CN114423619A

CN114423619A - Method for producing a security element and security element

Info

Publication number: CN114423619A
Application number: CN202080064316.0A
Authority: CN
Inventors: T.萨特勒; T.格哈特; M.拉姆; C.富斯; K.森德雷-特梅西; K.戈特施林; U.肖姆伯格; M.R.J.谢勒
Original assignee: Giesecke and Devrient GmbH
Current assignee: Giesecke and Devrient GmbH
Priority date: 2019-09-20
Filing date: 2020-09-16
Publication date: 2022-04-29
Anticipated expiration: 2040-09-16
Also published as: EP4031380A1; CN114423619B; WO2021052630A1; EP4031380B1

Abstract

The invention relates to a production method for producing a security element (4) for producing documents of value, such as banknotes (2) or checks, for example, wherein the production method comprises the following steps: a polymer film (12) is provided and, by the action of standing light waves (16, 16b) formed by interference of reference radiation and modulated object radiation, laterally structured microfibrous structures (13) are formed in the polymer film (12) in order to form position-dependent crosslinks in the polymer film (12) and are developed with a solvent such that the polymer film (12) produces a colored optically variable pattern in a top view onto the security element (4).

Description

Method for producing a security element and security element

The invention relates to a method for producing a security element which produces a colored, optically variable pattern (or graphic object). The invention also relates to such a security element.

Photosensitive layers are known in the prior art, whose physical and chemical properties change when they are irradiated with light. They are mainly used for manufacturing micro-nano structures. The structures are produced by means of UV lacquers or so-called photoresists, i.e. finally by microlithographic methods.

One possibility to produce layers with iridescence is the so-called color-shifting layer structure. Although they can be applied completely to the substrate by evaporation methods, multicolored structures require a plurality of working steps and therefore require complex production. The problem of register stability is particularly to be noted in the individual working steps.

Volume holograms are also known which produce images of a three-dimensional effect to the observer by light diffraction and interference. Volume holograms store the intensity and phase of an incident beam in a photosensitive medium. The large-scale production of volume holograms is complicated and expensive, since complex equipment is required for the exposure and very expensive photoresists must generally be used for the production.

It is known from the publication "Structural colour using organized micro-fibre polymer films" by m.ito et al, Nature, 2019, 6/20/570, page 363-367, to crosslink polymers by standing waves (or so-called optical standing waves) and thus form structures consisting of microfibers which produce a coloured pattern.

The object of the present invention is to provide a method for the simple production of a security element which produces a colored optically variable pattern, and to provide such a security element.

The invention is defined in the independent claims; the dependent claims relate to preferred further developments.

Optically variable security elements for producing documents of value, such as banknotes or checks, have a polymer film and optionally a reflector layer arranged below the polymer film. In the polymer film, transversely structured microfibers are formed, which give the polymer film a color effect that appears as a colored pattern. The microfibers are produced here according to the principles as described in said article by Nature. Thus, the microfibers are organized according to a standing wave. The standing light waves are, for example, interference from coherent light beams, which lead to the localization of constructive interference in the polymer film and thus to (usually locally altered) cross-links in the polymer, which are then exposed by means of a suitable solvent.

This microfibrillation method described in Nature is designed in variant 1 with a reflector layer which is laterally structured with respect to reflectivity and/or shaping profile, so that interference between incident and reflected radiation and ultimately a microfibrillar structure for the color pattern is formed in the exposure above the reflector layer in reflected light. The reflector layer is crosslinked in such a way that the microfibers have a transverse structure which forms a color pattern. The reflector layer can be designed, for example, as a metallic mirror layer and can also be removed after the microfibrillation process. In variant 2, interference occurs between two incident rays in the polymer film. Whereby no reflector layer is required and the volume hologram can be written directly into the polymer film, for example.

The polymer film is thus designed in particular as a volume hologram in terms of color effect by microfibrillation and has a sponge-like structure in which individual planes and/or holes are arranged periodically. Thereby producing a structural color.

