PAN-based fibers

Fig. 2. Young s modulus corrected for porosity as a function of preferred orientation curve is based on theoretical model where = rayon-based fibers Q — PAN-based fibers and A = pitch-based fibers (2). To convert GPa to psi, multiply by 145,000.

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. 

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. 

Figure 1 shows where mesophase carbon fibers fit in the rapidly developing field of structural carbon fibers. At present, fibers produced from polyacrylonitrile (PAN) constitute the bulk of the carbon fiber produced in 1981 all commercial PAN-based fibers fell below and to the left of the "1981 limit" for tensile strength and modulus. Since 1981, there have been major advances in the development of PAN-based fibers to achieve high strengths, particularly at the lower modulus levels (near 40 Mpsi, 276 GPa). 

Figure 16. Translation of fiber strength, after the first impregnation and carbonization (shaded bar), and after repeated densification cycles (32). Thomel 50, Modmor I, and Sigrafil HM fibers are high-modulus PAN-based fibers Sigrafil HF is a high-strength PAN-based fiber.

Figure 17. Transmission electron micrographs of PAN-based and mesophase-pitch-based carbon fibers, as delivered and after heat treatment (42,43). HT and HM refer to high strength and high modulus PAN-based fibers P55 and P100 refer to mesophase-pitch-based fibers, with tensile moduli of 55 and 100 Mpsi, respectively.

Figure 23. Cross-sectional shrinkage as a function of carbon yield in the first carbonization. Unidirectional composites fabricated with PAN-based fibers and various matrix precursors.

Figure 27. Flexural strength and bulk density of unidirectional composites subjected to graphitizing heat treatments either for each densification cycle or after the densification cycles were completed (20,52) The matrix was a coal-tar binder pitch with 10 wt.-% sulfur the high-modulus PAN-based fiber was Sigrafil HM.

Figure 35. Mechanical properties of carbon/carbon epoxy-resin hybrid composites, compared with the properties of the composite skeletons before resin impregnation (61,62). The composite skeletons were prepared from Sigrafil HM 3 PAN-based fiber, rigidized with a phenolic resin, and densified by four cycles with coal-tar pitch plus sulfur the carbonization temperature was 1000°C. (a) Young s modulus.

IPCL PAN-based fibers produced by Indian Petro Chemicals Ltd. P55, P75, P120 pitch-based fibers from Amoco, USA. (Reproduced with permission from Carbon, 32 (1994) 1485 [16]). 

Figure 8.11 Tensile strength and Young s modulus of PAN-based fibers as a function of high temperature treatment (after Watt, 1970).

PAN based fibers develop shrinkage forces during two different stages of stabilization entropic shrinkage in the early stages, and shrinkage due to chemical reaction in the later stages. 

Although the typical high tensile strength carbon fibers (obtained at heat treatment <1600 °C) are certainly considerably less ordered, and less densely packed, their structure may be assumed to be somewhere in-between the fibrillar structure with some pseudocrystalline lamellar order within the fibrils, as suggested by Warner for stabilized fiber, and the graphitic crystallites suggested by Crawford . The voids determine the relatively low density of the carbon fibers (for PAN based fibers... 

To stabilize PAN based fibers in a reasonable time (say one hour or less), higher entrance temperatures are required, and this inevitably dictates limitations concerning the type and the concentration of the comonomer. Evidently, a compromise between the beneficial influences of the comonomer with regard to the oligomerization reaction, and the danger of fiber fusion, has to be looked for. 

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