Abstract:
An inorganic phosphorescent article having a formed phosphorescent layer where the phosphorescent layer is not mixed with a frit and the majority of the layer comprises photoluminescent phosphors comprising rare earth doped alkaline earth aluminates, rare earth doped alkaline earth silicates, zinc sulfide doped with copper or mixtures thereof. The phosphorescent articles of the present invention may be formed as tile bodies and fired at high temperatures between 1000C and 1600C providing durable ceramic and porcelain tiles suitable for use in emergency lighting flooring systems, aqueous environments such as pools and spas, and outdoor pathway lighting.

Description:
[0001]    This application claims the benefit of U.S. provisional patent application Ser. No. 61/177,097 filed on May 11, 2009. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Buildings and subway stations display emergency and exit signs in order to direct people to a safe location in the event of a fire, natural disaster, or other event. In many cases electrical power is lost during such an event. If the building goes dark, another means is required to illuminate the emergency signage. 
         [0003]    One solution for such a problem is the use of photoluminescent materials. Photo luminescent materials can absorb light from the sun or from man made light sources. The light energy is then released, at a different wavelength. Some photo luminescent materials release the absorbed light energy at a slow rate, such as over several hours. The slow release of absorbed light energy is referred to as phosphorescence. Therefore, photoluminescent materials displaying phosphorescence can be charged with light from the sun or an artificial light source and then release the absorbed light energy in the dark over several hours providing illumination of emergency signage. 
         [0004]    Some examples of inorganic photoluminescent compositions include alkaline earth aluminates doped with rare earths, such as strontium aluminate, and zinc sulfide doped with copper. These inorganic photoluminescent materials are commercially available in a powder form under the brand name Lumilux® (Honeywell US, Morristown, N.J.). 
         [0005]    Conventional use of these inorganic photoluminescent material powders involves the dispersion of the powder into an organic or plastic matrix. This plastic matrix may then be formed using conventional plastics processing techniques such as compression molding, casting, extrusion and injection molding to create useful glow in the dark articles such as emergency signs, safety tape and toys. While numerous uses for this type of organic matrix photoluminescent article can be found there are important potential uses for photoluminescent articles for which this organic plastic matrix is poorly suited. The general use limitations associated with plastics in that they typically degrade when used in prolonged or sustained ultra violet (UV) radiation environments, degrade upon high temperature exposure and have poor wear resistance when subjected to an abrasive environment makes the use of organic plastic matrix photoluminescent articles unsuitable for applications such as emergency signage where fire is involved, outdoor tile subject to intense UV and floor tiling subject to high foot traffic. 
         [0006]    The highly desirous potential benefits of using photoluminescent materials in floor tile are described in U.S. Pat. No. 6,841,785 to Nolt, entitled, “Photoluminescent Floor Tile”. This patent discloses a photoluminescent floor tile system in which conventional inorganic photoluminescent phosphors are incorporated within a tile body containing limestone or clay. This construction requires that a substantial amount of photoluminescent material be added to the tile body to produce the desired phosphorescent properties. As an alternate embodiment, Nolt discloses a tile body having a wear layer of non-transparent inorganic material incorporating a resinous thermoplastic binder and photoluminescent material adjacent to the top surface of the wear layer. While the proposed floor tiles disclosed in the patent to Nolt are an improvement over previous photoluminescent tiles, these tiles are still subject to high cost (due to the amount of photoluminescent material incorporated in the tile body), high wear rates (related to the unfired or conventionally processed tile body) resulting in additional frequent replacement costs and UV degradation due to the thermoplastic resinous binder which incorporates the photoluminescent material. 
