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Materials

Plastics glow to warn of invisible cracks

Damage to plastics causes embedded microcapsules to rupture and release fluorescent molecules

by Prachi Patel, special to C&EN
October 3, 2016 | APPEARED IN VOLUME 94, ISSUE 39

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Credit: ACS Cent. Sci.
A scratch in an epoxy coating embedded with microcapsules blends in under visible light (left) but shines bright blue under UV light (right).
Credit: ACS Cent. Sci.
A scratch in an epoxy coating embedded with microcapsules blends in under visible light (left) but shines bright blue under UV light (right).

Microscopic cracks in a material can spread and grow into larger fissures—ones that can split apart the plastics and composites used in airplanes, spacecraft, electronics, and pipes. A simple new technique can reveal tiny, invisible cracks in a wide variety of plastics by making the cracks glow (ACS Cent. Sci. 2016, DOI: 10.1021/acscentsci.6b00198). Such an early-warning strategy could allow engineers to replace or repair critical components and prevent catastrophes. Nancy R. Sottos, Jeffrey S. Moore, and colleagues at the University of Illinois, Urbana-Champaign, embedded various polymeric materials with polyurethane microcapsules filled with a dilute solution of 1,1,2,2-tetraphenylethylene (TPE), which fluoresces brightly when TPE molecules aggregate. Formation of tiny cracks in the plastic ruptures the capsules, leading to solvent evaporation and growth of TPE crystals on the capsule shell. Ultraviolet light causes the crystals to shine bright blue. Scratches made with a blade in plastics loaded with microcapsules at a concentration of 10% by weight blended in under visible light but glowed when illuminated with a UV lamp. The researchers could detect cracks smaller than 2 µm in size up to 40 days after the damage occurred.

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