Self-healing fuel tanks under fire

InTech, Oct 2004

THERE IS NOTHING WORSE THAN a punctured fuel tank, and if you arc a fighter pilot, the problems just magnify incredibly.

Materials chemists and engineers are now working on plans to develop polymer films that can almost instantly close up holes ripped through fuel tanks by highspeed projectiles, said Christopher S. Coughlin, Ph.D., and colleagues at the Naval Air Systems Command in Patuxent River, Md.

The notion of "self-healing" polymers originates in biomimetics, the study and design of high-tech products that mimic biological systems. In this case, the system would be healing a wound.

"Our work started here when a group that oversees aircraft survivability came to us, saying they'd heard about an ionomcr [material] that could heal over holes if you shot at it. They wanted a polymer scientist to see whether we could use it in a fuel tank," said Coughlin. "Unfortunately, it degrades around jet fuel, but we started thinking, maybe we can modify this."

The material was Surlyn, made by DuPont, and a type of copolymer known as an ionomer. Its polyethylene chains are interspersed with methacrylic acid, to which ions attach.

Attractions between the ions form crosslinks of sorts within the material. Surlyn now sees use on golf ball covers, hockey helmets, and ski boots, among other products.

Coughlin said virtual crosslinks play a role in self-healing, and wants to either confirm the hypothesis or find the right one. His goal is to combine the mechanism with better fuel resistance in a single sturdy, lightweight, inexpensive material. "We do have a kind of self-sealing fuel tank now, but it's a multilayer system and thus heavier and more expensive than we'd like. And eveiy ounce of weight we save, that's one more ounce of fuel or weapons an aircraft can carry,"he said.

Researchers start with pellets of various Surlyn copolymers, molding and cutting them into sheets about 15 centimeters square and about 1.5 millimeters thick. Then it's out to the shooting range, where they fire at each square from about 20 meters away-currently with .308 rifle rounds, but 10-gauge shotgun, .50-caliber machine gun, and 23 mm anti-aircraft rounds also have been tested.

Afterward, most samples display one of three characteristics: The bullet hole either seals shut, the remaining hole has smooth edges that appear melted, or the material fractures around the bullet hole.