Carbon fiber
Carbon fibers - also called carbon fibers or carbon fibers (colloquially also incorrectly referred to as carbon fibers) - are industrially manufactured fibers made from carbon-containing raw materials, which are converted into graphite- like carbon through chemical reactions adapted to the raw material . A distinction is made between isotropic and anisotropic types: isotropic fibers have only low strengths and are of minor technical importance, anisotropic fibers show high strengths and stiffnesses with at the same time low elongation at break in the axial direction.
The most important property of carbon fibers as a stiffening component for lightweight construction is the modulus of elasticity ; the modulus of elasticity of the best fibers is close to the theoretical modulus of elasticity of graphite in the a-direction .
A carbon fiber or filament has a diameter of about 5-9 micrometers . Usually 1,000 to 24,000 filaments are combined into a multifilament yarn ( roving ), which is wound on. The further processing to textile semi-finished products such. B. woven fabrics , braids or multiaxial fabrics are carried out on weaving machines , braiding machines or multiaxial knitting machines or, in the area of the production of fiber-reinforced plastics, directly on prepreg plants , pultrusion plants or winding machines.
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 HT 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 combine such heavy tows 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 (CFK = C fiber reinforced plastic). The abbreviation CFRP (American English Carbon Fiber Reinforced Plastic ) is also used from English .
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% |
| Electronegativity (EN) χ | 2.50 |
Carbon fibers are electrically and thermally very conductive, the electronegativity EN has a very high value of 2.50. The difference to iron (EN = 1.64) is very high at 0.86, which is considerably corrosive in the presence of an electrolyte. For comparison, the difference in the material pairing iron to aluminum (EN = 1.47) is only 0.17. Carbon fibers also have a negative coefficient of thermal expansion in the longitudinal direction . As a result, when heated, they initially become shorter and thicker.
The two aforementioned properties make it imperative to isolate carbon fiber-based components from other metallic components both mechanically and electrically if, during use, temperature fluctuations and contact with outside air, water and, in particular, with sea water and other electrolytes (for example meltwater with road salt in the Road traffic) is to be expected. The rate of electrocorrosion of iron which is in direct contact with carbon fibers is high under a suitable electrolyte.
- Specific heat capacity : 710 J / (kg K )
- Thermal conductivity : 17 W / (m K )
- Thermal expansion coefficient : −0.1 · 10 −6 / K
- Specific electrical resistance : 1.6 10 −5 Ohm m (= 16 Ω mm 2 / m)
Carbon fiber types:
- HT - high strength ( high tenacity )
- UHT - very high strength ( Ultra High Tenacity )
- LM - Low Modulus
- IM - intermediate ( Intermediate Modulus )
- HM - highly rigid ( high modulus )
- UM - ( Ultra Modulus )
- UHM - ( Ultra High Modulus )
- UMS - ( Ultra Modulus Strength )
- HMS - high stiffness / high strength ( High Modulus / High Strain )
Manufacturing
As early as 1881, Thomas Alva Edison received a patent for the carbon fiber incandescent lamp he developed with filaments made from pyrolysed bamboo fibers .
A big step was taken in 1963 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 question that can first be converted into an infusible intermediate stage and then carbonized to carbon in a pyrolysis process while maintaining their shape. By stretching (applying a tensile stress ) during 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 from 1300 to 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 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 annealing treatment increases the modulus of elasticity due to the structural approach to the graphite grid, but this reduces the strength.
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 a departure 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 do not show a perfect carbon structure. They therefore have a comparatively low thermal and electrical conductivity. (When used as a filament, however, the high ohmic resistance was favorable.) They are therefore mainly used as insulating materials with high thermal loads (in the absence of air / oxygen) , for example 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 Niederfilament- and multifilament yarns ( English HeavyTow ). The latter uses the cheaper production technologies of the textile industry, which is why 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 carbonized, isotropic carbon fibers with lower strength values are obtained. Only the conversion into the so-called mesophase by a hydrogenation treatment allows 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 cost reasons.
Further processing
For further processing, the fibers are combined into so-called filament yarns. The types with 67 tex (1 K), 200 tex (3 K), 400 tex (6 K), 800 tex (12 K) and 1600 tex (24 K), rovings with a filament count of more than 24 K are common , e.g. B. 50 K, 100 K or 400 K are called heavy tows . The specification 200 tex stands for a weight of (200 g) / (1000 m) and 1 K means that 1000 filaments are combined into one yarn.
