Residual lignin distribution in bleached kraft pulp fibres

The distribution of residual lignin across the cell wall of kraft pulp fibres was measured after a variety of single bleaching stages. Measurements were taken using STEM-EDXA analysis of brominated fibre sections. The powerful oxidants ozone and isolated dimethyldioxirane reacted more rapidly with outer cell wall material (S-1 and S-3 layers) while reaction with lignin in the interior wall (S-2) appeared to be slowed by mass transfer limitations. The reaction front however, was not sharp for these powerful oxidants; a maximum difference of 30% lignin concentration was observed across the cell wall. Peroxide, chlorine dioxide, monoperoxysulphate and in-situ generated dimethyldioxirane produced uniform residual lignin distributions. Following oxygen bleaching, the residual lignin in the S-1 and S-2 layers were higher than in the S-2 layer, with a ratio very close to that of the original kraft pulp.