Thermochromic inks first appeared in the 1970s, as a form of mood ring, which used the wearer's body heat as a sign of his or her emotional state. Later, they were used in food packaging such as cans or labels to indicate the temperature changes, for example, on beer cans to tell people the drink is cool enough and ready to serve. Today, their application was widened and used in thermometers, clothing, paint, toys, architecture, plastics and so on.
There are two major categories of thermochromic inks: thermochromatic liquid crystals (TLCs) and leuco dyes.
Liquid crystals have many properties of a liquid crossed with structural elements inherent to crystals. Peer through a microscope at a liquid crystal and you'll see a fluid that exhibits evident textures. Their properties change depending on environmental conditions.
TLCs exhibit different colors in response to temperature changes. At lower temperatures, these liquid crystals are mostly in a solid and crystalline form. In this low temperature state, TLCs may not reflect much light at all, thus, appearing black. The TLCs shift from black to just about every color of the rainbow with the warmth increase. This happens because as temperature rises, spacing between the crystals changes, and as a result, they reflect light differently.
The liquid crystals must first be micro-encapsulated into billions of tiny capsules that are just a few microns in size. This encapsulation process offers some protection for the TLCs and maintains the thermochromic properties. Then, these capsules are blended with other materials and used in products, such as room thermometers. Hang the thermometer in a bedroom and you'll see a rapid change in color that indicates an accurate temperature.
Temperature accuracy is a strong suit for TLCs. Their color consistency means they can indicate heat levels to within a few degrees. However, TLCs are a touchy technology. Their performance can suffer with repeated exposure to UV light, water and chemicals. What's more, they require specialized equipment for proper integration into various products, and that equipment (as well as the TLCs themselves) often adds significant expense to a manufacturer's production costs.
Liquid crystal-based TLCs are a temperamental bunch and rather difficult to incorporate into labels, clothes or other goods. Leuco dye inks, though, feature more durable chemistry that lets product designers employ these inks for all sorts of fun applications.
As with TLCs, leuco dyes are also micro-encapsulated into tiny droplets that are only about 3 to 5 microns in size, which prevents them from reacting with or being damaged by other chemicals.
Leuco dyes are colored when they're at a cool temperature. As heat rises, they become translucent, which lets them reveal any colors, patterns or words that may be printed on an underlying layer of ink. In other products, leuco dyes can be blended with another color so that as temperatures change, a two-tone effect occurs. Mix blue with yellow, for example, and you have an ink that looks green at lower temperatures and yellow when heat rises.
When it comes to temperature accuracy, leuco dyes are more ham-handed than TLCs, so you can't depend on them for applications where you really need a precise temperature reading. But leuco dyes can be integrated into all sorts of fascinating and amusing products.
The temperature range of thermochromic liquid crystals is around -22 to 194 degrees Fahrenheit (-30 to 90 Celsius). For the most vivid colors, they require a black background, and in part because of this, the best use of TLCs is in plastic products such as thermometers. Because they're water-based, these inks are harder to work with than leuco dyes.
Leuco dyes can be incorporated in a much broader range of products, so long as engineers keep temperature specifications in mind. With the right tweaking, the dyes are capable of shifting from one color to another in temperature ranges of 5 to 140 degrees Fahrenheit (-15 to 60 Celsius). Again, the point at which the transition happens isn't terribly accurate with leuco dyes, but it generally occurs within 6 to 18 degrees Fahrenheit (-14 to -7 degrees Celsius) of the intended temperature.