Experimental and Statistical Modelling of Gelation in Aspartate-Based Polyurea Networks

This research investigates the complex crosslinking mechanism and kinetics of aspartate-based polyurea networks for high-performance coatings, focusing on a puzzling “gelling paradox”: we observed that the addition of dibutyltin dilaurate (DBTDL) dramatically extends the gel time (by 7×), while counterintuitively decreasing the gel conversion. This behaviour contradicts the Flory–Stockmayer theory, which predicts the gel conversion independent of reaction rate in simple gelling systems.Through a detailed investigation employing FTIR, NMR, and MALDI-TOF techniques, the complex underlying crosslinking chemistry has been elucidated. We found that beyond the main, urea-forming reaction, a consecutive aminolysis between urea amine groups and aspartate esters occurs, leading to the formation of hydantoin rings but also to additional branching and crosslinking.Aminolysis produces ethanol, which consumes the isocyanate groups of the crosslinker, decreasing its functionality. The DBTDL plays a multifaceted role, not only slowing the main NCO–NH reaction, but also enhancing aminolysis and promoting the NCO–OH reaction.A comprehensive theoretical model incorporating these chemical mechanisms and linking their kinetics to a statistical model of network formation, based on the theory of branching processes, has been developed and validated. The model accurately predicts gel critical conversion and time, effectively resolving the gelling paradox. The model thus provides a powerful tool for understanding and controlling PU-ASPE network formation and structure for optimal performance.