The First Carbon Fibers
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.
Near the end of World War II, Union Carbide began investigating a replacement for tungsten wire in vacuum tubes by carbonizing rayon, another cellulose-based polymer (like cotton) that became popular in clothing. The end of the war brought an end to the government’s funding for this project, but carbon fibers were still raising interest in the commercial sector. Barnebey-Cheney Company, in 1957, briefly manufactured carbon fiber mats and tows (rope-like threads without the twists) from rayon and cotton. These were used as high temperature insulation and filters for corrosive compounds. A year later, Union Carbide developed a carbonized rayon cloth and submitted it to the U.S. Air Force as a replacement for fiberglass in rocket nozzle exit cones and re-entry heat shields.
While finding a certain degree of success in their respective niches, all of these early carbon fiber materials had poor mechanical properties, making them unsuitable for structural use. It took a chance discovery to set the age of high performance carbon fibers in motion.
Early Applications of Carbon Fibers
As early as 1959 scientists at Parma had taken a step toward producing high performance carbon fibers. Curry Ford and Charles Mitchell patented a process for making fibers and cloths by heat-treating rayon to high temperatures, up to 3,000 °C. They had produced the strongest commercial carbon fibers to date, which led to the entry of carbon fibers into the “advanced composites” industry in 1963.
Composites are reinforced materials consisting of more than one component. The industry had been dominated by fiberglass and boron fibers, which were extremely popular in the late 1950s and early 1960s. Boron fibers, which contained a tungsten core, were especially strong and stiff, but they were also expensive and heavy. Carbon fibers were much lighter, so the appearance of relatively affordable carbon composites was a welcome development, and they found widespread use in gaskets and packaging materials.
While the tensile strength of these materials was increasing, all commercial carbon fibers to this point were still of relatively low modulus. The first truly high modulus commercial carbon fibers were invented in 1964, when Bacon and Wesley Schalamon made fibers from rayon using a new “hot-stretching” process. They stretched the carbon yarn at high temperatures (more than 2800° C), orienting the graphite layers to lie nearly parallel with the fiber axis. The key was to stretch the fiber during heat up, rather than after it had already reached high temperature. The process resulted in a ten-fold increase in Young’s modulus—a major step on the way to duplicating the properties of Bacon’s graphite whiskers.
Union Carbide developed a series of high modulus yarns based on the hot-stretching process, beginning in late 1965 with “Thornel 25.” The trade name was derived from Thor, the Norse god for strength, and the Young’s modulus of the fibers—25 million pounds per square inch (psi), to about 172 GPa. The Thornel line continued with increasingly higher levelswhich is equivalent of modulus for more than ten years.
The U.S. Air Force Materials Laboratory supported much of Union Carbide’s research into rayon-based fibers during this period in an attempt to develop a new generation of stiff, high strength composites for rocket nozzles, missile nose tips and aircraft structures. The fibers were also used in spacecraft heat shields to reinforce phenolic resin—plastics that solidify upon heating and cannot be re-melted. As a missile or rocket returns to the atmosphere, the phenolic resin decomposes slowly while absorbing the heat energy, allowing it to survive the trip through the atmosphere without destroying itself. Carbon fibers kept the phenolic resins intact and they have been an important ingredient in aerospace materials ever since.
Polyacrylonitrile (PAN)-based Carbon Fibers
While researchers in the United States were reveling in rayon, scientists overseas were busy creating their own carbon fiber industries based on polyacrylonitrile, or PAN, which had been passed over by U.S. producers after unsuccessful attempts at making high modulus fibers.
A quiet study by Japanese researchers in 1961—largely unknown to Western scientists—demonstrated high strength and high modulus fibers from PAN precursors. Akio Shindo of the Government Industrial Research Institute in Osaka, Japan, made fibers in the lab with a modulus of more than 140 GPa, about three times that of rayon-based fibers at the time. Shindo’s process was quickly taken up by other Japanese researchers, leading to pilot-scale production in 1964. In that same year, just a few months before Bacon and Schalamon debuted their hot-stretching method, William Watt of the Royal Aircraft Establishment in England invented a still higher-modulus fiber from PAN. The British fibers were rapidly put into commercial production.
The secret behind these developments was better precursors. In both Japan and England, researchers had access to pure PAN, with a polymeric backbone that provided an excellent yield after processing. The continuous string of carbon and nitrogen atoms led to highly oriented graphitic-like layers, eliminating the need for hot stretching. Chemical manufacturers in the United States, however, generally inserted other compounds in the polymer backbone that could account for up to 20 percent of the product, making them totally unsuitable for carbonizing.
The Japanese eventually took the lead in manufacturing PAN-based carbon fibers, effectively beating the British at their own game. Japan’s Toray Industries developed a precursor that was far superior to anything seen before, and in 1970 they signed a joint technology agreement with Union Carbide, bringing the United States back to the forefront in carbon fiber manufacturing.
PAN-based fibers eventually supplanted most rayon-based fibers, and they still dominate the world market. In addition to high modulus fibers, British researchers in the mid-1960s also developed a low modulus fiber from PAN that had extremely high tensile strength. This product became widely popular in sporting goods such as golf clubs, tennis rackets, fishing rods and skis; it is also extensively used for military and commercial aircraft.
Carbon Fibers Today
All commercial carbon fibers produced today are based on rayon, PAN or pitch. Rayon-based fibers were the first in commercial production in 1959, and they led the way to the earliest applications, which were primarily military. PAN-based fibers have replaced rayon-based fibers in most applications, because they are superior in several respects, notably in tensile strength. Fibers from PAN fueled the explosive growth of the carbon fiber industry since 1970, and they are now used in a wide array of applications such as aircraft brakes, space structures, military and commercial planes, lithium batteries, sporting goods and structural reinforcement in construction materials. In the late 1970s, Union Carbide formed a separate division as its primary carbon fiber producer; the business has since been sold to Amoco and then to Cytec, which is among a group of major carbon fiber manufacturers that spans the globe.
Pitch-based fibers are unique in their ability to achieve ultrahigh Young’s modulus and thermal conductivity and, therefore, have found an assured place in critical military and space applications. But their high cost has kept production to a minimum; only a few Japanese companies in addition to Cytec are currently making commercial mesophase fibers. A lower modulus, non-graphitized mesophase-pitch-based fiber, which is much lower in cost, is used extensively for aircraft brakes.
The cost of making carbon fibers has been reduced drastically in the last 20 years, and researchers are bringing that cost down every day. As they do, many of the applications once considered impossible will become reality. Carbon fibers are used sparingly in automotive applications, but someday entire body panels may be made from them. All high speed aircraft have carbon fiber composites in their brakes and other critical parts, and in many aircraft they are used as the primary structures and skins for entire planes. Carbon fibers could even be used to develop earthquake-proof buildings and bridges.