Feasibility study of thermochromic inks for the packaging industry

Abstract: A thermochromic ink is defined as an ink that exhibits colour changes depending on temperature. Thermochromic inks used in printing generally consist of an encapsulated three part system which includes a dye that changes colour, a compound that acts as a colour developer and a solvent. Due to the properties of the dye, the inks are always colourless above a certain temperature and coloured below a certain temperature. This study seeks to investigate the feasibility of using thermochromic inks in the existing packaging manufacturing process at Tetra Pak. Inks that have these properties could be relevant for companies in the packaging industry to create smart packaging materials to differentiate products. To analyse the properties of thermochromic inks,suitable thermochromic inks for water based flexographic printing were ordered from two different ink suppliers along with printed samples from a third ink supplier. By using L'a'b' measurements and calculated ΔE## values, different flexographic printing parameters including anilox volume and substrate were tested in lab scale tests. The measurements were carried out at different temperatures to analyze the colour changes in detail. After the lab tests, pilot tests were carried out to verify if the process behaves accordingly at a greater scale. The results from the pilot tests corresponded well with the lab results. When testing high temperature stability it was found that high temperatures, above 300 °C that is prevalent during lamination, did not have an impact on any of the inks tested in this study. It was observed that for all inks tested in this study the ink layer is visible at room temperature and not transparent to the paper surface. The inks tested in this study follows major food safety regulations but the next step would be to perform internal migration tests at Tetra Pak.