Early Applications of Carbon Fibres

As early as 1959 scientists at Parma had taken a step toward producing high performance carbon fibres. Curry Ford and Charles Mitchell patented a process for making fibres 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 fibres into the “advanced composites” industry in 1963.

Composites are reinforced materials consisting of more than one component. The industry had been dominated by fibreglass and boron fibres, which were extremely popular in the late 1950s and early 1960s. Boron fibres, which contained a tungsten core, were especially strong and stiff, but they were also expensive and heavy. Carbon fibres 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 fibres to this point were still of relatively low modulus. The first truly high modulus commercial carbon fibres were invented in 1964, when Bacon and Wesley Schalamon made fibres 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 fibre 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 fibres 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 fibres 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 fibres kept the phenolic resins intact and they have been an important ingredient in aerospace materials ever since.

Polyacrylonitrile (PAN)-based Carbon Fibres

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 fibres.

A quiet study by Japanese researchers in 1961—largely unknown to Western scientists—demonstrated high strength and high modulus fibres from PAN precursors. Akio Shindo of the Government Industrial Research Institute in Osaka, Japan, made fibres in the lab with a modulus of more than 140 GPa, about three times that of rayon-based fibres 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 fibres 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 fibres, 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 fibre manufacturing.

PAN-based fibres eventually supplanted most rayon-based fibres, and they still dominate the world market. In addition to high modulus fibres, British researchers in the mid-1960s also developed a low modulus fibre 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 Fibres Today

All commercial carbon fibres produced today are based on rayon, PAN or pitch. Rayon-based fibres were the first in commercial production in 1959, and they led the way to the earliest applications, which were primarily military. PAN-based fibres have replaced rayon-based fibres in most applications, because they are superior in several respects, notably in tensile strength. Fibers from PAN fueled the explosive growth of the carbon fibre 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 fibre producer; the business has since been sold to Amoco and then to Cytec, which is among a group of major carbon fibre manufacturers that spans the globe.

Pitch-based fibres 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 fibres. A lower modulus, non-graphitized mesophase-pitch-based fibre, which is much lower in cost, is used extensively for aircraft brakes.

The cost of making carbon fibres 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 fibres are used sparingly in automotive applications, but someday entire body panels may be made from them. All high speed aircraft have carbon fibre 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 fibres could even be used to develop earthquake-proof buildings and bridges.