About the Carbon Materials Used by Sound Composites

Carbon fiber and graphite fiber composites offer a reinforcement option that allows higher strength with less comparative weight, due to the inherent stiffness of the fibers. The fibers are produced in a process using one of two precursors derived from either PAN (polyacrylonitrile, a thermoplastic), or pitch that is derived from either coal or oil byproducts. These precursors are heated and then spun into thin filaments using textile-type equipment. It is then oxidized around 260ºC under tension, allowing the carbon chains to align. This is followed by carbonization in nitrogen above 1000ºC, to produce carbon fibers. The fibers are composed of 93 to 95% carbon, and have a stiffness (tensile modulus) of 20 to 30 msi. These are the carbon fibers that are primarily found in sporting goods. To achieve fibers where the carbon crystals are further stretched and aligned, and thus can achieve higher stiffness, graphitization takes place around 2000ºC, even as high as 3000ºC. These graphite fibers are over 99% carbon, and have a tensile modulus of 60 msi to as high as 140 msi. The best of these graphite fibers are used primarily in aerospace applications.

Carbon and graphite fibers are subsequently sized and either woven as textiles, or bundled together and spun onto spools in unidirectional form. Combined with either epoxy or cyanate ester resins, they can be cured with heat and pressure to form composite structures that are aligned to optimize their strength in one or more directions. Pre-impregnated composites have resins combined with the fibers in unidirectional sheets. They are spooled and stored frozen until close to the time of use.

Graphite is a polymorph of the element carbon and diamond is another polymorph. The two share the same chemistry, carbon, but have very different structures and very different properties. Diamond is the hardest mineral known to man, Graphite is one of the softest. Diamond is an excellent electrical insulator, Graphite is a good conductor of electricity. Diamond is the ultimate abrasive, Graphite is a very good lubricant. Diamond is usually transparent, Graphite is opaque. Diamond crystallizes in the Isometric system and graphite crystallizes in the hexagonal system. Graphite is the stable form of carbon. All diamonds at or near the surface of the Earth are currently undergoing a transformation into Graphite. This reaction is extremely slow. All of the differences between graphite and diamond are the result of the difference in their respective structures. Graphite has a sheet-like structure where the atoms all lie in a plane and are only weakly bonded to the graphite sheets above and below. Diamond has a framework structure where the carbon atoms are bonded to other carbon atoms in three dimensions as opposed to two in graphite. The carbon-carbon bonds in both minerals are quite strong, but it is the application of those bonds that make the difference.

Graphite is one of the softest minerals (a very slippery lubricant) and is the high-strength component in composites used to build automobiles, aircraft, and golf club shafts and many other products. It is the weakly bonded sheets that slide by each other to yield the slipperiness or softness. Yet when those sheets are rolled up into fibers, and those fibers twisted into threads, the true strength of the bonds becomes apparent. The threads are molded into shape, and held in place by a binder (such as an epoxy resin). The resulting composites have some of the highest strength-to-weight ratios of any materials (excluding diamond crystals and carbon nano tubes).