Readers of a certain age might remember Hypercolor or Hypergrafix clothing, the color-changing T-shirts produced by Generra that were all the rage in the early '90s. Now that the '90s are back, it's a good time to finally figure out the science behind the clothes.
Blinding You With Science
The secret to Hypercolor shirts and products like them is thermochromism, the ability of a substance to change color due to a change in temperature. The shirts are manufactured with two dyes: one regular dye that provides the constant "true" color of the fabric, and a thermochromic dye enclosed in microcapsules bound to the fabric's fibers. The thermochromic dye is usually a mixture of a leuco dye (a dye whose molecules can take on two forms, one colorless; Hypercolor shirts often used crystal violet lactone), a weak acid, and a dissociable salt dissolved in the fatty alcohol 1-dodecanol.
At low temperatures, the dodecanol is solid and the dye exists in its colorless leuco form. At warmer temperatures (>75.2°F), the salt dissociates, the pH is lowered, and the dye's lactone ring opens, allowing the dye to become colored, producing a color change in the warmed area. The new color is dependent on the combination of the color of the fabric and the color of the non-leuco form of the dye—so blue fabric and yellow leuco dye make for a green warm spot.*
More Than Nostalgia
Leuco dyes and other thermochromic applications, of course, have many uses beyond novelty T-shirts. Leuco dyes are used on Duracell batteries along a resistive strip to indicate their heating and gauge the amount of current flowing through it. Thermochromic dyes are also used on food vessels to indicate the temperature of their contents, or monitor their time-temperature storage history. They're also used in building materials, where solar heat turns the material white and results in the reflection of solar radiation and maintenance of the building's temperature. Other thermochromic materials are used in thermal sensors designed for immersed applications, like in fish tanks or washing machines.
* Sometimes, the opposite effect, where the thermochromic dye changes from its colored to non-colored form in response to heat, is desired. In these cases, phenolphthalein, thymolphthalein, or other compounds that are colorless in acidic ranges are often used.