Abstract:
A blast-resistant barrier comprising a plurality of units each including a panel having a thickness of greater than 20 to less than 40 millimeter is disclosed. The panel is in the form of a monolithic polycarbonate sheet or laminate that is positioned vertically between the source of a blast and the blast target, the laminate including at least two polycarbonate sheets and an optional image layer interposed therebetween. The panel is fixedly attached to a frame which is firmly embedded in concrete in a manner calculated to provide stiffness sufficient to absorb and withstand external forces resulting from said blast. 
     In a preferred embodiment the panel includes at least two polycarbonate sheets laminated one to the other, optionally including an image layer interposed therebetween. In an additional embodiment, the frame is anchored securely to the target enabling dissipation of the blast force through the target&#39;s structure. The height of the blast-resistant barrier is preferably proportional to the height of the target.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a blast-resistant barrier and in particular to a barrier comprising at least one polycarbonate panel. 
       TECHNICAL BACKGROUND 
       [0002]    Government and commercial buildings (e.g., hotels, casinos, malls, airports and stadiums) have proven attractive targets for bombing attacks throughout the world. The attacker, in most cases, is a politically motivated terrorist using, as a weapon, a high explosive device transported and detonated inside a vehicle adjacent to the targeted building. The explosive device carried in such vehicles is typically capable of generating a shock wave of sufficient force as to shear the face off unprotected buildings, leading to tremendous loss of life and property damage. The resulting debris field surrounding the building is often several feet thick blocking entrances. In addition, glass remnants dangle precariously, potentially falling from great heights to the ground in the slightest breeze. Consequently, both hazards hinder and threaten the safety of emergency response teams as they attempt to enter the damaged building to render aid to the injured. 
         [0003]    The simplicity and stealth of vehicular weapons make them a complex foe. It&#39;s virtually impossible to screen all the cars and trucks that rumble past critical buildings. Defending against such an explosive device involves keeping such vehicles at a distance from vulnerable targets, often using Jersey barriers, blocks, bollards and other concrete structures (U.S. Pat. Nos. 7,144,186 and 6,767,158, and U.S. Patent Application 2004/0261332). This can be difficult where public roads pass immediately outside these structures. Closure of roads, or protecting buildings with concrete barriers is not always practical, it can be unsightly and is generally undesirable. 
         [0004]    Existing buildings rarely have blast resistant construction and thus much emphasis has been placed on retrofits for windows to mitigate glass hazards. The use of so-called safety glazing or penetration-resistant glazing for windows, using multiple layers of polycarbonate, glass, and other resinous materials is well known. For example, glass-polycarbonate resin laminates adhering together with ethylene-vinyl copolymers are described in U.S. Pat. No. 3,666,614. In U.S. Pat. No. 3,520,768, there are described laminates of relatively thick glass having a comparatively thin polycarbonate foil as the adhering material. Also relevant is U.S. Pat. No. 4,027,072 that disclosed certain polysiloxane-polycarbonate block copolymers as an adhesive in preparing polycarbonate containing laminates. U.S. Pat. No. 3,624,238 concerns a bullet resistant laminated structure that includes outer faces or plies of safety glass with an intermediary ply formed of a polycarbonate resin. U.S. Pat. No. 4,312,903 deals with an impact resistant double glazed structure made of glass and polycarbonate and is concerned in particular with the thickness of the layers of the laminated window panes, and their chemical compositions. 
         [0005]    U.S. Pat. No. 5,059,467 is concerned with a protective ballistic panel including a first-impact, front layer and a second rear layer, the layers being spaced from one another by a semi-elastic material, defining a sealed space. The panel is used as a personnel protective shield. 
         [0006]    U.S. Pat. No. 6,266,926 describes a flexible apparatus that is deployed by inflating a protective barrier adjacent to windows to reduce the quantity of debris hazard in the event of an explosion. U.S. Pat. No. 6,349,505 discloses a louver system mounted adjacent to the inside and/or outside of a glass window and reinforced using high elongation cables or straps attached to the floor and ceiling. The louver system would immediately close upon detection of an explosion, reducing the quantity of debris hazard in the building. 
