New Insights into Lignin Modification during Chlorine Dioxide Bleaching Sequences (III): The Impact of Modifications in the (EO) versus E Stage on the D1Stage

Chlorine dioxide delignification (D0) modifies kraft residual lignin by oxidizing phenolic groups to both quinone and muconic acid structures. Alkaline extraction (E), in addition to removing solubilized lignin, converts quinone moieties to polyphenols. These polyphenols are easily oxidized by oxygen in an (EO) stage or by ClO2 in a D1 stage to hydroxyquinones (∼1.8 mmol/g lignin). Pulps treated by D0E consume considerably more ClO2 in the D1 than D0(EO), and have lower bleachability, as was quantified by a simple bleaching model. Both D0E and D0(EO) pulps approach a common brightness ceiling (∼83 ISO) when excess ClO2 is applied. Examination of the post‐D1 b* values indicates that D0E and D0(EO) also have similar asymptotic b* values (∼6), indicating that both pulps have similar residual chromophores. Hydroxyquinone structures appear to be eliminated in the D1 stage for D0(EO) pulps, and at high ClO2 levels for D0E pulps. *Dedicated to Drs. Donald R. Dimmel and Thomas J. McDonough on their retirement from the Institute of Paper Science and Technology.