I n their laboratory the authors have found that the use of improperly emulsified alkyd is the greatest single cause of freezethaw failure in a latex paint. One way of overcoming this deficiency is by pre-emulsifying the alkyd with soap and protective colloid prior to mixing with pigment; a second and probably preferred method is the addition of a suitable alkali to the pigment-alkyd paste phase. The alkali is not effective as a freezethaw stabilizer unless added to the alkyd phase: this indicates some possible reaction with the oil itself, possibly a partial saponification of the oil to form an effective emulsifier in situ, Table VI1 shows how a commercial paint latex reacts in three typical laboratory paint formulas. Addition of alkyd (long oil linseed-modified) ruins freeze-thaw, and the addition of an alkali t o the alkyd phase restores the freezethaw stability. ~
Table VII.
~
Effect of Alkyd on Freeze-Thaw Stability of Laiex Paint
Paint Paint Paint Wet Parts Form 1 F o r m 2 Form3 1. 10% ammoniated casein 90.0 90.0 90.0 2. Water 100.0 100.0 100.0 3. Lithopone 60.0 50.0 50.0 4. Clay 75.0 75.0 75.0 5 . Mica 50.0 50.0 50.0 6. Titanium dioxide 178.0 175.0 175.0 7. Alkyd. 15.0 15.0 ... 8. Driers 0.12 0.12 , .. 9. Ammonia (26’ B&) 1.0 1.0 1.0 10. Alkali (100%) Approx. 6 . 0 3QO:0 11. DGY-159 paint latex KO. 330:O 330.0 Freeze-thaw stability OK 0 I< Coag. a 28% phthalic anhydride drying oil-modified alkyd (100% solids, supplier, C. J. Osborn). b General Latex & Chemical Corp. il
CONCLUSIONS
For the best freeze-thaw resistance in a paint one may choose any polymer that confers the desired film properties, but i t should have been emulsified on a soap type that provides freeze resistance. The paint paste should be formulated to be compatible with the latex. A completely compatible latex paint should freezethaw several cycles a t least without radical change in
viscosity properties, when tested by any of the usual freezethaw methods. I n a practical way, however, the freeze-thaw test should be used more as a means of determining whether or not a compatible system has been established in the paint, or to establish minimum standards, than as a plan to confer an absolute property to the paint. These results may enable one to extrapolate the premises that a gallon can of paint would outperform the laboratory samples in freeze-thaw tests, that the ordinary times for shipment of paints would exceed the laboratory control periods of test and so be more dangerous to the paint, and finally that the variations of temperature and the ultimate lowest temperatures to which a paint may be subject during transit again render the test something less than positive. If the paint industry will label latex paint cans ‘Xeep from Freezing” and endeavor to ship and store under safe conditions, the freeze-thaw test will provide one type of measurement of stability. LITERATURE CITED
American Polymer Corp., private communication, July 2, 1952. Calcott, W. S., and Starkweather, H. W., E.S. Patent 2,187,846
(1940). Dewey & Almy Chemical Co., private communication, July 7, 1952. Dow Chemical Co , private communication, July 2 , 1952. Du Pont de ITemours & Co., Inc., E. I., Brit. Patent 504,446 (1939). Goodyear Tire & Rubber Co., private communication, July 23, 1952. Ibid., August 7 , 1952. Konrad, E., et al., U. 8 . Patent 1,851,546 (1932); Brit. Patent 311,381 (1929). Lecher, H., et aE., U. 8. Patent 1,851,104(1932). Marchionna, F., “Rutalastic Polymers,” p. 405, New York, Reinhold Publishing Corp., 1946. Maron, S. H., and Moore, C., India Rubber V o r l d , 116, Iio. 6, 789 (1947). Maron, S. H., Moore, C., Kingaton, J. G., Ulevitoh, I. N., Trinastic. J. C..and Bornernan. E. H.. IND.ENG.CHEM .. 41.. 156 (1949). Moiisanto Chemical Co., private communication, July 15, 1952, RECEIVED for review October 8, 1952.
ACCEPTED February 4, 1953.