         [0007]    U.S. Pat. No. 4,625,659 disclosed a bullet and explosion proof window or door system comprising two spaced apart panels, whereby the outer panel is spaced from a support soffit such that a gap is formed for providing a ventilation channel. However, peripheral portions of the panels are fitted with a security layer in order to prevent projectiles from entering through the ventilation gap. U.S. Pat. Nos. 6,177,368 and 4,642,255 disclosed blast-resistant panels produced from PVC and woven fiberglass, and polyvinyl acetal, glass and a fibrous layer encapsulated in the polyvinyl acetal layer. U.S. Pat. No. 3,191,728 disclosed a barrier consisting of welded metal strips, as protection for workers in aircraft parking areas from the exhaust of jet engines. 
         [0008]    U.S. Pat. No. 5,277,952 disclosed a decorative, cracked mirrored, glass panels created from glass bonded together with a polymeric interlayer. U.S. Pat. Nos. 5,643,666, 5,894,048, 5,958,539, 5,998,028 and 6,025,069 disclosed panels consisting of laminated copolyester sheets and containing decorative interlayers and high relief surfaces. 
         [0009]    Retrofits to protect building facades have traditionally involved strengthening of walls. To be truly effective, wall-strengthening is often an invasive operation which adversely affects the appearance of the structure and impacts building operations. It is, therefore, desirable to have a structure that is unobtrusive, easy to install, and at the same time protective of the entire building from the devastating effects of a vehicular bombing attack. 
       SUMMARY OF THE INVENTION 
       [0010]    A blast-resistant barrier comprising a plurality of units each including a panel having a thickness of greater than 20 to less than 40 millimeter is disclosed. The panel is in the form of a monolithic polycarbonate sheet or a laminate that is positioned vertically between the source of a blast and the blast target, the laminate including at least two polycarbonate sheets and an optional image layer interposed therebetween. The panel is fixedly attached to a frame which is firmly embedded in concrete in a manner calculated to provide stiffness sufficient to absorb and withstand external forces resulting from said blast. 
         [0011]    In a preferred embodiment the panel includes at least two polycarbonate sheets laminated one to the other, optionally including an image layer interposed therebetween. In an additional embodiment, the frame is anchored securely to the target enabling dissipation of the blast force through the target&#39;s structure. 
         [0012]    The height of the blast-resistant barrier is preferably proportional to the height of the target. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    The inventive panel comprise at least one monolithic, preferably two or more superposed polycarbonate sheets that are laminated and/or adhesively bonded one to the other to form a laminate. 
         [0014]    The inventive panel may optionally include at least one image layer in the form of wood, stone, glass, textile, metal, paper, plastic, plants, flowers or vegetation and their products and each of these may be of any color. The image layer may be laminated to or between any two of the layers. The thickness of the panel is in the range of 20 to 40 millimeters. 
         [0015]    In the embodiment where the panel includes a laminate it is preferred that it includes a first polycarbonate sheet 10 to 20, preferably 12-18 millimeter (mm) in thickness, a second polycarbonate sheet 10 to 20, preferably 12-18 mm in thickness and at least one image layer interposed between the first and second sheets. Other embodiments entail a plurality of polycarbonate sheets, typically three of four sheets of identical thicknesses or differing thicknesses. 
         [0016]    The several sheets making up the inventive panel may be bonded one to the other by lamination or by the use of an adhesive. A suitable adhesive layer includes 0.025″ thick A4700 Dureflex polyurethane film, a product of Deerfield Urethane. It is imperative that the adhesive be sufficiently heat resistant to withstand the thermal conditions encountered in lamination without degradation and distortion. Naturally, in circumstances where transparency of the panel is desired, the adhesive must be transparent. 
         [0017]    In one embodiment of the invention, the panel may be prepared by (a) providing a first polycarbonate sheet having a thickness of 10 to 20 mm; and (b) providing a second polycarbonate sheet having a thickness of 10 to 20 mm; and (c) placing at least one image layer between the first and second sheets to form a sandwiched structure and (d) pressing the structure at elevated temperature for a time sufficient to form a laminate. Suitable thermal conditions are generally 18 to 249° C., preferably 32 to 227° C. under pressure of 69 to 2069, preferably 448 to 662 kPa, for a time at maximum temperature and pressure of 0.1 to 20 preferably 0.1 to 5 most preferably 0.17 to 3 minutes. Temperatures exceeding 249° C. and pressures exceeding 2070 kPa are undesirable in hot press bonding since the sheet layers may squeeze out of the aligned image layer. It is preferred to apply pressure before the application of heat. Optionally the laminate thus formed may be cooled at pressure between 7 and 2065 kPa. In yet an additional embodiment the inventive laminate further includes a protective hard-coat layer. 
