Dynamic Colour Possibilities and Functional Properties of Thermochromic Printing Inks

Thermochromic printing inks change their colour regarding the change in temperatureand they are one of the major groups of colour-changing inks. One of themost frequently used thermochromic material in printing inks are leuco dyes. Thecolour of thermochromic prints is dynamic, it is not just temperature-dependent,but it also depends on thermal history. The effect is described by colour hysteresis.This paper aims at discussing general aspects of thermochromic inks, dynamiccolorimetric properties of leuco dye-based thermochromic inks, their stability andprinciple of variable-temperature colour measurement. Thermochromic materialis protected in round-shaped capsules. They are much larger than pigments in conventionalinks. The polymer envelopes of pigment capsules are more stable againstoxidation than the binder. If these envelopes are damaged, the dynamic colour isirreversibly lost. Our aim is to analyse the colorimetric properties of several reversiblescreen-printed UV-curing leuco dye thermochromic inks with different activationtemperatures printed on paper. A small analysis of irreversible thermochromicinks will be presented for comparison with reversible thermochromic inks.Moreover, so as to show interesting possibilities, a combination of different inkswas made, an irreversible thermochromic ink was printed on top of the red andblue reversible thermochromic inks. Special attention was given to the characterizationof colour hysteresis and the meaning of activation temperature.