Flame-retardant Paints

Progress in Organic Coatings, 7 (1979) 279 - 287 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

FLAME-RETARDANT

279

PAINTS

A. G. WALKER

Anzon Ltd., Willington Quay, Tyne and Wear (Gt. Britain)

Contents 1 Introduction.

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

279 280

2 Fire tests applicable to paints. .................................. 2.1 Flame spread tests. ....................................... 2.2 Heat release tests ........................................

286 281

3 Properties required of flame-retardant paints. ........................

281

4 Formulation of flame-retardant paints ............................. 4.1 General principles. ....................................... 4.2 Intumescent paints ....................................... 4.3 Antimony oxide based paints ................................

281 281 282 283

5 Properties of commercially available flame-retardant paints ............... 5.1 Flame-retardant properties .................................. 5.2 Decorative properties. .....................................

286 286 286

References.

286

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

1. Introduction In recent years there has been a rapid increase in fires resulting in damage to structures, limitation or cessation of manufacturing capability and death or incapacitation of occupants of buildings. It is therefore not surprising that considerable effort has been expended on studies of the factors which affect both the initiation and growth of fires. The results of this work and data obtained from records of actual fires have indicated that it might be necessary to place limitations on the use of certain products. This in turn has resulted in the formulation of regulations, particularly those related to the building industry. In general the regulations themselves limit materials which can be used by specifying particular performance requirements, and often products which might be eliminated because of their flammability may be modified so that they can be permitted, There is sufficient evidence to show that surface linings used in building construction can make a major contribution to the rate of growth of a fire [l] . This applies not only to the lining material itself but also to a decorative finish which may be applied to it. It is thus important, when considering the possible effect on fire growth of a particular material, that the effect of paints and other coatings be taken into account.

280

2. Fire tests applicable to paints

In view of the fact that, as far as paints are concerned, the surface spread of flame characteristics are likely to be the dominant ones, the most important group of tests is that which measures rate or distance of surface flame spread. It is generally accepted that fire tests for any particular property should not be designed specifically for the type or class of material. They should be suitable for testing all materials which are likely to be used for the specific situation. Thus tests for surface spread of flame have been designed to measure this characteristic of lining materials, irrespective of their chemical or physical make-up. Thus, for paints which are always applied to a substrate, they must be tested on the specific substrate at the thickness used in practice. In addition to flame spread properties, it is generally agreed that the heat evolved from a burning lining can itself constitute a hazard by causing ignition of nearby flammable materials. 2.1 Flame spread tests In the U.K. the flame spread test applicable to building materials is .described in B.S. 476: Part 7 [2]. A specimen is mounted at right-angles to a radiant panel which is adjusted to produce a specified heat gradient along the specimen. A pilot flame is applied to the surface of the specimen close to the radiant panel and the progress of surface flaming is recorded. From the results the test material is classified into one of four classes in respect of flame spread. In Germany, specification DIN 4102 [ 31 relates to tests for the fire characteristics of building materials, and included in Part 1 are tests applicable to surface linings. The apparatus consists of a vertical tunnel, the specimens in effect forming the sides. A standard burner allows flames to impinge on the faces of the specimens, near the bottom edge, and the distance the flame travels is determined by measuring the charred length,of the specimen. From the results the materials are classified either as A2 or Bl. In France the most important procedure is the Epiradiateur test described in Specification NF-P92-601 [ 4 ] . The specimen is subjected to a specified radiation intensity and propane flames impinge on its surface. In addition to flame distance, the times for ignition and the temperature of exit gases are measured. From the results, indices for burning, flame development and combustibility are calculated and from these materials are classified Ml, M2, M3, M4 or M5. In the U.S.A. the most accepted test is that described in ASTM E84 [ 51. The apparatus consists of a 25 ft tunnel, the test specimen constituting the roof. The underside is subjected to the effect of gas burners and the flame movement is measured visually. Results are compared with those obtained on testing red oak, which is given a flame spread rating of 100. As all the above tests differ both in detail and in principle, results from one cannot be expected to correlate with results from another. Some of the

281

test methods do seem to favour particular materials, for example, intumescent paints always give better results when tested to ASTM E84 than nonintumescent flame-retardant paints. At the present time there is no international agreement on a method of test for flame spread, although Committee ISO/TC92 has published a draft proposal which is based on the same principles as B.S. 476: Part 7. 2.2 Heat release tests In the U.K., the closest approach to a heat release test is that described in B.S. 476: Part 6 [6]. The specimen represents one face of a closed box and is subjected to the combined effects of electric heaters and gas flames. The temperature of the effluent gases is measured. By comparison of the temperature/time curve obtained on testing the specimen with that on testing an asbestos calibration panel indices of performance are calculated. Committee ISO/TC92 is developing a heat release test based at least on some of the principles of B.S. 476: Part 6. It is, however, at an early stage of development and no formal documents have been published.

