Self sufficient material

Researchers at the University of Illinois have developed a synthetic material that can heal itself when cracked or broken.

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.

‘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.

‘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 is said to begin 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,’ said White. ‘As the healing agent contacts the embedded catalyst, polymerisation 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, said White.

The ability to self-repair and restore structural integrity could also extend the lifetime 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, said White, 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,’ said White. ‘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.’