Synergistic reinforcement strategy toward a multifunctional soy protein adhesive with enhanced wet bonding, water resistance, and mold resistance

Soy protein (SP) adhesives are promising formaldehyde-free alternatives for wood bonding, but their practical application remains limited by inadequate water resistance, weak wet bonding strength, and susceptibility to microbial deterioration. In this work, a synergistic reinforcement strategy was developed to construct a multifunctional SP adhesive by incorporating carvacrol-loaded zeolitic imidazolate framework-8 (ZIF-8-CAR). Structural and performance analyses showed that ZIF-8-CAR promoted intermolecular interactions within the adhesive matrix, leading to a denser cured structure, improved thermal stability, and reduced moisture absorption. At an optimal loading of 0.4 wt% (based on the total mass of SPI and water), the modified adhesive achieved dry and wet shear strengths of 3.90 and 3.24 MPa, respectively, which are 2.68 and 3.70 times that of the unmodified adhesive. In addition, the onset of visible mold growth was delayed from 2 to 28 days. These improvements can be attributed to the combined effects of Zn²⁺-mediated coordination interactions and the hydrophobic/antifungal contribution of carvacrol, which together enhanced the cohesion, water resistance, and biological stability of the soy protein adhesive. This work provides a practical route for designing multifunctional bio-based adhesives with improved wet bonding, water resistance, and mold durability.