Strong, Tough and Flame-Retardant Bio-Adhesives of Soybean Meal with High Penetrability by Hierarchical Dual Bio-Inspired Strategy

The development of sustainable, high-performance bio-adhesives to replace formaldehyde-based resins remains a critical challenge for the wood industry. However, the low water resistance, poor penetration, and weak adhesion strength severely bottleneck the practical application of bio-adhesives. Here, we present a strong, tough, and flame-retardant soybean meal (SM)-based adhesive with high penetration through a hierarchical dual bio-inspired strategy, combining the hard-soft phase combination of bivalve hinges and the root-like topological entanglement of plants. In this system, calcium sulfoaluminate (CSA) acts as rigid fillers to prompt crack deflection and energy dissipation, while polyacrylamide (PAM) entanglement further enhances the toughness of the SM matrix and mechanical interlocking at the interface. The prepared SM-adhesive achieves significant enhancements in dry shear strengths and wet work of adhesive, which were 6.0 folds and 37.5 folds higher than those of the primary SM. Furthermore, prepared adhesive exhibits excellent flame retardancy with limiting oxygen index of 32.5%, attributing to the gas-phase protective layer and condensed-phase flame retardancy of CSA. This work establishes a sustainable pathway for replacing traditional petroleum-based adhesives in plywood, and it has potential to develop on industrial-scale and achieve eco-friendly alternatives in the future.