Coating kinetics of fluoropolymer processing aids for sharkskin elimination: The role of droplet size

The ability of fluoropolymer based polymer processing aids (PPA) to eliminate surface melt fracture (sharkskin) during extrusion of polyethylene is studied in relation to blend morphology (PPA droplet size) and processing conditions (shear rate). Under a constant throughput, the die entrance pressure, the PPA coating thickness and the degree of fracture are simultaneously monitored. The thickness of the fluoropolymer coating on the die surface is measured in situ through frustrated total internal reflection. A substantial enhancement in the coating rate and the magnitude of the steady state coating thickness as well as the faster elimination of sharkskin upon increase of PPA droplet size have been determined. These results are consistent with a recently developed model of the coating process.