Formation properties of solid‐state extruded high density polyethylene fibers

Extrusion rates have been measured for high density polyethylene fibers prepared by solid‐state extrusion through a conical die with maximum draw ratio of 52, using constant pressure and temperatures ranging from 0.12 to 0.49 GPa and 60° to 140°C and molecular weights from 5 × 104 to 6 × 106. The processing variables are reported in terms of an apparent elongational viscosity. The logarithm of the extrusion rate was proportional to the applied pressure. Deviation from the Eyring theory of viscosity was found, since lowering of the extrusion temperature to 60°C produced an anomalous decrease in the extrusion rates. The anomaly was explained by the increased friction between die and polymer, the pressure dependence of the yield stress, and by the pressure dependence of the α‐relaxation mechanism. Apparent activation energies of 60 and 100 kcal/mole were calculated for the solid‐state extrusion in the region 100°–130°C and 80–100°C. Three modes of deformation were observed for each extrusion, consisting of an unsteady state, a second stage of drawing in which the extrusion rate approached a constant value, and a fracture region. The first mode corresponded to orientation and rotation of the crystalline lamellae, as shown by the rapid increase and plateau region of the crystalline and noncrystalline orientation functions. During the second stage of deformation, the apparent elongational viscosity ηE increased linearly with draw ratio, as did the modulus, and this mode was found necessary for the attainment of high modulus fibers. The second mode of deformation was associated with the deformation of the crystalline lamellae and strain hardening in which ηE‐approached that for a glass. Strain hardening readily occurred with decreasing temperature, and increasing draw ratio and molecular weight. Continuous extrusion producing uniform modulus fibers was only possible when the maximum draw ratio of the die was less than the fiber draw ratio, where strain hardening occurred. The third mode of deformation limited the maximum modulus obtainable and consisted of fracture or kink planes oblique to the fiber axis. The relations between the processing variables used in the solid‐state extrusion and the resulting fiber properties are discussed.