Compositions containing daylight fluorescent pigments are usually quite transparent unless an opaque formulation or pastel tint is the objective. This means that the loss of daylight fluorescence is a layer by layer process. Obviously, the higher the concentration of pigment and/or the thicker the coating, the longer the persistence of color and fluorescence. But there are limitations.
Firstly, there's a problem of concentration quenching. Once the dye concentration reaches a certain level, some of the emitted radiation is captured by neighboring dye molecules. Over a pigmented surface, the degree of fluorescent emission vs. dye concentration starts to decrease above a concentration limited maximum. The visual effect is one of a reduction in luminance and/or a dulling of color.
Brilliant can advise you on pigment type and formulation in order to achieve the maximum desired fluorescence intensity for your application.
Another problem encountered in dye degradation is spectrum color shift on fading (Figs. 1, 2). Brilliant's standardized fluorescent pigment colors are the result of many hours of color optimization, including minimizing color shifts upon extended exposure. Nevertheless, such shifts do occur over time and this requires that the user be aware of such changes and select colors that not only best meet the initial appearance requirement but also one that sustains that appearance within those set requirements over time. This is one of the reasons that it is suggested that users not attempt to mix their own daylight fluorescent colors without first consulting Brilliant's technical specialists for advice on what is likely to work best in your application.
Fig. 1. Overlay of a daylight-adjusted xenon and solar spectra
Properly filtered, specially designed xenon light sources are considered the optimal approach for approximating the fading effects of pigment exposure to sunlight.
Fig. 2. 8-Hour Fading Profile for Four Representative Brilliant Daylight Fluorescent Pigments
Differential fading results from a combination of effects including relative dye concentrations, intrinsic susceptibility to loss of fluorescence, post-faded residual color, nature of the pigment matrix and any protective additives or overlay coatings.
Note: In photographically capturing these samples much of the daylight fluorescence effect is lost. What is more apparent in these images is color shift after a short interval of exposure.
Fade-Resistant Additives and Coatings
Table 3. The Blue Scale, developed by the American Association of Textile Chemists and Colorists, is a generally recognized standard measurement of color permanence of coloring dyes. In this system, daylight fluorescent coatings and colorings are rated 4 or less*, indicating that they are extremely fugitive. Moreover, even if some color is retained after exposure, daylight fluorescence tends to be more rapidly lost for the reasons previously noted.
Because of these fugitive characteristics upon exposure to daylight, Brilliant recommends a number of approaches which can be undertaken to extend both color and daylight fluorescence in an application. However, as a general rule, given the fact that pure daylight fluorescent pigment coatings are transparent, the thicker the coating layer, the longer the persistence of fluorescent color.
While many fluorescent pigments depend, in part, on UV absorption and subsequent emission of visible wavelengths of light, a significant portion of the shorter, more energetic UV component of daylight can actually be destructive of the fluorescent dye molecule without enhancing fluorescence. Moreover, for certain applications, there may be acceptable tradeoffs in lower fluorescent emission in exchange for a reduced fading profile by the incorporation of UV absorbers which do overlap the absorption region of the fluorescent dye. Here, the ratio of absorber to dye becomes important so as to minimize fading while maximizing the fluorescent effect.
What UV absorber to consider, at what concentration and the method of incorporation may be a matter of multiple trials and testing. However as a starting point, improvements of 10 to 30% in fading resistance in the coloring of plastics has been observed with the incorporation of 0.5 to 1.0% benzophenone or benzotriazole UV absorbers in some formulations. In coatings applications, UV absorbers may be added directly to the formulation or be applied in the form of one or more overcoats.
The use of UV absorbers and the method of incorporation will depend on your application. Clear acrylic top coatings containing high levels (2 to 4%) of UV screens have proven effective in enhancing exterior color stability.
Finally, fluorescent pigments may be used in tinting applications with conventional coatings such as indoor paints. In such cases, high loadings of opacifiers such as titanium dioxide, which both scatters and reflects the incident radiation, can go a long way in reducing fading over longer periods. Brilliant has much history in such testing and can be of significant help in making recommendations to help minimize fading in your application.