Microwave sintering of polypropylene/carbon nanotube/hydroxyapatite composites

Abstract

The microwave sintering technique was developed for the fabrication of microwave transparent polypropylene (PP) based hydroxyapatite (HA) composites as an alternative to conventional polymeric material processing. Several studies were conducted on the microwave sintering of PP composites due to their microwave transparency and low dielectric properties. It was hypothesized that the incorporation of microwave susceptors into the polymer matrix would allow the polymer matrix to be sintered by microwave absorption. In this study, carbon nanotubes (CNTs) and HA particles were used as the microwave susceptors and the bioactive fillers, respectively. The objectives of the study were to perform microwave sintering of polypropylene/carbon nanotube/hydroxyapatite (PP/CNT/HA) composites, to examine the structure and to investigate the mechanical properties of the resultant composites. The microwave sintering of PP/CNT/HA composites was made feasible by the incorporation of 1 wt% CNT. Moreover, PP/CNT/HA composites with 0, 5, 15 and 30 wt% HA filling were fabricated successfully. In some cases, rapid microwave sintering, i.e. with a sintering time of less than a minute, was achieved. It was also found that the sintering time decreases with increase of the HA content. The existence of the PP and HA compositions in the microwave sintered composites was confirmed by X-ray diffraction experiments. Homogeneous distribution of HA within the polymer matrix was confirmed by scanning electron microscopy. In order to evaluate the elastic modulus of the fabricated samples, a novel finite element based small punch testing method was developed, which was shown to be more sensitive and accurate than the small punch bending technique that is typically used for small samples. The elastic modulus of the PP/CNT/HA composite was determined as 1.4 ± 0.1 GPa. To conclude, incorporation of microwave susceptors allows microwave transparent polymer powders to absorb heat and to be subsequently sintered, forming a pre-defined shape. By applying the proposed dispersion method and the microwave sintering technique, PP/CNT/HA composites were prepared successfully.