The use of surface preparations and fillers in the promotion of cohesive failure of EA 946/aluminum bondlines

A model amine cured structural epoxy adhesive (EA 946) that tends to fail interfacially when bonded to aluminum adherends when tested in mode I fracture, is the subject of this manuscript. Two methods of promoting cohesive failure were investigated, surface modification and adhesive filler formulation. In the former, a new metasilicate/silane surface treatment was applied to 7075 aluminum. This treatment significantly improved the zero time bond fracture toughness and durability of adhesive bondlines. The effect of silane chain length and processing time lines on joint mode fracture toughness were also examined. In the second part of the study the filler composition was changed from M5 to TS-720 Cab-O-Sil. This had a significant effect on the failure mode. It was shown that TS-720 produced a significant increase in cohesive failure compared to M5 Cab-O-Sil. Electron micrographs suggest that failure was nucleated at the interface between the epoxy and the particles of TS-720. The presence of silane adhesion promoters on the M5 Cab-O-Sil was thought to preclude this failure mechanism in the M5 formulations. A non-linear viscoelastic model and biaxial constitutive modeling showed that the volume increase for the TS-720 formulation was approximately three times that of the M5 formulation.