Light fastness of liquid crystal-based thermochromic printing inks

Thermochromic inks are known for their poor stability against UV light, appearing differently in leuco dye-based and liquid crystal-based ink types. The leuco dye-based inks lose the colour contrast between the coloured and discoloured states but the temperature at which the colour changes stays the same. These inks retain thermochromic functionality, i.e. the relation between colour and temperature, as long as the colour contrast between both states remains visible. Liquid crystal-based inks reflect a narrow band of light with wavelengths equal to the pitch length of the helical structure formed in the chiral-nematic mesophase. While the length of this helix depends on temperature, the wavelength of the reflected light shifts across the spectrum. This unique thermochromic effect is also called the colour play effect and is used for visual determination of temperature. However, the helical structure is highly sensitive to UV light which changes the temperature-related colours; consequently, such an ink loses its thermochromic functionality. Almost complete deterioration of the colour play effect occurs on unprotected samples in less than 60 days of exposure to natural indoor daylight and after 10 h under the artificial daylight. UV-protective foil or UV filter protects the ink against detrimental effects of UV light and keeps its functionality even after 10 h of exposure but the colour play effect is shown at lower temperatures.