CA2667704A1

CA2667704A1 - Security element

Info

Publication number: CA2667704A1
Application number: CA002667704A
Authority: CA
Inventors: Christof Baldus; Max Voit; Christoph Mengel
Original assignee: Individual
Current assignee: Giesecke and Devrient GmbH
Priority date: 2006-10-27
Filing date: 2007-10-26
Publication date: 2008-05-02
Also published as: EP2084016A1; EP2084016B1; WO2008049632A1; ATE529269T1; DE102006051524A1

Abstract

A security element or a security document provided therewith comprises a substrate (1), a recess (3) which passes through the substrate, and an imprint (4) which adjoins the recess (3) and with the recess completes an information-conveying symbol or pattern (~20"). At least the recess (3) is covered by a film (5). A security feature (6), which can be visually checked and is preferably provided at least partially on the film (5), conveys a colour impression which varies due to an external stimulus and reverses on its own accord, and is arranged relative to the information-conveying symbol or pattern (~20") such that the symbol or pattern (~20") is obscured without the stimulus and the visual recognizability of the symbol or pattern is facilitated by way of the stimulus.

Description

Security element [0001] This invention relates to a security element for product protection, for ex-ample in the fonn of a label, seal, package or other product security element, but in particular for use in security documents, such as bank notes, shares, deeds, postage stamps, checks, check cards, credit cards, identity cards, passports, admission tick-ets, travel tickets, air tickets and the like. The security element can be integrated as such in the security document or be applied to the security document in the fonn of a patch or security strip.

[0002] The invention relates in particular to those security elements or security documents that possess a see-through window. A see-through window is fonned by an opening penetrating the substrate material and closed by means of a transparent film. WO 2006/018171 A2 describes such a security element in connection with a bank note. The bank-note substrate, which normally consists of paper but can also consist of plastic, has an information-conveying print provided thereon. The gap penetrating the substrate is restricted to partial areas of the infonnation-conveying print. Since the base surface or the surroundings of the print are light, normally white, the information is difficult to recognize when the bank note is viewed against a light background. Only against a dark background do the open areas appear dark and supplement the non-open areas of the information-conveying print so that the infonnation becomes recognizable in total. A reverse effect is achievable when the information-conveying print is printed with light ink on a dark base surface.
Then the total information is clearly recognizable when the bank note is viewed against a light background.

[0003] In WO 2006/018171 A2 it is furthermore proposed to cover the total print including the gaps with a transparent film which is provided with a liquid-crystal layer. The liquid-crystal layer is transparent and substantially colorless in transmit-ted light. However, over the dark print it appears in a characteristic dark color while it appears transparent against the light base surface. When the document is viewed against a dark background, the dark color in the area of the gaps, and thus the in-formation in total, also becomes visible. Alternatively, a partial area of the infonna-tion adjacent to the gap can also be realized as a dark print with a liquid-crystal layer printed thereover, and the other partial area of the infonnation as a liquid-crystal layer on the film covering the gap. Upon viewing against a dark background, the two parts then complement each other again to form the total infonnation and show the dark colors characteristic of liquid crystals.

[0004] The previously described security element permits an authenticity check by being viewed first in transmitted light and then against a dark background.
It can be imitated only with great effort, since various technical challenges must be over-come, such as production tolerances of the gaps relative to the information-conveying print, and exactly registered positioning of the film strip relative to the infonnation-conveying print, and the like. The security element is thus easy to check and hard to imitate.

[0005] It is the object of the present invention to further improve the security ele-ment with regard to checkability and protection from imitation.

[0006] This object is achieved according to the invention by a security element having the features of claim 1 and by a security document equipped with such a security element. Advantageous developments and refinements of the invention are stated in dependent claims. Furthermore, an inventive method for producing the security element is also stated.

100071 According to the invention, the combinational effect between the print and the gap (whereby there may also be a plurality of gaps) which complement each other to form an information-conveying sign or pattern, for exainple a code, or at least make a contribution thereto, is combined with a further security feature which exerts an additional decisive influence on the recognizability of the infonnation.

[0008] Specifically, the information-conveying sign or pattern is veiled by the further security feature, so that it not, not completely or at least not readily, recog-nizable. The further security feature is characterized in that it conveys a color im-pression changeable by an external stimulus, so that the simple visual recognizabil-ity of the information-conveying sign or pattern is made possible or at least facili-tated only by means of the stimulus.

[0009] The term "stimulus" will hereinafter be used synonymously with the Ger-man term "Reiz", as is also known from the free encyclopedia Wikipedia (http://de.wikipedia.org/wiki/Reiz). The stimuli to be used in the context of this ap-plication are primarily physical stimuli such as temperature, pressure, light, sound, magnetism, electrical voltage, etc. It is self-evident that e.g. the stimulus "tempera-ture" means in physical terms a heat transfer to the security feature or a heat re-moval from the security feature. Accordingly, the stimulus "light" is understood physically to mean the action of electromagnetic radiation of the visible and invisi-ble wavelength ranges on the security feature. Besides the physical stimuli, it is ba-sically also possible that chemical stimuli change the color impression of the secu-rity feature. For example, the color impression can be effected by the moisture in the form of water or other liquids acting on the security element or by any chemical compounds in the gaseous state of aggregation. Hereinafter a chemical stimulus will therefore be understood quite generally as the action of liquid, gaseous or solid chemical compounds on the security element.

[0010] The color impression conveyed by the security feature is independently reversible, i.e., in the absence of the external stimulus the security feature changes its color until it has the color prior to action of the stimulus again, i.e.
evokes the original color impression for the viewer again. According to the invention, the in-dependent reversibility applies to all forms of stimuli, in particular physical and chemical stimuli.

