handbook of carbon, graphite, diamond and fullerenes

3.0 CARBON FIBERS FROM PITCH
3.1 Pitch Composition
Carbon Fibers 183
As seen in Ch. 5, Sec. 2.1, pitch is a by-product of the destructive
distillation of coal, crude oil, or asphalt. It meets the conditions for carbonfiber
production stated above (Sec. 2.1) and is inexpensive and readily
available. Its carbon yield can exceed 60%, which is appreciably higher
than the yield of PAN (~ 50%).
The composition of pitch includes four generic fractions with variable
proportions.^ 11 ] These are:
• The saturates, which are aliphatic compounds with low
molecular weight similar to wax
• The napthene aromatics, which are low-molecular- weight
compounds with saturated ring
• The polar aromatics, which are medium-molecular- weight
compounds with some heterocyclic molecules
• The asphaltenes, which have high molecular weight and
a high degree of aromaticity. The higher the ratio of
asphaltene, the higherthe softening point, thermal stability,
and carbon yield.
3.2 Carbon Fibers from Isotropic Pitch
Low-cost carbon fibers are produced from an isotropic pitch with a lowsoftening
point. The precursor is melt-spun, thermoset at relatively low
temperature, and carbonized. The resulting fibers generally have low
strength and modulus (~ 35 - 70 GPa). They are suitable for insulation and
filler applications. Their cost dropped to less than $20/kg in 1992J 61
3.3 Carbon Fibers from Mesophase Pitch
Carbon fibers from mesophase pitch have high modulus and medium
strength. They are presently more costly than PAN-based fibers ($65/kg in
1992).
Precursor Pitch. The precursor material is a mesophase pitch,
characterized by a high percentage of asphaltene. Table 8.5 shows the
approximate composition of three common mesophase compounds.