Green Synthesis of Biobased NIPUrea–Acrylate Hybrids for Versatile, Fast-Curing Hot-Melt Coating Development

Polyureas are a category of polymers widely used in the coating industry for their high performance and the variety of substrates they can efficiently protect. However, for decades, polyureas have been synthesized using toxic diisocyanates and petro-based building blocks, often formulated into solvent-based coatings that release volatile organic compounds (VOCs) upon drying. To address these issues, partially biobased nonisocyanate polyurea (NIPUrea) resins were developed for use in VOC-free hot-melt coating formulations. Since NIPUreas react slower with nucleophilic cross-linkers than isocyanates, NIPUrea-acrylate hybrids were introduced, leveraging rapid photopolymerization to achieve fast curing (30 s). However, the functionalization of polyureas with acrylate groups often requires hazardous reagents (e.g., acrylamide, 2-hydroxyethyl acrylate), emphasizing the need for safer and more sustainable methods. In this study, we developed partially biobased NIPUrea bearing controlled amounts of reactive primary amine end-groups via melt polycondensation. These end groups were functionalized via the aza-Michael reaction, at room temperature and using a nontoxic diacrylate compound. The developed NIPUrea-acrylate hybrids are semicrystalline and are mixed with potentially biobased isobornyl acrylate as a reactive diluent to obtain VOC-free hot-melt coating formulations. The formulations were cured for a maximum of 30 s via radiation curing, obtaining gel contents >96%, and yielded coatings with exceptional performance (>200 water and MEK double-rub resistance, 6H+ scratch hardness, 5B grade adhesion) on both glass and wooden substrates. This work offers a greener approach to the synthesis of fast-curing, high-throughput NIPUrea coating formulations by considerably reducing the toxicity (no acid chlorides, triethylamine, or dichloromethane for acrylate modification; no isocyanates; VOC-free coating application) of the products while maintaining interesting coating performance.