         [0018]    Importantly, the first and second sheets are not necessarily the outermost sheets of the inventive panel. As noted above the panel may contain a plurality of sheets (layers) on each side of the image layer as well as several image layers. It is however required that the total thickness of the panel be greater than 20 and less than 40 mm. The panel is preferably 4 feet wide and 8 feet long but these are not limiting dimensions. 
         [0019]    The polycarbonate sheets independently may be transparent, translucent, or opaque. Moreover the sheets may differ one from the others in their respective degrees of transparency or translucency and color. 
         [0020]    Polycarbonate is well known thermoplastic, aromatic polymeric resin (see German Offenlegungsschriften 2,063,050; 1,561,518; 1,570,703; 2,211,956; 2,211,957 and 2,248,817; French Patent 1,561,518; and in particular the monograph by H. Schnell, “Chemistry and Physics of Polycarbonates”, Interscience Publishers, New York, N.Y., 1964, which is incorporated herein by reference). The polycarbonate suitable in the context of the invention has weight average molecular weight of 8,000 to 200,000, preferably up to 80,000 and an intrinsic viscosity of 0.40 to 1.5 dl/g as measured in methylene chloride at 25° C. Preferably, the glass transition temperature of polycarbonates ranges from 145 to 148° C. 
         [0021]    Polycarbonate sheets suitable in the context of the invention are available in commerce. Preferable for their good mechanical properties and excellent transparency are sheets made of a homopolycarbonate based on bisphenol A. 
         [0022]    Such suitable sheets are available under the MAKROLON trademark from Sheffield Plastics Inc.; a Bayer MaterialScience company. 
         [0023]    The image layer(s) preferably includes fabric, metallic wire, rod and/or bar, papers or photographic images, and vegetation, such as grasses, flowers, wheat, and thatch. The image layer may display images or designs or may be of a solid color and should be sufficiently thermally resistant, e.g. of sufficiently high melt temperature to avoid any degradation or distortion of the image during the manufacture or processing of the panel. Preferably, the image layer(s) are substantially continuous. The thickness of the image layer is advantageously 0.0254 to 1.524 mm, preferably 0.0254 to 0.05 mm, and is most preferably 0.04 mm. However, polymeric films thinner or thicker may be used in the decorative image layer depending on the equipment available, and under such conditions the thickness is limited only by functionality. 
         [0024]    In a preferred embodiment the panel includes at least one first image layer positioned between the first and the second polycarbonate sheet and at least one second image layer positioned between the second and the third polycarbonate sheet. 
         [0025]    In one embodiment of the present invention, the image layer comprises a fabric of textile fibers. The fabric may display images or designs produced, e.g., by weaving or knitting techniques, in the fabric. The fabrics may be textile fibers, (i.e., fibers of natural-occurring, semi-synthetic or synthetic polymeric materials). For example, the fabrics may be prepared from cotton, wool, silk, rayon (regenerated cellulose), polyester such as polyethylene terephthalate, synthetic polyamides such as nylon 66 and nylon 6, acrylic, methacrylic, and cellulose acetate fibers. The melting point of the textile fibers should be sufficiently high to avoid any degradation or distortion of the fabric during the manufacture or processing of the laminate of the invention. 
         [0026]    The fabric may be woven, spun-bonded, knitted, or prepared by other processes well known in the textile trade and may be uncolored, e.g., white, or colored by conventional dyeing and printing techniques. Alternatively, the fabrics may be produced from dyed yarn or from filaments and yarn derived from mass colored polymers. Preferably, the fabrics present within the decorated laminate structure are substantially continuous and constitute a distinct image layer or laminate. In an embodiment of the invention, the image layer comprises metallic wire, rod, or bar. The metal wire may be formed by a variety of techniques to produce metal mesh fabric, screens, or open mesh having high transparency. The metal wire, rod or bar may be woven, welded, knitted, or fabricated by means of other processes well known in the metal wire fabrication art. The metallic wire, rod and bar may be of any color. The metallic element of the image layer may be of different metallic materials such copper, aluminum, stainless steel, steel, galvanized steel, titanium, etc. or combinations thereof. The metallic component of the image layer may be prepared from wire filaments, rods and bars having various cross-sectional areas and geometries, e.g., generally circular, oval or relatively flat. The thickness or diameter of the wire, rod and bar is not critical. It is however critical that the metallic surfaces are smooth so as avoid creating of propagating cracks that may weaken the panel. Hence, embedding the metallic surfaces in a polymeric material, such as polyvinyl chloride, copolyester or polyurethane, may be advantageous. The only requirement relative to this embodiment is that the embedding polymeric materials have sufficient heat resistance so as not to be thermally degraded or distorted by the panel lamination and forming processes. 
