The objective of this research was to develop a rapid curing, non-isocyanate erosion-resistant coating containing low to no volatile organic chemicals (VOC) or hazardous air pollutants (HAP). This coating must meet or exceed critical performance properties specified in SAE AMS-C-83231 Revision A to include peel strength, flexibility, water resistance, aromatic fuel resistance, rain erosion resistance, electrical transmission, surface resistivity, weather resistance, and the ability to strip. In addition, resistance to lubricating oil and hydraulic fluid degradation is desirable. Proposals for use of alternative materials must include a preliminary toxicological assessment of the alternative materials.
This research effort was successful in developing a platform of non-isocyanate polyurethane (NIPU) building blocks for high-performance coatings by effectively leveraging cyclic carbonate/polyamine chemistry. One of the major intermediates—multi-functional cyclic carbonate (MFCC)—was not commercially available. A single-step, safe, energy-efficient, and carbon-negative process for deriving MFCCs with a wide range of chemical structures, molecular weights (MW), and functionalities was established using commercially available epoxy compounds and carbon dioxide gas as starting materials.
A library of functional NIPU derivatives with varying functionality types and contents, MW, and chemical structures were synthesized. These functional NIPU building blocks were custom designed for suitability for formulating two types of NIPU coatings: (a) two-component high solids ambient temperature curable coatings (2K-HS-NIPU); and (b) ultraviolet (UV)-curable 100% solid coatings (UV-NIPU).
a) The 2K-HS-NIPU coatings were formulated using amine-terminated NIPU intermediates as one component and aliphatic di- or higher functional epoxy compounds as the second component. Among various coatings developed and tested, two 2K-HS-NIPU coating candidates that passed all the tests for critical requirements were tested for rain erosion resistance. These coatings could not hold up to the rain erosion testing requirement of 30-60 seconds and failed at 10-11 seconds.
b) The UV-NIPU coatings were formulated from (meth)acrylate functional NIPU oligomers and with other conventional components. While high viscosities of NIPU-acrylates were one of the major constraints, the project team could successfully formulate 100% solid compositions. None of these coatings could pass all the critical requirements: specifically, balancing their % elongation, tensile strength, and low temperature flexibility was not possible. Therefore, none of UV-NIPU could make it to the Rain Erosion Test.
In this effort, the NIPU coating development was confined to a specific and unique Rain Erosion Resistant coating, as per the objective of this project. Rain Erosion Resistant coatings have unique and stringent performance requirements that require balancing and optimizing many opposing mechanical properties, exterior durability, protective properties, ambient temperature cure within acceptable time, and above all low VOC and no usage of HAP. Due to the nature of the NIPU chemistry adopted in this project, specifically the relatively lower reactivities of the functional groups involved (relative to isocyanate-based chemistry), these stringent requirements could not be sufficiently met. More specifically, these NIPU coatings could not pass the Rain Erosion Test that required coating to have extremely high tensile strength (~6.5 Mpa) and higher than 500% elongation, among other requirements. Due to the restrictions on VOC and HAP, it was not possible to use high MW polymers. Constraints on NIPU functional group low reactivities also resulted in high MW not being achieved during the coating curing process. These factors ultimately could not make these NIPU coatings complaint to the above-stated requirements.
Nevertheless, the outcome of this research project has provided an excellent NIPU-coating platform that can be effectively leveraged for not only a broad range of sustainable military coatings but also myriads of industrial and protective coatings with their significantly lower environmental impacts. The NIPU coating platform is amenable to developing advanced coating formulations on contemporary as well as emerging technologies. Moisture-cure, water-borne, and heat-curable coatings can be successfully formulated, besides the two-component ambient temperature cure and UV-cure coatings that have been extensively studied and demonstrated in this project. NIPU platform can also be used to develop advanced and sustainable additive manufacturing of three-dimensional printing materials.