CA2788413C - Security element with expanded color-shift effect and additional thermochromic function - Google Patents

Abstract

The invention relates to a security element for increasing protection against the forgery of security documents, such as banknotes, value papers, identifications, credit cards, debit cards, or the like. The security element consists of a mixture of color-shift effect pigments and micro-encapsulated thermochromic liquid crystal pigments. According to the invention, the color-shift effect pigments are anisotropic interference pigments. According to the invention, the security element thus consists of a mixture of isotropic, micro-encapsulated thermochromic pigments with anisotropic interference pigments.

Description

Security element with expanded color-shift effect and additional thermochromic function [0001] The invention relates to a security element for increasing the protection from forgery of security documents, such as banknotes, value documents, identity cards, credit cards, debit cards or the like, whereby the security element consists of a mixture of liquid crystal pigments of color-shift effect pigments and micro-encapsulated thermochromic liquid crystal pigments.

[0002] For protection from imitation, in particular using color copiers or other reproduction methods, data carriers, such as for example banknotes or cards, are equipped with optically variable security elements. There the protection from forgery is based on the circumstance that the visually and easily and clearly recognizable optically variable effect is not or not sufficiently reproduced by the above-mentioned reproduction apparatus.

[0003] Optically variable security elements are for example thin-layer elements consisting of a reflector layer, a dielectric layer and a partly transmissive layer. When viewing the security element from the side of the partly transmissive layer, the viewer perceives a certain color that changes upon changing the viewing angle.

[0004] The cause of the color alteration is an interference effect between the light beams that are reflected by the surface of the outer partly transmissive layer, and the light beams that pass through the outer partly transmissive and the middle dielectric layer, and are reflected back by the inner reflector layer to the partly transmissive layer. At the partly transmissive layer the light beams are thereupon either transmitted outwardly or reflected again, so that in this case the light beams are reflected back and forth several times between the reflective layer and the partly transmissive layer. Thus the light beams which have passed through the thin-film layer have covered a greater distance than the light beams reflected on the surface of the thin-film layer, so that they are phase-shifted in relation to these when they interfere with them.

[0005] When the light beams incident on the thin-film layer impinge on the thin-film layer at different angles of incidence, the distance covered by the light beams in the thin-film layer is of different length. This difference results from the difference in distance of the beams multiply reflected inside the thin-film layer that is changed by the angle of incidence. Therefore the phase relationship of the interfering light beams is different depending on the angle of incidence, so that, in dependence of the angle of incidence, there result different colors or color tones of the resulting light beam perceived by the viewer.

[0006] Such thin-layer elements can be used in the form of foils, such as described e.g.
in WO 2005/108110. From WO 2005/108110 a security element is known having a thin-film layer in which the individual layers are arranged over the entire surface on the security element. Into the layer sequence there are incorporated, by action of laser, markings in the form of patterns, letters, numbers or images. For this purpose the layer sequence contains a marking layer of a color mixture having a mixture component absorbing the laser radiation as well as a mixture component that is transparent for the laser radiation. The markings are visually recognizable and/or machine-recognizable due to an irreversible change of the optical properties of the color mixture caused by the action of the laser radiation.

[0007] Likewise pigment-shaped thin-layer elements are known that are added to a printing ink. EP 0 227423 B1 describes thin-layer elements with a symmetrical structure so as to ensure that the pigment looks the same from every side. Each of the thin-layer elements consists of a five-layer thin-film constructed from an inner metal reflector layer, a dielectric layer that is arranged below and above and has a refractive index of 1.65 or less, and of respectively one outer semi-opaque or partly transmissive layer. The five-layer thin film, on the side respectively facing the viewer, causes a color alteration, i.e. a change between two different colors or color tones at a first and a second viewing angle. Thus the optically variable color appears magenta-colored from a certain viewing angle and green from a different viewing angle. When a viewer tilts the data carrier, onto which such an optically variable color is applied, he perceives a color change from magenta to green or vice versa.

