Scientists develop self-healing rubber

LONDON -- Scientists have developed a kind of rubber that when snapped into two can heal itself back into a stretchy piece of rubber again.

The self-healing rubber was built up from simple starting materials -- fatty acids from vegetable oil and urea (a component of urine) in the lab of Ludwik Leibler at the Industrial Physics and Chemistry Higher Educational Institution in Paris, Nature reported Wednesday on its website.

The resulting material made by Ludwik Leibler and his colleagues is a cross between silly putty and a rubber ball, it can stretch, but when snapped in two it can be stuck back together again, according to the report.

The self-healing rubber could have applications in anything from adhesives to bicycle tyres, scientists said.

The material works at room temperature and just requires the two broken bits to be held together gently while other existing self-healing materials' bonds knit back together only when heated or a strong force is applied.

Conventional rubber is made of a single, continuous, stretchy molecule, held together with strong chemical links called covalent bonds, once the bonds are cut by a break in the material, the rubber can not be reassembled.

The self-healing rubber was made by using small molecular groups instead -- the fatty acids from vegetable oil. Reacting these molecules with urea in a two-step process stuck nitrogen-containing chemical groups (amides and imidazolidones) onto the ends of the fatty acids.

The fatty acids link to each other using hydrogen bonds -- a strong attractive force between hydrogen and another atom, and the bond responsible for holding water molecules to each other.

The resulting molecular system is very non-uniform with some acids having three protruding groups and some having two, meaning that the compound can't crystallize into a hard, shatterable material. Instead it can be stretched to five times its original size and then return to normal in a slower pace than an elastic band would.

The scientists said if rubber is cut, the end groups on the acids become exposed, and the hydrogen bonds to neighboring groups are broken. It is in the amide group's nature to seek out a partner to link up to, and this happens when the cut surfaces are brought back in contact -- the hydrogen bonds can form again. The longer the cut ends are held together, the more of these partnerships are made, and the more completely healed the rubber is.

A freshly cut sample can heal enough that the re-stuck rubber can be stretched to twice its size again after just 15 minutes, according to the scientists who said the broken rubber does not have to be mended immediately, it can still efficiently reform up to 18 hours after being severed.

However, the scientists said the system is not perfect yet as healed rubber that hasn't been left long enough will break again at the original "wound" site, and the broken rubbers will stick only to the other broken piece.

Leibler, who would like to see the rubber used in toys, has a deal with French chemical company Arkema to develop and commercialize the material.