Visualizing the elimination of sharkskin through fluoropolymer additives: Coating and polymer-polymer slippage

We developed a capillary rheo-optics technique to visualize how fluoropolym er polymer processing additives (PPA) eliminate a surface distortion called "sharkskin" in the extrudate of linear low-density polyethylene (LLDPE). T he measurements were carried out in a transparent sapphire tube located at the exit of a twin-screw extruder. Depth-resolved optical microscopy was us ed to measure both the polymer velocity profiles and to image the coating p rocess of the PPA onto the capillary wall. In the absence of PPA, no slippa ge occurs between the capillary wall and the polyethylene; sharkskin was ob served at all flow rates. Upon addition of the PIPA to the LLDPE, the PPA m igrates to the capillary wall where it sticks and induces slippage between itself and the LLDPE, concomitant with the elimination of sharkskin. The in terface between the PPA and LLDPE is characterized by long stripes in the f low direction. Large values of the polymer-polymer slippage parameter were found which indicate that the fluoropolymer and LLDPE are fully disentangle d at their interface. The PPA acts by dramatically reducing the extensional deformation of the LLDPE at the exit surface. (C) 2001 The Society of Rheo logy.