Entrance pressure instability of LLDPE and its composites

Abstract

Oscillating flow, one of the most important instability phenomena in capillary flow, has long been identified to be related with the whole capillary pressure oscillation under constant piston speed. Although the appearance of surface distortions has been theorized by Weill since l980 to originate in high-frequency oscillatory flow created at the die entry, the entrance pressure instability has not yet been studied experimentally. In this study Bagley correction was applied to calculate the instability of entrance pressure drop in capillary flow via both a constant-pressure capillary rheometer and a constant-rate capillary rheometer. The rheology of LLDPE and its composites with different kinds of fillers was measured to elaborate the phenomenon of flow instability. The entrance pressure fluctuation was clearly observed during the extrusion process, which was revealed to closely correlate with stress perturbation and extrudate morphologies. Furthermore, the flow instability was found to be depressed by increasing temperature and could be explained by the local stick-slip transition and disentanglement.