Page 184 - Fiber Fracture

FRACTURE OF CARBON FIBERS 169

Pitch processes have been under continual development for the last two decades, and
are now at the stage where high molecular weight, uniform pitches can be produced in
continuous processes. A detailed review of this subject will be found in Lavin (2001a).

A Paradox

The requirements for a strong polymer fiber are well known. They start with ex-
tremely pure ingredients which are polymerized to very high molecular weights. Once
spun, the crystallites are oriented parallel to the fiber axis by stretching. In the case
of pitch-based carbon fibers, the situation is very different. The ingredients come from
a waste stream of unknown and variable composition. Since the molecular weight of
a pitch is positively correlated with its melting point, molecular weight must be kept
down, so that fiber can be spun below about 300°C. Above this temperature, seals are
unreliable, and equipment becomes very expensive. Finally, the as-spun pitch-based
carbon fiber is too weak to stretch. These failings are compensated by the wonderful
self-organizing properties of aromatic carbon; particularly its ability to orient crystallites
along the fiber axis by heat treatment in the relaxed state.

Fiber Formation
Melt spinning of mesophase pitches, as described by Edie and Dunham (1989), is the
preferred method of obtaining high-performance fibers. The controlled drawing process
provides the most uniform continuous filament products, while the wound product form
necessitates uniform treatment of bundles of fibers in downstream processing. However,
processing rates are generally low and greatly depend upon the quality of the pitch
feedstock. Pitch rheology and the arrangement of the discotic liquid crystal was found
to determine mesophase pitch structure and resultant product responses in a study by
Pennock et al. (1993). This structure can be defined on a macroscopic scale by scanning
electron microscopy (SEM), whereas microscopic structure on the atomic scale requires
use of other techniques. such as transmission electron microscopy (TEM). Bourratt et
al. (1990) effectively used these techniques to determine the structure of pitch fibers.
Ross and Jennings (1993) and Fathollahi and White (1994) showed that the orientation
of discs relative to one another and the fiber axis is an important element to control in
the filament formation step.
By utilizing filament formation geometry to establish preferred flow profiles and spin
conditions that complement them, structure can be manipulated and controlled. Exam-
ple geometries, when coupled with appropriate feedstocks and operating conditions,
conducive to structure control and resultant product responses, are shown in Fig. 16.
Fiber cross-sectional structure, as defined by SEM, are schematically represented while
product categorizations of physical and thermal properties are noted. The typical fiber
structures illustrated here have been labelled by several researchers as ‘pacman’ radial,
wavy radial and severe ‘pacman’. Other structures such as random, onion-skin and ‘Pan
Am’ have also been produced and categorized. An illustration of the most common
types is shown in Fig. 17. The fibers with ‘pacman’ cross-sections have longitudinal
splits which may adversely affect physical properties. Downstream processing, within