Catching the cracks with color-changing material coatings

Written by: Ritu Raman

Original article: Li et al. Advanced Materials 2016
The Gist of It:
Intuitively, we all know that a small tear or crack in a material, if left unrepaired, can turn into a very large hole that makes the material nonfunctional. This is perhaps best represented by the squirrel in Ice Age who makes a tiny crack in a block of ice that eventually triggers an avalanche. Imagine if that crack occurred in the wing of a plane – if left undetected, it would lead to catastrophic failure with devastating consequences on human life. In this study, Li and colleagues developed a novel coating for materials that changes color when a crack forms, making it easy to detect and repair cracks as soon as they occur. The coating is an epoxy (a reactive polymer) containing tiny micro-scale beads. The beads are filled with a liquid that changes color from light yellow to bright red when it comes into contact with certain chemical groups in the epoxy. When something scratches the surface of the epoxy coating, the microcapsules break, releasing the color-changing liquid and clearly showing the size and location of the crack on the coating surface. The color intensity gets brighter when the crack gets deeper, as more and more microcapsules break. The researchers showed this coating could detect cracks as small as 10 microns (significantly smaller than the thickness of a human hair) and could be applied on a variety of different materials to enable autonomous damage detection. I know I’d feel a lot safer using machines built with smart damage-detecting materials! Would you?

Scratching the novel coating developed in this study ruptures the microcapsules within the coating, releasing a color-changing liquid that marks the size and location of the scratch,

The Nitty Gritty:
Li et al formulated double-walled microcapsules from polyurethane/poly(urea-formaldehyde) using a single batch process. The capsules contained 2ʹ,7ʹ-dichlorofluorescein dissolved in ethyl phenyl acetate, and only capsules in the size range of 48 ± 13 µm in diameter were used for the experiments reported. The capsules were mixed homogeneously in an amine-cured epoxy material, formed from a mixture of EPON 813 and the curing agent EPIKURE 3233. The coating was then applied to a range of substrate materials, including glass and steel. Damage indication experiments were conducted after rupturing the capsules in the coatings using a test panel scratcher, and results were visually monitored by a combination of optical microscopy and scanning electron microscopy.
Original Research Article: Li, W., et al. “Autonomous indication of mechanical damage in polymeric coatings.” Advanced Materials 28.11 (2016): 2189-2194.

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