Improvement of the fiber-bonding capacity of unbleached recycled pulp by the laccase/mediator treatment

The authors studied the oxidative and delignifying effects of the Trametes hirsuta laccase-HBT (1-hydroxybenzotriazole) system (LMS) on the papermakmg properties of an unbleached recycled softwood kraft pulp. The effects of pulp washing with a non-ionic surfactant (Tween 20) as a way to recover the enzyme and the effects of a final alkaline treatment were also analyzed. LMS treatment was applied to the fibrous fraction of a kappa-92 recycled pulp. The kappa-number reduction, the formation of radicals, and the increase in absorption coefficient that resulted verified that the LMS was active on this kind of pulp. Compared with the control treatment, the LMS treatment reduced the kappa number of the pulp by 8% and increased fivefold the number of radicals as analyzed by electron spin resonance, The LMS treatment also increased the light-absorption coefficient by 19%, suggesting that the fibers were oxidized. Alkaline treatment subsequent to the LMS treatment did not produce additional delignification and slightly decreased the light-absorption coefficient. The authors also found that LMS treatment or LMS treatment followed by alkaline treatment increased the fiber tensile and compressive strengths. It can be concluded that enzymatic delignification, oxidized residual lignin, or both contributed to improving the bonding capacity of the recycled fibers. Application: Enzymatic delignification or lignin oxidation could be useful for increasing the strength of high-kappa recycled unbleached kraft papers.