Carbon fiber is a material valued for its combination of lightness and strength. It is composed of thin filaments of carbon atoms, typically 5 to 10 micrometers in diameter, bonded together in a crystalline structure. This structure gives carbon fiber its unique properties, making it useful across various industries.
Precursor Materials
Carbon fiber production begins with precursors, raw materials rich in carbon content. Polyacrylonitrile (PAN) is the most widely used precursor, accounting for approximately 90% of carbon fiber production. PAN is a synthetic polymer, a long chain of molecules formed from acrylonitrile monomers, whose chemical structure allows for efficient conversion into carbon fiber.
Another precursor is pitch, a viscoelastic material derived from petroleum or coal. While less common than PAN, pitch-based carbon fibers can offer high stiffness. The choice of precursor influences the final properties of the carbon fiber produced.
The Manufacturing Process
The creation of carbon fiber is a multi-stage thermal process that transforms precursor material into a fibrous form. The initial step involves spinning the precursor into long, continuous fibers. For PAN, this involves extruding a solution through small holes and stretching it to align the molecules. This mechanical alignment is important for the fiber’s eventual strength.
Following spinning, the fibers undergo stabilization. They are heated in an oxygen-rich atmosphere, usually air, at temperatures from 200°C to 400°C for 30 minutes to two hours. During this stage, the molecular chains undergo chemical changes, preparing the fibers for subsequent high-temperature steps by preventing them from melting.
After stabilization, the fibers proceed to carbonization. They are heated to high temperatures, 1,000°C to 3,000°C, in an inert atmosphere like nitrogen or argon to prevent combustion. Non-carbon atoms are expelled as gases, leaving carbon atoms arranged in crystalline structures aligned parallel to the fiber’s axis. The temperature and duration dictate the fiber’s final carbon content, which can exceed 90%.
The next step is surface treatment, which modifies the fiber’s exterior to improve its bonding with resins in composite materials. This involves oxidizing the fiber surface through methods like exposure to air, carbon dioxide, nitric acid, or electrochemical processes. This enhances adhesion between the carbon fiber and the matrix material it will reinforce.
Finally, the fibers receive a protective coating called sizing. This thin layer of organic polymer, typically 0.5% to 5% of the fiber’s weight, protects the fibers from damage during handling and processing. Sizing also optimizes the interface between the carbon fiber and the resin matrix, which is important for the composite’s mechanical properties. Once sized, the fibers are wound onto bobbins, ready for use.
Key Properties
Carbon fiber possesses a combination of properties that make it highly desirable for advanced materials. It has a high strength-to-weight ratio, meaning it is very strong for its relatively low mass. This property allows for the creation of lighter structures that can withstand substantial forces. Carbon fiber also exhibits high stiffness, or modulus of elasticity, indicating its ability to resist deformation under load. It can be stiffer than many metals, providing structural rigidity.
Carbon fiber exhibits corrosion resistance, as it does not rust or degrade easily when exposed to chemicals or environmental elements. This durability contributes to the longevity of components made from it. It also demonstrates fatigue resistance, meaning it can withstand repeated cycles of stress without degradation. These combined properties enable the design of structures that are both lightweight and robust.
Common Applications
Carbon fiber’s distinct characteristics have led to its widespread adoption across many industries. In aerospace, its high strength-to-weight ratio allows for the manufacture of lighter aircraft and spacecraft components, contributing to improved fuel efficiency and performance. Aircraft fuselages, wings, and interior structures use carbon fiber composites.
The automotive industry utilizes carbon fiber for body panels, chassis components, and structural elements to reduce weight and enhance handling. This material finds extensive use in sports equipment, including bicycle frames, golf club shafts, tennis rackets, and fishing rods, where its lightness and strength translate into improved performance for athletes. Carbon fiber is also used in wind turbine blades for structural stiffness and reduced weight, and in military equipment for its strength and durability.