The polymer film is "exposed" to some extent by light applied in the form of standing waves, wherein, in variant 1, the lateral structuring of the reflector layer also structures the exposed portions and is subsequently developed by solvent-based removal of the non-crosslinked components. By using the laterally structured reflector layer for the exposure step, the pattern can be produced very simply.

Particularly advantageous and environmentally friendly is the use of water-soluble polymers, such as PVA, PA, PVP, PAA or PAM. In these polymers, the uncrosslinked regions can be removed with the aid of water only, and therefore no solvent is required.

In an embodiment, the reflector layer is laterally reflectance modulated, in particular pixelated. This can be realized, for example, in the form of a pixel structure. Additionally/alternatively, the distance of the reflector layer from the polymer layer can also be modulated laterally. The degree of reflection can be varied, for example, by the thickness of the metal layer, which can fluctuate between the maximum reflection layer thickness and zero. Metals such as Al, Cu, Cr, Ni, Au, Ag, etc. and alloys thereof, such as ZnS, SiO2, MgF2, or thin-film interference layer systems known from the prior art, are preferably used as materials for the reflector layer. Special dielectrics or layer systems composed of the mentioned materials are also possible.

The reflector layer, when microfibrillated, produces an intensity modulation of the pattern shown in the polymer, which in turn leads to the formation of laterally structured microfibers during the development process. The microfibers may be considered as laterally structured bragg planes, the action of which produces a color component. This makes it possible in particular to dispense with the structuring of the polymer film with lateral modifications. These embodiments have the particular advantage of simple manufacturability, since the reflector layer can be produced and structured laterally relatively simply. Nevertheless, they are difficult to imitate or counterfeit.

It is of course likewise possible, as an alternative or as an additional further development, to structure the polymer film laterally with respect to the color effect. This can be achieved, for example, by corresponding exposure with standing light waves that differ laterally, spectrally and/or in intensity.

In an embodiment, the microfibrous structure provides a volume hologram. To this end, in an embodiment, the polymer film is exposed as is known for reflection holograms, transmission holograms or Denisjuk holograms. In a further embodiment, the reflector layer is designed as a relief structure with corresponding reliefs for displaying the pattern, so that, on exposure, the light reflected by this laterally structured surface in the form of object rays overlaps in the polymer layer with the incident rays used as reference and thus interferes. The surface relief serves to achieve phase modulation of the light reflected by the reflective layer, so that this relief structure influences the color effect.

The relief structure may in particular act to create a laterally varying distance between the polymer layer and the reflective layer. The relief structure can be produced by means of micro-nano lithography and transferred or reproduced by means of different embossing methods. The relief structure may in particular have: platforms at different height levels affecting the color effect, micro mirrors, blazed grating structures, fresnel structures, moth-eye structures, sub-wavelength structures with sharp or rounded edges, sinusoidal grating structures, pillar structures or echelle grating structures with slanted or perpendicular sides, at different distances from the polymer structure. Different ones of these grating structures may also be used in side-by-side regions. The structures can also be superimposed, mainly in the case of medium periods or periods of similar or individual structure sizes having different orders of magnitude. In particular, the relief structure can be designed as a free-form surface. For example, a pattern common to banknotes can be displayed, wherein a three-dimensional pattern is also possible. When appropriately illuminated, the optically variable effect can also be observed without the laterally structured reflective layer.

In other embodiments, the microfiber structure produces a multicolor pixel image, wherein the pixel structures that function to produce the pixel image are designed into the reflector layer (if not removed), the polymer layer, or both the reflector layer and the polymer layer.

In another embodiment, the structuring of the microfibrous structure is effected perpendicularly to the surface of the polymer film. Here, the sponge-like structure of the polymer film, in which the individual layers and/or the pores are arranged, is produced in such a way that a standing wave of the exposed polymer film is applied in such a way that a microfibrous structure is formed aperiodically when the polymer film is brought into contact with a solvent. This may be achieved, for example, by irradiating the polymer layer with standing waves of different wavelengths.