E. K . STILBERT AND I . J . CUMMINGS Plastics Department, Coatings Technical Service, The Dow Chemical Co., Midland, Mich.
A
I
N INTUMESCENT coating is ” a type of fire-retardant coating n-hich has the special property of forming an insulating foam when exposed t o high temperature or direct flame. One such type of fire-retardant coating is reported by Kornerup and Lassen (9) t o have been developed prior to World War I1 in Germany. Chemical Industries reported in 1945 (3) that this type of coating material was developed in the United States during the early part of World War I1 by Grinnell Jones and Walter Juda. Fire-retardant coatings have been in existence for many years. A recent search of the patent literature has revealed over 100 United States patents relating to fire-retardant coatings. In the past, fire-retardant coating formulators have developed the following basic systems: 748
High Digment-volume concentration utilizing inert, inorganic pignient types. Chlorinated resinous-type vehicle. Combinations of chlorinated resin vehicle and amphoteric metal oxide. Fusible, inorganic ingredients such as sodium borate, magnesium sulfate, and sodium and potassium silicates. Compounds which decompose a t elevated temperature t o give off water vapor, carbon dioxide, chlorine, ammonia, and the like. These systems include a rather broad field of work and relate to a multitude of different formulations. The work described herein, however, is limited to a single type of fire-retardant coating which is designated as intumescent coating. In March 1945 Chemical Industries reported that a commercial
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intumescent paint, Albi R, retarded the spread of flame 60 t o 70% a s compared t o untreated wood, in accordance with tests made by the Underwriters Laboratories (3). I n another brief comment on this type of paint ( a ) a coating material is described as a powder thinned with water t o resemble a clear varnish in appearance. For color and protection, the report suggests t h a t a coating of conventional paint be applied over the fire-retardant coating. The only source of information on the composition of this type of coating t o date has been the patent literature ( 7 , 8 , 1 0 , 11). Intumescent coatings are of some interest for protecting metal surfaces, as the insulating effect of the foam greatly reduces the temperature on the metal side opposite the source of heat. However, such coatings are of most interest for protecting combustible surfaces such as wood and fiber insulating board. On these surfaces, the insulating effect of the carbonaceous foam developed by the heat or flame prevents burning of the coated surface even when exposed t o direct flames from a blow torch for many minutes. Intumescent coating formulations may be developed either with an organic solvent or with water as the diluent. Use of organic solvents permits formulation of a one-package, ready-toapply paint. I n this type of coating, the water-sensitive ingredients are maintained in a n inactive state. Standard paint vehicles such as alkyd resins or chlorinated resins are generally used as the binder. To date, no information has been published on the formulation of this type of coating which provides intumescence as well as protection in the normal paint sense. To date, no formulation has been developed with water as the diluent t h a t will cure t o a scrub-resistant film when dried a t room temperature. However, many aqueous intumescent coating formulations contain a resin binder, which is acid curing, and an acid fire-resistant salt. Such coatings must be used within a short period after mixture, as the presence of the acid salt causes gelation of t h e coating due t o its catalytic effect on polymerization of theresins. The degree of polymerization is great enough t o cause the undesired viscosity increase of the coating but not great enough t o cause the coating t o set up t o a scrub-resistant film at room temperature. Important intumescent coatings were developed in the United States ( 7 , 8) and marketed as two-package materials t h a t were blended together with water t o form the paint. Because of the presence of an acid salt and an acid-curing resin binder, the mixed coating had t o be applied within a few hours after the two parts were mixed. A typical formulation of this type of intumescent coating is: Ingredients
Urea Paraformaldehyde
Parts
61.6 1.8 7.3 0.9 3.6 13.7 11.1
