A knowledge of fundamental principles of polymerization and the role of such emulsion polymerization variables as pH, agitation, and recipe ingredients is of importance in the preparation of emulsion polymers for paint use. Such variables as the copolymer composition, the emulsifying system, and the particle size have a profound effect upon the properties of the ultimate paint. The copolymer determines such properties as adhesion, gloss, and scrub resistance. The emulsifying system, in addition to in-
fluencing these properties, is the major factor in control of the chemical stability, mechanical stability, and viscosity of the emulsion. The particle size of the emulsion may be shown to have a unique effect on the scrub resistance, gloss, and tensile strength of the polymer film. At best, preparation of emulsions for general paint use involves the proper balance of a large number of factors, which results in a compromise that may be overcome only by designing specific emulsions for specific end uses.
HAROLD NAIDUS American Polymer Carp., Peabody, Mass.
THE
widespread use of emulsion polymers and emulsion polymerization techniques to obtain products of great industrial potential leads to the belief that this field has a mature and well recognized background. It might be assumed that the overwhelming success of emulsion polymerization technology bespeaks a detailed knowledge of the fundamental physicochcmical mechanisms. It is well known, however, that the field of emulsion polymerization is still in its infancy. Less well known is the fact that the principles of emulsion polymerization axe not understood in all details. The understanding of these principles is essential to effective synthesis and utilization of emulsion polymers. POLYMERIZATION THEORY
The following discussion is concerned only with that type of polymerization known as free radical addition polymerization. The concepts discussed are equally applicable to bulk, solution, bead, and emulsion polymerization. Free radical addition polymerization is characterized by activation of a vinyl-type monomer, such as styrene, by a free radical and subsequent rapid addition of further monomer until the polymer chain is completed. The polymer contains the same structural units as the original monomer. The polymerization proceeds through a series of steps knon-n as initiation, propagation, and termination. Initiation. A free radical niay be formed by the decomposition of an initiator.
+2R. (RC02)z -%2RC02. Peroxide Free Radicals
+ 2C02 t
Propagation, The free radical reacts with the monomer, opens the double bond, and forms a new free radical which reacts with further monomer, etc.
The radical from the initiator actually participates in the polymerization reaction, and therefore cannot properly be termed a “catalyst.” This has been demonstrated conclusively by Price and coworkers (5, 6 ) , who used substituted peroxides, such as tribromobenzoyl peroxide, and isolated polymers containing chemically combined fragments of the initiator. Termination. The free radical at the chain end is deactivated by reaction with another radical and a stable polymer is formed. Deactivation may occur in a number of ways, two of which are illustrated.
112
OF POLYMER AND INITIATOR FREERADICALS 1. COWBINATION R +CHZ-CHZ+~ CHz-CHz. R. --+
+
R +CH2-CH2+,
CHZ-CHQR
2. DISPROPORTIOXATIOX R +CH?-CHz+, CH2-CHz. ~CHz--CH2fCH2--CHz+, R---t R +CHz-CH,+a CHz-CHs R +CHr-CHz+U CH=CH?
+
1
The three polymerization steps occur very rapidly. The formation of a single polymer molecule is complete in seconds or hundredths of seconds. Thus, one should not misinterpret polymerization data which indicate that a considerable length of time (hours or days) is required t o attain high conversion. Each polymer molecule is formed rapidly, but not all the polymer molecules are initiated a t once. They are initiated over a considerable length of time, and therefore a long reaction time is required for high conversions. Several other reactions also play a role in polymerization chemistry. Chain Transfer. This concept arose out of experimental data which indicated that certain polymerizations gave products of lower molecular weight than expected. It was ascertained that a free radical may abstract an atom from an othervise stable molecule and thereby terminate itself. The free radical activity is simultaneously transferred to the previously stable molecule. The growing chain, therefore, is terminated a t a lower molecular weight than would otherwise be expected. R -f-CH2-CHz+--,: CHz-CHz. R’Y +R +CHz-CHzfz CCL +R +CH2-CHz+s R’SH+R fCHz--CH2+*
+ + +
CHz-CHz-Y CHr-CHzCI CHn-CHa
+ R’. + ChC. + R’S.