In a first variant of this embodiment, the defects are specifically embedded in the microfibrous structure. For example, the individual layers in the microfibrous structure, which become thicker or thinner in a targeted manner than the other layers of the microfibrous structure by irradiation with a standing wave and subsequent solvent-based removal of the uncrosslinked components from the polymer film, are referred to as defects. In this way, minima/maxima in the reflection spectrum are formed in the regions of high reflectivity (band gap), and the color impression can be altered in comparison with the periodic design of the microfibrous structure.

In a second variant of this embodiment, the thickness of the layers of microfibrous structures is formed by irradiating the polymer structure with a standing wave in such a way that the thickness of the layers increases continuously or gradually as the layers are at a greater distance from the substrate. This will broaden the reflection spectrum. An increase in the layer thickness can be achieved, for example, by using different photoinitiators as the layer is located further from the substrate. Photoinitiators are compounds which decompose after absorption of light and thus form reactive particles which can initiate a reaction. The photoinitiators are exposed by light sources of different wavelengths and thus form different layer thicknesses in different layers, for example a photoinitiator a for the layer closer to the substrate, another photoinitiator B for the layer further away from the substrate and another photoinitiator C for the layer furthest away from the substrate. Photoinitiators are compounds which decompose upon absorption of light and thereby form reactive particles which can initiate a reaction. The photoinitiators are exposed by light sources of different wavelengths and thus form different layer thicknesses in the different layers.

In a variant, different polymer materials are used for each individual polymer layer and the same photoinitiator is used. Different polymers will produce different bandgaps because the layer thickness and refractive index vary depending on the polymer.

In an embodiment, multiple polymer films of different polymers are applied in multiple processes, and the exposing and developing are performed in one process. Development here means solvent-based removal of the uncrosslinked components of the polymer. Different developer mixtures, for example concentrations of acetic acid in water or mixtures of acetic acid in different solvents, develop different layers of the microfibrous structure in the polymer film at different speeds. In a variant, the layer thickness variation is achieved by setting the development time for the layers differently, i.e. the time required for the solvent-based removal of the non-crosslinked components of the polymer for each individual layer, for example, the upper layer has been completely developed and the lower layer has not yet been developed, or has only been partially developed. Here, one polymer and photoinitiator may be used all the time, and exposure and development may be performed in one process.

This use of different developer mixtures can be used not only for structuring the microfibrous structures perpendicular to the surface of the polymer film, but also for structuring in the transverse direction. Different developer mixtures can be used side by side in the transverse direction on the polymer film to achieve different degrees of transverse development and thus different structural colors.

In an embodiment, the developer mixture is laterally applied to the polymer film offset in time. This can be done, for example, in two working steps. A developer mixture is first applied to a first location on a polymer film. The same developer mixture is then also applied to a second location on the polymer film. Thus, the developer mixture has more time for interaction with the polymer film at a first location on the polymer film than at a second location, and the development is one step earlier at the first location, thereby producing different structural colors at laterally different locations.

According to a further embodiment, the polymer layer is designed to be elastically or plastically deformable. A reversible or irreversible change in the color of the polymer layer can thus be produced by varying the mechanical pressure exerted on a part or the entire surface of the polymer layer, by varying the layer thickness of the polymer layer. The color or the reflected (or transmitted) wavelength of the polymer layer is changed by changing its layer thickness, the thinner the layer the more the color impression is shifted towards blue. For example, if the polymer layer is red and mechanical pressure is applied thereto with a finger, the polymer layer is compressed in this area, the layer thickness decreases and the structural color can thereby change from red to blue, for example via yellow and green.

The plasticity and thus the permanent deformation of the polymer layer is effected, for example, by an embossing process, preferably by engraving.

The spongy structure of the polymer film is composed of microfibers and cavities. If these cavities are at least partially open-pored, according to another embodiment, they can be filled with a medium in liquid or gaseous state. For example, if a part of the cavity is filled with water and air is left in another part, the region in the range of the filled cavity has a different color because the refractive index of air is 1 and the refractive index of water is 1.33. The polymer layer may also become transparent if the refractive index of the microfibers and the refractive index of the medium in the cavity are of the same magnitude.

