Color-Changing Indicators Highlight Microscopic Damage to Composite Materials
Ian Wright posted on January 18, 2016 |
Polymer damage indication system is designed for aerospace, automotive and petroleum applications.
When cracks form, microcapsules embedded in the material rupture and cause a chemical reaction that highlights damaged areas. (Image courtesy of Nancy Sottos.)
When cracks form, microcapsules embedded in the material rupture and cause a chemical reaction that highlights damaged areas. (Image courtesy of Nancy Sottos.)

A new polymer damage indication system automatically highlights areas that are cracked, fractured, scratched or stressed, enabling engineers to address potential problems visually before they become concrete calamities.

This system is based on embedding tiny microcapsules of a pH-sensitive dye in an epoxy resin. The capsules rupture if the polymer cracks or is scratched, stressed or fractured. This releases the dye within the capsule which reacts with the epoxy to cause a dramatic color change: from yellow to red.

The deeper the scratch or crack, the more microcapsules are broken, resulting in a more intense color. This helps to assess the severity as well as the extent of the damage.

The researchers claim that even micrometer-scale cracks are enough to cause a visible color change, making even minute compromises to structural integrity abundantly clear. The system has also been shown to be stable in the long-term with no microcapsule leaking for false positives and no color fading.

Thus far, research has demonstrated that the damage indication system works well for a variety of polymer materials which can be applied as coatings to different substrates, including metals, polymers and glasses.

Detecting and Repairing Polymer Damage

This new system is designed for use in the petroleum, aerospace and automotive industries: all applications where one part’s failure can have costly or even catastrophic consequences.

“Polymers are susceptible to damage in the form of small cracks that are often difficult to detect,” said Nancy Sottos, professor of materials science and engineering at the University of Illinois.

“Even at small scales, crack damage can significantly compromise the integrity and functionality of polymer materials. We developed a very simple but elegant material to autonomously indicate mechanical damage," said Sottos.

 
The coating, applied to a steel plate, brightly highlights a thin zigzag scratch. (Image courtesy of Nancy Sottos.)
The coating, applied to a steel plate, brightly highlights a thin zigzag scratch. (Image courtesy of Nancy Sottos.)

“Detecting damage before significant corrosion or other problems can occur provides increased safety and reliability for coated structures and composites,” said Scott White, professor of aerospace engineering at the University of Illinois and Sottos’ co-author.

In addition to its safety-related benefits, the system is low-cost:

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

However, as these microcapsules will be in such a high concentration within the material they might affect the van der Waals attractions and molecular entanglement of the plastic molecules. As a result, significant testing is required to ensure that they will not negatively impact the structural properties of the composite materials.

On its own, this dye-marking system is an intriguing concept, but the possibility of combining this system with self-healing materials makes for truly exciting possibilities.

“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.”

For more information, see the research findings published in Advanced Materials.

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