Constructing SiO2 nanohybrid to develop a strong soy protein adhesive with excellent flame-retardant and coating ability

Developing an environment-friendly soy protein adhesive with high bonding strength and flame retardancy has drawn interest and simultaneously presented a challenge in the wood panel industry. Prompted by the organic–inorganic hybrid of oysters, this study proposes a nanohybrid ([email protected]2) consisting of the nano silica (SiO2) and polyethyleneimine (PEI) via electrostatic interaction and grafting 3, 4-dihydroxybenzaldehyde (DBA) by Schiff base reaction. The obtained structure was then combined with a multifunctional cross-linker hexachlorocyclotriphosphazene (HCCP) and soy protein isolate (SPI) to develop a plywood adhesive. The resultant adhesive exhibited good coating ability and prepressing properties, which were attributable to the catechol groups of [email protected]2 and the reaction between protein and HCCP. Compared with that of the SPI adhesive, the prepressing strength of plywood with the SPI/HCCP/[email protected]2 adhesive was increased by 304%. The dry and wet shear strengths of the plywood adhesive were 110% and 143% higher than those of the SPI adhesive, respectively, which were attributable to the formation of the cross-linking and organic–inorganic hybrid structure, and improvement in adhesive toughness caused by the combination of covalent and multiple hydrogen bonds. The limiting oxygen index (LOI) of the resultant adhesive was 39.4%, reflecting an increase of 71% relative to that of the SPI adhesive, indicating that the adhesive has excellent flame retardancy. This efficient organic–inorganic hybrid strategy can promote the enhancement and functional modification of underwater adhesives, hydrogel, and composite materials.