engleski

Processing of Ultrahigh Molecular Weight Polyethylene in the Solid State

Due to its high molecular mass (over 3*10^6 according to ASTM definitions D4020), UHMW-PE possesses excellent mechanical properties, notably abrasion resistance. For this reason it is used in demanding applications like artificial hip-joints, as the interface between the bone of the hip and the metal bar which is inserted in the leg. Such high values of molecular mass make the polymer rather intractable via conventional routes usually encountered in processing of thermoplasts. The products of UHMW-PE contain unfused powder particles usually referred to as grain boundaries. Due to the existence of the grain boundaries in parts of hip prosthesis, loosening of UHMW-PE particles occurs during active use causing undesirable effects in human body. In a study concerning processability of UHMW-PE, an unusual observation related to polymorphism induced by pressure was perceived by in-situ X-ray diffraction experiments performed on solution cast UHMW-PE. The experimental observation is that polyethylene crystals transform from the stable orthorombic crystal structure into a transient hexagonal phase depending on the polymer crystal size, i.e. smaller crystals which are more metastabile transform into the transient hexagonal phase at temperatures and pressures much below the thermodynamic triple point located at P=360MPa and T=230°C. The main feature of the hexagonal crystal structure in polyethylene is enhanced chain mobility within the phase. Starting with the proper choice of initial material i.e. initial crystal size it would be possible to process UHMW-PE via mobile hexagonal phase at acceptable pressure for polymer processing. It was confirmed experimentally, with the help of light microscopy, that compacting nascent UHMW-PE powder via transient hexagonalphase results in products which do not contain grain boundaries due to the enhanced fusion of the molecular chains within the phase.

UHMW-PE; processing; chain mobility

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