Corrosion protection for metals used in the fabrication of aircraft, munitions, and military vehicles generally consists of a pretreatment, primer, and topcoat. The pretreatment and primer often contain chromate compounds in which the chromium is in the hexavalent state. Hexavalent chromium is a known carcinogen and minimizing its use is a current policy of the Department of Defense (DoD). Additionally, topcoats often contain volatile organic compounds (VOCs) that are hazardous and released to the environment during application.
The objective of this project was to develop a novel, environmentally benign, coating system for metal alloy structural components in DoD systems.
The coating system consists of one or more organofunctional silanes, organic resins, and nanoparticle fillers. It is amenable to dipping, spraying, wiping, or brushing any clean metal surface. No conversion coating, either phosphate or chromate, is required. The key innovation underlying this technology is the hydrophobicity transition exhibited by organosilanes. While the uncondensed silanes are hydrophilic and water soluble, they become hydrophobic on deposition, resulting in extremely low water-transmission rates. When combining a water-based organofunctional silane system with a water-based resin, the properties of the components can be manipulated so that the hydrophilic-to-hydrophobic transition is strongly enhanced.
To achieve the required performance in a single-step coating system, every element of film formation must be carefully engineered. This project integrated a systematic study of the chemistry and physics of complex films with the coating development tasks. Neutron reflectivity, small-angle x-ray scattering, 29Si-NMR spectroscopy, and imaging ellipsometry were used to study the silane polymerization chemistry, phase separation in hybrid films, the physics of film formation, and the film-failure mechanisms.
The results of this project include two workable superprimer formulations, based on different types of resins, and six anti-corrosion pigments. Due to the availability of these pigments, the two systems can be used on almost any metal. Numerous invention disclosures on technologies developed from this project have been filed. Major applications that are envisioned for the technology include automobile bodies, automotive components, the coil coating industry, and the aerospace industry.
This project developed an entirely novel series of coatings that reduce VOC emissions and eliminate chromate handling and waste for a broad range of coating applications for military platforms. The costs of these new formulations are comparable or lower than current coating systems. Corrosion protection is equal to or better than current coating systems. Visual signature and defect-healing properties are maintained.