Page 178 - Fiber Fracture

FRACTURE OF CARBON FIBERS 163

PAN-BASED CARBON FIBERS

Polyacrylonitrile (PAN) fibers are made by a variety of methods. The polymer is
made by free-radical polymerization either in solution or in a solvent-water suspension.
The polymer is then dried and re-dissolved in another solvent for spinning, either
by wet-spinning or dry-spinning. In the wet-spinning process the spin dope is forced
through a spinneret into a coagulating liquid and stretched, while in the dry-spinning
process the dope is spun into a hot gas chamber, and stretched. For high-strength carbon
fibers, it is important to avoid the formation of voids within the fiber at this step.
Dry-spun fibers are characterized by a 'dog-bone' cross-section, formed because the
perimeter of the fiber is quenched before much of the solvent is removed. The preferred
process for high-strength fiber today is wet-spinning. Processes for melt-spinning PAN
plasticized with water or polyethylene glycol have been developed, but are not practiced
commercially. A significant improvement in carbon fiber strength was obtained by
Moreton and Watt (1974) who spun the PAN precursor under clean room conditions.
The strength of fibers spun in this way and subsequently heat treated was found to
improve by >SO% over conventionally spun fibers. The mechanism is presumed to be
removal of small impurities which can act as crack initiators. This technology is believed
to be critical for production of high strength fibers such as Toray's T800 and T1000.

Polyacrylonitrile 7^r('(*

CN

Cyclize

Stretch

H H H
O P O
Oxidize

H H H
Fig. 8. PAN-based carbon fiber chemistry: cyclization and oxidation.