FLUOROPOLYMERS AND THEIR EFFECT ON PROCESSING LINEAR LOW-DENSITY POLYETHYLENE

A comparison is made of flow curves for the extrusion of a LLDPE melt through clean metal capillaries and when overcoated by fluoropolymer ( FE) process aids. Characteristic differences between the flow traces i nclude a shear-rate dependent reduction in flow resistance due to the presence of FE at the die surface. The FE preferentially wets high ene rgy die surfaces, but interacts very weakly with LLDPE melts, thus act ing as a lubricant at the polymer/stationary phase interface, and prom oting the slip of LLDPE melts. Arguments are presented showing that th e percent slippage time must attain an equilibrium value at high extru sion rates. Flow curves for extrusion through FE-coated dies are divid ed into distinct regions and the slopes of these have been rationalize d by equations that combined the concepts of molecular dynamics and of adhesive failure at the die wall/polymer interface as the origins of slip-stick flow.