*
.
in which the plasticizer redistributed itself in the polymer particle to give a uniform concentration throughout, as the second stage of the process. The first stage only of the aging process would be measured by sedimentation measurements in the centrifuge, such as those described in a previous section, since no free plasticizer would remain a t the end of this period. The existence of such a second period in the aging would be more difficult to demonstrate but might be important in imparting satisfactory film-forming properties to the latex. Particles with a soft shell and hard core might form films with a fused yet lumpy surface such as that shown in Figure 4. Because of the complexity of diffusion in the polymer phase no attempt has been made t o treat this problem in detail. The problem is more complex, mathematically, than diffusion in the water phase, and additional experimental data on some of the phenomena involved would be desirable in order t o know what assumptions can reasonably be made in the mathematical development and what reference cases would be most useful to treat. A pseudo-steady state for diffusion in the polymer phase represents a very special case. In general, an explicit solution of the differential equation for diffusion giving the time-dependent, transient state will be required. SUMMARY
'
.
ties of good swelling agents or solvents t o the plasticizers, which swell the polymer structure and accelerate diffusion rate. I n analyzing the plasticizing process in terms of basic theory, diffusion through the water phase can be treated as a steady-state process. Diffusion in the polymer phase, however, must in general be treated as a transient phenomenon. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7)
(8) (9) (10) (11) (12) (13) (14)
Barrer, R. M., Kolloid-Z., 120, 177 (1951). Boyer, R. F., J . A p p l . Phys., 2 0 , 5 4 0 (1949). Boyer, R. F., J . PoEymer Sci., 5 , 139 (1950); Crank, J., and Park, G. S., Ibid., 5 , 1 4 0 (1950). Bradford, E. B., J . A p p l . Phys., 23, 609 (1952). Crank, J., and Park, G. S., Trans. Faraday Soc., 4 5 , 2 4 0 (1949). Crank, J., and Robinson, C . , Proc. Roy. Soc., 204A, 549 (1951). Dillon, R. E., Matheson, L. A., and Bradford, E. B., J . Colloid Sci., 6, 108 (1951).. Doty, P., J . Chern. Phys., 1 4 , 2 4 4 (1946). Frenkel, J., J . Phys. (U.S.S.R.), 9, 385 (1943). Hartley, G. S., Trans. Faraday Soc., 42B, 6 (1946). Hartley, G. S., Ibid., 4 5 , 8 2 0 (1949). Liebhafsky, H. A., Marshall, A. L., and Verhoek, F. H., IND. ENO.. CHEM.,3 4 , 7 0 4 (1942). McLoughlin, J. R., Ph.D. thesis, Princeton University, 1951. McLoughlin, J. R., and Tobolsky, A. V., J . Polymer Sci., 7,
658 (1951). (15) Mandelkern, L., and Long, F. A., Ibid., 6 , 4 5 7 (1951). (16) Park, G . S., Trans. Faraday Soc., 4 6 , 6 8 4 (1950). (17) Ibid., 4 8 , l l (1952). ENG.CHEM.,35, 896 (1943); Reed, M. C., (18) Reed, M. C., IND. and Connor, L., Ibid., 4 0 , 1 4 1 4 (1948). (19) Robinson, C., Trans. Faraday Soc., 42B, 12 (1946). (20) Robinson, C., Proc. Roy. SOC.,204A, 339 (1950); Crank, J., and Robinson, C., Ibid., 204A, 549 (1951). (21) Small, P. A., J . SOC.Chem. Ind., 66, 17 (1947). (22) Smekal, A., Kolloid-Z., 120, 189 (1951). (23) Wheaton, R. M., and Harrington, D. F., IND. ENG.CHEM.,44, 1796 (1952). (24) Williams, R. C., and Wyckoff, R. W. G., J . A p p l . Phus., 17, 23 (1946).
Plasticizers dispersed in a synthetic latex form emulsion droplets in the latex. The plasticizer then apparently diffuses through the water phase and is absorbed by the polymer. The rate of the process is largely determined by the rate at which the plasticizer is absorbed by the polymer. Plasticizers which are readily absorbed plasticize the latex quickly and permit the formation of continuous films soon after formulation. Plasticizers which are slowly absorbed by the polymer require an aging period after formulation before comparable films can be cast. The aging period is considerably shortened by the addition of small quanti-
RECEIVPID for review October 13, 1952.
