Preparation of Soy Protein–Chitosan Adhesive Inspired by Marine Arthropod Shells with Superior Bonding Strength and Water Resistance

Soy protein-based adhesives have garnered significant attention in the wood adhesive industry as eco-friendly alternatives to the traditional aldehyde-based adhesives for producing formaldehyde-free, green-engineered wood products. However, such adhesives, derived from natural soybean protein flour, commonly suffer from poor water and mold resistance, necessitating an appropriate modification. Inspired by the composition of marine arthropod shells (e.g., shrimp and crab)─chitin, protein, and calcium carbonate, this study developed a defatted soy flour (DSF)–chitosan adhesive via oxidative treatment at ambient temperature. Calcium carbonate was further incorporated to form an organic–inorganic hybrid system, resulting in a high-performance biomimetic wood adhesive. The preparation process and molecular structural characteristics of the soy protein-based adhesive were thoroughly investigated. The results indicate that the addition of NaIO4 effectively oxidizes the carbohydrates in DSF and the chitosan chains themselves to aldehyde groups, which subsequently react with the amino groups present in both soy protein and chitosan. Under hot-pressing conditions (160 °C, 5 min), the biomimetic adhesive exhibited optimal performance. The resulting plywood achieved dry strength, 24 h cold water (20 °C) strength, and 3 h hot water (63 °C) strength of 2.81, 1.63, and 1.54 MPa, respectively. These values represent improvements of 33%, 219%, and 258% compared to the unmodified soy protein adhesive.