R’Y may be one of a wide number of compounds including monomer, polymer and solvent, and Y may be, among others, a hydrogen or halogen atom. Inhibition. Many polymerizations do not proceed in t h e -CH,. presence of air, quinones, amines, etc. This inhibition of polymerization is useful in stabilizing monomers for storage and in ‘ (short-stopping” polymerizations a t the desired conversion. It may, however, cause serious difficulties in the polymerization reaction by creating chains of low molecular weight and by increasing conversion cycles. Cross Linking. This phenomenon is generally associated with the addition of monomers of high functionality to the polymerization system. I n general, monomers containing one active double bond are considered difunctional and give rise to linear polymers as previously described. On the other hand, certain monomers, such as divinyl benzene and butadiene,
INDUSTRIAL AND ENGINEERING CHEMISTRY
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contain two active double bonds and are, therefore, tetrafunctional. They give rise to branched or cross-linked polymers, illustrated below: CH-CH-CH-CH2
I
+CH-CH2+
111 1
1
*
6
+CH-CH,tCH-CH&CH-CHlf
II
I
1
-
CH-CHI
I
9
The bracketed groups are divinylbenzene molecules in a styrene polymer. The wavy lines indicate polymer chains. Copolymerization. This term may be defined as the polymerization of two (or more) monomers to yield a polymer containing both monomers. No polymer molecules derived from either pure monomer alone can be isolated from a true copolymerization system. This is in contrast to homopolymerization, in which only one monomer participates. As a general rule, the properties of true copolymers differ substantially from those of a simple admixture of the two homopolymers which may often form a heterogeneous system. Theoretical and experimental work to elucidate the underlying principles of copolymerization has been recently carried out, notably by Price and Alfrey ( 4 ) , Mayo (3), and Wall (8). Based on this work certain basic principles of copolymerization may be summarized. All monomers do not enter the polymer.chain at the same rate. Moreover, the reactivity of a monomer m homopolymeruation does not necessarily coincide with its reactivity in copolymerization. An excellent example of this type of monomer is maleic anhydride, which under normal polymerization conditions does not polymerize by itself. However, a one-to-one copolymer m t h styrene forms readily. Except in ver special caseR, the composition of a copolymer varies with the g g r e e of conversion, so that the polymer formed at the beginning of the polymerization will contain a higher proportion of the more active monomer. Therefore, 8, copolymerization carried to complete conversion will consist of a spectrum of polymer molecules having different compositions. It is often possible to prepare co olymers of more uniform composition by the simple expedient adding the more reactive monomer slowly during the polymerization cycle. I n certain cases, two monomers, which are reactive in homopolymerization, will retard or inhibit one another in copolymerization. Vinyl acetatestyrene is an example of such a pair of monomers.
OF
EMULSION POLYMERIZATION THEORY
s
Although the preceding discussion is applicable to emulsion polymerization, a number of other concepts must be considered because of the complexity of the heterogeneous systems involved. An emulsion polymerization system may, and often does, contain one or more compounds in each of several classes of addends. The various classes are listed separately below and an attempt is made to indicate their function in emulsion polymerization. Reaction Medium. Water is the universally used medium. Distilled, deionized, or softened water is generally preferred. Occasionally, t h e medium will be modified with a small percentage of an alcohol or polyhydric alcohol to act as an antifreeze for a low temperature reaction. Emulsiiiers. The dominant characteristic of a good emulsifying agent is the existence in the molecule of B polar “head” and a long-chain, nonpolar “tail,” or a water-soluble head and oil-soluble tail. These agents are generally classified according to three main groups: anionic, cationic, and nonionic or negative, positive, and uncharged. I n recent years, the role of soap and synthetic emulsifiers in emulsion polymerization has been clarified, notably by Harkins and coworkers (1). Above a certain critical aqueous concentration, soap forms oriented bundles termed “micelles.” When an April 1953