[0011] Depending on the material used for the security feature, the external stimulus can be temperature, pressure, electrical voltage, light (electromagnetic ra-diation), sound and/or magnetism, i.e. the security feature can thus comprise ther-mochromic, piezochromic, electrochromic, photochromic material or material react-ing with a color change to sound waves and/or magnetism. It is evident that the color of a security feature used might also be changed by different stimuli.
It is e.g.
known to the person skilled in the art that certain piezochromic compounds also have thermochromic properties at the same time. For chemical stimuli a suitable material is accordingly used, e.g. a hygrochromic compound.

[0012] The further security feature can be linked combinationally in a great vari-ety of ways with the information-conveying sign or pattern formed by the print and the gap. As a development of the above-described exemplary embodiment from WO 2006/018171 A2 wherein the information-conveying signs including their gaps are covered by a liquid-crystal-coated film, the liquid-crystal coating can e.g. be replaced by a thermochromic layer which is opaque at the normal temperature of use, generally ambient temperature, and becomes transparent upon a temperature increase or optionally also upon a temperature reduction. The information-conveying signs or patterns are only recognizable when at the same time the neces-sary temperature increase or temperature reduction has taken place to make the se-curity feature transparent, and the security element is viewed against a suitable background upon which the print complements the background visible through the gaps to form the information-conveying signs or patterns. It is therefore advanta-geous when the color impression of the security feature changes from opaque to transparent under the influence of the stimulus.

[0013] Alternatively, the liquid-crystal coating known from the prior art can also itself possess thermochromic properties, by the liquid-crystalline material being in the liquid state and thus transparent at the normal temperature of use, and passing into the liquid-crystalline state and conveying the color impression characteristic of the particular liquid-crystalline material at an accordingly reduced temperature. The information-conveying signs or patterns are in this variant only completely recog-nizable with the typical color impression when both the temperature reduction has taken place and viewing against a suitable background is done.

[00141 The "veiling" of the information-conveying signs or patterns by the secu-rity feature in the absence of the stimulus does not necessarily require a complete covering of the signs or patterns by the security feature. It is also possible for only a part of the gap and/or a part of the print contributing to infonnation conveyance to be covered by the changeable security feature.

[0015] It is even possible that neither the gap nor the print is covered by the se-curity feature. For example, the security feature can immediately adjoin the infor-mation-conveying print and e.g. convey the same color impression as the print it-self, for example black or any another color, at the usual temperature of use.
Under the action of the external stimulus the color impression of the security feature then changes from black to e.g. white or from a first color to a second color such that the information-conveying print stands out therefrom in contrasting fashion and, when the security element is then viewed against a suitable background, the print now no longer veiled complements the background color visible through the gap to forin the complete information.

[0016] The "color impression" according to the invention is thus not to be under-stood as solely involving true, i.e. pure, colors, but also black and white as well as transparent. A change from cloudy to clear can also be used for veiling and is there-fore to be understood as a changing color impression according to the invention.
[0017] According to a variant, (only) the part of the print contributing to infor-mation conveyance can have a negative edge contour line, for example a light con-tour line in otherwise dark surroundings. Then the information-conveying signs or patterns are poorly recognizable upon viewing of the security element against a dark background, since the dark background visible through the gap does not stand out clearly enough from the dark surroundings of the information-conveying sign or pattern. Against a light background, however, the gap then appearing light comple-ments the light edge contour line of the part of the print contributing to infonnation conveyance to form the total information.

100181 When the gap additionally also has a light edge contour line, the complete information is recognizable both against a light and against a dark background, in the case of a dark background as a complete dark sign or pattern with a complete light negative edge contour line.

[0019] The print is preferably an offset print or Nyloprint typical of security documents such as bank notes. This permits one- or multi-colored filigree guilloche patterns to be printed.

[0020] The substrate material used is primarily paper, in particular cotton vellum paper. It is of course also possible to use paper containing a proportion x of poly-meric material in the range of 0< x < 100 wt%.

[0021] Further, it is possible that the substrate material is a plastic film, e.g. a polyester film. The film can further be uniaxially or biaxially stretched. One result of the stretch of the film is to give it polarizing properties, which can be used as a further security feature. The aids required for utilizing said properties, such as po-larizing filters, are known to the person skilled in the art.

[0022] It can also be expedient if the substrate material is a multilayer laminate which has at least one layer of paper or a papery material. Such a laminate is char-acterized by exceptionally high stability, which is of great advantage for the dura-bility of the security feature and furthermore increases the forgery resistance.

[0023] However, it is also possible to use a multilayer, paper-free composite ma-terial as a substrate material. Such materials can also be used advantageously in certain climatic regions of the earth.

[0024) All materials used as the substrate material can have additives that serve as authenticity features. These can primarily be luminescent substances which are preferably transparent in the visible wavelength range and can be excited by a suit-able aid, e.g. a radiation source emitting UV or IR radiation, in the invisible wave-length range to produce a luminescence that is visible or at least detectable with aids. Other security features can also be used advantageously provided they do not, or hardly, impair the viewing of the print.

[0025] The film can be configured as a patch covering a partial area of the sub-strate or as a strip extending over the total length or width of the security document.
Materials to be used for the film are primarily plastics, in particular PET
(polyeth-ylene terephthalate), PBT (polybutylene terephthalate), PEN (polyethylene naphtha-late), PP (polypropylene), PA (polyamide), PE (polyethylene). The film can further be uniaxially or biaxially stretched.

