A New Class of Fibers: Composites Made with Carbon Nanotubes Offer Improved Mechanical & Electrical Properties

For example, composite fibers made from single-walled carbon nanotubes (SWNTs) and polyacrylonitrile - a carbon fiber precursor - are stronger, stiffer and shrink less than standard fibers.

Nanotube-reinforced composites could ultimately provide the foundation for a new class of strong and lightweight fibers with properties such as electrical and thermal conductivity unavailable in current textile fibers.

Researchers from the Georgia Institute of Technology, Rice University, Carbon Nanotechnologies, Inc. and the U.S. Air Force have been developing new processes for incorporating nanotubes into fibers and films. The results of that work were presented March 28 at the 227th national meeting of the American Chemical Society in Anaheim, California.

"We are going to have dramatic developments in the textile materials field over the next 10 or 20 years because of nanotechnology, specifically carbon nanotubes," predicted Satish Kumar, a professor in Georgia Tech's School of Polymer, Textile and Fiber Engineering. "Using carbon nanotubes, we could make textile fibers that would have thermal and electrical conductivity, but with the touch and feel of a typical textile. You could have a shirt in which the electrically-conducting fibers allow cell phone functionality to be built in without using metallic wires or optical fibers."

Thanks to the work of Kumar and researchers at the Air Force Research Laboratory, nanotubes have already found their way into fibers known as Zylon, the strongest polymeric fiber in the world. Research has shown that the strength of the fiber was increased by 50 percent through incorporation of 10 percent nanotubes.

Recently, Kumar's research team has been collaborating with Richard Smalley, a Rice University professor who received a 1996 Nobel Prize for his work in developing nanotubes, which are of great interest because of their high strength, light weight, electrical conductivity and thermal resistance.

The researchers have developed a technique for producing composite fibers containing varying percentages of carbon nanotubes, up to a maximum of about 10 percent. Produced by Rice University and Carbon Nanotechnologies, Inc., single-walled nanotubes exist in bundles 30 nanometers in diameter containing more than 100 tubes.

To produce composite fibers, the bundles are first dispersed in an organic solvent, acid or water containing surfactants. Polymer materials are then dissolved with the dispersed nanotubes, and fibers produced using standard textile manufacturing techniques and equipment. The resulting composite fibers have a touch and feel similar to standard textile fibers.

Addition of carbon nanotubes to traditional fibers can double their stiffness, reduce shrinkage by 50 percent, raise the temperature at which the material softens by 40 degrees Celsius and improve solvent resistance. Kumar believes these properties will make the composite fibers valuable to the aerospace industry, where the improved strength could reduce the amount of fiber needed for composite structures, cutting weight.

"If you can increase the modulus (stiffness) by a factor of two, in many applications you can also reduce the weight by a factor of two," Kumar noted.

But the greatest impact of carbon nanotubes will be realized only if researchers can learn how to break up the bundles to produce individual nanotubes, a process called exfoliation. If that can be done, the quantity of tubes required to improve the properties of fibers could be reduced from 10 percent to as little as 0.1 percent by weight That could help make use of the nanotubes - which now cost hundreds of dollars per gram - feasible for commercial products.

Including individual nanotubes in composite fibers could help improve the orientation of the polymer chains they contain, reducing the amount of fiber entanglement and increasing the crystallization rate. That could introduce new properties not currently available in fibers.

"If we can do this, that would conceptually change how fibers are made," Kumar said. "Having a very tough temperature resistant material with a density of less than water seems like a dream today, but we may be able to see that with this new generation of materials."

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