We investigated the bleaching efficiency of the final chlorine dioxide (D2) stage in the D0(EP)D1D2 bleaching sequence, focusing on the effect of pH in relation to the bleaching history of pulp samples. The samples used were unbleached kraft Eucalyptus grandis pulps with kappa no. 14.8 and the same pulp oxygen-delignified to kappa nos. 12 and 9.8. The samples were bleached according to the D0(EP)D1 sequence to a brightness of about 86% ISO and then submitted to the final D2 stage under identical conditions (e.g., chlorine dioxide charge, time, temperature, and final pH). The target final brightness was 90.5% ISO. Changes in the kappa number, brightness, viscosity, and contents of hexenuronic acid, 4-O-methylglucuronic acid, and the total amount of carboxylic acid (COOH) groups in pulps were monitored during the bleaching sequence. The final brightness of eucalyptus kraft pulps increased when the terminal pH of the D2 stage was raised from 3.0 to 6.5. The 90.5% ISO target brightness in the D2 stage was achieved for all pulps within a pH range of 4.5–6.5, but this required adjusting final pH for individual samples. The optimal pH value with respect to pulp viscosity was between 3 and 5. Despite dissimilar conditions applied in previous bleaching stages, the samples after the D1 stage revealed similar residual lignin contents as shown by kappa number analysis. The content of hexenuronic acid in the samples, however, varied broadly, from 2 to 26 mmol/kg. Conductometric titration showed different amounts of carboxylic acid groups in pulps after the D1 stage, of which hexenuronic acid accounted for only a minor part. The variations in the fiber charge resulted from the different bleaching conditions applied before the D2 stage. The fiber charge affected the alkali demand in the final D2 stage, whereas variations in the alkali demand affected the initial pH and associated process kinetics. Lower total fiber charge was found to be beneficial for improved final brightening and viscosity when bleached at higher final pH.

Ten industrial fully bleached hardwood pulps representing very different brightness reversion profiles were evaluated. The pulps were characterized before and after brightness reversion. The aging method (dry, wet, high/low RH, high/low temperature, short/long time) had, in most cases, no effect on overall brightness reversion trends, but affected the absolute reversion values significantly. Relative humidity in the range of 50 to 90% influenced reversion much more than temperatures in the range from 30 to 45°C. Dry and wet heat-induced brightness reversion correlated positively and significantly with hexenuronic acid (HexA) content in the pulp. The brightness reversion causes a significant decrease in pulp viscosity values, and HexA thus causing decrease of kappa number. Modern pulps produced by TCF and light ECF bleaching technologies were more prone to brightness reversion than regular ECF pulps, partly due to the higher content of HexA. At a pH level above 8, the brightness reversion process was strongly retarded for pulps with a very high HexA content.