Soybean Meal–Oxidized Lignin as Bio-Hybridized Wood Panel Adhesives with Increased Water Resistance

Soybean meal (SM) adhesive is widely acknowledged as a viable substitute for traditional formaldehyde-based adhesives, given its ability to be easily modified, the utilization of renewable sources, and its eco-friendly characteristics. However, the application of SM adhesive in manufacturing has been impeded due to its restricted bonding capacity and inadequate water resistance. Researchers in the wood industry have recognized the significance of creating an SM-based adhesive, which possesses remarkable adhesive strength and resistance to water. This study endeavors to tackle the issue of inadequate water resistance in SM adhesives. Sodium lignosulfonate (L) was oxidized using hydrogen peroxide (HP) to oxidized lignin (OL) with a quinone structure. OL was then used as a modifier, being blended with SM to prepare SM-based biomass (OLS) adhesives with good water resistance, which was found practically through its utilization in the production of plywood. The influence of the HP dosage and OL addition on plywood properties was examined. The changes in the lignin structure before and after oxidation were confirmed using gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The curing behavior and thermal stability of OLS adhesives were analyzed using dynamic mechanical analysis (DMA) and thermogravimetric (TG) analysis. The reaction mechanism was also investigated using FT-IR and XPS. The outcomes indicated a decrease in the molecular weight of L after oxidation using HP, and, at the same time, quinone and aldehyde functionalized structures were produced. As a result of the reaction between the quinone and aldehyde groups in OL with the amino groups in SM, a dense network structure formed, enhancing the water resistance of the adhesive significantly. The adhesive displayed exceptional resistance to water when the HP dosage was set at 10% of L and the OL addition was 10% based on the mass of SM. These specific conditions led to a notable enhancement in the wet bonding strength (63 °C, 3 h) of the plywood prepared using the adhesive, reaching 0.88 ± 0.14 MPa. This value represents a remarkable 125.6% increase when compared to the pure SM adhesive (0.39 ± 0.02 MPa). The findings from this study introduce a novel approach for developing adhesives that exhibit exceptional water resistance.