blowing ~ Final blowing ------+) Stripping off the core and ejection Injection ~ Temperature----+ Stretch-blowing ----~ Stripping off the conditioning * core and ejection ) Surface decoration Stripping off the Injection ~ Blowing· (e.g. printing) core and ejection

Dip blow-moulding equipment currently available is single-stage and typically one- or two-station,1° although the process involves several steps (see (a) above). BLOWING ARRANGEMENTS

In extrusion blow moulding of containers, such as bottles, the parison is most often blown by means of a blowing mandrel (blowing 'pin') through the neck opening, with the bottle formed either in the upright position ('top blow') or upside-down in the mould ('bottom blow'). A

* This can be useful with PVC (ct. Fig. 17.2B), but not absolutely necessary because PVC can be cooled directly to optimum orientation temperature, unlike PET which needs a separate 'tempering' treatment of the preform (normally at a separate station) for greatest property improvement on stretching, or PP which must be conditioned for stretching within a narrow temperature range.

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hollow needle may also be used, being introduced-at any convenient point-into a completely sealed parison through a suitable opening in the mould. Needle blowing is useful with some container designs, e.g. ones with neck openings too small to admit a regular blowing pin, or where a portion is trepanned out after moulding (e.g. a domesometimes called a 'lost blowhead'-moulded, for operational convenience, over a very wide opening) in which case the blowing needle is introduced through that portion. As with bottom blow, needle blowing can start as the mould closes. The bottom-blow arrangement is the simplest mechanically. The open lower end of the parison descends over a blowing pin (often called a spigot in this context). The mould closes around the parison, its neck part (at the bottom) clamping on the lowest portion of the tube which encloses the pin, and the top pinch-sealing the opposite end. Blowing is commenced immediately, sometimes even before the mould has completely closed (this can counteract any tendency for parison sag, and help in the formation of a strong nip weld). The general advantages of bottom blowing include fast operation, positive forming of the outside of the neck (by the neck part of the mould cavity against the blow pin), smooth internal neck surface, and considerable latitude in the relationships between the thicknesses and diameters of neck and body. The main limitations are that the internal diameter of the parison must be large enough to fit, with reasonable clearance, over the blowing pin, the external diameter must be greater than the internal diameter of the neck portion of the mould, and that-as a result-there is always flash on the sides and upper part of the neck. This has to be removed, and the finish in this area is never completely 'clean'. In a top-blowing arrangement the parison must be severed from the parent extrudate and clamped in a mould, completely closed, before the blowing pin is brought into action. Typically (but not invariably) the cutting follows the closing of the mould. If extrusion is continuous, the mould enclosing the parison is moved away from the extruder die. A blowing pin is introduced into the open top of the parison: the pin is usually shaped and dimensioned to participate in forming the neck of the bottle. Blowing, cooling and demoulding then proceed in the usual way (often at separate, consecutive stations). It is an advantage of top-blow arrangements that the bottle neck can be free from flash (if the parison diameter is not greater than that of the neck part of the mould cavity). However, the time lapse between parison formation

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and commencement of blowing increases the length of the cycle: the possibilities are also increased (although these are adequately counteracted in good modern equipment) of parison sag and undesirable surface cooling of material in the neck region. In injection and dip blow moulding, blowing takes place through the core rod on which the preform is moulded. The arrangement thus broadly corresponds (but is not strictly analogous) to top blowing in extrusion blow moulding. The blowing air pressures used in all blow-moulding processes are typically within the general range of 0·5-1·0 MPa. OTHER FEATURES

Mould venting: This is necessary to avoid impairment of surface finish of the moulding by air trapped in the mould (in extreme cases a pronounced 'orange peel' effect may be caused). Good surface contact between the moulding and the cavity-without intervening air layers-is also necessary for effective cooling. Vent slits are usually provided along the mould parting line, and-if required-at other points, according to the shape and design features of the article. Slight plate-out (which may not be otherwise discernible) may collect in the vents with some formulations. Such blockage should be removed with a suitable solvent (e.g. a chlorinated hydrocarbon). Multicavity moulds: These are used on some versions of blowmoulding equipment. In extrusion blow moulding they are operated in conjunction with multiple heads on the extruder. In injection blow moulding the number of cavities that can be operated has been increasing over the years: equipment with as many as 8, 12 and 14 cavity moulds is in current industrial use. 8 ,20 Waste material removal: In extrusion blow moulding some waste is always created as the mould grips the parison. This usually consists of the 'tail' beyond the bottom nip closure, often with flash in the neck region and-in containers with handles-handle pinch-off waste and flash. The waste has to be removed. The removal operations are automated to a large extent in modern equipment. 'Top and tail' waste is trimmed in the mould in several systems: in some the removal is a secondary operation carried out in-line-this is often the case with handle pinch-off waste. In some equipment for stretch-blow moulding

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of PVC the tail waste is torn off the preforms clamped in their moulds, by sliding grippers. Examples of systems with automatic waste trimming are the Hoover* Uniloy 300 CE line, the Automat Speed 3000 D and Maximat machines, and the Krupp-Kautex* KEB machine range. Part removal (take-off) systems: These too are largely automated, especially in modern extrusion blow-moulding equipment. Typically, mouldings trimmed of waste material (which may be automatically routed to a granulator and from there to the extruder) are positively deposited, in the required orientation, on a conveyor on which they proceed to leak-testing, decorating or labelling, and packing. (d) Cooling Methods The blown article must be cooled in the mould before ejection. Cooling is effected by circulation of a heat-exchange liquid (commonly water and glycol) through drilled channels or milled labyrinth cavities in the mould block. Blowing pins in extrusion blow moulding, and the core rods in injection and dip blow moulding, are also channelled for coolant flow. Note: Preform temperatures prior to stretch-blowing can be controlled via the mould cooling system. As in any moulding process, rapid cooling of the blow-moulded article is desirabie for maximum output. However, in the interest of product quality, the cooling should also be uniform and not so fast as to set up stresses in the moulding. The relatively low wall thickness of blow mouldings is conducive to quick cooling, whilst stresses arise less readily in thin than in thick sections. The rate of cooling of uPVC is faster than that of many other thermoplastics (including PET and PP which, as materials of stretch-blown bottles, compete with uPVC in some applications). Note: The principal factor in this is uPVC's comparatively high thermal diffusivity, which is the parameter governing the rate of transfer of heat through a plastics material in transient (i.e. non-steady) flow conditions, such as obtain during the cooling *Hoover Universal, Plastics Machinery Div., Manchester, Michigan, USA. t Automa SpA, Bologna, Italy. t Krupp-Kautex Maschinenbau GmbH, Bonn, West Germany.

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of a moulding when the temperature is continually falling. Heat flow in steady-state conditions is governed by thermal conductivity. The thermal diffusivity (D) of a plastic can be calculated from the relationship: D

= (thermal conductivity)/(specific heat

x density)

The following calculated average values of D (in cm2 S-1) are fairly representative for the temperature range over which a moulding would be cooling. uPVC, 1·2 x 10- 3 ; PP (homopolymer), 0·6 x 10- 3 ; PET, 0·8 x 10- 3 . Another practical advantage of uPVC over the polyolefins and PET is its substantially lower mould shrinkage. The typical ranges are: 0·1--0·7% for uPVC; 1·5-5% for PE (all densities); 1·0-2·5% for PP (homo- and copolymers); 2,0-2·5% for PET (blow-moulding grades). It can be shown 21 that, under given moulding conditions, with no undue fluctuation of the relevant operational parameters, the cooling time (t) of a part in a mould will be proportional to the square of the wall thickness (wZ), * i.e. t = const. x w2 . A plot of this equation (t versus w) for a real situation (i.e. only positive values of w) will be the positive arm of a parabola whose axis is the t-axis and whose vertex is at the origin. A typical plot-given in Ref. 21-of actual cooling time versus part thickness for preforms injection-moulded on an injection blow-moulding machine has this form. Removal of heat from a moulding via the mould walls by means of a circulating coolant is the traditional and still the basic method of cooling. Chilled water, at temperatures down to about 5°C, can be successfully used with PVC blow mouldings. However, the colder the mould the higher the risk-especially in humid atmospheres-of moisture 'sweating' on the cavity surface, with consequent impairment of the surface finish of the moulding. Factors in the efficiency of such cooling, more important than mould temperature alone, are the rate and nature of the flow of coolant through the mould channels: for * The derivation of this relationship in Ref. 21 is based on thermal conductivity rather than thermal diffusivity (which is the proper parameter to consider). However, in this particular case, this does not invalidate the method or the equation derived. The author also suggests a practical way of using the relationship to predict the cooling cycle for any thickness of moulding (in his example an injection-moulded preform) on the basis of a demonstration with a given thickness.

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effective heat transfer the flow should be fast and turbulent. These flow characteristics are achieved by a combination of appropriately high pressures in pumping the coolant through, with suitable design of the channels. Since the shortest practicable cooling time is desirable for increased outputs, various methods are available of supplementing the mould cooling with internal cooling of the moulding. Reductions of cooling time by up to 30% are claimed for some of these. The cost of the techniques has been a restricting factor on their industrial use, although they are of definite practical interest, particularly in the blow moulding of large articles which normally require long cooling dwell times in the mould. The following internal cooling methods may be mentioned. Liquid nitrogen or carbon dioxide cooling: In this, N2 or CO2 is injected into the interior of the moulding directly after normal blowing. Vaporisation and expansion of the originally liquified gas, and its warming to the temperature of the cavity, abstract a substantial amount of heat from the moulding. Chilled-air cooling: With this system, a shot of air, cooled to a low sub-zero temperature (about -50°C), is injected under pressure into the blown article through a special insulated nozzle. Cooling with air and water mist: In the Hunkar variant of this method (the Hunkar ILC process)* an amount of highly compressed air and a metered quantity of water are simultaneously injected into the interior of the moulding. The rapid expansion of the air as it leaves the nozzle causes sharp cooling: the atomised water freezes, and further heat is then extracted from the moulding as the ice particles melt and the resulting water droplets evaporate. Air flushing: There are several embodiments of this approach to internal cooling. In the 'Interval Blowing' systemt a minimum air pressure is maintained inside the moulding, whilst cool, compressed air is additionally introduced and then vented out at intervals, 'flushing out' * Hunkar Laboratories Inc. Also relevant here: the 'Frost Air' system (Ryder Associates, Whippany, NJ, USA). t Battenfeld-Fischer Blastformtechnik GmbH, Lohmar, West Germany.

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some heat each time. In a simple continuous flushing system blowing air is circulated through the moulding (after that has been blown in the normal way): by controlled air feed and venting, a pressure near the blowing pressure is maintained during the cooling period whilst heat is continually removed. The necessary modifications to the blowing circuit are relatively simple. A continuous cooling system has also been developed* which can reduce substantially the time for cooling the neck of a blow-moulded bottle. 17.2.2 Industrial Blow Moulding of PVC

(a) Some Process and Equipment Considerations Both in the commercial and the technical sense, bottles are by far the most important among blow-moulded PVC articles. Extrusion blow moulding is the predominant method, a considerable proportion of the mouldings being stretch-blown. Injection blow moulding of PVC is less widespread, principally because the greater thermal severity of this process has delayed and complicated its application to all heat-sensitive plastics materials from which bottles are blown (modified polyacrylonitrile, multi-acrylic copolymers and PVC). However, PVC bottles are being produced by this technique on an increasing scale, and the problems are eased by the advent of special moulding compounds (see Chapter 15). Relatively recent machine developments include injection units in which the preform mould is filled at low pressure by rotating the screw during injection:t this makes the treatment received by the stock less drastic, and also reduces cycle time because the next consecutive shot can be commenced before full screw recovery. Multi-cavity injection blow moulding of PVC bottles has been operated commercially for several years, with, for example, eight-cavity production of round, 120 ml bottles at the rate of 48 per minute as early as 1978. 8 The extruders used in the extrusion blow moulding of PVC are of the general kind suitable for PVC extrusion (d. Chapter 14): LID ratios around 25: 1 and compression ratios of 1· 8 : 1-2·5 : 1 are fairly typical for single-screw machines. The advantages and disadvantages (mainly higher cost) of twin-screw extruders in the processing of PVC are discussed in Chapter 14: the points made apply also to their use in * By FGH Systems Inc., Dover, NJ, USA. tRainville Co. Inc., Middlesex, NI, USA.

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blow moulding. The relatively gentler processing and good stock temperature control they afford are of particular interest in the production of high-clarity bottles. Some blow-moulding machines are available with a choice of twin-screw extruder (for PVC) or a single-screw one (for polyolefins). One example is the Cincinnati Milacron* BB3 equipment. Some equipment suppliers make a special point of their twin-screw extruders' ability to process bottle compounds based on PVC polymer of relatively high molecular weight (e.g. up to K value 65 with the conical twin-screw machine of the Bellt blow-moulding plant6). Certain features of equipment arrangement or design, serving to reduce the risk of material hold-up and decomposition, are characteristic of extrusion blow moulding. Thus, in some equipment, the extruder is mounted end-down for straight-through vertical extrusion. With the more common horizontal extrusion set-up, axial-flow crosshead dies are usual, equipped with a 'swan neck' curve (cf. Fig. 17.2A) which preserves smooth melt flow at a constant rate whilst changing the flow direction from the horizontal to the vertical. Side-entry crosshead dies as used with polyolefins are not suitable for PVC. The general, basic configuration of a typical die is similar to that in pipe extrusion, with a spider~supported core to form the parison tube. As in pipe extrusion, the spider legs should be suitably profiled and the melt temperature high enough to ensure full merging of the melt after its passage through the spider zone: otherwise flow lines can result in the parison and the blown article (see Section 19.3.2(a) of Chapter 19). The die head temperature is usually kept slightly below that of the melt. All internal parts of the die should be streamlined and smooth, with the number of joints kept to a minimum. A channel in the core, opening into the interior of the parison, serves as a passage for air which is blown in (usually through a connecting channel in one of the spider legs) to provide shape support during extrusion. Multi-parison extrusion-whether by means of multiple heads, each producing one parison, or one multi-parison head-is attractive in principle as a means of increasing production without a proportional increase in machine space requirements. However, in practice it also presents special problems of melt temperature and pressure control. Close, accurate control is necessary, not only because of the thermal sensitivity of PVC, but also to ensure even parison lengths (a hotter

* See Table 14.3 in Chapter 14.

t Bell Engineering Works Ltd,

Lucerne, Switzerland.

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portion of the melt will normally flow faster), and to prevent parison curling (outward splaying) which may occur if the outside of the tube is colder than the inside. The flow paths and lengths must also be suitably balanced. Multi-parison heads are available on modern extrusion blow-moulding equipment for PVC, e.g. four-parison ones on some Bell* and Plastimact models. A production rate of 2400 I-litre bottles per hour is claimed for the Bell two-station machine with the multi-parison head. 6 'Universal' die heads suitable for both PVC and polyethylene are also available, in single- or multi-parison versions. Resistance of working surfaces to acid corrosion is as important in blow moulding as it is in other melt-processing of PVc. Thus the relevant points made in the chapters on extrusion and injection moulding also apply here. With regard to the material of moulds for blow moulding of PVC articles (including preform moulds), good heat transfer and wear resistance are additional considerations. An appropriate grade of stainless steel:j: would be the first choice for its combination of very good resistance to corrosion and wear, unless cost (high for stainless steel moulds), and/or the highest possible thermal conductivity, and/or light weight were paramount considerations in the particular conditions. In such cases the alternatives would be an aluminium alloy (for light weight and fast heat transfer), or a zinc alloy (for similar reasons) or beryllium/copper (for high heat transfer rate with some corrosion resistance). The following examples illustrate something of the features and performance of some equipment in current industrial use for the blow moulding of PVC containers. (b) Extrusion Blow-moulding Equipment An example of the smaller-size, continuous-extrusion machine is the Bekum BMO 8, the smallest unit in the BMO§ range. Equipped with a 50-mm (2-in), 24: I LID extruder with a capacity of 40 kg (90 lb) of PVC per hour it can produce single containers of up to I litre, or-with a twin head-two containers up to about 113 litre. Production in a

*Bell Engineering Works Ltd, Lucerne, Switzerland. t Plastimac SpA, Milan, Italy. fA useful review of mould steels has been published recently by Hoffmann. 22 § High capacity extrusion stretch-blow-moulding machines for the production of biaxially oriented PVC and polyacrylonitrile bottles of up to 2 litre capacity. The BM and BAE series comprise single-station machines producing unstretched blow mouldings in the range 5-20Iitres.

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typical run, with a twin head, of 230 ml (8 oz) PVC bottles (material weight 12 g) can be at the rate of up to 2100 bottles per hour (3,4 s cycle: dry cycle claimed 1·4 s). The containers are fully de-flashed in the machine and leave in upright position for the secondary operations of testing, surface decoration and filling and/or packing. The single-station machines of the Battenfeld-Fischer VKl series range from the VK1 07 model (maximum blown container size 0·7 litre) to the 30 litre VK130 model. Capable of operation with a single extrusion head or twin heads, these machines also provide complete in-machine trimming, and delivery in upright position for secondary operations. A typical output for the smallest unit (with twin extrusion heads) on 12 g PVC bottles would be about 2000 per hour. The Automa Speed 1500 SB machine, designed primarily for PVC, can blow mould biaxially oriented containers up to 1·5 litre with single-head and up to 1 litre with twin-head extrusion. Typical output of I-litre (32 g weight) PVC bottles in a single-cavity mould would be 480 per hour. Other machines produced by Automa (in the Speed and Maximat ranges) produce containers between 2 and 10 litres. Some interesting features are embodied in the fully automatic blow-moulding line produced by Sidel* (see also Table 17.1), and in the Krupp-Kautex KEB 2-2 unit. In the former, rotary arrangements of 24 moulds are operated to mould and temperature-condition preforms (produced from continuously extruded parisons) and then to blow them into bottles. The process is run under closed-loop computer control involving automatic monitoring and adjustment of extrusion rate, blowing pressure, temperatures, and product wall thickness. The KEB 2-2 machine is a two-station unit equipped for both the conventional blow moulding and stretch-blow moulding of PVC bottles (typically about 1 litre capacity). In the former mode of operation, twin heads are used with two two-cavity moulds: the problems of constant parison delivery and uniform wall thickness and temperature distribution which-as has been mentioned-ean arise in multi-head extrusion of PVC, are overcome by the use of two extruders, each feeding one head. (c) 1njection Blow-moulding Equipment Two examples of commercial equipment of this kind suitable for PVC are the Battenfeld-Fischer FiB injection stretch-blow-moulding equip-

* The Side) Division of SMTP, Paris, France.

b

a

VK1-2ESB BM04 BM04D KEB 2 MSFBO 3000 MSF BO 5000

b

Model

Material weight, about 30 g. Can be converted for direct blow moulding.

Battenfeld-Fischer Bekum Bekum Krupp-Kautex Sidel (SMTP) Sidel (SMTP)

Producer

Equipment

2

1

2

1 2

Cavities per mould

24 preform moulds and 24 bottle moulds on two rotary wheels

2

1

2

1 1

Number

Moulds operated

500 900 1800 500 2600 10000

Typical production rate: I-litre bottles per hour

TABLE 17.1 Typical Performance of Some Commercial Extrusion Stretch-blow-moulding Machines in Producing 1-litre PVC Bottlesa

--.)

~

o

:::l

~

~

ClO ClO

17 Blow Moulding of pvc

789

ment range, and the Tri-Delta machines (Tri-Delta Technology, Inc., Middlesex, NJ, USA). Both types are four-station units. In, for example, the Fischer FIB 517 machine the preform is temperatureconditioned at a station between the injection and the blowing stations: the longitudinal stretch is imparted by pre-blowing (not mechanically): the output (for bottle sizes up to 0·5 litre) is 500-600 per hour. (d) Dip Blow-moulding Equipment Industrial equipment for this process has been mentioned in Section 17.2.1(a) above. Another example is the Gamma series of machines produced by Plastimac: this equipment is intended primarily for the manufacture of small containers for pharmaceutical products. The version specifically recommended for PVC (and polycarbonate) is a three-cavity unit in which the melt shots are deposited by three vertically mounted 35-mm screw extruders each driven by a 5-hp variable-speed motor. (e) Sources of Information on Blow-moulding Equipment A survey of blow-moulding machines (with indications, inter alia, of suitability for PVC proces&ing) has been published in Plastics and Rubber Weekly for the 13th September, 1980. Up-to-date information will also be found in the general sources mentioned in Section 14.6 of Chapter 14. 17.3 PVC COMPOSmONS FOR BLOW MOULDING

The discussion in this section is centered on compositions for blow-moulded bottles, since bottles are the most important PVC products manufactured by this process. Like any other PVC composition, a bottle compound is formulated in the light of three key considerations: behaviour in processing, properties for service, and cost. The formulation is normally a compromise reflecting the relative importance of these three factors in a particular situation.

17.3.1 The Processing Aspect The PVC compositions used as feedstock in the production of extrusion blow-moulded bottles may be in powder (dry blend) or pellet form. A twin-screw extruder is preferable with powder feeds (see Section 19.1 of Chapter 19). Pellet feeds are more common in injection

790

W. V. Titow

blow moulding of good quality bottles. The respective merits of the two forms of feedstock are mentioned in the introduction to Chapter 19 and, passim, in Chapter 14 (Section 14.2.2(d». Ambient conditions (especially possible high atmospheric humidity in transport and/or storage) can be a factor in the choice of feed form, in that powder blends absorb moisture more readily than pellets, and this can affect their dry-flow properties as well as cause bubbles in the mouldings (unless completely effective de-gassing can be guaranteed). The basic processing considerations in formulating a bottle compound are substantially as those applicable to PVC extrusion or injection-moulding compounds generally (see Chapters 14, 15 and 19), although some points may acquire particular emphasis. These include the use of relatively low K value polymer for ease of processing; need for particularly good dynamic heat stability of the stock in the manufacture of clear products; and desirability of complete freedom from plate-out to avoid blockage of vents in blowing moulds. Recommendations of the suppliers of the polymer and other formulation constituents (or of the compound if purchased ready made) can be useful as initial guidance on processing. In the light of such recommendations the compound should be tested for processing behaviour (as well as product quality) to establish the best operating conditions. Some of the preliminary tests may usefully be conducted on a suitable torque rheometer. The following properties are of direct interest (especially in connection with extrusion blow moulding, or dip blow moulding with extrusion feed). Dynamic heat stability: A useful assessment of this can be made in a torque rheometer test of the kind outlined in Section 9.8.2 of Chapter 9. The test conditions (temperature, shearing rate, and duration) should be worked out in preliminary trials so that they relate to those of production. Melt viscosity: This determines the ease of melt flow in processing (and the temperature for good flow: increasing temperature reduces viscosity). It is also a factor in the amount of working the stock will experience (which in turn influences the work-heat generation, degree of homogenisation and fusion). Determinations can be carried out in a torque rheometer or another suitable instrument. * * A concise review of melt rheometry (with 78 references) has been published recently by Dealy.z4

17 Blow Moulding of pvc

791

Rate of fusion: A bottle compound should be fast-fusing for good homogeneity of melt with minimum heat history. Rates of fusion can be determined in a torque rheometer (see Chapter 11, Section 11.1). Melt swell: This is relevant in extrusion blow moulding, where the extent to which the walls of a parison expand on leaving the die is a factor in the ultimate weight of the moulding. The degree of swell is determined by comparing the diameter of a rod produced under relevant conditions from an appropriate die, with that of the die. Melt fracture: This can occur, usually within a range of relatively high rates of shear, in the extrusion of plastics compositions. In extrusion blow moulding of PVC bottles it can cause varying degrees of surface roughness. Tests may be carried out directly on the extrusion equipment, or the relevant range of shear rates may be covered in determinations with a rheometer. Evolution of volatiles: The extent to which this occurs is a function of the composition of the material and, to some extent, the processing conditions. If the conditions are not so excessively severe as to cause appreciable decomposition, the volatiles are often vapours of the more labile components of the stabiliser system. The volatiles can condense on the cold surface of the mould cavity: this can cause marring of the bottle surface, with possibly also staining if the condensate darkens in colour, as occasionally happens. Condensation of the fumes inside the bottle can lead to tainting of the ultimate contents: this can be counteracted by flushing with clean air directly after moulding; 'natural' airing in the course of storage can also help to reduce the smell in some cases. However, the composition should in any case be formulated with a view to preventing such problems. A test for evolution and nature of volatiles can be run: this may consist in processing the compound, under relevant conditions, in a torque rheometer, collecting the condensate on a cold metal surface, determining the amount produced, and taking an IR spectrum to identify its nature. A version of the test, employing a Brabender Plasti-Corder fitted for condensate collection, has been described by Latham and Mendham. 23 These authors also give some information on the application of this rheometer in the evaluation of the other processing characteristics just mentioned.

792

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17.3.2 The End-use Aspect Rigidity is a cardinally important property in blow-moulded containers, since their relatively thin-walled structure is the better able to resist distortion in service (under external forces, or the pressure of own contents) the more rigid the walls. Moreover, the higher the rigidity of the wall material the lower the thickness necessary (Le. the less material need be used) to achieve the required degree of distortion resistance. This is one of the main reasons why only unplasticised PVC compositions are used for the production of PVC bottles. Other important considerations are the better barrier properties and general chemical resistance of uPVC in comparison with pPVC, and its greater stability to extraction of formulation constituents by the bottle's contents. The only plasticisers used commonly in PVC bottle compounds are epoxy compounds (usually epoxidised soyabean oil) which are incorporated, in low phr, as components of some stabiliser systems: thus whilst they do ease melt flow to some extent, plasticisation is not their primary function. Small proportions of dibutyl phthalate are also occasionally included in some PVC compositions for the production of bottles for mineral water: the functions of this additive in such formulations (in which it may be associated with benzoic acid) are to act as co-stabiliser and fungistat, and to assist fusion. Properly formulated uPVC has a number of properties which qualify it as a good material for bottles for a large number of uses (see Section 17.4 below). These include a high degree of rigidity, toughness (in impact-modified or biaxially stretched mouldings), resistance to environmental stress cracking, high clarity and 'sparkle' of transparent compositions, good barrier properties, and suitability (when formulated with approved constituents) for food contact. The points on which uPVC compares less favourably with PET, its main competitor in many blown-container applications, are relatively low maximum service temperature, susceptibility to ketone and chlorinated hydrocarbon solvents, lower impact strength, greater susceptibility to stress whitening (of some impact-modified grades) and permeability to some penetrants. PVC bottle compounds for the packaging of oil, beverages, and pharmaceuticals must fulfil the requirements of the appropriate regulatory authorities with regard to the suitability of all formulation components. They must also be formulated to minimise odour and taste effects and to resist microbiological infestation.

17 Blow Moulding of pvc

793

17.3.3 PVC Bottle Formulations The basic formulation will comprise the following. (a) PVC Polymer Usually a suspension (sometimes mass) polymer, with particle structure and size characteristics suited to ease of processing and rapid fusion. Relatively low K value: normally 50-60 (number-average molecular weight approximately 36000-55 OOO)-for low melt viscosity. The polymer should be of good quality to secure freedom from gels ('fish-eyes' or 'nibs'), and its VCM content must be acceptably low (in most countries under 1 ppm).

Note: The gel content is sometimes classified with processing features. Its determination in bottle compounds can be carried out on a suitably fitted torque rheometer, e.g. a Plasti-Corder with extrusion attachment and bubble die assembly. A free-blown thin-walled bubble is produced, in which the gels should be plainly visible. 23 For a quantitative determination the bubble size may be standardised and the number of gels counted. Whilst gels may not be readily visible from the outside in an opaque production bottle, they usually show up as small protrusions on the inside surface. Their presence may significantly impair strength properties, as they can act as stress concentrators. (b) Stabiliser System This is usually liquid, to avoid increasing the melt viscosity. Most often either a thiotin for good heat stability and high clarity (permitted compounds for food contact), or a Ca/Zn system for reasonable clarity and non-toxicity. In either case an epoxy co-stabiliser is usually included, sometimes with a phosphite or another synergist. (c) Impact Modifiers Although in some cases the impact resistance of a conventionally blown uPVC bottle of a particular design and wall thickness may be considered sufficient for a particular end-use (and intended contents) without impact modifiers in the formulation, these toughening additives are important ingredients of most PVC compositions for the blow moulding of bottles without biaxial stretching. Their nature, and

794

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V. Titow

effects in uPVC, are discussed in Section 11.2.2 of Chapter 11. As in the compositions for other transparent products, the impact modifiers in clear bottle compounds are of the MBS type. ABS, and some of the other types, may be used for opaque bottles. Impact modifiers can adversely affect, in varying degrees, such properties as permeability and colour ,25,26 softening point, chemical resistance, resistance to stress whitening,25 and resistance to photochemical degradation. It is, therefore, an important selection criterion that the modifier chosen should combine the best toughening effect with the least impairment of those of the properties which are the most significant in the particular situation. In most cases, and especially with clear bottle compositions, stress whitening potential will be a factor in the selection. The impact resistance of blown bottles normally increases with the proportion of modifier in the composition, up to about 15 phr. Higher loadings produce no further significant improvement, and may be increasingly detrimental to other properties. As has been mentioned, biaxial orientation imparted by stretchblowing can have a pronounced toughening effect, so that the amount of impact modifier in compositions for stretch-blown bottles may be substantially reduced, or the additive left out altogether in some cases. (d) Lubrication Both internal and external lubricants are employed in bottle formulations. Some proprietary composite lubricants, specially developed for this application (e.g. Irgawax 368-Ciba-Geigy), can be particularly useful. In general, all the points of lubricant action (and interactions with other formulation components, especially stabilisers) discussed in Section 11.1 of Chapter 11 are relevant to the formulation of lubricant systems for bottle compositions. The question of permanence (i.e. resistance to extraction and migration) is important: in this connection (and quite apart from cost considerations) it is a good general principle to keep the lubricant content to the necessary minimum. Impact strength can also be significantly reduced by excess of internal lubricant, whilst avoidance of the risk of plate-out through proper choice of the nature, amounts and balance of the components of the lubricant system is a further important consideration. Internal lubrication is often provided in bottle compositions by calcium stearate, whether incorporated solely for that purpose or as part of a Ca/Zn stabiliser system: the addition level should preferably not exceed 1 phr, and the effect on transparency of the moulding should

17 Blow Moulding of pvc

795

not be overlooked. Among lubricants with external action, polyethylene waxes, synthetic waxes, and some fatty acid esters and alcohols are of interest. (e) Other Additives Processing aids (commonly acrylic) are often included in bottle compound formulations at levels of up tl) about 1 phr. At such levels they can effectively discharge their functions in processing without significant effect on the properties of the finished mouldings, except for possible slight clouding of transparent products. For this reason any processing aid to be used in a clear composition should have a refractive index as close as possible to that of the PVC resin. Colourants and pigments are incorporated in some bottle compounds, as appropriate. Two examples of basic bottle formulations are given in Section 4.6.6 of Chapter 4. These, and the further examples in Table 17.2, illustrate some of the points made in the present section. The effects of some formulation and processing factors on the quality of bottles blow-moulded from powder blends based, respectively, on a PVC homopolymer, and vinyl chloride/alkyl vinyl ether and vinyl chloride/propylene copolymers, were investigated-with the aid of dynamic stability tests-by Taylor and King. 27 These workers found that the copolymer-based compositions gave bottles with a lower light absorbance and yellowness index, and that certain product imperfections were related to the particle size distribution and volatiles content of the resins. Suppliers of PVC compounds market some blow-moulding grades. One example is compound 9275 CI38 of the Ethyl Corp., formulated, like others of its kind, for good heat stability and low melt viscosity in processing, and for such product properties as good clarity (light transmittance about 74%), impact strength and UV barrier action (to protect light-sensitive contents in bottles).

17.4 PVC BLOW MOULDINGS 17.4.1 Applications The vast majority of PVC blow mouldings comprises containers, used for the packaging of a variety of products. The containers are mostly

Corvic D50116a Stanclere 80C 2·2 phr Paraplex G62f 2·8 phr a--phenyl indole 0·6 phr Kane Ace Bl2g 11 phr Estol294h Synthetic wax

Medium impact

1 phr 2phr

Kane Ace Bl2 14 phr 1·1 phr 0·1 phr

100 parts Mel/ite 831c;d Paraplex G62

High-clarity, high-impact

Fruit squash bottles

MBS lrgawax 368; PE wax

lOphr 1·2 phr 0·05 phr

Breon S90110 b 100 parts Irgastab 17 Moe 1·2 phr

Clear bottles (general purpose)

a

ICI. K value 50. Low MW polymer for blow moulding and film production. A special grade with low 'fish-eye' level available. b BP Chemicals. K value 57-60. A blow-moulding resin. C AKZO. Liquid octyltin. BPF (British Plastics Federation) and BGA (Bundesgesundheitsamt-West German Federal Health Office) approved for food contact. d Albright and Wilson. Liquid octylin. BPF, BGA, and FDA (US Food and Drug Administration) approved. e Ciba-Geigy. Di-n-octyltin bis(2-ethylhexyl thioglycolate) with 25% epoxidised soyabean oil. f Rohm and Haas. Epoxidised soyabean oil (high MW grade). BPF approved. g Kanegafuchi Chemical Industry Co. h Joseph Crosfield and Sons Ltd, Warrington, Lancs. ; Ciba-Geigy. Composite lubricant for blow-moulding applications.

Impact modifier Lubricants

PVC polymer (suspension type) Stabilisers

Formulation components

TABLE 17.2 Examples of Basic Outline Formulations for Bottle Compounds

~

:::'! 0'

:0:::

~

~

-.J

17 Blow Moulding of pvc

797

bottles of various kinds, but wide-mouthed jars and the like are also blow-moulded and find diverse applications in the packaging of some foodstuffs (e.g. dried vegetables, syrup, honey), cosmetics and toiletries (e.g. creams and ointments), small hardware items (e.g. nails, screws) and others. Large-scale application areas of PVC bottles comprise their use for packaging consumable liquids (including fruit squashes, edible oils, vinegar, cheaper varieties of table wine, mineral and natural spring water, and carbonated drinks), liquids for household use (detergents, bleaches, disinfectants, cleaners), cosmetics and toiletries (e.g. lotions, oils, shampoos and other hair-care preparations, liquid soaps), and pharmaceuticals. In several of these outlets PVC competes with polyethylene terephthalate (e.g. bottles for carbonated drinks, edible oil, some cosmetics and toiletries), polyethylene (e.g. detergent and other household liquid bottles, bottles for pharmaceuticals), polypropylene (bottles for household liquids, some toiletries and pharmaceuticalse.g. mouthwash), and modified polyacrylonitrile (applications in which barrier properties are of particular importance). As an illustrative example, it may be mentioned that the consumption of uPVC for the production of blow-moulded containers in France in 1972 and 1973 was reported 28 as, respectively, 110000 and 130000 metric tonnes. The main items making up the figure for the former year were stated28 to be bottles for still mineral water (70000 tonnes), for cooking oil (17000), for table wine (14000), for vinegar (3000), for cosmetics (2000), and miscellaneous (1000). 17.4.2 Properties and Tests

The material properties of uPVC blow-moulding compositIOns, which-together with the relatively moderate price-make containers blown in this material attractive for many applications, have already been mentioned. Several processing factors also play a part in the properties of a blow-moulded PVC container. Among these, perhaps the most important single effcd is that of the nature and extent of molecular orientation, especially when that is deliberately imparted in a substantial degree by biaxial stretching. Some other factors have been mentioned in Sections 17.2.1(c) and 17.3.1. Still others of considerable importance are the temperature of the parison or preform (and

798

W. V. Titow

the temperature distribution within it) during moulding, and the rate and efficiency of cooling. The former factor can affect the dimensional conformity and strength of the article and the surface finish; in extrusion blow moulding it is instrumental in the strength of the nip weld (also affected by the speed of mould closure, shape of the nipping edges, and the blowing time and pressure). The cooling factor can also influence the strength and dimensions. Two properties normally monitored in the course of routine quality control in production are the integrity (freedom from leaks) and wall thickness of containers. Microprocessor-operated leak detectors are available from several suppliers for on- or off-line use, in some cases as part of the regular blow-moulding equipment (e.g. with Bekum or Battenfeld-Fischer machines). A typical leak tester applies accurately controlled air pressure to the interior of the container and measures the rate of decay: this indicates the presence and size of any leaks. Apparatus of this kind can also be used to test the dimensions of small moulded orifices. An example of a fully automatic, computerised leak tester is the BLT 1000 Leak Finder (South Bend Lathe Inc., USA 5 ), capable of testing up to 50 containers per minute (test pressure about O·llbf in- 2): the device can be programmed for automatic removal of rejects from the line. Thickness gauges, working on the basis of absorption of IR radiation, are available (e.g. from Oy G. W. Sohlberg AB., Espoo, Finland): these can measure and record the wall thickness of a blow-moulded container at 100 points within 2 s. Service-related quality control tests widely performed on blowmoulded containers are those for impact resistance, resistance to crushing, and permeability (barrier effect). Many manufacturers have their own test specifications. ASTM test methods are also available in the following standards: ASTM D 2463 (drop impact resistance), ASTM D 2659 (column crush test), ASTM D 2684 (container permeability). Another ASTM standard (D 2911) lays down dimensional tolerances for blow-moulded containers. Compatibility with the intended contents is also sometimes determined, usually by relevant mechanical property tests on containers before and after a prescribed period of contact. The above tests are normally carried out on samples of actual production containers, although where the object is to compare or evaluate moulding compounds the container specimens may be ones specially produced in a prescribed design and size (e.g. the standard 750 cc Lesieur test bottle25 ). Compound property tests involved in such

17 Blow Moulding of pvc

799

comparison or evaluation may also include determinations of some or all of the following properties, carried out on appropriate standard moulded test specimens: deflection temperature under load; brittle temperature; Vicat softening point; tensile strength and elongation; flexural strength and modulus; density; hardness; and light transmittance (for clear compositions). In the practical service context, the drop impact resistance is possibly the most significant among the properties of a container. It is usually determined by dropping the container-filled with water and closedonto a standard, hard surface. Note: Some specifications include the same kind of test for a multi-container pack, if the containers involved are normally transported and stored in such groups.

In some versions of the drop test the height of the drop is fixed; in others it is increased by prescribed increments until a stated percentage (often 50%) of the drops results in failures, or-in some variants of the method-until all specimens fail, the failure criterion being fracture or defined damage. Both approaches are represented in ASTM D 2463 (together with a third procedure, known as the 'Bruceton staircase method'). It may be noted that, as illustrated, for example, by Sisson's data,25 whilst increasing the K value of the polymer (within the range acceptable for processing) does raise somewhat the impact resistance in a drop test, that rise is much less than one brought about by incorporating in the composition an effective impact modifier in suitable proportion. The design of the container is also a factor in drop impact tests. A non-destructive dynamic compression test of a few seconds' duration may also be used to obtain a numerical index of the resistance to flexure in the test conditions. With appropriate allowance for the design, wall thickness distribution, and material of construction, the index is claimed to be a reasonable measure of a bottle's relevant mechanical properties, and to correlate well with the results of some conventional destructive tests. 7 Bottles for carbonated drinks have to combine resistance to internal pressure with a low enough permeability to CO2 to provide a sufficiently long shelf life before 'carbonation loss' occurs. For a 2-litre bottle in PET (a material somewhat superior to PVC in this application) many soft-drink companies specify a carbonation-loss shelf life of 16 weeks (with 8-12 weeks for a !-litre bottle). Biaxial

800

w. v.

Titow

orientation, combined with pressure-resistant design and material weight for extra wall thickness, are necessary to meet such specifications with PVC bottles, especially where they are to be used in a hot climate (PET carbonated drink bottles are also biaxially oriented). An example is a round-shouldered, heavy weight (29 g as against 23 g for a conventional design), biaxially oriented O'33-litre clear PVC bottle with integrally moulded, internally rounded base (incorporating four 'feet' for stable standing without external support). This is capable of containing-without distortion-drinks with up to 8 g litre- 1 CO 2 at temperatures up to SO°c. The bottles, and injection stretch-blowmoulding equipment on which they are produced, have been developed by Voith-Fischer (now Battenfeld-Fischer) in collaboration with 4P Rube Gottingen GmbH. 16,29 Apart from the pressure resistance and barrier requirements, bottles for carbonated drinks and mineral water (as indeed those for wine and other beverages) must be made from PVC compositions formulated for the greatest possible resistance to development of taints and odours, through the highest purity and resistance to extraction of all constituents, and discouragement of bacterial growth. Some comments on this formulation aspect are provided in a paper by Sahajpal. 30 REFERENCES 1. Anon. (1980). Plast. Techno!., 26(13), 66-7. 2. Couzens, E. G. and Yarsley, V. E. (1968). Plastics in the Modern World, Penguin Books Ltd, Harmondsworth, Middlesex, England, p. 298. 3. British Patent No. 821 173. 4. Smoluk, G. R. (1981). Mod. Plast. Int., 11(2), 25-7. 5. Brockschmidt, A. (1982). Plast. Technol., 28(5), 78-81. 6. Anon. (1980). Mod. Plast. Int., 10(1), 25-7. 7. Hunkar, D. (1976). Plast. Rubb. Wkly, 12th November, pp. 30-1. 8. Crabtree, D. R. and Hart, R. J. (1978). Plast. Rubb. Int., 3 (6), 247-8. 9. Anon. (1975). Plast. Rubb. Wkly, 7th November, pp. 22-3. 10. Anon. (1979). Plast. Techno!., 25(2), 13-17. 11. Anon. (1982). Eur. Plast. News, 9(7), 8-9. 12. De Vries, A. J. and Bonnebat, C. (1976). Polym. Engng. Sci., 16(2), 93-100. 13. Brady, T. E. (1976). Polym. Engng. Sci., 16(9), 638-44. 14. Anon. (1981). Eur. Plast. News, 8(9), 64. 15. Anon. (1981). Plast. Rubb. Wkly, 26th September, p. 12. 16. Anon. (1980). Mod. Plast. Int.. 10(4), 36-8. 17. Anon. (1967). Packaging, 38(11), 95-9.

17 Blow Moulding of pvc

18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

801

Anon. (1976). Plast. Rubb. Wkly, 12th November, p. 37. Grant, R. and Foster, R. (1965). Mod. Plast., 43(2), 122-6,129,199. Brockschmidt, A. (1981). Plast. Technol., 27(13), 55-60. Valyi, E. I. (1981). Plast. Technol., 27(10),133-45. Hoffmann, M. (1982). Plast. Techno!., 28(4), 67-72. Latham, J. R. and Mendham, W. E. (1973). 31st ANTEC SPE Proceedings, pp. 458-60. Dealy, J. M. (1983). Plast. Engng, 39(3), 57-61. Sisson, W. B. (1968). Plast. Polym., 36(125), 453-63. 'The blow moulding of Welvic PVC', ICI Technical Service Note W. 110. Taylor, W. and King, L. F. (1970). Polym. Engng. Sci., 10(4),204-8. Anon. (1974). Plast. Rubb. Wkly, 26th July, p. 19. Anon. (1983). Eur. Plast. News, 10(1), 29. Sahajpal, V. K. (1978). 'PVC compounding for low organoleptics and controlled bacterial growth', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978.

