Abstract:
A security substrate for a document of value comprises a laminate including a generally transparent core layer and generally transparent outer layers on opposite sides of the core layer. At least one interference filter is embedded within the laminate. The interference filter includes a highly reflective opaque layer on a surface of the core layer and an optically variable thin film multilayer overlying at least a portion of the opaque layer. The interference filter is covered by one of the outer layers.

Description:
FIELD OF THE INVENTION 
     The present invention relates to document security and in particular to a security substrate for documents of value such as for example, banknotes, credit cards, identification cards and the like. 
     BACKGROUND OF THE INVENTION 
     Documents of value such as identification cards, credit cards, banknotes, etc. are in many cases formed of polymer-based laminates. For example, U.S. Pat. No. 5,935,696 to Benoit et al. discloses a laminated multilayer film substrate having high-density polyethylene layers on each side of an oriented polypropylene layer. The multilayer film substrate exhibits good embossability, dead-fold characteristics and other properties making it suitable for the production of banknotes and other security documents. 
     Marking documents of value is common practice to deter forgery and allow counterfeit cards and banknotes to be readily detected. For example, U.S. Pat. No. 3,858,977 to Baird et al. discloses an optical interference filter having an optical interference layer with a known characteristic of spectral reflectance and a different known characteristic of spectral transmittance, both of which vary with the angle of incident light on the interference filter. The interference filter is disposed on the banknote substrate over a coloured portion thereof designed to absorb some of the light transmitted by the interference filter so that the interference filer exhibits a colour change with a change in the angle of incident light. 
     In order to inhibit forgery and counterfeiting, it is desired that security features used to mark documents of value be difficult to copy and highly secured against alteration. Accordingly, improved substrates for documents of value are desired. 
     It is therefore an object of the present invention to provide a novel security substrate for a document of value and method of forming the same. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a security substrate for a document of value comprising: 
     a laminate including a generally transparent core layer and a generally transparent outer layer on at least one side of said core layer; and 
     at least one interference filter embedded within said laminate, said interference layer including a highly reflective opaque layer on a surface of said core layer and an optically variable thin film multilayer overlying at least a portion of said opaque layer, said interference filter being covered by said outer layer. 
     In a preferred embodiment, the opaque metal layer is patterned on the core layer at spaced locations. The opaque metal layer can be patterned to form continuous stripes or discrete elements that may or may not be intricately patterned. 
     Preferably, the optically variable thin film multilayer includes one or more dielectric film layers of low refractive index and a semi-transparent, medium reflective layer. 
     Preferably, generally transparent outer layers are provided on opposite sides of the core layer. Opacifying coatings can be applied to the outer layers and have windows formed therein that are in registration with the interference filter. 
     According to another aspect of the present invention there is provided a security substrate for a document of value comprising: 
     a laminate including a generally transparent balanced biaxially oriented core layer, said core layer being oriented in at least a first direction at an orientation ratio of at least 4:1 and oriented in a second direction substantially normal to the first direction at an orientation ratio of at least 6:1; generally transparent imbalanced biaxially oriented outer layers on opposite sides of said core layer, said outer layers being oriented in at least a first direction to a degree which is at least three times less than the degree of orientation present in a second direction substantially normal to the first direction; and a laminating adhesive resin disposed between the outer layers and the core layer to secure the outer layers to the core layer so that the first directions of orientation of the outer layers are substantially aligned; 
     at least one optically variable device embedded within the laminate, the at least one optically variable device including a highly reflective opaque layer on a surface of said core layer and an optically variable thin film multilayer overlying at least a portion of the opaque layer, said at least one optically variable device being covered by one of said outer layers; and 
     opacifying coatings on said outer layers, at least one window being formed in at least one of said opacifying coatings that is in registration with said at least one optically variable device. 
