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high-performance fiber (estimated as $16/kg). First, the starting material (petroleum or coal-tar pitch) costs 40 to 50 percent less than the monomers used to form PAN. Second, because pitch-based carbon fiber begins with a structure closer to graphite than PAN does, less energy is required to convert it to graphite. Because of this, lower carbonization temperatures and/or shorter carbonization times are required in the production of pitch-based carbon fibers. Third, mesophase precursor fiber contains a smaller percentage of nitrogen, hydrogen, and other noncarbon elements than PAN precursor fiber and, therefore, less material is driven off during carbonization. Because of this, the percent yield (in kilograms of carbon fiber per kilogram of precursor fiber) is approximately 75 percent for mesophase pitch precursor fiber compared with only 40 to 45 percent for PAN precursor fiber.
Mesophase pitch can be produced by the thermal or catalytic polymerization of a suitable petroleum or coal-tar pitch. When a highly aromatic pitch, such as a decant oil pitch, is heated to temperatures of 400°C to 450°C for approximately 40 hr, 45 to 65 percent of it will transform from an isotropic material to an optically anisotropic fluid phase—a mesophase or liquid crystal (ref. 10). A free-radical mechanism is believed to be responsible for polymerization of the carbonaceous material (ref. 11). Another method is to use solvents such as benzene, heptane, and toluene to first extract a portion of the isotropic pitch. The solvent insoluble portion can be converted to an anisotropic pitch by heating to between 230°C and 400°C for less than 10 min (ref. 12). The anisotropic, or oriented, phase is composed of stacked, polynuclear aromatic hydrocarbon molecules. These molecules tend to be disc-shaped with an average molecular weight of approximately 1000 (although the molecular weight can vary considerably). The molecular structure of the mesophase produced from coal-tar pitch is characterized by higher aromaticity, whereas the structure of the petroleum-derived mesophase is more open because of its higher content of aliphatic side chains (ref. 13). Figure 8 shows the structure of a typical polynuclear aromatic hydrocarbon in mesophase. Initially, small spheres of mesophase form in the isotropic pitch when heated for an adequate time at a sufficiently high temperature. Upon further heating, the concentration of mesophase spheres increases and causes the spheres to collide and coalesce, creating a mosaiclike, nematic liquid-crystal structure (ref. 14).
Mesophase products that have a high average molecular weight and no side groups or small molecular components to cause disordering often decompose before becoming fluid enough to flow. Because of this, the mesophase used to melt spin fibers is normally a mixture of high molecular weight molecules that still have a small number of side groups. Therefore, commercial mesophase precursors have certain characteristics of both mixtures and solutions: they soften over a range of temperatures and orient under an applied stress.
Figure 8. Typical poly nuclear aromatic hydrocarbon in mesophase (ref. 13). (Molecular weight given in glmol.)
Numerous studies have concluded that, in general, the flow behavior of mesophase pitches is shear thinning at low shear rates but approaches Newtonian at high shear rates (refs. 13 and 14). In addition, it is the large response time for changes in flow rate which indicates that mesophase pitch is somewhat viscoelastic (ref. 14). However, the most unusual characteristic of mesophase pitch is the extreme temperature dependency of its viscosity (ref. 15). Even though mesophase pitch can be formed into fibers by conventional melt-spinning techniques, extremely precise temperature control is required (ref. 16).
Melt-Spinning Mesophase Precursor Fibers
Figure 9 shows a schematic of a process for melt-spinning mesophase precursor fibers. Normally, the extruder screw consists of three zones: solid feed, melting, and pumping. The initial zone transports the solid mesophase feed to the melting zone, where it is heated to a temperature at which its viscosity is approximately 200 Pas (the optimum viscosity depends on the exact composition of the mesophase being extruded). Then, the pumping zone of the extruder forces this molten precursor into the top of a die head. The die head often contains a filter for removing solid impurities from the precursor. Finally, the molten mesophase exits through a multiple-hole spinnerette attached to the bottom of the die head. An initial orientation develops as the liquid crystalline precursor flows through the small capillaries in the spinnerette. As the precursor exits the capillaries, it is simultaneously cooled by the quench air and drawn before wind-up, yielding a precursor fiber with a high degree of molecular orientation. Because the aromatic, sheet-like molecules are already oriented in the direction of the fiber axis, additional drawing of the as-spun fiber is unnecessary.
