Search

Purpose: This paper aims to analyse the tack stability measurement readings of thermochromic offset inks. For this purpose, three reversible leuco-dye formed thermochromic inks were used to evaluate their tack. The used inks differ in their activation temperature, colour, drying mechanism, viscosity and chemical content. Thermochromic offset inks differ from conventional inks in formulation and size of colourants due to the presence of the microcapsules. Design/methodology/approach: Printing inks in offset lithography must have sufficient tack so they can be transported from a reservoir onto the substrate. The ink transfer takes place through the ink splitting by inking rollers, printing plate and blanket transport. An IGT Tackoscope device was used to produce the dynamic readings of ink tack as a function of distribution and splitting time. The data can be used for prediction of the ink press stability under controlled settings, such as temperature. Findings: This research explains the temperature and velocity influence on the tack stability measurement readings of thermochromic inks. It also gives an insight of the chemical compounds that dictate the tack measuring reading results. Originality/value: Development of new printing inks, such as thermochromic printing inks, may affect the printing of different products. The research was conducted due to a lack of available data for the tack of thermochromic inks. [ABSTRACT FROM AUTHOR]

Thermochromic inks are known for their poor stability against UV light, appearing differently in leuco dye-based and liquid crystal-based ink types. The leuco dye-based inks lose the colour contrast between the coloured and discoloured states but the temperature at which the colour changes stays the same. These inks retain thermochromic functionality, i.e. the relation between colour and temperature, as long as the colour contrast between both states remains visible. Liquid crystal-based inks reflect a narrow band of light with wavelengths equal to the pitch length of the helical structure formed in the chiral-nematic mesophase. While the length of this helix depends on temperature, the wavelength of the reflected light shifts across the spectrum. This unique thermochromic effect is also called the colour play effect and is used for visual determination of temperature. However, the helical structure is highly sensitive to UV light which changes the temperature-related colours; consequently, such an ink loses its thermochromic functionality. Almost complete deterioration of the colour play effect occurs on unprotected samples in less than 60 days of exposure to natural indoor daylight and after 10 h under the artificial daylight. UV-protective foil or UV filter protects the ink against detrimental effects of UV light and keeps its functionality even after 10 h of exposure but the colour play effect is shown at lower temperatures. Image 1 • Thermochromic liquid crystal inks have poor lightfastness. • Exposure to light shortens the pitch length of liquid crystals inside the pigments. • The exposed samples loose functionality after 10 h under artificial daylight. • A good UV protection preserves the colour play effect and prolongs usage. [ABSTRACT FROM AUTHOR]