     According to another aspect of the present invention there is provided a method of forming a security substrate for a document of value comprising the steps of: 
     depositing a highly reflective opaque layer on a surface of a generally transparent core layer; 
     depositing an optically variable thin film multilayer on said core layer to overlie at least a portion of said opaque layer; and 
     overlying at least one side of said core layer with a generally transparent outer layer to cover said opaque layer and optically variable thin film multilayer thereby to embed an optically variable device within said security substrate 
     The present invention provides advantages in that the optically variable interference filter is embedded in the security substrate protecting it from mechanical and chemical attack as well as wear and tear. As a result, the security substrate exhibits high longevity and is highly secured against alternation. Also, since the optically variable interference filter exhibits a sharp color shifting characteristic with changes in the angle of view, the security substrate is virtually impossible to counterfeit using digital methods, laser scanning or photocopying. Furthermore, the security substrate exhibits high tensile strength in the oriented and unoriented directions, good folding and crumple resistance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view of a security substrate in accordance with the present invention; 
     FIG. 2 is a top plan view of a core layer having highly reflective opaque metal stripes thereon forming part of the security substrate of FIG. 1; 
     FIG. 3 is a cross-sectional view of the core layer of FIG. 2 coated with an optically variable thin film multilayer; and 
     FIG. 4 is a top plan view of another embodiment of the core layer having discrete highly reflective opaque metal elements thereon. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to FIG. 1, a security substrate in accordance with the present invention is shown and is generally indicated to by reference numeral  10 . As can be seen, security substrate  10  includes a generally transparent central core layer  12  formed of oriented polypropylene (OPP) disposed between generally transparent, high-density polyethylene (HDPE) layers  14  and  16 . The HDPE layers  14  and  16  are secured to the OPP core layer  12  by laminating adhesive resin. Opacifying coatings  18  and  20  designed to accept printed indicia cover the HDPE layers  14  and  16 . Optically variable devices in the form of interference filters  30  are embedded within the security substrate  10 . The interference filters  30  in this example are disposed on one surface  12   a  of the OPP core layer  12  and are covered by the HDPE layer  14 . It will however be appreciated that the interference filters may be disposed on either or both sides of the OPP core layer  12 . Windows  32  are formed in the opacifying coatings  18  and  20  at locations corresponding to the positions of (i.e. in registration with) the interference filters  30 . Depending on the desired effect, the windows can be formed in one or both opacifying coatings  18  and  20 . 
     In the present embodiment, the OPP and HDPE layers  12 ,  14  and  16  respectively form a multilayer substrate similar to that disclosed in U.S. Pat. No. 5,935,696 to Benoit et al, the contents of which are incorporated herein by reference. Thus, the HDPE layers  14  and  16  are imbalanced biaxially and are oriented in a first direction to a degree that is at least three times less than the degree of orientation in a second direction normal to the first direction. The OPP core layer  12  is balanced biaxially and is oriented in a first direction at an orientation ratio of at least 4:1 and oriented in at least a second direction normal to the first direction at an orientation ratio of at least 6:1. The HDPE layers  14  and  16  are arranged such that their orientations in the first directions are aligned. 
     In the present embodiment, the interference filters  30  include highly reflective opaque metal stripes  50  formed of aluminum that are disposed on the surface  12   a  of OPP core layer  12  (see FIG. 2) and optically variable thin film multilayers  52  disposed on the opaque metal stripes  50 . The opaque metal stripes  50  preferably have a reflectance greater than 90% at a 500 nm wavelength and an optical density in the range from about 2.0 and 3.0. 
     The opaque metal stripes  50  are patterned on the surface  12   a  of the OPP core layer  12 . A number of techniques can be used to pattern the opaque metal stripes  50  on the OPP core layer  12 . For example, the opaque metal stripes  50  may be patterned on the OPP core layer  12  using an oil patterning process such as those described in U.S. Pat. No. 4,749,591 to Ronchi; U.S. Pat. No. 4,952,420 to Walters; U.S. Pat. No. 4,962,725 to Heinz et al.; U.S. Pat. No. 5,223,038 to Kleyer; and U.S. Pat. No. 5,350,598 to Kleyer. During this process a layer of suitable oil is deposited on the OPP core layer  12  prior to vacuum evaporization of the opaque metal stripes on the OPP core layer. The oil is applied to the OPP core layer  12  in a vacuum in the evaporization system to create areas where the opaque metal will not nucleate. In this manner, an etchless patterning method for the opaque metal is achieved. 
