Mechanical fabrication of high-strength and redispersible wood nanofibers from unbleached groundwood pulp

In the past, the direct production of lignin-containing nanofibers from wood materials has been very limited, and nanoscale fibers (nanocelluloses) have been mainly isolated from chemically delignified, bleached cellulose pulp. In this study, we have introduced a newly adapted, heat-intensified disc nanogrinding process for the enhanced nanofibrillation of wood nanofibers (WNF) with a high lignin content (27.4 wt%). The WNF produced this way have many unique and intriguing properties in their naturally occurring form, for example, being able to be dispersed in ethanol and having ethanol solution viscosities higher than water solution viscosities. When WNF nanopapers were formed with ethanol, the properties of the nanofibers were recoverable without a notable decrease in the viscosity or mechanical strength after redispersing them in water. The preservation of lignin in the WNF was noticed as an increase in the water contact angles (89°), the rapid removal of water in the fabrication of the nanopapers, and the enhanced strength of the nanopapers when subjected to high pressure and heat. The nanopapers fabricated from the WNF were mechanically stable, having an elastic modulus of 6.2 GPa, a maximum stress of 103.4 MPa, and a maximum strain of 3.5%. Throughout the study, characteristics of the WNF were compared to those of the delignified and bleached reference cellulose nanofibers. We envision that the exciting characteristics of the WNF and their lower cost of production compared to that of bleached cellulose nanofibers may offer new opportunities for nanocellulose and biocomposite research.