Visualisation and analysis of polyethylene coextrusion melt flow

Abstract

Polymer melts experience complex, time variant, stress and deformation fields on their passage through fixed geometries in many conversion operations. Flow complexity is further increased in operations involving the co-joining of two or more melt streams where one confining boundary is moving and viscoelastic. Such a complex situation arises in coextrusion processes. This work covers experimental studies on polyethylene melt flows in complex coextrusion geometries with a view to understanding the stress fields involved and their effects on flow stability. A 30°1coextrusion geometry is studied using two extrusion arrangements. In one arrangement a single extruder is used to feed a 'bifurcated' die design wherein the melt stream is split prior to, and rejoined after, a divider plate in the die. In the other design melt streams are delivered to, and converged at 30°, using two independent extruders. In a second die melt streams are brought together at 90°. In each die arrangement melt flow in the confluent region and die land to the die exit was observed through side windows of a visualisation cell. Velocity ratios of the two melt streams were varied and layer thickness ratios producing instability are determined for each melt for a variety of flow conditions. Stress and velocity fields in the coextrusion arrangements were quantified using stress birefringence and particle image velocimetry techniques. The study demonstrates conclusively that wave type interfacial instability occurred in the coextrusion geometries when the same low density polyethylene melt is used in each stream. This observation occurred at specific, repeatable, stream layer ratios in each die arrangement. The complex flows were numerical modelled using a modified Leonov model and Flow 2000P software. There was reasonable agreement between modelled at experimentally determined stress fields. Modelling however provided far more detailed stress gradient information than could be resolved from the optical techniques. A total normal stress difference (TNSD) sign criterion was used to predict the critical layer ratio for the onset of the interfacial instability in one die arrangement and good agreement between theory and experiment has been obtained. The study conclusively demonstrates wave type interfacial instability in the coextrusion process is not caused by process perturbations potentially introduced by extruder screw rotation but is associated with process-history dependant differences in melt elasticity. © 2009 American Institute of Physics.