Carbon PAN-based

Fig. 3. Schematic illustration of PAN-based carbon fiber microstmcture based on microscopic observations (3).

Producers of PAN-based carbon fiber include Toray, Toho Beslon, Mitsubishi Rayon, and Asahi Kasai Carbon in Japan Hercules, Amoco Performance Products, BASE Stmctural Materials, Eortafil (Akzo), and Mitsubishi Rayon in the United States and Akzo, Sigri, and Soficar in Europe. Primary suppHers of high performance pitch-based carbon fibers include Amoco Performance Products, Mitsubishi Kasai, and Tonen Corp. 

Fig. 4. Process flow diagrams for (a) PAN-based and (b) pitch-based carbon fiber processes.

Fig. 6. Each of carbonization temperature on PAN-based carbon fiber strength and modulus (31). To convert GPa to psi, multiply by 145,000.

There are two mechanisms of PAN-based carbon fiber oxidation dependent on oxidation temperature ((67,68). At temperatures below 400°C, oxygen diffuses into the fiber and attacks at pores resulting in significantly increased fiber surface area. At higher temperatures impurities catalyze the oxidation reaction. 

W. Johnson, "The Stmcture of PAN-Based Carbon Fibers and Relationship to Physical Properties," in W. Watt and B. V. Perov, eds., ELandbook of Composites, Vol. 1, Elsevier Science Pubhshers, New York, 1985. 

Fibers produced from pitch precursors can be manufactured by heat treating isotropic pitch at 400 to 450°C in an inert environment to transform it into a hquid crystalline state. The pitch is then spun into fibers and allowed to thermoset at 300°C for short periods of time. The fibers are subsequendy carbonized and graphitized at temperatures similar to those used in the manufacture of PAN-based fibers. The isotropic pitch precursor has not proved attractive to industry. However, a process based on anisotropic mesophase pitch (30), in which commercial pitch is spun and polymerized to form the mesophase, which is then melt spun, stabilized in air at about 300°C, carbonized at 1300°C, and graphitized at 3000°C, produces ultrahigh modulus (UHM) carbon fibers. In this process tension is not requited in the stabilization and graphitization stages. 

Pitch-based fibers generally have higher moduh but lower strengths than theh PAN-based counterparts. The specific properties of the various types of carbon fibers are compared in Figure 4. Pitch-based fibers also have higher electrical conductivity, which can be an important consideration in certain circumstances, for example, for use in electromagnetic inductance (EMI) shielding. 

Polymers mesophase pitch polyacrylonitrile carbons" mesocarbon microbeads, carbon fibers PAN-based carbon fibers ... 

Fig. 6. Breakthrough curves for aqueous acetone (10 mg 1" in feed) flowing through exnutshell granular active carbon, GAC, and PAN-based active carbon fibers, ACF, in a continuous flow reactor (see Fig. 5) at 10 ml min" and 293 K [64]. C/Cq is the outlet concentration relative to the feed concentration. Reprinted from Ind. Eng. Chem. Res., Volume 34, Lin, S. H. and Hsu, F. M., Liquid phase adsorption of organic compounds by granular activated carbon and activated carbon fibers, pp. 2110-2116, Copyright 1995, with permission from the American Chemical Society.

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. 

In addition to their exceptional tensile strengths, PAN-based carbon fibers are far more resistant to compressive failure than are their pitch-based counterparts or polymeric high-performance fibers. However, because the PAN precursor is not... 

Since PAN-based carbon fibers tend to be fibrillar in texture, they are unable to develop any extended graphitic structure. Hence, the modulus of a PAN-based fiber is considerably less than the theoretical value (a limit which is nearly achieved by mesophase fibers), as shown in Fig. 9. On the other hand, most commercial PAN-based fibers exhibit higher tensile strengths than mesophase-based fibers. This can be attributed to the fact that the tensile strength of a brittle material is eontrolled by struetural flaws [58]. Their extended graphitic structure makes mesophase fibers more prone to this type of flaw. The impure nature of the pitch preciusor also contributes to their lower strengths. 

In contrast, there is also current interest in investigating PAN-based fibers in low thermal conductivity composites [62], Such fibers are carbonized at low temperature and offer a substitute to rayon-based carbon fibers in composites designed for solid rocket motor nozzles and exit cones. 

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. 

Fitzer, E., PAN-based carbon fibers - present state and trend of the technology from the viewpoint of possibilities and limits to influence and to control the fiber properties by the process parameters. Carbon, 1989, 27(5), 621 645. 

Johnson, D. J., Structural studies of PAN-based carbon fibers. In Chemistry and Physics of Carbon, Vol. 20, ed. P. L. Walker. Marcel Dekker, New York, 1987, pp. I 58. 

Fig. 3 The natural logarithm of the average filament strength (n=40) as a function of the natural logarithm of the test length for an intermediate-modulus, PAN-based carbon fibre with an impregnated bundle strength of 5.7 GPa [8]...

---