Because of the extreme temperature dependency of mesophase. fiber diameters can vary widely if the spinnerette temperature is not accurately controlled. Edie and Dunham (ret. 15) showed that only a ±3.5°C variation in temperature across the face of the spinnerette can result in a ±15 percent variation in the diameter of the as-spun fibers (ref. 16). Their analysis also indicated that, even when process conditions during spinning are controlled, the tensile stress in the mesophase filament is almost one-half of its ultimate strength.
Heat Treatment of Mesophase Precursor Fibers
Oxidation of Mesophase Precursor Fibers
After spinning, the pitch-based fiber must be thermoset, in a manner similar to the PAN process, to render it infusible. The exact temperature and time required depend on the chemical composition and diameter of the mesophase fiber. The temperature must be below the softening point of the mesophase to minimize any fiber-to-fiber sticking. However, higher temperatures increase the rate of the stabilization reactions, decreasing the time required for this step. Commercially, the temperature selected for stabilization is a compromise between minimizing the required time for this process step and maximizing the mechanical properties of the final carbon fiber. Typically, the as-spun mesophase fibers are heated to temperatures of approximately 300°C for a period ranging from 30 min to 2 hr to be adequately stabilized for final heat treatment. Because the as-spun mesophase fiber already possesses a high degree of molecular orientation, applying tension during stabilization is unnecessary.
The low tensile strength of the mesophase fiber, both before and after stabilization, makes fiber handling during this step extremely difficult. Even though the final carbon fiber exhibited a tensile strength of 2.1 GPa, Mochida et al. (ref. 17) tested mesophase fibers before carbonization and found their tensile strength to be only 0.04 GPa (less than 2 percent of its final strength after carbonization). This lack of fiber strength restricts the design of the oxidation ovens used for mesophase fibers. Obviously, fiber handling must be minimized to avoid fiber breakage.
Figure 10 shows an apparatus for oxidizing the as-spun fiber without removing it from the spool used for spinning. Air, heated to the proper oxidation temperature, is forced through the porous wind-up spool and then passes through the fiber bundle. Because the oxidation reaction is exothermic, this flow geometry is important for heat as well as mass transfer. Designs such as this minimize damage to the as-spun fiber by completely eliminating fiber handling during oxidation. Commercially, many other processes are used to oxidize mesophase fibers. Although the designs vary considerably, all attempt to minimize handling of the fragile, uncarbonized mesophase fiber (ref. 16).
Carbonization and Graphitization
After thermosetting, mesophase fibers (like PAN and cellulose precursor fibers) are either carbonized or graphitized in an inert atmosphere to develop their final properties. However, when mesophase fibers are carbonized the principal gases that evolve are CH4 and H2. Like PAN precursor fibers, most of these gases are evolved below 1000°C. Thus, normally the stabilized mesophase fibers also are precarbonized for a few minutes at 900°C to 1000°C. After precarbonization, they are either carbonized or graphitized at the desired temperature. Here again,
hydrogen is the principal gas evolved above 1000°C. While still not extremely strong, after oxidation the mesophase fiber can be handled if the tow is sufficiently large. Thus, the ovens used to carbonize and graphitize mesophase fibers (fig. 11) are similar to those used to process PAN precursor fibers.
The process used to surface-treat and size mesophase pitch-based carbon fibers is similar to that used for PAN-based fibers. However, because pitch-based fibers are less reactive to surface oxidation, more severe reaction conditions are used during surface treatment.
Figure 11. Schematic of carbon resistance furnace used to carbonize mesophase precursor fibers.