Effect of the oxygen delignification on physical and mechanical properties of kraft pulps

The effect of oxygen delignification on physical properties was evaluated for four different types of kraft pulps, namely: (1) a softwood sample comprised of a mixture of Norway spruce and pine, (2) a softwood sample comprised of 100% southern pine, (3) a sample of eucalyptus pulp produced by the conventional process, and (4) another eucalyptus sample produced by the isothermal cooking process. These pulp samples were oxygen delignified with single and double stage processes and further bleached with the sequences DEopDD. The physical properties of the original, oxygen delignified, and bleached samples were determined. All properties were reported as a function of the tensile index. It was observed that the effect of oxygen delignification is not equal for all four types of pulps ad depends on whether or not the pulps are bleached. The impact is more intense on the unbleached pulps with the effect being substantially reduced after the pulps are ECF bleached. For the unbleached pulps, oxygen delignification had a positive effect or no effect on those physical properties that are more related to fiber flexibility and bonding potential. However, those properties that largely depend upon the fiber rigidity were negatively affected by the oxygen treatment. In general the oxygen effects that were apparent on the unbleached fibers were either substantially minimized or completely disappeared after bleaching. The small differences detected on the bleached samples were not significant. Furthermore, the effects of the oxygen delignification carried out in simple or double-stage, on the physical properties were similar both for the unbleached and the bleached samples.