Self-healing plastic

Self-healing plastic is a man-made material, capable of repairing its own surface.

Development
Aeronautical engineering professors Scott White and Philippe H. Geubelle; applied mechanics professor Nancy R. Sottos; and chemistry and materials science professor Jeffrey S. Moore; along with graduate students Michael R. Kessler, Suresh R. Sririam, Eric N. Brown, and Sabarivasan Viswanathan, reported in 2001 that they were the first people to invent a self-healing man made material. This material was a plastic that could fix microscopic cracks that formed on its surface.

Plastic over time naturally gets weaker, no matter how well made it is, because over time microscopic cracks form in the plastic that become weak points in the structure that will only degrade into ever larger cracks. White and his team have invented a way to heal these microscopic cracks before they can grow larger and become an actual problem.

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 mm 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.

Uses
The potential uses of this plastic are many, such as for things that are impractical to fix on a regular basis, like parts deep inside air plane wings or space ships. It is due to the possible applications of this plastic in the field of aeronautics that major funding for this research came from the United States Air Force Office of Scientific Research.