Carbon Fiber from Pitch

Carbon fiber fabrication from pitch generally involves the following four steps  

Carbon fibers made from the spinning of molten pitches are of interest because of the carbon yield approaching 99% and the relative low cost of the starting materials. 

The formation of melt-blown pitch webs is followed by stabilization in air and carbonization in nitrogen. Processes have been developed with isotropic pitches and with anisotropic mesophase pitches. The mesophase pitch-based and melt-blown discontinuous carbon fibers have a structure comprised of a large number of small domains, each domain having an average equivalent diameter from 0.03 to 1 mm, and a nearly unidirectional orientation of folded carbon layers assembled to form a mosaic structure on the cross section of the carbon fibers. The folded carbon layers of each domain are oriented at an angle to the direction of the folded carbon layers of the neighboring domains on the boundary. 

Carbon fibers from isotropic pitch Isotropic pitch or a pitch-like material, such as molten polyvinyl chloride, is melt spun at high strain rates to align the molecules parallel to the fiber axis. The thermoplastic fiber is then rapidly cooled and carefully oxidized at a low temperature ( 100 °C). The oxidation process is rather slow, so as to ensure stabilization of the fiber by cross-linking to make it infusible. However, upon carbonization, relaxation of the molecules takes place, producing fibers with no significant preferred orientation. This process is not industrially attractive due to the lengthy oxidation step, and because only low-quality carbon fibers with no graphitization are produced. These fibers are used as fillers in various plastics to form thermal insulation materials. 

Carbon fibers from anisotropic mesophase pitch High molecular weight aromatic pitches that are mainly anisotropic in nature are referred to as mesophase pitches. 

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Otani, S., Kokubo, Y. and Koitabashi, T., The preparation of highly-oriented carbon fiber from pitch material. Bull Chem Soc Japan, 1970, 43(10), 3291 3292. Otani, S., Watanabe, S., Ogino, H., ligima, K. and Koitabashi, T., High modulus carbon fibers from pitch materials," Bull Chem Soc Japan, 1972, 45(12), 3710 3714. 

In order to make high-modulus carbon fibers from pitch, petroleum- or coal-tar pitch is first filtered to remove solids which inhibit the growth of mesophase. This is followed by heat treatment, which results in a mesophase content of 70 to 95% then the mesophase pitch is spun into monofilaments with a fiber diameter of around 5 pm. Optimum viscosity and minor gas liberation of the pitch are of fundamental importance. The pitch monofilament is oxidized in the second stage to make it infusible. Oxygen is used as the oxidation agent. Unlike in the production of PAN fibers, it is not necessary to stretch the fibers before oxidation. 

In the early 1960s, polyacrylonitrile (PAN) fibers afforded a total carbon yield after pyrolysis that was higher, and high strength carbon fibers were obtained by stretching PAN fibers in steam and oxidizing them under stress before carbonization. Carbon fibers from pitch precursors are a more recent development. Pitches are low value residues of the petroleum industry. 

Singer LS, The mesophase and high modulus carbon fibers from pitch. Carbon, 16, 408, 1978. 

When producing carbon fibers from pitch, a critical processing parameter is the viscosity of the pitch, which is extremely dependent on the spinning temperature. If pitch was a truly Newtonian fluid, the viscosity would be independent of shear rate, attaining its value almost instantaneously. It would be expected that the ratio of the hot filament diameter to the orifice diameter (die swell ratio) would be less than 1.1 and the molten fluid would not climb the stirring rod (the so called Weissenberg effect) [228]. 

Barr JB, Chwastiak S, Didchenko R, Lewis IC, Singer LS, High modulus carbon fibers from pitch precursor, Appl Polym Symp, Wiley, New York, 29, 161, 1976. 

Amoco Performance Products Inc., Greenville, South Carolina, USA—acquired Union Carbide s carbon fiber production at Greenville, where a range of carbon fibers from pitch, cellulosic and PAN precursors were made under the trade name of Thornel. PAN precursor is also produced there. In the early 1990s, also acquired BASF s carbon fiber production. BASF, in 1985, had acquired the Celion carbon fiber production from Celanese at Rockhill, North Carolina. Meltspun PAN was also produced at Rockhill, but this was discontinued in favour of using precursor from Toho Rayon. Became BP Amoco and was purchased by Cytec Industries Inc. 

Osaka Gas Co. Ltd., Osaka, Japan—plant was a joint venture with Dainippon Ink and completed in 1984. Produce Donacarbo carbon fiber from pitch. 

Singer, "High Modulus Carbon Fibers from Pitch Precursor," J. Appl. Polym. 

Elongational behavior is induced in the entrance of the spinning hole and in the transition region from backhole to actual capillary. In practice hardly any permanent orientation is built up in this way, however, because molecular relaxation is rapid. Spinning hole profiles are smoothened only to prevent the formation of vortices which would lead to extrudate distortion. Promoting orientation already in the spinning holes is not common for melt spinning. It could be beneficial for the orientation of melt-spun liquid-crystalline polymers, however, for example in the production of carbon fiber from pitch. 

In the years following the Brooks and Taylor discovery, many researchers attempted to produce a mesophase pitch suitable for carbon fiber production. Otani et al. [21] were first to report producing a high-modulus carbon fiber from a "specific pitch-like material." The precursor used was tetrabenzophenazine, and thus, the resulting material might be considered a synthetic pitch. 

Fig. 7. Processing of carbon fibers from mesophase pitch.

Liu, G. Z., McHugh, J. J., Edie, D. D. and Thies, M. C., Processing carbon fibers from three sources of mesophase pitch. In Carbon 92 Proceedings of International Carbon Conference, Essen, Germany, 1992, pp. 795 797. 

Hamada, T., Nishida, T., Sajiki, Y. and Matsumoto, M., Structures and physical properties of carbon fibers from coal tar mesophase pitch, J Mat Res, 1987, 2(6), 850 857. 

A wide variety of carbon materials has been used in this study, including multi-wall carbon nanotubes (sample MWNT) chemically activated multi-wall carbon nanotubes (sample A-MWNT)16, commercially available vapor grown carbon nanofibers (sample NF) sample NF after chemical activation with K.OH (sample A-NF) commercially pitch-based carbon fiber from Kureha Company (sample CF) commercially available activated carbons AX-21 from Anderson Carbon Co., Maxsorb from Kansai Coke and Chemicals and commercial activated carbon fibers from Osaka Gas Co. (A20) a series of activated carbons prepared from a Spanish anthracite (samples named K.UA) and Subituminous coal (Samples H) by chemical activation with KOH as described by D. Lozano-Castello et al.17 18 activated carbon monoliths (ACM) prepared from different starting powder activated carbons by using a proprietry polymeric binder from Waterlink Sutcliffe Carbons, following the experimental process described in the previous paper13. 

Schulz D. A. Method of making carbon cloth from pitch based fiber. US Patent 4014725 (1977). 

In order to produce carbon fibers from polyacrylonitrile) (PAN) and various pitches, stabilization is essential after the spinning, which consists of a chemical reaction using different oxidizing gases, such as air, oxygen, chlorine, hydrochloric acid vapor, etc. [91]. The stabilized fibers are then... 

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