Carbohydrate-free and highly soluble softwood kraft lignin fractions by aqueous acetone evaporation fractionation

Technical lignins are complex mixtures of polymers with molecules of different molar masses, number of functional groups and carbohydrate residues. This heterogeneity induces variance in lignin properties and thus limits lignin's applicability in products where constant quality is required. Lignin fractionation into more homogeneous fractions is an attractive approach to valorize lignin feedstock. In this paper, solvent fractionation of lignin was applied to produce lignin fractions with improved purity and homogeneity. The main lignin fraction consisted 86% of the original lignin feedstock and this fraction was practically free from carbohydrates. In addition, a lignin fraction with very low molar mass, high solubility, high number of reactive free phenolic hydroxyl groups and a low glass transition temperature, was produced. The water solubility of lignin fractions depended not only on the molar mass and functional groups, but also on carbohydrates bound to lignin. The molecules with the highest solubility were most likely covalently linked to arabinogalactan. The applied aqueous acetone fractionation method with solvent evaporation required significantly less solvents than precipitation fractionation processes with nonsolvent addition and therefore is considered technically, economically and environmentally more sound process than alternative methods.