Although this method is very similar to suspension polymerization, the two schemes differ in two important aspects; the polymerization initiator is located in the aqueous phase and the particles of polymer produced are typically in the order of 0.1 micron in diameter, some ten times smaller than the smallest encountered in suspension polymerization.
Typical recipes include a significant amount of emulsifier (soap) necessary to maintain the emulsion identity and water soluble polymerization initiators.
In the initial part of the reaction sequence, the polymerization takes place around seed sites called micelles which are aggregates of soap molecules. As polymer is formed, these micelles grow by the addition of monomer from the water phase. At about 3% polymerization, further polymerization takes place within the polymer particles already formed. The monomer droplets are unstable at this stage and, if the agitation is stopped, they would coalesce into a continuous phase containing no polymer. Droplets of monomer act as reservoirs which feed the growing polymer particles by diffusion through the water phase. Monomer droplets disappear at about 80% completion and generally the polymerization is terminated at about 90% completion. The agitation intensity and impeller tip speed are vital factors in a successful emulsion polymerization reactor.
The recovery of the unreacted monomer and the isolation of the resultant polymer are handled in standard ways by stripping and spray drying. Frequently, the emulsion may be directly unstable.
The advantages of emulsion polymerization are that its use permits a rapid polymerization to a high molecular weight with narrow molecule weight distribution, the viscosity is typically lower than other methods, heat is relatively easy to remove from the reactor with water as the continuous phase and suitable for the production of soft and sticky polymers. The disadvantages of emulsion polymerization is that the resin produced frequently becomes contaminated with the emulsifier, almost inevitably leading to poor clarity and other undesirable properties.
As with other methods of polymerization, heat removal and polymer stickiness must be overcome for a satisfactory production scheme.
Products made by this method include polyvinyl acetate for paint and adhesives, carboxylated styrene-butadiene copolymer, SBR and Buna-N rubbers, and ABS (Acrylonitrile-butadiene-styrene polymer).