2.4 The magic of oxygen delignification
Oxygen delignification brings several benefits to the pulp and paper industries. Due to lignin and carbohydrates oxidation, the amount of carboxylic acid groups on the fibers can be increased and outstanding properties can be achieved. The higher the delignification time and alkali charge, in the oxygen delignification process, the higher will be the increase in the carboxylic acid groups on the fibers, which will be tremendously important for the pulp properties.
The swelling properties are extremely affected by the fiber charge increase, therefore water retention value and Schopper-Riegler degree were compared for oxygen delignified and kraft delignified pulps. It was concluded that water retention values depend highly on the fiber charge rather than lignin and that Schopper-Riegler degree increases significantly more for pulps with very high fiber charges after oxygen delignification. It is believed that the increase in the fiber swelling potentiates greater internal and external fibrillation on the fibers after the refining process, which will be extremely beneficial to fiber conformability and higher pulp strength. The tensile index was seen to increase up to 18 % with oxygen delignification, when fiber charges were increased more than 90%, compared to the kraft delignified pulps.
Fiber morphology is also another property that can be shaped by oxygen delignification. It has been established that oxygen delignification tends to increase the fiber's deformations compared to the cooking process, however, after refining, the opposite is seen - oxygen delignified pulps tend to have lower deformations.