The effect of delignification strategies on cellulose nanofiber and nanopaper properties and functionalities

In the field of cellulose nanomaterials, there has been an increased tendency toward the use of lignin-containing nanofiber, and therefore, limiting the delignification process in the production of lignocellulose nanomaterials in recent years. However, various theories have been proposed, and conflicting findings have been reported with respect to the role of residual lignin. To evaluate the different theories, lignin-containing nanofibers with various delignification levels from poplar wood chips were produced via the ultrafine grinding technique. Several lignin-containing nanofibers and nanopapers were produced and analyzed. Moreover, the relationship between delignification strategies and the functional properties was comprehensively investigated. The results revealed that a higher lignin content is not necessarily resulted in better nanofibrillation; rather, an optimal lignin level exists, the value of which depends on the raw material preparation conditions and the mechanical nanofibrillation process. However, based on the conditions of this study, it can be deduced that to optimize nanofibrillation, a range of delignification in which the lignin structure is disrupted and fragmented but not totally removed can be proposed. Moreover, residual lignin does not act as an independent variable-the functional properties of the nanofibers and nanopapers are, in fact, shaped by the total of delignification strategy adopted. Moreover, Principal Component Analysis (PCA) demonstrated that the nano-sized fraction index, tensile strength, water retention value, turbidity and, to some extent, tensile modulus are correlated and categorized around the PC1 axis (the same category of properties) as a combined indicator of nano-structural quality, mechanical performance, and network integrity.