Thermal Conversion of Cellulose-Lignin Precursors into Carbon Fibres

Abstract: Carbon fibres are used in many applications thanks to their exceptional tensile properties in relation to their relatively low weight. These thin strands of at least 90% carbon are mixed with a matrix such as epoxy to form composites used for cars, wind turbines, prosthetics, and many other things. Carbon fibres are often manufactured from polyacrylonitrile (PAN) based on fossil oil, an expensive and non-renewable resource. Developing carbon fibres from renewable resources, like products from the forest industry such as lignin and cellulose could reduce the environmental impact of carbon fibre production. Cellulose precursors have long been used for carbon fibre production, but adding lignin could potentially increase the yield and improve the tensile properties of the carbon fibres. The transition from PAN carbon fibres to lignin-cellulose carbon fibres might also lower the cost. Currently, the properties of cellulose- and lignin-based carbon fibres are poor compared to other fibres on the market. Research like this work focuses on improving the tensile properties of the carbon fibres to make them viable alternatives. The focus of this work was to further develop the three main parts of thermal conversion: stabilisation, low-temperature carbonisation and high-temperature carbonisation, by evaluating the impact of temperature and tension on the tensile properties of the fibre. The fibres were converted using a continuous setup for each process step. The stabilisation profiles tested ranged between 245-260 ⁰C, low-temperature carbonisation between 460-600 ⁰C and high- temperature carbonisation between 1100-1600 ⁰C. Batch conversion was conducted to compare conversion methods. The results showed that the temperature of the low-temperature carbonisation had a large impact on the ability to stretch the carbon fibre and increase the tensile properties with 600 ⁰C being the optimal temperature out of the temperatures tested on the setup used in this thesis. The stretching during the high-temperature carbonisation and the temperature of the stabilisation had less impact on tensile properties. The properties of the precursor fibre are not always indicative of the properties of the carbon fibre, but the methods used for creating the precursor fibres have a great impact on the behaviour during thermal conversion.