According to another preferred embodiment, the reflector layer may remain on the polymer layer. Alternatively, the polymer layer may be coated with a dark color or black on the surface facing away from the viewing (direction). Alternatively, the polymer layer may be modified by the addition of coal ash or carbon black to optimally perform the interference color function. To further increase the color intensity, transparent high refractive particles, for example made of TiO2, are added to the polymer, thereby further increasing the difference in refractive index between the fiberized layer and the continuous layer. An underprint with diffusely scattering colors (Unterdrucken) also results in different colors, especially complementary colors, when viewed in a specular or non-specular manner.

Of course, the aspects mentioned for the security element also apply equally to the production method which, with regard to the production of microfibers, follows the principle mentioned in the article by the cited Nature. In particular, a security element is specified which is produced or can be obtained by one of the production methods.

According to a further preferred embodiment, the polymer layer according to the invention is hardened or cured in a subsequent process step. This has the particular advantage that the stability of the polymer layer is increased, for example with respect to mechanical wear, subsequent mechanical stress, solvents or other environmental influences. The hardening is particularly preferably achieved by irradiating the polymer layer with ultraviolet radiation. The stability of the polymer layer and the reflector layer remaining on the polymer layer is further improved by embedding the polymer layer between the protective layer and/or the film.

The invention also relates to a value document having a security element of the above-mentioned type. In one embodiment, the document of value is, for example, in the form of a banknote or a check.

The security element according to the invention can be combined with any other security element of the document of value, such as a hologram, for example a micromirror with a running effect or a 3D surface (fresnel type), a micro-concave mirror or a subwavelength structure. This is preferably achieved in each case in such a way that the reflective layer generates reflections for generating standing waves and generates the other features described in the other lateral partial regions. Alternatively, the portion where the standing wave is generated is removed after the exposure, which may be done by etching, for example. Furthermore, combinations with the following features are possible: magnetic, conductive, fluorescent, phosphorescent. These optional further security elements are particularly preferably arranged laterally next to the microfibers.

The invention is explained in more detail below on the basis of embodiments with reference to the attached drawings, which also disclose features important for the invention. These examples are for illustration only and are not to be construed as limiting. For example, a description of an embodiment with multiple elements or components should not be construed as requiring that all such elements or components be present for implementation. Rather, other embodiments may include alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different embodiments may be combined with each other as long as not otherwise specified. The modifications and variations described for one of the embodiments can also be applied to the other embodiments. To avoid repetition, identical or corresponding elements are denoted by the same reference numerals in the different figures and are not explained again. In the drawings:

figure 1 shows a schematic view of a banknote having a plurality of security elements,

figure 2 shows a cross-sectional view through one of the security elements of figure 1,

figure 3 shows a schematic cross-sectional view of the polymer layer in the security element of figure 2,

figures 3A to 3D show different possibilities for exposing a polymer layer to form a structure according to figure 3,

fig. 4 shows a security element similar to that of fig. 2, but with a pixel-like structured reflector layer,

fig. 5 shows a sectional view similar to fig. 2, but with a polymer layer structured in a pixel-like manner,

fig. 6 shows a view similar to fig. 4 and 5, wherein both the reflector layer and the polymer layer are structured pixel-like,

fig. 7 shows an embodiment of a security element with a reflector layer, which has different platforms,

FIG. 8 shows a view similar to FIG. 2 of an embodiment for providing a security feature in the form of a volume hologram, and

fig. 9 shows a view of an embodiment for providing a security feature in the form of a volume hologram, wherein two interfering rays are used during manufacturing.

Fig. 1 schematically shows a top view of a banknote 2 with a plurality of security elements. One security element 4 is designed in the form of a patch and the other security element is designed in the form of a security strip or security thread 6. The specific areal design of the security element can be selected depending on the application. The following description refers purely by way of example to the security element 4.

Fig. 2 shows a sectional view through the security element 4. The security element is applied to a substrate 8, for example the banknote paper of the banknote 2, wherein an intermediate carrier can also be used as the substrate 8, and then the intermediate carrier is applied to the banknote paper of the banknote 2, so that the security element 4 is thus designed as a so-called transfer element (transfer).

