The strength of alumina trihydrate filled epoxy resins

The tensile and fracture properties of alumina trihydrate (ATH) filled epoxy resins have been determined up to a maximum volume fraction of 0.33 with a cold setting curing agent (Epikure T) and up to V[sub]f=0.54 with a hot curing agent (piperidine). The effect of a reactive rubber on these properties, on its own and in ATH-filled composites have also been considered. All filled compositions showed a lower tensile strength than the unfilled resin. The largest effect occurred at low volume fractions, up to V[sub]f = 0.1, loadings higher than this did not reduce the strength any further. The addition of a rubber toughener (ATBN), to the ATH-filled composite, reduced the strength still further. The elongation to failure was also reduced. Young's modulus was increased and was in agreement with other studies and predictions of theoretical models. The Lewis and Nielsen equation was found to give a satisfactory approximation for both the ATH-filled and ATH-rubber compositions. The fracture energy (G[sub]lc) exhibited a maximum at a volume fraction of 0.1. The fracture toughness (K[sub]lc) increased linearly with increasing volume fraction of filler. Addition of ATBN rubber to ATH-filled composites increased G[sub]lc and K[sub]lc further. The results correlated well with the crack-pinning prediction of Green at high volume fractions but showed some deviation at low loadings. The theory of Green gave a break-away value, r, of 0.75 (compared to r=1.8-2.0 for alumina). Fracture toughness results are discussed in terms of crack-pinning and crack-blunting mechanisms. Experiments using model specimens suggested that failure in composites containing weak particles, such as ATH, was initiated by failure of the particle itself. These experiments showed that a polymer coating around the particles could improve the tensile strength by delaying fracture and then blunting any subsequent crack. Various methods for applying a polymer coating to the filler were investigated. These included: Spray drying, solvent deposition, mechanical mixing and in-situ polymerisation. The in-situ polymerisation route showed the most potential, but this needs further optimisation.