Stretching rayon-based fibers

Two categories of pitch-based fiber exist isotropic carbon fiber produced from an isotropic pitch precursor, and an oriented, anisotropic fiber produced from a mesophase pitch precursor. Isotropic fibers were developed from low melting point isotropic pitches The precursor was melt-spun into fibers, which were oxidized to render them infusible, and then carbonized. Their low strengths and moduli make these fibers unsuitable for use in advanced composites. Orientation was accomplished by a hot-stretching process (>2200°C), but it is accompanied by the same processing difficulties encountered in the rayon precursor process. A different approach was suggested by the discovery of carbonaceous mesophase. ... 

The properties of stretched-graphitized rayon-based carbon fibers are shown in Table 8.9. The data is to be considered for its historical value, since the material is no longer produced commercially (data from Union Carbide Corp.)... 

Rayon-based carbon fibers have a more isotropic structure than similarly heat-treated PAN or MPP-based carbon fibers. Their Young s modulus values are therefore drastically lower ( < 100 GPa, cr < 100 MPa). Rayon-based carbon fibers can be transformed into anisotropic carbon fibers with high strength and Young s modulus values by hot-stretching treatment at temperatures of approximately 2800 K. 

The overwhelming success of PAN-based carbon fibers over rayon and pitch can be attributed to several key aspects.f Structurally, PAN has a faster rate of pyrolysis without much disturbance to its basic structure and to the preferred orientation of the molecular chains along the fiber axis present in the original fiber. By contrast, carbon fibers from rayon suffer from extremely low carbon yield (20-25%) due to chain fragmentation, which eliminates the orientation of the precursor fiber. While improved properties can be achieved by stretch graphitization, this process is expensive and does not compensate for the low yields. 

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