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

Abstract

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.

Keywords:

• Chlorine dioxide delignification
• extraction
• lignin modification
• alkali reactions
• kappa number
• klason lignin
• kraft pulps
• bleaching
• bleachability
• bleaching sequences
• quinones
• muconic acids
• phenolics
• oxygen

Acknowledgments

The authors thank Mark Turner of the Institute of Paper Science and Technology (IPST) at Georgia Tech for his assistance with the laboratory digester cooks of the black spruce employed in this study. Portions of this work were presented at the 2003 International Symposium of Wood and Pulping Chemistry in Madison, Wisconsin and the 2003 TAPPI Fall Technical Conference in Chicago, Illinois.

Notes

*Dedicated to Drs. Donald R. Dimmel and Thomas J. McDonough on their retirement from the Institute of Paper Science and Technology.