Study on the Soy Protein-Based Adhesive Cross-Linked by Glyoxal

Based on the ESI-MS and C-13-NMR analysis of the forms of glyoxal in acidic and alkaline solutions, the soy-based adhesive cross-linked by glyoxal was prepared in this work. The results showed that glyoxal existed in water in different forms at different pH levels. Under alkaline conditions, glyoxal transformed to glycolate through the intramolecular disproportionation reaction. Under acidic conditions, although some of glyoxal transformed to glycolate as what happened under alkaline conditions, most of glyoxal molecules existed in the form of five-or six-membered cyclic ether structure. No ethylene tetraol or free aldehyde group was actually detected under these conditions. Although glyoxal reacted with soy protein under both acidic and alkaline conditions, alkaline conditions were more favorable for the improvement of mechanical performance and water resistance of soy-based adhesives than acid conditions.