School of Engineering and Materials Science
Research Student Awards
PhD Thesis: Development of an injection moulding grade hydroxyapatite polyethylene composite
Author: JOSEPH, Roy
Supervisor(s): Liz Tanner
High density polyethylene (HDPE) filled with 40 vol.% hydroxyapatite (HA) ceramic has been proven as a minor-load bearing bioactive bone substitute. Two current applications of this material are as otological implants and for orbital floor reconstruction. Injection moulding of the composite is being sought to permit the manufacture of components in large numbers at reduced cost. Owing to the high molecular weight polyethylene matrix used in the original development, the composite could not be injection moulded. The current component manufacturing technique is to compression mould a plaque of composite and then machine components. However, this method is not economical for large-scale production of implants.
In this study, a successful approach to develop injection moulding grade HA-HDPE composite is presented. This approach involved the selection and characterisation of raw materials which comprised of three grades of HDPE and two grades of HA. From these starting materials composites were prepared and the following properties studied: (1) Shear rheology of the composite as a function of polymer molecular weight and hydroxyapatite surface area, (2) viscoelasticity studies of the above melts, (3) processability tests by injection moulding, (4) effect of processing conditions on filler distribution and polymer molecular weight, (5) mechanical characterisation of injection moulded tensile bars by monotonic, creep and fatigue tests, (6) fracture surface examination, (7) thermal characterisation for Pressure-Volume-Temperature dependence, specific heat, thermal conductivity etc. and (8) dynamic mechanical analysis. Finally, middle ear prostheses were injection moulded successfully by using two selected compositions.
The results obtained following the various characterisation techniques showed that the molecular weight of the HDPE matrix was the major factor controlling the flow behaviour. The processability of the composite melt can substantially be improved by incorporating HA filler with low surface area. The processing conditions selected gave good HA dispersion in the matrix with minimum degradation. They were able to produce composites with comparable modulus and fracture strain to those previously reported.