Nano-SiO2: A Key Beneficial Additive that Significantly Improves Cellulose Solubility and Reinforces Regenerated Cellulose Fibers in the Alkali/Urea System

Cellulose has the advantages of renewability and degradability, while cellulose's rigid structure and hydrogen bonding limit its dissolution and application. Low-temperature alkali/urea solvents offer a green, low-cost solution but suffer from low solubility, high coagulation costs, and poor fiber strength. In this work, to address these problems, the high specific surface area and surface-rich hydroxyl groups of nano-SiO2 powders were used to modify the low-temperature alkali/urea solvent system. There are two alkali/urea solvent systems: LiOH/urea has better solubility but expensive and NaOH/urea is cheap but of less solubility. In the LiOH/urea system, it was found that the solubility of cellulose could be increased by 60.1% by addition of nano-SiO2 powder, and the gel temperature of the solution remained basically unchanged. By using the phytic acid coagulation system, the strength and elongation of regenerated cellulose fibers could reach 547.6 MPa and 18.7%, respectively. More importantly, in the low-cost NaOH/urea system, nano-SiO2 boosts solubility by 45.2%, regenerated cellulose fibers with 404 MPa strength, and 20.18% elongation by using the H2SO4/Na2SO4 coagulation system. Finally, the mechanism of action of nano-SiO2 is revealed, which provides a new idea for green dissolution and the high-performance regeneration of cellulose.