Flexible fibers from rayon

As early as 1959 — just one year after Bacon’s discovery — 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, despite Bacon’s demonstration of their mechanical potential. 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), which is equivalent to about 172 GPa. The Thornel line continued with increasingly higher levels 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 resins — 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.

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The first carbon fibers | Bacon’s breakthrough | Flexible fibers from rayon | Polyacrylonitrile: a concurrent development | Singer’s taffy pull | Carbon fibers today | Landmark designation | Acknowledgments