ABSTRACT
The demand for strong, durable, lightweight structures for both military and commercial applications has placed an increased reliance on adhesively bonded joints rather than those that are mechanically fastened. Generally, in order to obtain the highest levels of joint strength and durability, the surface chemistry of the substrate(s) (i.e. adherends) must be altered prior to the bonding process. In the case of most metallic adherends, this is normally accomplished with chemical surface treatments involving strong acids/bases and significant amounts of water to rinse excess solution from parts. The rinse water and spent process solutions must be treated prior to discharge or reuse, generating hazardous waste materials. For maximum joint durability, an adhesive bonding primer that typically contains hexavalent chromium is applied to the treated parts prior to bonding. The entire process presents significant problems in terms of worker health and safety as well as hazardous waste management costs.
Previous work performed by the U.S. Air Force and Boeing as part of a Strategic Environmental Research and Development Program (SERDP) project entitled “Non-Chemical Surface Preparation” (designated “PP 130”) identified sol-gel technology as an environmentally compliant process which would provide structural adhesive bonds with potentially improved joint durability on aluminum and titanium substrates. This paper discusses the status of an ongoing Tri-Service (Air Force, Navy, and Army)/DOE/industry investigation which extends the scope of the original effort. Critical aspects of this initiative include surface activation criterion and characterization of interphase regions for a variety of metal alloys. The principal goal of the program is to develop a sol-gel system that is effective on a wide range of metal substrates and robust enough for both production and repair scenarios. The short-term vision of the sol-gel system would be to replace existing hazardous metal surface treatments and be compatible with waterborne primer technology. However, the longterm view is the development of a sol-gel system that obviates the need for a structural bonding 210primer. This paper discusses recent promising results obtained with chemical formulations utilizing γ-glycidoxypropyltrimethoxysilane, tetra-n-propoxyzirconium and glacial acetic acid on aluminum, titanium, and stainless steel substrates.
