New insights into lignin modification during chlorine dioxide bleaching sequences (III):: The impact of modifications in the (EO) versus E stage on the D1 stage

Chlorine dioxide delignification (DO) 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 ClO(2) in a D(1) stage to hydroxyquinones (similar to 1.8 mmol/g lignin). Pulps treated by DOE consume considerably more ClO(2) in the D(1) than D(0)(EO), and have lower bleachability, as was quantified by a simple bleaching model. Both D(0)E and D(0)(EO) pulps approach a common brightness ceiling (similar to 83 ISO) when excess ClO(2) is applied. Examination of the post-D(1) b* values indicates that D(0)E and D(0)(EO) also have similar asymptotic b* values (similar to 6), indicating that both pulps have similar residual chromophores. Hydroxyquinone structures appear to be eliminated in the D(1) stage for D(0)(EO) pulps, and at high ClO(2) levels for D(0)E pulps.