         [0007]    Another example of photoluminescent tile is described in U.S. Pat. No. 7,297,416 to Lee, entitled, “Photoluminescent Tile and Method for Fabricating the Same”. This patent discloses a photoluminescent tile construction in which a tile body has been machined or grooved to create a recess in the tile surface or body and a photoluminescent glaze powder (photoluminescent phosphor expressed by MAl 2 O 4  (M: metal) in 50-90 wt % mixed with glass frit in 10-50 wt %) is used to fill the recessed areas, covered with a protective frit layer and then conventionally fired at a temperature between 500 C and 1200 C. The photoluminescent glaze powder when fired at temperatures up to about 1050 C as provided in the examples produces a glassy mixture which is contained within the recessed areas. This method of producing a tile with machined or formed recessed areas and subsequently filling with a photoluminescent glaze is an improvement over previous photoluminescent tiles employing thermoplastic resins however this type of processing is very expensive. As an alternate embodiment, Lee discloses a photoluminescent tile in which a tile body having no machined recesses or grooves has a photoluminescent glaze powder placed on the surface of the tile in a pattern, a wear glaze covering the patterned photoluminescent glaze and firing a temperature between 500 C and 1200 C. While this method of producing a tile is lower cost than the machined groove method, the flowing nature of the photoluminescent glaze powder while firing can result in a distorted photoluminescent pattern. Additionally, conventional firing of these articles above 1050 C for an extended period of time, typically result in a degradation of photoluminescent properties. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed towards a phosphorescent article and a method of producing said phosphorescent article having a formed layer made up of essentially inorganic photoluminescent phosphor comprising a rare earth doped alkaline earth aluminate, rare earth doped alkaline silicate or zinc sulfide doped with copper 
         [0009]    There are numerous inorganic rare earth doped alkaline earth aluminate photoluminescent phosphors suitable for use in a phosphorescent article according to embodiments of the present invention. Some of these rare earth doped alkaline earth aluminates include formulations of strontium aluminate (SrAl 2 O 4 ), calcium aluminate (CaAl 2 O 4 ) and barium aluminate (BaAl 2 O 4 ) or mixed combination. Photoluminescent phosphors of aluminates such as these may be doped with rare earth elements such as europium (Eu), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), dysprosium (Dy), holomiun (Ho), erbium (Er), thulium, (Tm), ytterbium (Yb), lutetium (Lu) or combinations thereof. Suitable inorganic photoluminescent phosphor materials are commercially available in a powder form under the brand names Lumilux®, produced by Honeywell US, or LumiNova® produced by Stone Nemoto Co., Ltd. 
         [0010]    In accordance with an aspect of the present invention there is provided a method for producing a phosphorescent article comprising the steps of providing a photoluminescent phosphor comprising a rare earth doped alkaline earth aluminates, forming said photoluminescent phosphor into a shape using conventional ceramic processing techniques, firing said formed shape at a temperature between 1000 C and 1600 C in a controlled atmosphere. 
         [0011]    Conventional ceramic processing techniques are used form the photoluminescent phosphor powders into a layer or shape. These techniques include pressing, slip casting, tape casting, injection molding, screen printing, etc. Organic processing aids, such as binders, dispersants, defoamers, etc may be used during the forming step to assist in the process. During the firing step these organic processing aids are volatilized and completely removed from the finished phosphorescent article. Firing takes place in a furnace or kiln capable of maintaining sustained temperatures of 1600 C and providing a controlled atmosphere. The firing can be carried out in air, an inert atmosphere such as Argon, or in a reducing atmosphere such as an atmosphere containing some portion of hydrogen. The type of atmosphere suitable for a phosphorescent article of the present invention is determined by the particular photoluminescent phosphor and preservation of the valence state of dopants within the phosphor. 
         [0012]    In accordance with another aspect of the present invention there is provided a phosphorescent article having a composite structure. The composite structure includes a substrate layer bonded to a phosphorescent layer where the phosphorescent layer comprises greater than 90% by volume of photoluminescent phosphors. A phosphorescent article according to this aspect of the present invention may be formed providing a substrate and using ceramic processing techniques to apply the phosphorescent layer then firing the composite structure at a temperature between 1000 C. and 1600 C. 
         [0013]    In accordance with yet another aspect of the present invention there is provided a phosphorescent article having a composite structure with at least three layers. These layers include a substrate layer, a wear layer and a phosphorescent layer positioned between and bonded to the substrate and wear layers. The composition of the phosphorescent layer is such that the photoluminescent phosphor makes up a substantial majority of the layer and is not mixed with a frit. Each layer can be formed separately and then stacked to form the composite structure, or the each layer can be formed by coating a previously formed layer. Each layer can be formed from a powder using conventional ceramic powder processing techniques. Layers may be coated onto previously formed layers by spraying, brushing, dipping, or other coating method. Once the layers have been formed, the composite structure can be fired to temperatures between 500 C and 1600 C for up to several hours. All of the layers can be fired together at one time, or the forming and firing can be divided into several steps were some layers are fired before forming subsequent layers. 
