handbook of carbon, graphite, diamond and fullerenes

78 Carbon, Graphite, Diamond, and Fullerenes
well-defined melting point. The carbon yield is not fixed and varies with the
composition (see Ch. 5, Sec. 1.2).
Petroleum pitch is the residue from the distillation of petroleum
fractions. Like coal-tar pitch, it has a varying composition, consisting mostly
of aromatic and alkyl-substituted aromatic hydrocarbons. A solid at room
temperature, it also has a broad softening range and no well-defined melting
point. The formation of its mesophase is related to the chemical constituents
and the asphaltene fractions J 12 ' Coal-tar and petroleum-pitches do not
crystallize during cooling but can be considered as supercooled liquids.
Single hydrocarbons of interest as precursor materials are anthracene,
phenantrene and naphthalene. The latter is a precursor in the
production of carbon fibers. These compounds are obtained by recovery
from the distillation of coal-tar between 170 - 230°C for naphtalene, and
>270°C for anthracene and phenantrene. Other coal-tar derivatives are
biphenyl and truxene (see Sec. 3.2 below).
These materials can be processed singly or modified by the addition
of solvents or other materials to alter their carbonization and graphitization
characteristics, as will be shown in Ch. 8J 13 '
2.4 Carbonization of Polymers
Polymers are organic materials consisting of macromolecules, composed
of many repeating units called "mere", from which the word polymer
is derived. The major polymers in the production of synthetic graphite are
listed in Table 4.1 above and their chemical structures are shown in Fig. 4.4.
As a rule, polymers have a lower carbon yield than aromatic hydrocarbons
since they contain, in addition to hydrogen and carbon, other elements
with higher molecular weight, such as chlorine, oxygen, or nitrogen. These
elements must be removed. The carbonization mechanism of polymers is
usually more intricate than that of aromatic hydrocarbons since the composition
is more complex. The carbon yield is unpredictable in many cases. f2]
One of the oldest polymer precursors is cellulose which has been used
for generations in the production of charcoal from wood and, early in the
twentieth century, for lamp filaments from cotton or bamboo by Thomas
Edison.
In linear polymers (that is non-aromatic), such as polyethylene and
polystyrene, the polymeric chain breaks down into increasingly smaller
molecules which eventually gasify. As a result, these materials have a low