The effect of introducing ozone in elemental chlorine free bleaching of eucalyptus kraft pulp

ZDED and DEDD sequences were applied on an oxygen-delignified eucalyptus (mixture of E. grandis and E. saligna) kraft pulp from Brazil. When ZDED and DEDD were compared, the displacement ratio calculated as the quantity of pure chlorine dioxide replaced by 1 kg ozone was found between 2.5 and 3.5 kg, as compared to 1.7 kg in theory—indicating that less wasting reactions occur in the ZDED sequence. The cellulose viscosity decreased substantially with the increase in the ozone charge (down to 50% at 0.8% ozone). Other typical characteristics of the ZDED pulps were easier beating, lower water retention value, higher proportion of kinked fibers, and lower wet zerospan breaking length (seen only at 0.8% ozone charge), as compared to DEDD. The viscosity loss due to the introduction of ozone in an elemental chlorine free sequence did not translate into a similar loss in strength. A comparison with the depolymerizing effect of cellulase suggests that the determining factor is not the extent of the drop in viscosity but rather the heterogeneity of the attack along the fibers. It is concluded that ozone oxidation of cellulose in the fibers would be rather homogeneous. Moreover, the appearance of more curls and kinks would not be necessarily related to the viscosity loss. For example, a (2,2,6,6-tetramethylpiperidin- 1-yl)oxidanyl (TEMPO) treated DEDD pulp, enriched in carboxyl groups, had more straight fibers than the original DEDD pulp, despite its lower viscosity. The presence of ionic groups would be a more important factor; the lower content in carboxyl groups in the ZDED pulps, compared to the DEDD pulps, could promote the easier formation of curl and kinks during the ZDED sequence.