Moisture-Triggered Biooxazolidine Hydrolysis for the Generation of Mechanically Robust and Self-Healing Polyurethane Coatings Without CO2 Emissions

Single-component wet-curing polyurethane (SPU) coatings offer high curing efficiency; however, their practical application is hampered by suboptimal appearance, mechanical shortcomings, and limited thermal stability. To overcome these limitations, we synthesized novel biooxazolidines—HDI-IHO and MDI-IHO—via the reaction of 2-isopropyl-3-hydroxyethyl-1,3-oxazolidine (IHO) with hexamethylene diisocyanate (HDI) and 4,4′-diphenylmethane diisocyanate (MDI), respectively. When incorporated into the SPU matrix, these biooxazolidines effectively mitigate surface defects, such as bubbles and pinholes, thereby significantly enhancing coating smoothness and overall appearance. Moreover, the introduction of these latent curing agents not only improves the thermal stability of the cured system but also substantially boosts its mechanical properties, as evidenced by increased tensile strength and elongation at break. An optimal biooxazolidine mass fraction of 8% was determined, yielding the best mechanical performance. In addition, the modified SPU exhibits remarkable self-healing capabilities, with visible repair of coating cracks achieved at 60°C.