Materials chemists and engineers with the U.S. Navy are to develop materials that will improve the survival odds of military planes and helicopters hit by anti-aircraft fire and shrapnel.
Christopher S. Coughlin, and colleagues at the Naval Air Systems Command in Patuxent River, Md., are trying to develop polymer films that can quickly close up holes ripped through them by high-speed projectiles.
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.
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, moulding and cutting them into sheets about 15 centimetres square and about 1.5 millimetres thick. Then it’s out to the shooting range, where they fire at each square from about 20 metres away — currently with .308 rifle rounds, but 10-gauge shotgun, .50-calibre 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 are also 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.
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