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Pigments comprising zinc sulfide or various compounds of strontium aluminates have been used for several decades to achieve a glow-in-the-dark (photoluminescent) effect. However, both of these materials possess drawbacks that have hampered widespread commercial use. While pigments containing strontium aluminates are some of the most efficient (and safe) photoluminescent materials, they are extremely expensive to produce and also typically have a specific gravity of around 4 (i.e., they are four times heavier than water of equal volume), which limits their commercial application. These compounds also tend to have a large particle size, typically 80 to 100 microns, and they provide an inherent abrasiveness due to their hardness (they are 8 on the Mohs' scale, while diamonds are 10) and crystalline structure. As a result, the production costs for products containing these pigments are often quite high. Moreover, these compounds break down and deteriorate rapidly in the presence of retained moisture, which restricts their use to solvent-based formulations and limits their long-term effectiveness outdoors.
The major drawbacks to zinc sulfide technology are its afterglow duration and deterioration by ultraviolet (UV) radiation. These pigments rarely achieve more than 20 minutes of effective afterglow, and they break down rapidly with exposure to UV radiation such as sunlight. Although they cost significantly less than strontium aluminates, zinc sulfide pigments are mainly restricted to cheap novelty items such as children's bedroom ceiling stars, where greater afterglow duration is not required and outside use is limited.
Recently, a new generation of photoluminescent pigments* has been developed that claims to offer both cost effectiveness and high performance for a broad range of product applications, including glass and ceramic items. As companies continue to look for ways to set their products apart in the global market, special effects such as photoluminescence are creating new opportunities.
*Developed and supplied by VisionGlow Global Ltd., Melbourne, Australia, and VisionGlow of North America, LLC, Sanford, Fla.
The Alumina Silicate AdvantageThe new pigments are based on photoluminescent alumina silicate crystals that are processed using a proprietary post-treatment technology. According to the supplier, the treatment enables the specific gravity of the pigments to match that of the host medium, ranging anywhere from 0.18 to 4.2. As a result, the pigments can be integrated into a variety of products-including water- and solvent-based ceramic glazes, decorative inks and decals-and smaller quantities of the pigment (compared to strontium aluminate compounds) are typically required to achieve the desired effect. They can also withstand temperatures of up to 1000°C, which allows them to be added directly to glass body formulations before the melting stage. And a small particle size (6 to 26 microns, and even smaller in some cases), combined with their unique crystalline structure and proprietary processing technology, substantially reduces the abrasiveness of the pigments, which minimizes wear on production machinery.
Unlike conventional photoluminescent pigments, which typically see a substantial reduction in afterglow time at small particle sizes, the alumina silicate pigments have demonstrated an afterglow duration of up to 12 hours, and possibly even longer under the right conditions.
Another drawback of conventional photoluminescent pigments is an undesirable green tint or "greening effect" in the daylight. As a higher loading is used to achieve a stronger glow, the greening effect becomes more pronounced. With the new pigments, "the small particle size and post-treatment process enables manufacturers to achieve a high level of photoluminescence without greening, thereby increasing the value of their finished products," says Gary Lamphier, president of VisionGlow of North America.
Why Photoluminescence?Photoluminescent pigments have been used to create beautiful glow-in-the-dark designs on ceramic vases and other decorative items, as well as unique promotional products, such as barware. But with all of the special effects available to today's ceramic and glass producers, photoluminescent properties would seem to fall fairly low on the list of desirable attributes. After all, just how many glow-in-the-dark vases or promotional products can a company actually sell?
According to Lamphier, the novelty aspect of the pigments is just one potential market. "Of course these pigments can be used in decorative pottery or promotional products. But they also have significant applications in the life safety field," he says. "For instance, New York City's building code has recently been amended to include the requirement of photoluminescent exit signs and evacuation guidance systems in buildings greater than 75 ft tall. As a result, architects and builders are looking for creative ways to meet these specifications, such as through the use of photoluminescent glass or ceramic tile." Such tile could also be used in subways or swimming pools to provide an additional light source.
Another potential safety application is on the road. Tri-E Technologies, in Cincinnati, Ohio, has developed a proprietary process in which the pigments are used to coat spheres of glass that are added to road paint. Unlike conventional road paint, in which the glass beads simply reflect light when the conditions are optimum, road paint containing the photoluminescent alumina silicate-coated beads would actually transmit its own light. "This would provide a significant benefit in fog, rain and other conditions where normal reflective paint might not be visible," says Larry Lough, chief executive officer of Tri-E Technologies.
According to Lough, the U.S. Department of Transportation is currently evaluating the new technology in several states, including Ohio, West Virginia and Michigan. "In addition to the enhanced effects, the cost is also lower than conventional products being used in this application, so that makes it a very attractive option," he says.
A Cost-Effective AlternativeCeramic and glass products incorporating conventional photoluminescent pigments are already on the market, but their high cost and performance limitations often make sales-and profitability-challenging. With the alumina silicate pigments, manufacturers have a new option to create unique, high-quality products with a broader market appeal.
For more information about the photoluminescent alumina silicate pigments, contact VisionGlow of North America at 131 Commerce Way, Sanford, FL 32771; (877) 799-1628 or (407) 330-1628; fax (407) 330-0068; or e-mail VisionglowNA@aol.com .
For more information about Tri-E Technologies, contact the company at 96 Security Dr., Cincinnati, OH 45014; (513) 870-3840; fax (513) 870-9194; or visit http://www.trietechnologies.com .
SIDEBAR: Potential Applications in the Ceramic & Glass IndustriesNovelty
- Promotional ceramic and glass products
- Ceramic and glass tile
- Dinnerware and decorative pottery/glass
- Landscaping (glow-in-the-dark glass landscape chips, landscaping pavers, etc.)
- Bathroom ceramics (soap/toothbrush holders, tile, etc.)
- Ceramic or glass tile used as low level pathway markings, safety egress systems (using photoluminescent decals) or in/around swimming pools
- Glass beads for road paint applications
- Pavers for walkways and driveways
- Terrazzo for commercial buildings