Abstract:
A security film adapted to display a message indicative of tampering is disclosed. A light transmitting sheet has a first layer on one surface, a release layer on a portion of the opposite surface, and a second layer covering the release layer and another portion of the opposite surface. The first layer is a metal and the second layer is a metal or a non-metal. An adhesive layer on the second surface adheres the film to a substrate. Removal of the sheet from the substrate causes the sheet to separate from the second layer under the release layer making a release layer pattern visible on both the sheet and substrate as reverse images. A method disclosed includes metalizing a light transmitting sheet with a metal layer on one side and a release layer and a metal or a non-metal layer on the opposite side.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and claims priority to U.S. Provisional Application No. 60/793,585, filed Apr. 20, 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a multi-layer tamper evident film adherable to a substrate, the film displaying permanent, visible evidence when it is removed from the substrate. 
       BACKGROUND OF THE INVENTION 
       [0003]    Tamper evident security film is used to provide security against tampering, theft and counterfeiting. The film may be in the form of an adhesive tape applied to seal packaging and provide visible evidence that a package was opened, thereby providing an indication of potential tampering with the package contents. The film may be in the form of a decal or sticker, such as those affixed to automobile license plates, that are displayed to indicate compliance with registration requirements. Upon removal of the decal, an otherwise hidden message may be revealed on the decal that indicates unauthorized removal thereby voiding the decal, or the indicia displayed on the decal may be disrupted to render them unreadable and thereby deter theft. The film may also be in the form of a label applied to a product which reveals an otherwise hidden code when removed, absence of the label or the proper code being an indication of counterfeited goods. These products could also be used to hide user information on gaming cards, lottery cards and prepaid telephone cards with or without scratch-off inks that would indicate tampering. 
         [0004]    Films having tamper evident characteristics according to the prior art may comprise a transparent face stock made of a polymer such as polyethyleneterepthalate on which a release layer, such as polyvinyl alcohol, is printed in the form of a pattern or indicia. A primer layer, for example, a polyester varnish, is applied over the release layer and the face stock. A frangible metal layer, such as vapor deposited aluminum, covers the primer layer. An adhesive layer covers the metal layer and is used to apply the film to a substrate. The total thickness of the film is between 2 and 3 mils. 
         [0005]    The weakest adherence between the various layers exists between the face stock and the release layer. If the face stock is removed from the substrate, the face stock separates from the release layers, leaving the primer and metal layers behind still adhered to the substrate by the adhesive layer. In regions where there is no release layer, the primer layer, the metal layer and the adhesive layer pull away from the substrate and remain with the face stock. 
         [0006]    The release layer is printed in the form of a pattern or indicia. When the layers are intact (i.e., before removal from the substrate), the pattern or indicia are invisible. When the layers are disrupted by removal of the face stock from the substrate, the pattern becomes visible on the substrate as a positive image and on the face stock as a negative image. 
         [0007]    One disadvantage of films according to the prior art is the complexity of the different layers. For example, for the hidden pattern or indicia to be invisible, it is necessary to match the index of refraction of the release layer closely with that of the primer layer. This is often difficult to achieve and maintain throughout a large production run, resulting in significant product wastage and added expense. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention concerns a security film attachable to a substrate and adapted to display evidence of removal therefrom. The security film comprises a light transmitting sheet having a first and a second surface oppositely disposed. A first layer is attached to the first surface. A release layer is attached to but a portion of the second surface. A second layer overlies the second surface, the second layer including a first portion attached to the release layer and a second portion attached to at least a portion of the second surface not covered by the release layer. The second layer is attachable to the substrate. The first layer has a higher light transmissibility than the second layer. The release layer has a greater adherence to the second layer than to the light transmitting sheet such that upon removal of the light transmitting sheet from the substrate, the first portion of the second layer remains attached to the substrate, and the second portion of the second layer remains attached to the light transmitting sheet. 
