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D. D. Edie and R. J. Diefendorf
Clemson, South Carolina
Manufacture of PAN-Based Carbon Fibers 20
Manufacture of Rayon-Based Carbon Fibers 26
Manufacture of Pitch-Based Carbon Fibers 27
Melt-Spinning Mesophase Precursor Fibers 29
Manufacture of Vapor-Grown Carbon Fibers 33
Mechanical Properties of Carbon Fibers 35
The high strength, superior stiffness, and light weight of carbon fibers have made them the dominant reinforcing fibers used in high-performance polymer matrix composites. However, the same fibers can also reinforce brittle materials, such as ceramics and carbon, thus creating a unique class of high-temperature composite materials. When properly protected from oxidation, these carbon fibercarbon matrix composites can withstand extended exposure to temperatures of up to 2500°C, making them attractive for many aerospace applications. In addition, because of their improved friction performance and high wear resistance, carboncarbon (CC) materials are used in high-performance brakes of aircraft and racing cars. Using these CC brakes in passenger cars and trucks is currently being evaluated.
At present, all commercial carbon fibers are produced by the thermal decomposition of various organic fiber precursors. The most popular precursor materials are fibers of polyacrylonitrile (PAN), cellulose (rayon), and pitch (ref. 1). A proposed alternate process, which produces a discontinuous, high-performance carbon fiber, is called vapor-growth. This chapter describes the similarities of these four fiber processes and discusses their differences.
Manufacture of PAN-Based Carbon Fibers
Today, approximately 90 percent of all commercial carbon fibers are produced from a PAN precursor fiber. Normally, PAN is copolymerized with a small amount of another monomer, such as methylacrylate, to lower its glass transition temperature and control its oxidation rate. Figure 1 lists a few of the many monomers copolymerized with acrylonitrile to produce commercial PAN precursor fibers. Typically, the precursor fiber would contain 93 to 95 percent acrylonitrile units, with the remainder being one or more of these monomers. Because PAN decomposes below its melt temperature, it is normally extruded into filament form using various solution spinning techniques.
Solution Spinning of PAN Precursor Fibers
Figure 2 shows the process schematic of a typical solution spinning process (ref. 2). In this process, the copolymer first is dissolved in a suitable solvent, such as dimethylacetamide, and loaded into a storage tank. Typically, the spinning solution is quite concentrated (from 15 to 30 percent polymer by weight). The solution is pumped through a die head, where it is filtered to remove impurities before being extruded through a spinnerette containing a large number of small (approximately 100-/xm) capillary holes. In the process shown in figure 2, the solution immediately enters a coagulating bath as it exits the capillary. This is termed wet spinning.
Figure 2. Schematic of process for wet-spinning PAN precursor fibers (ref: 2).
In a wet-spinning process, the fiber is formed by using a coagulant (such as
ethylene glycol) which extracts the solvent from the polymer. The rate at which the solvent is extracted from the polymer as it passes through the coagulation bath governs the final cross-sectional shape of the fiber. Thus, the temperature, concentration, and circulation rate of the fluid in the coagulation bath all can affect the structure of a wet-spun fiber. Rapid extraction of the solvent during coagulation allows the outer portion of the fiber to solidify before the solvent can diffuse out of the fiber center. The large concentration gradient across the fiber cross section makes the initial density of the center portion of a rapidly formed, wet-spun fiber much less than that of the fiber skin. As the solvent eventually diffuses out of the fiber center, the density of the inner portion of the fiber increases, causing the fiber skin to collapse, yielding a kidney bean (or dog-bone-shape) fiber. However, wet spinning can produce PAN precursor fibers with a circular cross section and a minimum of internal voids if the rate of solvent extraction is properly controlled.
In another variation of solution spinning, the polymer solution is extruded into a hot gas environment. In this case, the temperature and composition of the gas must be carefully monitored to control the rate of solvent evaporation and, thus, the structure of the fiber. This process variation, called dry spinning, produces an as-spun fiber with a dog-bone-shaped cross section.
Often, both wet- and dry-spun fibers are washed after fiber formation to remove the final traces of solvent. Then the fibers are passed through one or two stages in which they are stretched to further align the polymer molecules parallel to the fiber axis. Finally, this fully drawn PAN precursor fiber is dried and packaged.
Melt-Assisted Spinning of PAN Precursor Fibers
Solution-spun PAN fibers can be converted to carbon fibers with excellent mechanical properties. However, large amounts of solvent are required for solution spinning, and ultimately this solvent must be completely removed from the fiber and recovered. This process adds to the cost of solution-spun precursor fibers, and the trace impurities ultimately can limit the properties of the final carbon fiber. To overcome some of these problems, a melt-assisted process for producing PAN precursor fibers has been developed by BASF Structural Materials, Inc. (ref. 3). In this process, the acrylonitrile copolymer is polymerized in an aqueous suspension. After polymerization, the copolymer is purified and dewatered before extrusion. The PAN copolymer then is pelletized and fed to an extruder. Excess water effectively plasticizes the polymer, allowing it to form a homogeneous melt well below its degradation temperature. Figure 3 shows a flow diagram of this novel fiber-forming process, termed melt-assisted spinning.
In melt-assisted spinning, the plasticized PAN copolymer is extruded through a multiple-hole spinnerette directly into a steam-pressurized solidification zone. After passing through this steam environment, the fiber is stretched and dried in a series of steps similar to those found in solution spinning processes. The meltassisted process offers several advantages over conventional solution spinning, including completely eliminating the need for expensive solvents and reducing waste water treatment requirements. In addition, because the polymer content of the plasticized PAN is much higher than that of the solutions used in wet or dry spinning, coalescence during fiber formation is simplified. Thus, the cross-sectional structure of fibers formed by melt-assisted spinning should be more uniform.