CHAPTER 18

Calendering of pvc W. V. TITaw

18.1 INTRODUCTION According to Griffin 1 the word 'calender' shares a common Greek antecedent with 'roller', and the first recorded reference to calendering appears to be a description, in an 18th century publication, of a smoothing treatment for textile fabrics, carried out by means of individually operated, weighted rolls. The multi-roll machine of the general type still represented by the modern calender, was developed for the processing of rubber around the mid-1800s. Today, in the thermoplastics context, calendering is very much a PVC-processing operation, in that relatively little in the way of other thermoplastics is processed on calenders, although some thermoplastic rubbers, certain polyurethane compositions, talc-filled polypropylene, and ABS have been calendered into sheeting. 2 ,3 More recently chlorinated polyethylene sheeting produced by calendering has been finding application as roof-lining material. In this context, therefore, calendering may be defined, in general terms, as a process whereby a hot mass of a thermoplastic is fashioned into a continuous sheet by passage through a system of heated rolls (the calender). The sheet may then simply be cooled (possibly after embossing-see below) and wound up, or it may be deposited, while still hot, on a continuous base material (e.g. fabric, paper) fed through the appropriate part of the calender, to form an adherent coating. A calender may also be used to laminate together externally fed sheets: however, this kind of operation is lamination by calender, not calendering proper. The general thickness range for typical calendered PVC products is 803

804

W. V. Titow

about 75-900,urn, although sheet up to about 1·5 mm thick can be produced. Thus, whilst the products are often referred to as 'film and sheet', according to the systematic terminology discussed and adopted in Chapters 19 and 20, all the unsupported products of this kind should properly be described as sheeting (or sheet), the top thickness limit for PVC film being about 75/lom (and its normal method of production the blown film variant of the extrusion process-d. Chapter 19, Section 19.5.2). The main advantages of calendering over extrusion as a method of sheet production are high outputs and production rates, good product thickness control (and its relatively ready automation), and suitability for long, continuous runs. These features are largely responsible for the fact that most PVC sheeting is manufactured by calendering despite the much higher capital cost of equipment and lower operational flexibility for short runs in comparison with extrusion. Suppliers of calenders and calender line equipment are included in most of the main sources of information on commercially available machinery for plastics processing. These sources are mentioned elsewhere in this book (see, for example, Chapter 14, Section 14.6). The following may be quoted as non-selective examples: The Berstorff group: Berstorff Corp., Charlotte, NC, USA; Herman Berstorff Maschinenbau GmbH, Hanover, West Germany. BKMllndustrieanlagen GmbH, Munich, West Germany. The Farrel group: Farrel Bridge Ltd, Rochdale, Lancs, UK; Farrel Conn. Div., Emhart Machinery Group, Ansonia, CT, USA.

18.2 THE CALENDER The machine comprises the arrangement ('stack') of rolls (also known as bowls) mounted in bearing blocks supported by side frames ('gables'), equipped with roll drives, nip-adjusting gear, and heating arrangements. Calenders used for the general production of PVC sheeting are commonly four-roll machines. Whilst there is no basic reason (other than cost, space, and structural complexity considerations) for limiting the number of rolls to four, five-roll calenders are not common, and

18 Calendering of pvc

805

only used for special purposes, such as the production of some types of thin rigid sheeting where the extra nip can substantially improve the surface finish. 1 ,4 Note: Three-roll (two-nip) calenders find some use in the manufacture of PVC flooring, and sequences of two-roll (single-nip) units are used for processing certain highly filled flooring compositions-see Section 18.4.2(a) below.

The various roll arrangements of a four-roll calender are shown in Fig. 13.41 of Chapter 13. Their respective merits and drawbacks have been discussed in some detail by Elden and Swan. 1 The advantages of the 'L' configuration, widely favoured (especially in the inverted form-d. Fig. 18.1(i» for PVC processing, are usually quoted as reasonable cost; good visibility and accessibility of all the rolls in the stack; good rigidity imparted by the vertical superimposition of three of the four rolls; good range of the available total lap ('wrap') of the materials round the rolls (up to 540°, depending on the positioning of the offset roll of the stack and the stripper rolls); and ability to apply thickness corrections at two material banks by crossing just one roll (No.3 roll-the middle one in the vertical arrangement).

(ij)

(i)

' Cf3 1

(iii)

2

"""

(iv)

3

Fig. 18.1 Customary roll numbering in calenders. (i) Inverted 'V type; (ii) inclined 'z' Jype, downstack; (iii) 'V type; (iv) three-roll offset.

806

w.

V. Titow

Note: It is customary to designate the rolls of a calender by

numbers, starting from the first roll at the feed nip and ending with the last one at the take-off (ct. Fig. 18.1). Another roll configuration popular with manufacturers of PVC sheeting is the inclined Z type (cf. Fig. 18.1(ii)): this too offers a relatively wide range of the degree of lap, as well as easy aCCtSS for fabric coating. Note: In principle, the 'L', inverted 'L', and 'Z' configurations are

also convenient for bringing an embossing unit close up to the last nip. Formerly this constituted an advantage, but is less significant with the multi-roll stripper arrangements favoured in modern practice (see Section 18.4.1(c) below). The following typical features of modern calenders are instrumental in their operational versatility and effectivity (including certain aspects of product quality). Roll face lengths up to about 3 m are available: large working face length (sheet width) is a factor in output rate, and provides the option of slitting the product into any lower widths required. Individual roll drives, standard on modern machines, enable the roll speed ratios to be widely varied, so that the calender can cope with a range of compositions differing in rheological behaviour: the corresponding available variation of friction ratios between the rolls makes possible a high degree of control over the material in the rolling nip which, inter alia, promotes a good surface finish in the product. In the modern drilled rolls, the close proximity of the drilled channels for circulation of the heating medium (high-pressure hot water or special heatexchange liquid), both to the working surface and the two ends of the roll provides good temperature control, with fast response to adjustments, which makes for improved processing and product quality in comparison with those obtainable with older type cored rolls. Another important feature is the provision for counteracting the deflection of the rolls by the PVC compound being passed through the nips. Because of its relatively high viscosity and compression resistance the material tends to force the rolls apart, the effect being most pronounced in the middle where the restraining effect of the bearings is least, and also increasing with roll face length (sometimes also called face width). The distorting forces developed can be quite high-up to about 1 MN per linear metre of working face length in the production

a

18 Calendering of pvc

807

of thin rigid sheeting: 1·5 MN over the roll face of a 1·68-m (i.e. 66-in) calender has been quoted as not unusual in such conditions. 1 ,2 With a typical plasticised composition a representative range would be about O· 2-0·5 MN m -1. These forces actually deflect the rolls, so that-if the deflection is not corrected or compensated for-the sheet produced can be substantially thicker at the centre than at the edges. The rolls can be suitably contoured (crowned) to counteract this effect on the product, where the deflecting forces are known and remain reasonably constant. In practice this means that roll crowning can be sufficiently effective only over a rather limited range of compositions and processing conditions. Outside this range the crowning will either undercom}.lensate (i.e. with deflecting forces significantly higher than those for which the crowning was designed, the sheet will again be thicker in the middle) or overcompensate (i. e. with deflecting forces significantly lower than those allowed for, the sheet will get thicker towards the edges). Systems are also available in which-to cope with a wider range of deflecting force values-roll crowning is associated with special arrangements for nip adjustment by a combination of lateral and vertical movements of some of the rolls in the stack. * However, the means of counteracting the effects of roll distortion that are the most operationally flexible and widely used in current industrial practice are the techniques known as roll bending and roll crossing (sometimes also referred to as roll skewing or cross-axis roll adjustment). A modern calender is normally equipped with facilities for both these procedures. Roll bending is effected by applying a hydraulic load to the roll journal ends (extended for this purpose beyond the regular bearings); this exerts leverage on the roll (against the bearings) which makes it slightly concave or convex at the nip, the direction of the imparted curvature depending on that of load application. The maximum extent of effective compensatory crown increase or decrease with this method is limited (typically to about 0·075 mm 1 ) by the extra load that may be safely exerted on the bearings. Roll crossing is an angular shift of one or both rolls of a nip-forming pair so that their axes, whilst remaining in their original horizontal planes, are no longer parallel, but form a slight angle: this increases the end clearance between the rolls and hence the amount of material * e.g. the ES Profit Steering calender, developed by Wiik and Hoeglund, Finland. s

808

w.

V. Titow

that can be accommodated in the resulting enlarged gaps, making for a thickening towards the edges of the sheet being formed. The degree of correction achievable in this way can-in particular conditions-be equivalent to an apparent crown increase on radius of about 0·75 mm. 1 The roll-crossing facility is usually available at least on the last nip of the calender (the gauge-determining or 'gauging' nip).

18.3 THE CALENDERING OPERATION: GENERAL FEATURES, AND THEIR EFFECTS ON THE STRUCTURE AND PROPERTIES OF CALENDERED SHEET The operation of a four-roll calender has been said6 to constitute a form of extrusion with rotating die lips, the material being moved forward by friction against the roll surfaces in its passage through the three consecutive nips, said to function, respectively, as the feed, metering, and final sheet forming and finishing stages. Whilst the calendering operation may indeed be considered in this way in general terms, there is no close analogy with screw extrusion as, quite apart from the different mechanical formats of the two processes, there are essential differences between them in the state of the PVC material and the way in which this is influenced by the processing. Thus, as pointed out in Section 13.4.2(b) of Chapter 13, unlike the extruder, the calender is not required to de-aerate, homogenise and fuse the PVC composition: these functions are performed by the compounding (upstream) part of the calender line, whose make-up and operation are discussed in some detail in the section just mentioned. The calender itself essentially only forms into sheet the homogenised fused (gelled), hot PVC composition which it receives as feed. Note: The amount of shearing and the temperature of treatment on the calender can, in principle, be sufficient to homogenise and gel even a powder feed, and some experiments have been carried out in that direction. 1 However, quite apart from such problems as ensuring adequate, uniform intake of powder feedstock, and effective removal of entrapped air, the residence time required to achieve a homogeneous, fully gelled melt makes for a relatively slow rate of material passage through the machine, and thus works against one of the main advantages of calendering, viz. a high output rate.

18 Calendering of pvc

809

Furthermore, because of the way the final product is formed by the calender roll nips it differs from extruded sheeting in some morphological features and in certain properties associated therewith. This is brought out by the following brief consideration of the material's passage through an inverted 'L' calender. In operation, three banks of material are maintained at the three nips (d. Fig. 18.1): a large relatively narrow feed bank at the first nip (between No.1 and No.2 rolls) and progressively smaller but wider banks at the subsequent two nips (with a full width bank between rolls No. 3 and 4). Because the material is shaped essentially by surface contact with the rolls. the surface of the ultimate product may be regarded as having undergone two re-forming treatments (in the second and the last nips), while the 'core' material inside the sheet is not substantially re-worked after being given its laminar shape by the first nip. In this sense, therefore, the core may be considered to be formed by this nip. The properties of the core-and hence of the whole product-are thus influenced by the uniformity of feed to the calender, and uniformity and homogeneity of the first material bank. Differential cooling of the sheet· surfaces and the core tends to set up stresses in the latter. These can be aggravated locally by non-uniform feed and irregularities in the feed bank, such as, for example, parts of the feed strip finding their way directly (without equalising residence in the bank) into the material being formed by the feed nip: this can give rise to cold streaks in the core with consequent creation of local stress. The extent of development of skin-and-core morphology together with the additional adventitious local strains and stresses, are factors in the strength properties of the sheet and affect its ability to lie flat. They can also cause problems in service (e.g. wrinkling) due to strain recovery. 18.4 CALENDER LINES 18.4.1 General-purpose Line A fairly typical calender line is schematically represented in Fig. 13.40 of Chapter 13, and something of its operation mentioned-with special detailed reference to the pre-calender compounding train-in Section 13.4.2(b) of that chapter. Line arrangements, and in particular those of the pre-calender

W. V. Titow

810

compounding and feed section, can differ in some respects, depending on the type and range of calendering operations undertaken. The schematic flow diagram of Fig. 18.2 illustrates a versatile arrangement (of the kind shown in part in Plate K(1)) of a pre-calender section which incorporates two alternative material paths for respective use with plasticised and rigid (or semi-rigid) compositions. Polymer storage

Plasticiser storage

~

I

Other additives storage

Metering (weighing) devices

111

High-speed (hot) mixer

1 1

Cooling mixer

Feed (metering) hopper

0

r

...... ....

Internal mixer

1

l

®

BussKneader

j

Two-roll mill

1

Metering extruder/strainer

Two-roll mill

.)

~

feed conveyor (with metal detector)

1

CALENDER Fig. 18.2 An arrangement of the compounding and feed section of a calender line providing two alternative material routes. A, Plasticised compound processing route; B, rigid and semi-rigid compound processing route.

18 Calendering of pvc

811

(a) Pre-calender (Compounding and Feed) Section Typical features of a compounding section of a calender line, and main variations of the arrangements used, include the following. The PVC formulation components can be metered in various ways for the initial mixing (blending) operation (see Chapter 13). Of the two main materials, the polymer is normally dispensed by weighing: plasticisers can be metered by volume, but weights should also be monitored for any dispensing adjustments necessitated by variation of density with temperature. The efficiency of the initial mixing, in which a powder blend is produced, is a factor in the uniformity and homogeneity of the calendered sheet. For this reason high-speed mixers are widely employed with both rigid and plasticised compositions. However, ribbon blenders are also sometimes used as the means of first combining the polymer with plasticiser(s) and the other, minor components of the formulation, especially where the subsequent melt-compounding operation (sometimes also referred to as 'fluxing' in this context) is carried out in an internal mixer. As indicated in Fig. 18.2, the melt-compounding unit may be a continuous-operation one, like a Buss-Kneader (or another one of the suitable machines discussed in Chapter 13), or an internal mixer of the Banbury type. Modern versions of these batch-processing mixers are equipped with microprocessor controls which automate the mixing cycle. Various basic models are available, with chamber capacities within the approximate range 40-5001itres: in many cases a standard model can be modified by the makers to suit a client's special requirements. In comparison with a continuous compounder, the advantages of an internal mixer in this application have been listed7 as a relatively wide capacity range (although the output of a batch mixer can be matched by that of a continuous compounder of appropriate size); ease of loading re-work* (promoted by the large loading port and ram operation) and its thorough re-dispersion; generally greater heat transfer capacity; and comparatively moderate routine maintenance costs. The main comparative disadvantages include more drastic working of the stock (a factor instrumental in confining the use of these batch mixers in calender lines to those processing pPVC), and the need to handle large discharged material batches; the connected-power requirements are also relatively high, although the total energy *In some operations re-work is freeze-ground and added to the high-speed mixer-see Chapter 13, Section 13.4.2(b).

812

W. V. Titow

consumption is roughly comparable with that of an equivalent continuous compounder. In general, the batch mixers are best suited to calendering operations involving relatively short runs on different formulations. In an arrangement much used with flexible compositions in the past, but now no longer very common, an internal mixer is followed by two two-roll mills. The batch of compounded material is dumped directly onto the first ('dump') mill where it is sheeted and taken off as an unsupported strip into the nip of the second mill (the 'strip' mill). The strip taken off this mill is carried on a conveyor to the feed bank of the calender. A metal detector would be positioned either over the strip between the two mills, or over that on the conveyor. Note: In the early days of PVC calendering the material off the dump mill might be fed to the calender manually in the form of 'dollys'*, and even the melt-compounding might be carried out on a mill.

Nowadays the arrangement of Fig. 18.2(A) would be fairly typical for processing flexible compositions. In this, the two-roll mill which receives the hot batches from the internal mixer links the batch-wise operation of the latter with the continuous operation of the calender by maintaining a reserve bank of hot, homogenised stock (the working the material receives on the mill can further enhance homogeneity). A continuous strip of material from the mill is fed to a short-barrelled extruder (the extruder/strainer): the functions of this are to remove from the stock extraneous contaminants and any material lumps and particles (e.g. of pigment, filler) that may be present; to maintain the material at a uniform, correct temperature; and to providecontinuously and at the appropriate rate-ealender feed in the form of a flat or round strip (see also Section 13.4.2(b), Chapter 13). Continuous compounders are equally suitable-and widely usedfor both plasticised and rigid PVC calendering compositions. With uPVC this is the type of unit normally employed, in preference to an internal mixer, ,because of its less severe processing action and very good stock temperature control afforded by such machines as the Buss* This spelling is sometimes favoured over 'dollies' (also called 'pigs')-pieces of hot hide cut from the mill and tightly rolled up to reduce heat loss: can cause excessive bowl deflection in the first nip of the calender if not fully integrated into the feed bank.

18 Calendering of pvc

813

Kneader. In some pre-calender line arrangements the compounding machine may feed the calender directly, without the interposition of another unit (see Chapter 13, Sections 13.4.2(b) and 13.4.4(a) for a discussion of this and many other relevant points). It is quite usual, however, for a continuous compounder to be followed in the line by a metering extruder (extruder/strainer) or sometimes a mill (ct. Fig. 18.2(B», either acting in the capacity already mentioned. Where a mill is used it makes a convenient addition point for edge-trim that is to be re-worked, because of the manipulative ease, and to keep down the heat history of such material (which has already experienced a full heat processing cycle). The Buss-Kneader is very popular as the continuous compounder in modern pre-calender trains. Planetary extruders have also proved their worth in this application (see Chapter 13). An interesting example of the use of a planetary extruder in a purpose-designed but fairly basic calendering set-up is the Berstorff* 'Rollex' line,3 intended for what by general calendering standards is comparatively small-scale manufacture of rigid calendered sheet for thermoforming (at a quoted typical rate of up to 500 kg h -1), speciality sheeting, and a wide range of rigid and flexible PVC sheeting for limited local markets (e.g. in developing countries). The meltcompounding and feed section of the line comprises a planetary extruder feeding directly into a single-screw metering extruder (LID ratio 8: 1) which feeds its hot extrudate (formed into a strip by a simple slot die) into the nip of a three-roll calender. Both extruders are available in several sizes (the planetary screw size ranging from 100 mm to 290 mm, with rated outputs for the latter size of about 3300 kg h- 1 for uPVC and 4000 kg h- 1 for pPVq,3 but the metering extruder always has the larger screw size (e.g. 200 mm if the planetary machine's screw is a 140 mm one): this permits running at screw speeds low enough to avoid substantial work-heat input into the stock. The compounding extruder takes a powder blend feed introduced by a hopper-mounted vertical force-feeder. The hopper also houses a metal detector, particularly important if scrap is being processed (although metal detectors are standard equipment in pre-calender trains-see below). A special screw design featured by the planetary extruder is said to make for good output rates with compositions based on PVC polymers of high K value, and with highly filled compounds. * Berstorff Maschinenbau GmbH, Hanover, West Germany-see also Table 14.3 in Chapter 14.

814

W. V. Titaw

Plate K Calender train (AECI Ltd Vinyl Products Division Midland Factory, RSA-eourtesy Mr. W. B. Duncker). (1) Part of the compounding and feed section.

As indicated in Fig. 18.2 (and discussed also in Section 13.4.2(b) of Chapter 13), the final member of a typical pre-calender train is a conveyor which carries the feed strip up to the calender feed bank. The conveyor may consist of one or more sections. The final section is usually swivel-mounted, so that the end can traverse the length of the feed nip for uniform feed distribution. To help maintain its temperature, the strip should have the lowest practicable specific surface (i.e. it should preferably have a circular cross-section or, if flat, be thick and narrow rather than thin and wide), and/or should be heated by some

18 Calendering of pvc

Plate K-contd.

815

(2) Rear view of calender.

means (for example by IR heaters) if it is carried over a long distance (say more than about 2 m). Where heating is employed the material of the conveyor should be suitably heat resistant. The metal detector, always employed in the pre-calender train to guard against damage to the calender rolls by any fragments of metal that may find their way into the PVC composition, is commonly positioned over the feed conveyor (although-as mentioned in passing above-other locations may also be used): see also discussion of the role of metal detectors in Section 13.4.2(b) of Chapter 13.

816

W. V. Titow

Plate L Calender train: part of the cooling-roll and take-off section (AECI Ltd Vinyl Products Division Midland Factory, RSA-eourtesy Mr W. B. Duncker).

(b) Calendering Several features of the calender and its operation have already been discussed in Sections 18.2 and 18.3 above. A few further important points should be mentioned. As the PVC composition fed to the calender is normally already gelled and molten, the machine's main task is to form it into a uniform sheet of the required thickness (although the material also receives a certain amount of mechanical working-see below). The forming is carried out gradually, in the course of passage through the consecutive

18 Calendering of pvc

817

roll nips. Two factors actuate the passage, their operation also governing the route the material follows through the machine (referred to as the 'sheet path') in that they determine whether the sheet is or is not transferred from one roll of a nip-forming pair to the other. These factors are the material's adhesion to the roll surfaces, and the ratio of the roll speeds at a nip, usually called the friction ratio. Unless it is grossly over-lubricated, a hot PVC calendering compound will adhere to a hot roll surface: the adhesion is always stronger to a matt than to a polished surface; it also usually increases with the roll temperature, although some compositions may be formulated to minimise or even reverse this effect by suitable selection of the lubricant system. Where the speeds of the nip-forming rolls are different, the material will run on the faster roll, if both have the same surface finish. Otherwise the effect of the finish is strongly dominant, that is to say if one of the nip-forming rolls is polished and the other matt, the hot sheet will tend to remain on, or transfer to, the matt roll irrespective of differences in speed and/or temperature. In practice the sheet path is conveniently established-and changed where necessary-by suitable settings of roll temperatures and friction ratios. Save in the special case of the so-called low-temperature mode of operation (see Section 13.4.2(b) of Chapter 13) it is usual to have the roll temperatures (as well as the speeds) going up with roll numbers. Note: If the sheet does not lap the last roll (e.g. roll No.4 in the

arrangement of Fig. 18.1(ii» then that roll will normally be run at a lower temperature, and usually a lower speed, than its partner (roll No.3 in the figure) which is required to retain the sheet. Some examples of roll temperature settings in particular runs on particular machines are given in Tables 18.1 and 18.2. As can be seen, the individual settings for a nip-forming pair can be very close: as a general rule, the maximum difference between them should not normally exceed lO°e. If, with optimum friction ratio, a difference of this order still does not make for satisfactory operation, the temperature measuring and control equipment should be checked and/or the lubricant system of the composition re-formulated. The nature of the compound, and in particular its lubrication, is always a factor in the roll temperature settings: in general, for normal industrial

TABLE 18.1

Some mustrative Features of Industrial Production of PVC Sheeting on an 'L' Type Calender with Highly Polished RoDs (Based on data supplied by Mr D. J. Sieberhagen of Vynide Ltd.) Composition type Rigid (crystal clear)

Flexible (opaque)

Basic formulation PVC homopolymer resin Stabilisers Lubricants Plasticiser Processing aid Impact modifier Filler Pigment

Processing conditions High-speed mixer Tool tip speed Mix temperature on discharge Cooler mixer Tool tip speed Mix temperature on discharge Buss-Kneader Compound temperature at die head Internal mixer Discharge temperature Two-roll mill Roll 1 temperature Roll 2 temperature Calender Roll 1 temperature Roll 2 temperature Roll 3 temperature Roll 4 temperature Stripping device Cieneral temperature Embossing roll Temperature Cooling train Temperature

87 pbw (K value 57) 2·5 pbw (organotin) 1·5 pbw (internal! external)

100pbw (K value 71) 2·5 pbw (BalCd liquid) 0·5 pbw (external) 46 pbw (DIDP)

5pbw 8pbw 5 pbw (coated CaC0 3) 8pbw

TABLE 18.1-eontd. Composition type Rigid (crystal clear)

Flexible (opaque)

Sheet properties

Tensile strength (yield)" Elongation at breaka •b Tear strength b (ASTM D 1922): in machine direction in transverse direction Tensile impact strength (DIN 53448)

49-60MPa

22-29MPa 206-273%

4·5--6·9 kgfmm- 1 6·(}-8·4 kgf mm- 1

8·7-11·3 kgf mm- 1 12·1-12·9 kgf mm- 1

323--555 kgf cm cm- 2

a Dumbbell b

specimens, with parallel central portion 10 mm wide. Sheet 251-350 11m.

TABLE 18.2 Some Wustrative Features of a Laboratory Preparation of Flexible PVC Sheeting on an Inverted 'L' Type, 61-cm (24-in) Calender Producing at 3mmin- 1 Formulation

PVC resin-Corvic R65181 a Stabilisers: BalCd liquid complex epoxidised oil Plasticiser: DAP Lubricant: stearic acid Calender

Offset (No.1) roll temperature (0C) Top (No.2) roll temperature COC) Middle (No.3) roll temperature (0C) Bottom (No.4) roll temperature (0C) Mechanical properties of sheeting

Tensile strength (MPa): in machine direction in transverse direction Elongation at break (%) in machine direction in transverse direction Tear strength (N per mm of thickness): in machine direction in transverse direction

100 phr 2 phr 3 phr 47phr 0'5phr For 0·125 mm sheeting

For 0·500 mm sheeting

155 155 160 165

158 164 168 170

0·125mm sheeting

0·500 mm sheeting

16·2 16·4

20·5 19·4

225 250

305 300

63 63

67 68

A high molecular weight VCNA copolymer containing 2% VA; porous, easyprocessing particles; recommended for extrusion and calendering of thick sheet.

a

820

W. V. Titow

production, these will be within the extreme overall range of 15O-195°C. The material of the banks at the roll nips experiences an amount of shearing and friction increasing with roll speeds and speed ratios: when these are sufficiently high the energy input can result in substantial temperature rises. Such work-heating must be allowed for in the roll temperature settings, to ensure that the composition is not overheated (in some cases the rolls may have to be kept at temperatures below the material temperature aimed at). Subject to this general consideration, and any special ones that may arise in individual cases, it is normally desirable to operate at the highest practicable material temperature for ease of processing (lowest material viscosity) and good ultimate sheet properties. With roll temperature settings and friction ratios increasing from feed to delivery, the material temperature also rises: in the final nip it can, for a short time, be as high as 200°C (although accurate measurement is difficult).8 For material coming off the last bowl a fairly typical range is 17o-175°C. (c) The Post-calender Train The post-calender section of a modern line is shown schematically in Fig. 18.3, with the principal components clearly labelled. The figure is largely self-explanatory, but the following additional points may be mentioned. STRIPPER ROLLS, AND OFF-THE-CALENDER STRETCHING

The hot sheet is removed from the calender bowl on which it runs after the final nip by a stripper (pick-off) roll. Whereas formerly the use of a single pick-off roll was quite common, in modern practice a set of rolls is employed, in which the first stripper roll is backed by a large number of similar rolls (see Fig. 18.3), temperature-controlled in groups of two or more: this makes for good control over sheet thickness and for uniform, gradual cooling, or close maintenance of temperature (with provision for an extra heat boost by an IR heater as shown in Fig. 18.3, or by passage over a hot drum) if the sheet is to be embossed in-line. The fact that each side of the sheet is in contact with alternate rollers (see Fig. 18.4) is a factor in the uniformity of temperature control. In the common arrangement where the first pick-off roll counter-rotates with respect to the calender roll, the stripping action takes place by virtue of the greater peripheral speed of the stripper and the lapping of the sheet round it (and the other rolls of the set-see Fig. 18.4).

18 Calendering of pvc

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The running of the stripper rolls may be so regulated that the sheet coming off the calender is stretched to a predetermined extent whilst it is still at a temperature significantly higher than any that will be reached in subsequent processing or in service (in conditions where retraction of the stretch with consequent undesirable distortion is possible). This procedure offers the following advantages. Greater operational flexibility, in that a calender profiled, roll-loaded and set to produce one thickness of sheet can also be used for a range of lower thicknesses: furthermore, the lowest thickness limit is effectively brought down below that for which the machine can be set. Note: For example, sheet of thickness about 75 J.lIIl (a typical lower thickness limit for calendered sheeting) would normally be produced by stretching down a sheet calendered at a higher thickness, say about 100 11m.

A setting of the final calender nip (the gauging nip) wider than would be required for direct production of the sheet thickness that is ultimately achieved by stretching, means lower distorting forces on the rolls and somewhat less drastic working (and heating) of the material. Calendering at greater thickness can also reduce the overall power requirements. Stretching of the sheet should not be allowed to cause a substantial reduction in width, or be carried to the point of excessive uniaxial orientation of the ultimate product: uniformity of thickness should also be maintained. These considerations are factors which-together with the composition of the sheet and the actual calendered thickness-limit the extent of sensible stretch in a given situation. For suitably formulated sheet, calendered at a thickness reasonably above the minimum for which the machine can be set, stretching (and the corresponding thickness reduction) by a factor of about 2 would not be uncommon: say a sheet calendered at a nominal 280l1m might be stretched down to 14(}-120l1m. EMBOSSING

Although calendered sheeting can be embossed away from the calendering line in an entirely separate operation (which involves re-heating to a suitable temperature), embossing is often carried out in-line. This obviates the need to set up separate equipment (which, inter alia, has to duplicate sheet heating and cooling), saves heating energy (as the sheeting does not have to be re-heated, but merely kept

822

W. V. Titow

Fig. 18.3 An 'L' calender with post-calender section. Schematic hot on leaving the last nip of the calender), and avoids the acquisition of extra heat history. Before the advent of multiple stripping rolls with close temperature control it was usual to have the embossing station positioned as closely as possible to the calender, for minimum heat loss from the sheet, although a heat boost would still normally be given at least to semi-rigid or rigid material which, for optimum embossability,

A

B

Fig. 18.4 Schematic representation of some stripper roll arrangements. A, The lapping of hot sheet around the stripper rolls; B, a reverse stripping arrangement.

18 Calendering of pvc

T'''''

823

r

,....-,--------,

representation. (Courtesy of Mr D. J. Sieberhagen, Vynide Ltd.) must be at a higher temperature than that suitable for flexible sheeting. A typical embossing unit consists of a chilled, metal roller engraved with the emboss pattern, and a back-up roller-usually of substantially larger diameter-covered with a synthetic rubber to provide the requisite degree of resilience. It is normal for the backing roller to be also cooled-internally and/or externally-to prevent distortion and possibly deterioration of the rubber, and to maintain a reasonably constant level of resilience. Other factors being equal, the definition of the emboss pattern is the better the lower the temperature of the embossing roller and the higher the nip pressure, although the optimum pressure will vary with sheet formulation, as well as its temperature and rate of passage through the unit. One (either one) of the unit's two rollers is positively driven: the drive is independent of those of the other parts of the line, but must be suitably synchronised to avoid stretching the sheet. Note: Prevention of uncontrolled stretching, and generally of introduction of strain into the sheet in any part of the post-calender train (after the initial, planned hightemperature stretching off the calender-see above), and especially avoidance of such straining at relatively low temperature (below about 100°C), is an important considera-

824

W. V. Titow

tion in the running of the train. The presence of lowtemperature strain impairs the dimensional stability of the sheeting in any subsequent heat processing (e.g. heat lamination) and in service. A typical example4 of a problem that may be caused by such strain is the wrinkling or puckering of interior car door trim produced from calendered PVC sheeting: during its manufacture, or in subsequent use, the temperature inside a car can reach about 80a C (or even higher in hot countries), giving rise to these unsightly faults as a result of the reversion of any substantial strains originally introduced at or below such temperatures.

COOLING

Since the early days of calendering the most common method of cooling the sheet has been by passage around cooled drums (also known as 'cans'), although air-cooling on conveyors has been tried, and thick sheet has been cooled by passage through a water bath. l The temperature of the drums is kept at the required level by circulation of water through the annular space between the outer wall (which constitutes the working surface) and an inner shell. Hollow drums cooled internally by water sprays have been used in the past, but both their temperature control and their balance in running are less satisfactory for the precise cooling rate adjustment and fast, stable rotation required with high production rates. Apart from the wide adoption of the 'double-skinned' drum just mentioned, the other-and indeed the main-developments over the years in the typical cooling section of the post-calender train have been an increase in the number of cooling drums, with improvement in their temperature control (in small groups, or even individually), and improvements in the speed drive. These developments were aimed at a better, more complete attainment of the objects of the cooling operation, which are to bring the temperature of the sheet gradually and evenly down to that of the surroundings (or even somewhat below ambient), without thermal shock (which can impair the sheet properties, especially with rigid compositions) and without introducing low-temperature strain. In typical modern practice these objects are achieved by passing the sheet around a large number of cooling drums, running uniformly (under an independent drive but in synchronisation with the rest of the train), and temperature-controlled at gradually decreasing values: as an

18 Calendering of pvc

825

example, in the cooling section shown in Fig. 18.3, drums 1-3 might be kept at 50°C, drums 4-6 at 40°C, drums 7-9 at 25°C, and drums 10-12 at a temperature about 5-lOoC below ambient. As indicated in the figure, the sheet is lapped around the drums in such a way that each of its surfaces is alternately in contact with consecutive drum surfaces: this promotes even, uniform cooling. SHEET THICKNESS MEASUREMENT AND CONTROL

The monitoring of sheet thickness is a necessary part of production control. For many years a ~gauge has been virtually standard equipment for on-line thickness measurement. This device determines continuously the extent to which the passage of a beam of electrons from a radioactive source scanning across the moving sheet is obstructed by the sheet material. Thus the sheet property actually measured is the mass per unit area; but as this is directly proportional to thickness (since the density of the material is fixed by the composition) the read-out is in terms of thickness (and its variation across the sheet).

Note: The nature of the measurement also means that the thickness obtained is gravimetric thickness. * For a plain sheet this is the same as the directly measurable 'geometrical' thickness. For an embossed sheet-where the geometrical thickness cannot be measured because of the surface contouring produced by the emboss-the gravimetric thickness is the appropriate one, and it is thus particularly useful that this is what is measured by the ~gauge. More recently, the scanning ~gauge has been integrated into a complete, computerised on-line control system incorporating feedback control loops: the thickness data from the gauge are utilised by microprocessor-based controls which automatically make continuous adjustments to the roll-crossing and roll-bending devices to keep the * The notional average thickness (t) which, for a given piece or section of uniform sheet is determined by the area (A), mass (m) and density (p), in accordance with the relationships: m/At = p;

t = m/Ap

Standard laboratory methods of measuring gravimetric thickness9-normally prescribed for embossed sheeting-involve weighing a specimen of known area and determining, separately, the density of the material.

826

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V. Titow

sheet thickness and profile within acceptable tolerances around the appropriate set target values. This kind of sophisiticated, integrated control system is exemplified by the well-known Measurex 2001/25 Vinyl Calender Control System *.10 In addition to the automatic closedloop control this provides multi-colour visual data displays and print-outs of production and control data (including sheet profile plots and figures in relation to the target values), process summaries, trend plots, etc. A particular feature is the 'target adaptive control' (TAD): this automatically adjusts the sheet thickness very close to a specified percentage-of-nominal limit, so that the finished sheeting area or weight per roll is optimised. EDGE TRIMMING

The edges of the sheet are trimmed, to eliminate unevenness which commonly arises in calendering. The trimming is normally done towards the end of the line, after cooling (cold trimming), although hot trimming on the calender is also possible. It is sometimes claimed that the latter procedure is advantageous because of the particular ease of re-circulating the hot trim by feeding it directly to the final two-roll mill or metering extruder of the calender feed section, or even straight into the calender feed nip. However, with a feed section of the kind illustrated in Fig. 18.2 there is normally no great problem about feeding the cold trim from rigid sheeting directly to the two-roll mill (of the B route in the flow diagram), or that from flexible sheeting into the internal mixer. In the former case, an end of the strip of rigid trim (which is wound into a roll during the trimming operation) is fed into the nip of the mill, where it is continuously melted and incorporated into the bank of hot stock as it is being unwound. The rolls of flexible trim strip can simply be dropped, without unwinding, into the internal mixer, where the heat and shearing action during the mixing operation will normally be sufficient to disperse them completely into the charge of virgin material, providing that the proportion of re-work so introduced is not excessive. Thus hot trimming on the calender is not really essential for fairly direct re-processing of trim, whilst it does present certain problems. One is that a bead tends to form on the hot-trimmed sheet edges: this can cause difficulties in the ultimate winding and-in any case-makes the edge finish less neat than that produced by cold trimming. Another *Measurex Corp., USA.

18 Calendering of pvc

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disadvantage is marking of the calender roll surface by the trimming knives. Cold trim can also be comminuted (e.g. by freeze grinding) and re-circulated by addition, in suitable proportions, to the high-speed mixer, where it is incorporated into the powder blend being produced from the components of the formulation (see Chapter 13, Section 13.4.2(b)). WIND-UP

Most calendered sheeting is wound up into rolls for subsequent storage, handling in post-calendering operations (e.g. printing or lamination), transport, etc. Note: The sheeting may occasionally be slit in-line prior to winding, where a number of specified, narrower widths is required. It is also sometimes cut into lengths or panels for further processing (e.g. press polishing or lamination) or for use: in such cases the roll-winding gear is replaced by cutting devices (rotary kn:ves or guillotines) followed by any necessary stacking and/or take-off arrangements.

The winding arrangement widely used in modern practice for both rigid and flexible sheeting is a centre core wind-up unit, in which the sheeting is wound onto a core (of wood or thick cardboad) mounted on a mandrel turned by a constant-torque drive that keeps the winding tension in the sheet at a constant, set value despite the rise of peripheral speed as the roll builds up on the core. It is important that the tension should be both constant and as low as possible (consistent with the production of a reasonably tight, stable roll), to minimise the introduction of low-temperature strain into (and its fluctuation in) the sheeting. It is self-evident that the drive of any wind-up unit, whilst independent, must be synchronised ('tracked-in') with the calender speed and the other drives in the post-calender train. A cheaper system, formerly in common use (and still employed in some lines) for flexible PVC sheeting is contact batching. In this, the core is not rotated directly, but driven round by surface friction between the roll of sheeting being built up upon it and a revolving wind-up drum. With this arrangement the tension in the sheet is more variable, and the control over introduction of low-temperature strain less good, than with a centre-core wind-up. Rigid sheeting is not wound up by contact batching, because the surface friction between

828

W. V. Titow

the driving drum and the roll of sheeting is not sufficient to maintain-in the relatively stiff material-even tension at the required level, so that slipping occurs which disrupts and impairs the operation. 18.4.2 Special Lines and Arrangements

(a) Calendered Flooring Lines Heavily filled vinyl/asbestos flooring compositions are difficult to process on a conventional calender because of their stiffness, hardness and relatively low resin content. Moreover, the thickness in which the material is required may range up to 5 mm. For these reasons material of this kind (for use as continuous flooring or cut into tiles) is calendered on a sequence of two or three individual two-roll calendering units, with vertical or inclined roll arrangement. The compounding and feed section of a typical line may be similar to that shown in Fig. i8.2(A), but with two mills between the internal mixer and the first calendering unit. In the production of the familiar mottled PVC flooring or tiles, multi-coloured vinyl chips are added either on the first (dumping) mill, or the second (sheeting) mill which feeds the first calendering unit. The functions of this and the subsequent units is essentially to roll out the sheet as it makes its straight passage through the nip, so that the sheet thickness is progressively reduced. The operation is thus somewhat akin to the rolling out of ingots in sheet metal production. The sheet emerging from the nip of the final unit is cooled and wound or cut up, as required. Fully flexible flooring compositions, whose relative resin content is substantially higher, can be processed in the normal way: three-roll calenders are quite often used, or four-roll calenders operated with a 'spinning' bank or a 'spewing' bank of stock before the second nip (both these promote maintenance of material and temperature homogeneity in the stock). Sheets of suitable flooring compositions calendered in the normal way on a three- or four-roll calender can be laminated-by various techniques-to produce multi-layer flooring of the type conventionally, if somewhat incongruously, styled 'homogeneous'. The individual plies may be of the same or different composition. For example, a three-layer laminate (which is fairly common, although flooring with a higher number of plies is also produced), may be made up of a highly filled base layer, a medium-filled middle layer with a decorated surface, and a clear, tough, top (wear) layer: alternatively, three layers

18 Calendering of pvc

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of sheeting of the same reasonably wear-resistant formulation, and calendered to the same thickness (fairly typically about 0·5 mm), may be laminated together, the sheeting with the best looking surface being used as the top layer. (b) Lamination on or at the Calender Calendered PVC films can be laminated-on the calender or in-line-to other continuous web materials, such as textile fabrics, felt, paper and films. Lamination in an entirely separate operation is mentioned in Section 18.7 below. In principle, the range of PVC-coated fabrics and papers which can be produced by lamination with flexible calendered film is equivalent to that of similar materials made by paste coating. In practice the coating route offers greater formulation versatility and operational flexibility, especially for relatively short runs, for multi-layer coatings, and where frequent formulation changes are made. Calender lamination can be economically attractive where long, continuous runs without formulation changes are involved. Product properties are broadly comparable, but the range and quality of cellular coating layers are better in paste-produced coatings, whilst solid coatings of this kind are completely free from orientation and strain, which may not invariably be the case with some laminated film coatings. There are three basic ways in which a freshly calendered PVC film can be laminated to a continuous web of another material. These are schematically illustrated in Fig. 18.5, which indicates the modifications to the normal calender set-up entailed by each method. Additional equipment is also normally necessary for unwinding the web to be laminated (substrate to be coated) and its conveyance to the laminating nip, as well as-especially with fibrous webs (fabrics and paper)arrangements for drying and pre-heating the web. The in-line lamination method shown in Fig. 18.5(C) is of special interest with heat-sensitive SUbstrates, or where an adhesive layer is to be applied prior to lamination. In the calender nip lamination arrangement illustrated in Fig. 18.5(A) the degree of penetration of the hot film into a porous substrate, say a fabric being coated by this method, increases with decreasing gap between the nip-forming rolls and with increasing friction ratio between them: zero friction and a suitably wide gap should be employed for minimum penetration. Similar considerations apply to fabric lamination on the calender roll with a squeeze roller, as shown in Fig. 18.5(B).

830

W. V. Titow

A

B

Fig. 18.5 Lamination or web coating with PVC film on an inverted 'L' calender. Schematic representation. A, Nip lamination; B, lamination, with a squeeze roller, against calender bowl; C, in-line lamination.

18.5 THE FORMULATION ASPECT

In formulating PVC compositions for calendering-as indeed with any kind of PVC formulation for industrial processing-it is necessary to take into account the considerations and requirements arising from the nature and conditions of the process, together with the end-use requirements and cost considerations, and to arrive at the best practicable compromise. The nature of the calendering process makes the lubricant system of cardinal importance, especially the extent and balance of external lubrication which is the formulation factor directly instrumental in the degree of adhesion of the composition to (and hence also ease of its release from) the calender rolls in the course of processing. External lubrication is necessary in both plasticised and unplasticised formulations. The role of internal lubricant additives in rigid compositions, whilst important, is more in line with the relevant general requirements of melt processing (see Chapter 11, Section 11.1). The same is broadly true of processing aids.

