In the last Tech-Tidbits column we discussed carbon and graphite fibers in general, and then touched upon the PAN-based polyacrylonitrile family. In this extension of the discussion, we will cover another very popular family, the pitch-based carbon and graphite fibers.
The pitch carbon fibers start off as a petroleum-based by-product that initially started off with a 1000C carbonization of a melt-spun precursor. The “pitch” itself is essentially a very high molecular weight, highly aromatic by-product of the petroleum distillation process. The process converts the pitch material from an amorphous, isotropic material into an ordered crystalline material from which it eventually derives significant tensile stiffness due to the alignment of internal structure. In the mid-1960s the tensile modulus was typically in the range of 5-10 Msi but the price predictions indicated they might be much lower in cost than the PAN-based systems. As the process improved, using mesophase state spinning and PAN process-type pyrolysis, the tensile modulus range expended to much higher levels (55-120 Msi) and improved tensile strengths (200-550 ksi by the mid- to late-1970s.

Pitch “tow count” differs a bit from PAN carbon fibers. While there is a 12K tow count held in common with PAN, the pitch materials generally come in 2K, 4K, 10K and 12K offerings – with nothing greater than the 12K materials. Hence, there is no “large tow” pitch based carbon fiber materials per se. Fiber diameter in the pitch fibers tends to run slightly larger – running 7-10 microns vs. 5-8 microns for the PAN materials. In general, the PAN fibers typically run in a density range of about 1.76-1.91 gm/cm3 whereas pitch fibers weigh in at a higher density – ~2.10-2.20 gm/cm3. Some typical properties are shown in Table 1.

The most notable performance property different between PAN and pitch carbon fibers are in their range of thermal conductivity properties. Pitch fibers have significantly higher thermal conductivity values. This makes them ideal replacements in thermal management design situations where copper materials are essentially the “heavy gorilla” on the block – an undesirable situation for design areas where “lightness is truly the desire.

Pitch carbon fiber stiffness, typically at the expense of tensile strength, still provides opportunities for stiffness-driven design composite components such as satellite structures where both stiffness and thermal conductivity play an important combined role in overall performance.

Figure 1 shows the regions where PAN and pitch carbon/graphite typically satisfy structural design needs. As can be seen in the figure, PAN provides “strength” and pitch provides “stiffness” – generally speaking.

General References:
1. Handbook of Composites, 2nd Edition, Edited by S.T. Peters, Chapman & Hall, 1998
2. Fundamentals of Composites Manufacturing: Materials, Methods and Applications, 2nd Edition, A. B. Strong, Society of Manufacturing Engineers, 2008.
3. Carbon and High Performance Fibres: Directory and Databook, 6th Edition, Edited by T. F. Starr, Chapman & Hall, 1995.
4. Carbon Fibers and Their Composites, P. Morgan, CRC Press/Taylor & Francis, 2005
5. Primary Carbon and Graphite Fiber Web Sites:
a. AKSA: www.aksa.com
b. Cytec Carbon Fibers: www.cyteccarbonfibers.com
c. Grafil Inc.: www.grafil.com
d. Hexcel: www.hexcel.com
e. Mitsubishi Rayon Company Ltd.: www.mrc.co.jp
f. Nippon Graphite Fiber Corporation: www.ngfworld.com
g. SGL Technologies GmbH: www.sglgroup.com
h. Toho Tenax: www.tohotenax-eu.com
i. Toray Industries: www.toray.com
j. Zoltek Corporation: www.zoltek.com