Physical and mechanical properties of ultra‐oriented high‐density polyethylene fibers

Abstract

AbstractUltra‐oriented high‐density polyethylene fibers (HDPE) have been prepared by solid‐state extrusion over 60–140°C range using capillary draw ratios up to 52 and extrusion pressures of 0.12 to 0.49 GPa. The properties of the fibers have been assessed by birefringence, thermal expansivity, differential scanning calorimetry, x‐ray analysis, and mechanical testing. A maximum birefringence of 0.0637 ± 0.0015 was obtained, greater than the calculated value of 0.059 for the intrinsic birefringence of the orthorhombic crystal phase. The maximum modulus obtained was 70 GPa. The melting point, density, crystallinity, and negative thermal expansion coefficient parallel to the fiber axis all increase rapidly with draw ratio and at draw ratios of 20–30 attain limiting values comparable with those of a polyethylene single crystal. The properties of the fibers have been analyzed using the simple rule of mixtures, assuming a two‐phase model of crystalline and noncrystalline microstructure. The orientation of the noncrystalline phase with draw ratio was determined by birefringence and x‐ray measurements. Solid‐state extrusion of HDPE near the ambient melting point produced a c‐axis orientation of 0.996 and a noncrystalline orientation function of 0.36. Extrusion 50°C below the ambient melting point produced a decrease in crystallinity, c‐axis orientation, melting point, and birefringence, but the noncrystalline orientation increased at low draw ratios and was responsible for the increased thermal shrinkage of the fibers.