Fabrication of an Ultrastrong Formaldehyde-Free Wood Adhesive with an Organic–Inorganic Cross-Linking Network

The wood industry faces challenges in producing eco-friendly, high-performance, and formaldehyde-free adhesives. In this study, carboxylated styrene–butadiene rubber (XSBR) was blended with polyamidoamine–epichlorohydrin (PAE) resin, and a controlled amount of CaCO3 powder was incorporated to create an adhesive with exceptional strength. The resulting three-layer plywood demonstrated remarkable dry and wet shear strengths of 3.09 and 2.36 MPa, respectively, and of 2.27 MPa after boiling water tests, comparable to that of phenolic resins. Additionally, the adhesive exhibited strong adhesion across various materials including glass, metal, etc. This exceptional performance was due to two primary factors: (1) the high-density chemical cross-linking reaction and the physical entanglement between XSBR and PAE; (2) the organic–inorganic hybrid involving metal ion complexation developed by CaCO3, which fostered molecular chain connections and enhanced the adhesive–material interface. These findings offer valuable references for further research in the field of wood adhesives.