A synergistic strategy for formulating a facile and cost-effective soybean protein-based adhesive via co-crosslinking and inorganic hybridization

Soybean protein-based adhesives (SPAs) are environmentally friendly, sustainable, and renewable, but their wide applications are limited by moderate bonding strength, poor water resistance, undesirable flame retardancy, and high costs. Inspired by the organic-inorganic hybridization of oysters, a facile and cost-effective SPA was developed in this study via co-crosslinking branched melamine-urea-glyoxal (MUG) and polyamidoamine-epichlorohydrin (PAE) resins, as well as inorganic hybridization by using the mineral bentonite (BT). Low-cost branched MUG resins that could efficiently co-crosslink with PAE resin and substitute for 40 % expensive PAE resin were optimally copolymerized from melamine, urea, and glyoxal. This approach provided more co-crosslinkable sites for constructing denser multiple covalently-crosslinked SP-PAE-MUG networks via MUG-protein crosslinking and MUG-PAE co-crosslinking. 30 wt% of low-cost layered mineral BT was intercalated to further reinforce the mechanical properties and flame retardancy of SPA by forming a compact organic-inorganic hybridized structure. The coordination of MUG-PAE co-crosslinking and BT inorganic hybridization resulted in a cost-effective and robust SPA with a 61.2 % higher dry strength, a soaked wet strength of 126.0 %, an aged wet strength of 86.0 %, a residual carbon content after combustion of 94.3 %, and reduced the raw material cost for formulating adhesive by 19.5 % compared with controlled commercial SPA. This novel strategy highlights the potential applications of the SP in bio-adhesives and bio-composites with excellent bonding strength, water resistance, and flame retardancy.