[0008] However, it is a disadvantage of the interference pigments that in certain illumination situations no optical effect can be perceived any longer. This is the case in particular when the light source and the viewer are located on the same side -with reference to the printing surface normal - for example at a point of sale.
Here the light usually comes from the ceiling, while the banknotes are put obliquely downwardly into the cash drawer. Doing so, the security features appear dark, so that for an authenticity check the banknote usually must be raised to eye level and held obliquely against the light source. This represents a noticeable additional effort and is moreover conspicuous for a customer.

[0009] For a pronounced color-shift effect, light as directional as possible is moreover necessary. Particularly indirect, diffuse light, as is frequently used for the uniform illumination of goods, leads to a strongly reduced color effect, since the light impinges on the pigments simultaneously from many different angles.

[0010] From EP 0357844 a thermochromic effect coating is known, whereby the coating comprises an aqueous binding agent component containing a color-shift effect pigment and an encapsulated thermochromic liquid-crystal pigment. The described color-shift effect pigments here are aluminum pigments and mica pigments, whereby the aluminum pigments preferably consist of plate-shaped aluminum particles and the mica pigments are preferably coated with metal oxides. As is well-known, mica pigments coated with silicon dioxide and titanium dioxide, i.e. with inorganic interference layers, have been used since the mid-1980s and produce the so-called pearl luster effect.
Examples for this are the so-called Iriodin pigments of the company Merck. The mentioned color-shift effect pigments, however, are freely available on the market and thus have only a very low or no protection from forgery.