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Much calendered sheeting is surface printed, whilst some largeoutlet applications (e.g. as reservoir lining, upholstery material, luggage covering) involve welding of the sheeting (ct. Chapter 20). Print adhesion and bond formation in welding can both be impaired by excessive external lubrication (as can the fusion in press laminationsee Section 18.7.2 below). The intensive working and relatively high temperatures experienced by calendering compositions in modern high-rate production entail the need for good heat stabilisation. This is especially important with rigid compounds, and most particularly those used in the production of high-clarity transparent colourless foil and sheeting for packaging applications. For these reasons clear, rigid PVC calendering compositions (as well as some pigmented ones) are commonly stabilised with organotin stabilisers (where relevant, ones permitted for food contact applications) . Barium/cadmium systems, with or without zinc, are the stabilisers most widely used in flexible calendering compositions. The liquid versions are of particular interest for ease of metering for compounding, and good dispersibility. However, in highly plasticised compositions a solid system may be preferable to avoid lowering the melt viscosity. This is the current position, although the question of toxic hazards associated with the use of cadmium compounds has been receiving increasing attention and development effort, which has already produced some viable (albeit still not really fully operationally equivalent) alternative stabiliser systems (see Chapter 9, Section 9.4.3(b)). Lead stabilisers, historically the first to be used in calendering formulations, are no longer employed on any scale because of their toxicity, somewhat inferior effectivity in high-speed processing, and generally limited suitability for clear formulations. The PVC resins used in calendering compositions are grades of suspension or mass polymers (the latter favoured for some high-clarity sheeting), of particle size, size distribution, porosity, and bulk density appropriate to the requisite high-rate processing suitability, inter alia, with regard to such features as ease and uniformity of dry flow in metering and dispensing, ready blending with other formulation components in high-speed mixing (including fast, uniform plasticiser absorption), and ease of fluxing and gelation. Flexible compositions are normally based on high K value resins (see, for example, Tables 18.1 and 18.2). This promotes good physical properties important in

832

W. V. Titow

service, whilst the associated increase of melt viscosity in processing (which prevents the use of high molecular weight resins in rigid compositions) is actually helpful-especially with highly plasticised formulations-as it compensates, at least to some extent, for the opposite effect of plasticisers. Indeed, one of the functions of a filler in a strongly plasticised material may be to bulk ('sludge') it up for higher melt viscosity. In rigid compositions, the customary use of polymers of relatively low K value is dictated (as also in other melt processes, e.g. injection moulding, blow moulding-d. Chapters 15 and 17) by the need to ease processing behaviour by all available means. The same consideration lies behind the use of copolymer resins in some types of composition-e.g. VCN A copolymers in calendered flooring, or in sheeting for thermoforming-(although acrylic-modified, homopolymer-based compositions are also widely used for the latter), and copolymers with vinylidene chloride (e.g. Breon CS lOOl30-BP Chemicals Ltd) in some special formulations. The choice of plasticiser(s) for a flexible calendering composition is primarily influenced by end-use and cost requirements, but consideration should also be given to correct melt rheology in processing and to possible effects (desirable or otherwise) of the plasticiser(s) upon the degree and balance of lubrication. Chlorinated polyethylene of appropriate (relatively high) chlorine content (cf. Chapter 11, Section 11.2.2) is being increasingly used as a solid plasticiser in calendered sheeting, because of the resulting combination of permanence of plasticisation with good resistance to weathering, important in such products as, for example, reservoir linings. The cost of blends of this kind can be reduced-without too drastic an impairment of properties-by the incorporation of phthalate plasticisers. An interesting study by Young l l indicated that some compositions based on a blend of PVC resin of relatively high molecular weight (Diamond 450Diamond Shamrock Chemical Co.) with a chlorinated HOPE (containing 42% or 46% chlorine: respectively, XO 2243.49 and 2243.51-Dow Chemical Co.) incorporating diundecyl phthalate (Monsanto) or linear phthalate esters (Santicizer 71l-Monsanto) compared favourably in mechanical properties and long-term performance with ones plasticised with polymeric plasticisers. The fillers most commonly encountered in calendered products are asbestos fibres in PVC flooring, and calcium carbonate, also used as a particulate filler in some flooring compositions, and as a cheapening filler with some processing-aid and reinforcing effects in certain other

18 Calendering of pvc

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types of sheeting (e.g. supported or unsupported vinyl upholstery material). The general use and effects of both these kinds of filler are discussed in other chapters (see in particular Chapter 4, Section 4.6.2; and Chapter 8, Sections 8.2.1, 8.3 and 8.4), with reference, inter alia, to the special stabilisation requirements and rheology of asbestoscontaining flooring compositions, and to the principal effects of the surface treatment and particle size characteristics of calcium carbonate fillers. With regard to the latter, it may be recalled that whilst with untreated grades plasticiser demand (and such reinforcing effects as may arise) is normally an inverse function of paticle size, this effect can be counteracted by surface treatment with stearic acid, some stearates, and certain other substances. Apart from reducing plasticiser absorption by the filler (and hence the amount of plasticiser required in the formulation) surface treatment can improve the filler's dry-flow characteristics and dispersibility in dry blending and melt compounding. However, possible lubricating effects of the surface coating must be considered and allowed for in the choice and formulation of the lubricant system. Some useful data on the effects of particle surface area and surface treatment of CaC0 3 fillers on the properties of calendered flexible PVC sheeting have been published by Mathur et al. 12 The basic make-up of some calendering compositions is indicated by the formulation examples in Section 4.6 of Chapter 4, and in Tables 18.1 and 18.2 here. However, successful formulating for the calender calls for considerable skill on the part of the formulator as well as knowledge not only of calendering and PVC materials generally, but also of the special requirements (especially with regard to lubrication) of the particular equipment concerned, as these may vary significantly from one calender line to another. 18.6 SOME FAULTS AND DEFECTS OF CALENDERED SHEETING

Several of these can also occur in extruded sheeting, in the same or similar form (see Chapter 14, Section 14.2.2(e)). 18.6.1 Simple Dimensional Faults

These are rare with good modern equipment and correct operation. However, the following may be mentioned for the sake of completeness.

834

W. V. Titow

Excessive thickness variation and profile irregularity: This fault can result, possibly temporarily, from faulty operation of the monitoring and/or control equipment, but would not continue undetected for long on a modern line properly run. Local dimensional irregularities: These are associated with 'bagging' of the sheet in the transverse direction, or longitudinal sagging, on stripping or in further passage through the post-calender train. These faults, which are due to uneven or inadequate support of the sheet while still hot, do not normally arise with modern equipment properly operated. Thickness generally too low or too high: Such faults can be caused by incorrect calender setting, or-with plasticised compositions-may sometimes arise as a result of, respectively, over- or underplasticisation, even if the setting is basically correct (for a properly plasticised composition). 18.6.2 Structural Defects

Presence of strain imparted at relatively low temperature: The origin and consequences of this fault are mentioned in various parts of Section 18.4.1(c) above. Unsatisfactory lay-flat behaviour: This can cause problems in winding and in subsequent handling, processing and use of the sheet. The fault may be associated with development of excessive skin-and-core structure in the sheet (and/or irregularities of such structure) during processing on the calender and subsequent cooling (see Section 18.3 above); it may also be caused or contributed to, by the kind of uneven stretching or inadequate support of the hot sheet that leads to bagging and sagging (ct. Section 18.6.1). Excessive uniaxial orientation: This can be caused by over-stretching the sheet on stripping from the calender. Although imparted at a temperature which should be high enough not to lead to retraction troubles in subsequent use (as in the case of low-temperature strain) such stretch can result in unduly high differences in strength in the machine and transverse directions (low tear strength lengthways; low transverse tensile strength) and excessive longitudinal retraction at

18 Calendering of pvc

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high temperature (high-temperature strain release). This fault is rare in modern calendering practice. Below-par mechanical properties: These are manifested as generally low values of the properties most commonly measured, viz. tensile strength, extensibility, and tear strength of sheeting (both flexible and rigid), and low impact strength of rigid sheeting. The most likely cause of this fault is insufficiently high processing temperature on the calender. 18.6.3 Faults Manifested in Appearance 'Fish-eyes' (nibs): These hard, undispersed particles of polymer may be introduced with substandard PVC resin, or persist in consequence of incomplete gelation (see also Section 14.2.2(e) of Chapter 14). Flecking ('fleck marking'): The flecks are a characteristic manifestation of an incorrectly functioning lubricant system (often excess of external lubricant). The fault may be aggravated by (and in extreme cases even due to) under-gelation of the composition, and/or incorrect calendering temperatures. Plate-out, on sheeting and/or equipment: The origins and nature of this fault are discussed in Chapter 9 (Section 9.7) and Chapter 14 (Section 14.2.2(d». Some of the main factors responsible are the same as those involved in flecking, although plate-out is a more complex phenomenon. Pluck marks: These are manifestations of poor release of the hot sheet from the calender roll surface. The release difficulty can be c~used by under-lubrication of the composition, or sometimes by overheating in compounding and/or too high roll temperatures in calendering. Heat lines: These marks take the form of continuous fairly close-lying lines running in the machine direction. They are usually associated with difficulty of sheet release, commonly due to excessively high calender roll temperatures; over-processing of the composition in the compounding section may also be a factor. In severe cases heat lines may be succeeded by pluck marks.

836

W. V. Titow

Surface roughness: In severe cases this may take the form of pronounced rough surface marks. The fault is usually due to under-gelation and attendant incomplete homogenisation of the sheet material. The manifestation is broadly in line with extrusion experience, in that surface roughness in extruded products (if not caused by purely rheological factors) can be reduced by increasing the processing temperature and residence time, both of which promote gelation. Bareich's discussion 13 of the form and measurement of surface roughness in pPVC extrudates, and of the processing factors instrumental in its origin, is of some interest here as part of the relevant background. In some cases surface roughness may be aggravated, or even caused, by incorrect operation of the calender. Bank marks: Typically these have the form of irregular areas of slight surface roughness, reminiscent of water marking of paper. In severe cases the marks may merge into an overall orange peel effect. The fault is caused by patches of compound cooler than the bulk of the stock going through the roll nips: this can be due to incorrect size (too large or too small) of the stock banks, or to temperature variation in the stock as delivered to the feed nip. Colour streaking (in coloured sheet): The streaks are due to poor homogenisation of the composition, usually associated with undergelation (which may also be manifested in some of the ways mentioned above). Overall discoloration: Commonly this is a result of incipient or substantial polymer degradation (the severity being reflected in the depth of colour developed), attributable to inadequate stabilisation, or overheating of the composition in compounding. Dark specks: These may be either foreign particles small enough to pass through any straining device employed in the feed section (e.g. the screen of an extruder/strainer), or particles of partly degraded material resulting from overheating at the compounding stage. 'Crow foot' marks: The marks (in some cases also referred to as 'pine trees'), which resemble a bird's footprint, are usually attributable to poor dispersion of particulate additives (fillers or pigments).

18 Calendering of pvc

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Pinholes: This is a fairly common fault in calendered sheeting, especially the flexible type, often due to faulty gelation. Pinholing adversely affects the barrier, strength and other properties of the sheeting (including the appearance, in more severe cases). Large pinholes, or substantial incidence of smaller ones, are normally detectable at the inspection panel before the final wind-up (see Fig. 18.3). However, special automatic inspection by laser scanning 14 offers a particularly effective means of detecting the presence and loation of pinholes, as well as gauging their size; the time elapsing before any necessary remedial action can be taken is also minimised. 18.7 FURTHER PROCESSING OF CALENDERED SHEET 18.7.1 Press Finishing Substantial quantities of sheets produced by cutting up calendered sheeting (especially rigid and semi-rigid) are finished by hot pressing between suitably surfaced metal plates (polished or matt). A stack of sheets, interleaved with the plates, is usually processed in one cycle.

Note: A typical cycle, which comprises a heating and a cooling period of roughly equal durations, may last about 40 min, with the set temperature peaking at about 175°C, and the actual material temperature only slightly lower. The operation is sometimes referred to as press surfacing. The term 'planishing' has also made its appearance: 15 the question of its purely linguistic appeal and merit apart, it is the more accurately descriptive in the technical sense, in that the hot pressing not only imparts particularly good surface finish to the sheets, but also regularises sheet thickness and relieves internal stresses and residual strains. Pressfinished sheets are used for products in which the resulting properties are important, e.g. offset printing plates, draftsman's instruments (transparent curves, set squares, etc.), name plates, calculator cases, computer floppy discs, and some thermoformed products.

18.7.2 Press Lamination As will be mentioned in Chapter 20 (Section 20.1) this method is used to combine a number of calendered sheets into one of greater thickness

838

w. v.

Titow

than the top limit for direct calendering. The process is similar to press finishing, except that only two metal plates are used, on the top and bottom of the stack, so that the sheets forming the stack are laminated together and the surfaces of the resulting thick sheet acquire the desired high-quality finish. The sheets to be combined, including the outermost ones, need not be of particularly good quality except for uniformity of thickness, necessary for straight, parallel-faced product. Moreover, the pressed sheet may be matt on one side and glossy on the other-a surface finish combination not obtainable directly by calendering. This combination can also be obtained on individual thin sheets by press finishing. 18.7.3 Surface Treatments

(a) Printing Much calendered sheeting is printed with decorative designs, e.g. in the production of vinyl wall coverings, self-adhesive decorative surface-facing sheets (including imitation veneers), vinyl shower curtaining, etc. The methods used are mentioned in Section 20.3.5(a) of Chapter 20. Of these, rotogravure printing (with solvent-based inks) is the most widely used, especially for plasticised sheeting. (b) Coating

Coatings are applied to calendered sheeting-by variants of the common roller or doctor blade techniques-for various purposes. Plasticised sheeting is often coated with a lacquer for surface protection (especially against soiling and abrasion) and/or to provide a decorative finish. In general, the objects of lacquer coating, and the lacquers and coating methods used, are the same as those described for paste-produced PVC layers on fabric and other support in Chapter 22 (Section 22.2.6) and mentioned in Chapter 25 (Section 25.4). Various adhesives are also coated onto sheeting as a preliminary to lamination with certain other materials (see below), and in the manufacture of self-adhesive surfacing sheeting, tapes, plasters, flocked wall-coverings, and the like. (c) Embossing In some operations it is convenient to emboss away from the calender line. The two rollers forming the embossing nip are essentially like

18 Calendering of pvc

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those used in the post-calender train: the sheeting must be pre-heated to a suitable temperature, and subsequently cooled evenly with minimum strain. 18.7.4 Continuous Lamination

Continuous lamination of calendered sheeting to similar sheeting, other plastics sheet materials, and various fibrous substrates (fabric, paper) is often carried out on laminating equipment entirely separate from the calendering line. This alternative to lamination on the calender in many cases offers greater convenience and operational versatility. The key element in a typical operation is the bringing together of the sheeting and the other web material and their joint passage, under suitable pressure, through a laminating roller assembly. In heat lamination the sheeting (and, where appropriate-as, for example, with a textile fabric-also the other component) is pre-heated, and the lamination takes place under further heating. With adhesive lamination, the adhesive layer may also require pre-heating (e.g. for activation of a melt adhesive, or gelation and softening of a PVC paste coating serving as a bonding layer). An example of heat lamination of two calendered PVC sheets is provided by a common method of production of veneer-style facing sheeting for furniture and panels. In this, flexible sheeting with a wood-grain pattern printed on the surface is laminated with transparent sheeting to serve as a wear layer and give good surface finish. Typically, the clear facing sheet is pre-heated by passage over a hot roller, and then brought into contact-under suitable tension and/or pressure-with the base sheet, on the surface of a drum kept at about 170°C. The resulting laminate, in which the two components should be completely merged (so that it has the appearance and characteristics of a single-layer sheet) is stripped off the drum by stripper rolls and may be either cooled directly or first passed through an embossing station. Cellular leathercloth may be produced by lamination of a base fabric with calendered sheeting (as an alternative to the paste coating methods discussed in Chapters 22 and 25). Various procedures are used. In one type of process, specially formulated thin sheeting which will act as a wear layer is assembled with sheeting containing a blowing agent, and with the base fabric: the three are laminated together under pressure at a suitable high temperature at which the blowing agent is

840

W. V. Titow

activated and controlled expansion initiated of the film constituting the middle layer between the surface 'skin' and the fabric. Special machines are available for this process, the 'Lembo' laminating unit being a typical example. In this kind of composite lamination the two PVC layers merge at the interface, but the fabric is usually bonded to the middle layer with the aid of a thin coating of PVC paste deposited on the surface of the middle-layer sheeting by a roller and fused in the laminating step. Note: The expandable sheeting may also be calendered directly onto the base fabric and the 'skin' layer applied in a separate operation. Expansion of the middle layer may also be carried out after lamination, in a separate passage through an oven.

An interesting example of lamination by adhesive bonding is the facing of calendered PVC sheeting (with or without fabric support) with thin polyvinyl fluoride sheet. Outstanding resistance to soiling and weathering is claimed for such laminates in use as, for example, wall coverings, greenhouse sheeting and other outdoor applications.

18.8 PROPERTIES AND APPLICATIONS OF CALENDERED MATERIALS In addition to the continuous on-line monitoring and control of sheet thickness and profile, and inspection for faults on- and off-line, some properties of calendered sheeting are commonly determined in the course of production quality control and for characterisation purposes generally. These properties are listed in Tables 18.1-18.4 together with numerical values that are either fairly typical of the product to which they relate (Tables 18.1 and 18.2) or representative of minimum requirements laid down in relevant standards (Tables 18.3 and 18.4). Similar information relating to PVC-coated fabrics is contained in Table 18.5. Other properties are also assessed in tests, where they are of interest in connection with the quality or suitability for service of particular kinds of sheeting. Some examples are: surface hardness of plasticised sheeting; print adhesion on printed sheeting; resistance to blocking of flexible sheeting; ply adhesion of laminated sheeting; emboss retention by embossed sheeting; electrical resistivity of sheeting for hospital use. Standard test methods are available and-in some cases-minimum

15 50

8 50

21 9 (plain-surfaced sheet) 11 (other surfaces, or laminated sheet)

160

250-900 14 180 21 8

Up to 400 (inclusive) 13

8

11 (laminated sheeting)

21 9 (single sheeting)

160

Up to 900 (inclusive) 13

7

35

75-250 14·5-15·9 150

b

a

Each numerical property value quoted relates to the determination method appropriate to the standard specification concerned. Medium stiffness general-purpose single or laminated thick sheeting with plain or embossed surface (one of the eight types of sheeting covered by the specification). c PVC sheeting for hospital-use; production method not specified. d The calendered sheeting covered by this specification, which also covers extruded sheeting (Type II) and cast sheeting (Type III).

Low-temperature extensibility (%, minimum) Extensibility after heat ageing (%, minimum)

Thickness (JlIIl) Tensile strength (MPa, minimum) Elongation at break (%, minimum) Tear strength (N mm- 1 , minimum) Dimensional stability: change in a linear dimension (%, maximum)

BS 3878:1982 c

(Type 1 sheetingd )

sheetin!/,)

(Type 103 BS 1763:1975

ASTM D 1593-81

BS 2739:1975

TABLE 18.3 Some Standard Property Requirements for Calendered Flexible PVC Sheeting

00 .f>.

~

"1:l

~

I

~

..... 00

Softening point (minimum) Dimensional change at 120°C (maximum) Heat deflection temperature (at 264lbf in- 2 fibre stress) (maximum) Hardness (Rockwell R) (minimum)

Tensile strength (minimum) Flexural strength (minimum) Elastic modulus in f1exure c (minimum) Impact resistance d

-

%

MPa MPa Number of failures in test °C

Units

--

-

-

60 15

38 2000 0

45 2500 Negotiable'

70 15

Type C3

Type Cl

-

50 Negotiable'

38 2000 Negotiable'

Type D

Numerical value for sheet type b

BS 3757:1978"

Ibf in- 2 Ibf in- 2 Ibf in- 2 ft-Ib per in. of notch (Izod)

Units

70 110

7000 11 000 400000 0·5

Type I

66 100

5000 8500 300000 3·0

Type Il

Numerical value for sheet type b

CS201-55"

°C R scale units

TABLE 18.4 Some Standard Property Requirements for Rigid PVC Sheeting

No failure in the mechanical splitting test prescribed in the specification

% % % %

% % % % increase decrease increase decrease

increase decrease increase} decrease

0 0 55

10

15

5 5

20 0 0 80

25

15 0

No delamination or disintegration in the acetone immersion test prescribed

a Both standards are directed to rigid sheeting generally, but Types Cl, C3 and 0 of BS 3757 are specified as calendered or extruded sheets. CS 201 is a US commercial standard. b BS 3757: Type Cl-general-purpose sheet suitable for most applications and fabricating techniques. Type C3--similar to Cl but with specific impact strength and possibly lower chemical resistance. Type D-particularly suitable for deep vacuum forming. CS 201: Type I-1:hemical resistant, normal impact resistance. Type II-1:hemical resistant, high impact resistance. C In BS 3757 applicable to sheet of minimum nominal thickness of 0·5 mm. dIn BS 3757 applicable to sheet of minimum nominal thickness of 1·0 mm. e Between supplier and purchaser.

Change in flexural strength (maximum)

After immersion in 100% acetic acid Change in weight (maximum)

Change in flexural strength (maximum)

Property retention After immersion in 80% sulphuric acid Change in weight (maximum)

Resistance to delamination

Total mass per unit area (gm- Z , minimum) Base cloth mass per unit area (g m- z , minimum) Coating mass per unit area (gm- Z , minimum) Tear strength (N per 50 mm, minimum) Lengthways Transverse

Property"

Standard

~

-

-

-

75 685

110 480

-

760

Grade V

-

-

110 685

795

Grade X

Type 2 (with PVC coating incorporating an expanded layer)

590

Type 1 (with solid PVC coating

BS 5790:Part 1:1979 (for PVC-coated knitted fabrics)

TABLE 18.5 Some Standard Property Requirements for PVC-coated Upholstery Fabrics

40 40

300

550

Grade A

29 29

240

420

Grade B

BS 5790:Part 2: 1979 (for PVC-coated woven fabrics)

1·27 1·14 700

1·09 0·97 700

-

-

-

3

3

33 400000 30 5

10

40

26

690

3

-

33 400000 30 5

26

380

33 400000 30 5

15 50

26

690

10 40

-

0·4

3

400000 30 5

580 26

0·4

3

300000 30 5

450 26

a Properties tested by the relevant methods of BS 3424, except for print wear, for which modified test methods are specified in both parts of BS 5790. b Requirements applicable to both lengthways and transverse strengths. COn a Martindale-type abrasion apparatus, under prescribed test conditions.

Bursting strength (kPa, minimum) Breaking strength (N, minimum b ) Coating adhesion (N per 50 mm, minimum) Elongation (%, minimum) Lengthways Transverse Tension set (% of actual elongation, maximum) Flex cracking (cycles, minimum) Surface drag angle (degrees, maximum) Heat ageing (% coating mass loss, maximum) Print wear (chan~e of appearance) (grey scale rating, mmimum) Thickness (mm, minimum) Mean Individual reading _ Abrasion resistance (cycles,C minimum)

w.

846

V. Titow

requirement specifications (d., for example, BS 1763, BS 2739, BS 3878, and the relevant standards among those listed in Appendix 1 and Appendix 3). However, producers, processors and purchasers of calendered sheeting sometimes use their own tests. For example, whereas a standard test for emboss retention (BS 1763 and 2739) prescribes immersion in water at 100°C for 10 min as the test treatment (whereupon the emboss pattern should remain substantially unaffected), other test treatments, involving different conditions and higher temperatures (up to 180°C in some cases), are also in use where they are relevant to particular processing or service conditions that the embossed sheet may experience. 1 The properties which are of interest in flexible calendered sheeting for reservoir lining are listed in Table 18.6, together with some relevant standard test methods and typical minimum requirements. TABLE 18.6 Calendered Flexible PVC Sheeting 1 IDOl Thick, for Reservoir Lining and Similar Applications: Typical Minimum Property Requirements Property

Specific gravity Tensile properties: Tensile strength Elongation at break Modulus at 100% elongation Tear resistance Brittleness temperature Dimensional stability Q

Test method

ASTM D 792, Method A ASTM 882

ASTM D 1004 (Die C) ASTM D 1790 ASTM D 1204 (15 min at 100°C)

Volatile loss

ASTM D 1203 Method A

Water extraction

ASTM D 3083

ASTM D 751 Method A Hydrostatic resistance Resistance to soil burial: original property retention Tensile strength Elongation at break Modulus at 100% elongation Q

In both the machine and transverse directions.

Numerical value

1·20 17MPa 300% 9MPa 50Nmm- 1 -30°C Linear dimensional change Q not to exceed 5% Not to exceed 0·5% by weight Weight loss not to exceed 0·35% 690 kPa

95% 80% 90%

18 Calendering of pvc

847

The following are some of the main application areas of calendered flexible PVC sheeting: Seepage barriers (swimming pool liners; lining for water reservoirs, effluent lagoons and the like); film-packaging applications; production of inflatables (both supported and unsupported film); production of baby pants; production of adhesive tapes and labels; motor car trim (door panels, head liners, crash pads); decorative surface coverings; awnings; furniture-facing sheet (veneer effects and others); wall coverings; facing sheet for metal and board panels for partitioning and building applications; production of book bindings, document cases, folders; shower curtains; tablecloths; mattress covers; floor coverings (continuous or tiles); luggage. Rigid calendered PVC sheeting finds substantial outlets in thermoformed packs and containers (blister packs; packs and trays for confectionery and sweets, pharmaceuticals, margarine tubs); lining and trim for public transport vehicles, aircraft and marine craft; display signs; production of venetian blinds; film-packaging applications. Press-laminated sheeting is used in chemical plant construction; wall cladding; tank lining; corrosion-resistant ducting; tunnel lining. Uses of PVC sheeting are also discussed in Chapter 26.

REFERENCES 1. Elden, R. A. and Swan, A. D. (1971). Calendering of Plastics, Iliffe Books and The Plastics Institute, London. 2. Stackhouse, N. (1978). 'Calendering and paste processing,' paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 3. Anon. (1977). Mod. Plast. Int., 7(7), 18. 4. Elden, R. A. (1977). In Developments in PVC Production and Processing-I, (Eds A. Whelan and J. A. Craft), Applied Science Publishers, London, Ch. 10. 5. Anon. (1980). Plast. Rubb. Wkly, 13th December, p. 7 and (1981). Plast. Rubb. Wkly, 24th January, p. 6. 6. Eighmy, G. W. (Jr) (1982). In Modern Plastics Encyclopedia 1982-1983, pp.220-2. 7. Watkins, W. D. (1976). Plast. Engng, 32(6),23-5. 8. Private communication from Mr T. Hodgson, Storey Brothers and Co. Ltd, Lancaster, England (1978). 9. ISO 4591-1979. Plastics-Film and sheeting-Determination of average thickness of a sample and average thickness and yield of a roll by gravimetric techniques (gravimetric thickness). BS 2782: Part 6: Method 631A: 1982. Determination of gravimetric thickness and yield of flexible sheet. (Identical with ISO 4591.)

848

10. 11. 12. 13. 14. 15.

w.

V. Titow

ASTM D 1593-81. Non-rigid vinyl chloride plastic sheeting. ASTM E 252-78. Thickness of thin foil and film by weighing. Anon. (1979). Plast. Techno!., 25(5), 24-9 and 49; and Technicalliteature of Measurex Corporation, Cupertino, California 95014, USA. Young, W. L. (1978). 36th ANTEC SPE Proceedings, pp. 750-3. Mathur, K. K., Greenzweig, J. E. and Driscoll, S. B. (1978). Ibid., pp. 732-5. Bareich, G. (1970). 28th ANTEC SPE Proceedings, pp. 569-72. Anon. (1982). Eur. Plast. News, 9(10), 44. Rusincovitch, G. (1982). Plast. Engng, 38(11), 31-3.

CHAPTER 19

Rigid PVC: Main Products-Production, Properties and Applications B. J.

LANHAM

and W. V. TITOW

19.1 INTRODUCTION The properties, processing and applications of rigid PVC are mentioned, in various contexts, in several parts of this book. Those references are accessible via the Index. In the present chapter, some salient technological and applicational aspects of rigid PVC materials are focused more directly in a brief review, with special reference to the most important products. The rigid PVC compositions discussed are in fact almost exclusively unplasticised (uPVC) materials. Although many compositions containing plasticisers in relatively low amounts might be classified as rigid PVC on a strict, formal definition, * and could thus qualify for inclusion here, the ways in which the presence of plasticisers affects the properties of a PVC composition (including 'antiplasticisation' by small proportions of plasticisers) are adequately covered in other parts of the book (see, in particular, Chapters 5,6 and 7, and Appendix 3). It may be noted in passing that in semi-rigid PVC compositions (which may typically contain up to about 20-30 phr of plasticiser) the

* See Chapter 1, Section 1.1 in conjunction with ISO 472-1979 and ASTM D 883-80: both these standards define a 'rigid plastic' essentially as a plastics material which, at a standard temperature and humidity (about 23°C and 50% RH respectively), has a modulus of elasticity (in flexure or in tension) greater than 700 MPa (= 1Q5 Ibfin- 2). A 'semi-rigid' plastic is similarly defined as one having a modulus of elasticity between 70 and 700 MPa (approximately 2 1Q4-1Q5 Ibfin- ). Typical modulus values for the material of main uPVC products are given in Appendix 3. 849

850

B. J. Lanham and W. V. Titow

relationships between plasticiser content and degree of modification of mechanical properties are often relatively complex and non-linear ('antiplasticisation' can be a factor in this). An early summary of some of these relationships was published by Jacobson. 1 The effect of the proportion of plasticiser in a semi-rigid composition (based on Pevikon R335 PVC resin-Kema Nord, Sweden) upon the impact strength is illustrated, for four plasticisers, in Fig. 19.1. Note: Whilst, as shown by the curves of Fig. 19.1, the impact strengths of these compositions pass through minima (not quite reached by the TIP curve within the plasticiser content range covered), the corresponding tensile strength plots pass through maxima (at about 20 phr for TIP, and 5-10 phr for DOP, DOA and DBS). The curves for elongation at break in the tensile test also exhibit minima at about 10-15 phr plasticiser.

As has been mentioned elsewhere in this book, uPVC compositions

120

DBS

11

DOA

:550 01

c

~40

+' Ul

+,30 u

E C1J

20 10

o

10

20

30

Plasticisar conttlnt I phr

Fig. 19.1 Effect of some plasticisers, at low content levels, on the impact strength of a PVC composition.

19 Rigid PVC: Main Products-Production, Properties and Applications

851

may be based on homopolymers (in conjunction with suitable modifiers where shear and heat processing is involved) or copolymers, which are easier to process (but normally have lower softening temperatures and are somewhat inferior in certain properties, especially mechanical and thermal). The copolymers most frequently used are VCNA (e.g. in gramophone records, flooring compositions, some vacuum-forming sheet especially in the UK) and VCNDC (e.g. in some calendered sheeting, some packaging films, and as extender polymer in special paste compositions for rigid products). Copolymers of vinyl chloride with propylene are also used in some uPVC compositions for ease of processing (see Chapter 1, Section 1.5.2, and Sections 19.3.1 and 19.5.3 here). Chlorinated PVC is often the polymer in rigid compositions for use at temperatures higher than the service temperature of ordinary uPVC (ct. Chapter 1, Section 1.6). The feedstocks in the processing of uPVC are nowadays mainly dry-blend powders, although melt-compounded pellet feeds are also used, especially in the injection moulding of parts, and compression moulding of gramophone records. Some extruded and blow-moulded products are also made from pellet stock, particularly where processing is by single-screw extruder, albeit dry blends are successfully run on single-screw machines (suitably vented). For the production of-for example-pipe or window frame profiles from powder (dry blend) feedstocks, this kind of equipment may advantageously incorporate: such features as special screw design (e.g. a 'wave' screw-see Chapter 14), and special feed arrangements (e.g. cram-feeders and vacuum hoppers). A few examples of the uses of commercial uPVC compounds representing both kinds of feedstock are given in Table 19.1. The principal advantage of pellet feedstocks is that, having been melt-compounded, they are fully homogenised (gelled) and thus the processing they undergo in the course of conversion into products (by extrusion or moulding) is not required to effect homogenisation (with complete gelation-see Chapter 14, Section 14.3) but only melting: this places less exacting demands on the processing effectivity of the equipment. The advantages of a dry-blend feed over pellets are: (i) lower cost of the compound; (ii) lower capital cost of production equipment for the compound; (iii) less extensive heat history acquired by the compound.

Q

Powder Pellets Powder

BS 3504 and 3506 BS 4514 BS 4576

Pressure pipe

Soil pipe fittings

Rainwater goods

Pellets Powder

Pellets

BS 4607 Part 1}

High-impact conduit High-impact profiles

Pellets Powder

Form

-

Relevant standard

Profiles (normal impact)

Nature

Product

--

B.I.P. Ltd: VX 105 B.I.P. Ltd: VI 109

B.I.P. Ltd: VX 115 B.I.P. Ltd: VI 109

BP Chemicals Ltd: PA 182

BP Chemicals Ltd: RA 166

BP Chemicals Ltd: RA 126 BP Chemicals Ltd: PA 181

Source and gradeQ

PVC feedstock

1·44-1·46 1·48-1·52

1·47-1·49 1·48-1·52

1·42

1·48

1·48 1·46

SG

By way of non-exclusive, illustrative example. Numerous feedstock compounds available from many different sources.

Injection moulding

Extrusion

Process

TABLE 19.1 Examples of the Uses of Some Commercial uPVC Feedstocks

'"

is

:::1

~ :0:::

;::, l:>...

l:>

;:

l:>

;::, ;::,-

t-< l:>

~

~

~

00

19 Rigid PVC: Main Products-Production, Properties and Applications

853

Various formulation aspects of uPVC compositions and their significance in processing, material properties, and product performance, are considered in many of the other chapters. However, two points may be reiterated here, viz. that in the absence of plasticisers the molecular weight (K value) of the polymer (as well as its nature-i.e. whether a homo- or copolymer) acquires additional importance vis-a-vis processing, whilst the role of polymeric modifiers (and hence their correct choice) with regard to effects in processing and service also assumes primary significance, especially in homopolymer-based compositions. Note: The physical characteristics of the polymer powder (including

particle structure, size and size distribution, bulk density) are also important in such connections as flow and mixing behaviour in dry blending, ease of gelation in melt compounding or melt processing from dry blend. Other factors being equal, the K value of the PVC polymer influences the ease of fusion and the melt rheology of a uPVC composition. Hence relatively low K values are chosen where ease of flow at reasonably non-degradative processing temperatures is particularly important (e.g. in injection moulding, extrusion blow moulding), although a high K value is always desirable for best mechanical properties of the product. Copolymers (in preference to homopolymers) are also used for ease of processing and good melt flow (as required, for example, of gramophone record compositions), and special purity grades of resin where high quality (high clarity in transparent products) is a requirement-see Chapter 4, Sections 4.4-4.6 for more detailed discussion. Some of the characteristics of PVC resins used in common types of uPVC compositions are illustrated by the examples given in Table 19.2. The role and functions of polymeric modifiers in uPVC compositions are discussed in some detail in Chapter 11 (Section 11.2). Both the increased ease of processing imparted by polymeric processing aids and the improved impact resistance conferred on the ultimate product by impact modifiers are important in all uPVC compositions. In the case of some products for outdoor use-e.g. window-frame profiles, cladding, some kinds of pipe-it is particularly desirable that the polymeric modifier(s) should provide maximum process-aid effects at the high rates of extrusion normally aimed at, with the greatest weathering resistance in service. Some acrylic modifiers achieve this

100 9 0·2 0·15

H S or M 57 580

Injection or blow moulding; calendered sheeting

-

99·5 10 0·5

65-67 500-550

Sb

Hb

Largediameter pressure pipe

100 5 0·2 0·22

He se 66 54{)

General extrusion

100 99 0·2

H E 72 480

b

D

99·9 5 0·2

C S 62 510

Extruded or calendered sheeting (for thermoforming)

= emulsion polymer.

Battery separators

H = homopolymer; C = copolymer (VCNA); S = suspension polymer; M = mass polymer; E e.g. Breon 5110110 (BP Chemicals Ltd). e e.g. Corvic 5 6617 (AECI). dAfter 30 min at 135°C.

Polymer natureD Polymer typeD K value (Fikentscher: DIN 53726) Apparent density (ISO 60) (g litre-I) Particle size (ASTM D 1705): % below 250 /lm % below 75 /lm Heat IOSSd (%) Porosity (ASTM D 2873) (cm 3 g-I)

Polymer characteristics

Typical application

TABLE 19.2 Some Characteristics of PVC Polymers Typically Chosen for Various Applications (Based largely on data from Ref. 2)

99·7 15 1·8

C S 47 780

Gramophone records; flooring

'l:

0

:::1

: ;::

;:l

s:> ;:: ;::s:>

t-<

~

~

00 1Il 00

19 Rigid PVC: Main Products-Production, Properties and Applications

859

TABLE 19.7

Some Electrical Properties of Two Rigid PVC Compounds (Breon RA 124 and RA 170") Volume resistivity (Qcm)

Temperature eC)

RA 124 23 40

60 80

90 100 aA

1·0 1·0 1·0 1·0 1·0 1·5

x 1014 x 1014 X

1014

x 1014 x 1014 x 1013

RA 170 1·0 X 2·5 X 7·0 X 2·0 X 9·0 X 5·0 X

1014 1013 1012 1012 1011 1011

Power factor at

Permittivity at

800 Hz

800 Hz

RA 124

RA 170

RA 124

RA 170

0·02 0·01 0·12 0·10 0·10 0·13

0·02 0·01 0·07 0·12 0·14 0·15

3·0 3·9 8·5 7·9 7·9

3·3 3·0 3·5 5·1 6·1 7·5

7-8

high-impact composition.

CHEMICAL PROPERTIES

These are discussed in Chapter 12 (Section 12.8), where data on resistance of PVC to various chemicals are tabulated. OPTICAL PROPERTIES

The light transmission characteristics of transparent uPVC materials are similar to those of PMMA plastics. Compositions of high clarity and 'sparkle' can be formulated (for use in, for example, the production of transparent bottles or packaging films and sheeting). Light transmission curves for clear and translucent (white) pigmented uPVC (mass-polymerised resin) films, 0·025 in thick, are shown in Fig. 19.2. LONG-TERM MECHANICAL PROPERTIES

With the use of thermoplastics for engineering applications (i.e. ones entailing service under stress, continuous or intermittent) now widespread, their long-term mechanical properties-creep (including creep rupture) and fatigue-have been extensively studied and the results used to aid design, and prediction of behaviour in service.

Creep: Because of their viscoelastic rheological behaviour, polymers-and the plastics materials based upon them-are subject to creep, i.e. increase of strain (deformation) with time under continued stress. For a given polymeric material the creep strain will normally be the higher the greater the applied stress, and the higher the temperature.

860

B. J. Lanham and W. V. Titow 100 A

80

..

c"60

o "iii

III

'EIII c

......~40 ~

01

:J

20

o0""""'3~:--."L-,---0~''='5---0~"'="6---0~.=7---='0'-=8 WavlZllZngth.l!m

Fig. 19.2 Light transmission of O·025-in uPVC films. A, Clear formulation; B, translucent formulation (Ti0 2). The creep rate-i.e. the slope at a given time of a plot of creep strain versus time (cf., for example, Fig. 19.3)-ean also increase with increasing temperature and/or stress. The creep strain or rate can sometimes be influenced by other factors, e.g. relative humidity (which can affect the moisture content of the material). For many thermoplastic materials the rate of creep can increase relatively suddenly after remaining virtually constant for a considerable period under the same applied load (cf., for example, the top curve in Fig. 19.4). Creep can take place in any of the modes of deformation encountered by polymeric materials in service, or employed in tests, i.e. tension, flexure, compression, shear or torsion. Formal definitions of creep and creep strain differ somewhat depending on whether the context is scientific or engineering. 9 The 'engineering' type of definition is favoured in standard plastics terminology, as represented by the following versions. Creep: 'The time-dependent strain resulting from stress'. 10 'The time-dependent increase in strain in a solid resulting from force' Y

c

o~

I&l

>C

~

2

o iii 3·0 c

4·

Fig. 19.3

0'1

100

Time in Hours

'000

0'1

10

At 51°C

100

Creep of a high-impact uPVC composition (Breon RA 170) at two temperatures.

10

4·

5·0

5.0

1

6·0

6·0

1000

g;

-

i;

5"

2

'.. ~

§

~

.g ...;:t.'"

l'>.. ;:

"g. :s ::p

~

~

S·

~

~

"tl

l'>..

;:.;, ~:

..... '0

896

B. J. Lanham and W. V. Titow

American-type cladding (siding) composition: Homopolymer PVC resin (K value 66) Stabiliser: a butyltin mercaptide Impact modifier: acrylic type (or chlorinated polyethylene) Lubricants: calcium stearate paraffin wax (MP 74°C) fatty acid ester Pigment (UV screen): TiO z

100 1·6 phr 6phr 1·9 phr 0·8 phr 0·9 phr 12phr

A basic formulation for extruded (blown) uPVC film is given in Section 4.6.1(c) of Chapter 4. Apart from the use of VCNA copolymers as the resin components in some thermoformable sheet compositions, two copolymers of vinyl chloride find their way into certain packaging films. Vinyl chloride/ propylene copolymers have been used (for their greater ease of processing-d. also Chapter 1, Table 1.2) in films for food wrapping (especially for fresh meat) and in over-wrap films for cigarette packets. 59 Vinyl chloride/vinylidene chloride copolymers are used as components of some packaging films where very low permeability is a requirement (see also Chapter 1, Section 1.5.2). Laminated sheets of polyvinyl fluoride and PVC have been used in the production of vacuum-formed containers for some packaging applications where the superior barrier properties and stability of PVF were of particular benefit. 59 Many national and international standards relating to uPVC sheet and film are listed in Section 5 of Appendix 1. An ISO standard for the designation and classification of PVC film and sheeting is in preparation. * 19.6 GRAMOPHONE RECORDS

Gramophone records are one of the only two uPVC products of industrial significance to be manufactured by compression moulding (the other being pressed sheets). The dominant requirements for record compositions are reasonable * ISO draft proposal 6236: Plastics-PVC film and sheeting-Part 1: Designation.