oil-soluble monomer is added to such a system, i t is distributed in three ways. A very small amount dissolves in water to form a true solution, a small amount is solubilized in the nonpolar section of the micelle, and the balance of the monomer is dispersed as comparatively large oil droplets. When a water-soluble initiator is used, initiation of polymerization occurs primarily in the micelle-solubilized monomer. As polymer forms, propagation continues by adsorption of monomer from the monomer droplet reservoir. Thus, the emulsifier acts as a suspending agent for the monomer droplets and the polymer particles and as a solubilizing agent for the monomer. When insufficient emulsifier is present to accomplish these ends, coagulation may occur. Harkins further points out that for soaps and alkyl sulfates, the critical concentration for micelle formation (an inverse measure of soap efficiency) increases fourfold for every decrease by two in the number of carbon atoms. Thus, soaps with 12 or less carbon atoms are very poor emulsifying agents when used alone. However, the critical concentration is greatly reduced by the addition of inorganic salts, such as potassium chloride, and 10- to 12-carbon soaps are suitable emulsifiers in their presence. Protective Colloids. Water-soluble polymeric materials, such as natural gums, cellulose derivatives, and polyvinyl alcohol, are often used in conjunction with emulsifying agents. Their function is not too clear, although they may serve to envelop the sticky monomer-polymer particles to prevent agglomeration and coagulation. Initiators. The compounds used as initiators in emulsion polymerization are generally the water-soluble, or partially watersoluble hydroperoxides, persulfates, etc. When oil-soluble initiators such as benzoyl peroxide are used, the particle size of the emulsion may be adversely affected, as initiation will occur in the large monomer droplets and thereby create larger polymer particles. This cannot occur with water-soluble monomerinsoluble initiators, aa no initiation can take place in these large droplets. Modifiers. Oil-soluble chain transfer agents, such as mercaptans (most widely used), chlorinated aliphatics, and higher aldehydes are generallyfmployed, although water-soluble members of the series are also used. I n general, the water-soluble types are less satisfactory because they overmodify the product and may inhibit polymerization. In diene polymerization, the mercaptan appears to act as an accelerator as well as a chain transfer agent. It has been suggested that it acts as a typical reducing agent in a redox system. The chain transfer action of the modifiers is particularly important in diene polymerization, because dienes are tetrafunctional monomers, and, therefore, may cause cross linking. This may occur in the later stages of the polymerization, when the available free radicals are more likely to activate the residual double bonds in the polymer chain than the m a l l quantity of monomer left in the system. Activation of these double bonds will lead to branched and cross-linked polymers, except when the polymer radical is terminated by chain transfer with the mercaptan. Accelerators. These compounds are distinguished from initiators in that they function to increase the reaction rate to a much greater extent than an equivalent amount of initiator. Accelerators are generally reducing agents and their interaction with initiators, which are oxidizing agents, has resulted in the term “redox polymerization.” By the use of accelerators, conversions of 70 t o 80% can be obtained in 24 hours at temperatures of -5” to 5 ” C., whereas similar conversions in the absence of accelerators require temperatures of 40’ to 60’ C . Short-Stops. It is often desirable to terminate a polymerization before complete conversion occurs. This is particularly true in diene polymerization where the tendency to form crosslinked polymers, and therefore products of inferior physical properties, is much greater a t the end of the polymerization cycle.
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713
The compounds used to short-stop a polymerization at the desired conversion are those which are efficient inhibitors for the polymerizat,ion reaction. These would include quinones, amines, etc. Other Emulsion Polymerization Variables. pH. The effect of pH is extremely significant in emulsion polymerization but, unfortunately, is more closely related to empirical know-how than to any fundamental work which has yet been carried out. It appears certain that most emulsion polymerization systems have a definite pH for optimum conversion. As yet, no experimentally verified explanation for this phenomenon has been advanced, although its major effect may be on the efficiency of initiator decomposition. Figure I illustrates the effect of pH in several typical systems.
The second major problem of viscosity control occurs in emulsion polymerization systems in which a viscosity peak takes place during the course of the reaction. This is a particularly important industrial problem because the viscosity peak often occurs a t the period during the polymerization cycle where monomer is being converted to polymer a t the highest rate. Obviously, this creates a serious heat transfer problem. Innovations in reactor design to achieve better heat transfer have been one approach to the solution of this problem. Polymerization chemists have approached this problem by modifications in the polymerization recipe. I n some cases, the addition of emulsifier shortly before or a t the viscosity peak markedly decreases the viscosity of the system. Inorganic salts and certain dispersing agents also serve to reduce the,viscosity. EMULSION POLYMERIZATION TECHNIQUES
1
1
1
*-EO% BUTADIENE-bO%
STYRENE AT 60.C
AFTER 6
HRS.
