PEP Review IX-1-6
PolyvinylChloride, Vapor Phase Bulk Polymerization
Published: September 1973
Polyvinyl chloride is produced mainly by suspension polymerization. The suspending agent, water, facilitates the removal of heat of polymerization but also has drawbacks: it increases the reactor volume required, it has to be removed from the polymer by drying, and it has to be disposed of. This all adds to the investment and operating cost.
Bulk polymerization of PVC was a long time goal of researchers. The technical problems were not solved, however, until the mid-1960s when Pechiney-St. Gobain announced a liquid phase bulk polymerization process that was further improved during the following years, Pechiney-St.Gobain's process operates in two steps: first, the vinyl chloride is polymerized to about 10% conversion at which point the reaction mass is still fluid. Then, the mixture is transferred to special horizontal autoclaves equipped with heavy duty agitators. There, the polymerization is completed. The Pechiney-St. Gobain process, as evaluated by SRI, requires a significantly lower investment than the corresponding suspension process and has also some advantages in utilities consumption.
The development of a superactive initiator by PPG Industries may benefit the suspension process. The new initiator is capable of cutting the reaction time to two hours from five to six hours. Thus, the superfast initiator would further increase the productivity of the very large suspension reactors and further narrow the investment gap between the two process routes, Whether the liquid phase bulk process can profit from a superfast initiator is not clear. The bulk polymerizers are limited in size to about 3,800 gal by fabricating considerations. The heat from these bulk polymerizers is removed through the jacket; it is not clear whether a reflux type cooling could be used effectively with the solidifying polymer mass. On the other hand, a vapor phase bulk polymerization would most likely benefit from the superfast initiator since no equipment size limitation is apparent.