Luminescent properties of fluorescent colour water-based ink films

Abstract

The article presents the luminance results of printouts for eight fluorescent inks intended for various applications, e.g., securing documents and packages, safety signs, or decorative effects. The research involves inks with 30% pigment concentration. The novelty is to show the luminescence depending on the ink film thickness and colour properties. The range of printout ink layer thicknesses is from 0 to 100 μm. The article presents scanning electron images and energy dispersive analysis of tested pigments, spectrofluorometric measures of lifetime, quantum yield, and Stokes shift. It also shows the rheological properties of pigment powder and inks and the optical properties, i.e., colour parameters, transparency, gloss, luminescence intensity, and the fluorescence expressed by the ΔCh, Δb∗, and ΔL∗ parameters. The surface topography and morphology, grain size, pigment shoreline, and roughness of the particles influence the fluorescent effect. The elemental composition, the b∗ colour parameter value in the CIELAB space, and lightness also affect the fluorescence. Pigments with a small grain and a smoother surface, such as chrome, orange, and yellow, shine much better than those with a more significant pigment grain and a rougher structure, such as violet and blue. The topography of pigments also affects their rheological properties and ink rheology. An increase in the ink layer thickness resulted in a decrease in its gloss and transparency values. However, the gloss and transparency do not affect the level of fluorescence, which increases with the ink film thickness to a certain maximum specified for a given ink colour. The equations in the article enable the fluorescent ink users to predict and adjust the appropriate thickness of its layer to obtain desired glowing effects.