EP3466711A1

EP3466711A1 - Two-sided transparent window feature with dichroic dyes

Info

Publication number: EP3466711A1
Application number: EP17194937.3A
Authority: EP
Inventors: Winfried HOFFMÜLLER; Manfred Heim; Mario Keller; Beatriz Cerrolaza; Carlos Carrasco
Original assignee: Giesecke and Devrient Currency Technology GmbH; Alise Devices SL
Current assignee: Giesecke and Devrient Currency Technology GmbH; Alise Devices SL
Priority date: 2017-10-05
Filing date: 2017-10-05
Publication date: 2019-04-10
Anticipated expiration: 2037-10-05
Also published as: WO2019068655A1; AU2018344427B2; AU2018344427A1; ES2814449T3; EP3466711B1

Abstract

The present invention is directed towards a method for producing a foil security element for securing a document of value by means of a liquid crystalline mixture containing dichroic dyes and is furthermore directed towards the foil security element as such. Furthermore, a document of value is suggested comprising the foil security element along with an apparatus for provision of such a security element and a computer program product.

Description

The present invention is directed towards a method for producing a foil security element for securing a document of value by means of a liquid crystalline mixture containing dichroic dyes and is furthermore directed towards the foil security element as such. Furthermore, a document of value is suggested comprising the foil security element along with an apparatus for providing such a security element and a computer program product.
EP 2 508 358 A1 teaches a procedure for manufacturing sheets having several latent images, fabricated from a polymerizable liquid crystal.
EP 1 894 736 A2 describes a model based on a liquid crystal featuring chiral behavior (liquid crystal molecules show orientational order and optical activity, generating a rotation of the light as it travels through). Two liquid crystal layers are placed on a support substrate, the second one being chiral. Interaction between the chiral layer and the first layer, whose molecules are partially oriented, generates a variable interlacing between layers that produces a color variation by the tilt/twist of the liquid crystal molecules.
EP 1 894 736 A1 teaches a security element using liquid crystals and furthermore providing a color-shift effect.
In the technical field of providing security elements for banknotes there is a need for further security elements, which provide an unusual optical effect being recognizable by the unarmed eye. For instance, holograms are known as a very usual security element, while windows inside banknotes are becoming a growing trend. It is furthermore required to provide techniques, which allow manufacturers to combine already known security elements such that they provide a surprising optical effect. Such security elements must allow security personnel to verify authenticity of security documents. For instance, liquid crystalline material is known, which is able to show different optical effects based on its orientation.
A polymerized liquid crystalline layer contains at least one substance, whose orientation depends on the orientation of the molecules of the liquid crystals surrounding it. This substance may be a dye, whose absorption of light depends on its orientation in relation to the orientation of the polarization of the incident light.
Liquid crystals in their liquid crystalline state may be oriented by outside forces. The orientation may be induced by electric fields, magnetic fields, electrostatic, chemical composition of surfaces and their topography.
The desired two-sided image window feature requires a special orientation of the liquid crystals. The two major faces of the liquid crystal layer have to be oriented independently of each other, yet the orienting force has to be applied until (or almost until) the orientation is finally fixed by crosslinking of the liquid crystals. Crosslinking can take place by UV curing. There are different means for achieving orientation of the liquid crystals. One way is applying an electrical field with electrodes. These may be planar. They may contain a structured layer of ITO on glass. Structuring is done according to an aspect of the present invention in such a way that the ITO on each glass plate (or other carrier) forms both poles for the electric field. This means that it is neither necessary nor desired to have an electric field crossing the whole thickness of the liquid crystal layer. The field is used to orient the liquid crystal molecules near the respective surface. Another way to orient the liquid crystals is using rubbed and/or embossed foils which may carry an embossing lacquer.
While several security elements are already known, there is a need for combining such security elements and furthermore produce such security elements in a large scale. For the production of foils for banknote application, a much higher productivity is necessary on the one hand, and on the other hand the combination with further security features is required.
Accordingly, it is an object of the present invention to provide a method for producing a foil security element along with a respectively arranged apparatus for producing such security elements. It is furthermore an object of the present invention to provide the foil security element as such and furthermore a banknote or in general a document of value comprising the foil security element. It is furthermore required to suggest a computer program product being arranged to perform the suggested method or operate the apparatus. The suggested production techniques shall be able to combine several security documents with little technical effort.
The object is solved by subject matter being provided by independent claim 1. Further advantages are provided by dependent claims.

