I d
LL
I
2
4
6
8
10
12
14
16
TI/: Figure 3
is changing rapidly-that is, i n the region of the second order transition temperature. In view of the complexity of these changes, the effects of temperature have not been studied in detail as yet. I n general, polymers of higher modulus whiten more slowly, which is in accord with the mechanism proposed. Exact comparisons are not allowable because of wide variations in emulsifiers and the permeability of the various polymers. For certain copoIymer systems, water resistance changes markedly in the narrow concentration region where the modulus changes rapidly and is little affected by equivalent alterations in composition where the modulus is essentially unaffected. This effect would seem t o dissociate the changes which can be attributed to hardness and those attributable t o the change in monomer composition. These effects are being investigated further. Films of increasing thickness whiten more slowly, at least
initially. Increasing film thickness is equivalent t o increasing polymer modulus, since it opposes expansion of a water cell near the polymer surface. This effect is illustrated in Figure 4, where the intial rate of whitening of Rhoplex WN-80 films are plotted against film thickness. Values presented are averages of two or more determinations on films of the same thickness. Similar effects have been demonstrated for the rate of swelling of a polymer in solvent. Although t h e rate of water absorption of Rhoplex WN-80 could be accounted for by either capillary penetration or difI 5 IO 15 20 25 30 f u s i o n , b o t h of FILM T”Ess MLS which bear the Figure 4 same dependence on t h e aauare root of time, capillary penetration is clearly unimportant. This is best proved by rates of absorption of water by films immersed in salt solutions. A Rhoplex WN-80 film does not swell or whiten when exposed t o a saturated sodium chloride solution (75% relative humidity). LITERATURE CITED
(1) Barrer, R. M., and Ibbitson, D. A,, Trans. Faraday Soc., 40, 206 (1 944). (2) Lowry, H. H., and Kohman, G. T., J. Phys. Chem., 31,23 (1927). (3) Taylor, R. L., Herrman, D. B., and Kemp, A. R., IND. ENG. CHEM.,28,1255 (1936). RECEIYBD for review OCTOBER 17, lS52
ACCEPTED January 29, 1953.
F. A. DICIOIA A N D R. E. NELSON General Latex & Chemical Corp., Cambridge, Mass.
-
Aqueous latex emulsions are frequently irreversibly coagulated by freezing. This paper reports a study of freeze-thaw properties of several polymers being used or proposed for use in latex paints. The effects of particle size, emulsifying agents used in the latex polymerization, and variations in monomers utilized are reported. A
latex which in itself has excellent freeze-thaw properties may be altered in this respect by compounding for paint. The effect of varying the freeze-thaw method has also been studied and observations are reported. Some suggestions are given regarding compounding t o obtain good freezethaw in a latex paint.
A
be functions of the protective colloids on the particle. I n certain instances, advantage has been taken of the phenomenon of coagulation of latices by freezing. I. G. Farben was the first to try this method. Patents were issued t o Lecher et al. (9) for coagulation of isoprene latex by freezing. Konrad (8) described coagulation of a polyisoprene or polybutadiene sodium soap-emulsified latex by freezing at 15 ’ to -20’ C. Neoprene is normally produced in the solid form b y a freezing technique (%, 5 ) . Synthetic latices have been microflocculated by freezing as part of a procedure for solids concentration (11,18).
