Effect of rotation extrusion and molecular weight on ductile failure of polyethylene pipes under hydrostatic pressure

Abstract

In this study, two kinds of polyethylene (PE) with different molecular weight were processed into the pipes via rotation extrusion and the failure behaviours under hydrostatic pressure as well as molecular relaxation were investigated. The experimental results showed that the high molecular weight PE exhibited slower relaxation behaviour with higher relaxation activation energy to facilitate the formation of shish-kebab under flow field, while for the low molecular weight one, the stretching molecules easily relaxed back coil state and spherulites were prone to form. Therefore, the low molecular weight PE pipes via convention and rotation extrusions had similar isotropic spherulite morphology and short failure times. In the case of high molecular weight PE, during the convention extrusion, polymer melts flowed along the axial direction to induce the alignment of shish-kebab accordingly, which went against to the hoop stress in hydrostatic pressure. With the rotation of mandrel and die, the direction deviated from the axis so that PE pipe exhibited better resistance to the hoop stress. The failure time was 182 h, 264% longer than the convention-extruded one. Accordingly, a new strategy to prepare high hydrostatic pressure PE pipe under cooperative effects of rotation extrusion and long molecular relaxation time was proposed.