It may sound surprising, but the strongest material human beings have ever created comes not from metal, but from carbon, the same material that makes up the tip of a pencil. Of course, much technology goes into transforming it into a material with such immense strength, but the fact remains; our planet’s most prevalent element can be used to create a material with such unique properties.
What is carbon fibre?
In general, the term “carbon fibre” refers to polymer chains of aromatic carbons that are between 5–10 micrometres in diameter. The aromatic carbon molecules are heated and bond into a crystallised structure along an axis, forming fibres that possess remarkable mechanical and thermal properties .
What are the properties of carbon fibre?
When holding something made out of carbon fibre, the most easily recognisable characteristic is its low weight. Carbon has half the density of aluminium and one fifth the density of steel . Carbon fibre composites can also be machined and used to create specialised parts, making it an attractive alternative to metal in many cases, as it can decrease weight without sacrificing mechanical properties.
Through mechanical testing, one can truly appreciate the considerable strength of the material. Carbon fibre has high stiffness, high tensile strength, and a high Young’s modulus, and when combined with its low weight, results in a strength-to-weight ratio that is unparalleled by other materials . Additionally, carbon fibre has excellent thermal properties; it has a high temperature tolerance, exhibits low thermal expansion, and is a poor conductor of heat (making it an effective insulator) .
How is carbon fibre made?
Although the specific parameters of industrial carbon fibre production will differ from process to process, each shares the same general scheme for transforming aromatic hydrocarbons into fibre. It all begins with a carbon-rich polymer precursor material. The most commonly used precursor is polyacrylonitrile (PAN), a synthetic polymer resin derived from oil, but rayon (regenerated cellulose fibres) and petroleum pitch can also be used .
The first step is to spin the precursor into a fibre; in manufacturing terms, spinning refers to a process of extruding a polymer through a specialised device called a spinneret. There are a wide variety of spinning methods (ex. wet spinning, dry spinning, melt spinning, electrospinning), each of which differs in terms of the specific fibre formation mechanism and should be selected with respect to the precursor being used and the end use of the fibre .
After being spun, the fibre enters the stabilisation stage, which occurs at mild temperatures and results in the cross-linking of multiple carbon fibres to one another. This is followed by the carbonisation stage, which occurs at elevated temperatures (300-1200°C) and results in the release of much of the undesired, non-carbon elements present in the fibre.
Depending on the class of carbon desired, there may be an additional graphitisation phase, which occurs at up to 2800°C and releases recalcitrant non-carbon elements that remain after carbonisation .
What do we make out of carbon fibre?
Once a carbon fibre has been spun and stabilised, its potential applications are vast and varied. The filaments can be directly woven into textiles, or they can be mixed with resins to create composite materials. The primary consumers of carbon fibres are the aerospace, wind energy, and automobile industries, as well as a niche market for low-volume, high-value products like performance sporting goods (e.g. bicycles, tennis rackets, hockey sticks) .
Currently, the high cost of PAN precursor limits carbon fibre application to speciality products where the high mechanical performance and low weight are more important than the overall cost . The next anticipated carbon fibre market poised for explosive growth is in the automotive composite sector, where the high strength allows it to be used as a steel replacement that improves fuel economy (due to the dramatic weight reduction) .
Yet, as process engineering technologies improve and allow for cheaper and faster production, carbon fibre consumption is expected to increase and the market is expected to broaden to include lower-value products that were previously not viable, due to precursor and production cost .
How can we make carbon fibre cheaper?
Considering the high cost of PAN, much research is being done to find cheaper sources of aromatic polymer precursors. One promising option is to use lignin, a naturally occurring aromatic compound found in trees. This biopolymer is currently processed in large quantities by the pulp and paper industry but is treated essentially as a waste product.
The papermaking process removes lignin from wood, but instead of utilising it as a high-value stream of aromatics, paper mills burn it to create process energy for the operation . Some paper mills have begun isolating a portion of their lignin instead of burning it, creating a stream of cheap, renewable biopolymers that can be spun into fibres.
Lignin-derived aromatics are more heterogeneous than petroleum-derived PAN aromatics, so the focus of much research is towards chemical modifications of lignin biopolymers that improve its processability and increase its elemental and functional group homogeneity (which results in fibres with better thermal and mechanical properties)  .
How can we make it stronger and lighter?
Instead of using a conventional polymer matrix to form carbon composites, a metal matrix can also be used. Due to its potential to further chemically react and corrode, metal-matrix composites (MMC) are generally coated for stabilisation, which results in increased production cost . For certain applications, however, such as in aerospace and aeroplane technologies, the improved properties and performance make up for the additional cost of production.
Relative to conventional polymer-matrix carbon fibre composites, MMCs are stable at higher temperatures, display increased electrical and thermal conductivity and are resistant to radiation . Carbon nanotubes, or allotropes of carbon that have a cylindrical nanostructure and an extremely high length-to-diameter ratio (i.e. 132,000,000:1), have proven to be a promising coating for both polymer-matrix and metal-matrix composites, as they reduce fibre degradation that can be caused by weathering and UV exposure .
Graphene is a carbon allotrope that is comprised of a single layer of carbon atoms arranged in a hexagonal lattice, which results in a material with extraordinary properties. As such, it can be considered a one atom-thick form of carbon fibre. It is roughly 200 times stronger than steel, yet it is incredibly lightweight and flexible. It is highly conductive, but at the same time, nearly transparent .
A property of graphene that is particularly interesting is its ability to store electrical charge. To produce thin-film supercapacitors that can be used in miniature circuits, a material like graphene, with a high surface-to-volume ratio, is needed to efficiently store and quickly deliver electric charge . Developments in graphene could catalyze a move away from conventional battery technology and towards the incorporation of supercapacitors into modern electronics. Ultimately, as demonstrated by graphene, the more time and energy is invested in processing carbon fibres, the more impressive the properties of the fibre.
Due to its material properties and the already established market, carbon fibre is poised to continue playing a role in building tomorrow’s materials. The cost of the PAN precursor currently assumes half of the cost of manufacturing carbon fibre; if a cheap precursor can be developed, carbon fibre applications can move beyond the current speciality product market into more widespread applications .
As nanotechnology progresses and allows for increased control in every aspect of fibre production, we will be able to create carbon fibres with ever more specialised properties than are already being created. With the recent developments of carbon nanotubes and graphene, the frontier of carbon fibre is only beginning to be understood.
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