         [0014]    In accordance with still another aspect of the present invention there is provided a phosphorescent article having a composite structure including a substrate layer, a reflective layer bonded to the substrate layer and a phosphorescent layer bonded to the reflective layer. The phosphorescent layer comprises photoluminescent phosphor. An optional wear layer may be bonded to the phosphorescent layer. The reflective layer is designed to reflect light emitted from the phosphorescent layer back through the phosphorescent layer providing a more pronounced and sustained phosphorescent effect. Suitable compositions for the reflective layer are those that reflect the wavelength of emitted light from a particular photoluminescent phosphor and include materials such as alumina (Al 2 O 3 ), titania (TiO 2 ) and mixtures thereof. Each layer can be formed separately and then stacked to form the composite structure, or the each layer can be formed by coating a previously formed layer. Each layer can be formed from a powder using conventional ceramic powder processing techniques. Layers may be coated onto previously formed layers by spraying, brushing, dipping, or other coating method. Once the layers have been formed, the composite structure can be fired to temperatures between 500 C and 1600 C for up to several hours. All of the layers can be fired together at one time, or the forming and firing can be divided into several steps were some layers are fired before forming subsequent layers. 
         [0015]    Suitable wear layers for any of the aforementioned embodiments may include low or high fire glazes. The wear layer may include decorative patterns or colorants. Alternatively decorative layers may be added to the composite structure to provide improved aesthetic properties. 
         [0016]    In accordance with still yet another aspect of the present invention there is provided a phosphorescent article where the phosphorescent layer takes the form of various indicia, graphics, designs or lettering to convey concepts to those who may view them. For example, the shaped phosphorescent phosphor may be formed into an arrow to convey a directional path for someone to follow in the case of emergency signage. A schematic design of a person taking a step may indicate a nearby flight of stairs. Wording may be formed such as “EXIT” denoting an exit location in the case of fire. 
         [0017]    The phosphorescent articles of the present invention are well suited for creating dense, highly wear resistant, UV stable, high temperature use tiles for flooring, walls, pools, spas, outdoor pathway lighting, general low level lighting, backlighting of indoor or outdoor signage and other decorative uses. Additionally, photoluminescent articles of the present invention may be combined with other technologies for use in displays, electroluminescent devices and scintillation technology. 
         [0018]    These aspects of the invention and the advantages thereof will be more clearly understated from the following description and drawings of preferred embodiments of the present invention: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a partially enlarged view of a photoluminescent article according to an embodiment of the present invention; 
           [0020]      FIG. 2  is a partially sectioned view of a photoluminescent article according to another embodiment of the present invention; 
           [0021]      FIG. 3  is a partially sectioned view of a photoluminescent article according to still another embodiment of the present invention; 
           [0022]      FIG. 4  is a partially sectioned view of a photoluminescent article according to yet another embodiment of the present invention; 
           [0023]      FIG. 5  is a diagram illustrating a method of forming a photoluminescent tile according to an embodiment of the present invention; 
           [0024]      FIG. 6  is a diagram illustrating a method of forming a photoluminescent tile according to another embodiment of the present invention; 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Generally, an inorganic phosphorescent article of the present invention may be used in any location where it is desirous to have low level lighting such as a part of an emergency lighting system.  FIG. 1  generally illustrates an inorganic phosphorescent article  10  of the present invention which includes a formed phosphorescent layer  12 . Phosphorescent layer  12  is preferably made where the majority of the layer  12  comprises photoluminescent phosphors powders of rare earth doped alkaline earth aluminates or rare earth doped alkaline earth silicates or zinc sulfide doped with copper or combinations thereof. Some of these rare earth doped alkaline earth aluminates include formulations of strontium aluminate (SrAl 2 O 4 ), calcium aluminate (CaAl 2 O 4 ) and barium aluminate (BaAl 2 O 4 ) or mixed combination. Of the aluminates, strontium aluminate is preferred. Dopants for the aforementioned aluminate formulations include rare earth elements such as europium (Eu), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), dysprosium (Dy), holomiun (Ho), erbium (Er), thulium, (Tm), ytterbium (Yb), lutetium (Lu) or combinations thereof. Suitable commercially available photoluminescent phosphors powders are supplied under the trade name of Lumilux® by Honeywell US. Conventional ceramic powder processing techniques may be used to form phosphorescent layer  12  into a desired shape. These techniques include pressing, slip casting, tape casting, injection molding, screen printing, etc. Organic processing aids, such as binders, dispersants, defoamers, etc may be used during forming to assist in the process of forming. Phosphorescent layer  12  is then fired in a furnace at temperatures ranging between 1000 C. and 1600 C preferably in a controlled atmosphere. During the firing process the organic processing aids are volatilized and completely removed from the finished phosphorescent article  10 . The controlled atmosphere during the firing process may be provided by supplying air, or creating an inert atmosphere using Argon or a reducing atmosphere using Hydrogen. The controlled atmosphere during the firing process ensures that the valence state of the dopants within the photoluminescent phosphor can be maintained in their proper state to allow optimal phosphorescent properties of phosphorescent article  10 . 
