Photochromic ink applications (UV-sensitive)
One of the most popular uses for photochromic inks is on screen-printed garments. As this design illustrates, the inks are transparent until exposed to a UV light source (sun, black light, etc.), at which time they take on their specific color characteristics. They return to a transparent state when UV exposure is discontinued.
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 left. 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.