Microfibrillated cellulose (MFC) is an important industrial nanocellulose product and material component. New MFC grades can widen the materials property range and improve product tailoring. Microfibrillated lignocellulose (MFLC) is investigated, with the hypothesis that there is an optimum in lignin content of unbleached wood pulp fibre with respect to nanofibril yield. A series of kraft fibres with falling Kappa numbers (lower lignin content) was prepared. Fibres were beaten and fibrillated into MFLC by high-pressure microfluidization. Nano-sized fractions of fibrils were separated using centrifugation. Lignin content and carbohydrate analysis, total charge, FE-SEM, TEM microscopy and suspension rheology characterization were carried out. Fibres with Kappa number 65 (11% lignin) combined high lignin content with ease of fibrillation. This confirms an optimum in nanofibril yield as a function of lignin content, and mechanisms are discussed. MFLC from these fibres contained a 40–60 wt% fraction of nano-sized fibrils with widths in the range of 2.5–70 nm. Despite the large size distribution, data for modulus and tensile strength of MFLC films with 11% lignin were as high as 14 GPa and 240 MPa. MFLC films showed improved water contact angle of 84–88°, compared to neat MFC films (< 50°). All MFLC films showed substantial optical transmittance, and the fraction of haze scattering strongly correlated with defect content in the form of coarse fibrils. Graphic abstract: [Figure not available: see fulltext.]

Unbleached lignocellulosic wood fiber materials of low porosity are of great interest as eco-friendly load-bearing materials because their yield is much higher than that for "pure" wood cellulosics. The difference between comparable materials based on lignocellulose fibers or nanocellulose is investigated. The structure, surface area, mechanical properties, moisture sorption, and optical properties of films based on fibers or microfibrillated lignocellulose (MFLC) were characterized as a function of lignin content, and the environmental impact was compared. The modulus and tensile strength of comparable fiber and MFLC films (approximate to 25% porosity) increased up to an optimum lignin content (11-17%) and then decreased at a very high lignin content. Hot-pressed MFLC films with little porosity showed excellent properties, 230-260 MPa strength, 17-20 GPa modulus, and 81 MPa wet strength. The mechanical property values of hot-pressed wood fibers with 25% porosity were also as high as 154 MPa strength and 13.2 GPa modulus, which are higher than those of comparable materials reported in the literature. Because hot-pressed lignocellulose fibers can be readily recycled and show low cumulative energy demand, they are candidates for semistructural engineering materials. MFLC is of great interest for coatings, films, adhesives, and as additives or in high-technology applications.

Low-porosity materials based on hot-pressed wood fibers or nanocellulose fibrils (no polymer matrix) represent a new concept for eco-friendly materials with interesting mechanical properties. For the replacement of fossil-based materials, physical properties of wood fiber materials need to be improved. In addition, the carbon footprint and cumulative energy required to produce the material also needs to be reduced compared with fossil-based composites, e.g., glass fiber composites. Lignin-containing fibers and nanofibers are of high yield and special interest for development of more sustainable materials technologies. The present mini-review provides a short analysis of the potential. Different extraction routes of lignin-containing wood fibers are discussed, different processing methods, and the properties of resulting fiber materials. Comparisons are made with analogous lignin-containing nanofiber materials, where mechanical properties and eco-indicators are emphasized. Higher lignin content may promote eco-friendly attributes and improve interfiber or interfibril bonding in fiber materials, for improved mechanical performance.