crystals Article Identifying a Unique Communication Mechanism of Thermochromic Liquid Crystal Printing Ink Maja Striži´ c Jakovljevi´ c 1, *, Branka Lozo 1 and Marta Klanjšek Gunde 2, *   Citation: Striži´ c Jakovljevi´ c, M.; Lozo, B.; Gunde, M.K. Identifying a Unique Communication Mechanism of Thermochromic Liquid Crystal Printing Ink. Crystals 2021, 11, 876. https://doi.org/10.3390/cryst11080876 Academic Editors: Kohki Takatoh, Jun Xu and Akihiko Mochizuki Received: 13 July 2021 Accepted: 27 July 2021 Published: 28 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Faculty of Graphic Arts, University of Zagreb, Getaldi´ ceva 2, 10000 Zagreb, Croatia; branka.lozo@grf.hr 2 National Institute of Chemistry, Hajdrihova 19, 1001 Ljubljana, Slovenia * Correspondence: maja.strizic.jakovljevic@grf.unizg.hr (M.S.J.); marta.k.gunde@ki.si (M.K.G.); Tel.: +385-2371-080 (ext. 255) (M.S.J.); +386-147-60-291 (M.K.G.) Abstract: Thermochromic liquid crystal materials are commonly used in printing inks, opening up a wide range of possible applications. In order to ensure and control the most accurate application, the occurrence of the so-called colour play effect, i.e., the appearance of iridescent (rainbow) colours as a function of temperature, must be determined precisely. For this purpose, the temperature-dependent reflection of a sample must be measured using a spectrometer with an integrating sphere. The same values should be obtained for each sample containing the same thermochromic liquid crystalline material, irrespective of the spectrometer used, integrating sphere, layer thickness and the surface properties of the substrate. To describe this intrinsic property of the thermochromic liquid crystal material, the term communication mechanism might be considered. The research has shown how this mechanism is obtained experimentally. Keywords: thermochromic liquid crystal inks; temperature; colour play effect; communication mechanism 1. Introduction Thermochromic liquid crystal (TLC) inks respond to temperature change with a change of colour [1]. In order to achieve numerous possible applications, the TLC func- tional material is usually microencapsulated, so as to protect its unique properties and to “pigment” the ink or some other host material. The TLC material inside the microcapsules determines the colour, mechanism of colour change, and temperature at which the change occurs, but the binder of the ink defines its printing and curing technology [2,3]. TLC inks are coloured within the temperature activation range of several degrees, also referred to as the “bandwidth” or “colour play interval” [4,5]. At temperatures below or above the activation range, the TLC ink is colourless. During heating, at the point of reaching the activation temperature, the red colour appears first, followed by orange, yellow, green, blue, and violet. This effect is referred to as “colour play” [4,6]. Each of the colours is limited to a narrow temperature interval [3]. 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” [4,7,8]. Our previous experiments have shown the colour cycles of TLC inks to be reversible. Microcapsules are the functional pigments and can be used in a number of different applications. They are usually contained in a binder system and are incorporated in various places including wearable devices [9–11]. In this study, we used the TLC inks in which the binder of the ink defines the printing and curing technology, whereas the pigment defines the thermochromic functionality [2,3]. Such inks can be used to print arbitrary designs on various surfaces, including direct printing on curved ones. In this special state, the adjacent sheets of equally oriented molecules twist and the corresponding director (i.e., the direction of the long axis of the molecules) traces a helical path. The distance required for the director to complete a 360 ◦ turn is called the pitch length. The thermochromic effect of Crystals 2021, 11, 876. https://doi.org/10.3390/cryst11080876 https://www.mdpi.com/journal/crystals