Chapter 20 Carbon Fiber Technology

“Go, go, let’s go to the space station.” In an elevator, a boy shouted, excited by TV news of Shenzhou No. 7 with which a Chinese astronaut firstly went into the space. Suddenly, he questioned, “Daddy, why don’t we build an elevator between the earth and the space station? If so, we can go to the space everyday.” Such a childish question maybe also emerges time by time in the adults’ minds, but rapidly disappears for somewhat perceptual and rational reasons. Experts with different background of knowledge may give different reasons of impossibility by today’s knowledge and technology, but for a non-expert father, it is not curious that he naturally associates this problem with the strength of ropes lifting the elevator. The ropes may be sacrificed by their own weights before the elevator goes out of the earth. It is true. For example, the tensile strength of steel is commonly 1-2 GPa (1-2 k N/mm2) and the density is about 7.8 g/cm3. That is to say, a suspended steel rope with a radius of 1 mm will break at the point of about 4.2-8.3 km away from the ground suffering from its own weight. Combining the second powered increase of weight with radius, the father answered, “There is no rope including the strongest steel rope tough enough to lift the elevator to so far away.”“But, can we make a rope much, much stronger than the steel one?” asked the boy. The answer is definitely “Yes, we can. But the problem is when and how we make it.” This is the story of this unit.

The 1940s and 1950s saw popularity of university-style laboratories in industry. Of the major laboratories Union Carbide’s basic research program opened its Parma Technical Center in the state of Ohio in 1956. Dozens of young, brilliant scientists were recruited from a variety of backgrounds and allowed to run their own favorite research projects. Roger Bacon, newly graduated Ph.D. in physics, joined the research center. His project was to study the melting of graphite at high temperatures and pressures, by the purpose to determine the triple point of graphite, where the liquid, solid, and gas are all in thermodynamic equilibrium. The experimental setup resembled the early carbon arc streetlamps, but operating at much higher pressures. During the experiment, Bacon observed that small amounts of vaporized carbon would travel across the arc and then deposit as liquid. As the pressure in the arc decreased, the carbon vapor phase would deposit directly to the solid phase, producing a stalagmite-like layer on the lower electrode. He examined those deposits and found they were all whiskers imbedded like straws in brick. They were up to an inch long with only a tenth of the diameter of a human hair, but they were soft. They were the long filaments consisting of carbon fibers. Two years later, Bacon demonstrated the carbon fibers with a tensile strength of 20 Giga pascals (GPa) and Young’s modulus of 700 GPa, much stronger than the common steel (tensile strength of 1-2 GPa and Young’s modulus of 200 GPa). It became a milestone in materials science.

Carbon fibers (alternatively called graphite fiber, or carbon graphite) are polymers of graphite, a pure form of carbon where the atoms are arranged in large layers of hexagonal rings. Each carbon fiber is a bundle of thousands of carbon filaments. A single such filament is one of Bacon’s graphite whiskers, a sheet of graphite rolled into scroll with a diameter of 5-8 micrometers and continuous over the entire length of the filament. Carbon atoms bonded in microscopic crystals are more or less aligned parallel to the long axis of the fiber. This makes the fiber very strong for its size. In addition, the density of carbon fiber is much lower than that of steel, endowing it incomparable high ratio of strength to weight.

Although Bacon’s processes were costly and not applicable for the commercial production, his discovery was a significant starting point. One year later, Bacon’s colleagues, Curry Ford and Charles Mitchell, managed to produce high performance carbon fibers. The process for making fibers and cloths was by heating rayon up to 3,000℃. These were the strongest commercial carbon fibers in 1963. The next year, the first high-modulus carbon fibers were commercialized. A new “hot-stretching” process was developed for making fibers from rayon. The carbon yam was stretched at high temperatures during heating, which forced the graphite layers to orient along the fiber axis. By this process, the fiber with Young’s modulus ten times higher was produced - a major step on the way to duplicating the properties of Bacon’s graphite whiskers.

In 1961, a Japanese research team independently demonstrated fibers with both high modulus and high strength from polyacrylonitrile (PAN) precursors. Akio Shindo, a national research institute researcher in Osaka, was able to make fibers with a modulus of more than 140 GPa, that was roughly three times that of rayon-based fibers. A pilot-scale production started in 1964 by Japanese researchers. The key to their success was better precursors. PAN and rayon are both non-graphitizing materials, thus carbon fibers from these precursors will never be truly graphitic, even after heat treatment to high temperatures. Thus, a new type of precursor materials was necessary to make the next generation of carbon fibers.

