Christopher Paul Buckley

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

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 degrees 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 degrees 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 degrees C to a mean of 560% at 175 degrees 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 degrees 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 degrees C. They persisted up to 200 degrees 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 degrees C.

Development of an Integrated Approach to Modelling of Polymer Film Orientation Processes

An integral part of many manufacturing processes for polymer products is the deliberate introduction of preferred molecular orientation by means of uniaxial or biaxial stretching. An integrated scheme is under development for the modelling of biaxial orientation processes and has been applied to the case of poly(ethylene terephthalate) (PET) films. It consists of: experimentation under process-relevant conditions using a purpose-built biaxial testing machine; development of a physically-based constitutive model based on the data obtained; incorporation of the constitutive model within a Finite Element continuum model; and validation tests of the model. A first pass has been made through the complete sequence, and the application of the model to a section of the process for production of biaxially oriented PET film has been demonstrated.

Processing of Ultra-High Molecular Weight Polyethylene: Modelling the Decay of Fusion Defects

A problem in applications of ultra-high molecular weight polyethylene (UHMWPE) is the tendency for components to contain fusion defects, arising during processing of the as-polymerized powder. These defects have been implicated previously in failures of UHMWPE load-bearing surfaces, in knee and hip prostheses. Recent work of the authors has recognized two forms of defect: voids (Type 1) and particle boundaries deficient in diffusion by reptation (Type 2). To assist process and product design, a method has now been developed for predicting the decay of severity of Type 2 defects during processing, for a component of given shape and process history. A new quantifier was introduced for characterizing the progress of diffusion at Type 2 defects in UHMWPE—the maximum reptated molecular mass M ˆ . This was computed using results from reptation theory, embedded within a Finite Element thermal model of the process. The method was illustrated by simulating compression moulding trials already carried out experimentally by the same authors. It was discovered that M ˆ never reached the viscosity average molecular mass of the polymer, indicating incomplete boundary diffusion, and explaining the previous observation of Type 2 defects even in fully-compacted, apparently perfect mouldings. The method described has potential as a design tool, especially for optimizing manufacture of UHMWPE prosthesis components.