Semi-continuous emulsion copolymerization of styrene-butyl acrylate with methacrylic acid: Structure-property relationship (2008)

Styrene-acrylic emulsion polymers are widely used for exterior coatings because of their good exterior durability. Different monomer combinations can produce a variety of copolymer products for coating applications. A broad range of functional chemical groups can also be incorporated into the polymer chain to improve coating adhesion to the substrate, sites for crosslinking, as well as compatibility with other polymer materials. The versatility of styrene-acrylic latex chemistry, from the point of view of polymer chain microstructure, polymerization process manipulation, and molecular weight and particle size control, leaves much to be exploited to meet the challenges of producing products with desired structures and application-related material properties and minimizing cost and waste. Systematic studies, as well as newer and more sophisticated chemical, physical, colloidal and computer-oriented analytical and modelling tools will allow the correlation of synthesis, microstructure and performance properties. This study looks at the semi-continuous emulsion polymerization of styrene (St)-butyl acrylate (BA) with methacrylic acid (MAA) to produce latex for coating applications. The objectives were to investigate the factors influencing the polymerization process, latex polymer and coating adhesion properties. Statistical experimental design techniques were used to study their relations in order to improve coating adhesion. An experimental screening design was first used to investigate the effects of five variables, i.e., St/BA ratio, MAA contents, pH, emulsifier content, and chain transfer agent (CTA) content on various product properties. The impact of these variables on the carboxylic acid distribution in the latex, the average latex particle diameter, the molecular weight and distribution of the polymer products, and the glass transition temperature of the polymer were studied using a 25-1IV fractional factorial experimental design. The results from the screening design also showed that inside the chosen experimental region, the pull-off adhesion between the latex coating and a polycarbonate substrate was influenced by the pH of the emulsion polymerization. Reactions under increased pH produced latexes with higher adhesion forces to the polycarbonate substrate. Following the screening design of experiments, three variables, i.e., St/BA ratio, MAA content, and pH, were further investigated through a Box-Behnken design over a wider experimental region. Their influence on the latex polymer properties when the CTA and the emulsifier contents were held constant was studied. Copolymers with a broader range of Tg were produced. Experimental results also showed how the pH affected the distribution of carboxylic acid in the polymer particles. An empirical model was obtained through the regression analysis to correlate pull-off adhesion to the three variables. The viscoelastic behaviour of the polymer products was studied by dynamic mechanic analysis (DMA). This was a step to correlate the polymer structural properties, which were affected by variables in the emulsion polymerization with their performance properties such as the adhesion to polycarbonate substrate. It was found that ionic and hydrogen bonding interactions which led to the interfacial crosslinking of particles improved the cohesive strength of the latex film and hence the coating adhesion performance. This was also affected by the carboxylic group distribution in different St/BA/MAA emulsion polymerization systems. This work shows that a combination of techniques offers the opportunity to investigate the influence of different variables to improve the coating adhesion properties of St/BA/MAA based latexes. The empirical model based on the extensive experimental program has led to an improved understanding of the St/BA/MAA emulsion polymerization and its latex polymer product properties.