Hole new approach to repairs

US Navy engineers are testing polymers with self-healing properties, which could be used to improve the survival prospects of aircraft hit by anti-aircraft fire and shrapnel.

Surlyn – an ethylene-methaclyric acid (EMAA) ionomer – is already used in various forms for coating golf balls and ski boots.

According to Dr. Christopher Coughlin, materials engineer at the Aerospace Materials Division, Naval Air Systems Command, NASA researchers began investigating Surlyn 18 months ago when the space agency was looking into the survivability of aircraft.

Research initially focused on using the material in lightweight fuel tanks for planes, ships and ground vehicles, to take advantage of its self-healing properties. But during the project it was discovered that the material degrades upon contact with jet fuel.

So Coughlin decided to investigate why the material heals, to allow the team to find a polymer that is resistant to jet fuel. ‘As a separate effort, we decided to take a step back from the application and work on the basic mechanism, why it is self-healing and what is causing it,’ he said.

Surlyn consists of chains of polyethylene, which are interspersed with methacrylic acid, to which ions are attached. Coughlin believes the attractions between these ions form crosslinks that allow the material to heal itself following an impact such as a bullet strike.

Coughlin tested the material by taking various Surlyn copolymers and shooting at them from 18m with 7.62mm bullets, equivalent to an AK47. The results displayed varying degrees of success, he said. ‘It turns out even within this family of ethylene-methacrylic ionomers, some of them heal completely, some fracture and with some you get a melted hole. You need something in the middle range to close up the hole.’

The results are subjected to thermal analysis to discover the melting transitions of the material, which enables Coughlin to separate the ionomers into different fractions and determine why some grades heal and others do not. Studies show that a lot of energy is required for Surlyn to heal itself, limiting its applications to military fields. Coughlin aims to combine the mechanism with better fuel resistance in a single, inexpensive material.