Overview

The flow properties of molten polyethylene (PE) are of primary practical concern in forming, moulding, or extrusion processes. Deviating considerably from ideal Newtonian flow, melts of PE show non-ideal viscoelastic behaviour; their shear rate versus shear stress plot is non-linear. Very closely related to this non-linear behaviour of pseudoelastic materials is their elastic nature, in that some recovery may be observed when an applied stress is relaxed. Molecularly, chain disentanglement in the course of increasing stress can explain these phenomena.

Melt fracture, another phenomenon typical of PE melts, occurs on extrusion through a capillary die at high shear rates, resulting in an irregular and rough extrudate. The critical shear rate or stress is that at which a rough surface is first observed. Since this irregularity increases with increasing extrusion rate, it poses limitations on processing rates and requires control in the manufacture of sheet and film.

Rheological properties can be evaluated from the data obtained using rotational rheometers, which are of two types: the rotating cylinder and the rotating flat disc or cone. Rheological parameters may be calculated from measurements of torque, speed of rotation, and the approximate geometrical dimensions of the system. A device for measuring the processability is the Brabender Plastograph, which measures the torque required to mix a molten resin in a heated chamber by dual mixing blades. The equilibrium torque is a measure of processability. Stability can be measured by the decrease of equilibrium torque as a function of milling time, while the variation...