Preparation and Characterization of a Water-Soluble Kraft Lignin

Kraft lignin can be extracted from black liquor generated during wood digestion. At present, water soluble kraft lignin is prepared chemically. Here, a method for the preparation of a purified form of CO2-precipitated softwood kraft lignin, which forms a stable aqueous solution at a pH value of 8.9 is described. The lignin samples are characterized by thermogravimetric analysis, IR spectroscopy, nuclear magnetic resonance spectroscopy, and gel permeation chromatography. The comparison of water-washed lignin with reference samples of unwashed- and acid-washed lignin shows that the water-washed lignin has a higher molecular weight compared to the reference samples, while the content of hydroxyl and methoxyl groups is similar. Solubility studies show that drying of washed lignin has a significant effect on its dissolution in water. Charge titration and dynamic light scattering experiments show that water-washed lignin forms a stable colloid in aqueous solution.