carbon fiber

An extremely strong thin fiber made by pyrolyzing synthetic fibers, such as rayon, until charred. It is used to make high-strength composites.

Background

A carbon fiber is a long, thin strand of material about 0.0002-0.0004 in (0.005-0.010 mm) 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 makes the fiber incredibly strong for its size. Several thousand carbon fibers are twisted together to form a yarn, which may be used by itself or woven into a fabric. The yarn or fabric is combined with epoxy and wound or molded into shape to form various composite materials. 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.

Carbon fibers were developed in the 1950s as a reinforcement for high-temperature molded plastic components on missiles. The first fibers were manufactured by heating strands of rayon until they carbonized. This process proved to be inefficient, as the resulting fibers contained only about 20% carbon and had low strength and stiffness properties. In the early 1960s, a process was developed using polyacrylonitrile as a raw material. This produced a carbon fiber that contained about 55% carbon and had much better properties. The polyacrylonitrile conversion process quickly became the primary method for producing carbon fibers.

During the 1970s, experimental work to find alternative raw materials led to the introduction of carbon fibers made from a petroleum pitch derived from oil processing. These fibers contained about 85% carbon and had excellent flexural strength. Unfortunately, they had only limited compression strength and were not widely accepted.

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.

Classification of Carbon 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. The English unit of measurement is pounds of force per square inch of cross-sectional area, or psi. Carbon fibers classified as "low modulus" have a tensile modulus below 34.8 million psi (240 million kPa). Other classifications, in ascending order of tensile modulus, include "standard modulus," "intermediate modulus," "high modulus," and "ultrahigh modulus." Ultrahigh modulus carbon fibers have a tensile modulus of 72.5-145.0 million psi (500 million-1.0 billion kPa). As a comparison, steel has a tensile modulus of about 29 million psi (200 million kPa). Thus, the strongest carbon fiber is about five times stronger than steel.

The term graphite fiber refers to certain ultrahigh modulus fibers made from petroleum pitch. These fibers have an internal structure that closely approximates the three-dimensional crystal alignment that is characteristic of a pure form of carbon known as graphite.

Raw Materials

The raw material used to make carbon fiber is called the precursor. About 90% of the carbon fibers produced are made from polyacrylonitrile. 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.

During the manufacturing process, a variety of gases and liquids are used. Some of these materials are designed to react with the fiber to achieve a specific effect. Other materials are designed not to react or to prevent certain reactions with the fiber. As with the precursors, the exact compositions of many of these process materials are considered trade secrets.

The Manufacturing
Process

The process for making carbon fibers is part chemical and part mechanical. The precursor 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.

Here is a typical sequence of operations used to form carbon fibers from polyacrylonitrile.

Spinning

• 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. In some methods, the plastic is mixed with certain chemicals and pumped through tiny jets into a chemical bath or quench chamber where the plastic coagulates and solidifies into fibers. This is similar to the process used to form polyacrylic textile fibers. In other methods, the plastic mixture is heated and pumped through tiny jets into a chamber where the solvents evaporate, leaving a solid fiber. The spinning step is important because the internal atomic structure of the fiber is formed during this process.
• The fibers are then washed and stretched to the desired fiber diameter. The stretching helps align the molecules within the fiber and provides the basis for the formation of the tightly bonded carbon crystals after carbonization.

Stabilizing

• Before the fibers are carbonized, they need to be chemically altered to convert their linear atomic bonding to a more thermally stable ladder bonding. This is accomplished by heating the fibers in air to about 390-590° F (200-300° C) for 30-120 minutes. This causes the fibers to pick up oxygen molecules from the air and rearrange their atomic bonding pattern. The stabilizing chemical reactions are complex and involve several steps, some of which occur simultaneously. They also generate their own heat, which must be controlled to avoid overheating the fibers. Commercially, the 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

• Once the fibers are stabilized, 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. The gas pressure inside the furnace is kept higher than the outside air pressure and the points where the fibers enter and exit the furnace are sealed to keep oxygen from entering. 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 de heating during carbonization.

Treating the surface

• After carbonizing, the fibers have a surface that does not bond well with the epoxies and other materials used in composite materials. To give the fibers better bonding properties, their surface is slightly oxidized. 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. The fibers can also be coated electrolytically by making the fibers the positive terminal in a bath filled with various electrically conductive materials. The surface treatment process must be carefully controlled to avoid forming tiny surface defects, such as pits, which could cause fiber failure.

