Model polymer fluid systems

Abstract

The development of non-shear thinning elastic liquids and their impact in bridging the gap between predictions from continuum theory and practical reality in problems of interest in polymer processing is described. It is shown that the highly elastic, constant viscosity fluids represent a class of fluid behaviour consistent with molecular theory, which can be described at low shear rates by a constitutive equation developed by Oldroyd in 1950. Experimental observation with these materials has allowed elastic effects to be observed in flow fields for the first time in the absence of any effects due to a varying viscosity, and has also allowed direct comparison to be made with predictions from continuum theory in these flows. In squeeze film flows good agreement between observation and prediction is obtained. In exit flows (extrudate swell) significant elastic effects are observed and predicted but the agreement between observation and numerical simulation is poor at high levels of elasticity. In tubular entry flows (die entry flows) large vortex enhancement due to elasticity is observed which is not predicted by the appropriate continuum analysis. It is not clear why continuum theory is adequate in some flow and inadequate in others. Nevertheless great strides have been made in the solution of flow problems of importance in polymer processing. © 1985 IUPAC