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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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Photochromics: Light-sensitive inks

Photochromic inks change from clear when indoors to colored when taken outdoors. Specifically, they exhibit color in response to exposure to UV light from sunlight, black lights, or similar sources. UV light changes the chemical structure of the photochromic material and makes it absorb color like a dye. It then reverts to a clear state when the UV source is removed. The color change can occur thousands of times, depending on the application. A photochromic can also change from one color to another when it is combined with a permanent-pigment ink, similar to the leucodye-ink manufacturing process described earlier. These inks are also available in a full range of colors, including a four-color process system.

The most famous photochromic application is found in Transitions eyeglasses that darken in sunlight. Photochromics are also found in novelty applications such as ad specialties, stickers, and nail polish, and industrial applications that include security printing. Perhaps the most popular application is the color-changing T-shirt. An example is shown in Figure 3.

In their pure state, photochromics are powdered crystals that must be dissolved in the inks to which they are added. Some manufacturers microencapsulate the photochromics in their own system, as with leucodye microcapsules. Microencapsulating photochromic systems enables them to be used in inks that cannot dissolve them, such as water-based systems.

Even on cloudy days, photochromics exhibit bright color changes when taken outdoors. The color you see may differ slightly on very hot days or if a UV lamp, rather than sunlight, is used to excite the materials. These "quirks" in performance are a function of the unique materials that make up the photochromic system, and can be learned by most printers in a relatively short period of time.

Photochromic material is inherently unstable, and actually changes its chemical structure when exposed to UV light. In fact, the colored, or excited, form of a photochromic appears to be nearly broken in half, as shown in Figure 4. Because the dye is so vulnerable in its excited state, stabilization is the prime challenge for photochromic ink manufacturers. Without stabilization, most photochromic inks would not last even a few days in sunshine and may even expire before being printed. If shelf life is an important consideration, you should evaluate the stability of any photochromic ink before approving it for production.

The degradation of photochromic inks is more a function of UV exposure than the number of times it changes color. A properly stabilized photochromic ink will last for years on the shelf, but even the best of them will withstand only a few months of outdoor exposure after printing. The best photochromic textile inks will withstand about 20 washings after printing and are even more susceptible to the negative effects of chlorine bleach than their thermochromic counterparts. Bleach must never be used on garments printed with photochromic inks.

Photochromics are relatively new, having been introduced in the early 1990s, and their use has steadily increased as manufacturers have gained control over the stabilization process. A step up in this technology took place in 1995, leading to the most stable formulations to date. They are also available in flexographic inks, powders, and plastics.

Processing considerations for photochromic inks

The types of photochromic inks and their general processing considerations are found in Table 2. These inks need to be printed on lightly colored surfaces, preferably white, to display the strongest color change. This is because photochromics are dyes, not pigments, and they absorb light. This absorption requires a light-colored background to reflect the remaining light so that the color-changing effect is maximized. A photochromic ink would still change color when printed on a black substrate, but you just wouldn't be able to see it. Normal inks use pigments, which reflect light, allowing their colors to be seen when printed over any colored substrate.

Because unexposed photochromic inks are usually invisible indoors, press setup can be more difficult than usual. The easiest solution is to have a black light at the press to excite the ink.

Table 2 Photochromic Inks
Photochromic inks are available in all ink types except for UV formulations (UV curing could damage the UV-sensitive photochromic crystals). These crystals form a dye that is relatively transparent, so the inks should be used on lighter color garments and printed as thickly as possible.

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