May 2021 – C. O’Donovan
We explore the virtues of carbon fiber composite material; what makes it so desirable, costly and most importantly, how recyclable is it?
Carbon fiber is a material made of fibers spun from filaments of graphite. The fibers can be woven into a fabric and impregnated with epoxy resin to take on complex shapes and forms as a composite material. The carbon atoms in the filaments are arranged in sheets in a semi-regular structure that allows the material to take on immense tensile loads with low deformations. As the filaments are extraordinarily thin (around 6 times thinner than a human hair), they cannot carry compressive loads on their own and this is where a carrier, such as epoxy resin, provides the three-dimensional matrix in which the carbon can take on shapes and profiles. The carbon fibers are stiff, lightweight and strong and once impregnated with an epoxy can act together to create rigid structures in endless shapes and sizes. The two constituents of carbon fiber composites complement one another to produce a material that exhibits the strength and rigidity of steel at the density of plastic. Furthermore, the carbon fibers can be placed with directional bias and in higher density in critical areas to create strength where it is needed and reduce mass where it is not. At impact zones and high-stress areas like the underside of the bottom-bracket and downtube or the joints around the headtube, additional carbon fiber layers can be added. Extending the idea of composites further, lower modulus materials, such as Kevlar, can be introduced into the fabric to absorb energy and reduce crack propagation. In essence, carbon fiber composites are ideal limit state materials, strong, stiff and light. There are, however, drawbacks to this wonder material. The first being its difficult and costly production process, both in an economic and environmental sense, and the second being what happens to the material at the end of its useful life.
One might assume that as carbon fiber is made of carbon there is little harm in producing the material and allowing it to exist in the environment. Carbon is, after all, an abundant element in nature that forms the base building block of organic chemistry and life on Earth. While the long-term effects of carbon fiber composites and other plastics in the environment is uncertain, it is true that manufacturing carbon fiber composites is not green – a carbon fiber composite part requires roughly fourteen times more energy to produce than a similar steel part. Only a handful of commercial manufacturers worldwide produce the carbon fibers required in high strength and stiffness applications, such as bicycle frames. These manufacturers use similar processes, too. They all start off with some type of precursor organic polymer, generally derived from petroleum, such as polyacrylonitrile (PAN). Carbon atoms form the base of these precursor molecules with other elements bound to the carbon backbone. These precursor polymers must be sent through a variety of industrial processes to manipulate the original chemistry and structure to create filaments of almost pure carbon. Another requirement is that the final carbon atoms are arranged in a semi-crystalline structure, which is important for the strength characteristics of the material. The precursor polymer is drawn into filaments then ‘stabilized’ and finally ‘carbonized’. To achieve ‘high-grade’ carbon used in critical applications a further heat treatment step is required. The carbon filaments can then be spun or wound into fibers which can be woven into a fabric. The industrial processes must use heat energy and inert gas atmospheres in the presence of solvents to strip away non-carbon atoms from the base molecules. The result is a space age material, albeit with a large footprint out of the factory gate.
The practical lifespan of carbon fiber composites is indefinite. As fatigue of composite materials is extremely difficult to model, engineers usually resort to overdesigning carbon fiber composite structures to eliminate the chance of fatigue failure. This equates to many decades of useful service life, usually longer than expected by engineers. The other side of the coin is that the composites are made using a thermoset plastic carrier resin, which does not readily break down in the environment, especially if buried in a landfill. The carrier resin will not melt when heated which is important in applications that demand materials retain strength through extreme conditions. It also means that the composite cannot be readily recycled by melting down and extracting the constituents. As a result, the predominant composite recycling practices entail mechanical delamination of the matrix and reinforcing fibers – an energy intensive and inefficient process. This process is essentially mechanical destruction of the original part and results in shortening of the carbon fibers, which reduces how useful the fibers are once extracted from the carrier. The very reason carbon fibers are so strong in composites is because of their continuous nature. This common recycling method destroys this property and, therefore, recycled carbon fibers cannot be used to reconstruct the products from which they were recycled. Pyrolysis or volatilization to ‘burn off’ the carrier resin and leave behind the carbon fiber strands are also potential recycling techniques9. Although, also entail large amounts of waste heat energy.
All things considered, there is a definitive reduction in the usefulness of carbon fiber once it has been used in its first or virgin application. The second application of the fibers will need to be a markedly smaller or less critical application. For instance, carbon fibers recycled from bicycle frames may only be suitable for use constructing items such as bottle cages or headset spacers the next time the fibers are used. In addition, current recycling processes discard or destroy the carrier resin. At present there are limited commercially viable recycling processes to recycle thermally setting resins, such as epoxy and polyester,. Therefore, these materials will be a byproduct of the composite recycling process and will likely end-up as a gas in the atmosphere or a solid in a landfill.
Carbon fiber composite is remarkably strong, corrosion resistant and does not fatigue in a meaningful sense, when considering bicycling applications. In addition, carbon fiber composites can be repaired – in some cases more readily than aluminum alloys. Integrity testing methods of the composite have come a long way and hopefully we should soon see acoustic and thermal imaging tests being made available at the retail level to inspect parts. Considering this, to reduce the environmental footprint of carbon fiber composites we need to capitalise on the material’s strengths. Why throw something away that still holds so much value? Let’s keep this material in use for as long as possible. Service and inspect your bicycles regularly and get expert advice when you’re not sure. By keeping our machines going for as long as possible we can reduce our toll on the planet.