3. Properties required of flame-retardant paints Most conventional decorative paint films are flammable when dry. Thus, when decorating a building a potential fire hazard can be introduced and this increases as the thickness of the paint film increases with redecoration. Paints, however, are not the only source of fire hazard in a modem building. Many lining materials will burn, and flame movement across their surfaces is a factor which can decide the rate at which a fire will travel within a building. Flame-retardant paints have a dual purpose: to ensure that decoration itself does not present a fiie hazard and to enable the hazard of flammable linings to be reduced. It must not be forgotten that flame-retardant paints are decorative materials and thus ideally must have the same application and weathering properties as the conventional paints with which the decorator is fully familiar.

4. Formulation of flame-retardant paints 4.1 General principles Before outlining the principles of flame-retardant paint formulation it is necessary to consider the processes which occur when a fire starts. Obviously a source of ignition is essential, and the initial stage of a fire is for this source to heat the material with which it is in contact. The reactions which can then take place and which could result in fire growth are illustrated in Fig. 1. Thus, to reduce flame spread once ignition has taken place can be achieved by one or more of the following means.

282 OXYGEN

FLAM,NG

y-y

GASEOUS PRODUCTS

/ HEAT

I/.p>

POLYMER

-->

+

\

\

GLOWING

\

/

SOLIDPRODUCTS

“XYGEN

Fig. 1. Reactions

affecting

flame propagation.

(i) Ensure that heating is not sufficient. to cause evolution of flammable gases. (ii) Modify the substrate so that its initial mode of decomposition is changed, resulting in the evolution of less flammable gases. (iii) Ensure that conditions are not established such that the flammable gases propagate flame. (iv) Prevent access of air. The second of these is more suitable for the modification of synthetic polymers or impregnation of timber products than for use in paint formulations. Some of the first flame-retardant paints were based on alkaline silicates or borates which when heated formed a glass-like layer effectively sealing the flammable substrate from air [ 71. Although the film gave effective protection when remaining in place, it had a tendency to flake under fire conditions. In addition, most paints based on this principle are water-soluble and thus have limited durability. Intumescent flame-retardant paints are those which swell up to form a solid coherent foam when heated, and this insulates the substrate. The other major group of flame-retardant paints is that where the paints contain antimony oxide and a halogenated binder. N 4.2 Intumescent paints Each of the components is incorporated for a specific purpose. There needs to be a source of carbon, a dehydrating agent and a blowing agent [ 81. (i) Carbon so&e. This is usually a carbohydrate, e.g. starch or glucose, or a polyfundional alcohol, e.g. pentaerythritol. (ii) Dehydmting agent. This needs to be a material which decomposes forming an acid which will esterify the hydroxyl group in the polyalcohol. Qpical materials include ammonium ortho- and polyphosphates and melamine phosphate. (iii) Blowing agent. This must decompose giving a non-combustible gas to expand the foam. Typical materials include urea, melamine and dicyandiamide.

