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BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to environmental seal technology for spaced transparent armor and, more particularly, to environmental seal technology incorporated into a double-paned window having an inner desiccant seal combined with a structural spacer. 
         [0002]    Spaced armor has been used for many years in opaque armor applications. The challenge for using it in transparent armor has been related to the environmental durability of the seal. The air between the glass laminate blocks must be kept clean and dry throughout the life of the armor. 
         [0003]    In the insulated glass market, two or more panes of glass are used, with an air space defined between the panes. The primary purpose of the air space is for insulation. The primary purpose for the air space in transparent armor is for improved ballistic protection, although the air space also improves the product&#39;s insulation. 
         [0004]    Several approaches have been used in the insulated glass industry to keep the air between the glass panes clean and dry. None of these approaches appears to use a seal having sufficient strength for environmental and mechanical durability in military applications. In the insulated glass industry, a seal of significant strength is not required. Thus, the primary seal is typically a low modulus elastomer. In a military vehicle application, the seal needs to be able to withstand substantial environmental and mechanical loading. 
         [0005]    A few approaches that use an air gap have been tried in ballistic applications. Many of these approaches have been designed for use in the periscope market. An example is U.S. Pat. No. 4,149,778. This patent states that the void between the spaced blocks is preferably filled with an inert gas such as dry nitrogen or may have a vacuum formed therein. Having a vacuum or inert gas in the gap, however, has been found to be cumbersome and costly in larger military applications, such as double-paned windows for military vehicles. 
         [0006]    Transparent armor for a passenger vehicle application is disclosed in U.S. Pat. No. 4,316,404. In this patent, a polycarbonate layer is bonded to a glass laminate using a double-sided tape, thus creating a thin air gap between the polycarbonate layer and the glass laminate. This approach, however, fails to use a seal having sufficient strength for environmental and mechanical durability in military applications, and does not address the issue of moisture within the air gap. 
         [0007]    It should thus be appreciated that there is a need for environmental seal technology for spaced transparent armor that combines a strong, durable seal with a means for keeping the air between the glass laminate blocks clean and dry. The system should function without the need for inert gas or a vacuum in the gap between the glass laminate blocks. The present invention fulfills this need and provides further related advantages. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is embodied in a spaced transparent armor structure comprising a desiccant system and a structural spacer. The subject invention solves the problem of moisture between the window panes of the spaced transparent armor structure by keeping the internal gap dry using the desiccant system. The invention also incorporates a durable structural spacer that will not rupture under normal military vehicle loads and environmental conditions. The spacer is bonded to the window laminates using pressure-sensitive adhesives, thus allowing for easy manufacture. 
         [0009]    In one embodiment, the spaced transparent armor structure comprises a first transparent laminate configured as a pane having a first face, a second face, and an edge; a second transparent laminate configured as a pane having a first face, a second face, and an edge; a structural spacer bonded to the second face of the first transparent laminate and to the first face of the second transparent laminate; and a desiccant. The first and second transparent laminates are spaced in a substantially parallel relationship so that an air gap is defined therebetween. The desiccant is positioned to absorb moisture trapped in the air gap. 
         [0010]    In one embodiment, the desiccant is contained in an inner desiccant seal that circumscribes the air gap and that extends between the second face of the first transparent laminate and the first face of the second transparent laminate. The inner desiccant seal comprises a polymer binder. The desiccant is supported in the polymer binder. The polymer binder is selected from the group consisting of silicone foam, ethylene propylene (EPM), ethylene propylene diene (EPDM) rubber, styrene-butadiene rubber (SBR), nitrile, and polyurethanes. The inner desiccant seal comprises at least twenty percent desiccant. 
         [0011]    In one embodiment, the structural spacer has an elastic modulus greater than 300 psi. The polymer binder has an elastic modulus greater than 200 psi and less than the elastic modulus of the structural spacer. The structural spacer circumscribes the inner desiccant seal and comprises a material selected from the group consisting of polyurethanes, polymethyl methacrylate, and metals. 
         [0012]    In one embodiment, the desiccant is embedded in the structural spacer. In another embodiment, the structural spacer is configured as a hollow tube. The desiccant is embedded within the hollow of the tube. 
