Buzz
If you remember the '90s, you likely recall the buzz around Hypercolor and Hypergrafix clothing—those color-shifting T-shirts from Generra. With the '90s back in style, now's the perfect time to uncover the science behind these iconic shirts.
The Science Behind the Magic
The key to Hypercolor shirts lies in thermochromism, the process by which a material changes color in response to temperature fluctuations. These shirts are made with two types of dyes: a permanent one for the base color and a thermochromic dye sealed in microcapsules that bind to the fabric's fibers. This thermochromic dye usually includes a mix of leuco dye (often crystal violet lactone), a mild acid, and a dissociable salt dissolved in fatty alcohol like 1-dodecanol.
When the temperature is low, the dodecanol solidifies, keeping the dye in its colorless leuco state. As the temperature rises (above 75.2°F), the salt separates, the pH drops, and the dye's lactone ring opens, turning the dye into its colored form and changing the fabric's color in the heated areas. The resulting color depends on both the fabric's base color and the activated dye's hue. For instance, blue fabric and yellow dye produce a greenish tint in the warmed section.*
Beyond a Blast from the Past
Leuco dyes and other thermochromic technologies have far-reaching uses beyond fun T-shirts. For example, they’re employed in Duracell batteries along a resistive strip to signal heat and measure current flow. Thermochromic dyes are also utilized in food containers to indicate the temperature of contents or to track their temperature history during storage. In construction, these dyes help building materials change color in response to solar heat, reflecting radiation and regulating internal temperatures. They also find use in thermal sensors for submerged applications, like in aquariums or washing machines.
* In some cases, the reverse effect is desired, where the thermochromic dye shifts from its colored form to a colorless one when exposed to heat. For this, compounds like phenolphthalein or thymolphthalein, which are colorless in acidic conditions, are often employed.
