FORMIC-ACID PEROXYFORMIC ACID PULPING .4. LIGNINS ISOLATED FROM SPENT LIQUORS OF 3-STAGE PEROXYFORMIC ACID PULPING

Lignins isolated from the spent liquors obtained from three-stage peroxyformic acid and formic acid pulpings have been investigated by chemical analyses, NMR spectroscopy and molar mass distribution measurements. The lignins investigated in the present work are isolated by precipitation with water. The greatest amount of lignin is obtained from the spent liquor of the second stage, during which hydrolysis of beta-aryl ether bonds occurs. However, in the case of softwoods this reaction is restricted by condensation reactions, during which inter- and intramolecular alpha-5 bonds may be formed. Going from the first stage to the third stage the molar masses of the lignins increase, suggesting that the highest molar mass part of the lignin in the wood is the last to go into solution. However, intermolecular condensation reactions may also cause an increase in molar mass. Carbohydrate analysis reveals that for birch the glucose content is high in lignins from the first stage, during which hydrogen peroxide is used, and much lower in the second-stage lignin. A high concentration of xylose occurs in the second-stage lignins. For softwoods the highest concentration of glucose is observed in the second-stage lignins. Pulping experiments show that birch is more easily delignified than pine with the three-stage peroxyformic acid method. This may at least partly be explained by the fact that guaiacyl-type lignin undergoes condensation reactions.