19 Rigid PVC: Main Products-Production, Properties and Applications

897

rigidity and high compositional homogeneity of the product, combined with ease of flow in production. The latter two requirements are dictated by the need for the best possible replication of the intricate groove pattern of the record mould, necessary for fidelity and good quality of sound reproduction. For the same reasons it is important that the polymer should be free from fish-eyes and particulate impurities, and the carbon black used as the pigment (as well as all other formulation components) should be well dispersed with no coarse aggregates of particles, lumps, or other inhomogeneities, for maximum freedom from surface defects. The frictional characteristics of the composition must also be such as to ensure freedom from 'hiss' in playing, and smooth running of the stylus generally-this, as well as the processing characteristics, are factors in the choice of lubricant. To meet this combination of requirements, record compound formulations are based on a high-purity grade vinyl chloride/vinyl acetate copolymer of a low K value and vinyl acetate content of about 15%-say Breon AS 60/40 (K value 47-50), or Corvic R46/88-with the minimum number and amounts of additives. An example of a basic formulation is given in Chapter 4, Section 4.6.5: this example shows the use of a lead stabiliser-other stabiliser systems are also employed (liquid ones for best melt flow and completeness of dispersion), e.g. calcium stearate/antimony mercaptide (see Chapter 9, Section 9.4.3(b)). Melt compounding is invariably used. The original processstill practised in some places-involves an internal mixer~ mill~ extruder sequence to produce pelleted compound from the formulation components; a weighed amount of the pellets is fluxed to form a coherent, hot slug, which is fed to the mould and pressed into a record with little or no flash; the moulding is briefly pre-cooled in the mould, and cooling completed (in an air stream) after removal. A typical slug temperature would be 165-170°C, and mould temperature about 95°C. Flow behaviour of the compound is checked with a plastometer to ensure uniform quality. In more modern versions of the process a dry blend is melt-compounded in a compounding extruder, and metered portions of the melt fed to the mould.

19.7 INJECTION·MOULDED uPVC ARTICLES These are mainly fittings for pipe systems of various kinds. Some other mouldings are mentioned in the summary review of PVC products in

898

B. J. Lanham and W. V. Titow

Chapter 26. Injection moulding of uPVC is discussed in Chapter 15. An example of a basic formulation for injection-moulded pipe fittings is given in Section 4.6.7(a) of Chapter 4. The fine structure of uPVC injection mouldings is influenced by the moulding conditions in a way generally similar to that observed with other materials. For example, a study by Copsey and Gooding60 indicated a 'skin-and-core' structure, (with some molecular orientation in the skin and little in the core), increasing density of the mouldings with higher melt temperatures and injection rates, and a correlation between the processing conditions and shear moduli of the mouldings (see also Section 15.3 of Chapter 15, and Section 1.5.1 of Chapter 1).

REFERENCES 1. Jacobson, U. (1959). Brit. Plast., 32(4), 152. 2. Corvic Vinyl Chloride Polymers (1982). Technical publication of AECI Chlor-Alkali and Plastics Ltd, Johannesburg, RSA. 3. Howie, J. A. (1971). Chem. Brit., 7(10), 428-33. 4. Mining: Why PVC Now? Technical booklet; AECI Ltd, Plastics Division, Johannesburg, RSA. 5. Properties of Plastics (1976). Technical booklet, Shell Chemicals UK Ltd, London, England. 6. Titow, W. V. and Lanham, B. J. (1975). Reinforced Thermoplastics, Applied Science Publishers, London. 7. Plastics: The Principles of Injection Moulding. ICI Technical Service Note G103 (2nd Edn), ICI Plastics Division, Welwyn Garden City, Herts, England. 8. Press, J. B. and Trebucq, D. A. (1977). In Developments in PVC Production and Processing-i, (Eds A. Wheelan and J. Craft), Applied Science Publishers, London, Ch. 9. 9. Handbook of Plastics Test Methods (1981). (Ed. E. Brown), George Godwin Ltd and The Plastics and Rubber Institute, London, 2nd Edn, p.189. 10. ISO 899-1981. Plastics-Determination of tensile creep. 11. ASTM E 6-81. Standard definitions of terms relating to methods of mechanical testing. 12. BS 4618. Recommendations for the presentation of plastics design data. Part 1: Mechanical properties. Section 1.1: 1970: Creep. 13. ASTM D 2990-77. Tensile, compressive, and flexural creep and creep rupture of plastics. 14. Turner, S. (1964). Brit~ Plast., 37(6), 322-4, and (12), 682-5. 15. Ogorkiewicz, R. M. and Bowyer, M. P. (1969). Brit. Plast.. 42(9), 125-8. 16. Bucknall, c., Gotham, K. V. and Vincent, P. I. (1972). In Polymer Science, (Ed. A. D. Jenkins), North Holland, Amsterdam, Vol. 1, Ch. 10.

19 Rigid PVC: Main Products-Production, Properties and Applications

899

17. ASTM E 206-72 (Reapproved 1979). Standard definitions of terms relating to fatigue testing and statistical analysis of fatigue data. 18. Andrews, E. H. (1968). In Testing of Polymers, (Ed. W. E. Brown), Interscience, New York, Ch. 6. 19. Andrews, E. H. (1973). In The Physics of Glassy Polymers, (Ed. R N. Haward), Applied Science Publishers, London, Ch. 7. 20. 'Spotlight on fatigue' (7 brief papers by different authors), ASTM Standardisation News (1980), 8(2), 8-27. 21. Gotham, K. V. (1974). In Thermoplastics Properties and Design, (Ed. R. M. Ogorkiewicz), John Wiley and Sons, London, Ch. 4. 22. Gotham, K. V. and Hitch, M. J. (1975). Pipes Pipelines Int., February, pp.10-17. 23. Gotham, K. V. and Turner, S. (1973). Polym. Engng. Sci., 13(3),113. 24. Moore, D. R, Gotham, K. V. and Hitch, M. J. (1978). 'The mechanical properties of PVC and how these are influenced by changes in processing, formulation and polymer type', paper presented at PRI International Conference on PVC Processing, Egham Hill, Surrey, England, fr-7 April, 1978. 25. Benton, J. L. and Brighton, C. A. (1965). IGE J., March, pp. 185-202. 26. Benjamin, P. (1979). Plast. Rubb. Int., 4(5), 269-73. 27. Swiss, M. (1976). Plast. Rubb. Wkly, 17th September, pp. 33-4. 28. Stephenson, D. (1979). Pipes Pipelines Int., December, pp. 9-17. 29. Cist, J. D. and Smith, J. G. (1978). 36th ANTEC SPE Proceedings, pp. 548-50. 30. Smith, M. (1976). Plast. Rubb. Wkly: 24th September, pp. 25-7; 1st October, pp. 1fr-17; 8th October, p. 29. 31. Anon. (1981). Mod. Plast. Int., 11(11), 35. 32. Smoluk, G. R (1982). Mod. Plast. Int., 12(11),44-7. 33. Zechinati, J., Harvey, B. and Despain, C. R (1978). 36th ANTEC SPE Proceedings, pp. 740-4. 34. Marshall, G. P. and Birch, M. W. (1982). Plast. Rubb.: Process. and Appln, 2(4), 369-79. 35. Hucks, R. T. (Jr) (1981). AWWA J., July, pp. 384-

5000

a

5

Ag. of

10 IIlStl,

15

days

Fig.21.13 Ageing characteristics of Breon P130/l :CS100/30 blends (polymer: plasticiser, 60: 40).

21

PVC Pastes: Properties and Formulation

965

21.3.2 Plasticisers In addition to the properties important from the point of view of end-use, which are discussed in Chapter 7, the choice of plasticiser for pastes is governed by the paste viscosity and general rheological characteristics which the plasticisers will impart; this includes also the gelation and fusion properties, as has been illustrated in Section 21.2.5. In addition to the studies of these effects already mentioned in that section, there are several earlier publications on the effect of plasticisers on the rheology of pastes. 43--47 Other factors being equal, the initial viscosity of the paste is significantly influenced by the bulk viscosity of the plasticiser(s), but this may be overshadowed by the effect of plasticiser affinity (solvating power). As would be expected in the light of the relevant considerations already discussed, highly solvating plasticisers will normally tend to produce higher paste viscosities. In those normal pastes which are subject to appreciable ageing effects, the main viscosity increase will commonly take place within a few hours from the completion of mixing; it is therefore reasonable to measure paste viscosities for routine control purposes 12-24 h after preparation. As has been mentioned in Chapter 7, a plasticiser mixture will normally impart to the compound physical properties and end-use characteristics intermediate between those imparted by its individual components. This is so also in pastes where, moreover, the principle extends to such properties of the liquid pastes as flow properties and ageing characteristics. An illustration of this is provided by the examples in Table 21.1. The effect of some individual plasticisers on the viscosity of PVC pastes is illustrated by Table 21.2. Solvating power of plasticisers as a factor in paste ageing is discussed in a paper by Bigg and Hill. 49 The following further comments may be made on the effects of various plasticisers in pastes. In general, the phthalate plasticisers give medium-viscosity pastes, with low to medium setting (gelation) temperatures. Most phthalates, and especially general-purpose Cs phthalates, impart some thixotropy to the paste even when used alone in a relatively low concentration (down to about 50 phr) so that the paste is very viscous. This feature is useful where pastes are required which will not flow easily without being worked (i.e. without shear), but which must spread readily under

w. V. Titow

966

TABLE 21.1 Viscosity of Pastesa with Mixed Plasticisers48 Plasticiser composition

Relative apparent viscosity at indicated shear stress (DOP 18 h viscosity = 1(0) 0·159Ibfin- 2

DOP-l00% DOP- 90%: Polyester (Flexol R2Hy DOP- 80%: Polyester (Flexol R2H) DOP- 70%: Polyester (Flexol R2H) DOP- 90%: 1TP DOP- 80%: 1TP DOP- 90%: Chlorinated paraffin (Halowax 4004Y DOP- 80%: Chlorinated paraffin (Halo wax 4(04) DOP- 70%: Epoxy compound (Paraplex G60)d

30days b

2·23Ibfin- 2 30days b 18h 30days~

18h

30 days

l8""h

10% 20% 30% 10% 20% 10%

100 141 246 259 123 133 158

175 196 351 351 196 186 214

1·75 1-40 1·43 1·36 1·60 1·37 1·35

100 174 344 309 217 213 173

145 281 354 398 295 301 202

1·45 1·61 1·03 1·29 1·36 1·41 1·17

20%

158

196

1·24

186

217

1-17

30%

175

205

1·16

149

186

1·25

• PVC resin Bakelite QYNV 100 pbw, total plasticiser 60 pbw. b This ratio is an index of the viscosity stability. C Union Carbide. d Rohm and Haas.

TABLE 21.2 Effect of Plastieisers on Ageing Characteristics of Pastes (ICI 'Come' Polymer) Plasticiser

Viscosity (Gardner) 1 day 28 days

DBP DAP DOP Bisoflex 791 a DNP

TIP

TXP TOP

DAS HexapLas PPL b HexapLas PPA

a

b

B.P. Chemicals. ICI.

Too viscous 72 120 80 124 68 110 69 69 410 1200 320 480 13 48 6 4 1030 300 Too viscous

21

PVC Pastes: Properties and Formulation

967

shear, as, for example, in coating operations. However, it should be noted that some plasticisers thixotropic in their effect at moderate rates of shear (e.g. DOP, DOS) may promote dilatant behaviour when the paste is subjected to high shear rates. As would be expected, plasticisers whose own viscosity is high tend to make viscous pastes. Paste viscosity is also promoted by plasticisers with good solvating power (e.g. DBP-see Table 21.2). Moreover, such plasticisers usually tend to promote dilatancy, which may be pronounced at low rates of shear: triaryl phosphates, and some aromatic esters of glycols (e. g. diethylene glycol dibenzoate) are examples of plasticisers with this kind of action. These general considerations are relevant in paste formulation, as neither high viscosity (unrelieved by thixotropy) nor dilatancy is normally desirable in a paste. Note: Some pastes for the production of flexible PVC foams

constitute an exception here: a relatively high viscosity, either natural to the formulation or-as may be preferableresulting from dilatant behaviour in processing, can promote uniformity and stability of the foam cells during their formation, and help to maintain these features until the structure is fixed by gelation. Promotion of a measure of dilatancy to these ends is one of the advantages of inclusion of suitable, rapidly solvating plasticisers (usually BBP; in some cases DBP as part of the plasticiser system-d. Chapter 25) in pastes for foaming, although rapidity of gelation and fusion to solidify the foam quickly may be regarded as their main function. Viscous and/or dilatant pastes are more difficult to stir, de-aerate and transfer in production and directly prior to application; they do not flow or spread easily. This can make for problems in casting and coating operations, and cause difficulties where smooth, even coatings (especially thin ones) are required. Both high viscosity and dilatancy can be counteracted by incorporating in the paste a suitable plasticiser (usually as part of the plasticiser system); some aliphatic diester ('low-temperature') plasticisers are particularly effective for this purpose and selected phthalates can also be useful, e.g. DINP (which, in addition to lowering paste viscosity, also offers lower volatility in comparison with Cs phthalates). Other phthalate plasticisers noteworthy from the point of view of advantageous paste rheology and

968

W. V. Titow

viscosity stability in storage are dicapryl phthalate (DCP) and Hexaplas

OPN. * Where relatively high temperatures and long times of setting

and fusion may be required, such higher phthalates as DDP and DLDP should be considered: their comparatively low solvating power also confers good ageing stability on the paste. The strong solvating ability of triaryl phosphate plasticisers accounts for the fact that their inclusion in a paste formulation makes for increased viscosity and, in most cases, dilatant behaviour. At the same time they promote rapidity and ease of gelation and fusion, and their tolerance of extenders is good. The 'low-temperature' (aliphatic diester) plasticisers lower the paste viscosity, very considerably in some cases (e.g. DIDA; some 'nylon acid' esters-d. Section 21.4.2), and impart thixotropic properties (some dialkyl sebacates and adipates are particularly effective). The ability of several members of this group to promote a large drop in paste viscosity with increasing temperature makes them useful in formulations for rotational casting. Pastes containing aliphatic diesters (normally-in view of their secondary-plasticiser character-as one component of the plasticiser system) tend to have good ageing resistance and relatively high setting temperatures. Epoxy plasticiser/stabilisers are employed in pastes for the same end-use effects as in solid compounds. Some (epoxidised soyabean oil) will generally tend to increase paste viscosity, whilst others (epoxy alkyl esters) normally have the opposite effect. Polymeric plasticisers tend to give fairly high viscosity and may promote dilatancy. However, the ageing properties imparted are good. These plasticisers are used in pastes essentially for the permanence of properties in end-use which they confer. Crosslinkable and polymerisable plasticisers have a special application in some pastes, where their use permits a relatively low initial viscosity to be combined with conversion to a semi-rigid or even fully rigid ultimate product. However, their incorporation in paste formulations gives rise to special considerations. Unless radiation can be used as a means of effecting polymerisation or cross-linking of the plasticiser in the fused product (and this additional treatment, usually expensive in any case, may not be practicable) an initiator or cross-linking agent, commonly a peroxide, will usually be included, and this may affect the stabiliser system or have some effect on properties. These specialist * Proprietary product of ICI: see Chapter 6, Section 6.5.5.

21

PVC Pastes: Properties and Formulation

969

plasticisers and their uses are mentioned in Section 6.10.4 of Chapter 6. Examples of such constituents of PVC compositions include diallyl phthalate (e.g. Bisomer DALP-BP Chemicals International Ltd), certain glycol dimethacrylates,50 trimethylol propane trimethacrylate, Santoset (a proprietary product of Monsanto), and the Sartomer plasticisers (Ancomer Ltd, UK). Some other paste-relevant properties and effects of various plasticisers and extenders are pointed out in the discussion of these materials in Sections 6.5 and 6.10 of Chapter 6.

21.3.3 Stabilisers In this respect the service-suitability aspect of paste formulation is very similar to that of solid compositions, whilst the effects of stabilisers on paste rheology are associated largely (though not exclusively) with their physical state: liquid stabilisers and stabiliser systems are used where the least effect on the flow properties (minimal viscosity increase) is desired. A comparison of some common stabilisers in a paste compound (Breon P13011, 100 parts; TIP, 33 phr; DOP, 33 phr; stabiliser, 1·67 phr) on the basis of the Congo Red test is given below. Dibasic lead phosphite Basic lead carbonate Calcium stearate Barium ricinoleate Lead stearate Cadmium stearate Control (no stabiliser)

Time to breakdown (min) 101 83

39 27 19 14

4

Stabiliser systems based on combinations of Ba, Cd and Zn compounds (see Chapter 9) are more widely used in PVC pastes than any other stabiliser type. Liquid Ca/Zn and Mg/Zn systems are useful in pastes where risk of sulphur staining in end-use is a consideration: they are also advantageous to paste rheology. Some are approved for non-toxic applications. Some Ca/Zn and Cd/Zn as well as certain lead stabilisers function also as 'kickers' for the blowing agent in flexible foam production from pastes. Liquid organotin stabilisers (especially the sulphur-free systems) are of interest where maintenance of

970

w.

V. Titow

reasonably low paste viscosity, or viscosity reduction, is an important consideration. Good heat and light stability can be obtained in paste products stabilised with basic lead carbonate and dibasic lead phosphite; the former stabiliser has been useful for opaque formulations in the absence of non-toxicity requirements. Calcium or lead silicate have been used in translucent formulations. Calcium stearate functions as stabiliser/lubricant for transparent compositions. However, in formulating with stearates it should be remembered that they tend to increase initial paste viscosity and can intensify viscosity increase on ageing.

21.3.4 Fillers Fillers are included in pastes for the same end-use purposes as in solid compounds, but their effects on paste rheology and ageing characteristics must be considered, as most can increase the viscosity and reduce the shelf life of the paste, or modify the response to shear. Films and coatings produced from filled pastes are usually less tacky and harder than those made from unfilled ones, but other physical properties may be inferior. In some cases the optical effects of a filler in the finished product are particularly important, e.g. the achievement .of the correct degree of translucency to impart a life-like appearance to pastemoulded PVC dolls. Other factors being equal, paste viscosity tends to rise with increasing filler loading and with decreasing particle size. The former factor increases viscosity partly as a result of the increase in the particulate phase content (and modification of its size distribution) in the paste (cf. Section 21.2.4), and partly because more particles are available to adsorb some plasticiser onto their surface. Particle size reduction also increases plasticiser adsorption by the filler because the specific surface (surface-to-volume ratio) goes up. The surface adsorption effects can be considerably modified if the filler particles are coated (see Chapter 8, Section 8.3.2). The amount, distribution and chemical nature of the coating all play their role, the most common overall effect being a reduction of plasticiser adsorption-and hence of viscosity increase-by the presence of the coating. Some of the effects of organic titanate coatings on fillers upon the viscosity and other properties of filled PVC pastes are discussed in a paper by Monte and Sugerman. 51

21

971

PVC Pastes: Properties and Formulation

TABLE 21.3

on Absorption of Fillers Filler

Oil absorption number 16 30 33 36

Barytes Slate powder Dolomite Whiting China clay (medium fine) China clay (very fine) Silica Water-ground mica

35 55

42

79

An indication of the likely magnitude of the effect of a filler on paste viscosity is given by its oil absorption avalue (cf. Chapter 8, Section 8.3.3), which is influenced by the factors just mentioned. A few oil absorption values of common fillers are shown, by way of example, in Table 21.3, and others in Table 21.4 where some effects of fillers on the viscosity of a paste and on the mechanical properties of the ultimate product are also illustrated. The table indicates, inter alia, something of the paste viscosity response to the incorporation of precipitated calcium carbonate fillers of, respectively, low and high oil TABLE 21.4 Effect of Some Fillers on a PVC Pastea and its Ultimate Product Filler Nature

None Calcined clay Blanc fixe Ppted CaCO/ Ppted CaC0 3c Barytes Diatomaceous earth Lithopone

Viscosity Tensile Modulus Elongation Oil (cP: 1 day) strength at 100% at break, absorption (lbfin- 2 ) extension phr (%) (No.) (lbfin- 2 )

0 20 31 21 20 34 18 33

10000 17000 15000 18000 40000 7000 40000 40000

2400 2050 2250 2100 2250 2220 1900 1750

1000 1125 1100 1000 1100

950 1175 1000

350 340 320 350 370 420 270 320

66

38 36 45 16 148 37

a Breon P1301l, 100 parts; DOp, 66·7 phr; filler as shown (each weight represents a volume equivalent to that of 20 phr calcined clay). b Low oil absorption grade. c High oil absorption grade.

W. V. Titow

972 1500

Sturcal L _ - - - - - - - - 3 3 phr

_ - - - - - - - - - - C a I O f i l A.4 33phr

ll. >, .... "iii

ou

III

:>

__

---------Sturcal L 8phr ~_ _- - - - - - - - - - - - C a l o f i l A.4 8phr

__------------NO filhzr

o

5

10 TimlZ, days

15

Fig. 21.14 Effects of two CaC03 fillers on viscosity of a PVC paste. Sturcal L (John E. Sturge Ltd), medium oil absorption filler; Calofil A4 (John E. Sturge Ltd), low oil absorption, resin-coated filler. Paste formulation: PVC resin (Breon P130/1) DOP

TIP

Filler Stabiliser (white lead paste)

lOOpbw 33 phr 33 phr as shown 1.7 phr

absorption. The rheological effects of two commercial grades of CaC03 filler are also shown in Fig. 21.14; the main features demonstrated by the curves of this figure are fairly typical, viz. higher paste viscosity with higher loadings of both fillers, and generally lower viscosity with the coated filler (except for the slight initial reversal at the higher level of filler loading, where the early viscosity of paste containing the uncoated filler is somewhat lower, because of the slower rate of wetting out). Filler grades specially recommended for pastes are available from many suppliers. As with other major components of pastes (e.g. plasticisers) the effect of fillers on viscosity stability is of interest to the formulator and processor: this is often expressed in

21

PVC Pastes: Properties and Formulation

973

terms of the ageing index (sometimes called the viscosity stability index--d. Table 21.1) calculated as a ratio VL: V S , where VL is the viscosity measured after an appropriate, long period (say 15 days) and Vs is the viscosity value obtained in an earlier measurement (say the 24 h or 48 h viscosity). 21.3.5 Thickening Agents (for Thixotropic Plastisols and Plastigels) Thixotropic plastisols are used in various dipping processes, where one-dip coatings are required or where no-drip coatings are essential, and in spreading processes to reduce penetration and mobility. Normal pastes can be made thixotropic or converted into plastigels by the addition of various thickening agents. These include certain grades of precipitated or fumed silica, e.g. Gasil 23 and Neosyl (Joseph Crosfield and Sons Ltd), Cab-o-Sil (Cabot Corp.), Aerosil 200 (Bush, Beach and Segner Bailey), special bentonites, some grades of aluminium stearate, e.g. Higel No.1 (Albright and Wilson Ltd), and Sylodex (W. R. Grace (UK) Ltd)---ehrysotile asbestos fibres in the form of 'crumb' produced by wetting out with plasticiser (2 parts DIBP to 1 part of asbestos). Some fillers can also have similar effects, e.g. very fine whitings and carbon blacks. The normal method of working is to pre-gel the plasticiser with the agent and use the resultant product for preparing the paste. The process is very simple and consists of dispersing the agent in the plasticiser and warming until a clear solution is obtained (with soluble thickening agents). A marked increase in viscosity also takes place. After cooling the plasticiser is ready for use. In the case of aluminilJm stearate (Higel No.1), a 25% solution (pre-gel) is made in the plasticiser and the amount on the plastisol is 2-10%, a good average being 5%. DOP and DOS can readily be pre-gelled in this way but TIP cannot be. The sort of increase in viscosity which can be achieved (Breon P130/1 100; DOP 66·7 phr; white lead paste 1 phr and 5% pre-gel Higel No. 1 in DOP) is from 10 000 cP to 106 000 cPo Aluminium stearates G and 2027 (Durham Raw Materials Ltd) behave in a similar way. The latter will gel at a lower temperature, the actual gelation temperature depending on the type of plasticiser used. The recommended use levels for both grades are 1% upwards. Aerosil is a useful material for plastisols. It increases the degree of thixotropy by about 60%, it can be easily dispersed (by stirring into the

974

w.

V. Titow

plastisol or dispersing in the plasticiser) and it gives a stable product. Sedimentation of filler is also prevented and the addition of 0'5-1 % is required. Bentone 27 (F. W. Berk and Co. Ltd) is another suitable material. Sometimes plastigels of putty-like or even modelling consistency are required. Suitable materials here are Santocel C (Monsanto Co., USA), a fine diatomaceous silica, and Neosyl C, used to the extent of 5-15%. For modelling purposes 30-50% of fillers such as French chalk or kieselguhr are employed. Again 10-30% of a finely divided china clay or calcium carbonate may be used or titanium dioxide in plasticiser. Some recipes are (in phr): Corvic P65150 Dialkyl 70 phthalate or similar phthalate Tritolyl phosphate Aluminium stearate Santocel Cor 54 Lead carbonate Pigment

A 100 40 40 4 6

4

B

C

100

100

80

55

4 6

4 As required

2·5 2·5 4

As indicated by the above formulations, in general the higher the plasticiser content, the more thickening agent is required. Mixing procedure is as follows. As aluminium stearate does not dissolve in tritolyl phosphate, the soap must first be dispersed in a phthalate plasticiser. This is done by making a paste with the soap and some of the plasticiser, adding the remaining plasticiser to form a slurry, then heating this to about 120-140°C (248-284°F) with slow stirring. When this temperature is reached, a clear mobile solution is formed, which on cooling changes to a soft jelly. To this jelly is added any additional plasticiser necessary, the Corvic P65150, heat stabilisers and pigments, mixing being carried out in a dough mixer which can be water-cooled. About 10-15 min mixing is required and the temperature of the mix should not be allowed to exceed 35-37°C (95-98°F). In the later stages of mixing, Santocel C should be added, the result being a firm putty-like mass. The mixed compound should be stored for 24 h or so at room temperature before use, to allow full thixotropic properties to be developed. Within this period marked thickening occurs, but thereafter changes are slow. Over a period of months plastigels may harden slightly but they may be used safely some months after manufacture.

21

PVC Pastes: Properties and Formulation

975

The gelled material has the same good ageing properties as normal PVC compounds. 21.3.6 MisceUaneous Paste Components These include the following. (a) Viscosity Depressants These are used when it is essential, for end-use properties, to employ a particular plasticiser system and plasticiser: polymer ratio which at the same time give unduly high paste viscosities. Viscosity depressants are usually surface-active agents of various kinds. The effect of a particular depressant in a particular formulation cannot be entirely predicted and therefore tests are necessary. Most viscosity depressants also retard the ageing of the paste. These compounds are added in low quantities (of the order of 1%) in order not to upset the formulation, and to avoid exudation. Polyethylene glycols and Lubrol MOA (non-ionic surfaceactive agent: fatty alcohol/ethylene oxide condensate-ICI) are examples. (b) Diluents These are usually organic solvents producing a thinning effect on the paste, and are mainly used in organosols (see Section 21.4.1).

(c) Other Minor Additives Occasionally Included These include colourants, blowing agents, fungicides and bactericides, tire-resisting agents, odourants and deodorants. 21.4 PASTES FOR RIGID PRODUCTS: ORGANOSOLS AND RIGISOLS Apart from the use of cross-linking plasticisers in plastisols, the requirements of initial low viscosity and semi-rigid tinal products can be met by organosols and rigisols. Brief reference has been made to both in this chapter; a few more details may now be given. 21.4.1

Organosols

These are essentially plastisols containing additional diluents. The diluents may be aromatic hydrocarbons (e.g. xylene and toluene), aliphatic hydrocarbons (e.g. white spirit), or certain ketones (e.g.

976

W. V. Titow

methyl ethyl ketone, methyl isobutyl ketone); alcohols, glycol ethers and chlorinated hydrocarbons have also been used. The diluents are employed in circumstances somewhat similar to those in which viscosity depressants are used, but, unlike the depressants, they are volatilised orf in the course of processing. Accounts of their use were first given in early literature. 52-54 Organosols have their place where the process demands a paste combining low viscosity with high solids content, to yield a relatively rigid final product. For example, sprayable compositions can be formulated, with the aid of suitable volatile thinners, containing as much as 60% non-volatile matter, with up to 150 phr filler, and very low plasticiser content (down to a few per cent if necessary).

21.4.2 Rigisols As mentioned in Section 21.1, these are plastisols specially formulated to achieve low viscosity for processing, but combining this with high polymer content and hence rigidity of the ultimate products. The factors which are adjusted and controlled to achieve this result are the following.! (i)

Selection (and if necessary blending) of paste-grade PVC polymers with particle size and size characteristics best suited to give the kind of particle packing system in the paste that will promote the lowest practicable viscosity combined with highest polymer content. Selected suspension-grade homopolymer resins may be included, or some vinyl chloride/vinylidene chloride copolymers, as extenders for the paste-grade resin(s) to reduce the viscosity of the system still further. (ii) Selection of plasticisers which promote low viscosity and thixotropy in a paste, so that the plasticiser content can be reduced to a minimum. Several aliphatic ester 'lowtemperature' plasticisers are especially suitable, in particular the AGS ('nylonate') esters. Note: It has been suggested! that the packing of the polymer particles achieved by suitable blending is a close multimodal packing with the smaller particles almost filling the interstitial spaces among the larger ones, and that the amount of plasticiser should be that which then fills the remaining inter-particle voids, just preventing the particles from touching. It is further suggested that such a 'lubricated' system

21

PVC Pastes: Properties and Formulation

977

is considerably different from an ordinary plastisol in which the polymer particles may be regarded as floating in the plasticiser: it obviously contains proportionately less plasticiser. ! (iii) Careful attention to the choice of stabilisers and fillers. The stabilisers should preferably be liquid with no thickening effect on the plasticiser, and the filler loading should not exceed 15 phr. (iv) Use of viscosity depressants: polyethylene glycol (400) monolaurate has been recommended as particularly effective.! (v) Use of diluents. It is an academic point whether the inclusion of a solvent in a paste otherwise formulated as a rigisol makes it into an organosol. In practice, white spirits and aliphatic naphtha (free from aromatics) are diluents useful in lowering the viscosity of a rigisol. It has also been stated! that the speed of mixing in the course of preparation is a factor in successful production of rigisol pastes. High-speed mixing is preferable in the first ('dry' or 'thick' stage-see Chapter 22, Section 22.1.1) to ensure that any aggregates that may tend to form are thoroughly broken up, but once the composition has been homogenised the mixing speed should be reduced. Useful advice on basic rigisol formulations for specific purposes is available from manufacturers of paste resins and plasticisers. One 2 Parts by weight example is given below: PVC resins: paste-grade emulsion polymer, low viscosity (K value* 65) 50 50 100 suspension polymer (K value* 55) 50 50 Plasticisers (AGS esters): Reomol MDt 25 30 Reomol MNt 25 Stabilisers: BalCd liquid system 2 2 2 epoxy co-stabiliser 2 2 2 2 2 2 Viscosity depressant: Lubrol MOA Apparent' viscosity of paste (at low shear rate) (P) 18 22 42 after 1 day after 7 days 18 20 98 BS softness No. of ultimate product 6 7 9 * 0·5 g polymer in 100 ml dichloroethane at 25°C.

t Ciba-Geigy.

978

W. V. Titow

REFERENCES 1. Goodier, K. (1960). Proceedings of the International Congress on the Technology of Plastics Processing, Amsterdam 17th-19th October, N.C. V't Raedthuys, Amsterdam. 2. Ciba-Geigy Technical Service Bulletin PL 3.3 (1975). 3. Johnston, C. W. and Brower, C. H. (1970). SPE J., 26(9), 31-5. 4. Underdal, L., Lange, S., Palmgren, O. and Thorshaug, N. P., (1978). 'PVC paste technology and polymer characteristics', paper presented at the PRI International Conference on PVC Processing, Egham Hill, Surrey, England, 6-7 April, 1978. 5. Bjerke, O. (1966). 'Relation between the distribution of size of the primary particles and the rheological properties of PVC plastisols', SCI Monograph No. 26, pp. 370-80. 6. Ram, A. and Schneider, Z. (1970). Ind. Eng. Chem., Prod. Res. Develop., 9(3), 286-91. 7. Mooney, M. (1951). J. Colloid Sci., 6, 162. 8. Eilers, H. (1941). Kolloid Z., 97,313. 9. Breon Pl30II Paste Resin. (1969). BP Chemicals International Ltd., Technical Manual No.2, pp. 5-10. 10. Cawthra, c., Pearson, G. P. and Moore, W. R. (1965). J. Plast. Inst., 33, 39-44. 11. Mendelson, R. A. (1968). Encyclopedia of Polymer Science and Technology, Vol. 8, (Eds H. F. Mark et al.), Interscience, New York, pp. 588-90. 12. Lodge, A. S. (1964). Elastic Liquids, Academic Press, New York, pp. 228, 246-8. 13. Reiner, M. (1960). Lectures on Theoretical Rheology, North Holland Publishing Co., Amsterdam. 14. Fredrickson, A. G. (1964). Principles and Applications of Rheology, Prentice-Hall Inc., New York, pp. 27, 142-4. 15. Gillespie, T. (1966). J. Colloid Interface Sci., 22,554-9. 16. Hoffman, R. L. (1974). J. Colloid Interface Sci., 46,491-7. 17. Strivens, T. A. (1976). J. Colloid Interface Sci., 57,476-80. 18. Rangnes, P. and Palmgren, O. (1971). J. Polym. Sci.-Part C, 33, 181-7. 19. Thinius, K., Reicherdt, W. and Hosselbarth, B. (1963). Plaste u. Kaut., 10,339-41. 20. Goodrich, J. E. and Porter, R. S. (1967). Polym. Engng. Sci., 7, 45-8. 21. Lee, G. C. N. and Purdom, J. R. (1969). Polym. Engng. Sci., 9, 360-;;. 22. Schreiber, H. P. (1970). Polym. Engng. Sci., 10, 13-15. 23. Shah, P. L. and Allen, V. R. (1970). SPE J., 26,56-60. 24. Buckley, C. D. (1971). J. Cell. Plast., 7,23. 25. Cayrol, B., Klason, C. and Kubat, J. (1974). Polym. Engng. Sci., 14(12), 868-72. 26. Greenhoe, J. A. (1961). Plast. Technol., 7(2), 35-8. 27. Hoy, K. L. (1966). J. Appl. Polymer Sci., 10(12), 1871-93. 28. Johnston, C. W. and Brower, C. H. (1966). SPE J, 22(11),45-52. 29. Alter, H. (1959).1. Appl. Polym. Sci., 2(6),312-17.

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30. Nakajima, N., Ward, D. W. and Collins, E. A. (1979). Polym. Engng. Sci., 19(3), 210-14; and Nakajima, N. and Ward, D. W. (1983). J. Appl. Polym. Sci., 28(2), 807-22. 31. Bauer, W. H. (1961). SPE J., 17(2), 174--7. 32. Greenhoe, J. A. (1960). Plast. Technol., 6(10),43-7. 33. McKenna, L. A. (1958). Mod. Plast., June, pp. 142-5. 34. Titow, W. V. Unpublished work. 35. ISO 2555-1974. Resins in the liquid state or as emulsions or dispersionsDetermination of Brookfield RV viscosity. 36. ASTM D 1824-66 (Reapproved 1980). Apparent viscosity of plastisols and organosols at low shear rates by Brookfield viscometer. 37. Ciba-Geigy Technical Service Bulletin PL 9.1 (1977). 38. ISO 4575-1978. Plastics-Polyvinyl chloride pastes-Determination of apparent viscosity using a Severs rheometer. 39. ASTM D 1823-66 (Reapproved 1979). Apparent viscosity of plastisols and organosols at high shear rates by Castor-Severs viscometer. 40. Lewis, T. B. and Nielson, L. E. (1968). Trans. Soc. Rheol., U(3), 421-4. 41. Farris, R. J. (1968). Trans. Soc. Rheol., U(2), 281-93. 42. Park, R. A. (1975). Plast. World, 33(1), 48. 43. Severs, E. T. and Austin, J. M. (1954). Ind. Eng. Chem., 46(2), 369. 44. Darby, J. R. and Graham, R. R. (1955). Mod. Plast., 32, 148. 45. Newton, D. S. and Cronin, J. A. (1958). Brit. Plast., 31, 426. 46. Werner, A. C. (1957). Mod. Plast., 34, 137. 47. Todd, W. D., Esarove, D., and Smith, W. M. (1956). Mod. Plast., 34, 159. 48. 'Bakelite Vinyl Dispersion Resin QYNV' (1956). Union Carbide International Co., Technical Release No. 14. 49. Bigg, D. C. H. and Hill, R. J. (1976). J. Appl. Polym. Sci., 20(2),565-8. 50. British Patent No. 694444, Union Carbide and Carbon Corp. 51. Monte, S. J. and Sugerman, G. (1978). 36th ANTEC SPE Proceedings, pp.781-4. 52. Werner, A. C. (1959). Mod. Plast., 36, 126. 53. Nielson, E. R. (1950). Mod. Plast., 27, 97. 54. Powell, G. M., Quarles, R. W., Spessard, C. I., McKnight, W. H. and Mullen, I. E. (1951). Mod. Plast., 28, 129.

CHAPTER 22

Preparation, Processing and Applications ofPastes W. V. TITaw

22.1 INTRODUCTION 22.1.1 Preparation In the early days, with old-type resins,. grinding was essential to produce satisfactory pastes. With modern paste polymers 'stir-in' techniques in good mixing equipment are effective. The method and equipment used in the industry for paste preparation are discussed in considerable detail in Chapter 13 (Sections 13.4.3 and 13.4.4(f)). The use of an effective mixer and proper technique should ensure that the paste is free from lumps, with all constituents uniformly distributed, and not excessively aerated (to facilitate the postpreparative de-aeration which will usually be carried out). Of the two possible general modes of operation, continuous mixing is in practice confined to industrial processes: batch mixing is practised on both industrial and laboratory scale. Modern equipment (e.g. a planetary mixer) makes it possible to prepare a satisfactory paste batch in a one-step operation, and versions of this kind of technique are laid down in standard specifications (ISO 4612-1979; DIN 54800-1979) for the preparation of standard pastes to be used in the control and evaluation of formulation factors (especially the nature of the PVC resin) and the rheological properties of pastes. However, this does not detract from the practical value of a good two-stage method of paste preparation, especially for low-viscosity pastes, both in the laboratory and for production purposes. The following is a brief, general outline of one such method. 981

982

w.

V. Titow

All dry components (pigments, fillers, solid stabilisers, etc.) are thoroughly pre-dispersed in a small quantity of the plasticiser (say between 1 and 3 pbw plasticiser to 1 pbw dry materials). This is best done on mill-type equipment (say three-roll or five-roll mills, cone mills) or by ball milling. The polymer is then placed in the mixer with the addition of plasticiser in quantity just sufficient to form a paste, and into this the pre-dispersed dry components are added. This is the first stage of mixing known as the 'thick stage' and the aim is to have just enough plasticiser to ensure mixing at maximum shear. The shearing action will produce heat, and care should be taken that the temperature of the paste should not exceed about 30°C, otherwise the plasticiser will start to attack the polymer and the ultimate viscosity of the paste will be higher than it should. The mixer can be water-cooled if necessary. The 'thick-stage' stirring should be continued for some time (roughly 10-30 min depending on conditions). At the end of this period the gradual addition of the rest of the plasticiser (and any diluents that may be employed) is commenced. This marks the beginning of the 'thin stage' of the mixing process. The plasticiser addition may conveniently be made continuous from, say, a suitable funnel arrangement. The thin stage mixing may take very roughly about 1 h. The stirring action traps air in the paste, which is undesirable for most processes, and it must be removed. One of the best ways of effecting this is to mix under vacuum. If this is not possible the finished paste is subjected to vacuum or, for the thinner types, allowed to stand for 24 h. Storage is important and should receive careful consideration. If the pastes are prepared and formulated correctly they will store satisfactorily for long periods. They must, of course, be stored in a cool place. The material of the container is also important. Iron or zinc should be avoided unless the former is protected by a lacquer which is not attacked by the paste. Glass, aluminium and tinplate are satisfactory. The above method is suitable for both laboratory and work-scale preparation of pastes. For very small quantities the whole operation can be effected on a mill mixer (say a triple-roll mill) by a suitable number of passages. 22.1.2 Conversion to Products

As discussed in Section 21.2.5 of the preceding chapter, the conversion of the liquid paste to a solid PVC product involves gelation and fusion, actuated by heat. The initial drop in viscosity which precedes the

22

Preparation, Processing and Applications of Pastes

983

'setting' of the paste can also be significant in some processes. The state reached by a paste which has set to a considerable extent but has not fused, is sometimes called 'semi-gel' or 'pre-gel'. These essentially practical terms are not very precise, and in fact relate to what is a fairly advanced degree of gelation, in which the paste may be sufficiently cohesive to withstand some handling: this is sometimes utilised in, say, multiple dipping or coating techniques, where each consecutive paste layer is 'pre-gelled' after application, and then the whole coating is finally fused at an appropriate, higher temperature. As has been mentioned, the temperature necessary for complete fusion cannot be lower than 160°C (and will be appreciably higher for many paste compositions). Heating above the appropriate lowest temperature of complete fusion will effect fusion in a shorter time. If the temperature is too high, some plasticiser loss will first occur, and then decomposition of the polymer may follow if heating is prolonged. It is important for the purposes of process and quality control to test the completeness of fusion of paste-derived products. Determination of relevant physical properties (tensile strength, hardness, etc.) is one obvious way of doing this, but it can be cumbersome as a routine check. Solvent tests have been proposed from time to time, particularly for use with PVC coatings on fabrics, and some of those are convenient to use and quite reliable. The tests include the ethyl acetate immersion methods of Kling! and Schimke? and the acetone test widely used with PVC-coated gloves. The general principle of such tests is that a test strip, or a complete article (e.g. a PVC-coated glove), is immersed in the solvent either in a strained condition (e.g. a strip bent into a loop) or freely suspended, and the PVC is watched for signs of flaking, i.e. rising of small fragments of the solvent-swollen material away from (i.e. above) the surface (in the case of a coated fabric, sometimes also away from the fabric surface). In some versions of the test the flaking site is predetermined by a nick made in the specimen, but spontaneous flaking will normally occur in material which is incompletely fused. If they form, the flakes are clearly visible and unmistakable in appearance: severe flaking may result in the flakes becoming completely detached: in the case of under-fused PVC coatings partial stripping from the supporting layer may occur. In solvent tests of this kind it is usual to regard the PVC material as satisfactorily fused if no flaking is observed in a specified period of time: this may be anything from about 5 to 30 min, depending on the aggressiveness of the solvent used and the presence or absence of imposed strain in the specimen.

