handbook of carbon, graphite, diamond and fullerenes

Carbon Fibers 187
However, this high degree of crystallinity also results in low shear and
compressive strengths. In addition, these carbon fibers tend to have flaws
such as pits, scratches, striations, and flutes. These flaws are detrimental
to tensile properties but do not essentially affect the modulus and the
thermal conductivity.! 161
4.0 CARBON FIBERS FROM RAYON
Rayon-based fibers were the first carbon fibers produced commercially.
They were developed in the 1960's specifically for the reinforcement
of ablative componentsfor rockets and missiles. However, they are difficult
to process into high-strength, high-modulus fibers and have been replaced
in most structural applications by PAN or pitch-based fibers.
4.1 Rayon Precursor
A number of rayon fibers are available, but the most suitable is the
highly polymerized viscose rayon. The molecular structure is asfollows:! 11 '
OH OH
CH2
CH2
CH-O CH-O
/ \ / \
-CH CH-O-CH CH-O
\ / \ /
CH-CH CH-CH
II II
OH OH OH OH
As can be seen, this structure has many heteroatoms (O and H) which
must be removed. Moreover, many carbon atoms are lost due to the
formation of volatile carbon oxides during pyrolysis. As a result the carbon
yield is low (