Makers of color-morphing products love thermochromic inks, but they have to choose these inks carefully to ensure that they'll work well in their current manufacturing processes. Usually, companies will acquire samples from ink suppliers and then follow a process of trial and error until the results are stable and visually mesmerizing.
Both types of thermochromic inks have pros and cons. Chemists must weigh the properties of each before choosing an ink that's best for their application.
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.
Unlike TLCs, 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.
Leuco dyes are far sturdier than TLCs. Because of that, they're used in many kinds of inks, including water-, solvent-, epoxy-based inks and more. Their adaptability means they're integrated into all sorts of printing processes, such as screen, offset, gravure and others. You'll also find them in plastics, on metals and blended into a variety of paints.
Leuco dyes appear in a plethora of paper products as well, such as quirky, direct-mail advertisements and promotional materials. Security-minded products rely on leuco dyes, too, including prescription drug pads and check security features. Simply touch a temperature icon embedded in the document and its appearance will change, confirming its authenticity.
Thermochromic inks aren't currently used for hardcore security purposes. For example, they aren't great for preventing counterfeiting because the inks are available for purchase on the open market, meaning crooks could easily make their own temperature-sensitive cash.