         [0013]    In one embodiment, the spaced transparent armor structure further comprises a film adhesive. The structural spacer is bonded to the second face of the first transparent laminate and to the first face of the second transparent laminate by means of the film adhesive. The film adhesive is a pressure-sensitive tape adhesive, such as acrylic foam tape. 
         [0014]    In one embodiment, the spaced transparent armor structure further comprises a gasket bonded to the edge of the first transparent laminate and to the edge of the second transparent laminate by means of a sealant. The sealant is selected from the group consisting of silyl modified polymer sealants, urethane sealants, polysulide sealants, silyl-terminated-polyether sealants, acrylic sealants, and silicone sealants. The gasket extends from the second face of the first transparent laminate to the second face of the second transparent laminate. 
         [0015]    In one embodiment, the spaced transparent armor structure further comprises a urethane backfill that circumscribes the structural spacer and that extends between the second face of the first transparent laminate and the first face of the second transparent laminate. The urethane backfill additionally extends at least partially between the structural spacer and the second face of the first transparent laminate, and at least partially between the structural spacer and the first face of the second transparent laminate. The urethane backfill further extends around the edge of the first transparent laminate and the edge of the second transparent laminate. 
         [0016]    In one embodiment, the spaced transparent armor structure further comprises a frame into which the first transparent laminate and the second transparent laminate are potted. 
         [0017]    Other features and advantages of the invention should become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a cross-sectional view of a spaced transparent armor structure, in accordance with an embodiment of the present invention. 
           [0019]      FIG. 2  is a partial cross-sectional view of a spaced transparent armor structure having a desiccant-filled structural spacer, in accordance with an embodiment of the present invention. 
           [0020]      FIG. 3  is a partial cross-sectional view of a spaced transparent armor structure having a desiccant-filled structural spacer and a urethane backfill, in accordance with an embodiment of the present invention. 
           [0021]      FIG. 4  is a partial cross-sectional view of a spaced transparent armor structure having a solid structural spacer and inner desiccant seal, in accordance with an embodiment of the present invention. 
           [0022]      FIG. 5  is a partial cross-sectional view of a spaced transparent armor structure having a solid structural spacer, inner desiccant seal, and urethane backfill, in accordance with an embodiment of the present invention. 
           [0023]      FIG. 6  is a partial cross-sectional view of a spaced transparent armor structure having a pair of transparent laminates, a structural spacer, and a urethane backfill that extends partially between the structural spacer and the transparent laminates, in accordance with an embodiment of the present invention. 
           [0024]      FIG. 7  is a partial cross-sectional view of a spaced transparent armor structure having a pair of transparent laminates, a structural spacer, and a urethane backfill that extends around the edges of both transparent laminates as well as partially between the structural spacer and the transparent laminates, in accordance with an embodiment of the present invention. 
           [0025]      FIG. 8  is a partial cross-sectional view of a spaced transparent armor structure having an elastomer gasket, in accordance with an embodiment of the present invention. 
           [0026]      FIG. 9  is a partial cross-sectional view of a spaced transparent armor structure  180  having a pair of transparent laminates and a metal frame into which the transparent laminates are potted, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    With reference to  FIG. 1 , there is shown a cross-sectional view of a spaced transparent armor structure  10 , in accordance with an embodiment of the present invention. The spaced transparent armor structure comprises a pair of transparent laminates  12 , each transparent laminate configured as a substantially rectangular pane having an outer face  14 , an inner face  16 , and an edge  18 . The transparent laminates may comprise glass and/or other transparent or translucent materials. 
         [0028]    The inner faces  16  of the transparent laminates  12  are spaced in a substantially parallel relationship so that an air gap  20  is defined therebetween. The air gap is configured as a substantially rectangular space, the edges of which are bounded by an inner desiccant seal  22  that extends between the inner faces of the transparent laminates. Although  FIG. 1  shows the spaced transparent armor structure  10  as having two transparent laminates, the present invention encompasses spaced transparent armor structures having more than two transparent laminates. 
