Effect of Raw Material Powder on Melt-Drawing Behavior of Ultrahigh-Molecular-Weight Polyethylene

Abstract

Ultrahigh-molecular-weight polyethylene (UHMW-PE) can be drawn even from the molten state owing to its high melt viscosity. Molten UHMW-PE exhibits elastic deformability, which is homogeneously transmitted within the entire sample through molecular entanglements, resulting in oriented crystallization. Thus, entanglement characteristics are a dominant factor for the drawability of molten UHMW-PE. The entanglement characteristics of a UHMW-PE reactor powder will be retained for a film prepared by melt-processing due to the long relaxation time of UHMW-PE chains. Therefore, in this study, the melt-drawing behavior of UHMW-PE film prepared from different raw material powders was investigated. Three kinds of commercially available UHMW-PE reactor powders with comparable MW (viscosity average MW of 3×106) were used as a raw material. The UHMW-PE film was melt-drawn at 150℃, well above the sample melting temperature. The stress-strain curves recorded during melt-drawing exhibited a yield point, plateau stress region, and strain-hardening region for all films, although the stress level was different, indicating that the entangled state depends on the kind of raw material powder. In-situ wide-angle X-ray diffraction (WAXD) measurements during melt-drawing revealed two oriented crystallization mechanisms : the characteristic orthorhombic crystallization via transient hexagonal crystallization derived from deep entanglements, and the direct orthorhombic crystallization derived from shallow entanglements. Consequently, the entanglement characteristics of a UHMW-PE reactor powder dominate the melt-drawing behavior of the film. The obtained results are promising for developing the industrial melt-processing of UHMW-PE starting from a supplied UHMW-PE raw material powder.