Only0.21perAr-freePh-OHwasfoundinDWL,sincemostofPhOHareoccupiedinlinkagestoneighboringphenylpropaneunits.In contrast,thefrequencyoffreephenolichydroxylgroupswasrather high in all kraft lignin samples. A substantial increase in phenolic hydroxyl group content was observed with decreasing precipitation pH and increasing cooking severity due to the formation of phenolic OH groups as a result of the extensive cleavage of the and -ether bonds.
5. Conclusions
Selectiveacidprecipitationwasshowntobeanefficient,simple and inexpensive method to separate and fractionate lignin from softwood black liquor. Our results indicated that up to 85% of the lignin solubilized in softwood black liquor can be recovered in high purity by lowering the pH to 5. A further pH reduction leads to increased precipitation of carbohydrates. Extended cooking resulted in increased molecular weight of the precipitated lignins; a consequence of progressive condensation reactions. The increase in Mw was especially pronounced for the pH 10.5 and pH 2.5 precipitated lignins, respectively. The demethylation of kraft lignin was found to be particularly high in the beginning of cook. Additionally, it seems that kraft lignin with a low molecular weight had significantly high phenolic and aliphatic hydroxyl group and low methoxyl group contents. Based on the results, the pulping stage and precipitation acidity should be carefully selectedtotailortheligninpropertiestothoserequiredforthefinal application.
Acknowledgements
ThefinancialsupportforthisworkbyTEKESandtheForestCluster as part of the Wood Wisdom project is gratefully appreciated. TheauthorsacknowledgeDrKariKovasinandMetsäFiberforsupplying the BL samples. We thank Hannes Moosbauer (Lenzing AG) forprovidingassistancewithATR-IRanalysisandDrHendrikWetzel (Fraunhofer IAP) for his help with the GPC analysis. We also thank Dr Miro Suchy, Dr Marc Borrega and Dr Agnes Stepan for helpful discussions.