The Pressure-Volume-Temperature Behavior Polyethylene Melts

Abstract

Abstract The pressure-volume-temperature (PVT) melt behavior of 12 polyethylene resins was evaluated at pressures up to 200 MPa, using both isothermal and isobaric measurements in a GNOMIX high pressure dilatometer. The resins included high density polyethylene (HDPE), low density polyethylene (LDPE) and linear low density polyethylene (LLDPE). They were produced using a variety of catalysts, including Ziegler-Natta (ZN) and metallocene catalysts. The PVT data were used to evaluate two empirical equations of state (the Tate and Inverse Volume equations) in predicting PVT behavior of the melt, the isothermal compressibility, and the thermal expansion coefficient. The dependence of the melting and crystallization temperatures on pressure was also evaluated, and compared to existing equations. It was not possible to identify structural effects on the PVT melt behavior, as well as the isothermal compressibility and thermal expansion coefficient. However, some slight dependence on density was observed for the parameters of the equations of state. The isobaric experiments revealed that the pressure effect on the melting and crystallization temperatures was similar for all resins. While the melting and crystallization temperatures varied widely for the various resins, the pressure coefficients of the melting and crystallization temperatures were approximately equal (0.25°C/MPa.) The main differences in the behavior of the resins were in the transitional region during melting and crystallization.