STRONGER than steel, lighter than aluminium and with superior corrosion resistance. If carbon-fibre composites are so great, why are we not seeing its widespread use on mainstream production cars? The historic inhibiters are cost and high-volume-production difficulties, but that is all about to change. Stringent emissions laws and fuel-consumption targets have resulted in extremely efficient powertrains. And the best way to further improve the total efficiency of a vehicle is to reduce its mass. How then has the process of producing carbon-fibre composites evolved, and what are the possible applications in high production-volume vehicles?
What is carbon-fibre?
A single carbon-fibre strand is between 0,004 and 0,010 mm in diameter and consists of carbon molecules in microscopic crystals that are mostly aligned parallel with the long axis of the fibre. This structure endows it with incredible tensile-strength qualities. Thousands of carbon-fibres can be twisted together to form a yarn that is used in the weaving process to create the signature carbon-fibre cloth.
How is carbon-fibre made?
The process of producing the fibre is partly mechanical and partly chemical. In most cases, the raw material (precursor) used to produce carbon-fibre is polyacrylonitrile (PAN). This organic polymer is characterised by long strings of molecules bound together by carbon atoms (the building blocks of carbon-fibre). The first process is called spinning, where the precursor is spun into fibres. One method is heating the raw material and pumping it through nozzles into a chamber where the fibre solidifies. The next step is to wash and stretch the fibre to align the molecules and create the desired diameter.
An interim process before the fibres are carbonised is to stabilise them at around 250 degrees Celsius to create a more stable (and stronger) molecular structure. Then the carbonising can commence at around 2 000 degrees Celsius for several minutes. It is important that no oxygen is present during this process, as the carbon would combust and burn at these elevated temperatures. The result is that non-carbon molecules leave the fibre in the form of gases, ensuring a final product with a carbon content that exceeds 90 per cent.
After carbonising, it is important to treat the surface of the fibre to ensure it bonds well with epoxies, as used in composite materials. This involves allowing oxygen molecules to bond with the fibre’s surface under controlled conditions.
Lastly, the fibres are coated (sizing) to form a protective layer and further improve the bonding properties in composites. Some of the coating materials include epoxy, polyester and nylon. The fibres are then twisted in yarns and woven into cloth before being used in composite applications.
A composite usually consists of two or more materials with significantly different properties. In the case of carbon-fibre-reinforced polymers (CFRP), the most common binding polymer is epoxy. The end product has properties that are different (and sometimes superior) to the constituents (see table of material properties) to meet the specific application requirements. A composite material is normally non-isotropic (it has different strength qualities depending on load direction), which is the case with CFRP. As fibre orientation (or the weave of the cloth) plays such a critical role in the stress capability of the component, it is important to consider this when designing CFRP components and laying the cloth direction during manufacturing.
The most common method of producing CFRP components used on supercars involves pre-impregnated (with epoxy) carbon cloth that is placed in an autoclave mould (or mould with vacuum bag) for the curing process. The “pre-preg” cloth needs to be kept at minus-20 degrees Celsius to prevent curing during storage and to ensure it occurs only under controlled pressure and temperature in the mould.
To speed up the production rate of CFRP components, a resin-transfer method is employed, in which the carbon-fibre cloth is laid robotically into the cavity of a two-piece mould. After the mould is closed, a matrix material (mostly epoxy) is injected before the autoclave curing process can commence. This process is still much slower than the stamping of metal body panels, though.
The costs of CFRP components are still roughly 10 times higher than those made from steel. This additional cost is impossible to recoup in the budget-vehicle segment, but the potential mass-saving benefit is difficult to ignore.
A further problem of CFRP components is recyclability. Steel can easily be reused through the melting of scrap metal, but epoxy is solid after the curing process during manufacturing. One option is to cut the CFRP components into small sections and reuse them in other composite applications. The recycling process might get overloaded with the high-volume numbers of budget vehicles when they reach the end of their service lives. Although CFRP technology is filtering down the price spectrum of new vehicles (see Carbon-fibre cars heading for SA), the informal term “tin boxes” that describes budget cars looks set to remain.