     Alternatively, the opaque metal stripes  50  may be evaporated as thin films onto the surface  12   a  of the OPP core layer  12  through a physical mask as is well known. 
     The opaque metal stripes may also be patterned on the surface  12   a  of the OPP core layer  12  through demetallization using one of two techniques. In one technique, an alkaline etchant solution is printed directly onto the opaque metal in selected areas. The residue is then washed away leaving the patterned metal, which is then evaporated onto the OPP core layer  12 . In another technique, the opaque metal is evaporated onto the OPP core layer  12 . A mask in the shape of the desired pattern is then placed over the opaque metal and an alkaline etchant solution is printed directly onto the exposed opaque metal. The residue is then washed away and the mask is removed leaving the patterned opaque metal. Using either of these demetallization techniques allows intricate opaque metal patterns to be placed on the OPP core layer  12  with features less than 100 microns in size. 
     The opaque metal stripes  50  may also be printed on the surface  12   a  of the OPP core layer  12  using metallic high reflectance inks. 
     Each optically variable thin film multilayer  52  includes one or more layers of a low refractive index dielectric film identified collectively by reference numeral  60  such as for example, SiO 2 , MgF 2 , Al 2 O 3  etc. or acrylate-based organic compounds, together with a semi-transparent, medium reflective metal layer such as aluminum or alloy such as Inconel (Ni/Cr/Fe alloy)  62  (see FIG.  3 ). The layers  60  of dielectric film are evaporated, sputtered or deposited by chemical vapour deposition (CVD) on the OPP core layer  12  over portions of the opaque metal stripes  50 . If an Inconel layer is used, the thickness of the layer is preferably about 10 nm. If SiO 2  dielectric films are used, the thickness of the films is preferably in the range of about 170 nm to 480 nm. If Al 2 O 3  dielectric films are used, the thickness of the films is preferably in the range of from about 200 nm to 470 nm. At locations where the optically variable thin film multilayers  52  cover the opaque metal stripes  50 , interference filters  30  are formed that exhibit sharp color shifting characteristics with change in the angle of view. 
     The interference filters  30  have well defined characteristics of spectral reflectance and transmittance, both of which vary with the angle of incident of light. As a result, the interference filters  30  provide a sharp color shift that changes with the angle of view. The spectral characteristics are of course dependent on the optical constants, refractive indices, absorption coefficients and thicknesses of the security substrate layers. Depending on whether the interference filters  30  are exposed by a window  32  in one opacifying coating  18  or  20  or windows  32  in both opacifying coatings  18  and  20 , different optical effects are achieved. If windows  32  are provided in both opacifying coatings  18  and  20 , an observer sees an optically variable effect through the window  32  in opacifying coating  18  and a reflective metallic image through the window  32  in opacifying coating  20  that is in registration with the optically variable effect. If a window  32  is only provided in the opacifying coating  18 , only the optically variable effect is visible. If a window  32  is only provided in the opacifying coating  20 , only the reflective metallic image is visible. 
     The opaque metal disposed on the OPP core layer  12  need not take the form of stripes  50  as shown in FIG.  2 . For example, as shown in FIG. 4, the opaque metal can be deposited on the surface  12   a  of OPP core layer  12  at discrete locations as discrete elements  150  or in intricate patterns  250 . 
     Although the security substrate  10  is described as including an OPP core layer  12 , other biaxially oriented polymers of comparable tensile strength such as linear low-density polyethylene (LLDPE), nylons or polyesters may be used. In cases where high evaporation temperatures are used, PET is preferred. 
     Also, although preferred embodiments of the present invention have been described, those of skill in the art will appreciate that variations and/or modifications may be made without departing from the spirit and scope thereof as defined by the appended claims.