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Tough Coatings for Airplanes

A strong material inspired by abalone shells could be applied over large areas.

For decades, materials scientists have looked to naturally existing composites as inspiration for tough, lightweight materials that could lighten vehicles. Such materials could save on fuel costs, protect airplanes, and be used in engine turbines that run more efficiently. The material that lines abalone shells, called nacre, has been of particular interest: it’s lightweight and strong, yet shatter-resistant. But mimicking the microscale structures responsible for its properties has been difficult, and hasn’t resulted in materials that can be manufactured on a large scale.

Paper for airplanes: This paper (top), made from layers of tiny clay discs and a polymer (seen under the microscope at bottom), might be used as a strong, lightweight coating for buildings and airplanes.

Now researchers in Helsinki, Finland, have developed a simple method for making large-area, nacre-like papers and coatings that could be painted on building walls and airplane skins for lightweight reinforcement. The researchers will work with the Finnish paper company UPM to commercialize the material.

“The excitement with nacre is that its properties are impressive when you consider what it’s made out of: calcium carbonate and a protein,” says Robert Ritchie, chair of the materials science and engineering department at the University of California, Berkeley, who is not involved with the coatings research. Nacre’s combination of interconnected plates of a very hard but shatter-prone material with an infill of a very soft but ductile material results in a composite whose properties are better than the sum of its parts. By starting with better materials, such as industrial ceramics and polymers or metal, it should be possible to make a synthetic composite whose properties are even better than those of nacre.

Most efforts to mimic the nacre structure’s combination of hard and soft materials have centered on structural materials that could provide a lightweight alternative to steel in building and vehicle frames and engine turbines. Steel is tough–that is, it doesn’t fracture when it’s stressed. Materials such as ceramics can’t be used for structural applications because they’re not tough. They can hold up under the stress of a great weight, but they’re prone to shattering. Last year, for example, Ritchie’s group made a nacre-like material that is the toughest ceramic ever made. In the form of a coating, such a strong, tough material could reinforce walls and airplane skins without adding significant weight. Previous work on making tough biomimetic coatings has stayed in the lab because these materials involved very laborious processes, such as dipping a glass slide in two solutions 1,800 times, to make thin coatings over small areas.

In the journal Nano Letters, researchers at the Helsinki University of Science and Technology describe a process for combining strong, disc-shaped clay platelets with the soft polymer polyvinyl alcohol. When mixed together in water, the polymer coats the discs to create a slurry that can be made into paper or painted over a surface such as a wall. The resulting paper or coating is made up of discs of the so-called nanoclay stacked in rows like plates in a cupboard with the polymer surrounding them, a structure very similar to that found in nacre.

The Helsinki coatings are very strong and lightweight, but still too prone to fracture to be used in the way structural materials such as steel are used today. Their material properties are similar to those of fiberglass, says Andreas Walther, one of the Helsinki researchers. The first application for the material may be as a reinforcing coating for walls. Experiments with flamethrowers showed that the coatings can act as a heat and fire shield.

“Stiffness and strength are a start, but it needs to be tougher to be interesting,” says Berkeley’s Ritchie of the material. Walther says the method is compatible with a range of polymers, which provide the toughness in these composites, and that the group is experimenting with softer polymers in hopes of making tougher coatings. If they succeed in making the material tougher, it could be suitable for armor and for replacing structural materials in airplanes.

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