The self-healing coating uses chitosan, a substance found in the shells of crabs and shrimp. This is incorporated into traditional polymer materials, such as those used in coatings on cars to protect paint.When a scratch damages the chemical structure, the chitosan responds to ultraviolet light by forming chemical chains that begin bonding with other materials in the substance, eventually smoothing the scratch. The process can take less than an hour.
Crab chemical could give cars a self-healing 'shell'
Kurt Kleiner, New Scientist 12 Mar 09;
You might never have to fear for your car's paintwork again if a new kind of polyurethane that is able to heal its own surface scratches makes it to market.
Small scratches to the surface of the material close up in only a few minutes when the material is exposed to the ultraviolet light in sunlight. This life-like healing occurs because the damaged polymer molecules around the edges of a scratch use the energy from the UV to form new cross-links and recreate the network that makes up the material.
The material could make a good top coat for an automobile, says Marek Urban, a polymer scientist at the University of Southern Mississippi, Hattiesburg, who led the study.
Crab chemical
Previous self-healing materials have mostly used some form of liquid epoxy that is encapsulated in spheres or fibres, or delivered by an engineered vascular system. When the material is damaged, the epoxy is released into the fissure and sets when it contacts a hardening agent in the material.
Taking a different approach with their new self-healing polymer, Urban and his team combined polyurethane with a molecule made up of chitosan, a carbohydrate found in the shells of crustaceans like crabs and lobsters. The researchers modified the chitosan slightly with the addition of the structures composed of four carbon atoms called oxetane rings.
It is the oxetane rings that give the material its ability to heal, says Urban. When a scratch is made, some of the rings are broken, leaving chemically reactive free ends. However, while he has worked out which bonds are involved in the reaction, the exact details of the chemical process are so far unknown.
Exposure to UV light creates reactive spots on sections of the chitosan molecules which then combine with the broken oxetane rings to form new chemical cross links that close up the damage. The process appears to begin at the bottom of a scratch, pulling it closed like a zipper.
Deep healing?
Urban says that scratches about 10 micrometers wide and 50 deep – just visible with the naked eye – heal over after 30 minutes of exposure to UV light. The fact the process starts at the bottom the two sides of a scratch are closest together suggests it will work for larger scratches, too, he adds.
Urban says that the material could be useful for a number of applications, including vehicles and furniture, or electronic devices like cellphones. "Anything you can think of," he told New Scientist, "If you scratch it, let it sit in Sun for some time and it's cured."
"I think this is a great new techology," says Michael Kessler, a materials scientist at Iowa State University. But he said it isn't clear how thoroughly the material would heal from deeper and wider scratches. "It's a first step," he says, pointing out that the material needs to have other desirable characteristics for different applications, such as heat tolerance or hardness.
Journal reference: Science (DOI: 10.1126/science.1167391)
New coating makes scratches on cars disappear
Julie Steenhuysen Yahoo News 12 Mar 09;
CHICAGO (Reuters) – Scientists have developed a polyurethane coating that heals its own scratches when exposed to sunlight, offering the promise of scratch-free cars and other products, researchers said on Thursday.
"We developed a polymeric material that is able to repair itself by exposure to the sun," said Marek Urban of the University of Southern Mississippi in Hattiesburg, whose study appears in the journal Science.
"In essence, you create a scratch and that scratch will disappear upon exposure to the sun," Urban said in an interview on the Science website.
The self-healing coating uses chitosan, a substance found in the shells of crabs and shrimp. This is incorporated into traditional polymer materials, such as those used in coatings on cars to protect paint.
When a scratch damages the chemical structure, the chitosan responds to ultraviolet light by forming chemical chains that begin bonding with other materials in the substance, eventually smoothing the scratch. The process can take less than an hour.
Urban said the new coating uses readily available materials, offering an advantage over other self-repairing coatings, which he said were "fairly elaborate and economically unfeasible."
The team tested the compound's properties using a razor-blade-thin scratch. "We haven't done any of the tests to show how wide it can be," Urban said in a telephone interview.
He said the polymer can only repair itself in the same spot once, and would not work after repeated scratches.
"Obviously, this is one of the drawbacks," he said, adding that the chances are low of having two scratches in exactly the same spot.
Howell Edwards, who leads the chemical and forensic sciences division of the University of Bradford in Britain, said the findings were novel.
"Clearly, there are future applications of this work in the repair of automotive components, which extensively use polyurethane polymers, that have suffered minor damage," Edwards said in a statement.
Urban said the coating could be used in packaging or furniture or anything that requires a high-performance type of coating.
"You can dream up anything you desire," he said.
Urban said his team has patents pending on the material and is considering commercialization.
(Editing by Patricia Zengerle)
