Synthesis, Characterization, and Performance Evaluation of Highly Reactive and Sustainable Hot-Melt Polyurethane Adhesives Derived from Lignin-Based Polyols

The current research was conducted to study the use of waste materials to isolate lignin, which was further used to prepare polyurethane hot-melt adhesives. To enhance the reactivity of lignin, its hydroxyl content was increased by hydroxymethylation, and then it was polymerized with 4,4′-methylenediphenyl diisocyanate (MDI), polyethylene glycol (PEG), and 1,4-butanediol (BDO) to prepare polyurethane hot-melt adhesives. SiO₂ nanoparticles were added to provide mechanical strength to the final polymer. The structure of the final polymer was confirmed by FT-IR spectroscopy. Morphological behaviors were analyzed by using SEM and XRD. Thermomechanical characteristics were studied by using thermogravimetric analysis (TGA\DSC). Finally, the adhesion properties were analyzed by using melting viscosity, softening temperature, and a T-peel strength test. The absence of the -NCO peak of monomers in the FT-IR spectra confirmed the completion of the reaction. The gradual decrease in thermal transition and increasing crystallinity in the DSC\TGA thermograms represent its stable thermal behavior. SEM and XRD show that the prepared hot-melt polyurethane adhesives have distinct crystallinity and peaks. Melting viscosity, softening temperature, and T-peel strength test exhibit the strong adhesive behaviors of the prepared biobased polymers in different time intervals under stress and heating conditions. HMPUA-4 with optimum values of lignin and SiO₂ nanoparticles is considered to be the best product due to its high adhesive properties and sustainability. Lignin’s strong antibacterial activity is also evidence that a promising polymeric adhesive was prepared that can be used in various fields.