         [0009]    In a particular embodiment, the security film according to the invention comprises a light transmitting sheet having a first and a second surface oppositely disposed. A first metal layer is attached to the first surface. A transparent release layer is attached to but a portion of the second surface. A second metal layer overlies the second surface. The second metal layer includes a first portion attached to the release layer and a second portion attached to at least a portion of the second surface not covered by the release layer. The second metal layer is attachable to the substrate. The first metal layer has a higher light transmissibility than the second metal layer. The release layer has a greater adherence to the second metal layer than to the light transmitting sheet such that upon removal of the light transmitting sheet from the substrate, the first portion of the second metal layer remains attached to the substrate, and the second portion of the second metal layer remains attached to the light transmitting sheet. 
         [0010]    In an alternate embodiment, the second layer is a non-metal layer, for example, a pigmented layer. 
         [0011]    In another embodiment, the security film comprises a light transmitting sheet having a first and a second surface oppositely disposed. A first layer is attached to the first surface. A release layer is attached to but a portion of the second surface. The first layer has a higher light transmissibility than the release layer such that, when viewed through the first layer, the release layer is not visible. A second layer overlies the second surface. The second layer includes a first portion attached to the release layer and a second portion attached to at least a portion of the second surface not covered by the release layer. The second layer is attachable to the substrate. The release layer has a greater adherence to the second layer than to the light transmitting sheet such that upon removal of the light transmitting sheet from the substrate, the release layer remains attached to the substrate. The release layer may be tinted, and the second layer may comprise an adhesive. 
         [0012]    The invention also encompasses a method of making a security film attachable to a substrate and adapted to display evidence of removal of the security film from the substrate. The method comprises: 
         [0013]    (a) providing a light transmitting sheet having a first and a second surface oppositely disposed; 
         [0014]    (b) metalizing the first surface with a metal layer having a first light transmissibility; 
         [0015]    (c) coating but a portion of the second surface with a release layer; and 
         [0016]    (d) applying to the second surface and the release layer a second layer having a second light transmissibility, the first light transmissibility being higher than the second light transmissibility. 
         [0017]    The method may further include coating the second layer with an adhesive layer for attaching the security film to the substrate. 
         [0018]    Another method for making a security film comprises: 
         [0019]    (a) providing a light transmitting sheet having a first and a second surface oppositely disposed; 
         [0020]    (b) metalizing the first surface with a metal layer; 
         [0021]    (c) coating but a portion of the second surface with a release layer, the first metal layer having a higher light transmissibility than the release layer such that, when viewed through the first metal layer, the release layer is not visible; and 
         [0022]    (d) applying to the second surface and the release layer a second layer, the second layer being attachable to the substrate, the release layer having a greater adherence to the second layer than to the light transmitting sheet such that upon removal of the light transmitting sheet from the substrate, the release layer remains attached to the substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a cross-sectional view of a security film according to the invention shown on an enlarged scale; 
           [0024]      FIG. 1A  is a cross-sectional view of an alternate embodiment of a security film according to the invention shown on an enlarged scale; 
           [0025]      FIG. 2  is a cross-sectional view of the security film shown in  FIG. 1  being disrupted by removal from a substrate; 
           [0026]      FIG. 3  is a perspective view of a security film being removed from a substrate and displaying visible evidence of removal of the film in the form of indicia; 
           [0027]      FIG. 4  is a perspective view of a security film being removed from a substrate and displaying evidence of removal visible under ultraviolet or infrared light; 
           [0028]      FIG. 5  is a perspective view of a security film being removed from a substrate and displaying a radio frequency antenna; 
           [0029]      FIG. 5A  is a cross-sectional view of a security film in which a radio frequency antenna is embedded; 
           [0030]      FIG. 6  is a perspective view of a security film being removed from a substrate and displaying visible evidence of removal in the form of a pattern to include tagents; 
           [0031]      FIG. 7  is a cross-sectional view of another embodiment of a security film according to the invention shown on an enlarged scale; and 
           [0032]      FIG. 8  is a cross-sectional view of the security film shown in  FIG. 7  being disrupted by removal from a substrate. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0033]      FIG. 1  shows a cross-sectional view of a security film  10  according to the invention. The security film is applied to a substrate  12  using an adhesive layer  14 . The security film  10  comprises a light transmitting sheet  16  having oppositely disposed surfaces  18  and  20 . Surface  18  faces outwardly away from substrate  12 . An outer layer  22  is attached to this surface and forms the layer through which the security characteristics of the film will be viewed. The outer layer  22  comprises a metal layer. 
