Study: Polymers to make fuel safer

In Hollywood films, cars seem to burst into flame as soon as there's even a minor collision. If they hit a tree or roll over, a flaming explosion is inevitable.

In reality cars explode much less often. That doesn't mean, though, that blazing explosions never happen - they do, with cars and planes as well. In 1977, for example, two 747's collided on the runway at Tenerife airport. The resulting fireball from jet fuel ignition killed 583 passengers.

According to the research team around Ming-Hsin Wei of the California Institute of Technology in Pasadena, fires initiated by aviation crashes cause on average 500 to 1000 deaths each year.

No wonder: one liter of jet fuel contains the explosive energy of 18 dynamite sticks. And a plane is loaded with hundreds of thousands of liters.

A much-wanted product

Since the accident in 1977, researchers have been looking for methods to reduce the flammability of gasoline, diesel and jet fuel.

More precisely, they've been trying to minimize the 'misting' of these volatile substances. Liquid hydrocarbons evaporate even at room temperature and form flammable mixtures with air. This can lead to their ignition, and within fractions of a second after ignition, a fireball rips through the air.

It turns out that adding small quantities of very long polymers - very thin plastic filaments - to jet fuel can reduce misting and make handling fuels much safer. Kerosene imbued with antimisting polymers contains less than 0.3 percent of a high molecular weight polymer.

There is, however, one major drawback: As soon as this antimisting kerosene is passed into a pump, for example when it's pumped into the plane's fuel storage tanks, the long polymer chains break - like very long sticks of dry spaghetti you try to put into a pot that's too small. And that means the fuel loses its antimisting property.

In a recent issue of the journal "Science", Ming-Hsin Wei and his colleagues presented a possible solution to this problem: Very long polymers that break under shearing forces, but repair themselves later on.

Really really long molecules made of chained segments

In a video, lead author Julia Kornfield demonstrates how the molecules work.

She puts several metal chains onto the table. Each chain has colored plastic ends which represent chemical groups that will form a bond with each other. The chemical groups at each chain-end act like hook-and-loop tape ("Velcro" is a brand of hook-and-loop tape). This way, several metal chains will stick together and form one extremely long super-chain. The nature of the complementary chemical groups is designed to ensure the chains cannot form rings.

"When these molecules move through pumps, filters and long pipelines, they fall apart", explains Kornfield, demonstrating the effect by pulling on the chain so that it falls apart into several smaller ones.

And this falling-apart is reversible: "When they find themselves in a quiet part [...] again, they reassemble," she says, and puts the chains back together to a large one. Now they're once again "ready to do their job in [...] controling mist."

Other large polymers, in contrast, break irreversibly.

The researchers call their invention "megasupramolecules".

Crucial experiment

Only 0.3 percent of the polymer, as a percent of the volume of jet fuel, is needed to reduce the flammibility of jet fuel dramatically, the researchers demonstrate in another video.

They disperse a stream of jet fuel mist over continuosly burning torches. The fuel ignites almost instantanously, forming a fireball.

When they use jet fuel with 0.3 percent polymer in the same experiment, though, no fireball occurs. Instead, you see small local ignitions in the stream of fuel. They flare up for a fraction of a second and then extinguish again.

These effect remains even after the fuel-polymer mixture has passed through a fuel pump, the researchers say.

Still flammable

A fuel should not to ignite if it's spilled on the road or on a runway - however, it must ignite at the higher temperatures and pressures in an engine. Those are must-have characteristics of transportation fuels.

"Misting is critical for jet engines," write Michael Jaffe and Sahitya Allam of the New Jersey Institute of Technology in an accompanying comment in "Science."

"The polymer must not inhibit the desirable mist formation that affects combustion properties like flash point."

To put it in a nutshell: misting yes, but only a little.

The Caltech researchers claim that their polymer-enhanced fuel is compatible with current engines.

"The most exciting aspect of these results is [...] the recognition that an important commercial application can be derived directly from a quantitative theoretical prediction," write Jaffe and Allam.

Since 1977 researchers have been looking for a product with the property to reduce misting even after passing through pumps and pipelines. Ming-Hsin Wei and his team used modeling and computer simulations to calculate the optimal length for such a polymer.

Now further experiments are needed to find out if the finely calibrated anti-misting performance of their megasupramolecules will bear up under large-scale production, and whether the polymers can meet their developers' commercial and financial hopes.