The Department of Defense (DoD) spends millions of dollars each year utilizing solvent-borne organic coatings to protect weapon system substrates. These coatings include chromated primers and topcoats containing high volumes of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) and present a significant burden for environmental compliance, permitting, tracking, storage, operations, disposal, and reporting requirements. The objective of this project was to demonstrate, validate, and implement a VOC/HAP-free Low Temperature Cure Powder Coating (LTCPC) on DoD weapon system components in a depot production environment.
Low temperature cure powder coatings were developed through scientific analysis of powder coating chemistries and the discovery that an acid/epoxy reaction could facilitate a low temperature cure of powder coating. These studies led to the down selection of an acid functional polyester resin with a triglycidylisocyanurate crosslinker. Coating material development involved optimizing parameters such as catalyst, resin type, corrosion inhibitor, flow additive, pigment concentration, powder particle size, and the ratio of resin to crosslinker. The resultant coating developed under SERDP project WP-1268 is a VOC/HAP-free alternative to solvent-based coatings, and it features performance properties equal to or better than those of coating systems governed by commonly used specifications such as MIL-PRF-23377, MIL-P-53022, MIL-PRF-85285, and MIL-PRF-22750. The coating can be applied directly to the component substrate, eliminating the need for primers. This low temperature cure powder coating is fully curable at 250°F and can replace other powder coatings, which require temperatures between 280°F and 400°F. Decreasing the curing temperature enables temperature-sensitive substrates, such as tempered aluminum, to be powder coated safely, without compromising their structural integrity and performance. Although all powder coatings offer superior barrier protection against corrosion, the powder coating examined also contains a corrosion inhibitor, barium metaborate, to further improve corrosion protection in the event of coating damage.
This project completed the work associated with transitioning the LTCPC into use at DoD maintenance facilities. Additional testing and evaluation of the candidate material was conducted to more thoroughly characterize performance (beyond the testing and substrates used in SERDP project WP-1268) utilizing a Joint Test Protocol. The improvements in the coating process and the superior operational performance of the powder coating on aircraft components and ground support equipment were demonstrated. The project also validated the environmental benefits associated with the LTCPC on aircraft components and ground support equipment. The cost, logistics, and performance parameters of baseline coating methods for Air Force and Navy logistics centers were quantified, and the cost-savings potential for transitioning to LTCPC was demonstrated. Finally, the project coordinated and facilitated technology transition of the low temperature process into governing documents (e.g., MIL-PRF-24712 and coatings related technical orders) and actual depot operations.
A combination of laboratory test results and actual field evaluations confirmed the suitability of LTCPC as a direct replacement for several wet coating systems that are currently in use on DoD aircraft and ground support equipment components. LTCPC demonstration results support the current stakeholder efforts directed at implementing this technology at DoD maintenance facilities. Although this coating material will not be used initially on a wide-scale basis, Air Force and Navy acceptance will increase LTCPC usage through the modification of specifications and technical orders regarding approved coatings. This will facilitate adoption of the process by other services and original equipment manufacturers. In addition to the previously identified military uses for LTCPCs, technology transition opportunities exist within general aviation and other industries looking to reduce existing powder cure energy requirements or to apply uniform, high-performance coatings to temperature-sensitive substrates.