# Description of Thermochromic Offset Prints Morphologies Depending on Printing Substrate
## 1 Introduction  
Materials that change color are called chromogenic materials. Thermochromic materials respond to changes of temperature by changing their color [1, 2]. Thermochromic (TC) systems based on leuc dyes are widely used as a carriers of thermally induced color changes in different applications [3, 4, 5, 6, 7]. These TC materials can be encapsulated in a polymer envelope or in the form of a pigment deposited in other polymer materials [1]. For example, encapsulated TC composites lose their color while heated and regain it again by cooling. Reversible change of color in encapsulated leuco dye system appears through two competitive reactions, between the colorant and developer (which predominates on lower temperatures, resulting in production of colored complex) and between the solvent and developer. At lower temperatures, organic solvent is in a solid state, while, with the increase of temperature,it becomes liquid. The complex colorant-developer is decomposed by the action of solvent in liquid form. When the thermochromic complex cools down again, the color returns to its original state due to the transition of the solvent into a solid state and formation of colorant-developer complex [8]. Crystal violet lactone (CVL) is the most common example of the spirolactone type of thermochromic dyes. In its ground state, it is colorless, while, in the presence of a weak acid or electron acceptor (color developers), the lactone ring opens and forms a triphenylmethane (TPM) type system (high color density) [9]. Weak acids (phenolic compounds, for example, bisphenol A), which are commonly used as color developers, assist opening the lactone ring of the leuco dye and stabilize the open form. Color development occurs when the developer and dye are fused or mixed in a solvent [9]. Since the thermochromic mixture is about 75-95 mol% solvent, the solvent's melting point largely controls that of the mixture. Commonly used solvents are long-chain alkyl alcohols, esters or ethers, and carboxylic acids [9]. In thermochromic (TC) printing inks, thermochromic pigments that encapsulate a leuco dye-developer-solvent system in a polymer shell are dispersed in a vehicle and are about ten times larger than the particle size of conventional pigment particles, which usually have medium particle size of a few micrometers [10]. TC inks can be applied on different kinds of substrate materials (paper, cardboard, plastic materials) through various printing techniques (screen printing, sheet-fed and web offset printing, gravure, pad printing, flexography, and rod coating). The most widely used systems for microencapsulation of thermochromic pigments in printing inks involve urea or melamine and formaldehyde [11], gelatin-gum arabic, and epoxy resins [12]. In the past few years, the research and the development of different novel technologies incorporating thermochromic materials have attracted much attention. Recently, some of the thermochromic technologies for application as thermal sensors and indicators in architecture [13, 14] and smart packaging [15] and as smart textiles [16, 17, 18] were studied. Despite that, most of the studies were conducted to determine the interactions inside the TC microcapsules between the solvent-developer and color former on the colorimetric properties of the thermochromic composite [4, 19, 20, 21], but there is a lack of studies regarding printing substrates and thermochromic printing ink interactions. In the field of graphic technology, most of the studies recently were devoted to thermochromic printing inks end-of-life solutions [22, 23, 24] and their health safety [25, 26, 27], UV stability [28], and novel approaches in measurements of thermochromic dynamic color change [29]. The increasing packaging added value through combination of printed electronics with the thermochromism phenomenon for development of thermochromic displays on cardboard packaging [30] and development of thermochromic microencapsulated inks for smart packaging [31] was reported, as well.