Lighting time analysis of water-based flexographic inks containing phosphorescent pigments

Abstract

The research investigates the illumination time of four luminescent pigments (green, blue, violet, and orange) dedicated to security printing and other special effect covers. It involves printout preparation on paper bases with and without optical brighteners with nine k-bar applicators and flexographic water-based inks with three different pigment concentrations (5, 15, and 30%). The printouts assessment is in the range of wild ink film thickness concerning the time of illumination. Achieved results led to the modelling of these properties. The Zeiss SEM microscope presents the images of the pigments tested.The innovation in this research is to show the nature of luminescent pigments and inks and to demonstrate its application possibility. The mathematical models presented in this article can help to adjust the appropriate ink layer's thickness and pigment concentration to achieve the desired glow time.The article shows that the green pigment glows the longest, successively blue and violet. The changes in the glow time from the thickness of the layer look similar for green and blue, for violet only at the highest pigment concentration. In contrast, the orange differs significantly from the others and shines approx. 20 times as short as the green one. The article explains how the size of the pigment can affect its glow time and why the samples obtained on a substrate containing optical brighteners glow longer. The optical brighteners intensify the shining time for the longer light wavelengths of pigments, such as green and orange, than for the shorter ones – blue and violet. The influence of the substrate on the glow effect is less visible for higher pigment concentration or greater print layer thickness. Applying a luminescent pigment on a dark-coloured substrate diminishes the lighting effect and eliminates it on a black one.