Oyster-inspired organic-inorganic hybrid system to improve cold-pressing adhesion, flame retardancy, and mildew resistance of soybean meal adhesive

Soy protein adhesives have aroused widespread interest as a replacement for traditional formaldehyde synthetic resins in the wood industry. However, how to improve the poor cold-pressing adhesion to the industrial level while retaining decent integrated performances is a bottleneck of soy protein adhesives for practical applications. Inspired by the strong wet adhesion of marine oysters, we propose a novel strategy for developing excellent adhesion performance soybean meal (SM) adhesive based on an organic-inorganic hybrid system. The introduction of poly (vinyl alcohol) (PVA) and hydroxyapatite-tannin acid (HT) nanohybrids construct a densely-crosslinked coacervate network through non-covalent interactions to enhance the adhesion and cohesion of the adhesives, which penetrated the wood interfacial water layer. Compared with the pristine SM adhesive, the cold-pressing bonding strength of the novel adhesive increased from 167.3 kPa to 557.0 kPa. The adhesive possessed excellent toughness of 4.99 MJ/m3 because the multiple non-covalent bonds provided sacrificial interactions that dissipated the fracture energy. Moreover, due to the synergistic effect of the inorganic components and phenolic components, the adhesive also displayed good flame resistance and mildew resistance, which stored for 15 days without mildew formation and showed self-extinguishing behavior during combustion tests. This work demonstrates a novel and effective method for developing other wood adhesives with high performance.