Wall slip of molten high density polyethylenes. II. Capillary rheometer studies

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Abstract

Above the critical stress for slip, the procedures normally used to analyze the results of capillary flow data give anomalous results. In particular, the Bagley plots are curved, even when a variation of viscosity with pressure is not anticipated, and the Mooney technique used to calculate the slip velocity gives results that indicate that the slip velocity depends on the L/D ratio. It is proposed that these phenomena arise from the dependence of the slip velocity on the wall normal stress, which implies a dependence on pressure. Based on this hypothesis, an approximate method is developed for interpreting the results of capillary flow experiments to determine the slip velocity as a function of both the wall shear stress and the pressure. The large available data set was used to incorporate into the model the effects of molecular weight parameters and temperature on the slip velocity. Finally, a detailed model for slip flow in a capillary was formulated that takes into account that the slip velocity and wall shear stress vary along the flow direction due to the pressure gradient. This model was used to evaluate the validity of the approximations used in the approximate data analysis technique for determining the slip velocity.

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