A sip of water in your thermoset: the role of H2O as an accelerator of the crosslinking reaction of polyhydroxyurethane networks

The impact of water, added as a diluent, on the sol-gel transition of a model polyhydroxyurethane (PHU) thermoset was investigated using multiwave rheology. An impressive 2 to 5 fold decrease in gel-time, tgel, was observed for water content ranging from 2 wt% to 10 wt% at temperatures close to r.t. (30°C), indicating that water strongly accelerates the crosslinking process. Interestingly, multiwave rheology gives access to the power law exponent of the transition, n, a characteristic that has never been considered for PHU thermoset so far. In the frame of the percolation theory, the gradual increase of n with water content suggests an increase of the dimension of the PHU clusters at the sol-gel transition, likely due to the strong hydrogen bonding of water with the hydroxyl groups of the growing PHU. This is an unprecedented illustration of the specific interaction of water with the PHU during the crosslinking reaction, that is expected to contribute to an increase of the molecular mobility, and thus of the crosslinking rate. This is well supported by comparing water to other solvents with different hydrogen-bonding ability and polarity. On top of that, model reactions and DFT calculations were conducted to investigate the catalytic role of water on the aminolysis. For the first time, they indicate that water is an effective catalyst, acting as a proton shuttle during the ring opening of the 5cc, and provide an additional explanation for the tremendous impact of water on tgel. In the end, this work introduces water as an accelerating agent for the curing of PHU thermosets at (near)-ambient temperatures, and suggests that multiple mechanisms – both physical and chemical – are at play during the crosslinking reaction.