Polymer composites designed for high-performance applications are mostly based on carbon fibre reinforcement, produced from fossil originating raw materials such as poly(acrylonitrile) and pitch (petroleum- or coal-based). As of today, lignin has proved to be one of the most promising alternatives. Lignin is a substance that is found in wood but is removed during kraft pulp production. Thanks to LignoBoost technology*, the pulp mill can extract extremely pure lignin which could be used and thus increasing the access to a raw material for carbon fibre. Previous research has mainly focused on the use of hardwood lignin which is easier to meltspin. However, the presented work by Ylva Nordström states that precursor fibres also can be produced from kraft lignin from softwood, which is the most common industrial wood raw material in the northern hemisphere, such as the Nordic region and North America.
The aim of this work was to find a way for melt spinning of softwood kraft lignin which was managed by adding small amounts of low-molecular lignin. Both unfractionated hardwood and softwood lignin were used with addition of their fractionated counterparts acting as softening agents. As previously reported in Ida Norberg's PhD thesis, the stabilisation step in the process was also further developed as a first step towards a process optimised for softwood kraft lignin based carbon fibres. In Ylva Nordström's work, mechanical characterisation of the final carbon fibres was carried out. The experimental results and predictions showed a good fit, although the strength of the produced fibres is still significantly lower than that of commercially available carbon fibres
"This thesis reports the first mechanical characterisation of softwood kraft lignin based CFs. This will hopefully enable further process optimisation", says Ylva Nordström.
Ylva's licentiate work was carried out within the framework of the LigniCarb/LigniCarb Add research project, with the aim of examining and demonstrating the possibilities of making carbon fibre from kraft lignin. The work constitutes a part of Innventia's new field of research on production processes for lignin-based carbon fibres. "There's a lot to be gained from better equipment. Thanks to the recent promising findings in this field, investment in larger scale, more advanced equipment is now being planned," adds Elisabeth Sjöholm, who was Ylva's supervisor at Innventia and the project manager for LigniCarb.
* LignoBoost has been developed in partnership between Innventia and Chalmers University of Technology. The process has been successfully demonstrated at Innventia's demonstration facility in Bäckhammar. The LignoBoost process, including patent rights, has been owned by Metso since 2008.