Several studies have shown that fractionation of technical lignins provides a way to gain starting materials with enhanced properties such as better spinnability, which result in higher-quality carbon fibers. (20,21) Unfortunately, the mechanical properties of these fibers are not satisfying, and the cost of the final products is still highly dependent on the price of PAN. Lignin has also been combined with PAN to gain partially bio-based carbon fibers. (18,19) However, the mechanical properties of fully lignin-based fibers do not meet the standards, which are required from the materials intended for the automotive industry. These lignins comprise 90% of the annually produced technical lignins, and they have been studied as a starting material for carbon fiber production. Kraft lignins are side products of the kraft pulping process that is the principal method for the production of wood pulp today. (15) Moreover, lignin-based carbon fibers have high electrochemical activity and serve as an excellent electrode material in the batteries and supercapacitors needed for the increasing electrification of transportation. (14) The weight reduction of cars is urgently needed to comply with the EU emission target of 95 g CO 2/km effective in 2021. (1,12) Lignin-based carbon fibers could represent more than 50% cheaper option than the current polyacrylonitrile (PAN)-based fibers (13) and could be utilized in various applications with moderate strength requirements such as in light-weight composites for the automotive sector. (11) Especially, the lignin-based carbon fibers have gained a lot of attention because lignins have a high carbon content (approximately 60%), and they are rich in aromatic moieties, which are both desired features when aiming for carbon fibers with good mechanical properties. (4) Lignins have shown great potential as a raw material, for example, in energy storage applications, (5−7) carbon fibers, (8−10) and nanoparticles for drug delivery. The interest of applying lignin in more sophisticated and higher value applications is growing. This work shows how white-rot fungal treatment can be used to modify the structure of lignin to be more favorable for the production of bio-based fiber materials. The stronger incorporation of modified lignin in the precursor fibers was confirmed by composition analysis, thermogravimetry, and mechanical testing. The modified lignin leaked less to the spin bath compared with the unmodified lignin starting material, making the recycling of spin-bath solvents easier. When the modified lignin was mixed together with cellulose, the mixture could be spun into intact precursor fibers by using dry-jet wet spinning. Furthermore, the ratio of remaining aliphatic hydroxyls to phenolic hydroxyls was increased, making the structure more favorable for carbon fiber production. This modification is assumed to be beneficial when aiming for graphite materials such as carbon fibers. Enzymatic radical oxidation reactions were hypothesized to induce condensation of lignin, which increased the amount of aromatic rings connected by carbon–carbon bonds. In this study, we were able to polymerize kraft lignin and reduce the amount of hydroxyl groups by incubating it with the white-rot fungus Obba rivulosa. The kraft lignin’s low molecular weight and too high hydroxyl content hinder its application in bio-based carbon fibers.
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