Direct ram extrusion of polyethylene; a correlation between chain-folding and tensile modulus

Abstract

The paper concerns the production of highly oriented polyethylene by means of pressing the softened but essentially still solid material through a reducing capillary between 90 and 120° C. The objectives were to explore this route towards the achievement of high tensile modulus and to assess its practicability, both of which are presented in the paper. In addition, however, information was obtained relevant to deformation processes and to the relation between tensile properties and microstructure in general. Thus the induced deformation in the solid state and its dependence on processing variables was found to be interpretable in terms of Eyrings theory of viscosity with simple shear as an important element. Both changes in the fold length and fold content could be followed most informatively by low frequency Raman spectroscopy and it was observed, in the course of the present solid-state deformation, that the chains partly refolded to a fold length appropriate to the processing temperature, and partly unfolded. The former is in accord with earlier analogous observations on cold-drawing [13], the latter, as assessed through the reduction of fold content, correlated in an apparently uniquely defined way with the measured tensile modulus where the lowering of the fold content corresponds to an increase in modulus. Thus, this Raman spectroscopically determined fold content represents a promising new parameter in the correlation of structure with properties, it is applied further in the present paper for assessing modulus variation across the width of the extrudate. Other observations on the extrudate include a quantitative analogy with wood, the fibrous structure element being the common denominator.