T h e detailed mechanism of the film formation of latex paint is not yet understood, but this paper presents a skeleton outline to assist in obtaining better understanding of the subject. A two-stage mechanism is proposed:
first, irreversible contact brought about by evaporation of water and, second, fusion of dispersed resin particles to form a strong continuous film. Photographs and tabulated data illustrate basis of this concept of film formation.
W . A . HENSON, D . A . TABER,
AND
ACCEPTED February 9, 1953:
E. B . BRADFORD
The Dow Chemical Co., Midland, Mich.
T
HERE is every indication that latex paints are permanently
.
established commercially. The basic phenomenon of film formation and pigment binding by latex, either natural or synthetic, is not new. However, recent developments make it clear t h a t a very wide variety of synthetic latexes will ultimately be used as binders for paint and related coating materials. It is timely, therefore, t o consider what is known about the mechanism involved in the formation of a relatively impermeable film from the discrete colloidal particles as they exist in a latex. It is the purpose of this paper to present one concept of this mechanism. A great deal more work must be done before there will be a clear understanding of this mechanism, but a systematic study of the principles involved will make possible the use of April 1953
latexes now unsuited as binders for paints and the deFign of new latexes superior t o those in use at this time. PAINT INGREDIENTS
Basically, any paint consists of one or more pigments dispersed in a fluid medium suitable for spreading on a surface and of such a viscosity as t o prevent sagging if the surface happens t o be vertical. I n the case of an oil-based paint (the word oil is here used loosely t o include drying oils, alkyd resins, varnishes, etc.) the oil performs two functions, I n t h e wet state it provides part of the fluidity of the system and hence acts as part of the vehicle. However, after the paint is applied, the oil, undergoing
INDUSTRIAL AND ENGINEERING CHEMISTRY
735
paints t o date has been based on styrene-butadiene copolymer latexes. Some of the other monomers now used for latex production and which will be included in the continuing flow of new latexes into paint development are acrylonitrile, chloroprene, ethyl acry-
In the field of organic coatings it has
Figure 1.
Hiding Power Cards
a chemical reaction, solidifies in place and acts as a pigment ander. The extent to which pigment is dispersed in the oil bimains essentially unchanged as the drying process proceeds end the oil solidifies. I n a latex paint substantially the entire vehicle is water. While this is a very prominent advantage of latex paint, it brings ua face t o face v i t h the first major difference between this paint and the oil paints. In the wet state both the pigment and the material which is later to become the binder are dispersed discretely in the volatile vehicle. I n the relatively short time required for the evaporation of water during drying, the synthetic resin particles dispersed in the latex must arrange themselves in the most efficient packing configuration and distribute themselves as efficiently as possible around the dispersed pigments. During the loss of water, packing of the particles proceeds and eventually they come into contact Kith one another. From this point onward some fairly reliable physical chemical principles can be brought to bear upon the process. * I n recent years there have become commercially available a large number of monomeric materials which when polymerized or copolymerized result in families of plastic materials of a broad variety of physical properties, The art of preparing polymers and copolymers in emulsion, while not new, has undergone its greatest progress within the past 10 or 15 years. Therefore, taking the additional step of designing latexes based on a broader variety of chemical copolymers and adapted for easy film formation should not be difficult. once the principles involved in film formation have been clarified. Most of the activity in latex
Figure 2.
736
Electron Micrographs of Contact and Distortion of Spherical Particles
toughness, and durability are improved 1% ith increase in molecular weight, making it desirable t o use coating materials of the highest molecular weight feasible for a given type of coating. -4s a corollary to this, however, high molecular weight usually has brought with it the difficulty of high viscosity and difficulty of application and hence has imposed a molecular weight barrier. A very basic advantage to be had from using organic coatings in the latex form is that the molecular weight of the synthetic resin contained in the latex bears no relation to the viscosity of the latex. The molecular weight of the resin in the latex does have a direct bearing on the ease of film formation, but here again if the mechanism of film formation is clearly understood it should be much easier t o compensate for any difficulties this might impose. A paint consists basically of a pigment dispersed in a binder. I n addition to these two basic ingredients, there are in all paints, both oil-based and latex-based, other ingredients present in minor percentages. Tables I and I1 are formulas for a latexbased paint and an oil-based paint, which have been selected t o illustrate only the presence of these additional ingredients. Each of these minor ingredients performs a certain specific function t o contribute t o improved quality or performance of the paint. In the case of the oil-based paint many years of laboratory study have been devoted to discovering the relative advantages or disadvantages involved in their use. I n the case of the latex-based paint not enough is known as yet of the effect of these minor ingredients on the character of the finished film. For the purpose of this paper, therefore, the discussion is centered around the mechanism of film formation of the binder only, with some additional information and comments on the pigment present. The formation of a film by latex paint, if knorrn in every detail, would probably be found to consist of a complicated sequence of
Figure 3.