Summary of the invention

1. (First aspect of the invention) A method for producing a foil security element for securing a document of value by means of a liquid crystalline mixture containing dichroic dyes providing latent images, comprising:
- arranging (100) a lacquer layer on a carrier substrate layer;
- performing an embossing (101) of the lacquer layer to provide a first alignment structure;
- at least partly covering (102) the lacquer layer by the uncured liquid crystalline mixture containing dichroic dyes; and
- performing an embossing (103) of the uncured liquid crystalline mixture to provide a second alignment structure, wherein the first and the second alignment structure cause the liquid crystalline mixture to show different images from respectively opposite sides in a cured state.
According to a preferred embodiment the embossing is performed by the use of a circular molding tool (or rather a circular embossing tool) having a structured surface for providing the embossing of the first or the second alignment structure. The circular molding tool is preferably selected from a roll, a cylinder, a belt, a film or a metal plate. According to a particularly preferred embodiment, the circular molding tool comprises a structured surface consisting of a plurality of identical, repeatedly arranged embossing patterns for manufacturing several embossed items. As an example, the circular molding tool may comprise a plurality of stamping dies, wherein said stamping dies are mounted on a cylinder.
2. (Preferred embodiment) The method according to clause 1, characterized in that covering (102) the lacquer layer by the uncured liquid crystalline mixture is performed after embossing (101) of the lacquer layer, wherein the uncured liquid crystalline mixture at least in part covers the embossed first alignment structure.
3. (Preferred embodiment) The method according to clause 1 or 2, characterized in that the lacquer layer is curable by means of ultraviolet irradiation.
4. (Preferred embodiment) The method according to any one of the preceding clauses, characterized in that the first alignment structure and the second alignment structure are arranged on different sides of the liquid crystalline mixture.
5. (Preferred embodiment) The method according to any one of the preceding clauses, characterized in that at least one primer layer is provided for providing a homogeneous thickness of the foil security element.
6. (Preferred embodiment) The method according to any one of the preceding clauses, characterized in that a heat-sealing layer, a print-receptive layer, a coating layer, a laminating adhesive layer, a metal layer, a color-shift effect layer and/ or a protective layer are provided.
7. (Preferred embodiment) The method according to any one of the preceding clauses, characterized in that the foil security element is arranged to form a patch or a stripe.
8. (Preferred embodiment) The method according to any one of the preceding clauses, characterized in that a metal layer is provided being partly demetallized to provide a writing or a negative writing.
9. (Preferred embodiment) The method according to any one of the preceding clauses, characterized in that the carrier substrate layer is provided by means of PET.
10. (Preferred embodiment) The method according to any one of the preceding clauses, characterized in that arranging (100) the lacquer layer on the carrier substrate layer is performed using printing techniques.
11. (Second aspect of the invention) A foil security element for securing a document of value by means of a liquid crystalline mixture containing dichroic dyes providing latent images, comprising:
- a lacquer layer being arranged on a carrier substrate layer;
- the lacquer layer is embossed to provide a first alignment structure of the liquid crystalline mixture;
- the lacquer layer being at least partly covered by the uncured liquid crystalline mixture containing dichroic dyes; and
- the uncured liquid crystalline mixture is embossed to provide a second alignment structure, wherein the first and the second alignment structure cause the liquid crystalline mixture to show different images from respectively opposite sides in a cured state.
12. (Third aspect of the invention) A document of value, in particular a banknote or an identity document, comprising the foil security element according to clause 11.
13. (Preferred embodiment) The document of value according to clause 12, characterized in that the foil security element is inserted into a window portion.
14. (Fourth aspect of the invention) An apparatus for producing a foil security element for securing a document of value by means of a liquid crystalline mixture containing dichroic dyes providing latent images, comprising:
- an arrangement unit for arranging (100) a lacquer layer on a carrier substrate layer;
- a covering unit for at least partly covering (102) the lacquer layer by the uncured liquid crystalline mixture containing dichroic dyes, characterized in that
- a first embossing unit for embossing (101) of the lacquer layer is provided to provide a first alignment structure of the liquid crystalline mixture; and
- a second embossing unit for embossing (103) of the uncured liquid crystalline mixture to provide a second alignment structure, wherein the first and the second alignment structure cause the liquid crystalline mixture to show different images from respectively opposite sides in a cured state.
According to a preferred embodiment the first and/or the second embossing unit is based on a circular molding tool (or rather a circular embossing tool) having a structured surface for providing the embossing of the first and/or the second alignment structure. The circular molding tool is preferably selected from a roll, a cylinder, a belt, a film or a metal plate. According to a particularly preferred embodiment, the circular molding tool comprises a structured surface consisting of a plurality of identical, repeatedly arranged embossing patterns for manufacturing several embossed items. As an example, the circular molding tool may comprise a plurality of stamping dies, wherein said stamping dies are mounted on a cylinder.
15. (Fifth aspect of the invention) A computer program product being arranged to perform a method in accordance with any one of the preceding clauses 1 to 10, when being executed on a computer.