carbonaceous foam which is formed during burning. The monoammonium phosphate is the fire-resistant salt, which is also a source of ammonia gas and of sufficient acidity t o catalyze the polymerization of the urea and formaldehyde into a resinous binder. Many research and plant laboratories throughout the United States have worked diligently on the formulation of intumescent coatings, using water as a diluent. The water-base system is of special interest in the coating of fiber insulating board, because the manufacturers are interested in applying the coating t o t h e board during the prbcess of manufacture t o improve its properties for use in home and commercial construction. A U. S. patent (11) covers a water-base intumescent coating especially formulated for use on fiber insulation board and wallboard. Commercial development of aqueous intumescent coatings has been very slow, owing t o the poor scrub resistance and lack of flexibility of the fully cured coatings. These drawbacks might be overcome by the application of an organic solvent-base intumescent coating. This is not desirable in a manufacturing plant because of the danger of fire from t h e solvent thinner in the coating. For this reason, the primary interest continues t o lie in the development of a water-thinned intumescent coating suitable for application at the manufacturer's plant. The work herein recorded was undertaken as a result of this interest. Because the principal deficiencies of these coatings are lack of scrub resistance and poor flexibility of the cured coating, work has been limited t o the study of an auxiliary binder in the form of a synthetic latex which could be added directly t o the known type of intumescent coating formula t o develop the desired properties. The addition of the synthetic latex t o this type of intumescent coating has presented difficulties due t o the presence of a relatively large quantity of water-soluble salt and the acidic nature of the coating formula. Latexes are dispersions of colloidal polymer particles in water. They are irreversible in nature and must be handled very carefully t o avoid upsetting the colloidal equilibrium. The tolerance of a synthetic latex t o changes of p H and t h e addition of water-soluble ingredients depends on the procedure used t o manufacture the latex and the type of emulsifying agents and monomers which comprise the latex solids. Most commercially available synthetic latexes are anionic. They are sensitive t o divalent and trivalent cations such as calcium, magnesium, and aluminum, and somewhat sensitive t o low pH. The degree of sensitivity t o metal ions and t o p H varies with each latex. For this reason t h e use of a synthetic latex in the known type of intumescent coating presents a definite formulation problem. TYPE OF LATEX
I n this system, the urea and paraformaldehyde act not only as the sources of ammonia and carbon but also as the binder for the system, The starch contributes coherence and fine texture t o the
Before the detailed evaluation of properties, a number of types of synthetic latexes were added t o the following typical intumescent coating formula t o determine which ones were stable t o the low pH and high salt concentration:
An intumescent fire-retardant coating composition is described, which has the inherent disadvantages of lack of wet scrub resistance and excessive brittleness. The degree of intumescence and wet scrub resistance of the oven-dried coating film were studied. The coating film was modified by the addition of synthetic latexes. Out of ten types of synthetic latexes evaluated, a vinyl chloridevinylidene chloride latex, Dow Latex 744-B, was the most satisfactory latex modifier. Addition of minor amounts of the plasticized latex to the intumescent coating composition markedly improved flexibility and wet scrub resistance with a minor loss in degree of intumescence. The compositions discussed did not have sufficient wet
stability for consideration of one-package systems. Relatively short drying times at elevated temperatures were utilized t o indicate the feasibility of the latex-modified intumescent coating compositions for factory applications. Low density fiber insulating board was utilized for coating applications of the latex-modified intumescent compositions. The inclusion of a plasticizer with the latex is necessary t o attain maximum latex film properties. Useful pigmented coating compositions, however, were obtained over a range of 0 to 20% plasticizer based on latex solids. Suggested optimum coating weights as well as optimum coating compositions were proposed for mill application t o low density fiber insulating board.
April 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
749
Parts by Weight 47.0
Ingiedients
1.5 5.6 15.0 3 9 7 8 10.4 8.5
Lithopone Urea Paraformaldehyde Water Synthetic latex solids
60.0 15.0
Table I lists the test results obtaiiied 11hen ten different types of latexes were added to the above formula. Only lour types of the latexes used were stable; however, stability might be built into all these latex types if sufficient research work were done t o tailor them specifically for this use. Because standard commercial products were used in most instances, the four stable types possessed the required stability as basic properties without modification.
during room temperature drying Table I1 lists typical properties of the latex. Table 111 lists tipical properties of the latex solids. The latex has a relatively low surface tension. K h e n dried, a nonflammable, high-melting, white powder is deposited. I n order t o form a film the latex must be modified with film-forming latexes, plasticizer emulsions, or certain liquid plasticizers. The plasticizer selected for use with the latex was butyl phthalyl butyl glycollate. This material, commercially designated as Santicizer B-16, is a chemical plasticizer that can he stirred directly into the latex.