         [0029]    The inner desiccant seal  22  is a composition of a desiccant supported in a polymer binder. The desiccant absorbs moisture trapped between the inner faces  16  of the transparent laminates  12 . An advantage to this approach is that the spaced transparent armor structure  10  can be assembled without the need for an inert gas or vacuum, which gives this approach a cost advantage over other forms of fabrication. 
         [0030]    The desiccant material can be embedded within one of a number of elastomers to create a seal. For example, Super Spacer® Triseal™ from Edgetech I.G. of Cambridge, Ohio, is a desiccant embedded in silicone foam. Other suitable elastomers include ethylene propylene (EPM), ethylene propylene diene (EPDM) rubber, styrene-butadiene rubber (SBR), nitrile, chloroprene, Epichlorohydrin, polyacrylic, fluorosilicone, perfluroelastomers, polyether block polyamides, chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplastic elastomers, thermoplastic vulcanizates, thermoplastic polyurethane (TPU), thermoplastic olefins, and polysulfide rubber. The desiccant could additionally or alternatively be embedded within one of a number of plastics, including polypropylene, polystyrene, acrylonitrile/butadiene/styrene (ABS), polyethylene terephthalate, polybutylene terephthalate, polyester alloys, nylons, poly(vinyl chloride), polyurethanes, polycarbonate, polyethylene, polymethyl methacrylate, polytetrafluoroethylene, polyetheretherketone, polyetherimide, and phenolics. In one embodiment, the desiccant is embedded within a polymer having an elastic modulus greater than 200 psi and less than the modulus of the structural spacer (described below). 
         [0031]    Desiccants that may be used in the present invention include activated alumina, aerogel, benzophenone, bentonite clay, calcium chloride, calcium hydride, calcium sulfate, copper(II) sulfate, lithium chloride, lithium hydride, lithium bromide, magnesium, magnesium sulfate, magnesium perchlorate, sodium potassium alloy, phosphorus pentoxide, potassium carbonate, silica gel, sodium chlorate, sodium hydroxide, sodium sulfate, sodium benzophenone, and molecular sieves. In one embodiment, the inner desiccant seal  22  comprises twenty percent desiccant. 
         [0032]    The spacing between the pair of transparent laminates  12  is maintained by a structural spacer  24 , which surrounds the inner desiccant seal  22  and extends between the inner faces  16  of the transparent laminates. The structural spacer may comprise hard elastomers, composites, plastics, metals, and/or metal alloys. Suitable hard elastomers include ethylene propylene monomer (EPM) rubber, ethylene propylene diene monomer (EPDM) rubber, styrene-butadiene (SBR), nitrile, chloroprene, Epichlorohydrin, polyacrylic, fluorosilicone, perfluroelastomers, polyether block polyamides, chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplastic elastomers, thermoplastic vulcanizates, thermoplastic polyurethane (TPU), thermoplastic olefins, and polysulfide rubber. Suitable plastics include polypropylene, polystyrene, acrylonitrile/butadiene/styrene (ABS), polyethylene terephthalate, polybutylene terephthalate, polyester alloys, nylons, poly(vinyl chloride), polyurethanes, polycarbonate, polyethylene, polymethyl methacrylate, polytetrafluoroethylene, polyetheretherketone, polyetherimide, and phenolics. Suitable metals and metal alloys include iron, aluminum, copper, stainless steel, nickel, magnesium, zinc, and titanium alloys. In one embodiment, the material comprising the structural spacer has an elastic modulus greater than 300 psi. 
         [0033]    Composites, including fiber reinforced composites, would be suitable for lightweight structural spacers. Suitable fiber reinforced composites may comprise glass and/or carbon fibers, and matrices of epoxy, vinyl ester, polyester, phenolics, and/or polyimides. The structural spacer  24  also acts as a moisture barrier. Low permeability materials such as polyurethanes, polymethyl methacrylate, and metals are also suitable for the structural spacer. 
         [0034]    In one embodiment, the structural spacer  24  is an extrusion of any of the previously stated materials. The extrusion may comprise polymer, composite, elastomer, plastic, and/or metallic extrusions. A desiccant may be embedded within the extrusion. The structural spacer may have a solid cross-section or a hollow cross-section (hollow tube). 
