###### Abstract  
In the present research, properties and performance of special effect printing inks were observed with the aim of obtaining a printed product with dual functional properties. Thermochromic liquid crystal-based printing ink (TLC) and UV-visible (daylight invisible) fluorescent inks (UVF), pure and as hybrid ink systems, were printed using a screen-printing technique on two types of uncoated paper substrates. Characterization of the paper substrates was performed, as well as detailed analysis of printed layers. Thickness, surface roughness, surface free energy, and adhesion parameters of printed layers were analysed. Spectral reflectance of pure UVF and TLC printing inks, as well as the spectral reflectance of the proposed hybrid ink systems were measured. The thermochromic effect of the TLC ink and hybrid systems was analysed. Microscopy was used to display the visual colour play effect and the effect of the fluorescence. Results of the measurements showed high compatibility of used materials in the proposed hybrid ink systems. Since the effect of luminescence and the colour play effect in the hybrid systems were preserved, it can be concluded that TLC/UVF hybrid ink systems can find their application in the development of functional packaging and in all other applications with special requirements for temperature monitoring and hidden information for different products. thernochromic liquid crystal-based inks; UV-visible fluorescent ink; colour play effect; luminescence; screen printing +
Footnote †: journal: Applied Sciences  
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[PERSON], University of Zagreb, 10 000 Zagreb, Croatia; [EMAIL_ADDRESS]  
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[PERSON], University of Zagreb, 10 000 Zagreb, Croatia; [EMAIL_ADDRESS]  
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[PERSON]  
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[PERSON], [PERSON] Properties and Color Performance Performance of Screen-Printedchanges colour inside this region, but is colourless outside of it. Every TLC ink has its activation temperature \(T_{A}\), the point at which, during heating, the colour starts to change. This type of printing ink is the only one so far known for this colour play effect. After reaching the \(T_{A}\) and with further heating, the effect starts with the appearance of red colour, followed by orange, yellow, green, blue, and violet [7; 9]. Each of these colours is limited to a narrow temperature interval (White MA, [PERSON], 1999). Above the upper threshold of the activation range, the violet colour disappears, and the TLC ink becomes colourless again. The temperature required to reach the colourless stage is called the \"clearing point\" [7; 10]. Our previous experiments have shown the colour cycles of TLC inks to be reversible [11]. The microencapsulated cholesteric or chiral nematic liquid crystals reflect the light on the helical structure formed inside the activation region (i.e., chiral nematic mesophase). This occurs at wavelengths equal to the optical value of the pitch length of the helix. In TLCs, the length of the helix depends on the temperature, which shifts the reflection peak across the spectrum-producing structural colour [5; 7]. Microencapsulation prevents the adjacent of TLC drops from fusion or splitting and protects the active material from degradation by chemicals such as fats, greases, and solvents [5]. The protection from harmful effects of UV radiation is very poor and previous research has shown that TLC ink deteriorates in a short time if the prints are unprotected. Therefore, applications with unprotected TLC inks are suitable only for indoor purposes and for lighting conditions where no prolonged exposure to UV light is present [12]. The colour play effect of TLC inks is only clearly visible when the ink is printed on a black substrate, which can absorb the greatest part of the light transmitted through the ink layer [10; 13]. In such conditions, the spectral reflection obtains clearly visible iridescent colours [14; 15]. On a white substrate, most of this light is backscattered, virtually obscuring the low light intensity reflected from the molecular pitch [10].