Traditional coating systems contain hexavalent chromium [Cr(VI)] in both the conversion coating and primer, volatile organic compounds (VOC) and hazardous air pollutants (HAP) in both the primer and topcoat, and several other heavy metals for corrosion protection and color. Approaches to coating evaluations, rather than investigating the entirety of the system, generally have focused on individual components while maintaining the presence of harmful materials throughout the rest of the coating system.
The objectives of this project were threefold: (1) to evaluate combinations of chrome-free conversion coatings and primers to maintain high corrosion resistance and good physical properties; (2) to develop and evaluate coating systems that have near zero VOCs (less than 25 grams per liter) and no HAP emissions, while improving performance; and (3) to develop and evaluate environmentally friendly coating alternatives as a complete system with regard to various performance criteria.
Researchers further developed and optimized the Naval Air Systems Command (NAVAIR) non-chromated process (NCP) and trivalent chromium process (TCP), which have undergone field testing on Navy, Marine Corps, and Army assets. Combinations of non-chromate pretreatments and chromate-free primers were evaluated for performance and corrosion prevention. Water-dispersible topcoats and primers were utilized and formulated with recently exempt solvents such as tert-butyl acetate to eliminate HAPs and reduce VOCs. Nanoparticles were added to provide longer recoat times with improved barrier properties. In order to enhance durability, minimize repainting, and reduce solvent emission, a new class of high reflectance pigments was also explored. The pigments, which are stable and inert mixed metal oxides, provide excellent near-infrared (IR) reflection while appearing visibly identical to their conventional counterparts, yet providing optimum durability. Lastly, the chromate-free pretreatments/primers were combined with the no-VOC/HAP coatings and evaluated as a whole for performance, corrosion reduction, survivability requirements, and affordability.
TCP is the leading candidate to replace Cr(VI) pretreatments in the military. This project worked to modify and improve this process to increase performance and function well on a variety of substrates. Research was conducted on the application of TCP to aluminum as a pretreatment and steel substrates as a wash primer. Analysis indicated that this pretreatment functions through the deposition of chromium(III), zirconium, and zinc onto the surface. Analysis of the chromium deposited indicated that it is in the Cr(III) state rather than the carcinogenic and dangerous Cr(VI) state. Experiments showed that this Cr(III) was very stable and did not form Cr(VI) under normal conditions. However, results showed that it may have been possible to generate Cr(VI) under severely corrosive/oxidizing conditions, but the results were unclear because similar testing of the base metal without the TCP coating also produced Cr(VI), which came from oxidation of Cr deposits within the substrate. Chromium-free primers have been used with TCP pretreatments. Their performance is good, although not as good as that for chromated systems.
Commercially available zero VOC coatings from Deft have proven effective for NAVAIR applications. Zero VOC coatings were formulated using standard epoxy monomers and solvated using VOC-exempt solvents. These coatings performed well relative to standard Army primers. Powder coat epoxies with no volatile solvents have also been approved for military use. The use of water-dispersible topcoats for NAVAIR systems has resulted in lower coating system performance, likely due to an increase in hydrophilicity of the topcoat. Zero VOC powder coat topcoats have not been effective for Army application, failing chemical agent resistance testing and weathering performance. On the other hand, new zero VOC topcoats have been developed with excellent performance all around. The use of fluorinated additives in Army coatings has potential because they do successfully segregate to the surface and reduce the surface energy, but the existing fluorinated polyols do not currently have sufficient environmental stability. Polymeric bead flattening agents have been applied to all Army topcoats to reduce solvent loading and improve weatherability. New low solar loading pigments have been developed and used to produce excellent coatings with improved weathering performance.
This project identified critical Department of Defense (DoD) environmental needs and developed practical solutions to reduce VOC emissions and Cr(VI) content from military coatings systems. Future work must still be done to further develop these new coatings systems and make them widely applicable to DoD weapons platforms.