Transport of organic vapors and liquids in poly(vinyl chloride)

Abstract

AbstractThis paper reviews research since 1980 on the equilibria and kinetics of transport of small organic molecules in rigid and plasticized PVC. The forms of both the solubility isotherms and the sorption kinetics are shown to change as the PVC/penetrant system undergoes a glass‐rubber transition with an increase of either temperature or penetrant concentration. The isotherms are of “dual‐mode” form (concave to the activity axis) for the glassy state, and show an inflection to Flory‐Huggins form when the penetrant concentration exceeds Cg, the transition composition at the experimental temperature. The solubility at a given penetrant activity is governed primarily by the PVC/penetrant interaction parameter, χ. Sorption kinetics are Fickian for conditions producing small changes of concentration in either the glassy or rubbery state. For sorption into initially unplasticized PVC, kinetics are anomalous if the final penetrant concentration is between about Cg/2 and Cg, and Case II if Cg is exceeded. The magnitude of the Fickian diffusion coefficients depends largely on the geometric factors of molecular size and shape of the penetrant; this dependence is much steeper in the glassy than in the rubbery state. Recent results show that carbon dioxide displays both high diffusivity and substantial solubility in PVC under high pressure; this combination makes compressed CO2 uniquely useful in accelerating the absorption of low‐molecular‐weight additives into PVC.