Sulfuric acid bleaching of kraft pulp III: reactivity of kraft pulping-resistant structures under acidic conditions

To investigate the bleaching mechanism, a lignin-carbohydrate complex (LCC) model compound, a vinyl ether-type lignin model dimer, and a hexeneuronic acid model compound were treated with dilute sulfuric acid of different pHs . Beech kraft pulp and red pine kraft pulp were also treated with dilute su lfuric acid and then extracted with aqueous alkali. The amount of hexeneuro nic acid degradation products in acid effluents and lignin dissolved in alk ali effluents were determined. It was found that the benzyl ether-type LCC bond and the vinyl ether bond in lignin were effectively cleaved under the pH where sulfuric acid bleaching of kraft pulp was effective. Hexeneuronic acid group was also effectively degraded during sulfuric acid bleaching. In beech kraft pulp bleaching, both lignin removal and hexeneuronic acid remo val contributed to the kappa number reduction. In red pine bleaching, the c ontribution of hexeneuronic acid removal was negligible, and most of the ka ppa number reduction was achieved by the lignin removal.