In the 1950s, it was recognized that materials that combine light weight, high strength, and high stiffness are needed to produce ultralightweight structures.3 Carbon fibers were developed to fill this need. Carbon fibers are typically produced by high-temperature processing of one of three types of precursor fibers: polyacrylonitrile (PAN), pitch, or rayon. Depending upon the type of precursor and processing method, the finished carbon fiber has somewhat different properties. Figure 1.2 compares some of the properties of pitch-based and PAN-based carbon fibers. Typically, PAN-based fibers have a higher specific strength and lower specific modulus than fibers made from pitch or rayon; pitch-based fibers have lower electrical resistivity and higher thermal conductivity. In general, PAN-based fibers are less expensive than pitch-based or rayon-based carbon fibers.
Carbon fibers generally are delivered in "tows," or bundles of fibers. These are found in two forms based on the starting precursor: special acrylic fiber (SAF) or textile tow. Textile tow as referred to here is a modified form of an acrylic fiber used in the textile and carpet industry. The world capacity is around 5 billion pounds. It is made as a large-tow material, with 120k to 320k (k = 1,000) filaments that may be split into smaller tows of around 36k. In most cases, a crimp is applied to allow easier handling of the tow. The textile tow is most often piddled into a box containing 300 to 1,000 pounds for shipping. The crimp is needed to keep the piled fiber together in tows.
The SAF tow, conversely, is manufactured solely for conversion to carbon fiber. It can be a compound similar to a textile tow, but it is made on a special spinning line. The filament count ranges from 3k to 24k or larger if required by a specific manufacturing process, such as pultrusion. This fiber is normally provided on a spool without crimp or twist. The conversion process for the two materials is the same except for the feed end and the take-up end of the line. It is more difficult to maintain a uniform tension on the textile tow precursor, which results in the large tow producing a slightly lower tensile strength and greater variability when compared to SAF materials.
More than 90 percent of all commercial carbon fibers are produced by the thermal conversion of PAN precursor fibers. PAN-based carbon fibers are produced in three distinctly separate process steps: polymerization and wet spinning, stabilization, and carbonization.