Material of the Future: Carbon Fiber

Carbon fiber is a highly strategic material offering innovative solutions, involving efficient usage of resources, to the world’s primary problems like increasing efficiency of wind turbines, reducing fuel consumption in vehicles, and strengthening infrastructure and structures against earthquakes.

Very strong and lightweight, carbon-fiber-based materials are used in a variety of applications where weight savings, emissions reduction, durability, and energy efficiency are key performance factors.

Q.What is Carbon Fiber?

Carbon fiber is an extremely strong, stiff and lightweight material, superior in strength to conventional metal materials. It is technically known as a fiber containing at least 95% carbon atoms bonded together. It consists of very thin strands of carbon (typ. 5-7 µ in dia.), even thinner than human hair. The strands can be twisted together, like yarn. The yarns can be woven together like cloth, which is used to make composite parts. Carbon fibers are produced in tows (yarns) ranging from 1.000 filaments (1k), to 3k, 6k, 12k, 24k, 50k, etc.

Q.What are its advantages over other materials?

When used with a bonding polymer, like a stiff resin or plastic, the strength-to-weight ratio (as well as stiffness-to-weight ratio) of carbon fiber is much higher than either steel or aluminum.

5 times stronger than steel
2 times stiffer than steel, aluminum and glass fiber
4 times lighter than steel
2 times lighter than aluminum and glass fiber
Corrosion and fatigue resistance

Q.How is carbon fiber made?

Most plastics (polymers), like polyester, nylon or polyethylene melt and then decompose when heated up. A few polymers (e.g. polyacrylonitrile and cellulose) do not melt and instead, they react with themselves to char or carbonise. An example being bread, which carbonises when heated to give toast rather than melts or decomposes. In general, polyacrylonitrile is normally used to make carbon fibre as it gives higher strength and stiffness fibres.

The first step of the process involves the conversion or polymerisation of the starting material, acrylonitrile (monomer) into a polymer (chain of linked monomers) by addition of a free radical catalyst. It is possible to carry out this polymerisation in a solvent (solution polymerisation) or in a slurry of the acrylonitrile in water but the slurry polymerisation is the most common. In solution polymerisation the dissolved polymer is fed directly to the spinning step, whereas in slurry polymerisation, the polymer is filtered from the slurry and dissolved in an appropriate solvent and sent to spinning; similar to solution polymerisation.

In general, the conversion of the polymer solution into a fibre involves the extrusion of the viscous solution through an array of tiny holes (spinneret) into bath of non-solvent (water) where the fibre precipitates and forms a fibrous gel. A series of washing, stretching and drying steps then converts the gelatinous fibres into resilient polyacrylonitrile fibres (precursor fibre). This white precursor fibre is then ready for conversion into carbon fibre.

The first step of the carbonisation process involves the careful and controlled crosslinking of the precursor by heat and oxygen into a thermally stable, black fibre. This material is referred to as oxidised fibre. It still contains 35 - 40% of other elements (hydrogen, nitrogen and oxygen) and the crosslinked structure is random (amorphous) and not crystalline as in carbon fibre.

The oxidised fibre is then heated treated in a series of high temperature furnaces to firstly volatilise the non-carbon elements, which results in a 50% reduction in mass and then a series of carbon to carbon bond breaking and forming steps occur to turn the random/amorphous carbon structure into a highly crystalline, graphite structure held together by amorphous regions. The higher the conversion temperature and time the more amorphous carbon is converted to crystalline carbon.

The resultant carbon fibre has a very smooth and inert surface and therefore difficult for glues / adhesives / resins to stick to the surface, so the surface needs to be made rougher and more reactive by an etching process that can be electrochemical, plasma, strong acid etc. A surface coating (size) is normally applied to the etched carbon fibre to protect the carbon fibre, lubricate the carbon fibre in subsequent processing and act as an interface with the composite resins.

The resultant carbon fibre is then wound onto a tube to give a known weight or length, which is referred to as a bobbin, spool or cheese of carbon fibre.