Self-Healing Plastic : Nano Technology

Researchers at the University of Illinois at Urbana-Champaign (UIUC) have developed a nanotechnology polymer that can "heal" itself by filling in cracks and tears automatically. Although self-healing plastic is not an entirely new concept, the UIUC material is different because it can repair itself multiple times without any intervention.

The material could have important uses where making repairs is difficult, where materials are under enormous stress and/or where material failure would be catastrophic -- such as in implanted medical devices, airplane and spacecraft components, and microprocessors. The UIUC researchers emphasize, however, that practical applications are years away, and that initial products will be highly expensive.

How it works

The theory behind it is to put microcapsules filled with dicyclopentadiene, a liquid tricyclic diolefin, into the plastic along with a catalyst during the production of the material. So when a lengthening crack reaches a microcapsule, it bursts the microcapsule, allowing the dicyclopentadiene to seep into the crack through capillary action, where it will come into contact with the catalyst. The catalyst mediates gelation of dicyclopentadiene by ring-opening metathesis polymerization. The highly cross-linked polymerization of dicyclopentadiene in the crack heals it.

In making their composite system, the Illinois researchers stir 10% by weight of resin microcapsules 100 µm in diameter into the epoxy formulation. They cure the molded epoxy for 24 hours at room temperature followed by a 24-hour bake at 40 ºC. The polymerization catalyst dispersed throughout is a ruthenium carbene complex invented by chemistry professor Robert H. Grubbs of California Institute of Technology. The Grubbs' catalyst is ideal because it remains active even on exposure to air, moisture, or most organic functional groups.

Probably the greatest challenge when making self-healing plastic is how to make the microcapsules. They must be small enough that they won’t adversely affect the strength of the epoxy but if they are too small they don’t carry enough dicyclopentadiene. The walls of the microcapsules must be thin enough that they will crack when they meet a crack but thick enough that they will survive the production of the material. Not to mention the stiffness of the microcapsules is also important, microcapsules that are too stiff cause distributions of stresses in the plastic that force the crack to grow away from them, while a certain amount of resilience will guide developing cracks toward the microcapsules.

They make the microcapsules by stirring of an aqueous solution of urea and formaldehyde, dicyclopentadiene, resorcinol acid catalyst, and ethylene-maleic anhydride resin emulsifying agent at very high speeds. The product is microcapsules of urea-formaldehyde resin containing dicyclopentadiene liquid.

Source:wiki