[0026] The gap can be formed either during or after the production of the sub-strate. In the former case, a further security feature is created due to the fibers pro-truding irregularly into the gap, as described more closely in WO 03/054297 A2.
The disclosure of WO 03/054297 A2 is thus incorporated in the present application by reference. In the latter case, the gap is produced after the production of the sub-strate, e.g. by punching or laser beam cutting. Suitable aids for this purpose, such as punching and cutting apparatuses as well as lasers, are basically known to the per-son skilled in the art.

[0027] The application of the security feature to the film is effected before or preferably after the application of the film to the substrate. In the latter variant, the security feature advantageously extends beyond the film onto the substrate.
This makes imitation more difficult, since ink adheres differently to a film, on the one hand, and to a substrate preferably of paper, on the other hand, and not every ink composition is suitable therefor.

[0028] The security feature is preferably present at least partly on the film.
It is particularly preferable if the security feature is printed on the film and optionally furthermore on the adjacent substrate by the intaglio printing process.
Firstly, this makes it possible to produce tactile structures characteristic of intaglio printing, which come, on the one hand, from the pasty printing inks used which are applied to the printed substrate and project tactilely from the substrate surface. On the other hand, the tactile structures come from the printed substrate being pressed during the printing operation into the depressions filled with printing ink in the intaglio print-ing plate used for printing, and thereby being "embossed". Secondly, the printing of films by the intaglio printing process requires considerable expert skill, in particular to avoid tears in the film through the action of the engraving plate, and therefore offers additional protection against forgery.

[0029] The part of the print contributing to information conveyance can advanta-geously be applied on both sides of the substrate either congruently or complement-ing each other in transmitted light. In both cases a simultaneous printing machine is required for obtaining the desired congruence. This gives the security element addi-tional protection against forgery.

[0030] The film can comprise further layers. Thus, the security feature and a transparent or semitransparent effect layer can cover each other at least partly to thereby integrate a supplementary security feature into the security element.
The effect layer can comprise thin-film pigments with interference effects, such as so-called IRIODIN from the company Merck. But other optically variable security features can also be used, such as transparent and/or non-transparent holographic elements, diffractive structures and the like.

[0031] Furthermore, the film can be partially metallized, the inetallization being aligned in an accurate fit with the print or with a separate background print upon application of the film to the substrate. This measure also makes forgery more diffi-cult, since machines that work particularly exactly are required for the accurately fitting application of such a security strip. The fit factor of the film relative to the print is preferably 0.3 mm to 0.5 mm or therebelow.

[0032] The formation of the gap in an accurate fit relative to the print is also of special importance, since even small tolerances in the interaction with the part of the print contributing to information conveyance are striking to the viewer.
If the gap is punched, the fit factor of the gap relative to the print should be less than 0.2 mm, at a punching width of preferably at least 2 mm to about 3 mm.

[0033] The inventive security element can be used independently, for example as a label, seal, package or other product security element. However, it is in particular suitable for covering a through hole in a security document.

[0034] Security documents to be used are in particular bank notes, but also shares, deeds, postage stamps, checks, check cards, credit cards, identity cards, passports, admission tickets, travel tickets, air tickets and the like.

[0035] Instead of applying the security element to the security document as a separate element, it can also be integrated in a security document, in which case the substrate of the security element is formed by the security-document material and the gap of the security element realized as a through hole in the security document.
[0036] The invention will hereinafter be described by way of example with ref-erence to the accompanying drawings. Therein is shown:

Fig. 1 a bank note having a security feature according to a first exemplary em-bodiment, Fig. 2 the bank note from Fig. 1 in cross section, Fig. 3 a bank note having a security element according to a second exemplary embodiment, Fig. 4 a bank note having a security element according to a third exemplary embodiment, Fig. 5 a bank note having a security element according to a fourth exemplary embodiment, Fig. 6 the bank note from Fig. 5 in cross section, and Fig. 7 a bank note having a security element according to a fifth exemplary embodiment.

[0037] Fig. 1 shows a security document, here by the example of a bank note, which is equipped with a security element 2. It is evident that the security docu-ments described hereinafter have, in addition to the security element 2, further secu-rity elements or security features (not shown), such as security prints, color-changing coatings, holograms, diffraction structures, see-through registers, etc. The security element 2 of Fig. 1 is integrated into the bank note in so far as a through hole 3 in the substrate of the bank note is a component of the security element 2.
The through hole 3 constitutes a see-through area or a "see-through window" in the document. The security element 2 furthermore comprises a print 4 on the bank-note substrate 1, a film 5 completely covering the through hole 3 and the print 4, and a coating 6 constituting a special security feature by its color impression being re-versibly changeable by an external stimulus.

[0038] First, the print 4 is printed on the substrate 1 with a color contrasting with the substrate 1, for example with a black printing ink. For this purpose, there can be used a printing method usual in bank-note printing, such as offset printing or Nylo-print. Subsequently, the through hole 3 (here a plurality thereof) is punched out of the substrate 1 in an accurate fit with the print 4, so that the corresponding gaps to-gether with the print 4 convey the information "20". It is of course basically also possible to form the gap prior to application of the print, e.g. according to WO
03/054297 A2 during production of the substrate, e.g. papermaking, or after the production of the substrate but before printing of the substrate with the print. A
transparent film 5 is applied thereover so that the information "20" is particularly clearly visible when the bank note is viewed against a dark background.

