Carbon Fibers From Athabasca Oil-Sand Asphaltenes

Carbon fibers possess ultra-high specific strength and modulus. Therefore, the material is popular in aerospace, automotive, and sports industries. Approximately 95% of all carbon fibers are spun from a high-cost precursor, polyacrylonitrile (PAN). There is a need for a cheaper grade of carbon fiber for low-cost applications because about half of carbon fiber cost is associated with precursor cost. A promising alternative to the high-cost PAN precursor is petroleum-based asphaltene. Asphaltenes are highly aromatic and have a high carbon content but lack the purity of the PAN precursor. Processes like the molten- sodium desulfurization upgrading have been used to harvest impurities such as sulfur and produce a cleaner asphaltene. The goal of this study was to utilize such refined Athabasca oil-sand-asphaltenes to produce precursor fibers and convert them into carbon fibers. Various such grades were investigated to determine the effect of impurities and thermal treatment on the processing and properties of carbon fibers.\nThe samples used for this study were provided by Alberta Innovates as a part of the Carbon Fiber Grand Challenge (CFGC). Two types of samples were studied: (i) asphaltenes removed as vacuum tower bottoms (VTB) from the oil-sand refining process and subsequently desulfurized, and (ii) VTBs that were also heat-treated (HT-VTB) by a proprietary process that too were desulfurized. These precursors had sulfur contents ranging between 0.3 and 7.0 wt%.\nThe asphaltenes were characterized by using softening point and thermogravimetric analysis to determine the volatile content and melting characteristics of the asphaltenes. Several of these asphaltenes grades could be melt-spun into precursor fibers that werethermo-oxidatively stabilized and successfully carbonized at 1000°C, 1500°C or 2000°C. The microstructure was analyzed with Raman microscopy, EDX and SEM. The tensile strength and electrical resistivity were also measured for various fibers produced using different carbonization temperatures for different impurity contents.\nThe results showed that for untreated VTB group, a small increase of softening point was observed, 188°C to 203°C, when samples were desulfurized and refined (sulfur content reduced from 1.7wt% to 0.3wt%). In contrast, the heat-treated vacuum-tower- bottoms (HT-VTB) asphaltenes without sulfur removal had the highest softening point of 280°C (sulfur concentration of 7 wt%). When subjected to the desulfurization refining step, the softening point decreased significantly to 208°C (about 1.4 wt% S). However, due to its high softening point, HS-VTB could be thermo-oxidatively stabilized in about 24 hours at a high temperature and fast rate. In contrast, untreated VTBs had a lower softening temperature and needed 72 hours to be adequately stabilized as a slower rate had to be used. Because the softening point of untreated VTBs was not significantly different, no trend between sulfur concentration and stabilization time could be inferred.\nThe VTB asphaltenes showed a char yield of approximately 50%, whereas the HT- VTB asphaltenes had char yields between 55-65%. For both types of asphaltenes, the graphitic content moderately increased with carbonization temperature. The disordered to graphitic carbon ratio (ID/IG) ratio from Raman spectroscopy was measured at 1.2 for carbonization at 2000°C. The EDX analysis of the VTB and HS-VTB asphaltenes showed a reduction in sulfur concentration when carbonization temperature was increased from 1000 to 1500°C and further from 1500 to 2000°C. For all carbon fibers obtained at 2000°C, he sulfur concentration was undetectable\nThe HS-VTB asphaltene-based fibers had the highest tensile strength of 1.1 GPa when carbonized at 1000°C. The strength reduced to 0.3 GPa for a carbonization temperature of 2000°C. The tensile strength for all asphaltene-based fibers decreased with increasing carbonization temperature but was largely unaffected by the sulfur concentration. Electrical resistivity decreased from 60 to 40 µΩ-m as carbonization temperature was increased from 1000°C to 2000°C. This corresponds to an increase in electrical conductivity, which is consistent with increasing graphitic content inferred from Raman spectroscopy.\nIn summary, several Athabasca oil-sand-asphaltenes were successfully melt spun, stabilized, and carbonized into carbon fibers. It was found that fibers obtained from heat- treated asphaltenes (HT-VTBs) stabilize faster and have higher tensile strength than those from untreated VTB asphaltenes. It was found that 1000°C was the optimal carbonization temperature for asphaltene-based carbon fibers with tensile strength reaching 1 GPa. Also, these asphaltene-based carbon fibers are electrically conducting (in contrast to insulating glass fibers), which makes them useful for applications where electrostatic dissipation and electromagnetic shielding is necessary.

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