Flexible fibers from rayon
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.