Production of Sulfur Containing Kraft Lignin Products

Kraft lignin is produced in great quantities in many countries, but is mainly used as an energy source. To valorize its potential end-use applications, softwood kraft lignin was modified via sulfuric acid and sodium sulfite treatments in this study. The modification of kraft lignin through a sulfuric acid treatment resulted in a modified lignin (SA-lignin) with a charge density of 0.8 meq/g but with a limited water solubility. The sulfonation of the kraft lignin through a sodium sulfite treatment induced a soluble sulfonated lignin (SS-lignin) with a charge density of 1.4 meq/g, which was obtained under the conditions of 90 °C, 4 h, and 0.67 Na2SO3/lignin molar ratio. The elemental compositions, molecular weights, and thermal and rheological properties of modified lignin samples were characterized.