## 4 Conclusions  
The aim of this research was to obtain a functional printed product in hybrid printing ink systems, by using two special effect printing inks, i.e., TLC ink and UV-visible fluorescent pigment/ink, on two types of black uncoated paper substrates (UT and CB). Results of adhesion performance showed that higher work of adhesion was achieved between CB paper and TLC ink than between UT paper and TLC ink. The adhesion of UVF ink was better on CB paper as well. In hybrid ink systems, the highest work of adhesion was achieved between CB paper and UVFp in TLC ink, pointing to the highest level of interactions at the interface among hybrid samples. Interfacial tension and wetting coefficient were not optimal for most interfaces, pointing to the possibility of the separation of the layers in contact. In future research, this problem, generally present for TLC inks, should be addressed by additives and/or protective coatings. Spectral reflectance of UVF ink and hybrid ink systems showed that TLC ink does not cause any changes in the reflectance spectra of the UVF printing ink. The highest spectral reflectance was measured on the UVFp in TLC prints compared to the other ink systems. It is concluded that used TLC ink is suitable for the combination with used UVF pigment in the proposed hybrid ink system. Results of this research show that both pure TLC and hybrid ink systems based on combination of TLC and UVF inks/pigments have full colour play effect on both UT and CB paper substrates. This is important knowledge because it shows compatibility of highly sensitive TLC microcapsules and UVF ink/pigment, as well as their combination with chosen uncoated black paper substrates. Since UVF ink/pigment does not diminish the colour play effect of the TLCs in hybrid ink systems, it can be concluded that these systems have a special effect with several distinctive features in visible and UV parts of the spectrum. Spectral reflectance measurements of printed samples showed that UT paper substrate is less stable than CB. These results are confirmed with colorimetric results of CIELAB and \(L^{*}\)/T values. These results are related to the characteristics of paper substrate, regarding processing of black papers during production. The samples of pure TLC and hybrid ink systems at defined temperatures (27, 29, and 31 \({}^{\circ}\)C) showed higher stability, i.e., mutual uniformity for CB paper substrate than for UT, for which this effect occurs only at higher temperature (31 \({}^{\circ}\)C). The value of the offered printing ink systems is the dual functional-thermochromic effect used in smart packaging that is visible in daylight in a specific temperature range; and the second added value of the thermochromic print (or brand) protection is in terms of the authenticity given by UVF component of the hybrid system. Both added value features were applied on the same print (and in the same prepared ink in one of hybrid ink systems) for the first time. In this way, a new thermochromic ink with hidden security feature has been produced. Presented hybrid ink systems could find their application in development of functional packaging and all other applications with special requests for temperature monitoring (TLC) and possible hidden information (UVF); such applications could imply security and/or brand protection. Future research will go in direction of investigating and improving the light fastness of hybrid prints and their adhesion properties, as well as measuring the influence of UV radiation, temperature, and humidity on hybrid ink systems, as functional packaging in today' s supply chains is exposed to different conditions, and needs to fulfil many strict demands.