Sizing

• After the surface treatment, the fibers are coated to protect them from damage during winding or weaving. This process is called sizing. 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 called bobbins. The bobbins are loaded into a spinning machine and the fibers are twisted into yarns of various sizes.

Quality Control

The very small size of carbon fibers does not allow visual inspection as a quality control method. Instead, producing consistent precursor fibers and closely controlling the manufacturing process used to turn them into carbon fibers controls the quality. Process variables such as time, temperature, gas flow, and chemical composition are closely monitored during each stage of the production.

The carbon fibers, as well as the finished composite materials, are also subject to rigorous testing. Common fiber tests include density, strength, amount of sizing, and others. In 1990, the Suppliers of Advanced Composite Materials Association established standards for carbon fiber testing methods, which are now used throughout the industry.

Health and Safety Concerns

There are three areas of concern in the production and handling of carbon fibers: dust inhalation, skin irritation, and the effect of fibers on electrical equipment.

During processing, pieces of carbon fibers can break off and circulate in the air in the form of a fine dust. Industrial health studies have shown that, unlike some asbestos fibers, carbon fibers are too large to be a health hazard when inhaled. They can be an irritant, however, and people working in the area should wear protective masks.

The carbon fibers can also cause skin irritation, especially on the back of hands and wrists. Protective clothing or the use of barrier skin creams is recommended for people in an area where carbon fiber dust is present. The sizing materials used to coat the fibers often contain chemicals that can cause severe skin reactions, which also requires protection.

In addition to being strong, carbon fibers are also good conductors of electricity. As a result, carbon fiber dust can cause arcing and shorts in electrical equipment. If electrical equipment cannot be relocated from the area where carbon dust is present, the equipment is sealed in a cabinet or other enclosure.

The Future

The latest development in carbon fiber technology is tiny carbon tubes called nanotubes.

These hollow tubes, some as small as 0.00004 in (0.001 mm) in diameter, have unique mechanical and electrical properties that may be useful in making new high-strength fibers, submicroscopic test tubes, or possibly new semiconductor materials for integrated circuits.

Where to Learn More

Books

Brady, George S., Henry R. Clauser, and John A. Vaccari. Materials Handbook. McGraw-Hill, 1997.

Kroschwitz, Jacqueline I. and Mary Howe-Grant, ed. Encyclopedia of Chemical Technology. John Wiley and Sons, Inc., 1993.

Periodicals

Ebbesen, T.W. "Carbon Nanotubes." Physics Today (June 1996): 26-32.

Other

American Carbon Society website. www.americancarbonsociety.org

Carbon Composites website. http://www.carb.com.

[Article by: Chris Cavette]

A cloth of woven carbon filaments

Carbon fibre or carbon fiber can refer to carbon filament thread, or to felt or woven cloth made from those carbon filaments. By extension, the term is also used informally to mean any composite material made with carbon filament, such as carbon fiber reinforced plastic. Carbon fibers find many uses because of their strength and light weight. Carbon fiber was invented in the early 1960's at the Royal Aircraft Establishment at Farnborough, Hampshire (England).

Structure and properties

A 6 μm diameter carbon filament compared to a human hair.

Each carbon filament thread is a bundle of many thousand carbon filaments. A single such filament is a thin tube with a diameter of 5–8 micrometers and consists almost exclusively of carbon.

The atomic structure of carbon fiber is similar to that of graphite, consisting of sheets of carbon atoms arranged in a regular hexagonal pattern. The difference lies in the way these sheets interlock. Graphite is a crystalline material in which the sheets are stacked parallel to one another in regular fashion. The chemical bonds between the sheets are relatively weak, giving graphite its soft and brittle characteristics. Carbon fiber is an amorphous material: the sheets of carbon atoms are haphazardly folded, or crumpled, together. This interlocks the sheets, preventing slippage and greatly increasing the strength of the material.

The density of carbon fiber is 1750 kg/m³. It has high electrical and low thermal conductivity. When heated, a carbon filament becomes thicker and shorter. Carbon fiber is naturally a glossy black color. Recently, however, colored carbon fiber has become available.

Carbon fiber thread or yarn is rated by either linear density (mass per unit length, with the unit 1 tex = 1 g/1000 m), or by the number of filaments per yarn, in thousands.