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In the course of a study3 of gelation and fusion effects in the coating of fabric-backed PVC gloves produced by a dip-coating process, the author showed that an acetone immersion test* normally used to check~n a pass or fail basis-whether satisfactory fusion has been achieved in an industrial operation, could also provide data on the degree of fusion (where fusion complete in terms of the test had not been attained), and an indication of the minimum temperature necessary for complete fusion of the particular paste composition under the heating conditions employed. Substantially identical fabric glove liners were dip-coated (on production formers, in a standard way) with the same PVC paste (a standard production formulation). Groups of three of the paste-coated liners withdrawn at random from the batch so produced were subjected to heat treatment, each group at a different temperature/time combination. After the heat treatment, followed by a cooling and conditioning period, the acetone test was carried out on two gloves from each group (the third being retained for reference). Where flaking occurred, the time for the first flake to appear was noted (these times were in excellent mutual agreement for the duplicate determinations on gloves from the same group). The test results were plotted as shown in Fig. 22.1. In the figure each point represents the result of one test (duplicate determination) on gloves from the group which had undergone heating at the temperature and for the time given, respectively, by the relevant abscissa and ordinate values. Points marked with crosses correspond to tests in which flaking occurred: the time (minutes) of appearance of the first flake is shown by each point. The circled points represent those tests in which no flaking took place during an immersion period of 24 h: i.e. where the PVC coating could be regarded as properly fused in terms of the test. The regions of the plot occupied by the circles and the crosses correspond to heat treatment conditions producing, respectively, complete and incomplete * The specimen used in this test is a complete glove with a circular hole of 2·5 cm diameter cut in the middle of the back, about 2 cm above the finger base line (to admit the solvent to the interior of the glove). The specimen is suspended, fingers downwards, in a beaker, and acetone is quickly poured in until it surrounds and fills the glove, with the surface level standing about 2·5 cm above the edge of the hole. Timing is commenced from the moment this level is reached, the surface of the glove being closely watched for signs of flaking. In a popular industrial version of this test the fusion of the PVC coating is considered complete if no flakes appear within 30 min.

985

22 Preparation, Processing and Applications of Pastes 45 40

x

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35

.••..

30

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25

.!

•

20

»

15

'j

-.•.

0

10 5

o ---- \

)(

H

x 4-5

x

2·5

x

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x HI

1"4 160

170

I

180

190

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210

I

220

230

I

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(·C l

Fig. 22.1 Effects of heating time and temperature on the completeness of fusion of PVC coating on dip-coated gloves. Each point on the plot relates to an acetone immersion test on gloves 'cured' under the conditions (time/ temperature combination) given by the appropriate ordinate and absicissa values. x, Tests on gloves 'cured' under these conditions resulted in flaking after the number of minutes indicated by the numericals (e.g. x3.:r-first flake appeared after 3·3 minutes); 0, gloves 'cured' under these conditions did not flake during 24 h continuous immersion.

fusion. The curve constituting the boundary between the two regions is thus the locus of the minimum time/temperature conditions for complete fusion in the particular process. The steeply rising portion of this curve may be regarded as asymptotic to the 172°C temperature line: this indicates that 172°C is the minimum temperature for complete fusion of the paste (and in the conditions) used. Figure 22.1 also demonstrates that the test is sensitive enough to distinguish between degrees of fusion where fusion had not been complete. This is shown by the fact that the times to flaking increase with increasing oven temperature and with increasing oven time.

986

W. V. Titow

22.2 APPLICATIONS 22.2.1 Rotational Casting This is the main process for producing hollow moulded articles (dolls' heads and body parts, hollow 'squeaky' toys, playing balls, etc.) from PVC paste, offering several advantages over slush moulding which is the alternative method (see the next section). The development of rotational casting began around 1950, and the first descriptions of machinery4-6 are of that vintage. The principles and early practice of this process were described by Meazey.7 Modern rotational casting equipment, of various sizes, numbers of moulds and capacities, is available from several manufacturers in the UK, USA and Europe, most of whom will be found in the current editions of the relevant directories. 8 Rotational casting (also known as rotary casting, roto-casting, or rotational moulding) has gained popularity for the following reasons, most of which represent advantages over slush moulding. Rotationally moulded hollow articles can be produced with uniform wall thickness and fine surface detail, from a pre-metered amount of paste. Virtual absence of flash minimises trimming operations, and the amount of material waste is also minimal. The equipment and operation are automated to a large extent, whilst the capital expenditure on machinery and moulds is moderate by present-day standards. The principle and basic process of rotational casting are simple, although in their modern embodiments the equipment and technique have attained considerable sophistication and versatility. Typically, multiple moulds-earried on one or more arms indexed to move between operational stations-are each charged with the appropriate, metered amount of paste at the filling station, and closed. Transfer to the heating/cooling station follows, the moulds being spun in one plane and simultaneously rotated at right angles to the spin direction at the outset and throughout the heating period. The heat treatment takes place in an oven powered by electricity, gas or oil, employing hot air or IR radiation as the heating medium. The complex two-directional rotation of the moulds spreads the paste evenly over their interior surfaces, where it is gelled and fused by the heat. The rotary movement is continued during cooling (by an air stream and/or water spray) in a cooling chamber, after which the moulds are returned to the filling station where they are unloaded prior to the commencement

22 Preparation, Processing and Applications of Pastes

987

of the filling step of the next cycle. A typical cycle time may be about 15 min. Spinning rates of 5-16 r min- 1 are fairly representative, with the ratio of these to the rotation rates between about 1: 1 and 1 :4. For a given product, the best rates and their combinations are established on the basis of trials. Simple moulds can be quite satisfactory, made by the electrodeposition technique (e.g. nickel on copper for good release properties and thermal conductivity), or cast or machined in aluminium. Some rotational mouldings (e.g. playing balls) are inflated after manufacture with air introduced through a hypodermic needle, the resulting hole being heat-sealed. Rotationally cast products may also be filled with foam generated from a polyurethane composition poured in through a hole foamed in situ whilst the hollow moulding is supported in a retaining mould. This method has been widely used in the production of car arm-rests with outer 'skins' rotationally moulded from PVC paste. However, in that particular application, it is being strongly challenged by essentially one-step moulding of the arm-rests in structural polyurethane foam. The following basic formulations illustrate the kind of pastes used for some rotationally cast products. Rotational casting formulation for soft dolls or playing balls (paste viscosity about 12P):

PVC resin: Corvic P65/54* Plasticisers: DAP DNP Stabiliser: Cd/Ba liquid system

l00pbw 40phr 50phr 3phr

Rotational casting formulation for harder objects:

PVC resins: Corvic P65/54 Corvic D55/3t Plasticiser: Pliabrac 810t. Stabiliser: CdlBa liquid system Viscosity depressant: Lubrol MOA§

50pbw 50pbw 25 phr 3phr 2phr

* Medium-viscosity, paste-grade resin (ICI).

t Suspension resin

(leI) here used as extender polymer.

:j: Proprietary plasticiser of Albright and Wilson (a dialkyl phthalate, prepared

from a blend of straight-chain octyl and decyl alcohols). § Non-ionic surface-active agent (ICI).

988

W. V. Titow

Rotational casting formulation for car arm-rest 'skins':

PVC resins: Corvic P65/54 Corvic D55/3 Plasticisers: DNP Reomol MN* Paraplex G62t Stabiliser: Irgastab TI50* Viscosity depressant: Lubrol MOA

70pbw 30pbw 50phr lOphr 2phr 1·5 phr 2phr

Rotational mouldings are sometimes surface-finished by spraying with a lacquer to provide a decorative, tack-reducing finish which also improves resistance to soiling. The lacquers used are the same as those applied, for similar purposes, to PVC coatings on fabrics (see Section 22.2.6). 22.2.2 Slush Moulding This technique may be regarded as the precursor or rotational casting, to which it has yielded considerable ground over the years. In comparison with rotational casting, slush moulding is more labour-intensive, whilst repeated handling of the same paste, which also experiences warming in the process (see below), can cause aeration and adversely affect the rheological stability. These and other points of comparison will be apparent from the following brief ol,ltline of the process. The principle of slush moulding is that the paste is poured into a lightweight, open mould, and then poured out leaving a layer on the inside surface of the mould. The thickness of the retained layer may be governed entirely by paste rheology, or it may be increased by heating to effect a degree of gelation. In either case the gelation and fusion of the retained material (which becomes the hollow slush moulding) is completed by a final heat treatment. The mould is cooled sufficiently to enable the moulding to be removed without damage or permanent distortion, but not to a temperature so low that removal may become

* An AGS ester 'low temperature' plasticiser (Ciba-Geigy).

t Epoxidised soyabean oil (Rohm and Haas).

:j: A modified, sulphur-free tin carboxylate stabiliser (Ciba-Geigy).

22

Preparation, Processing and Applications of Pastes

989

difficult: in practice a temperature of about 35°C will usually be suitable. The moulding is most often blown out of the mould by a jet of compressed air. With shapes which make removal more difficult, vacuum can be applied to the opening of the mould to collapse the moudling first. The main practical variants of the basic method are outlined below. In one, sometimes referred to as the 'one-pour' method, the moulds are filled with paste and passed on a conveyor through an oven or heating bath in such a way that only the body of each mould is heated (not the paste surface exposed in the open top). The object is to gel the desired thickness of paste adjacent to the mould wall, and the heating time (dwell period) and temperature will be adjusted to this end in accordance with the gelling properties of the paste and the relevant thermal characteristics of the mould. After the heat treatment the moulds are emptied of the ungelled paste (this may be by inversion on the conveyor, so that the paste runs into a catchment trough underneath) and conveyed to a second heating station-normally an oven-operated at a temperature required to complete, in the course of a convenient dwell period, the gelation and fusion of the material remaining in the mould. The arrangement should be such that both the mould and the material inside it (accessible to heat through the mould opening) are uniformly and effectively heated: slow rotation of the moulds may assist in this, but should not be necessary with an efficient heating set-up. The temperatures of treatment will vary considerably depending on the paste composition and conditions, but will normally be within the range 17D-230°C. After leaving the oven the moulds are cooled (by water or air) in a cooling chamber to a temperature most appropriate to easy removal of the mouldings (see above), which is then effected. In another variant the moulds are pre-heated at around 170°C for a time depending on their size and thickness. They are then filled with paste and allowed to stand (this may be on a moving conveyor) for 1-2 min. After this the moulds are inverted so that the paste, other than that of the layer gelled on the mould walls, drains out. The gelled layer, which may be up to 3-4 mm thick, is then fused by heating the moulds in a fusion oven. Cooling and de-moulding follow as before. A method combining elements of both above variants, known as the 'two-pour' method, is also used. In this, cold moulds are filled with paste, and inverted immediately after filling so that whilst some paste remains on the interior wall surface the layer is thin. The draining may

990

W. V. Titow

take place on a conveyor, into a catchment trough, as described previously. The moulds are placed in (passed through) an oven where the paste layer is pre-gelled. In practice this treatment is short (a few minutes) at a temperature of 17~220°C. On leaving the oven, the moulds are re-filled with paste and then emptied almost directly, but-because they are hot-a paste layer of fairly substantial thickness is gelled onto the previous, thin coating. The actual thickness of the second layer will be determined by the characteristics of the paste, its residence time in the hot mould and the temperature of the mould. The combined paste layer inside each mould is then fused by heating in a second oven, and the moulds cooled down and emptied as before. In slush moulding, moderate variation in various paste and process factors is not absolutely critical, but once suitable conditions have been established they should be adhered to as closely as possible. Attention must also be paid to the condition of the paste which is being re-used, particularly with regard to air bubbles and adventitious contamination that may have been introduced, and any gelled particles, as well as to the prevention of any water from the cooling process entering the paste (as this can cause blistering in the mouldings). The re-circulated paste should be passed through a fine-mesh screen on its way into the holding tank, where it should be de-aerated, preferably under vacuum. The following formulation exemplifies a basic composition for the production of PVC overboots by slush moulding. PVC resins: Corvic P65/54 Corvic D55/3

Plasticisers: DIOP Pliabrac 810

Stabiliser: Colourant:

Cd/Ba liquid system Blue Masterbatch C

70pbw 30pbw 55phr 15phr 3phr 0·75 phr

Blue Masterbatch C

PVC resin: Plasticiser: Stabiliser: Colourants:

Corvic P65/54

DNP Calcium stearate Reckitt's: ultramarine blue M 6925 blue 8651

100pbw 60phr 4phr 1 phr 2phr

22 Preparation, Processing and Applications of Pastes

991

22.2.3 Paste Casting This is a relatively straightforward technique, employing comparatively simple moulds; however a large number are needed for mass operation. The main types of product manufactured by paste casting are shock-absorbent pads moulded onto the ends of air and oil filters for vehicle engines, printing rollers, and various kinds of mats (table mats, doilies, anti-slip under-mats for rugs). The end-pads for filters are made in open dish moulds, by pouring the required amount of paste into the mould in which the filter is standing on one of its ends, gelling and fusing the paste around the filter 'insert' by placing the mould with its contents on a hot plate or in an oven, and finally cooling and removing the filter with the moulded-on pad. The whole operation is then re-run to mould a pad onto the other end of the filter. In the production of printing rollers, a metal core is first coated with a thin priming layer (typically an acrylic and/or epoxy resin composition) to promote positive interfacial adhesion to the ultimate paste-produced pPVC roller body. The core is placed axially in a cylindrical mould, the paste is poured in, and the assembly transferred to an oven where the paste is gelled and fused, and then removed for cooling and de-moulding. The heating conditions must ensure complete fusion throughout the PVC body of the roller, to develop maximum structural uniformity and durability: these, together with the correct degree of softness and resilience (which, for properly fused material, are determined by the paste formulation), are important in this application. The roller (in particular the ends) may require final machining for correct size: this may necessitate freezing the PVC if it is too soft. Like the filter mounting pads, mats and like products are produced by open casting, usually into a shallow dish mould. Excess paste may be removed by passing a doctor blade across the top of the mould, and the moulds subjected to a vacuum if the filling operation has introduced air bubbles. Gelation and fusion are carried out in an oven (or sometimes on a suitable hotplate or heated conveyor) as for the filter mounting pads. In the heating operation, a longer treatment at a lower temperature (providing that this is not below the minimum temperature for complete fusion) will normally be preferable to a shorter period at a higher temperature, to avoid under-fusing the inside with possible overheating of the outside of the products. PVC films are cast or spread from paste onto a suitable base (e.g. release

992

W. V. Titow

paper) in fabric coating operations: this application is discussed in Section 22.2.6.

22.2.4 Dip Coating and Moulding (a) Hot-dip Coating This is a useful method of producing ppve coatings on metal objects suitable for dipping in paste. In essence, the article to be coated is heated, dipped into the paste, allowed to drain for an appropriate time, and heated to complete the gelation and fusion of the coating layer. Articles of thickness substantially below 3 mm (e.g. thin wire) will not normally be suitable for treatment by this process, because low heat capacity hampers satisfactory fusion. For similar reasons it is difficult to obtain a good coating on sharp edges. The two essential items of equipment are an oven (with air circulation or other means of ensuring maintenance of uniform temperature distribution) and a dipping tank equipped with a slow-speed stirring arrangement. Oven temperature of 175-200o e will usually be satisfactory. Stirring the contents of the tank, whilst necessary to maintain general paste uniformity, cannot prevent some lumping and viscosity rise brought on by the continual dipping of hot objects: the state of the paste must therefore be monitored. The objects to be coated must be thoroughly cleaned and degreased before dipping. This gives some adhesion but of relatively low degree: if positive interfacial bonding is required the surface of the object must be primed by application of a priming coat of the kind mentioned in Section 22.2.3, deposited from solution in a solvent. Two commercial examples are Deckor Primer (Scott Bader, UK) and Vinatex Adhesive MP7A (Vinatex Ltd, UK). The primed article to be coated is first heated in an oven to a temperature between 900 e and 130oe: depending on the thickness this may take 5-10 min. It is then dipped in the paste and left long enough to build-up the desired (or maximum practicable) thickness of coating: the time required for this is normally quite short-typically 1-2 min. On wire, coatings 0·5-0·8 mm can usually be obtained in one dip. The temperature of the paste may be between about 17°e and 300 e depending on the composition. The speeds of entry into the paste and of withdrawal are factors in the process. If the object is withdrawn too quickly a poor coating will

22 Preparation, Processing and Applications of Pastes

993

result-ideally the rate should be the same as that at which the paste drains off. Not much heat should be left in the object: the final fusion is effected in a separate step. 'Dip marks' (paste drips) usually formed at the bottom of the article may be brushed off before the heat treatment for fusion. This will be appropriate to the paste composition, coating thickness and other relevant process factors. An under-fused coating which may appear matt and be weak or even crumbly, may be returned to the oven for further treatment, providing that the fault is not due to the oven temperature being generally below the minimum necessary for complete fusion. Articles PVC-coated by the hot-dip process include fence posts and fittings, handles, thick wire baskets and trays, mounting brackets and the like. (b) Hot-dip Moulding This process is closely similar to hot-dip coating, except that the object being dipped is a suitably shaped metal former (which may be treated with a release agent), and the coating becomes a hollow moulding when it is stripped from the former after fusion and cooling. Two procedural variants are in use. In one, the formers are pre-heated in an oven and dipped into the paste in the dipping tank at a controlled rate: this is usually quite fast (say about 10 s to submerge the former to the required depth) to avoid differential cooling with consequent uneven paste layer build-up. The immersed formers are left for the time necessary for the coating layer thickness required: this residence period may be quite short. They are then withdrawn at a uniform rate (normally slower than the dipping rate) so regulated as to avoid excess paste being dragged out of the bath through viscosity effects. Once out of the bath, the formers are inverted to let any drips merge back into the coating layers, and transferred to a second oven to fuse the coatings. This treatment is followed by cooling (which may be forced), stripping, and any trimming operations that may be necessary. The second variant involves the same dipping and withdrawal procedures, but the pre-heating of the formers and gelation/fusion of the paste coatings picked up are both done by immersing the formers in a tank of a liquid heating medium. More effective heating for shorter periods (e.g. completion of gelation and fusion about 2-3 min in many cases) are claimed as the main advantages over the method involving oven heating, with consequent faster cycles and reduced

994

W. V. Titow

tendency to the formation of 'drips' and 'runs' of paste on the articles directly before and in the early stages of the gelation/fusion treatment. Disposable, unsupported PVC gloves (typically produced from a basic plastisol composition containing about 100 phr DOP), tool and handlebar grips, golf club covers, and covers for cable terminals, are examples of mouldings produced by the hot-dip process.

(c) Cold-dip Coating In this process the object undergoing dipping is cold. The advantage of this is that the viscosity and general condition of the paste in the dipping tank remain stable, and there is no accumulation of partly gelled lumps and particles. Although on some metal objects the finish obtainable may not be as good as that from hot dipping, cold dipping can be useful in some cases where the object is of irregular thickness: in hot dipping the thicker parts, having greater heat capacity, will tend to build up a thicker coat. Cold-dip coating is important as the method of production of fabric-lined PVC work gloves. These are made by drawing knitted fabric gloves (the 'liners' for the ultimate composite articles) onto hand-shaped formers (usually metal, but sometimes also ceramic), cold dipping the liners on their formers fingers-downwards into the PVC paste, withdrawing, allowing to drain, inverting to let any drip marks at the ends of fingers and thumbs flow back into the coating, and then gelling and fusing the paste layer by passage through an oven under suitable conditions of oven temperature and dwell time. Operation in modern plants is continuous and highly automated. The thickness of PVC coating and degree of its penetration into the fabric of the liner are influenced by the fabric's construction, the rheological properties of the paste, the rates of dipping and withdrawal of the formers, the length of the draining period, and the gelation/fusion conditions. An appreciable degree of penetration is desirable for good union between coating and liner, but a layer of free fabric should remain on the inside of the glove to fulfil the moisture absorption and cushioning functions important to the wearer's comfort. For these reasons any extensive 'strike through' of the paste to the inside of the glove is a fault. A glove-dipping plant is shown in Plate P. Other fibrous products cold-dip coated with PVC paste, in which a degree of mechanical keying through partial penetration of the coating into the surface of the substrate contributes to the strength of union

22

Preparation, Processing and Applications of Pastes

995

Plate P Fully automatic dipping plant for PVC-coated industrial gloves. (Comasec SA, Paris, France-Courtesy Mr A. Charnaud.)

between the two, are household clothes lines and some types of cords and ropes. Cold-dip coating of metal objects-where there is no surface anchorage effect of the kind afforded by a fibrous substrate-is generally more difficult to operate and control than the hot-dip process. A low-viscosity, non-dilatant paste may be used to build up the required coating thickness, by multiple dipping, from a number of thin layers, each pre-gelled before the application of the next one. In this kind of procedure enough heat must be applied at each gelling step to soften the previous coat sufficiently to ensure good merging with the one undergoing gelation. The combined layer is ultimately fused as a whole in a final heat treatment. In some cases a one-dip coat may be given with a viscous paste formulated to be strongly thixotropic at the relatively low shear rates involved in the dipping process: such a paste will 'set-up' in the form of a fairly thick coating, with very little draining, directly after withdrawal from the tank.

996

W. V. Titow

An example of a basic formulation for a low-viscosity, cold-dip paste is given below: PVC resin: Breon P 130/1 Plasticiser: DOP DOS Stabiliser: White lead paste

100pbw 65phr 16phr 2phr

22.2.5 Spray Coating The area of application of this method is similar to that of dip coating. It is, however, particularly useful for objects which are either too large to be easily manipulated in dipping, or of intricate shape. Plastisols for spraying should have low viscosity and be non-dilatant. A definite yield point for flow (Bingham body behaviour) is also desirable as it restricts flow after deposition, although it also makes levelling more difficult: this is, in any case, not normally as easy as with paints for spray application where the solvent vehicle promotes the necessary degree of mobility. Raising the temperature of application can assist the levelling of the paste coating by reducing its viscosity. The incorporation of a small proportion (say about 10 phr) of solvent (e.g. white spirit) can also be helpful in this connection. However, if too much solvent is used a two-stage heat treatment may be necessary, to remove the solvent and then to gel and fuse the coating. Limiting the proportion of solvent added to well below the level usual for a true organosol also enables a thicker coating to be achieved in one operation: as an example, say 0·25-1·25 mm depending on the paste formulation and application conditions, as against 0·03-0·06 mm with some true organosols. For thick organosol-applied coatings, multiple application may be necessary, with solvent evaporation after the deposition of each layer to avoid blistering or orange-peel effects in the final product. The spraying method and equipment may be of the air-spray or airless variety. In the former, the spray gun used should be of the external mix type. A commercial example is the De Vilbiss JGA gun fitted with a No. 306 air cap and size D fluid tip, and connected to a pressure feed container for the paste. Pressure feed is normally more efficient than gravity feed, although the latter type can be used satisfactorily in relatively small-scale operations for spraying organosols of suitably low viscosity. A typical capacity range for pressure feed containers would be 0·5-40 gal. The airless method uses a high hydraulic pressure, in the region of

22

Preparation, Processing and Applications of Pastes

997

2000 lbf in- z, to force the paste through a small spray orifice (typically O·OB-in bore) and thereby obtain the right atomisation. Electrostatic spraying of PVC paste may be carried out by either of the above two general methods, with appropriate arrangements for charge generation. This mode of operation, which is particularly useful with metal objects, can offer economies in paste consumption and coating time: uniformity of coating of intricately shaped surfaces is another advantage of a properly run electrostatic spray-coating process. Problems which may be experienced in spray coating with pastes are generally similar to those encountered in paint spraying. Some of the more common ones are: a 'pebble' finish caused by excessively high line pressure or the gun being held too far from the work; runs which may form if the gun is too close or the paste too fluid; wrinkles or sags on vertical surfaces where the coating has been applied too thickly. After spraying, the coating is gelled and fused by a heat treatment. This may be preceded by drying if an organosol has been used. The drying temperature should be kept to a reasonable minimum to prevent bubbling by the departing solvent, and to keep down the polymer's heat history. The gelation/fusion of coatings on metal substrates may take about 5-10 min at an oven temperature of around 180aC. Higher temperatures may be used to shorten the time, but caution is necessary, as hot spots can develop (especially in a large oven) which can lead to local overheating. The following are two examples of basic formulations for spraycoating pastes: Plastisol PVC resin: Plasticisers:

Stabiliser:

Corvic P 65/54A DAP DNP Paraplex G62 Stanclere80(AKZO, Chemie, Sarl, France)

100pbw 40phr 50phr 2phr 0·75 phr

Breon P 130/1 DOA Mellite131 (Albright and Wilson Ltd, UK) MIBK white spirit TiO z paste (1 : 1 in DOP)

100pbw 25phr 2phr 15phr 15 phr 5phr

Organosol

PVC resin: Plasticiser: Stabiliser: Solvents (diluents) : Pigment paste:

998

W. V. Titow

22.2.6 Coating of Sheet Materials (Fabrics and Paper) PVC pastes find a very important commercial outlet in the coating of fabrics and papers in the manufacture of such products as tarpaulins, tent fabrics, awnings, upholstery materials, wall-coverings, bookbinding fabrics and papers, leathercloth for travel and fancy goods and garments (protective and fashion), floor coverings, conveyor and drive belts, and adhesive tapes. The coating procedures fall into two general groups-direct coating and transfer coating (also known as reverse coating*). In procedures of the first kind the paste is applied directly to the substrate to be coated (textile or non-woven fabric, paper), whilst the essential features of a transfer method are that the coating is first deposited on a carrier material with an easy-release surface, the substrate proper is laminated to the deposited layer (which may be composite), and the carrier removed. Extrusion coating, and lamination to a preformed PVC film, are alternatives to paste coating as methods of manufacture of some of the above mentioned products (as well as others-e.g. decorative surface coverings such as imitation veneers and the like). In the film lamination method, a calendered film may be laminated to the appropriate substrate in-line, i.e. as the hot material leaves the calender (see Chapter 18). A finished film (manufactured by calendering or extrusion) can also be applied to a substrate in an entirely separate operation. In this case the film will be heated or treated with an adhesive for bonding to the substrate: the latter may itself be carrying a tie coat to facilitate bonding, or a coating (say a foamable layer) directly applied, so that the operation becomes a combination of direct coating and film lamination techniques (see also further on in this section). The common basic arrangements for applying a coat of PVC paste to a continuous sheet material are schematically illustrated in Figs 22.2 and 22.4. Their practical embodiments form the paste-application sections of various industrial coating units and lines. In the laboratory, paste layers can be hand-cast with a simple film-coating frame, with a fixed or adjustable gap under the spreading edge. * Not to be confused with reverse roller coating which is one of the ways whereby paste is applied to a sheet material by means of an arrangement of rollers-see Fig. 22.4 and the associated text.

22

Preparation, Processing and Applications of Pastes

999

The suitability of each basic type of industrial coating arrangement in a particular situation will depend on several mutually interacting factors, especially the nature of the substrate (with particular reference to its permeability, and extensibility under the tension experienced in processing), paste rheology, thickness of coating and degree of penetration required. Note: In some cases (e.g. certain tarpaulins and ground sheets, conveyor belts, some types of rainwear and protectivegarment fabrics) complete impregnation of the substrate may be required. With most true coatings on fabrics a depth of penetration of 1/3-1/2 of the fabric thickness will normally be aimed at to combine good 'keying' of the coating with the preservation of satisfactory flexibility and 'handle' of the finished product. For a given application system and paste rheology, the thickness and degree of penetration of the coat (and, to some extent, the rate of coating) are influenced by the size and configuration of the gap between the substrate and the coating element (doctor blade or roller) as governed by the setting and (especially with a doctor knife) the profile of the latter, the tension applied to the substrate, and the nature and positioning of any support under the substrate at the coating point. Some of the ways in which the factors just mentioned will affect the casting operation and its results are fairly obvious. Thus a thick, heavily filled, non-thixotropic paste will tend to produce thick coatings with relatively limited penetration, and will make for comparatively slow rates of application. A light substrate with an open structure and substantial extensibility will be more prone to penetration by a given paste in a given coating process than a dense, heavy, stiff one. A doctor knife with large radius of the leading edge, set at an angle to the substrate, will tend to produce a heavier coating than a vertically set, finely radiused blade. Such general considerations provide useful broad guidelines, and a few more are mentioned below among further comments on the knife and roller coating arrangements. However, the individual factors do not operate in isolation: they invariably interact, and the effects of the interaction must be taken into account (and the results confirmed by practical trials) in any given case.

1000

w. v.

Titow

(a) Paste Coating (Spreading) by Doctor Knife The basic variants of this method are represented schematically in Fig. 22.2. In coating with a doctor knife the blade is set over the substrate and the paste is poured or pumped from a reservoir so that it forms a constantly replenished uniform 'roll' or 'coil' in front of the blade across the width of the substrate 'web'. Depending on the properties of the latter, and the coating characteristics required, the consistency of the paste may range from that of a fairly free-flowing liquid to a thick dough: the knife blade profile and setting (i.e. whether vertical or inclined), and the effective spreading gap will also be chosen accordingly. Some typical profiles are illustrated in Fig. 22.3, with an indication of their applications. The coating speed is subject to the factors just mentioned: depending on the conditions, the speeds for a fairly representative range of operations may vary between about 10 and 100 it min-I. In the knife-over-roller and knife-over-plate arrangements, the support roller may be of steel or rubber, and the plate is usually steel.

Fig. 22.2 Knife coating (spreading) arrangements: schematic representation. A, Knife over roller; B, air knife; C, knife over plate; D, knife over blanket.

B

c

D

l. I E

(

Fabrics: over rubber roll or blanket

Fabrics: as D

D

F

C

B

Fabrics: as air knife, or over rubber roll or blanket Heavy fabric: over blanket Paper: over steel roll Paper: over metal roll

A

Substrate, and coating arrangement

Typical uses

May be angled (up to about 4°) AsD

Vertical, or angled up to 3° Usually vertical

Vertical

Usual knife setting

Heavy (suitable for thick, heavy pastes)

Suitable for range of coating weights (with range of paste viscosities) Heavy and/or penetrating

Thin (including antiqueing on embosssee text) Thin (especially priming or tie coats)

Thickness and/or weight of coating

Good surface finish; typical products: floor coverings, brattice cloth

Appropriate for highspeed coating; good surface finish

Remarks

Fig. 22.3 Some common doctor blade profiles: schematic representation. Substrate movement from left to right. Other factors being equal the amount of coating will increase with increasing radius of the leading edge of the blade.

A

r r $::>

8

--

~

-~

.Q.,

~

5·

$::>

. Similar to VAGH, food-contact applicabut higher solubility tions; crosslinkable and maximum solids by virtue of hydroxyl contents in solutions content As VAGH, but lower Similar to VAGH Similar to VAGD, costs in coatings as but still easier and higher applied solids greater solubility and cheaper solvent systems can be used

a

The former. Bakelite (Vinylite) range of vinyl chloride copolymer resins for solution applications (Union Carbide Corp. in the USA, and associate companies elsewhere). Table based on data from Union Carbide technical literature. b 15% resin in 1: 1 MEK: toluene at 25°C. C Formed by hydrolysing part of the vinyl acetate component. d See Note on pp. 1058-9.

Approx. 12-16

2.~5.3

14·4-17·7

81·5-84·5

VMCC

w.

1052

V. Titow

tolerance and maximum achievable concentration, all increase, and solution viscosity decreases, with decreasing molecular weight. Although all PVC polymers are susceptible, in varying degrees, to many ketone, chlorinated hydrocarbon, and ester solvents, as well as to some aromatic hydrocarbons (cf. Chapter 12, Section 12.8), the homopolymer will give solutions of reasonable concentrations and workable viscosities in only relatively few solvents. Of those, tetrahydrofuran and cyclohexanone are the most important in the technological context. Methylcyclohexanone and isophorone are also relevant, especially for mixed solvent systems, and dimethylformamide is an effective 'booster' for solutions of PVC polymers of high molecular weight. Methyl isobutyl ketone is a useful co-solvent in some systems. For copolymers used in solution applications, the range of effective solvents increases, as does general ease of solution, with increasing proportion of co-monomer(s) present and activity of chemical groups brought thereby into the polymer chain. Solvents effective at room temperature include many ketones, some aliphatic esters, chlorinated hydrocarbons, and nitro compounds. Certain other compounds also act as solvents or co-solvents, e.g. the cellosolves (ethylene glycol monoalkyl ethers), and some aromatic hydrocarbons (benzene, toluene, xylene) can have a marked swelling action. Some solubility data for commercial copolymers are given in Tables 24.2 and 24.3. Tolerance for diluents in solvent systems also becomes greater in the main (although it varies from one copolymer to another): for example, considerable proportions of toluene or xylene can be included in some TABLE 24.2 Saturation Concentrations of a Commercial Vinyl Chloride/Acetate Copolymer (Breon AS 70/42) in Various Solvents Solvent

Weight (g/lOO g solution)

Solvent

Weight (g/lOO g solution)

Acetone Methyl ethyl ketone Methyl isobutyl ketone Cyclohexanone Methylcyclohexanone Mesityl oxide

25 30 25 30 27 32

Isophorone Benzene Toluene Xylene 50/50 acetone/xylene 50/50 MEK/xylene

28 1 1 1

28 28

24 PVC Solutions and their Applications

1053

TABLE 24.3 Solubility (in 10% Concentration) of a Commercial Vinyl Chloride/Acetate Copolymer for Solution use (Vinylitea VYHH) Solubility

Solvent

Acetone Butyl acetate Carbon tetrachloride Cellosolve solvent Cellosolve acetate Dibutyl phthalate Dichloroethyl ether Dimethyl phthalate Dioxane Ethanol Ethyl acetate Ethylene dichloride Ethylene glycol Ethyl ether Dioctyl phthalate Isophorone

at 25°C

at 95°C

S S I I S S S S S I S S I I I S

S S I I S S S S S I S S I I PS S

Solvent

Solubility at at 25°C 95°C

Isopropanol (anhydrous) Isopropyl acetate Mesityl oxide Methanol Methyl acetate Methyl ethyl ketone Methyl cellosolve acetate Methyl isobutyl ketone Propylene dichloride Propylene oxide Tricresyl phosphate Hydrogenated naphtha (diluent) Xylene (diluent) Toluene

I S S I S-CI S S S S S I SO

I S S I S-CI S S S S S PS PS

SO SW

PS S

Key: S, soluble; I, insoluble; PS, partially soluble; CI, cloudy solution; SO, softens. a Materials of this range now marketed under the trade name Ucar-see footnote to Table 24.1. surface-coating formulations. All these points are illustrated by the examples of compositions given in Section 24.4, which also demonstrate that the choice of solvent, or solvent/diluent, system in any given case is determined by the nature, mode of application and end-use of the composition. Aliphatic hydrocarbons, alcohols, and water have precipitant action on PVC solution copolymers, but alcohol diluents can be tolerated in moderate amounts by hydroxyl-containing polymers (especially when dissolved in good solvent(s) in relatively low concentrations) . In general, the solubility of PVC solution resins tends to increase with rising temperature. Highly concentrated solutions may become thixotropic or gel permanently (especially when prepared at an elevated temperature and then cooled down to room temperature). Viscosity increases with the solute content (cf. Fig. 24.1), and-for

1054

W. V. Titow 10000

l

I

.i

:

I

0..1000 u

/

'iii 0

u

l/l

;;: c

(f)

/

. /

I .:/ :/

/

-:

.: / /

~

I I.

I'

1/ I •• •

.• ••

:1

,. '" . ' " . • !/ ,. •

//

100

:

,

,

....Q (5

!

,

....» ::J

:

..

<

",,7

/

....-

.... ..-7 ....-

/

10 0

5

10

15

20

010 Polymer

25

30

35

Fig. 24.1 Solution viscosity (Brookfield) versus concentration for some commercial copolymers. Ucar VYNSIMIBK; _ . Ucar VYNSIMEK; Ucar VAGHIMIBK; + + + + Ucar VAGDlMIBK; ----Breon AS 70142 in MEK:xylene, 1: 1.

many systems, but not invariably-with iQcreasing percentage of diluent at the same solute concentration. The requirements applicable to solvent systems are those for surface-coating or film-casting solutions generally, embodying such considerations as applicational functionality (including evaporation characteristics), health and fire hazards, and cost. 24.2.3

Other Solution Constituents

Like any other PVC composition, a solution may incorporate several of the additives commonly included in PVC formulations. However, the nature and applications of PVC solutions modify considerably the need for, or relative importance of, some of the usual formulation constituents. Thus heat stabilisers, essential in heat-processed compounds, may be omitted from many solution formulations, except

24 PVC Solutions and their Applications

1055

those for stoving finishes or where the product may experience significant heat in service. In these cases the choice of stabiliser should be made in the light of advice from suppliers (of both stabilisers and solution resins) and its suitability verified by tests. With polyvinyl chloride/acetate copolymer, or modified copolymer, solution resins the following general points are relevant. Selected tin or BalCd stabiliser systems can be particularly suitable, preferably used in conjunction with an epoxy co-stabiliser. However, BalCd stabilisers are not recommended for solutions of hydroxyl-modified copolymers,2 and metal soap stabilisers generally can impair adhesion in surfacecoating and adhesive compositions; blooming can also be a problem in some cases, and possible opacifying effects should be borne in mind where transparent final products (e.g. coatings) are required. Basic lead carbonate, satisfactory for some applications (in the absence of nontoxicity requirements) is not suitable for clear formulations: a further consideration with this kind of s.tabiliser is possible development of alkalinity which may be troublesome particularly with acid-containing terpolymer resins. Urea- or melamine-formaldehyde resins can have a stabilising action in clear baking finishes, when present at the level of about 3 phr. A tin mercaptide in conjunction with an epoxy resin (e.g. Bakelite Resin ERL 2774-Union Carbide) has also been recommended2 as a typical stabiliser system for a baked surface coating (in the respective proportions of 1 phr and 5 phr). With polyvinyl chloride/acetate copolymer resins, including hydroxyl-modified grades, zinc stabilisers can cause rapid colour development on heating, especially in unplasticised compositions, or those of low plasticiser content. Stabilisation against light is normally effected by suitable pigmentation, in particular with titanium dioxide used in substantial proportions in many coating formulations, or with carbon black (up to about 6 phr) where a black colour is acceptable. In the absence of protective pigments the resistance of copolymer and modified copolymer solution resins to photochemical degradation is not sufficient for prolonged outdoor service, although a clear film or coating may withstand limited exposure if an effective light stabiliser system is included in the formulation, say about 1 phr of a good UV absorber (e.g. Cyasorb UV-24*-see Chapter 9, Section 9.5) in conjunction with 3-5 phr of a suitable epoxy co-stabiliser (e.g. Bakelite Resin ERL 4221-Union Carbide). * American Cyanamid Co., USA, and associated companies elsewhere.

1056

W. V. Titow

Plasticisers may be incorporated in PVC solutions, for increased flexibility of the ultimate products. The compatability of the usual PVC plasticisers both with the solutions and the PVC polymers or copolymers in the final, solid products is adequate for the purposes of the established applications. In practice, therefore, the plasticiser(s) will, as usual, be chosen for the final properties required (in the light of cost considerations), the main technical limitation on the amount incorporated being development of tackiness in films and coatings and impairment of the adhesion of such products to substrates (especially metals). To the extent to which a meaningful generalisation is possible, about 40 phr may be regarded as the top plasticiser content limit for tack-free coatings based on vinyl chloride/acetate copolymers. The amounts used in solution-deposited coatings based on hydroxylmodified copolymers may typically range between about 15 phr and 30 phr. In some cases, plasticised films and coatings may be improved by a short heat treatment at about 70°C. The already mentioned UV-protective action of pigments (best with carbon black and titanium dioxide, but exerted by all otherwise suitable pigments at sufficiently high loading levels) is one of the reasons for their inclusion in PVC solution-coating formulations, additional to the functions of colouring the end product and imparting covering power to the coatings. Many of the pigments discussed in Chapter 11 (Section 11.3.5) are used, several in relatively high loadings. A few examples of pigment contents of surface coating solutions based on vinyl chloride/acetate copolymers (including hydroxyl-modified grades) are given below. Selected soluble colourants can be used in solutions for clear coatings or films, in rather lower concentrations, as their role is confined to imparting colour.

Pigment Titanium dioxide Carbon black Lead chromate Lead sulphate (white lead) Chrome orange Chrome green (chromic oxide) Iron oxide brown Phthalocyanine green Phthalocyanine blue Aluminium powder

Typical loading range (phr) 50-90 4-6 75-125 75-125 75-100 50-75 40-75 10-20 10-20 30-50

24 PVC Solutions and their Applications

1057

For applications involving heating (e.g. baked finishes), pigments containing zinc should be avoided, as should-preferably-also iron pigments, because of possible promotion of polymer degradation mentioned above in connection with zinc stabilisers. Similarly, alkaline pigments can cause problems in compositions based on acid-modified vinyl chloride/acetate copolymers. Conversely, some lead pigments can have a stabilising action (e.g. white lead). At the high loading levels at which many pigments are used in some PVC solution-coating compositions they may, in some respects, also be regarded as fillers, in that their modifying effects on the properties of the resin in the final product will be broadly in line with those of particulate fillers, as discussed in Chapter 8. In view of the rather specialist applications of PVC solution compositions, fillers with a purely extender (cheapening) function are used only to a relatively limited extent (e.g. clay in some coating formulations) although there are no general technical reasons precluding their incorporation in that role. The properties of PVC solutions and their ultimate products are also influenced by polymeric materials, other than the PVC solution resins, that may be included in some formulations (e.g. for many surface coatings), which may incorporate, for example, epoxy resins, certain cellulose derivatives, or alkyd resins (see also Section 24.4). Such formulations are, however, more properly in the province of surfacecoating technology rather than that of PVC. Lubricants are not included in PVC solution formulations since the processing of this special type of PVC composition does not involve the kind of manipulation of the polymer under heat and shear in which these additives are beneficial (see Chapter 11, Section 11.1.1). Indeed, the presence of a lubricant in, say, a solution-coating composition could impair the ultimate adhesion to substrates. 24.3 PREPARATION OF PVC SOLUTIONS AND SOLUTION COMPOSITIONS FOR PARTICULAR APPLICATIONS

The viscosities of simple PVC solutions used directly (e.g. for film casting) or in the preparation of composite surface coatings are, in most cases, sufficiently low for paddle or impeller mixers to be employed in their preparation, both on the laboratory and industrial

1058

W. V. Titow

scale. The mixer should preferably be covered to reduce solvent loss, and a reflux facility is desirable where heating is applied to assist dissolution of the polymer and keep solution viscosity down for ease of stirring (although heating for these purposes will normally be mild-say at about 35°C). Where the solution is to be heated to assist solvent removal after application, and especially if a deposited film will be post-treated by heating (as with baked finishes), consideration must be given to inclusion of a heat stabiliser in the formulation; in any case, the temperature and duration of heating should not exceed the lowest values necessary for good results. The generally lower heat stability of copolymer resins in comparison with homopolymers should be borne in mind in these connections. Since thermal degradation of PVC resins can be promoted by the presence of iron, the working surfaces of mixing equipment or those of storage containers should not be of mild steel: stainless steel, glass or enamel are the preferred materials. In simple solution preparation involving only the PVC resin and solvent(s), with no other solid constituents (other polymers, pigments) to be incorporated, a useful technique is to add the resin portionwise into the vortex produced in the solvent(s) by the rotation of the stirrer, each portion being allowed to dissolve before the next is added, to avoid lumping. If the solvent system includes also a diluent, the resin may be wetted out with this first, before being stirred into the solvent(s). Alternatively, the diluent may be added slowly, with vigorous stirring, to the solution of resin in the solvent(s); but the pre-wetting method can be particularly useful in preventing lump and gel formation. Where the amounts of solvent(s) and diluent called for by the formulation permit it, the resin should first be dispersed in the diluent, and the solvent added gradually, with vigorous stirring to the resulting suspension, the stirring being continued until solution is complete. As with any polymer solution for clear products, or products of low thickness-e.g. films, fibres-where the presence of even small particulate impurities, gels, or bubbles can seriously affect the properties and appearance, PVC solutions for film casting and clear coatings should be filtered and de-aerated. Note: Ready-made solutions of some vinyl chloride copolymers are commercially available. Two examples are Ucar* vinyl resin solution VYNC (a clear, 40% solution of a hydroxyl-modified

* Formerly Bakelite.