1
I
0
2
4
6
l l l 1l0 l 1l2 l 14l l
8 INITIAL pH
RATEOF AGITATION is another emulsion polymerization variable which falls in the category of industrial know-how. A recent study by Shunmukham and coworkers (7) demonstrates that the reaction rate decreases and the inhibition period increases with increased agitation. Kolthoff and Harris (2') also demonstrated that increased agitation rate caused an increase in the rate of mercaptan consumption for a butadiene-styrene copolymerization. Agitation in an emulsion polymerization system must serve a twofold purpose: It must maintain the uniformity of the batch and provide sufficient shear to prevent agglomeration of particles and i t must provide sufficient movement of the batch for adequate heat transfer. A large number of polymerization systems proceed best with the least possible agitation in order to minimize induction periods and control the rate of modifier consumption, as indicated above. On the other hand, a reasonable agitation rate is required for uniformity and heat transfer purposes. An empirical compromise between these two demands must be determined for actual plant operations. Frequently, this compromise is best worked out on plant scale equipment, as agitation data based on laboratory or pilot plant equipment cannot be extrapolated easily to large scale operations. VISCOSITYCONTROL in emulsion polymerization is an important industrial problem. A wide variety of phenomena could be included under this heading. This paper is confined to two major problems of viscosity control. The first is concerned with the preparation of an emulsion polymer system at some arbitrarily stated viscosity. It is often possible to attain this desired viscosity by simple addition of the proper protective colloid or thickening agent to the emulsion system before polymerization. In most cases, the viscosity of the resultant polymer emulsion is dependent solely upon the molecular weight of the protective colloid. For example, two 55% vinyl acetate emulsions can be prepared by identical procedures in which one has a viscosity of 600 cp. and the other has a viscosity of 10,000 cp. I n the former, a polyvinyl alcohol of low molecular weight is used as the protective colloid, whereas a higher molecular weight grade of polyvinyl alcohol is used in the latter. 114
Emulsion polymerizations may be carried out by simply adding all the ingredients, heating to reaction temperature, and cooling when the reaction is complete, but other techniques may be used. Each technique has certain inherent advantages and disadvantages. Batch Process. This process is the simplest in concept in that it requires only that the ingredients be charged, the agitator started, the temperature adjusted, and the reaction carried out to the desired conversion. This polymerization process is particularly sensitive to inhibition by air unless the reactor is purged by an inert gas. Moreover, the rate of heat liberation is often excessively high and the temperature may not be readily controlled by convection to the cooling jacket. In many cases, the reaction can be carried out under reflux conditions to reduce contaniinatiori by air and to provide additional heat transfer capacity. hlost butadiene-styrene copolymerizations are carried out as batch operations without reflux, whereas acrylic ester polymerizations are usually carried out under reflux conditions. Monomer Addition. It is often desirable, and even necessary, to add one monomer continuously during copolymerization to ensure a homogeneous composition in the resultant polymer. Frequently, in homopolymerization, monomer is added continuously to the aqueous phase during polymerization. Several desired effects are brought about by this technique: Extremely exothermic polymerizations-Le., the lower acrylate esters-are conveniently run by this technique t o yield high solids emulsions. The concentration of unreacted monomer can be kept low and the added cool monomer aids in dissipating the heat of polymerization. I n general, using similar charges, this technique will give emulsions of smaller particle size than the batch technique, as a large emulsifier-monomer ratio exists throughout the polymerization. The monomer addition technique can be carried out either in an inert atmosphere or under reflux conditions. This technique is not universally applicable-for example, in the polymerization of sluggish monomers, it may lead to very slow and uneconomical reaction rates. Emulsion Addition. In this technique, a seed containing water, emulsifier, and/or initiator is added to the reaction vessel and brought to reaction temperature. An emulsion, containing the monomer, additional water and emulsifier, catalyst, modifier, etc., is prepared and slowly added to the reaction vessel throughout the polymerization cycle. This technique has advantages similar to those of the monomer addition technique and generally leads to the most stable emulsions with the least amount of coagulum. This is probably due to the fact that the monomer is added in emulsion form, whereby adequate emulsification is not dependent upon the agitation and other mechanical features of the reaction vessel as is the case in the monomer addition technique. The emulsion addition technique can be readily used as the precursor to a continuous process, as it is possible to add a mono-
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Vol. 45, No. 4
.
