and in these areas the smaller particles have packed themselves hexagonally. This photograph illustrates among other things the undesirability of having present in a latex a few particles appreciably larger in siae than the average. An examination shows that the efficiency of packing is disrupted for a large area surrounding each large particle. CONCLUSION
This is not an attempt to give a detailed mechanism, as too much about it is not yet understood. Pertinent questions, such
as how capillarities are maintained for the exudation of fluid, still exist. It is believed, however, that the description given here will provide a skeleton outline that will assist in obtaining better understanding of the mechanism of film formation. LITERATURE C I T E D
(1) Dow Chemical Co., Midland, Mich., Tech. B$1. “Dom Latex 762-K for Paint Use,” 1951. (2) Von Fisoher, W., “Paint and Varnish Technology,” New York, Reinhold Publishing Corp , 1948.
RECEIVED for rewew October 17, 1952.
-4CCEPThD December
16, 1952.
Effect of Unhydrolyzed Soybean Protein I n many cases latex paints lose viscosity during storage. These investigations were planned to study the effects of certain handling variables in protein usage on the stability of latex paints. Concentration of ammonia in the dispersion, the temperature used in preparing the dispersion, the protein concentration in the dispersion, and the order or manner of mixing have no effect on the viscosity stabil-
ity. Stable paints may be made using up to about 2% protein. Ammonia is the most satisfactory dispersant of several tried. Latex paints with viscosities over the entire desirable range were made with small amounts of protein. While the actual starting viscosities could be changed, depending upon some of the factors discussed, the stability of the finished paints was not impaired.
DEAN A . BIXLER Buckeye Cotton Oil Co., Cincinnati 17, Ohio
x
I
K T H E manufacture of latex paints the problem of viscosity
stability is widespread and critical. I t s study is complicated by the fact that every component in the system has a bearing on it. The author has taken one component and studied its varied application in relation to viscosity stability i n a single type of formulation. The work reported in this paper is restricted to information obtained from one particular soybean protein, manufactured by the Buckeye Cotton Oil Co. 1200
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5
10
15
20
MINUTES
Figure 1.
Thixotropic Build-Up of Viscosity after Stirring in Typical Latex Paint
The dispersant for the protein and the concentrations of dispersant and of protein in the paint were investigated for their effect on viscosity stability. The concentration of the protein dispersion, the temperature for dispersion, age of the dispersion, and a number of ways of incorporating the protein were also studied.
April 1953
VISCOSITY MEASUREMENT AND SIGNIFICANCE
With non-Newtonian materials, including latex paints, only the apparent viscosity a t a given rate of shea1 can be measured (4). Some examples show how this applies specifically to latex paints and why special precautions must be taken to assure that the viscosity measurements are comparable. Thixotropy. Thixotropy is defined as a reversible gel-sol transformation ( 4 ) or as the variation of viscosity with time ( 2 ) . Latex paints exhibit this property to a varying degree, and when the viscosity of a latex paint is measured, the amount of stirring and the time after stirring before measurement should be considered. Figure 1 is a typical plot of the change in viscosity of a latex paint soon after stirring. Anomalous Viscosity. Anomalous viscosity (8j or pseudoplasticity (4)is defined as a lower apparent viscosity at increasing rates of shear. This property, responsible for smoothing out under brushing or rolling, is important to the application characteristics of a paint; and to its viscosity measurement. Figure 2 is a plot of the apparent viscosity of a latex paint from measurements a t different r.p.m. on a Brookfield LVF viscometer. An instrument such as the Hercules high shear viscometer will plot such a figure or rheogram directly. Temperature Coefficient of Viscosity. Temperature control for viscosity measurements is, of course, understood. Viscosity measurements reported here were made with the Brookfield instrument at 60 r.p.m. and 27” C. and at these conditions the temperature coefficient of some latex paint was as high as 70 centipoises per degree. GENERAL EFFECTS OF PROTEIN I N LATEX P A I N T S
The principal subject of this paper is the effect of soybean protein on the viscosity stability of latex paints; while this is a primary function of protein, some other effects are important. Protein thickeners €or latex paints are generally believed to
INDUSTRIAL AND ENGINEERING CHEMISTRY
739
give better brushing and leveling characteristics than nonprotein thickeners. This property is related to the rheological behavior of the paint, but while there is as yet no generally accepted method of mathematically correlating them, some progress has been reported (1). Obvious defects in viscosities much too high or low are easily seen and poor aging stability may cause the viscosity to change out of a usable range.
5000
of specific ingredients does not constitute a recommendation of these over other available materials. It is merely a record of exactly horn- the experiments were carried out. The experimental paints were all formulated so that the latex, pigment, and protein solids constituted 50% of the weight of the paint and the latex solids-pigment weight ratio v a s constant a t 32.5 to 67.5. Xhen the protein content was increased, it replaced pigment and latex in this ratio (Table I). Add the protein to the water with stirring, mix for 5 minutes, add the ammonia, and stir for an hour a t room temperature. Add the dispersant and ammonia to the water in a pony mixer followed by the pigments and premix for half an hour. Tranefer to a ball mill and grind for 4 hours. Mix the paint by adding the protein dispersion to the pigment dispersion, followed by the preservative solution and the antifoam. hText mix in the latex emulsion, followed by the water. Mix the entire paint, screen, and mix again before packaging.
c
The paint is:
1000
t I
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-7 I
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30
60
~
BROOKFIELD RPM
Figure 2.
