Melt Elongational Flow Rheometry in Converging Channels.

Abstract

The basic problem in polymer melt elongational rheometry is obtaining a stable and constant elongational flow field free from shear effects. The objective of this study is to present a technique to generate plug flow and elongational flow of polymer melts in a channel geometry. The results of the study included the development of analytical and numerical solutions for flow behavior in the coextrusion of a core polymer melt encapsulated by a sufficiently low viscous skin polymer melt for linear and converging die geometries. Studying the effects of the power-law index, viscosity ratio, and flow rate ratio on the velocity profile and shear behavior indicated that essentially all of the shear gradient could be pushed into the skin layer. In converging geometries, it was thus possible to generate nearly pure elongational flow in the core by choosing an appropriate skin material. This study also demonstrated that by designing a hyperbolic die, a constant steady elongational strain rate, which was a linear function of the core flow rate, could be induced in the core. Experimental data for polypropylene as a core and polyethylene as a skin material showed good agreement of the elongational flow behavior and the predicted constant elongational rate with the model. Finally, elongational viscosity was calculated using the experimental data.