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CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. application Ser. No. 08/808,143 filed Feb. 28, 1997, now U.S. Pat. No. 5,960,606, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to penetration resistant windows, and more particularly relates to laminated window glass which resists damage from hurricanes and the like. 
     BACKGROUND INFORMATION 
     Hurricane damage to residential and commercial buildings is a major problem, particularly in coastal regions. For example, Hurricane Andrew which hit Florida in 1992 caused tens of billions of dollars in property damage. A large amount of hurricane damage to buildings is caused by penetration of the building envelope, allowing forceful winds and weather to penetrate or enter the interior of the building. In a typical hurricane scenario, high winds initially throw debris through a building window. The wind then enters through the broken window, increasing internal air pressure within the building. The increased internal pressure places extremely high stresses on the walls and roof, typically causing the remainder of the windows to blow out and, in some cases, the roof to blow off. Hurricanes also exhibit a push-pull effect which creates an alternating pressure differential inside and outside the building. 
     Recognizing the importance of maintaining window integrity, stringent codes and regulations have recently been enacted in an attempt to reduce hurricane damage. For example, certain regions of Florida have enacted codes which require windows to pass a flying missile and wind load test before an occupancy certificate will be granted. During the test, windows are impacted by a 9 pound 2 by 4 board travelling at 34 mph. After impact, the windows are subjected to alternating push-pull forces comprising 9,000 inward and outward pressure cycles every 1 to 3 seconds, thereby simulating the alternating forces experienced in a hurricane. The windows must withstand penetration during this test in order to meet the code. 
     One solution to hurricane-induced window damage is to fit metal storm shutters over the windows. While this approach meets the above-noted code, the shutters must be installed prior to a hurricane. If a building is unattended, or if a storm is not anticipated, the metal shutters may not be installed in time to adequately prevent damage. Furthermore, shuttering of unattended buildings sends a signal to potential burglars and vandals that a building is unoccupied. 
     Another approach for preventing hurricane damage which meets the above-noted code is the use of window glass laminated with several co-extensive polymeric layers. The polymeric layers include a polyvinyl butyral (PVB) layer attached to the window glass, a polyester middle layer, and an abrasion resistant coating. The composite layers are sold by Dupont under the designation SentryGlas. While window glass laminated with such composite layers meets the above-noted code, the exposed inner surface of the window is relatively susceptible to cuts, abrasions and scratches. Attempts have been made to replace the coextensive abrasion resistant coating with an inner layer of glass in order to improve abrasion resistance. However, windows incorporating such glass inner layers have not adequately prevented penetration, and have not been able to meet the above-noted code. 
     The present invention has been developed in view of the foregoing, and to overcome other deficiencies of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention provides a penetration resistant window including a frame, window glass constrained within the frame, a penetration resistant sheet substantially co-extensive with the window glass constrained within the frame, and a floating sheet supported on the penetration resistant sheet and substantially unconstrained by the frame. 
     The present invention also provides a sheet of laminated window glass including a layer of window glass, a penetration resistant sheet adhered to the layer of window glass and substantially co-extensive therewith, and a floating sheet adhered to the penetration resistant sheet having a perimeter that is set back from the perimeter of the window glass layer. 
     The present invention will more readily understood from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially schematic sectional perspective view showing a conventional penetration resistant window. 
     FIG. 2 is a partially schematic sectional perspective view showing a penetration resistant window in accordance with an embodiment of the present invention. 
     FIG. 3 is a partially schematic front view of penetration resistant window glass in accordance with an embodiment of the present invention. 
     FIG. 4 is a partially schematic sectional perspective view illustrating the penetration resistance of a window in accordance with an embodiment of the present invention during the application of alternating inward and outward pressure. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 schematically illustrates a conventional penetration resistant window  10  including a window frame  12 . A sheet of window glass  14  is secured about its perimeter by the frame  12 . Multiple polymeric layers are laminated on the window glass  14  such that the layers are co-extensive with the window glass and secured about their perimeters by the frame  12 . The polymeric layers include a polyvinyl butyral (PVB) layer  16 , a polyester (PET) layer  17  and an abrasion resistant coating layer  18 . The layers  16 ,  17  and  18  form a composite laminated sheet commercially available from Dupont under the designation SentryGlas. 