[0039] In the shown exemplary embodiment, the transparent film 5 is expedi-ently already coated with the security feature 6 at the time of application of said film to the substrate 1. However, the coating can also be applied, e.g.
printed, only in a subsequent step and can then extend in particular beyond the film 5 onto the substrate 1. The security feature 6 can furthermore also be restricted to one or more partial areas of the film 5. It is also possible that the security feature has at least in partial areas a piece of information individualizing the security element or security document in the form of an alphanumeric character, bar code, pattern, microwriting, etc. Such a piece of information is omitted in the drawing, however, for reasons of clarity.

[0040] The security feature 6 has thermochromic, piezochromic, electrochromic, photochromic, hygrochromic properties or ones reacting with a color change to sound waves and/or magnetism, namely such that in normal use it is opaque or cloudy and obstructs the view of the information "20" therebelow, and in the stimu-lated state it reveals the view of the information "20". However, the infonnation "20" is optimally recognizably only when the bank note is viewed against an appro-priately dark background so that the see-through areas 3 complement the print 4 to form the total information "20".

[0041] Hereinafter, variants of the security feature 6 will be explained.
Micro-encapsulated, organic, thermochromic three-component dye system [0042] A first example of a thermochromic material of the security feature 6 with which the film 5 can be printed or coated is a micro-encapsulated, organic three-component dye system. The dye system comprises an electron-rich organic dye, the color former, which is also referred to as an "electron donor", a color developer, which is also referred to as an "electron acceptor", and a suitable solvent.
Said three components are micro-encapsulated, typically in gelatin. The electron donor used is a so-called leuco dye, in particular spirolactone, fluoran, spiropyran or fulgide. The electron acceptor used is a weak acid, for example bisphenol A (4,4'-methylethylidene-bisphenol), alkyl-p-hydroxybenzoates, 1,2,3-triazoles or 4-hydroxycoumarin. The solvent used is a polar solvent, for example alcohols, ke-tones, esters or ethers. Such three-component mixtures are typically colored or opaque at lower temperatures and colorless at higher temperatures. The color change temperature can be adjusted by modifying the components. Preferably, the color change temperature of the micro-encapsulated three-component mixture is in the range of -10 C to 60 C, particularly preferably in the range of 25 C to 60 C. It is ideal when the color change temperature is between a usual ambient temperature of e.g. 25 C to 30 C and the body's temperature of 37 C, so that the color change is attainable by transfer of body heat. The above-described organic thermochromic mixtures are described in greater detail, with reference to further literature, in White/Leblanc "Thermochromism in Commercial Products", Journal of Chemical Education, 1999, page 1202.

[0043] The micro-encapsulated three-component mixture can be contained in printing ink coordinated with the intaglio printing process to thereby produce high-security intaglio printed products. Printed images produced by intaglio printing can be characterized in particular by tactility. Furthermore, intaglio printing can realize very fine line structures or microwriting that screen printing does not permit as such.

[0044] The printing ink is comparatively viscous and "stands" on the printed sub-strate surface, i.e. projects from the substrate surface in tactile fashion.
To obtain this, the printing ink comprises not only the micro-encapsulated three-component mixture but also a suitable binder, optionally furthermore filler, dryer, varnish and thinner. The proportion of micro-encapsulated three-component mixture in the printing ink is 10 wt.% to 40 wt.%, preferably 25 wt.% to 35 wt.%, the weight ratio of the three-component mixture to the binder being in the range of 1 to 6 to 2 to 1.
Suitable binders are rosin resins and rosin oils.

[0045] The dryer used can be a mixture of Mn/Co octoates, the thinner may be high-boiling mineral oils, and the varnish may be boiled-down resins. Suitable fill-ers are CaCO3, BaSO4 and Ti02. For producing the intaglio printing ink, the micro-encapsulated three-component mixture is mixed as a pigment, directly or as an aqueous suspension, with a suitable binder and optionally additives, such as fillers, solvent, varnish, thinner, dryer, etc. The weight ratio of colored pigments to the three-component mixture can be in the range of 1 to 40 to 1 to 1, and the weight ratio of effect pigments to the three-component mixture in the range of 1 to 40 to 1 to 2.

[0046] It is thus possible to obtain for example with a yellow colored pigment and a three-component mixture which changes its color from blue to colorless upon a temperature increase, a printing ink that appears green below the color change temperature and yellow above the color change temperature. This can be particu-larly suitable for an exemplary embodiment to be described more closely hereinaf-ter.

[0047] To be suitable for the intaglio printing process, the inventive printing ink has a pasty consistency and possesses accordingly high viscosity values of prefera-bly 100 Pa=s to 600 Pa=s at 20 C and a shear rate of 4 s"l, particularly preferably of 200 Pa=s to 500 Pa=s at 20 C and a shear rate of 4 s-'.

[0048] The printed image produced by intaglio printing is preferably solid or characterized by very fine lines. Suitable printing plates for producing solid printed images produced by the intaglio printing process are described in EP 1 117 537 and in EP 1 119 457 B l.

[0049] Hereinafter, two concrete exemplary embodiments will be stated for the engraving ink.

Exemplary embodiment 1 [0050] The starting substance is the micro-encapsulated three-component mix-ture Matsui 25 Fast Black. This is a suspension of 70 wt.% pigment in 30 wt.%
wa-ter. It is concentrated so that the pigment is present. 23 wt.% of the dried pigment is ground with 62.5 wt.% transparent white (binder with filler, no. 9 SL 0700 from the company SICPA). This mixture was furthermore mixed with 2.3 wt.% of a Mn/Co octoate mixture (dryer, no. 870950 from SICPA), with 9.3 wt.% HT varnish (no.
850090 from SICPA) and with 2.9 wt.% high-boiling mineral oils (thinner, no.
859041 from SICPA).

