The objective of this project was to establish a non-isocyanate polymerization-crosslinking platform for a high-performance exterior topcoat to replace the use of environmentally hazardous isocyanate chemistry used in state-of-the-art polyurethane (PU) topcoats. This project developed a new chemistry platform for ultraviolet (UV) and weathering-stable aircraft topcoats based on organosilane chemistry traditionally used for silicone-based sealants and elastomers. The topcoat was to be a thermoset hybrid resin coating consisting of nonaromatic organic and siloxane segments that are cured at room temperature (RT) or very moderate temperatures.
The original technical approach included the synthesis of adequate intermediate building blocks for achieving the desired topcoats utilized the highly efficient hydrosilylation reaction between silicon hydride (Si-H) and terminal alkene sites, a reaction also employed to cure the derived coating formulations. This synthesis approach can utilize many commercially available reagents, can be scaled up, and is applied using common manufacturing methods.
The technical approach was modified in the last year of the project to overcome several drawbacks of the original approach identified during the program. The modified approach began with the synthesis of multiple Si-OH-containing intermediates via hydrosilylation (as was the case in the original approach), followed by dehydrocoupling reactions to efficiently convert Si-H groups to Si-OH in the presence of water and/or Si-OR in the presence of alcohol. The intermediates were then cured by catalyzed Si-OH condensation. This modified approach overcame major challenges associated with the original approach, and further development is recommended using the modified approach.
The project focused on nonaromatic formulations known to be significantly more stable during long-term exposure to UV light, as required for aerospace applications. The project team synthesized more than 100 intermediates and screened more than 450 formulations as clear and pigmented topcoats over aircraft aluminum alloys or aircraft-certified primers deposited on aluminum alloys. Most of the hydrosilylation syntheses were found to be very versatile and simple to process (even in the absence of solvents) if the team included 10 to 20 ppm of an extremely efficient platinum (Pt) catalyst. The catalyzed reaction was less useful in amine and isocyanate functional groups (complete deactivation of the catalyst) and acrylic and other vinyl groups in proximity to carbonyl group (much slower reactions at low temperatures). However, the main problems identified with the hydrosilylation system occurred during the curing of the evolved formulations as coatings. Nevertheless, the reactions could be easily scaled up from 10 to1000 g quantities.
Curing in this system relies on additional hydrosilylation reaction in the presence of the live platinum remaining from the intermediate synthesis step. In most cases, the researchers had to add about 30 ppm of a fresh catalyst to accelerate the curing reaction at RT and 50°C. The original approach demonstrated curing capability at 50°C with adequate hardness and adhesion when down-selected formulations of intermediates were deposited as coatings over aluminum and chromated aluminum surfaces. However, the team encountered various challenges, which required them to seek solutions and redirect the project’s approach at month 22. The main shortcoming associated with curing the coating formulations occurred during their deposition over primers. The amine content at the surface of the cure primers was sufficient enough to poison the Pt catalyst. An additional major hurdle was the very short pot life after mixing the Si-H and vinyl intermediates once the additional catalyst required for curing was added. In contrast, the lack of additional catalyst led to insufficient curing even at 50°C. These shortcomings are described in detail in this report. Although the team has solved or mitigated some of these problems, the solutions were not elegant and added more steps, solvents, and surface pretreatments than desired.
The team switched to the modified approach in the last year of the project. They took a few of the multiple Si-H-bonded intermediates and efficiently converted all their Si-H into Si-OH via a dehydrocoupling reaction with water. This was a very efficient reaction, in which a commercially available ruthenium carbonyl catalyst was used at the level of 100 ppm. During the synthesis of the best Si-OH intermediate (down-selected for the final sets of testing), some condensation of the Si-OH into Si-O-Si (+H2O) was observed, and it extended the degree of polymerization and consequently the molecular weight of the Si-OH intermediate and its viscosity. The higher viscosity was found to be advantageous for paint formulations and applications. Curing the formulation based on the Si-OH intermediates was very efficient at RT when a base (amine) catalyst was added; this yielded high hardness and adhesion values that far exceeded the values achieved with the hydrosilylation system. The pot life was extended to the range of four to 20 h, which was a very realistic time frame for thermoset coatings. The amine content of the primers was so detrimental to Pt-catalyzed hydrosilylation, curing was advantageous for curing and bonding of the Si-OH system.
Researchers had only enough time to evaluate six different Si-OH intermediates, and they down-selected one for scale-up and formulation. Thus, they were clearly far from an optimization stage. Indeed, many other formulas can be synthesized and evaluated if additional studies are performed. The down-selected intermediate was scaled up to 2 L of ready-to-use solutions. Although the team evaluated a single intermediate in the standard tests of aviation-grade topcoats (according to MIL-PRF-85285E and Boeing’s commercial standards), some of the early formulations passed many of the required tests. The team still needed to improve the bending and impact resistance of the coatings according to the military specifications (MIL specs) (even though they were adequate for Boeing’s tests). Weathering stability can also be improved by using appropriate additives that have not yet been studied in a systematic manner.
The hydrosilylation-deydrocoupling intermediate approach provides intermediates that are good candidates for further R&D activities focusing on topcoats for aircraft and non-aircraft applications. In contrast, the hydrosilylation-only system is not adequate for top-coating applications. Nevertheless, researchers encourage assessment of the intermediates (which were simple to make with the hydrosilylation reaction) because they believe they are excellent candidates for gas separation of organic compounds when tailored-affinity membranes are used.
The benefits of the proposed hydrosilylation-dehydrocoupling system are its versatile intermediate synthesis capabilities, the chemical stability of the derived coatings, and the fact that there are many suitable commercially available reagents for obtaining good intermediate candidates based on this approach. Given the very low levels of transition-metal catalysts and the capability to recover them via sorbing materials, the overall cost of the formulations will be similar to or less than that of currently produced topcoats (polyurethanes) for aircraft applications.