Bright Ideas for Aqueous Dispersions of Fluorescent Orange
Written by: Dr. Cathy Cooper, Technical Manager
Executive Summary
Daylight fluorescent pigments offer exceptional brightness and visual impact, making them valuable in coatings, inks and high-visibility applications. However, they can behave very differently from conventional organic pigments. Lankem screened a range of dispersant chemistry in fluorescent orange aqueous dispersions, in order to obtain the best combination of wetting, particle stabilisation and optical properties.
Why Fluorescent Pigments Behave Differently
Widely valued for their bright, eye-catching appearance, fluorescent pigments absorb shorter wavelengths of light and re-emit this energy at longer visible wavelengths, creating the intense fluorescent effect associated with high visibility colours. This can be as much as four times the light intensity reaching the eye of the observer, providing a significant impact from the coated item.
Although described as pigments, many are particles in which fluorescent dyes have been incorporated into a polymer resin matrix. This gives excellent brightness but also means their dispersion behaviour can be different from crystalline organic pigments.
For formulators, this changes the challenge. The objective is not simply to grind down a hard pigment crystal. Instead, the system must:
• Wet a resin-based fluorescent pigment efficiently
• Break down soft agglomerates without over-processing
• Prevent excessive settling or hard sediment formation
• Maintain brightness and drawdown appearance in the final coating
• Ensure any structure formed during storage can be redispersed
In this work, the fluorescent orange pigment was evaluated at 40% pigment loading in water, with performance assessed through wetting tests, millbase appearance, drawdowns in a clear gloss paint and storage testing at room temperature and 50°C.\
Initial Screening: Wetting Behaviour
Two experimental grade dispersants were compared with the effect of no dispersant, and a conventional polymeric dispersing agent. Figure 1 shows the initial testing in which 0.2g pigment was used in a total of 15g millbase with a high dispersant loading of 7.5% agent on weight pigment (AOWP). This was solely to test the wetting capabilities of the dispersant options, with Dispersant B clearly showing the best wetting of the dry powder. All three dispersant options gave some benefit over that of no dispersant, showing the ability of the molecules to displace air from the powder surface and promote incorporation of the resin-based pigment into the liquid.
Dispersion Performance at 5% AOWP
At 40% pigment loading on a 40 g scale, all three dispersant candidates produced waterthin dispersions when processed using a Thinky planetary mixer for two one-minute cycles at 2000 rpm with no milling beads.
The control sample, without dispersant, remained powdery at first and then milled to a viscous, foamy paste, demonstrating the need for a suitable surfactant package.
Room-temperature storage then began to differentiate the candidates. Dispersant A settled to a hard layer over the course of a week, while the other two showed easier redispersion. This showed that even when initial viscosity was low, the quality of the sediment and ease of remixing were important indicators of practical stability. It was noted that Dispersant B showed less settling than the other samples at room temperature, leading to the initial thought that it was the preferred option.
However, high temperature storage revealed that the lack of settling shown by Dispersant B may have arisen from slight gel structuring of the system. In the sample using Dispersant B, a hard layer of sediment was shown to form upon storage for 1 week at 50°C. Dispersant A also displayed this irreversible sedimentation, but Dispersant C gave a soft gel that could easily be mixed back to a liquid.
Dispermat Scale-Up
Dispersant B and C were further evaluated on a larger 220 g scale using a Dispermat highspeed stirrer with a sawtooth blade, for 10 minutes at 40% pigment loading and 4.5% agent on weight of pigment. Significant differences were noted during processing, where the wetting of the pigment was a much longer process with Dispersant C.
The storage stability showed the same trend as the smaller samples. Dispersant B gave good room temperature storage with little settling. However, on elevated temperature storage it was apparent that a significant structure was formed with a failure of the dispersion. Dispersant C displayed settling at room temperature, but this could potentially be resolved with the use of an additional thickener in the final formulation. After storage at 50°C for one week, a gel was formed but this returned to liquid with mixing.
Based on these observations, a blended approach was developed to give a new product Lansperse FX30. This product predominantly contains Dispersant C that gave the softer high-temperature gel behaviour, with a smaller proportion of Dispersant B to give a boost to the initial pigment wetting performance. Another significant advantage of this blend is the estimated bio-based content, with a calculated renewable carbon index of approx. 64% from the raw materials used.
Lansperse FX30
The new dispersant was tested with the screening pigment wetting test, and then at a loading of 4.5% agent on weight pigment with the Dispermat high-speed mixer. It was seen that the performance regarding pigment wetting was similar to Dispersant B, whereas the settling at room temperature and the formation of a reversible gel structure on storage was more similar to Dispersant C.
The fluorescent orange dispersion was combined with a clear acrylic gloss in a ratio of 1:4 parts by mass and drawn down on Leneta black & white card with a wet film thickness of 50 µm. This gave an indicative comparison of performance in a coatings system, however, a commercial base paint is likely to have a different surfactant and co-solvent package than potentially used in a final fluorescent formulation.
Conclusions
Fluorescent pigments can provide outstanding visual impact in waterborne coatings and inks, but they require careful formulation. Their brightness comes from a unique polymerdye structure, and that same structure can create challenges in wetting, dispersion stability and high-temperature storage. The work carried out by Lankem shows that successful dispersion of fluorescent orange pigment is not simply about finding the strongest wetting agent. It is about balancing wetting, drawdown quality, storage behaviour and ease of redispersion. Lansperse FX30 represents a practical route toward this balance, offering good pigment wetting and improved handling compared with single-component systems, along with a renewable carbon content of over 60%.
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