24 PVC Solutions and their Applications

1059

vinyl chloride/acetate copolymer in isopropyl acetate) and Ucar vinyl resin solution VERR-4(j3 (a 40% solution of an epoxy-modified vinyl chloride/acetate copolymer of low molecular weight in MEK: toluene). The VYN C solution is recommended for use in coatings incorporating nitrocellulose or nitrocellulose/alkyd blends, as well as coatings modified by reaction (via the hydroxyl groups) with amino resins or isocyanates. The VERR-40 solution, in combination with an acid-modified VCNA copolymer (e.g. Ucar YMCA) can be used for high-solid baked coatings with very good adhesion to metal, hardness and gloss. Examples of another kind of commercial PVC solutions are proprietary solvent cements for bonding pipes and fittings (see Chapter 20, Section 20.3.2). Because of the nature of the solvents and diluents used, the precautions to be taken in the preparation, storage, transport and handling of PVC solutions are substantially the same as those called for with flammable liquids generally. The preparation of pigmented PVC solution compositions (for use as paints and the like), which may also contain other constituents, e.g. plasticisers (cf. Section 24.4), may be carried out in more than one way, depending on the equipment available. Advice is readily obtainable from the resin or pigment suppliers in particular cases. A useful general approach, which can give compositions producing coatings of good gloss, is to prepare first a solution of the PVC resin(s) in the solvent(s) as outlined above. The pigment (and stabiliser, if used) is then pre-dispersed in the diluent and plasticiser(s), withpreferably-some grinding aid, in the ball mill. The resin solution is finally added to the mill and grinding continued for the requisite time. For coatings of maximum gloss, a suitable pigment concentrate should be used. This may be a solid masterbatch (highly pigmented chips of the appropriate resin) or the appropriate pigment concentrate paste. The paste need only be diluted with the resin solution in the required proportion to make the complete coating composition (if plasticisers are to be included, these can be stirred into the resin solution before blending with the pigment concentrate). Masterbatch chips must be dissolved in solvent (part of the total amount called for by the formulation): the quantity of solvent used should be the minimum necessary to form a thick, pasty (but complete) solution. This solution is then blended with the resin solution (by stirring-in the latter). Where

1060

W. V. Titow

good gloss of the final coating is not a primary requirement, the plasticiser(s), pigment, and grinding aid (and stabiliser if used) may be pre-dispersed directly in the resin solution (in a high-speed disperser, two-roll mill, or other suitable equipment), and the dispersion ground in a paint or ball mill in the usual way. Details of the preparation of paints in which PVC resins are modified by, or serve as modifiers for, other polymeric binders, and oils, belong to the field of surface coatings, and are thus outside the scope of this book. It may be mentioned, however, that the following types of paint binder materials are among those with which vinyl chloride/acetate copolymer and terpolymer resins are compatible (in varying degrees, but generally adequate for paint formulation): urea-formaldehyde resins, phenolic resins, alkyd resins, epoxy resins, polyketone resins, castor oil and other oils, urethane prepolymers. Note: The urethane prepolymers react with the -OH groups of hydroxyl-modified VCNA copolymers, giving tough coatings with very good adhesion. The reaction can proceed at a significant rate at toom temperature, so that the storage life of mixtures is limited to 8-24 h,z For this reason, surface-coating compositions based on urethane prepolymers and the modified copolymers (e.g. Ucar VAGH or VAGD) are formulated as two-component systems, to be mixed directly before use.

24.4 APPLICATIONS Solutions of vinyl chloride/acetate copolymer resins find a number of applications as surface coatings, of which the following are typical: overlacquers for PVC coatings on fabric and paper in such products as leathercloth, floor-coverings and vinyl wallpapers (see also Chapter 22, Section 22.2.6); strippable coatings (sprayed or dipped) for temporary protection of metal surfaces and various products during transport and storage, cocooning compositions (applied by spraying) for equipment and article protection in similar circumstances; protective coating (resistant to chemicals and moisture) for concrete, wood and metal in the building industry. Some relevant starting formulations are given, by way of example, in Table 24.4. Solutions of the modified copolymers (whose carboxyl or hydroxyl groups variously improve adhesion to substrates and reactivity with

1061

24 PVC Solutions and their Applications

TABLE 24.4 Vinyl Chloride/Acetate Copolymer Solutions for Some Coating Applications: Examples of Basic Formulations (Based on data from the technical literature of Union Carbide Corp.)

Component

Formulation (Pbw) Overiacquer for vinyl coatings

Ucar VYNS Ucar VYHH Methyl methacrylate resin Cellulose acetate-butyrate resin' Plasticiser Aluminium powder Methyl ethyl ketone (MEK) Methyl isobutyl ketone (MIBK) Toluene

5·0-7·5 a 2·5-5·0 0·2-0·4

40d 60d

Strippable Cocooning coating solution solution 10 5

16b

4

8 6 70

27 27 27

a Toughness,

flexibility and adhesion improve with increasing resin content. For bridging large gaps (say about 40 cm) 2 parts of the resin should be replaced with polyvinylidene chloride resin of high molecular weight. 'May be omitted if surface slip and blocking resistance not required, or replaced with amorphous silica if the attendant matting effect acceptable. d The amount of the solvent system used may be varied according to viscosity required for application and the desired solids content (but the ratio of 4: 6 MEK:MIBK should be preserved). b

other resins) alone or in combination with one another and/or with the unmodified copolymers, are used in a variety of coating applications where moisture and chemical resistance, toughness and gloss are of interest. The applications include primers and top coats for wood and fibre or particle board (in which the copolymers-especially the hydroxyl-modified resins-are usually combined with other resins, e.g. alkyd, nitrocellulose, urea-formaldehyde, urethane prepolymer), and various paper coating uses, notably coatings for food-packaging papers in which the hydroxyl-modified copolymers (offering particularly good adhesion) may be used alone or in blends with vinyl acetate and other polymers. The same copolymers are also especially useful in decorative/protective coatings for paper labels in view of their excellent adhesion to a variety of print types, and the gloss obtainable after drying at about 105-120°C (enhanced further by a short baking treatment). 2

W. V. Titow

1062

Industrial maintenance and marine paints for steel and other substrates constitute a long-established, important area of application of solution systems based on the vinyl chloride/acetate copolymers and terpolymers. 4 Such systems, nowadays normally of the high-build variety, offer some adyantage over each of the rival types of coating (alkyd, epoxy, or chlorinated rubber) in handling, performance (especially corrosion and weathering resistance) or costs. Something of the nature of the formulations, and the suitability of different resins for particular kinds of these finishes, is indicated by the examples given in Table 24.5. Baked finishes (solution-applied by roller or spray) for tinplate or sheet iron, based on hydroxyl-modified VeNA copolymer in conjunction with the previously mentioned co-reactive resins (UF, MF, epoxy, TABLE 24.5

High-build Vinyl Marine and Maintenance Paints: Examples of Basic Formulations (Based on data from Ref 4)

Formulation (Pbw)

Component

Cellosolve acetate Methyl butyl ketone Toluene Xylene VM & P naphtha Ucar resin VAGH Ucar resin VYHH Ucar resin YMCA Ucar resin VYHD Tricresyl phosphate Didecyl phthalate Thixotropic agent Red lead Rosin Cuprous oxide Dispersant Titanium dioxide Clay extender Zinc phosphate

Red lead primer

Shipbottom antifouling

White anticorrosive primer

White topcoat

44·45 6·35 6·98 4·44 1·27 13·18

13·15 1·88 2·07 1·32 0·38

36·76 5·25 5·78 3·67 1·05

38·20 5·46 6·00 3·82 1·09

2·49

1·36

2·27

1·96 20·01

1-39

21·90

15·66

3·92 0·92

3·90 1·05

11·53

0·27 10·95 13·60

9·75 65·30

9·22

24 PVC Solutions and their Applications

1063

urethane prepolymer) offer excellent protective properties combined with adhesion good enough to withstand drawing, stamping and forming operations involved in the formation into containers, closures and the like. 2 Solutions of vinyl chloride homopolymer of relatively high molecular weight (for good mechanical properties) are used (at about 30% solids concentration) for the production-on a limited scale (because of the relatively high cost in comparison with calendering or extrusion)-of high clarity PVC film for special packaging applications. The solution is cast onto a stainless steel band in the way originally developed for casting cellulose acetate film. 5 ,6 Lack of molecular orientation, and hence freedom from mechanical and optical anisotropy (which, inter alia, virtually eliminates retraction on heating and affects optical properties) is a special feature of cast polymer films. The polymeric components of PVC solutions used as solvent cements for bonding pipes, pipe fittings, and certain other PVC products may be homopolymers, copolymers, standard PVC compounds,7 or the actual compositions of the mouldings, etc., to be bonded. Solvent cements for PVC are discussed in Section 20.3.2 of Chapter 20, where attention is drawn to some relevant standard specifications. Two further ones, not mentioned either in that section or in Appendix 1, are: ASTM D 3138-80 Solvent cements for transition joints between acrylonitrile-butadiene-styrene (ABS) and poly (vinyl chloride) (PVC) non-pressure piping components. ASTM F 493-80 Solvent cements for chlorinated poly (vinyl chloride) (PVC) plastic pipe and fittings. PVC fibres are spun from solvent solutions of suitable copolymers. 8 24.5 ADHESION OF SOLUTION·APPLIED PVC COATINGS TO SUBSTRATES

In general, the adhesion of vinyl chloride homopolymers and vinyl chloride/acetate copolymers to smooth substrates (especially metal, but also others) is poor, although it can be improved in some cases by a baking treatment (cf. Table 24.6). The presence of carboxyl or hydroxyl groups in the modified copolymers improves the adhesive properties of these resins substantially, as indicated in Table 24.7.

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W. V. Titow

TABLE 24.6 Adhesion of Vinyl Chloride/Acetate Copolymer Resin (Breon AS 70/42) Film Solution -deposited onto Aluminium Sheet Q

Stoving conditions

Solution solids

Resin (20 g)

None 30 min. at 50°C 15 min. at 150°C

Resin (15 g) + maleic acid (5 g)

Resin (10 g)

None } 30 min. at 50°C 30 min. at 150°C

+ maleic acid (10 g)

None 30 min. at 50°C } 30 min. at 1500C

Q

Adhesion

Air-dried film peeled easily Slight adhesion Adequate adhesion Air-dried film peeled easily Adequate adhesion Air-dried film peeled easily Adequate adhesion

Solution: solids, 20 g; acetone, 40 g; toluene, 40 g; phosphoric acid, 0·5 g.

TABLE 24.7 General Adhesion Characteristics of Commercial Vinyl Copolymer Resins for Solution Use (Based on data from Ref. 2) Resin type

Surface

Metal (clean, smooth) Phosphated metal Wood Glass Paper Cloth PVC Polyvinyl butyral Phenolic resins Urea-formaldehyde resins Methacrylates and acrylates Q

Vinyl chloride acetate (Ucar VYHHor VYHD)

Carboxylated VClVA (Ucar YMCA, VMCC or VMCH)

Hydroxylated VCIVA (Ucar VAGH or VAGD)

P P P P P P E P P P

E E

P F

F E G F-E E F F F

F F G G E E G G

E

E

E

1065

24 PVC Solutions and their Applications

TABLE 24.7-eontd.

Surface

Resin type Vinyl chloride acetate (Ucar VYHHor VYHD)

Carboxylated VCIVA (Ucar VMCA, VMCC or VMCH)

Hydroxylated VCIVA (Ucar VAGH or VAGD)

P P F P

F F F P

P E F

P

G

G

E

G G

Nitrocellulose Alkyd resins Chlorinated rubber Oleoresinsa Shellac Concretea Plastera Key: E

P

G

E

F-E

= excellent; G = good; F = fair; P = poor.

a Adhesion

can vary with type.

Notably, the air-dried adhesion of carboxyl-modified copolymer is particularly good, e.g. Ucar VMCH adheres well, on air-drying, to iron, steel, cadmium and brass, although on zinc a phosphate pretreatment or a wash primer is desirable (wash primers are useful in many other cases). Admixing with the VMCH resin can improve the adhesion of other vinyl chloride/acetate copolymer coatings.

REFERENCES 1. Bakelite Vinyl Resin Solution VYNC, Union Carbide Corp. Technical Bulletin, 1971. 2. Bakelite Hydroxylated Vinyl Resins VAGH and VAGD, Union Carbide Corp. Technical Bulletin, 1971. 3. Bakelite Vinyl Chloride-Acetate Resin Solution VERR-40, Union Carbide Corp. Technical leaflet, 1978. 4. High-build Vinyl Maintenance Paints, Union Carbide Corp. Technical Bulletin, 1973. 5. Yarsley, V. E. and Flavell, W. (1956). Cellulosic Plastics: Part 1: Cellulose Acetate, Cellulose Ethers, and Regenerated Cellulose, Plastics Monograph No. C6, Plastics Institute, London. 6. Couzens, E. G. and Yarsley, V. E. (1968). Plastics in the Modern World, Penguin Books Ltd, Harmondsworth, Middlesex, England. 7. ASTM D 2564-80: Specification for solvent cements for poly (vinyl chloride) (PVC) plastic pipe and fittings. 8. Cook, J. G. (1964). Handbook of Textile Fibres, Merrow Publishing Co., Watford, England.

CHAPTER 25

Cellular PVC Materials and Products w.

V. TITaw

25.1 INTRODUCTION Neither the terminology nor its usage in the field of cellular plastics is as yet fully standardised. The principal nomenclature standards* offer definitions of some basic terms, but these are by no means mutually identical. Moreover, none of the definitions can be said to be fully comprehensive, and they are not adhered to strictly in industrial usage. For the purpose of the present chapter the most important relevant terms are defined as follows. Cellular PVC: A PVC material or product whose apparent density is significantly lower than that of its parent PVC composition by virtue of the presence of numerous cells (voids) dispersed throughout its mass.

This definition conforms closely with those of 'cellular plastic' given in the ISO and ASTM standards. * Cellular vinyl:t A cellular PVC whose solid material is a flexible PVC composition. PVC foam = expanded PVC: A cellular PVC in which the cells have

* ISO 472-1979; ASTM D 883-80; BS 1755: Part 1: 1967.

t As

mentioned in Chapter 1, the use of 'vinyl' for 'PVC', especially 'flexible PVC', is widespread, although it might be questioned on strict terminological grounds (see Chapter 1, Section 1.1). In the context of cellular PVC the 'flexible' connotation of 'vinyl' is particularly strong. 1067

1068

W. V. Titow

been formed by a gas evolved in, or introduced into, the parent PVC composition in the course of production or processing. Vinyl foam = expanded vinyl: A PVC foam whose solid material is a flexible PVC composition. Blown PVC: A PVC foam in which the cell-producing gas has been generated or expanded within the parent PVC composition (and not, say, mechanically admixed at atmospheric pressure). Blown vinyl: A blown PVC whose solid material is a flexible PVC composition.

In both the commercial and the technical sense PVC foams form the most important group among cellular PVC products. They are also significant among plastics foams generally, with several applications where their processability and/or properties offer special advantages over alternative cellular materials including those (like, for example, polyurethane foams) whose volume of use is much greater in many areas. Two characteristic examples are the foam layers of composite coatings on vinylleathercloth (d. Chapter 22), and rigid foam cores of GRP or other sandwich panels used in the construction of vehicle bodies (e.g. refrigerated vans, modern high-speed train cabs l ). In the former application, processability in paste form-unique to pPVCand good bonding to the skin and anchor layers of the coating are cardinal advantages; as are the strength of rigid PVC foam (greater than that of polyurethane or polystyrene foam) and its low flammability in the constructional sandwich panels. Note: The floor in the cab of the British Rail high-speed train is a GRP/PVC foam panel. In the prototype a more complicated sandwich was used, with a composite PVC/polyurethane foam core layer in which the PVC foam (of density about 40 kg m-3) was the strength-imparting component.

Like other expanded plastics PVC foams are usually classified by reference to type, i.e. flexible or rigid, and density. The size and nature (open or closed) of the cells is also a significant characteristic, influencing such properties as, for example, the resilience and compression behaviour of flexible foam or the heat conductivity and moisture permeability of rigid foam insulation. The description of a plastics foam as low, medium, or high

1069

25 Cellular PVC Materials and Products

density, very common in practice, is not exact, as the density ranges corresponding to each of these terms are not strictly defined. The following figures can, however, serve as a rough, general guide. Low density range Medium density range High density range

kg m-3

lb /t- 3

10-50 50-350 350-900

0·6-3 3-21 21-54

Expanded PVC can be produced in a variety of densities. Most of the commercial rigid foams which find application in vehicle and marine construction, cold-store insulation and some aircraft interior fitments, are of low to medium density, say, in general terms, 30-400 kg m -3, with densities of 80-120 kg m -3 fairly typical for the PVC foam cores of constructional sandwich panels for vehicle bodies. Many types of flexible foam fall within the same general density range, but higher densities, up to about 800 kg m -3, are also practicable. Such high densities are fairly typical for (and may even be exceeded in) flexible injection-moulded microcellular shoe soles, or certain types of permeable coatings on leathercloth (cf. about 1250 kg m- 3 for solid flexible PVC). Some rigid structural foam products (e.g. extruded cellular PVC profiles, injection-moulded parts) are also in the high density range. The production of vinyl foam from plastisols provides the basis of important industrial processes and such large-tonnage commercial products as coated fabrics, flooring and foam-backed carpets. These processes and outlets are dealt with in the following sections, as are other applications and production methods of cellular PVc. The production of expanded vinyl coatings on fabrics is also mentioned in Chapter 22. Microporous PVC sheets with intercommunicating cell structure, which are used as battery separators and filter media, may be mentioned as an example of a cellular PVC material manufactured by methods other than foaming (see Section 25.2.2 below).

25.2 PRODUCTION METHODS AND PROCESSES 25.2.1 Foams (a) Dispersed-gas Blowing The general principle of this method is the incorporation of an inert gas, usually a halogenated hydrocarbon or CO2 , into a PVC plastisol at

1070

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V. Titow

low temperature and high pressure (the gas usually dissolves in the plasticiser(s) under these conditions), followed by pressure release and heating, carefully controlled to bring about synchronised expansion and solidification (fusion) of the composition, which is finally cooled. The principle has been utilised in several commercial processes, including the Elastomer process2 ,3 (originated in the USA, but also used in Europe), the Dennis process4 (sometimes known as the 'Fay Foamer' process) and the German Trovipor process of the Dynamit Nobel company. 5 The PVC foam manufactured by processes of this kind is flexible, mainly open-cell (about 90% of the cells intercommunicating), and of low to medium density (typically 60-270 kg m- 3). It is normally produced in the form of continuous sheet (slab) several inches thick, which is usually slit into thin layers (e.g. for use in upholstery). The sequence of operations in the Trovipor process is schematically shown in Fig. 25.1. Machines have been available since the mid-1960s capable of slitting the foam slab into layers of thickness down to 1·5 mm (0,050 in) with a tolerance of about 0·25 mm (0,010 in).6 The layers may be bonded to textile fabrics with adhesives, for use, for example, in the proquction of foam-backed clothing~ However, such techniques will not produce cellular PVC leathercloth, partly because of the comparatively coarse nature of the foam, and partly because the thickness of coating on many types of leathercloth is only about 0·040 in or even less. It is also very difficult (and costly) to produce cellular layers of this thickness by direct deposition of 'wet' dispersed-gas-blown foam onto a fabric. Cellular leathercloth is not, therefore, made from this type of foam, although it is a major field of application for foams produced by other processes, as described below. The most important application of foams made by dispersed-gas blowing was for many years in the automobile industry as padding and cushioning material, especially in car seat upholstery, where the open-cell nature (and hence 'breathability') of the foam, its good resilience, and-perhaps most importantly-its suitability for highfrequency welding to PVC sheet materials used for seat surfacing, were important advantages. However, over the past ten years the use of vinyl seating and trim in motor cars has declined drastically in favour of fabrics, now widely used for this purpose (in conjunction with polyurethane foam cushioning). Vinylleathercloth is currently installed in only a few per cent of European cars (the percentage is substantially higher in cars for export to, or made in, some overseas countries, notably in Africa). Vinyl foam produced by dispersed-gas blowing still

25

1071

Cellular PVC Materials and Products

(B)

m

I

____ --.l...-

~I ()

Inlrt gu

(C)

0

!

p

I

I

_

P s

,q

r

I

()

Fig. 25.1 Schematic representation of the Trovipor process. * A. Paste preparation: a, PVC; b, plasticiser; c, additives; d, mixer. B. Gasification plant: e, gas circulation pump; f, cooler; g, autoclave; h, paste; i, gas stream; k, perforated plate; I, paste feed pump; m, paste feed container. C. Spray and fusion plant: n, spray tower; 0, h.f. heating; p, after-heating; q, cooling tunnel; r, to transverse or longitudinal cutters; s, conveyor belt.

features prominently in such major end-uses as travel and fancy goods, certain types of foam-backed clothing, embossed quilting, and some types of furniture upholstery (especially where low flammability is of particular importance). (b) 'Chemical' Blowing This method is used industrially in the manufacture of many cellular PVC products (see Table 25.1), but it is also convenient for small-scale preparation of PVC foams, e.g. in laboratory operations. A blowing • H. V. Finkmann et al. (1969). The foaming of plastics, Kunststoffe, 59(9), 695; reproduced by kind permission of the publishers, Carl Hanser Verlag.

Free ~ blowing

The sheet may be laminated with a fabric and topped with a dense skin. Sheet or

Calendered sheet (produced by compounding and pro-

Sheet blowing

Solid (gelled) skin usually formed first by rotational casting from ordinary plastisol. Mould (vented to prevent pressure build-up during blowing) then partially filled with foaming composition (amount adjusted to give foam filling mould but with atmospheric pressure within cells), heated in hotair oven, and finally cooled

Plastisol

Deposition of plastisol layer on sheet support (e.g. release paper or conveyor band) for subsequent lamination with a fabric, or direct onto a fabric; heating to expand and gel (hot-air or IR oven); cooling. Posttreatments: embossing, lacquering, printing

Cavity filling

Q

Principal production operations or process stages

Plastisol

Form of composition (starting material)

Direct expansion

Method type

Carpet and flooring underlays, embossed wallpapers, cellular leathercloth, upholstery, cushioning

Typical products

Largely closed cell at medium densities, largely open-cell at low densities

Foam flooring, cellular leathercloth

Soft: low to medium density, Foam filling for arm rests, mainly open-cell structure, handlebar grips, dolls and but mixed in some products soft toys

Largely closed cell at medium densities, largely open-cell at low densities (could be described as mixed cell structure)

Characteristics of foam produced

TABLE 25.1 Main Production Methods Involving Chemical Blowing

~~

~

: Kevlar 49 cloth/the glass filament cloth/the Kevlar 49 cloth/gel coat. 12

Materials of some types of buoys and floats; cellular profiles and certain types of pipe (see Chapter 19, Sections 19.3.4 and 19.4.1). (b) Flexible Foam Once widely used as the cushioning material in car upholstery: this use has declined drastically in the last ten years. Still used widely as the foam layer in coated-fabric flooring and cellular leathercloth (see Section 26.2.1 below). Material of injection-moulded microcellular soles, certain types of carpet-backing, crash pads, and some toy fillings.

26.2 COMPOSITE PRODUCTS (COATED, LAMINATED, OR FILLED) 26.2.1 Coated Fabrics (see also Chapters 22 and 25) This is a big outlet for flexible PVC. The main types of product (mostly produced by paste coating) are: leathercloth of all kinds (both solid and cellular coatings) used in the production of upholstery, protective and foul-weather clothing, travel and fancy goods; (ii) tarpaulins (usually with a synthetic-fibre base, commonly nylon or polyester), inflatable buildings, * other inflatables, t brattice cloth; (iii) some types of water barriers, hovercraft skirts, life rafts, bulk liquid freight bags (some produced by hot lamination of PVC/nitrile rubber blend sheeting to nylon fabric 13);

(i)

* Very large air-inflatable structures (usually from PVC-coated nylon) may be delivered to the site in sections, and finally fabricated by welding before erection. t e.g. large inflatable air bags for breaking the fall in rescue situations-an example is the bag developed (originally for use by a film stuntman) by Greengate Polymer Coatings Ltd in the UK14 (similar products are also available in continental Europe and the USA).

1112

W. V. Titow

(iv) PVC backing on carpets (see Chapter 23); (v) continuous fabric- or felt-backed flooring; (vi) fabric-backed PVC adhesive tapes for electrical insulation, medical and other uses; (vii) PVC-impregnated sectional blinds for picture windows; (viii) PVC-coated gloves; (ix) PVC-coated glass-fibre fabric, used, for example, in the building industry for ducting (wrapped over flexible wire helical frames). 15

26.2.2 Conveyor Belting This might be classified as a kind of coated fabric, but is in fact much more substantial and heavy than coated fabrics for all other uses. The main application of PVC conveyor belting is in mines, although some grades are also used in manufacturing processes. The two main structural types of conveyor belting are known as multi-ply and solid-core belts. Multi-ply belting is produced from layers of fabric (cotton duck, and/or synthetic fibre) individually coated on both sides with flexible PVC (applied in paste form) and laminated together: the resulting multi-layer band is over-coated on both sides with an outer coat (typically about 1 mm thick) formulated for toughness, abrasion resistance, and low electrical resistivity (antistatic property). The outer coverings may also be calendered sheets heat-laminated to the interply assembly: the latter may typically contain 3 piles (common with synthetic fibre fabrics) or 5 plies (with cotton duck fabric). To obtain the necessary shape and smoothness of edge contour, an extruded flexible profile may be applied under heating, or a paste of suitable composition moulded on. A common continuous method of producing multi-ply belts is the Rotocure process,16 in which the pre-coated plies (with the coating pre-gelled by passage through an oven) are combined, under pressure, on the surface of a slowly rotating, heated drum of large diameter, where the fusion of the paste layers in the resulting laminate is completed. Stepwise lamination in a daylight press may also be used, but this is generally slower than the continuous method. Solid-core belting consists of a fabric layer encapsulated by coating/impregnation in paste-applied flexible PVC, over-coated with a harder surfacing composition. In some versions the yarns of the core fabric may be pre-impregnated with PVC. 16

26 Applications of pvc

1113

26.2.3 Sheet-type PVC Interior Wan-Coverings The salient features of this kind of wall-covering have long been known: the main ones are-in combination-distinctive appearance with many possible decorative effects, durability, and washability (in all but a few cases of special composite effects, like a flocked or metallised finish). The majority of PVC wall-coverings consist of clear vinyl coatings on the more traditional materials such as paper, or cotton fabric, but laminates of PVC with woven acrylics, hessian, and even silk, or metallised layers, are also represented. Further variants include wall-coverings with embossed PVC layers, which mayor may not be cellular. Note: Non-coated, special wall-coverings are also made; these are panels thermoformed in rigid PVC, and might thus be classified as wall cladding for interior use. The panels are made-in three-dimensional effects-to resemble brickwork or stone walls. A commercial example is Stone Decor (Triplastic Gesellschaft, Niederheim, West Germany). In general, the wall-coverings fall into two categories: supported and unsupported products. In the supported products the base material carrying the polyvinyl chloride layer may be paper (of varying thickness and substance) or fabric of varying weave and consisting of various types of fibre (synthetic fibres, cotton, jute or silk). The PVC layers of the supported products, and the 'body' of the unsupported products, are essentially the same in nature, appearance and physical characteristics. They consist of plasticised PVC compositions and may be either solid or cellular (expanded). The PVC composition may be bulk-coloured, or natural, or white. The surface of the PVC layer, or of the PVC sheet in the unsupported wall-coverings, may be plain or embossed. It may also be printed with a design. The wall-covering, whether supported or unsupported, may carry a separate surface layer which may be a thin coating of metal or a protective plastics film or a lacquer coating. The commercial products represent many combinations of the above principal structural features. The following may be quoted as illustrative examples: Mural Mousse Somvyl Decor 5106 (Sommer Exploitation, Neuilly,

1114

w.

V. Titow

France): An unsupported, cellular, bulk-coloured material with a printed embossed surface, produced by paste-coating on a carrier (cf. Chapter 22).

Suwide Placo 180-182 (Helmonde Textile Co., Helmond, The Netherlands): Fabric-supported solid PVC layer, bulk-coloured, embossed, surface-protected by Tedlar (Du Pont) fluoropolymer layer. The Ferralon range (Chamberlain Plastics, Rushden, Northants, UK): Paper-supported, thin, solid, colourless PVC layer with metallised surface (plain or embossed). The Ferralon range is not recommended for large-scale wall covering, but is fully illustrative of the similar (but cotton scrim-backed) Metalon range which is. The Galan range (Galon AB, V. Frolunda, Sweden): Fabric-based PVC wall-covering produced by extrusion of a plasticised sheet and integral bonding to the fabric support.

26.2.4 PVC Coatings and Coverings on Metal Substrates (a) Wire and Cable Insulation and Coverings Some aspects of this subject are also covered in Chapters 12 (Section 12.3), 13 (Section 13.4.2.4), 14 (Section 14.8), and mentioned elsewhere in the book. Suitably formulated PVC is a good electrical insulator: * this fact coupled with the relative ease of application (by extrusion), good mechanical properties, and low flammability, have been instrumental in making wire insulation and cable covering into major applications of pPVC compositions. The relatively low heat resistance and maximum service temperature of PVC is a limitation, but these can be substantially increased by cross-linking: specially formulated compositions containing radiation sensitisers are applied by normal extrusioncoating procedures, and the coatings are subsequently cross-linked by exposure to high-energy radiation (an electron beam, or emission from a cobalt-60 source). Some cross-linked coatings suffer no cut-through after application for 5 min of a weighted soldering iron at 350°C. 18 Other property improvements include increases in toughness, abrasion * The factors affecting the volume resistivity of a PVC composition have been discussed in some detail in a paper by Wingrave. 17

26 Applications of pvc

1115

resistance, tensile strength at elevated temperatures (e.g. about 3 MPa at 150a C for a Type 5 hard insulation to BS 6747 18), and resistance to cracking under stress, as well as a reduction in creep and improved solvent resistance. Examples of some applications of wire with cross-linked PVC insulation are: flexible cord for hot domestic electrical appliances, jumper wires in telephone exchanges (specified, for example, by the UK Post Office); under-the-bonnet wiring in some quality motor vehicles (including, for example, Jaguar cars).18 A useful brief review of the service requirements for modern cables, mentioning several applications of PVC in this area, has been published by Nye. 19 In certain types of power cables (10 and 20 kV) corrugated polyester film is used as a binder and thermal insulation in the form of a helical wrapping between the inner PVC covering of the core and the outer PVC sheathing. 2o Note: Wire for the production of chain-link fencing is also extrusion-coated with PVC compositions.

(b) PVC/Metal Sheet Laminates As with any other PVC coatings on metal where positive adhesion is required, a primer is first applied to the metal: typically, the primers are epoxy- and/or acrylic-based. The main functions of the PVC layer are decorative and protective. With suitable priming the metal sheet may be formed without dislodging the PVC layer. Steel plate and aluminium sheeting are available with PVC coating (e.g., respectively, Hishi-metal-Mitsubishi Plastics Industries Ltd, Japan,21 and Bondene22-Storey Brothers and Co. Ltd*): applications of the former include electronic equipment casings, electric cookers, refrigerators, lighting apparatus and show cases: 21 the PVC/aluminium sheeting is also used for electronic equipment casings, interior and exterior signs and display panels, and road-tunnellinings. 22 26.2.5 Laminates of PVC with Non-metallic Materials (a) Sandwich Panels PVC-sheet-faced panels with insulant filling (non-PVC foam or other porous material) are used for some constructional and partitioning

* See footnote :j: on p. 905.

1116

W. V. Titow

applications (for use of PVC foam cores in composite panels see Section 26.1.4(a) above). (b) PVC/Polystyrene Sheet Laminate This type of laminate, comprising a decorative PVC film on a polystyrene sheet backing carrying a pressure-sensitive adhesive layer is manufactured* for use as surface finish (wood and other effects), e.g. in the radio and television industries, furniture and interior fitting applications.

(c) PVc/Polyacetal Laminated Sheeting has also been produced. 23

26.2.6 Unsupported PVC Flooring and Floor Tiles This is a substantial application. The flooring (continuous, or cut into tiles) is produced by calendering from compositions based on VCNA copolymers and filled with asbestos fibres (see also Chapter 4, Section 4.6.2; Chapter 8, Section 8.2.1; Chapter 9, Section 9.4.3(b); and Chapter 18). Note: These PVC flooring materials can occasionally suffer embrittlement, shrinkage and cracking under conditions of heavy use in industrial applications (some of these effects have been attributed to the use of solvent-gel cleaners and strong detergents). However they are widely used in hospitals and schools, and domestic kitchens and bathrooms, where they offer wide choice of colours and patterns, ease of cleaning, good cushioning, insulation and reasonable price (the last three features represent advantages over linoleum).

A special type of dust-capturing flooring with a permanently tacky surface (achieved by deliberate formulating for limited plasticiser migration) is commercially availablet for use in premises where dust-free atmosphere is important-e.g. production premises for pharmaceuticals or electronic components, operating theatres and intensive-care units in hospitals. The material is also offered as an alternative to chemically impregnated rugs in offices, with the claim

* In the UK, for example, by Matcon Ltd, Mayland, Essex.

t In the UK from

Dycem Ltd, Bristol.

26 Applications of pvc

1117

that the latter require shampooing, on the average every 6-8 weeks, whereas the special flooring should perform satisfactorily if cleaned about every 20 months. 24

26.3 PVC FIBRES AND FIBRE PRODUCTS Apart from the vinyl chloride copolymer fibres mentioned in Chapter 1 (Section 1.5.2) homopolymer fibres are also produced (by solution spinning). There are many commercial products. Some examples are: Rhovyl (continuous filament and staple-originally produced by Societe Rhovyl SA, France) and other fibres from the same makers (e.g. Thermovyl, Fibravyl, Retractyl); Movil (Montecatini, Italy) and PeCe (Agfa Wolfen, West Germany) fibre. PVC fibres have been used in the production of filter cloths, wadding and braiding for use in the chemical industry. Other applications include protective clothing (in which the low flammability and chemical resistance of the fibres are utilised), tarpaulins, fishing nets, and awnings. Blends with other fibres are also used: one example is the use of shrinkable Retractyl fibre yarns in cloque fabrics, where retraction under heating of this PVC yarn produces special effects. The use of fibres of VCNA copolymer modified with maleic acid has also been proposed 25 in heat-sealable thermoplastic papers to improve fibrillation, sheet formation and sealing.

26.4 MISCELLANEOUS PRODUCTS AND APPLICATIONS 26.4.1 Gramophone Records These are discussed in Section 19.6 of Chapter 19. An example of a modern record-production plant-daimed to be the most modern in the EEC-is that operated by CBS Records at Aylesbury, Bucks, UK. The automatic compounding line, utilising a Buss KoKneader KG 20-25 compounder and other Buss AG equipment (supplied by Buss-Hamilton Ltd, Cheadle Hulme, UK) has a capacity of 1625 kg h- 1 for free-flowing granulate: output (in mid-1982) was 100000 per day of both 7-in and 12-in discs (produced by 66 presses).26

1118

W. V. Titow

26.4.2 Blown Bottles and Containers (see Chapter 17)

The development and growth of this application was given impetus first by the development of suitable uPVC moulding compositions combining stability in processing and service with good clarity, strength, and suitability for food-contact applications (cf. Chapters 9, 10, 11 and 17), and then by developments in processing (cf. Chapter 17). 26.4.3 Footwear (see also Chapter 11, Section 11.2.2, and Chapter 25)

Microcellular moulded pPVC, and PVC blend soles have been in use for a considerable time, as have pPVC uppers in some types of footwear. The main synthetic materials competing with PVC in the former application are polyurethane, some thermoplastic rubbers, and EVA (as well as natural rubber), and in the latter mainly polyurethane. 27 ,28 Complete units (boots, shoes and sandals) are produced from PVC by injection moulding,29 including all-weather golf shoes (from a PVC/nitrile rubber blend30). 26.4.4 Battery Separators (see Chapter 25, Section 25.2.2)

The main competitive materials in this application are resin-bonded papers and porous polyethylene. 26.4.5

Powder-coated Products and Mouldings Produced by Powder-coating Methods

Along with other thermoplastics, PVC is suitable for powder-coating applications. The two main techniques by which PVC powders are applied are dip-coating (in fluidised beds )31-33 and electrostatic deposition (electrostatic spraying) ,31,33,34 which started to be practised more recently, with the advent of compositions of suitable particle size and high resistivity. * The formulational versatility of PVC is a factor in its applicability by the powder techniques. Plasticised compositions are

* e.g. in the UK, Vyflex PC 80 ES-Plascoat Systems, (formerly Plastic Coating Systems). Typically sprayed (with object earthed) at a negative potential of 60-100 kV, followed by oven treatment at 220-260°C to fuse the powder deposit.

26 Applications of pvc

1119

normally used, commonly in the form of dry blends, but meltcompounded compositions-eomminuted by freeze-grinding-have been employed: these offer the usual advantages of this type of compound but the cost differential is even higher than between dry blend and pellet feedstocks, because of the expense of the grinding operation. Because of the lack of adhesion between PVC and metal surfaces priming is necessary in powder coating as in other coating techniques. The main considerations in the two techniques, brought out well in Newton's summary,31 may be listed as follows: Electrostatic deposition Typical coating thickness (mm)

Preferred particle size of powder (Ilffi)

Pre-heating of part to be coated

Automation

0·075-0·2 (tends to be selflimiting) 30-80 (oversize particles tend to accumulate on recirculation) Not normally necessary (powder coating held by electrostatic attraction until sintered) Readily effected with suitable equipment

Fluidised bed dipping 0·25-0·75 (not self-limiting) Over 100 (some oversize particles acceptable) Required

Possible, but needs careful design

The useful features of powder coating in comparison with other methods are the 'dry' nature of the process (absence of liquid media like solvents or plasticisers), feasibility of required thickness build-up in a single pass, and suitability for coating difficult shapes (especially sharp corners and edges-particularly with electrostatic spray). Articles powder-coated with PVC include wirework (special coating formulations are available with the requisite resistance to hot detergent solutions, for caoting dishwasher baskets), metal furniture and vehicle seat frames,32 pipework,35 road signs, railings and posts of balustrading. * An interesting application is a clear coating on

* Including ones for use in heavy weathering conditions, e.g. the safety balustrading and fog detector hood of the Bull Point lighthouse at Woolacoombe, Devon, UK have a PVC powder-applied protective coating. 36

w.

1120

V. Titow

laboratory glassware l4 to retain the pieces in the event of breakage: the coating also increases the resistance of the object to impact (e.g. in accidental dropping) and provides some first-line protection against abrasion. Compositions can be formulated for the production of cellular coatings by the fluidised-bed technique. Mouldings can be produced with PVC powders by methods analogous to those used with PVC pastes (see Chapter 22, Sections 22.2.1, 22.2.2 and 22.2.4), i.e. rotational moulding, slush moulding (usually referred to as 'static moulding' when a powder is used), and dip moulding (sometimes also called 'fusion moulding'3? when practised with a powder composition) employing a fluidised bed. 26.4.6 Medical Applications

Plasticised PVC compositions are the materials of many kinds of tubing (drainage, endotracheal, infusion), blood-collection and transfusion sets, stopcocks, aprons and sheeting. Note: In one case of blood-storage containers made of DOP-

plasticised composition (with a Ca/Zn stabiliser) some plasticiser was leached out, and even detected in patients' bodies after repeated, large-scale transfusions (no adverse effects were found attributable to its presence). 38 Special extrusion lines are available for the production of medical-grade PVC tubing. * Bags fabricated from clear, flexible PVC sheeting have been used for enclosing wounded limbs to provide an environment controlled in terms of bacteria content, humidity, temperature and pressure, to secure suitable conditions for healing without ordinary dressing (whilst the transparent nature of the bag allows the wound to be observed).t Thermoformed PVC trays, designed specially to accommodate the * e.g. Model 116 line of Betol Machinery Ltd, Luton, Beds, UK designed for the production of soft, radio-opaque PVC tubing for surgical use: the tubing, with an inside diameter of 1·2mm and wall thickness of O'12mm, has an internal web, off-set so that it divides the interior into two unequal compartments. 39 t Sterishield bag, developed in the UK by the Biomedical Research and Development Unit of the Department of Health and Social Security.40,41

26 Applications of pvc

1121

contents (which may be sealed-in with a transparent PVC film) are available with single-use medical devices-e.g. anaesthetic handventilation circuit assemblies for chest operations. Rigid, transparent compositions based on vinyl chloride/propylene copolymers are used for moulding and extrusion of components for medical applications.3° Flexible PVC film tapes with a pressure-sensitive adhesive layer are widely used as water-resistant sticking plasters. 26.4.7 Applications in Motor Cars

Apart from its virtually universal use as insulation on electrical wiring, the usage of PVC in motor cars tends to vary somewhat from country to country, and also to a certain extent from model to model. * However it is one of the three materials which-on the averagejointly account for about 75% of the plastics content of a car (the other two are polyurethane and ABS). With the decline in popularity of PVC leathercloth upholstery (ct. Chapter 25, Section 25.2.1)although it is still used in some parts of the world-the main interior applications of PVC in a typical western motor car are door panels (in some cases a 2-mm PVC foam-backed sheeting adhesive-bonded to a filled polypropylene materia!), rear parcel-shelf covers, head-linings (usually with a polyurethane foam backing), crash pads (also foam-backed), seat trim, and-in some cases-arm rests (ct. Chapter 22, Section 22.2.1). In some models the steering wheel and gear-lever knob may also be PVC-covered. PVC paste compositions are used as underseals and for corrosion protection in wheel arches and other vulnerable areas. 26.4.8 Tubular·frame Furniture and Related Applications

Interest has been growing on both sides of the Atlantic,42 and also overseas (e.g. in Australia and South Africa)43 in furniture-mainly for outdoor use-playground equipment, balustrades, and the like, made from rigid PVC pipe specially formulated for impact and weathering resistance. The pipe may be plain or externally patterned-in some cases to resemble bamboo, cane, reed or wicker. t Special 'furniture * e.g. the total PVC content (including electrical insulation) in the current model of the VW Golf motor car is about 15 kg. t e.g. the Ramboo tubing of Plastirama, Mexico.