         [0027]    In an additional embodiment the panel may comprise an image layer of wire, rod, or bar that reinforce the polycarbonate. In further embodiment, the image layer comprises a printed or colored image. Preferably, the printed or colored image layer has opposed surfaces wherein an image is printed on one of the surfaces and/or the decorative image layer contains coloration. More than one printed or colored decorative image layer may be used in the decorated laminate structure of the present invention. The use of multiple decorative image layers may provide a 3-dimensional or “floating” appearance to the decorative images or lettering in the printed or colored image layers. Each of the printed or colored image layers is joined to a first sheet on one of its surfaces such that the image or coloration may be viewed through the first sheet without significant distortion. The printed or colored image layer may comprise any suitable polymeric material which is compatible with the materials used for the first and second sheets, inks, or other materials used in fabricating the laminate. Preferably, the image layer comprises polyvinylchloride, copolyester, polycarbonate or polyurethane thermoplastic. 
         [0028]    In another embodiment, the image or coloration is printed on the bottom side of the image layer in which case the polymer used to prepare the image layer is transparent. 
         [0029]    The printed image may be prepared according to conventional photographic printing processes or with a digitized database generated from a photographic image. Digitizing and storing the image may be accomplished through any of a number of processes well known in the computer art such as scanning. 
         [0030]    In yet another embodiment, the image layer comprises vegetation, such as grasses, thatch, flowers, for example rose petals, wheat, grains, natural papers and others, such that the natural color of vegetation is preserved. More than one image layer comprising vegetation may be used in the decorated laminate structure of the present invention. The use of multiple image layers may provide a 3-dimensional or “floating” appearance to the decorative vegetation in the image layers. Each of the image layers is joined to a first sheet on one of its surfaces such that the vegetation can be seen through the first sheet without significant distortion. 
         [0031]    The laminate structure may optionally comprise a protective hard-coat layer, which is a transparent, hard, scratch-resistant or abrasion resistant coating or layer laminated to the top surface of the first sheet. Such coating may also increase the chemical resistance of the laminate and provide an anti-graffiti surface. The protective layer may be a bi-layer film comprising a protective layer on top of a sheet layer. The protective layer is preferably selected from the UV-cured or electron-beam-cured crosslinked acrylic, vacuum-cured or UV-cured urethane, UV-cured or electron-beam-cured silicon with acrylic or heat cured urethane or plastisol. A layer of polyurethane may be applied over the exterior surface to provide abrasion resistance. Alternatively, a biaxially oriented polyethylene terephthalate, such as MYLAR® or a TEFLON® film, such as TEDLAR® both available from DuPont Chemical Company, may be laminated to the top surface of the first sheet as a protective layer. More preferably, the protective layer comprises a thermal-cured, UV-cured or electron-beam-cured silicon to achieve glass appearance. 
         [0032]    Lamination of the inventive panel is conventional. In one laminating method a plywood laminating press that features efficient heat transfer and even distribution of heat is preferably used. 
         [0033]    To augment the reduction in pressure, a vacuum may be applied in order to remove trapped air between the layers. During the bonding process, if necessary, the polycarbonate materials may be bonded or fused together with the use of adhesive. 
         [0034]    Preferably, the laminating method comprises hot press bonding or cold press bonding. As is well known, hot press bonding methods include, but are not limited to, hot steam, electric heat, hot oil heated and other methods known in the art. Cold press bonding methods include, but are not limited to, cold water and glycol cooled method. The lamination may be performed either with or without a vacuum press. Generally, the formation of bubbles in the laminated panel is less likely if the air is evacuated prior to applying heat and pressure. In any event it is critical that sufficient pressure is applied to rid the system of air prior to bonding. Following the hot press bonding, the bonded structure is allowed to cool by being held at 10 to about 148° C. (50° F. to about 298° F.), preferably 21.1 to 32.2° C. (70 to 90° F.) and pressure of 7 to 2069 preferably 448 to 662 more preferably 552 to 662 most preferably 634 kPa until it cools below the glass transition temperature of the polycarbonate. Optionally, in the course of press bonding texture may be applied to one or both surfaces of the panel. 
