Fully bio-based soybean adhesive in situ cross-linked by interactive network skeleton from plant oil-anchored fiber

The fiber-reinforcement is an attractive strategy to enhance composites under the green development concept. However, the poor dispersion/interactivity of natural fiber and most petroleum-based modification strategies impede development of fiber-reinforced environmental composites. In this study, we employed a soybean oil derivative as a multi-functional cross-linker that reacted with kenaf fiber (KF) to synergistically improve the soybean flour (SF)-based adhesive. The biobased cross-linker, phosphate and epoxy groups-containing soybean oil (PESO), was synthesized via phosphoric acid ring-opening reaction from epoxidized soybean oil in certain extent. To provide reaction sites, the KF was surface-coated by poly (tannic acid) (PTA) to obtain a PTA-coated KF (TKF). The PESO cross-linker, with the role of epoxy groups, constructed an interactive network skeleton with TKF and formed in situ cross-linking reactions with SF matrix. In addition, the incorporated phosphate groups of PESO gave rise to interactions with hydroxyl of TKF and polysaccharide of SF further stabilizing cross-linking network as confirmed by fourier transform infrared (FTIR), nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. As expected, compared to pristine SF adhesive, the modified adhesive showed a significant increment in wet shear strength by 236.4% and also exhibited a favorable thermal stability. We envisage that this design may provide a new avenue for developing high-performance fully sustainable biomass composites.