Tough, waterproof, and mildew-resistant fully biobased soybean protein adhesives enhanced by furfuryl alcohol with dynamic covalent linkages

The adhesives formulated from soy protein (SP) exhibit desirable dry bonding strength needed to eradicate over-reliance on unsustainable petroleum-based formaldehyde adhesives. However, the adhesive full industrial adoption is limited by its inherent poor water resistance, short shelf-life, and proneness to mildew. This study explored a feasible, versatile, green, and sustainable strategy to engineer superior SP-based adhesives using furfuryl alcohol (FA) and 2,5-diamino-1,4-benzenedithiol hydrochloride (DBH). FA/DBH was conjugated via the carbodiimide-mediated addition of DBH under a light-proof condition. The acidity was controlled by NaOH, creating a mildly acidic medium for FA/DBH to develop a strong crosslinked network. The FA and DBH were hybridized by an amide bond, with subsequent SP integration via dynamic covalent linkages and amide bonds. The synergistic interactions between FA/DBH hybrid and the protein chains remarkably enhanced the adhesive properties. Consequently, the added FA/DBH hybrid (6 wt% of SP) achieved high dry and wet shear strengths of 2.38 and 1.43 MPa (plywood), which were 63 % and 200 % higher than the unmodified soy protein adhesive (USPA), respectively. In addition, the constructed organic network improved the adhesive thermal stability, mildew resistance, and shelf-life. Therefore, this sustainable, versatile, and green approach could serve as an alternative crosslinking pathway for developing highly competitive SP-based adhesives.