Viscosity Measurements. Viscosity measurements were made immediately after mixing and again in a few days, at which time it was assumed that equilibrium had been attained. A typical pattern is seen in Figure 3. The eauilibrium value was taken to base aging values on, as it is much more reproducible than the initial. Paint samples were aged a t laboratory temperature and 50".
Anomalous Viscosity of Typical Latex Paint
The adhesion of straight latex paints and their hardness are generally increased with increasing protein. An increase in the protein portion of a paint at the expense of pigment hnd latex will give a slight increase in hiding power.
Table I.
Base Formula
Paint Protein dispersion, 1870, grams Pigment dispersion, 76770, grams Latex 48%LLgrams Preservative', 36% aq. s o h . *, ml ilntifoam c , grams Water, ml. Protein Dispersion Unhydrolyzed soybean proteind, grams Water, grams Concd. aqueous ammonia, grams Pigment Dispersion Water, grams Dispersante, grains Concd. aqueous ammonia, grams Titanium dioxide/, grams Lithoponeo, grams Aluminum silicateh, grams a
50.0 262.0 196.5 10.0 3.0 75.0 18.0 80.4 1.6
250.0 7.4 2.1 515.0 150.0 75.5
Dow Latex 762K Dow Chemical Co.
* Dowicides A 8: G', 1: 1, Dow Chemical Co. Defoamer E D El Dorado Oil Co.
d Buckeye prote'in, Buckeye Cotton Oil Co.
e R 8: R 551 Ross a n d Rowe Co. f Unitane OR540, Calco Chem. Div., American Cyanamid Co. Q
Albalith 11, New Jersey Zinc Co.
h ASP 400, Edgar Brothers Co.
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14 DAYS
Figure 3.
AFTER
1
MIXING
Early Viscosity History of Typical Latex Paint
Latex solids-pigment ratios of 30 to 70 or higher are required for good water resistance (in the particular formulation used). With sufficient latex the water resistance is not affected by the protein concentration. EXPERIMENTAL
Preparation of Latex Paints. The information in this paper was obtained from paints made from a formulation adapted from one recommended by a latex manufacturer ( 3 ) . The mention
740
Protein Dispersant. Any of a wide range of substances, chiefly alkalies, may be used to disperse soybean protein, but from B practical standpoint some are more usable than others. Protein dispersions made using six of these 1%-erecompared as to their effectiveness in maintaining paint viscosity (Table 11). Paints were made using fresh and day-old dispersions and dispersions made at different temperatures. After aging a t room and elevated temperatures, comparisons were made of the apparent viscosities and the average per cent loss was calculated, including all experiments. Sodium hydroxide, ammonia, and triethanolamine dispersions gave the most stable paints. blorpholine, borax, and sodium carbonate gave poorer stability in that order. There is a general belief that sodium sake should be avoided wherever possible because they can cause effloresence in the dry
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 4
paint film. Ammonia is generally recommended by latex manufacturers and seems to be entirely satisfactory here. Ammonia Concentration. Because protein is believed t o be mainly responsible for viscosity control of the paint, it may be valuable to observe how protein solutions age as a function of I
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I20
“t 0‘
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. . ’
.
Viscosity of Preserved Protein Solutions during Aging
ammonia concentration. The aging of preserved protein solutions is seen in Figure 4. It is not possible t o separate the effects of the preservative, as a preservative is essential and it most certainly reacts with the protein. These solutions were 15% protein preserved with 1% of the total as a sodium 2,4,5trichlorophenate. This amount of preservative was not completely successful, but a t this protein concentration any more phenolate would have caused gelation in the higher pH samples. Very similar patterns were obtained with 12 and 18% protein solutions. The ammonia concentration is based on the protein content, as is common with this type of dispersion, and parts of concentrated aqueous ammonia per hundred parts of protein by weight is referred t o as per cent ammonia on protein. The protein dispersions show the most stable viscosities a t from about 6 to 10% ammonia on protein. Groups of paints were made from protein dispersions having ammonia concentrations of from 6 t o 16%. There was no significant difference in viscosity stability up to 14% ammonia, and 1500 -
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Table 11.
