What is carbon fibre?
Carbon fibre contains carbon in various modifications. A carbon fibre is a long thin strand of material with a diameter of 5–8 μm. It is made up mainly of carbon atoms and is used in the manufacture of strong and resistant fabrics that are used in various fields.
The atoms in a carbon fibre are bonded in a crystalline structure oriented in parallel with the long axis of the fibre. Carbon fibre gains its great strength, despite its thinness, thanks to the arrangement of the microscopic crystals.
Carbon fibre and its uses
The vast majority of carbon fibre manufactured is used to strengthen composites. Carbon fibre is also used in many branches of industry, from the production of sports equipment to materials for the manufacture of aircraft.
Carbon fibre – examples of use:
- Aviation – fuselages and wings made from carbon fibre
- Wind turbines – rotor blades and sometimes offshore platforms are made from carbon fibre
- Various types of composite – military and civilian boats, gas tanks, etc.
- Sports equipment – dinghies, golf clubs, ski coatings, sports arrow shafts and much else that also makes use of carbon fibre
- Automotive industry – carbon fibre is used for example for bumpers and body parts
The manufacture of carbon fibre step by step
- Preparation of precursors – treatment of the raw material either by melt spinning or solution spinning. The structure of the precursor influences the structure and strength of the carbon fibre. The carbon fibre is then drawn to the required fineness.
- Stabilisation – before carbonisation it is necessary to create from the long carbon fibres a thermostable interwoven structure. This is carried out in an atmosphere heated to the relatively low temperature of 200 - 450 °C for 20 – 30 minutes. This causes the carbon fibre to absorb oxygen molecules from the air resulting in the rearrangement of the atomic structure of the fibre. The stabilisation leads to the release of excess heat from the fibre and this has to be controlled to avoid overheating. In practice sometimes fibre is drawn through a series of heated chambers or the carbon fibre is passed through heated rollers and bulk materials which absorb excess heat.
- Carbonisation – the transformation of the precursor to carbon fibre. It is carried out in an inert atmosphere (usually nitrogen), at temperatures of 1000 °C – 2000 °C. Without the presence of oxygen carbon fibre cannot burn. In place of this the high temperatures lead the atoms in the fibre to vibrate in such a way that the majority of those that are not carbon are removed. The resulting fibres contain 85 - 95% carbon.
- Graphitisation – this does not need to occur but if it does it results in the formation of so-called graphite fibre. It takes place in an inert atmosphere at temperatures in the range 2400 °C – 3000 °C. This leads to an increased carbon content of about 99 % and more. This also leads to a layered structure.
- Surface treatment – the surface of a carbon fibre does not bond well to epoxides and other substances used in composite materials. For this reason the surface of the fibre is slightly oxidised. The addition of oxygen atoms on the surface makes for better adhesion of other substances and roughening the surface leads to better mechanical bonding with these substances. Oxidation can be achieved through exposure to the gas from air, carbon dioxide or ozone, or by submerging in various liquids such as sodium hypochlorite or nitric acid. Carbon fibre can also be covered in a protective layer to prevent damage from further processing.