Dynamic Colour Possibilities and Functional Properties of Thermochromic Printing Inks
Thermochromic printing inks change their colour regarding the change in temperature and they are one of the major groups of colour-changing inks. One of the most frequently used thermochromic material in printing inks are leuco dyes. The colour 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, dynamic colorimetric properties of leuco dye-based thermochromic inks, their stability and principle of variable-temperature colour measurement. Thermochromic material is protected in round-shaped capsules. They are much larger than pigments in conventional inks. The polymer envelopes of pigment capsules are more stable against oxidation than the binder. If these envelopes are damaged, the dynamic colour is irreversibly lost. Our aim is to analyse the colorimetric properties of several reversible screen-printed UV-curing leuco dye thermochromic inks with different activation temperatures printed on paper. A small analysis of irreversible thermochromic
inks will be presented for comparison with reversible thermochromic inks. Moreover, so as to show interesting possibilities, a combination of different inks was made, an irreversible thermochromic ink was printed on top of the red and blue reversible thermochromic inks. Special attention was given to the characterization of colour hysteresis and the meaning of activation temperature.
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