In suspension, a method usually applied to very large polymers, monomers that are relatively insoluble in the suspending medium (generally water) are dispersed as liquid droplets. The suspension is maintained by agitators with the aid of suspending agents, and the polymer leaves the reactor in the form of a slurry. Actually polymerization proceeds as a bulk polymerization in manufacture; each globule being a separate reactor with a large surface to volume ratio facilitating heat transfer. The water suspending medium is a good heat transfer agent being easily cooled by contact with the heat transfer surface (generally the reactor walls), initiators, monomer soluble, are used. The system is supplied with soap-type stabilizers to assist in keeping the polymerizing droplets distinct from each other.
Removal of the heat of polymerization is easy in small reactors, but becomes a problem in larger ones due to the declining ratio of side wall heat transfer area to volume. Two methods are employed to increase heat transfer. A suspending medium reflux is available if the refluxing temperatures will yield a satisfactory molecular weight distribution. Internal cooling coils can be used if the fouling is not an excessive problem.
During the course of the reaction, the polymerizing droplets exhibit various degrees of “stickiness” which foul the vessel’s heat transfer surfaces and threaten destruction of the reacting system if these sticky droplets are not kept apart by the agitation intensity.
At the completion of the reaction, the polymer is freed from the stabilizer by washing, commonly accomplished during the phase where the unreacted monomer is stripped from the polymer. Since the polymer granules are of good size (typically 100 mesh), they can frequently be used directly.
This method of polymerization is particularly suitable for the production of hard, glass-like vinyl polymers from vinyl chloride and vinylidene chlorides such as PVC (Polyvinyl Chloride) as well as most if not all thermoplastic resins.