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Carbon Fibre

Carbon Fibre

Carbon fibres or carbon fibres (alternatively CF, graphite fibre or graphite fibre) are fibres about 5–10 micrometres in diameter and composed mostly of carbon atoms. Carbon fibres have several advantages including high stiffness, high tensile strength, low weight, high chemical resistance, high-temperature tolerance and low thermal expansion. These properties have made carbon fibre very popular in aerospace, civil engineering, military, and motorsports, along with other competition sports. However, they are relatively expensive when compared with similar fibres, such as glass fibres or plastic fibres.

To produce a carbon fibre, the carbon atoms are bonded together in crystals that are more or less aligned parallel to the long axis of the fibre as the crystal alignment gives the fibre high strength-to-volume ratio (in other words, it is strong for its size). Several thousand carbon fibres are bundled together to form a tow, which may be used by itself or woven into a fabric.

Carbon fibres are usually combined with other materials to form a composite. When impregnated with a plastic resin and baked it forms carbon-fibre-reinforced polymer (often referred to as carbon fibre) which has a very high strength-to-weight ratio, and is extremely rigid although somewhat brittle. Carbon fibres are also composited with other materials, such as graphite, to form reinforced carbon-carbon composites, which have a very high heat tolerance.

History

In 1860, Joseph Swan produced carbon fibres for the first time, for use in light bulbs.[1] In 1879, Thomas Edison baked cotton threads or bamboo slivers at high temperatures carbonising them into an all-carbon fibre filament used in one of the first incandescent light bulbs to be heated by electricity.[2] In 1880, Lewis Latimer developed a reliable carbon wire filament for the incandescent light bulb, heated by electricity.[3]

In 1958, Roger Bacon created high-performance carbon fibres at the Union Carbide Parma Technical Center located outside of Cleveland, Ohio.[4] Those fibres were manufactured by heating strands of rayon until they carbonized. This process proved to be inefficient, as the resulting fibres contained only about 20% carbon and had low strength and stiffness properties. In the early 1960s, a process was developed by Dr. Akio Shindo at the Agency of Industrial Science and Technology of Japan, using polyacrylonitrile (PAN) as a raw material. This had produced a carbon fibre that contained about 55% carbon. In 1960 Richard Millington of H.I. Thompson Fiberglas Co. developed a process (US Patent No. 3,294,489) for producing a high carbon content (99%) fibre using rayon as a precursor. These carbon fibres had sufficient strength (modulus of elasticity and tensile strength) to be used as a reinforcement for composites having high strength to weight properties and for high-temperature-resistant applications.

The high potential strength of carbon fibre was realized in 1963 in a process developed by W. Watt, L. N. Phillips, and W. Johnson at the Royal Aircraft Establishment at Farnborough, Hampshire. The process was patented by the UK Ministry of Defence, then licensed by the British National Research Development Corporation to three companies: Rolls-Royce, who were already making carbon fibre; Morganite; and Courtaulds. Within a few years, after successful use in 1968 of a Hyfil carbon-fibre fan assembly in the Rolls-Royce Conway jet engines of the Vickers VC10,[5] Rolls-Royce took advantage of the new material’s properties to break into the American market with its RB-211 aero-engine with carbon-fibre compressor blades. Unfortunately, the blades proved vulnerable to damage from bird impact. This problem and others caused Rolls-Royce such setbacks that the company was nationalized in 1971. The carbon-fibre production plant was sold off to form Bristol Composites.

In the late 1960s, the Japanese took the lead in manufacturing PAN-based carbon fibres. A 1970 joint technology agreement allowed Union Carbide to manufacture Japan’s Toray Industries product. Morganite decided that carbon-fibre production was peripheral to its core business, leaving Courtaulds as the only big UK manufacturer. Courtelle’s water-based inorganic process made the product susceptible to impurities that did not affect the organic process used by other carbon-fibre manufacturers, leading Courtaulds ceasing carbon-fibre production in 1991.

