EP1491357A2 - Printing substrate having an optically anisotropic layer - Google Patents

Abstract

The print medium has an at least partial transparent anisotropic coating, especially a colorless doubly refracting coating, especially arranged on a layer-oriented structure. The anisotropic coating contains nematic liquid crystals. The medium has a partial area with a block embossing (BP) and/or an uncoated relief and/or a partial area with a conventional, optically isotropic clear lacquer. An independent claim is also included for a method of manufacturing a print medium.

Description

The invention relates to a print carrier, in particular labels, labels, information or data carriers, admission tickets, prepaid cards etc. and a method for producing such a print carrier.

It is known in the prior art to use print media e.g. to secure and authenticate any goods, e.g. Software products, money cards etc. Here it is known, among other things, to use embossing prints, also in the form of blind embossing or in connection with embossing holograms, which are difficult to forge.

The published patent application DE 198 45552 A1 describes a data carrier, such as securities, banknotes, identity cards or the like, which is provided with an embossing in a predetermined area. At least part of the embossing is in the form of an inclined plane. The embossed area of the data carrier is additionally provided with at least one color layer or a color layer sequence, the optical impression of which varies depending on the viewing angle due to the inclined plane, in order to make embossing more perceptible to a viewer depending on the viewing angle.

The print carriers known in the prior art all have the disadvantage that the securing of a product can be recognized immediately with the naked eye, since the print carrier stands out conspicuously from the background, or an embossment in the print carrier strikingly stands out from the rest of the print carrier. A counterfeiter is therefore always aware that to counterfeit the product, he must always also counterfeit the specific print medium. Counterfeiting of such print media can be carried out so professionally that it is sometimes difficult for both laypersons and experts to distinguish counterfeit products from original products.

The invention has for its object to provide a print carrier and a method for producing such that when viewed with the naked eye shows no distinguishable areas or a security print is not recognizable when viewed simply, so that e.g. a product backup with such a print carrier is inconspicuous. Due to the fact that product security with such a print carrier is not clearly recognizable, counterfeiting is made considerably more difficult for a counterfeiter and, at the same time, it is immediately and easily possible to identify a counterfeit without the inventive feature.

This object is achieved according to the invention in that the print carrier is at least partially provided with a transparent, anisotropic layer, in particular optically colorless birefringent layer, in particular which is applied to a layer-oriented structure.

Such a print carrier can be produced by using an anisotropic layer, in particular a birefringent layer of e.g. nematogenic liquid crystals, is applied to at least a partial area of the print carrier which has at least one layer-orientating structure. Smectic and chiral nematic liquid crystals can also be used.

In contrast to the prior art, for example in accordance with laid-open specification DE 198 45552 A1, printing or embossing is carried out by the inventive method The method is not highlighted and cannot be recognized with the naked eye, or cannot be easily recognized, since the anisotropic layer is transparent, preferably colorless, and therefore the visual impression is essentially given by the printing medium which shines through the layer, i.e. through its color or structural appearance.

There is no color effect dependent on the viewing angle, but inclined planes that are complicated to produce to justify a color effect dependent on the viewing angle can, but do not have to be present. Rather, according to the invention, it is a print, which is also to be understood as an embossing which, without optical aids in particular, is in no way distinguishable from blind embossing or embossing on the basis of commercially available, optically isotropic clear lacquers. In this way, hidden information can be integrated or displayed in the print, which results from optically recognizable differences between an anisotropic layer and other areas or also from differences within the anisotropic layer.

The invention can e.g. when printing security documents such as B. banknotes, securities, credit cards and ID cards are used. Here, the print carrier itself can already be the product to be protected, as is the case with banknotes or credit cards, for example, or the print carrier can be applied as an additional security feature, or the print carrier can be attached to or attached to any goods in the form of a so-called security tag.

The transparent anisotropic layer has, for example, optical polarization-dependent effects which, for example, cannot be perceived by the eye, but which can be detected by aids, for example in the case of a birefringent property, by means of polarization filters of a linear or circular type, in particular visible to the eye of an observer with such an aid can be made.

Particularly preferred as an anisotropic layer with birefringent property are liquid crystals, e.g. Nematic liquid crystals are used, or varnishes or the like, which contain such liquid crystals and form such a liquid crystal film on a print carrier when printing or embossing. Such radiation-curable liquid-crystalline mixtures are produced, for example, by Merck KGaA. These mixtures are practically invisible after they have been applied to the print carrier, but they show on a suitable surface, e.g. a reflective print carrier and with the help of linear or circular polarizers pronounced visual optical effects.

