1. Field of the Invention
The present invention relates to a method for efficiently and economically removing unreacted monomer from an aqueous dispersion of vinyl chloride polymerizate produced by the suspension or emulsion polymerization of vinyl chloride monomer or a mixture of monomers mainly composed of vinyl chloride in an aqueous medium and a system for the purpose.
2. Description of the Prior Art
Polyvinyl chloride (hereinafter referred to as PVC for brevity) is usually produced by the suspension or emulsion polymerization of vinyl chloride monomer in an aqueous medium, optionally with one or more comonomers. The aqueous dispersion of PVC, i.e. slurry or emulsion, formed following the completion of polymerization contains a considerable amount of residual vinyl chloride monomer remaining unreacted. Also in the finished PVC product obtained from the aqueous dispersion through separation and drying processes, a significant amount of unreacted residual monomer is contained. These facts tend to cause environmental pollution and adversely affect workers' health.
For the purpose of reducing residual monomer contents in the finished PVC product to a satisfactorily low level, it has been proposed to employ one or more means before or during the processing of the aqueous PVC dispersion, including the step of separating PVC from the aqueous dispersion and the step of drying the wet PVC cakes. The various methods practiced hitherto have failed to give satisfactory results. The complete removal of unreacted residual vinyl chloride monomer is an extremely difficult problem owing, on the one hand, to the porous structure of the particles of PVC produced especially by suspension polymerization, which is apt to absorb and retain more unreacted monomer; and, on the other hand, to the relatively low thermal stability of PVC, which makes it impossible to process the PVC with heat at a desirably high temperature.
The aqueous PVC dispersion, for example, is heated in a vessel under reduced pressure for a length of time, as disclosed in Japanese Public Disclosure No. 51-17288 of Japanese Patent Application based on U.S. Patent Applications Ser. Nos. 482,112, 482,113 and 482,115. In this method, however, it is not practical to conduct the heating at a sufficiently high temperature for a sufficiently long period of time for the complete removal of unreacted monomer, since discoloration tends to occur to the resulting PVC product at temperatures above a certain limiting point.
As a further example, the aqueous PVC dispersion is dehydrated by a dehydration apparatus, such as centrifugal separators, to form wet cake and the wet cake is brought into contact with steam, as disclosed in U.S. Pat. No. 3,956,249. This method is disadvantaged by a similar unsolved problem and also by difficulties encountered in the construction and operation of the apparatus to handle the wet cake.