The Effect of Pressure on the Shear Viscosity of Polymer Melts

Abstract

In many polymer processing operations like injection molding, the polymer melt flows under the influence of high pressure, sometimes exceeding 20,000 psi. It has been suggested by a number of workers, on the basis of both experimental and theoretical considerations, that the effect of hydrostatic pressure on viscosity cannot be ignored in such instances. However, most of the proposed techniques to determine the effect of pressure on viscosity suffer from limitations especially at high shear rates. The present work proposes a more general and quantitative method for obtaining the effect of pressure on the viscosity of polymer melts from capillary viscometer data. In addition to its applicability over a wide range of practical shear rates, the proposed method employs less restrictive assumptions than previous techniques and contains corrections for the effect of viscous heating. The results obtained for the effect of pressure on the viscosity of polystyrene at 180°C are compared to rheological data obtained by conventional methods. Preliminary results on the pressure coefficient of viscosity appear to be in agreement with predictions based on the W-L-F equation.