         [0034]    A transparent release layer  24  is attached to the opposite surface  20  which faces the substrate  12 . Release layer  24  is applied in a pattern or in the form of indicia, such as a trademark, and may be continuous or non-continuous over the surface  20 . The pattern of the release layer contains the intelligence or “message” that is conveyed when the security film is disrupted by attempted removal from the substrate as described in detail below. 
         [0035]    An inner layer  26  is attached to the release layer  24  and to at least a portion of the substrate-facing surface  20  of the light transmitting sheet  16  that is not covered by the release layer  24 . The inner layer  26  may comprise a metal layer or a non-metal layer. The release layer  24  has greater adhesion to the inner layer  26  than to the light transmitting sheet  16  to permit separation of the release layer from the sheet as described below. The adhesive layer  14  is applied between the inner layer  26  and the substrate  12  to adhere the film  10  thereto. The adhesive layer may be included as a part of the film  10 , or it may be applied to the film or the substrate separately to effect attachment of the film to the substrate. 
         [0036]    In one embodiment, the outer and inner layers  22  and  26 , are preferably formed of the same metal, although different metals having similar optical properties with respect to light reflection (i.e., metals that are similar in color) may also be feasible. For example, it may be possible to have the outer metal layer be aluminum and the inner metal layer be silver. When the layer  26  is a non-metal layer, for example, a pigmented or opaque color layer, then it is preferable that the pigments or colors of the inner and outer layers also have similar optical properties (i.e., similar in color). 
         [0037]    The light transmissibility of the inner and outer layers, which is a measure of the amount of incident light that is transmitted through these layers, is controlled as described below to render the release layer  24  invisible when the film  10  is intact, and visible when the layers are disrupted. To that end, the light transmissibility of the outer layer  22  is greater than the light transmissibility of the inner layer  26  such that the outer layer transmits a significant portion of incident light and the inner layer reflects a significant portion of incident light. When the film  10  is intact and attached to a substrate  12  as shown in  FIG. 1 , incident light  28  is transmitted through the outer layer  22 , through the light transmitting sheet  16  and the transparent release layer  24  to the inner layer  26  where it is reflected back through the layers of the film. (When the layer  26  is an opaque pigment, a portion of the incident light is absorbed.) In this condition, with the various layers intact, the pattern or indicia formed by the release layer  24  is not visible by the light  28  reflected from the inner layer  26 . The inner and outer layers  22  and  26  work together to camouflage the pattern or indicia formed by the release layer  24 . 
         [0038]    However, when the layers of film  10  are disrupted by attempted removal of the film from the substrate  12  as shown in  FIG. 2 , the light transmitting sheet  16  separates from the release layer  24  due to the lower adhesion of the release layer to the light transmitting sheet as compared with the release layer&#39;s adhesion to the inner layer  26 . Where release layer is present, the inner layer  26   a  beneath the release layer  24  remains attached to the substrate  12 , as shown in the lower half of  FIG. 2 . Where there is no release layer, however, the inner layer  26   b  remains attached to the light transmitting sheet  16  as shown in the upper half of  FIG. 2 . This disruption of the layers renders the pattern or indicia formed by the release layer visible as a positive image on the substrate  12 , and as a complementary negative image on the light transmitting sheet  16 . 
         [0039]    As shown in  FIG. 2  for the disrupted layers, incident light  28  passes through the outer layer  22  and the light transmitting sheet  16 . Where the inner layer  26   b  still adheres to portions  30  of the sheet  16 , light  28  is reflected. Light is not reflected from sheet portions  32  where release layer  24  was attached to the sheet. This absence of reflected light results in the pattern defined by the release layer being visible as a “hole” or negative image on the light transmitting sheet  16 . Similarly, light  28  incident on the substrate  12  reflects differently from the inner layer  26   a  beneath the release layer  24  that remains with the substrate than from the substrate portion  34  surrounding the inner layer where the inner layer was removed with the light transmitting sheet  16 . The inner layer  26   a  that remains on the substrate is visible as a contrasting positive image of the pattern or indicia. 