     The conventional penetration resistant window shown in FIG. 1 is capable of meeting stringent code requirements. In particular, such windows pass the Florida flying missile and wind load test which involves projecting a 9 pound 2 by 4 board at the window at 34 mph, followed by 9,000 inward and outward pressure cycles. While prior art windows as shown in FIG. 1 exhibit extremely good penetration resistance, the inner surfaces of such windows are susceptible to abrasion, scratching and cutting. Although an abrasion resistant coating  18  is provided on the interior side of the windows, the coating does not adequately protect against many types of abrasions, cuts and scratches. 
     FIG. 2 schematically illustrates a penetration resistant window  20  in accordance with an embodiment of the present invention. The window  20  may be used in architectural, residential and institutional applications. A window frame  22  secures a sheet of window glass  24  about its perimeter. A penetration resistant sheet  25  is laminated on the window glass  24  such that the penetration resistant sheet is substantially co-extensive with the window glass. Both the window glass  24  and penetration resistant sheet  25  are secured about their perimeters within a channel of the window frame  22 . The glass  24  and penetration resistant sheet  25  are secured within the frame  22  by any suitable means such as adhesives, mechanical pressure, fasteners or the like. In a preferred embodiment, silicone glue is used to secure the glass  24  and penetration resistant sheet  25  within the frame  22 . 
     The window glass  24  may be of any suitable thickness, typically from about {fraction (1/16)} to about 1 inch. The window glass  24  may be made of any suitable material including glasses such as annealed, heat strengthened or tempered glass. A particularly preferred window glass  24  comprises annealed glass having a thickness of from about ⅛ to about ⅜ inch. While the use of glass is preferred, the window glass  24  may alternatively comprise other materials such as polycarbonate or other types of polymers in accordance with the present invention. The window glass  24  preferably comprises a single layer, but may alternatively include multiple layers. 
     The penetration resistant sheet  25  is preferably made of an ionoplast polymeric material having satisfactory penetration resistance and light transmission characteristics. The ionoplast penetration resistant sheet  25  may comprise ethylene/methacrylic acid copolymers, and may have a thickness of from about 1 to about 200 mils. A suitable ionoplast penetration resistant sheet is sold by Dupont under the designation SentryGlas Plus. Single or multiple layers of the ionoplast material may be used to form the penetration resistant sheet  25 . Alternatively, the penetration resistant sheet may comprise PVB laminates commercially available from Solutia and DuPont, or PET laminates available from 3M. 
     As shown in FIG. 2, a floating sheet  32  is supported on the penetration resistant sheet  25  but is not co-extensive therewith. Instead of extending into the frame  22  with the penetration resistant sheet  25  and window glass  24 , the floating sheet  32  has a perimeter which is set back from at least a portion of the perimeter of the window glass and penetration resistant sheet. The floating sheet  32  is thus substantially unconstrained by the frame  22 . The surface of the floating sheet  32  preferably extends across substantially the entire sight line of the window  20 . The gap between the edge of the floating sheet  32  and the frame  22  may optionally be filled with caulking or the like. 
     The floating sheet  32  may be of any suitable thickness, typically from about {fraction (1/32)} to about ½ inch. The floating sheet  32  may be made from any suitable material having sufficient abrasion resistance. The floating sheet  32  is preferably made of glass such as annealed, heat strengthened or tempered glass. However, other materials such as hard polymers may be used. The floating sheet  32  may also include multiple layers. For example, the floating sheet  32  may comprise multiple layers of plastic that can be individually removed from the window as desired. In many applications, the floating sheet  32  is substantially clear. Alternatively, colored, etched, roller coated, spray painted or silk screened floating sheets may be used. In a preferred embodiment, the floating sheet  32  comprises annealed glass having a thickness of from about {fraction (1/16)} to about ⅜ inch. Additional thermal insulation layers may be mounted adjacent the floating sheet  32  or adjacent the window glass  24 . 
     In the embodiment shown in FIG. 2, an adhesive layer  34  is used to mount the floating sheet  32  on the penetration resistant sheet  25 . The adhesive layer  34  is preferably substantially co-extensive with the floating sheet. Any suitable type of adhesive layer  34  may be used, provided that it adequately secures the floating sheet  32  to the penetration resistant sheet  25  and does not decrease light transmission through the window  20  to an undesirable extent. The adhesive layer may comprise polyurethane, PVB or silicone, with polyurethane being preferred in many applications. 