         [0035]    The frame to which the panel is fixedly attached is preferably made of carbon steel, i.e. steels having up to about 2 percent carbon content, stainless steel or aluminum. For increased durability and aesthetic appeal, frames of carbon steel may be treated with corrosion resistant coatings and/or paints. Stainless steels are preferred for outdoor applications because they are more resistant to rusting and staining than carbon and low alloy steels, thus maintaining their aesthetic appeal. It is imperative that in the instances where the image layer is capable of absorbing moisture, the edges of the panel are sealed to prevent wicking. Suitable sealing may be by the application of silicone or by gluing to the edge a thin polymeric film, e.g. polycarbonate film. 
         [0036]    The steel frame comprise shaped members (e.g., a “C” cross section shaped members) providing sufficient stiffness and strength to absorb the external forces applied by the blast without major distortion. The frame may be extended vertically at its bottom so that the extensions can be embedded in reinforced concrete foundation. As an alternative, the steel frame may be attached to the steel skeleton of the target (e.g. building) in a manner to dissipate the shock wave. 
         [0037]    The panel may be attached to the frame by either a structural adhesive or by a plurality of bolts. The bolts, preferably shoulder bolts, are 0.75 to 1.25 inches, preferably 1.0 inch in diameter, with flat heads so that upon tightening, the bolt head and nut place the area of the panel around the bolt hole in compression without creating cracks or notches. The bolts may be spaced 4 inches to 8 inches, preferably 6 inches, apart and offset approximately 1.0 inch to 1.5 inches from the panel edge. The bolt holes in the panel are preferably produced with smooth, elongated edges to allow for thermal expansion and to mitigate stress. Rubber or elastomeric washers or spacers may be used between the panel and frame to further absorb impact energy and dampen forces transmitted to the building. 
         [0038]    The mechanical properties for the “C” section steel channels preferably exhibit a final yield strength in tension of approximately 300 MPa. Otherwise, for higher or lower modulus materials such as aluminum, equivalent section properties are preferably followed through use of thicker or thinner walls. Overall, the inventive panel is preferably placed at a distance of at least 12 inches from the surface of the protected target to avoid the polycarbonate panels striking the building while bending as a result of being hit with the shock wave resulting from a blast. Shorter distances may be used for lower threat levels or smaller panels. 
       EXAMPLES 
     Example 1 
       [0039]    A panel in the form of a laminate that includes a colored textile was prepared. A hot press platen was preheated to 475° F. The cold press platen temperature was set at 65° F. Next the following were assembled in the following order (top to bottom): steel press plate, Nomex pad (Nomex pressure distribution pads), or another suitable medium to attain even distribution of pressure, aluminum separation plate, release paper (patina finish Ultra-cast release paper), 0.060″ polycarbonate sheet, image layer in the form of textile (sheer nylon textile), 0.060″ polycarbonate sheet, release paper, aluminum separation plate, Nomex pad, steel press plate. 
         [0040]    A thermocouple was inserted in-between the first sheet of polycarbonate and the textile. The assembly was then inserted in the hot press, the press was closed and the pressure was increased to 94 psi. The temperature was closely monitored until the thermocouple read 420° F. Once the temperature was reached, the pressure was released and the press opened. The assembly was then transferred to the cold press set to cold press platen temperature of 65° F. Next, the pressure in the cold press was increased to 94 psi. This transfer and re-pressurizing was completed in less than 3 minutes. The temperature was closely monitored until the thermocouple read 90° F. at which point the decorated laminate structure was removed from the press. 
       Example 2 
       [0041]    An additional panel in the form of a laminate was prepared. A hot press platen was preheated to 475° F. The cold press platen temperature was set at 65° F. Next the following were assembled in the following order (top to bottom): steel press plate, Nomex pad (Nomex pressure distribution pads), aluminum separation plate, release paper (patina finish Ultra-cast release paper), hard coated polycarbonate film (0.005″ thick film), oriented with the hard-coat against the release paper, 0.118″ polycarbonate sheet, image layer in the form of textile, 0.118″ polycarbonate sheet, release paper, aluminum separation plate, Nomex pad, and steel press plate. The “hard coat” used was a flexible aliphatic polyurethane coating. 
