Effective role of eco-friendly acetyl tributyl citrate in large-scale catalyst-free synthesis of waterborne polyurethanes without volatile organic compounds

Abstract N-methyl pyrrolidone (NMP)-free waterborne polyurethane (WPU) in a two-tons scale was prepared without catalyst in the presence of a non-toxic acetyl tributyl citrate (ATBC) with high boiling point and extremely low vapor pressure. It is found that a small amount of ATBC is key to the obtainment of WPU nano-sized dispersions with proper viscosity, good storage stability, and low volatile organic compounds (VOC). Macromolecular structure, molecular weight and its distribution, mechanical and thermal properties, and water-resistance of the cast films of the ATBC-based WPUs have been systematically evaluated. The ATBC-based WPUs exhibit several advantages including higher molecular weight, much lower VOC, higher thermal stability, and stronger water resistance than the NMP-based ones. ATBC plays six roles as hydrophilic chain extender's medium, emulsifier, and viscosity thinner in the polymerization stage, plasticizer, water- and heat-resisting additives in the WPU films, which is responsible for the performance improvement. The tough problem of -NCO group residue in PUs that result from slightly excessive isocyanates, has been tackled here. This has been confirmed by a simultaneous disappearance of the characteristic carbon peak at 122.7 ppm and IR absorbance at 2252 cm−1 of -NCO group. Usage of ATBC and the phase transition in water tackle the toxic problems from NMP and -NCO residue, opening up a large-scale and eco-friendly method for cost-effectively producing VOCs-free WPU films.