Description of the preferred embodiments

Accordingly, a method for producing a foil security element for securing a document of value by means of a liquid crystalline mixture containing dichroic dyes providing latent images is suggested, comprising arranging a lacquer layer on a carrier substrate layer, performing an embossing of the lacquer layer to provide a first alignment structure, at least partly covering the lacquer layer by the uncured liquid crystalline mixture containing dichroic dyes, performing an embossing of the uncured liquid crystalline mixture to provide a second alignment structure, wherein the first and the second alignment structure cause the liquid crystalline mixture to show different images from respectively opposite sides in a cured state. In this context, respectively opposite sides refer to both sides of the foil security element containing the crystalline mixture containing dichroic dyes.
The method proposed here achieves a polymerized liquid crystal sheet with several latent images on each side. The manufacturing procedure employs confinement plates that induce an alignment pattern onto a polymerizable liquid crystal doped with a dichroic dye. The resulting sheet will show two or more latent images when shined on with polarized light from either side, or when the sheet is observed through a polarizer.
A foil security element is a security element, which is arranged to form a foil comprising several layers, which can be joined together. Joining layers is preferably accomplished using laminating processes. The respective foil security element as such forms a foil, which can be used in combination with further security elements and can, for instance, be introduced in a banknote. A document of value is not restricted to a banknote but may also be present in the form of a passport. The respective document of value is secured such that security personnel or any user may verify the respective document of value.
The main advantage of the present invention is that several images can be formed using liquid crystalline mixture materials such that the user can see a different appearance of a single image or even several images when viewing the foil security element from opposite sides. It may be the case that the images can be seen with the unarmed eye or, more preferably, using additional tools such as a polarizer. Accordingly, such images serving as security features are named latent images.
Arranging a lacquer layer on a carrier substrate is first accomplished, wherein the carrier substrate may be formed by PET. The arranging of the lacquer layer can be performed using any known techniques comprising for instance printing techniques. The lacquer layer may be sensitive to ultraviolet radiation such that curing is performed using such irradiation. The lacquer layer is at least partly covered by uncured liquid crystalline mixture, which means that the liquid crystal is not crosslinked yet. Accordingly, it is possible to emboss both the lacquer layer on its top side and the liquid crystalline mixture on its top side. The notion of top side refers to an arrangement as, for instance, being depicted throughout the figures. Accordingly, the liquid crystalline mixture is arranged above the lacquer layer and two different alignment structures are caused, one being created on top of the lacquer layer and one being created on top of the liquid crystalline mixture. The liquid crystalline mixture is at least partly covering the lacquer layer, which means that the liquid crystalline mixture may be formed by an additional layer completely covering the lacquer layer or by means of specific areas, which merely partly cover the lacquer layer.
The resulting structure is that the liquid crystalline mixture comprises a first and a second alignment structure being arranged on both sides of the liquid crystalline mixture. Accordingly, each alignment structure causes the liquid crystalline mixture to arrange in a specific fashion, thereby providing respectively different images. Due to this arrangement, it is possible that the liquid crystalline mixture provides two different images, which are observable from respectively one side. An optical effect is established such that when viewing the foil security element from one side, a first image is visible and when viewing the foil security element from the opposite side, a second, different image is visible. This is due to the fact that the liquid crystalline structure can be created differently and especially separately from each other using both alignment structures. Accordingly, each alignment structure causes a specific behavior of the crystalline mixture to show a specific image.
According to the suggested arrangement, it may also be possible to view not different but same images from both sides, which would not require such a specific arrangement. Hence, it is preferred that the two alignment structures cause different images to appear.
Furthermore, the present invention is directed towards a document of value, especially a banknote, with a window portion, in which the security element is arranged, the security element comprising dichroic dyes being inserted into a liquid crystalline layer or material. The liquid crystal feature can be seen in transparent light using the front side or the back side. However, the security feature is not visibly recognizable, according to an aspect of the present invention. Using the display of a cell phone or a computer display, the observer may identify the security feature. Furthermore, a polarizer may be used to identify the latent images. Accordingly, linear polarized light may be used to identify respectively different security information on the front side or respectively the rear side of the banknote.
The foil security element comprises an embossing being applied on a so-called UV-lacquer. A UV-lacquer is a lacquer layer being sensitive to ultraviolet light. On such a lacquer layer, the crystalline layer may be printed along with the dichroic dyes. This is performed in a not yet crosslinked state. Furthermore, in addition to the liquid crystalline feature, the arrangement may comprise a metallized layer comprising a cavity (herein also referred to as a gap) or a relief. Such features may form patterns, characters, numbers and suchlike. In particular, a metallized layer forming a positive text or a negative text is obtainable. The metallizing can be performed using evaporating techniques. In advance of such a procedure, a demetallizing ink can be applied. Furthermore, a print receptive layer can be applied such that additional security elements can be printed on the security feature. Such print receptive layers typically are based on binders, optionally containing fillers such as Al₂O₃ or SiO₂, wherein additional leveling agents or stabilizer materials can be used.
