First Self-Healing Coatings

A paint additive will protect cars, bridges, and ships from corrosion.

When a car's underbody or a ship's hull begins to corrode, it usually ends up junked. New protective coatings developed at the University of Illinois heal over their own scratches with no external intervention, protecting the underlying metal. The self-healing elements, enclosed in microcapsules that rip open when the coating is scratched, are compatible with a wide range of paints and protective coatings. The coatings, being marketed by Autonomic Materials of Champaign, IL, may be on the market in as soon as four months.

Heal thyself: Steel painted with a conventional coating (left) rusts where scratched after immersion in salt water. At right, scratched steel protected by a new self-healing coating shows no corrosion. Inset images from a scanning electron microscope show that the self-healing coating has filled in the scratch with a protective caulk. Credit: Paul Braun

The materials, described online this week in the journal Advanced Materials, were developed by Paul Braun and Scott White, both professors in the Beckman Institute at the University of Illinois at Urbana-Champaign. The self-healing system consists of two kinds of microcapsules: one filled with polymer building blocks, the other with a catalyst. Because the capsules, made of polyurethane, keep the reactive chemicals inside isolated, they can be mixed into a wide range of coatings. When the coatings are scratched, the microcapsules are torn open and their contents flow into the crack and form siloxane, a polymer that Braun likens to bathroom caulk. Unlike other self-healing systems, the Illinois coatings don't require elevated temperatures or moisture to mend.

The Illinois researchers scratched steel plates, some coated with the material and some with a conventional coating, then immersed them in salt water for five days. The metal covered by the new coating was protected against rust, while scratches in the conventional coating allowed significant rusting. "They make a very compelling case that the system is working as advertised," says Christopher Bielawski, an assistant professor of materials science and engineering at the University of Texas at Austin.

Bielawski points to the practical aspects of the Illinois coatings, which are made up of cheap, readily available chemicals. And Braun says that the new additives could be used in a wide range of applications in coatings that are cured up to temperatures of about 150 °C. The group demonstrated the self-healing system in various coatings, including in a commercial military ship paint.

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Most work on self-healing materials, including those developed by the Illinois group, has been aimed at incorporating them into various structures, restoring mechanical properties to walls so that they won't crumble or to airplane wings so that they won't fracture. The key to the coating technology, says Braun, was encapsulating the catalyst. If unprotected, the catalyst could degrade the coating itself; encapsulating it makes the system compatible with a wide range of paints and coatings.

These paints won't be suitable for places where aesthetics are important, like the top of a car, says Braun. The capsules are 10 to 100 micrometers in diameter, so thinner coatings incorporating them would be rough to the touch. Larry Evans, CEO of Autonomic Materials, says that the first target markets include industries in which performance is key, such as ships, oil rigs, and pipelines, where metals are exposed to harsh environments and taking systems offline for frequent repainting is costly. Evans says that the self-healing system is ready for commercialization and that the company has partnerships with major coating companies.