Free Radical Polymerization: Heterogeneous Systems

Abstract

Bulk free-radical polymerizations are carried out in the absence of a dispersion medium, and without (or with a very little) solvent. Many technical polymers such as LDPE, LLDPE, PVC, PS, HIPS, PMMA, nylon, and polyester are manufactured in bulk processes. Compared with suspension, emulsion, or solution processes, bulk polymerizations exhibit a higher reactor performance, higher product purity, and reduced transfer reactions to solvents or additives. The disadvantages are related with the high process viscosity, which in turn generates problems of heat removal, mixing, pumping, and reactor wall fouling, by film formation. Some bulk polymerizations (e.g., the production of crystal or general purpose PS) are homogeneous because the produced polymer is amorphous and completely soluble in the monomer. In contrast, PVC soon precipitates from its monomer. In the HIPS process, the heterogeneity results from the incompatibility between the initial PB prepolymer and the generated PS chains. Many thermoplastics are heterogeneous (or heterophase), because they contain liquid or rubber dispersions that improve their physical properties with respect to those of the continuous brittle phase. Examples of this are the softening of PVC by the presence of phthalate droplets, and the improved toughness of HIPS or ABS by addition of PB-based rubber particles. This chapter will focus on the (heterogeneous, bulk, and free-radical) polymerizations leading to the production of HIPS and PVC.