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Carbon fibers (CFs) have excellent mechanical properties and a low density, making them attractive as a reinforcing fiber in composites. The use of CFs is limited to high-end applications,since they are produced from an expensive fossil-based precursor via an energy-intensivemanufacturing process, explaining the need for cheaper CFs from renewables. CFs can be madefrom strong cellulosic precursors, but the low carbon content of cellulose results in a lowconversion yield, and thus an expensive CF. Lignin has a higher carbon content than cellulose butCFs from melt spun lignin precursors have presented challenges, since these precursors have a lowstrength and are difficult to convert to CF in a realistic conversion time.In the present work, CFs from solution spun precursors consisting of blends of softwood Kraft lignin and cellulose have been developed. The lignin-cellulose precursors (up to 70% lignin) were prepared with air-gap spinning and wet spinning, using an ionic liquid and a water-based solvent system for co-dissolution, respectively. Co-processing of cellulose and lignin was beneficial as theformer made the precursor strong and easy to handle, whereas the latter gave a higher conversionyield than precursors based solely on cellulose. The precursors were converted to CFs via both batchwise and continuous conversion, using industrially relevant times ( less than 2 h), with a yield up to 45 wt% after incorporation of a flame retardant. These CFs have a moderate Young’s modulus and tensile strength up to 75–77 GPa and 1.2 GPa, respectively, i.e. similar to the values for CFs from fossil-based isotropic pitch and they can thus be classified as general-grade CFs. These biobased CFs have a disordered turbostratic graphitestructure, and their tensile properties are affected by the precursor structure, the conversionconditions, and the final diameter. These CFs can potentially be used as a sustainable component in non-structural and semi-structural applications.

Professor Monica Ek, KTH, Dr Maria Sedin, RISE, Docent Elisabet Brännvall, KTH, and Docent Elisabeth Sjöholm, KTH

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