Modification mechanism of soy adhesives based on furfuryl alcohol and protein modelcompound

Abstract: Nowadays, formaldehyde-based adhesives are still widely used in wood industry, such as urea formaldehyde resin, phenol formaldehyde resin, melamine formaldehyde resin. Emission of toxic formaldehyde is still one of the biggest problems for these formaldehyde-based adhesives in recent years for the development of wood adhesives. Lately, there has been a considerable industrial interest in the development of natural or green wood adhesives, such as lignin, tannin, protein, starch, and polysaccharides, to substitute synthetic thermosetting resins as adhesives for wood panels because of the mounting environmental pressure worldwide. As one kind of bio-based adhesives, soy protein-based adhesive has been always a hot topic since 1990s. As it is well-known that the soy-based adhesive with no modification cannot be used directly for wood panels for its poor bonding strength and water resistance especially, and the latter one is so bad. Cross-linking modification has been proved to be an effective modification method to improve the water resistance of soy-based adhesive. Some cross-linkers have to be used for this modification, such as formaldehyde-based resins, polyacrylic ester, epoxy. As a biomass material, furfuryl alcohol is more friendly than formaldehyde-based crosslinkers for protein-based adhesive. To guide the preparation of protein-based adhesive, especially the soy-based adhesive, the reaction between a simple dipeptide N-(2)-L-alanyl-L-glutamine (AG), used as a model compound of protein, and its cross-linker furfuryl alcohol was studied in this paper. The products prepared with furfuryl alcohol and AG under different pHs were analyzed by ESI-MS (electrospray ionization mass spectrometer), 13C-NMR (carbon-13 nuclear magnetic resonance) and FT-IR. It was found that the medium environment had great effects on the competition of the co-condensation reaction between furfuryl alcohol and AG and self-condensation reaction of furfuryl alcohol molecules in the mixing system with furfuryl alcohol and AG. Under alkaline conditions, both co-condensation and self-condensation were not obviously detected. Only when the value of pH was higher than 11, a few co-condensation reaction products were gotten. The reaction occurred mainly between furfuryl alcohol and the primary amido groups of AG. Under acid conditions, both co-condensation and self-condensation were observed. The more acid the preparation conditions, the easier to be observed the self-condensation of furfuryl alcohol molecules than the co-condensation between furfuryl alcohol and AG. And when the value of pH was higher than 5, both co-condensation and self-condensation were not outstanding. In this study, under pH value 3, the co-condensation and self-condensation found equilibrium. There was a great possibility for the primary amido and aliphatic amino groups of AG molecules to react with furfuryl alcohol molecules. No reaction was detected between the secondary amido groups of AG and furfuryl alcohol. Besides, the reaction mechanism between furfuryl alcohol and model compounds of soy protein had correspondence with the performance of plywood with soy protein-based adhesive, namely, soy protein-based adhesive crosslinked by furfuryl alcohol showed better bonding performance and water resistance under the pH value of 3 than that of others. In conclusion, the research has great significance to realize the scientific use and improve the market competitiveness of soy protein-based adhesives.