Cellulose nanofiber extraction from unbleached kraft pulp for paper strengthening

CNF-based materials have developed into the current global research and development hotspot, while the high cost of preparation and the difficulty of industrialization restrict their large-scale application. Therefore, how to efficiently and greenly prepare CNFs is still the key challenge to promote the development of CNF-based materials. In this work, we propose a few-chemical, low-energy process for the direct extraction of cellulose nanofibers (CNFs) from unbleached kraft pulp (UBKP), which simultaneously realize delignification and defibrillation by potassium permanganate oxidation. It is worth mentioning that the residual lignin and hemicellulose in the pulp also have been converted into active components during the oxidation process, which can promote defibrillation (nanofibrillation efficiently achieved in 0.3-1 h). Meanwhile, the activated residual lignin significantly improved the UBCNF properties, especially tensile strength (stress reached 151 MPa, elastic modulus was 4.1 GPa) of the UBCNF films. Finally, the UBCNFs were used as an eco-friendly paper strengthening agent and prominent enhancement was achieved with only 3% addition of UBCNFs (17.37%, 17.36%, and 16.20% increase in tensile strength, breaking strength, and tear strength, respectively, and the brightness improved from 72.34 to 74.89%).