The Influence of Initial Morphology on the Physical and Mechanical Properties of Extruded High Density Polyethylene Fibers

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Abstract
Ultra-oriented high density polyethylene fibers have been produced by solid state extrusion of various polyethylene morphologies prepared in the reservoir of an Instron Rheometer. A constant extrusion temperature and pressure were used in the following studies: (a) high density polyethylene melted at 170°C and then cooled at ∼1°C min−1 to the desired extrusion temperature below the ambient melting point; (b) a molten sample removed from the rheometer and quenched into an ice-water bath and subsequently extruded; (c) a chain-extended polyethylene prepared by crystallization at 0.49 GPa and 200°C; (d) various chain-folded morphologies prepared by isothermal crystallization between 134°C and 200°C and pressures from 0.12 to 0.49 GPa. The properties of the fibers have been assessed by birefringence, thermal expansivity, differential scanning calorimetry, small angle X-ray scattering, modulus, and tensile strength. Fibers produced from the various isothermally-crystallized morphologies attained, at deformation ratios 20–30, a birefringence (in the draw direction) greater than a perfect polyethylene single crystal value (c-axis) of 0.059 and a maximum modulus of 70 GPa. Maximum birefringence was obtained with increasing crystallization temperature and molecular weight of the chain-folded morphology. Extrusion of chain-extended HDPE of molecular weights 5 × 104–147 × 103 produced brittle fracture of the extrudate. This was explained by the high fraction of segregated low melting polymer between the chain-extended lamellae.