*
.
mer emulsion to the polymerizing batch and continually transfer the product to a finishing reactor. It is not always possible to determine in advance the most desirable technique for a particular polymerization, as many unknown factors are involved. Some polymerizations can be carried out in the laboratory by any of the three techniques. However, because of heat transfer problems, and coagulation, molecular weight, and reaction rate considerations, it is often preferable t o use one particular technique in plant processing. PAINT APPLICATION
In order t o relate these brief fundamentals of polymerization to the actual preparation of a paint latex, the problem has been oversimplified by assuming that there are only three major factors t o be considered-the polymer or copolymer composition, the emulsifier type and content, and the particle size of the latex. Copolymer Composition. The copolymer composition affects many paint properties, one of which is adhesion. Certain polymers and copolymers have excellent adhesion to specific surfaces. Obviously, it is preferable to formulate a paint with a copolymer having superior adhesion to the surface to be painted. For example, polyvinyl acetate has much better adhesion t o glass surfaces than do butadiene-styrene copolymersand, in formulating a paint for such surfaces, it is desirable to choose a polyvinyl acetate emulsion as the vehicle. Another property which depends upon the copolymer system is the scrub resistance of the paint. Again, a direct comparison of the polyvinyl acetate and butadiene-styrene vehicles may be made. Polyvinyl acetate is inherently water-sensitive, whereas butadiene-styrene copolymers are not. The proper choice of film former t o obtain good scrub resistance is, therefore, selfevident.
BLACK GLASS READING
.
20
40
60
:9
s)
80
100
% STYRENE IN POLYMER
. e
’
The property of gloss may be consideredahnctionof the copolymer system. Figure I1 illustrates the effect of butadiene-styrene copolymer composition upon gloss. I n this example, the gloss increases with increased styrene content until film discontinuity occurs a t a copolymer composition of about 70/30 styrene-butadiene. The dark circle in Figure I1 represents a blend of a 50/50 styrene-butadiene copolymer emulsion and a polystyrene emulsion t o give a composition of 65/35 styrene-butadiene. The gloss reading is appreciably lower than for the true copolymer and this may be attributed to incompatibility of the polystyrene with the styrene-butadiene copolymer. Fusion of the film of this blend will increase the gloss, but it will not reach that of the true copolymer. Thus, the choice of copolymer type and copolymer composition resides in the specific requirements and end use of the paint. Emulsifier. The emulsifiers in a paint latex influence the few April 1953
properties mentioned above and have a more specific effect upon such properties as chemical stability, mechanical stability, and viscosity of the paint. CHEMICAL STABILITY. This section is concerned with the chemical stability of the latex in contrast t o the chemical stability of the film. Such film properties as acid, alkali, and solvent resistance are of great interest in the specialty paint field and suitable copolymers generally may be chosen from past experience or from the voluminous literature available on this subject. A typical data sheet published by a producer of butadienestyrene latex states: The latex tolerates appreciable quantities of solvents, acids, salts, and polyvalent metal ions without coagulation. Latex X, for example, is not affected by the addition of equal volumes of 5y0 aqueous solutions of sodium chloride, isopropyl alcohol, calcium chloride, or alum. This statement defines the problem of chemical stability as covered by this paper. It has significance in that these simple tests indicate, in a qualitative way, the stability of the latex to a wide variety of pigment types, including those containing a high proportion of heavy metal ions. In formulating a latex to have suitable chemical stability, the primary variable is the emulsifying system. It may be assumed that this stability can be achieved by the use of a nonionic emulsifier which would not be affected by the reagents noted above. Unfortunately, there are a number of reasons why this approach has not yet been effective in butadiene-styrene systems.
