WALL SLIP IN THE EXTRUSION OF LINEAR POLYOLEFINS

A molecular model, based on the concept of the activation rate theory, is developed for slip of a polymer as it flows through a die. The model correctly predicts the dependence of the critical shear stress for the onset of slip and the slip velocity of high molecular weight, linear polyethylene resins on temperature, pressure, wall shear stress, and molecular weight. It also predicts the increased slip which occurs when die surfaces are coated with a fluoropolymer having a low surface tension. It is not applicable at and above the critical shear stress for the onset of ross melt fracture.