Hypercolor
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Hypercolor was a line of clothing, mainly T-shirts and shorts, that changed color with heat. They were manufactured by Generra (now a division of Public Clothing Company) and marketed in the United States as Generra Hypercolor or Generra Hypergrafix and outside the US as Global Hypercolor. They contained a thermochromic (temperature sensitive) pigment made by Matsui Shikiso Chemical of Japan, that changed between two colors–one when cold, one when warm. The shirts were produced with several color change choices from the late 1980s until the early 1990s. The effect could easily be permanently damaged, particularly when the clothing was placed in a hotter than recommended wash.
[edit] Principle
Substances that can change colour due to a change in temperature are called thermochromes. There are two types of thermochromes: liquid crystals (used in Mood rings) and leuco dyes (used in hypercolour 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. The dye is enclosed in microcapsules, tiny (few micrometers in diameter) drops of liquid sealed in a transparent shell, bound to the fibers of the fabric.
When it is cold the mixture in the capsule is solid and the carbon molecules in the leuco dye are attached to each other in a ring shape. When we heat up, just like liquid crystals, the molecules change shape. They break open the carbon ring. This change means that the dyes absorb and release light differently, and the colour we see is different.
The liquid is a leuco form of a dye (in this case crystal violet lactone), a weak acid (benzotriazole), and a quaternary ammonium salt of a fatty acid (myristylammonium oleate) dissolved in a solvent (1-dodecanol). At low temperatures, the weak acid forms a colored complex with the leuco dye, interrupting the lactone ring. At high temperatures, above 24-27 °C, the solvent melts and the salt dissociates, reversibly reacts with the weak acid and increases the pH. The pH change leads to closing of the lactone ring of the dye, which then regains its colorless (leuco) form.
Therefore at the low temperature the color of the shirt is the combination of the color of the microcapsules with the color of the dyed fabric, while at higher temperatures the capsules become colorless and the color of the fabric prevails.
