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When applying a transparent coating or inkjet ink there might be a need to visualize the coating in order to determine its presence on the substrate. A fluorescent additive allows the coating to become visible under a UV-black light. This additive must however not impair the transparency of the coating. There are 2 possibilities of instilling fluorescence in a coating: the addition of a fluorescent dye or a fluorescent pigment. In this case study, we shall focus on the fluorescent pigments. In order to keep the coating transparent, the fluorescent particles must be reduced in size. This size reduction can be accomplished by milling the particles in a pearl mill. During the milling, samples were taken at regular intervals in order to evaluate the dispersion quality. The samples were evaluated using fluorescent spectroscopy and particle size analysis.


A blue organic fluorescent additive was milled in water in a bead mill (Dynomill). The particle size of the dispersed particles was obtained using a Malvern Particle sizer (See Figure 1).​


Figure 1: Measuring particle size distribution

Figure 2 shows that the largest particles are reduced after 30’ of milling. Beyond this time the average particle size remains the same, only the size distribution is decreased.​


Figure 2: Particle size measurements of the dispersion samples at different milling times. The volume % of the particles is plotted as a function of the particle size. The arrow denotes the narrowing of the particle size distribution after 30' of milling

The fluorescence of these dispersion samples was determined using a Jaz spectrophotometer (Ocean Optics). The samples were excited with a 365nm LED and their fluorescent response was recorded perpendicularly. The fluorescence was then plotted by determining the intensity of both fluorescent peaks of the additive at 451 and 482nm. Figure 3 shows that the fluorescence of the samples decreases as a function of milling time, and disappears completely after 105’ of milling. The blue line denotes the fluorescent spectrum of a blank water sample.


Figure 3: Fluorescence of the samples taken at different milling times. The fluorescence is plotted using the intensity of the fluorescent peaks at 451 and 482nm. The blue line denotes the fluorescence of a blank water sample
Right top corner: TLC analysis of different dispersion samples. The fluorescent additive is present in all three dispersions but the appearance of 2 new components is denoted by the arrows

At first glance we assumed that the additive was totally destroyed in the pearl mill. Thin layer chromatography (TLC) however demonstrated the creation of a contaminant during the milling process (see Figure 3 right top corner). At time zero 1 main component is visible, while after progressive milling 2 extra components appear. Care must therefore be taken when milling this particular additive: milling for too long at too high shear and impact forces (pearl mill) promotes the formation of new species that absorb all the incoming light, thus leaving none for the original fluorescent additive. 

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