SHARKSKIN DEFECTS OF POLYMER MELTS - THE ROLE OF COHESION AND ADHESION

An explanation is given for the surface defect, commonly referred to as shark-skin, considered in the present study to occur at the die exit. It will be shown numerically that large negative pressures can exist at the die exit by simulating the flow of a linear polydimethylsiloxane melt, which showed significant shark-skin, through a capillary using a finite element program. Furthermore, a series of photographs indicates that the cracks on the surface of the extrudate originate at the die exit and not further down the extrudate. It will then be postulated that negative pressure (hydrostatic tension) cavitates the polymer melt very close to the die lip (within a micron) at the die exit on the surface of the die or in the bulk of the polymer depending on the respective strengths of adhesion to the die surface at the exit or of the cohesion of the bulk of the polymer. The growth and coalescence of these voids would then lead to cracks (sharkskin) at the die exit.