Singer’s taffy pull

Singer’s taffy pull

Any material containing carbon can be “carbonized” by heating it to around 1,000 °C, producing a substance that is roughly 99 percent carbon. Upon further heating, typically to about 2,500 °C, such a material can be coverted to 100 percent carbon, while transforming the internal structure from a poorly ordered to a more ordered form. But not all carbon materials heat-treated to these high temperatures are truly graphitic. Only certain carbons start with an adequately ordered structure to form nearly perfect graphite crystals, and only these graphitic substances can approach the excellent properties of pure graphite — high thermal and electric conductivities combined with high stiffness (Young’s modulus).

PAN and rayon are both non-graphitizing materials, so carbon fibers from these precursors will never be truly graphitic, even after heat treatment to high temperatures. To make the next generation of carbon fibers, scientists needed a new starting material. Once again, research at the Parma Technical Center led the way.

Leonard Singer came to Parma in the mid-1950s with little experience in carbon or graphite. He was attracted to the “utopian flavor” of the place, and he planned to continue his work with electron paramagnetic resonance. He was using this research technique to study the underlying mechanism of carbonization, which involved heating various petroleum- and coal-based materials. Heating organic substances like these inevitably leads to the formation of a pitch — a tar-like mixture of hundreds of branched compounds with different molecular weights. Pitch is an important high carbon organic precursor used in the manufacture of a number of carbons and graphites.

Two Australian scientists had recently made an important discovery involving pitches. Most pitches are isotropic, having identical properties in all directions, but these researchers showed how a pitch can be polymerized slightly further to orient the molecules in a layered form. “This happens because of the existence of a liquid crystal state, which is also called a mesophase,” Singer says. “That really solved the orientation mystery which had been bothering me for a long time.” Fiber research was going on all around him at Parma, so Singer couldn’t help but be pulled in. “It occurred to me that one could probably make a fiber out of this,” he says. “That’s when I decided to try orienting a fiber by elongation of the carbonaceous mesophase.”

Singer and his assistant, Allen Cherry, designed a “taffy-pulling” machine that applied stress to the viscous mesophase to align the molecules, and then heated the material to convert it to a highly oriented carbon fiber. The process worked, and subsequent analyses verified that they had made highly-oriented graphitizable carbon fibers.

The physical properties of these graphitized mesophase pitch fibers were astounding. Not only did they have an ultrahigh elastic modulus, approaching 1,000 GPa, but these were also the first carbon fibers with ultrahigh thermal conductivity. This made them especially useful for any application where stiffness and heat removal were important — such as aircraft brakes and electronic circuits. Most mesophase pitch-based fibers did not achieve the high tensile strengths of some PAN and rayon fibers, except in the laboratory.

Singer’s initial discovery was made in 1970, but a patent for both the fiber and the process was not issued until 1977. The patent was an incredible amount of work, a 42-page document with 47 illustrations.

Pitch is a fairly inexpensive raw material. However, depending on the form and properties of the desired product, the cost of the final product, mat, strands, or cloth, can vary widely. On the one hand, the mesophase pitch-based farbon fibers used in aircraft brakes and reinforced concrete are relatively inexpensive. On the other hand, due to the extremely high graphitizing temperatures required, the ultrahigh modulus, high thermal conductivity fibers required in satellites and other spacecraft can be expensive.

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