Comparison of microwave and thermal curing on toughening behaviors of SiCw-filled PBZ

Abstract

This research aims to investigate the effect of microwave cure and thermal cure in silicon carbide whisker (SiC[subscript w]) filled in polybenzoxazine (PBZ) composite on its corresponding thermal and mechanical behaviors. The application of microwave has been found to be an alternative method for curing thermosets with an increase in the rate of reaction. However, microwave curing has not widely used in polymeric material because of the non-polar nature of most polymers. Polybenzoxazine is a novel class of phenolic resins that possesses some outstanding properties such as low viscosity and producing no volatile by-product during polymerization. Therefore, polybenzoxazine is a highly attractive matrix for composite and bulk applications. Like most polymeric materials, the low dielectric constant of polybenzoxazine means it can not be cured easily by microwave radiation. SiC whisker is thus, used as a filler of the polybenzoxazine to enhance the microwave absorption capability of the molding compound and to reinforce the material. The experimental results reveal that the benzoxazine resin (BA-a) was hardly cured at a high power of 1 kW for 15 min. However, with a relatively low content at 4% by weight SiC[subscript w], the benzoxazine resin was found to be cured relatively well. Therefore, the strong microwave interaction of the benzoxazine molding compound, due to the presence of SiC[subscript w], significantly helps reduce the processing time of the molding compound from few hours at 200 ํC to be less than thirty minutes at 4% by weight of SiC[subscript w] using the irradiation power of merely around 300 Watts. To achieve the maximum cure, polybenzoxazine/SiC[subscript w] composite cured by microwave oven consumed less energy than that required by the traditional thermal cure by approximately 20 to 30 times. At 4 wt% of SiC[subscript w] content, the mechanical and thermal properties of the polybenzoxazine composites cured by thermal and microwave heating showed no significant change. The flexural modulus and strength of the thermal cured composites at 4 wt% SiC[subscript w] were determined to be 5.3 +- 0.48 GPa and 119 +- 28 MPa while those of the microwave cured specimens were about 5.7 +- 0.2 GPa and 118 +- 7 MPa respectively at the same filler content. The neat polybenzoxazine and the SiC[subscript w]-filled polybenzoxazine composites possess a glass transition temperature, determined by DSC, ranging from 155 to 160 ํC for both thermal and microwave cured. Moreover, the thermal and mechanical properties were increased with increasing the SiC[subscript w] content. Their fracture surface morphology could be observed by SEM studies, which revealed substantial bonding between the two phases. The fracture surface of the composites was significantly rougher than that of the pure matrix. The microwave cure for SiC[subscript w]-filled polybenzoxazine composite could drastically reduce the curing time and energy-consumption while the microwave-cured specimensstill maintained the good thermal and mechanical properties.