         [0040]    An example of this is further illustrated in  FIG. 3 , wherein the film  10  is shown being removed from the substrate  12 . The release layer is applied to the light transmitting sheet  16  in the form of the letter “V” which appears as a positive image on the substrate and as a negative image on the light transmitting sheet. 
         [0041]    The light transmitting sheet  16  may be transparent or translucent and is preferably formed from polymers such as polyester, olefin, vinyl, styrene, acrylic, polyvinylchloride or polyimide. 
         [0000]    Polyethyleneterepthalate is one example of a particular polymer that is advantageous for use as the light transmitting sheet. Alternately, the sheet may be formed of glassine or translucent paper to provide a translucent appearance. The sheet is preferably flexible and conformable so that it may be readily applied to a curved or irregular surface. The sheet may range in thickness between about 0.00025 inches (¼ mil) to about 0.02 inches for many practical applications. 
         [0042]    The outer layer  22  is a metal layer and may be, for example, formed from aluminum, silver, gold, indium, tin or zinc that is applied directly to the sheet  16  by metalizing techniques such as vacuum deposition, vapor deposition and sputter processes. This allows the light transmissibility of the outer metal layer  22  to be in the range between about 85% to about 12%. 
         [0043]    As shown in  FIG. 1A , the outer layer  22  may also have a light transmitting primer layer  23  applied to it. The primer layer  23  functions to enhance the printability of the security film and may comprise polyester or a PVC resin for example. The primer layer  23  is preferably coated onto the outer layer  22  by techniques such as flood coating, offset or gravure printing and Meyer rod coating. 
         [0044]    The inner layer  26 , when a metal layer, may be formed from the same metals as the outer layer and deposited on the sheet and the release layer using the same techniques. Its light transmissibility ranges between about 10% to about 0%. The thickness of this inner metal layer ranges between about 30 to about 250 angstroms for practical applications. The inner metal layer  26  may also be a different metal from the outer metal layer  22 , as long as both metals have similar optical properties such as the color of the light reflected. 
         [0045]    The inner layer  26  as a non-metal layer may be formed from resins such as polyester, polyvinyl, acrylics, urethane, waxes, cellulosic resins or polyimides. The resins can have various colors and take on the appearance of metal. The colors may be imparted by dyes, tints or pigments. The thickness of the non-metal layer may range between 0.1 mils and 0.5 mils. The light transmissibility may range between about 10% to about 0%. 
         [0046]    The release layer  24  is preferably applied to the sheet  16  in liquid or gel form allowing it to be conveniently printed on the sheet  16  in the desired pattern or indicia by known printing techniques such as offset printing and gravure. In one embodiment, the release layer forms a solid transparent layer that bonds weakly to the sheet  16  so that the sheet separates from the inner layer where the release layer is present. The release layer is preferably a urethane resin, but may also comprise cellulosic resins such as nitrocellulose, as well as polyvinyl alcohols, silicones, fluorinated compounds, electrically conductive compounds and waxes. It is advantageous that the release layer be as thin as possible yet still remain effective at providing a separation point between the layers. Release layer thicknesses between about 0.01 mils and about 0.5 mils are considered practical. 
         [0047]    The adhesive layer  14  is preferably a pressure sensitive adhesive that is coated or laminated onto the inner layer and protected with a release paper. The release paper is removed before use, exposing the adhesive, which effects a bond between the film  10  and a substrate when pressure is applied to the film against the substrate. Other types of adhesive are also useable, including heat activated adhesives and glues. 
         [0048]    As shown in  FIGS. 4-6 , information may be encoded within the layers in many ways.  FIG. 4  shows a film  36  having hidden indicia  38 , for example, a trademark, that become visible when the layers are disrupted only when viewed under light outside of the visible spectrum, such as ultraviolet or infrared light  40 . This is achieved by incorporating dyes or pigments  42  into the release layer that reflect or fluoresce under ultraviolet light. Such a film would be useful to deter counterfeiting. 
