Rheological characterisation of hydroxyapatite filled polyethylene composites Part 1 - Shear and extensional behaviour

The shear and extensional properties of injection moulding grade hydroxyapa tite-polyethylene composites developed for orthopaedic applications have be en studied. The composite was prepared without processing aids owing to con cerns over the potential biological responses to such additives. The compos ite containing 20vol.-% hydroxyapatite filler showed typical pseudoplastic behaviour. However, that containing 40 vol.-% hydroxyapatite filler tended to exhibit yield. The Maron-Pierce equation was found to be useful in predi cting the viscosities of the composite systems. The activation energy of th e composite and the unfilled polymer were equal, indicating that the 20 vol .-% system exhibits the same flow mechanism as the unfilled polymer. A qual itative assessment of extensional properties was made following Cogswell's method. The extensional stress of the unfilled polymer decreases with incre asing temperature whereas the composites behave in a complex manner. For al l the systems the Trouton ratios tend to increase with apparent shear rates . The Trouton ratio also indicates that at higher temperatures the flow of these composites is dominated by extensional properties.