QUEOUS latex emulsions are frequently irreversibly coagu-
lated by freezing (10). Normal natural Hevea latex is very difficult t o freeze, b u t it too can be irreversibly coagulated by freezing, especially after being compounded. A concentrated latex made from a freeze-resistant normal latex, by a method t h a t involves removal of natural protective colloids-Le., by centrifuging-becomes very much less resistant t o freezing. On the other hand, a normal latex concentrated b y evaporation retains its stability against low temperature. The properties governing low temperature resistance may therefore be said to April 1953
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
-
745
The paint industry wants a latex that will be freeze-resistant. Latices for the paint industry are generally synthetic, containing a predetermined type and amount of protective colloid. This paper reports a study of freeze-thaw stability of some 17 polymer latices, including some being used or proposed for use in paint. It does not describe methods of producing a freeze-thaw-resista n t latex; this study merely presents findings on freeze-thaw properties of the particular latices examined. The freeze-thaw test is primarily a stability measurement on a latex or compound, not unlike the mechanical stability test. Freeze-thaw has been a controversial subject ever since the acceptance of latex for paints by the industry. Conflicting reports have been made on the freeze-thaw resistance of specific latices by different paint formulators. It has been suggested that the freeze-tham properties of latex are functions of the tack of the polymer, the butadiene-styrene ratio, the particle size, etc. A search of the literature reveals that very little has been published on this phenomenon. In this study the authors have tried to clarify these reports and t o reach conclusions regarding particle size, emulsifving agents used in latex polymerization, variations in monomers utilized, the effect of various freeze-thaw methods, and the effect of compounding the latex paint on the ultimate freezethaw pioperties (1, 3,4,6. '7, I S ) .
Effect of Varying Testing Method on FreezeThaw Stability of Latices
Table 11.
[Freeze method (one cycle) ] Paint Latex Base A B C D E
F
Genera Latex OK Coag. Coag. Coag.
OK
Coag.
Dow Amei,ican Chemical Polymer OK OK Coag. Coag. Coag. Coag. Coag. Coag. OK OK Coag. Coag.
Table 111. Effect of Varying Freeze Time or Volume on Freeze-Thaw Stability of Latex Paint at -10" C. Fieeze Time, Hours
Volume, Ounces
Because no standard method has been accepted for determining freeze-thaw stability, a series of tests \T as made on several commercial latices now in use in latex paints, in M hich the freezethaw stability was determined bv several different methods as suggested by latex suppliers. Table I shows six of the suggested methods, indicating test freezing temperatures from - 12' C. ( + 10" F.) t o -25" C. ( - 1 2 " F.), time of freezing from 15 to 24 hours, and sample size from 2 to 6 ounces, as well as var) ing thaw times before examination of the sample. The containers are generally larger than the sample and the nature of the container, whether glass or metal, does not seem important Undoubtedly, paint formulators are using tests which fall n ithin these limits, probably as well as larger scale "practical" tests of some manufacturers.
2
6
4
6
S
2 6
Method General Latex Dow Chemical
Freeze-Thaw Testing Methods
Freeae Temp., E C.
Freeze Time, Hours
Approx. Volume ContainerFrozen, Vol.. Type Ounces ounces 2 4 Glass
-18
24
- 18
16
6
8
American Polymer 1Ionsanto
- 12
16
4
4
18
6
8
Dewev8 Almy
-20 & 2
16 & 1
2
4
Goodyear
-23
16 t o 17
5
8
-19
; r
2
= 2
Glass or lined can Glass Glass or can Glass Glass or can
Thaw Time,
16
2
2 6 2
Size of Glass Jar, Ounces 8 4 8 4
8 4 8
4
Freeze-Thaw OK 0K 0 I< OK
OK
Coag. Coag. Coag.
In Table IT' the time was held fixed a t a Ion period, normally considered safe. and two temperatures were employed against the extreme sample sizes Again the smaller sample was in failure a t the critical temperature. This indicates that louerpd tempeiature has a direct bearing on the ability of the paint to pass the test.
Table IV. Effect of Varying Freeze Temperature or Volume on Freeze-Thaw Stability of Latex Paint Freeze
Hours
Temperature, C.
4 , room temp.
7 , room temp.
- 10
8, 25' C.
- 18
(4-hour freeze time) Size of T'oliunie. Glass J a r . Ounces Ounces 6 8 2 4 6 2
Freeze-Thaw OK
S
0 I< 0K
4
Coag.
8, room
teiiip.
~
0 . 5 . room
temp. 2, 60' C. 2 t o 4 , room temp.
