The Colorimetric Properties of Thermochromic Inks

Thermochromism is defined as reversible or irreversible color change during the defined range of temperature. This temperature range is limited by the boiling point of eachliquid, the boiling point of the solvent in the case of solution or the melting point for solids. Thermochromic (TC) printing inks have been used fordifferent applications such as security printing, smart packaging and marketing activities. The systems that are used in printing inks are mostly organic compounds, since the organic compounds could be made in many colors which display color change at different temperatures.Reversible thermochromic organic systems usually consist of three components called a color former, color developer andsolvent. Leuco dye–developer–solvent systems consist of an electron-donating compound (leuco dye), an electron acceptor (developer) and a component as a matrix which can be asolvent. The leuco dyes could have lactone group and when the lactone ring is closed these so-called leuco dyes or colorformers are colorless. Opening of the lactone ring occurs when leuco dye reacts with an electron-accepting developer. Because of this reaction, the dye become colored since theconjugated п-electron system of the dye will extended and the maximum wavelength absorption peak shifts from the ultraviolet to the visible range. The temperature that decolonization/colorization occurs is specified by the melting temperature of the solvent. In fact,solvent component enables the colour-forming reaction to be reversible with temperature. Leuco dye–developer–solvent systems are coloured in the solid state and when is heated above their melting temperature, transform into a colorless liquid. In the molten state the solvent acts as an inhibitor of the colour-forming reaction. The present study aims on investigation the colorimetric properties of thermochromic inks which could be used for printing.