The Multifaceted\nRole of Water as an Accelerator for\nthe Preparation of Isocyanate-Free Polyurethane Thermosets

Polyhydroxyurethane (PHU) thermosets are the most promising\nisocyanate-free\nsubstitutes to polyurethane ones when targeting the main applications\nof the PU business (coatings, adhesives, and foams). However, the\ncuring of their solvent-free formulations at near-room temperature\nis slow and requires the utilization of organocatalysts, limiting\nthe large-scale deployment of the technology. Herein, we study the\nimpact of water, introduced as an additive (2 to 10 wt %), on the\ncross-linking rate of common solvent-free thermosetting PHU formulations\ncomposed of a polycyclic carbonate and a diamine. Rheology measurements,\nusing a multifrequency approach, indicate that even small amounts\nof water (<5 wt %) impressively shorten gel times by up to 5-fold\nat 40 °C. The multifrequency methodology highlights for the first\ntime the strong interactions of water with the growing PHU network\nduring the cross-linking reaction. It suggests that water breaks down\nthe multiple intra/intermolecular H-bond interactions within the PHU\nclusters, consequently enhancing the molecular mobility and delaying\nthe vitrification (hydroplasticization), both phenomena that contribute\nto accelerating the cross-linking rate. On top of that, through a\ncombination of model reactions and computational calculations, we\ndemonstrate that water is an actual catalyst of cyclic carbonate aminolysis.\nEventually, thanks to its multifaceted role, water can efficiently\nsubstitute the organocatalysts (e.g., TBD, DBU) usually required to\ncure PHU formulations at near-ambient temperature. This work demonstrates\nthat the use of additive amounts of water in solvent-free formulations\nis a cost-effective, nontoxic, and robust solution to accelerate the\nproduction of PHU materials, free of any organocatalyst. It offers\npromising prospects for the deployment of PHU-based materials requiring\nmild processing conditions.