Carbon fibers have now been available for some 35 years and in that time, many significant improvements have been made. The replacement of cellulose based precursors with PAN precursors provided carbon fibers with good strength and improved the moduli, followed later by the intermediate modulus PAN based range, whilst the pitch based fibers were available with very high moduli and were truly graphitic.
As the world political situation changed, the requirement for carbon fibers in military applications has decreased, entailing major defense cuts. The emphasis has shifted to commercial applications, which have grown extensively. Therefore, it is not surprising that carbon fibers are involved in a whole gamut of applications. Developments do occur very rapidly in the composites field and some of these applications may now have been discontinued or replaced, but sever to illustrate the diverse applications of carbon fiber.
Although this represents the first stage of making carbon fiber from a PAN precursor, the production of opf does not follow exactly the same route and is tailor made for a given end use.
Flameproof applications: The opf, with an oxidized density greater than 1.38g/cm3, is non-flammable and finds many uses as a non-flammable material, which includes a replacement material for asbestos. Initially, a large opf tow is converted into an opf yarn via a modified worsted spinning route, the opf spun yarn can then be woven into a fabric.
The measurement of relative flammability can be ascertained by determining the Oxygen Index, often referred to as the Limiting Oxygen Index (LOI), when a carefully regulated, known mixture of N2 and O2 is passed over the sample, which has been ignited with the highest oxygen value at which the specimen does not burn for more than 3 min, or over a length of 50mm. In general, LOI values for flammable fibers are <25, whilst flame retardant fibers are 25-28 and flame resistant fibers have LOI values in excess of 28. Flameproof fibers are completely unaffected by heat. The LOI value can only be used as an indication, since the presence of additives, or the way the sample is supported, can significantly affect the result. Although the LOI value for opf is high, when it is admixed with an aramid to improve the abrasion resistance and/or wool to confer a better feel, then this will reduce the overall LOI.
Aviation and aerospace: There is no doubt that aircraft are an extremely safe mode of mass transport but, unfortunately, when an accident does occur, it tends to be a major disaster and possibly, the two main sources of fatality are crash impact and fire. The fire may be initiated by an electrical fault, neglected cigarettes, or the ignition of the fuel spilling from the aircraft following the crash. Danger from fire can be a result of the heat of the fire itself, from asphyxiation, or from poisoning by toxic gases released by the combustion process.
Since it was necessary to retain the inner foam core and the outer decorative fabric, it was decided to use a series of protective layers between the two components that would protect the foam from the effects of the heat.