Rheological Implications of the Exit Pressure and Die Swell in Steady Capillary Flow of Polymer Melts. I. The Primary Normal Stress Difference and the Effect of L/D Ratio on Elastic Properties

Abstract

Apparatus which permits accurate measurement of shear stress and “exit pressure” has been constructed by the authors. Data has been obtained for high density polyethylene and polypropylene and the primary normal stress difference has been calculated by revising an equation advanced by Metzner et al. Measurements were obtained using capillaries of various L/D ratios inasmuch as there is evidence to indicate that elastic properties require a considerable flow distance to fully develop. Furthermore, the “exit pressure” has been correlated with the melt die swell which is known to depend on the L/D ratio. In addition to to the experiments with polymer melts, studies were also performed with a Newtonian fluid (Indopol H‐1900) which, at room temperature, has a viscosity comparable to that of a polymer melt. These experiments demonstrated that the technique gives a zero (gauge) “exit pressure” for a Newtonian fluid. Finally, a special die was used to test Lodge's “Hole Pressure” hypothesis inasmuch as the existence of this effect would considerably influence the results. Experiments were performed using polyethylene, polypropylene, and Indopol H‐1900. In no case was the “Hole Pressure” effect observed.