Abstract (summary)
This research explores the utilisation of softwood Kraft lignin as an alternative precursor for the production of fibrous preforms for the subsequent production of carbonised fibres. Currently, only 1-2% of lignin is used for low valued-added applications and the rest is burned as a fuel for energy generation. Electro-spinning was used to transform lignin solutions into fibrous preforms. Acetone-soluble softwood Kraft lignin was electro-spun successfully using a non-toxic binary solvent and without the use of polymer blends. Prior to electro-spinning, the impurities from lignin were removed to aid electro-spinning and carbonisation. Initially, the as-received lignin was characterised, including determining the inorganic content, lignin content and the molecular weight distribution. The inorganic content in the as-received lignin was 1.195 ± 0.021% and the two major elements present in the ash were sulphur and sodium. As-received lignin was purified using the following methods: (i) treatment with acidified water; and (ii) fractionation using acetone. The experimental matrix for the treatment of lignin with acidified water was optimised using Taguchi analysis. The inorganic content of as-received lignin was reduced to 0.354 ± 0.020% after treatment with acidified water. Whereas the acetone-soluble lignin obtained from fractionation with acetone had an an ash content of 0.055 ± 0.021%. Hence, due to the lower inorganic content in the acetone-soluble lignin fraction, it was used for fibre formation studies using electro-spinning. A novel approach for electro-spinning of acetone-soluble softwood Kraft lignin without polymer blending was carried out. The acetone-soluble lignin was dissolved in a 2/1 (v/v) mixture of acetone and dimethyl sulfoxide. Bead-free and electro-spun lignin fibres with a circular cross-section were produced when a concentration of the acetone-soluble lignin was 53 wt%. The average fibre diameter was 1.16 ± 0.21 µm. Subsequent to electro-spinning, procedures for (i) removing the residual solvent, (ii) thermo-stabilisation and (iii) carbonisation were undertaken to enable the production of carbonised lignin fibres. In each procedure, properties of the resultant fibres were characterised using range of analytical techniques. Randomly orientated and short-lengths of axially-aligned carbonised fibres from 100% acetone-soluble lignin were demonstrated for the first time.