Abstract:
A security film for shatterproofing windows comprises: at least two oriented polyethylene terephthalate self supporting strata having critical thicknesses for shock absorption and tensile strength, of which at least one optionally has a semi-transparent vapor deposited aluminum coat for solar control; and at least two relatively soft bonding strata having critical thicknesses for adhesion and cohesion, of which at least one is an internal copolyester stratum for bonding self supporting strata together and one is a pressure sensitive stratum at the outer surface of the film for application to window glass.

Description:
BACKGROUND AND SUMMARY 
     The present invention relates to a security film for shatterproofing windows against impact or explosion without normally detracting from their optical clarity, and, more particularly, to a security film for ready application to the inner face of a window as a safeguard against fragmentation of glass notwithstanding its splintering or cracking when damaged from outside. Prior optically clear security films, when adhered to window faces, have tended to rip in the event of impact or explosion to the accompanyment of flying glass, which is the primary danger that safety film is intended to prevent. 
     In accordance with the present invention, it has been found that 21/2 to 6 mils (62.5 to 150 microns) thick laminated assemblage (not including any release stratum) of alternate polyethylene terephthalate strata and inner and outer bonding strata, provides for radically improved shatterproofing when applied to the inner face of an architectural window, if (1) each of at least two of the polyethylene terephthalate strata is at least 1 mil thick, (2) each of the inner bonding strata between the polyester terephthate strata is composed of from 3.2 to 9.6 grams per square meter of a copolyester bonding agent from 0.1 to 0.3 mil (2.5 to 7.5 microns) thick, and (3) the outer bonding stratum for adhering the assemblage to a glass window is composed of from 9.6 to 38.4 grams per square meter of a pressure-sensitive acrylate polyene from 0.4 to 1.5 mil (10 to 37.5 microns) thick. All of these strata are optically clear and water vapor permeable and, optionally one or more of the polyethylene terephthalate strata is coated with a vacuum vapor depositied semi-transparent aluminum stratum. Based on the overall thickness of the assemblage, the copolyester stratum ranges from 6 to 18 percent and the pressure-sensitive stratum ranges from 13 to 26 percent. While the basis for the shatterproofing efficacy of this assemblage is not understood with certainty, it is postulated that this assemblage possesses a critical balance of physical properties including: sufficient overall flexibility to permit high incremental pressure when squeegeed onto a glass window; sufficient tensile modulus, impact strength and tear strength imparted by the oriented polyester strata to preclude rupture while distributing shock; particular adhesion, cohesion, compatibility and resilience to withstand shock without failure; sufficient modulus of rigidity discontinuities between the polyester terephthalate strata and the inner bonding strata to damp vibratory energy transfer; and particular adhesion, cohesion, compatibility and resilience imparted by the pressure-sensitive bonding stratum to catch and retain glass chards that are produced by deformation of the security film-window combination at the moment of initial impact. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     For a fuller understanding of the nature and objects of the present invention, reference is made to the following detailed description, taken in connection with the accompanying drawing, wherein: 
     FIG. 1 is a flow diagram illustrating a security film, in exaggerated cross-section, undergoing a process of the present invention, and an assemblage resulting therefrom; 
     FIG. 2 illustrates a window incorporating the present invention; 
     FIG. 3 is an alternative product, in exaggerated cross-section, analogous to the product of FIG. 1; 
     FIG. 4 is another alternative product, in exaggerated cross-section, embodying the present invention; and 
     FIG. 5 is a further alternative product, in exaggerated cross-section, embodying the present invention. 
    
    
     DETAILED DESCRIPTION 
     Generally, each of the security films ranges from 21/2 to 6 mils (62.5 to 150 microns) in overall thickness and comprises from 2 to 5 self-supporting strata, inner bonding strata for laminating them together and an outer bonding stratum for laminating the security film to a glass window. Each of at least two of the self supporting strata are composed of biaxially oriented polyethylene terephthalate and range in thickness from 1 to 3 mils (25 to 75 microns). Each of the inner bonding strata is composed of a polyester terephthalate copolymer of low molecular weight relative to that of the self supporting strata, particularly a hydrocarbon or polyol copolymer of polyethylene terephthalate such as polyethylene glycol terephthalate, polyethylene propylene terephthalate and polyethylene butylene terephthalate, and ranges in thickness from 0.1 to 0.3 mil (2.5 to 7.5 microns). The pressure sensitive adhesive stratum is composed of an acrylic base resin and a polyene tackifier, and ranges in thickness from 4 to 15 times the thickness of one of the inner bonding strata, typically from 0.4 to 1.5 mil (10 to 37.5 microns). A typical formulation of the pressure sensitive stratum incorporates from 15 to 30 parts of a straignt chain acrylate, particularly, a copolymer, homopolymer or interpolymer of methyl acrylate, ethyl acrylate, propyl acrylate or butyl acrylate, and from 2.5 to 7.5  parts of a low molecular weight rubber, particularly, a polyhydrocarbon such as polybutene or polyterpene. Optionally one of the polyethylene terephthalate strata is coated with a vacuum vapor deposited stratum of aluminum ranging in thickness to no more than 300 angstroms and characterized by a visible light transmission of from 5 to 60%. 
     Typically the pressure sensitive stratum is characterized, in accordance with the tests at 72° F. (22° C.) described below: by a rolling ball test value (PSTC Test No. 7) ranging from 1/16 to 6 inches (0.15 to 15 centimeters); by a static sheer test value (PSTC Test No. 6) ranging between 1 and 5 hours; and by a 180 degree static peel test value (PSTC Test No. 1) ranging between 7.5 and 75 ounces (213 to 2130 grams). These tests, which refer to the Pressure Sensitive Tape Council (PSTC), are described in space of 2 inches. 
