Effect of commercial polymerization conditions on rubber particle size and efficiency in high impact polystyrene

Abstract

High impact polystyrene (HIPS) is a heterogeneous polymer alloy produced by dissolving rubber in styrene and polymerizing the solution. The final product consists of rubber particles dispersed within a polystyrene matrix. For a HIPS resin, mechanical properties depend on the rubber particle morphology, rubber particle size, and adhesion to the polystyrene matrix. Previous studies have shown that these rubber phase properties are related to feed formulations, polymerization conditions, and process type. The current research provides data on the effect of polymerization conditions on rubber phase properties. Specifically, the effect of polymerization temperature, initiator concentration, chain transfer agent concentration, and polybutadiene rubber molecular weight on rubber particle size and morphology were examined in a 1250 ml laboratory continuous stirred tank reactor (CSTR). Also, the effect of continuing polymerization and devolatilization, varying agitation, and using a preinversion CSTR on rubber particle size and morphology were examined in a 1 00 lb/hr pilot plant. Rubber particle size was shown to depend on the viscosity ratio of the polystyrene and rubber phases, interfacial tensions and shear rate. Rubber particle size is increased by: (1) increasing the viscosity ratio by increasing styrene molecular weight, and increasing CSTR conversion, decreasing poly rubber molecular weight, (2) increasing HIPS polymerization solution viscosity, (3) decreasing interfacial tension through grafting, and (4) increasing shear rate through agitator speed. Rubber efficiency was shown to depend on polystyrene occlusion size, rubber membrane thickness and the amount of disintegrating particles. Rubber efficiency is increased by: (1) increasing polystyrene occlusion size through the use of a preinversion reactor, (2) decreasing rubber membrane thickness through grafting and (3) decreasing the amount of disintegrated particles by preventing excessive grafting.