Legacy military protective coatings utilize chemistries and application techniques that require significant amounts of labor while having negative regulatory and operational impacts. Coating systems often incorporate or employ hazardous precursors, constituents or processes during their lifecycle. These constituents frequently exhibit heavy metal or organic toxins and/or high organic laden chemistries incorporating multiple human and environmental risk factors.
The military spends annually as much as $20B on weapon system corrosion prevention and control coatings, a significant percentage of which is due to topcoat weathering and failure. Current and legacy DoD CPC coating(s) fail to fully meet worldwide military protection requirements. Existing coating systems involve multiple compound processes and material systems where manufacturing, re-manufacturing, field and depot application, maintenance and removal techniques directly contribute to and significantly affect military weapon system lifecycle costs and operational readiness.
Many high performance military grade exterior topcoats use or are based on polyurethane and/or fluorinated chemistries to improve physical, operational and material characteristics. Target topcoats include MIL-DTL-53039 and MIL-DTL-64159 used on ground vehicles and Army helicopters and the US Navy’s extended weathering (MIL-PRF-85285 Type IV) aircraft coating used on Navy and Air Force helicopters and airplanes.
The target coatings exhibit excellent military exterior applications properties. However, the organic coating products and precursors are experiencing increased negative focus regarding their lifecycle human health and environmental risk going forward. In a previous assessment, the military coating market was estimated at well over 2,000,000 gallons per year. Even at 0.1% hexamethylene di-isocyanate (HDI) by weight, it represents more than 2,000 pounds HDI per year. Topcoats formulated without di-isocyanates would make DoD topcoat operations more sustainable with respect to human health and the environment.
Military polyurethane coatings are formed through reaction of polyols with poly-isocyanate compounds, typically HDI. Di-isocyanate compounds are coming under increased regulatory scrutiny as dermal and inhalation sensitizers/irritants, and chronic exposure to di-isocyanates has been associated with asthma and lung damage. Many current and proposed regulations target the aromatic compounds methylene diphenyl di-isocyanate (MDI) and toluene di-isocyanate (TDI) rather than the aliphatic HDI, primarily because MDI and TDI usage and emission is in significantly larger volumes. However, the impetus for regulation is the fact that MDI and TDI are common to spray-applied adhesives, sealants, and coatings, so it is reasonable to anticipate similar regulatory actions against spray-applied HDI coatings in the future. All three compounds (MDI, TDI, and HDI) are regulated hazardous air pollutants (HAPs) under the Clean Air Act. It should be noted that existing topcoats could be labeled accurately as “organic HAP-free” if the weight percent of HDI monomer is less than 1%, which is the required reporting threshold for non-carcinogenic ingredients on safety data sheets. Still, the continued use of HDI-based topcoats is not conducive to the long-term sustainability of military weapons systems. Gaps exist in coatings formulations and processes/techniques that can produce nearly undetectable amounts of low molecular weight isocyanates. Additionally, gaps exist in alternative chemistries. Various alternative chemistries have been explored, such as cyclocarbonate-amines, but slow cure times without the use of regulated toxic catalysts remain problematic.
Despite numerous attempts to find alternative compositions, limited transition progress has been made in reducing military coating lifecycle regulatory risk. Subsequently, regulatory and political interest has increased.
