Elastomeric coatings used for aerospace applications typically contain volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) such as methyl ethyl ketone, methyl isobutyl ketone, toluene, or xylene at levels as high as 600 g/L. Despite this fact, these coatings are currently exempt from 1998 National Emissions Standards for Hazardous Air Pollutants (NESHAP) due to the lack of a suitable low-VOC substitute as well as their low usage volume at the time the regulation was drafted and passed. Since that time, the requirement for use in aerospace applications of these coatings has substantially increased. Over the next decade, the U.S. military plans to deploy several new weapons systems that use elastomeric coatings and technology to retrofit several existing systems, including the use of elastomeric coatings to improve the performance of the aircraft. As a result, the emission of VOC from elastomeric coatings is expected to increase to about 2 million pounds per year.
In addition to environmental issues, the process for applying elastomeric coatings is time and labor intensive due to the relatively thick coatings that are applied. The required thickness is achieved by applying multiple layers. Applying these coating to an aircraft or missile weapon system is a very cumbersome process and usually requires multiple shifts.
The objective of this project was to demonstrate and validate innovative technologies that will result in a nearly 100% reduction of VOC emissions from an elastomeric coating spray application. The coating resin was developed by Foster-Miller, Inc. in part under Strategic Environmental Research and Development (SERDP) funding (WP-1180). This specific resin was selected based on its potential ability to allow cure of thick layers of filled formulations. The ultraviolet (UV) coating technology has the potential to provide 90% reduction in application and cure time, thus reducing life-cycle costs and improving mission readiness of the aircraft.
Specific objective of the demonstration were to:
- Demonstrate that the UV curable resin can be tailored to meet specific weapon system requirements through the addition of appropriate fillers and additives
- Validate that the material can be spray-applied and cured in order to demonstrate compatibility with production and field application methods
- Demonstrate that the coating is amenable to field repair and the repaired coating maintains its aerospace performance characteristics.
The material met most of the performance requirements. This demonstrates that the alternative material has potential as a viable replacement for the baseline material. The coating adhesion following fluid exposures to MIL-DTL-83133 (JP-8) and MIL-PRF-85570 Type II (alkaline cleaner) showed some deviation from the material requirements. The test coupons exposed to alkaline cleaner were tested and resulted in tensile strength that deviated less than 10% from the goal. This deviation in performance may be due to the test method used for these specific test coupons. Since the adhesion testing of the UV curable material following these exposures were within 10% of the requirement, it is likely that the testing of the material at the correct test speed would yield results that meet the material requirements. The UV cured material will be retested under alternate funding to establish the material adhesion following exposure to MIL-PRF-85570.
Following exposure to JP-8 the material failed the flatwise tensile test. These coupons were tested with the same method as described above. This discrepancy in test method may account for some of the loss of adhesion; however, the test method is not the only factor affecting the material’s adhesion following fluid exposure. Additional testing will be performed on test coupons following MIL-DLT-83133 exposure.
Compatibility with the baseline material was also an area where the UV curable coating did not meet the full requirement. Again, flatwise tensile strength was used as the test method to demonstrate compatibility to the baseline material. The same discrepancy in test speed was repeated in this testing. Also, the results showed partial adhesive failures to the pull member bonded onto the UV curable material for test. This shows failure of the surface preparation to the test coupon, which does not represent the platform application. These two factors are potentially large factors that would affect the tensile strength results from the testing. These test configurations will be retested under alternate funding to establish the UV repair material adhesion.
The UV curable material demonstrated has equivalent or better technical performance when compared to the baseline coating for most of the material requirements. Once the adhesion properties can be verified, the UV curable material could be used as a viable alternative to the baseline coating and repair material. The UV curable coating provides significant benefits in reductions of cycle time, VOC and HAP emissions, and overall cost. The UV curable material costs more per gallon of material; however, the overall quantity of the UV curable material is less than the baseline material. The baseline material contains VOCs and exempt solvents, which flash off after applications. However, with the UV curable system, there are no VOCs and no solvents; therefore, the wet film thickness of the material is equivalent to the dry film thickness of the material, thus reducing material volume loss. Robotic application is dependent on development of a cure system for this type of large-scale application.