Novel Isocyanate-Free Moisture-Curable Polymers for Single-Component Polysiloxane Topcoats

By Erick Iezzi, Ph.D.

The U.S. Naval Research Laboratory (NRL) recently devel-oped novel alkoxysilane-terminated N-substituted urea polymers for use in moisture-curable, single-component (1K) polysiloxane topcoats for military assets. These polymers demonstrate similar mechanical properties and chemical resistance to polyurethanes, yet do not contain harmful isocyanates. The polymers can be synthesized in a variety of polar and aromatic solvents to provide low viscosity solutions, thus allowing for the generation of topcoats that are free of hazardous air pollutants (HAPs) and contain low levels of volatile organic compounds (VOCs).

These polymers have been primarily utilized to formulate 1K polysiloxane topcoats for use on the exterior of Navy surface ships and aircraft, yet they are applicable to a variety of non-military markets where exterior durable topcoats are required. For Navy ships, the 1K polysiloxane topcoat is designed as a haze gray colored system that is <250 g/l VOCs, provides a semi-gloss finish once cured, will adhere to high solids epoxy primers, and offers enhanced weatherability and hydrocarbon resistance compared to legacy Navy topcoats.

The coating can be brush- and roll-applied by sailors for touchup/repair applications or spray-applied by contractors during depot-level maintenance. For Navy aircraft, the 1K topcoat is designed as a camouflage gray colored system that is <300 g/l VOCs, is spray-applied via high-volume low-pressure (HVLP) equipment, provides a low-gloss finish once cured, and demonstrates comparable flexibility, weatherability, and hydro-carbon resistance to qualified polyurethane topcoats. Gloss topcoats for Navy aircraft have also been formulated using these novel polymers. Laboratory testing of these 1K polysiloxane topcoats to military specifications (MIL-Specs.) and demonstrations on military assets will be discussed herein.