The coarser yarns ( called “ rovings ” for textile glass ) are used, for example, as reinforcing fibers for flat structures. In aircraft construction, yarn sheets or fabrics, the so-called prepregs, which are pre-impregnated with resin, are used with a low or medium weight per unit area. The most common product used in automotive engineering is a multiaxial fabric.
application
In order to be able to use the mechanical properties of the fibers, they are further processed in the production of fiber composite materials , in particular fiber-plastic composites , and for some time also in ceramic fiber composite materials . The importance of carbon-fiber-reinforced plastics in high-performance mechanical engineering has been increasing significantly for some years; they were already used in aircraft construction before that. In general usage, especially in sports equipment for all sports, terms such as carbon , graphite (s) and carbon fiber typically stand for carbon fiber-reinforced thermoset plastics .
Compared to glass fibers, carbon fibers are characterized by a lower weight and a higher price. They are therefore particularly in the aviation and aerospace as well as sports equipment (such as fishing rods, racing bikes, mountain bikes, tennis rackets, speed skates used, rowing boats, windsurfing equipment). Thus, also the so-called, for example monocoque and other parts of Formula 1 - racing cars from carbon fiber-reinforced plastic produced.
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 carbon fiber reinforced concrete that can withstand enormous tensile and compressive forces.
Carbon fiber-reinforced carbon is mainly used in space travel as a material for heat shields or booster nozzles , but it is also used in the hollow glass industry as a replacement for asbestos or as a lining for fusion reactors .
Carbon fiber reinforced plastic components are now widely used in some bicycles , such as B. racing bikes / mountain bikes. In the meantime, not only the frames, but also other components such as cranks, wheels, handlebars, seat posts, etc. a. made of 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 water skiing micromechanics, in high quality fishing rods and bows for string instruments and even instruments of the violin itself carbon fibers are another possible application.
In dentistry, carbon fibers are glued into the roots for splinting teeth, but also in the form of pins for the retention of abutments for destroyed teeth.
The electrical conductivity and the small size (diameter) of carbon fibers in graphite bombs are used for military purposes. The short carbon fiber sections placed in a bomb are distributed over the respective object by a decomposing charge . The fibers are distributed in electrical systems and devices by air currents, as well as by fans or ventilation and cooling systems, and can 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.
Testing of materials reinforced with carbon fibers
Both destructive and non-destructive testing methods are used to test carbon fiber-reinforced materials. A destructive test (e.g. notch impact test) is used to test the breaking load of the material or the breaking behavior, for example. Non-destructive testing methods, such as ultrasonic or acoustic testing, are primarily used to test defects in the polymer component of the composite (delaminations, cavities, bubbles).
Effects in the fiber structure itself (alleys, cracks, undulations, folds, overlaps, fiber accumulations or misorientations) are measured using high-frequency eddy current methods.
Similar eddy current methods are used for the local determination of the basis weight in CFRP components and textiles.
Manufacturer
The largest manufacturers by production capacity in 1000 t (as of 2018) are:
| Manufacturer | capacity |
|---|---|
| Toray (with Zoltek ) | 47.5 |
| SGL carbon | 15th |
| MCCFC | 14.3 |
| TohoTenax | 12.6 |
| Hexcel | 12.5 |
| Formosa Plastics | 8.8 |
| Solvay ( Cytec ) | 7.0 |
| Zhongfu-Shenying | 6th |
| Hengshen Fiber Material | 5 |
| DowAksa | 3.6 |
Disposal and recycling
The disposal and recycling of materials containing carbon fibers are still under development and have not been finally resolved. There are various approaches that are being pursued, from re-using the fiber in fiber-reinforced components to thermal recycling. The Fiber Institute Bremen e. In 2008–2010 V. experimented with a technique in which directed fibers with a fiber length of about 60 mm were applied to a thermoplastic polypropylene film and thus pressed into high-strength mats (so-called organo film ). With this technology, as with other recycling methods (e.g. in the production of so-called organic sheets ), the first step is to shred the waste. Conventional mechanical methods can be used for this. However, small amounts of dust from fibers and matrix material are generated. These dusts are undesirable because they are not suitable for recycling, the fibers they contain are electrically conductive and can lead to the failure of electrical systems. In addition, dusts are harmful to health, so appropriate protective clothing must be worn. The mechanical processing of CFRP does not, however, create “WHO fibers” (fibers that are considered to be potentially carcinogenic). In the case of thermal recycling in waste incineration plants for municipal waste, the residence time of the waste in the hot zone of the plants is usually too short for the fibers embedded in a matrix to be completely burned off. This can lead to technical problems with electrostatic precipitators . In hazardous waste incineration plants, the residence time of the materials is longer and the temperatures are higher. Nevertheless, fibers were found in the slag that is dumped, so that no complete recycling takes place in the WIP.
