Carbon fibers are typically made from polyacrylonitrile (PAN). Upon heating to 300C, the cyano side groups form cyclic rings with each other.
Further heating at 700:C causes these rings to become aromatic pyridine groups due to the loss of hydrogen from the carbon atoms.
By slowly applying heat between 400-600:C, adjacent chains fuse together to form ribbons, expelling more hydrogen gas.
In order to form wider ribbons, the temperature is increased to 600-1300:C, and nitrogen gas is expelled.
These ribbons contain carbon in its hexagonal graphitic structure, and do not have the long range ordering, so carbon fibers are amorphous. Graphite is simply planes of carbon atoms in a hexagonal lattice, with only weak van der Waals forces between parallel planes. These weak forces allows the planes to easily slide over each other, which makes graphite very brittle, but useful as a lubricant and in pencil lead.
The carbon ribbons described above are oriented parallel to the fiber axis. Carbon fibers achieve their exceptional strength due to the interlocking and folding of these ribbons.
There is a lot of confusion between the usage of the terms carbon fiber and graphite fiber. Many companies claim their product contains graphite fibers, but what they really mean is carbon fiber. People confuse these terms because the structure of carbon fiber contains carbon in its graphitic form, but a carbon fiber is not simply a fiber of graphite.
Conversion of Carbon Fiber to Graphite
Upon prolonged heating at higher temperatures (above 1300:C), carbon fibers will eventually become pure graphite. The graphitic form of carbon is a more stable form of carbon than carbon fibers, due to extensive resonance stabilization from single and double carbon bonds. Carbon fibers are also stabilized by resonance, but since they are amorphous, this effect is lessened due to the lack of long range ordering.
Kevlar and Nomex are two examples of aromatic polyamides. The synthesis of these polymers is done through the nucleophilic acyl substition between a diamine and diacid, both substituted with a phenyl group. (The acyl group is Cl-C=O) The nucleophile is the nitrogen from the diamine, which then seeks out an electron poor site, the carbon from the diacid. The nitrogen substitutes itself for the chlorine atom, which forms HCl with the hydrogen from the diamine. The reaction is catalyzed with LiCl and DMF (dimethylformamine).
The structures of Kevlar and Nomex only differ in the phenyl group linkage orientation. Kevlar is para-substituted, while Nomex is meta-substituted. A para-substitution lead to increased rigidity along the polymer chain axis, but this also makes Kevlar difficult to process because it's not soluble in typical solvents. The meta-substitution compromises a little rigidity, but allows the material to be easily made into paper form, from which a honeycomb structure can be formed by gluing and pulling apart multiple sheets of Nomex fiber. Kevlar and Nomex are both wholly aromatic polyamides, meaning they don't contain any flexible aliphatic chains that would compromise their stiffness.
An epoxy resin is made through the ring opening reaction between Bis-phenol-A and epichlorohydran.
The resin is combined with a hardener, or crosslinking agent to create a network of interconnected polymer chains. Common hardeners are based upon diamine-substituted monomers.
A stiffer substituted group delays the time in which the resin reacts with the hardener. The time in which the mixture of the resin and hardener is still in liquid form is known as the pot life. An aliphatic diamine hardener typically only has a pot life of 30 minutes or less, while an aromatic diamine may have a pot life of up to 60 minutes. After the pot life has passed, the mixture becomes a gel, which is the first stage of the curing process. The transformation from liquid to gel occurs due to the formation of crosslinks between chains. The cure progresses as more crosslinks are formed between chains, until the mixture has completely solidified into a rigid matrix. The TTT diagram shows how applying heat can accelerate the cure process.