Carbon Fibres from Kraft Lignin
Abstract: Kraft lignin has a high potential for use in more valuable applications than its current use as fuel in pulp mills and integrated pulp and paper mills. The possibility of using kraft lignin, a green material with a carbon content of more than 60 %, for the manufacturing of carbon fibres was investigated in this thesis. The strong and lightweight carbon fibre material has many potential application areas, e.g. in cars; the main obstacle limiting its demand is the high production cost, with the raw material (petroleum pitch and polyactrylonitrile) and fibre spinning constituting approximately 50 % of the cost.Industrial kraft lignins originating from both softwood (spruce/pine) and hardwood (birch/aspen) were isolated with the LignoBoost technique and then purified and characterized to determine the best suitable lignin for the production of carbon fibre. Using ultrafiltration of the black liquor before isolation using the LignoBoost technique, a kraft lignin with satisfactory high purity was obtained. The fractionated kraft lignin can be used either as such or as a softening agent during melt spinning to obtain continuously spun kraft lignin fibres.The behaviour during thermal treatment was found to differ depending on the type of kraft lignin used. After oxidative stabilisation, the studied lignins became more stable, and thus, the final yield after carbonisation was increased by 10-20 % in comparison to stabilisation in absence of oxygen. The identified products indicate that the main reactions during oxidative stabilisation are radical, oxidation, condensation and rearrangement reactions.The structural differences between softwood and hardwood kraft lignins facilitated the stabilisation of the softwood lignin fibre as compared with the hardwood lignin fibres. Thermal stabilisation in an inert atmosphere using only heat was successfully achieved for the softwood kraft lignin fibres. Stabilisation and carbonisation was successfully performed in a one-step operation on softwood kraft lignin fibres. Thus, it seems possible that the separate stabilisation step can be omitted, which may reduce the processing costs of softwood kraft lignin-based carbon fibres.
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