Lignin oligomers from mild base-catalyzed depolymerization for potential application in aqueous soy adhesive as phenolic blends

Abstract

We report, for the first time, that lignin oligomers prepared with base-catalyzed depolymerization have great potential to be used as phenolic blends for enhancing the tensile shear strength of soy protein adhesive. In this work, kraft lignin was used as a model for lignin waste feedstock for the preparation of lignin oligomers. The resulting lignin oligomers were directly used as the phenolic blend to soy protein adhesive. The tensile shear strength of the resulting soy protein–lignin oligomer adhesive was tested on plywood samples. The results showed that 50 wt% loading (by solid content) of lignin oligomers prepared with the base catalyzed depolymerization process at 170 °C has enhanced the bonding strength by 13.2%. It indicated that those oligomers were highly promising to be used as phenolic blends for the soy protein adhesive system. The lignin oligomers obtained by the same process at different temperatures: 140 and 200 °C, were also tested for shear strength but showed lower bonding strength. In addition, the lignin oligomer blended adhesive was characterized for single lap shear strength after the water-soaking process, Fourier transform infrared spectroscopy, a simulated curing process in a thermogravimetric analyzer, and thermal stability. The work showed that the base-catalyzed depolymerization with a mild reaction temperature of 170 °C could produce lignin oligomers with the best suitable molecule size, for crosslinking with the soy protein molecules, and result in improved shear strength of soy protein adhesive. The resultant soy protein–lignin adhesive had a decent dry shear strength of 1.46 MPa and wet shear strength of 0.62 MPa. The FTIR characterization of lignin before and after treatment showed that the C–O intensity has changed significantly during the depolymerization process, indicating the cleavage of the β-O-4 bond in the raw lignin structure. Most importantly, the bio-adhesive is aldehyde-free and water-based, which makes this adhesive highly promising in interior applications and the wood products industry.