[0011] It is therefore the object of the invention to further develop a generic security element in such a fashion that the disadvantages of the prior art are remedied, the protection from forgery is further increased, and the recognizability of forgeries is improved.
[0013] According to the invention the color-shift effect pigments are anisotropic interference pigments. According to the invention the security element thus consists of a mixture of isotropic, micro-encapsulated thermochromic pigments with anisotropic interference pigments.
[0014] Anisotropic pigments within the meaning of this invention are pigments which do not or only to a small measure reflect back impinging light to the viewer when the light source and the viewer are located on the same side of the surface normal (viewed two-dimensionally).
[0015] Isotropic pigments within the meaning of this invention are pigments which reflect back impinging light in almost all spatial directions, inter alia also to the viewer.
The light source there can take any desired position relative to the viewer.
This signifies that the position of the light source relative to the viewer does not play a role. The isotropic pigments, however, can e.g. contain anisotropic structures on their inside. For this case isotropic within the meaning of this invention signifies that the pigments are present in the printing layer in a largely isotropically oriented fashion. It is then also achieved as effect that the light source can take any desired position relative to the viewer without resulting in a loss of color effect.
[0016] The described unfavorable behavior of security elements of the state of the art results from the plate-shaped structure of the used pigments. Impinging light cannot or only to a small measure be thrown back to the viewer when the light source and the viewer are located on the same side of the surface normal (viewed two-dimensionally).
By the combination according to the invention of anisotropic color-shift pigments with isotropic pigments thus a color shift effect can be achieved in a particularly advantageous fashion also in the unfavorable illumination situations. This results in an increased security vis-à-vis forgery and the possibility to produce new color-shift effects. Further the feature can be individualized in several stages by laser radiation.
[0017] According to the invention the isotropic thermochromic pigment is formed by a micro-encapsulated thermochromic liquid crystal. A micro-encapsulated thermochromic liquid crystal is characterized in that it has a temperature-dependent color reflection characteristic: Starting at a certain trigger temperature the capsules reflect visible light of a certain circular polarization direction. With rising temperature this reflection is shifted from the long-wave to the short-wave range. Thus a changed color shift effect is achieved above the trigger temperature of the liquid crystal.
[0018] Such micro-encapsulated thermochromic liquid crystals are for example known from DE 10243 650 Al.
[0019] The micro-encapsulated thermochromic pigments are preferably partly transparent and are particularly preferably configured to be transparent.
Translucence within the meaning of this invention signifies that an object such as the substrate or the plastic foil allows a certain proportion of impinging light to pass through.
When light impinges on one side of the object a certain proportion of the light is allowed to pass through to the other side of the object and exits there again. The greater the percentage of passing light in relation to the impinging light is, the more translucent the object is.
When the percentage is at least 90%, i.e. when the object allows the impinging light to pass almost unattenuated like in the case of a window, the object is referred to as transparent. An object which allows between 90% and 20% of the impinging light to pass is referred to as partly transparent. In contrast, an object which allows less than 20%, preferably under 10% and particularly preferably around 0% of the impinging light to pass through, i.e. in which the proportion of the passing light in relation to the impinging light is small or almost or equal to zero, is referred to as opaque or light-non-transmissive.
[0020] The anisotropic interference pigments or pigment-shaped thin-layer elements according to the invention are preferably constructed in three layers and consist of a reflective layer, a dielectric layer and a partly transmissive layer. When the security element is viewed from the side of the partly transmissive layer, the viewer perceives a certain color, which changes upon changing the viewing angle. As materials for these layers there are used in particular:
for the reflective layer all reflecting substances, in particular metals, such as aluminum or copper, for the dielectric layers Si02 (silicon dioxide), Zr02 (zircon dioxide) or TiO2 (titanium dioxide) or other transparent materials, for the partly transmissive layers chromium and/or nickel.
[0021] The anisotropic interference pigments or pigment-shaped thin-layer elements according to the invention are configured particularly advantageously in five layers. Here they consist of a middle reflective layer, two dielectric layers surrounding the middle reflective layer on each side, and two outer partly transmissive layers. A
five-layer pigment thus consists of a lower partly transmissive layer of chromium and/or nickel, followed by a dielectric layer of silicon dioxide or magnesium fluoride, a middle reflective layer of aluminum, to which there adjoins on the opposite side again a dielectric layer of silicon dioxide or magnesium fluoride, as well as finally of an upper partly transmissive layer of chromium and/or nickel. Light impinging on the upper or lower side of the pigment is partly reflected and partly transmitted by the partly transmissive layer. The latter proportion passes through the dielectric layer, which has a refractive index that differs from that of the partly transmissive layer, and is reflected back by the reflective layer and, after passing the dielectric layer, split again into a transmitted and a reflected proportion.
[0022] It is a special advantage of the five-layer anisotropic interference pigments in comparison to the three-layer ones that the five-layer anisotropic interference pigments show a color-shift effect both when viewed from the upper side and the lower side. In the three-layer anisotropic interference pigments in contrast no color-shift effect takes place when viewed from the lower side, i.e. from the reflective layer.
[0023] The anisotropic interference pigments or pigment-shaped thin-layer elements according to the invention are advantageously constructed in seven layers and consist of a middle layer, two reflective layers surrounding the middle layer on each side, two dielectric layers respectively surrounding the reflective layers on the side respectively facing away from the middle layer, and two outer partly transmissive layers.
The middle layer is particularly preferably configured magnetically, i.e. its material has magnetic properties, whereby the magnetic property can be detected preferably by corresponding sensors. Such materials with magnetic properties are preferably ferromagnetic materials, and here preferably magnetically soft or hard materials or ferromagnetic materials and here preferably ferrites. As material for the reflective layers, the dielectric layers and the partly transmissive layers in particular the same materials are used as for the above-mentioned three- or five-layer anisotropic interference pigments or pigment-shaped thin-layer elements.
