Carbon fiber is a textile consisting mainly of carbon. It is produced by spinning various carbon-based polymers into fibers, treating them to remove most of the other substances, and weaving the resulting material into a fabric. This is usually embedded in plastic — typically epoxy — to form carbon fiber reinforced plastic or carbon fiber composite. The most notable features of the material are its high strength-to-weight-ratio and its relative chemical inertness. These properties give it a wide range of applications, but its use is limited by the fact that it is fairly expensive.
The production of this material is usually based on either polyacrylonitrile (PAN), a plastic used in synthetic textiles for clothing, or pitch, a tar-like substance made from petroleum. Pitch is first spun into strands, but PAN is normally in fibrous form to start with. They are converted to carbon fiber by strong heating to remove other elements, such as hydrogen, oxygen, and nitrogen; this process is known as pyrolysis. Stretching the fibers during this procedure helps remove irregularities that might weaken the final product.
The raw fibers are initially heated to around 590°F (300°C) in air and under tension, in a stage known as oxidation, or stabilization. This removes hydrogen from the molecules and converts the fibers into a more mechanically stable form. They are then heated to around 1,830°F (1,000°C) in the absence of oxygen in a stage known as carbonization. This removes further non-carbon material, leaving mostly carbon.
When high quality, high-strength fibers are required, a further stage, known as graphitization takes place. The material is heated to between 1,732 and 5,500°F (1,500 to 3,000°C) in order to convert the formation of the carbon atoms to a graphite-like structure. This also removes the majority of the residual non-carbon atoms. The term "carbon fiber" is used for material with a carbon content of at least 90%. Where the carbon content is greater than 99%, the material is sometimes called graphite fiber.
The raw carbon fiber that results does not bond well with the substances used to make composites, so it is mildly oxidized by treatment with suitable chemicals. The oxygen atoms added to the structure enable it to form bonds with plastics, such as epoxy. After being given a thin protective coating, it is woven into yarns of the required dimensions. These in turn can be woven into fabrics, which are then usually incorporated into composite materials.
Structure and Properties
A single fiber has a diameter of about 0.0002 to 0.0004 inches (0.005 to 0.010 mm); yarn consists of many thousands of these strands woven together to form an extremely strong material. Within each strand, the carbon atoms are arranged in a similar way to graphite: hexagonal rings joined together to form sheets. In graphite, these sheets are flat and only loosely bonded to one another, so that they slide apart easily. In a carbon fiber, the sheets are folded and crumpled, and form many tiny, interlocking crystals, known as crystallites. The higher the temperature employed in manufacture, the more these crystallites orient themselves along the axis of the fiber and the greater the strength.
Within a composite, the orientation of the fibers themselves is also important. Depending on this, the material can be stronger in a certain direction or equally strong in all directions. In some cases, a small piece can withstand an impact of many tons and still deform minimally. The complex interwoven nature of the fiber makes it very difficult to break.
In terms of strength-to-weight ratio, carbon fiber composite is the best material that civilization can produce in appreciable quantities. The strongest are approximately five times stronger than steel and considerably lighter. Research is underway into the possibility of introducing carbon nanotubes into the material, which may improve the strength-to-weight ratio by 10 times or more.
Other useful properties it has are its ability to withstand high temperatures and its inertness. The molecular structure is, like graphite, very stable, giving it a high melting point and making it less likely to react chemically with other substances. It is therefore useful for components that may be subjected to heat and for applications that require resistance to corrosion.
Carbon fiber is used in many areas where a combination of high strength and low weight are required. These include public and private transport, such as cars, airplanes, and spacecraft; sports equipment, like racing bicycles, skis, and fishing rods; and construction. The material’s relative inertness make it well suited for applications in the chemical industry and in medicine — it can be used in implants as it will not react with substances in the body. In civil engineering, it has been determined that old bridges may be spared from destruction and rebuilding through simple carbon fiber reinforcements, which are comparatively cheaper.
As of 2013, the uses, and demand, for carbon fiber have been limited by its cost. A bicycle made from composite typically costs around a few thousand US dollars (USD). Formula One racing cars, which travel at speeds over 200 mph (320 kph), may cost over $1 million USD to build and maintain, a cost determined in no small part by the generous use of this material. Demand has risen significantly, however, due largely to the increase in production of large commercial airplanes. If the cost can be significantly reduced, it may become a universal material for vehicles and small products designed for extreme durability and lightness.