Exe!nplar_y embodiment 2 [0051] 32.5 wt.% of the micro-encapsulated three-component mixture Matsui 25 Fast Black is mixed with the following components: 39.2 wt.% transparent white (binder with filler, no. 9 SL 0700 from SICPA), 2.3 wt.% of a Mn/Co octoate mix-ture (dryer, no. 870950 from SICPA), 23.2 wt.% HT varnish (no. 850090 from SICPA), 2.8 wt.% high-boiling mineral oils (thinner, no. 859041 from SICPA).
[0052] Further details on the engraving inks used for the inventive security ele-ment can be found in the unpublished German patent application DE 10 2006 016 118.1, whose disclosure is thus incorporated in the present application by reference.
Thermochromic liduid-crystalline materials [0053] As a second example, the security feature color-changeable by the exter-nal stimulus comprises a micro-encapsulated, thermochromic, cholesteric liquid-crystalline substance. Cholesteric liquid crystals are supplied for example by the manufacturers Merck and TMC. In cholesteric liquid crystals the molecules are ori-ented in preferred directions, the preferred directions changing periodically over the layer thickness. The distance between these differently oriented layers changes with temperature, thus changing their color effect, which shifts through red, orange, yel-low, green, blue, violet with the march of temperature.

[0054] For use in inks, the liquid crystals are micro-encapsulated in gelatin or other encapsulating media. In White/Leblanc "Thermochromism in Commercial Products", Journal of Chemical Education 1999, pp. 1201 ff., cholesteric liquid crystals are explained in more detail with reference to further literature.
They usu-ally comprise a nematic host with chiral dopant. The rainbow-like color gradation becomes visible only upon heating and also normally only upon viewing against a dark background. At normal temperature the material is colorless. Upon further temperature increase, when the material passes from the liquid-crystalline state to the liquid state, the material that is colored in the liquid-crystalline state becomes transparent again.

Thermochromic cloudy-clear [0055] As a third example, polymer solutions can serve as the material for the security feature. A polymer solution breaks down into a polymer-rich and a poly-mer-poor solution phase when a critical value Xk is exceeded or undershot. The value of Xk depends on degree of polymerization, on temperature and of course on the system. Systems to be mentioned by way of example are the solution of poly(N-acryloyl-pyrrolidine) in water and polystyrene in cyclohexane. The value Xk nor-mally decreases with increasing temperature, i.e. the polymer solubility becomes greater. As of a critical temperature TucsT (upper critical solution temperature) the appearance of the polymer solution changes from cloudy to clear. However, it is also possible that the value X rises with increasing temperature, or with increasing temperature first drops and then rises again. The temperature at which the value X
reaches the critical limit Xk for the second time is referred to as the lower critical solution temperature TLCST. In the temperature interval between TUCST and TLCST the polymer is completely soluble and therefore clear. When the temperature drops be-low the value TucsT or increases above the value TLCST, then the polymer precipi-tates and the polymer solution becomes cloudy. This phenomenon is also referred to as temperature-induced segregation. The temperature interval in which the polymer is completely soluble is greater the smaller the molecular weight is.

[0056) By a suitable choice of polymer solution system, molecular weight and mixing ratio it is possible to adjust the color change temperature and the tempera-ture interval. By encapsulation of these systems e.g. in gelatin they can be admixed to printing inks. Printed over a window this permits the window to be switched al-ternatively transparent and cloudy by temperature variation. In the same way it is possible to use temperature variation to expose information previously applied to the window or to the substrate. The physical relations of thermochromic cloudy-clear polymers are described in Lechner/Gehrke/Nordmeier, "Makromolekulare Chemie" 2nd ed., pp. 183 ff.

Piezochromic and thermochromic metal complexes [0057] As a fourth example, compounds that change their color tone under pres-sure ("piezo") can also be used. Known piezochromic materials are metal com-plexes in a solvent. A slight increase of the dielectric constant of the solvent by pressurization influences the energy distribution within the metal coinplexes.
Due to the pressure the metal complexes assume a different electronic state (from "high spin" to "low spin" and vice versa). This causes a perceptible color change.
This is explained physically with reference to palladium complexes for example in Ta-kagi/Noda/Itoh/Iwatsuki "Piezochromism and Related Phenomena Exhibited by Palladium Complexes" in Platinum Metals Revision, 2004, 117 ff. Other known complexes are iron or nickel complexes.

[0058] The effect of the spin transition is a reversible, electrodynamic phenome-non which can be observed in numerous coordination compounds of elements of the first transition metal series with 4 to 7 electrons in the valency shell (configuration d4 to d7). This phenomenon occurs particularly frequently in iron(II) coordination compounds. The spin transition is not only pressure-dependent, however, but can also be caused by change of temperature, so that such materials possess both ther-mochromic and piezochromic properties. Furthermore, the spin transition at the same time involves a drastic change in the magnetic properties of the coordination compounds concerned. While the color change is recognizable with the naked eye, the spin transition can therefore also be detected by measuring magnetism.
Since the spin transition is completely reversible and the color of the complex compounds is based on light absorption by the valency electrons of their metallic central atom, the substances remain light- and colorfast even upon constant use.

[0059] In DE 103 07 513 Al there are described various iron(II) coordination compounds with different ligands. The color change associated with the spin transi-tion in iron(II) complexes with tetrazole, triazole and oxazole ligands from "high spin" to "low spin" is accompanied by an effortlessly recognizable color change from white to red or violet.

