Bleachability Improvement of Eucalypt Mechanical Pulps Using Hydrogen Peroxide in Ethanol-water Media

Alkaline darkening has a negative effect on chemimechanical pulps produced from various eucalypt chips. For instance, the pulp’s brightness has been found to drop rapidly when alkali addition exceeds its optimum amount. Therefore, to compare the effects of different media on brightness gain, eucalypt mechanical pulps were bleached using a typical peroxide bleaching sequence in a water medium and in an ethanol-water (E/W) medium. Various pulp brightness levels and changes of residue chemicals in the spent bleaching liquors were investigated. Compared to water medium, the net brightness gain notably increased when using the E/W medium at the same chemical dosages. The rate of pulp brightness loss caused by increasing alkali dosage dropped in the E/W medium. Analyses based on ultraviolet-visible diffuse reflectance spectra of bleached pulps, as well as gel permeation chromatography and pyrolysis-gas chromatography-mass spectrometry spectra of the residues in the spent bleaching liquors, indicated that the contributions to brightness improvement from ethanol-water media could be explained by the selective dissolution and removal of phenolic- or guaiacyl-structured lignin fragments that result in the formation of chromophoric groups under alkaline conditions.