Six polybutadiene-styrene copolymer latices, coded A to F, tested according to each of the above procedures gave freezethaw tests as shon-n in Table 11. The results show that a latex by itself will pass or fail uniformly under the conditions of all except the Goodyear test. Only one sample passed by all methods. Four commercial latex paints, purchased on the open market, were put to the same tests; three coagulated and one thawed out satisfactorily in one cycle. In an endeavor to ascertain what particular feature of the tests might be the limiting factor, and with the hope of setting up conditions that would more accurately define the freeze resistance of a latex paint, other than "coagulated" or "OK," one
746
OK
Coag.
Dewey & Almy Goodyear OK OR Coag. Coag. Coag. Coag. Coag. Coag. OK Coag. Coag. Coag.
of the freeze-sensitive paints was subjected to each of the variables in turn. Table 111 indicates the results when temperature was fixed a t - 10' C. and time and sample size were varied. The authors find that time a t the test temperature has a direct relationship to the ability of the paint to regain its original condition. A t the critical time period it is evident that a small sample is more sensitive to the effect of low temperature than a larger sample.
FREEZE-T HA W METHODS
Table I .
Nonsanto OK Coag. Coag. Coag.
These tests were also carried out varying the container. No different results were obtained whether glass jars, resin-lined cans, or terne-plated cans were used in the test. Slow freezing os. fast freezing was tried also. In slow freezing the sample mas exposed for 3 hours a t each decrement of 10" C. until -18" C . was reached and then held for the test period. Fast freezing as b y immediate exposure to the - 18' C. temperature. The reactions of slow freezing, however, have not as yet been completely explored. There is some evidence that a paint sample held just a t or below its freezing point is more damagingly affected by slow ice formation. I t may 1% ell be that in some of these tests, equilibrium was not established and the larger samples did not attain the refrigerator temperature in the shorter time tests. Nevertheless, the above observations indicate that i t is not sufficient to state t h a t a latex or latex paint passes or does n o t pass the freeze-thaw test. A4more accurate definition of the condition of the test is neces-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 4
,
sary. It is hoped that ultimately a standard test will be accepted that will not be too harsh or too easy, b u t will define a fair average factor of stability. EFFECT OF VARYING TEST CONDITIONS
,
.
Effect of Monomer Ratio. Trade comments have been made concerning the reported freeze resistance of the so-called “dry” polymers, because such polymers did not adhere t o themselves. T o study the polymer effect on freezing, 13 commercial latices were cBosen with butadiene-styrene ratios ranging from 100% polybutadiene t o 100% polystyrene. T h e results, using the General Latex test method, are shown in Table V. There is no trend due to the softness or hardness of the copolymer; indeed, within any monomer ratio range there appear t o be random results.
Table V. Latex
Effect of Monomer Ratio on Freeze-Thaw Stability (General latex freeze-thaw method) Butadiene-Styrene Ratio
Freeze-Thaw Coag.
OK
0K Coag. OK Coag. OK
9 18 12
OK
13
. .
COMPOUNDING OF FREEZE-THAW LATEX PAINTS
Coag. Coag. Coag.
11
Effect of Emulsifier Used in Latex Polymerization. What then might account for freeze-thaw resistance of latex? If not inherent in the polymer itself, it must be in the protective colloid. Seventeen polymers, including many in Table V, were studied further and a correlation was made with reference to the type of emulsifying agent used. The results shown in Table VI indicate that the use of rosin soap as latex emulsifier results in poor freeze resistance on all samples tested. Fatty acid soap-emulsified latices generally give good freeze-thaw stability. Nonionic emulsifiers appear to give poor freeze-thaw stability. The choice of emulsifier thus appears t o be the essence of the freeze-thaw problem with latex and probably in the further compounding of the latex into a paint.
Table VI. Latex Polymer A B C D E F
G
n I
J K L M N 0
:
Effect of Emulsifier Used in Latex Polymerization on Freeze-Thaw Stability (General latex freeze-thaw method) FreezeEmulsifier Thaw Anionic (rosin) Coan. Anionic (rosin) Coa;. Anionic (rosin) Coag. Anionic (rosin) COEg. Anionic rosin) Coag. Anionic [rosin) Coag. Anionic mixed-rosin and f a t t y acid Coag. Anionic mixed-rosin and fatty acid OK Anionio mixed-rosin and fatty acid OK OK Anionic mixed-rosin and fatty acid OK Anionic mixed-rosin and fatty acid Anionic (undisclosed) OK Anionic (undisclosed) Coae. Anionic/nonionic (undisclosed) OK Anionic/nonionic (undisclosed) OK Nonionic (undisclosed) Coag. Nonionic (undisclosed) Coag.
