The application of the multi-pass rheometer for precise rheo-optic characterisation of polyethylene melts

Abstract

This paper presents an overview of the application of the Multi-Pass Rheometer (MPR) for the study of polymer processing with particular reference to flow birefringence measurements. The MPR is a two-piston capillary rheometer recently developed at the University of Cambridge. We review previous experiments that have been carried out on molten polymers using the MPR and we report on recent results of flow birefringence experiments for a polyethylene (PE) melt. Using a specially designed optic flow cell, molten PE was passed within a Double Cavity Die. A field-wise flow-induced birefringence technique was used to obtain the stress field in a complex flow of a flowing molten polymer. In general, for the PE tested, the entry stress field in the second cavity was found to be different from that in the first. The differences observed in the overall entry stress fields in the two slit configuration could be explained as resulting from the extensional and shear strain history experienced by the polymer. We have carried out matching K-B.K.Z.–Wagner numerical simulations and found that reasonable consistency can be achieved for certain polymers. As a consequence of these experiments, we have found that the MPR is a very good platform for experimental flow birefringence studies that can be matched with numerical simulation and in addition experiments can be carried out with small 20 g quantities of material.