Car research could boost carbonated beverage packaging

A discovery that improves the practicality of vehicles powered by compressed natural gas could have unexpected benefits for carbonated beverages.

Rice University chemist James Tour has enhanced a polymer material, making 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, but could also find a market in food and beverage packaging.

By adding modified, single-atom-thick graphene nanoribbons (GNRs) to thermoplastic polyurethane (TPU), the Rice lab made it 1000 times harder for gas molecules to escape, Tour said. This is 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.

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, Tour said.

But graphene nanoribbons are already there. Tour’s breakthrough ‘unzipping’ technique for turning multiwalled carbon nanotubes into GNRs, first revealed in Nature in 2009, has been licensed for industrial production. “These are being produced in bulk, which should also make containers cheaper,” Tour said.

The researchers, led by Rice graduate student Changsheng Xiang, 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% of the composite’s weight. But the overlapping 200- to 300-nanometre-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, Tour said.

The team tested GNR/TPU films 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 100 seconds as nitrogen escaped into the vacuum chamber. With GNRs at 0.5%, 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% 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.

“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.

“Beer has a bigger problem and, in some ways, it’s the reverse problem,” Tour said. “Oxygen molecules get in through plastic and make the beer go bad.” Bottles that are effectively impermeable could lead to brew that stays fresh on the shelf for far longer, Tour said.

Tour and his colleagues at Rice University and in Hungary, Slovenia and India reported their results in the online edition of the American Chemistry Society journal ACS Nano.