The role of pressure annealing in improving the stiffness of polyethylene/hydroxyapatite composites

The effect of the combination of pressure annealing and subsequent hydrosta tic extrusion on some mechanical properties of composites of hydroxyapatite in a polyethylene matrix is examined. Both linear and branched polyethylen es have been used as the matrix and it is found that composites with the fo rmer can be processed to give the higher modulus. An important practical fi nding is that products with a Young's modulus well into the range shown by cortical bone can be produced. The critical step in the enhancement of the modulus is pressure annealing which alters the morphology of the linear pol yethylene, encouraging the development of both crystallinity and crystallit e size. The presence of butyl branches along the molecular backbone limits the extent to which these can be developed by pressure annealing and accoun ts for the failure of the process to improve the modulus of composites usin g the branched material. Comparison with similarly prepared samples of the pure polyethylenes shows that the development of orientation in the polyeth ylene is considerably restricted by the presence of hydroxyapatite particle s, irrespective of whether pressure annealing is performed prior to extrusi on. Consequently, the properties of these composites are less than might be expected from studies on the isolated polyethylenes. (C) 2001 Kluwer Acade mic Publishers.