Effect of plasticization and pH on film formation of acrylic latexes

The effect of several variables relevant to film formation in 49:49:2 poly(methyl methacrylate-co-butyl acrylate-co-methacrylic acid) latexes were studied. Plasticization of the terpolymer by both an anionic [sodium dodecyl benzene sulphonate (NaDBS)] and a nonionic [nonyl phenol ethylene oxide adduct (NP40), forty ethylene oxide units on average] surfactant were investigated. Dynamic mechanical measurements indicated that NP40 plasticized the polymer, whereas NaDBS did not. This was attributed to the compatibility of the nonionic emulsifier and the polymer. The influence of the surfactants on the film formation process was studied using minimum film temperature (MFT) measurements and scanning electron microscopy (SEM). As expected, the MFT was independent of NaDBS concentration. However, the MFT was also independent of NP40 concentration. Film formation was further investigated using SEM. The series of micrographs at varying NaDBS concentrations showed no effect on the degree of film fusion. However, surface exudates were observed. The micrographs revealed that increasing NP40 content resulted in an apparently greater degree of film coalescence. That is, there was bridging of the particles in the interstitial regions. This was attributed to localized plasticization of the polymer in these regions by NP40. The effect of water plasticization of the latex polymer was studied using dynamic mechanical measurements. The physical character of the terpolymer was varied using mol wt modifiers (CBr4 chain transfer agent and ethylene glycol dimethacrylate crosslinker). It was found that all of the polymers were plasticized by water. Finally, the film forming behavior was investigated as a function of latex pH. MFT measurements indicated an independence of pH.