Study of the stick-slip phenomenon of linear low-density polyethylene in a brass die by using electrical measurements

Extrusion through brass dies has been found to be useful to eliminate sharkskin and stick-slip instabilities in linear low-density polyethylene. However, the physicochemical interaction of polyethylene melts with brass dies is not well understood. In the present work, such an interaction is analyzed by using electrical measurements. Experiments were performed in a single screw extruder at a temperature of 190°C with a metallocene linear low-density polyethylene and a brass die. The interaction between the polymer melt and the die changed with time. Different flow curves were obtained as the extrusion time was increased to reach an invariant flow curve. An irreversible transition from slip to no-slip at the die wall under stable flow was observed, consistent with a change in the sign of the measured electric charge. The critical shear stress for the onset of the stick-slip also increased with the extrusion time. Electrical measurements reproduced the stick-slip dynamics with fidelity. Pressure oscillations...