CARBON FIBERS (Graphite Fibers)

Carbon fibers are special reinforcement types having a carbon content of 92–99 wt%. They are prepared by controlled pyrolysis of organic materials in fibrous forms at temperatures ranging from 1,000–3,000°C. The commercial fibers are produced from rayon, polyacrylonitrile, and petroleum pitch. When acrylonitrile is heated in air at moderate temperatures (≈ 220°C ), HCN is lost, and a ladder polymer is thought to be the intermediate:

Further heating above 1700°C in the presence of nitrogen for a period of 24 hours produces carbon fiber. Carbon fibers are characterized by high strength, stiffness, low thermal expansion, and thermal and electrical conductivity, which makes them an attractive substitute for various metals and alloys.  

 These fibers have longitudinal tensile strengths and moduli ranging from 2.5–7.0 GPa and 230–590 GPa, respectively. A bending beam force detector was developed to measure longitudinal compressive strengths of polyacrylonitrile-based carbon fibers.

Most carbon fiber composites are based mainly on thermosetting epoxy matrices. Current U.S. production of carbon fibers is approximately ten million pounds/year.