Onset of the sharkskin phenomenon in polymer extrusion

A specific form of melt flow instabilities associated with surface defects for polymer extrudates, and commonly referred to as the "sharkskin effect", is modeled. When this effect occurs, a more or less regular pattern of ridges on the surface is observed resembling the skin of a shark if bent. It is shown that the relaxation oscillation model of Molenaar and Koopmans [J. Rheol. 38, 99 (1994)] developed to describe "spurt" defects — in this perturbation not only the surface but the extrudate as a whole shows distortions — can be expanded to include a description for the dynamics of surface defect appearance. By introducing a nonlinear viscoelastic constitutive equation (Kaye-Bernstein-Kearsly-Zapas model) into the relaxation oscillation model a boundary layer can develop which shows oscillating behavior. Explicit criteria for the onset of this behavior are derived. The relations between these criteria and experimental parameters are pointed out. This allows for an experimental verification of the supposition that this kind of solution is the origin of the sharkskin effect. The current macroscopic approach may form the basis for the reconciliation of the debate on the origin of melt flow instabilities as either a "slip at the wall" or a nonmonotone "constitutive equation" phenomenon.