Process and properties of continuous fibers based on cellulose nanocrystals and nanofibers
Abstract: In recent years, composites made from natural fibers based on cellulose have received increasing attention since they have a low environmental impact and good mechanical properties. However, these fibers are short and discontinuous and the conventional spinning techniques used for these fibers results in continuous yarns with mechanical properties considerably lower than that of the single fibers. The aim of this work was to prepare continuous fibers where nano-sized cellulose crystals and cellulose nanofibers were used to improve the fiber properties. Two different strategies have been used to reach this aim. In the first study, bio-based fibers of cellulose acetate butyrate (CAB) and cellulose nanocrystals (CNC) using triethyl citrate (TEC) as plasticizer were prepared by melt spinning. Two different dispersion techniques were studied. In the first technique, the water content of the CNC suspension was reduced and exchanged to ethanol using centrifugation. In the second, the water in the CNC suspension was completely exchanged to ethanol by a sol-gel process. Results showed that tensile modulus and tensile strength of the nanocomposite fibers produced with the first technique were lower than CAB-TEC fibers, but the fibers produced by the sol-gel process showed an increase in the tensile modulus and had no decrease in the strength. Optical microscopy of the fibers indicated less aggregations in the sol-gel prepared materials. The results indicate that the sol-gel process is enhancing the dispersion of cellulose nanocrystals and can be a suitable way to prepare nanocomposite fibers. The second study is an extension of the first study. Here the effect of weight concentration of CNC and fiber drawing was studied. The microscopy studies showed that the addition of CNC in CAB resulted in defect-free and smooth fiber surfaces. An addition of 10 wt% CNC enhanced the storage modulus and increased the tensile strength and Young’s modulus. Fiber drawing improved the mechanical properties further. In addition, a micromechanical model of the composite material was used to estimate the stiffness and showed that theoretical values were exceeded for the lower concentration of CNC but not reached for the higher concentration. In conclusion, this dispersion technique combined with melt spinning can be used to produce all-cellulose nanocomposites fibers and that both the increase in CNC volume fraction and the fiber drawing increased the mechanical performance. In the third study a different strategy was used. Here low cost and environmentally friendly continuous fibers of native cellulose were prepared by dry spinning an aqueous suspension of cellulose nanofibers (CNF). The CNF were extracted from banana rachis, a bio-residue from banana cultivation in Columbia. The effect of spinning rate and CNF concentration on the mechanical properties of the fibers were investigated. The results showed that there was a relationship between the spinning rate and concentration. The modulus of the fibers was increased from 7.7 to 12.6 GPa and the strength increased from 131 to 222 MPa when the lowest concentration and highest speed was used. This improvement is believed to be due to an increased orientation of the CNF in the fiber. A minimum concentration of 6.5 wt% was required for continuous fiber spinning. However, this relatively high concentration is thought to limit the orientation of the CNF in the fiber. The process used in this last study has a good potential for up-scaling providing a continuous fiber production with well-controlled speed but further work is required to increase the orientation and subsequently the mechanical properties.The results from these three studies shows that it is possible to spin continuous fibers where nanocellulose is used as a reinforcing agent. It is also shown that the dispersion and alignment of the nanocellulose plays a key role in improving the mechanical properties.
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