[0060] The thermochromic properties of the metal complexes can be observed without the metal complexes having to be in solution. However, the piezochromic properties require the presence of a solvent. In this case a micro-encapsulation of the metal complex solution e.g. in gelatin is required.

Piezochromic polymers [0061] As a fifth example, piezochromic polymers can be mentioned. In poly-mers whose molecules have a certain order and orientation, the color impression of the polymer compound can be changed by pressure. For example, liquid crystals rearrange themselves under pressure. This can be observed in the commonly known effect occurring when one presses on an LC display. The orientation of the mole-cules can be shaped such that interferences are produced that show an optically per-ceptible change by pressure. Suitable material classes are for example thermotropic as well as lyotropic liquid-crystal systems.

Electrochromic compounds [0062] As a sixth example, it is in principle also possible to use electrochromic compounds as a security feature in connection with the present invention.
Electro-chromic compounds are known in connection with so-called intelligent windows and automatically dimming rear-view mirrors. The glass is in such cases coated with electrochromic compounds that are able to change their color or transparency upon applied voltage. For checking the security feature it is therefore necessary to apply a voltage. It is therefore suitable in particular as a machine-readable authen-ticity feature.

[0063] The security feature is in this case of multilayer structure, the electro-chromic material being disposed adjacent to an electrolyte layer between two elec-trodes. The electrolyte is required for charge mobility. When a voltage is applied to the electrodes, ions migrate from the electrolyte layer into the electrochromic mate-rial or vice versa. This causes the electrochromic material to be reduced or oxi-dized, and thereby the color to change at the same time.

[0064] The most popular electrochromic material is W03 which is colorless when uncharged and blue colored when oxidized. Inorganic complex compounds and also a multiplicity of organic, conductive polymers are also electrochromic, such as polyaniline, polypyrrole, polythiophenes and derivatives thereo PEDOT
from Bayer (Baytron ) is also electrochromic. Organic materials such as so-called viologens (bipyridinium salts) are likewise electrochromic.

Photochromic compounds [0065] Finally, as a seventh example, photochromic compounds are to be men-tioned, such as fulgides, fulgidimides, spiro-dihydroindolizines and others.
Such compounds are described for example in EP 0 327 788 B 1 for use in printing inks.
The photochromic property (phototropy) is based on a change of the absorption be-havior of visible light when the compound is subjected to electromagnetic radiation of a given wavelength. Systems are known for example that change their color when they are irradiated with ultraviolet light. The original absorption spectrum reappears either independently after a certain time, in particular in the dark, or can be restored by heating. The material is micro-encapsulated or broken up into mi-croparticles of 1 gm to 20 m. It is suitable for most printing methods, including intaglio printing.

[0066] Hereinafter, further exemplary embodiments of the invention will be de-scribed. They constitute only a fraction of the possibilities of combination that re-sult from the fact that the above-mentioned materials suitable for the security fea-ture can be combined with each other, by for example being mixed into printing ink or being printed over each other and/or beside each other. Furthermore, possibilities of combination result from the fact that the materials can be provided completely or partly over the gap contributing to the information-conveying sign or pattern and/or completely or partly over the print contributing to the information-conveying sign or pattern, or only adjoining the gap and/or the print. Furthermore the multiplicity of possibilities of combination increases due to the fact that the security feature formed of the above-described materials can pass either from cloudy/opaque to col-orless or from colorless to cloudy/opaque or from one color to another color by means of the external stimulus depending on the application case.

[0067] Fig. 3 shows an exemplary embodiment similar to Fig. 1 and Fig. 2 but with the transparent film 5 provided with the security feature 6 only in partial areas, so that partial areas of the see-through areas 3 remain visible in any case.
The visi-ble partial areas of the see-through areas 3 constitute a first easily checked security feature. By means of the external stimulus it is additionally possible to lift the veil-ing of the information "20" temporarily, making this information well recognizable only against a dark background.

[0068] Fig. 4 shows a further exemplary embodiment wherein the security fea-ture 6 is disposed immediately adjoining the print 4 and the gaps 3 which together form the information-conveying sign "20". The color impression of the security feature 6 is in the unactivated state, i.e. without the external stimulus, identical or almost identical to the color impression of the print 4, for example green, so that a viewer normally recognizes only the see-through windows 3. However, the security feature 6 could additionally cover the see-through windows. When the color im-pression of the security feature 6 is now changed by an external stimulus, for exam-ple from green to yellow, or to transparent if the window areas 3 are also covered by the security feature 6, the print 4 first stands out in contrasting fashion from the security feature 6. The complete information "20" is then again well recognizable against an accordingly colored or dark background.

[0069] The security feature 6 can be applied to the film 5 before or after the film is applied to the substrate 1. In the case that it is applied therebefore, it can be lo-cated between the substrate 1 and the film 5 in the final product so that it is pro-tected by the film 5, or it can be located on the exterior side of the film 5.
This also applies to the exemplary embodiments described hereinabove and hereinafter. In the last-named case it can be expedient to cover the security feature 6 with an outer protective layer (not shown), for example a lacquer layer or a laminated film.
There may also be further effect layers laminated or printed on (not shown), which contain for example interference layer pigments or other optically variable components.
[0070] The security feature 6 is preferably printed on the film externally by the intaglio printing process, so that the haptics of the intaglio print can be perceived as an additional security feature.

