Mesophase pitch 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. 

A very unusual characteristic of mesophase pitch is the extreme dependency of its viscosity on temperature [19,34,35]. This factor has a profound influence on the melt-spinning process (described above), as a mesophase pitch fiber will achieve its final diameter within several millimeters of the face of the spinnerette, in sharp contrast to most polymeric fibers. 

Matsumoto, T. and Mochida, I., Oxygen distribution in oxidatively stabilized mesophase pitch fiber, Carbon, 1993,31(1), 143 147. 

The future remains bright for the use of carbon materials in batteries. In the past several years, several new carbon materials have appeared mesophase pitch fibers, expanded graphite and carbon nanotubes. New electrolyte additives for Li-Ion permit the use of low cost PC based electrolytes with natural graphite anodes. Carbon nanotubes are attractive new materials and it appears that they will be available in quantity in the near future. They have a high ratio of the base plane to edge plain found in HOPG. The ultracapacitor application to deposit an electronically conductive polymer on the surface of a carbon nanotube may be the wave of the future. 

Figure 8. Melt extrusion of mesophase pitch fibers with a rotation screw and multi hole spinneret [6] reproduced with permission from Noyes Publications, Park Ridge, NJ.

The precursor is melted in an extruder which pumps the melt into a die head equipped with a filter and a multihole spinneret [5-6] [24]. As the precursor fibers exit the spinneret holes they cool and solidify, and are drawn before windup. The window for achieving successful and continuous fiber formations is small. The temperature dependence of the viscosity is large and the failure strength of the mesophase pitch fibers is low (30-40 MPa). Thus, the extrusion temperature must be precisely controlled. 

As-spun mesophase pitch fibers consist of large and elongated anisotropic domains aligned almost parallel to the fiber axis [15] [25]. Since a high degree of orientation is achieved during melt extrusion, they do not need to be further stretched. Moreover, the transverse microtexture of these fibers can be tailored by controlling the flow of the discotic molecules in... 

As-spun and stabilized mesophase pitch fibers have no functional utility but serve as the solid precursor fibers from which mesopitch (MP) based carbon fibers are derived. The conversion of mesophase pitch fibers into carbon fibers is carried out in an inert atmosphere at temperatures above 500 C. Initially, free radicals [17] are formed by a dehydrogenation condensation mechanism and the formation and evolution of gaseous species is accompanied by weight loss. Most of the gaseous species are formed and evolve at lOOO C, except hydrogen which is still formed in small amount above this temperature. 

Figure 7.4 Oxygen diffusion along ihe diameter from the surface to the center of a 30 nm diameter mesophase pitch fiber after stabilization at 300 = C for 15 min. Source Reprinted from Mochida I, Toshima H, Korai Y, Hino T, Oxygen distribution in the mesophase pitch fiber after oxidative stabilization, J Mater Sci, 24, 389-394, 1989.

Figure 7.5 Diagram of a hairpin element furnace used to carbonize mesophase pitch fibers. Source Reprinted with permission from Buckley JD, Edie DD Eds., Carbon Materials and Composites, Noyes Publications, Park Ridge, NJ, 32, 1993. Copyright 1993, William Andrew Publishing.

Mochida I, Toshima H, Korai Y, Hino T, Oxygen distribution in the mesophase pitch fiber after oxidative stabilization, J Mater Sci, 24, 389-394, 1989. 

Mochida I, Zeng SM, Korai Y, Toshima H, The introduction of a skin core structure in mesophase pitch fibers by oxidative stabilization. Carbon, 28, 193, 1990. 

Matsumoto T, Mochida 1, A structural study on oxidative stabilization of mesophase pitch fibers derived from coal tar. Carbon, 30, 1041, 1992. 

MESOPHASE PITCH BASED CARBON FIBERS (MPP BASED CARBON FIBERS) are CARBON FIBERS obtained from MESOGENIC PITCH after it has been transformed into MESOPHASE PITCH (MPP) at least during the process of spinning, after the spun MESOPHASE PITCH fibers have been made non-fusible (stabilized) and carbonized. 

Carbon fibers are either the strongest or stiffest materials available, when corrected for density, as illustrated in Fig. 1. The mesophase pitch fibers akso have high levels of thermal conductivity, as shown in Fig. 2. 

Mesophase pitch fibers are produced through melt spinning that is essentially the same as that used to spin commercial polymers. Figure 13 shows this process in which an extruder melts pitch particles and pumps the molten pitch through a multiholed spinnerette. The high extensional shear orients the liquid crystalline mesophase molecules as it approaches and flows through the spinnerette. The flbers emerging from the spinnerette are drawn by a windup spool. 

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