Temperature-dependent instabilities in the capillary flow of a metallocene linear low-density polyethylene melt

Abstract

The capillary flow behavior of a metallocene linear low-density polyethylene was studied in a wide temperature range. The critical shear stress for the onset of the unstable spurt flow was found to be dependent on temperature in a nonlinear fashion and it showed a minimum value at a critical temperature, at which unusually long period pressure oscillations were observed. For temperatures above the critical one, the observed decrease of the critical shear stress with decreasing temperature is explained on the basis of an increase in the distance between entanglements. At temperatures below the critical one, the increase in the critical shear stress and the eventual suppression of pressure oscillations as the temperature is further decreased are suggested to be the result of a flow-induced phase change that ends on complete crystallization and suppression of flow. The flow-induced crystallization phenomenon and the extrudate quality were dependent on the contraction ratio. Elimination of surface extrudate distortions took place at low temperatures when using a contraction ratio of 30, this fact can be attributed to the flow-induced phase change. Finally, a decrease in the activation energy for flow or “easy flow” was observed at temperatures below the critical one. Such “easy flow” is perhaps the precursor of the “temperature window” of low-flow resistance reported by Keller and co-workers [Kolnaar, J. W. H. and A. Keller, Polymer 35, 3863–3874 (1994); Waddon, A. J. and A. Keller, J. Polym. Sci. Polym. Part B: Polym. Phys. 28, 1063–1073 (1990)].