The increasing demand for sustainable wood adhesives has motivated researchers to explore alternatives to petroleum-based phenol-formaldehyde (PF) resins, which have long dominated the wood adhesive industry due to their exceptional thermal and chemical stability. However, environmental concerns and the depletion of fossil resources necessitate the development of renewable, bio-based adhesives. This thesis investigates the potential of lignin and glyoxal as sustainable replacements for phenol and formaldehyde, respectively. First, a bio-based phenolic adhesive was developed by entirely replacing phenol with unmodified corn stover lignin and formaldehyde with glyoxal, a bio-based dialdehyde. The resin formulation process was optimized to achieve strong chemical, thermal, and mechanical properties. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) confirmed the successful integration of lignin and glyoxal into the polymer network, resulting in adhesives with high dry adhesion strength suitable for interior-grade wood composites. Further studies were focused on developing lignin-based resins by replacing 80-100% phenol with commercially available unmodified kraft softwood and organosolv wheat straw lignins. After extensive optimization of the resin formulation, the curing time for lignin (80%)-phenol (20%)-formaldehyde adhesives was significantly reduced from the initial 11 minutes to 5 minutes, while maintaining comparable performance to commercial PF adhesives. Additionally, replacing 80% of phenol with lignin reduced formaldehyde consumption by at least 63%, making the adhesive more environmentally friendly and sustainable. Advanced spectroscopic techniques, including FT-IR and NMR, provided valuable molecular insights into the curing mechanisms of lignin-based adhesives. Solid-state NMR confirmed the formation of methylene bridges between lignin chains when formaldehyde was used as a cross-linker. Similarly, when glyoxal was used, the analysis revealed the formation of glyoxylene bridges, highlighting the potential of both lignin and glyoxal as sustainable substitutes for phenol and formaldehyde in adhesive formulations. This work highlights the feasibility of lignin and glyoxal-based adhesives as fully biobased, eco-friendly alternatives for phenol-formaldehyde adhesive used currently in various engineered wood products, such as plywood, oriented strand board (OSB) and laminated veneer limber (LVL), contributing to the broader application of renewable materials in the adhesive industry. The findings emphasize the importance of understanding lignin's properties and its reactions with co-monomers, such as formaldehyde or glyoxal. The successful optimization and detailed characterization contribute to the development of high-performance bio-based adhesives and paves the way for their practical application in the wood adhesive industry.
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