Carbon and graphite fibers are produced using PAN-based or pitch-based precursors. The precursor undergoes a series of operations. In the first step, the precursors are oxidized by exposing them to extremely high temperatures. Later, they go through carbonization and graphitization processes. During these processes, precursors go through chemical changes that yield high stiffness-to-weight and stength-to-weight properties. The successive surface treatment and sizing process improves its resin compatibility and handleability.
PAN refers to polyacrylonitrile, a polymer fiber of textile origin. Pitch fiber is obtained by spinning purified petroleum or coal tar pitch. PAN-based fibers are most widely used for the fabrication of carbon fibers. Pitch-based fibers tend to be stiffer and more brittle. The cost of carbon fiber depends on the cost of the raw material and process. The PAN-based precursor costs around $1.50 to $2.00/lb. During oxidation and carbonization processes, the weight reduces to almost 50% of the original weight. Considering the weight loss, the cost of fibers based on raw material alone becomes $3.00 to $4.00/lb. The fabrication method for the production of carbon fibers is slow and capital intensive. Therefore, higher tow count is produced to lower the cost of the fibers. There is a limitation on increasing the tow size. For example, a tow size more than 12K creates processing and handling difficulties during filament winding and braiding operations.
Pitch-based carbon fibers are produced in the same way as PAN-based fibers but pitch is more difficult to spin and the resultant fiber is more difficult to handle. Pitch itself costs pennies a kilogram, but processing and purifying it to the fiber form are very expensive. Generally, pitch-based fibers are more expensive than PAN-based fibers.
The cost of carbon fibers depends on the strength and stiffness properties as well as on the tow size (number of filaments in a fiber bundle). Fibers with high stiffness and strength properties cost more. The higher the tow size, the lower the cost will be. For example, 12K tow (12,000 filaments per fiber bundle) costs less than 6K tow.