While “carbon fiber” sounds relatively simple to analyze, which is to say “fibers made out of carbon,” it is deceptively more complex than that. In reality, there are countless materials that simple fibers can be combined with, in order to achieve the durability and lightness that many designers prefer for innovative projects.
Precursors
Precursors are the building blocks of carbon fiber, also known as an organic polymer. In most cases, the precursor used is polyacrylonitrile (PAN). Alternatively, rayon or petroleum pitch is used. Organic polymers comprise of long strands of molecules which are bound together by carbon atoms. While these elements compose precursors, the specific makeup of a precursor varies with each manufacturer, and the specific measurements are often undisclosed.
For certain projects, a specific effect within the carbon fiber is called for. Similarly, a project may call for a distinct reaction or the prevention of a distinct reaction. In order to create these distinct properties – glasses, liquids, and other materials might be added to the carbon fibers during the manufacturing process. These specifications can vary and are often not available to the public.
The Carbon Fiber Manufacturing Process
The manufacturing process begins with the balance of process materials is determined. The first step is to achieve carbonization by pulling the precursor into long strands (or fibers), then heated to extremely high temperatures in an inert atmosphere (pyrolysis). This process drives out most of the non-carbon atoms, leaving only long, tightly connected cables of carbon atoms, with very little non-carbon material left behind. Typically, carbonization is completed in five steps
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Spinning – The precursor is combined with other process materials, and is then spun into fibers. The fibers must then be washed and stretched out.
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Stabilizing – Carbon fibers must be thermally stable for the carbonization process to continue, so they must be chemically altered by converting their linear atomic bonds to ladder bonds. In order to do this, the fibers are heated to temperatures between 200-300°C for half an hour to two hours. This process causes the carbon atoms to pick up the oxygen atoms from the air and then rearranges the molecules into a bonding pattern that is more thermally stable. It is important that the fibers not be overheated, so this exothermic process must be meticulously controlled. There are numerous processes which can be used to stabilize carbon fibers.
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Carbonizing – Once the fibers are thermally stable, they are once again heated, this time between 1,000-3,000°C, for a shorter period of time, and without oxygen. It is this lack of oxygen that allows the fibers from incinerating in such high temperatures. While this takes places, the gas pressure inside of the furnace must be kept higher than the air pressure outside of the furnace, and the fiber entry and exit points must be sealed so oxygen cannot enter the furnace. At these extreme temperatures, the fibers get ride of their non-carbon atoms, and the carbon atoms that remain form carbon crystals, which are tightly connected, and run parallel to the long axis of the carbon fiber.
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Surface Treatment – As a result of the carbonization process, the fibers are left with a smooth surface that epoxies, and other process materials, don’t bond well with. In order to improve its chemical bonding properties, the surface must be oxidized, as it also etches the surface to allow other chemicals to adhere to it better. To oxidize the fiber surface, they can be saturated in gases such as carbon dioxide, oxygen, ozone, or liquids like nitric acid or sodium hypochlorite. Alternatively, electrolysis can achieve oxidization by immersing the positively charged fibers in a pool of electrically conductive materials. Regardless of the method chosen for surface treatment, it must be conducted under meticulous, professional care, in order to mitigate defects in the surface, which could potentially lead to material failure.
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Sizing – Once the fibers are oxidized, they are wound into spools or woven into fabrics in order to prevent damage. Coating the fibers is the process known as sizing, which can be achieved with materials like polyester, nylon, urethane, or epoxy. These sizing materials must be carefully chosen to ensure their compatibility with the adhesives used to develop composite s
tructures.
Once the sizing process is complete, the carbon fibers are wound onto spools, then spinning machines twist them into various sizes of yarn. This yarn can either be woven into fabrics, or formed into composites.
