Cellulose–lignin blends are proposed as alternative precursors for carbon fiber (CF) production, offering a potential sustainable and cost-effective alternative to the expensive fossil-based polymers currently used. The characteristics of the precursor fibers including their crystallinity, the incorporated chemical structures and the distribution of the biopolymers have a significant influence on their carbonization behavior and the properties of the CFs. They are partly determined by the composition of the bio-based resources and the conditions used during the fiber fixation, i.e. the coagulation, an important processing step. In this work, 13C solid and 2D solution NMR methodologies were applied to investigate the impact of coagulation and thermostabilization conditions on cellulose and cellulose-lignin blends using a thin film model. Solid state NMR spectroscopy showed that the choice of the anti-solvent influenced the proportion of cellulose II versus amorphous regions in the coagulated films. Independent of the presence of lignin, the choice of anti-solvent seems to impact the rate of thermal reactions. After thermostabilization at 245 °C, the samples were investigated using a solution NMR protocol devised for cellulosic materials. At 275 °C, most of the samples became insoluble for solution NMR. However, solid state NMR revealed further changes in the chemical composition, which were dependent on both the presence of lignin and the choice of anti-solvent. This multi-faceted approach combining solid state and 2D solution NMR techniques provides a comprehensive understanding of the cellulose structure and the products formed for cellulose-lignin-based CFs, which is crucial for optimizing their properties and potential applications.

Carbon fibers (CFs) are fabricated by blending hardwood kraft lignin (HKL) and cellulose. Various compositions of HKL and cellulose in blended solutions are air-gap spun in 1-ethyl-3-methylimidazolium acetate (EMIM OAc), resulting in the production of virtually bead-free quality fibers. The synthesized HKL-cellulose fibers are thermostabilized and carbonized to achieve CFs, and consequently their electrical and mechanical properties are evaluated. Remarkably, fibers with the highest lignin content (65%) exhibited an electrical conductivity of approximately 42 S/cm, surpassing that of cellulose (approximately 15 S/cm). Moreover, the same fibers demonstrated significantly improved tensile strength (∼312 MPa), showcasing a 5-fold increase compared to pure cellulose while maintaining lower stiffness. Comprehensive analyses, including Auger electron spectroscopy and wide-angle X-ray scattering, show a heterogeneous skin-core morphology in the fibers revealing a higher degree of preferred orientation of carbon components in the skin compared to the core. The incorporation of lignin in CFs leads to increased graphitization, enhanced tensile strength, and a unique skin-core structure, where the skin’s graphitized cellulose and lignin contribute stiffness, while the predominantly lignin-rich core enhances carbon content, electrical conductivity, and strength.

This article elucidates the dissolution of cellulose from different raw materials in NaOH aqueous solution via the combination of synchrotron-radiation-based SAXS/WAXS characterization. The X-ray measurements probed the mesostructure of the cellulose samples during the freeze-thawing cycle allowing tracking the initial swelling of the structure, the kinetics of disintegration of the cellulose crystallites as well as controlling the final state of the cellulose solution, i.e. presence or absence of cellulose aggregates. The individual SAXS and WAXS measurements were fitted and modelled to enable visualisation and tracking of the changes in the structure in relation to temperature during cooling and warming phases. To further increase the understanding of the parameters affecting dissolution different cellulose samples and solution compositions were considered. For this purpose the effect of increasing the concentration of NaOH and adding Zn2+ has been carefully investigated as well as the importance of the cellulose origin. We found consistent development that the dissolution occurs faster at higher concentrations of NaOH and with Zn2+ regardless the origin. Nevertheless, SAXS data show that materials with a larger amount of cellulose I show more apparent swelling in mesoscopic structure than bleached agricultural containing cellulose II. Despite few crystalline residues after the complete cooling-heating cycle shown by WAXS, some cellulose was not completely dissolved as some network structure remained in the samples under the test condition as suggested by SAXS. © The Author(s) 2024.

There are limited methods available for measurement of the porosity of cellulose fibers, even more so for obtaining a pore size distribution. Conventional pore analysis methods require dry samples, with intact pores. However, pores in cellulose fibers collapse when dried from water and thus present a challenge for sample analysis. Furthermore, the pore collapse is partially irreversible which should be accounted for in the analysis. In this study, analysis of pore structure was carried out in the wet state with thermoporometry and also for critical point dried samples, analyzed with N2 sorption. This study determines the effect of fiber lignin content and certain spinning parameters on the pore size distribution of spun fibers before and after drying. It could also be concluded that solvent exchange, drying from a non-polar solvent will result in an altered pore size distribution, with a total pore volume greater than if dried from water, however not representative of the never-dried state. It is concluded that thermoporometry together with the water retention value (WRV) measurement is a powerful combination to acquire insights to the pore size distribution of spun fiber. 

The potential for using discarded viscose textiles to produce high-quality viscose fibres is limited by the low molecular weight of the cellulose and its continued reduction in the recycling process. Herein, we present a straightforward approach of reprocessing discarded viscose textiles while achieving high-quality recycled viscose fibres. Discarded viscose textile was defibrated and centrifuged, and the resulting fibres were reprocessed under industrially relevant conditions. The produced viscose dope was fluid and resulted in viscose fibres with properties comparable to fibres made from commercial wood cellulose pulp (titer 2 dtex; dry elongation 16%, dry tenacity 15 cN/tex). To explore the potential for a more environmentally friendly production process, the steeping step was performed twice (double-steeping), thereby producing a more homogeneous viscose dope. Through double-steeping, the consumption of carbon disulfide (CS2) could be reduced by 30.5%. The double-steeping method shows to be a suitable approach to reprocess discarded viscose textiles while reducing the environmental impact of the viscose process associated with the use of CS2. Our work demonstrates that discarded viscose textile has the potential to be part of a circular textile value chain. 