Table 11.
Typical Properties of Dow Latex 744-B
PH Surface tension. dvnes Der cm.
8.0 + 0.5 34 0-35.0 50 0 f 0 . 5 1.195 i 0 004 10 < 20 5
Table I.
-
700
f
0
v, > e
600
5000
Figure 3.
April 1953
4
8
2b
II 16 AGING T I M E (HOURS)
24
g8
500
Effect of Aging Latex-Plasticizer Mixture on Coating Properties
.Figure 5. Comparison of Properties of Intumescent Coatings on Natural and Prime Coated Insulating Board
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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KEY:
% PLASTICIZED L A T E X % PLASTICIZER ON LATEX SOLIDS
15 IO COATING WEIGHT (LBS. DRY CoAT'NG~OOO~ ~ . 2 ) - - 3 0 DRYING TEMPERATURE 300° F. (FORCED AIR L A B ORATORY OVEN)
-
BOARD SURFACE
1
Coated Panel Drying and Conditioning. For all the laboratory work a Despatch electric laboratory oven equipped with temperature controls and a blower t o maintain a uniform temperature was used. A drying schedule of 10 minutes at 70" C. followed by 5 minutes at 150" C. was adopted t o ensure uniform, complete fusion of all coatings. A standard conditioning procedure was adopted for all test panels prepared. -4fter drying, the coated panels were allowed to condition 18 to 20 hours a t 75' F. and 50% relative humidity. They were then tested for wet scrub resistance and fire-retardant properties.
In order to vie>\- the drying time more realistically for mill application of intumescent coatings, tests were made to determine the minimum conditions necessary for satisfactory results. Figure 6 shows the oven dmell time necessary t o develop in the coating a certain degree of scrub resistance for the two types of board surfaces a t one particular temperature and coating weight level. Attempts were made t o utilize heavier coating %-eights and higher temperatures, but difficulties were encountered. The equipment available in the laboratory was not designed t o remove large amounts of moisture efficiently. Because of this limitation, the information shown in Figure 6 is only an indication of what might be accomplished in the large multiple-stage drying units utilized in the insulating board industry.
2 3 DWELL TIME (MINUTES)
Figure 6.
4
Effect of Elevated Drying Temperature on Oven Dwell Time
due t o the presence of the prime coat as a major proportion of the total coating. The crossing of the curves above 30 pounds indicates that this effect is being overcome by the superior film properties of the intumescent coating. A coating weight of 40 pounds of dry coating per 1000 square feet was selected as the level at which the rest of the work was t o be carried out. The degree of difference in intumescent properties between primed and unprimed board as shown in Figure 5 mag not be significant, because the ratings are visual. Laboratory evaluation has indicated that ratings within a value of =t0.25 cannot be precisely distinguished.
Figure 8. Visual Comparison of Intumescent Ratings Test Methods. VET SCRUBRESISTANCE. \Tet scrub tests mere made on the coated fiber insulating board using the Gardner straight-line scrub tester from the Henry A. Gardner Laboratory, Inc., Bethesda 14, Md. This instrument was used in t h e procedure outlined in ( 6 , Section E-9, paragraph F-31). A 1-pound weighted brush is drawn horizontally across the specimen a t approximately 40 oscillations per minute. The 0.5% soap solution required by the specification mas used t o maintain the board in a wet condition. I n general, the coated board waa scrubbed to 10% of the board surface showing as a n end point.
Figure 7. 752
Apparatus for Inclined Panel Test
FIRETEST,Conventional test methods for measuring flame spread resistance of coatings have been described by Van Heuckeroth, Cook, and Hill ( I S ) . The tests of greatest general interest and use have been described ( 4 , 5 ) .
