Effective and environmentally safe self-antimildew strategy to simultaneously improve the mildew and water resistances of soybean flour-based adhesives

Wide applications of soybean-based adhesives in the production of wood composites are significantly restricted by mildew attacks. Increasing mildew resistance without increasing costs and causing environmental risks and/or loss of bonding properties remains challenging. This study proposes a self-antimildew strategy that involves the easy conversion of mildewing primary amines of soybean proteins of soybean flour (SF) into mildew-tolerant secondary amine linkages based on the structural design and synthesis of polyamidoamine–epichlorohydrin (PAE) resins. This effectively and environmentally safely improves the mildew resistance of SF-based adhesives without using additional mildew preventives and simultaneously enhances the bonding properties. NMR and FTIR results confirmed that the azetidine content of the PAE resin increased by 123.1% as the epichlorohydrin ratio increased from 0.75 to 1.2. The SF-based adhesive fabricated from the optimal PAE resin improved the mildew resistance by ≥ 367% and water resistance by 61.3% owing to the effective conversion of primary amines into secondary amine linkages via sufficient protein-PAE cross-linking. Furthermore, the addition of 0.45 wt% of environmentally safe and low-cost sodium formate into self-antimildew SF-based adhesives could maintain the desired mildew resistance and reduce the dosage of expensive PAE resin by 33% without compromising the bonding properties. This novel antimildew strategy allows cost-effective and environmentally safe improvements in both antimildew and mechanical performances, thereby allowing cleaner production of protein-based adhesives and composites.