The objective of this project is to develop a novel method for producing polyurethanes that are based on innovative non-isocyanate chemistries. The resulting nonisocyanate polyurethanes (NIPU) will be sustainable, non-toxic, water-dispersible and/or dissolved in volatile organic compound (VOC)-exempt solvents, and will be applied to substrates via conventional high volume, low pressure (HVLP) equipment. These polyurethanes will eliminate VOCs and hazardous air pollutants (HAPs) during the application process, but will still maintain, or exceed, the critical performance requirements specified in SAE AMS-C-83231.
The production of the proposed NIPU will follow two distinct synthetic strategies. The two routes for the formation of NIPU that will be explored are: (1) a two-step polycondensation approach that involves the reaction between cyclic carbonates and fluorinated diamines to produce an intermediate that is then reacted with a fluorinated diol to produce the NIPU, and (2) the polyaddition, by ring-opening polymerization, of fluorinated bifunctional cyclic carbonates with fluorinated diamines. Both routes will be explored concurrently.
The linear NIPUs produced using the polycondensation approach are pre-polymers that will be treated with dimethylolpropionic acid (DMPA) followed by a base, which is a common procedure that is used to form water-dispersions of polymers without any VOCs or HAPs during application. The pre-polymer dispersion will be the basis for the proposed HVLP spray coating system. The pre-polymers will be sprayed onto a substrate and a crosslinking reaction will be initiated with triethylenetetramine (TETA). The optimum film fabrication conditions will be determined using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). These techniques will also be used to analyze the kinetics of the cross linking reaction after the addition of TETA. After the team determines the optimum film fabrication processes using the polycondensation route, the NIPU erosion-resistant films will be fully characterized.
The polyaddition synthetic scheme permits in-situ polymerization of the monomers on the substrate, and will produce no byproducts, thus eliminating all VOCs and HAPs from the coating process. Dispersions of the monomers will be prepared in VOC-exempt solvents and applied via HVLP spray. The curing rates, viscoelastic properties, glass transition temperature (Tg), and crystallization temperature will be determined by DSC and DMA. The thermal stability of the polymers will be determined by thermogravimetric analysis (TGA).
After the optimum coating and curing parameters for the NIPUs produced via polycondensation and polyaddition are determined, the team will analyze the coatings per the specifications in SAE AMS-C-83231.
The benefits to the Department of Defense from this work include: (1) the reduction of environmental liabilities associated with isocyanate and VOC/HAP use in current erosion-resistant coatings, (2) a decrease in production time-costs, both during application and curing, (3) the reduction and/or elimination of production/repair backlog, which will reduce equipment downtime, and (4) a significant reduction in the disposal costs associated with the current process. All of this can be achieved while ensuring mission readiness and worker safety. (Anticipated Project Completion - 2018)