(66e) Water-Based Wood Bio-Adhesive Made from Soy Protein Isolate and Depolymerized Lignin

Soy protein was once a dominant industrial adhesive for bonding wood products. However, it has suffered low bonding strength and poor water resistance. The soy protein adhesive was gradually phased out by the introduction of phenol-formaldehyde and urea-formaldehyde adhesives. Biobased phenolics could be derived from lignin by base-catalyzed depolymerization process under 200 °C. These phenolic compounds, including monomers and oligomers, could be used for crosslinking with soy protein isolate (SPI) molecule, resulting in a protein-lignin adhesive with higher bonding strength as well as better water resistance. After forming crosslinked network with depolymerized lignin, the soy protein-based bio-adhesive has a less water-soluble structure, which helps improve the water resistance. This could be a highly promising way to valorize lignin efficiently and economically. Our results have showed that while adding the untreated lignin decreased the single lap shear strength of the soy protein adhesive, incorporating the depolymerized lignin could enhance the bonding strength. Our bio-adhesive had a single lap shear strength of 912.5 N (equals 1.46 MPa over 625 mm² adhesion area), which was at a relatively high strength level when compared with similar soy protein adhesives from the literatures. It is worth noting that 50% of the bio-adhesive solid was from depolymerized lignin, implying the economic benefit due to the use of low-cost biomass derived lignin. More importantly, the solvent was water only. Most of the references had lignin loadings less than 50% in solid content and environment-unfriendly solvents. Additionally, urea was introduced in the adhesive system to unfold the natural tertiary structure of soy protein, which was based on the idea that unfolded protein molecule structure could reduce the hindering effect due to the tertiary structure. The results showed that the urea did not improve the bonding strength.