Extrusion of Polyethylene Single Crystals

Abstract

The solid (crystalline)-state extrusion of unoriented single-crystal aggregates and oriented single-crystal mats of high-density polyethylene (HDPE) has been studied. Irrespective of the initial state of aggregation, uniform, transparent, highly oriented and high modulus fibers and films were produced. Single crystal aggregates extruded at a rate faster than melt-crystallized spherulitic morphology. Surface cracks were observed but shear fracture seldom occurred on single-crystal extrudates. The maximum attainable extrusion draw ratio (EDR) and hence the modulus was higher for the extrusion of single crystals than for spherulitic samples. Thus, the initial morphology had a marked effect on the extrusion of HDPE but no appreciable effect on the properties (birefringence, modulus, and transparency) of the resultant extrudates measured as a function of EDR. These results clearly show that intercrystalline networks and entanglements of the original sample have a remarkable effect on extrusion but are not important in achieving efficient draw as evaluated by the properties as a function of EDR. Moreover, the results indicate that interfacial friction-adhesion between crystalline lamellae is the primary deformation mechanism.