Page 183 - Fiber Fracture

168 J.G. Lavin

CH3
Fig. 14. Typical pitch molecule.

"Pole"

Trace of lamellae
direction

Edges of disk
of sphere
"Pole"
Fig. 15. Brooks and Taylor mesophase sphere.

vents; for example toluene, pyridine, quinoline. The highest molecular weight fractions
are not soluble in any known solvent. It is believed that the smaller molecules in the pitch
are solvents for the larger ones, and allow the pitch to flow at elevated temperatures.
Petroleum-based pitches are typically made from the same slurry or decant oils used
to make isotropic pitches. The earliest processes for making mesophase pitches are sim-
ilar to that described by McHenry (1977). They used a long heat soak (typically about
30 h at 400°C) under an inert atmosphere, while a gas sparge was used to take away
volatile compounds. Such pitches might typically have a molecular weight of about 1000
Dalton, and melt at about 300°C. They would also be characterized by high quinoline
insolubles.
Coal tar pitches are a by-product of coke ovens associated with steel-making
operations. They differ from petroleum pitches in their rheological properties; for a
given molecular weight the flow viscosity is much higher. Coal tar pitches also have
fewer aliphatic groups on the molecules, which makes for longer stabilization cycles.
A breakthrough in preparation of coal tar pitches came when the Japanese Agency of
Industrial Science and Technology (1983) developed a process for hydrogenating them,
significantly reducing viscosity and reducing quinoline insolubles to zero. The physical
properties of fibers from coal tar pitches are generally competitive with fibers from
petroleum pitches, except that, so far, they have not been capable of making the highest
modulus products (800 GPa and higher).