Mesophase pitch carbon fibers from

Carbon fibers from mesophase pitch have high modulus and medium strength. They are presently more costly than PAN-based fibers ( 65/kg in 1992). 

Precursor Pitch. The precursor material is a mesophase pitch, characterized by a high percentage of asphaltene. Table 8.5 shows the approximate composition of three common mesophase compounds. 

Processing. The processing of mesophase-pitch fibers is similar to that of PAN fibers, except that the costiy stretching step during heat-treatment is not necessary, making the process potentially less expen-sive.[ l[ The processing steps can be summarized as follows and are represented schematically in Fig. 8.11  

Polymerization of the isotropic pitch to produce mesophase pitch. 

Spinning the mesophase pitch to obtain a green fiber . 

A relatively new class of high-performance carbon fibers is melt-spun from mesophase pitch, a discotic nematic liquid crystalline material. This variety of carbon fibers is unique in that it can develop extended graphitic crystallinity during carbonization, in contrast to current carbon fibers produced from PAN. 

The mesophase pitches used for high-modulus carbon fiber production can be formed either by the thermal polymerization of petroleum- or coal tar-based 

pitches and oils are classified into four general fractions saturates, 

It has been established that, when mesophase pitch is carbonized, the morphology of the pitch is the primary factor [20] in determining the microstructure of the resulting graphitic material. This may be attributed to the stacking behavior of mesophase molecules (quite similar to the planar stacking in turbostratic graphite), which may be visualized as shown in Fig. 5. 

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. 

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

Regarding the intensity, the higher value corresponds to the intermediate component, ti, which represents approximately the 90% of the total intensity. This agrees with the results obtained in previous studies carried out with porous carbons [12] and carbon fibers from mesophase pitch [11]. In the first study [12] only the intermediate component (ti) was found from the lifetime spectrum. These results indicate that, in carbon materials with high surface area, most of the positron annihilation takes place on the surface of the porosity. In the second case [11], i.e., PALS in carbon fibers, two components in the lifetime spectrum were found. The first component with high intensity (97%) and lifetime of 367 ps was attributed to positron annihilation in pores. The second one with a lifetime of 1130 ps corresponds to the annihilation of positronium atoms (i e., o-Ps). 

Didchenko R, Barr JB, Chwastiak S, Lewis IC, Lewis RT, Singer LS, High modulus carbon fibers from mesophase pitches. Extended Abstr 12 Biennial Conf on Carbon, 329, 1975. 

Lewis IC, McHenry ER, Singer LS, Process for producing carbon fibers from mesophase pitch, U.S. Pat., 3,976,729, 1976. 

Other companies which have developed carbon fibers from mesophase pitch include Exxon Enterprises (who sold their carbon fiber technology to DuPont) and many Japanese companies. The Exxon work was based on extensive research at Rennselaer Polytechnic institute by Riggs and Diefendorf [32]. No commercial product, other than Union Carbide s, has appeared on the market except in sample quantities. However, based on technology developed by the Government industrial Research Institute, Kyushu, Japan, fourteen Japanese companies are trying to develop products of similar nature [331. 

The mesophase phenomenon, discovered by James D. Brooks and Geoffrey H. Taylor in 1965, made a decisive contribution to the understanding of the production of high-value carbon products from mesophase pitch, such as premium coke by delayed coking and carbon fibers by spinning. 

Korai Y, Ishida S, Watanabe F, Yoon SH, Wang YG, Mochida I, Kato I, Nakamura T, Sakai Y, Komatsu M, Preparation of carbon fiber from isotropic pitch containing mesophase spheres. 

Edie DD, Fain CC, Melt spun non-circular fibers from mesophase pitch. Carbon 86, Proceedings of the International Conference on Carbon, Baden-Baden, FRG, 629-631, 1986. 

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. 

In the previous symposium, we reviewed mesophase mechanisms involved in the formation of petroleum coke ( 2 ). Since 1975, two significant developments have been the use of hot-stage microscopy to observe the dynamic behavior of the carbonaceous mesophase in its fluid state (3-6), and the emergence of carbon fibers spun from mesophase pitch (7-9) as effective competitors in applications in which high elastic modulus or good graphiticity is important. This paper focuses on mesophase carbon fibers as an example of how the plastic mesophase can be manipulated to produce fibers with intense preferred orientations and elastic moduli that approach the theoretical limit for the graphite crystal in the a-direction. 

Figure 1 The status of carbon fiber development in 1984 based primarily on product data supplied by manufacturers. The solid line refers to commercial grades of fiber spun from mesophase pitch ( 7) the dashed line refers to an experimental mesophase fiber spun from solvent-extracted pitch (10). Experimental (noncommercial) fibers are indicated by triangular symbols.

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