The procedure of producing a metallized layer containing cavities by means of a demetallizing ink is known, for example, from EP 1 972 462 A2 .
Furthermore, so-called spacer materials can be used to make sure that at least the liquid crystalline layer is of a constant thickness. Moreover, a tilt effect can be implemented. Furthermore, it is possible to print a so-called metallic mask as a further security feature. Furthermore, a reflective coating can be arranged comprising metal flakes and/or metal pigments. Such a metallized layer may form a reflective layer.
According to an aspect of the present invention, the step of covering the lacquer layer by the uncured liquid crystalline mixture is performed after embossing of the lacquer layer, wherein the uncured liquid crystalline mixture at least in part covers the embossed first alignment structure. This provides the advantage that first an embossing is performed on the lacquer layer such that the respective structure arises, as such a structure is covered in further steps with the liquid crystalline mixture. Accordingly, it is possible to provide a first alignment structure under the liquid crystalline mixture.
According to a further aspect of the present invention, the lacquer layer is curable by means of ultraviolet irradiation. This provides the advantage that the lacquer layer can be cured and therefore hardened at any state of the production process using known curing processes.
According to a further aspect of the present invention, the first alignment structure and the second alignment structure are arranged on different sides of the liquid crystalline mixture. This provides the advantage that different images as security features can be applied on a single liquid crystalline mixture layer. Accordingly, both sides of the liquid crystalline mixture layer can be created differently and apart from each other such that it is possible to implement different optical informations on both sides.
According to a further aspect of the present invention, at least one primer layer is provided for providing a homogeneous thickness of the foil security element. This provides the advantage that the different thickness arising from different features, such as the liquid crystalline mixture material, can be remedied to provide a foil security element which can easily be handled and inserted in a document of value.
According to a further aspect of the present invention, a heat sealing layer, a print-receptive layer, a coating layer, a lamination-adhesive layer, a metal layer, a color-shift effect layer and/or a protective layer are provided. This provides the advantage that different coatings can be applied and especially further security elements can be integrated. For instance, a color-shift effect layer provides an optical effect such that different colors are provided as a function of a viewing angle.
According to a further aspect of the present invention, the foil security element is arranged to form a patch or a stripe. This provides the advantage that several application domains are possible for the suggested foil security element.
According to a further aspect of the present invention, a metal layer is provided being partly demetallized to provide patterns, characters, numbers and suchlike. In particular, a metal layer forming a positive text or a negative text is obtainable. This provides the advantage that metallization can at least be partly applied on one of the layers such that a writing becomes visible. A negative writing may be performed using demetallizing ink.
According to a further aspect of the present invention, the carrier substrate layer is provided by means of polyethyleneterephthalat, PET for short. This provides the advantage that PET provides an easy-to-use carrier layer which can be easily inserted, for instance into a banknote.
According to a further aspect of the present invention, arranging the lacquer layer on the carrier substrate layer is performed using printing techniques. This provides the advantage that the lacquer layer can be applied with little technical effort. According to the present invention, any known printing technique is suitable for applying the lacquer layer.
The object is also solved by a foil security element for securing a document of value by means of a liquid crystalline mixture containing dichroic dyes providing latent images, comprising a lacquer layer being arranged on a carrier substrate layer, the lacquer layer being at least partly covered by the uncured liquid crystalline mixture containing dichroic dyes, wherein the lacquer layer is embossed to provide a first alignment structure of the liquid crystalline mixture, and the uncured liquid crystalline mixture is embossed to provide a second alignment structure, wherein the first and the second alignment structure cause the liquid crystalline mixture to show different images from respectively opposite sides in a cured state.
The object is also solved by a document of value comprising the foil security element according to the suggested features.
According to an aspect of the present invention, the foil security element is inserted into a window portion of the document of value. This provides the advantage that the foil security element can be made part of any document of value, especially banknotes, and furthermore that the security feature is visible through the window.
The object is also solved by an apparatus being arranged in accordance with the suggested method. Accordingly, an arrangement unit, a covering unit as well as a first embossing unit and a second embossing unit are suggested. The respectively suggested units provide features for accomplishing the suggested method. Accordingly, the method is arranged to operate the suggested apparatus. By performing the suggested method and operating the suggested apparatus, the foil security element as currently suggested can be established.
The object is also solved by a computer program product implementing the respective method steps and furthermore being able to operate the suggested apparatus.
Further advantages will be shown by means of the accompanying figures, which show:

Fig. 1:: a product after a first step on an embossing machine with two embossing stations according to an aspect of the present invention;
Fig. 2:: the product after a metallizing and demetallizing procedure according to a further aspect of the present invention;
Fig. 3:: the product wherein a primer, heat sealing, and a print receptive layer is applied according to an aspect of the present invention;
Fig. 4:: the product after a preliminary step on an embossing machine with two embossing stations and making use of a spacer according to an aspect of the present invention;
Fig. 5:: the respective product without demetallizing ink according to an aspect of the present invention;
Fig. 6:: the product comprising a heat-sealing layer according to an aspect of the present invention;
Fig. 7:: a transfer foil with color-shift effect according to a further aspect of the present invention;
Fig 8:: a foil being arranged as a patch according to an aspect of the present invention;
Fig. 9:: a foil with additional masking of the LC element according to a further aspect of the present invention; and
Fig. 10:: a flowchart illustrating the method for producing the foil security element according to an aspect of the present invention.

Figures 1-8 show respective intermediate steps of the suggested method, where a layer arrangement is provided for producing the foil security element.
The following process may be done either in one step or according to the capabilities of the machines used, in several steps. The examples are explained referring to machines typically used in the production of security foils.
Figures 1 to 3 demonstrate example 1: Production of a foil to be used as a foil stripe applied to a security paper with a window (hole).
A polyester foil (according to an aspect of the present invention, 6 µm to 36 µm, preferably 12 µm, pretreated for ink adhesion) is coated with an embossing lacquer (any coating/printing process, e. g. flexo, gravure, slot die ...), embossed and UV cured. If there is the need for registration of one of the motives to the position of the liquid crystals (later to be applied), it is preferably this.
The mixture to be used according to an aspect of the present invention as the liquid crystal component is used in a printable state. This may be achieved by thinning with a suitable solvent (physical drying is necessary) or preferably by heating. Most preferable is a temperature between 50°C and 80°C (of course depending on the mixture). Besides the pressure, the temperature of the mixture in the deposition step, and the foil and roller temperatures are key factors to control the thickness of the layer.
The application of the mixture is done in register to the embossing structure applied in the first step. Horizontal movement of the liquid crystal mixture during further processing is taken into account.
Printing processes/coating processes which can be easily used in register are preferred. This can be use of a piezo system (like inkjet), application with other nozzles, silk screen, gravure or flexo.
Especially preferable are nozzles/inkjet and silk screen as the excess volume of liquid crystal material needed is small. For printing systems using anilox rolls, a chamber blade system should be used to minimize the volume of material needed. Reducing wasting material as much as possible should also be key to select the printing process.
Parts of the machinery in direct contact with the liquid crystals should be kept at a controlled temperature to prevent the liquid crystals from solidifying/ crystallizing and also to guarantee the LC phase in which the molecules will be oriented in right direction (overheating the mixture will be also adequate). A heating of the foil may be necessary for higher process speeds as the speed of alignment depends on the temperature.
Depending on the printing process used, additives may have to be added to the liquid crystalline mixture. E. g. the pulling of threads is of disadvantage for coating with nozzles. This has to be taken into account when deciding on the pressure distribution in the nozzle area during and after dosing. The threads may have to be removed mechanically or by the use of additives to the formulation like defoamers.
Irradiation with light of the right wavelength and intensity (e. g. laser) in the area between nozzle and substrate may break the thread in the right moment.
As described according to an aspect of the present invention, the liquid crystal mixture is directly applied to the embossed surface of the foil. In principle, an application on the embossing roll described hereafter is also possible, it can be done in an area shielded from light before the foil touches the cylinder. Prior to any posterior procedure, it may be necessary to introduce a step of thickness/leveling control. This may be done by introducing the film with the mixture between two pressure rollers at a given temperature. Potentially the roll or the foil in the material's side may include pillars (spacers grown) to ensure a controlled material's thickness. By carefully controlling the temperature, i. e. the material's viscosity, a well-established and uniform material thickness can be obtained prior to subsequent processing.
In the second embossing station, for instance as shown by Figures 4-6, the foil containing the liquid crystalline mixture locally is brought into contact with the embossing roll which is being kept, according to an aspect of the present invention, at a suitable temperature. The foil may either just enter the nip between embossing roll and impression roller, or it may be led to the embossing roll by the impression roller. The impression roller is pressed against the embossing roll with a pressure suitable for preventing the incorporation of larger amounts of air bubbles into the liquid crystal mixture in the nip between the rolls. Besides this requirement for the minimum pressure, the pressure must be suitable for achieving the ideal thickness of the liquid crystal material if the thickness is not exactly defined by the coating/ printing process. E. g. the application of one or more drops from a nozzle may lead to a very high thickness at the points of application and zero thickness in the other areas. The leveling can take place between embossing roll and impression roller.
If the temperature of the liquid crystal mixture is not homogeneous when it enters the embossing area, the coating will be hazy in the areas not reaching the ideal temperature in time.
Another way to achieve a homogeneous thickness is the use of spacers in the formulation. Spacers are hard particles with a defined diameter (e. g. 8 µm). However, they can be used advantageously only in combination with a hard impression roller. Growing spacers, pillars or columns in the lacquer structure is also suggested as an alternative way to fix the final thickness.
The embossing roll may be made from different materials. It may be made from nickel, diverse plastics, silicones, or quartz. In contrast to the established UV embossing process, where the positioning of the lamps is only of limited importance, it is here preferred for the alignment process and the thermal conditioning to perform curing only after some time on the embossing roll. The respective area in the embossing station is shielded from UV light to avoid disturbing the alignment early.
The thickness of the liquid crystals after this process according to an aspect of the present invention is 2 µm to 30 µm, preferably 3 µm to 14 (µm, most preferably 4 µm to 8 µm.
Preferably the embossing rolls (1^st step and 2^nd step) should contain structures suitable for alignment covering the areas which either directly (second roll) or indirectly (first roll; structure transferred to embossing lacquer) come into contact with the liquid crystal mixture. Theses structures should be designed in a way that each of the embossing rolls induces alignment in the form of a motive/background containing alignment structures inducing alignment in at least two directions, giving a good contrast in polarized light, and containing various alignment areas preferably covering the whole surface with at least two alignment directions (different orientations could be used to generate more than one latent image and also to generate grey levels).
Failure to do so leads to areas without any special alignment. Many small domains with different alignment appear. Light scattering increases. These areas show increased haze and reduced/no contrast in polarized light. However, this effect can also be used as a design feature, as this difference in transparency can be seen from both sides without the help of a polarizer.
Orienting structures may have very small depth and still be efficient as shown in rubbing processes. For the use of established embossing processes, alignment structures with a line distance of 50 nm to 1500 nm and a depth of 50 nm to 1500 nm may be used. The structures do not have to be periodic to be effective. The chosen structures may be adapted to the other requirements of the production process of the other security features as a wide range of structures is effective for alignment. E. g. standard holograms with a period of 400 nm and a depth of 150 nm have been shown to be effective for this process.
UV-crosslinking of liquid crystals can be done either in one step or in several steps. An iron-doped medium pressure mercury lamp is especially effective for curing on the embossing roll. When choosing the UV source, it has to be considered that at least a substantial part of the emitted light should be transmitted by the carrier substrate (in the case of PET, 300 nm and longer wavelength) and be able to crosslink the liquid crystals. This condition is also met by gallium-doped lamps and especially in recent years by LED sources. Sources emitting at 365 nm are preferred.
If an additional curing step after removing the foil from the embossing cylinder is used, any medium-pressure mercury lamp can be used for this step. Most liquid crystal mixtures are cured under inert conditions (nitrogen or CO₂ inerting).
After this step, the new optical feature is functional already (apart from the open structures on the surface which should be closed to render them optically inactive). Depending on the way of application of the liquid crystal mixture, liquid crystalline material may cover a larger area than necessary for the optical effect, possibly even larger than the optical window in the substrate. The amount of excess material has to be minimized as it would also fill embossed structures needed for other optical effects. If registration between the embossing steps is possible in the respective machine, the embossing structure outside the window of the second embossing step may be chosen in a way that it matches or harmonizes with the first embossing. Another solution is to mask this area e. g. by later coatings or an opaque layer (e. g. very thin/small metal pigments useful for holograms) in this area before applying the liquid crystals.
Optical features on security foils often contain structured PVD layers and/or further liquid crystal layers. PVD layers can easily be structured in a washing process.
According to an aspect of the present invention, the foil is coated with a special coating. The application is usually done by gravure or flexo printing. However, almost any process is possible. This coating is a mixture that is applied to a substrate and usually physically dried before a further coating process, usually PVD metallizing. After this, a washing process takes place, in which this coating is removed together with the coatings which have been applied on top of it. This washing can be done using solvents, water and mechanical tools like brushes or felts. This removable coating is used to cover substantial parts of the newly produced optical security feature.
One or more PVD layers may be applied which may be structured individually or altogether by washing processes (special removable coating before PVD) or etching processes (resistive coating after PVD, before etching).
E. g. aluminium-metallizing may be used as a mirror for holograms (adhesion of aluminum may be achieved by using a small amount of chromium as a first "layer" or other adhesion promoting measures (corona and/or plasma treatment).
After the metallizing and demetallizing/ washing processes further layers may be applied:

If the window is supposed to look more interesting, the embossed structures of the liquid crystals can be kept visible by applying a coating that does not match the refractive index of the liquid crystals. It may be a HRI coating. The coating with primers and heat, sealing layers may cover either the whole element or only part of it. Preferentially the liquid crystal element should be coated with coatings with good transparency.

The top of the foil may be coated using primers and ink receptive coatings. In this process, decorative coatings are also possible. They may be used to cover areas covered with liquid crystals due to technical reasons. These decorative layers may also contain metallic pigments
Squashings of the liquid crystals may be minimized by achieving a good uniformity of the layer, e. g. by silk screen printing. In addition to this, the grammage of the liquid crystalline mixture near the borders may be lowered so much that during embossing it is not squashed beyond the predefined area.
An attractively look with well-defined edges can be achieved using careful deposition of the material, such as silk-screen printing, potentially in combination with features limiting the extension of the materials in the layer.
Alternatively, any less well-defined edges can be masked by additional, opaque, or semi-opaque, layers deposited on top and/or below the area with material. If the masking is done prior to curing, and if UV curing is employed, care has to be taken to ensure that the material is exposed adequately to the curing light.
This can be done by applying UV opaque masking layers after the curing of the material. Or it can be done by employing a UV transparent secondary roll.
Another way of getting an attractive feature is, according to an aspect of the present invention, applying a metallizing/ demetallizing process to the embossed foil before applying the liquid crystal or producing a kind of mask by a printing process. By this, an optically attractive frame can be generated through which the liquid crystals can be seen. In addition to this, imperfections likely to be concentrated near the borders of the liquid crystal area can be masked. On the other side, the liquid crystals can be seen only through transparent areas of the substrate whose borders are structured separately.
The applied mask has to be produced in a way that the covered areas transmit at least enough light of the wavelength needed for crosslinking the liquid crystals. In some cases, it may be necessary to use a multistep process, to add some more layers after the first masking layer (which is not completely opaque) and the liquid crystal layer. In some cases, another curing station for completely curing the liquid crystals (probably under inert atmosphere, e. g nitrogen) unhindered by a mask may be necessary. In a different approach ebeam curing can be used if curing by UV is not possible.
The end user of the banknote may not know what kind of motive he is supposed to see when verifying the banknote. The motives shown in the see-through situation may be repeated elsewhere on the side of the banknote facing the viewer or on other parts of the foil. The see-through feature may also be completed by other design elements on the respective side on the banknote. In the case of a windowed security thread such as a so-called "Pendelfaden" there may be three windows with one of the windows giving a see-through situation and the other windows showing the thread only from one side each. So e. g. the embossing motive to be seen by the viewer on the respective side may be repeated in the windows as see-through element.
If the feature has to be produced on simpler machinery with just one embossing station, the first embossing is just done in the established way. Optionally a removable coating is applied. Optionally a metallizing/demetallizing process is done (separate machines).
Then the embossed film is again used in the embossing machine. The liquid crystals are somehow applied and embossed. The rest of the process stays the same.
The embossing structures may contain holograms, microlenses, micro-mirrors, microcavities, moth eyes, in principle any structure which can be embossed. The embossed structures may be rendered visible by applying coatings with a different refractive index (e.g. high refractive index (HRI)), metals (e. g. aluminium, chromium, copper), multilayer PVD coatings showing color shift or defined colors, multilayer PVD coatings giving other see-through features like a first color (e.g. gold) in reflection and a second color (e.g. blue) in transmission, which may advantageously be used in the case of banknotes with at least two windows or one window covering both areas (liquid crystals area and PVD area).
Figure 7 demonstrates example 2 as follows:

Shown is a production of a foil to be used as a transfer element on paper with a hole and an additional foil (hybrid) or on polymer substrate. In both cases the thickness of the transferred element should be as low as possible.