         [0042]    A thermocouple was inserted in-between the first sheet of polycarbonate and the textile. The assembly was then inserted in the hot press, the press was closed and the pressure was increased to 94 psi. The temperature was closely monitored until the thermocouple read 420° F. At that temperature the pressure was released and the press opened. The assembly was then split between the first release paper and hard-coated polycarbonate film and then transferred to the cold press (press platen temperature of 65° F.) and the pressure in the cold press was increased to 94 psi. This transfer and re-pressurizing was completed in less than 3 minutes. The temperature was closely monitored until the thermocouple read 90° F. at which point the laminate was removed from the press. Surface finishes on the bottom of the product were uniform and even. 
       Example 3 
       [0043]    An additional panel in the form of a laminate that including botanical matter with clear resin, flat texture, thatch reeds embedded on multiple layers, and patina finish on both sides, was prepared as follows: A hot press platen was preheated to 475° F. The cold press platen temperature was set at 65° F. Next the following were assembled in the following order (top to bottom): steel press plate, Nomex pad (Nomex pressure distribution pads), aluminum separation plate, release paper, 0.118″ polycarbonate sheet, thatch (thatch reeds), 0.236″ polycarbonate sheet, thatch, 0.118″ polycarbonate sheet, release paper, aluminum separation plate, Nomex pad, and steel press plate. 
         [0044]    A thermocouple was inserted in-between the first thatch and 0.236″ polycarbonate sheet. The assembly was then inserted in the hot press, the press was closed and the pressure was increased to 10 psi. The temperature was closely monitored until the thermocouple read 410° F. At that temperature the pressure was increased to 30 psi. The temperature was closely monitored until the thermocouple read 420° F. At that temperature the pressure was increased to 94 psi. The temperature was closely monitored until the thermocouple read 435° F. Next, the pressure was released and the press opened. The assembly was then transferred to the cold press (press platen temperature of 65° F.) and the pressure in the cold press was increased to 94 psi. This transfer and re-pressurizing was completed in less than 3 minutes. The temperature was closely monitored until the thermocouple read 90° F. at which point the decorated laminate structure was removed from the press. The resulting laminate was thermally fused around and through the thatch, resulting in its intimate encapsulation. Surface finishes on the bottom of the product were uniform and even. 
       Example 4 
       [0045]    A yet additional panel in the form of a laminate that includes textile as the image layer, was prepared. In a clean room, the sheet was unmasked, cleaned with 50/50% (by volume) water/isopropyl solution, air dried and the static electricity was removed from the sheet using deionized air. The following were assembled on a table in order (top to bottom): 0.5″×4′×8′ polycarbonate sheet, image layer (sheer nylon textile), 0.025″ aliphatic TPU film (Deerfield A 4700), 0.5″×4′×8′ polycarbonate sheet. 
         [0046]    The assembly was then inserted into a vacuum bag which was subsequently evacuated to 29″ of mercury. This vacuum was maintained 1 hr prior to, and subsequently throughout, the autoclaving cycle. The vacuum bag with its contents was then placed in an autoclave and heated 2.5° F./min for 96 min to 240° F. At the same time the pressure was increased 3.8 psi/min to 171 psi over 45 minutes. The 240° F. temperature was then held for 90 min at 171 psi. Next, the vacuum bag was gradually cooled at 2.0° F./min to 105° F., and the pressure was then reduced at 3.8 psi/min to ambient. The assembly was allowed to sit undisturbed for another hour. Finally the assembly was removed from the vacuum bag. 
       Example 5 
       [0047]    A virtual barrier structure produced in accordance with the invention was tested in a ABAQUS computer model simulating vehicular bomb blasts. The model simulated a blast using the equivalent of 2000 pounds of trinitrotoluene (TNT) against a panel wherein the panel was a 4′ by 8′ sheet of polycarbonate 25 to 35 mm thick at distances of 100 feet, 80 feet and 50 feet, and against 2′ by 8′ sheets of polycarbonate 25 mm thick at 80 feet, 50 feet and 40 feet from the blast resistant barrier. The data show that for panels 4 by 8 feet the stand off distance (the distance from the blast) should be greater than 50 feet. For panels 2 by 8 feet the stand off distance should be greater than 40 ft. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Wall 
                 Inward Force to 
                 Outward Force from 
               
