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

Abstract

In this paper we determine the ability to obtain values of the first normal stress difference, N 1, 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, P h, 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 P h 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 P h were made even with commercial pressure transducers. Values of N 1 obtained from P h data agreed well with values of N 1 and twice the storage modulus (2 G′) 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 N 1 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.