It is extremely difficult to prepare a nonionic butadipnestyrene latex which does not result in excessive coagulation and excessive reaction times, without adversely affecting a number of the other properties of the latex. A large increase in nonionic emulsifier content permits the preparation of a butadiene-styrene latex with minimum prefloc. However, the water resistance of the resultant latex is very poor and the flow and viscosity characteristics in the com ounded paint are inferior. TEe use of nonionic emulsifiers generally results in a latex of lar er particle size. This has a deleterious effect upon the gloss of &e ultimate paint. Most suitable nonionic emulsifiers for butadiene-styrene latices appear to have both an inhibiting effect on polymerization and some plasticizing action on the resulting film. Other copolymer systems do not present the same obstacles; excellent paints have been formulated from nonionic vinyl acetate emulsions and nonionic, plasticized styrene emulsions, among others. In butadiene-styrene copolymerizations, the polymerization chemist must turn t o other types of emulsifiers t o overcome these objections, The use of cationic emulsifiers is not recommended, as the latex would be incompatible with the conventional anionic paint addends, such as casein and sodium polyacrylate. Furthermore, such cationic emulsions do not wet well or level well on the very surfaces upon which the paint is t o be used-paper, wood, metal, etc. Anionic emulsifiers, therefore, offer most promise in butadienestyrene systems. Soap is eliminated as a possibility, because it has very poor chemical stability. Therefore, only the synthetic sulfonates and sulfates are left to be considered. A search of the literature will eliminate many of these, as they are stated to be unstable in the presence of acids, alkalies, or salts. Thus, the vast list of anionic emulsifiers can be reduced t o a choice of 10 or 15, as many of the others will be unavailable commercially or uneconomical. This short list of anionic emulsifiers may then be screened by such simple techniques as titration to the cloud point with salts, acids, and other reagents of interest. These tests will eliminate from initial consideration those emulsifiers which have low tolerance to the reagents. The final step, or rather series of steps, then involves carrying out a broad series of polymerization experiments utilizing mixtures of the nonionic and anionic emulsifiers over a wide range of compositions. This
INDUSTRIAL AND ENGINEERING CHEMISTRY
715
work is preferably carried out in consultation with a skilled statistician, as such considerations as reaction rate, amount of coagulation, and particle size, as well as the other requirements under discussion, will play a part in the final choice of the emulsifying system. One must also consider, in setting up such a series of polymerization experiments, the emulsion polymerization fundamentals discussed above. Such changes as the addition of salts to increase emulsifier efficiency, a close control of pH, or a change from batch technique to monomer addition technique may have sufficient influence upon a polymerization reaction to make an otherwise unsatisfactory formulation perform in a satisfactory manner.
EFFECT OF AD V 0
N in
S ON VISCOSITY OF
6
s a v)
0
4
>-
6
! 9 2 w
8
m 0
I
2
3
4
5
6
7
8
9
PARTS IO% EMULSIFIER SOLUTION /IO0 PARTS LATEX-CASEIN BLEND
VISCOSITY. This section is not concerned with the viscosity characteristics of the latex as such, but rather with these factors in the paint formulation. For obvious reasons, good brushing qualities and good leveling qualities are major requirements for a satisfactory paint. These and other properties related to viscosity may be controlled by proper formulation of the latex itself. It is most desirable, for example, that the addition of the latex to the other ingredients of the paint be accompanied by a suitable increase in viscosity, which should remain constant from batch to batch, and the viscosity should neither increase nor decrease appreciably upon aging. I n certain instances, it is possible to add a suitable thickener, such as sodium polyacrylate, to achieve this purpose. In other paint formulations, this technique is not desirable. I t is in the latter case that the formulation of the latex plays a most important role. The ultimate viscosity properties are largely controlled by the emulsifier system used (Figure III). The series containing nonionic wetting agent tends to drift in viscosity with age, whereas the series containing the anionic wetting agent tends to level out the viscosity of the mixture. At 4 parts of added wetting agent, this observation is particularly true. These effects are applicable, in a qualitative sense, to the compounding of a complete paint, because the viscosity characteristics are largely controlled by the interaction of the emulsifiers in the latex and the casein. From these data, it need not be concluded that the wetting agents must be added after polymerization to obtain the desired viscosity characteristics. Similar results are obtained by adding the wetting agents prior to polymerization. MECHANICAL STABILITY may be defined as the ability of a latex or paint t o withstand very high shear stresses, such as are encountered in pumping, grinding, or mixing. A standard test for evaluating this property involves agitation of 200 grams of latex on a high speed mixer for 30 minutes, followed by filtration through 100-mesh screen to determine the amount of coagulum.
