Existing primers and topcoats are solvent-based systems that offer excellent protection against corrosion or other operating environment conditions. However, the industrial use of solvent-based technology has numerous drawbacks, which include evaporation of toxic volatile organic compounds (VOC), release of toxic isocyanates, low transfer efficiency, and relatively long cure times. Powder coatings have been developed as an alternative technology. These coatings typically involve the electrostatic application of powdered metal to a grounded part, followed by a curing cycle. Advantages of the powder metal technology include a reduction in toxic material use and generation, increased transfer efficiency, and reduced costs of environmental safety, compliance, and energy use.
The goal of this project was to provide powder materials and technology to improve aircraft coating performance and increase environmental acceptability. These powder materials will enable the minimization or elimination of VOCs and hazardous air pollutants (HAP) used in manufacturing and coatings applications. Furthermore, the coatings will exhibit improved performance including improved durability; cleanability; impact, solvent, mar, and corrosion resistance; and enhanced appearance.
The project approach was to focus on the formulation, development, optimization, and production of powder materials that would provide a reduction in VOCs and HAPs and improvements in overall coating system performance. Characterization of previously developed materials was undertaken to determine the causes of performance limitations and to identify avenues to improved performance. Material research and development included the following: (1) crosslinkable resin powders; (2) pigmented polymer beads; (3) microballoon-based pigments; and (4) hollow fibrillar pigments.
Four different polymeric powder coatings were optimized for validation testing. The powder polymeric resins that were optimized included: (1) polyether blocked polyimide - Autochem 5533 Pebax; (2) fluoropolymer - 2850 Kynar; (3) ultra-high molecular weight polyethelene coathyene - Clariant UHMWPE; and (4) polyamide - Vestosint 2154 Nylon 12.
The optimized powders were designed for demonstration on a mobile communications shelter. The successful coatings were then evaluated for use on ground support equipment. Shot-blasted 1010 steel panels underwent more than 3,000 hours of salt spray testing. No visual signs of undercutting or surface corrosion outside of the 0.8 inch bare metal scribe lines were observed. An 8-inch wide bare metal sample without surface treatment, primer, or corrosion inhibitor was scribed prior to salt spray testing. No coating surface corrosion or undercutting was noted after 1600 hours of testing. Additionally, efforts with powder vendors to obtain the required particle size distribution to optimize this process were successful. This project was completed in FY 1997.
The research and development will lead to low- or zero- VOC coatings that will be easier to clean, have a more uniform appearance, and will be less susceptible to weathering effects. As a result, repainting and touch-up requirements will be reduced, thereby producing significant life-cycle cost savings. The effort also is applicable to the commercial aircraft and automotive industries. The materials and technology will be incorporated into various low/zero- VOC coating systems under development by the Air Force and industry.