Vinyl chloride monomer in PVC: from problem to probe

Study of the transport of vinyl chloride monomer -(Vc) in PVC was initially undertaken to help design processes for removing residual monomer from the polymer. Vapor sorption experiments on PVC powder samples obey simple Fickian kinetics at low VCM concentrations and provide a convenient method of determining the diffusivity of VCM in PVC. From a simple model of diffusion in uniform spherical particles, it was' predicted that essentially complete monomer removal from particles of diameter under 5 im could be achieved in a few minutes at temperatures near 100°C. The design of continuous steam stripping systems which exploit this rapid desorpti'on rate, combined with a progressive reduction in non-porous particle content, has re-' sulted in a remarkable reduction in residual VCM concentration of commercial suspension PVC over the past five years. In addition to their practical value in helping overcome "the VCM problem", vapor sorption experiments have proven to be highly useful in characterizing the structure and properties of PVC powders. Analysis of sorption kinetics through a Fickian diffusion model for heterodisperse spheres has been employed to. determine particle size distributions for all common types of PVC resins. Variations of sorption isotherms have been found to reflect history-dependent changes in the free volume of PVC, and study of non-Fickian sorption kinetics has led to new methods of observing glassy-state relaxation processes. Vapor sorption techniques have now been applied to a variety of polymer powder/vapor combinations and have shown that effects observed in the VCM/PVC system are quite characteristic of the general interactions between organic vapors and glassy polymers. The use of small organic molecules as probes in vapor sorption experiments represents a broadly applicable technique for the characterization and study of polymer structure and properties.