Effects of the addition of inorganic nanoparticles on the adhesive strength of a hybrid sol–gel epoxy system

A combination of inorganic nanoparticles was added as reinforcement to a diglycidyl ether of bisphenol-A (DGEBA)-based epoxy resin modified by a hybrid sol–gel method. Different epoxy/sol–gel systems were prepared to evaluate their adhesive strength. The mechanical performance of different formulations was characterised by shear and tensile tests to define the influence of nano-fillers on adhesive strength performance of the modified epoxy/hybrid sol–gel. The results obtained indicate that the incorporation of a selected ratio of inorganic nanoparticles in the epoxy/sol–gel adhesive improves the adhesion performance between substrate surfaces. A significant increase in adhesive lap shear strength of the sol–gel modified epoxy, compared with that of the neat epoxy, was observed. Butt joint strengths of the modified epoxy/sol–gel were also recorded, showing good adhesion behaviour to mild steel surfaces. Tensile strength of joints up to 28.5 MPa for 16 h/150 °C cure time/temperature was observed. The modified system exhibited a high yield point and large extension compared with that of the unmodified epoxy. The study further showed that doping with small amounts of one type of nanoparticle to the system increases adhesive cross-linking. Epoxy/sol–gel adhesive strength was also evaluated as a function of cure temperature for mild steel and Al2024-T3 substrates. Results showed adhesive strength decreased with increased cure temperature on the Al substrate, while lap joint strength of the mild steel exhibited no significant changes at three different cure temperatures. This may be attributed to good interfacial bonding of the sol–gel adhesive to the mild steel over the designated temperature range.