Fire Safety Journal, 4 (1981/82) 163 - 167 163

Clear Intumescent Fire Retardant Coating Consisting of Chlorinated Paraffins and Polyurethanes

VIJAY MOHAN BHATNAGAR* and JEAN-MAURICE VERGNAUD

Laboratory of Materials and Chemical Engineering, University of Saint-Etienne, U.E.R. of Sciences, 23, Dr. Paul Michelon. 42023 Saint-Etienne Cddex (France)

(Received June 1, 1981;in revised form September 17, 1981)

SUMMARY

Clear, intumescent fire retardant coatings for wood were tested by exposure to flame. The coatings consisted of a polyurethane film incorporating chlorinated paraffins, which, when exposed to flame, produced a foam. The decrease in the relative flame spread rating was found to be about proportional to the paint thickness. The flame spread rating was not found to decrease with the chlorowax percentage according to a simple law; the maximum effect was reached with about 1 7% chlorowax 70.

INTRODUCTION

It is well known that an intumescent fire retardant coating, when properly applied, can effectively control fire spread and damage. It will provide sufficient time for evacuation of personnel from buildings and allow for the arrival and efficient use of fire-fighting equip- ment. Nothing additional can be expected of fire retardant coatings. Thus the dual purpose of an intumescent fire retardant coating is:

(i) to confine the surface spread of flame along the coated surface to the boundaries of the already established fire;

(ii) to retard penetration of heat and flame to, and through, the coated surface.

Most of the coatings described in the literature were pigmented [1, 2]. Other coatings were clear, but they were different from the composition in our work. For exam-

*Permanent address: Flammability and Fire Retar- dant Institute, Alena Enterprises of Canada, P. O. Box 1779, Cornwall, Ontario K6H V7, Canada.

ple, Quelle [3] described a clear fire retardant system based on a condensate resin formed from urea, formaldehyde and phenol; this resin was blended with mono and dibutyl phosphate. A clear fire retardant system based on a proprietary phosphoramide was studied by Ellis [4], and an isano oil system by Neville Chemical Co. [5]. In 1968, Clark [6] discussed the formulation of clear fire retar- dant coatings: aromatic diisocyanates were reacted with halogenated phenoxyether diols, polyhydric alcohols, and tris haloalkyl phos- phates. In 1969, Thomas [7] produced a clear coating using a hydroxy-alkylated pyrophosphate, methylol melamine and a chlorinated paraffin. Juneja ]8] described the preparation of stabilized aqueous solutions of formaldehyde, melamine, dicyandiamide and phosphoric acid for wood coatings. These clear coatings were more suitable for impreg- nating the wood, but their application was accompanied by the release of unacceptable levels of formaldehyde vapour.

Thus we must accept that clear fire retar- dant coatings have not been extensively studied due to difficulty in preparation and the choice of suitable ingredients. Several early coatings were ineffective in retarding fire and flame spread and they imparted some discoloration during protection of the sub- strates; in many cases the discoloration was such that further pigmentation had to be added during production of the coating to mask the fire retardant agent and produce a pleasing effect. Several other clear fire retar- dant coatings had little or no flow quality and, hence, did not provide a desirable film smoothness. Another serious limitation of the earlier reported clear fire retardant composi- tions was their lack of washability.

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Bearing in mind the limitations of earlier reported coatings, the present work was carried out to develop a clear fire retardant coating having the application characteristics of high quality, organic solvent coatings. Our composit ion had permanent latent fire retardant qualities due to the ability of the applied film to expand and provide an insula- ting layer between the flame source and the combustible substrate, as well as to liberate a quanti ty of primarily non-flammable decom- position products which tend to extinguish the flame.

EXPERIMENTAL

A. Preparation o f fire retardant coating

(i) Description o f raw materials The main ingredients of the coating were:

cellulose acetate butyrate, methyl isobutyl ketone, chlorinated paraffin, and hexa- methylene diisocyanate, but the formulation could be varied and extended by the addition of other ingredients to obtain a fire retardant coating.

Polyisocyanates. The hexamethylene di- isocyanate was found to be particularly effe- tive in providing color stability and gloss retention. It also provides good weather resistance and light fastness due to the lack of absorbance of u.v. In fact, the product used in this work was not actually HMDI but Desmodur N resulting from the reaction of 3 molecules of HMDI with 1 of water and the release of 1 mode of COz.

