Radar Adsorbing Material (RAM) coatings typically contain volatile organic compounds (VOC) and hazardous air pollutants (HAP) at levels as high as 600 grams per liter. Despite this fact, RAM coatings currently are exempt from the 1998 National Emissions Standards for Hazardous Air Pollutants due to a lack of suitable low-VOC RAM substitutes and the comparatively low volume of their usage at this time. Over the next decade, the U.S. military plans to deploy several new weapons systems that utilize low observable (LO) technology and to retrofit several existing systems to render them more stealthy. As a result, the emission of VOCs from RAM coatings is expected to increase to about 2 million pounds per year. In addition to environmental issues, the process for applying RAM coatings is time and labor intensive due to the relatively large coating thickness, multiple spray passes, and down time between passes that allow for solvent flash and cure. Applying RAM coatings to a typical aircraft or missile weapon system requires more than 10 hours of material application time and multiple working shifts to complete.
The objective of this project was to develop a MILSPEC compliant RAM coating that contains no VOCs or HAPs.
The proposed No-VOC coating was a 100% reactive, addition curing, one-part polyvinyldioxolane-based polyurethane that could be cured using ultraviolet (UV) light photoinitiators. Foster-Miller’s No-VOC binder resin was combined with Boeing Phantom Works proprietary RAM filler, which provides the RAM coating with the same electrical properties as conventional conductive fillers at 40% lower loading by weight. The Boeing filler greatly enhances the sprayability of the RAM coating, and the filler does not block incident UV and visible light radiation needed to promote cure as strongly as the conventional filler. The key enabling technology was Foster-Miller's sprayable, No-VOC polyurethane coating originally developed for clear automotive and aircraft topcoats. The basic chemistry was modified to produce a resin binder system that will meet the stringent coating requirements for RAM applications. Coating formulations were evaluated using various environmental and performance-related screening tests. The project also investigated the feasibility of employing Foster-Miller's “Lock & Key” (release on demand) coating system by incorporating a biodegradable component in the backbone of the binder resin.
Foster-Miller developed an environmentally friendly LO coating that met mechanical, chemical, and electrical property requirements. The formulation has less than 1% VOCs and contains no HAPs or free diisocyanates. It is single component and does not decompose into environmentally noxious materials upon storage. Filled formulations were successfully cured under UV irradiation in thickness ranging from 1-4 mils. Multilayer application was successfully demonstrated up to 100 mils, and layers were found to be compatible and demonstrated good adhesion. The feasibility of significantly reducing application and cure of multilayer LO coatings was demonstrated. Resin synthesis was scaled up to the liter level. It also was shown that the LO coating can be formulated and the fillers can be dispersed uniformly to promote electrical and mechanical continuity. The cured coating has no voids, because there is no solvent evaporation involved. In addition, there is no settling of fillers because the coating is frozen in place by the quick cure mechanism, overcoming issues in electrical continuity encountered with solvent-borne systems. This will make the process viable for repair applications. No-VOC RAM coatings have been transitioned to the Environmental Security Technology Certification Program (ESTCP) for full-scale demonstration and validation under ESTCP project WP-200303.
Implementation of the innovative RAM coatings developed in this project should result in a nearly 100% reduction in VOC emissions generated during spray application. Cost savings related to the elimination of VOCs are estimated to be between $9 and $30 million annually. Radical reductions in labor hours for RAM application also will result. These No-VOC coatings may lead to a rapid, effective HAP-free coating removal process. (Project Completed - 2004)