Modeling and Estimation of the Pressure and Temperature dependent Bulk Density of Polymers

Abstract

The bulk density of polymers is a crucial factor for the throughput and the pressure build-up of single screw plasticating units. It depends not only on the density of the polymeric material itself, but also on the geometry of the single pellets, on the dimensions of the screw channel and, furthermore, on the temperature and the pressure. A review of the existing models shows that there is potential for improvement, as their applicability is restricted. It is difficult to obtain model parameters, as they can only be determined by experiment, but there are only a few devices which enable temperature- and pressure-dependent bulk density to be measured. Furthermore, these devices are not standardized. This work presents a new model which can be used to calculate the bulk density as a function of pressure and temperature and an adapted approach that considers the dimensions of the single pellets and the screw channel. The model is verified by both, new data derived from experiments and data from the literature. Compared to existing models, this new model fits the experimental data better, requires fewer parameters and shows much better extrapolation behavior as well as a continuous changeover behavior at the melting temperature. Another big advantage of the new model compared to the existing ones is that the model parameters of this new model can be estimated from the stress at yield and from the melting temperature. This enables a rough estimation of the pressure and temperature dependent bulk density without having to measure it.