Wood chemists have been investigating how various oxidants modify residual lignin in softwood kraft pulps for decades.  A better understanding of the fundamental chemistry involved during bleaching delignification and brightening stages is crucial to making improvements to current elemental chlorine-free bleaching.  In this presentation, the author will review published work of the last decade which utilized novel in-situ techniques for probing structural changes occurring to softwood kraft residual lignin when sequentially treated with chlorine dioxide delignification (D0) and caustic extraction (E1) or oxidant-reinforced extraction  (e.g., (EO), ((EOP) or (EP)).  These in situ techniques involve the examination of the pulp's permanganate consumption during the kappa number test and its Klason lignin content.  Analysis of laboratory data suggests that roughly two-thirds of the phenolic groups in residual softwood kraft lignin are oxidized by chlorine dioxide to quinones, with the remainder being converted to muconic structures.  The data also suggest that simple caustic extraction converts quinones of D0 pulps to polyphenols, while oxidant-reinforced extraction modifies these polyphenols to hydroquinones and/or  dibasic organic acids.  The Hydroquinones represents a newly formed chromophore during bleaching delignification, which can be resistant towards further oxidation by chlorine dioxide brightening stages.  Residual hydroquinone structures have been detected in ECF bleached softwood pulps and have been implicated as a potential chromophore limiting the pulp's brightness ceiling.

Observing lignin modifications during the early stages of elemental chlorine-free (ecf) bleaching of a softwood Kraft pulp utilizing in situ techniques: Reviewing a decade of research and its implications

244th National Meeting (2012)

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