An untreated polyester foil may be used as a temporary carrier. This may be (not necessarily) coated with an additional release layer that makes removal of the foil possible later on under more defined conditions. This foil is coated with an embossing lacquer. The embossing lacquer is embossed with the desired structure. If a one-layered UV curing system is used, a precuring step (UV) can be used before embossing. The UV intensity is so low that complete cure is not effected. In a multilayer system, the first layer is cured at least to a degree that contamination of rolls within the machine is avoided. As long as good adhesion by the following embossing layer is achieved, any degree of cure is possible. In this process embossing can usually take place in a casting process (that means without precuring), which is an advantage for the replication of the structures.
In any case, the embossing lacquer has to be chosen in a way that the liquid crystalline substance does not attack the layer too severely as this would be detrimental to the aligning effect of the structures.
Further processing may take place similarly to example 1. In this case, an extra print receptive layer is not always necessary as the top layer (after removal of the carrier foil) is already print-receptive enough.
Figure 8 demonstrates example 3 as follows, namely the production of a foil to be applied as a patch on a banknote or to be laminated in a card.
The main difference to the cases considered before is that here only a part of the whole area of coatings/foil shall be transferred and not a complete stripe, because the additional thickness has to be limited and the introduction of a weakness (over a wider area) in the laminate structure has to be avoided.
Starting with the structure of example 2 being depicted by Fig. 7, at any time an additional foil is laminated to the carrier foil already there. After the first embossing, any additional layers may be introduced (e.g. metallizing, demetallizing, high refractive index(HRI)-layers, any pigmented layers) under the precondition that the layers do not permanently cover the area important for alignment of the liquid crystals (example: special coating for demetallizing process later on in the window), or at least do not disturb the alignment of the liquid crystals.
Then, according to an aspect of the present invention the liquid crystals are deposited and brought into contact with the embossing roll (a suitable temperature has to be chosen) and crosslinked. The geometry is chosen in such a way that curing is effected only after the liquid crystals have had some time for alignment according to the structure on the embossing roll and the structure on the carrier film (alignment).
Printing of additional decorative layers may take place, however, the essential areas of the area covered with liquid crystals and placed on a transparent window to be seen from both sides are not covered by opaque layers.
Then, according to an aspect of the present invention another foil is laminated to the side of the element the liquid crystals are on. This foil is to remain in the final product.
This foil is coated with primer(s) and heat-seal coatings. The foil is punched from the side of the heat-seal coating. Punching takes place in a way that at least the embossing coating is punched and at most the temporary carrier foil and the laminating adhesive between the temporary carrier foils is punched.
The matrix containing the parts not needed is stripped away, the patches needed for the final product and the patches needed for technical reasons stay on the temporary carrier foil.
Application of the patches can be done on an "OVS" or an Optinota H machine.
For integration into cards, the patches may be applied to a suitable foil, e. g. polycarbonate. This foil can be laminated together with the patches.
If foils containing material other than polycarbonate should be avoided in the step in which the permanent foil is laminated, a polycarbonate foil is chosen.
Either a primer and heat-seal coating is applied in the next step and then punching a matrix stripping is done, or punching and matrix stripping is done directly as polycarbonate may be used as heat-seal coating to polycarbonate. Then the original temporary carrier foil is removed. Then one major surface contains polycarbonate and the other major surface (of the patches) contains a release coating, or more likely a UV lacquer.
In the following, general information on security features to be realized in this way is provided. The embossing structure according to an aspect of the present invention has to contain structures inducing alignment just in the area to be covered with liquid crystals. In addition to these structures in this area, and especially in all the other areas, other structures are possible. Examples are micro mirrors, holographic structures, micro cavities, micro lenses, Fresnel structures and the like.
Depending on the structures and on the desired effect, they may be

a layer obtainable by physical vapor deposition (PVD), e. g. aluminium, chromium, ZnS, SiO2, TiO2, silver, gold, copper;
a layer system (obtainable for example by PVD) comprising several layers, e.g. (i) a color shifting layer system showing an angle dependent color change, particularly comprising a reflecting layer, a dielectric layer and an absorbing layer, e.g. Al/SiO₂/Cr; or (ii) a semi-transparent layer system showing a first color in reflection (e.g. gold) and a second color in transmission (e.g. blue), described, for example, in WO 2011/082761 A1 ;
or coated with an ink comprising platelet-shaped metal pigments, enabling the metallization of a surface relief (e.g. a surface relief containing microstructures) by using a conventional printing system such as flexography, rotogravure, offset printing, silkscreen printing, digital printing, and ink jet printing; such platelet-shaped metal pigments, e.g. thin aluminium or silver pigments, enable angle-dependent color shifting effects or reflection/ transmission-depending color change effects and are known in the art, e.g. WO 2013/186167 A2 , WO 2010/069823 A1 and WO 2005/051675 A2 .

