Optimum Conditions for Selective Separation of Kraft Lignin

Abstract

Lignin has been investigated over the last few decades as a promising natural alternative to phenolic resin (phenol-formaldehyde), due to their structural similarity. In the present work, lignin was precipitated from the Kraft black liquor of Eucalyptus camaldulensis Dehn. at pH 2, 4 and 6, respectively in order to determine the optimal separation conditions. A comparison study was performed among the lignin samples obtained, based on yield, average molecular weight, chemical structure and thermal stability. The results showed that the best conditions occurred with lignin precipitation at pH 4. For low molecular weight (MW) lignin, each selective extraction was carried out using dichloromethane, acetone and methanol to study the effects of the lignin to solvent ratio, and extraction time on the lignin yield. Physico-chemical characterization of the low MW lignin was also carried out to evaluate the most suitable lignin for adhesive purposes. The results showed a ratio of 1:5 (lignin:solvent) and an extraction time of 1 h were sufficient for extraction. Dichloromethane-soluble lignin with a molecular weight of approximately 940 g mol^-1 would be best for the adhesive formulation in future work.