Effects of Pressure and Supercritical Fluids on the Viscosity of Polyethylene

Abstract

A high-pressure sliding plate rheometer was used to determine the effects of supercritical fluids and pressure on the viscosity of molten high-density polyethylene (HDPE). In this instrument, the shear strain, temperature, pressure, and gas concentration are all uniform, and a shear stress transducer senses the stress in the center of the sample to eliminate edge effects. The effect of pressure alone at 180 °C was determined up to 70 MPa. The sample exhibited piezorheologically simple behavior, and the Barus equation was found to describe the pressure-shift factor. The effects of gas concentration and pressure on the viscosity were determined up to 23 wt % CO2 concentration and 34.5 MPa. Because the gas of interest is the pressurizing fluid, it is necessary to ensure that the sample was saturated before measurements were made. The saturation time was estimated by use of Fick's law, and the prediction was confirmed by monitoring the viscosity as a function of time. This implies that the rheometer can be us...