Gln-Lys isopeptide bond and boroxine synergy to develop strong, anti-mildew and low-cost soy protein adhesives

The development of renewable soy protein (SP) adhesives has aroused growing interest in the wood industry. This research aims to develop a low-cost, sustainable, and high-performance SP adhesive without a petrochemical crosslinker. The hard skin barrier benefits from the dense cross-linked layer of cuticle cells constructed by the peptide bonds between membrane proteins and the dehydration condensation reaction between adjacent hydroxyl groups of polysaccharides. Inspired by the above skin mechanism, a biomimetic strategy of commercially available transglutaminase (TGase) was applied, which constructed Gln-Lys isopeptide bonds within the SP chains. The covalent boroxine structure formed by dehydration of the hydroxyl groups of boronic acid engineered a dense and robust network like skin barrier, converting the SP into a strong, water-resistant, anti-mildew, and flame-retardant adhesive. The resultant adhesive showed outstanding wet shear strength of 1.55 MPa, about 138.5% greater than the pure SP adhesive, and the residual rate increased from 86.19% to 95.99%. The small content of TGase achieved satisfactory performances and low-cost SP adhesive, establishing its usefulness as a crosslinker to replace the petroleum-based modifiers. Notably, the boronic acid mounted on the SP chains by TGase protected proteins from microbial consumption, thus extending the anti-mildew shelf-life of the liquid adhesives from 2 to 15 days. Moreover, the dense and rigid structure containing Gln-Lys isopeptide bonds and boroxine network effectively improved the flame resistance of the cured adhesives, which reduced the peak heat release rate (PHRR) from 95.1 to 84.2 W/g. Therefore, this simple bionic strategy achieved the goal of formulating green, low-cost, high-performance, and multifunctional SP-based adhesives.