Mesophase pitch carbon fiber properties

Mochida I, Zeng SM, Korai Y, Hino T, Toshima H, Structure and properties of mesophase pitch carbon fiber with a skin core structure carbonized under strain, J Mater Sci, 27(7), 1960-1968, 1992. 

Isotropic pitches are used to make a general purpose (GP) grade of pitch carbon fiber, which is not graphitic and has poorer properties than the high performance grade (HP), which requires a special treatment process to convert the pitch to a mesophase grade, i.e., an optically anisotropic and graphitic material. 

PAN-based carbon fibers develop exceptional tensile strength [in excess of 7000 MPa (23)] and are more resistant to compressive failure than high performance polymers (24). These factors combine to make PAN-based carbon fibers the ideal choice for applications requiring significant fiber strength. However, PAN-based fibers are less appropriate than mesophase pitch-based fibers for applications in which molecular order-dependent properties are key. 

Unlike PAN-based carbon fibers, mesophase pitch-based fibers experience significant graphitization during which dislocations in the turbostratic carbon stacks are gradually annealed, resulting in the formation of a three-dimensional lattice. Fischbach (75) presented a detailed study of graphitization which characterizes the process as a combination of atomic diffusion and crystallite growth. Table 4 provides a comparison of properties for some commercial pitch- and PAN-based carbon fibers. 

Because of their unique blend of properties, composites reinforced with high performance carbon fibers find use in many structural applications. However, it is possible to produce carbon fibers with very different properties, depending on the precursor used and processing conditions employed. Commercially, continuous high performance carbon fibers currently are formed from two precursor fibers, polyacrylonitrile (PAN) and mesophase pitch. The PAN-based carbon fiber dominates the ultra-high strength, high temperature fiber market (and represents about 90% of the total carbon fiber production), while the mesophase pitch fibers can achieve stiffnesses and thermal conductivities unsurpassed by any other continuous fiber. This chapter compares the processes, structures, and properties of these two classes of fibers. 

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. 

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. 

Low density, carbon fiber-carbon binder composites are fabricated from a variety of carbon fibers, including fibers derived from rayon, polyacrylonitrile (PAN), isotropic pitch, and mesophase pitch. The manufacture, structure, and properties of carbon fibers have been thoroughly reviewed elsewhere [3] and. therefore, are... 

The surface properties of carbon fibers are intimately related to the internal structure of the fiber itself, which needs to be understood if the surface properties are to be modified for specific end applications. Carbon fibers have been made from a number of different precursors, including polyacrylonitrile (PAN), rayon (cellulose) and mesophase pitch. The majority of commercial carbon fibers currently produced are based on PAN, while those based on rayon and pitch are produced in very limited quantities for special applications. Therefore, the discussion of fiber surface treatments in this section is mostly related to PAN-based carbon fibers, unless otherwise specified. 

Figure 9. Mechanical properties, as a function of heat treatment temperature, of carbon fibers spun from mesophase pitch prepared by the Kyukoshi method.

Table 8.4 Tensile properties of mesophase pitch-based carbon fibers."...

Solvent fractionated mesophase pitches can be made which meet these three requirements. As spun strength of pitch fibers is low but in the same range as some other commercially produced textile fibers. Subsequent processing to carbon fiber Is similar to that used for polyacrylonitrileprecursor carbon fiber. A more complete description of this process and fiber properties are presented In the paper of Riggs Dlefendorf In this volume (17). 

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