During the 1960s, experimental work to find alternative raw materials led to the introduction of carbon fibres made from a petroleum pitch derived from oil processing. These fibres contained about 85% carbon and had excellent flexural strength. Also, during this period, the Japanese Government heavily supported carbon fibre development at home and several Japanese companies such as Toray, Nippon Carbon, Toho Rayon and Mitsubishi started their own development and production. Since the late 1970s, further types of carbon fibre yarn entered the global market, offering higher tensile strength and higher elastic modulus. For example, T400 from Toray with a tensile strength of 4,000 MPa and M40, a modulus of 400 GPa. Intermediate carbon fibres, such as IM 600 from Toho Rayon with up to 6,000 MPa were developed. Carbon fibres from Toray, Celanese and Akzo found their way to an aerospace application from secondary to primary parts first in the military and later in civil aircraft as in McDonnell Douglas, Boeing, Airbus, and United Aircraft Corporation planes.

Structure and properties

Carbon fibre is frequently supplied in the form of a continuous tow wound onto a reel. The tow is a bundle of thousands of continuous individual carbon filaments held together and protected by an organic coating, or sizes, such as polyethylene oxide (PEO) or polyvinyl alcohol (PVA). The tow can be conveniently unwound from the reel for use. Each carbon filament in the tow is a continuous cylinder with a diameter of 5–10 micrometres and consists almost exclusively of carbon. The earliest generation (e.g. T300, HTA and AS4) had diameters of 16–22 micrometres. Later fibres (e.g. IM6 or IM600) have diameters that are approximately 5 micrometres.

The atomic structure of carbon fibre is similar to that of graphite, consisting of sheets of carbon atoms arranged in a regular hexagonal pattern (graphene sheets), the difference being in the way these sheets interlock. Graphite is a crystalline material in which the sheets are stacked parallel to one another in a regular fashion. The intermolecular forces between the sheets are relatively weak Van der Waals forces, giving graphite its soft and brittle characteristics.

Depending upon the precursor to make the fibre, carbon fibre may be turbostratic or graphitic, or have a hybrid structure with both graphitic and turbostratic parts present. In turbostratic carbon fibre, the sheets of carbon atoms are haphazardly folded, or crumpled, together. Carbon fibres derived from polyacrylonitrile (PAN) are turbostratic, whereas carbon fibres derived from mesophase pitch are graphitic after heat treatment at temperatures exceeding 2200 °C. Turbostratic carbon fibres tend to have high tensile strength, whereas heat-treated mesophase-pitch-derived carbon fibres have high Young’s modulus (i.e., high stiffness or resistance to extension under load) and high thermal conductivity.

Applications

Carbon fibre is most notably used to reinforce composite materials, particularly the class of materials known as carbon fibre or graphite reinforced polymers. Non-polymer materials can also be used as a matrix for carbon fibres. Due to the formation of metal carbides and corrosion considerations, carbon has seen limited success in metal matrix composite applications. Reinforced carbon-carbon (RCC) consists of carbon fibre-reinforced graphite and is used structurally in high-temperature applications. The fibre also finds use in the filtration of high-temperature gases, as an electrode with high surface area and impeccable corrosion resistance, and as an anti-static component. Moulding a thin layer of carbon fibres significantly improves fire resistance of polymers or thermoset composites because a dense, compact layer of carbon fibres efficiently reflects heat.

The increasing use of carbon fibre composites is displacing aluminium from aerospace applications in favour of other metals because of galvanic corrosion issues.

Carbon fibre can be used as an additive to asphalt to make electrically-conductive asphalt concrete. Using this composite material in the transportation infrastructure, especially for airport pavement, decreases some winter maintenance problems that led to flight cancellation or delay due to the presence of ice and snow. Passing a current through the composite material 3D network of carbon fibres dissipates thermal energy that increases the surface temperature of the asphalt, which is able to melt ice and snow above it.

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