Carbon Fibre - The Material of the Future?

by Katie O'Flaherty

With technology advancing at an almost unimaginable rate, and globalisation causing industry to expand exponentially, the materials of yesterday are already becoming old and obsolete, their properties not up to scratch for the new tasks of today. And yet, could the materials of the past be re-thought and re-imagined for the future? Aerogel was first synthesised as a fun project in 1929, and spent the first fifty years of its life unused and unable to be commercialised. Then, in the 1980s, suddenly it was found to be the perfect material for capturing ‘stardust’ in space to analyse the composition of comets. This re-emergence of a previously disregarded substance is becoming a reoccuring theme in modern day materials science, and carbon fibre is no exception.

The first carbon fibres were in fact created in 1879 by Thomas Edison, to be used as a filament in his lightbulbs, however tungsten filaments quickly replaced them, and carbon fibers slipped into the dark. The modern era of carbon fibre began nearly 80 years later, when in 1958 a scientist called Roger Bacon stumbled upon the first high performance carbon fibers by accident when trying to find the triple point (point where the solid, liquid, and gas phase all exist simultaneously) of graphite. After testing these fibres, Bacon found them to have a tensile strength (resistance to elastic deformation) of 20 Gigapascals (GPa), and a Young Modulus (measure of stiffness) of 700 GPa. For comparison, steel usually has a tensile strength of 1-2 GPa, and Young Modulus of 200 GPa. These amazing results started to gain carbon fibre the grand reputation it has today.

Carbon fibre is a type of reinforced polymer (commonly regarded as a plastic), and can be seen as a polymer of graphite, which is a form of carbon in which each carbon atom is bonded to three other carbon atoms to form giant sheets of hexagons, the sheets held together by relatively weak intermolecular forces, thus are able to slide over each other. In more modern times, ultrahigh modulus carbon fibres have been synthesised, with a tensile modulus of up to 500-1000 GPa, unlocking the material for plethora of uses.

The production of carbon fibre is, at heart, a four step process,
with 90% of carbon fibres made from PAN (polyacrylonitrile) (C₃H₃N)_n, which is itself a polymer, of which the monomer unit consists of 3 carbon atoms, one of which is a branch chain, triple bonded to a nitrogen atom. The production process begins with stabilisation, where the precursor is heated in air, causing the fibres to ‘pick up’ oxygen atoms, rearranging the molecules from linear bonding to a more thermally stable ladder bonding. These molecules then undergo carbonisation, where they are heated to very high temperatures without contact to oxygen, thus are not able to burn. Instead, this causes the molecules to vibrate until nearly all of the non-carbon atoms are expelled. After this, the surface of the carbon fibre is then treated so that it can bond well with the epoxies (adhesives or plastics), and other materials used in the composite. Finally, the fibres are coated to protect them from damage during winding or weaving, in a process called sizing.

There are many properties of carbon fibre that make it attractive to industry, including its high stiffness and strength, being lightweight, corrosion resistance, X-Ray transparency, low coefficient of thermal expansion, resistivity to chemical corrosion, and thermal and electrical conductivity, among many others. This makes carbon fibre ideal for use in alternate energy, enabling production of longer blades on wind turbines without them snapping. In addition to this, it has plethora of uses in the aerospace industry, construction and infrastructure industries, such as in lightweight pre-cast concrete, and in fuel-efficient transport, as already shown in its use in F1 with high performance, small production vehicles, and is being looked into for large production series cars.

Carbon fibre’s properties come almost entirely from its structure, which can therefore be adjusted in the production by changing the production environment. The amazing strength and stiffness of carbon fibre come from its crystalline structure, with the very thin fibres of carbon chains bonding together to form microscopic crystals, which are aligned parallel to the long axis of the fiber. Each individual fibre has a diameter of just 0.005-0.01mm, and these fibres, bonded with a crystalline structure, can then be twisted together to form a yarn, which can then in turn be woven together to form a sheet or fabric.

Carbon fibre is defined as a fibre containing at least 92% wt carbon, with graphite fibre containing at least 99% wt, however these are often confused, and people will often assume carbon fibre’s electrical conductivity is due to its graphite-based nature, this, however, would be incorrect. It is, in fact, the aromatic nature of the electrons that enables carbon fibres to conduct electricity, with electrons being able to ‘jump’ from one carbon to the next using clouds of overlapping electrons. This, in turn, produces heat, and in equal measure, heating the carbon fibres improves their electrical conductivity, as it causes the electron clouds to increase their size, thus increasing the size of the pathway for the mobile electrons.

Though this article has only shown a small fragment of the many impressive properties of carbon fibre, hopefully it has served to show part of the reason it has seemingly infinite possibilities in the future, in addition to the many it serves at the moment. Supporting us from behind the scenes, carbon fibre can provide a strong yet lightweight backbone, assisting us in everything from transport to energy, to the very buildings we live and work in.