Lignin-containing cellulose nanofibrils (LCNF): processing and characterization

Abstract

Currently, the global dependence on oil as a source of energy and raw materials has dramatically increased, creating practical and environmental concerns. This has led to several efforts towards replacing this non-renewable with environmentally friendly alternatives. In this sense, lignocellulosic materials have gained attention over the years as an alternative for the replacement of oil-based products due to their abundancy and renewability. Such lignocellulosic materials can be found in nature as a combination of polysaccharides, such as cellulose and hemicelluloses –the structural components of biomass-, embedded in lignin, which act as an adhesive for the structure. Renewable, biodegradable and biocompatible lignocellulosic materials with nano-scale dimensions are known as nanocellulose. The methods applied to obtain nanocellulose usually involve chemical-, mechanical-, and enzymatic treatments, or a combination of thereof, giving rise to different types of nanocellulose. Most commonly, the term nanocellulose refers to cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF), the latter being the object of the present study. Both types of nanocellulose can be utilized in novel applications such as packaging, functional nanocomposites, emulsion stabilizers and in the pharmaceutical and medical fields, due to their unique properties such as high aspect ratio, high strength, low density, and high capacity for chemical–modification. The chemical composition and properties of the starting lignocellulosic material utilized for CNF production will play an important role in the behavior of the resulting materials after fibrillation, as the individual components interact at a very fundamental level. The main source for CNF is bleached cellulose pulp, where lignin and non-cellulosic polysaccharides have been removed by chemical treatments. During the pulping processes the fibers undergo chemical reactions that will affect their surface properties, which are the main factors responsible for their interactions with the medium in which they will be dispersed. Understanding the final properties of cellulose fibers after the pulping processes, in terms of functional groups and composition, allows a deeper understanding of the final CNF properties such as morphology, thermal stability, and chemical composition. Although, the removal of the cell wall components has been widely used, the presence of residual lignin in raw materials for the production of lignin containing cellulose nanofibrils (LCNF) might be beneficial as demonstrated by the reduction of dewatering time and oxygen permeability of films. Additionally, the utilization of unbleached cellulose pulp to produce LCNF results in higher yields while reducing costs, due to the lower energy consumption during the manufacture process. Furthermore, from an environmental point of view, the production of LCNF could be beneficial since the processes of lignin removal as well as the following bleaching steps are no longer necessary. The main objective of this work is to study how the remaining lignin on the starting cellulose fibers, affect the properties of the resulting CNFs, allowing for a better utilization of LCNF and selection of final applications. Different characterization techniques were utilized to assess the relationship between the lignin content and the characteristics of the nanofibrils. Morphology of the CNF was studied using optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Fibril diameter and diameter distributions for CNF with different levels of residual lignin were investigated based on AFM images. Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and X-ray powder diffraction (XRD) were used to analyze chemical composition, thermal decomposition, and degree of crystallinity, respectively. The stability of the colloidal suspensions was assessed by zeta-potential and charge density analyses. Finally, rheological behavior of the samples was evaluated and correlated with all the above mentioned properties of LCNF.