Visual observation of development of sharkskin melt fracture in polybutadiene extrusion

Abstract

We studied the sharkskin melt fracture phenomena of polybutadiene (PBD), which exhibits similar flow properties and instabilities to linear low density polyethylene (LLDPE). The advantages of using PBD are that it exhibits larger distortions and slower development due to its high viscosity. By using a video camera with close-up lenses focused at the die exit, we observed the development of sharkskin in profile. It was shown that the sharkskin melt fracture develops with the cohesive failure at the die exit due to a peeling of surface layers as described by Howell and Benbow (1962). Oddly enough, this mechanism is similar to that proposed by Ovaici et al. (1998) for the extrusion of chocolate. A soap solution coating around the die exit to induce a slippery interface eliminated the sharkskin fracture, similar in the effect of fluoro-elastomer coating with LLDPE. Based on our visual observations, we qualitatively modeled the sharkskin fracture by modifying Cogswell‘s (1977) idea on exit stretching at the die exit and the force balance of Ovaici et al. on the ring formation of the chocolate extrusion.