Synthesis

A common method of making carbon filaments is the oxidation and thermal pyrolysis of polyacrylonitrile (PAN), a polymer based on acrylonitrile used in the creation of synthetic materials. Like all polymers, polyacrylonitrile molecules are long chains, which are aligned in the process of drawing continuous filaments. When heated in the correct conditions, the non-carbon constituents evaporate away, the chains bond side-to-side (ladder polymers) and form narrow graphene sheets which eventually merge to form a single, jelly roll-shaped or round filament. The result is usually 93–95% carbon. Lower-quality fiber can be manufactured using pitch or rayon as the precursor instead of PAN.

The carbon fiber can become further enhanced by heat treatment processes. Carbon heated in the range of 1500-2000 °C (carbonization) exhibits the highest tensile strength (820,000 psi or 5,650 MPa or 5,650 N/mm²), while carbon fiber heated from 2500 to 3000 °C (graphitizing) exhibits a higher modulus of elasticity (77,000,000 psi or 531 GPa or 531 kN/mm²). For further literature see Rose, Ziegmann and Hillermeier. ^{[citation needed]}

Uses

Carbon fiber is most notably used to reinforce composite materials, particularly the class of materials known as carbon fiber reinforced plastics. This class of materials is used in aircraft parts, high-performance vehicles, sports equipment such as racing bikes, radio controlled vehicles, wind generator blades and gears and other demanding mechanical applications; a more thorough discussion of these uses, including composite lay-up techniques, can be found in the carbon fiber reinforced plastic article.

Carbon fiber is one of the leading materials used in Formula One car production since the introduction of the fiber into common commercial use in the early 1980s.

Non-polymer materials can also be used as the matrix for carbon fibers. Due to the formation of metal carbides (i.e., water-soluble AlC), bad wetting by some metals, and corrosion considerations, carbon has seen limited success in metal matrix composite applications; however, this can be improved by proper surface treatment, e.g., for carbon-aluminium MMCs a vapor deposition of titanium boride on the fibers is often employed. Reinforced carbon-carbon (RCC) consists of carbon fiber-reinforced graphite, and is used structurally in high-temperature applications, such as the nose cone and leading edges of the space shuttle.

The fiber also finds use in filtration of high-temperature gases, as an electrode with high surface area and impeccable corrosion resistance, and as an anti-static component in high-performance clothing.

Generally, within the realm of design and marketing there is a trend toward use of carbon fiber to imply a technical construction (for the given item) or associate it with traditional uses (i.e. military, or high performance) to attract a certain demographic. This is best noted in the increasing prevalence of carbon fiber in jewelery (e.g. Montblanc), [pen|pens]] (e.g. Caran d'Ache), and watches (e.g. TAG Heuer).

Musical instruments

Some string instruments, such as guitars and members of the violin family are being fabricated of carbon fiber reinforced composite. One revolutionary company, XOX Audio Tools, makes a Monocoque carbon fiber electric guitar, the Handle. There is also a company specializing in Carbon Fiber wind instruments offering bagpipe chanters, tin whistles and flutes currently manufacturing product.

For over a decade, a number of drum companies such as Tempus and Rocket Shells have been using carbon fiber in their kits and individual snare drums. Carbon fiber solves a number of problems inherent to wood drums and is especially beneficial when used in drums used by drum corps and marching bands, which are typically worn while played. There is also a company called Carbonlite making a lightweight drum rack and cymbal boom arms out of carbon fiber.

Utensils

Knife manufacturers sometimes use carbon fiber for functional purposes, such as creating lightweight handles and scales. Some also use carbon fiber inserts for decorative purposes. An entire knife can be constructed from carbon fiber.

Vehicles

Many high-end frames for road bikes and mountain bikes are made of carbon fiber reinforced composite. Some velomobiles use a monocoque body constructed with carbon fiber.

It is also widely used to enhance the look of automobiles and reduce weight. Many of the "tuner" style cars have carbon fiber hoods or other components to reduce weight. Another use is in the increasingly popular hobby of RC cars, many high-end kits come with many carbon fiber parts due to their light weight and attractive appearance.

In motorsports, carbon fiber is often used to construct bodywork or a monocoque chassis. This trend started in Formula 1 and has gradually been adopted in other forms of motor racing.

Aprilia RSV Nera, with full carbon fiber body

Disassembled fuselage section of the Boeing 787 Dreamliner

Newer designs of aircraft are beginning to make increased use of carbon fiber composities. For example, the Airbus A380 uses many CFRP components. The even newer Boeing 787 Dreamliner will be the first passenger jet with a main wing and fuselage (body) made entirely out of carbon fiber. This is one of the main causes of the current worldwide shortage as Boeing have bought up most of the world's output for the new 787.

