Extensional deformation, cohesive failure, and boundary conditions during sharkskin melt fracture

Abstract

We measure the flow kinetics of a polyethylene extruded through the exit of a sapphire capillary tube in order to understand the nature of sharkskin, a surface roughness in the extruded material. Optical velocimetry shows that sharkskin can occur under a variety of polymer/wall boundary conditions; stick, slip, or oscillating stick/slip, demonstrating that the flow boundary condition is not the direct cause of sharkskin. Downstream of the exit, high-speed video microscopy reveals two distinct material failures during each sharkskin cycle, the first is cohesive and splits the material into two regions, the second one occurs at the polymer–wall interface. Upon modification of the surface with a polymer processing additive (PPA), we confirm strong slip at the wall and a suppression of sharkskin, but find that sharkskin does return at sufficiently high flow rates. The extensional strain rate at the onset of sharkskin is significantly higher in the case with PPA than that without. We then empirically define a “reconfiguration rate” and find it is comparable at the onset of sharkskin for the two surface conditions. We use data in the literature to show that the reconfiguration rate also predicts the relationship observed between the onset of sharkskin and the capillary radius.