               
                 Thickness 
                 Structure 
                 Structure 
               
               
                 [mm] 
                 [N/(Unit Length)] 
                 [N/(Unit Length)] 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 −14200 
                 5400 
               
               
                 15 
                 −11900 
                 12200 
               
               
                 20 
                 −9012 
                 12943 
               
               
                 25 
                 −7810 
                 12858 
               
               
                 30 
                 −7076 
                 9489 
               
               
                 35 
                 −6788 
                 5833 
               
               
                 40 
                 −9093 
                 3961 
               
               
                 45 
                 −9587 
                 2622 
               
               
                 50 
                 −9056 
                 984 
               
               
                 55 
                 −7970 
                 4263 
               
               
                   
               
             
          
         
       
     
         [0048]    Data presented in the above table show that for panels having wall thickness of 10 to 20 mm, the inward force to the structure is greater than 9,000 units. For panels having thickness greater than 20 and less than 40 mm the inward force decreases below 8,000 units. At wall thicknesses of 40 mm to 50 mm, the inward force to the structure again tends to exceed 9,000 units. While the performance of the panel at a wall thickness of 55 mm increases, the thickness and inward force to the structure is reduced again as a consequence of the added thickness and weight stiffening the panel. 
         [0049]    The above description is not to be construed as limiting the invention, since those of ordinary skill in the art will realize that various modifications, changes and substitutions can be made in various materials and methods disclosed herein, without departing from the spirit or the scope of the present inventive discovery. Instead, the present invention is defined by the claims appended hereto and the equivalents encompassed thereby.