Page 182 - Fiber Fracture

FRACTURE OF CARBON FIBERS 167

Fig. 13. PAN-based carbon fiber fractured in vacuum. The fibrils are approximately 100 nm in diameter.
(Copyright 1982, reproduced with permission from Elsevier Science.)

A typical commercial pitch is Ashland Aerocarb 70, which has a softening temperature
of 208°C and a viscosity of 1 Pas at 278°C. Additional treatments to selectively reduce
low molecular weight components are described by Sawran et al. (1985).
General purpose fibers are prepared by two different spinning methods, centrifugal
spinning and melt blowing, both of which are high-productivity processes. A more
detailed discussion of these processes will be found in Lavin (2001b).
High-Performance Pitch-Based Carbon Fibers

High-performance fibers are made from mesophase pitch, which is a discotic
liquid crystalline material. While mesophase pitches can be made from many starting
materials, there are only a few which are of commercial interest. These are dealt with in
the sections which follow. These fibers are typically melt spun, and spinning technology
is the same for all pitch types.
There are three common elements in pitch preparation: first, a highly aromatic feed-
stock; second, a process for polymerizing the molecules; third, a process for separating
out the unreacted feed molecules. The feedstock is typically a decant oil from cat cracker
bottoms. When polymerized, the pitch molecule will have characteristics similar to the
molecule shown in Fig. 14. When they get sufficiently large, the pitch molecules ag-
gregate to form spheres, as shown in Fig. 15. The spheres are named for their discov-
erers, Brooks and Taylor (1965). The spheres in turn coagulate to form larger spheres
and then, as polymerization continues, there is a phase inversion and a continuous ne-
matic liquid crystalline phase, typically called mesophase (Greek for changing phase), is
formed.
Pitches are characterized by their fractional solubility in increasingly powerful sol-