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Optical density of printing substrate required for clearly visible color play effect of thermochromic liquid - crystal printing inks

Thermochromic printing inks change color in dependence of surrounding temperature. The color change of thermochromic liquid-crystal (TLC) printing inks occurs inside the microcapsules containing thermo-responsive material (Seeboth et al., 2007). It starts at the defined activation temperature (TA), occurring in several degrees wide region above the TA where the color changes throughout the whole visible spectrum from red, orange, yellow, green, blue to violet, the effect known as "color play" (Bamfield & Hutchings, 2010). The color activation region can also be called "the color play interval" (Christie & Bryant, 2005). The color change inside this interval appears as the result of a reorientation of the molecules in the liquid crystal structure with change in temperature, and the effect this has on the incident light (Christie & Bryant, 2005). Outside the activation region TLC printing ink is colorless. Previous research showed that the "color play" effect of TLC printing ink is clearly visible if the ink is printed on a black substrate but not on the white one (Jakovljević et al, 2016). Within "the color play interval", the spectral color with given wavelength λ appears when the elongated molecules of the active material inside the microcapsules develop a helical superstructure with pitch equal to λ. As the temperature raises the helical pitches shrink causing the shift of the peak in the reflectance spectra towards shorter wavelengths, therefore the color of the material changes towards blue shades (White and LeBlanc, 1999 ; Seeboth et al., 2007 ; LCR Hallcrest, 2014). The currently available data do not show what optical density of the substrate is needed for the observation of the color pay effect. The presented research aims to answer it. For this purpose, white and black papers were used as printing substrates. The white paper was a semi gloss paper digitally printed in six different optical density of black color. Black paper printing substrates were uncoated and coated black papers. The water-based TLC printing ink was screen-printed on all mentioned printing substrates. The TA of the TLC printing ink was 25°C, the activation region from 25°C to 30°C. Optical properties of printed samples were determined by spectroscopic measurements. The "color play" effect was quantified by spectral reflectance applying (8°:di) measuring geometry. The samples were heated and their reflectance spectra were measured in dependence on temperature. The measured temperature dependent iridescent colors were calculated and presented as CIELAB colorimetric values, showing the "color play" effect throughout the visible spectra. The results confirm optimal "color play" effect of the samples printed on black paper as well as on the grey substrate with optical density of at least 0, 72. The measured optical effect of TLC printing ink is shown as closed curve in (a*, b*) graph. If the "color play" effect is optimal, the mentioned curve extends across all quadrants. Samples printed on grey substrate with optical density between 0, 30 and 0, 42 have resulted with closed curves, but only across first quadrant of (a*, b*) graph. Samples printed on uncoated black paper result in stronger temperature dependent optical properties of TLC printing ink compared to the samples printed on coated black paper. This result is best seen in (a*, b*) and L*(T) graphs. This effect is related to drying mechanism of TLC printing ink and absorption properties of printing substrate.

thermochromic liquid-crystal printing inks, "color play" effect, optical effect of liquid crystals, spectroscopy and measurement geometry

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