Protein Dispersions %
% AMMONIA ON PROTEIN
Figure 4.
a t 16% the stability was very slightly less. This is shown in Figure 5. It was demonstrated in other instances that a large excess of alkali used t o disperse the protein would result in gradual loss of paint viscosity on aging. Therefore, about 9% ammonia was used t o disperse the protein for most experiments. Concentration of Protein in Paint. Increasing the protein in a given paint formulation will increase the initial viscosity. However, upon aging, paints with more protein will lose more body (Figure 6). This has occurred consistently throughout the research. It occurred qualitatively this way a t other levels of initial viscosity obtained by different solids content, different latex-pigment ratios, and different pigment compositions. Because the desirable range of viscosity for latex paints is within that produced by 2% protein in normal formulations, any higher protein concentration is not usually of concern. Age of Protein Dispersion. Protein dispersions which have been kept unpreserved for a day or more do not make as stable paints as when used fresh. Paints made from protein dispersions stirred up to 8 hours did not show any’significant difference. For maximum stability of the paints the protein dispersion should be used or preserved immediately after making. A portion (about a fourth) of the preservative solution used in the paint would be satisfactory for this preservation.
I
Dispersant Triethanolamine Sodium hydroxide Conod. aqueous ammonia Morpholine Borax Sodium carbonate
Dispersion PH 7.2 7.3 9.5 8.6 7.2 9.1
0 :
Protein 10.0 2.5 10.0 10.0 12.0 10.0
Age of Pigment Dispersion. Paints made from pigment dispersions that were several weeks old demonstrated very poor viscosity stability. In the lecithin-dispersed pigments there is organic material present for microorganisms t o attack, and even in polyphosphate-type dispersions there is enough organic dirt t o allow many types of microorganisms t o develop. As in the case of the protein, pigment dispersions should be preserved if stored. Concentration of Protein Dispersion. Paints of the same final composition were made from protein dispersions of 9, 12, 18, and 24% protein. The viscosity stability was not affected by the concentration of the protein dispersion. This will enable its use a t any practical concentration in order t o conform with the requirements of the other ingredients. Results of such an experiment are seen in Figure 7.
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EQUILIBRIUM
> t v)
sv,
-
500-
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looor
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1.5
1.0
I
2.0
% PROTEIN
Figure 6.
April 1953
Effect of Protein Concentration in Paint on Stability
INDUSTRIAL AND ENGINEERING CHEMISTRY
74 1
Temperature of Protein Dispersion. Protein dispersions were heated for 0.5 hour at 25", 37", 50", and BO", and after cooling were made into paints. The paints from the dispersion heated to 60" had somewhat higher base viscosities. The stability was not significantly different in any case. Dispersions are generally more easily and quickly prepared a t temperatures of 25' to 40' than at more extreme conditions. The results of these esperiments are summarized in Figure 8. I
I
amounts of various preservatives were being determined. The preservative used here has been successful in laboratory experiments. At least as important as the preservative is the practice of cleanliness whenever aqueous dispersions are handled. This should be especially emphasized for paint manufacturers unused to handling materials subject t o putrefaction. Tanks and lines should be emptied and cleaned after each use, and wherever possible, equipment should be designed to eliminate traps and pockets where inoculum for later batches may build up. Seeding from even small spots like this can easily overpower otherwise adequate preservation. 1500
I MONTHS
2
3
AGING
Figure 7. Effect of Protein Concentration in Dispersion on Paint Stability
Method of Incorporating Protein. The general method used here-to mix a protein dispersion with a milled pigment dispersion followed by the other ingredients-is not the only way in which protein may be successfully incorporated into the paint. Paints have been made here in a number of ways, some of which are listed, and in no case was a change in mixing procedure or method responsible for a change in stability.
MIXINGAND MANUFACTURIKG PROCEDTRES. Protein dispersion mixed with pigment dispersion followed by other ingredients. Protein dispersion, latex, preservative, antifoam, and water mixed and added to pigment dispersion. Protein dispersion added to a mixture of all other ingredients. Pigment dispersed in protein solution in pony mixer, followed by other ingredients. ,411 ingredients except latex added to ball mill and ground together. Pigment dispersions with or without protein made in a kneadertype mixer and added to latex in a change can-type mixer. SPOILAGE A S A N AGING DIFFICULTY
Spoilage, a very obvious reason for stability difficulties often associated with protein, has been left out of the experimental portion because it can be prevented. Hundreds of experimental batches of paint have been made in the laboratory and virtually the only spoilage has been IT ith paints where minimum effective
742
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3
MONTHS
Figure 8.
AGING
Effect of Temperature Used t o Disperse Protein on Paint Stability CONCLUSION
The work reported here is a start in the systematic study of aging characteristics of latex paints. Obviously, all possible variations of all possible factors could never be worked out. Much work has been done here also with different latices, other variations in formulations. The qualitative results have been consistent throughout all the paints. LITERATURE CITED
(1) Asbeck, W. K., Lauderman, D. D., and Van Loo, M , , J . Colloid Sci., 7, 306 (1952). ( 2 ) Chadwiok, E., Ofic. Dig. Paint & Varnish Production Ctubs, KO. 321, 636 (1961). (3) Dow Chemical Co., Dow Coatings Technical Service Bulletin, Dow 762K Latex. (4) Smith, J. W., and Applegate, P. D., Papel. Trade J . , 126, 60 (1948). RECEIVED for review November 8 , 1952.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTED January 30, 1953.
Vol. 45, No. 4
