Interphase Composition in Aluminum/Epoxy Adhesive Joints

Abstract

Model epoxy/aluminum adhesive joints were constructed with a geometry that allowed cracks to be propagated extremely close to the adhesive/adherend interface. The joints were fractured in air and the fracture surfaces analyzed using angle resolved X-ray photoelectron spectroscopy. Fracture occurred in a manner that left a significant amount of aluminum oxide on the epoxy side of the fracture surface and very little epoxy on the aluminum side. Aliphatic amine curing agent found associated with the aluminum oxide on both the adhesive and the adherend sides of the fracture surface was protonated by the acidic hydroxyls present in the aluminum hydroxide. Moreover, catalysis of the curing reaction by these hydroxyls resulted in an increased degree of crosslinking in the regions of the adhesive very close to the oxide surface. Thus, the aluminum oxide surface modified the structure of the adhesive in the near surface regions and resulted in the formation of a distinct interphase region with a composition different from that of the bulk adhesive.