716
Latices with satisfactory mechanical stability will have less than 0.5 % coagulum. This property is a primary function of the emulsifier system of the latex. It can be demonstrated that a latex which has poor mechanical stability can be converted to one of excellent mechanical stability by the addition. after polymerization, of the proper amount and type of emulsifying agent. It is not possible, however, to add this excess wetting agent to the polymerization recipe itself, since the emulsifier apparently becomes “tied up” and does not function t o increase the mechanical stability. Thus, it is the free emulsifier, unassociated with the polymer particle, which functions to increase mechanical stability. Particle Size. The size of the polymer particles in the latex affects the properties discussed above and, although particle size is a function of both the copolymer composition and the emulsifiers in a latex formulation, it is possible to characterize its effect on such film properties as water resistance, tensile strength, and gloss. As a typical example, the property of gloss has been chosen for closer scrutiny. In Figure IV, the effect of particle size upon gloss is illustrated for a series of carefully controlled polyvinyl acetate emulsions in which the emulsifier content and type have been kept essentially constant. It is evident that an increase in gloss results from a decrease in the particle size of the resin emulsion. Figure 3 also illustrates the effect of better film continuity upon gloss. The incorporation of a small amount of plasticizer in the polyvinyl acetate emulsion or the addition of a small amount of acrylic monomer prior to polymerization tends to improve the continuity of the film, which results in a higher gloss reading. The influence of particle size upon gloss may be explained by the better “packing” of the particles and, thereby, improved continuity of the film.
FIGURE 3 T EFFECT OF PARTICLE SIZE AND FILM CONTINUITY ON GLOSS
so
V N Y L ACETATE- ACRYLIC COPOLYMERS
PARTICLE SIZE
IMICROYSI
To achieve the most desirable particle size and particle size distribution in a paint latex, several variables must be carefully controlled. The polymerization technique has a profound effect upon the particle size. I n particular, the average particle size for an identical emulsion recipe decreases in the following order: batch technique, emulsion addition technique, monomer addition technique. However, the batch and emulsion addition techniques give more uniform particle size distribution than the monomer addition technique. The amount and type of emulsifier will affect particle size in that the higher the quantity of emulsifier present in the initial formulation, the greater is the number of micelles formed, hence the greater is the number of polymer particles formed. The greater the number of particles formed for a particular quantity of monomer available in the emulsion, the smaller will be the particle size of the final product. Under comparable conditions, the ionic emulsifiers, both anionic and cationic, will yield emulsions of finer particle size
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 4
than will emulsions formulated with nonionic emulsifiers. This is probably a function of the ease with which micelle formation occurs with these emulsifier types. As a corollary, it should be pointed out that the mode of addition of emulsifier during the polymerization will affect the particle size of the final product. Thus, it is possible to formulate an emulsion polymerization recipe such that the amount of emulsifier initially present is very small. This results in the formation of relatively few micelles. If emulsifier is subsequently added in a way to avoid further micelle formation-i.e., in amounts below the critical micelle concentration-the initial emulsifier will determine the number of particles to be formed during the polymerization. Therefore, to obtain large particle size, a low initial emulsifier content is required. The rate of agitation, the choice of oil- or water-soluble initiators, the pH, etc., will also have an influence upon particle size and these variables must be balanced appropriately.