Such a liquid crystal layer can e.g. be applied by means of an embossing print to a preferably high-gloss, metallic printing medium, the resulting, e.g. nematic films with appropriate procedures, e.g. B can be permanently fixed by irradiation with UV light.

When viewed with the naked eye, these embossed prints do not differ in any way from corresponding blind embossings or those embossed prints that have been applied using commercially available clear coats. They therefore have the usual three-dimensional optical impressions caused by light and shadow effects, but do not allow the embossing to emerge more visually in any way, i.e. not by creating an additional contrast or viewing angle-dependent color effect. A purely tactile distinction is also not possible.

Only when viewed through a linear or circular polarizer do the embossed prints, which were produced using the nematogenic mixtures, appear more or less optically highlighted, for example, in a shiny color. The color impressions can also be highly dependent on the (angular) position of the polarizer.

The differences in appearance can be detected not only with the eye of an observer, but also by machine, e.g. by means of detectors for different polarization directions of the reflected light, so that an automatic test of a print carrier according to the invention is also possible.

The reason for this behavior of the liquid crystal components is their spatial orientation, which in turn is particularly determined by the forces acting during the embossing process, in particular shear forces, and also by the respective fine structures of the printing medium or embossing tools.

If you divide an embossed print image into different, spatially separated (partial) areas, and when creating the embossed print image, orienting forces are used in the individual areas that differ in their directions, or are individual, defined areas of the print carrier or the Stamping tools structured in different directions result in an embossed printed image, the areas of which are distinguished by different optical effects when viewed through a polarizer.

The embossed prints according to the invention are particularly suitable for appearing unobtrusively to the naked eye in the presence of blind embossing and embossing based on commercially available clearcoats. In fact, however, they represent optical information which, with the help of e.g. of a polarizer can be made visible or detectable. This allows the invention in security printing from z. B. use securities, banknotes and credit cards or to increase the security against counterfeiting of corresponding documents.

A print carrier according to the invention is therefore preferred in addition to at least one partial area with an anisotropic layer and also at least one partial area with a Include blind embossing and / or an uncoated relief and / or at least a partial area with a commercially available, optically isotropic clear lacquer.

The printing or embossing structures according to the invention can be particularly easily e.g. using a modified flexographic printing process. Here, z. B. unrolling a hard cliché e.g. with a Shore hardness D of approx. 60 ° - 70 ° over the preferably reflective, permanently deformable printing medium or substrate, whereby the impression cylinder with an elastic rubber blanket e.g. Shore hardness A of e.g. approx. 50 ° - 60 ° can be equipped.

The depth of the embossments is controlled via the level of the contact pressure. In addition, e.g. B. by varying the plate thicknesses in one and the same print areas of different embossing depth can be obtained. Depending on whether and which printing medium is printed over the cliché, there are either blind embossing or embossing, which can be done with z. B. isotropic clear coats or the optically birefringent e.g. nematic liquid crystal films are coated.

The latter are e.g. B. based on the corresponding nematogenic liquid crystal mixtures, for example from Merck KGaA e.g. are available and e.g. can be used in the form of their melts at about 60-70 ° C or in the form of their solutions in organic solvents.

In addition, the embossing according to the invention can be produced using any embossing tool. You can e.g. B. raised in intaglio printing, the embossed structures are carved into a metal plate by known methods. An electronic method for producing such intaglio printing plates is described, for example, in WO 97/48555. During the printing process, the printing material is pressed into the recesses of the engraved metal plate and thus permanently deformed. To create blind embossing, these printing plates are used during the Printing process not filled with printing medium, but only used to deform, ie to emboss the printing material.

Regardless of the question of whether the method results in a deepened or raised embossing, it is not possible for the viewer to see with the naked eye, e.g. B. to distinguish between blind embossing, embossing using commercially available (optically isotropic) clear coats and embossing using nematogenic liquid crystal mixtures. Rather, the viewer is presented with a uniform embossed structure, which conveys the usual three-dimensional optical impressions as a result of light and shadow effects.

