Hydroxypropyl Sulfonated Lignin as Dye Dispersant: Effect of Average Molecular Weight

Sulfonated lignin obtained from pulping waste liquor is a nontoxic and renewable polymer that can be used as a dispersant in the dyeing industry. In order to reveal the effect of the lignin dispersant's molecular weights on disperse dye, three hydroxypropyl sulfonated alkali lignin (HSL) samples with different molecular weights were obtained by controlling the dosage of etherification to cross-link lignin molecules. The molecular weight of HSL can be adjusted from 8,100 to 14,830 Da. More than 80% of phenolic hydroxyl groups of HSL samples were blocked by etherification compared to that of AL and which decrease with increasing molecular weight. The increasing molecular weight of HSL causes a considerable reduction in the staining effect of HSL on fiber since the adsorption amount of HSL on the fiber decreases by reducing the phenolic hydroxyl group. HSL with Mw of 11,020 Da contains 2.10 mmol·g–1 of the sulfonic group and as low as 0.46 mmol·g–1 of the phenolic hydroxyl group (compared to 2.32 mmol·g–1 of AL), providing excellent dispersive ability and high temperature stability on dye. More importantly, the dye uptake with added HSL with Mw of 11,020 Da is the highest of 85.17% among all dispersants here. Therefore, the etherification modification is a promising approach to increase the molecular weight of lignin and for widespread applications of lignin as a highly efficient dye dispersant.