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Coating Signals Critical Unseen Damage

Thursday, February 11, 2016

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Researchers at the University of Illinois are developing a coating meant to deliver a clear indication of structural damage so small that it would otherwise be undetectable to the naked eye yet still capable of catastrophic failure.

The team—led by Illinois professor of materials science and engineering Nancy Sottos, aerospace engineering professor Scott White, and postdoctoral researcher Wenle Li— is working on a new polymer coating specifically for use on structural materials, the university said in a statement.

When those materials suffer even the slightest damage, the coating changes color to alert inspectors to the problem. Potential applications include airplanes, bridges and pipelines.

Nancy Sottos/University of Illinois
Images: Nancy Sottos

The coating, applied to a steel plate, brightly highlights a thin zigzag scratch.

“Polymers are susceptible to damage in the form of small cracks that are often difficult to detect. Even at small scales, crack damage can significantly compromise the integrity and functionality of polymer materials,” Sottos said.

“We developed a very simple but elegant material to autonomously indicate mechanical damage.” 

The team recently published its findings in a paper titled “Autonomous Indication of Mechanical Damage in Polymeric Coatings” in the scientific journal Advanced Materials.

Eliciting a Reaction

In the lab, the scientists placed microcapsules filled with a yellow pH-sensitive dye in an epoxy resin. A crack, scratch, fracture or any kind of stress on the material will break the capsule and release the dye.

When the dye reacts with the epoxy, an unmistakable color change occurs—the dye changes from light yellow to a bright red. A crack as small as 10 micrometers is enough to cause the color change, the team said, indicating a loss of structural integrity.

This helps to assess the extent of the damage, as a deeper abrasion or crack will break open more microcapsules, leading to a more intense red.

“Detecting damage before significant corrosion or other problems can occur provides increased safety and reliability for coated structures and composites,” White said.

Nancy Sottos/University of Illinois

When cracks form, microbeads embedded in the material break open and cause a chemical reaction that highlights the damaged area.

Testing showed that the coating is effective on a variety of materials, including metals, polymers and glasses; possesses long-term stability (i.e., no false positives as a result of leaking microcapsules and no color fade); and offers a low-cost solution to preventing structural failures.

“A polymer needs only to be 5 percent microcapsules to exhibit excellent damage indication ability,” Sottos said. “It is cost effective to acquire this self-reporting ability.”

Future Developments

The research team is now turning its attention to finding additional applications for its “damage indication system.” Possibilities include applying it to fiber-reinforced composites and integrating it with the group’s previous work in self-healing systems.

“We envision this self-reporting ability can be seamlessly combined with other functions such as self-healing and corrosion protection to both report and repair damage,” Sottos said.

“Work is in progress to combine the ability to detect new damage with self-healing functionality and a secondary indication that reveals that crack healing has occurred.”

White and Sottos are affiliated with the Beckman Institute for Advanced Science and Technology at the University of Illinois, which supported the work.

Postdoctoral researcher Li was the first author of the work, and graduate students Christopher Matthews, Michael Odarczenko and Ke Yang were co-authors.


Tagged categories: Asia Pacific; Coating Materials; Coatings; Coatings Technology; Colleges and Universities; EMEA (Europe, Middle East and Africa); Epoxy; Latin America; North America; Polymers; Protective Coatings; Research and development; Resins

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