High pressure capillary rheometry of polymeric fluids

The application of superposition theory to capillary and entry pressure dro p data for a number of polymer melts, measured at elevated pressures, is in vestigated in order to gain information on their pressure dependencies in b oth shear and elongational flows. To facilitate the study a capillary rheom eter has been modified, by fitting a second chamber and valve arrangement b elow the main die, which allows the pressure downstream of the relevant cap illary and orifice dies to be raised so that the mean pressure associated w ith each die can be varied. Five polymer melts are investigated: high-densi ty polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP) , polymethyl methacrylate (PMMA) and polystyrene (PS). Each of these are te sted at three temperatures within the normal processing range, at apparent shear rates between 50 and 2500 s(-1) and at mean pressures ranging from at mospheric up to 80 MPa. Time-temperature-pressure superposition is applied to the capillary and orifice pressure drop data for each of the polymers an d the resulting pressure coefficients are found to be independent of temper ature. The superposition is found to hold for all of the samples considered in both shear and elongational flow, although the degree of fit is best fo r the HDPE and LDPE. The resulting pressure coefficients for the shear and elongational flows then order the pressure dependencies of the polymers as follows: PS > PMMA > PP > LDPE > HDPE. It is demonstrated how this ordering is determined by the molecular structure of the polymers. However, the mos t significant result is that for each polymer the shear temperature and pre ssure coefficients are of similar value to those of elongation, with the ex ception of PS that has considerably greater coefficients in elongation part icularly for temperature. Complementary results for single and multigrade o ils are also included, in the appendix. (C) 2000 Elsevier Science Ltd. All rights reserved.