283

It is important to choose these ingredients such that they react in the correct sequence. The dehydrating agent must decompose at about the same temperature as the blowing agent. Thus if monoammonium phosphate is used, urea is a suitable blowing agent, melamine being unsuitable because its decomposition temperature is too high. Other ingredients of intumescent paints have to be chosen with care. Many basic extenders often reduce the height of intumescence considerably and should thus be avoided. Titanium dioxide appears to be inert in most intumescent systems and is therefore the recommended opacifying pigment. The paint vehicle must be thermoplastic throughout its life. This is the major restriction when formulating intumescent paints. Hence most of them have been formulated using PVA or similar emulsion binders. Solvent-based paints can be formulated but the resin used must be at least partly non-convertible. The following resins have been successfully used: Vinyl toluene/butadiene Vinyl toluene/acrylic Non-oxidising-oil alkyd Ethyl hydroxycellulose Chlorinated rubber Melamine formaldehyde and urea formaldehyde resins, although not thermoplastic, have been used as intumescent paint binders and presumably have been successful because they also act as blowing agents when they are heated. Typical formulations for emulsion-based intumescent paints are given in Table 1 [9, lo] and a typical formulation for a solvent-based paint in Table 2 [lo]. Many attempts have been made to formulate a clear intumescent paint. The major problem has been to find resins and intumescent agents which produce a film which is not significantly affected by moisture. There has only been limited success in overcoming the problem, but a formulation which at least partially meets these requirements is given in Table 3 [ 11,121. 4.3 Antimony oxide based paints Paints based on antimony oxide inhibit flaming by a chemical mechanism. It is generally accepted that propagation of flames progresses by a free-radical mechanism involving active H and OH free radicals [ 13 - 151. It has been shown that the injection of the much less active chlorine or bromine radicals has a marked inhibiting effect on flame propagation [16] . Antimony trichloride or antimony tribromide appear to be more effective than the hydrogen halides, and it is believed thatin the flame the halogen free radical is released thus inhibiting growth [ 17,181. This would result in the formation of antimony oxide in a very fine form which itself will act as a flame retardant as a result of the well-known dust or wall effect. It is not possible to incorporate the antimony halides as such into a paint film, and thus it is necessary to ensure that they are formed by reaction of antimony oxide and hydrogen halide when a flame is applied to the film.

284 TABLE

1

Emulsion

baeed intumeecent

paint8

PVA em&ion Monoammonium phoephate Ammonium polyphoephate I* Dicyandiamide Pentaerythritol Tripentaerythritol Chlorinated paraffin Starch Pota88ium tripolyphosphate Diethylene glycol mono-ethyl ether acetate Melamine formaldehyde resin Melamine Titanium dioxide Surfactant.8, antifoams Thickener8 etc. Water

18% 22 16 12 -

3

25

Solventbas&dintumescentpaint

TABLE

4.4% 6.4 4.0 6.1 6.4 17.7 66.0

3

Clear intumescent

coating

Butylated UF resin in butanol/xylol Monobutyl phosphoric acid Dibutyl phosphoric acid Monoethylamine Ethanol &Chlorometaxylenol Glyceryl tolyl ether

23.0 -

-

4

TABLE 2

Ethyl cellulose Chlorinated paraffin Titanium dioxide Dipentaerythritol Melamine Ammonium polyphoephate Toluene

16.8% -

76.0% 7.3 3.1 1.5 6.6 6.0 2.6

3.6 4.6 1.0 2.0 1.9

7.7 5.6 14.0 28.9

285

Thus, when formulating antimony oxide based flame-retardant paints it is necessary to incorporate a halogen compound which at flame temperatures will decompose to give the appropriate hydrogen halide. Compounds which have been successfully used include the following: Chlorinated paraffins Chlorinated rubber Polyvinyl chloride Polyvinylidene chloride Pentabromotoluene Chlorinated alkyd Some of these can also be used as both the paint binder and as the source of halogen. The principle can be used for both emulsion and solvent based paints and there are no specific restrictions on the types of binders which can be used. Typical formulations are given in Table 4 and 5.

TABLE 4 Antimony

oxide based emulsion

painte

Antimony oxide Titanium dioxide Whiting

10.9% 21.8 -

8.9% 5.5 30.2

23.4% 11.5 -

Mica PVA co-polymer emulsion Acrylic emulsion

9.4 21.2 -

11.1 8.0

-

PVDC co-polymer Thickeners etc. Water Liquid chlorinated

-

-

3.0 29.4 4.3

3.0 31.2 2.1

TABLE Antimony

paraffin

7.8

30.5 5.8 21.0 -

5 oxide based emulsion

paints

Undercoats Antimony oxide Titanium dioxide Micronised talc Whiting Mica China clay Solid chlorinated paraffin Liquid chlorinated paraffin Long oil soya alkyd Chlorinated alkyd* White spirit, driers etc. *Based on chlorendic

6.1% 28.6 6.7 11.5 5.3 1.7 22.2 17.9

anhydride

6.1% 25.6 11.1 10.3 -

31.7 15.2 and containing

Glose finiehee

Luetre finiehee

7.7% 25.4 -

6.2% 28.4 -

-

7.7

45.0

-

14.2 13% chlorine.

