Properties of Rayon-Based Carbon Fibers

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.) 

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The first commercial carbon fibers were based on viscose rayon, a cellulosic precursor, but Polycarbon is now the only current producer of this type of carbon fiber. The properties of rayon-based carbon fibers are listed in Table 20.1. A difficult fiber to produce with a low yield, its main use is in existing space programs. 

Table 8.9. Summary of Physical Properties of Rayon-Based Carbon Fibers...

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. 

Cellulose based carbon fibers tend to be used for ablative applications, which require lower thermal conductivity and enhanced insulating properties than other carbon-carbon materials. The precursor for this product is rayon, which when carbonized, produces rayon based carbon fibers (RBCF). This type of fiber is usually more porous and weaker, which lowers its effective thermal conductivity. At high temperatures, it oxidizes and/or evaporates without disintegration, producing an evaporative cooling effect. 

For applications that do not require exceptional mechanical properties, carbon fibers made from high performance aramid polymers show considerable potential. These aramid fibers do not require stabilization prior to carbonization, which substantially simplifies the production process. Rayon-based carbon fibers continue to appear in some composite applications, but have become key substrates for the development of activated carbon fibers. These ACFs develop a microp-orous surface structure that is ideal for adsorption of low levels of volatile organic compounds. 

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. 

Pitch as a precursor material is cheaper than PAN as a precursor fiber, but the conversion of pitch into mesophase pitch and subsequent fiber formation is complex and costly. When a pitch is not transformed into a mesophase and is spun as an isotropic liquid, the resulting carbon fibers have extremely poor mechanical properties. These considerations explain why more than 90% of today s carbon fibers are fabricated from PAN based precursors. Processes for fabricating carbon fibers from PAN or pitch based precursor fibers differ in important aspects, but also share important commonalties (Figure 2). Finally, the carbon yield from PAN based precursor fibers is 50%, that from mesophase pitch is 70-80%, and that from rayon is 25%. 

The majority of commercial carbon fibers are produced from polyacrylonitrile (PAN) fibers. In fact, HTA-12K PAN-based carbon fibers are the most commonly used commercial carbon fiber (15). PAN-based fibers are the strongest commercially available carbon fibers and dominate structural applications. Mesophase pitch-based carbon fibers represent a smaller but significant market niche. These fibers develop exceptional moduli and excel in lattice-based properties, including stiffness and thermal conductivity (1). Rayon-based fibers are used in heat shielding and in missile nosecones (16). Carbon fibers made from high performance pol5oners (17-19) or from chemical vapor deposition of hydrocarbons, such as benzene or methane, display imique properties that make them potentially attractive futime alternatives (20-22). 

For rayon fiber based composites (Sections 3 and 4) the fiber and powdered resins were mixed in a water slurry in approximately equal parts by mass. The isotropic pitch carbon fiber composites (Section 5) were manufactured with less binder, typically a 4 1 mass ratio of fiber to binder being utilized. The slurry was transferred to a molding tank and the water drawn through a porous screen under vacuum. In previous studies [2] it was established that a head of water must be maintained over the mold screen in order to prevent the formation of large voids, and thus to assure uniform properties. The fabrication process allows the manufacture of slab or tubular forms. In the latter case, the cylinders were molded over a perforated tubular mandrel covered with a fine mesh or screen. Moreover, it is possible to mold contoured plates, and tubes, to near net shape via this synthesis route. 

Rayon-based ACFs are used in the adsorption of many volatile organic compounds including formaldehyde (80), methyl ethyl ketones (81), and benzene (81). ACFs are also finding uses in natural gas storage (82), electrodes for batteries (83), catalyst supports (84), and NO removal (85). Stabilized rayon fibers are carbonized and then activated with air (80), steam (86), or carbon dioxide (87), much as in granular carbon activation. The extent of pyrolysis governs the pore structure, carbon yield, and surface area of the fiber, while activation impacts the presence of functional groups on the pore surface (12). Properties of some commercial ACFs are summarized in Table 6. 

Rayon-based fibers are not as strong as PAN-based fibers. They are used in insulation and some Ccirbon-carbon and ablative applications because of a good match of properties with the carbonized matrix. 

Thus, the 2nd fiber category encompasses carbon and carbonized polymers. The foundation materials of this group are the heat convertible, fiber forming polymers, the most common of which today is PAN (polyacrylonitrile). Others are rayon, PBI, and pitch tar. One of the earliest to report results of PAN pyrolysis was Goodhow, et. al.(5i) in 1975. Later, in 1979, Fischbach and Komaki(5 and Brehmer, et. al.(di) in 1980 reported on the electrical properties of carbon fibers made fi om various polymers and described a dependency of resistivity upon the heat treat temperature (HTT) employed to carbonize the fiber which is now well known. The studies by Swift, et. al(52) in 1985 and more recently reported herein were undertaken to expanded upon this base of knowledge and to initiate studies of the stability of the electri properties of fibers made on commercially viable platforms. These studies have led to a launch point for what is believed to have been the first, relatively large scale, commercial application for partially carbonized PAN fibers(50), as resistive carbon fiber based static eliminator brushes. 

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