Effect of coupling agent on mechanical properties of hollow carbon microsphere/phenolic resin syntactic foam

Various hollow carbon microspheres/phenolic resin syntactic foams were prepared by varying the content of hollow carbon microspheres. To improve the compressive and flexural strength and fracture toughness of syntactic foam, the hollow carbon microspheres were oxidized in nitric acid and followed by the treatment with a coupling agent of glutaric dialdehyde. Compression, flexural, and fracture toughness tests were performed to observe the mechanical behavior of the foams. Results demonstrated that compressive and flexural strength decrease with increasing filler content. On the other hand, it is found that better interfacial adhesion could be induced from coupling agent treated hollow carbon microspheres, hence resulted in better mechanical properties. The fracture toughness results revealed that the use of coupling agent increases the fracture toughness and the maximum fracture toughness values occurred at 28.12 vol.% for both untreated and coupling agent treated hollow carbon microspheres. The toughening mechanisms, such as crack deflection, crack bowing, and debonding mechanism, were discussed.