The multifaceted role of water as an accelerator of the crosslinking reaction of isocyanate-free polyurethane networks

Polyhydroxyurethane (PHU) thermosets are the most promising isocyanate-free substitutes to polyurethane ones when targeting the main applications of the PU business (coatings, adhesives, foams). However, the curing of their solvent-free formulations at near room temperatures is often very slow and requires the utilization of organocatalysts, limiting the large-scale deployment of the technology. Herein, we study the impact of water, introduced as an additive (2 to 10 wt.%), on the crosslinking rate of common solvent-free thermosetting PHU formulations composed of a polycyclic carbonate and a diamine. Rheology measurements, using a multi-frequency approach, indicate that even very small amounts of water (< 5 wt.%) impressively shorten gel times, by up to 5 folds at 40 °C. The multi-frequency methodology highlights for the first time the strong interactions of water with the growing PHU network during the crosslinking reaction. It suggests that water breaks down the multiple intra-/inter-molecular H-bond interactions within the PHU clusters, consequently enhancing the molecular mobility and delaying the vitrification (hydroplasticization), both phenomena contributing to accelerate the crosslinking rate. On top of that, through a combination of model reactions and computational calculations, we demonstrate that water is an actual catalyst of the cyclic carbonate aminolysis. Eventually, thanks to its multifaceted role, water can efficiently substitute the organocatalysts (e.g. TBD, DBU) usually required to cure PHU formulations at near ambient temperature. This work demonstrates that the use of additive amounts of water in solvent-free formulations is a cost-effective, non-toxic, and robust solution to accelerate the production of PHU materials, free of any organocatalyst. It offers promising prospects for the deployment of PHU-based materials requiring mild processing conditions.