     In the static sheer test, a substrate is coated with the pressure sensitive adhesive to a dry thickness of approximately one mil (25 microns). A one inch (2.5 centimeter) square of the coated substrate is applied to a clean stainless steel plate and rolled twice with a 5 pound (21/4 kilograms) rubber roll. The plate is positioned in a jig. A weight is attached to the bottom of the substrate and the assembly is subjected to a selected temperature. The time to complete failure or the distance the substrate moves after a set time is recorded as the value. 
     In the 180° static peel test, the pressure sensitive adhesive is applied to and cured on a flexible substrate, which is reversely bent. One of the exposed pressure sensitive faces is pressed onto a vertical rigid substrate and the other is attached to a depending weight. The weight required to cause peeling is the test value. 
     In the rolling ball test, a 7/16 inch (0.9 centimeter) diameter stainless steel ball is rolled down an approximately 45 degree inclined plane onto the pressure sensitive surface. The distance from the bottom of the inclined plane to where the ball stops is measured in inches to provide the indicated value. 
     EXAMPLE I 
     The security film 36 of FIG. 1 comprises in laminated sequence: a polyethylene terephthalate stratum 10 that is 1 mil (25 microns) thick; a polyethylene glycol terephthalate stratum 12 that is 0.2 mil (5 microns) thick; a polyethylene terephthate stratum 14 that is 2 mils (50 microns) thick; a polyethylene glycol terephthalate stratum 16 that is 0.1 mil thick; a polyethylene terephthalate stratum 18 that is 1 mil (25 microns) thick; a methyl acrylate polybutene pressure sensitive stratum 20 that is 0.6 mil (15 microns) thick; and a silicone impregnated paper release stratum 22 that is 2 mils (50 microns) thick. All of these strata are interrelated chemically and mechanically and are optically clear as discussed above. In use, release stratum 22 is peeled from pressure sensitive stratum 20 and a lubricating coat of aqueous detergent 24 is applied to pressure sensitive stratum 20. Thereafter, the film is squeegeed onto the inner face of a glass window 26 with aqueous stratum 28 interposed between the window and the film. When allowed to dry by permeation of moisture through film 36 and from the free edges of glass window 26 and film 36, the resulting assemblage of glass window 26 and security film 36, when secured within a conventional architectural frame 30, has an extraordinary ability to withstand shock originating from direction 32 and to prevent glass chards from being projected in direction 34. In this example, the inner bonding strata more specifically are described in U.S. Pat. No. 3,170,833, issued on Feb. 23, 1965 in the name of Paul R. Noyes for Adhesive Compositions And Laminates Prepared Therefrom. In this example, the methyl acrylate polybutene pressure sensitive stratum has the following characteristics: a rolling ball test index of 4.5 inches (11.6 centimeters); a 180° peel test index of 12.5 ounces (3.5 kilograms); and a static sheer test index of 24 hours. 
     EXAMPLE II 
     Another security film of the present invention is shown in FIG. 3 as comprising: a first polyethylene terephthalate stratum 38 that is 2 mils (50 microns) thick; a polyethylene glycol terephthalate stratum 40 that is 0.2 mil (5 microns) thick; a polyethylene terephthalate stratum 42 that is 2 mils (50 microns) thick; a methyl acrylate polybutene pressure sensitive stratum 44 that is 0.8 mil (20 micron) thick; and a silicone impregnated paper release stratum 46 that is 2 mils (50 microns) thick. All of these strata are interrelated chemically and mechanically and are optically clear as discussed above. This security film is processed in the same manner as the security film of Example I. 
     EXAMPLE III 
     Another security film of the present invention is shown in FIG. 4 as comprising: three polyester terephthalate strata 48, 52, 56, each 1 mil (25 microns) thick; two interposed inner bonding strata 50,54 which are composed of polyethylene glycol terephthalate and are 0.2 mil (5 microns) thick; a methyl acrylate polybutene pressure sensitive stratum 58 that is 1 mil (25 microns) thick; and a silicone impregnated paper release stratum 60 that is 1 mil (25 microns) thick. All of these strata are interrelated chemically and mechanically and are optically clear as discussed above. This security film if processed in the same manner as the security film of Example I. 
     EXAMPLE IV 
     Another film, which combines security and solar control functions, is shown in FIG. 5 as comprising: three polyester terephthalate strata 64, 68, 74, of which each of strata 64, 74 is 1 mil (25 microns) thick and stratum 68 is 2 mils (50 microns) thick; two interposed inner bonding strata 66, 70, which are composed of polyethylene glycol terephthalate and are 0.2 mil (5 microns) thick; a vapor deposited aluminum coat 72, on a surface of stratum 74, which is interposed between this surface and stratum 70 and is approximately 100 angstrom units thick; a methyl acrylate polybutene pressure sensitive stratum 76 that is 1 mil thick; and a silicone impregnated paper release stratum 78 that is 1 mil (25 microns) thick. All of these strata are interrelated chemically and mechanically and are optically clear as discussed above. This security and solar control film is processed in the same manner as the security film of Example I to produce a security and solar control composite assemblage incorporating an architectural window 80 in the manner shown in FIG. 2. 
     It is to be understood that the vapor deposited aluminum stratum of FIG. 5 may be applied optionally to any of the inner faces of the polyethylene terephthalate strata of the films of FIGS. 1, 2, 3 and 4. The present invention thus comprises the security films of FIGS. 1, 3, 4 and 5 and the security film and glass window assemblage of FIG. 2. Since certain changes may be made in the foregoing disclosure without departing from the scope of the invention herein involved, it is intended that all matter shown in the accompanying drawing or described in the foregoing specification be interpreted in an illustrative and not in a limiting sense.