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.