The production of commercial carbon fibres is expensive and has a negative impact on the environment due to the use of the costly, petroleum-based and toxic polymer polyacrylonitrile (PAN) and harmful, expensive conventional organic solvents. Lignin is an abundant biopolymer present in wood biomass and a promising renewable raw material for carbon materials. Lignin-based carbon fibres are an attractive alternative to PAN fibres if robust and cost-effective manufacturing technologies can be developed.
In this thesis, the development of a wet-spinning technique using a low-cost (<$1/kg) ionic liquid, N,N-dimethylbutylammonium hydrogen sulfate, [DMBA][HSO4] for lignin fibre production is described. First, the dissolution of technical lignins (including ionoSolv lignins and Kraft lignins) and a fibre forming polymer poly(vinyl alcohol) (PVA) in aqueous ionic liquid mixtures was examined, and the spinnability of the solutions was tested. Following these, fibres were produced continuously with exceptionally high lignin solid contents (75 wt% - 90 wt%) for four different lignins, using an environmentally benign aqueous Na2SO4 solution or simply pure water as a coagulant. The lignin-rich fibres were characterised to assess their morphology, mechanical properties, and carbonisation yields, and were subsequently oxidatively stabilized and carbonised, producing lignin-based carbon fibres via a low-cost IL-based approach. The produced carbon fibres had mechanical performance comparable to the previously reported lignin/PVA-derived carbon fibres which were produced from more expensive solvents and precursor fibres with a lower lignin content. This demonstrates the potential of this low-cost and ‘greener’ approach to be applied in the future for lignin fibre production.
This thesis also explores, for the first time, the integration of biomass fractionation and lignin-based carbon fibre production. The lignin-rich extract (liquor) obtained in the biomass fractionation was transformed directly into a fibre spinning dope by adding the PVA solution, producing high lignin content fibres which were later converted to carbon fibres. By this preliminary, not yet optimised, integrated approach, the produced carbon fibres outperformed the fibres produced from the isolated lignin dissolved in the same solvent. This shows a clear potential to reduce the cost of lignin-based carbon fibres by avoiding the costly lignin precipitation, drying and redissolution step, while increasing revenues of ionic liquid-based biorefineries through direct production of a value-added product.