         [0035]    The structural spacer  24  is bonded to the inner faces  16  of the transparent laminates  12  using a film adhesive, such as pressure-sensitive tape adhesive  26 . Bonding with a pressure-sensitive tape adhesive allows for a much quicker and cleaner application method than a paste adhesive, and the cleanliness of the pressure-sensitive tape adhesive eliminates a source of potential messes in the air gap  20 . A pressure-sensitive tape adhesive also has near immediate bond strength, thus allowing for a quicker and easier application method. Furthermore, a pressure-sensitive tape adhesive can act as an additional moisture barrier. Suitable pressure-sensitive tape adhesives include acrylic foam tapes, such as HyperJoint H8000 series (Nitto Denko Corporation of Osaka, Japan), HyperJoint H9000 series (Nitto Denko Corporation of Osaka, Japan) and Very High Bonding (VHB) tapes (3M Company of Maplewood, Minn.). Other suitable film adhesives include urethane, polyvinyl butyral (PVB), and epoxies. Although  FIG. 1  shows the use of a pressure-sensitive tape adhesive, the present invention encompasses the use of other adhesives, including paste adhesives. 
         [0036]    In one embodiment, a pressure-sensitive tape adhesive is not used, and the structural spacer  24  is instead bonded to the inner faces  16  of the transparent laminates  12  using a hot-melt adhesive. A hot-melt adhesive allows for good control of adhesive placement, without the need for cleanup. Suitable hot-melt adhesives include Jet-melt™ Adhesive by 3M Company of Maplewood, Minn. 
         [0037]    To mount the transparent laminates  12  into a vehicle, the transparent laminates can be potted into a frame, or a gasket  28  can be bonded to the edges  18  of the transparent laminates and the structural spacer  24  using a sealant, such as pressure-sensitive tape adhesive  26  and/or urethane adhesive  30 . Either of these methods adds another sealant layer between the air gap  20  and the outside environment, and thus further improves the environmental and mechanical durability of the spaced transparent armor structure  10 . Suitable sealants include silyl modified polymer sealants, urethane sealants, polysulfide sealants, silyl-terminated-polyether sealants, acrylic sealants, and silicone sealants. 
         [0038]    The gasket  28  is configured to extend around the edges  18  of both transparent laminates  12 , extending from the outer face  14  of one transparent laminate to the outer face of the other transparent laminate. In one embodiment, pressure-sensitive tape adhesive  26  is used to bond the gasket to the outer faces of the transparent laminates, while urethane adhesive  30  is used to bond the gasket to the edges  18  of the transparent laminates and the structural spacer  24 . The spaced transparent armor structure  10  may comprise a single gasket or a plurality of gaskets as needed to mount the transparent laminates into a vehicle. 
         [0039]    In some embodiments, a desiccant is embedded within the structural spacer extrusion, so that a separate inner desiccant seal is unnecessary. With reference to  FIG. 2 , there is shown a partial cross-sectional view of a spaced transparent armor structure  40  having a pair of transparent laminates  42 , an air gap  44  defined therebetween, and a desiccant-filled structural spacer  46  bonded to the transparent laminates using pressure-sensitive tape adhesive  48 , in accordance with an embodiment of the present invention. With reference to  FIG. 3 , there is shown a partial cross-sectional view of a spaced transparent armor structure  60  having a pair of transparent laminates  62 , an air gap  64  defined therebetween, a desiccant-filled structural spacer  66  bonded to the transparent laminates using pressure-sensitive tape adhesive  68 , and a urethane backfill  70 , in accordance with an embodiment of the present invention. The urethane backfill  70  surrounds the outer perimeter of the desiccant-filled structural spacer  66  and extends between the transparent laminates  62  to provide additional sealing for the air gap  64 . 
