Influence of Residual Lignin on Mechanical Fibrillation and Colloidal Stability of Cellulose Nanofibrils from Eucalyptus Kraft Pulp

Eucalyptus kraft pulping represents an important source for the production of lignin-containing cellulose nanofibrils (LCNF); however, the influence of controlled residual lignin levels on fibrillation energy demand and suspension stability remains poorly understood. This study evaluated the effect of residual lignin on the chemical properties of Eucalyptus urograndis kraft pulps, energy consumption during mechanical fibrillation over 20 passes in a Super Masscolloider, and the colloidal stability of the resulting suspensions. Brown kraft pulp was subjected to delignification treatments under mild (60 °C, 3 min) and severe (95 °C, 60 min) conditions, resulting in pulps with residual lignin contents of 3.63%, 1.77%, and 0.46%, respectively. Delignification reduced the kappa number from 29.59 in the brown pulp to 16.16 and 1.63 after mild and severe treatments, respectively, while pulp viscosity varied according to treatment severity. Energy consumption increased progressively with the number of passes and was strongly affected by residual lignin content. After 20 passes, the brown pulp (3.63% lignin) showed the highest energy consumption, reaching approximately 0.109 kWh, compared with 0.016 and 0.018 kWh for pulps containing 1.77% and 0.46% residual lignin, respectively. Despite these differences in energy consumption, all LCNF suspensions exhibited higher absolute zeta potential values under neutral to alkaline conditions, ranging from approximately −24 to −37 mV. These findings indicate that controlled delignification was associated with lower energy consumption during mechanical fibrillation while maintaining favorable electrokinetic behavior of the colloidal suspensions. Therefore, controlling residual lignin content may represent a strategy for balancing lignin removal, fiber preservation, and energy consumption during mechanical fibrillation under the conditions evaluated in this study.