Structural Defects Generated upon Two-Stage Solid-State Coextrusion Drawing of Ultrahigh Molecular Weight Polyethylene Reactor Powder.

Abstract

A film of ultrahigh molecular weight polyethylene reactor powder, prepared by compaction below the melting point, was drawn by two-stage coextrusion at 110-130°C to a total extrusion draw ratio (EDRt) of 24. EDRt is a product of the EDRs of the two stages. Two kinds of flaws were observed; kink bands running at an angle of 30-60° to the fiber axis and periodic variations in the sample thickness along the fiber axis. The effects of extrusion variables on the formation of these flaws were studied by polarized light microscopy, atomic force microscopy (AFM) and wide-angle X-ray diffraction recorded with normal beam (diameter; φ = 0.5mm) and micro-beam (φ = 10 or 100μm). The number of flaws of both types increased with decreasing coextrusion temperature and/or decreasing die EDR used for the second-stage coextrusion. It was found that the kink bands were generated by the spring back associated with the die swell. In contrast, the thickness variation along the draw axis was found to form within an extrusion die during the second-stage draw. The effect of each of the two kinds of flaws on the stress/strain behavior is also discussed.