![]() 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. In order to function, a leucodye requires a combination of chemicals working together in a system. The types of thermochromic leucodye inks and general process considerations are shown in Table 1. Processing considerations for leucodye thermochromics A wide range of colors is also available. Leucodyes can be designed to change color at various temperature ranges, from as low as -13☏ (-25☌) up to 150☏ (66☌). In its cool state, the printed ink layer is green, and once warmed, reverts to yellow as the leucodye becomes clear or translucent. For example, the ink manufacturer may formulate a green ink by adding a blue leucodye to a yellow ink. This is achieved with an ink that combines a leucodye with a permanent-colored ink formulation. Some products printed with leucodye thermochromic inks change from one color to another, rather than transitioning from colored to clear. For this reason, liquid crystal thermochromics, rather than leucodyes, are used in the production of thermometers. It takes a 5-10☏ (3-6☌) shift to bring about a change in color, making leucodyes suitable for novelty items and general-purpose products not requiring distinct temperature readouts. In its cool state, a leucodye exhibits color, and when warmed, it turns clear or translucent. For example, in Figure 1, a leucodye thermochromic label indicates when the syrup is heated to the proper temperature. Many of these applications go beyond novelty status, using thermochromic technology for functionality of the printed part. ![]() Some of the applications include security printing, novelty stickers, product labels, advertising specialties, and textiles. Leucodye thermochromics are used in a wide range of applications because they add value in unique ways. ![]() Because of these processing difficulties, we will limit our discussion to the other type of thermochromic ink, the leucodye. Unfortunately, liquid crystal thermochromics are very difficult to work with and require highly specialized printing and handling techniques. Today, liquid crystals are used in many products, including aquarium thermometers, stress testers, and forehead thermometers. The most famous thermochromic application ever, the “mood ring,” was a liquid crystal. The two types of thermochromic inks are liquid crystals and leucodyes. Thermochromics: Temperature-sensitive inks 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. 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. Our discussion will be limited to thermochromics and photochromics, as they are the most widely used and most easily applied technologies available today.ĭepending on the application, color-changing inks can be applied with a number of printing processes, including offset lithography, flexography, gravure, and screen printing. Other, emerging color-changing technologies include hydrochromics, which change in response to water, and piezochromics, which change color in response to pressure. Both materials are reversible and will change colors over and over again with the appropriate exposure. 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). They are rapidly becoming functional parts of manufactured industrial products in product labeling, the medical field, and security applications–and that may be just the beginning. Color-changing inks are not just used in novelty items anymore. Technology is advancing the use of these inks, leading to more and more interesting applications. You may have seen printed garments that change color outdoors, and you may not even realize that those convenient on-package battery testers are screen-printed color-changing inks. You’ve seen color-change technology in a variety of forms: in the “mood rings” of the ’70s, the stress testers of the ’80s, and the forehead thermometers of the ’90s. Color-changing inks might be a great place to start. Breaking away from “price-only” competition involves a commitment to expanding your capabilities. Increasing profits and expanding product lines are two issues on the minds of many business owners in today’s competitive world of screen printing.
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