However, these can e.g. Due to the miniaturization and crossover of the individual print areas due to multiple printing, they have a considerable, difficult to forge fine structure, which is only revealed when viewed through a linear or circular polarizer in the form of different optical effects depending on the viewing angle.

In a typical application where e.g. B. a high-gloss, silver-colored, non-stretched polyethylene film was embossed as a print carrier using a nematogenic, 60 ° C hot liquid crystal melt, the viewer appears with the help of a linear polarizer in the 0 ° position only the embossed areas in the color blue, which with a nematic Liquid crystal film are provided. All other areas have no difference compared to the observation without a polarizer. When the polarizer is turned through 45 °, the blue color impression becomes a yellow-red one.

The color impressions are similar when analyzing the embossed image with a circular polarizer. Here the color impressions change depending on the polarizer position e.g. B. between a shiny gold and a shiny silver blue. However, cases are also possible in which the colors do not experience a significant change depending on the polarizer position, or cases occur on, where it is not every 45 °, but in particular every 90 ° to only a slight color change between z. B. comes a rather dark and a rather light brown.

In general, this (dynamic) color behavior depends on a large number of factors, including, for example, the print carrier properties, the printing process used, the leveling and wetting properties of the liquid crystal paint and the thickness, homogeneity and fine structure of the liquid crystal film produced.

In general, the e.g. nematic films are much more reflective when viewed through a circular polarizer than when using a linear polarizer. A variation of the viewing angle has no influence on the color impression obtained.

The method is given a special configuration if, for. B. the above-mentioned modified flexographic printing process or similar processes can be used, which in the course of the embossing process with the application of a force, e.g. a shear force is associated with the (nematogenic) liquid crystal films and their embossing tools are structured in such a way that a microscopic orientation of the components of the resulting liquid crystal film in a preferred direction is supported.

If, for example, using nematogenic liquid crystal mixtures, the printed image is rotated through an angle, preferably by 45 °, after a first embossing process and then followed by a second embossing process, the viewer reveals a two-color embossed printed image when analyzed with a linear or circular polarizer. Multi-colored embossing is possible by taking advantage of the entire range between the possible color impressions.

The contact pressures and thus the embossing depths can also be made arbitrarily small, so that although no embossed structures can be seen with the naked eye, an orientation of the liquid crystals is nevertheless produced, which means that when using a polarizer, at least corresponding color impressions occur.

For all the process variants according to the invention, it is essential that an anisotropic layer, in particular a birefringent layer of e.g. nematogenic liquid crystals, is applied to at least a partial area of a print carrier which has at least one layer-orientating structure.

Due to the structure, a force can act on the liquid crystals of an anisotropic liquid crystal layer in at least one direction, which causes alignment of the liquid crystals, in particular along the force acting in each case.

One or more of such structures can be applied to an area of a print carrier to be printed before or during the printing of the anisotropic layer. Print carriers that are used here can therefore already be supplied with such a structure or are only provided with such a structure in the printing press, e.g. during the application of the printing medium.

The origin and nature of the structure are essentially irrelevant, provided that they have the property of causing the anisotropic layer to be layer-oriented, that is to say, for example, a crystal orientation of the liquid crystals. A print carrier can thus be provided with a mechanical structure and / or an electrostatic structure or charge distribution. Separate orientation layers can also be applied in front of the liquid crystal layer. Changes or targeted orientations of the crystal orientation can also be done by locally heating the applied liquid crystal layer or by local application of electrical and / or magnetic fields.

