I
D. B. FORDYCE, JEAN DUPRE, and WALTER TOY Research Laboratories, Rohm & Haas Co., 5000 Richmond St., Philadelphia, Pa.
Alkali-Soluble Acrylic Emulsions ALKALI-soluble copolymer emulsions of methacrylic acid (MAA) with lower acrylate and methacrylate esters can be prepared by emulsion polymerization techniques. These emulsions have the advantages over the usual acrylic watersoluble salts prepared as solutions, of high solids content and low viscosity, ease of dilution and solubilization by a variety of bases, and low manufacturing cost. Polymers offered as dry powders are usually hygroscopic, dusty, and relatively slowly soluble. The properties of solutions and films obtained from these emulsions can be varied \videly by proper choice of polymerization conditions and monomers. Polymerization can be conducted under redox or reflux conditions with watersoluble initiators and 1 to 5% of anionic or nonionic emulsifiers. Alkali-soluble emulsions containing 8 to 7070 methacrylic acid have been prepared. They can be cross-linked with polyfunctional monomers and the molecular weight varicd at will by use of conventional chain transfer agents. These small (ca. 0.1-micron) particle size emulsions are solubilized by neutralization of the essentially un-ionized carboxylic acid groups by typical bases. One equivalent of base will solubilize the polymer essentially instantaneously to give a clear solution. The solubilization phenomenon is shown schematically in Figure 1. Linear polymers, prepared with about 25 to joy0 MA.4 and with viscosityaverage molecular weights of approximately 600,000, are efficient latex thickeners in the salt form for many compounded and uncompounded latices. BEFORE SOLUBILIZATION
They are generally used as 0.23 to 2% of the total system and do not act merely by thickening the aqueous phase. Their primary thickening action results from adsorption of individual polyelectrolyte molecules on two or more latex particles. This interparticle adsorption or molecular bridging builds structure into the latex system, and this results in increased viscosity. In weakly stabilized latices, where these linear polymers exhibit exceptionally high thickening efficiency, thixotropic, plastic flow is generally obtained. With more highly stabilized latices, less efficient thickening results because of less thickener-latex interaction. I n these cases, pseudoplastic flow with no thixotropy or yield stress is observed. By means of chain transfer agents, linear emulsions of low molecular weight can be prepared which exhibit useful deflocculating properties in aqueous pigment suspensions. By adsorbing on single particles these polymers in the salt form produce a large negative charge on the particle surface. An electrostatic repulsion between particles results, which overcomes ordinary flocculation forces. Linear alkali-soluble emulsion polymers have been prepared which are suitable for film-forming applications. Low molecular weight polymers can be obtained which give low viscosity, high solids solutions in the salt form. Approximately 8 to 1Sy0MAA is required for alkali solubility. Films obtainable with the ethyl acrylate-methyl methacrylate-methacrylic acid system vary from soft and tacky to extremely hard and brittle, depending on monomer ratios.
AFTER SOLUBILIZATION
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By using ammonia or other volatile base, water-insoluble films can be obtained upon air drying. Alkali-soluble emulsion polymers can be efficiently cross-linked by incorporating a di-, tri-, or other polyfunctional monomer. Lightly cross-linked polymers upon dilution and neutralization with alkali produce highly swollen polymer particles rather than true solutions. However, these “solutions” are clear and extremelv viscous. The extent of cross linking will markedly affect solution properties. Optimum aqueous viscosity of l.Oyo sodium salt solutions of polymers containing 40:g MAA is obtained at about 0.40y0 cross linker. An increase in cross linking beyond 0.40Y0 causes a marked reduction in aqueous viscosity. This reduction is a result of the inability of the polymer molecules to swell because of the restrictive forces imposed by the added cross links. The viscosity of solutions of the cross-linked polymers, compared to linear polymers, is much less temperature-dependent. These lightly cross-linked polymers are also efficient thickeners for latex arid pigment systems. They are particularly useful in highly stabilized latices where latex thickener interaction is comparatively low and efficient aqueous thickeners are needed. The cross-linked emulsions can be used as in situ thickeners for latices arid pigment dispersions. The acidic, waterinsoluble emulsions at high solids can be added directly to an alkaline system. The polymer then dissolves and exhibits its normal thickening action. By this means, the handling of a dilute, viscous thickening agent solution is entirely avoided. In situ thickening is demonstrated in Figure 2.
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Figure 1. Base sohbilizes the polymer almost instantly to form clear solution
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Figure 2. A crosslinked emulsion is used for in situ thickening VOL. 51, NO. 2
FEBRUARY 1959
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