PROCESSING OF SEMICRYSTALLINE POLYMERS BY HIGH-STRESS EXTRUSION.

In the reported experiments, a study has been conducted on the solid-state extrusion of three semicrystalline polymers: PP, poly(vinylidene fluoride) (PVDF), and HDPE. HDPE has been extruded in continuous lengths with area reductions up to 25 multiplied by at temperatures substantially below the melting region. Such extrusion has been identified as a solid-state process, since measurements of the temperature of the polymer during extrusion indicate the absence of significant heating due to the deformation. In contrast, continuous lengths of PP and PVDF could not be obtained substantially below their melting temperatures, indicating that crystallization during extrusion is an important process for these polymers. Under severe extrusion conditions (low temperatures, high area reductions, etc.), all three polymers failed within the tapered region of the extrusion die. Two modes of failure have been identified, brittle fracture and, surprisingly, necking. Grid-line distortion patterns and a highly simplified upper-bound plasticity analysis both indicate that shear deformations are a major factor during high-stress extrusion.