Electrical charging during the sharkskin instability of a metallocene melt

TL;DR

This study investigates electrical charging during sharkskin instability in metallocene polyethylene extrusion, revealing that electrification correlates with wall slip and molecular features, and could serve as an indicator of flow instabilities.

Contribution

It provides the first detailed measurements of electrical charges during sharkskin instability and links electrification to wall slip and molecular properties of the polymer.

Findings

Electrical charges appear during sharkskin instability but not in smooth extrudates.

Electrification correlates with wall slip and can be used as an indicator.

Charge levels follow a time-temperature superposition, indicating molecular influence.

Abstract

Flow instabilities are widely studied because of their economical and theoretical interest, however few results have been published about the polymer electrification during the extrusion. Nevertheless the generation of the electrical charges is characteristic of the interaction between the polymer melt and the die walls. In our study, the capillary extrusion of a metallocene polyethylene (mPE) through a tungsten carbide die is characterized through accurate electrical measurements thanks a Faraday pail. No significant charges are observed since the extrudate surface remains smooth. However, as soon as the sharkskin distortion appears, measurable charges are collected (around 5 10-8 C/m2). Higher level of charges are measured during the spurt or the gross-melt fracture (g.m.f) defects. This work is focused on the electrical charging during the sharkskin instability. The variation of the electrical charges versus the apparent wall shear stress is investigated for different die geometries. This curve exhibits a linear increase, followed by a sudden growth just before the onset of the spurt instability. This abrupt charging corresponds also to the end of the sharkskin instability. It is also well-known that wall slip appears just at the same time, with smaller velocity values than during spurt flow. Our results indicate that electrification could be a signature of the wall slip. We show also that the electrification curves can be shifted according to the time-temperature superposition principle, leading to the conclusion that molecular features of the polymer are also involved in this process.