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

Abstract

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.