One goal of materials chemists and engineers with the U.S. Navy is to improve the odds that military planes and helicopters will survive hits by anti-aircraft fire and shrapnel. To that end, Christopher S. Coughlin, Ph.D., and colleagues at the Naval Air Systems Command in Patuxent River, Md., are trying to develop polymer films that can almost instantly close up holes ripped through them by high-speed projectiles.
The research was described today at the 228th national meeting of the American Chemical Society, the world's largest scientific society.
The notion of 'self-healing' polymers originates in biomimetics, the study and design of high-tech products that mimic biological systems — in this case, wound healing.
"Our work started here when a group that oversees aircraft survivability came to us, saying they'd heard about an ionomer [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 are attached. Attractions between the ions form crosslinks of sorts within the material, a feature that conveys specific properties. For one, DuPontTMSurlyn® is tough without being brittle — various forms of it are used for golf-ball covers, hockey helmets, ski boots and many other products.
Coughlin suspects the virtual crosslinks play a role in self-healing, and he wants to either confirm the hypothesis or find and elucidate the right one. Ultimately, he hopes 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 every ounce of weight we save, that's one more ounce of fuel or weapons an aircraft can carry," he said.
The researchers begin 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.
Afterwards, most samples display one of three characteristics: the bullet hole either seals shut, or the remaining hole has smooth edges that appear melted, or the material fractures around the bullet hole.
The results give clues to the mechanism, said Coughlin, by relating to fundamental differences among the materials' compositions. The material's thermal properties somehow influence its self-healing performance as well.
"There seems to be a balance between all these properties," Coughlin asserted. "When the hole heals, something is sucking the edges back together. How much restoring force, or elasticity if you will, is the right balance? Too high, and the material just fractures. Too low, and it melts instead of snapping back to fill the hole."
A few groups outside the Navy also are trying to answer the question, with a particular focus on space applications: structural polymers that can seal punctures from impacts with micrometeorites or space debris, for example, or even conductive ones that can restore computer circuits hit by ionic or solar radiation.
The American Chemical Society is a nonprofit organization, chartered by the U.S. Congress, with a multidisciplinary membership of more than 159,000 chemists and chemical engineers. It publishes numerous scientific journals and databases, convenes major research conferences and provides educational, science policy and career programs in chemistry. Its main offices are in Washington, D.C., and Columbus, Ohio.
The poster on this research, PMSE 261, will be presented Monday, Aug. 23, 8:00 – 10:00 p.m., during Sci-Mix, and Tuesday, Aug. 24, 6:00 – 8:00 p.m., during a joint PMSE/POLY poster session; both poster sessions are in the Pennsylvania Convention Center, Hall D.
Christopher S. Coughlin, Ph.D., is a materials engineer with the Polymer and Composites Branch, Aerospace Materials Division, Naval Air Systems Command in Patuxent River, Md.
Surlyn® is a registered trademark of DuPont or its affiliates. Only DuPont makes Surlyn®.