Epoxy oxalic acid antifungal agent reinforced soy protein adhesive with long mold resistance and high bonding strength

Applications of eco-friendly soy protein (SP) adhesives in the wood industry are restricted by their poor wet bonding strength and mold resistance. In this study, we noted that epoxy crosslinking agents not only have crosslinking properties, but also antifungal properties and can be used as cross-linkable antifungal agents to simultaneously improve the mold resistance and strength of SP adhesives. In order to develop sustainable and highly active epoxy cross-linkable antifungal agents, biomass-derived oxalic acid (OA) was converted into epoxy oxalic acid (EOA) with two electron-withdrawing ester groups, with 99.93% antifungal activity. The 1% ultra-low amount of EOA (when used as crosslinking agent) was uniformly dispersed in the uncured adhesive, prompting the anti-mold life of the adhesive to increase from 2 to 26 days. After curing, the tighter and more stable cross-linked network structure of the adhesive was constructed by the reaction of low reactive groups such as amide II bonds with ultra-low amounts of EOA, and the dry and wet shear strengths were increased by 49.0% and 139.7%, respectively, compared with pure adhesives. In addition, the anti-mold life of the cured adhesive increased from 4 to more than 30 days under the action of residual EOA. This work demonstrates the novel antifungal effect of epoxy crosslinkers, providing a controllable, green, and sustainable idea for the high-efficient improvement of mold resistance and strength of bio-based adhesives.