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W. V. Titow

grade' uPVC compounds have come onto the market formulated on lines similar to those of window-frame profile compositions.

Note: For example, Geon 80 x 3 (B. F. Goodrich) is said to be42 an acrylic-modified composition with about 7 phr Ti0 2 . Another example is Conoco RPlOO (Conoco Chemicals).

In fabricating the pipe into the products it is cut into lengths and suitably shaped (after pre-heating). Shaped surface effects, e.g. bamboo, etc., can be produced by expanding the hot pipe inside a mould by air pressure. The products are then assembled with the aid of solvent bonding or special couplings. Good weathering performance with respect to appearance and strength retention over two years' exposures in Florida and Arizona has been reported. 42 26.5 SOME SPECIAL, UNUSUAL, OR MINOR PRODUCTS AND APPLICATIONS Baby pants: These are made from thin pPVC sheeting (about 0·1 mm thick), and usually incorporate some form of foam (polyurethane) padding, fabric-covered elastic band, and fastening studs. The sheeting is formulated for resistance to hardening by loss of plasticiser through washing and wear in use, tear and staining resistance, and good weldability. Typically, DIDP is used as the plasticiser together with Cd/Ba/Zn stabiliser systems with very low susceptibility to sulphide staining. A tear strength (Elmendorf test, e.g. to BS 1763) of 180 g per 0·1 mm of thickness is a reasonable practical minimum value. Fishing lures: PVC lures moulded in the shapes of sand eel, lugworm, squid and pilchard have been reported to be highly successful as aids to commercial fishing in Cornwall. 44 The lures are produced from metal-pigmented compositions, in moulds made from highly accurate carved acrylic models of the animals. Coupled with the paint-spray finish this results in very realistic appearance. Hooks larger than practicable with live bait can be used. Simulated skin: A PVC composition developed by the UK Ministry of Defence (Stores and Clothing Research and Development Establishment) has been successfully used-in the form of a sheet about 0·15 mm thick (produced by Storey Bros, Lancaster, UK)-as a

26 Applications of pvc

1123

simulant for human skin in the evaluation of clothing for protection against molten metal. 45 The material has a weight per unit area of 230 g m- 2 . The formulation is given as: PVC resin Di-alphanaphthyl phthalate

TIP

Chlorinated paraffin White pigment Lead tartrate Lead stearate Calcium stearate

100 26phr 13·75 phr 12·50 phr 10phr 1·75 phr 1·00 phr 0·75 phr

pPVC selective membranes: Films of PVC plasticised with tributyl, dibutylcresyl, and dicresylbutyl phosphates have been found to have selective permeability for uranyl nitrite, with the diffusion rate of the compound strongly dependent on the chemical nature of the plasticiser. 46 ,47 PVC-based plastic from waste products: Bags, biogas (methane) generators, pond linings, moulded solar collector panels and other sheet and moulded products have been produced from a composition* combining scrap PVC with two waste products-'red mud' (a residue from the processing of bauxite ore in the production of aluminium) and used engine oil. The 'red mud' is claimed to provide considerable reinforcement and act as stabiliser for the resin (by virtue of its metal salt and oxide content).48 PVC cricket pitch surface: Green pvc sheeting (Ruberoid, UK), laid between the stumps in two 33 ft lengths, each 6 ft wide, on a smooth concrete base, has been shown-in extensive use 49-to provide a robust, wear-resistant surface with playing characteristics similar to a medium to fast grass pitch. Industrial duckboard: Embossed or smooth heavy pPVC duckboard is commercially availablet for use in industrial premises, milking sheds, trucks for animal transport, and the like. *Developed at the Taiwanese Industrial Technology Research Institute. t e.g. in the UK the Heron duckboard manufactured by Plastic Extruders Ltd: a softer, hollow-section version is also made for use on worktops where a cushioning effect is required.

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W. V. Titow

Rug and mat underlay: Two special PVC products are noteworthy. A rayon mesh coated with PVC fibre flock which provides a surface grip (Ako-Stop, developed in West Germany50), and a highly plasticised PVC material in the form of sheet or netting for use as dust-capturing floor mat or non-slip underlay (Protect-A-Mafl-Dycem Plastics Ltd, Bristol, UK). Ball valves: These have been produced in PVC (for use in food and chemical industries)52 and PVC/polyacetal combinations (for use with water and other non-corrosive liquids).53 PVC bricks: Several designs have been proposed in the UK, Germany and Sweden, most with channels to take cement (so that the brick structure ultimately becomes a framework strengthened by a cement core). The British-developed Inca-Brick (Inca Construction Company Ltd) aroused particular interest. Despite claims that costs compare favourably with those of conventional brickwork, with building times cut by a factor of 10 or more,54 the bricks have not come into widespread use. Protective sheath for hydrophones: Specially formulated PVC tubing has been used as protective sheathing for lines of hydrophones (about 100 m long) towed by geological survey vessels engaged in mapping the sea-bed by echo-sounding techniques.55 The equipment, sealed inside the tubes, is surrounded by paraffin which acts as a protective medium: the tube material must thus be resistant to paraffin and sea water, and retain its properties at near-zero temperatures (for use in the North Sea and other cold waters). Various minor applications and products: Reflective, self-adhesive vinyl tape for clothing, walls, etc., for use in mines and other sites and work situations. Hazard-warning labels (self-adhesive, PVC-film backed) bearing symbols for radio-activity, poison, inflammable material, and other hazards. Ear-protection pads covered with flexible PVc.56 Decoy dummy aircraft fabricated in PVC (some equipped with radar reflectors for correct response).57 A low-density core material in sheet form, for decorative and some constructional applications has been made by a novel sheet-drawing process, from various thermpolastics, including PVC (Nor-Core, Norfield Corp., Conn., USA).58 Heat-shrinkable

26 Applications of pvc

1125

end-caps and harness clips for cables (e.g. the Heatshrink fittings range, Thomas Ness Ltd, UK).59 Cellular PVC monofilament has been used as the bristles in rough-duty brushes. 60 Safety goggles, moulded in high-clarity PVC compound (We/vic X16/909-ICI). Composting bins for garden-use, made of interlocking, perforated rigid PVC profiles. 61 Extruded, fluted PVC curtain rods covered (by an in-line operation during production) with vacuum-metallised polyethylene terephthalate sheet. 62

REFERENCES 1. Plastics in Building, ICI Plastics Division, Technical Publication G.25 (revised periodically). 2. Fischer, P. (1968). Kunststoffe, 58(1), 21-5. 3. Smith, P. I. (1969). Appl. Plast., 12(7), 17-19. 4. Anon. (1976). Plast. Rubb. Wkly, 2nd April, p. 10; 25th June, p. 11. 5. ASTM D 2846-81. Chlorinated poly(vinyl chloride) (CPVC) plastic hotand cold-water distribution systems. 6. ASTM D 3309-81b. Polybutylene (PB) plastic hot- and cold-water distribution systems. 7. Petzetakis, A. G. British Patent 984247, and corresponding patents in other countries. 8. Anon. (1981). Eur. Plast. News, 8(9), 109. 9. Anon. (1976). Plast. Rubb. Wkly, 20th August, p. 7. 10. Anon. (1981). Plast. Rubb. Wkly, 29th August, p. 5. 11. Johnson, F. M. and Langford, A. J. (1982). Plast. Rubb. Int., 7(6), 217-20. 12. Anon. (1980). Plast. Rubb. Int., 5(1), 11. 13. Anon. (1976). Plast. Rubb. Wkly, 17th December, pp. 12-13. 14. Anon. (1982). Eur. Plast. News, 9(7), 29. 15. Anon. (1982). Plast. Rubb. Int., 7(6), 205. 16. Breon P. 130/1 Paste Resin: Technical Manual No.2, B.P. Chemicals (UK) Ltd, 1969, p. 61. 17. Wingrave, J. A. (1978). 36th ANTEC SPE Proceedings, pp. 580---5. 18. Matheson, A. F. (1981). Electrical Times, 5th June. 19. Nye, H. F. (1976). Plast. Rubb., 1(2), 87-90. 20. Anon. (1976). Plast. Rubb. Wkly, 5th November, p. 26. 21. Anon. (1976). Plast. Rubb. Wkly, 10th September, p. 9. 22. Anon. (1976). Plast. Rubb. Wkly, 16th April, p. 11. 23. Kubitzki, C. and Schulz, G. (1965). Kunststoffe, 55(9), 727-8. 24. Anon. (1982). Mod. Plast. Int., 12(11), 6. 25. US Patent 2899351; Morse, E. A. (Assignor to Personal Products Corp.). 26. Anon. (1982). Eur. Plast. News, 9(8), 6.

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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.

W. V. Titow

Anon. (1977). Plast. Rubb. Wkly, 20th May, p. 18. Pettit, D. (1981). Plast. Rubb. Int., 6(5), 205-10. PVC for Footwear, ICI Technical Service Note W.109. Anon. (1976). Plast. Rubb. Wkly, 17th December, p. 10. Newton, D. S. (1978). Plast. Rubb. Int., 3(5), 203-6. Anon. (1976). Plast. Rubb. Wkly, 16th April, p. 9. Anon. (1979). Mod. Plast. Int., 9(2), 8-11. Cross, U. (1981). Chern. Britain, 17(1), 24-6. Anon. (1976). Plast. Rubb. Wkly, 5th November, p. 36. Anon. (1976). Plast. Rubb. Wkly, 30th April, p. 1. Anon. (1979). Plast. Technol., 25(5), 135. McHattie, G. V. 'General aspects of use ot polymers in biomedical applications', Paper presented at the Symposium on Polymers in Biomedical Applications, BruneI University, 2nd May, 1974. Anon. (1982). Eur. Plast. News, 9(12), 41. Anon. (1977). Plast. Rubb. Wkly, 28th January, p. 1. Anon. (1977). Plast. Rubb. Wkly, 21st January, p. 14. Anon. (1982). Mod. Plast. Int., U(12), 50-1. Anon. (1982). Plast. Rubb. News, November, p. 24. Anon. (1975). Plast. Rubb. Wkly, 3rd October, p. 19. Metha, P. N. and Willerton, K. (1977). Text. Inst. Ind., 15(10), 334-7. Bloch, R., Finkelstein, A., Kedem, O. and Vofsi, D. (1967). Ind. Engng. Chern.: Process Design Develop., 6,231. Vofsi, D., Kedem, 0., Bloch, R. and Marian, S. (1969). J. Inorg. Nucl. Chern., 31,2631-4. Hao, L. c., Tang, H. S. and Hsu, W. W. (1978). 36th ANTEC SPE Proceedings, p. 768. Anon. (1976). Plast. Rubb. Wkly, 19th November, p. 4; and Anon. (1979). Plast. Rubb. Int., 4(6), 248. Anon. (1982). Plast. Rubb. News, November, p. 43. Anon. (1975). Plast. Rubb. Wkly, 27th June, p. 14. Anon. (1975). Plast. Rubb. Wkly, 5th September, p. 18. Anon. (1976). Plast. Rubb. Wkly, 23rd July, p. 11. Anon. (1968). Australian Plast. Rubb. J., 23(2), 12. Anon. (1975). Plast. Rubb. Wkly, 17th October, p. 21. Anon. (1979). Plast. Rubb. Wkly, 27th July, p. 1. Anon. (1976). Plast. Rubb. Wkly, 17th September, p. 35. Anon. (1975). Plast. Rubb. Wkly, 5th September, p. 16. Anon. (1976). Plast. Rubb. Wkly, 26th November, p. 16. Anon. (1976). Plast. Rubb. Wkly, 23rd January, p. 12. Anon. (1975). Plast. Rubb. Wkly, 16th May, p. 16. Anon. (1975). Plast. Rubb. Wkly, 10th January, p. 12.

APPENDIX 1

Standards Relevant to PVC Materials and Products Compiled by N. HERBERT and W. V. TITOW

With the exception of Sections 1, 7 and 8 (and a few individual entries in other sections) the standards listed in this appendix are those relating specifically to PVC (and-in some cases-major constituents, e.g. plasticisers). Many general 'plastics' standards which are used in the testing of PVC are mentioned in the relevant chapters and (some) in Appendix 3. Copies of ISO standards and national standards of countries foreign to one's own are available from (and in fact must be ordered through) the local national ISO member body which acts as sales agent in that country for ISO and for all other ISO member bodies. These bodies are normally the individual countries' own national standardising bodies. As new standards are being reviewed, revised or amended, standardising bodies provide the means to maintain up-to-date records. The four bodies regarded as the most important (cf. Chapter 1, Section 1.7) are given below together with some observations on certain general aspects of the standards listed in this appendix.

International Organization for Standardization (ISO) Headquarters: 1, Rue de Varembe, Case Postale 56, CH-1211 Geneve 20, Switzerland. The ISO publishes a Catalogue of all its standards annually. The ISO Bulletin is published monthly and includes a list of newly published 1127

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N. Herbert and W. V. Titow

standards and draft international standards. The ISO technical committees dealing with plastics are TC 61 Plastics and TC 138 Plastics pipes, fittings and valves for the transport of fluids. Plasticisers are found under TC 47 Chemistry.

United Kingdom British Standards Institution (BSI), Linford Wood, Milton Keynes MK14 6LE. The annual British Standards Yearbook gives a list of all British standards with a short summary of each one, and an alphabetical index. The BSI News is published monthly and may be used for manual updating of the Yearbook. In addition to lists of all new and revised standards and amendments to standards published during the month, draft standards and 'new work started' are also given. A comprehensive list of standards received from all parts of the world is contained in BSl's Worldwide List of Published Standards which is issued monthly. A large number of standard specifications is contained in BS 2782:1970 Methods of testing plastics, which is currently (1984) under revision. Many of the specifications in this collection have already been published in the revised form, and some entirely new ones have been added. Work is in progress on others. The relevant revised and new specifications have been included in this appendix, as have those originally published in the 1970 edition which have not yet been revised. Note: In the body of this appendix the degree of equivalence of a British Standard to an ISO standard is indicated where possible according to the coding used in the BSI Yearbook, viz.

= identical in every detail;

1- technically equivalent though the wording and presentation may differ quite extensively; ± a related standard; covers subject matter similar to that covered by a corresponding international standard.

Al Standards Relevant to PVC Materials and Products

1129

United States of America

ASTM

American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pennsylvania 19103.

Until recently, the Annual Book of ASTM Standards consisted of 48 parts, each part containing the standards covering a specific main subject or an aspect of that subject. The division has now been restructured into 16 sections subdivided into 66 volumes. Section 8 contains the volumes directly relevant to plastics:

Volume

~::~:} 08.03

08.04

Subjects covered

Plastics-General test methods, nomenclature Plastics-Materials, films, reinforced and cellular plastics; high modulus fibres and composites Plastic pipes

Corresponding parts in former classification

Part 35 Part 36 Part 34

Other volumes of interest in connection with some PVC materials and products, and the Index volume, are:

::::: } 10.03

00.01

Electrical insulation-Test methods: solids and solidifying fluids Electrical insulation-Specifications: solids, liquids, and gases; protective equipment Index-Subject index; alphanumeric list

Part 39 Part 40 Part 48

The parts and the individual standards are obtainable from the ASTM. Note: Where an ASTM designation is followed by a number in brackets, this indicates the year of the latest reapproval by the committee concerned. ANSI: The national standardising body in the United States of

1130

N. Herbert and W. V. Titow

America is the: American National Standards Institute (ANSI), 1430 Broadway, New York, New York 10018. A great many individual standards issued by the ASTM have been adopted by the ANSI as national standards. These adopted standards may be purchased through a local national ISO member body. West Germany DIN Deutsches Institut fUr Normung, Burggrafenstrasse 4-10, 1000 Berlin 30. The DIN-Katalog is published annually, most of the German titles being given in English as well. The catalogue may be kept up to date by Ergiinzungen (Supplements) which are published monthly and are cumulative. Further information on standards is given in DINMitteilungen (DIN-reports), which is also a monthly publication. The latter two are in German only. DIN standards on plastics which have been translated into English are available in ring binders in the following collections: Sales no. 10053 Plastics 1. Test standards for mechanical, thermal and electrical properties 10056 Plastics. Duroplast resins and duroplast moulding materials 10705 Plastics 2. Test standards on chemical, optical, usability and processing properties 10789 Plastics. Semi-finished products of thermoplastic plastics 10790 Plastics. Pipes, pipeline components and pipe joints of thermoplastic plastics The collections of DIN standards in German are reduced in size and bound into Taschenbiicher (Pocketbooks) in A5 size, and cover the same classification of the collections translated into English, viz. TAB 18 Kunststoffe 1. Prtifnormen tiber mechanische, thermische und elektrische Eigenschaften

Al Standards Relevant to PVC Materials and Products

1131

TAB 21

Kunststoffe 3. Duroplast-Kunstharze und DuroplastFormmassen TAB 48 Kunststoffe 2. Prtifnormen tiber chemische, optische Gebrauchs- und Verarbeitungs-Eigenschaften TAB 51 Kunststoffe 8. Halbzeuge aus thermoplastischen Kunststoffen TAB 52 Kunststoffe 5. Rohre, Rohrleitungsteile und Rohrverbindungen aus thermoplastischen Kunststoffen

Japan Japanese Industrial Standards Committee, c/o Standards Department, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, 1-3-1 Kasumigaseki, Chiyodaku, Tokyo 100. In view of Japan's standing as a major producer and consumer of PVC, the existence of many relevant Japanese Standards should not be overlooked (e.g. JIS K 6720 (1977), Polyvinyl chloride resins; JIS K 6741 (1975), Unplasticised polyvinyl chloride pipes. It has not been practicable to include them in this appendix. However, from the technological standpoint, they do not cover any important specifications or methods or materials not dealt with by standards from one or more of the four sources represented here. It is useful to remember that Japanese Standards are regularly included in the already mentioned BSI Worldwide List of Published Standards and that they are available (generally in English translation) from the BSI.

1 PLASTICS TERMINOLOGY, PROPERTIES AND TESTING: GENERAL 1.1 Terminology (a) General ISO 472-1979 Plastics-Vocabulary BS 1755 Glossary of terms used in the plastics industry

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Part 1: 1982 (=t= ISO 472) Polymer and plastics technology Part 2: 1974 Manufacturing processes BS 3558: 1980 Glossary of rubber terms BS 4815: 1972 Glossary of generic names for man-made fibres BS 5168: 1975 Glossary of rheological terms ASTM C 168-80 Definitions of terms relating to thermal insulating materials ASTM C 274-68 (1980) Definitions of terms relating to structural sandwich construction ASTM D 16-80 Definitions of terms relating to paint, varnish, lacquer and related products ASTM D 883-80 Definitions of terms relating to plastics ASTM D 907-82 Definitions of terms relating to adhesives ASTM D 1418-81 Recommended practice for rubber and rubber latices-Nomenclature ASTM D 1566-82 Definitions of terms relating to rubber ASTM E 6-81 Definitions of terms relating to methods of mechanical testing ASTM E 12-70 (1981) Definitions of terms relating to density and specific gravity of solids, liquids and gases ASTM E 41-79 Definitions of terms relating to conditioning ASTM E 206-72 (1979) Definitions of terms relating to fatigue testing and the statistical analysis of fatigue data ASTM E 284-81 Definitions of terms relating to appearance of materials

ASTM F 17-76 Definitions of terms relating to flexible barrier materials

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ASTM F 141-79 Definitions of terms relating to resilient floor coverings ASTM F 412-81 Definitions of terms relating to plastic piping systems DIN 7732 Part 1 (1963) Standardized terms and definitions relating to plastics; summary (b) Common Names and Abbreviations ISO 1043-1978 Plastics-Symbols

BS 3502 Schedule of common names and abbreviations for plastics and rubbers Part 1: 1978 Principal commercial plastics Part 3: 1978 Rubber and rubber lattices

BS 4589: 1970 Abbreviations for rubber and plastics compounding materials ASTM D 1600-83 Abbreviations of terms relating to plastics DIN 7723 (1971) Abbreviations for plasticizers DIN 7728 Part 1 (1978) Symbols for terms relating to homopolymers, copolymers and polymer compounds Part 2 (1980) Symbols for reinforced plastics (c) Equivalent Terms in Various Languages ISO 194-1981 Plastics-List of equivalent terms DIN 7730 Part 1 (1965) Plastics, equivalent terms in German, English, French and Russian following ISO/R 194-1961

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1.2 General Test Conditions and Methods (a) Conditioning and Testing Conditions ISO 291-1977 Plastics-Standard atmospheres for conditioning and testing ISO/R 483-1966 Plastics-Methods for maintaining constant relative humidity in small enclosures by means of aqueous solutions ISO 554-1976 Standard atmospheres for conditioning and/or testing-Specifications ISO 558-1980 Conditioning and testing-Standard atmospheres-Definitions ASTM D 618-61 (1981) Conditioning plastics and electrical insulating materials for testing DIN 50013 (1979) Climates and their technical application; preferred temperatures DIN 50014 (1975) Atmospheres and their technical application; standard atmospheres DIN 50015 (1975) Atmospheres and their technical application; constant test atmospheres DIN 50016 (1962) Testing of materials, structural components and equipment; method of test in damp alternating atmosphere DIN 50017 (1982) Climates and their technical application; stress in condensed water containing climates DIN 50018 (1978) Testing of corrosion; methods of test in condensation water alternating atmosphere containing sulphur dioxide DIN 50019 Part 1 (1979) Climates and their technical application; climates with regard to technology, sign and cartographical graph of the open-air climates Part 2 (1963) Testing of materials, structural components and equipment; open air climates; data on climates

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Part 3 (1979) Climates and their technical application; climates with regard to technology; climatic patterns based on statistics Supplement to Part 3 (1979) Climates and their technical application; climates with regard to technology, geographical survey for open-air climate patterns based on statistics (b) Some General Test Methods ISO 62-1980 Plastics-Determination of water absorption ISO 171-1980 Plastics-Determination of bulk factor of moulding materials ISO 3451/1-1981 Plastics-Determination of ash-General methods 2 VINYL POLYMERS AND COPOLYMERS (Chapter 2) 2.1 General (Designation, Coding, Characterisation Tests)

ISO 1060/1-1982 Plastics-Homopolymer and copolymer resins of vinyl chloride. Part 1: Designation ISO 1060/2-1978 Plastics-Homopolymer and copolymer resins of vinyl chloride. Part II: Determination of properties ISO 1163/1-1980 Plastics-Unplasticized compounds of homo- and copolymers of vinyl chloride. Part 1: Designation ISO 6186-1980 Plastics-Determination of pourability ASTM D 1755-81 Specification for poly(vinyl chloride) resins Note: This specification gives test methods referred to in some of the ASTM standards listed in the sections that follow, e.g. Sections 2.2 to 2.9, etc.

ASTM D 2474-81 Specification for vinyl chloride copolymer resins

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ASTM D 2873-70 (1982) Test for interior porosity of poly(vinyl chloride) (PVC) resins by mercury intrusion porosimetry ASTM D 3591-77 Recommended practice for determining logarithmic viscosity number of poly(vinyl chloride) (PVC) in formulated compounds DIN 7746 Part 1 (1979) Vinyl chloride (VC) polymers; homopolymers; classification and designation Part 2 (1979) Vinyl chloride (VC) polymers; homo- and copolymers, determination of properties DIN 7747 (1979) Vinyl chloride (VC) polymers; copolymers, classification and designation

2.2 Viscosity ISO 174-1974 Plastics-Determination of viscosity number of PVC resins in dilute solution ISO/R 1628-1970 Plastics-Directives for the standardisation of methods for the determination of the dilute solution viscosity of polymers ISO 3219-1977 Plastics-Polymers in the liquid, emulsified or dispersed stateDetermination of viscosity with a rotational viscometer working at defined shear rate BS 2782 Part 7 Rheological properties Method 730A: 1979 (± ISO/R 1628) Determination of reduced viscosity (viscosity number) and intrinsic viscosity of plastics in dilute solution Method 730B: 1978 (= ISO 3219) Determination of the viscosity of polymers in the liquid, emulsified or dispersed state using a rotational viscometer working at a defined shear rate ASTM D 1243-79 Test for dilute solution viscosity of vinyl chloride polymers

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DIN 53726 (1983) Testing of plastics; determination of viscosity number and K-value of vinyl chloride (VC) polymers 2.3 Chlorine Content ISO 1158-1978 Plastics-Vinyl chloride homopolymers and copolymers-Determination of chlorine ASTM D 1303-55 (1979) Test for total chlorine in vinyl chloride polymers and copolymers DIN 53474 (1976) Testing of plastics, rubber and elastomers; determination of the chlorine content 2.4 Vinyl Acetate Content in VCIVA Copolymers ISO 1159-1978 Plastics-Vinyl chloride-vinyl acetate copolymers-Determination of vinyl acetate 2.5 Ash and/or Sulphated Ash Content ISO 1270-1975 Plastics-PVC resins-Determination of ash and sulphated ash BS 2782: Part 4 Method 454A: 1978 Determination of ash Method 454B : 1978 (= ISO 1270) Determination of sulphated ash 2.6 Volatile Matter (Including Water) ISO 1269-1980 Plastics-Homopolymer and copolymer resins of vinyl chlorideDetermination of volatile matter (including water) BS 2782: Part 4 Method 454D: 1978 (= ISO 1269) Determination of volatile matter (including water) of PVC resins ASTM D 3030-79 Test for volatile matter (including water) of vinyl chloride resins

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2.7 Impurities and Foreign Matter

ISO 1265-1979 Plastics-PVC resins-Determination of number of impurities and foreign particles ASTM D 2222-66 (1978) Test for methanol extract of vinyl chloride resins 2.8 Bulk Density

ISO 60-1977 Plastics-Determination of apparent density of material that can be poured from a specified funnel ISO 61-1976 Plastics-Determination of apparent density of moulding material that cannot be poured from a specified funnel ISO 1068-1975 Plastics-PVC resins-Determination of compacted apparent bulk density BS 2782: Part 6 Method 621A: 1978 (= ISO 60) Determination of apparent density of moulding material that can be poured from a funnel Method 621B : 1978 (= ISO 61) Determination of apparent density of moulding material which cannot be poured from a funnel Method 621D : 1978 (= ISO 1068) Determination of compacted apparent bulk density of PVC resins ASTM D 1895-69 (1979) Tests for apparent density, bulk factor, and pourability of plastic materials NB Method A equivalent to ISO 60 Method C equivalent to ISO 61 2.9 Particle Size

ISO 1624-1978 Plastics-Vinyl chloride homopolymer and copolymer resinsSieve analysis in water

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ISO 4576-1978 Plastics-Aqueous dispersions of homopolymers and copolymers-Determination of gross particle content by sieve analysis ISO 4610-1977 Plastics-Vinyl chloride homopolymer and copolymer resinsSieve analysis using air-jet sieve apparatus BS 2782: Part 4 Method 454F: 1978 (= ISO 4610) Sieve analysis of vinyl chloride homopolymer and copolymer resins using air-jet sieve apparatus ASTM D 1705-61 (1980) Particle size analysis of powdered polymers and copolymers of vinyl chloride 2.10 Bromine Number

ISO 3499-1976 Plastics-Aqueous dispersions of homopolymers and copolymers of vinyl acetate-Determination of bromine number 2.11 pH of Aqueous Extract

ISO 1264-1980 Plastics-Homopolymer and copolymer resins of vinyl chlorideDetermination of pH of aqueous extract BS 2782: Part 4 Method 454C: 1978 (= ISO 1264) Determination of pH of aqueous extract of PVC resins 2.U Miscellaneous Properties Relevant to Processing

ASTM D 2396-79 Recommended practice for powder-mix test of poly(vinyl chloride) (PVC) resins using a torque rheometer ASTM D 2538-79 Recommended practice for fusion test of poly(vinyl chloride) (PVC) resins using a torque rheometer ASTM D 2873-70 (1982) Test for interior porosity of poly(vinyl chloride) (PVC) resins by mercury intrusion porosimetry

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ASTM D 3367-75 (1980) Test for plasticizer sorption of poly(vinyl chloride) resins under applied centrifugal force ASTM D 3596-77 Recommended practice for determination of gels (fish eyes) in general purpose poly(vinyl chloride) (PVC) resins 2.13 Methanol Extract ASTM D 2222-66 (1978) Test for methanol extract of vinyl chloride resins 2.14 VCM Content DIN E 53743 (1979) Draft Testing of plastics; gas chromatographical determination of vinyl chloride (VC) in polyvinyl chloride (PVC)

3 VINYL COMPOUNDS (Chapter 3) 3.1 General (Designation, Coding, Characterisation Tests) (a) Rigid Compounds ISO 1163/1-1980 Plastics-Unplasticized compounds of homopolymers and copolymers of vinyl chloride. Part 1: Designation ASTM D 1784-81 Specification for rigid poly(vinyl chloride) (PVC) compounds and chlorinated poly(vinyl chloride) (CPVC) compounds ASTM D 2124-70 (1979) Analysis of components in poly(vinyl chloride) compounds using an infrared spectrophotometric technique ASTM D 3010-71 (1981) Recommended practice for preparing compression-moulded test sample plaques of rigid poly(vinyl chloride) compounds DIN 7748 Part 1 (1979) Plastic moulding materials; unplasticized PVC moulding materials; classification and designation Part 2 (1979) Plastic moulding materials; unplasticized PVC moulding materials; determination of properties

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Standards Relevant to PVC Materials and Products

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(b) Flexible Compounds ISO 289811-1980 Plastics-Plasticized compounds of homopolymers and copolymers of vinyl chloride. Part 1: Designation ISO 2898/2-1980 Plastics-Plasticized compounds of homopolymers and copolymers of vinyl chloride. Part 2: Determination of properties BS 2571 : 1963 Flexible PVC compounds ASTM D 2124-70 (1979) Analysis of components in poly(vinyl chloride) compounds using an infrared spectrophotometric technique ASTM D 2287-81 Specification for non-rigid vinyl chloride polymer and copolymer molding and extrusion compounds DIN 7749 Part 1 (1979) Plastic moulding materials; plasticized polyvinyl chloride (PVC) moulding materials; classification and designation Part 2 (1979) Plastic moulding materials; plasticized polyvinyl chloride (PVC) moulding materials; preparation of specimens and determination of their properties (c) Pastes ISO 4612-1979 Plastics-PVC paste resins-Preparation of a paste DIN 54800 (1979) Testing of plastics; preparation of polyvinyl chloride (PVC) paste for testing purposes DIN 54801 (1979) Testing of plastics; determination of apparent viscosity at high rates of shear of polyvinyl chloride (PVC) paste by Severs capillary viscometer

(d) Miscellaneous ASTM D 729-81 Specification fOr vinylidene chloride molding compounds ASTM D 3364-74 (1979) Test method for flow rates for poly(vinyl chloride) and rheologically unstable thermoplastics

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3.2 Properties and Tests (a) Bulk Density and Pourability

I

ISO 60-1977 ISO 61-1976 BS 2782: Part 6: Method 621A: 1978 BS 2782: Part 6: Method 621B: 1978 ASTM D 1895-69 (1979)

For titles see Section 2.8 above

DIN 53466 (1960) Testing plastics. Determination of bulk factor of moulding materials DIN 53467 (1960) Testing plastics. Determination of apparent density of moulding material that cannot be poured from a specified funnel DIN 53468 (1974) Testing plastics. Determination of apparent density of moulding material that can be poured from a specified funnel (b) Water Absorption BS 2782: Part 5 Method 502e: 1970 Water absorption and water soluble matter of polyvinyl chloride extrusion compound See also DIN 7748: Part 2 (1979) in Section 3.1(a) of this Appendix. (c) Temperature Effects (i) PHYSICAL BS 2782: Part 1 Method 122A: 1976 Determination of deformation under heat of flexible polyvinyl chloride compound Method 150B: 1976 Determination of cold flex temperature of flexible polyvinyl compound Method 150C: 1983 Determination of low temperature extensibility of flexible polyvinyl chloride sheet ASTM D 746-79 Test for brittleness temperature of plastics and elastomers by impact

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ASTM D 1043-72 (1981) Stiffness properties of plastics as a function of temperature by means of a torsion test ASTM D 1593-81 Specification for non-rigid vinyl chloride plastic sheeting

(ii)

CHEMICAL

ISO/R 182-1970 Plastics-Determination of the thermal stability of polyvinyl chloride and related copolymers and their compounds by splitting off of hydrogen chloride ISO 305-1976 Plastics-Determination of thermal stability of polyvinyl chloride, related chlorine-containing polymers and copolymers, and their compounds-Discoloration method BS 2782: Part 1 Method BOA: 1976 (=f:. ISO/R 182) Determination of the thermal stability of polyvinyl chloride by the Congo red method Method BOB: 1976 (=1= ISO/R 182) Determination of the thermal stability of polyvinyl chloride by the pH method ASTM D 793-49 (1976) Test for short-time stability at elevated temperatures of plastics containing chlorine ASTM D 2115-67 (1980) Recommended practice for oven heat stability of poly(vinyl chloride) compositions DIN 53381 Testing of plastics; determination of the thermal stability of polyvinyl chloride and related copolymers and their compounds; Part 1 (1971) Congo red method Part 2 (1975) Discoloration method Part 3 (1971) pH method (d) Mechanical Properties BS 2782: Part 3: Method 365A: 1976 Determination of softness number of flexible plastics

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(e) Miscellaneous Properties and Analysis ASTM D 2124-70 (1979) Analysis of components in poly(vinyl chloride) compounds using an infrared spectrophotometric technique ASTM D 2151-68 (1977) Test for staining of poly(vinyl chloride) compositions by rubber compounding ingredients ASTM D 2538-79 Recommended practice for fusion test of poly(vinyl chloride) (PVC) resins using a torque rheometer ASTM D 3421-75 Extraction and analysis of plasticiser mixtures from vinyl chloride plastics ASTM D 3596-77 Recommended practice for determination of gels (fish eyes) in general purpose poly(vinyl chloride) (PVC) resins

4 PLASTICISERS (Chapters 5-7)

4.1 Bulk Properties Specifications for individual plasticisers (as chemical materials) may be found in the various catalogues of standards under the appropriate compound names. Some of these are given here, as are some general tests on plasticisers and some specifications covering groups of compounds used as plasticisers.

ISO 1385/1-1977 Phthalate esters for industrial use-Methods of test-Part 1: General ISO 1385/2-1977 Phthalate esters for industrial use-Methods of test-Part II: Measurement of colour after heat treatment (Diallyl phthalate excluded) ISO 1385/3-1977 Phthalate esters for industrial use-Methods of test-Part III: Determination of ash

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ISO 1385/4-1977 Phthalate esters for industrial use-Methods of test-Part IV: Determination of acidity to phenolphthalein-Titrimetric method ISO 1385/5-1977 Phthalate esters for industrial use-Methods of test-Part V: Determination of ester content-Titrimetric method after saponification ISO 2520-1974 Tritolyl phosphate for industrial use-List of methods of test ISO 2521-1974 Tritolyl phosphate for industrial use-Determination of acidity to phenol red-Volumetric method ISO 2522-1974 Tritolyl phosphate for industrial use-Determination of apparent free phenols content-Volumetric method ISO 2523-1974 Adipate esters for industrial use-List of methods of test ISO 2524-1974 Adipate esters for industrial use-Measurement of colour after heat treatment ISO 2525-1974 Adipate esters for industrial use-Determination of acidity to phenolphthalein-Volumetric method ISO 2526-1974 Adipate esters for industrial use-Determination of ash-Gravimetric method ISO 2527-1974 Adipate esters for industrial use-Determination of ester content-Volumetric method BS 573, 574, 1995, 1996, 2535, 2536, 3647: 1973 Plasticizer esters Comprises: BS 573 Dibutyl phthalate BS 574 Diethyl phthalate BS 1995 Di-(2-ethylhexyl) phthalate BS 1996 Dimethyl phthalate BS 2535 Dibutyl sebacate BS 2536 Di-(2-ethylphenyl) sebacate BS 3647 Dimethoxyethyl phthalate BS 1998: 1970 Triphenyl phosphate

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BS 1999: 1964 (± ISO 2520/2) Tritolyl phosphate BS 4835: 1973 (± ISO 1385, ISO 252417) Methods of test for plasticizer esters BS 4968-70: 1973 Di-isobutyl phthalate, di-isooctyl phthalate and di-isooctyl sebacate ASTM D 1045-80 Sampling and testing plasticizers used in plastics ASTM D 1249-81 Specification for octyl ortho-phthalate ester plasticizers ASTM D 2288-69 (1980) Test for weight loss of plasticizers on heating DIN 53400 (1970) Testing of plasticizers; determination of density, refractive index, flash point and viscosity DIN 53401 (1975) Determination of saponification value DIN 53402 (1973) Determination of acid value DIN 53404 (1952) Testing of plasticizers; determination of saponification rate DIN 53409 (1967) Testing of plasticizers and solvents; determination of Hazen colour (platinum cobalt colour, APHA method) 4.2

Properties in Association with PVC (Compatibility, Volatility, Migration)

ISO 176-1976 Plastics-Determination of loss of plasticizers-Activated carbon method ISO 177-1976 Plastics-Determination of migration of plasticizers BS 2782: Part 4 Method 465A and 465B: 1979 (= ISO 176) Determination of loss of plasticizers (activated carbon method) BS 2782: Part 5 Method 51lA: 1970 Effect of polyvinyl chloride compound on the loss tangent of polythene

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ASTM D 1203-67 (1981) Tests for loss of plasticizer from plastics (activated carbon methods) ASTM D 2134-66 (1980) Test for softening of organic coatings by plastic compositions ASTM D 2383-69 (1981) Recommended practice for testing plasticizer compatibility in poly(vinyl chloride) (PVC) compounds under humid conditions ASTM D 3291-74 (1980) Test for compatibility of plasticizers in poly(vinyl chloride) plastics under compression ASTM D 3421-75 Extraction and analysis of plasticizer mixtures from vinyl chloride plastics DIN 53405 (1981) Testing plasticizers; determination of migration of plasticizers DIN 53407 (1971) Testing plastics; determination of loss in weight of plasticized plastics by the activated carbon method DIN 53408 (1967) Testing plastics; determination of solubility temperature of polyvinyl chloride in plasticizers

4.3 Effects on PVC ISO 4574-1978 Plastics-PVC resins for general use-Determination of hot plasticizer absorption ISO 4608-1977 Plastics-PVC resins for general use-Determination of plasticizer absorption at room temperature BS 2782: Part 4 Method 454E: 1978 (= ISO 4608) Determination of plasticizer absorption at room temperature of PVC resins for general use ASTM D 2396-79 Recommended practice for powder-mix test of poly(vinyl chloride) (PVC) resins using a torque rheometer ASTM D 3367-75 (1980) Test for plasticizer sorption of poly(vinyl chloride) resins under applied centrifugal force

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DIN 53408 (1967) Testing of plastics; determination of solubility temperature of polyvinyl chloride in plasticizers 5 PVC SHEETING AND FILMS (Chapter 20) 5.1 Rigid BS 3757: 1978 Rigid PVC sheet BS 4203: 1980 Extruded rigid PVC corrugated sheeting ASTM D 1927-81 Specification for rigid poly(vinyl chloride) plastic sheet ASTM D 2123-81 Specification for rigid poly(vinyl chloride-vinyl acetate) plastic sheet DIN 16927 Part 1 (1977) Sheets of rigid polyvinyl chloride (rigid PVC), normal impact strength; technical delivery specifications Part 2 (1977) Sheets of rigid polyvinyl chloride (rigid PVC), raised impact strength; technical delivery specifications DIN 16929 (1965) Tubes and sheets of rigid PVC (rigid polyvinyl chloride); resistance to chemicals; recommended practice 5.2 Flexible BS 1763: 1975 Thin PVC sheeting (calendered, flexible, unsupported) BS 2739: 1975 Thick PVC sheeting (calendered, flexible, unsupported) BS 2782: Part 6 Method 643A: 1976 Shrinkage on heating of film intended for shrink wrapping applications BS 3878: 1982 Flexible PVC sheeting for hospital use

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ASTM D 1239-55 (1971) Test for resistance of plastic films to extraction by chemicals. Discontinued in 1980 ASTM D 1593-81 Specification for non-rigid vinyl chloride plastic sheeting ASTM D 1893-67 (1978) Test for blocking of plastic film ASTM D 3083-76 Flexible poly(vinyl chloride) plastic sheeting for pond, canal and reservoir lining ASTM D 3354-74 (1979) Test for blocking load of plastic film by the parallel plate method DIN 16937 (1971) Sheets of bitumen-resistant nonrigid PVC (nonrigid polyvinyl chloride) for waterproofing of buildings; requirements, testing DIN 16938 (1971) Sheets of non-bitumen-resistant non-rigid PVC (non-rigid polyvinyl chloride) for damp-proofing DIN 53372 (1970) Testing of plastics films; determination of break at low temperature of films of nonrigid polyvinyl chloride The following list gives further DIN specifications on the testing of plastics sheet and film (short titles): DIN 53353 DIN 53365 DIN 53366 DIN 53369 DIN 53370 DIN 53374 DIN 53375 DIN 53377 DIN 53378 DIN 53380 DIN 53488

(1971): Tear test (1974): Mass per unit area (1975) Draft: Blocking (1976): Shrinking stress (1976): Thickness (1959): Flexure (1972): Coefficients of friction (1969): Dimensional stability (1965): Colour fastness to hydrogen sulphide (1969): Gas transmission rate (1963): Hole test

5.3 Sheet and Film Fabrication and Products BS 1776: 1951 Fabrication of lightweight articles (other than rainwear) from polyvinyl chloride sheeting

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BS 3501: 1962 Dinghy buoyancy equipment ASTM D 1789-65 (1977) Test for welding performance of poly(vinyl chloride) structures DIN 1910 Part 3 (1977) Welding; welding of plastics, processes DIN 16930 (1964) Welding of rigid PVC (rigid polyvinyl chloride); recommended practice DIN 16931 (1959) Welding of nonrigid PVC (nonrigid polyvinyl chloride); recommended practice DIN 16995 (1976) Packaging materials; plastics films, main properties, special properties, test methods Note: Some properties of PVC films and sheeting are covered by general plastics film and sheeting standards. Such standards are mentioned in the appropriate chapters and in Appendix 3. Some examples are water vapour permeability, gas permeability, electrical properties and flammability. BS 1133 Packaging Code; Section 21: 1976 Regenerated cellulose film, aluminium foil and flexible laminates is also relevant to plastics films for packaging (cf. DIN 16995 above).

