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IntroductionThe environmental regulations imposed on the pulp andpaper industry make the implementation of new bleachingprocesses necessary in which the use of chlorine or itsderivatives is partially or totally eliminated. The develop-ment of these new bleaching processes, namely ElementalChlorine Free (ECF) and Total Chlorine Free (TCF)processes, relies on the improvement of the action of bleach-ing agents. This is particularly crucial in the case of oxygensince the oxygen bleaching process is usually limited to50% delignification because of the simultaneous degrada-tion of carbohydrates in the pulps, reducing their strengthproperties. In order to make the lignin degradation processmore efficient and more selective, it is of primary impor-tance to have a better understanding of the chemical reactions involved in oxygen bleaching as well as of thestructures of residual lignin and their reactivity.The main purpose of this investigation is to study thechemical changes brought about by oxygen bleaching onthe structure of residual lignin in Eucalyptus globuluskraftpulp. Among Eucalyptus species,Eucalyptus globulushas a low lignin content (Pereira and Sardinha 1984; Pereira1988). In addition, it is the most important species for pulpproduction and the only hardwood used for this purpose inPortugal.Samples examined in this paper were residual ligninsprepared by acid hydrolysis of unbleached and oxygenbleached kraft pulps, namely AcRL and AcRLO, respec-tively. The water-soluble residue (AcRLOs), recovered fromthe aqueous phase after acid hydrolysis and precipitationwith water, was also characterised. The structural changesbrought about by kraft cooking and oxygen bleaching in thecorresponding dissolved lignins, KDL and KDLO respec-tively, were investigated as well. The samples were charac-terised by |
