State-of-the-art metal surface preparations for adhesive bonding consist primarily of anodizing or etching processes using strong acids or bases that rely on mechanical interactions for improved adhesion. These processes typically utilize hexavalent chromium and generate contaminated wastewater during rinsing. The surface preparations are followed by application of a chromated bond primer that can also contain high levels of volatile organic compounds (VOCs). Specific waterborne sol-gel chemistry, now commercially available, was developed under SERDP projects WP-130 and WP-1113. No-rinse processes utilizing sol-gel formulations were developed and optimized in the laboratory, demonstrating excellent adhesion via chemical bonding. The objective of this ESTCP project was to validate and demonstrate these high-performance sol-gel surface preparations that are environmentally friendly as well as simple and inexpensive to perform.
Sol-gel coatings act as an interface between the metal substrate and the polymer-based adhesive or primer. The molecules in the sol-gel solution contain certain functionalities that chemically bond to the polymer and other functionalities that are attracted to the metal substrate. This direct chemical bonding mechanism results in superior adhesion that, in turn, helps the treated surface resist moisture attack and achieve long-term durability.
This project addressed surface preparation for adhesive bonding of aluminum, titanium, and steel substrates. The emphasis was on defining tolerances for the steps involved in sol-gel processing under real-world constraints to support implementation in a variety of applications. Processes using simple abrasion techniques, grit-blast pretreatment, and those used in conjunction with and without waterborne primer were included. Demonstrations of several sol-gel processes were conducted for Air Force, Navy, and Army depot- and field-level applications. To support transition of this technology, processing steps were refined as necessary, data was generated to facilitate end-user implementation, and procedures and savings were documented.
The sol-gel technology was demonstrated for F/A-18, CH-46, and C-5 aircraft applications. In each of these demonstrations, sol-gel was proven to be an effective alternative, with tests showing sol-gel bond performance to be as good as or better than the performance of existing surface preparation methods. The demonstrations verified that the sol-gel technology was feasible and could be implemented at maintenance facilities across DoD for a wide variety of adhesive bonding applications with significant cost savings.
The unique sol-gel solution developed through SERDP and demonstrated under ESTCP is commercially available as AC-130 from AC Tech in Garden Grove, California. The sol-gel technology is applicable to many weapons systems, not only in the Services but also with original equipment manufacturers (OEM), and implementation provides substantial environmental and cost benefits. Using sol-gel instead of the current surface preparation process, DoD will decrease hexavalent chromium exposure, VOC emissions, and generation of hazardous wastes. Because the sol-gel process is faster than existing technologies, repairs are performed more rapidly—increasing throughput at maintenance facilities and decreasing equipment downtime. This streamlining translates to substantial cost savings. It is estimated that sol-gel surface preparation is resulting in an annual savings of $660,000 for C-5 aircraft depot-level bonding applications. For CH-46 aircraft repair, estimates show a cost avoidance of $300,000 per year over a 10-year period. A similar cost avoidance would be realized on the F/A-18 for bonded repairs, and F/A-18 horizontal stabilizer applications have already produced substantial payoffs.