State-of-the-art metal surface preparations for adhesive bonding mainly consist of anodization or etching processes that use strong acids and contain hexavalent chromium. The surface treatment is followed by the application of a corrosion-inhibiting adhesive primer that typically contains high levels of volatile organic compounds (VOC) and hexavalent chromium. As a result of new regulations, increased costs of hazardous waste disposal, and an increased awareness of the costs associated with employees’ health and safety, it became imperative that low-VOC/nontoxic surface treatments and primers be developed for structural adhesive bonding and sealant applications.
The primary objective of this project was to develop and transition processes that eliminate the VOCs, chromates, and strong acids typically found in the metal surface treatment and priming steps conducted prior to application of adhesives and/or sealants.
This project was built on previous work using sol-gel technology to deposit thin organic-inorganic coatings on metal surfaces to develop good adhesion between the metal and subsequently-applied polymers (primer, adhesive, or sealant) via covalent chemical bonding. The first stage of research sought to optimize a sol-gel surface preparation that would be compatible with experimental waterborne adhesive bond primers. This was accomplished through sol-gel chemistry optimization and the development of application procedures with an emphasis on the surface activation and the drying/cure steps. A one-step process then was developed that combines the adhesive primer and sol-gel surface treatment into one consolidated interfacial layer. These sol/primer mixtures were evaluated as traditional adhesive primers. The leading low-VOC primers were used without their chromate constituents. Finally, the sol-gel work for adhesive bonding was leveraged to develop adhesion promoters for sealant operations.
This project successfully developed and initiated the transition of new metal surface preparations based on sol-gel technology for adhesive bonding. Multiple processes were developed using the same chemical formulation with alternate pretreatment, application, and post-treatment steps. These similar approaches yield bond strength and moisture durability performance that is equivalent to or better than currently approved procedures for a variety of field-level and depot/production applications using aluminum, titanium, and steel alloys. The waterborne sol-gel chemistry, designated as Boegel-EPII, is now commercially available in four-component kits from Advanced Chemistry & Technology in Garden Grove, CA under the product name of AC-130. Some implementation has already occurred, primarily for adhesive bonding of titanium and stainless steel in the production setting. This project was completed in FY 2001 and has since successfully transitioned to the Environmental Security Technology Certification Program.
Development of new non-chromated, zero-VOC adhesive and sealant surface preparation and primer technologies impacts both the cost and performance of military and commercial aircraft. Eliminating VOCs and chromates from these processes results in a considerable cost savings by avoiding the need for hard controls and/or fines for noncompliance. The new sol-gel processes also can provide increased bondline strength and/or durability for many applications, which will improve aircraft performance, decrease downtime, reduce maintenance labor hours associated with reworking poor repairs, and enhance operational readiness.