Demonstration and Validation of Siloxane-Based Aircraft Topcoats that are Isocyanate-Free and Provide a Reduced Environmental Impact
ESTCP 2020 Project of the Year Award for Weapons Systems and Platforms
The entire exterior surface of Department of Defense (DoD) aircraft (i.e., fixed and rotary wing) and ground support equipment (GSE) are currently painted with two-component (2K) polyurethane topcoats. These topcoats, which are qualified to MIL-PRF-85285 performance requirements, provide visual camouflage for the assets, in addition to flexibility, hydrocarbon resistance, and other properties to protect the underlying anti-corrosive primer. As 2K systems, these topcoats require the mixing of a clear hardener component with a pigmented base component in a specific volumetric ratio to form a solid, cross-linked coating. However, the hardener contains hazardous isocyanate-based chemicals, such as hexamethylene 1,6-diisocyanate (HDI) and homopolymers thereof, which are harmful to the environment and can cause serious health issues for coating applicators and those in proximity to airborne mists during spray applications. Exposure to isocyanates can result in severe irritation of the eyes, nose, and skin, symptoms of asthma, and sensitization, and they are listed as a potential human carcinogen by the Occupational Safety and Health Administration (OSHA). Many of these polyurethanes also contain hazardous air polluting (HAP) solvents, such as xylenes, which are potentially carcinogenic.
To address these issues, the Naval Research Laboratory (NRL) recently developed novel organosilicon polymers that are free of hazardous isocyanates and can be used to provide single- (1K) and 2K polysiloxane topcoats (Fig. 1). When formulated into coatings for aircraft and GSE, the topcoats demonstrate similar properties (e.g., flexibility, hydrocarbon resistance, weatherability) as the qualified polyurethane topcoats. These polysiloxane topcoats are also HAP-free and lower in volatile organic compounds (VOCs) than polyurethanes, thereby improving air quality. More details on this effort can be found the project webpage.
During this project, Dr. Erick Iezzi and a team of materials engineers and chemists from the Naval Air Systems Command (NAVAIR), Air Force Research Laboratory (AFRL), and NCP Coatings, Inc. evaluated various 1K and 2K polysiloxane topcoats to MIL-PRF-85285, Type II and IV performance requirements in the laboratory, in addition to testing their corrosion resistance over chromate and non-chromate primers at outdoor exposure sites. After meeting these requirements, the topcoats were demonstrated and validated on the exterior of active DoD assets at various locations across the country.
This project represents the first time that isocyanate-free topcoats have demonstrated equivalent or greater performance than qualified polyurethanes. The 1K polysiloxane topcoat is a user-friendly system because it eliminates the need for metering and mixing of multiple components, which reduces preparation time and prevents mixing errors. In addition, the container of 1K polysiloxane can be resealed for future use once opened, thereby reducing the generation of hazardous paint waste.
Through their efforts on the project titled Demonstration and Validation of Siloxane-Based Aircraft Topcoats that are Isocyanate-Free and Provide a Reduced Environmental Impact, Dr. Erick Iezzi and his team have been awarded the 2020 ESTCP Project of the Year from the Weapons Systems and Platforms Program Area.
- Erick Iezzi, Ph.D. – Naval Research Laboratory
- Mike Brindza, Ph.D. – Naval Air Warfare Center – Aircraft Division
- Maria Hatch – Naval Air Warfare Center – Aircraft Division
- Brenna Skelley – Naval Air Warfare Center – Aircraft Division
- Julia Russell – Naval Air Warfare Center – Aircraft Division
- Frank Pepe – Naval Air Warfare Center – Aircraft Division
- Diane Buhrmaster – Air Force Research Laboratory
- Mike Spicer – Air Force Research Laboratory
- Chris Gladson – Naval Air Systems Command, Fleet Readiness Center East
- Jack Benfer – Naval Air Systems Command, Fleet Readiness Center Southeast
- Atish Gupta – Naval Air Systems Command, Lakehurst
- Randy Terrill – NCP Coatings, Inc.