Solvent-free Synthesis of Room-Temperature Curable Nonisocyanate Polyurethane Adhesives and Coatings

Because of the high risks to human health that arise from the use of isocyanates, new regulations concerning the use of these chemicals were implemented in recent years to improve health and safety standards in the workplace. Correspondingly, the attention of polymer chemists is rapidly shifting from conventional polyurethanes (PUs) to more sustainable and safer nonisocyanate polyurethanes (NIPUs). For applications such as adhesives and coatings, solvent-free NIPU formulations are attractive due to their reduced environmental impact and their potential for use in consumer good applications, but curing typically requires an external heat source due to the low reactivity of the typically applied cyclic carbonate precursors. In this work, solvent-free and ambient-temperature-curing NIPU adhesives and coatings are reported. Two-component (2K) NIPU formulations were designed, exploiting the increased reactivity of a liquid CO2-sourced bis(α-alkylidene cyclic carbonate) monomer, which was polymerized with a low-viscosity polyamine cross-linker under solvent-free conditions. The composition was varied, adding coreagents to the mixture to diversify the chemical structure of polymer networks. The properties of the thermosets were evaluated at different curing time, revealing high adhesion on aluminum joints already after 1 day of room-temperature hardening. The bonding performance was also tested on different substrates, further evaluated after the incorporation of catechol groups into the polymer matrix, and benchmarked against commercial solvent-free PU glues. The easy handling, prolonged pot life, negligible VOC, and fast curing at low temperature make the new 2K formulations a competitive alternative to isocyanate-based products, paving the way to new room-temperature NIPU-based adhesives and coatings.