Effect of composition on the fracture toughness and flexural strength of syntactic foams

Syntactic foams are strong, lightweight materials that can be useful in marine and aeronautical applications. However, these materials are brittle, which can limit their usefulness. The fracture toughness of several epoxy based syntactic foams, as affected by foam composition, was evaluated. The parameters of interest were the curing agent type (anhydride and amine) and elastomer type and level. For the anhydride curing agents studied, the addition of elastomer improved the fracture toughness, and no reduction in flexural strength was observed upon elastomer addition. Examination of the fracture surface showed that the failure mechanism varied with foam composition, from crack propagation around the microballoon filler to crack propagation through the filler, possibly explaining the strength results. For a syntactic foam cured with a cycloaliphatic amine curing agent, the addition of elastomer did not significantly affect the fracture toughness. Examination of the fracture surface showed that the failure mechanism for the syntactic foam compositions with and without added elastomer was crack propagation through the matrix rather than through the microballoon filler.