Who Created Carbon Fibres?
Development of Carbon Fibres
Difference between Rayon, Pitch and PAN Carbon Fibres
Manufacture of Carbon Fibres
PAN Carbon Fibre Production through Pyrolysis
High-Strength Fibres
High-Modulus Fibres and Graphitisation
The first known use of carbon fibres dates back to 1860 when an English physicist by the name of Sir Joseph Swan created the world’s first incandescent light bulb. He initially created it with a carbonized paper filament in a vacuum tube but later improved on his design in 1878 by using a carbon fibre filament derived from cotton.
When Thomas Edison took out a patent for the manufacture of his light bulb in 1879 of the same design he proposed using carbonised bamboo as his filament. Edison took bamboo and heated it in a controlled atmosphere (a method known as ‘pyrolysis’ which is used in the manufacture of carbon fibres today) which produced fire-resistant carbon fibres. The fibres produced by Swan and Edison had no tensile strength but could withstand high temperatures making them perfect for use in his bulbs.
In 1883 Swan produced the world’s first synthetic fibre when he squeezed nitrocellulose – obtained from tree bark- through holes to produce fibres which he could use as a new filament. This revolutionised the textile industry and led to his knighthood in 1904.
All the filaments produced proved inefficient, requiring high currents to produce light and soon they were replaced with tungsten.
It would be around 50 years before experiments with carbon fibres would take place. In order to find a replacement for tungsten as light filaments the Union Carbide Corporation in the USA began to produce carbon fibres through the method of pyrolysis using a material called rayon. At the same time a Governmental Research institute in Japan (known as GIRIO) were experimenting with a substance called polyacrylonitrile or PAN for short. The fibres produced by these materials (particularly PAN) showed to have many useful qualities.
Difference Between Rayon, Pitch and PAN Carbon Fibres
PAN carbon fibres account for 96% of the carbon fibre market. Because fibres made from this material are normally 93-95% carbon their molecular structure makes them very strong and allows three dimensional structures such as nanotubes. Since the 1960’s PAN carbon fibres had a new role as a high tensile strength material for use in the aerospace industry.
Rayon and pitch carbon fibres have greater thermal and electrical conductivity but are also more brittle making them impractical for structural applications in the aerospace industry. Rayon and pitch carbon fibres are used in the production of sports equipment, civil infrastructure or marine craft.
Model of the molecular structure of carbon nanotubes.
Within the textile industry the term carbon fibre is generally given to a material that contains a minimum of 90% carbon and the term Graphite Fibre is given to materials containing more than 99% carbon.
Carbon Fibres are manufactured by the controlled thermal treatment of organic precursors. This basically means that certain materials like polyacrylonitrile, pitch, or rayon; are heat treated in an inert atmosphere (an atmosphere filled with a non reactive gas such as nitrogen, carbon dioxide, or helium). At certain temperatures various impurities are boiled off leaving a high concentration of the carbon component. During the heating process the materials form minute, natural, ribbons that increase in width and these ribbons are then packed together forming fibres which are then combined with a matrix material like epoxy. This creates a lightweight, high tensile material which is used in a variety of applications.
The majority of carbon fibres are made from polyacrylonitrile or PAN which before being processed into carbon fibre must be stabilised to prevent the material decomposing in the high temperatures required (ordinarily polyacrylonitrile melts at 319°C). The Fibres are heated to approximately 300°C in an oxygen containing atmosphere whilst being stretched.
Once stabilised, the material is then heated in an inert atmosphere to temperatures of 400 – 600°C causing the cyano repeat units to form rings (see Diagram below for full process).
PAN Carbon Fibre Production through Pyrolysis
Stage1
Heated in an inert atmosphere to temperatures between 400 and
600˚C.
Stage2
Heated to 700˚C to boil off hydrogen.
+ Hydrogen
Stage3
Heated to approximately 600˚C the chains form bonds and link together
+ Hydrogen
Stage4
Now heated to temperatures between 600 – 2,000˚C. The ribbons join together getting wider and wider and with each increase in width nitrogen is released purifying their carbon content.
+ Nitrogen
When the purity reaches 90% carbon then it is ready for the final stage.
Heating the ribbons to temperatures of 1,500 to 1,600˚C will produce fibres with a carbon purity of 93-95%. This is optimal strength for the carbon ribbons but they are also more rigid. These ribbons produce high-strength fibres.
High-Modulus Fibres and Graphitisation
Adding an additional stage, known as graphitisation, can produce a purer carbon ribbon, at least 99% which has more elastic but has less strength. The process requires additional heating to 2,000˚C. These ribbons produce high-modulus fibres.
The long ribbons of carbon are then packed together forming fibres that are combined with a matrix material like epoxy producing the final product, Carbon Fibre.
