Evaluation of the use of a semi-hyperbolic die for measuring elongational viscosity of polymer melts

The semi-hyperbolic (SHPB) die with and possibly without wall lubrication has been proposed as a device for measuring the elongational viscosity of polymeric fluids. Using numerical simulation under the condition of complete wall slip, it was found for two polyethylenes (LDPE and LLDPE) that the calculated elongational viscosity values agreed well with strain-averaged values, <ƞe>, obtained from independent measurements in stretching type rheometers. This is in agreement with the original hypothesis of Everage and Ballman (E-B). Numerical simulations showed that the Baird and Huang (B-H) approach for calculating <ƞe>, which accounts for the shear stress due to geometric considerations in the presence of complete slip, agreed with data better than did the E-B approach. Numerical simulations using varying degrees of wall slip indicated that reasonable values of <ƞe> could be obtained using the B-H approach with wall slip levels which could be most likely reached using a coating such as a flouroelastomer. The numerical simulations provided an explanation as to why the elongational viscosity values determined in the SHPB die for resins such as LDPE, which are extensional-strain hardening, are less sensitive to wall slip than non-strain-hardening resins such as LLDPE.