Spectroscopic evaluation of the colour play effect of thermochromic liquid crystal printing inks

Abstract

Colour changes of a thermochromic liquid crystal (TLC) ink printed on a black substrate are clearly observed by the naked eye, and therefore it is reasonable to expect that every approach recommended for colour measurements of samples with solid colour would be appropriate. In addition, the published spectroscopic analyses of TLC inks do not provide any guidance or limitations for the measurement geometry. On the other hand, our study demonstrates that spectroscopic measurements of such samples are not so straightforward. When using the most simple and commonly used measurement equipment with a directional illumination and viewing approach, the temperature‐dependent spectral features resolve in a narrow spectral region that gives only a small part of the colour play effect. Detection of the entire effect requires the use of integrating spheres, which spatially integrate the radiant flux reflected on a sample in any direction; better results are obtained by spheres of larger diameter. The comparision of spectroscopic and colorimetric results obtained when different measurement geometries are applied on the same sample may demonstrate certain properties of the liquid crystal material inside microcapsules and help in understanding some details of the optical properties of the ink layer.

Effect of the Concentration of Pigment Slurry on the Film Performances of Waterborne Wood Coatings

“…When the temperature drops to the original temperature, the solution of thermochromic microcapsules returns to the original color [2]. Temperature-sensitive reversible systems of thermochromic microcapsules can be divided into organic reversible [3], inorganic reversible, [4] and liquid crystal reversible categories [5]. Solvents in a thermochromic system determine the thermochromic temperature of temperature-sensitive materials [6,7].…”

Identifying a Unique Communication Mechanism of Thermochromic Liquid Crystal Printing Ink

“…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.…”

“…The "colour play effect" of TLC inks is only clearly visible when the ink is printed on a black substrate [7,14]. The reflection of light from the helical structure is nearly imperceptible, because most of the light passes through the ink layer and hits the substrate.…”

“…The reflection of light from the helical structure is nearly imperceptible, because most of the light passes through the ink layer and hits the substrate. On a white substrate, most of this light is backscattered, virtually obscuring the low light intensity reflected from the molecular pitch [7]. To prevent this obscuring effect, TLCs should be deposited on a black substrate, which can absorb the greatest part of the light transmitted through the ink layer.…”

Polycaprolactone primers with zinc oxide and silicon dioxide nanoparticles for paper substrates: Influence on the properties of cyan and magenta offset prints

“…Various coatings are often used on paper to improve its properties and printability 6,7 . Therefore, new functional materials such as temperature sensors, oxygen‐scavenging films, food freshness indicators, and coatings with various barrier properties, are increasingly applied 8‐10 . Protective coatings/films can be applied to printed products (often paper substrates) either before or after the printing process 11,12 .…”