In process measurement of apparent extensional viscosity of low density polyethylene melts using flow visualisation

Abstract

Flow visualisation has been used to study the in process flow behaviour of a low density polyethylene melt as it is processed through planar hyperbolic and abrupt entry slit dies on a commercial scale extruder. The former die profile consisted of a planar hyperbolic section that gradually merged with a parallel slit and was designed to promote constant extensional strain rates at the centreline of flow. The melt was processed through these dies at several flowrates. Extensional strain rates were determined by performing particle velocimetry at the centreline of melt flow in the contraction regions of each die. Constant extensional strain rate conditions were approached at low flowrates in the hyperbolic die. Constant strain rates were not attained for the hyperbolic die at high flowrates nor, as expected, for the abrupt entry die. Analysis of flows using birefringence showed significant shear boundaries developed at the wall of the hyperbolic die at high flowrates. Such boundaries, in combination with the non-Newtonian behaviour of the viscoelastic polymer melt, lead to non-constant strain rates along the centreline of the die at higher flowrates. Stress, strain, and strain rate data for the low density polyethylene melt are presented which, although derived under flow conditions that are not strictly steady in the Lagrangian sense, are experimentally accessible and informative. Stresses and strains derived from the flow visualisation technique are compared with constant strain rate data obtained from a Rheometrics RME elongation rheometer. Close agreement was found between data from the two techniques.