Car, heal thyself

Direct sunlight is bad for your skin, but could be your car's best friend.

Researchers have developed a new polyurethane material that heals its own scratches in less than an hour when exposed to ultraviolet light.

"It has a lot of potential. This is the first time ever that we've developed material that as a result of UV light will do what it does," said researcher Marek Urban, a professor of polymer science at the University of Southern Mississippi.

The research, published in today's issue of the journal Science, holds much promise for the automotive industry, which is on the lookout for coatings with high scratch resistance.

"Because of their hardness and elasticity, polyurethanes exhibit good scratch resistance but can still suffer from mechanical damage. An ideal automotive coating would mend itself while a vehicle is driven," the paper states.

The self-repairing agents could be integrated into any polyurethane paint. The way it works involves some chemical magic.

Prof. Urban and Biswajit Ghosh, a graduate student at the university, incorporated molecules called oxetane-substituted chitosan precursors into a polyurethane. Chitosan comes from a natural product found in the shells of crabs, shrimps and other sea crustaceans.

To achieve the self-repairing properties of polyurethane, the researchers chemically attached two components: oxetane and chitosan.

When a scratch damages the molecule, a ring of oxetane is ripped open and exposes two reactive ends, which act like arms wanting to grab onto something.

This mechanical break allows ultraviolet light to enter the scratch, which pulls apart a rod of chitosan, revealing another set of reactive ends.

The broken chitosan rods and oxetane ring start pulling toward each other to close and seal the gap, leaving your paint job as good as new.

Left in the sun, a scratch can heal itself in less than an hour, the researchers found.

Prof. Urban said the material can be used on cars, in packaging, furniture and even on buildings.

Chemical companies have shown interest in this work and in commercializing the product, Prof. Urban said. But he declined to give any names.

"This is a proved concept that this type of material can be self-repairing," he said.

"This project is part of the major effort in my research group dealing with stimuli-responsive materials. We have other projects in the pipeline that deal with self-repairing systems and the ultimate goal will be to develop new materials with sustainable properties."