Solvent extraction as a means of preparing homogeneous lignin fractions

Sequential extraction method was applied to lignins from hardwood and softwood isolated by kraft and VTT organosolv processes. Solvent extraction was found to fractionate lignin according to the molecular weight: small molecular weight lignin is dissolved in the organic solvents and the lignin with higher molecular weight is enriched into the residue. Isolated acetone fractions of lignin are more homogeneous with narrow molecular weight distributions. Based on the 31P NMR results, both total hydroxyl content and the content of phenolic hydroxyl units are higher in the acetone fraction than in the residue. Pyrolysis-GC/MS of all lignins showed that p-hydroxy phenols are enriched to the residues. Preferential dissolution of syringyl type lignin in acetone was observed for hardwood kraft lignin, whereas the opposite behavior was seen for the hardwood organosolv lignin. Glass transition temperatures of all acetone soluble fractions were notably low compared to starting and residue lignins, which gives possibilities for future applications as a material with specific properties.