     FIG. 3 is a front perspective view showing a sheet of laminated window glass in accordance with a preferred embodiment of the present invention. The window glass  24  and penetration resistant sheet  25  are substantially co-extensive. The floating sheet  32  is set back a distance D from the window glass  24  and penetration resistant sheet  25 . The set back distance D may be adjusted depending on the depth of the window frame into which the laminated window glass is to be installed. The set back distance D typically ranges from about {fraction (1/16)} to about 2 inch, preferably from about ¼ to about 1 inch. For many window frames, a set back distance of about ½ inch is preferred. The floating sheet  32  is preferably set back from the window glass  24  and penetration resistant sheet  25  around its entire perimeter. The set back may also act to align the window glass  24  in the desired location during installation in the frame  22 . The set back distance D may be substantially uniform around the perimeter of the floating sheet  32 , or may be varied. The set back distance D is preferably selected such that the floating sheet  32  substantially covers the window sight line when the laminated window glass is mounted in the window frame. 
     FIG. 4 schematically illustrates a window of the present invention similar to that shown in FIG. 2 subjected to alternating inward and outward pressure typical of the pressures encountered during a hurricane. In a typical hurricane scenario, flying debris would impact the window glass  24  causing it to break into many pieces as shown in FIG.  4 . The penetration resistant sheet remains intact upon impact by the flying debris. The impact of the flying debris may also fracture the floating sheet  32 . However, the adhesive layer  34  secures many of the broken pieces of the floating sheet  32  to the penetration resistant sheet  25   a . In this manner, upon impact by the debris, flying pieces of the floating sheet  32  are reduced or eliminated. After impact, inwardly applied pressure causes the penetration resistant sheet  25   a  to deform inwardly as shown in FIG. 4. A reversal from inward to outward pressure causes the penetration resistant sheet to deform outwardly as shown in phantom at  25   b . For purposes of clarity, the outwardly deformed penetration resistant sheet  25   b  is shown without the layer of window glass  24 , floating sheet  32  and adhesive layer  34  attached thereto. After repeated cycling between inward and outward pressure, the penetration resistant sheet  25   a ,  25   b  remains substantially intact. The window of the present invention is thus resistant to penetration when subjected to forces similar to those experienced during a hurricane. 
     The laminated window glass of the present invention may preferably be fabricated in the following manner. A first sheet of window glass  24  is cleaned and a penetration resistant sheet  25  is laid thereon. A smaller sheet of adhesive material  34  is placed on the penetration resistant sheet  25  such that a set back is formed. A floating sheet  32  is then laid on the adhesive sheet  34  with a set back from the perimeter of the window glass  24 . Heat and pressure are applied to at least partially liquify the penetration resistant sheet  25  and the adhesive sheet  34 . The resultant product comprises a laminated composite. 
     The following example is intended to illustrate various aspects of the present invention, and is not intended to limit the scope thereof. 
     EXAMPLE 
     A sheet of laminated window glass is formed as follows. A sheet of annealed window glass having a thickness of ¼ inch, a height of 48 inches and a width of 48 inches is cleaned and laid flat. Next, a sheet of Dupont SentryGlas Plus penetration resistant ionoplast material is cut to the same size as the first sheet of window glass. The penetration resistant laminate sheet is placed against the glass sheet. Next, a sheet of polyurethane having a thickness of 25 mils is step cut to 47 by 47 inches to the size of the sight line of a window frame opening and placed on top of the penetration resistant sheet. A sheet of annealed glass having a thickness of ⅛ inch is then step cut to a 47 by 47 inch window sight line and centered on the other layers. This lay up is then placed in a film type bag and the air is vacuumed out. The bag containing the lamination composition is then placed into a vessel and pressurized at about 150 psi at a temperature of about 280° F. for 2.5 hours. After this process is completed, the laminated composite window glass is removed. The resultant laminated composite window is mounted n a frame as shown in FIG.  2 . 
     While particular embodiments of the present invention have been described herein, it is to be understood that various changes, additions, modifications and adaptations may be made without departing from the scope of the invention, as set forth in the following claims.

Summary:
A penetration resistant window includes a sheet of window glass having a penetration resistant layer of ionoplast material adhered thereto. A floating sheet made of hard material such as glass is adhered to the penetration resistant layer in order to provide abrasion resistance. The perimeter of the floating sheet is set back from the perimeter of the window glass. This set back allows the window to be mounted in a window or door frame such that the perimeters of the window glass and penetration resistant sheet are secured to the frame, but the floating sheet is unconstrained by the frame. The penetration resistant window is useful in architectural, residential and institutional applications for resisting debris penetration during hurricanes.