Leonard Singer of the Parma research center was trying to elucidate the mechanism of carbonization by electron paramagnetic resonance (EPR), during heating various petroleum- and coal-based materials. However, he was soon attracted by the pitch, a byproduct of heating organic substances, noticing a new discovery involving pitches at the time. Most pitches are isotropic and can be polymerized slightly further to orient the molecules in a layered form. It reminded Singer of the possibility of the existence of a liquid crystal state, also called a mesophase, in pitches, which favored the orientation. In addition, fiber research was so hot at Parma that Singer could not help but being pulled in. “It occurred to me that one could probably make a fiber out of this,” he said. “That’s when I decided to try orienting a fiber by elongation of the carbonaceous mesophase.” This decision was finally proven to be correct. In 1970, the highly-oriented graphitizable carbon fibers were produced by their “taffy-pulling” machine. Such graphitized mesophase pitch fibers were astounding in their physical properties. They were the first carbon fibers with an ultrahigh thermal conductivity, accompanying to an ultrahigh Young’s modulus, near 1000 GPa. It rendered them valuable materials for applications where stiffness and heat removal were vital, such as aircraft brakes and electronic circuits. However, unfortunately the high tensile strengths of some PAN and rayon fibers were never achieved for most mesophase pitch-based fibers, except in the laboratory.

Today, all commercial carbon fibers are produced from rayon, PAN, or pitch. Rayon-based fibers were first commercialized and applied primarily in military during early times. Nowadays PAN-based fibers with superior tensile strengths have replaced most rayon-based fibers and dominated the world market. The continuous string of carbon and nitrogen atoms in PAN chains leads to highly oriented graphitic-like layers, eliminating the need for hot stretching. The explosive growth of the carbon fiber industry was largely promoted by the PAN-based process. Applications such as aircraft fuselage, wings, tail, doors and brakes, space structures, lithium-ion batteries, tennis rackets, and structural reinforcement in construction materials have become industrial norm. Each Boeing 787 plane uses approximately 35 tons of carbon fiber reinforced plastic, produced from 23 tons of carbon fiber supplied by Toray Industries. Carbon fiber composites have a higher strength to weight ratio than traditional aircraft materials, and help make the 787 more energy efficient. Pitch-based fibers can achieve ultrahigh Young’s modulus and thermal conductivity and have gained their market in critical military and space products. Their high cost has resulted in a relatively small production scale. However, lower modulus, non-graphitized mesophase-pitch-based fibers with much lower cost have commonly been used for aircraft brakes. On the other hand, the cost of making carbon fibers has been reduced drastically in the last 20 years, but it is still not cheap enough for extensive use in automobiles. Earthquake-proof buildings and bridges can be built from carbon fibers.

However, carbon fibers are not used by themselves. They are used to reinforce composite materials, particularly the polymers such as epoxy resins and other thermosetting materials. Carbon fiber reinforced composites are often stronger than steel, but a whole lot lighter, thus usually used as steel substitutes. However, carbon fiber has been seldom used in metal matrix composite applications, because of corrosion and formation of metal carbides (i.e., water-soluble A1C). Carbon fiber-reinforced graphite, also known as reinforced carbon-carbon (RCC), could be used for high-temperature structural applications.

Finally, in the Bacon’s old mine, a Japanese scientist, Sumio Iijima, dug out a big “diamond” in 1991, i.e., the great discovery of carbon nanotubes which are the hollow cylinders of graphite with diameters on the order of single molecules. The incredible properties have made nanotubes one of the most active areas of research in recent years, promising to revolutionize just about every area of science. In the aspect of mechanical properties, it has been estimated that the single-walled nanotube possesses an ultrahigh tensile strength of about 150 GPa and Young’s modulus of 1054 GPa, whilst for the multiwall nanotube, the Young’s modulus reaches as ultrahigh as 1200 GPa, almost 6 times that of the steel. However, they are still lying in the laboratory. Undoubtedly, the strong fiber consists of filaments of pure nanotubes will come in the market, but the only problem is “who, when, and how to make it.”