Synergistic Co-Cross-Linking and Organic–Inorganic Hybridization Strategy for Formulating Aqueous Polymer-Isocyanate Adhesives with Strong Bond Durability and Improved Flame Retardancy

The applications of traditional aqueous polymer-isocyanate (API) adhesives in engineered wood composites are limited due to their poor bond durability and flame retardancy. To overcome these limitations, an environmentally friendly API adhesive with excellent bond durability was formulated by using a polyamidoamine-epichlorohydrin (PAE) resin to replace a traditional polyalcohol to ensure sufficient co-crosslinking with a polyisocyanate (PAPI). Layered bentonite (BT) was also included to develop an organic–inorganic hybrid. The optimal PAE concentration (6%) and the designed molecular structure of the PAE-0.85-40% resin (synthesized with diethylenetriamine/adipic acid ratio of 0.85 and epichlorohydrin content of 50%) ensured sufficient co-crosslinking between PAE and PAPI resins to form a densely cross-linked network. Because the layered silicate was intercalated by the cationic PAE resin, adding 8 wt % of BT into the PAE/PAPI resins provided effective organic–inorganic hybridization and a cross-linking density of 96.4% for the cured 8BT@PAE-0.85-40%/PAPI adhesive. This optimized API adhesive, formulated by co-crosslinking PAE-0.85-40%/PAPI into a BT hybrid, showed a maximum aged wet strength of 1.46 MPa and a strength loss of only 19.3% after 7 hygrothermal-aging cycles, exhibiting strong bond durability comparable to that of a commercial PF resin. This study provides a strategy for improving the bond durability of API adhesives to enable their applications in engineered wood composites.