A Water Soluble Hydrated Metal Oxide primer for Adhesively Bonded Joints

Abstract Weight saving and manufacturing cost benefits have led to the increase in use of adhesively-bonded structures in the automotive, aerospace and marine industries. In order to be a viable alternative to, for example, metal fasteners, these adhesive bonds should maintain the strength typical of conventional fastener systems. In many applications, the bonds are put under a variety of environmental and mechanical stresses. For example, frequently these bonds are exposed over long periods of time to wet environments which can result in a loss of bond strength. The loss of strength can result from the extension of cracks and other deformations that occur in the adhesive or metal oxide which are accelerated by the moist environment. As a result of this deficiency, extensive research and development efforts have been undertaken to define methods and identify materials which improve bonded joint performance in humid conditions. For example, it is known that surface preparation is important in the bonding of aluminum and titanium, and cleanliness in the bonding of ceramic articles. Thus, it is essential that, before bonding, the adherend is cleaned and chemically pretreated to produce a surface which in combination with the adhesive develops the bond strengths which meet application requirements. The normal procedure after surface treatment is to apply a corrosion-inhibiting primer by a spray technique for surface protection prior to bonding and to insure resin penetration into the oxide structure which provides improved environmental resistance. A major drawback of spray application is the large volume of organic solvent (normally MEK) emitted to the atmosphere. A successful alternative is the recently-developed electrodeposited primer by Northrup Corp., which consists of water solubilized primer particles which migrate in an electric field to a conductive work piece where they are deposited in a dense, continuous coating.1 The primer was developed for use with 121°C (250°F) curing epoxy adhesives. An Air Force sponsored contract is currently under way, the objective of which is to develop an electrodeposited water-based primer for use with 177°C (350°F) curing epoxy systems.2 A water-based epoxy primer system for application using the more conventional spray techniques has also been decribed.3