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A recent discussion we had shows student interest in the material sciences. So we quizzed them on what is probably the most ubiquitous space-age material there is - carbon fiber. It's in our airliners, cars, computers, even furniture. But it seems that most people are quite ill-informed on it, irregardless of their interest.
It's a good thing that we're here to teach... |
“They were imbedded like straws in brick. They were up to an inch long, and they had amazing properties. They were only a tenth of the diameter of a human hair, but you could bend them and kink them and they weren’t brittle. They were long filaments of perfect graphite.”
Roger Bacon, 1956
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 carbon fiber compared to human hair 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 are classified by the tensile modulus of the fiber. 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 fibers are ten times stronger than steel and eight times that of aluminum, not to mention much lighter than both materials, 5 and 1.5 times, respectively. Additionally, their fatigue properties are superior to all known metallic structures, and they are one of the most corrosion-resistant materials available, when coupled with the proper resins.
Thirty years ago, carbon fiber was a space-age material, too costly to be used in anything except aerospace. However today, carbon fiber is being used in wind turbines, automobiles, sporting goods, and many other applications. Thanks to carbon fiber manufacturers who are committed to the commercialization concept of expanding capacity, lowering costs, and growing new markets, carbon fiber has become a viable commercial product.
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.
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 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.
History
The synthetic carbon industry had its official beginning in 1886 with the creation of the National Carbon Company. Based in Cleveland, Ohio, the company would eventually merge with Union Carbide in 1917 to form Union Carbide & Carbon Corp., which changed its name to Union Carbide Corp. in 1957. The carbon products division of Union Carbide Corp. became the independent UCAR Carbon Company in 1995, and was renamed GrafTech International Holdings in 2002.
Electricity was mostly a lab curiosity until the late 1800s, when carbon arc lamps began lighting the streets of major U.S. cities. The lamps were composed of two carbon rods connected to a current source and separated by a short distance. A blazing hot path of charged particles—the “arc”—formed between the two rods, giving off an intense light. National Carbon got its start by producing carbon electrodes for streetlamps in downtown Cleveland.
In 1879, Thomas Edison invented the first incandescent light bulb, which uses electricity to heat a thin strip of material, called a filament, until it glows. He may also have created the first commercial carbon fiber. To make his early filaments, Edison formed cotton threads or bamboo slivers into the proper size and shape and then baked them at high temperatures. Cotton and bamboo consist mostly of cellulose, a natural linear polymer made of repeating units of glucose. When heated, the filament was “carbonized,” becoming a true carbon copy of the starting material—an all-carbon fiber with the same exact shape. Tungsten wire soon displaced these carbon filaments, but they were still used on U.S. Navy ships as late as 1960 because they withstood ship vibrations better than tungsten.
Carbon fibers are classified by the tensile modulus of the fiber. 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 fibers are ten times stronger than steel and eight times that of aluminum, not to mention much lighter than both materials, 5 and 1.5 times, respectively. Additionally, their fatigue properties are superior to all known metallic structures, and they are one of the most corrosion-resistant materials available, when coupled with the proper resins.
Thirty years ago, carbon fiber was a space-age material, too costly to be used in anything except aerospace. However today, carbon fiber is being used in wind turbines, automobiles, sporting goods, and many other applications. Thanks to carbon fiber manufacturers who are committed to the commercialization concept of expanding capacity, lowering costs, and growing new markets, carbon fiber has become a viable commercial product.
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.
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 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.
History
The synthetic carbon industry had its official beginning in 1886 with the creation of the National Carbon Company. Based in Cleveland, Ohio, the company would eventually merge with Union Carbide in 1917 to form Union Carbide & Carbon Corp., which changed its name to Union Carbide Corp. in 1957. The carbon products division of Union Carbide Corp. became the independent UCAR Carbon Company in 1995, and was renamed GrafTech International Holdings in 2002.
Electricity was mostly a lab curiosity until the late 1800s, when carbon arc lamps began lighting the streets of major U.S. cities. The lamps were composed of two carbon rods connected to a current source and separated by a short distance. A blazing hot path of charged particles—the “arc”—formed between the two rods, giving off an intense light. National Carbon got its start by producing carbon electrodes for streetlamps in downtown Cleveland.
In 1879, Thomas Edison invented the first incandescent light bulb, which uses electricity to heat a thin strip of material, called a filament, until it glows. He may also have created the first commercial carbon fiber. To make his early filaments, Edison formed cotton threads or bamboo slivers into the proper size and shape and then baked them at high temperatures. Cotton and bamboo consist mostly of cellulose, a natural linear polymer made of repeating units of glucose. When heated, the filament was “carbonized,” becoming a true carbon copy of the starting material—an all-carbon fiber with the same exact shape. Tungsten wire soon displaced these carbon filaments, but they were still used on U.S. Navy ships as late as 1960 because they withstood ship vibrations better than tungsten.
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The modern era of carbon fibers began in 1956, when Union Carbide opened its Parma Technical Center just outside Cleveland. The complex was one of the major laboratories of Union Carbide’s basic research program, modeled after the university-style corporate labs that became popular in the late 1940s and 1950s. They gathered young, bright scientists from a variety of backgrounds and let them loose on their favorite projects, giving them an extraordinary degree of autonomy.
With a freshly minted Ph.D. in physics, Roger Bacon joined the Parma staff in 1956. “I got into carbon arc work, studying the melting of graphite under high temperature and pressures,” Bacon recalls. “I took on the job of trying to determine the triple point of graphite. That’s where the liquid, solid, and gas are all in thermal equilibrium.” The equipment was akin to the early carbon arc streetlamps, only operating at much higher pressures. Small amounts of vaporized carbon would travel across the arc and then deposit as liquid. As Bacon decreased the pressure in the arc, he noticed that the carbon would go straight from the vapor phase to the solid phase, forming a stalagmite-like deposit on the lower electrode. “I would examine these deposits, and when I broke one open to look at the structure, I found all these whiskers,” he says. “They were imbedded like straws in brick. They were up to an inch long, and they had amazing properties. They were only a tenth of the diameter of a human hair, but you could bend them and kink them and they weren’t brittle. They were long filaments of perfect graphite.” |
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Ponder this
Would it be possible to create fibers using other nonmetallic elements? What are the chemical or physical characteristics needed?
Discuss
How is this material better than other synthetic fibers that are renowned for their strength such as rayon, kevlar, or fiberglass? In what application do they each excel? What considerations in those application should we look for? And what will cause them to fail in their use?
How is this material better than other synthetic fibers that are renowned for their strength such as rayon, kevlar, or fiberglass? In what application do they each excel? What considerations in those application should we look for? And what will cause them to fail in their use?
Further readings
Roger Bacon, the inventor of carbon fiber
The chemical principles of carbon fiber production, a more technical guide from the Massachusetts Institute of Technology
Carbon by Design, for a sample of the applications of carbon fiber
"Making Carbon Nanotubes into Long Fibers", the next step, weaving carbon nanotubes into yarn. From MIT Tech Review.
