Mesophase pitch, development

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. 

To date, there has been relatively little work reported on the mesophase pitch rheology which takes into account its liquid crystalline nature. However, several researchers have performed classical viscometric studies on pitch samples during and after their transformation to mesophase. While these results provide no information pertaining to the development of texture in mesophase pitch-based carbon fibers, this information is of empirical value in comparing pitches and predicting their spinnability, as well as predicting the approximate temperature at which an untested pitch may be melt-spun. 

The properties of mesophase pitch-based carbon fibers can vary significantly with fiber texture. Inspection of the cross-section of a circular mesophase fiber usually shows that the graphitic structure converges toward the center of the fiber. This radial texture develops when flow is fully developed during extrusion through the spinnerette. Endo [48] has shown that this texture of mesophase pitch-based carbon fibers is a direct reflection of their underlying molecular structure. 

Further improvements in the properties of PAN-based carbon fibers are likely to emerge through improved stabilization, that is, by creating the ideally cross-linked fiber. On the other hand, as purer pitch precursors become available, further improvements in mesophase pitch-based carbon fibers are likely to arise from optimized spinnerette designs and enhanced understanding of the relationship between pitch chemistry and its flow/orientation behavior. Of course, the development of new precursors offers the potential to form carbon fibers with a balance of properties ideal for a given application. 

McHugh, J. J. and Edie, D. D., The orientation of mesophase pitch during fully developed channel flow. Carbon, 1996, 34(11), 1315 1322. 

These methods, described above, are to produce material (still loosely called mesophase) which essentially is a feedstock for other process developments. This "mesophase" prepared at temperatures below normal carbonization temperatures can be called low temperature mesophase pitch (LTMP). The term mesophase pitch has crept into the vocabulary of this subject and is thought to refer mainly to mesophase as a feedstock. Its anisotropy can be detectable by polarized light optical microscopy. 

The technology of mesophase-pitch-based carbon fiber has stimulated the rapid development of the chemistry of mesophase behavior and preparation. The carbonization schemes and mechanisms leading to optical anisotropy via the mesophase, the control of carbonization with emphasis on the preparation of spinnable mesophase, and the mesophase transition and reactivity in relation to the structure of its constituent molecules are summarized in this paper. 

Based on the creation of naphthenic structures in the condensation reaction, the modification by aluminum chloride increased carbon yield and improved the potential for anisotropic development. Oxidative pretreatment usually impairs anisotropic development although it increases carbon yield. The oxidized pitch is also modified by aluminum chloride (34) this may be used to prepare additives (38) and mesophase pitches (39). It should be noted that these processes allow the catalyst to be readily separated (in contrast to catalytic carbonization) since the modified product is still either soluble in some solvents or fusible. 

The relative magnitudes of the Tg values in the two-phase region should be related to the development of microstructure in mesophase pitch, as discussed earlier. Tg will also be a useful parameter in defining the maximum temperature at which fibres can be oxidized without molecular motion causing some decrease in extent of preferred orientation. 

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.

Similarly, PZ pitch as precursor for HPCF was replaced by other mesophase pitches (12). At this point in time, as is well-known, Singer (13) and Lewis (14) of the Union Carbide Corporation developed similar methods. Mesophase carbon fiber progressed more rapidly in the USA than in Japan because Japanese defense and aerospace needs were less demanding. Recently, however, the drive toward higher-added-value products from the heavy fractions of coal and petroleum has intensified, and pitch-based carbon fibers, including HPCF, are now the subjects of extensive investigation in many Japanese laboratories. 

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