A thermal-material recovery in steel recycling and a purely material recovery in calcium carbide production in an electric arc furnace and thus at significantly higher temperatures are currently being investigated. In appropriate pilot tests, the fibers were completely decomposed and no fiber discharge could be detected.
When recycling, after a fiber matrix separation z. B. in commercial pyrolysis plants from the fibers ground fibers, short fibers or nonwovens produced. Since almost continuous fiber properties are achieved from fiber lengths of 3–4 cm, high-quality products can be produced again with fiber fleeces and staple fibers.
In the case of a thermoplastic matrix, the components can be shredded directly and reused in injection molding; a fiber matrix separation is not necessary.
Web links
Individual evidence
- ↑ Hans-J. Koslowski: Chemical fiber - Lexicon. 12th, expanded edition, Deutscher Fachverlag, Frankfurt am Main 2009, ISBN 978-3-87150-876-9 , p. 118.
- ↑ a b c Erich Fitzer , Arnold Kurt Fiedler, Dieter Jürgen: For the production of carbon fibers with a high modulus of elasticity and high strength . In: Chemical Engineer Technology . tape 43 , no. August 16 , 1971, p. 923-931 , doi : 10.1002 / cite.330431607 ( PDF ).
- ↑ wiki.rg.de fiber composite materials (note: 1 ohmmeter = 100 ohm centimeter)
- ↑ Konrad Bergmeister: Carbon fibers in structural engineering . 2003, Ernst & Sohn, p. 39
- ↑ Hauke Lengsfeld, Hendrik Mainka, Volker Altstädt: Carbon fibers - production, application, processing. Hanser Verlag, Munich 2019, ISBN = 978-3-446-45407-1, p. 54ff.
- ↑ Patent US390462 : Process of making carbon filaments. Published October 2, 1888 , inventor: Thomas Alva Edison.
- ↑ patent GB1110791 : The production of carbon fibers. Registered April 24, 1964 , published April 24, 1968 , inventors: William Johnson, Leslie Nathan Phillips, William Watt.
- ↑ New Materials make their mark . In: Nature . tape 219 , no. 5156 , August 24, 1968, p. 818-819 , doi : 10.1038 / 219818a0 ( PDF ).
- ↑ How is Carbon Fiber Made? accessed on December 29, 2017
- ↑ Hauke Lengsfeld, Hendrik Mainka, Volker Altstädt: Carbon fibers - production, application, processing. Hanser Verlag, Munich 2019, ISBN = 978-3-446-45407-1, p. 52.
- ↑ Lecture notes on fiber-reinforced bridges (opened May 2018)
- ↑ Press release on fiber-reinforced bridge (opened May 2018)
- ↑ Methods of non-destructive carbon fiber testing ( English ) SURAGUS GmbH. Retrieved November 29, 2014.
- ↑ Composites Market Report 2018
- ↑ a b Tjark of speeches; Warzelhan: Current developments in the field of recycling and recovery of CFRP. In: 22nd International Dresden Lightweight Construction Symposium.
- ↑ Holger Fischer, Ralf Bäumer: Organo films made from recycled carbon fibers - new ways for CFRP semi-finished products in series production . Lecture, ThermoComp Chemnitz, June 30, 2011, online [1] (PDF; 2.3 MB)
- ↑ Saved? In: VDI-Nachrichten, May 10, 2018.
- ↑ MAI Recycling - Development of resource-efficient CFRP recycling processes and process chains for the future provision of high-quality rC semi-finished products: MAI Recycling final report
- ^ A b Marco Limburg, Jan Stockschläder, Peter Quicker: Thermal treatment of carbon fiber reinforced plastics. : Hazardous substances - cleanliness. Air . 77, No. 5, 2017, ISSN 0949-8036 , pp. 198-208.
- ↑ N. Bienkowski, L. Hillermann, T. Streibel, J. Kortmann, F. Kopf, R. Zimmermann, P. Jehle: Processing of carbon concrete - a construction process and medical consideration: DVI-Bautechnik, annual edition 2017/2018 s. 110-119
- ↑ Marco Limburg, Peter Quicker: Small parts, big problems. In: ReSource. 29, No. 2, 2016, ISSN 1866-9735 , pp. 54-58.
- ↑ Denny Schüppel, Jan Stockschläder, Tjark von Reden: End Of-Life CFRP as a Raw Material in Steel and of Calcium Carbide Production. In: European Conference on Composite Materials 2018 / Athens
- ↑ ELG website. Retrieved July 9, 2018 .
- ↑ CarboNXT website. Retrieved July 9, 2018 .
literature
Hauke Lengsfeld, Hendrik Mainka, Volker Altstädt: Carbon fibers - production, application, processing. Hanser Verlag, Munich 2019, ISBN = 978-3-446-45407-1.