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No more buffing: self-healing coatings

Photonics Spectra
May 2009
Rebecca C. Jernigan,

We’ve all had it happen at least once. You make your way out to the parking lot, figure out where you left your car, reach for the door handle – and freeze. That scratch wasn’t there when you left your vehicle earlier, and it doesn’t look as if anyone is going to take responsibility for the damage. So what do you do? Call your insurance company? Go home and dig out the buffer and wax? Park in a sunny spot and wait for the mark to vanish?

Option three sounds too good to be true, but researchers at the University of Southern Mississippi are working to make it a reality. Marek W. Urban and Biswajit Ghosh have developed a self-repairing polyurethane coating that could be used on automobile and other coated surfaces to make them even more durable.


Researchers scratched the polyurethane coating and exposed it to UV light for gradually increased amounts of time. The coating healed itself, as demonstrated by this series of images, with the scratch becoming unnoticeable after 30 minutes of exposure. Courtesy of Marek W. Urban.

The coating consists of an oxetane-substituted chitosan precursor incorporated into a two-component polyurethane. Chitosan is a natural substance found in the shells of crustaceans such as crabs and shrimp, while polyurethane is commonly used to make the paint on cars scratch-resistant.

The precursor molecules contain oxetane rings, which are made up of three carbon atoms and one oxygen atom. When the coating is scratched, the damage to the molecules prompts the rings to open, exposing two reactive ends. Those ends link with chitosan chains, created by exposing linear chitosan to ultraviolet light. The cross-linking of these molecules fills in the scratch so that, after less than an hour of exposure to sunlight, it disappears.

A scratch would heal three to four times faster during the summer in the southern US than in northern states because the process works more quickly when the ultraviolet light is stronger, the researchers noted. The cross-linking reaction is not affected by atmospheric moisture; the coating heals equally well in dry or humid climates.

The coatings are not yet perfect, however. Once a group of molecules has repaired a scratch, that group will not be able to fix more damage. A scrape in that exact spot will act just like a scratch in normal paint would; that is, it will stay there. The chance of two scratches damaging the exact same group of molecules is minuscule, however.

carbon atomscoatingsmoleculespolyurethane coatingResearch & TechnologyTech Pulse

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