Chlorinated hydrocarbon. Chlorowax 70 (Diamond -- Shamrock Chemical), a creamy- white powdered product, was preferred in this work due to its high chlorine content (about 70% of chlorine by weight). It dis- solved readily in the usual solvents, and also gave the following benefits:

(a) it produced non-flammable decomposi- tion products and gave a good fire retardant effect;

(b) it helped to glaze the film surface adjacent to the point of flame contact;

(c) it was thermoplastic and thus main- tained film flexibility and delayed film cracking during its destruction;

(d) the degree of intumescence was in- creased, and helped eliminate afterglow;

(e) the cellular structure of the residue was strengthened.

Solvents. In this work, care was taken to use anhydrous solvent, otherwise turbidities may develop after some time in the presence of isocyanate groups. The most useful sol- vents were found to be toluene, methyl ethyl ketone and cellosolve acetate.

Cellulose acetate butyrate. A product resulting from the treatment of wood pulp with acetic and butyric acids and anhydrides to form a resin suitable for coating purposes.

Additives and catalysts. Additions were made to improve the slip properties of the coating surface by reducing the block resis- tance of the film:

(a) dibutyltin dilaurate; (b) desmorapid PP.

(ii) Coating preparation When an hydroxyl-containing polymeric

material was present it was preferable to prepare the composit ion as a two compo- nent system. For instance, the chlorinated hydrocarbon and hydroxyl containing poly- meric material were blended together in a solvent as component A, and the organic polyisocyanate was dissolved in a solvent as component B. The components A and B, as separate materials, have a pot life of several months. The coating dried tack free with 30 min to 1 h and required about 2 days to reach maximum hardness. The following example of coating composi t ion preparation was used particularly in this work.

Component A cellulose acetate butyrate 48

chlorinated paraffin 18 mar and slip additive 0.6

Component B Desmodur N 33.4

Each component was prepared in a solvent and then components A and B were mixed together to produce a final coating composi- tion containing about 50% solvent. Variations of this composit ion were tested using Tenth-

second butyrate (Eastman Chemicals) instead of C A B and Byk 300 (Mallinckrodt) as additive.

All the coatings were applied to one-inch red oak panels which had been conditioned for about four weeks at 20 °C and 50% relative humidity prior to use. A minimum of 24 h was allowed between application of the two coats. The panels were dried for a mini- mum of 2 days after application of the last coat. A spray gun was used to coat the panels.

B. Fire retardancy test Many types of fire rating tests are currently

being used to classify the fire safety aspects of wood or building materials. In the U.S.A. the most widely recognized standard for the measurement of the fire hazard characteristics of wood is ASTM E-84 or 25-Foot Tunnel Test. Since 1967, the 2-Foot Tunnel has been used by a number of laboratories to screen ex- perimental fire retardant coatings [ 9]. There is a good correlation between the 25-Foot Tunnel and the 2-Foot Tunnel [10]. Vander- sail [11] has stated that the value obtained by the relationship (eqn. (1) in our paper) will be within + 4 units of the value obtained in the 25-Foot Tunnel for 95% of the time.

The 2-Foot Tunnel was used for our expe- riment because it is very practical. The sample holding rack utilizes a 30 in. by 4 in. specimen and is inclined 30 o from the horizontal. The coating was subjected to the flame for 4 min; during this time, the length of the flame front as it spread along the sur- face was recorded every 20 s. The flame spread rating, F S R, was calculated from the maximum front advance on the panel surface as compared with that of red oak and asbes- tos mill board, which arbitrarily were assigned values of 100 and 0, respectively

Lsample -- Lasbestos F S R = 100 (1}

Lo~ - - Lasbestos

where L is the average maximum length of flame front on the chosen material. The boards were conditioned for about 4 weeks at 20 °C and 50% relative humidity prior to use, in the same way as the coated panels.

RESULTS AND DISCUSSION

The effect of three parameters was investi- gated in this work: the type of wood used,

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the percentage of chlorowax 70 as chlorina- ted paraffin, and the coating thickness.

A. Effect of the type of wood All the coating formulations were tested on

several woods, e.g., poplar, red oak, Douglas fir, birch, and also on Masonite panels. The coated woods, using the described compo- sition and a dry film thickness of 10 mils (0.24 mm), showed much improved fire retardancy, as shown in Table 1.

