The material -- consisting of a microencapsulated healing agent and a special catalyst embedded in a structural composite matrix -- could increase the reliability and service life of thermosetting polymers used in a wide variety of applications ranging from microelectronics to aerospace.

"Once cracks have formed within typical polymeric materials, the integrity of the structure is significantly compromised," said Scott White, a UI professor of aeronautical and astronautical engineering and lead author of a paper published in the Feb. 15 issue of the journal Nature that described the new self-healing material.

"Often these cracks occur deep within the structure where detection is difficult and repair is virtually impossible." In the new material, however, the repair process begins as soon as a crack forms.

"When the material cracks, the microcapsules rupture and release the healing agent into the damaged region through capillary action," White said. "As the healing agent contacts the embedded catalyst, polymerization is initiated which then bonds the crack face closed."

In recent fracture tests, the self-healed composites recovered as much as 75 percent of their original strength. And because microcracks are the precursors to structural failure, the ability to heal them will enable structures that last longer and require less maintenance.

"Filling the microcracks will also mitigate the harmful effects of environmentally assisted degradation such as moisture swelling and corrosion cracking," White said. "This technology could increase the lifetime of structural components, perhaps by as much as two or three times."

The ability to self-repair and restore structural integrity also could extend the lifetimes of polymer composite circuit boards, where microcracks can lead to both mechanical and electrical failure.

One of the many challenges the researchers faced in developing the material was obtaining the proper size of microcapsules. They currently use spheres about 100 microns in diameter. Larger spheres could have weakened the matrix, White said, and work continues on creating ever-smaller capsules.

"We also had to determine the correct shell thickness so the capsules would open under the appropriate stress," White said. "Capsule walls that are too thick will not rupture when the crack approaches, while capsules with walls that are too thin will break during processing."

In addition to White, the research team included theoretical and applied mechanics professor Nancy Sottos, aeronautical and astronautical engineering professor Philippe Geubelle, chemistry professor Jeffrey Moore, and graduate students Eric Brown, Michael Kessler, Suresh Sriram, and Sabarivasan Viswanathan. The work was sponsored by a UI Critical Research Initiatives grant.

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