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. 

In recent years, there has been extensive research into the development of cheaper and more sustainable carbon fiber (CF) precursors, and air-gap-spun cellulose-lignin precursors have gained considerable attention where ionic liquids have been used for the co-dissolution of cellulose and lignin. However, ionic liquids are expensive and difficult to recycle. In the present work, an aqueous solvent system, cold alkali, was used to prepare cellulose-lignin CF precursors by wet spinning solutions containing co-dissolved dissolving-grade kraft pulp and softwood kraft lignin. Precursors containing up to 30 wt% lignin were successfully spun using two different coagulation bath compositions, where one of them introduced a flame retardant into the precursor to increase the CF conversion yield. The precursors were converted to CFs via batchwise and continuous conversion. The precursor and conversion conditions had a significant effect on the conversion yield (12–44 wt%), the Young’s modulus (33–77 GPa), and the tensile strength (0.48–1.17 GPa), while the precursor morphology was preserved. Structural characterization of the precursors and CFs showed that a more oriented and crystalline precursor gave a more ordered CF structure with higher tensile properties. The continuous conversion trials highlighted the importance of tension control to increase the mechanical properties of the CFs. © 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. 

Carbon fibers (CFs) are gaining increasing importance in lightweight composites, but their high price and reliance on fossil-based raw materials stress the need for renewable and cost-efficient alternatives. Kraft lignin and cellulose are renewable macromolecules available in high quantities, making them interesting candidates for CF production. Dry-jet wet spun precursor fibers (PFs) from a 70/30 w/w blend of softwood kraft lignin (SKL) and fully bleached softwood kraft pulp (KP) were converted into CFs under fixation. The focus was to investigate the effect of carbonization temperature and time on the CF structure and properties. Reducing the carbonization time from 708 to 24 min had no significant impact on the tensile properties. Increasing the carbonization temperature from 600 to 800 °C resulted in a large increase in the carbon content and tensile properties, suggesting that this is a critical region during carbonization of SKL:KP PFs. The highest Young's modulus (77 GPa) was obtained after carbonization at 1600 °C, explained by the gradual transition from amorphous to nanocrystalline graphite observed by Raman spectroscopy. On the other hand, the highest tensile strength (1050 MPa) was achieved at 1000 °C, a decrease being observed thereafter, which may be explained by an increase in radial heterogeneity.

There is an increasing demand for lightweight composites reinforced with carbon fibers (CFs). Due to its high availability and carbon content, kraft lignin has gained attention as a potential low-cost CF precursor. CFs with promising properties can be made from flexible dry-jet wet spun precursor fibers (PFs) from blends (70:30) of softwood kraft lignin and fully bleached softwood kraft pulp. This study focused on reducing the stabilization time, which is critical in CF manufacturing. The impact of stabilization conditions on chemical structure, yield, and mechanical properties was investigated. It was possible to reduce the oxidative stabilization time of the PFs from about 16 h to less than 2 h, or even omitting the stabilization step, without fusion of fibers. The main reactions involved in the stabilization stage were dehydration and oxidation. The results suggest that the isothermal stabilization at 250 °C override the importance of having a slow heating rate. For CFs with a commercial diameter, stabilization of less than 2 h rendered in tensile modulus 76 GPa and tensile strength 1070 MPa. Impregnation with ammonium dihydrogen phosphate significantly increased the CF yield, from 31-38 to 46-50 wt %, but at the expense of the mechanical properties.

A reference lignin separated from an industrial softwood kraft black liquor via an improved LignoBoost process was compared to four other lignins derived from the same liquor. The four lignins were produced by using a) pH-fractionation within the LignoBoost process, b) ultrafiltration of black liquor prior to the LignoBoost process, and c) solvent leaching of the reference lignin using methanol and d) ethanol.Lignin compositional characteristics and thermal properties were compared, and monofilament extrusion used to assess their potential for successful melt spinning at the 24 filament scale. The lignin prepared by ethanol leaching of the reference lignin was found to be most appropriate for potential pilot scale fibre production. This was owing to a high purity, lower comparative glass transition temperature (Tg), and good spinning performance.Thermal pretreatments of the ethanol leached lignin gave a selection of enhanced lignins which were characterized for comparison, and melt spun on pilot multifilament equipment. The enhanced lignins could be continuously melt spun giving filaments with diameters as low as 10 μm and with minimal defects. Conversion of selected filaments provided carbon fibres with a tensile strength of 1259 ± 159 MPa, tensile modulus of 67 ± 3 GPa and diameter of 7.3 ± 0.5 μm.

