Two-step process for post-chlorinating poly(vinyl chloride)

Abstract

A two-step process for the post-chlorination of PVC homopolymer and other predominantly vinyl chloride-containing polymers is disclosed, which process produces chlorinated PVC ('CPVC') containing at least 70% by weight Cl, in a fast, but controllable, aqueous, two-step process catalyzed using an organic peroxide catalyst instead of actinic radiation. Less than 10% excess Cl remains after the second stage, even when 75 wt% Cl CPVC is made. In the first step, an aqueous slurry of PVC is chlorinated in the absence of liquid chlorine and free oxygen, preferably at a starting temperature in the range from about 30.degree. C. to 75.degree. C., in the presence of at least an essentially stoichiometric amount of chlorine required to make the desired Cl content in the end product, and, a catalytic amount of the peroxy catalyst, at least sufficient to produce `first step CPVC` having an intermediate Cl level of chemically bound Cl in the range from 67% to 72% by weight. The temperature is maintained below the T.sub.g of the first step CPVC formed. In a second step, the first step CPVC formed is further chlorinated in the presence of less than a 10 wt % excess of chlorine, at a temperature in the range from 90.degree. to 130.degree. C. until the Cl content in the CPVC is in the range from 69% to 75% by weight. At least a 3% increase in Cl content of the first step CPVC is obtained within about 30 min irrespective of the volume of the slurry. Preferably, a substantial excess of chlorine is not used, and no remaining chlorine needs to be recovered. CPVC with a 75% Cl content is made in about 3 hr. The CPVC end product so obtained is unexpectedly more easily thermoformed than another CPVC end product having the same Cl content, but made by a prior art aqueous process.