An understanding of the relationships between shear and extensional
flows with temperature, processing rate and molecular
weight is a useful tool for process design, optimisation and troubleshooting.
The characterisations carried out during these studies
have shown that the flow behaviour of the HPC grades studied was
typical of many thermoplastic polymers of relatively high molecularweight,
up to a certain processing rate. Above this critical rate or
stress significant instabilities were generated, in particular for the
two higher molecularweight HPC-SL and HPC-L grades at lowtemperatures.
Two distinct instabilitieswere observed; the appearance
of a surface roughness at high shear rates whichwas followed by the
formation of a foamed extrudate core. The prior has been reported
by many workers for polymers such as polyethylene, polypropylene,
polystyrene and polybutadiene but formation of foam without
[25] addition of additive has not been reported. It is important to
understand the basic cause of this phenomenon, which is potentially
advantageous to drug delivery system design, and is therefore
the subject of ongoing research in our laboratories.