Pigmentation of latex paints requires pigments inherently resistant to alkalinity of the vehicle and surfaces to be painted. Latex paints respond to many principles of emulsion technology. Special requirements of low electrolyte content and neutrality to alkaline pH must be met. For most purposes, the more light-fast pigments are preferred. Pigments that are poorly dispersed or show strong flocculation tendency fail to produce maximum pigmenting or coloring efficiency and may cause streaking and poor color uniformity. The external or continuous phase generally consists of a n alkali-dispersed colloid with or without other surface active agents or sequestering agents. Certain anionic agents have proved effective aids in pigment wetting and, in combination with ammonium caseinate, have shown minimum liquid requirement values in absorption titrations. Selection of aqueous vehicle constituents to yield low absorption values has
CONCLUSION
The tools for the preparation of latices having specific properties are at hand and the near future may see the formulation of tailor-made latices for specific paint applications. LITERATURE CITED
(1) Harkins, W. D., J . Am. Chem. Soc., 69, 1429 (1947). (2) Kolthoff, I. M., and Harris, W. E., J . PoZgmer Sci., 2, 41 (1947). (3) Mayo, F. R., and Walling, C., Chem. Revs., 46, 234 (1950). (4) Price, C. C . , and Alfrey, T., J . PoZymer Scz., 2, 101 (1947). (5) Price, C. C., and Durham, D. A , , J . Am. Chem. Soc., 65, 757
(1943).
(6) Price, C . C., Kell, R. W., and Krebs, E., Ibid., 63, 2798 (1941). (7) Shunmqkham, S. R., Hallenbeck, F. L , and Guile, R. L., J . PoZymer Sci., 6 , 691 (1961). (8) Wall, F. T., J . Am. Chern. Soc., 6 6 , 2050 (1944). RECF~IVED for review November 4, 1952.
ACCEPTEDFebruary 2, 1963.
given the best pigment dispersion and greatest coloring efficiency. Casein is an excellent dispersant for extenders and inorganic pigments and for organic toner pigments. It improves the dispersion properties of the water phase and appears t o function well as a protective colloid, stabilizer, and thickener. Pigment mixtures, both organic toners and inorganic white and extender pigments, must be dispersed to simulate the anionically dispersed latex and must be compatible with each other and with the latex for maximum color efficiency and pigment stability. The dispersed film-forming binder and pigments are carried by the aqueous phase, which also carries the colloidally dispersed thickener. Overcrowding a t any stage of manufacture may cause flocculation or agglomeration; a careful procedure must be followed in dispersing pigments and blending ingredients to produce the final paint.
LOUIS A. MELSHEIMER AND WALLER H . HOBACK Calco Chemical Division, American Cyanamid Co., Bound Brook, N. J .
F
OR the purpose of this paper latex paint is considered to mean any pigmented surface coating in which the major film-forming constituent is a synthetic resin latex, with or without other film-foping constituents added in an oil-water emulsion type of system. Only systems in which water is the external or continuous dispersing phase are considered. This paper does not recommend specific paint formulations or special types of equipment for dispersing the pigments. Specific processing is given only for illustrative purposes. The experimental work reported has pointed out the importance of maintaining satisfactory dispersing and peptizing constitutents and proper balance throughout the entire process of preparing the paint from the individual undispersed materials through to the finished paint. Although i t is not suggested that the principles of emulsion technology may be applied directly for the formulation and processing of latex coatings, there is much in the literature on emulsion paints, colloids, and pigment surface chemistry that provides a suitable background for the subject. Extensive bibliographies are included in publications by Sutheim ( 1 4 ) and by the TechniApril 1953
cal Committee of the New York Paint and Varnish Production Club (11). Each reviews the field technologically, and includes much valuable information. While much of the literature on emulsion technology is highly relevant, i t is necessary to consider also the principles of pigment dispersion and liquid-solid interfacial relationships. Work by Harkins and coauthors ( l a ) and Bartell and coauthors (3, 10) has provided information of great value. The latex paint system is a dispersion with a continuous phase consisting of an alkali-dispersed hydrophilic colloid in water, containing two or more different phases or different types of particles suspended therein. I n most cases, there are several highly individualistic substances, in finely divided form dispersed and coexistent in the external phase. These particles should remain as separate entities without coalescence, coagulation, or flocculation until the paint has been applied. Then, and only then, should the pigment and vehicle coalesce. At that stage, the pigment particles must be receptive to the vehicle particles and form a well consolidated film (7). Generally, there are two or more types of pigment particles, frequently partly inorganic
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