Shear Flow Properties of Carbon Fibre-Filled Polyethylene Melts

Abstract

Abstract A fundamental investigation of the influence of fibre loading, fibre length and matrix properties on shear viscosity, complex viscosity and the first normal stress coefficient of filled polyethylene melts prepared by different compounding techniques was carried out. Carbon fibre was compounded into two different types of low-density polyethylene (PEA and PEB) at concentrations up to about 20% by volume using different compounding techniques: a single screw extruder, and a twin screw extruder. The main difference in flow behaviours for the two kinds of carbon fibre-filled systems (CF/PEA and CF/PEB) is not due to differences between the matrix polymers, but due to the difference in fibre length or its distribution caused by the difference in compounding technique. Increasing fibre loading and fibre length increases the shear viscosity and the normal stress coefficient, with the highest increase observed at low shear rates. Especially for longer fibre-filled melts (CF/PEA series) one can estimate the yield stress in the very low shear rate region from the steady shear and dynamic flow data. In the moderate and high shear rate regions the viscosity and normal stress coefficient are influenced greatly by shear flow. Data on the relative viscosities ηr, the relative complex viscosities |η*|r and the relative normal stress coefficients ψr of the two filled systems in the moderate and high shear rate regions were compared with the empirical equation derived previously for ηr for short fibre filled polymer melts. It was found that a form of the expression similar to that obtained previously could describe ηr, |η*|r, and ψr for relatively long carbon fibre filled systems.