YOU might never need fear for your car’s paintwork again if a new kind of polyurethane that can repair its own scratches makes it to market.
Previous self-healing materials have mostly used some form of liquid epoxy that is encapsulated in microscopic spheres or fibres, or is delivered by an engineered vascular system. When it is damaged, the epoxy is released into the gap and sets when it makes contact with a hardening agent contained in the material itself.
The new self-healing coating works in a different way. Scratches to its surface close up in only a few minutes when exposed to sunlight. This occurs because the damaged polymer molecules around the edges of a scratch absorb energy from the ultraviolet radiation in the sunlight and use it to form new cross-links, and so rebuild its molecular network.
Advertisement
The material could be useful as a clear coating for cars, electronic devices such as mobile phones, and even furniture, says , a polymer scientist at the University of Southern Mississippi, Hattiesburg, who led the study. “If you scratch it, let it sit in sun and it’s cured.”
Urban and his team combined polyurethane with a polymer similar to chitosan, a carbohydrate derived from the shells of shrimps, crabs and other crustaceans. The researchers modified the chitosan molecules by adding ring-shaped structures called , which are composed of one oxygen and three carbon atoms. It is the oxetane rings that give the material its ability to heal. When a scratch is formed, some of the rings break, leaving chemically reactive free ends, although the exact chemical mechanism is unknown.
Exposure to UV light creates reactive spots on other sections of the chitosan molecules which then bond with the broken oxetane rings to form new chemical cross-links that make good the damage. The process appears to begin at the bottom of a scratch, pulling it closed like a zipper ().
Urban says that scratches about 10 micrometres wide – just visible to the naked eye – heal after 30 minutes’ exposure to UV light.
, a materials scientist at Iowa State University in Ames, agrees that the material may have enormous promise but points out that it isn’t clear how well it can recover from deeper, more obvious scratches.
