The thermoplastic development of an Australian coking coal was investigated by linking thermal swelling with changes in molecular weight of its pyrolysis products. Coal thermal swelling was investigated together with volatiles evolution and characterisation of generated volatiles (including volatile tar and light gases). The molecular weight distributions of coal and its solvent extracts were measured by using Laser Desorption/Ionization Time of Flight Mass Spectroscopy (LDI-TOF-MS). Solvent extraction (by acetone and tetrahydrofuran (THF)) was initially used on the raw coal to aid interpretation of thermoswelling by volumetric expansion measurements. The removal of ~2% solvent soluble matter from the raw coal (the mobile phase) reduced its swelling extent during heating by up to 22% (from 86% down to 68% and 64% for acetone-and THF-residues, respectively). Volatile release after solvent treatment remained unaffected. This suggested that the majority of the coal's swelling behaviour could be attributed to the formation of heat-generated liquid matter (the metaplast) during pyrolysis. Broad molecular weight changes were found in the solvent extractable component (metaplastic material extracted by acetone and THF) of the semi-coke. Prior to softening (350C), the extractable components were composed of molecules mainly <500 Da. The upper limit in molecular weight distribution of solvent extracts increased significantly to 1800 Da at the onset of swelling (400-450°C) and decreased back to ~500 Da at the end of swelling (500°C). The spectra showed that the volatile tar and acetone extract (the light solvent extract) consisted of similar repeating structures separated 12-14 Da apart. As the treatment temperature increased, the molecular weight distribution of volatile tar increased in molecular mass approaching that of the acetone extract distribution (~600 Da). THF extract molecular weight distribution was a mixture of 12-14 Da and 24 Da repeating units which only became apparent at molecular weight above 600 Da. The LDI-TOF-MS analysis of the solid coal showed that it contained a distribution of molecular structures centred at 2000 Da and spanning between 500 and 7000 Da. This raw coal spectrum also contained multiple repeating mass lines every 24 Da apart. Overall, these results suggested that the coal consisted of complicated structures which subsequently degraded into smaller fragments capable of forming a complex intermediate liquid phase and a distribution of lighter volatile tar species.
A fundamental study was undertaken to characterize the changes in solvent-extracted matter formed during the thermoplastic phase of coking. Coal samples were heated to fixed temperatures within the pyrolytic plastic range of 400−500 °C, and the volatile material was extracted in a two-stage extraction with acetone (light extract) and then tetrahydrofuran (THF heavy extract). The extracted material was analyzed using laser desorption ionization (time-of flight) mass spectrometry (LDI-TOF-MS). The LDI-TOF-MS results showed that three extracted fractions could be broadly classified here as overlapping molecular weight ranges as volatile tars (200−450 Da), light acetone-soluble extract (250−500 Da), and heavy THF-soluble extract (300−1200 Da). A further class of compounds was identified from THF extraction of the raw coal in the range of 600− 2500 Da that required higher laser powers to ionize and was not observed in the thermally generated samples. Negligible changes were observed in the composition of the acetone-soluble extracts with temperature, while the THF-soluble extract showed smaller proportions of larger molecules with higher treatment temperatures. It was observed that each molecular weight spectrum showed repeating structural units forming peaks every 12−14 Da (homologous series), with distributions of species around each peak. The volatile tar and acetone-soluble material shared common repeating structures also evident in the raw coal extract. This suggested that the material in this fraction was thermally stable over the analyzed temperature range. The repeating features of the THF-soluble extract species appeared to be structurally different. Overall, this work has indicated that development of extractable matter expected to be associated with fluidity during coking and subsequent resolidification relies on <1000 Da compounds. The results showed that >600 Da compounds are thermally sensitive. Compounds with molecular weights of <450 Da may be removed during coking, possibly as a vapor, resulting in a reduction in fluidity. There has been speculation that the thermally stable (acetone-soluble) material identified in both raw coal extract and those from thermally treated samples may be capable of undergoing a phase change to initiate plastic deformation.
Preparation of high-value pitch-based carbon fibres (CFs) from mesophase pitch precursor is of great importance towards low-cost CFs. Herein, we developed a method to reduce the cost of CFs precursor through incorporating high loading of coal tar pitch (CTP) into polyacrylonitrile (PAN) polymer solution. The CTP with a loading of 25% and 50% was blended with PAN and their spinnability was examined by electrospinning process. The effect of CTP on thermal stabilization and carbonisation of PAN fibres was investigated by thermal analyses methods. Moreover, electrospun PAN/CTP fibres were carbonised at two different temperatures i.e., 850 °C and 1200 °C and their crystallographic structures of resulting such low-cost PAN/CTP CFs were studied through X-ray diffraction (XRD) and Raman analyses. Compared to pure PAN CFs, the electrical resistivity of PAN/25% CTP CFs significantly decreased by 92%, reaching 1.6 kΩ/sq. The overall results showed that PAN precursor containing 25% CTP resulted in balanced properties in terms of spinnability, thermal and structural properties. It is believed that CTP has a great potential to be used as an additive for PAN precursor and will pave the way for cost-reduced and high-performance CFs.
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