Reactors for suspension polymerization

The term suspension polymerization is used traditionally for polymerization in a dispersed monomer phase. Occasionally the term is also u for the type of process described in the preceeding section, but for that the term precipitation polymerization is more appropiate. The polymerization in a dispersed monomer phase might also be called dispersion polymerization , but this term is used for polymerizations where the monomer is originally dissolved, and the polymer is separated as a second liquid phase, usually absorbing some monomer. Suspension polymerization is used on a large scale for the manufacture of polyvinyl chloride (PVC), and for specific grades of polystyrene. 

There is an essential difference between suspension and emulsion polymerization, which is discussed in the next section. In emulsion polymerization Ae polymer forms a second dispersed phase, which consists of particles smaller than 1 Lim. In suspension polymerization die polymer is formed within the dispersed monomer phase. This consists of droplets that are usually larger than 10 fim. In both types of process the continuous phase is usually water. 

Suspension polymerization is mostly carried out batchwise, also on large scales. In a stirred reactor, filled with a certain amount of water, liquid monomer and a surfactant are added, and the monomer is dispersed. TTie catalyst or initiator, often an organic peroxide, is fed with the monomer. The mixture is heated till the reaction starts, then the temperature is allowed to rise to the desired level, and kept constant by cooling. 

In some processes, instead of a surfactant, a finely ground inorganic salt is used, that is insoluble in water, such as calcium carbonate or calcium phosphate. The solid should be partially wetted by each liquid phase, so that it will accumulate at the interface. When the particles have the right size (on the order of 10 )im) they will effectively stabilize droplets of a certain size range. 

In the first part of the process some coalescence and redispersion may occur, but as the polymerization proceeds, the droplets become more viscous and more rigid, and retain their identities. When the polymer and the monomer are completely miscible (as in the case of styrene), the droplets will gradually become solid particles. In the case of PVC, the polymer precipitates inside each particle. 

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