Carbon Nanotube Structural Composites - Implications and Impact

Some time ago we ran a series of articles on lightweight construction and how it might play in various forms of transport in our energy constrained future. We concluded that it was easier said than done due to limitations in existing materials technology and due to the generally high costs of strong light materials. We also predicted that lightweight construction techniques were unlikely to make rapid inroads into the automotive sector where they would have the greatest impact.

Those basic caveats are still in force, but within the last year the advanced composites industry has changed sufficiently as to alter the situation of the manufacturer striving to reduce the dry weight of any given vehicle.

The most significant change has to do with the emergence of commercial resin pre-impregnated fabrics and tapes utilizing carbon nanotubes rather than conventional carbon fiber. Carbon nanotube technology has been advanced as the ideal solution for reducing the mass of vehicles without compromising strength or durability almost since such materials were discovered in 1991, but, apart from a few cost-is no-object experiments involving military aircraft, carbon nanotubes have had almost no impact on commercial advanced composite construction. Carbon nanotubes of various sorts have themselves been commercially available for years, but the minute size of the filaments, the difficulty of aligning them and forming them into fabrics, and their extraordinarily high cost have effectively kept them out of the structural composite marketplace.

Today these limitations have been ameliorated if not entirely eliminated. Carbon nanotubes have been grown to lengths of hundreds of meters where a few millimeters was the norm a decade ago. Manufacturers have devised methods of aligning the fibers and bonding them to one another—a critically problem in the past because the fibers are chemically inert and difficult to form into composites. And, perhaps most important, the price has declined by three orders of magnitude. The lowest price nanotubes may be had today for a few hundred dollars per kilogram. Just a few years ago the tubes we're selling for over $1,000 per gram.

The first pre-impregnated tapes and fabrics are now entering the market from companies such as Zyvex and Nanocyl, and undoubtedly more vendors of fiber reinforced fabrics will add to carbon nanotube reinforcements to their products. Existing weaves are still a mixture of conventional carbon fiber and carbon nanotubes, but the admixture of nanotubes is sufficient to reduce weight by 30% per a given degree of tensile strength.

But that figure, while significant, does not in itself indicate the scope of performance improvements to be had by the use of these new materials.

Carbon fiber of the conventional sort has already been hailed as a revolutionary structural material, which in many ways it is, but it also suffers from certain critical limitations which have restricted its use in both consumer and industrial transportation products. Foremost among those limitations is its brittleness and low impact resistance. Carbon fiber, once seen as the logical replacement for fiberglass in boat building, remains confined to race boats where long term durability is unimportant or at best a secondary consideration. And, for similar reasons, its penetration into the automotive realm has been utterly negligible. But nanotube reinforcements change all that. Impact resistance surpasses that of Kevlar or Spectra, the ballistic fibers that are sometimes substituted for conventional carbon and tensile strength is far superior. Carbon nanotubes appear to be the strongest materials in the universe. Their recent availability in quantity cannot be without significance.

Will these new materials revolutionize the transportation industry? Surely not any time soon. The raw cost of the nanotubes is one thing but the cost of woven fiber is much higher, so much higher than sheet steel and aluminum that carbon nanotubes don't really qualify as a replacement technology. Furthermore, there is no clear path to further price reductions. The filaments still have to be grown in a reaction chamber, and it's hard to see how anyone could produce them by the ton. Worse yet, the production process is highly energy intensive, a major impediment to further adoption in the face of rising fossil fuel prices.

Still, for certain industries this is important news. Very likely these materials will be extensively used in aircraft eventually although conventional carbon fiber for a variety of reasons have had limited acceptance to date and have been confined for the most part to small experimental aircraft. We'll probably be seeing these in customade boats and limited production sports cars and almost certainly in high performance bicycles. But don't count on a wider adoption any time soon. The bulk of the transportation industry has an enormous fully amortized investment in rust belt era metal bending technology. They're not about to give it up so long as they can derive further profit from it.