| Abstract:Developing eco-friendly lignin-based phenolic resin adhesives is crucial for reducing the consumption of fossil resources. In this work, two novel lignin-based adhesives, namely demethylated lignin-glyoxal resin (DLPG) and demethylated lignin-glutaraldehyde resin (DLPGe), were synthesized by replacing phenol with demethylated lignin and replacing formaldehyde with low-toxicity aldehydes (glyoxal and glutaraldehyde). For comparison, adhesives using unmodified lignin as a phenol substitute were also prepared under the same conditions. The experimental results demonstrated that the lap shear strength of DLPG and DLPGe was 166% and 100% higher than that of the unmodified lignin-based counterparts, respectively. Notably, both adhesives exhibited excellent mechanical strength under various environmental conditions, including 100% relative humidity, low temperature (-55°C), and high temperature (60°C). Further analysis revealed that DLPGe showed superior shear strength compared to DLPG. The underlying mechanism was proposed as follows: DLPG possessed a loose and porous cross-linked structure, whereas DLPGe exhibited sufficient particle agglomeration with a more compact and flexible cross-linked network. The long carbon chain of glutaraldehyde endowed DLPGe with enhanced toughness, effectively resisting water penetration and buffering the stress caused by temperature changes. This study provides a theoretical foundation for the high-value utilization of lignin and the development of formaldehyde-free, environmentally friendly phenolic resin adhesives. |