Traditional formaldehyde-based adhesives rely excessively on petrochemical resources, release toxic gases, and pollute the environment. Plant-derived soybean protein adhesives are eco-friendly materials that have the potential to replace the formaldehyde-based adhesives used to fabricate wood-based panels. However, the poor water resistance, high brittleness, and poor mildew resistance of soybean protein adhesives limit their industrial applications. This article reviews recent research progress in the modification of soybean protein adhesives for improving the bonding performance of adhesives used for wood-based panel fabrication. Modification methods were summarized in terms of water resistance, solid content, and mildew resistance. The modification mechanisms and remaining problems were also discussed. Finally, the current industrial applications and the future research direction of soybean protein adhesives are discussed.

Soybean meal-based adhesive is a good wood adhesive mainly due to its renewable, degradable, and environmentally friendly features. To improve the enhancement efficiency for adhesives, diglycidyl resorcinol ether (DRE) containing a benzene ring and flexible chain structure was used as an efficient cross-linker to enhance the adhesive in the study. The physicochemical properties of adhesives, the dry shear strength, and wet shear strength of plywood were measured. Results suggested that DRE reacted with the functional groups of soybean meal adhesive and formed a cross-linking network during hot press process in a ring-opening reaction through a covalent bond. As expected, compared to adhesive control, the soybean meal adhesive with 4 wt% DRE incorporation showed a significant increment in wet shear strength by 227.8% and in dry shear strength by 82.7%. In short, soybean meal adhesive enhanced with DRE showed considerable potential as a wood adhesive for industrial applications.

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 core–shell nanohybrid (DP@SiO₂) consisting of the nano silica (SiO₂) core and polyethyleneimine (PEI) grafting 3, 4-dihydroxybenzaldehyde (DBA) shell via electrostatic interaction and Schiff’s 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 DP@SiO₂ and the reaction between protein and HCCP. Compared with that of the SPI adhesive, the prepressing strength of plywood with the SPI/HCCP/DP@SiO₂ 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.