The bead suspension polymerization process

In the bead type of suspension polymerization, the polymer is soluble in its monomer and, thus, the monomer-polymer mixture is homogeneous. In this case, the suspension system can be treated as an aqueous dispersion of a time-varying viscoelastic fluid. 

Polystyrene used in injection molding is mamrfactured by the suspension bead process. Poly(methyl methacrylate) and its copolymers containing small amounts of acrylate esters are also produced by the bead suspension process. Clear transparent polymers are often required, so formulations involving Pickering dispersants (e.g., MgCOa) that can removed from the polymer with dilute acid after polymerization, are particularly advantageous. 

In the case of styrene-acrylonitrile copolymers, the method of choice for batch suspension polymerization is normally that involving the azeotropic monomer/comonomer composition to minimize copolymer compositional drift. Nevertheless, complications often arise because considerably more acrylonitrile than styrene dissolves in the continuous aqueous phase. As conversion proceeds, acrylonitrile diffuses into the polymer particles and the monomer ratio in the bead changes, causing the composition of the copolymer to change as well [4]. 

High-impact polystyrene and acrylonitrile-butadiene-styrene copolymer are often prepared in a combined bulk-suspension process. This begins with a solution of polybutadiene in styrene or styrene/acrylonitrile. Subsequently, the polymerization of styrene or styrene/acrylonitrile is initiated and continues under stirring until phase inversion occurs (i.e., polybutadiene is dispersed in a continuous PS matrix - Chapter 4). In the final stage, water and dispersant are added to the system and the polymerization is completed in suspension. 

---

In general, the suspension polymerization can be distinguished into two types, namely, the bead and powder suspension polymerization [4]. In the former process, the polymer is soluble in its monomer and smooth spherical particles are produced. In the later process, the polymer is insoluble in its monomer and, thus, precipitates out leading to the formation of irregular grains or particles. The most important thermoplastic produced by the bead suspension polymerization process is PS. In the presence ofvolatile hydrocarbons (C4—C6), foamable beads, the so-called EPS, are produced. On the other hand, PVC, which is the second largest thermoplastic manufactured in the world, is an example of the powder type suspension polymerization. 

---