New aspects of the microhardness of ultraoriented polyethylene

Abstract

The influence of extrusion conditions on the microhardness of ultradrawn polyethylene (PE) has been investigated. The micromechanical behaviour of ultradrawn PE fibres can be defined in terms of a creep constant and a microhardness value (MH) at 0.1 min, relating to the size of indentation under load rather than to the residual dimension. The PE strands show, in addition, a conspicuous anisotropic shape of the indentation pattern (MH‖>MH⊥) which involves a local elastic recovery of the material parallel to the fibre axis. The hardening of the fibres in the parallel and perpendicular direction to the fibre axis is an increasing function of both extrusion temperature and extrusion pressure and can be explained in terms of a simultaneous improvement in the strength and the lateral packing of fibrils and microfibrils in fibre direction. For extrusion temperatures (Te) greater than 134° C the microhardness shows, however, a decrease which is thought to be related to the molecular relaxation occurring in the vicinity of the melting point. In the range of extrusion temperatures (90 to 137° C) and extrusion pressures (0.24 to 0.5 GPa) investigated, the hardness anisotropy on the fibre surface, ΔMH, is a unique increasing function of draw ratio λ. It is completely independent of both the extrusion temperature and pressure. ΔMH in the core of cleaved fibres shows a constant value which is independent of λ and equals the extrapolated value at the surface for the maximum attainable value of λ. The results can be satisfactorily explained if one considers the existing morphological differences between the outer sheath and the inner core of the fibres.