6 PVC PIPES, TUBING AND PIPE FITTINGS 6.1

Rigid Pipes and Fittings, Including Pressure Pipes

ISO DATA 7-1979 Unplasticized polyvinyl chloride pipes and fittings-Chemical resistance with respect to fluids ISO 580-1973 Moulded fittings in unplasticized polyvinyl chloride (PVC) for use under pressure-Oven test ISO 727-1979

Unplasticized polyvinyl chloride (PVC) fittings with plain sockets for pipes under pressure-Dimensions of sockets-Metric series

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ISO 2035-1974 Unplasticized polyvinyl chloride (PVC) moulded fittings for elastic sealing ring type joints for use under pressure-Pressure resistance test ISO 2043-1974 Unplasticized polyvinyl chloride (PVC) moulded fittings for elastic sealing ring type joints for use under pressure-Oven test ISO 2044-1974 Unplasticized polyvinyl chloride (PVC) injection-moulded solventwelded socket fittings for use with pressure pipe-Hydraulic internal pressure test ISO 2045-1973 Single sockets for unplasticized polyvinyl chloride (PVC) pressure pipes with elastic sealing ring type joints-Minimum depths of engagement ISO 2048-1973 Double socket fittings for unplasticized polyvinyl chloride (PVC) pressure pipes with elastic sealing ring type joints-Minimum depths of engagement ISO 2505-1981 Unplasticized polyvinyl chloride (PVC) pipes-Longitudinal reversion-Test methods and specification ISO 2507-1982 Unplasticized polyvinyl chloride (PVC) pipes and fittings-Vicat softening temperature-Test method and specification ISO 2508-1981 Unplasticized polyvinyl chloride (PVC) pipes-Water absorption-Determination and specification ISO 2536-1974 Unplasticized polyvinyl chloride (PVC) pressure pipes and fittings, metric series-Dimensions of flanges ISO 2703-1973 Buried unplasticized polyvinyl chloride (PVC) pipes for the supply of gaseous fuels-Metric series-Specification ISO 3114-1977 Unplasticized polyvinyl chloride (PVC) pipes for potable water supply-Extractability of lead and tin-Test method ISO 3460-1975 Unplasticized polyvinyl chloride (PVC) pressure pipes-Metric series-Dimensions of adapter for backing flange

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ISO 3472-1975 Unplasticized polyvinyl chloride (PVC) pipes-Specification and determination to resistance to acetone ISO 3473-1977 Unplasticized polyvinyl chloride (PVC) pipes-Effect of sulphuric acid-Requirement and test method ISO 3474-1976 Unplasticized polyvinyl chloride (PVC) pipes-Specification and measurement of opacity ISO 3603-1977 Fittings for unplasticized polyvinyl chloride (PVC) pressure pipes with elastic sealing ring type joints-Pressure test for leakproofness ISO 3604-1976 Fittings for unplasticized polyvinyl chloride (PVC) pressure pipes with elastic sealing ring type joints-Pressure test for leakproofness under conditions of external hydraulic pressure ISO 3606-1976 Unplasticized polyvinyl chloride (PVC) pipes-Tolerances on outside diameters and wall thicknesses ISO 4434-1977 Unplasticized polyvinyl chloride (PVC) adapter fittings for pipes under pressure-Laying length and size of threads-Metric series ISO 4439-1979 Unplasticized polyvinyl chloride (PVC) pipes and fittingsDetermination and specification of density BS 3505: 1968 (1982) (oF ISO 2505, ISO 3114, ISO 3472-3, ± ISO 3474) Unplasticized PVC pipe for cold water services BS 3506: 1969 Unplasticized PVC pipe for industrial purposes BS 3943: 1979 Plastics waste traps BS 4346 (± ISO 2035, ISO 2043-5, ISO 2048) Joints and fittings for use with unplasticized PVC pressure pipes Part 1: 1969 Injection moulded PVC fittings for solvent welding for use with pressure pipes, including potable water supply Part 2: 1970 Mechanical joints and fittings principally of unplasticized PVC

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Part 3: 1982 Solvent cement BS 4514: 1983 Unplasticized PVC soil and ventilating pipe, fittings and accessories BS 4576 Unplasticized PVC rainwater goods Part 1: 1982 Half-round gutters and circular pipe BS4607 Non-metallic conduits and fittings for electrical installations Part 1: 1970 Rigid PVC conduits and conduit fittings. Metric units Part 2: 1970 Rigid PVC conduits and conduit fittings. Imperial units Part 3: 1971 Pliable corrugated, plain and reinforced conduits of selfextinguishing plastics material Part 5: 1982 Rigid conduits, fittings and components of insulating materials BS 4660: 1973 Unplasticized PVC underground drain pipe and fittings BS 5481 : 1977 Specification for unplasticized PVC pipe and fittings for gravity sewers BS CP 312 Plastics pipework (thermoplastics materials) Part 2: 1973 Unplasticized PVC pipework for the conveyance of liquids under pressure ASTM D 876-71 Testing non rigid vinyl chloride polymer tubing used for electrical insulation ASTM D 1785-76 Specification for poly(vinyl chloride) (PVC) plastic pipe, schedules 40, 80 and 120 ASTM D 2152-80 Degree of fusion of extruded poly(vinyl chloride) (PVC) pipe and molded fittings by acetone immersion ASTM D 2241-80 Specification for poly(vinyl chloride) (PVC) plastic pipe (SDR-PR)

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ASTM D 2464-76 Specification for threaded poly(vinyl chloride) (PVC) plastic pipe fittings, schedule 80 ASTM D 2466-78 Specification for poly(vinyl chloride) (PVC) plastic pipe fittings, schedule 40 ASTM D 2467-76a Specification for socket-type poly(vinyl chloride) (PVC) plastic pipe fittings, schedule 80 ASTM D 2665-81 Specification for poly(vinyl chloride) (PVC) plastic drain, waste and vent pipe and fittings ASTM D 2672-80 Specification for bell-end poly(vinyl chloride) (PVC) pipe ASTM D 2729-80 Specification for poly(vinyl chloride) (PVC) sewer pipe and fittings ASTM D 2740-80 Specification for poly(vinyl chloride) (PVC) plastic tubing ASTM D 2846-81 Specification for chlorinated poly(vinyl chloride) (CPVC) plastic hot- and cold-water distribution systems ASTM D 2949-78 Specification for 3·25-in outside diameter poly(vinyl chloride) (PVC) plastic drain, waste, and vent pipe and fittings ASTM D 3033-81 Specification for type PSP poly(vinyl chloride) (PVC) sewer pipe and fittings ASTM D 3034-81 Specification for type PSM poly(vinyl chloride) (PVC) sewer pipes and fittings DIN 1187 (1982) Drain pipes of unplasticized polyvinyl chloride; dimensions, general requirements, test methods DIN 3441 Valves of unplasticized polyvinyl chloride Part 1 (1982): requirements and tests Part 2 (1977): ball valves, dimensions Part 3 (1977): diaphragm valves, dimensions Part 4 (1978): oblique pattern valves, dimensions

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Standards Relevant to PVC Materials and Products

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DIN 3543 Part 3 (1978) Tapping valves for plastics pipes, dimensions DIN 4279 Part 7 (1975) Internal pressure test of pressure pipelines for water, pressure pipes of unplasticized PVC DIN 6660 (1980) Pneumatic tube systems; conveyor tubes of rigid PVC DIN 6661 (1979) Pneumatic tube systems; sleeves of rigid PVC DIN 6664 (1980) Pneumatic tube systems, conveyor tube bends 900 of rigid PVC DIN 8061 Part 1 (1974) Pipes of rigid PVC; general quality requirements, test methods Part 2 (1971) Pipes of high impact PVC; general quality requirements, test methods DIN 8062 (1974) Pipes of rigid PVC; dimensions DIN 8063 Parts 1-11 Pipe joints and their elements for pipes of unplasticized PVC under pressure DIN 8079 (1974) (Preliminary Standard) Pipes of PVC-C (chlorinated polyvinyl chloride) dimensions DIN 8080 (1974) (Preliminary Standard) Pipes of PVC-C; general quality requirements and test methods DIN 16450 (1975) Socket-fittings for pressure main lines of rigid polyvinyl chloride; symbols DIN 16451 Parts 1-7 Socket-fittings of cast iron with lamellar graphite for pressure main lines of rigid polyvinyl chloride DIN 16929 (1965) Tubes and sheets of rigid PVC; resistance to chemicals, recommended practice DIN 19531 (1980) Pipes and fittings of unplasticized PVC, with rubber ring sockets,

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for waste and soil installation inside buildings; dimensions, technical specifications for delivery DIN 19532 (1979) Pipe-lines of unplasticized PVC for drinking water supply; pipes, pipe connections, fittings for pipe lines DIN 19534 Part 1 (1979) Pipes and fittings of unplasticized PVC (polyvinyl chloride) with sockets for elastic sealing ring joints for sewage; dimensions Part 2 (1979) Pipes and fittings of unplasticized PVC (polyvinyl chloride) with sockets for elastic sealing ring joints for sewage; technical delivery specifications DIN 19538 (1980) Draft Pipes and fittings of chlorinated polyvinyl chloride (PVC-C) with rubber ring sealed sockets for hot water resistant waste and soil installations inside buildings; dimensions, technical specifications for delivery DIN 86012 (1978) Pipe lines of unplasticized polyvinyl chloride (unplasticized PVC) on ships, cemented type joints; summary of components DIN 86013 (1978) Pipes of unplasticized polyvinyl chloride (unplasticized PVC) on ships with pipe fittings for solvent cement joints DIN 86015 (1976) (Preliminary standard) Pipe lines of unplasticized polyvinyl chloride on ships, cemented type joints; application, processing, laying 6.2 Flexible Tubing BS 1882: 1976 Specification for flexible polymeric tubing and drainage sheeting (radio-opaque) for medical use BS 3746: 1964 (1983) PVC garden hose BS 4607 Non-metallic conduits and fittings for electrical installations Part 3: 1971 Pliable corrugated, plain and reinforced conduits of selfextinguishing plastics material

Al Standards Relevant to PVC Materials and Products

1157

ASTM D 876-80 Testing nonrigid vinyl chloride polymer tubing used for electrical insulation ASTM D 922-80 Specification for nonrigid vinyl chloride polymer tubing ASTM D 3150-81 Specification for crosslinked and noncrosslinked poly(vinyl chloride) heat shrinkable tubing for electrical insulation DIN 16940 (1964) Extruded hoses of nonrigid PVC (nonrigid polyvinyl chloride); permissable deviations for dimensions for which tolerances are not indicated DIN 16942 (1966) Water-hoses of nonrigid PVC (nonrigid polyvinyl chloride); dimensions 6.3

Miscellaneous Standards Relevant to Pipes

ISO 3514-1976 Chlorinated polyvinyl chloride (CPVC) pipes and fittingsSpecification and determination of density ISO 3608-1976 Chlorinated polyvinyl chloride (CPVC) pipes-Tolerances on outside diameters and wall thicknesses ISO 4065-1978 Thermoplastic pipes-Universal wall thickness table BS 4962: 1982 Pipes for use as light sub-soil drains BS 5556: 1978 (± ISO 161/1) Specification for general requirements for dimensions and pressure ratings for pipe of thermoplastics materials (metric series) BS CP 312 Plastics pipework (thermoplastics materials) Part 1: 1973 General principles and choice of material ASTM D 2152-80 Test for degree of fusion of extruded poly(vinyl chloride) (PVC) pipe and molded fittings by acetone immersion ASTM D 2564-80 Specification for solvent cements for poly(vinyl chloride) (PVC) plastic pipe and fittings

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ASTM D 2855-81 Recommended practice for making solvent-cemented joints with poly(vinyl chloride) (PVC) pipe and fittings ASTM D 3036-73 Specification for socket-type poly(vinyl chloride) (PVC) plastic line couplings ASTM F 409-81 Specification for thermoplastic accessible and replaceable plastic tube and tubular fittings ASTM F 412-81 Definitions of terms relating to plastic piping systems DIN 16929 (1965) Tubes and sheets of rigid PVC (rigid polyvinyl chloride); resistance to chemicals, recommended practice

7 PVC-COATED MATERIALS AND PRODUCTS Several standards covering plastics-coated fabrics generally are relevant to PVC-coated fabrics, and have been included in this list. 7.1

Coated Fabrics, Including Conveyor and Transmission Belting

ISO 1419-1977 Fabrics coated with rubber or plastics-Accelerated ageing and simulated service tests ISO 1420-1978 Rubber and plastics coated fabric-Determination of resistance to penetration by water ISO 1421-1977 Fabrics coated with rubber or plastics-Determination of breaking strength and elongation at break BS 351: 1976 (=1= ISO 22) Specification for rubber, balata or plastics flat transmission belting of textile construction for general use BS 490 Conveyor and elevator belting BSO 490 Part 1: 1972 (=f:. ISO 282, ISO 283, ISO 703, ISO 1121) Rubber and plastics conveyor belting of textile construction for

Al Standards Relevant to PVC Materials and Products

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general use (of either multi-ply, single-ply or solid woven construction) Part 2: 1975 (± ISO 251, ISO 252, ISO 282, ISO 283, ISO 432, ISO 433) Rubber and plastics belting of textile construction for use on bucket elevators BS 3424 Parts 0-7: 1982 Testing coated fabrics BS 3546 Parts 1 and 2: 1981 Coated fabrics for water resistant clothing BS 5790 Coated fabrics for upholstery Part 1: 1979 Specification for PVC coated knitted fabrics Part 2: 1979 Specification for PVC coated woven fabrics ASTM D 751-79 Testing coated fabrics ASTM D 2136-66 (1978) Testing coated fabrics-Low-temperature bend test ASTM D 2137-75 Test for rubber property-Brittleness point of flexible polymers and coated fabrics DIN 16922 (1981) Flexible sheet materials, manufactured using plastics; technological classification. DIN specifications relating to coated fabrics generally-not specifically PVC-coated-(short titles): DIN 53352 (1982): DIN 53353 (1971): DIN 53354 (1981): DIN 53356 (1982): DIN 53357 (1982): DIN 53358(1971): DIN 53359 (1957): DIN 53360 (1982): DIN 53361 (1982): DIN 53362 (1980):

Mass per unit area Thickness Tensile test Tear growth test ('trouser-leg' specimen) Adhesion Mass per unit area of coating Cracking on repeated flexing Elongation Cold creasing Determination of stiffness in bending

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7.2 Other Coated Materials and Products (including chain-link fencing, powder-coated wire and other products) BS 1651: 1966 Industrial gloves BS 4102: 1971 Steel wire for fences DIN 3036 Part 1 (1978) Plastic coated steel wires Part 2 (1978) Plastic coated steel wires 8 CELLULAR VINYLS (Chapter 25) Standards relating specifically to cellular vinyls are still comparatively few, and those covering cellular plastics generally are frequently used. For this reason most of the latter standards have been listed here. Specifications relating to fabrics coated with cellular vinyls are given in Section 7 of this Appendix.

8.1 Rigid CeDular Materials ISO 844-1978 Cellular plastics-Compression test of rigid materials ISO 845-1977 Cellular rubbers and plastics-Determination of apparent density ISO 1209-1976 Rigid cellular plastics-Bending test ISO 1922-1981 Cellular plastics-Determination of shear strength of rigid material ISO 1923-1981 Cellular plastics and rubbers-Determination of linear dimensions ISO 1926-1979 Cellular plastics-Determination of tensile properties of rigid materials ISO 2581-1975 Plastics-Rigid cellular materials-Determination of 'apparent' thermal conductivity by means of a heat-flow meter

Al Standards Relevant to PVC Materials and Products

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ISO 2796-1980 Cellular plastics-Test for dimensional stability of rigid materials ISOrrR 2799-1978 Cellular plastics-Determination of the temperature at which fixed permanent deformation of rigid materials occurs under compressive load ISO 2896-1974 Rigid cellular plastics-Determination of water absorption BS 3869: 1965 Rigid expanded polyvinyl chloride for thermal insulation purposes and building applications BS 4370 Methods of test for rigid cellular materials Part 1: 1968 (=1= ISO 844, 845, 1923) Methods 1-5 Part 2: 1973 (=1= ISO 1922; ± ISO 1926) Methods 6-10 Part 3: 1974 Methods 11-13 ASTM D 1621-73 (1979) Test for compressive properties of rigid cellular plastics ASTM D 1622-63 (1975) Test for apparent density of rigid cellular plastics ASTM D 1623-78 Test for tensile properties of rigid cellular plastics ASTM D 2126-75 Test for response of rigid cellular plastics to thermal and humid aging ASTM D 2842-69 (1975) Test for water absorption of rigid cellular plastics DIN 53421 (1971) Testing of rigid cellular plastics; compression test DIN 53423 (1975) Testing of rigid cellular plastics; bending test DIN 53424 (1978) Testing of rigid cellular plastics; determination of dimensional stability under heat in the case of flexural load and compression load DIN 53425 (1965) Testing of rigid foams; creep-depending-on-time compression test under heat

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DIN 53427 (1976) Testing of rigid cellular plastics; determination of shear strength of rigid cellular plastics in the form of a sandwich between metal plates DIN 53430 (1975) Testing of rigid cellular plastics; tensile test DIN 53432 (1977) Selfskinning rigid foams; test methods DIN 53433 (1979) Testing of rigid cellular plastics; determination of water absorption by dipping in water 8.2 Flexible Cellular Materials

BS 4023: 1975 Flexible cellular PVC sheeting BS 4443 (~ ISO 485, 1794, 1798, 1856,3386) Methods of test for flexible cellular materials Part 1: 1979 Methods 1 to 6 Part 2: 1972 (= ISO 2439) Method 7-Indentation hardness test Part 3: 1975 Method 8--Determination of creep Method 9-Determination of dynamic cushioning performance Part 4: 1976 (;zf ISO 2440) Methods 10-12 Part 5: 1980 Test for dynamic fatigue by constant load pounding Part 6: 1980 Methods 14-16 ASTM D 1565-81 Specification for flexible cellular materials-vinyl chloride polymers and copolymers (open-cell foam) ASTM D 1667-76 (1981) Specification for flexible cellular materials-vinyl chloride polymers and copolymers (closed-cell vinyl) DIN 53570 (1981) Testing cellular materials; determination of linear dimensions DIN 53571 (1978) Testing of flexible cellular materials; test of the tensile strength

Ai Standards Relevant to PVC Materials and Products

1163

DIN 53572 (1979) Testing of flexible cellular materials; determination of compression set after constant deformation DIN 53574 (1977) Flexible cellular materials; test for dynamic fatigue by constant load pounding DIN 53576 (1976) Testing of flexible cellular materials; hardness testing by indentation techniques DIN 53577 (1976) Testing of flexible cellular materials; determination of compression stress value and compression stress-strain characteristics DIN 53578 (1974) Testing of flexible cellular materials; testing of ageing DIN 53579 Part 1 (1980) Testing of flexible cellular materials; hardness test on finished parts, compression test on shaped parts Part 2 (Draft) (1977) Testing of flexible cellular materials; hardness test on finished parts, compression test on profiles

8.3 MisceUaneous Standards (including those relevant generally to cellular plastics materials and products) (a) Definition and Classification DIN 7726 Part 1 (1966) Cellular materials; definitions of terms, classification (b) Physical Properties-General DIN 53420 (1978) Testing of cellular materials; determination of apparent density

(c) Thermal Properties-General ASTM C 177-76 Test for steady-state thermal transmission properties by means of the guarded hot plate ASTM D 696-79 Test for coefficient of linear thermal expansion of plastics

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(d) Flammability and Burning ASTM E 84-81 Test for surface burning characteristics of building materials ASTM E 162-81 Test for surface flammability of materials using a radiant heat energy source

(e) Chemical Resistance and Permeability ASTM E 96-80 Tests for water vapor transmission of materials DIN 53428 (1967) Testing of foams; determination of the resistance to liquids, vapours, gases and solid materials (f) Insulation Materials (i) THERMAL ASTM C 351-61 (1973) Test for mean specific heat of thermal insulation ASTM C 421-77 Test for tumbling friability of preformed block-type thermal insulation DIN 18164 Part 1 (1979) Foamed plastics as insulating building materials; insulating materials for thermal insulation Part 2 (1979) Foamed plastics as insulating building materials; insulating materials for impact sound insulation (ii) ELECfRICAL ASTM D 149-81 Tests for dielectric breakdown voltage and dielectric strength of electrical insulating materials at commercial power frequencies ASTM D 257-78 Tests for D-C resistance or conductance of insulating materials ASTM D 1673-79 Tests for relative permittivity and dissipation factor of expanded cellular plastics used for electrical insulation

Al

Standards Relevant to PVC Materials and Products

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(iii) ACOUSTICAL ASTM C 384-77 (1981) Test for impedance and absorption of acoustical materials by the impedance tube method ASTM C 423-81 Test for sound absorption and sound absorption coefficient by the reverberation room method (g) Cushioning Materials ASTM D 1596-78 Test for shock absorbing characteristics of package cushioning materials ASTM D 2221-68 (1979) Test for creep properties of package cushioning materials (h) Sandwich Structures ASTM C 273-61 (1980) Shear test in flatwise plane of flat sandwich constructions or sandwich cores ASTM C 393-62 (1980) Flexure test of flat sandwich constructions

9 PVC WIRE AND CABLE INSULATION, CABLE SHEATHING AND JACKETING BS 6004: 1975 PVC-insulated cables (non-armoured) for electric power and lighting BS 6231: 1981 Specification for PVC-insulated cables for switchgear and controlgear wiring BS 6346: 1969 (1977) PVC-insulated cables for electricity supply BS 6485: 1971 (1977) PVC-covered conductors for overhead power lines BS 6746: 1976 PVC insulation and sheath of electrical cables

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BS 6746C: 1969 (1979) Colour chart for PVC insulation and sheath of electric cables ASTM D 876-80 Testing nonrigid vinyl chloride polymer tubing used for electrical insulation ASTM D 922-80 Specification for nonrigid vinyl chloride polymer tubing ASTM D 1047-79 Specification for poly(vinyl chloride) jacket for wire and cable ASTM D 1755-81 Specification for poly(vinyl chloride) resins Section 13: Electrical conductivity of water extract (This test distinguishes between electrical and non-electrical grades of unprocessed PVC resin) ASTM D 2219-81 Specification for poly(vinyl chloride) insulation for wire and cable, 60°C operation ASTM D 2220-80 Specification for poly(vinyl chloride) insulation for wire and cable, 75°C operation ASTM D 2405-81 Specification for general-purpose acrylonitrile-butadiene/poly(vinyl chloride) (NBRlPVC) jacket for wire and cable ASTM D 2432-81 Specification for heavy-duty acrylonitrile-butadiene/poly(vinyl chloride) (NBRlPVC) jacket for wire and cable ASTM D 2633-82 Testing thermoplastic insulated and jacketed wire and cable ASTM D 2708-81 Specification for extra-heavy-duty acrylonitrile-butadiene/poly(vinyl chloride) (NBRlPVC) jacket for wire and cable

General: Test methods for electrical properties of plastics insulation materials are given in the following collections of standards: BS 2782: Part 2. Electrical properties ASTM Book of Standards: Volumes 08·02, 10·01-10·03 DIN Handbook Plastics 1. Mechanical, thermal and electrical properties

Al

Standards Relevant to PVC Materials and Products

1167

10 PVC FLOORING BS 3260: 1969 PVC (vinyl) asbestos floor tiles BS 3261 Unbacked flexible PVC flooring Part 1: 1973 Homogeneous flooring BS 4902: 1976 Specification for sheet and tile flooring colours for building purposes BS 5085 Backed flexible PVC flooring Part 2: 1976 Cellular PVC backing DIN 16950 (1977) Flooring materials; vinyl-asbestos-tiles, requirements, test methods DIN 16951 (1977) Floor coverings; polyvinyl chloride (PVC) flooring without backing, requirements, test methods

11 VARIOUS PRODUCT STANDARDS AND TESTS 11.1 Colour Bleeding and Staining ISO 183-1976 Plastics-Qualitative evaluation of the bleeding of colorants BS 2782: Part 5 Method 542A: 1979 (= ISO 183) Qualitative evaluation of bleeding of colourants ASTM D 2151-68 (1977) Test for staining of poly(vinyl chloride) compositions by rubber compounding ingredients

11.2 Miscellaneous ISO 580-1973 Moulded fittings in unplasticized polyvinyl chloride (PVC) for use under pressure-Oven test

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N. Herbert and W. V. Titow

BS 3887: 1965 Regenerated cellulose and unplasticized PVC pressure-sensitive closing and sealing tapes ASTM D 2301-73 (1979) Specification for vinyl chloride plastic pressure-sensitive electrical insulating tape ASTM D 3005-73 (1979) Specification for low-temperature resistant vinyl chloride plastic pressure-sensitive electrical insulating tape DIN 16941 (1964) Extruded profiles of nonrigid PVC (nonrigid polyvinyl chloride); permissable deviations for dimensions for which tolerances are not indicated DIN 53419 (1977) Extruded profiles of unplasticized polyvinyl chloride (unplasticized PVC); test method of the behaviour against methylene chloride

APPENDIX 2

Quantities and Units: The SI System: Unit Conversion Tables Compiled by W. V. TITOW

The measurement and expression of the properties of the great variety of PVC materials and products in existence today, as well as the description of their performance in their numerous applications, involve the use of various units from a number of scientific disciplines and technical fields. The conversion tables and other information provided in this appendix are offered as relevant and-it is hopedpotentially useful in these connections. Sources of further information include the following standards and publications: ISO 31* Covering the general principles concerning quantities, units and symbols (ISO 31/0-1974), the quantities and units of space and time (ISO 31/1-1978), periodic and related phenomena (ISO 31/2-1978), mechanics (ISO 31/3-1978), heat (ISO 31/4-1978), electricity and magnetism (ISO 31/5-1979), light and related electromagnetic radiations (ISO 31/6-1973), acoustics (ISO 31/7-1978), physical chemistry and molecular physics (ISO 31/8-1973), atomic and nuclear physics (ISO 31/9-1973), nuclear reactions and ionising radiations (ISO 31/10-1973), and solid state physics (ISO 31/13), as well as mathematical signs and symbols for use in the physical sciences and technology (ISO 31/11-1978), and dimensionless parameters (ISO 31/12-1975).

* ISO Standard Handbook 2: 'Units of Measurement', contains the texts of all relevant ISO standards. 1169

1170

W. V. Titow

ISO 1000-1973. * 'SI Units and recommendations for the use of their multiples and of certain other units'. BS 350. 'Conversion factors and Tables' (Part 1 :1974, Part 2: 1962 with Supplement No.1: 1967). BS 1991. 'Letter symbols, signs and abbreviations' Covering general aspects (Part 1: 1976), chemical engineering, nuclear science and applied chemistry (Part 2: 1961), fluid mechanics (Part 3: 1961), structures, materials and soil mechanics (Part 4: 1961), applied thermodynamics (Part 5: 1961), electrical science and engineering (Part 6: 1975) with a list of subscripts for electrical technology (Supplement No.1: 1973 to Part 6). BS 3763: 1976. 'The International System of Units (SI)' ASTM Metric Practice Guide (1976). US National Bureau of Standards. Changing to the Metric System. (1969). Anderton, P. and Bigg, P. H., HMSO. The International System of Units (1973). HMSO. The Use of SI Units (1969). British Standards Institution. Quantities, Units and Symbols (1975). The Royal Society. A Dictionary of Scientific Units. (1964). lerrard, H. G. and McNeill, D. B., Chapman and Hall.

Standardisation of units, with emphasis on the use of those of the SI system, has-for some time now-been strongly promoted in the science and technology of plastics. The SI units are, therefore, given prominence in this section. The SI system is based on seven so-called base units. It also contains two supplementary units, and a number of derived units with special names. Multiples and sub-multiples of all SI units are, of course, also within the system.

* ISO Standard Handbook 2: 'Units of Measurement', contains the texts of all relevant ISO standards.

A2

Quantities and Units: The Sf System: Unit Conversion Tables

1171

Named Units of the SI System Quantity (usual symbol(s) in brackets)

Unit Name

Common Equivalent in equivalent base (and Symbol in Sl supplementary) units Sl units

Length (L, I) Mass (M, m) Time (T, t) Electric current (I) Thermodynamic temperature (e, T) Luminous intensity (J, I) Amount of substance (N, n)

metre kilogram second ampere

m kg s A

kelvin candela

cd

mole

mol

Supplementary units

Plane angle (cr, {3, y, 0, Solid angle (Q, w)

radian steradian

Tad sr

Derived units with special names

Absorbed dose (ionising radiation) Electric capacitance (C) Electric conductance (G) Electric potential, tension (V, U) Electric resistance (R) Energy (E), Work (W, w), Quantity of heat (Q, q) Force (F) Frequency (v, f) illuminance (E) Inductance (L, M) Luminous flux (4)) Magnetic flux (4)) Magnetic flux density (B) Power (P), Energy flux (E",) Pressure (P), Stress (a, r) Quantity of electricity, Electric charge (Q, q)

gray farad siemens

Gy F S

J kg-I Cy-l Ay-l

m2 s- 2 s' A 2 m- 2 kg-I S3 A 2 m- 2 kg- 1

volt ohm

Y

WA- 1 YA- 1

m2 kgs- 3 A- 1 m2 kgs- 3 A- 2

joule newton hertz lux henry lumen weber tesla watt pascal

J N Hz Ix

Nm

m2 kg S-2 m kgs- 2 S-I

H

WbA- 1

1m Wb T W Pa

Ys Wbm- 2 J S-l Nm- 2

coulomb

C

Base units

!p,

etc.)

K

Q

cd srm- 2 m2 kgs- 2 A- 2 cd sr m2 kgs- 2 A- 1 kgs- 2 A- 1 m2 kgs- 3 kg m- I S-2 As

Decimal multiples and sub-multiples of units in the SI and other systems are denoted by adding directly (i.e. without a space or hyphen) the appropriate prefix to the name of the unit, or the appropriate symbol to the unit symbol. The use of multiples and sub-multiples which are not powers of 1000 (indicated by parentheses in the list below) is discouraged in the SI system.

1172

W. V. Titow

Multiple Prefix Symbol

x 1018 exa E

x 1015 pela P

Sub-multiple Prefix Symbol

x 10- 1 x 10- 2 (deci) (cenli) d c

1012 lera

X

T

10- 3 milli m

X

109 giga G

X

10- 6 micro

X

/.l

6 X 10 mega M

10- 9 nano n

X

x 1cP kilo k

x lOZ (hecla) h

10- 12 pica

X

X

P

10- 15 femlo f

xlO (deca) da

X

10- 18 atto a

Unit Conversions: Time Other common units

SI unit s 1 3·155 6926 x 10' 8·640 x 1Q4 3600 60 a

b

year 3·169 x 1 2·738 x 1·141 x 1·901 X

10- 8 10- 3 10-4 10- 6

d· (solar day)b

h· (hour)

min· (minute)

1·157 X 10- 5 365·24 1 4·167 X 10- 2 6·944 X 10- 4

2·778 X 10- 4 8·766 x 103 24 1 1·667 X 10- 2

1·667 X 10- 2 5·259 x lOS 1440

60 1

Units recognised for use with the International System. 1 sidereal day = 86164·090 6 seconds; 1 year = 366·25 sidereal days.

Unit Conversions: Electric Current Only the SI unit (ampere) or its multiples or sub-multiples in use. Unit Conversions: Luminous Intensity Only the SI unit (candela) or its multiples or sub-multiples in common use. Unit Conversions: Amount of Substance Only the SI unit (mole) now in common use (mainly in calculations in chemistry and physics). The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in exactly 0-012 kg of 12c. The entities (which must be specified in each particular case) may be atoms, ions, molecules, electrons, or groups of such units.

1000 10 1 1 X 10- 3 1 X 10- 7 914·4 304·8 25·4

100 1 0·1 1 X 10- 4 1 X 10- 8 91·44 30·48 2·54

1 0·01 1 X 10- 3 1 X 10- 6 1 X 10- 10 0·9144 0·304 8 0·0254 2·54 X

10

4

1 X 106 1 X 104 1 X 103 1 1 X 10- 4

jimQ

1X 1X 1X 1X

1010 108 107 104 1

(angstrom)

A

QThe name 'micron' for this unit (micrometre) is discouraged.

mm

cm

m

Sf units

Unit Conversions: Length

3·28084 0·032808 3·2808 X 10- 3 3 1 0·083333

1 0·333333 0·027778

ft (foot)

1·093610 0·010 936 1·093 6 X 10- 3

yd (yard)

Other common units

36 12 1

39·3701 0·393701 0·039370

in (inch)

?3

~

-.I

........ '"

if

c::r-

;:;'J

;:l

'"5't::l

-

~

'"

;;l

~.

~

~

~:

§

K:l

1 1 X lO-4 1 X lO-6 0·8361 0·09290 6·452 X lO-4

m

2

mm 1 X 1()6 100 1 8· 361 27 X lOS 9·2903 X 10' 645-16

1 X 10" 1 0·01 8·361 X 10' 929 6·4516

2

cm 2

SI units

Unit Conversions: Area

1·196 1·196xlO- 4 1·196 X 10- 6 1 0·1111 7·716 X 10- 4

yd

2

10·764 1·076 X 10- 3 1·076 X 10- 5 9 1 6·944 X lO-3

it>

Other common units

1·550 X lO3 0·1550 1·550 X lO-3 1·296 X 10' 144 1

in 2

~

0

~

:0::::

~

~

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

a

cm3 UK gal (UK gallon) 220·0 2-2 X 10- 4 2·2 X 10-7 1 0·8327 0-2200 6-229 3·605 X 10- 3

mm 3

1 X 109 1 X 103 1 4-546 X 106 3-785 X 106 1 X 106 2-832 X 107 1·6387 x 104

Unit recognised for use with the International System.

1 x 106 1 6 1 X 101 1 X 10-3 1 x 10- 9 4·546 X 10-3 4-546 X 103 3-785 x 10-3 3-785 X 103 1 x 10- 3 1 X 103 0-02832 2·832 x 104 1-6387 x 10- 5 16-39

m3

Sf units

Unit Conversions: Volume

264-2 2-642 X 10- 4 2·642 X 10-7 1·201 1 0·264 2 7-4805 4·329 X 10- 3

US gal (US gallon) 1 x 103 1 X 10- 3 1 X 10- 6 4-546 3·785 1 28·32 0·01639

(litre)a

I

fr3

35-32 3-532 X 10-5 3·532 X 10- 8 0-1605 0·1337 0-03532 1 5·787 x 10- 4

Other common units

~

6-1024 x 104 0·061024 6·1024 x 10- 5 277·4 231·0 61·024 1·728 x 103 1

-.l V1

........

1r

-

~

~

o· ;:s

~

~

~ ;,..

~

""

~

'"~

~.

~

in 3

~

~

S S. :::to

to

~

b

a

3·43775 x 1cP 60 1 0·0166667 54 5·4 X 103

57·2958 1 0·0166667 2·77778 x 10- 4 0·9 90

1 0·0174533 2·908 88 x 10- 4 4·84814 X 10- 6 0·0157080 1·57080

2·06265 x 3·6 X 60 1 3·240 x 3·240 X

5

10 105

3

10 103

" (secondt

Other units g

63·6620 1·11111 0·0185185 3·08642 x 10- 4 1 100

(grade)

Units recognised for use with the International System. The degree and the second may be subdivided decimally, e.g. 57.2958° = 57°17'44.8".

(minuteY

° (degree)ab

rad (radian)

S1 unit

Unit Conversions: Plane Angle

0·636620 0·011111 1·851 85 x 10- 4 3·08642 x 10- 6 0·01 1

(right angle)

L

~

~

:-=:::

~

a-

--.I

--

a

1·23457 x 10- 4 1

1 0·81

The solid angle of a sphere (=4n).

4·052 85 x 10

3·28281 x 13

0·079577 5 1

1 12·5664 3·046 17 x 10- 4 2·46740 x 10- 4

3

(square grade)

OK

00 (square degree)

sp (spat)a

Other units

sr (steradian)

SI unit

Unit Conversions: Solid Angle

-J -J

--

ff

0-

~

;:

0"

'"C::l"

;:

~

~ ::::

~

~

~

'"

;:J

~

S. ~

l:l

s:~

§

10

~ N

1 x lQ3 1 1 X 106

1 1 x 10- 3 1 x 103 1·01605 x 103 907·185 50·8023 0·453592 0·028350

4

f'

1 x 10- 3 1 X 106 1 1·01605 0·907185 0·05080 4·5359 x 10- 4

(tonne) 1·102 3 x 10- 3 1·102310 1·12 1 0·056 5·00 X 10-4

0·984207 1 0·892857 0·05 4·4643 x 10- 4

US tone

9·842 1 x 10- 4

UK ton b

19·6841 20 17·8571 1 8·9286 x 10- 3

0·019684

cwt (hundredweight)

Other common units lb

2·20462 2·20462 x 10- 3 2·20462 x 103 2·24 x 103 2 x 103 112 1 0·0625

(pound)

35·2740 0·035274 3·52739 x 104 3·584 X 104 3·2 X 104 1·792 x 103 16 1

oz (ounce)

b

a

Also known as 'metric ton' or 'metric tonne'. Unit recognised for use with the International System. 1 t = 1 Mg (which is an SI unit). The 'long' ton. e The 'short' ton.

5·08023 x 10 453·592 28·3495

g (gram)

kg

Sf units

Unit Conversions: Mass

~

0

:::1

~

~

-..l 00

..... .....

A2 Quantities and Units: The Sf System: Unit Conversion Tables

1179

Unit Conversions: Density 51 units

Other common units

kgm- 3

gcm- 3

kgl- 1

Ibin- 3

Ibft- 3

lb UKgal- 1

1 1 X 1 1;;

(continued)

~

-

f}

;:s

(ASTM C 177) ]

Wm- 1 K- 1

M (and other compounds): 0·14-0·17

M (and other compounds): 0·14-0·28

BS 874: 1973 (1980); BS 4618: Section 3.3: 1973; ASTM C 177-76; DIN 52612-1979

See relevant conversion table in Appendix 2

The quantity of heat which passes in unit time through unit area of a slab of uniform material of infinite extent and unit thickness, when unit difference of temperature is established between its parallel faces

Thermal conductivity

k;)"

M (and other ~ compounds): ~ General-purpose It plasticised com- ~ pound typically ~ about 40°, but i:!-. the range is wide ~ (ASTM D 1525 .Q., does not recom- ~ mend the test for ~ pPVC for that ~ reason) ~ M (and other compounds): 65-100°C (ISO 306:5 kg load)

ISO 306-1974; BS 2782 : Part 1: Methods 120A to E: 1976; ASTM D 1525-76; DIN 53460-1976

~

~

Vicat softening point

"'"

:t.

°Cor OF

Low at normal plasticiser contents

The temperature at which a flat-ended needle of specified dimensions penetrates a specimen to a prescribed depth under a prescribed load (usually 1 kg or 5 kg), in standard test conditions

M: 2·2-3·5 GPa (ASTMD 695)

ISO 604-1973; ASTM D 695-80; DIN 53457-1968

As for tensile (or compressive) strength

'Bulk modulus'. Ratio of change in external pressure to fractional change in volume, in reversible conditions

(d) in compression K

(BS 2782: Method 335A)

BS 4618: Section 3.1: 1970; ASTM D 696-79

K- 1 ; 0C- 1 ; °F- t

°C; of

%

Change in length per unit original (reference) length per degree temperature change; i.e. a= 6.LI(L o . 6.T)

The temperature at which, in specified test conditions, a test specimen (bar of prescribed dimensions) undergoes a specified deflection under a flexural load causing a maximum fibre stress in the specimen of either 1·82 MPa (2541bf in -Z) or 0·455 MPa (66lbf in- Z)

Percentage deformation of a sheet specimen of prescribed dimensions by a specified load at 70°C, under stated test conditions

Coefficient of linear thermal expansion

Deflection temperature under load

Deformation under heat of flexible PVC compounds k

a

BS 2782 :Part 1: Method 122A:1976

ISO 75-1974; BS 2782: Methods 121A and B: 1976; ASTM D 648-72 (1978); DIN 53461-1969

Some standards relevant to determination in plastics

Common units

Brief definition

Property and usual symbol

Very low at normal plasticiser contents (pPVC not normally tested for this property)

M (and other ·compounds) : 15-65% (BS 2782)

-

M (and other compounds) : 10 x 10- 5_ 25 x 10- 5 K- 1 (ASTMD 696)

Flexible PVC

M (and other compounds): 6O-80°Ci (ISO, BS, ASTMor DIN) M(G): 7085°Ci (ISO, BS, ASTMor DIN)

M (and other compounds): 5 x 10- 5_ 15 x 10- 5 K- 1 (ASTMD 696) M(G): 2·7 x 10- 5 (ASTMD 696)

Rigid PVC

Typical values or value ranges for pvc'

'l:

:::'l S

~

:

'0

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

Ratio of the capacitance (Cx ) of a given configuration of electrodes with the particular material as the dielectric, to the capacitance (C v ) of the same electrode configuration with vacuum (or air) as dielectric; Le. k' = C)Cv

The ratio of true power dissipated to the apparent power absorbed during the passage of an alternating current through a dielectric

Permittivity'" (dielectric constant) k'; £'

Loss tangentn (dissipation factorO) tan 0 None (a ratio)

None (a ratio)

J g-IK- 1 ; Ical g-l 0C-\ IBtu Ib- 1 °F- 1

BS 2782: 1970: Method 207A; BS 4618: Section 2.2: 1970; ASTM D 150-81; DIN 53 483-1969, 1970

BS 2782: 1970: Method 207A; BS 4618: Section 2.1 : 1970; ASTM D 150-81; DIN 53 483-1969, 1970; (NB ISO 1325-1973 PlasticsDetermination of electrical properties of thin sheet and film)

BS 4618: Section 3.2: 1973; ASTM C 351-61 (1973)

M (and other compounds): 4·5-8·5 at 50 Hz 3·5-4·5 at 1 MHz (ASTMor DIN)

M (and other compounds): 1.0--2.0 J g-l K- 1

j

~.

~ ~ ::t '"

§:

..,

~o

~

~

!i

a

f2

~

~

.s;,

~

'" i;.

.g

~

~ §.:

~

~

~

.......

)..

Fo

required to initiate sliding; = minimum force required to maintain it (at a particular speed); and L = the force (usually gravitational) acting normally to the surfaces to maintain their contact

/is

= FsIL Ilo = FoIL where Fs = the minimum force

BS 2782: 1970: Method 311A; BS 4618: Section 5.6: 1975; ASTM D 1894-78 (/is and Ilo of plastics film and sheeting); ASTM D 3028-72 (1978) (Ilo of plastics solids and sheeting)

None (a ratio)

The two coefficients of friction, static' (/is) and dynamic' (Ilo) are defined by the expressions:

Coefficients of friction

Il

ASTM D 1044-78 (Taber abraser); ASTM D 1242-56 (1981); ASTM D 673-70 (1982) (Mar resistance) ; DItol,53 754-1977

No conventional units. Measured in terms of mass loss by the specimen, or visual effects, e.g. marring of surface, loss of transparency.

Some standards relevant to determination in plastics

Common units

Resistance to surface damage or wear caused by rubbing by prescribed abrasives in strictly specified conditions. NB Abrasion resistance (adhesion) of print on thin PVC sheeting prescribed in BS 1763: 1975 (measured according to BS 2782 : 1970: Method 310B)

Brief definition

Abrasion resistance

Property and usual symbol

Flexible PVC

:::J