Aqueous ethanol fractionation of softwood and hardwood kraft lignins: Impact on purity and properties

Industrial kraft lignins are mixtures of macromolecular components variable in both structure and composition. To maximize their value in commercial applications, they often need to be homogenized and purified. Several fractionation methods have been reported to improve the properties of kraft lignins, but these reports have been mostly limited to kraft lignins of single origins. In this study, the physicochemical properties of fractions from four industrial kraft lignins were compared. Fractionation was performed on both softwood and hardwood lignins by partial dissolution in aqueous ethanol followed by precipitation with water. The yields of each fraction varied greatly between the lignins, with differences reaching up to 35% for a single fraction. All fractions were characterized which showed that fractions having remarkably similar properties and compositions can be obtained from different lignins. Organic and inorganic impurities were found to concentrate in specific fractions which allowed the isolation of highly purified fractions of kraft lignin. These results highlight the importance of matching individual kraft lignins with suitable applications.