Melt Viscosity of Low Pressure Polyethylene

I. Newtonian Viscosity and Dependence on Stress

Abstract

The relationships between shear stress and flow rate for the low pressure polyethylene were measured by a capillary viscometer. Two Hizex polyethylene samples, made by Ziegler process, having number-average molecular weights of 40, 000 and 61, 000, were investigated under the following experimental conditions; shear stress range 10⁵-10⁶ dyne⋅cm^-2; temperature range 180-340°C; diameter and length of the capillary 0.5 mm and 1mm, respectively. The rate of shear increases with shear stress more rapidly than would be expected from thedirect proportionality for the Newtonian liquid. The zero-shear viscosities ηo obtained by extrapolation to zero shear stress were found to be fairly large. Though ηa showed thesmaller value for the lower molecular weight and became smaller with temperature, η0 of the sample having molecular weight of 40, 000 was 2, 700 poise at 280°C. The apparent activation energy for viscous flow was 6-8 kcal/mol in the range of experimental conditions. The flow curves were well represented by the so-called power law for the flow curve; D=aτ_w^N. The value of the exponent N were the smaller one for the smaller molecular weight, decreased with increasing temperature and lay between 2.0 and 2.4 under the experimentalconditions.