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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Mechanical behaviour of hydroxyapatite reinforced polyethylene

Author: HEMACHANDRA, Khemchai

Year: 1992

Supervisor(s): Liz Tanner, Bill Bonfield

Composites of hydroxyapatite-reinforced polyethylene have been formulated with two hydroxyapatite powders, with a range of volume fractions up to 0.40. Such composites have been pioneered as analogue materials for cortical bone reconstruction and replacement and prototype of one particular volume fraction is already in clinical trial. The primary objective of this investigation was to evaluate the effect of hydroxyapatite particles of various mean particle size and surface morphology and of different moulding techniques on the subsequent mechanical properties of the composites. A variety of structural techniques, including scanning electron microscopy were utilized to assess the constituent polymer and ceramic powders, as well as the bulk composites. The associated mechanical properties of the composites were determined for a range of volume fractions of hydroxyapatite. The composites were compounded by co-rotating twin screw extrusion and then either compression or injection moulded into plaques for subsequent mechanical tests and structural characterization. Measurements were made to check the hydroxyapatite content. Tensile and microhardness tests were used to characterize mechanical properties of the various composites. Scanning electron microscopy was used to examine the initial morphology of the two hydroxyapatite powders, the dispersion of the hydroxyapatite in moulded samples and the composite fracture surface after testing.
It was established that the Young's modulus of the test composites increased with hydroxyapatite volume fraction, while both the strain to fracture and the tensile strength decreased over the same range. The Young's modulus increase was attributed to the introduction of the significantly stiffer hydroxyapatite particles. At the higher volume fractions the decrease in tensile strength and strain to fracture can be related to hydroxyapatite particles serving as local stress concentrators and/or flaws.