Double-Network Soy Protein Adhesives with Enhanced Water Retention and Bonding Strength.

Developing soy protein adhesives with high water retention and strong bonding strength is essential. Such materials can serve as a sustainable alternative to petroleum-based formaldehyde adhesives in wood panel manufacturing. However, a trade-off between precure water retention and postcure bonding strength remains a major challenge, and current strategies rarely achieve simultaneous enhancement of both properties. Inspired by the multi-enhancement mechanisms of double‑network systems, we established a hierarchical dual‑network adhesive system. It involves the self-assembly of chitosan into a compliant supramolecular framework via dynamic hydrogen bonding and electrostatic interactions, along with the additional formation of a stiff covalent crosslinked network during curing. This design achieves synergistic optimization of water retention and bonding strength. The "skeleton-tentacle" structure formed by functionalized chitosan significantly enhanced water retention by 430% and was crucial to forming the dual-network structure, improving wet shear strength by 445% and work of adhesion by 2133%. Additionally, the adhesive exhibited excellent mildew resistance and flame retardancy. The global warming potential was only 1.4 kg CO2 eq, significantly lower than urea-formaldehyde and phenol-formaldehyde resins. This design provides a novel approach for developing high-performance bio-based adhesives and is expected to advance its practical application.