A characterisation of the thermal curing- and mechanical properties of polymethylmethacrylate / hydroxyapatite composites
Aim The aim of this study was to investigate the changes in exothermic polymerisation characteristics and a range of mechanical properties in PMMA/HA composites (of varying HA concentrations) against a control sample of pure PMMA. Methods Specimens of pure PMMA, and 5, 10, 15, 20 and 25 percent HA composites were made according to the specification of appropriate testing standards using the flask and packing method. Exothermic polymerisation testing was conducted on respective samples using an internal j-type thermocouple temperature sensor. The rate of temperature change and maximum temperature in relation to time were recorded. Mechanical tests included tests of flexural strength and modulus, compressive strength and modulus, tensile strength and modulus and shear strength. All specimens were kept in a controlled environment prior to testing, which was performed on a LIoyd® LR30K universal testing machine, and recorded in computer-generated logs. Results Exothermic polymerisation testing revealed a decrease in mean maximum temperature values with increasing HA content. The mean exothermic temperatures of all six groups were above 100 ̊C, with small relative temperature reductions as the HA percentage increased. The results of mechanical testing revealed that there was a significant reduction in flexural strength in the range between pure PMMA and 15 percent HA and no statistical difference in flexural modulus. There was a notable trend toward a decrease in compressive strength as HA percentage increased, achieving statistical significance at 20 and 25 percent HA, with no statistical difference in compressive modulus between samples. The tensile strength test results no significant difference between pure PMMA and composites containing up to 15 percent HA. A significant difference was noted between the 20 percent- and 25 percent HA composites and those of lower HA concentration with an increased failure risk as HA concentration was increased above 10 percent. There was a tensile modulus peak at 15 percent HA, and a significant difference between 15 percent HA composites and pure PMMA and the 10 percent HA composite. Shear strength was noted to decrease with HA percentage, with significant reduced strength between the 15 percent HA composite and pure PMMA, as well as between the 20 and 25 percent HA composites and composites of less than 10 percent HA. Conclusions The study revealed that the addition of HA to pure PMMA negatively affects the mechanical strength measured in compression, bending or shear. Tensile, compression and flexural moduli showed a gentle increase with the addition of increasing amounts of HA. The peak values were noted at 15 percent for tensile modulus and 25 percent for compressive and flexural moduli. It was recommended that the best compromise across all properties (mechanical and thermal) should be based upon the context of composite use. It was further recommended that PMMA/HA composite materials with 10 – 15 percent HA be investigated further, with due cognisance of the limitations of the present study. The researcher recommended replication of the study using a larger sample size, more refined methodology and the incorporation of additional tests, including shear modulus testing, impact resistance, bioactivity and composite degradation.