         [0049]      FIG. 5  shows a film  44  wherein the pattern defined by the release layer is overprinted with an electrically conductive pattern forming an antenna  46  for a radio frequency identification tag or RFid. The antenna  46 , formed in between the release layer  24  and a non-metal inner layer  26  remains with the substrate when the sheet  16  is removed. The antenna is connectable to an integrated circuit  48  to form the RFid tag which responds to an interrogating radiofrequency signal to identify and track the item to which the film  44  is attached. This embodiment would be useful in deterring theft, as the RFid, when exposed by removal of the sheet  16 , will respond and trigger anti-theft alarms as are used in theft deterrent systems.  FIG. 5A  shows a cross-sectional view of the film embodiment  45  wherein the antenna  46  is printed on a non-metal inner layer  26 , and a non-conductive release layer  24  covers the antenna  46  and the inner layer  26 . The release layer  24  interfaces with the surface  20  of the light transmitting sheet  16 , the opposite surface  18  of that sheet having a metal outer layer  22 . Other types of electronic circuits could be substituted for the RF antenna, such as integrated circuits or larger scale conductive networks. 
         [0050]    Film  50 , shown in  FIG. 6 , has colored tagents  52  embedded within the release layer. The colored tagents become visible upon disruption of the layers. Instead of tagents, a hologram could also be used as a message indicating that the film has been tampered with. 
         [0051]    An example film embodiment according to the invention comprises a light transmitting sheet formed of polyethyleneterepthalate having a thickness of 2 mils and a light transmissibility approaching 100%. An outer metal layer of aluminum having a light transmissibility of 85% is vacuum deposited on the outwardly facing surface of the sheet. A transparent release layer formed of urethane resin having a thickness of 0.05 mils is printed on the inwardly facing surface of the sheet in the form of indicia, such as the word “VOID”. An inner metal layer, also of aluminum, is vacuum deposited over the release layer and the remainder of the inwardly facing surface of the sheet to a thickness of 173 angstroms and a light transmissibility of less than 1%. A pressure sensitive adhesive is applied over the inner metal layer, the adhesive being protected by a release paper that is easily removable when the film is to be applied to a substrate. 
         [0052]    In the example, it is also feasible to substitute a layer of polyester resin for the inner metal layer. The resin layer is about 0.25 mils thick and is tinted so as to have the same or a similar color as the outer layer  22 , and a light transmissibility of less than 1%. 
         [0053]      FIGS. 7 and 8  illustrate another embodiment  54  of a security film according to the invention. Film  54  comprises a light transmitting sheet  16  having oppositely disposed surfaces  18  and  20 . An outer metal layer  22 , as described above, is attached to the outwardly facing surface  18 . A release layer  56  is positioned on the inwardly facing surface  20 , and unlike the aforementioned embodiments, the release layer  56  is not transparent but tinted. Again, the release layer is applied as a pattern or in the form of indicia. An adhesive layer  14  overlies the inwardly facing surface  20  and covers the release layer  56  and the surface  20  where the release layer is absent. The outer layer  22  is a metal layer that has similar optical properties to the release layer. The optical properties, for example, the light reflectance and color of the outer layer, allows it to render the release layer  56  invisible when the intact film  54  is viewed. When the film is disrupted, for example, by removal of the light transmitting sheet  16  from a substrate  12  as shown in  FIG. 8 , the pattern or indicia become visible as described above for the other developments. The aforementioned description of the film embodiment  10  is largely applicable to the various components of the embodiment  54  and need not be repeated here. Again, the light transmissibility of the outer metal layer  22  is greater than the light transmissibility of the tinted release layer  56  to effect the invisibility of the pattern or indicia formed by the release layer. 
         [0054]    Security film according to the invention provides a means to detect tampering and counterfeiting while avoiding the complications associated with multi-layer films of the prior art, especially with respect to the optical properties of the various layers.