Monocomponent Non-isocyanate Polyurethane Adhesives Based on a Sol-Gel Process

Non-isocyanate polyurethanes (NIPUs) based on five-membered cyclic carbonates have emerged as the most promising alternative to replace more toxic, conventional polyurethanes. However, the low reactivity of five-membered cyclic carbonates has limited the preparation of one pot systems because of long curing times. This work focuses on the improvement and application of these materials as adhesives through the combination of NIPU chemistry and a sol-gel process that allows for curing by atmospheric humidity. The synthesis of NIPU prepolymers functionalized with (3-aminopropyl)triethoxysilane (APTES) is demonstrated, and their curing behavior and adhesion performance are investigated by means of rheological experiments and a lap-shear test, respectively. In spite of the ability of the alkoxysilane to cure under ambient conditions, our results show that the use of catalyst and elevated temperatures speeds up the curing process and leads to improved adhesion properties. Hence, it is demonstrated that the fastest curing and the best performance are achieved at 100 degrees C when acetic acid is employed as the catalyst. Finally, we demonstrate the importance of the soft (poly(propylene glycol)dicarbonate) to hard (resorcinol dicarbonate) ratio to achieve superior cohesion and adhesion properties in NIPU adhesives.