The military utilizes high-performing exterior topcoats with a wide range of stringent performance requirements. Military vehicles and aircraft require coatings which cure at ambient temperatures under a wide range of environmental conditions. These requirements have historically led to the use 2-component urethanes which utilize isocyanate-functional crosslinkers. These chemistries have known health hazards associated with their use, particularly in long term exposure situations. The objective of the SERDP funded project was to develop high performance coatings meeting one or more military specification without the use of isocyanate crosslinkers. The technical objective of this effort was to develop high performance coatings meeting one or more military specifications: MIL-DTL-53039E, MIL-DTL-64159B and MIL-PRF-85285E without the use of isocyanate crosslinkers.
The replacement of isocyanate crosslinkers was approached by evaluating three alternative chemistries, down-selecting a most preferred approach, and optimizing for performance properties. The alternative crosslinking chemistries had been investigated prior to this project and each had an attractive characteristic.
The first alternative chemistry evaluated was the cyclic carbonate-amine ring opening reaction which generates a urethane moiety as product without employing an isocyanate. The second was the reaction of polymeric uretidiones with hydroxyl functional compounds. Prior work had demonstrated good room-temperature cure with amidine base catalysis, although evaluation of performance versus military requirements was not done. The third chemistry was alkoxy silane hydrolysis and condensation which had the benefit of being the basis of a commercial technology used in the protective coatings market.
The general approach taken to evaluate the crosslinking chemistries was to identify initial coatings formulations and then investigate room temperature cure response. Two methods were employed:
- infrared spectroscopy following the increase or decrease of key functional groups on the reacting compounds
- development of solvent resistance of coated substrates as an indication of network formation.
If acceptable room temperature cure response could be established, further coatings evaluations were carried out including adhesion, flexibility and hardness and compared to isocyanate crosslinked controls. When the second tier performance characteristics were acceptable testing was carried out using military coating specifications as guides. The two that were used were MIL-DTL-53039E for military vehicles and MIL-PRF-85285E for Navy aircraft.
The cyclic carbonate-amine reaction did not proceed well at room temperature in thin films. A number of changes were tried, including polymer modifications for improved film mobility and increased functionality of the amine crosslinkers, but none were sufficiently effective to provide acceptable film properties. The polyuretidione-hydroxyl reaction proceeded very well at room temperature with amidine bases as catalyst. The deficiencies identified were very short pot life and low solids content at spray viscosity. The short pot life was significantly improved without compromising room temperature cure by blocking the base catalysts with volatile acids. The low solids at spray viscosity, however, could not be remedied without losing cure response. Extensive use of exempt solvents provided prototypes that would meet the Clean Air Act requirements but these were much softer than urethane counterparts.
Investigations into alkoxy silane crosslinking provided the most promising results. The starting point was a coating that was a high gloss product sold commercially in the protective coating market. Good room temperature cure was achieved as expected and low gloss formulations were identified using commercially available flatting pigments. Flexibility and dry time were identified as the major gaps and both were closed using alternative alkoxy silane functional reactants. As a result prototype formulations were evaluated versus the two targeted military specifications. In this evaluation, the principal remaining gap was fluid resistance.
At the conclusion of the project, it was evident that the formulation latitude provided by the alkoxy silane cured coatings was wide enough to close the remaining gaps in performance. Although a final formulation was not identified during the project performance, this chemistry appeared to represent a reduced hazard profile compared to isocyanate-crosslinked coatings. However, a careful evaluation of this issue should be conducted when final formulations are developed. An additional benefit of the project work was the identification of blocking agents for the strong amine catalysts that still permitted acceptable room temperature cure. It is possible this tool could be used for other amine catalyzed room temperature cure coatings such as epoxyamine primers, Michael Addition chemistry, and isocyanate crosslinking. As such, a patent application has been filed on the blocked amine catalyst.