THE PRESSURE-DEPENDENCE OF THE SHEAR AND ELONGATIONAL PROPERTIES OF POLYMER MELTS

A capillary rheometer has been modified, by the addition of a second c hamber and valve arrangement below the main die, in order to measure t he pressure drops associated with the capillary and entry flows of a n umber of polymer melts as a function of pressure. The five polymer mel ts investigated are high- and low-density polyethylene, polypropylene, polymethyl methacrylate and polystyrene, each of which is tested at t hree temperatures within the normal processing range, at apparent shea r rates between 50 and 2500 s(-1) and at mean pressures ranging from a tmospheric up to 70 MPa. The capillary pressure drop data are used to obtain shear viscosity functions using conventional capillary rheometr y expressions, whilst extensional viscosities are estimated from orifi ce pressure drop data via the Cogswell-Binding analysis. Both the shea r and extensional viscosity curves for all of the polymers are seen to exhibit an exponential pressure dependence that can be characterised by pressure coefficients that are found to be independent of temperatu re. Trouton ratios for the polymers can be specified by an expression with separable strain rate and pressure dependence terms, the latter o f which is again exponential. The pressure coefficients of the Trouton ratio terms then orders the pressure dependence: PS>PMMA>PP>HDPE>LDPE . Our major conclusion is that the Trouton ratio for some of the polym er melts can be a strong function of the pressure, indicating that the variation of extensional properties with pressure can be greater than that of the shear properties. (C) 1998 Elsevier Science B.V. All righ ts reserved.