Processing of ultrahigh-molecular-weight polyethylene reactor powders

Prof. Dr. Pieter Lemstra, Plempolco B.V. (The Netherlands)

Ultra-high-molecular-weight polyethylene, UHMW-PE, could become an interesting engineering plastic in view of the very beneficial properties like toughness, chemical resistance and notably the very high abrasion resistance, in fact superior amongst all (thermo)plastics. But the major problem is the processability via standard processing routes like injection-molding and extrusion is not possible due to the very high molar mass, >3 MDa. The melt-viscosity scaling with exponent 3.4 is prohibitively high, preventing any melt-flow.
UHMW-PE is processed via compression-moulding and/or ram-extrusion (‘hammering’ UHMW-PE through a heated tube into rods) and subsequently machined into parts. Consequently, UHMW-PE is currently only some 0,2% of the total PE market. The melt-viscosity in polymer systems is related to the presence of a physical network of entangled polymer chains and chain mobility is restricted to reptative motions as coined by De Gennes [1]. With an increasing number of entanglements per chain molecule the viscosity increases and hence the flowability of the molten polymer reduces and in the case of UHMWPE the well-known melt-index, MFI, an industrial measure of flowability, is close to zero, no flow.
The question is could removal of entanglements prior to processing (radically) improve flow hence processability? Past and ongoing research on this issue will be presented.
[1] P.G. De Gennes, Scaling concepts in polymer physics, Cornell University Press (1979)