Functional Properties of the Leuco Dye-Based Thermochromic Printing Inks

摘要

Leuco dye-based thermochromic printing inks are one of the candidates for active packaging of the future. Such an advanced application requires much more knowledge about these novel inks as it is currently provided by the manufacturers. Thermochromic inks reversibly change from colored to discolored state in a certain temperature region. Details of this change and the width of the temperature region required for it are characteristic properties of a selected ink. Inside this region the color of a sample depends on temperature and thermal history; this is a color hysteresis, which contains a sort of memory. The activation temperature, which is the only parameter given by the producer to describe the change of color, does not reveal any characteristic properties of this temperature dependent color loop. Therefore other parameters were introduced to characterize the analyzed thermochromic inks. Thermochromic material is leuco dye-based composite which is protected in round-shaped capsules. They are much larger than pigments in conventional inks. Their large size can seriously influence the appearance of very thin layers, being the size of capsules. The polymer envelopes of pigment capsules are more stable against oxidation than the binder. If these envelopes are damaged, the dynamic color is irreversibly lost. The lightfastness of thermochromic samples is low. Thicker layers retain their dynamic color up to longer light exposures. Light changes color of the fully colored and totally discolored states and of all intermediate states; the changes are larger in the colored state and smaller in the discolored one.