Adhesion and durability of metal-polymer bonds

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 surfaceanalytical techniques such as extended resolution scanning electron microscopy (XSEM), X-ray photoelectron 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.