In the examples, the alignment of the liquid crystals is described only on the second embossing roll. This was tested at the pilot line. In this way only limited line speeds are possible, as the distance between first contact with the cylinder and removal of the foil from the cylinder is limited by the circumference of the cylinder and the wrap angle possible, and alignment of the liquid crystals needs a certain amount of time. This limitation may be avoided by using an embossing foil as an endless belt taking over the function of the embossing roll. The contact time possible depends on the length of the belt. In this way, this step is not limiting for the line speed anymore.
One of the most common foils is polyester foil. Using the kind of foil for optical features involving polarizing effects has several shortcomings. The major one is that the foil is optically anisotropic and even has a certain dispersion. This may reduce the contrast of this kind of feature.
One solution is using a foil that has been produced without a stretching step, e. g. cast foils. If the mechanical stability of these foils is insufficient, they might be used only in the final step. The security feature is transferred to an isotropic foil. Another way to cope with this problem is using PET foils which have been stretched in a way to minimize optical anisotropy or even more important dispersion. Also several thinner layers may be used if these layers are selected for minimal optical interference with the feature or to compensate each other.
The inventive process is not limited to PET. Any transparent foil may be used. However, the machine parameters and the necessary primers have to be chosen accordingly.
Especially polycarbonate should be mentioned. UV curing coatings may show good adhesion to it if a certain temperature is reached before and/ or during curing and/or certain reactive diluents (like HDDA) are used to induce some swelling.
In the further processing to produce cards, polycarbonate is subjected to high temperatures which may have an influence on the optical properties (e. g. on birefringence).
Prior to any posterior procedure, it may be necessary to introduce a step of thickness/leveling control. This may be done by introducing the film with the mixture between two pressure rollers at a given temperature. Potentially the roll or the foil in the material's side may include pillars (spacers grown) to ensure a controlled material's thickness. By carefully controlling the temperature, i. e. the material's viscosity, a well-established and uniform material thickness can be obtained prior to subsequent processing.
Fig. 9 shows a further aspect of the present invention, namely masking of the LC element.
Fig. 10 shows a flowchart depicting a method for producing a foil security element for securing a document of value by means of a liquid crystalline mixture containing dichroic dyes providing latent images, comprising arranging 100 a lacquer layer on a carrier substrate layer, at least partly covering 102 the lacquer layer by the uncured liquid crystalline mixture containing dichroic dyes, wherein embossing 101 of the lacquer layer is performed to provide a first alignment structure of the liquid crystalline mixture and embossing 103 of the uncured liquid crystalline mixture to provide a second alignment structure, where the first and the second alignment structure cause the liquid crystalline mixture to show different images from respectively opposite sides in a cured state of the liquid crystalline mixture. The person skilled in the art recognizes that the above method steps can be performed iteratively and may comprise further substeps.

Claims

A method for producing a foil security element for securing a document of value by means of a liquid crystalline mixture containing dichroic dyes providing latent images, comprising:
- arranging (100) a lacquer layer on a carrier substrate layer;

- performing an embossing (101) of the lacquer layer to provide a first alignment structure;

- at least partly covering (102) the lacquer layer by the uncured liquid crystalline mixture containing dichroic dyes; and

- performing an embossing (103) of the uncured liquid crystalline mixture to provide a second alignment structure, wherein the first and the second alignment structure cause the liquid crystalline mixture to show different images from respectively opposite sides in a cured state.
The method according to claim 1, characterized in that covering (102) the lacquer layer by the uncured liquid crystalline mixture is performed after embossing (101) of the lacquer layer, wherein the uncured liquid crystalline mixture at least in part covers the embossed first alignment structure.
The method according to claim 1 or 2, characterized in that the lacquer layer is curable by means of ultraviolet irradiation.
The method according to any one of the preceding claims, characterized in that the first alignment structure and the second alignment structure are arranged on different sides of the liquid crystalline mixture.
The method according to any one of the preceding claims, characterized in that at least one primer layer is provided for providing a homogeneous thickness of the foil security element.
The method according to any one of the preceding claims, characterized in that a heat-sealing layer, a print-receptive layer, a coating layer, a laminating adhesive layer, a metal layer, a color-shift effect layer and/ or a protective layer are provided.
The method according to any one of the preceding claims, characterized in that the foil security element is arranged to form a patch or a stripe.
The method according to any one of the preceding claims, characterized in that a metal layer is provided being partly demetallized to provide a writing or a negative writing.
The method according to any one of the preceding claims, characterized in that the carrier substrate layer is provided by means of PET.
The method according to any one of the preceding claims, characterized in that arranging (100) the lacquer layer on the carrier substrate layer is performed using printing techniques.
A foil security element for securing a document of value by means of a liquid crystalline mixture containing dichroic dyes providing latent images, comprising:
- a lacquer layer being arranged on a carrier substrate layer;

- the lacquer layer is embossed to provide a first alignment structure of the liquid crystalline mixture;

- the lacquer layer being at least partly covered by the uncured liquid crystalline mixture containing dichroic dyes; and

- the uncured liquid crystalline mixture is embossed to provide a second alignment structure, wherein the first and the second alignment structure cause the liquid crystalline mixture to show different images from respectively opposite sides in a cured state.
A document of value comprising the foil security element according to claim 11.
The document of value according to claim 12, characterized in that the foil security element is inserted into a window portion.
An apparatus for producing a foil security element for securing a document of value by means of a liquid crystalline mixture containing dichroic dyes providing latent images, comprising:
- an arrangement unit for arranging (100) a lacquer layer on a carrier substrate layer;

- a covering unit for at least partly covering (102) the lacquer layer by the uncured liquid crystalline mixture containing dichroic dyes, characterized in that

- a first embossing unit for embossing (101) of the lacquer layer is provided to provide a first alignment structure of the liquid crystalline mixture; and

- a second embossing unit for embossing (103) of the uncured liquid crystalline mixture to provide a second alignment structure, wherein the first and the second alignment structure cause the liquid crystalline mixture to show different images from respectively opposite sides in a cured state.
A computer program product being arranged to perform a method in accordance with any one of the preceding claims 1 to 10, when being executed on a computer.