         [0026]      FIG. 2  illustrates a cross sectioned view of an alternate embodiment of the present invention where a phosphorescent article  20  includes a substrate  22  bonded to a phosphorescent layer  24 . This type of construction for phosphorescent article  20  allows a thin phosphorescent layer  24  to be supported by substrate  22 . Substrate  22  is typically a ceramic composition formed by conventional ceramic processing techniques. Phosphorescent layer  24  contains no frit and is substantially formed of photoluminescent phosphor powders. Photoluminescent phosphor powders make up greater than 90% by weight of phosphorescent layer  24 . Phosphorescent layer  24  may be formed onto the surface of substrate  22  as a thin layer using conventional ceramic powder processing techniques such a tape casting, slip casting or silk screening. Alternatively, phosphorescent layer  24  may be formed into a thin sheet separately and then stacked on substrate  22 . Photoluminescent article  20  may then be fired in a furnace a temperature between 500 C and 1600 C in a controlled atmosphere. The preferred firing temperature phosphorescent layer  24  is between 1000 C and 1600 C to form a dense sintered layer adherent to substrate  22 . 
         [0027]      FIGS. 3 and 4  illustrate additional alternate embodiments of phosphorescent articles of the present invention having composite structures.  FIG. 3  shows an enlarged cross sectioned view of phosphorescent article  30  having a substrate  32 , a phosphorescent layer  34  and a wear layer  36  where the phosphorescent layer  34  is disposed between the substrate  32  and wear layer  36 . Substrate  32  is typically a ceramic composition formed by conventional ceramic processing techniques. Phosphorescent layer  34  preferably has a composition that does not contain a frit and is substantially formed of photoluminescent phosphor. Phosphorescent layer  34  may be formed onto the surface of substrate  32  as a layer using conventional ceramic powder processing techniques such a tape casting, slip casting or silk screening. Wear layer  36  may be a frit containing layer and is intended to provide desirable surface properties for the fired phosphorescent article  30 . These desirable surface properties may include matte, gloss, textured, anti-slip, decorative, colorants and abrasion resistance finishes or combinations thereof. The wear layer  36  may be formed onto the surface of phosphorescent layer  34 . Alternatively, phosphorescent layer  34  and wear layer  36  may be formed into sheets separately and then stacked on substrate  32 . The layers of phosphorescent article  30  may then be co-fired in a furnace at a temperature between 500 C and 1600 C in a controlled atmosphere. 
         [0028]      FIG. 4  shows an enlarged cross sectioned view of phosphorescent article  40  having a substrate  42 , a reflective layer  44 , a phosphorescent layer  46  and a wear layer  48  where phosphorescent layer  46  is disposed between reflective layer  44  and wear layer  48 . Substrate  42  is typically a ceramic composition formed by conventional ceramic processing techniques. Reflective layer  44  is designed to reflect light emitted from the phosphorescent layer  46  back through the phosphorescent layer providing a more pronounced and sustained phosphorescent effect. Suitable compositions for the reflective layer are those that reflect the wavelength of emitted light from a particular photoluminescent phosphor and include materials such as alumina (Al 2 O 3 ), titania (TiO 2 ) and mixtures thereof. Reflective layer  44  may be formed onto the surface of substrate  42  using conventional ceramic powder processing techniques. Phosphorescent layer  46  preferably has a composition that does not contain a frit and the majority of the layer is formed of photoluminescent phosphor. Phosphorescent layer  46  may be formed onto the surface of reflective layer  44  as a layer using conventional ceramic powder processing techniques such a tape casting, slip casting or silk screening. Wear layer  48  may be a frit containing layer and is intended to provide desirable surface properties for the fired phosphorescent article  40  and is preferably transparent. These desirable surface properties may include matte, gloss, textured, anti-slip, decorative, colorants and abrasion resistance finishes or combinations thereof. Alternatively, reflective layer  44 , phosphorescent layer  46  and wear layer  48  may be formed into sheets separately and then stacked on substrate  42 . The layers of phosphorescent article  40  may then be co-fired in a furnace at a temperature between 500 C and 1600 C in a controlled atmosphere. 