[0071] A veiling of the information "20" can also be realized by means of a secu-rity feature 6 that is transparent in the unactivated state and can be switched opaque or cloudy by means of the external stimulus. This is shown by way of example in Fig. 5 and Fig. 6, with Fig. 6 showing a cross section of the bank note from Fig. 5.
There the polychromatic security element 6 is provided exactly and exclusively over the see-through areas 3. In the unactivated state the security feature is trans-parent, so that the total information "20" formed by the see-through windows 3 and the print 4 becomes recognizable only against a dark background.
Alternatively, when the print 4 is printed light on a dark base surface or in dark surroundings, the information "20" becomes completely visible upon viewing of the bank note against a light background (not shown). The security element 6 now permits the veiled in-formation "20" to be made completely visible, independently of a special back-ground, by the external stimulus switching the transparent appearance of the secu-rity feature 6 to cloudy or opaque.

[0072] Fig. 7 shows a further exemplary embodiment of the invention. In this case, the film 5 is applied to the substrate 1 of the bank note, not as a patch, but as a film strip extending across the total bank-note width. There is again printed on the substrate the print 4 which contributes with individual components to the informa-tion "20" and furthermore solidly encloses the information "20". An area 4' immediately adjoining the information "20" is unprinted. Said area 4' thus forms a kind of negative contour to the information "20". The information "20" is again completed by the see-through areas 3 in the substrate 1.

[0073] Due to the negative contour 4' the information "20" is recognizable re-gardless of whether the bank note is viewed against a dark or a light background.
Against a dark background, however, the information "20" is even more clearly recognizable because the negative contour 4' therefore has stronger contrast.
If the negative contour 4' is restricted only to the part of the print 4 contributing to infor-mation conveyance, i.e. does not extend along the see-through areas 3 (not sepa-rately shown), the complete information "20" is difficult to perceive upon viewing of the bank note against a dark background. Against a lighter background, however, it is well perceptible since the negative contour 4' then complements the window areas 3 to form the information.

[0074] The security element 6 is again applied over the infonnation and veils the information in the unstimulated state. The security feature 6 extends here partly over the print 4, partly over the see-through areas 3, and furthermore extends from the film 5 onto the substrate 1. The security feature 6 can again be advantageously realized as an intaglio print.

[0075] The substrate 1 furthermore has a background print 7 which is indicated here by five arched lines and typically realized as an intertwined line pattern ("guil-loche pattern"). The print 4 and the background print 7 can be produced in the same operation, for example by offset printing or Nyloprint. The film 5 can for its part have line patterns 8 corresponding to the background print 7. It must then be taken care that the film 5 is applied to the substrate 1 in exact register so that the patterns 7 and 8 complement each other in an accurate fit and/or cover each other congru-ently. This further increases the forgery resistance of the document. The line pat-terns 8 of the film 5 are preferably executed as metallization, thereby increasing the complexity and thus the forgery resistance, on the one hand, and facilitating a sim-ple optical check of the accurately fitting application of the film strip 5 by the un-practiced user, on the other hand. The patterns 7 and/or 8 can also be realized ad-vantageously in the other exemplary embodiments.

[0076] Upon application of the film 5 to the substrate 1 over the window areas it must be taken care that, if the adhesive layer also covers see-through areas 3, an adhesive is used whose bonding ability already diminishes after a very short time in the area of the see-through areas 3 so as to prevent adjacent sheets from sticking together upon stacking of the documents during production. Suitable adhesives for this purpose are known to the person skilled in the art.

[0077] In a further refinement of the exemplary embodiments, the print 4 can be printed on both sides of the substrate 1, either congruently with each other or com-plementing each other in transmitted light. This constitutes a further security feature that can only be imitated with special technical effort and can be realized by the technically elaborate simultaneous printing.

[0078] Furthermore, the security feature 6 or components of the information "20"
can be overprinted with transparent or semitransparent effect pigments, e.g.
with interference layer pigments, IRIODIN , Colorcrypt, etc.

[0079] Furthermore, the security feature 6 can advantageously be covered with a protective layer, in particular by means of a clear lacquer layer or a laminated trans-parent film, in order to protect the color-changing security feature against environ-mental influences. The protective lacquer layer can be applied all over or in partial areas. In the lacquer system there can be used e.g. UV lacquers, hybrid lacquers, oleographic lacquers or dispersion lacquers of the one- or two-component type.
The protective lacquer layer is preferably printed on, e.g. by flexographic printing or offset printing.

Claims

1. A security element, comprising - a substrate having a gap penetrating the substrate, - a print on the substrate adjacent to the gap, whereby the print and the gap jointly make a contribution to an information-conveying sign or pattern, - a film covering at least the gap and forming with the gap a see-through window in the substrate, and - a visually checkable security feature, characterized in that the security feature conveys an independently reversible color impression changeable by an external stimulus and is so disposed that the information-conveying sign or pattern is veiled without the external stimu-lus, and the visual recognizability of the information-conveying sign or pattern is facilitated under the action of the external stimulus.

2. The security element according to claim 1, characterized in that the color im-pression of the security feature is changeable by means of the stimulus from opaque to transparent.

3. The security element according to claim 1 or 2, characterized in that the secu-rity feature immediately adjoins the information-conveying sign or pattern upon viewing in plan.

4. The security element according to claim 1 or 2, characterized in that the secu-rity feature covers the gap at least partly, preferably completely.

5. The security element according to claim 1 or 2, characterized in that the secu-rity feature covers at least a part of the print contributing to information con-veyance.

6. The security element according to claim 1 or 2, characterized in that the secu-rity feature covers both at least a part of the gap and at least a part of the print contributing to information conveyance.

7. The security element according to any of claims 1 to 6, characterized in that at least the part of the print contributing to information conveyance has a nega-tive edge contour line.

