The objective of this project is to demonstrate, validate, and implement commercial off-the-shelf ultraviolet (UV)-curable coatings as an alternative to the hazardous and long cure time coatings currently used on aerospace equipment. Targeted applications include simple geometry off-aircraft components, exterior and interior panels, markings, and touch-up and repair beyond the technical capabilities of powder coatings. The project team will conduct a cost-benefit analysis, laboratory testing, and field testing to verify that UV-curable coatings exhibit the same or better performance, reduce the overall environmental burden, and reduce the overall process flow time as compared to currently used and approved coatings.
UV-curable coatings are volatile organic compound (VOC)-, hazardous air pollutant (HAP)-, and isocyanate-free, single-component high-solid (nearly 100%) cross-linked coatings cured by brief exposure to intense UV light. The chemical reaction, or polymerization, that occurs involves two major constituents, oligomers and monomers, and is set in motion by the photoinitiators blended into the coating. Oligomers are polymers that define the coating's physical properties, with examples being urethanes, epoxies, polyester epoxies and acrylates, and polyesters. In UV-curable coating formulation, the oligomers form the backbone of the structure and the monomers "link" to the oligomers to form a network during exposure to UV light. Increasing the number of reactive groups, called multifunctional monomers, will increase the cross-linked density of the coating. In addition, these monomers are generally low viscosity and can serve as a diluent to decrease coating viscosity for application purposes, reducing or eliminating the need for reactive solvents. In the presence of UV light (and dependent on the intensity of the light), these constituents are capable of reacting at very high rates (seconds). UV-curable coatings can be applied through traditional coating techniques (brush, roll, or spray) and are rapidly cured with exposure to a UV light source of the proper intensity and frequency. A lamp system capable of generating intense bursts of UV light must be supplied in order to successfully cure the coating. The nature of the lamp system being utilized is determined largely by the shape and area of the surface being coated, while the nature of the bulbs used is determined by the photoinitiators in the coating being cured.
Implementing UV-curable coatings at Department of Defense (DoD) maintenance and repair facilities will lead to significant environmental, occupational safety, and health benefits as well as productivity increases. Specifically, replacing current solvent-borne systems with UV-curable coatings could result in the reduction of 138 tons of VOC emissions and 28 tons of HAP emissions from U.S. Air Force depots alone. Additional reductions from other DoD depots and field-level painting operations will be substantial. Implementing a one-component, UV-curable system also will reduce hazardous waste collection since the unsprayed material will not cure in the spray guns and in lines such that it can be used in later applications. Furthermore, a reduction in the amount of solvents used for cleanup will be realized since the application equipment can be cleaned with water-based cleaners. Finally, occupational safety and health benefits will be realized by eliminating worker exposure to coatings containing isocyanate, and production improvements will result from significantly shorter coating cure times of minutes versus the hours and days currently required. (Anticipated Project Completion - 2011)