Newswise — For a fighter pilot, sealing a punctured fuel tank even temporarily can mean the difference between making it back to base, ditching a $60-million aircraft and its highly trained personnel, or worse.

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, DuPontÔ Surlyn® 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®.

PMSE 261

Mechanism of ballistic self-healing in EMAA ionomers

Certain ethylene-methacrylic acid (EMAA) ionomers have been shown to be self-healing to ballistic impact. Self-healing materials would have significant potential to improve the survivability of Navy aircraft by providing self-sealing characteristics to currently unprotected fuel, and fluid containing parts. A wide range of EMAA materials have been characterized for their thermal behavior as well as for their ballistic self-healing. Successive Self-Nucleation/Annealing (SSA) was employed to characterize molecular segregation during crystallization. These data were correlated with ballistic results to show that the distribution of melting fractions is an important determinant of the ability to self heal, though this phenomena also appears to be influenced by the counterion and melt index of the material. Rheological characterization is underway and a model relating the thermal, rheological and self-healing behavior of these materials is under development.

PMSE 261Mechanism of ballistic self-healing in EMAA ionomers

Briefly explain in lay language what you have done, why it is significant and its implications, particularly to the general public. There has been some work that shows that ethylene-methacrylic acid ionomers are self healing when shot with a bullet. These materials are commercially available from Dupont under the brand name Surlyn and are used in a wide variety of applications, including golf ball covers, for their toughness. We, the Navy, are interested because a material that can reseal after being shot has great potential for use as lightweight, self-healing fuel tanks for aircraft, ships and ground vehicles. Unfortunate Surlyn itself is not resistant to jet fuel so we can't use it directly. Our research is aimed at trying to determine exactly why Surlyn heals and why some grades of Surlyn heal better than others. With that understanding, we can then find or make other polymers that will heal, as well as be resistant to fuel. We have been making panels of different Surlyns and shooting them with a .308 full metal jacket round at a range of about 20 yards. We then evaluate how well they healed. We are also characterizing the thermal properties of the different grades to determine why some heal and other do not. In terms of application for the general public, hopefully they aren't getting shot at all the time. It appears that you need a fair amount of energy input, like a speeding bullet, to get a hole in the Surlyn to heal. So it probably won't be useful for general puncture resistance in the sense of a nail in the tire. Of course, there are many military applications, but not many places in everyday life where a high speed projectile is involved.

It is also important to note that this material is not bullet resistant. The bullet still goes through, the hole just closes up after the bullet passes. So this would not be good for a bullet proof vest. That seems to be the first application people think of.

How new is this work and how does it differ from that of others who may be doing similar research?

There has been some work on this material/technology at Virginia Tech and at NASA Langley. I have not seen much published, other than a couple of MS theses from VA Tech. I think we are taking a different approach in characterization from VA Tech by looking at the distribution of melting fractions and how that relates to healing. I believe we are also sampling a larger variety of Surlyn materials, all the same basic EMAA copolymers but with variations in copolymer composition, neutralization and counterion. And we are shooting with bigger, more powerful rounds. I haven't seen anything from the NASA group so I'm not sure the approach they are taking. I believe that Surlyn is also sometimes used as a material for targets because it can take a lot of shots before it has to be replaced.

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228th American Chemical Society National Meeting