"Tiny Tubes Tap Competitive Edge", Input/Output, ME Departments, September 2006

input/output

by Alan S. Brown, Associate Editor

Tiny Tubes Tap Competitive Edge

When golfer Geoff Ogilvy won the U.S. Open in June, he did it with clubs unlike any used previously in the tournament's 106 years. Their shafts were reinforced with nanotubes.

Nanotubes are hollow cylinders of carbon atoms bonded to one another in a hexagonal pattern that resembles chicken wire. Only a few nanometers in diameter, they would have to be stacked 20,000 to 100,000 deep to equal the thickness of this page.

After 15 years of development, funded mainly by the military, nanotubes are just reaching the market. Makers of sporting goods are among the first to embrace the technology. Their customers, after all, are willing to pay nearly as much as the Pentagon for a competitive edge.

As a result, nanotubes are taking star turns in baseball and softball bats, hockey sticks, and tennis rackets. Nanotube-reinforced bicycle frames ran the course at the Tour de France. At least in sports, "nano" has acquired the cachet once associated with such buzzwords as "atomic," "space age," "eco," and "online."

Tiny nanotubes in a club can give this ball a sharper sendoff. They helped golfer Geoff Ogilvy to his first win in this year's U.S. Open.

Yet even marketing hype cannot obscure compelling science. The carbon atoms of nanotubes form bonds that are stronger, more stable, and more uniform than those found in diamonds. This yields outstanding properties. Tensile strength of nanotubes reaches 150 gigapascals, and their elastic modulus exceeds 1 terapascal.

Harnessing those properties has not been easy. "Nanotubes love other nanotubes and hate everything else," said Lance Criscuolo, a business manager at Zyvex Corp., a nanotechnology company in Richardson, Texas. Nanotubes added to a polymer will not disperse uniformly like gravel in concrete, but will form useless clumps instead.

Zyvex is one of several companies that have found a solution for agglomeration. It has created polymer "bridges" that bond to nanotubes on one side and link to polymers on the other. This allows flowing resin to carry away and disperse nanotubes like logs in a stream.

The technology gives Aldila Inc., the Poway, Calif., company that made Ogilvy's golf shafts, a new way to attack old tradeoffs between weight and control. On the one hand, Aldila wants to shave weight from shafts so golfers can swing faster and hit longer drives. On the other, it needs to keep shafts strong and stiff enough so they retain accuracy instead of bending and twisting during each swing.

Engineers began attacking the problem in the 1920s, when they replaced wood with stronger, lighter steel tubes. Fifty years later, they switched to plastic reinforced with carbon fibers. The composites weighed less than metal and resisted twisting. Shaving even more weight off composite shafts by switching to stiffer carbon fibers poses problems.

"The hurdle with very light shafts made with high-modulus carbon fiber is that they are brittle," explained John Oldenburg, Aldila's vice president of engineering and new product development. "This has led to durability issues in consumer products. Nanotubes remove the weak link in the composite structure."

Carbon nanotubes reinforce the tapered area of this baseball bat.

That weak link is the resin matrix, which is much weaker than the carbon fibers it surrounds. "Normal wear and tear causes microcracks and microfractures to form in the resin," Criscuolo said. "As they propagate, they run into nanotubes. The nanotubes are too strong for the cracks to penetrate. In the images we have, the cracks just stop. The nanotubes are so evenly dispersed, there's nowhere for the crack to go."

This works in all sorts of carbon-reinforced sporting goods. Manufacturers have begun to tap them to provide longer life in baseball and softball bats, hockey sticks, tennis rackets, mountain bicycles, and other components that must withstand sudden impacts.

Designers achieve these gains with light loadings of nanotubes. Only 1 to 2 percent of an Aldila club, for example, consists of carbon nanotubes. Yet this is enough to double interlaminar shear strength, boost tensile modulus by 5 to 8 percent, and increase compression strength by 11 to 15 percent, Oldenburg said. "Call out a laminate property and nano- tubes make it better," he said.

For Oldenburg, the roadmap to better golf shafts is just becoming visible. "It took us two years to get to where we are today," he said. "The theoretical properties of nanotubes are tremendous and the application is still in its infancy."