Abstract

Here a single-step acidic thiourea delignification without and with sequential hydrogen peroxide bleaching were used as a pre-treatment for producing low-lignin-containing cellulose nanofibers (LCNF) and bleached cellulose nanofibers (BCNF), respectively. Without bleaching, LCNF contained 7 wt% lignin, resulting in a slightly yellowish color in both the water dispersion and film. Despite low gravimetric yield of nanosize fibers, 23 wt% after microfluidization, the LCNF films exhibited notable mechanical strength, with a tensile strength of 174 MPa and a modulus of 11 GPa. Upon introducing an additional bleaching step, the color was eliminated, reducing the lignin content to 3 wt%. This adjustment also led to a significant increase in the gravimetric yield of nanofibers, reaching 78 wt%. The BCNF films demonstrated even higher mechanical performance, with a tensile strength of 207 MPa and a modulus of 12 GPa. LCNF film exhibited a higher contact angle (≈70°) than BCNF (≈60°), presumably due to the higher lignin content of the nonbleached pulp. Furthermore, hot pressing of the films at 150 °C for 18 min substantially enhanced the wet strength of the films by over 50%, attributed to the residual lignin.