7.9% 26.1 -

32.6 21.7 11.7

-

5.6 6.3

7.3 ,2.3 25.2 18.7

6.2% 26.1 -

-

7.3 7.8

35.0

286

5. Properties of commercially available flame-retardant paints 5.1 Flame-retardan t propepfies

Both intumescent and antimony oxide based paints can be used on a variety of substrates to reduce spread of flame. Thus the application of either type of cellulosic based insulation board will raise its inherent Class 4 spread of flame properties to Class 1 when tested to B.S. 476: Part 7. On timber substrates it is necessary to use intumescent paints to achieve a.Class 1 spread of flame surface, but a Class 2 surface can be achieved by the use of antimony oxide based paints. It is essential to consider the combination of paint and substrate, and thus it is not possible to refer to flame-retardant properties of paints alone. Results obtained using commercial flame-retardant paints on a variety of substrates using the procedures described in B.S. 476: Part 6 and B.S. 476: Part 7 have been published [19 - 211. These publications should be consulted by potential users of any flame-retardant paint. 5.2 Decorative properties When formulating intumescent flame-retardant paints it is necessary to incorporate appreciable quantities of intumescent agents. In many cases these materials have an appreciable water solubility and in addition have little or no opacifying properties. In most cases the properties of the intumescent paint are determined by the properties of the intumescent agents. Thus, compared with conventional decorative paints, intumescent paints are more likely to be affected by washing down and exposure to moist conditions. Therefore, although initially flame-retardant properties will be good, they may deteriorate due to leaching of the intumescent agents. This effect is most marked when ammonium phosphate is used but is less likely when this is replaced by ammonium polyphosphate. Because of the limited quantity of opacifying pigments which can be incorporated into intumescent paints, they usually have poor opacification and thus fairly heavy film weights have to be applied to achieve adequate covering. Intumescent paints are available only with a matt or semi-matt finish and are normally not available in a range of colours. Flame-retardant paints based on antimony oxide are commercially available in a range of finishes and there are very few limitations on the colours available. The components are not water-soluble and thus the paints can be washed down or exposed to wet conditions without loss of properties. References 1 P. H. Thomas, British Wood Preserving Assoc. Conf., 1956, pp. 112 - 130. 2 Surface spread of flame test for materials, B.S. 476: Part 7, British Standards Institution. 3 Fire characteristics of building materials, DIN 4102, Deutsches Institute fiir Normung.

287 4 Building materials. Reaction to fiie test. Radiation test for rigid materials, NF-P92601, Association Francaise de Normaiisation. 5 Method of test for surface burning characteristics of building materials, E84: 61, American Society for Testing Materials. 6 Fire propagation test for materials, BS 476: Part 6, British Standards Institution. 7 Br. Pat. 1,076,909. 8 A. P. Rylea, New polymer applications in the building and construction industry, Symposium, Surrey University, 1973. 9 Technical leaflet, Monylith DC 20F, Farbewerke Hoechst. 10 Technical leaflet, Phoschek P30, Special Rep. No. 7088, Monsanto Company. 11 Br. Pat. 862,669. 12 V. M. Bhathagar, Farbe Lack, 78 (1972) 1076. 13 J. L. BoBand, Q. Rev. (London), 3 (1949) 1. 14 C. E. Frank, Chem. Rev., 46 (1960) 166. 16 L. Bateman, Q. Rev. (London), 8 (1964) 147. 16 Chemical Kinetics of Gas Reactions, Pergamon Press, Oxford, 1964. 17 J. W. Hastie, J. Res. Natl. Bur. Stand,, Sect. A, 77 (1973) 733. 18 J. W. Hastie, Combust. Flame, 21 (1973) 49. 19 Fire Test Results on Building Materials. Surface Spread of Flame, compiled by R. W. Fisher, publihed by Fire Protection Association. 20 Results of spread of flame tests on building materials, Build. Res. Estab. Rep., 1976, H.M.S.O. 21 Results of fine propagation tests on building materials, Build. Res. Estab. Rep., 1976, H.M.S.O.