Electron Micrographs of Contact and Distortion of Spherical Particles
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 4
Table I.
.
Flat Wall Paint (2)
Rutile titanium calcium pigment Lithopone Calcium carbonate Magnesium silicate Aluminum stearate gel Bleached linseed oil Blown linseed oil Kettle-bodied linseed (Body X) 25-gal. ester gum-castor oil varnish, 45% solids Kerosene 24% lead naphthenate 6% cobalt naphthenate Lb. per gallon Viscosity Binder-pigment ratio
Table 11.
Pounds Gallons 276.00 10.20 414.00 11.62 138.00 6.11 69.00 2.91 56.50 8.65 16.80 2.16 44.20 5.40 43.10 5.40 173.50 23.70 162.00 23.70 0.97 0.10 0.40 0.05 1194.47 100.00 11.94 90 Krebs Units 0.73/1
- -
Ingredients Titanium dioxide (rutile)
lasein (15% aqueous solution NHa cut) Antifoamer Dowicide A/G (1: 1) (15% aqueous soln.) Styrene-butadiene Latex (48% solids) Weight per gallon Pigment volume concentration Yo nonvolatile Specific gravity Binder-pigment ratio
Lb. of N. V. per 100 Gal.
78,50 11.77 3-5 36.99 6.52 447.20 214.8 1113.77 594.17 11.14 29.50% 53 29% 1.336 3:5
-
physical changes, which a t present would be too difficult t o unravel. Present evidence indicates, however, that an approach can be made t o the problem which will help t o explain, in a very simplified manner, some of the more important changes that take place. For this purpose, the transition from latex paint to film is considered to come about in two steps or stages: loss of water to a point of irreversible contact of the particles and a fusion or actual development of the film. To illustrate the first stage a series of simple experiments was conducted. A latex paint was prepared essentially according to the formulation given in Table I1 and films approximately 10 mils thick were cast on Morest (3.5 X 6 inch) hiding power cards. Initial weight of the wet film was obtained by subtracting the tare weight of the card from the weight taken immediately after casting. After varying lengths of drying time, the cards were
Figure 4.
April 1953
I n Table 111 are shown the weight losses due t o removal of film by water and the corresponding per cent water which had been removed by drying. Figure 1 is a picture of the cards from which the data of Table I11 were obtained. As 720/, is the maximum per cent solids which can theoretically be attained without distortion of the spherical particles, it can be approximated t h a t irreversible contact occurs when the first distortion of the particles takes place. DISTORTION OF SPHERICAL PARTICLES
Latex-Based Top Coat ( I ) Lb. per 100 Gal. of Paint
reweighed t o determine loss of water by evaporation. Immediately, 100 ml. of water were run over the card from a buret placed directly above the card while the card was supported at a 25" angle t o the vertical. Drying and reweighing then give the amount of film removed by this method,
Election Micrographs of Contact and Distortion of Spherical Particles
A series of electron micrographs (see Figures 2 t o 5 ) depicts the contact and the distortion of the spherical particles. I n the second step complete fusion takes place and a dense, continuous film is formed. During this step forces must be present in order t o account for the distortion of the particles which occurs as they lose their spherical shape. Figure 2 illustrates the pockets formed between particles after first contact is made. Although these pockets are initially irregular and nonspherical in shape, surface tension, working t o minimize surface area, will tend t o render them spherical. For this reason it is believed justifiable t o simplify the computation of forces which exist by treating tht pockets as though they were Fpherical in shape. The surface tension forces in dynes per square centimeter acting on the surface of a sphere of radius R can be expressed as
p =
Table 111.
y_f.
