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10/03/2011
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Researchers have announced a 'self healing' plastic that can automatically repair itself and stop the growth of microcracks. The OSIRIS project is being run by the German Federal Ministry of Education and Research with researchers at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen.
The source of inspiration was the caoutchouc tree hevea brasiliensis and plants that conduct latex, such as the Weeping Benjamin. The latex contains capsules filled with the protein hevein. If the caoutchouc tree is damaged, the latex escapes and the capsules break open to release hevein, which also links the latex particles in the latex to form a wound closure.
Indestructible things are a figment of the imagination of advertising. Even plastic components that have to stand up to major mechanical loads can break. The reason for this are microcracks that may be found in any component part. Researchers have now come up with elastic polymers that heal themselves to put an end to the growth of cracks.
By applying this principle to elastomers, Fraunhofer scientist, Dr Anke Nellesen, loaded microcapsules with a one component adhesive (polyisobutylene) and put it in elastomers made of synthetic caoutchouc to stimulate the same self healing process in plastics. "If pressure is put on the capsules, they break open and separate this viscous material," explained Dr Nellesen. "Then this mixes with the polymer chains of the elastomers and closes the cracks. We were successful at making capsules stable to production, although they did not provide the self healing effect we wanted."
However, the researchers obtained good results by putting the self healing component into the elastomer uncapsulised. Various test bodies from different synthetic caoutchoucs indicated clear self healing properties, since the restored tension expansion was 40% after a healing period of 24h.
Better results were achieved by supplying elastomers with ions – again by using the caoutchouc tree as the model. The hevein proteins that are released when there is damage link up to each other through ions and stick in this process so that the crack closes. Therefore, if the elastomer material is damaged, the particles with opposite charges are looking for a new bonding partner – a plus ion attracts a minus ion, which makes it adhere. Dr Nellesen explained: "We make sure that the wound closure is stable by charging the elastomers with ions, which means that the healing process can take place as often as needed. There are already duromers with self healing functions in the form of self repairing paints in cars. We still haven't developed elastomers that can close their cracks without interference from outside."
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Author
Chris Shaw
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