Surface Tension 66 58 44 40
38 33
56
50 54 54
38
33 66
32 56 30 33
Effect of Surface Tension. Table VI also shows figures for the surface tension of the latices tested. Surface tension is a n indication of the amount of soap used in the latex when particle size is approximately equivalent, and here again i t is the type, not the amount, which is the governing factor, provided at least the minimum necessary amount for the polymer is present. April 1953
Effect of Particle Size. Certain general tests have indicated t o the authors t h a t particle size range of the polymer has no effect on the freene-thaw stability. This belief could not be carried t o a firm conclusion because it was impossible in the time allotted t o secure a series of samples of a single polymer, with a minimum of suitable emulsifier, varying only in particle size range. However, certain people have indicated that improved freezethaw results from latices of larger particle size. It is the authors’ belief that this improved freeze-thaw was not merely a function of particle size; rather, it was a function of the amount of protective emulsifier on the particle. For example, in identical recipes in which only particle size is varied, actually the total amount of soap on each particle varies too. I n the case of emulsions of fine particle size the soap is distributed over a much larger total surface and consequently confers less protection upon the latex particle; thus, increasing the particle size has the same effect as increasing the total soap. This theory can be supported by the fact that certain latices of fine particle size, which are not freeze-thaw stable, can be made stable by increasing the total amount of soap on the individual latex particles. Effect of Viscosity on Freeze-Thaw. On suitably emulsified latices, viscosity of the latex appears to have no effect, nor was there a change in viscosity Ffter five cycles on a latex which had adequate freeze resistance, by tests shown in Table I.
A freeze-resistant latex does not necessarily compound into a freeze-thaw-resistant paint. Several of the above latices which test well by themselves produce poorly resistant paints. If a latex does not have adeqeate freeze resistance of itself, i t will be difficult t o make a paint satisfactory in this respect, without major modification during compounding. Some formulators have resorted to the addition of large amounts of emulsifiers, and/or antifreezes such as ethylene glycol. These, of course, react against washability. Different latices will react differently in a particular paint formulation as regards freeze resistance. If one latex is t o replace another in a paint, it is sometimes necessary to make certain modifications to assure maintenance of freeze-thaw stability. The paste must be formulated and dispersed or stabilized in a manner‘to be compatible with the particular latex being used for best results, as the latex is the most sensitive portion of the paint. Some of the more important factors influencing freeze-thaw stability in a latex paint have been found to be: Presence of alkyd, particularly oil-modified types. Pigment choice. p H of the system. Amount and type of protective colloid. Presence of certain antifoams in excessive amounts. I n working with latex paints one is dealing with rather complex colloidal systems. The stability of latex emulsions may be disturbed by the use of unemulsified or crudely emulsified alkyd or modified oil-alkyd phase; pigments containing water-soluble metallic multivalent ions or a low p H will similarly disturb the stability of these emulsions. These changes in stability are often accompanied by aviscosity increase, if not complete coagulation of the system. Sometimes, however, the changes in stability of the system may not be drastic enough t o show marked changes in these respects a t normal temperatures, b u t may be evidenced by a decrease in the freezethaw stability. By keeping the p H of the pigment dispersion adequate, by employing sufficient protective colloid, by using minimum amounts of defoamers, by avoiding pigments with water-soluble multivalent metallic ions, and b y using pre-emulsified oil or alkyd phase, one can usually maintain the freeze-thaw stability of almost any paint latex. If calcium pigments are used, however, a specific stabilizer for calcium ions must be used.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
747
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 175.0 175.0 178.0 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 . 0 330:O 11. DGY-159 paint latex KO. 3QO:0 330.0 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
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 4
.
