Mechanical integrity of compression-moulded ultra-high molecular weight polyethylene : effects of varying process conditions

Abstract

Ultra-high molecular weight polyethylene (UHMWPE) bearing surfaces in knee and hip prostheses are frequently manufactured by direct compression moulding of the as-polymerised powder. A study was made of the important role of the temperature–time sequence in the melt state during processing, in determining the mechanical integrity of mouldings at 37°C. Structural features were determined by calorimetry (for the degree of crystallinity), infra-red spectroscopy (for the degree of oxidation), density measurement, and scanning electron microscopy. Mechanical integrity was assessed by tensile tests at a constant nominal strain-rate of 10−3 s−1, with post-failure microscopic examination. For the whole range of melt temperatures 145–200°C and times 10–90 min, essentially the same stress–strain path was followed, reflecting invariance of the degree of crystallinity. However, there were dramatic changes in elongation-to-break, from ca 10% for some mouldings at 145°C to a mean of 560% at 175°C where, at the 86% confidence level, there was evidence for a peak. The rise was explained by microscopy, that revealed two distinct types of fusion defect, of reducing severity with increasing temperature. Type 1 defects were voids arising from incomplete powder compaction, and persisted up to 165°C. Type 2 defects were regions of enhanced deformability at inter-particle boundaries in apparently fully compacted mouldings, evidenced microscopically by localised relative displacements at particle interfaces, during the plastic deformation at 37°C. They persisted up to 200°C. Type 2 defects may be attributed to the slow self-diffusion of UHMWPE in the melt, leading to incomplete homogenisation, even after compaction is complete. The level of oxidation in the mouldings was small but rose with melt temperature, explaining the fall in elongation-to-break at temperatures higher than 175°C.