• die Erzeugung positiver und negativer Prägungen in ein und demselben Druckbild,
• die Veredlung optisch anisotroper oder verschiedenfarbiger Druckträger mit dem erfindungsgemäßen Verfahren,
• den Einsatz von nicht vorgeprägten Druckträgern mit vorgegebenen und lokal definierten, verschiedenartigen Orientierungsrichtungen für mesogene Systeme,
• die Bedruckung oder Beschichtung von vorgeprägten Druckträgern auch mit holographischen Strukturen oder anderweitig, z. B. über Methoden des Spritzgießens und anderer Abformtechniken erzeugten Reliefs mit z.B. nematogenen Flüssigkristallgemischen, insbesondere wobei die Strukturierungen der Prägebereiche oder Reliefs zur Orientierung der Texturen der optisch anisotropen Flüssigkristallfilme beitragen können,
• die Erzeugung von verschieden dicken, optisch anisotropen Flüssigkristallfilmen in ein und demselben Prägedruckbild, wodurch sich weitere Farbeffekte ergeben,
• das Aufbringen einer zusätzlichen transparenten, optisch isotropen oder anisotropen Decklackschicht, Folie oder dgl. zum Zwecke z. B. des Kratzschutzes oder der Erhöhung der Fälschungssicherheit der Prägung,
• die nachträgliche Prägung teilweise oder vollständig ausgehärteter, optisch anisotroper, z.B. nematischer Flüssigkristallfilme,
• den Prägedruck auf transparente Druckträger und die definierte Rückseitenbedruckung dieser so behandelten Druckträger mit z. B. reflektierenden Farben.
• Im ersten Schritt Bedruckung oder Beschichtung einer Trägerfolie mit einem bevorzugt vollständig gehärteten, nematischen Flüssigkristallfilm, wobei die Verfahrensparameter derart eingestellt werden, daß nur eine definierte geringe, jedoch ausreichende Kohäsion zwischen Trägerfolie und Flüssigkristallfilm besteht. Im zweiten Schritt Übertragung definierter Abschnitte des Flüssigkristallfilmes auf einen Druckträger durch rückseitige Bearbeitung der entsprechend bedruckten oder beschichteten Trägerfolie mit entsprechenden Prägewerkzeugen, wobei dieser Prozeß sowohl bei Raumtemperatur oder tieferen oder höheren Temperaturen als auch unter Einwirkung nur sehr schwacher Prägekräfte ausführbar ist. Verfahrensgemäß ist ein Druckträger bevorzugt, der verformbar ist, der eine gegenüber der Trägerfolie erhöhte Klebekraft verfügt und der fähig ist, das Licht so zu reflektieren, daß die erfindungsgemäßen, optischen Effekte mit Hilfe eines Polarisators sichtbar werden.

Further embodiments of the method, for example the print carrier according to the invention relate to, for. B .:

• the creation of positive and negative embossings in one and the same printed image,
• the refinement of optically anisotropic or differently colored print carriers with the method according to the invention,
• the use of non-pre-embossed pressure carriers with predefined and locally defined, different orientations for mesogenic systems,
• the printing or coating of pre-embossed print carriers with holographic structures or otherwise, e.g. B. Reliefs generated using methods of injection molding and other molding techniques with, for example, nematogenic liquid crystal mixtures, in particular where the structuring of the embossed areas or reliefs can contribute to the orientation of the textures of the optically anisotropic liquid crystal films,
• the production of optically anisotropic liquid crystal films of different thicknesses in one and the same embossed printed image, which results in further color effects,
• the application of an additional transparent, optically isotropic or anisotropic top coat layer, film or the like. B. the protection against scratching or increasing the security against forgery of the embossing,
• the subsequent embossing of partially or completely hardened, optically anisotropic, for example nematic liquid crystal films,
• the embossed print on transparent print media and the defined print on the back of this print media treated with z. B. reflective colors.
• In the first step, printing or coating of a carrier film with a preferably completely hardened, nematic liquid crystal film, the process parameters being set in such a way that only a defined low, but sufficient, cohesion between the carrier film and the liquid crystal film consists. In the second step, transfer of defined sections of the liquid crystal film to a print carrier by processing the correspondingly printed or coated carrier film on the back with appropriate embossing tools, this process being able to be carried out both at room temperature or lower or higher temperatures and under the action of only very weak embossing forces. According to the method, a print carrier is preferred which is deformable, which has an increased adhesive force compared to the carrier film and which is capable of reflecting the light in such a way that the optical effects according to the invention are visible with the aid of a polarizer.

Exemplary embodiments and advantages of the invention are explained with reference to FIGS. 1a, 1b, 1c and 2a, 2b, 2c. These are not to scale, only reproduce the color impressions schematically and only serve to illustrate the invention.

FIG. 1a shows a schematic representation of an embossing according to the invention on a silver-colored, high-gloss printing medium, and the perceived, simplified representation of the color impression without optical aids. Essentially only the embossed structure can be seen, but no color differences between the areas BP of blind embossing without any lacquer layer, P + LC of embossing with liquid crystal (Liquid Crystal) layer, P + KL of embossing with isotropic clear lacquer and the non-embossed area LC which has only one liquid crystal layer.