[0024] However, the invention is not limited to three-, five- or seven-layer pigments, but can be applied to all single- or multilayer pigments which produce an optically variable effect. In particular, from the state of the art optically variable layers are known which have more than two layers proceeding from the reflective layer in the direction of the viewer. The invention can also be applied to pigments which are made of such optically variable layers and thus have more than five layers.
[0025] The invention comprises both the security element itself and the security document onto which the security element is applied. Here the security document is for example a bank note, a paper of value, a credit card or an identity card, a passport, a certificate and similar, a label, a package or an element for securing products.
[0026] Preferably the security element according to the invention is printed directly onto the security document.
[0027] Alternatively the security element is printed on a substrate, which is in turn applied on the security document. The application on the security document preferably takes place by known transfer procedures, for example by means of thermal transfer, sealing, gluing or pressing.
[0028] Preferred printing methods are for example screen printing, flexographic printing and intaglio printing.
[0029] The substrate consists particularly preferably of paper of cotton fibers such as is used for example for banknotes. By a double-sided coating or lining with respectively one plastic foil a security paper is achieved as described in DE 102 43 653 A9.
[0030] Preferably the substrate can also consist of paper of other natural fibers, preferably likewise of synthetic fibers, i.e. a mixture of natural and synthetic fibers. or preferably likewise of at least one plastic foil, which consists of a polymer for example.
Further preferably the substrate consists of a combination of at least two different substrates arranged above each other and cormected with each other, a so-called hybrid, for example a combination of plastic foil - paper - plastic foil, i.e. a substrate of paper is covered on each of its two sides by a plastic foil, or paper - plastic foil -paper, i.e. a substrate of a plastic foil is covered on each of its two sides by a substrate of paper.
[0031] Specifications regarding the weight of the used substrate are for example specified in the document DE 10243 653 A9, whose explanations in this regard are included in their entirety in the present application. The document DE 10243 explains in particular that the paper layer usually has a weight from 50 g/m2 to 100 g/m2, preferably of 80 g/m2 to 90 g/m2. Any suitable weight can of course be used, depending on the application.
[0032] Preferably the security document or the substrate, in the region in which the security element is printed, has a dark color at least in a partial region.
Dark blue, violet, brown and dark red are particularly preferred. It is ensured thereby that the isotropic thermochromic pigment can be optically recognized particularly well. A further advantage of a dark background color is that the second checking plane (check of the reflected circularly polarized light, see below) shows a contrast that is the greater, the less unpolarized light is scattered back by the background.
[0033] When the security element is below the trigger temperature it shows the pure color shift effect of the interference pigment, for example from green to magenta, superimposed by the background color. By using different background colors or background grids special effects can be achieved here. Thus in the background coloring a color tone can be used which is complementary to a certain appearing color of the temperature pattern of the printing layer according to the invention, contributing to a stronger contrast and a better recognizability. When the security element is above the trigger temperature a changed color shift effect is shown, which is additionally also shown in a situation of illumination from behind.
[0034] The interference pigments are preferably configured to be opaque.
Within the meaning of this invention the term "opaque" signifies that the pigments have a light transmission below 20%, preferably below 10%. This execution is advantageous in that the background color does not cover the color shift effect of the pigment in the places in which there are opaque pigments.
[0035] According to a preferred embodiment it is provided that the thermochromic liquid crystal reflects right-handed or left-handed circularly polarized light. This is achieved in that the liquid crystal is present in a chiral nematic phase. Through this characteristic a second checking level can be realized. This second checking level can be implemented by means of a circular polarizer held in front of the security feature. If the latter is transmissive e.g. only for right-handed circularly polarized light, a security feature according to the invention that was configured with a thermochromic liquid crystal which reflects exclusively right-handed circularly polarized light appears light, whereas a security feature that was printed with a thermochromic liquid crystal which reflects exclusively left-handed circularly polarized light appears dark viewed through the circular polarizer.
[0036] According to a further preferred embodiment it is provided that the isotropic pigments consist of other spherical pigments. Preferably for example spherical liquid crystal pigments which do not show a thermochromic effect. These can for example be manufactured in that a cholesteric liquid crystal with cross-linkable functionalities is micro-encapsulated and is subsequently cross-linked at a defined temperature, so that the desired color is maintained.
[0037] According to a further preferred embodiment it is provided that several anisotropic, opaque pigment types are combined with several isotropically reflecting pigment types. Also the additional use of translucent anisotropic pigments is possible.
[0038] Since the security feature is determined exclusively by the anisotropic pigments below the trigger temperature, the combination with a thermochromic liquid crystal is very well suitable as an additional security feature complementing existing security features. Combinations are for example with commercially usual pearl luster pigments on the basis of mica or synthetic glass.
[0039] The quantity ratio of anisotropic interference pigments to isotropic liquid crystal pigments preferably amounts to 1:20 to 5:1 and particularly preferably 1:10 to 2:1. By adjusting the quantity ratio the optical effect can be changed. When e.g. an anisotropic interference pigment is employed that has a very strong optically variable effect and/or a high optical brilliance and/or a very strong opacity, the proportion of this pigment in the mixture can be smaller than in the case that a pigment with a hardly pronounced optically variable effect and/or minor brilliance and/or weaker opaque behavior is used.
[0040] With the help of the following embodiment examples and the additional figures the advantages of the invention are explained. The exemplary embodiments represent preferred embodiments, to which the invention, however, shall be in no way be restricted. Furthermore the representations in the figures are strongly schematic for easier understanding and do not reflect the actual conditions. In particular, the proportions shown in the figures do not correspond to the dimensions present in reality and exclusively serve for the improvement of clarity. Furthermore, the embodiments

