Influence of residual lignin on Acacia mearnsii De Wild nanofibrillated cellulose suspensions and films

Studies on plant-derived nanocellulose are mainly carried out with lignin-free materials. However, lignocellulosic materials can be used as an alternative raw material, mainly due to their low cost and low environmental impact. This work aimed to study and compare the influence of residual lignin on the nanofibrillation process, on the properties of the nanofibrils and the films formed from the cellulose nanofibrils of Acacia mearnsii De Wild. The cellulosic pulp was subjected to bleaching to remove the lignin and obtain the bleached pulp. Bleach and unbleached pulps were subjected to mechanical defibrillation to obtain cellulose nanofibrils. The effects of defibrillation and residual lignin content were analyzed by characterizing the suspensions and nanocellulose films formed from this material. In addition, an energetic study of the production process of different cellulose nanofibrils was carried out, considering the different levels of residual lignin present in the material. The results demonstrated that the residual lignin presence preserved the thermal properties and provided changes in the morphology of the films, the suspensions showed colloidal stability from pH 5.0 (between |25| and |30| kV), the increase of the lignin content in the pulp caused a slight expressive improvement in the antioxidant activity and a decrease in the suspension viscosities, which fitted the Herschel–Bulkley model well (R² > 0.98). The mechanical properties of the films [127 MPa (NBR) and 75 MPa (N16)] and the energy consumption to produce nanofibrils [0.324 kWh (NBR) and 0.180 kWh (N16)] showed significant differences, pointing to the influence of residual lignin present in the materials.