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These test methods are referred t o in the trade and are used as the bases for specifications covering the classification of fireresistant fiber insulating board. The results repdrted herein are based on the inclined panel test ( 4 ) , which is a modification of a British test ( 1 ) . The apparatus is pictured in Figure 7. The test requires a test specimen 12 inches square which has been conditioned a t 75" F. and 50y0 relative humidity. One cubic centimeter of absolute ethyl alcohol is pipetted into the brass cup located on the cork under the test specimen. This test is sometimes criticized as not being as severe as t h e
K fY %PLASTICIZER ON LATEX SOLIDS IO COATING WEIGHT DRY CoATING/lOOO ~ ~ 2 ) - 4 0
2500
0 SCRUB RESISTANCE 0
5
INTUMESCENCE
1
T
4 I
(3
5
5 W
IO
15
20
25
30
PERCENT PLASTICIZED LATEX
3a
Figure 10.
Effect of Latex Binder on Coating Properties
V
v)
2W
3i-
f
0
FiguPe 9.
5
IO 15 PO PERCENT PLASTICIZER
25
30
Effect of Santicizer B-16 with Latex on Coating Properties
more complicated one (6). Perhaps the most comprehensive correlation of the two procedures has been made by Van Kleeck and Martin (14), who have reported on their work at the Forest Products Laboratory, Madison, Wis. They concluded t h a t the two tests could be correlated with 80% accuracy. Panels with char area of less than 12 square inches by the inclined panel test passed the 20-minute exposure time t o flame for the slow burning classification of (6) in 34 out of 43 tests. Because of the simplicity, the relatively small test sample required, and the degree of correlation between tests, the inclined panel test ( 4 ) was adopted as standard for this work. DEGREEOF INTUNESCENCE. Intumescent coatings possess excellent resistance t o flame spread. Even coatings that intumesce only mildly will often give a char area of less than 12 square inches with the inclined panel test. For this reason the measurement of char area is not of special significance in differentiating the effectiveness of intumescent coatings. For the purposes of this study it seemed that the degree of intumescence was the important factor, as the insulating effect of the foam would depend on the thickness and texture of the insulating layer. Arbitrary ratings of from 0 t o 5 were established. The 0 rating was given t o coatings which did not intumesce, while t h e best result was rated 5 . A visual comparison of these ratings may be observed in Figure 8.
Depending upon which property is most important, it can be seen that a compromise is necessary between wet scrub resistance and degree of intumescence. For applications in the fiber insulating board industry t h e fire-retardant properties are of primary importance and improved film properties are next. The range of 5 t o 15y0 plasticizer content probably represents the most useful for the insulating board products. However, useful coatings with excellent scrub resistance can be formulated-without plasticizer when drying conditions as described in this-work are used. Effect of Latex Binder. Figure 10 indicates t h e range within which an optimum coating composition can be chosen based on plasticized latex content. As high as 17.5% latex binder may be utilized without making a significant sacrifice in intumescent quality. The optimum composition for application t o fiber insulating board would probably include 12.5 t o 15% plasticized latex solids.
INTUMESCENCE
I
1
I
I / T
1
EFFECT O F CHANGED C O N D I T I O N S ON COATING PROPERTIES
Effect of Plasticizer Content. Figure 9 shows the changes that take place in the coating film as the ratio of latex t o butyl phthalyl butyl glycollate is varied. As the per cent plasticizer increases, intumescence increases and scrub resistance rises t o a peak at 5% plasticizer, then falls off at a more or less constant rate. April 1953
Figure 11.
