In producing polyvinyl chloride by the suspension polymerization method, the monomer or monomer mixture is finely distributed in the aqueous phase by mechanical agitation, stabilized with the aid of protective colloids, and polymerized in the presence of free radical initiators. Suitable as protective colloids or suspension stabilizers are water-soluble, natural polymeric compounds, such as starch (amylose), gelatin, and the like; water-soluble cellulose derivatives, such as alkyl celluloses, hydroxyalkyl celluloses, and the like; water-soluble synthetic products, such as partially saponified polyvinyl acetates, copolymers of maleic acid anhydride and vinyl acetate, and also various combinations of the aforementioned substances.
Polyvinyl chloride resins produced with the mentioned suspension stabilizers are suitable for hard processing (processing into rigid components); but in soft processing (plasticizer-enhanced processing, processing into soft components), due to an unsatisfactory absorption of plasticizer, they produce finished parts having a large number of so-called gels or fisheyes. Furthermore, due to the low granular porosity of the products, the residual monomer can be removed only by expensive and difficult procedures. To counteract these disadvantages, surfactants have been added as porosity-enhancing agents, such as sorbitan monolaurate, for example (DOS [German Unexamined Laid-Open Application] 2,208,796). However, polyvinyl chloride resins manufactured according to this process still have an excessively high gel content (fisheyes), especially if the polyvinyl chloride resin is processed with a highly viscous plasticizer. In addition, such products still have porosity inadequate for conducting a vinyl chloride degasification step in a simple way. Moreover, the use of tensides, such as the above-mentioned sorbitan monolaurate, leads to products having unsatisfactory electric resistance values.
In suspension-polymerized polyvinyl chloride resins prepared by conventional processes, an interdependence exists between the porosity and the bulk density. An increase in porosity for the purpose of a satisfactory removal of monomers entails a reduction of the bulk density. However, a decrease in bulk density means a reduction in output of extruded polyvinyl chloride resin.