FIG. 1b shows the same embossing according to the invention from FIG. 1a on a silver-colored, high-gloss printing medium, and the perceived, simplified representation of the exemplary color impression when looking through a linear polarizer in the 0 degree position. Due to the crystal orientation, both the embossed area P + LC and the unembossed area LC now appear highlighted in color. This area is bold.

1c shows the same embossing 1a according to the invention on a silver-colored, high-gloss printing medium, and the perceived, simplified representation of the color impression when looking through a linear polarizer, now in the 45 degree position. Here the areas P + LC and the area LC have a different color impression than in FIG. 1b due to the changed polarizer position. This different color impression is represented by the bold dotted lines.

FIG. 2a shows an embossing according to the invention on a silver-colored, high-gloss printing medium and the perceived, simplified representation of the color impression without optical aids. Again it shows that without polarizing aids the color impression for the area KL (isotropic clear lacquer without embossing), P1 / P2 + LC (embossing 1/2 with liquid crystal), P + KL (embossing with isotropic clear lacquer), BP (blind embossing without Lacquer) and LC (liquid crystal without embossing) is the same everywhere.

FIG. 2b shows the embossing 2a according to the invention on a silver-colored, high-gloss printing medium and the perceived, simplified representation of the exemplary color impression when looking through a linear polarizer in the 0 degree position. The areas KL and P + KL show no change in the color impression, since only isotropic clear lacquer was applied here. In contrast, the areas P1 + LC and P2 + LC now have two different color impressions, since the embossing in these areas is such that different orientations of the liquid crystals have occurred. The color impression of the area LC can correspond to that of the area P1 + LC.

FIG. 2c shows the embossing 2a according to the invention on a silver-colored, high-gloss printing medium, and the perceived, simplified representation of the color impression when looking through a linear polarizer, now in the 45 degree position. Again, there are different color impressions in the liquid crystal coated areas P1 + LC, P2 + LC and LC. Here is due to the changed polarizer position, the color impression is exactly the opposite of that in FIG. 2b.

Claims (12)

Print carrier, in particular data or information carrier, characterized in that it is at least partially provided with a transparent, anisotropic layer, in particular an optically colorless birefringent layer, in particular which is arranged on a layer-oriented structure. Print carrier according to claim 1, characterized in that, the anisotropic layer comprises nematic liquid crystals. Print carrier according to one of the preceding claims, characterized in that it comprises a partial area with blind embossing and / or an uncoated relief and / or a partial area with a commercially available, optically isotropic clear lacquer. Print carrier according to one of the preceding claims, characterized in that all subregions, irrespective of the viewing angle when viewed with the naked eye, produce an indistinguishable, in particular three-dimensional, optical impression. Print carrier according to one of the preceding claims, characterized in that a partial area, in particular the partial area provided with the optically anisotropic layer, can be detected, in particular highlighted, by an optical aid, in particular a linear or circular polarizer. Print carrier according to one of the preceding claims, characterized in that at least one partial region with an optically anisotropic layer has defined, delimited regions of different layer orientation, in particular as a result of which, by means of an optical aid, in particular a linear or circular polarizer, defined, delimited regions with different Color impressions result. Method for producing a print carrier with an optically anisotropic layer arranged thereon at least in some areas, in particular according to one of the preceding claims, characterized in that by means of a printing process a layer comprising anisotropic layer, in particular birefringent nematogenic liquid crystals, is applied to at least a partial area of the print carrier, which has at least one layer-orientating structure. A method according to claim 7, characterized in that a force acts on the liquid crystals of an anisotropic liquid crystal layer in at least one direction by the structure, which causes an alignment of the liquid crystals, in particular along the respective acting force, in particular before the layer hardens. Method according to one of the preceding claims 7 or 8, characterized in that before or during the printing of the anisotropic layer, the area to be printed is provided with a mechanical structure and / or electrostatic structure or charge distribution, in particular with such a structure having one or more different orientations the anisotropic layer. Method according to one of the preceding claims 7 to 9, characterized in that the structure is produced by printing and / or embossing tools before and / or during the printing process or by injection molding or molding techniques, in particular wherein the molding tools have a corresponding structure. Method according to one of the preceding claims 7 to 10, characterized in that the layer-orientating structure is produced by a pressure roller. Method according to one of the preceding claims 7 to 11, characterized in that after a printing operation, the printing medium is rotated through an angle and then at least one further printing operation takes place.

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