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Polymer Research Could Mean Lighter CNG Tanks

One of the challenges to widespread adoption of natural gas vehicles (NGV) has been the weight of the metal tanks needed to hold compressed natural gas (CNG). A researcher at Houston's Rice University might have hit upon an answer.

The Rice lab of chemist James Tour has enhanced a polymer material that could make tanks more impermeable to pressurized gas and lighter than the metal in tanks now used, according to the university. Tour and his colleagues at Rice and in Hungary, Slovenia and India reported their findings in the online edition of the American Chemistry Society journal ACS Nano.

By adding modified, single-atom-thick graphene nanoribbons (GNR) to thermoplastic polyurethane (TPU), the Rice lab made it 1,000 times harder for gas molecules to escape, Tour said. That's due to the ribbons' even dispersion through the material. Because gas molecules cannot penetrate GNRs, they are faced with a "tortuous path" to freedom, he said.

A solid, two-dimensional sheet of graphene might be the perfect barrier to gas, but the production of graphene in such bulk quantities is not yet practical, Tour said. However, graphene nanoribbons can be produced. Tour's "breakthrough" technique turns multi-walled carbon nanotubes into GNRs, which was revealed in 2009 in the publication Nature, has been licensed for industrial production.

"The idea is to increase the toughness of the tank and make it impermeable to gas," Tour said. "This becomes increasingly important as automakers think about powering cars with natural gas. Metal tanks that can handle natural gas under pressure are often much heavier than the automakers would like."

The technology could also find a market in food and beverage packaging.

"Remember when you were a kid, you'd get a balloon and it would be wilted the next day? That's because gas molecules go through rubber or plastic," Tour said. "It took years for scientists to figure out how to make a plastic bottle for soda. Once, you couldn't get a carbonated drink in anything but a glass bottle, until they figured out how to modify plastic to contain the carbon dioxide bubbles. And even now, bottled soda goes flat after a period of months."

Research at Rice has targeted the other end of the energy value chain as well. Rice's Andrew R. Barron, a professor of materials science, has been working to commercialize lighter-weight ceramic proppants through Oxane Materials Inc. Lighter proppant means fluids used for hydraulic fracturing can be less viscous, which means they can contain fewer chemicals (see Daily GPI, Nov. 13, 2009).

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