Role of anionic and nonionic surfactants on the control of particle size and latex colloidal stability in the seeded emulsion polymerization of butyl methacrylate

The evolution of the main colloidal parameters in the seeded starved-feed semi-continuous emulsion polymerization of butyl methacrylate (BMA) was investigated, with the main purpose of assessing the effectiveness of the semi-empirical relationship S = K (.) S-s (.) Delta A/A(s) as a tool to define the surfactant/monomer feed ratio (proportional to K) best suited to achieve a target particle size. In particular, the effect of the type and amount of surfactant [i.e., anionic, sodium dodecyl sulfate (SDS), or nonionic, Brij 58P] added during the semi-continuous stage was considered. Coagulum formation was never observed under the adopted experimental conditions. To detect the occurrence of secondary nucleation or particle aggregation, or both, the particle size and number of particles, the surface tension and the particle surface coverage ratio were correlated. The best results were obtained with SDS and 0.8 <= K <= 3. In fact, under the selected experimental conditions, only with SDS did the number of particles remain nearly constant throughout the polymerization at the value defined by the seed latex; the particle size distribution was highly monodisperse, and the final particle diameter closely matched the calculated one (similar to 120 nm). The above semi-empirical relationship based on the adjustable parameter K was validated by running test polymerizations aimed at lattices with well-defined particle size. (c) 2006 Wiley Periodicals, Inc.