Effect of Coating Weight on Coating Properties
INDUSTRIAL AND ENGINEERING CHEMISTRY
153
Effect of Coating Weight. The degree of fire retardance imparted t o fiber insulating board by the latex-modified intumescent coating as measured by intumescent rating appears to be proportional to the weight of coating applied, as is shown in Figure 11. This relationship does not apply, however, t o the other property being measured, that of wet scrub resistance. Scrub resistance also increases with coating weight, but not linearly. A specific property of a coating, such as its ability to resist the spread of a flame across the surface of the coated object, is t o a large extent a function of the mass of the protective layer. I t then follows that the greater the coating weight applied the greater will be the resultant fire protection. The ability of such a protective coating to resist removal by scrubbing with a weighted brush and dailute soap solution, hoTTever, is dependent t o a greater extent on the film properties rather than the coating mass. When a coating of this type is applied to a panel of fiber insulat,ing board it is very important to have sufficient coverage to lay down the loose fibers and overcome the undesirable properties of the natural-board surface. Figure 11 shows that a t a coating weight of 20 pounds per 10'00 square feet t.he properties of the protective coat,ingfilm, as measured by a scrub test, are not realized ovving t o insufficient coverage. As the coating weight is increased and the minimum coverage point is overcome, the scrub resistance of t'he coating increases very markedly. For the particular type o f natural finish insulating board used for this series of tests the 30pound coating weight level vas about the minimum useful amount from the standpoint of both fire retardance and \vet, scrub resistance. CONCLUSIONS
The 1%-ork which has been discussed was undertaken to illustrate t h e utility of a synthetic latex as a modifier for intumescent coatings. The data presented indicat,e that the property of resistance of the dry coating t o wet abrasion can be significantly improved by the latex addition, To obtain that improvement it was necessary t o sacrifice a small degree of the intumescent quality of the coating composition. There are many problems involved in the translation t o mill practice of the coating system discussed in this work. The addition of the latex t o intumescent coatings does nothing to simplify plant application problems. Such addition, however, does not create new ones. The 1imitat.ions on application techniques imposed by the intumescent coating system are not significant'ly narrowed by the presence of t'he synt.hetic latex. The intumescent composition upon which this work is based
does not represent an ideal coating formulation. Indeed, many aspects of the preparation described would be objectionable to the mill operator and t,echnician. The handling of powdered paraformaldehyde or a formaldehyde solution is an odorous operation and would be prohibited in some plants. This objection can be overcome, hovever, by t.he use of commercially available urea-formaldehyde resins, either in the powder form or as a wat,er dispersion. Blthough the water dispersions contain free formaldehyde for stabilizing purposes, the content is usually kept to an absolute minimum by the manufacturer when the application requires it. The selection of 10% plasticizer content for the latex as being opt,imumwas based on the requirements of fiber insulating board. For this readily combustible material the requirement is for a coating which imparts maximum fire protection. For coating applications calling for fire resistance and a high degree of wet scrub resistance the useful plasticizer range may be from 10 t o 0%. Certain obvious advantages would be gained from the use of a formulation which contained no plasticizer. In addition to elimination of the operation of adding plasticizer, significant process time savings could be realized by not haviag to allow an aging period for the lates-plasticizer mixture. LITERATURE CITED
British Standard Specification 476, 193%. Chem. Eng., 56, No. 10, 156 (1949). Chem. Inda., 62, 383-4 (March 1948). Commercial Standard CS 42-49, Washington, Government Printing Office, 1949. (5) Federal Specification SS-A-11Sa, Washington, Government Printing Office, 1948. (6) Ibid., TT-P-88a. ( 7 ) Jones, Grinnell, Juda, Walter, and SOH,Samuel (to Aibi Manufacturing Co.), U. S. Patent 2,523,626 (Sept. 26, 1950). (8) Jones, Grinnell, and Soll, Samuel (to Albi Manufacturing Co.1, (1j (2) (3) (4)
Ibid., 2,452,054 (Oct. 26, 1948). (9) Kornerup, A,, and Lassen, €I., Ingeni$ren, 59, 345-6 (April 22, 1950). (IO) Lauring, E. 9.(to .Minnesota and Ontario Paper C o . ) , U. S. Patent 2,594,937 (April 29, 1952). (11) Scholz, H. A,, and Saville, E. E. (to U. 9. Gypsum Co.), I b i d . , 2,566,964 (Sept. 4, 1951). (12) Stilbert, E. K., and Clack, H. L., T a p p i , 34, SO. 8, (August 1951). (13) Van Heuckeroth, A . W., Cook, G. S.,and Hill, R. W., Armed Services Technical Information Agency, Tech. Data Digest, 15, No. 4, 27-30 (April 1 , 1950). (14) Van Kleeck, A., and Martin, T. J., Forest Produch Lab., Bull. D1756 (April 1950). RECEIVED for review October 13, 1952.
ACCEPTED February 2, 1963.
END OF SYMPOSIUM
754
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