12 described in the following embodiment examples are reduced to the substantial core information for the sake of greater clarity. In practical implementation substantially more complex patterns or images can be used.
[0041] In the Figures the following is schematically shown:
[0042] Fig. 1 a security feature of the state of the art at different illumination angles, [0043] Fig. 2 further embodiments of a security element according to the invention, [0044] Fig. 3 a microscopic image of a printing layer according to the invention;
[0045] Fig. 4 a multi-stage laser structuring of a security element according to the invention, [0046] Fig. 5 an embossed structuring after intaglio embossing.
[0047] Fig. 1 shows the appearance of a conventional security element 2 of the state of the art in dependence on different illumination situations, using the example of an information item 4 in the form of the number "50". This security element 2 consists exclusively of anisotropic interference pigments which are bound in an aqueous binding agent system and have been applied on a paper substrate. The security element 2 is below the trigger temperature. Merely a pure color shift effect of the interference pigment is shown, for example from green to magenta, superimposed by the background color.
[0048] Fig. la here shows the security element according to the invention in plan view, Fig. lb in an oblique view against the light and Fig. 1 c in an oblique view with illumination from behind.

13 [0049] According to Fig. la light from a light source 1 impinges obliquely on the security element 2 with the anisotropic interference pigments. A viewer 3 looking at the security element 2 perpendicularly, recognizes the number 50 in the color green.
[0050] When, according to Fig. 1 b, the viewer looks at the security element 2 obliquely, the number 50 appears in the color magenta.
[0051] When, according to Fig. 1c, the illumination now takes place obliquely from behind, i.e. when the light source 1 is located behind the viewer, who looks at the security element 2 obliquely, the number 50 appears black or in the background color. In the illumination situation according to Fig. lc impinging light thus is not or only to a small measure thrown back to the viewer, since the light source and the viewer are located on the same side of the surface normal (viewed two-dimensionally).
[0052] Fig. 2 shows further embodiments of a security element according to the invention. Here the security element is above the trigger temperature, so that a changed color shift effect shows, which additionally also shows in a situation of illumination from behind.
[0053] Fig. 2a here shows the security element according to the invention in plan view, Fig. 2b in an oblique view against the light and Fig. 2c in an oblique view with illumination from behind. An anisotropic effect pigment was used which causes a color flop from green (plan view) to magenta (against the light), in the mixture with a micro-encapsulated thermochromic liquid crystal. In plan view the thus fabricated security element appears in a green color tone (Fig. 2a), upon transition to an against-the-light situation the color flop takes place to an intensive blue-violet color tone (Fig. 2b). In the situation of illumination from behind represented in Fig. 2c a clear green-blue color is perceivable.

