Mechanical properties of carbon fibers -CFCCARBON LTD

The structure of carbon fibers, to a large extent, controls their tensile strength and modulus. Because of this, the manufacturers of both PAN-based and pitch-based carbon fibers are attempting to develop new methods that can modify this structure during the fiber formation or heat treatment steps. Currently, Pan-based carbon fibers exhibit higher tensile strengths, but lower modulus than mesophase pitch-based carbon fibers. However, the new varieties of mesophase pitch-based fibers recently introduced by some company, exhibit significantly improved tensile strengths. The reported modulus for vapor grown fibers are comparable to carbonized PAN fibers, but their tensile strengths are slightly lower. As expected, isotropic pitch fibers exhibit the lowest strengths and modulus of all carbon fibers. In the manufacture of both PAN-based and mesophase pitch-based carbon fibers, increasing the final heat treatment temperature improves the degree of preferred orientation within the fiber and, thus, the fiber modulus. Because of this, the various grades of fiber available from a particular manufacturer are normally the result of changes in this temperature.

The more perfect graphitic structure of mesophase pitch-based carbon fibers, compared with PAN-based fibers, accounts for its higher thermal conductivity. In fact, mesophase pitch-based fibers recently developed by some company, exhibit a thermal conductivity that is three times that of copper.

The precursor fibers used to produce current commercial carbon fibers are produced by melt, melt-assisted, and solution spinning. Melt spinning normally is the preferred fiber formation process because it eliminated the problems of solvent recovery and produces a purer precursor fiber. However, conventional melt spinning cannot be used for polymers, such as PAN and cellulose, which degrade below their melt temperatures. Nevertheless, melt-assisted spinning, a new process, permits PAN to be spun as pseudo-melt. The PAN-based and rayon-based precursor fibers are thermoset, carbonized, and graphitized with similar equipment and at similar conditions. However, their low tensile strength makes pitch-based carbon fibers much more difficult to handle before final heat treatment; therefore, special oxidation ovens are often used for this product. Short, vapor-grown carbon fibers represent the latest entry to the high-performance fiber field. If health issues can be adequately addressed, deisontinuous filaments could become a low-cost reinforcement for composite materials.


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