The three purified Sodium lignosufonate fractions with different molecular
weights were obtained using ultrafiltration. With the
increase in molecular weight, the content of phenolic
hydroxyl group, sulfonic acid group, and carboxyl group
decreases, the purity of Sodium lignosulfonate increases, color deepens, and guaiacol group content increases.

There are great differences in the performance of Sodium lignosufonate with different molecular weights used as dye dispersant. The results demonstrate that Sodium lignosulfonate with high molecular weight exhibits better dispersion and heat stability on dye. When the concentration of SL is more than 6.0 g=L, with the increase of molecular weight, fiber staining becomes weaker.

The main factors affecting fiber staining and dye
reduction are the molecular weight and phenolic
hydroxyl content of Sodium lignosulfonate. The Sodium lignosulfonate sample that has a
molecular weight above 2.5 kDa shows the minimum
dye reduction degree. Moreover, the dyeing rate gradually increases with increase in molecular weight of Sodium lignosulfonate.