The fractionation, carbonisation and characterisation of electro-spun lignin fibres

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Khan, Inam Ur Rahman ORCID: https://orcid.org/0000-0001-8836-1465 (2020). The fractionation, carbonisation and characterisation of electro-spun lignin fibres. University of Birmingham. Ph.D.

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Abstract

Polyacrylonitrile (PAN) continues to be the primary precursor that is used for the production of carbon fibres. However, PAN is derived from petroleum and it is not a sustainable precursor in the long-term. This current research was focused on identifying bio-based precursors that could be used as an alternative for the production of fibrous preforms that could subsequently be carbonised.

In the first instance, lignin was purchased from Domtar PC sales centre (Germany) and it is referred to as softwood Kraft lignin (BioChoice®) in the thesis. Lignin was chosen because of its high aromatic content, availability and it is known to cross-link. The as-received lignin was characterised using a range of analytical techniques. The analyses indicated the presence of impurities such as small concentrations of carbohydrates and inorganics. The carbohydrates and inorganic content in the BioChoice® lignin were found to be 4.4% and 1.3%.

Two methods were considered to reduce the carbohydrate and inorganic contents in the as-received and these were acid-washing and solvent fractionation. The acid-washing was carried out using nitric acid. The solvent fractionation of as-received lignin was carried out using acetone; this was extended to include ethanol and propanol. The inorganic content in the BioChoice® lignin was reduced from 1.3% to 0.1% after solvent fractionation in acetone. However, the inorganic content after acid-washing was 0.34%. Solvent fractionation was found to be simpler and time-efficient for removing the inorganic content in lignin when compared with acid-washing and hence, it was adopted as a preferred method to purify the lignin prior to electro-spinning.

The majority of the initial electro-spinning experiments were undertaken using acetone-fractionated lignin using a binary solvent of acetone and dimethyl sulfoxide but the reproducibility was not consistent. The majority of the studies in the literature with regard to producing fibres with lignin have involved the use of synthetic polymer blends. For example, polyvinyl alcohol, polyvinyl acetate, polyethylene oxide and PAN. In the current study, a novel approach of electro-spinning with a 100% blend of acetone-soluble and ethanol-soluble lignin was demonstrated for the first time. These fractions were electro-spun using a mixture (2:1) of acetone and dimethyl sulfoxide. The electro-spun fibres inspected in this study were found to be void-free with a relatively uniform and circular cross-section. From the numerous micrographs that were inspected, there was no evidence to suggest the presence of fused fibres.

The above-mentioned electro-spun fibres had a random orientation, as is the case with conventional electro-spinning using a ground conductor. In this study, a new ground electrode made from graphite was designed to enable the electro-spinning of aligned fibres. A series of detailed experiments were undertaken to derive the heat-treatment regime for the as-spun fibres. This was necessary to prevent fibre fusion during subsequent oxidative and carbonisation treatments.

The optimum heat treatment for the as-spun 100% lignin fibres was found to be drying at 140 °C in a vacuum oven for 6 hours prior to thermo-stabilisation at 250 °C in air and carbonisation in nitrogen. The sp2 graphitic content in the carbonised fibres, as inferred by Raman spectroscopy, was found to be in the order 1000 °C < 1200 °C < 1500 °C.

The BioChoice® lignin was obtained from a commercial source and the decision was made to extract lignin from coir. Coir was chosen because it has the highest lignin content of all plant matter. The procedures and protocols for the extraction were developed and the lignin was characterised as mentioned previously. The lignin was electro-spun using a binary solvent of acetone and dimethylsulfoxide. It is proposed that coir with its high lignin content (32%) can be used as a renewable and sustainable precursor for the production of preforms for the production of carbon fibres.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):