The effect of polymer network structure upon the bond strength of epoxy–aluminum joints

The bond strength to aluminum of Epon 828 cured with various amounts of methylene dianiline has been investigated as a function of the resin network structure. In order to meet this objective, a torsional test for bond shear strength was developed, and fully cured resins with different network structures were prepared. The effects of the rate of loading of the joint and the aluminum surface pretreatment on the bond strength were also examined. Very high bond shear strengths, in excess of 9000 psi, were found for joints which after machining had been polished, vapor degreased, and treated with dilute sulfuric acid–potassium dichromate solution. It was found for these joints that the average bond strength decreased as the molecular weight between crosslinks increased. For the joints without acid–dichromate treatment, the failures were adhesive, and the network structure did not seem to significantly affect the bond strength. There are indications that chemical bonding occurred in the case of the acid–dichromatetreated joints; the decrease in bond strength as per cent excess amine and Mc increased is possibly associated with a decreased amount of chemical bonding. The bond strength increased to a limiting value as the rate of testing increased.