Carbon fibers: Explore, innovate and replace traditional polymers and metals

Carbon fiber is a high-tensile fiber made by heating rayon or polyacrylonitrile fibers or petroleum residues to appropriate temperatures. Today, carbon fibers are an important part of many products, and new applications are being developed every year. The United States, Japan, and Western Europe are the leading producers of carbon fibers.

Carbon fiber, alternatively carbon graphite, is a material consisting of fibers about 510 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in microscopic crystals that are more or less aligned parallel to the long axis of the fiber. The crystal alignment gives the fiber high strength-to-volume ratio. Several thousand carbon fibers are bundled together to form a tow, which may be used by itself or woven into a fabric. Carbon fibers are considered very expensive. They are generally used together with epoxy, where high strength and stiffness are required such as automotive and space applications, sport equipment etc.

Main application areas: The two main applications of carbon fibers are in specialized technology, which includes aerospace and nuclear engineering. Other functional areas are textiles, microelectrodes, Carbon fiber-reinforced composite materials are used to make aircraft and spacecraft parts, racing car bodies, golf club shafts, bicycle frames, fishing rods, automobile springs, sailboat masts, and many other components where light weight and high strength are needed.

What makes them unique?

Carbon fibers are the stiffest and strongest reinforcing fibers for polymer composites, the most used after glass fibers. Carbon fibers are classified by the tensile modulus of the fiber. Tensile modulus is a measure of how much pulling force a certain diameter fiber can exert without breaking. Depending on the orientation of the fiber, the carbon fiber composite can be stronger in a certain direction or equally strong in all directions. A small piece can withstand an impact of many tons and still deform minimally. The complex interwoven nature of the fiber makes it very difficult to break.

Classification of Carbon Fibers: Based on modulus, strength, and final heat treatment temperature, carbon fibers can be classified into the following categories:
-> Based on carbon fiber properties
-> Based on precursor fiber materials
-> Based on final heat treatment temperature

Raw Material and manufacture process:
The raw material used to make carbon fiber is called the precursor. About 90% of the carbon fibers produced are made from polyacrylonitrile (PAN). The remaining 10% are made from rayon or petroleum pitch. All of these materials are organic polymers, characterized by long strings of molecules bound together by carbon atoms. The exact composition of each precursor varies from one company to another and is generally considered a trade secret.

Manufacturing process:
The process for making carbon fibers is part chemical and part mechanical. The precursor (fiber’s molecular backbone) is drawn into long strands or fibers and then heated to a very high temperature with-out allowing it to come in contact with oxygen. Without oxygen, the fiber cannot burn. Instead, the high temperature causes the atoms in the fiber to vibrate violently until most of the non-carbon atoms are expelled. This process is called carbonization and leaves a fiber composed of long, tightly inter-locked chains of carbon atoms with only a few non-carbon atoms remaining. Today the majority of carbon fibre is derived from polyacrylonitrile (PAN), made from acrylonitrile, which is derived from the commodity chemicals propylene and ammonia.

SEQUENCE OF OPERATIONS USED TO FORM CARBON FIBER

Polymerization and spinning: The process begins with a polymeric feedstock known as a precursor, which provides the fiber’s molecular backbone. Acrylonitrile plastic powder is mixed with another plastic, like methyl acrylate or methyl methacrylate, and is reacted with a catalyst in a conventional suspension or solution polymerization process to form a polyacrylonitrile plastic. The plastic is then spun into fibers using one of several different methods.  The spinning step is important because the internal atomic structure of the fiber is formed during this process. Lastly the fibers are then washed and stretched to the desired fiber diameter.

Stabilizing: The stabilizing process chemically alters spun fibers at the atomic level prior to carbonizing. Stabilization is accomplished by heating the fibers in air at lower temperatures (approx. 200-300° C). Heating causes the spun fibers to pick up oxygen molecules. Stabilization process uses a variety of equipment and techniques. In some processes, the fibers are drawn through a series of heated chambers. In others, the fibers pass over hot rollers and through beds of loose materials held in suspension by a flow of hot air. Some processes use heated air mixed with certain gases that chemically accelerate the stabilization.

Carbonizing: After fibers are stabilized, the carbonizing process expels non-carbon atoms and bonds carbon atoms in a crystalline structure. They are heated to a temperature of about 1,830-5,500° F (1,000-3,000° C) for several minutes in a furnace filled with a gas mixture that does not contain oxygen. The lack of oxygen prevents the fibers from burning in the very high temperatures. As the fibers are heated, they begin to lose their non-carbon atoms, plus a few carbon atoms, in the form of various gases including water vapor, ammonia, carbon monoxide, carbon dioxide, hydrogen, nitrogen, and others. As the non-carbon atoms are expelled, the remaining carbon atoms form tightly bonded carbon crystals that are aligned more or less parallel to the long axis of the fiber. In some processes, two furnaces operating at two different temperatures are used to better control the rate of heating during carbonization.

Treating the surface: After fibers are carbonized, surface treatment helps prepare the fibers for bonding to resins.The addition of oxygen atoms to the surface provides better chemical bonding properties and also etches and roughens the surface for better mechanical bonding properties. Oxidation can be achieved by immersing the fibers in various gases such as air, carbon dioxide, or ozone; or in various liquids such as sodium hypochlorite or nitric acid.

Sizing:
Sizining is the process where the fibers are coated to protect them from damage during winding or weaving and this is the step followed after treating the surface. When the sizing dries, the long process is complete. Coating materials are chosen to be compatible with the adhesive used to form composite materials. Typical coating materials include epoxy, polyester, nylon, urethane, and others. The coated fibers are wound onto cylinders which are later loaded into a spinning machine and the fibers are twisted into yarns of various sizes.

Safety Concerns: The principal health hazards of carbon fibre handling are due to mechanical irritation and abrasion similar to that of glass fibres. These micro fibres if uncontrolled have a potential to stick into human skin or the mucous membranes causing irritation. Protection of eyes and throat from carbon fibre dust is paramount.

Some Reason Why Carbon Fiber Composites Are Replacing Traditional Materials

High strength-to-weight ratio: Carbon fiber weighs about 25% as much as steel and 70% as much as aluminum, and is much stronger and stiffer than both materials per weight. High-end auto engineers use composites to decrease vehicle weight by as much as 60% while improving crash safety; multilayer composite laminates absorb more energy than traditional single-layer steel.

Durability: CF composites never rust, regardless of their environment. CF composites have less fracture toughness than metals but more than most polymers. High dimensional stability allows them to maintain their shape, whether hot or cold, wet or dry. It helps to reduce maintenance costs and ensure long-term stability.

New design options: CF composites offer design options that would be hard to achieve with traditional materials. It allows for part consolidation; a single composite part can replace a full assembly of metal parts. The surface texture can be altered to mimic any finish, from smooth to textured.

Future of Carbon fiber: The future of the carbon fiber reinforced plastic (CFRP) market looks attractive with opportunities in the aerospace, automotive, pressure vessel, and wind energy industries. The major drivers for this market are the growing demand for high performance and lightweight composite materials and increasing performance requirements in the various end-use industries.

Emerging trends, which have a direct impact on the dynamics of the industry, include the development of technologies to reduce the manufacturing cost of carbon fiber as well as part fabrication and reuse of CFRP as recycled carbon fiber reduces the cost of product and environmental impact.