In this investigation, a modified Poisson-Boltzmann approach has been used to estimate the mean ion concentration distributions of ionic species present in black liquors around charged kraft lignin macromolecules at industrially relevant conditions. The distributions were utilized to predict the double layer repulsion and, further, predict the overall pair interaction potentials between two kraft lignin macromolecules or particles immersed in the black liquor electrolyte. The properties of softwood black liquors were used as input data to the model. The numerical results predicted the pair interaction potentials to remain repulsive up to salt concentrations of about 2.5 M at pOH 1. The critical coagulation concentration of salt was predicted to decrease as the pOH of the black liquor was increased. The predictions at 65 °C and moderate levels of pOH (3.5) and salt concentration (1.9 M) were found to be in good agreement with previous observations reported in the literature.

To meet the demand for carbon-fibre-reinforced composites in lightweight applications, cost-efficient processing and new raw materials are sought for. Cellulose and kraft lignin are each interesting renewables for this purpose due to their high availability. The molecular order of cellulose is an excellent property, as is the high carbon content of lignin. By co-processing cellulose and lignin, the advantages of these macromolecules are synergistic for producing carbon fibre (CF) of commercial grade in high yields. CFs were prepared from precursor fibres (PFs) made from 70:30 blends of softwood kraft lignin (SW-KL) and cellulose by dry-jet wet spinning with the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate ([EMIm][OAc]) as a solvent. In focus was the impact of the molecular mass of lignin and the type of cellulose source on the CF yield and properties, while membrane-filtrated kraft lignin and cellulose from dissolving kraft pulp and fully bleached paper-grade SW-KP (kraft pulp) served as sources. Under the investigated conditions, the yield increased from around 22% for CF from neat cellulose to about 40% in the presence of lignin, irrespective of the type of SW-KL. The yield increment was also higher relative to the theoretical one for CF made from blends (69%) compared to those made from neat celluloses (48-51%). No difference in the mechanical properties of the produced CF was observed.

The influence of inorganic elements on lignin ‐based carbon fibre (CF) quality was studied using sulphates of Na +, K+, Mg2+, Fe2+, Al3+. The metal sulphates were added to wet spun prefibres made from softwood kraft lignin (SKL):cellulose (70:30) and melt spun prefibres made from low molecular mass SKL. An increase in concentration from 0.1 w% to about 0.4 w% did neither affect the mechanical properties nor the morphology as observed by SEM. In contrast, metal sulphates added to the initial 0.45 w% to a total range 1.5 to 5.0 w%, was found detrimental to the melt spinning and to the final CF quality. Thus, the recommendation of <0.1 w% ash in kraft lignin may be exceeded, but more research is needed to establish the upper concentration limit.

The GreenLight consortium is working to demonstrate a biobased, renewable and economically viable carbon fibre from lignin. The aim is to provide a basis for commercial production of lignin, lignin filaments, carbon fibre and carbon fibre composites. The most difficult boundary to success in the developing lignin as a precursor for continuous filament carbon fibre has been identified as melt extrusion of lignin. The consortium is working to develop a robust melt spinning platform for use up to the 1,000 filament scale. Methodical studies have been performed to examine lignin separation from differing black liquors derived from both softwood and hardwood and assess their viability in terms of thermal, compositional and structural properties. The move will then be made to pilot scale melt spinning at the 100 filament scale. The characteristics of some kraft lignin fractions obtained from the same Sodra Monsteras softwood kraft black liquor have been studied. The lignins were manufactured in quantities of approximately 10-20kg. Several variations of the LignoBoost process were used to provide lignins with improved melt spinning properties. The lignins were of high purity, each having low carbohydrate, extractives and inorganic contents. All four lignins could be melt spun and converted to carbon fibre.