         [0040]    In other embodiments, a separate inner desiccant seal is used. With reference to  FIG. 4 , there is shown a partial cross-sectional view of a spaced transparent armor structure  80  having a pair of transparent laminates  82 , an air gap  84  defined therebetween, a solid structural spacer  86  bonded to the transparent laminates using pressure-sensitive tape adhesive  88 , and an inner desiccant seal  90  bonded to the solid structural spacer using pressure-sensitive tape adhesive, in accordance with an embodiment of the present invention. Wither reference to  FIG. 5 , there is shown a partial cross-sectional view of a spaced transparent armor structure  100  having a pair of transparent laminates  102 , an air gap  104  defined therebetween, a solid structural spacer  106  bonded to the transparent laminates using pressure-sensitive tape adhesive  108 , and an inner desiccant seal  110  bonded to the solid structural spacer using pressure-sensitive tape adhesive, and a urethane backfill  112 , in accordance with an embodiment of the present invention. The urethane backfill  112  surrounds the outer perimeter of the solid structural spacer  106  and extends between the transparent laminates  102  to provide additional sealing for the air gap  104 . 
         [0041]    In some embodiments, the urethane backfill extends partially between the structural spacer and the transparent laminates. With reference to  FIG. 6 , there is shown a partial cross-sectional view of a spaced transparent armor structure  120  having a pair of transparent laminates  122 , an air gap  124  defined therebetween, a structural spacer  126  bonded to the transparent laminates using pressure-sensitive tape adhesive  128 , an inner desiccant seal  130  bonded to the structural spacer using pressure-sensitive tape adhesive, and a urethane backfill  132  that extends partially between the structural spacer and the transparent laminates, in accordance with an embodiment of the present invention. Wither reference to  FIG. 7 , there is shown a partial cross-sectional view of a spaced transparent armor structure  140  having a pair of transparent laminates  142 , an air gap  144  defined therebetween, a structural spacer  146  bonded to the transparent laminates using pressure-sensitive tape adhesive  148 , an inner desiccant seal  150  bonded to the structural spacer using pressure-sensitive tape adhesive, and a urethane backfill  152  that extends around the edges of both transparent laminates as well as partially between the structural spacer and the transparent laminates, in accordance with an embodiment of the present invention. 
         [0042]    As noted above, the transparent laminates can be potted into a frame, or a gasket can be bonded to the edges of the transparent laminates and the structural spacer. With reference to  FIG. 8 , there is shown a partial cross-sectional view of a spaced transparent armor structure  160  having a pair of transparent laminates  162 , an air gap  164  defined therebetween, a structural spacer  166  bonded to the transparent laminates using pressure-sensitive tape adhesive  168 , an inner desiccant seal  170  bonded to the structural spacer using pressure-sensitive tape adhesive, and an elastomer gasket  172  configured to extend around the edges of both transparent laminates and bonded thereto with a urethane adhesive  174 , in accordance with an embodiment of the present invention. With reference to  FIG. 9 , there is shown a partial cross-sectional view of a spaced transparent armor structure  180  having a pair of transparent laminates  182 , an air gap  184  defined therebetween, a structural spacer  186  bonded to the transparent laminates using pressure-sensitive tape adhesive  188 , an inner desiccant seal  190  bonded to the structural spacer using pressure-sensitive tape adhesive, and a metal frame  192  into which the transparent laminates are potted using a urethane adhesive  194 , in accordance with an embodiment of the present invention. 
         [0043]    It should be appreciated from the foregoing disclosure that the present invention provides environmental seal technology for spaced transparent armor that combines a strong, durable seal with a means for keeping the air between the glass laminate blocks clean and dry, the seal functioning without the need for inert gas or a vacuum in the gap between the glass laminate blocks. 
         [0044]    Although the invention has been disclosed with reference only to the presently preferred embodiments, those of ordinary skill in the art will appreciate that various modifications can be made without departing from the invention. Accordingly, the invention is defined only by the following claims.

Summary:
The present invention is embodied in environmental seal technology incorporated into a double-paned window, the environmental seal technology comprising an inner desiccant seal and a structural spacer. The subject invention solves the problem of moisture between the window panes by keeping the internal gap dry using a desiccant system. The invention also incorporates a durable structural spacer that will not rupture under normal military vehicle loads and environmental conditions. The spacer is bonded to the window panes using pressure-sensitive adhesives, thus allowing for easy manufacture.