A Close Look at the 'Sharkskin' Melt Fracture of Polybutadiene

Sharkskin melt fracture (SSF) refers to a fine surface distortion found on extradates of certain polymers at shear stress levels around 0.1 MPa. SSF can limit the productivity of extrusion lines. Two schools of thought concerning the mechanism of SSF have developed: one assigns the initiation of fracture to slip in the die land [e. g., Ramamurthy (1986)] whereas the other attributes the fracture to excessive stress at the exit as the flow profile redistributes [e. g., Howells and Benbow (1962), Kurtz (1984)]. Both may contribute to the problem. The former is supported strongly by the seeming invariant shear stress required for SSF and by the reported kink in the flow curve at this stress.