How Polymer Science Is Making Aviation Safer
By Kerry Connell
There’s a reason that nobody smokes cigarettes while pumping gas (besides declining smoking rates in industrialized nations). We are all aware that gasoline is dangerous around an ignition source, but not everybody knows that liquid gasoline will not burn. Flammable vapors are the real problem.
The Science of Combustion
When gasoline is at or over its specific flash point, or the temperature at which it will vaporize (-40°F), conditions are good for combustion. Vaporized gasoline is highly volatile and heavier than air, which means it will sink and possibly meet up with an electrical spark, a pilot light or a cigarette dropped carelessly next to the pump. But because our everyday interactions with gasoline are limited to a few gallons at a time, this danger is fairly manageable.
Combustion is a chemical reaction that results in the rapid oxidation of fuel: it produces heat, light and byproducts. Standard 921 of the National Fire Protection Association (NFPA) addresses the chemistry of combustion — the surprisingly complex interactions of heat, fuel, oxidizing agent and uninhibited chemical chain reactions that we generally call fire. The NFPA standard notes that “Some flammable liquids … are difficult to ignite in a pool but can ignite readily and burn rapidly when in the form of a fine spray or mist.” That includes jet fuel — and the sheer amount of jet fuel that is usually present in any one place can present a huge potential for disaster.
Jet Fuel: A Potentially Deadly Cocktail
Also known as aviation turbine fuel, jet fuel is composed of a mixture of many different hydrocarbons designed to power the gas turbine engines of modern aircraft. It is different from the fuel that powers reciprocating engine (or piston-powered) aircraft, which generally has a low-flash point. Turbine engines are less fussy when it comes to fuels — the greater flexibility allowed in fuel composition means that jet fuel typically has a higher flash point, making it less flammable and therefore safer. But, again, jet fuel is generally used in far greater quantities than gasoline. The tragedies of September 11th illustrated how jet fuel can be just as deadly as a bomb. In fact, fuel fires due to crashes are estimated to be the cause of 40 percent of aviation-related deaths.
Aviation safety has always been a top concern of the industry, and for good reason — although travel by aircraft is statistically safer than travel by automobile, it’s also more likely that a plane crash will result in fatalities. This is why we’re willing to put up with de-icing before takeoff. The time it takes for the ground crew to apply the proper chemicals is time well spent.
A Little of This, a Little of That
When it comes to jet fuel, ASTM D1655 specifications permit the addition of antistatic agents (to prevent sparking), antioxidants (to prevent gumming) and corrosion and icing inhibitors, among other additives. A compound known as anti-misting kerosene (AMK) interferes with the fuel’s ability to mist and therefore to ignite when dispersed over a wide area (e.g., during a plane crash). AMK is a blend of a high molecular weight, long-chain polymer called FM-9 and the most common variety of jet fuel, Jet-A; it works by reducing drag on liquids in solution, but it does allow sufficient misting to produce the combustion needed for gas turbine operation. The polymer cannot be introduced directly into a gas turbine engine because, during its passage through pumps, hydrodynamic tension breaks the polymer chain and reduces its mist-suppressing properties. Stability during fueling, therefore, is reduced.
Aviation Safety: Soaring to New Heights
After the terrorist attacks of September 11th, a group of Caltech scientists set out to create a mist-controlling polymer that remains stable during routine handling. They used statistical mechanics to design a polymer that can self-assemble at a low total polymer concentration when their backbones are long enough (>400 kg/mol) and end association strength is optimal. The individual links are short enough to avoid hydrodynamic chain scission, and reversible linkages protect covalent bonds.
The resulting telechelic polymer, called a megasupramolecule, links together with other megasupramolecules to form massive chains that can break apart and link back together automatically. These self-assembling polymer chains allow the fuel to flow easily through the engine while still reducing drag, preventing the fuel from misting, dispersing and igniting after impact.
The scientists published their research in Science in October 2015. The new polymer, still in the testing phase, promises a safer future for air travelers and aviation workers alike.