Effect of Anodic Aluminum Oxide Chemistry on Adhesive Bonding of Epoxy

Abstract

The transition to Cr­(VI)-free production is a great challenge in the global aerospace industry that currently still relays on it for the preparation of aluminum for bonding. Proper surface pretreatment is a prerequisite for strong and durable adhesive joint. Despite decades of experience, the nature and contribution of the different adhesion forces between the aluminum and organic adhesive remain under discussion. Herein we studied the adhesion of epoxy resin as a function of the surface chemistry of barrier-type anodic oxides prepared in sulfuric acid (SAA), phosphoric acid (PAA), and mixtures of phosphoric–sulfuric acids (PSA) and chromic acid (CAA) at different anodizing temperatures. X-ray photoelectron spectroscopy (XPS) data measured on model specimens were curve-fitted to calculate the relative amounts of O2–, OH–, PO4 3–, and SO4 2– species at the surface. The amounts of these species were then related to the mechanical performance of the joint measured by the floating roller peel test. Results show that significant initial adhesion is achieved without mechanical interlocking and independent of the type of electrolytes used for the pretreatment. Conversely, bonding stability under wet conditions is highly influenced by the surface chemistry. The wet adhesion strength increases with the hydroxyl concentration at the aluminum (oxide) surface, indicating that interfacial bonding is established through these surface hydroxyls. Phosphates and sulfates anions were not found to contribute to bonding with this type of adhesive.

Interface strength and degradation of adhesively bonded porous aluminum oxides

“…5c. The peel strength for such featureless oxides with an epoxy adhesive was previously reported in Abrahami et al 10 Their dry peel strength was in the same order of magnitude as porous oxides with small pores shown here. Once the pores are large enough and the resin penetrates into the pores, sufficient bonding is achieved and no further improvements in peel strength are registered.…”

“…In our earlier study it was reported that interfacial bonding between anodic oxides and epoxy resin proceeds through the hydroxyl groups at the surface of the oxide. 10 The incorporation of phosphate and sulfate anions did not change the bonding mechanism; it only affected the amount of hydroxyl groups available for bonding. It was found that the density of hydroxyl groups at the surface of SAA oxide is larger than for PSA oxides (approximately double, depending on the anodizing conditions).…”

“…Also in the absence of surface roughness, a higher anodizing temperature was found to increase interface bond stability in the presence of water. 10 This can be related to changes in the chemical properties of the oxide. 19 Unexpected observation concerns the fact that dry and wet peel strengths are independent of the oxide thickness.…”

“…In our previous study, we reported that the density of hydroxyl species at the surface of the oxide is closely related to the stability of bonding with an epoxy-based adhesive upon the ingress of water. 10 Since the former study has been conducted on thin, featureless barrier anodic oxide films, it does not take into account variations in the surface roughness that can lead to different levels of mechanical interlocking. Hence, in this study we explore the additional effect of changes in the oxide morphology on the resulting adhesion strength and stability.…”

Adhesive Bonding and Corrosion Performance Investigated as a Function of Aluminum Oxide Chemistry and Adhesives

“…As opposed to model molecules, adhesive films undergo curing at elevated temperatures and pressures and typically contain extra additives. In a recent work we demonstrated that bonding between barrier-type anodic aluminum oxides and FM 73 epoxy resin is distorted by water 26 . This was postulated to occur due to the reversible nature of hydrogen bonds that were formed between the oxide and epoxy.…”