Hypercolor – Change color by temperature

https://en.wikipedia.org/wiki/Hypercolor

Hypercolor was a line of clothing, mainly T-shirts and shorts, that changed color with heat. They were manufactured by Generra Sportswear Company of Seattle and marketed in the United States as Generra Hypercolor or Generra Hypergrafix and elsewhere as Global Hypercolor.

Substances that can change color due to a change in temperature are called thermochromes. There are two common types of thermochromes: liquid crystals (used in mood rings) and leuco dyes (used in Hypercolor T-shirts).

The color change of Hypercolor shirts is based on combination of two colors: the color of the dyed fabric, which remained constant, and the color of the thermochromic dye. Droplets of the thermochromic dye mixture are enclosed in transparent microcapsules, a few micrometers in diameter, bound to the fibers of the fabric.

At low temperatures, the mixture is a solid. The weak acid forms a colored complex with the leuco dye by causing the lactone ring in the center of the dye molecule to open. At high temperatures, above 24–27 °C, the solvent melts and the ammonium salt dissociates, allowing it to react with the weak acid. This reaction increases the pH, which leads to closing of the lactone ring of the dye to convert it to its colorless (leuco) form.

Therefore, at the low temperature the color of the shirt is the combination of the color of the encapsulated colored dye with the color of the dyed fabric, while at higher temperatures the capsules become colorless and the color of the fabric prevails.

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