Carbon fiber is also used by skateboard companies to make strong lightweight skateboards for all types of skating, mainly downhill speedboarding. It is also used in many composite longboards to stiffen an otherwise very flexible board.

In surfing, carbon fiber is emerging as a very high-end (and very expensive) board construction material, exhibiting even higher strength and lighter weight than epoxy boards.

A Bicycle helmet with carbon fiber inserts.

Sports equipment

Carbon fiber is used on racing yachts, rowing boats, kayaks and canoes, as well as on the paddles and oars used with them. Its use has allowed boat builders to produce stiffer and lighter boats. Carbon, along with other artificial fibers, has replaced more traditional laminated wooden or fiberglass constructions. As well as these water sports, carbon fiber is also used in the construction of water skis from Goode Ski Technologies of Ogden, Utah.

Carbon fiber is a prime material for use in archery. Many modern-day arrows are made as either aluminium-carbon composites or entirely of carbon fiber. Limbs can also have a laminate system, consisting of carbon fiber sheets alternated with materials such as foam. This makes the bow faster and smoother to shoot. Another use in archery is in the bow's riser, for both recurve and compound bows. More risers nowadays are beginning to have carbon fiber parts, which makes them lighter, and there are several companies that have made full carbon-fiber risers (including Carbofast and KG), with at least 3 currently in production (Win&Win, FiberBow the first in total carbon, and High Country Archery[compound]).

In cycling, the use of carbon fiber for both frames and different components became commonplace in the late 90s/early 2000s, a notable early example being Chris Boardman's full-carbon monocoque Lotus frame, which he rode in the 1992 Olympic Games in Barcelona in track events. This so-called 'carbon revolution' resulted in the production of bicycles with weight-to-stiffness characteristics that had never been seen before, eventually leading to the almost-exclusive use of carbon as a material of choice in high-end competition bicycles in all disciplines of the sport. Today, carbon-fiber frames are quite common even in the non-sponsored amateur ranks (nevertheless they are still quite expensive), along with many types of components, e.g. wheels, handlebars, seat pillars etc.

Fishing rods are often made of carbon fiber: this started during the 1970s. Prior to that, heavier glassfiber rods were used.

High-end sports kites are framed using carbon fibre spars, taking advantage of the low weight and high rigidity of the material. Cheaper pultruded spars are still popular but "wrapped" and conical spars are now standard in high-end kites. Manufacturers include Skyshark, Aerostuff and Avia.

In track and field events, carbon fiber has been used in newer designs of pole-vault poles, to add rigidity while reducing weight, javelins for the same purpose, and the discus, to increase the percentage of rim weight for higher spin.

Carbon fiber is used in lacrosse and hockey stick shafts and curling brooms. Pure carbon-fibre sticks are rare due to the brittleness of carbon fiber. In these sticks carbon fibre is often found in composites.^[1]

Carbon fiber is also used in many paintball products. Carbon fiber barrels, tanks, and triggers are not uncommon in aftermarket paintball parts.

Carbon fiber can also be found in shoes. Nike, in particular, uses foot-length carbon fiber spring plates in high-end basketball shoes like the Air Jordan XI. These plates are found between the outsole and the midsole, usually partially exposed along some stretches of the sole.

In sailing and yacht racing, traditionally wood, and then aluminum, were used to form components such as spars. Newer sailboat designs incorporate carbon fibre materials for these parts to reduce weight. An example of this is the Pixel, which sports a carbon-fibre mast.^[2]

Photography equipment

Several photographic tripod manufacturers, including Manfrotto and Gitzo, employ carbon fiber in their professional range tripods, due in large part to its lightweight properties and its comparable strength to aluminum, the material most consumer tripods are made from.

Toyo View also produces a 4x5 large format view camera made partially from carbon fiber, mostly used in this case to cut down on the high weight of a standard large format camera.

See also

• Carbon nanotube
• Tenax

References

1. ^ Hockey Stick Materials (2006). Retrieved on 2007-04-23.
2. ^ Pixel Sailboat.

External links

• Working with Carbon fiber for Robotics and R/C Aircraft
• Carbon Fiber in Formula One
• The Chemistry of Carbon Fiber
• Making Carbon Fiber
• Carbon Fiber Examples
• Luis and Clark Carbon Fiber Musical Instruments
• Carbon Fiber Musical Instruments XOX Audio Tools

Fibers
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Synthetic:	Acrylic · Aramid (Twaron, Kevlar, Technora, Nomex) · Carbon fiber · Microfiber · Nylon · Olefin · Polyester · Rayon · Spandex · Zylon

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