Scientific Information
(Caution, do not use heavy machinery when reading
this page....drowsiness may occur.)
The two major groups of color-changing inks are thermochromic, which change color in response to temperature fluctuations, and photochromic, which respond to variations in exposure to UV light (primarily sunlight). Both materials are reversible and will change colors over and over again with the appropriate exposure. Other, emerging color-changing technologies include hydrochromics, which change in response to water, and piezochromics, which change color in response to pressure. This page will be limited to thermochromics and photochromics, as they are the most widely used and most easily applied technologies available today. (Most if not all of the mugs visible on this site will be Thermochromics.)
The following is copied from a scientific site concerning the types of inks and should be considered only for the most interested of readers.
Depending on the application, color-changing inks can be applied with a number of printing processes, including offset lithography, flexography, gravure, and screen printing. These are highly specialized inks that combine standard ink components with one of several color-changing agents, which will be described in the following sections. Since these inks are used on a wide variety of screen-printing substrates, it follows that they are offered in the typical solvent-based, water-based, plastisol, and UV formulations.
Thermochromics: Temperature-sensitive inks
The two types of thermochromic inks are liquid crystals and leucodyes. The most famous thermochromic application ever, the "mood ring," was a liquid crystal. Today, liquid crystals are used in many products, including aquarium thermometers, stress testers, and forehead thermometers. Unfortunately, liquid crystal thermochromics are very difficult to work with and require highly specialized printing and handling techniques. Because of these processing difficulties, we will limit our discussion to the other type of thermochromic ink, the leucodye.
Leucodye thermochromics are used in a wide range of applications because they add value in unique ways. Some of the applications include security printing, novelty stickers, product labels, advertising specialties, and textiles. Many of these applications go beyond novelty status, using thermochromic technology for functionality of the printed part. For example, in Figure 1, a leucodye thermochromic label indicates when the syrup is heated to the proper temperature.
Color-Changing Inks
Fig. 1: Practical applications for thermochromatic inks
Today, novelty applications aren't the only use for thermochromatic leucodye inks. As this example illustrates, the inks, which are colored when cold and clear when heated, can be used to produce temperature labels on food products that require warming before use. (All photos courtesy of Color Change Corp.)
In its cool state, a leucodye exhibits color, and when warmed, it turns clear or translucent. It takes a 5-10°F (3-6°C) shift to bring about a change in color, making leucodyes suitable for novelty items and general-purpose products not requiring distinct temperature readouts. For this reason, liquid crystal thermochromics, rather than leucodyes, are used in the production of thermometers.
Some products printed with leucodye thermochromic inks change from one color to another, rather than transitioning from colored to clear. This is achieved with an ink that combines a leucodye with a permanent-colored ink formulation. For example, the ink manufacturer may formulate a green ink by adding a blue leucodye to a yellow ink. In its cool state, the printed ink layer is green, and once warmed, reverts to yellow as the leucodye becomes clear or translucent. Leucodyes can be designed to change color at various temperature ranges, from as low as -13°F (-25°C) up to 150°F (66°C). A wide range of colors is also available.
Processing considerations for leucodye thermochromics
The types of thermochromic leucodye inks and general process considerations are shown in Table 1. In order to function, a leucodye requires a combination of chemicals working together in a system. This special system of materials needs to be protected from the components of the ink to which it is being added, so it is microencapsulated. The microencapsulation process takes a small droplet of the leucodye and coats a protective wall around it, as shown in Figure 2. The leucodye microcapsules contain the complete color-changing thermochromic system, which, when added to inks, give them their color-changing properties.
The microcapsules are also large. At 3-5 microns, they are at least ten times larger than the average pigment particle. Special considerations are usually involved in printing inks with these relatively large particles and include coarser screen mesh, heavier ink laydown, etc.
