The influence of hydroxyl group concentration on epoxy–aluminium bond durability

Abstract

The influence of hydroxyl group (OH) concentration on the durability of adhesive bonds formed between an epoxy resin and aluminium adherend has been examined. Initially, surface analysis in combination with chemical derivatisation was employed to characterise the OH and epoxy functional groups present in FM-73, a structural epoxy adhesive. Bulk FM-73 indicated a higher degree of cure than the surface of FM-73 present at the interface of an epoxy–aluminium adhesive joint. Plasma and water treatment of the aluminium adherend was employed to alter the metal oxide's surface OH concentration. Despite a several-fold difference of aluminium surface OH concentrations for the different metal pre-treatments, there was no significant variation in the adhesive joint fracture toughness in a humid environment, G Iscc. In contrast, grit-blasting the aluminium prior to bonding increased G Iscc almost 15-fold. Simple calculations indicate that the aluminium surfaces used in the bonding experiments would have a large excess of OH groups available to react with a standard epoxy resin and that the influence of surface roughness on adhesion durability is not insignificant.