8. The security element according to any of claims 1 to 7, characterized in that the gap has an edge contour line.

9. The security element according to any of claims 1 to 8, characterized in that the security feature comprises thermochromic material, and the external stimulus is temperature.

10. The security element according to claim 9, characterized in that the thermo-chromic material comprises a micro-encapsulated, organic, thermochromic three-component dye system with at least one electron donor, at least one elec-tron acceptor and at least one solvent.

11. The security element according to claim 9 or 10, characterized in that the ther-mochromic material comprises a micro-encapsulated, thermochromic, choles-teric liquid-crystalline substance.

12. The security element according to claim 9, characterized in that the thermo-chromic material comprises a thermochromic polymer solution which breaks down temperature-dependently into a polymer-rich and a polymer-poor solu-tion phase and thereby passes from a transparent to a cloudy state.

13. The security element according to any of claims 1 to 8, characterized in that the security feature comprises piezochromic material, and the external stimu-lus is pressure.

14. The security element according to claim 13, characterized in that the pie-zochromic material comprises metal complexes in a solvent, whereby the color impression of the material changes due to a pressure-induced change of the dielectric constant of the solvent.

15. The security element according to claim 13, characterized in that the pie-zochromic material comprises oriented molecules, in particular crosslinked liquid crystals, which are reorientable by pressure such that a color impression conveyed thereby changes.

16. The security element according to any of claims 1 to 8, characterized in that the security feature comprises photochromic material, and the external stimu-lus is electromagnetic radiation.

17. The security element according to claim 16, characterized in that the absorp-tion spectrum of the photochromic material is changeable by electromagnetic radiation, in particular by means of UV radiation.

18. The security element according to claim 17, characterized in that the color impression of the photochromic material changes from transparent to colored upon action of the electromagnetic radiation.

19. The security element according to any of claims 1 to 8, characterized in that the security feature comprises electrochromic material, and the external stimu-lus is electrical voltage.

20. The security element according to claim 19, characterized in that the security feature is of multilayer structure, whereby the electrochromic material is dis-posed adjacent to an electrolyte layer between two electrodes.

21. The security element according to any of claims 1 to 20, characterized in that the security feature is applied as an intaglio print.

22. The security element according to any of claims 1 to 21, characterized in that the security feature and a transparent or semitransparent effect layer cover each other at least partly.

23. The security element according to claim 22, characterized in that the effect layer comprises interference layer pigments.

24. The security element according to any of claims 1 to 23, characterized in that the security feature is covered with a protective layer.

25. The security element according to claim 24, characterized in that the protec-tive layer is a clear lacquer layer.

26. The security element according to claim 24, characterized in that the protec-tive layer is a laminated transparent film.

27. The security element according to any of claims 1 to 26, characterized in that the print is an offset print or Nyloprint.

28. The security element according to any of claims 1 to 27, characterized in that the print is applied to both sides of the substrate congruently or complement-ing each other in transmitted light.

29. The security element according to any of claims 1 to 28, characterized in that the film is fixed to the substrate by means of an adhesive layer at least partly covering also the gap, whereby the adhesive layer has lost its bonding ability in the area of the gap.

30. The security element according to any of claims 1 to 29, characterized in that the film is partially metallized and placed on the substrate such that the metal-lization is aligned in an accurate fit with the print and/or a background print.

31. The security element according to any of claims 1 to 30, characterized in that the accurately fitting alignment of the film with the print and/or background print is 0.3 mm to 0.5 mm or therebelow.

32. The security element according to any of claims 1 to 31, characterized in that the gap is punched with a width of at least 2 mm to about 3 mm.

33. The security element according to any of claims 1 to 32, characterized in that the gap is aligned relative to the print with a fit factor of less than 0.2 mm, in particular 0.1 mm.

34. The security element according to any of claims 1 to 33, characterized in that the security element is selected from the group consisting of: label, seal, pack-age or other product security element.

35. A security document, characterized in that it has a security element according to any of claims 1 to 34.

36. The security document according to claim 35, characterized in that the sub-strate of the security element is formed by the security-document material, and the gap of the security element is realized as a through hole in the security document.

37. The security document according to claim 35 or 36, characterized in that the film extends as a strip over the total length or width of the security document.

38. The security document according to claim 35 or 36, characterized in that the film is applied as a patch over the through hole of the security document.

39. The security document according to any of claims 35 to 38, characterized in that the security document is selected from the group consisting of: bank note, share, deed, postage stamp, check, check card, credit card, identity card, pass-port, admission ticket, travel ticket, air ticket.

40. A method for producing a security element according to any of claims 1 to or a security document according to any of claims 35 to 39, comprising the steps of:

- printing a substrate with a print comprising at least a part of an informa-tion-conveying sign or pattern, - forming at least one gap penetrating the substrate, the gap being re-stricted to a partial area of the information-conveying sign or pattern, - applying a film so that the film covers at least the gap, and - applying a security feature before or after the step of applying the film, characterized in that the security feature conveys an independently reversible color impression changeable by an external stimulus, and is disposed upon application such that the information-conveying sign or pattern is veiled with-out the external stimulus, and the visual recognizability of the information-conveying sign or pattern is facilitated under the action of the external stimu-lus.

41. The method according to claim 40, characterized in that the at least one gap is effected after production of the substrate, in particular by punching or cutting, particularly preferably by laser beam cutting.

42. The method according to claim 41, characterized in that the printing of the substrate is effected before the at least one gap is formed.

43. The method according to any of claims 40 to 42, characterized in that the ap-plication of the film is effected before the application of the security feature.

44. The method according to any of claims 40 to 43, characterized in that the ap-plication of the security feature is effected by the intaglio printing process.