Synthesis, characterisation, and thermal degradation kinetics of lignin-based polyurethane wood adhesives

Polyurethane adhesives are widely employed in a range of industrial applications due to their exceptional bonding strength , flexibility, and chemical resistance. These materials play a crucial role in wood bonding technologies, where their versatility and durability make them ideal for creating strong, long-lasting joints. In this work, Four different polyurethane wood adhesives were synthesised using ligno-based bio-polyols obtained through microwave assisted liquefaction reaction of two wood species (hardwood and softwood) using polyethylene glycol and glycerol as solvents. The reaction conditions used for the synthesis of bio-polyols were optimised in a previous work. The synthesis of polyurethanes was carried out by one-shot method using Tetrahydrofuran (THF) as solvent and MDI as diisocyanate employing different NCO:OH ratios (2.0:1, 2.5:1, and 3.0:1). The chemical structure of polyurethanes was determined through ATR-FTIR and the shear strength was analysed using Automated Bonding Evaluation System (ABES) employing beech veneer strips. Through ABES it was concluded that an NCO:OH ratio of 2.5:1 was the formulation that showed the best shear strength for a pressing time of 120 s. Employing this ratio and the same synthesis procedure, two new polyurethanes were synthesised with the bio-polyols obtained using crude glycerol instead commercial glycerol. Finally, a study of thermal degradation kinetics employing the Ozawa–Flynn–Wall (OFW) and Kissinger–Akahira–Sunose (KAS) isoconversional methods of the polyurethanes synthesised with an NCO:OH ratio of 2.5:1 was carried out. On the one hand, the E a of each system were estimated for the different α ratios, obtaining slightly higher values for the adhesives produced using commercial glycerol than crude glycerol. In addition, the pre-exponential factor was determined, enabling an estimation of the lifetime of the polymers. This study highlights demonstrated that crude glycerol could replace commercial glycerol without compromising adhesive properties . The findings revealed that the lignin source significantly influences the adhesive's characteristics and stability, while addressing challenges in achieving industrial viability remains essential for broader application.