Aldehyde Free Bio-adhesive Prepared from Soy Protein Isolate and Partially Degraded Lignin

Lignin is an abundant natural polymer, existing widely in different kinds of biomass. Lignin could be produced from biomass by two typical pathways: the kraft process and fermentation. The kraft process dissolves lignin from biomass using alkali solution, at an elevated temperature, in a pressurized reactor. The fermentation uses microorganisms to ferment cellulose and hemicellulose to C6 and C5 sugars, leaving lignin as a residue. In both processes, as a by-product, lignin is currently not further refined. Industrially, lignin by-product is combusted to produce energy. Thus, lignin valorization is strongly desired. In this work, commercially available kraft lignin was partially degraded by a base-catalyzed depolymerized process. The partially degraded lignin was further used as a crosslinking blender for the soy protein adhesive. The solvent in the adhesive system was water only. No aldehyde was involved during the preparation of the adhesive. The lignin loading in the soy-lignin system was 50% by the mass ratio of solid content. Investigations on different depolymerization temperatures from 140~200 °C have shown that the depolymerization process at 170 °C has produced the most suitable lignin fragments, resulting in the highest bonding strength when blended with soy protein isolate. Fourier transform infrared spectroscopy has shown that the functional groups of the partially degraded lignin have no visible change when compared to untreated kraft lignin, indicating the fragments were mainly lignin oligomers. The results showed that the soy protein-lignin adhesive had a single lap shear strength of 1.46 MPa, which was higher than pure soy protein adhesive or soy protein-lignin with lignin degraded at 140 and 200 °C. After being soaked for 3 hours at room temperature, the single lap shear strength of the soy protein-lignin adhesive with lignin degraded at 170 °C has reduced to 0.62 MPa. It indicated that the soy protein-lignin had an acceptable water resistance. The base-catalyzed depolymerization broke down the lignin polymer into derivative oligomers and monomers. These lignin fragments could form crosslinking structures with soy protein molecules and improve the bonding strength as well as the water-resistance of the soy-based aldehyde-free bio-adhesive.

Changle Jiang
West Virginia University, Morgantown, WV, United States

Keywords: Lignin; Catalytic Depolymerization; Soy Protein; Adhesive

Changle Jiang - Department of Chemical and Biomedical Engineering, West Virginia University
Jianli Hu -  Department of Chemical and Biomedical Engineering, West Virginia University
Jingxin Wang -  Division of Forestry and Natural Resources, West Virginia University
Corresponding Author: Professor Jianli Hu, john.hu@mail.wvu.edu