Adhesion of fibers within a spun tow, including carbon fibers and precursors, is undesirable as it may interrupt the manufacturing process and entail inferior fiber properties. In this work, softwood kraft lignin was used together with a dissolving pulp to spin carbon fiber precursors. Lignin-cellulose precursors have previously been found to be prone to fiber fusion, both post-spinning and during carbon fiber conversion. In this study, the efficiency of applying different kinds of spin finishes, with respect to rendering separable precursors and carbon fibers, has been investigated. It was found that applying a cationic surfactant, and to a similar extent a nonionic surfactant, resulted in well separated lignin-cellulose precursor tows. Furthermore, the fiber separability after carbon fiber conversion was evaluated, and notably, precursors treated with a silicone-based spin finish generated the most well-separated carbon fibers. The underlying mechanism of fiber fusion post-spinning and converted carbon fibers is discussed. 

Regenerated cellulose fibers have been considered as potential precursor fibers for carbon fibers because of their balanced cost and performance. Increased attention has been paid to blending lignin with the regenerated cellulose to generate precursor fibers which render good mechanical properties and higher carbon yield. The mechanical properties of carbon fibers have been found closely correlated to the structure of precursor fibers. However, the effects of lignin blending on molecular- and morphological structure of the precursor are still unclear. This study aims at clarifying the structural information of lignin–cellulose precursor fibers from molecular level to mesoscale by scanning X-ray microdiffraction. We present the existence of a skin–core morphology for all the precursor fibers. Increase of lignin content in precursor fiber could reduce the portion of skin and cause obvious disorder of the meso- and molecular structure. By correlating structural variations with lignin blending, 30% lignin blending has been found as a potential balance point to obtain precursor fibers maintaining structural order together with high yield rate. 

Material recycling requires solutions that are technically, as well as economically and ecologically, viable. In this work, the technical feasibility to separate textile blends of viscose and polyester using alkaline hydrolysis is demonstrated. Polyester is depolymerized into the monomer terephthalic acid at high yields, while viscose is recovered in a polymeric form. After the alkaline treatment, the intrinsic viscosity of cellulose is decreased by up to 35%, which means it may not be suitable for conventional fiber-to-fiber recycling; however, it might be attractive in other technologies, such as emerging fiber processes, or as raw material for sugar platforms. Further, we present an upscaled industrial process layout, which is used to pinpoint the areas of the proposed process that require further optimization. The NaOH economy is identified as the key to an economically viable process, and several recommendations are given to decrease the consumption of NaOH. To further enhance the ecological end economic feasibility of the process, an increased hydrolysis rate and integration with a pulp mill are suggested.

Chemical recycling of textiles holds the potential to yield materials of equal quality and value as products from virgin feedstock. Selective depolymerization of textile polyester (PET) from regenerated cellulose/PET blends, by means of alkaline hydrolysis, renders the monomers of PET while cellulose remains in fiber form. Here, we present the mechanism and reactivity of textile PET during alkaline hydrolysis. Part I of this article series focuses on the cellulose part and a possible industrialization of such a process. The kinetics and reaction mechanism for alkaline hydrolysis of polyester packaging materials or virgin bulk polyester are well described in the scientific literature; however, information on depolymerization of PET from textiles is sparse. We find that the reaction rate of hydrolysis is not affected by disintegrating the fabric to increase its surface area. We ascribe this to the yarn structure, where texturing and a low density assures a high accessibility even without disintegration. The reaction, similar to bulk polyester, is shown to be surface specific and proceeds via endwise peeling. Finally, we show that the reaction product terephthalic acid is pure and obtained in high yields. © 2022 by the authors. 

The demand for carbon fibers (CFs) based on renewable raw materials as the reinforcing fiber in composites for lightweight applications is growing. Lignin-cellulose precursor fibers (PFs) are a promising alternative, but so far, there is limited knowledge of how to continuously convert these PFs under industrial-like conditions into CFs. Continuous conversion is vital for the industrial production of CFs. In this work, we have compared the continuous conversion of lignin-cellulose PFs (50 wt % softwood kraft lignin and 50 wt % dissolving-grade kraft pulp) with batchwise conversion. The PFs were successfully stabilized and carbonized continuously over a total time of 1.0-1.5 h, comparable to the industrial production of CFs from polyacrylonitrile. CFs derived continuously at 1000 °C with a relative stretch of-10% (fiber contraction) had a conversion yield of 29 wt %, a diameter of 12-15 μm, a Young's modulus of 46-51 GPa, and a tensile strength of 710-920 MPa. In comparison, CFs obtained at 1000 °C via batchwise conversion (12-15 μm diameter) with a relative stretch of 0% and a conversion time of 7 h (due to the low heating and cooling rates) had a higher conversion yield of 34 wt %, a higher Young's modulus (63-67 GPa) but a similar tensile strength (800-920 MPa). This suggests that the Young's modulus can be improved by the optimization of the fiber tension, residence time, and temperature profile during continuous conversion, while a higher tensile strength can be achieved by reducing the fiber diameter as it minimizes the risk of critical defects. © 2022 The Authors.