The Sharkskin Onset and Stability Diagram

Abstract

The onset of sharkskin extrudate was investigated in terms of molecular structure, molecular weight, temperature, and die geometry. The recoverable shear S_R or the ratio of wall shear stress over the storage modulus was determined and found to be independent of molecular weight, temperature, and die geometry for linear low-density polyethylenes (LLDPEs), but it depended critically on these factors and the molecular weight distribution for high-density polyethylenes (HDPEs). The plot of S_R versus λ _s /ϵ _s , or the sharkskin wavelength roughness amplitude ratio, constituted a sharkskin marginal stability curve common for all molecular weights, temperatures, and die geometry investigated; it is unique for each polymer type or molecular structure. Sharkskin instability occurred without hysteresis effect.

ACKNOWLEDGMENT

We are grateful to Professor R. Larson for providing comments on the manuscript and to Montara Thammachart for the molecular weight measurements. This work was supported by MTEC, grant MT-37-06-POL-09-04. We are pleased to acknowledge the materials donated by TPI and TPE.