Signally improvement of polyurethane adhesive with hydroxy-enriched lignin from bagasse

Acetic acid lignin (AAL) obtained from Bagasse was used as a partial substitute of polyols for polyurethane (PU) adhesive due to the presence of reactive sites, i.e. aliphatic and aromatic hydroxyl groups, in its structure. To improve its reactivity, lignin is partly demethylated before the copolymerization with polyisocyanate. The demethylation, catalyzed by hydrobromic acid solution under heating conditions, resulted in a decrease of methoxy groups on the phenyl ring and the increase of phenolic hydroxyl groups. It is found that 120 degrees C is the preferred temperature for the demethylation resulted in a 36.5 % increase in hydroxyl content of lignin up to 5.25 mmol/g. Lignin-based polyurethane adhesives were prepared in two steps including preparing a prepolymer based on mixtures of toluene-2,4-diisocyanate (TDI) and unmodified or demethylated lignin, and then the polymerization was completed by adding polyethylene glycol (PEG). The mass ratio (1:1) of TDI: (lignin + PEG) in the compositions was here investigated. The results indicated that the presence of lignin (with and without demethylation) in polyurethane adhesives increased the glass transition temperature, Young's modulus, and elongation at break. Particularly, the tensile strength was increased by 15-36 MPa with the addition of 10-30 wt% of lignin. Lignin with higher amount of hydroxyl groups prepared by partly demethylation is better integrated into the three-dimensional cross-linking networks of the covalent polymer as a substitute of polyols. Compared to those PU adhesives based on the raw AAL, the adhesives prepared by demethylated lignin have higher tensile strength and thermal stability. It indicates that the demethylation is effective and it is a good strategy to prepare lignin-based PU adhesive with favorable mechanical properties.