Engineering strong man-made cellulosic fibers: a review of the wet spinning process based on cellulose nanofibrils

With the goal of sustainable development, manufacturing continuous high-performance fibers based on sustainable resources is an emerging research direction. However, compared to traditional synthetic fibers, plant fibers have limited length/diameter and uncontrollable natural defects, while regenerated cellulose fibers such as viscose and Lyocell suffer from inferior mechanical properties. Wet-spun fibers based on nanocelluloses especially cellulose nanofibrils (CNFs) offer superior mechanical performance since CNFs are the fundamental high-performance building blocks of plant cell walls. This review aims to summarize the progress of making CNF wet-spun fibers, emphasizing on the whole wet spinning process including spinning suspension preparation, spinning, coagulation, washing, drying and post-stretching steps. By establishing the relationships between the nano-scale assembling structure and the macroscopic changes in the CNF dope from gels to dried fibers, effective methods and strategies to improve the mechanical properties of the final fibers are analyzed and proposed. Based on this, the opportunities and challenges for potential industrial-scale production are discussed.