Uses of Fluorescent Pigments

 Tiffany Ramsey,  Chemistry Student Missouri S&T

            Have you ever stopped to wonder what makes the lines on roads so easy to see when you are driving home late at night or the road signs so easy to read? Have you ever wondered what makes glow-in-the dark products glow? Many of these products are made from a type of paint, coating or dye.

            Coatings have many different functions in our everyday life and can be made millions of different ways. Some examples of coating uses include: buildings, roadways, signs, and vehicles, and many others. There are many different components that go into a coating such as: resin, solvent, extenders, wetting agent, defoamer, coalescing aid, surfactant, thickener, dispersant, pigments, etc.  There are also multiple compounds that can be used for the same purpose.

            Pigments are defined as fine particles that are used in coatings for several different reasons but mostly to provide color.Fluorescent pigments are pigments that absorb light that causes the excitation of electrons to a higher energy state and then emit light when the electrons relax back to a lower energy state. Fluorescence was first observed in 1567 by Nicolas Monardes during the infusion of wood.^[1] Some examples of different uses of fluorescent pigments in everyday materials are: black lights, glow in the dark paint, luminescent clothing, and fluorescent paint for roads.

 Glowing Paint

            One of the many examples of fluorescent pigments is bright or what appears to be glowing paint. It should be noted that fluorescent paint is not glow in the dark paint which uses phosphorescent pigments.  There are two significant uses for this type of paint, one is on roadways and one is for traffic signs. Before the 1950’s, roadway paint was not very visible at night with artificial light from vehicles. Companies then worked on different ways to produce a bright luminescence at night via artificial light. Besides the artificial light hurdle, they also had to find a way to keep the hiding pigments from blocking out the fluorescent ones. These pigments also needed to be protected from the UV radiation from sunlight that would cause unwanted degradation. The traffic signs had the same problem as the roadway paint, the only difference is that they needed to have daylight fluorescence as well as a colored return after dark. ^[2]

 One patented invention provides a detailed description of how an improved marker is obtained by coating the glass beads that are normally used in traffic paints with a relatively thin coating of a pigmented resin. After curing this resin and drying it until the coated beads are freely flowing, the beads can be incorporated into a binder (transparent or semi-transparent) that contains the fluorescent pigments. This is then applied to a base and then uncoated glass beads are applied to the surface while it is still tacky enough to bind them. This method results in the fluorescent pigment particles and the regular pigment particles within the coating on the glass beads not mixing in order to keep from killing the fluorescent effect. ^[2]

Toners, Developers, Powder Coatings, Inks, and Color Filters

            This industry was in the need of a fluorescent yellow colorant of which they were able to use one called Yellow 155 that was inexpensive, highly transparent, good dispersibility, high thermal stability, and very little to no hazards associated with it. This patent uses an azo pigment as a colorant and it is known for a neutral inherent electrostatic effect as well as a thermal stability of >300°C. With the use of this particular pigment, the disadvantage of an additional step to suppress an inherent triboelectric effect of the pigments typically used was avoided. Yellow 155 pigment has a neutral inherent triboelectric effect (charge system). ^[3] This pigment was used to enhance the brightness of the colors used in their products.

Multi-Use Coatings

            Mark Vukasovich formulated a coating for the production of a transparent, hard, silica particle with fluorescent dye and a solvent ‘sorbed’ into the pores. This pigment is used to impart bright fluorescent color to paints, paper, textile coatings, silk screen and printing inks, plastics, crayons, chalks, etc. It is also used to brighten already non-fluorescent conventional colors. This invention uses pigmentary silica gel and bringing it into contact with a liquid solution that is comprised of fluorescent dye, a fraction of non-volatile solvent, and volatile solvent. Deposition of this solution takes place onto the surface of the silica gel within its pores via adsorption/absorption. These two components are then kept in constant contact via agitation, which give uniform deposits of the pigment within the silica gel pores. The volatile solvent is then removed via a drying operation and results in a pigmented product that will then be processed further by heating, screening, and any other refining/recovery steps. ^[4]

Monitoring Functional Coatings and Compositions

            Kurt Melancon formed a method using Uvescers that absorb radiant energy and do not emit visible light, therefore they cannot change the appearance or color of an object that is used for monitoring the weight of a coating, uniformity, and any defects in the coating based on fluorescence. Melancon’s method  overcomes the problem of visible light ranges of monitoring fluorescent coatings due to interference of substrate that can mask the emission of the fluorescent pigments. They use of wavelengths between 240 and 400nm for this invention because it minimizes the interference of visible radiation coming from the substrate. ^[5]

Craig Libby formed a method to detect a silicone coating applied to paper webs and similar substances to provide an easy-release surface. This invention uses a fluorescent compound that absorbs UV light and emits it as a visible light. The resin used is an extremely friable organic glass formed by co condensing toluene sulfonamide-formaldehyde with a triazine (for example: melamine or benzoguanamine). ^[6]

Fluorescent Fabrics

            Fluorescent fabrics are used for heightened visibility of workers on construction sites in order to lower the number of injuries that are due to low visibility of coworkers. A production method consisting of non-hazardous fluorescent fabric made of three layers was made. The bottom layer is an opaque base, the middle layer is a transparent polyvinyl chloride resin that contains the fluorescent pigment, and the top layer is a transparent polyvinyl chloride resin that contains dissolved fluorescent colorant. It is important that the pigments remain as particles to achieve the maximum brightness and fluorescence of coatings and films. The plastisol process of preparing polyvinyl chloride coatings and film involves dispersing resin particles (polyvinyl chloride) in a plasticizer (acts as a non-solvent for PVC at room temperature) to form a thick, easily moldable/formable heavy paste. After forming this paste into the desired shape, it is heated to about 300-350°F causing the PVC resin particles to dissolve in the plasticizer that is then converted to a film after cooling.  When the fluorescent pigments are mixed in, the heating temperature will not melt the pigments, it only darkens them due to uneven heating temperatures. This is a problem because it causes uneven coloring, which is also caused by certain methods of application, or a combination of the fusing ovens heat being uneven and the application methods. This unevenness in color and the darkening can be overcome by applying a transparent surface layer of a vinyl chloride polymer solution that has a daylight fluorescent dye/colorant dissolved in it. ^[7] 

Conclusion

            Fluorescent pigments are mostly used for optical effects such as heightened visibility and brighter colors. On roadways we use them for brightening the painted lines and traffic signs for greater visibility during the daylight and at night with artificial light. We use them in clothing for greater visibility of workers on construction sites where heavy machinery is being used and safety is a must. This type of clothing is also worn by people that are working out via running, walking, or biking along the roadways so that they can be seen better by on-coming traffic. Fluorescent pigments are also being used in coatings to help check uniformity and thickness in a coating without affecting the color by using an uvescer pigments that will not emit visible light.