Oxygen delignification is widely used in the pulp and paper industry as a part of delignification process between the kraft cook and bleaching. However, its potential has not been fully utilized. Rather than an intermediate process between cooking and bleaching, oxygen delignification is a strong oxidizing agent with powerful effects on the pulp properties. In this work, the hypothesis that oxygen delignification has the potential to improve the pulp mechanical properties was investigated.

Several pulps were produced by either kraft cooking or kraft cooking combined with a subsequent oxygen delignification stage to a similar kappa number and their properties were analyzed and compared. This methodology assessed the real oxidative potential of oxygen on the final fiber properties. Total fiber charge, pulp mechanical properties, fiber morphology, swellability and fiber nanostructure, were studied.

The major part of this research investigated the relationship between the carboxylic acid groups, seen as total fiber charge, and the mechanical strength of the paper. The total fiber charge was evaluated by conductometric titration and correlated with the pulp swellability and mechanical properties. It was demonstrated that oxygen delignification could significantly increase the charge content and the swelling of the pulp when an extended oxygen delignification (i.e, higher delignification degree) was used. In addition, the tensile index of the sheets increased when the fiber charge after oxygen delignification was sufficiently high.

The swelling of the different pulps was investigated by Schopper-Riegler degree (SR°), water retention value (WRV) and fiber saturation point (FSP). It was determined that the higher the fiber charge, the higher the swelling ability, regardless of the lignin content.

High alkali impregnation was utilized in this study due to its potential to increase cooking yield. The yield was compared to kraft pulp cooked with standard and high alkali impregnation, followed by oxygen delignification and bleaching. It was observed that the increase in yield was preserved in both unit processes, i.e., after oxygen delignification and after bleaching.

During this work, pulp properties such as fiber morphology and fiber nanostructure were also important properties that were studied following each unit process and refining step. Oxygen-delignified pulps presented higher fiber deformation when compared to the kraft-cooked pulps. However, even with higher fiber deformation, oxygen-delignified pulps showed higher mechanical strength, contradicting previous reports that claimed lower pulp strength for oxygen-delignified pulps, due to fiber deformation. Additionally, it was found that fiber deformation tends to increase with PFI-refining for kraft-cooked pulps, while for oxygen and bleached pulps it tends to decrease. Fiber nanostructure was additionally studied by X-ray scattering, and the results obtained from pulp delignification by kraft and kraft followed by oxygen delignification were compared.

This thesis highlights the benefits of increasing fiber charge by performing an extended oxygen delignification after a reduced kraft cooking. The results indicate that when oxygen-delignified pulps achieve 80 % higher fiber charge than kraft-cooked pulps at a similar kappa number, the pulp tensile index can be improved by up to 18 %. The oxidation reactions that occur during the oxygen delignification lead to a significant increase in the carboxylic acid groups in the fibers which increases the fiber's swelling ability and improves the refining process efficiency. The combination of those effects results in a higher tensile index and lower refining energy required. However, to obtain mechanical improvement, the oxygen delignification must be sufficiently long (extended). Therefore, it is believed that an extended oxygen delignification will yield a more uniform distribution of the charged groups in the fibers, which will increase the fiber swelling and fiber flexibility leading to a more efficient refining process and stronger fiber bonding structure in the paper.