R
Weight Losses
% Hz0
Evap. from Paint Film 0 33 50 66 100
Figure 5.
om
% Solids Washed Away with 100 ml. HBO at 25' C . 46 30 11.5 0 0
Electron Micrographs of Contact and Distortion of Spherical Particles
INDUSTRIAL AND ENGINEERING CHEMISTRY
737
DISPERSION OF PIGMENTS I N LATEX
The manner in which pigments are dispersed in a latex is undoubtedly very important in the mechanism of film formation. This field is essentially unexplored a t present. 9 few generalizations seem justified from t,he exploratory data that have been gathered. Figure 6 shows latex particles with titanium dioxide pigment closely associated. This photograph mas made by diluting a sample of paint with water sufficient to cause isolated portions of binder and .pigment t o deposit upon the sample screen. The field shown in this photograph is typical of the entire field seen in the electroil microscope. The latex used in this sample contained a very soft copolymer of styrene and butadiene rhicb possesses very ready film formability a t room temperature. .I photograph of such a copolymer always shows the particles having very hazy and indistinct boundaries. Careful examination of this photograph shows pigment particles thoroughly embedded in each group of latex particles. The contrast is not striking, but t'he pigment particles are visible as the patches darken in color. It is known that the extreme dilution of any latex paint, with plain water usually detracts from the stability, a t least to the extent of permitting particles of widely different specific gravities to separate. The fact that t.his does not happen in the case of this sample is interpreted to indicat,e a very close union between the latex particles and pigment particles, or at least this particular pigment.
Figure 6.
Latex Particles with Titanium Dioxide Pigment
Where P = dynes pel sq. cm. and 2' = surface or interfacial tension characteristic of the materials involved. For the water interface with relatively nonpolar materials-e.g., for most organic liquids and plastics-2' = 25 to 30 dynes per em. In the case of a latex paint with an average particle diameter of 2000 -4,the approximate size of the larger holes to be expected betneen the particles is about 400 A. This gives an I? of 200 A. em.) and assuming T = 30 we have: (2 X
2T P = - = 3.0 X lo7 dynes per sq. em. R = 435 pounds per square inch
At this point it is evident that if the compression strength of the plastic is less than 435 pounds per square inch gradual shrinkage of the pockets will occur. As the pocket becomes smaller, a summation of the forces tending to contract the sphere is reduced less rapidly than is a summation of the strength forces tending to maintain the sphere. Because for the surface tension, 2T P = -, as R is reduced to R/2, the total force acting over the
R
entire surface of the sphere is reduced t o one half. At the same time the strength forces are reduced t o one fourth, so that as the pocket once starts to shrink its extinction is almost certainly assured. Following through this same type of calculation, it Till be seen that the smaller the particle size in the latex originally the quicker will be the film-forming process. For example, if the particle size were 1000 A., the pressure developed would be 8iO pounds per square inch. Over long periods of stress application the strength values obtained for plastic materials are considerably lower than the strength values obtained when stress is applied for short periods. For this reason, in attempting to predict film formation, a lower compression strength value than the one normally obtained by standard test methods would be applicable, as standard tests usually last only a few minutes, while the application of forces tending to form a film can be considered as acting, if necessary, over a period of dayu.
738
-1
Figure 7 .
1
I mfi
Replica of Latex Paint Film
Figure 7 is a photograph of a replica of a latex paint, film froin m-hich the water has evaporated but which has not fused. To make this photograph a latex u-as chosen which does not fuse a t ordinary room temperature but. in other respects behaves very similarly to the softer varieties of latex. The manner in Rhich the pigment is dispersed is somewhat different t,han in the case of t,he softer latex. Among the latex particles seen in this photograph are several much larger than the average. In all cases around these large particles the smaller particles arrange theniselves about as efficiently as the relative dimensions will permit. This, however, serves to destroy completely the efficient hexagonal packing which gives greatest strength of film. There are one or two areas in the photograph .where no large particles appear,
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 4
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 t h a t the efficiency of packing is disrupted for a large area surrounding each large particle. CONCLUSION
This is not an attempt t o 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, t h a t the description given here will provide a skeleton outline t h a t will assist in obtaining better understanding of the mechanism of film formation. LITERATURE CITED
(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 t h a t every component in the system has a bearing on it. The author has taken one component and studied its varied application in relation t o viscosity stability i n a single type of formulation. The work reported in this paper is restricted t o information obtained from one particular soybean protein, manufactured by the Buckeye Cotton Oil Co. 1200
1
I
I
. 1
I
I
I
I
0
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 t o latex paints and why special precautions must be taken t o assure that t h e viscosity measurements are comparable. Thixotropy. Thixotropy is defined a s a reversible gel-sol transformation ( 4 ) or as the variation of viscosity with time ( 2 ) . Latex paints exhibit this property t o 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 a t increasing rates of shear. This property, responsible for smoothing out under brushing or rolling, is important t o 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 IN LATEX PAINTS
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 t o
INDUSTRIAL AND ENGINEERING CHEMISTRY
739
