Exploration of the slip phenomenon in the capillary flow of linear low-density polyethylene via electrical measurements

The slip phenomenon and the electrification occurring in the capillary flow of a linear low-density polyethylene melt were studied in this work using dies made up of stainless steel and brass, respectively. The experiments we re carried out in a torque driven capillary rheometer at a temperature of 2 00 degreesC and spanned the different flow regimes observed in capillary fl ow, including stable and unstable conditions. Flow enhancement or slip was generated in the brass die, as compared to the stainless steel one, after a n appropriate cleaning treatment. The melt exhibited electrostatic charge a nd sharkskin distortions were eliminated during the processing with the bra ss die in the stable flow regime prior to the stick-slip. Electrification a nd elimination of sharkskin distortions on the extrudates were observed whe n the slip velocity was a significant part of the whole average melt veloci ty (more than 50% in this work). The observations in this work point to a t ribological origin for the electrification of the melt. Finally, an electri c charge of a different sign was measured on the extrudates obtained from t he stainless steel and brass dies, respectively, during the stick-slip, whi ch suggests a detachment mechanism for slip in this flow regime. (C) 2001 T he Society of Rheology.