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 were carried out in a torque driven capillary rheometer at a temperature of 200 °C and spanned the different flow regimes observed in capillary flow, including stable and unstable conditions. Flow enhancement or slip was generated in the brass die, as compared to the stainless steel one, after an appropriate cleaning treatment. The melt exhibited electrostatic charge and sharkskin distortions were eliminated during the processing with the brass die in the stable flow regime prior to the stick-slip. Electrification and elimination of sharkskin distortions on the extrudates were observed when the slip velocity was a significant part of the whole average melt velocity (more than 50% in this work). The observations in this work point to a tribological origin for the electrification of the melt. Finally, an electric charge of a different sign was measured on the extrudates obtained from the stainless steel and brass dies respectively, during the stick-slip, which suggests a detachment mechanism for slip in this flow regime.