Where is carbon fiber made

Carbon fiber

6 µm thick carbon fiber compared to 50 µm thick human hair.

Carbon fiber (also Carbon fibers) are industrially manufactured fibers made from carbonaceous raw materials that are converted into graphite-like carbon by pyrolysis. A distinction is made between isotropic and anisotropic types: isotropic fibers have only low strengths and less technical importance, anisotropic fibers show high strengths and stiffnesses with low elongation at break.

A carbon fiber has a diameter of about 5-8 micrometers. Usually 1,000 to 24,000 individual fibers (filaments) are combined to form a bundle (roving) that is wound onto spools. Further processing takes place, for example, on weaving machines or knitting machines into textile structures.

As short-cut fibers, they can be added to polymers and processed into plastic components using extruder and injection molding systems. In addition to these low-filament types, there are also so-called HeavyTow types with 120,000 to 400,000 individual fibers, which are mainly processed into short-cut fibers, but also into textile fabrics. It is also possible to have such HeavyTows with subtows e.g. B. in the form of seven times 60,000 individual filaments.

The fibers are mainly used for the production of carbon fiber reinforced plastic.

properties

Typical properties of HT carbon fibers
density 1.8 g / cm³
Filament diameter 6 µm
tensile strenght 3530 MPa (N / mm²)
Tensile modulus 230 GPa
Elongation at break 1,5 %
Typical properties of UMS carbon fibers
density 1.8 g / cm³
Filament diameter 6 µm
tensile strenght 4560 MPa (N / mm²)
Tensile modulus 395 GPa
Elongation at break 1,1 %

Carbon fibers have excellent electrical and thermal conductivity. They have a negative coefficient of thermal expansion in the longitudinal direction. As a result, they become shorter and thicker when heated.

Designations:

  • HT - high strength (High Tensity / High Tenacity)
  • IM - intermediate (intermediate modulus)
  • HM - extremely stiff (high modulus)
  • UM - (Ultra Modulus)
  • UHM - (Ultra High Modulus)
  • UMS - (Ultra Modulus Strength)
  • HMS - high stiffness / high strength

Manufacturing

Carbon fiber tubes, in the background carbon fiber scrims

Thomas Alva Edison received a patent as early as 1881 for the carbon fiber incandescent lamp he developed with filaments made from pyrolysed bamboo fibers.

A big step was taken in 1955 with the production of fibers with oriented crystal structures in the British Royal Aircraft Establishment.

Carbon fibers are made from organic raw materials. Primarily those compounds come into consideration that can first be converted into an infusible intermediate stage and then carbonized to carbon in a pyrolysis process while retaining their shape. By stretching (applying a tensile stress) in this temperature treatment step, the orientation of the atomic structure in the fibers can be changed in such a way that higher strengths and stiffnesses of the fibers are achieved during carbonization.

During this carbonization treatment, all elements except for the main part of carbon are split off in gaseous form. The relative carbon content increases with increasing temperature, which is usually in the range of 1300-1500 ° C. This achieves a carbon content of 96 to 98 percent by weight.

Graphitization is used above 1800 ° C. Above all, the structure of the graphitic carbon layers is being perfected more and more. The distance between the layers between these carbon layers, however, remains above the value known from the actual graphite. That is why the term “graphite fiber (fiber)” used in the English-speaking world is strictly speaking incorrect. This also applies to the terms "graphite fiber" and "carbon fiber" used in German-speaking countries.

The structural diversity of the fibers with the wide range of properties results from the anisotropy of the graphitic layers that can be controlled via the manufacturing parameters. With continuous fibers, depending on the fiber type, almost the theoretical stiffness value is achieved, but usually only 2-4% of the theoretical strength. In the case of fibers which, in deviation from the method described above, are deposited from the gas phase (so-called whiskers with a very short length), significantly higher strengths can be achieved.

