EVALUATION OF MELT FLOW INSTABILITIES OF HIGH-DENSITY POLYETHYLENES VIA AN OPTIMISED METHOD FOR DETECTION AND ANALYSIS OF THE PRESSURE FLUCTUATIONS IN CAPILLARY RHEOMETRY

An optimised method for the detection and analysis oft he time dependent pressure associated with the development of melt flow instabilities during extrusion through a capillary die was developed and validated. The magnitude and frequency of the developed quasi-periodic distortions, as well as the pressure profiles along the die length, were found to depend on the MWD, topology, melt elasticity and uniaxial extensional flow properties. Both the onset and magnitude of strain hardening in uniaxial extension appear to be related to the onset for the development of melt flow instabilities under capillary flow. For a better understanding of the role of the extensional properties (namely that of a purely elastic instability) the Hencky strain to failure was also determined and correlated to the observed flow instabilities. Time resolution of the capillary rheometer was improved by a factor of 1000, pressure resolution by a factor of 100 compared to the original set-up.