Carbon fibers are filaments that consist of almost completely pure carbon. The tensile strength of such fibers can be up to five times the strength of steel, making them extremely useful where a combination of strength and lightness is needed.
One of the earliest uses of a form of carbon fiber was in the incandescent lamp of the U.S. inventor Thomas Edison in 1879. Edison charred cotton thread to drive off water from the cellulose of the cotton. The result was a carboased material that glowed white hot as it conducted an electrical current.
Carbon is an excellent refractory material owing to its high vaporization temperature of 6700°F (3700°C) and its resistance to chemical and physical change even at high temperatures. These properties make carboiber felts, wools, and papers useful for the filtration of hot corrosive fluids, for higemperature insulation, and for catalytic purposes in industrial processes. Early carbon fibers were brittle and lacked mechanical strength, thus they had limited uses.
During the 1950s, a method was developed for making carbon fibers from viscose rayon filaments. The fibers so produced had sufficient mechanical strength to make them useful for reinforcing phenolic resins such as Bakelite. In the 1960s, advances in carboiber technology led to techniques for producing loensity fibers of great stress resistance and tensile strength.
Starting materials
All methods for producing carbon fibers start with polymeased fibers called precursors. These precursors are carbonized (converted into higarbon materials) by controlled pyrolysis (decomposition by heat). The carbonization pruct retains the fibrous structure of the precursor.
A variety of natural and synthetic materials have been used as precursors for the manufacture of carbon fibers. Natural cellulosic fibers, such as cotton, hemp, and flax, produce a low carbon yield and poouality fibers; viscose rayon—a semisynthetic cellulosic precursor—produces carbon fibers that are somewhat superior to those based on natural cellulose.
The most significant advances were made by using polyacrylonitrile fibers as precursors. Polyacrylonitrile, or PAN, is the material used to make commercial acrylic fabrics.
Other potentially important synthetic precursors include polyvinyl alcohol, polyimides, and polyamides. Various fibers based on pitch, wool, and bitumen are also practical starting materials.
Manufacture
The conversion of precursor fibers into carbon fibers of consistently high quality requires heat treatment under strictly controlled conditions of timing, temperature, atmosphere, and tension. In batch processing, a bundle of many thousands of fibers is subjected to a variable heating schedule in a single furnace; in continuous processing, the fibers travel through a sequence of heating zones at different temperatures.
A typical sequence for the conversion of a polymeric precursor begins with a pretreatment, such as stretching in steam. After pretreatment, the fibers are heated to between 390 and 570°F (200–300°C); this strengthens the fibers by linking together neighboring polymer chains through chemical bonds called crosslinks. The crosslinked fibers are then carbonized by heating to around 1800°F (1000°C) in an atmosphere of an inert gas such as argon. The decomposition of the precursor polymer causes the fibers to shrink and releases gases that are carried away by the inert atmosphere. During the carbonization of PAN, for example, ammonia, carbon dioxide, water vapor, hydrogen cyanide, and nitrogen are given off. The final stage is the graphitization of the fibers by heat treatment at 3600 to 5400°F (2000–3000°C), which promotes the formation of graphite crystals within the fibers.