Today there are three established starting materials for continuous carbon fibers:

Rayon / viscose (cellulose)

The cellulose-based viscose fibers produced using the viscose process are the starting material for the carbon fibers. Due to the raw material, these show a less than perfect carbon structure. They thus show a comparatively low thermal and electrical conductivity. (However, when used as a filament, the high ohmic resistance was favorable.) They are therefore mainly used as insulating materials with high thermal loads (in the absence of air / oxygen), e.g. in furnace construction.

Polyacrylonitrile (PAN)

Most of the high-performance fibers (HT / IM) in use today are manufactured from polyacrylonitrile through stabilization reactions in air and subsequent pyrolysis under protective gas. Their main feature is their high tensile strength. A distinction is made between low-filament and multifilament fibers (HeavyTow). For the latter, the cheaper production technologies of the textile industry are used; therefore they are the most cost effective.

Bad luck (different origins)

As a raw material, pitch is much cheaper than PAN, but the cleaning and processing costs are so high that fibers made from PAN are still cheaper.

If the pitch is merely melted, spun and graphitized, isotropic carbon fibers with lower strength values ​​are obtained. Only the conversion into the so-called mesophase by a hydrogenation treatment allows an orientation of the carbon network planes along the fiber axis by stretching during the manufacturing process.

This then also allows the production of fibers with high rigidity (HM). With high tensile strength (HMS) at the same time, these fibers are only used in special applications for reasons of cost.

Further processing

For further processing, the fibers are combined into so-called rovings. The types 1K, 3K, 6K, 12K, 24K and 48 or 50K are common here. In rare cases, 80K is also used. 1K means that 1000 individual fibers are combined into one strand.

The rovings are usually processed into fabric mats for use as reinforcement fibers. The most common fabric used in automotive engineering has a plain weave.

application

Carbon fiber winding pattern in the Carbon Obelisk of Emscherkunst. 2010

In order to be able to use the mechanical properties of the fibers, they are further processed in the production of fiber composites, in particular fiber-plastic composites, and for some time also in ceramic fiber composites.

In general usage, especially for sports equipment in all sports, there are terms such as Carbon, Graphite (s) and Carbon fiber typically for carbon fiber reinforced thermoset plastics.

Compared to glass fibers, carbon fibers are characterized by their lower weight and higher price. They are therefore mainly used in the aerospace industry and in sports equipment (for example fishing rods, racing bikes, mountain bikes, tennis rackets, speed skates, rowing boats, windsurfing equipment). For example, the so-called monocoque and other parts of Formula 1 racing cars are made from a carbon fiber-reinforced plastic.

Impressive examples from aviation are the vertical stabilizer of the Airbus A380 or the fuselage of the Boeing 787.

In England a bridge is made of a concrete reinforced with carbon fibers that can withstand enormous tensile and compressive forces.

Carbon fiber reinforced plastic components are now widespread in high-quality bicycles (racing bikes / mountain bikes). In the meantime, not only the frames, but also other components such as cranks, wheels, handlebars, seat posts, etc. are made from CFRP.

Carbon fibers are also used in archery. Modern sport arrow shafts are made with carbon fiber reinforcement, which are ideal for long distances due to their low weight.

In the area of ​​water ski production, high-quality fishing rods and bows for string instruments, carbon fibers can be used for another application.

In dentistry, carbon fibers are glued into roots for splinting teeth, but also in the form of pins to gain retention of abutments for destroyed teeth.

The conductivity and the small size (diameter) of carbon fibers are used militarily with graphite bombs. Short sections of fiber are distributed in a bomb by a decomposing charge over the respective object. The fibers are distributed in electrical systems and devices by air currents, as well as assisted by fans or ventilation and cooling systems, and even reach inaccessible places inside computers. The short circuits caused then lead to the failure of even large systems if the control devices are affected.

See also

Web links

supporting documents

  1. ↑ wiki.r-g.deFiber composites(note: 1 ohmmeter = 100 ohm centimeter)