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

Flow visualisation has been used to study the in process how 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 ra tes 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 regi ons of each die. Constant extensional strain rate conditions were approache d at low flowrates in the hyperbolic die. Constant strain rates were not at tained for the hyperbolic die at high flowrates nor, as expected, for the a brupt entry die. Analysis of flows using birefringence showed significant s hear boundaries developed at the wall of the hyperbolic die at high flowrat es. Such boundaries, in combination with the non-Newtonian behaviour of the viscoelastic polymer melt, lead to non-constant strain rates along the cen treline of the die at higher flowrates. Stress, strain, and strain rate dat a for the low density polyethylene melt are presented which, although deriv ed under flow conditions that are not strictly steady in the Lagrangian sen se, are experimentally accessible and informative. Stresses and strains der ived from the flow visualisation technique are compared with constant strai n rate data obtained from a Rheometrics RME elongation rheometer. Close agr eement was found between data from the two techniques. PRC/1610 (C) 2000 Io M Communications Ltd.