First normal stress difference measurements for polymer melts at high shear rates in a slit-die using hole and exit pressure data

In this paper we determine the ability to obtain values of the first normal stress difference, N1, at relative high shear rates (1.0–100.0 s−1) in a slit-die for several polymer melts of varying degrees of fluid elasticity using hole pressure data and the Higashitani–Pritchard–Baird (PHB) equation. Accurate measurements of the hole pressure, Ph, which is defined as the difference between the pressures measured by means of a flush-mounted pressure transducer and a transducer mounted at the base of rectangular slot (i.e. the hole), are extremely difficult to make because Ph is typically less than 2% of the absolute pressure measured. By calibrating the pressure transducers once mounted in the slit-die, accounting for viscous heating, and ensuring the transducers were properly aligned, precise measurements of Ph were made even with commercial pressure transducers. Values of N1 obtained from Ph data agreed well with values of N1 and twice the storage modulus (2G′) for four different polymer melts for shear rates approaching 70 s−1 and wall shear stresses approaching 80 kPa. Furthermore, exit pressure measurements were made simultaneously in the same slit-die device, and values of N1 extracted from the data were found to be as much as 100% higher than values measured by means of a cone-and-plate device and the hole pressure which was consistent with the reports of others.