Soy protein adhesive with bio-based epoxidized daidzein for high strength and mildew resistance

Soy protein based adhesives showed great potential for replacing petroleum derived formaldehyde adhesives in the commercial wood industry. However, soy protein adhesives have several problematic limitations, particularly with respect to low water and mildew resistence. As such, in order for soy protein-based adhesives to be practically applicable on an industrial level, a multi-function bio-based crosslinker that resolves both low water and mildew resistance must be designed. In this study, we synthesized a multifunctional crosslink agent (DDE) by reacting soybean-derived daidzein with epichlorohydrin (ECH) and incorporating it into soy protein to parpare a 100% bio wood adhesive with significantly improved water and mildew resistances. The results showed that by using 6 wt% DDE in the adhesive formulation, the dry and wet shear strength of plywood bonded with the SPI/DDE adhesive increased by 52.3% and 164.4%, respectively, compared with that of SPI adhesive; and was shown to be 22.3% and 69.6% higher than that of the industrially used SPI/PAE adhesive. These improvements were attributed to formation of double cross-linked network and elevated adhesive toughness. Furthermore, the resultant adhesive's mildew resistance property were also improved with the addition of DDE, which extended the shelf life of adhesive and durability of the resultant plywood. Thus, this resultant DDE can be used to improve the mechanical properties and mildew resistance for bio-hydrogels or bio-based composites and polymers.