Transport Properties of Oriented Poly(Vinyl Chloride)

Abstract

In commercial thermoplastic forming operations, poly(vinyl chloride), or PVC, is stretched to several times its original dimensions at fixed rates and temperatures. The stretching operation is usually followed by rapid quenching which freezes in the strain-orientation imparted to the polymer during forming. Since orientation is known to strongly influence physical properties, it was of interest to correlate several critical end-use properties with the range of processing conditions which PVC might experience during a typical forming operation. In addition to mechanical strength, gas and vapor transport are perhaps the most critical properties from the packager’s point of view. This report describes the influence of 1) degree of biaxial orientation, 2) stretching rate, and 3) stretching temperature on the oxygen and water vapor transport parameters of non-impact modified PVC. The diffusion and solubility coefficients, as well as permeability, were determined for oxygen transport, while only equilibrium permeability was measured for water vapor. Structural changes were monitored by measuring both birefringence and density as a function of orientation. Since processing temperature was found to have a pronounced effect on properties, separate annealing studies were carried out, where variations in the transport properties were correlated with structural changes as measured by both x-ray diffraction and differential scanning calorimetry, DSC.