Processing of continuous fibers based on nanocellulose: Influence of dispersion and orientation on mechanical properties

Abstract: The aim of the work was to prepare continuous bio-based fibers where nano-sized cellulose was used to improve the mechanical properties. Two different strategies were used to reach this aim, melt-spinning of thermoplastic fibers reinforced with nanocellulose and dry-spinning of cellulose nanofibers without solvent or chemicals. In the first strategy, melt-spun fibers were reinforced with cellulose nanocrystals. First, nanocomposite fibers of cellulose acetate butyrate (CAB) reinforced with cellulose nanocrystals (CNC) and plasticized with triethyl citrate (TEC) were prepared. Two different techniques for dispersing CNC were compared: a process of solvent exchange of the aqueous CNC suspension to ethanol by centrifugation and sol-gel process. The mechanical properties and microscopy results indicated that the sol-gel process enhanced the dispersion. Subsequently the effect of CNC concentration and solid-state drawing (SSD) was studied. The results were defect-free and smooth fiber surfaces, in which an addition of 10 wt% CNC and drawing increased the tensile strength and Young’s modulus by 43% and 134% compared to the as-spun unreinforced fibers. This melt spinning process was also used to process melt-spun nanocomposite fibers of polylactic acid (PLA) and CNC. In this study the effect of surface modification of the CNC as well as the melt draw ratio (MDR) was investigated. The results showed that the increased MDR together with the surface modification resulted in better mechanical properties. In the second strategy, continuous fibers of native cellulose nanofibers (CNF) were prepared by dry-spinning. First, the effect of the spinning rate and the CNF concentration on the mechanical properties were investigated. The highest orientation and mechanical properties were achieved by combining a low CNF concentration with a high spin rate. The modulus of the fibers increased from 7.7 to 12.6 GPa and the strength form 131 to 222 MPa. After this, to further improve the orientation of the CNF, a small amount of hydroxyethylene cellulose (HEC) was used as a binder and the fibers were cold drawn after the spinning. The results showed that the addition of the binder and cold drawing increased the modulus and strength by 76% and 73% being 15 GPa and 260 MPa respectively. The results also confirmed that dry-spinning has potential for up-scaling, providing a continuous fiber production with well-controlled speed. These studies demonstrated that the dispersion and alignment of nanocellulose in spun fibers play key roles in improving the mechanical properties of these continuous bio-based fibers.