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News

Graphene-polymer composite - light tanks to keep gases in check

Rice University : 17 October, 2013  (Technical Article)
A composite material created at Rice University is nearly impervious to gas and may lead to efficient storage of compressed natural gas for vehicles. It might also prolong the shelf life of bottled beer and soda. A 65 micron-wide polymer film contains a tiny amount of enhanced graphene nanoribbon that present gas molecules a tortuous path to escape.
Graphene-polymer composite - light tanks to keep gases in check
The Rice lab of chemist James Tour has enhanced a polymer material to make it far more impermeable to pressurised gas and far lighter than the metal in tanks now used to contain the gas. The combination could be a boon for an auto industry under pressure to market consumer cars that use cheaper natural gas. It could also find a market in food and beverage packaging.
 
Adding modified, single-atom-thick graphene nanoribbons (GNRs) to thermoplastic polyurethane (TPU) made it 1000 times harder for gas molecules to escape, due to the ribbons’ even dispersion through the material. Because gas molecules cannot penetrate GNRs, they are faced with a “tortuous path” to freedom.
 
The researchers acknowledged that 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.
 
But graphene nanoribbons are already there. Tour’s breakthrough “unzipping” technique for turning multi-walled carbon nanotubes into GNRs, first revealed in 2009, has been licensed for industrial production. “These are being produced in bulk, which should also make containers cheaper,” he said.
 
The researchers produced thin films of the composite material by solution casting GNRs treated with hexadecane and TPU, a block copolymer of polyurethane that combines hard and soft materials. The tiny amount of treated GNRs accounted for no more than 0.5 percent of the composite’s weight. But the overlapping 200 to 300 nanometer wide ribbons dispersed so well that they were nearly as effective as large-sheet graphene in containing gas molecules. The GNRs’ geometry makes them far better than graphene sheets for processing into composites.
 
GNR/TPU films were tested by putting pressurised nitrogen on one side and a vacuum on the other side. For films with no GNRs, the pressure dropped to zero in about 100s, as nitrogen escaped into the vacuum chamber. With GNRs at 0.5 percent, the pressure didn’t budge over 1000 seconds, and it dropped only slightly over more than 18 hours. Stress and strain tests also found that the 0.5 percent ratio was optimal for enhancing the polymer’s strength.
 
“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.”
 
He said the material could help to solve long-standing problems in food packaging, too. “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. Beer has a bigger problem and, in some ways, it’s the reverse problem. Oxygen molecules get in through plastic and make the beer go bad,” said Tour. Now, bottles that are effectively impermeable could lead to brew that stays fresh on the shelf for far longer.
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