14 [0054] Fig. 3 shows a microscopic image of a printing layer according to the invention, consisting of the plate-shaped anisotropic interference pigments 6 and isotropically shaped pigments 7 in the form of round thermochromic liquid crystal pigments.
[0055] Fig. 4 sketches a multi-stage laser structuring of a security element according to the invention, which consists i.a. of thin film pigments according to DE 10 2006 062281.
Proceeding form the unchanged security feature (Fig. 4a), consisting of the dark background 8 and the mixture of isotropic thermochromic liquid crystal pigments 7 and anisotropic interference pigments 6, three different stages can essentially be produced by suitably choosing the laser parameters, such as known for example from DE 10 2006062281:
= Fig. 4b: silver colored, thermochromic effect intact: low power density, the anisotropic effect pigments 6 no longer show a color flop, but appear silver gray, the isotropic liquid crystal pigments 7 are functional and the background 8 was not changed either, = Fig. 4c: color of the background 8 visible, thermochromic effect intact:
medium power density, the anisotropic pigments 6 were completely removed and thus no longer contribute to the color impression of the feature; the effect is now produced only by the thermochromic liquid crystals 7 in connection with the color of the background 8, = Fig. 4d: color of the background visible, thermochromic effect destroyed:
high power density, also the thermochromic effect pigments 7 were destroyed and are represented a destroyed micro capsules 10, the element now no longer shows a temperature dependence and color flop, but merely the color impression of the background 8.

[0056] Here the strength of the silver effect is adjusted in several stages, such as known for example from DE 10 2006 062281.
[0057] A printing layer according to the invention can be structured by an embossing, for example by intaglio embossing in a steel engraving machine. Fig. 5 shows such an embossing, carried out with a machine for steel engraving with embossed regions 11 in the form of rhomboid patters next to unembossed regions 12. The edges of the embossed structures of the embossed regions 11 were emphasized. In the regions that are subjected to the maximal pressure, the thermochromic liquid crystal pigments are destroyed and the pure effect of the interference pigment remains, whereas in the surrounding regions anisotropic interference pigments and isotropic liquid crystal pigments are statistically distributed.

List of reference numbers 1 light source 2 security element 3 viewer 4 information 6 anisotropically formed pigments 7 isotropically formed pigments 8 dark background 9 pigment with destroyed interference effect destroyed micro-capsule 11 embossed region 12 unembossed region

Claims (11)

Claims:

1. A security element for increasing the protection from forgery of security documents, such as banknotes, value papers, identity cards, credit cards, debit cards or the like, whereby the security element consists of a mixture of color-shift effect pigments and encapsulated thermochromic liquid crystal pigments, wherein the color-shift effect pigments are anisotropic interference pigments, which do not or only to a small measure reflect back incident light to a viewer when the light source and the viewer are located on the same side of the surface normal.

2. The security element according to claim 1, wherein the anisotropic interference pigments are constructed in three layers and consist of a reflective layer, a dielectric layer and a partly transmissive layer.

3. The security element according to claim 1, wherein the anisotropic interference pigments are constructed in five layers and consist of a middle reflective layer, two dielectric layers surrounding the middle reflective layer on each side and two outer.
partly transmissive layers.

4. The security element according to claim 1, wherein the anisotropic interference pigments are constructed in seven layers and consist of a middle, where applicable magnetic layer, two reflective layers surrounding the middle layer on each side, two dielectric layers respectively surrounding the reflective layers on the side respectively facing away from the middle layer, and two outer, partly transmissive layers.

5. The security element according to any one of claims 1 to 4, wherein the security element is printed on the security document.

6. The security element according to claim 5, wherein the security document has at least partly a dark color in the region in which the security element is printed.

7. The security element according to any one of claims 1 to 4, wherein the security element is printed on a substrate.

8. The security element according to claim 7, wherein the substrate is applied on the security document.

9. The security element according to any one of claims 7 or 8, wherein the substrate has at least partly a dark color in the region in which the security element is printed.

10. The security element of claim 1, wherein the light source and the viewer are located on the same side of the surface normal when viewed two-dimensionally.

11. The security element according to any one of claims 1 to 10, wherein the micro-encapsulated thermochromic pigments are configured preferably to be partly transparent and particularly preferably to be transparent.