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Color-Changing Inks

(January 2003) posted on Wed Jan 29, 2003

Brighten your bottom line


By J. Homola

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Increasing profits and expanding product lines are two issues on the minds of many business owners in today's competitive world of screen printing. Breaking away from "price-only" competition involves a commitment to expanding your capabilities. Color-changing inks might be a great place to start.

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. 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. Technology is advancing the use of these inks, leading to more and more interesting applications. Color-changing inks are not just used in novelty items anymore. 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.

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. Our discussion will be limited to thermochromics and photochromics, as they are the most widely used and most easily applied technologies available today.

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.

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.

Because the microencapsulation process cannot completely protect the leucodye system, ink manufacturers must take care to ensure compatibility with the inks to which they are added. This can involve slight modifications to standard ink formulations to make them more receptive to the addition of the leucodyes. So before adding any unapproved solvents or ink additives, you should confirm their compatibility by consulting with the thermochromic ink manufacturer.

Under normal conditions, thermochromic leucodye inks have a shelf life of six months or more. After they are printed, they function, or continue to change color, for years. The post-print functionality can, however, be adversely affected by UV light, temperatures in excess of 250°F (121°C), and aggressive solvents. Thermochromic textile inks will withstand about 20 washings before showing any significant deterioration and can last even longer when dried without heat. The use of chlorine bleach is not recommended, as this also will degrade the performance of the printed ink on textiles.

Table 1 Thermochromic Leucodye Inks
Thermochromatic leucodye inks incorporate large (3-5 micron) microcapsules of leucodye chemicals, and therefore require low mesh counts for printing. The leucodyes are available for all major ink types.
Ink Type Water base UV Epoxy Plastisol

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