         [0029]      FIGS. 5 and 6  illustrate additional embodiments of the present invention wherein methods of forming composite phosphorescent tile are schematically shown.  FIG. 5  depicts a method of forming a composite structured phosphorescent tile  50  according to an embodiment of the present invention including the steps of providing a substrate  52  that takes the form of a tile body, a phosphorescent layer  54  disposed on said substrate  52  and a wear layer  56  disposed on said phosphorescent layer and the step of firing phosphorescent tile  50  at a temperature between 500 C and 1600 C. Substrate  52  is typically a ceramic composition formed by conventional ceramic processing techniques. Phosphorescent layer  54  has a composition that does not contain a frit and is substantially formed of photoluminescent phosphor. Phosphorescent layer  54  may be formed onto the surface of substrate  52  as a layer using conventional ceramic powder processing techniques such a tape casting, slip casting or silk screening. Phosphorescent layer  54  takes the form of a graphic and in particular the form of an arrow to indicate a directional path for someone to follow in the case of emergency signage. Phosphorescent layer  54  may take other forms including schematic designs and words such as “EXIT” to denote an exit location in the case of fire. Wear layer  56  may be a frit containing layer and is intended to provide desirable surface properties for the fired phosphorescent tile  50 . These desirable surface properties may include matte, gloss, textured, anti-slip, decorative, colorants and abrasion resistance finishes or combinations thereof. The wear layer  56  may be formed onto the surface of phosphorescent layer  54 . Alternatively, phosphorescent layer  54  and wear layer  56  may be formed into sheets separately and then stacked on substrate  52 . The layers of phosphorescent tile  50  may then be co-fired in a furnace at a temperature between 500 C and 1600 C in a controlled atmosphere. Some layers may be fired at a temperature between 500 C and 1600 C sequentially prior stacking all the layers for the aforementioned co-firing. For example, the phosphorescent layer  54  may be fired with the substrate  52  at a temperature between 1000 C and 1600 C in a controlled atmosphere prior to the addition of wear layer  56 . 
         [0030]      FIG. 6  depicts a method of forming a composite structured phosphorescent tile  60  according to an embodiment of the present invention including the steps of providing a substrate  62  that takes the form of a tile body, a reflective layer  64  disposed on substrate  62 , a phosphorescent layer  66  disposed on said reflective layer  64  and a wear layer  68  disposed on phosphorescent layer  66  and the step of firing phosphorescent tile  60  at a temperature between 500 C and 1600 C. Substrate  62  is typically a ceramic composition formed by conventional ceramic processing techniques. Reflective layer  64  is designed to reflect light emitted from the phosphorescent layer  66  back through the phosphorescent layer providing a more pronounced and sustained phosphorescent effect. Suitable compositions for the reflective layer are those that reflect the wavelength of emitted light from a particular photoluminescent phosphor and include materials such as alumina (Al 2 O 3 ), titania (TiO 2 ) and mixtures thereof. Reflective layer  64  may be formed onto the surface of substrate  62  using conventional ceramic powder processing techniques. Phosphorescent layer  66  preferably has a composition that does not contain a frit and the majority of the layer is preferably formed of photoluminescent phosphor. Phosphorescent layer  66  may be formed onto the surface of reflective layer  64  as a layer using conventional ceramic powder processing techniques such a tape casting, slip casting or silk screening. Phosphorescent layer  66  takes the form of a graphic and in particular the form of an arrow to indicate a directional path for someone to follow in the case of emergency signage. Phosphorescent layer  66  may take other forms including schematic designs and words such as “EXIT” to denote an exit location in the case of fire. Wear layer  68  may be a frit containing layer and is intended to provide desirable surface properties for the fired phosphorescent tile  60  and is preferably transparent. These desirable surface properties may include matte, gloss, textured, anti-slip, decorative, colorants and abrasion resistance finishes or combinations thereof. Alternatively, reflective layer  64 , phosphorescent layer  66  and wear layer  68  may be formed into sheets separately and then stacked on substrate  62 . The layers of phosphorescent tile  60  may then be co-fired in a furnace at a temperature between 500 C and 1600 C in a controlled atmosphere. Some layers may be fired at a temperature between 500 C and 1600 C sequentially prior stacking all the layers for the aforementioned co-firing. For example, the phosphorescent layer  66  may be fired with the substrate  62  and the reflective layer  64  at a temperature between 1000 C and 1600 C in a controlled atmosphere prior to the addition of wear layer  68 . 
         [0031]    As is apparent, there are numerous modifications of the preferred embodiments described above which will become readily apparent to one skilled in the art, such as many variations and modifications of the inorganic phosphorescent article including many different variations of the photoluminescent phosphors, many variations of methods of phosphorescent article construction and controlled atmospheres. These modifications would be apparent to those having ordinary skill in the art to which this invention relates and are intended to be within the scope of the claims which follow.