TABLE 1

Dependence of F S R on the species of wood

Material Uncoated Coated Relation(2)

Poplar 128 65 0.53 Red oak 100 50 0.50 Douglas fir 115 45 0.61 Birch 110 43 0.59 Masonite 90 41 0.54

It is also interesting to note that the relative decrease in F S R defined by expres- sion (2), was about the same for different woods burnt under the same conditions:

F S Rwood - - F S Rsample F S Rwood

( 2 )

B. Effect of percentage of chlorowax 70 The general experience of chlorowax 70 as

a chlorinated paraffin indicated that its percentage is quite effective in reducing flame spread. Below about 17%, our overall experi- ence with chlorowax 70 indicated that the proportion by weight had a marked effect on flame spread. This is illustrated in Fig. 1 which shows flame spread rating against proportion by weight of chlorowax 70 in the range 0 - 40%. While it may appear to be logical that increasing the amount of chloro- wax 70 will provide more insulation or foam thickness, Fig. 1 shows that this is not the case. Beyond avalue of 17%, an increase in the percentage of chlorowax 70 does not decrease the flame spread rating to any great extent.

C. Effect of film thickness The effect of increasing the number of

coats, especially in commercial terms, will be to increase the cost of protecting the product. Figure 2 shows a significant increase in flame retardancy properties with increase in the

FSR

6 0

f

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o ; ,* ' : ' ~,, Fig. 1. Flame Spread Rating, F S R, as a function of per cent. chlorowax 70. Poplar, 6 mils coating thickness.

60-

8C

10C

12(

FSR

ch lo r l n .paraf.

, ; 2b 3~ 4b ~

Fig. 2. Flame Spread Rating, F S R, as a function of coating thickness (mils). Poplar, 18% chlorowax 70.

number of coats. Clearly, thick, strongly bonded surface films give bet ter pro tec t ion to the material in a combust ion situation. How- ever, an op t imum film thickness (5 - 6 mils) exists, and beyond this any increase in thick- ness has only a relatively small effect on flame spread rating.

At tempts were made to develop a single equat ion with which all the values agreed. The most simple was

d F S R - - , - g d T ( 3 )

F S R

where K is the kinetic constant of the overall reaction, and T is the film thickness.

While our experimental values agreed with this equation in general, accuracy was not good; Table 2 shows that the value of K also alters with thickness.

TABLE 2

Variation of K with film thickness

Thickness 1 2 3 1 5 6 (rail) K 0.25 0.22 0.18 0.16 0.14 0.12

D. Physical tests and household hazard tests Several tests were under taken to determine

the effectiveness of the coatings from a marketing point of view. The tests included: solvent resistance, household hazard ( tomato sauce, coca cola, coffee), and physical tests (cross hatch, frail). The fire retardant coatings passed all these tests.

CONCLUSIONS

This paper describes an intumescent, transparent, fire retardant coating compo- sed essentially of three components : cellulose acetate butyrate , chlorowax 70 as chlorinated paraffin, and a modified form of HMDI. The coating has improved fire retardancy, colour stability, water and solvent resistance, hardness, and general characteristics desirable for panel or furniture coatings.

When exposed to a flame, the coating underwent a chemical reaction with expan- sion and decomposit ion, foaming and frothing taking place to form a porous, solid, crusty, protective coating completely covering the surface of the substrate article.

The effect of several parameters has been studied: the type of wood used, the per cent. chlorowax 70 as chlorinated paraffin, and the coating thickness. Beyond a value of 17% an increase in the amount of chlorowax 70 does no t decrease the flame spread rating to any great extent. An opt imum film thickness (5 - 6 mils) was found, beyond which any increase had only a relatively small effect on flame spread rating.

REFERENCES

1 V. M. Bhatnagar, Advances in Fire Retardants, Technomic Publishing Co, 1973.

2 V. M. Bhatnagar, Thesis, Saint-Etienne, 1980. 3 G. Quelle, Brit. Pat. 862,569, March, 1961. 4 R. E. Ellis, U. S. Pat., 3,102,821, Sept. 1963.

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5 Neville Chemical Co, Pittsburgh, U.S.A. 6 C. C. Clark, U. S. Pat., 3,365,420, Jan. 1962. 7 F. H. Thomas, U. S. Pat., 3,422,046, Jan. 1969. 8 S. C. Juneja, Canad. Pat. 907233, Aug. 1972. 9 M. M. Levy, J. Cell. Plast., Apr. (1967).

10 H. Miller, Mater. Res. Stand., (1969) n 8, 9. 11 H. L. Vandersall, Wayne State Univ. Polym.

Conf., 1966.