Surface, interfacial and molecular aspects of polymer bioadhesion on soft tissues

A review is presented of those factors responsible for promoting the integrity and longterm durability of metal-polymer bonds used in the fabrication of aircraft and aerospace structures. Using a multidisciplinary approach and a variety of surface analytical techniques such as extended resolution scanning electron microscopy (XSEM), X-ray photo-electron spectroscopy (XPS), ellipsometry, and a new technique called surface behaviour diagrams (SBD), investigators at the author's laboratories have evolved several important concepts. First, it has been determined that the initial integrity of metal-polymer bonds depends critically upon the morphology of the surface oxide on the metal. For aluminium and titanium, the metals studied, it is demonstrated that certain etching or anodization pretreatment processes produce oxide films on the metal surfaces which, because of their porosity and microscopic roughness, mechanically interlock with the polymer forming much stronger bonds than if the surface were smooth. Second, the long-term durability of metal-polymer bonds is shown to depend strongly on the environmental stability (or lack of stability) of the same oxide which is responsible for good initial bond strength. For aluminium moisture intrusion at the bond line causes the oxide to convert to an hydroxide with an accompanying change in morphology and bond strength. For titanium the oxides appear to be much more stable than those on aluminium but under severe environmental conditions the oxide undergoes a polymorphic transformation which may lead to bond degradation. Third, it is observed that significant improvements in durability of adhesive bonds to aluminium can be achieved using an extremely simple treatment in which monolayer films of certain organic acids are applied to the adherend oxide to protect it against the effects of moisture.

Adhesion and Durability of Metal/Polymer Bonds

An anticorrosion layer of a smart polymer coating is developed. The nature and properties of the coating simultaneously provide three mechanisms of corrosion protection: passivation of the metal degradation by controlled release of an inhibitor, buffering of pH changes at the corrosive area by polyelectrolyte layers, and self-curing of the film defects due to the mobility of the polyelectrolyte constituents in the layer-by-layer assembly.

Self-healing anticorrosion coatings based on pH-sensitive polyelectrolyte/inhibitor sandwichlike nanostructures

Review of surface pretreatments for aluminium alloys

The previous paper, Part 1, reviewed surface and interfacial phenomena, including mechanisms of adhesion. The present paper, Part 2, reviews the mechanics and mechanisms of failure of adhesive joints and the effects of various operating environments on joint performance.

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The science of adhesion

Oxide morphologies on aluminum prepared for adhesive bonding

The hydration of aluminium surfaces prepared for adhesive bonding by anodization in phosphoric acid has been studied using surface behaviour diagrams These surface behaviour diagrams, which are similar to phase diagrams for equilibrium bulk phases, trace the evolution of the aluminium adherend surface composition, obtained by X-ray photoelectron spectroscopy, during the hydration process When supplemented with high-resolution scanning electron micrographs and Auger depth profiles, the surface behaviour diagrams show that hydration proceeds in three steps The first step is reversible and consists of the adsorption of water by the monolayer of AlPO4 initially present on the surface It involves no change in the oxide morphology The second, which appears to be rate-controlling, involves the slow dissolution of the phosphate followed by rapid hydration of the exposed alumina to the oxyhydroxide, boehmite During this stage, extensive morphological changes occur as the boehmite fills the pore cells and bridges the whiskers of the original surface The third step consists of the nucleation and growth of the trihydroxide, bayerite, on top of the boehmite Using these results as examples, we propose the surface behaviour diagram approach as a new tool for the study of surface reactions in general

Application of surface behaviour diagrams to the study of hydration of phosphoric acid-anodized aluminium

Interpretation of AES depth profiles of porous Al anodic oxides

Effects of chemical and thermal treatments on the composition of 2024 aluminum adherend surfaces

We have examined the stability of A1 oxide films on A1 alloys against attack by moisture and the relationship between oxide stability and bond durability. The surface composition of phosphoric-acid-anodized (PAA) aluminum alloys was determined using X-ray photoelectron spectroscopy (XPS) and these data were used to construct an Al2O3 - AlPO4 - H2O surface behavior diagram. The fresh PAA oxide surface consists of a monolayer of AlPO4 adsorbed on the porous Al oxide. Hydration of the oxide proceeds first by adsorption of water by the AlPO4 layer. This process is reversible, it can occur during storage, and it does not alter the Al2O3 morphology. Irreversible hydration occurs with the dissolution of the phosphate followed by the conversion of the AI2O3 to boehmite (AlOOH). Finally, bayerite (Al(0H)3) crystallites grow on top of the boehmite.

T. S. Sun

Papers

Oxide morphologies on aluminum prepared for adhesive bonding

Application of surface behaviour diagrams to the study of hydration of phosphoric acid-anodized aluminium

Interpretation of AES depth profiles of porous Al anodic oxides

Effects of chemical and thermal treatments on the composition of 2024 aluminum adherend surfaces

Correlation of Surface Chemistry and Durability of Aluminum/ Polymer Bonds