Crosslinking Mechanism of Soy Protein-based Adhesives based on Glyoxal and a Compound of Protein Model

Abstract

The crosslinking mechanism of glyoxal and asparagine was analyzed, and the relationship between the mechanism and practical performances of soy protein-based adhesives was also discussed. It is shown that when pH=1 and 3, glyoxal reacted with asparagine in the form of major cyclic ether compounds. When pH =5, glyoxal reacted with asparagine in two structural forms of sodium glycollate and cyclic ether compounds. However, amidogens of asparagine were easy to develop protonation under acid conditions. Supplemented by the instability of cyclic ether compounds, the reaction activity and reaction degree between glyoxal and asparagine were relatively small. Under alkaline conditions, glyoxal mainly reacted with asparagine in the form of sodium glycollate. With the increase of pH, the polycondensation was more sufficient and the produced polycondensation products were more stable. The reaction mechanism between glyoxal and asparagine had strong correspondence to the practical performances of the adhesives. Glyoxal solution could develop crosslinking reactions with soy protein under both acid and alkaline conditions. Bonding strength and water resistance of the prepared soy protein-based adhesives were increased significantly. When pH>7, glyoxal had relatively high reaction activity and reaction intensity with soy protein, and the prepared adhesives had high crosslinking density and cohesion strength, showing relatively high bonding strength, water resistance and thermal stability.

Keywords

soy protein / wood adhesives / glyoxal / model compounds / crosslinking

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