On the substrate 8 there is a polymer layer 12, which polymer layer 12 has been provided with a microfibrous structure 13 from its upper side 14 by means of a process described in the cited article by Nature. The microfibrous structure is schematically shown in fig. 3 and is organized according to a standing wave generated by the interference of the object rays with the reference rays. There are two variants for providing these rays: variant 1 works with a reflector layer underneath the polymer layer. The incident light beam or radiation is thus the reference radiation, and the radiation reflected at the reflector layer is the object radiation. In an alternative variant 2, the object ray and the reference ray are incident independently.

The microfibrous structure 13 comprises microfibres 13a and cavities 13b (see fig. 3). In variant 1, the microfibrous structure is produced by irradiating a commercially available flat polymer, for example a polystyrene or polycarbonate film or a corresponding film (preferably parallel to its surface normal) with radiation having a coherence length which is greater than the thickness of the polymer layer 12. Preferably UV radiation is used. During this illumination, a reflector layer is located below the polymer layer 12 (see fig. 3-8), forming object rays in back reflection. In variant 2 (see fig. 9), the object ray and the reference ray are incident independently. In both cases, the thickness of the polymer layer 12 is less than the coherence length of the radiation used, thereby forming a standing wave within the polymer layer 12. The polymer is crosslinked at the antinodes where the standing wave intensity is greatest and a periodic mechanical stress field is formed between the crosslinked and uncrosslinked regions, where the uncrosslinked regions are located at the nodes of the standing wave. Preferably, additional photoinitiators are added to the polymer.

By using a matching solvent, the microfibrous structure 13 exposed in this way is then formed according to fig. 3, wherein the individual planes of the microfibres 13a and of the cavities 13b are automatically periodically arranged according to a standing wave configuration. For details of the production, reference is made to the article "Nature" and to the supplementary materials published in Nature. The contents of these publications are hereby incorporated in their entirety.

When the polymer layer 12 has been exposed and developed in this way, the polymer layer 12 produces a laterally modulated color effect which, in variant 1, is influenced by the lateral structuring of the reflector layer 10 located beneath it on exposure, which also occurs when the polymer layer 12 is incorporated into a security element without the reflector layer 10 located beneath it.

The structuring can take place during exposure, i.e. the microfibrous structure 13, which is composed of the microfibres 13a and the cavities 13b, can be structured transversely, i.e. transversely to the surface 14. Fig. 3A illustrates an embodiment in which the far field exposure 16 uniformly exposes the entire polymer layer 12. The transversely structured microfibrous structure 13 and thus the color pattern are formed by the additionally transversely structured reflector layer 10.

The substrate 8 on which the polymer layer 12 is present is not shown here and below. The substrate may be arranged between the polymer layer 12 and the reflector layer 10, or on the upper side of the polymer layer 12 or on the side facing the exposure 16. In both cases, the substrate 10 must be transparent to the illumination wavelength required for structuring the polymer layer 12.

Fig. 3B shows that structured exposure can additionally be achieved by using a mask 18. The mask 18 blocks the far field exposure 16 at individual locations so that light is incident on the polymer layer 12 only at the gaps of the mask 18. Accordingly, light can only be reflected on the reflector layer in these regions and the incident light interferes, provided that there is reflection in these illuminated regions. The color effect is also formed only at the locations where light is irradiated through the mask and is simultaneously also reflected on the reflector layer. In this way, it is possible in particular to form structures, for example pixelations, in the polymer layer 12, in addition to or as an alternative to the structures produced by the action of the reflector layer 10, wherein such pixelations influence the color. The exposure may also be performed with different wavelengths, so that the hue produced by the polymer layer 12 is different laterally, for example with three-color pixels formed by sub-pixels of the primary colors (e.g. red, green, blue). For this purpose, a plurality of far-

field exposures

16a, 16b are used sequentially in time at different wavelengths or in different wavelength ranges. The additional lateral structuring is realized here by

different masks

18a, 18b, which respectively act on the respective far-

field exposures

16a, 16 b. Fig. 3C shows these exposures, which are carried out successively, in a common view. Of course, the pleochroic properties are not limited to two colors; likewise, three, four or more different exposure steps can also be carried out, wherein each exposure step exposes a different partial surface area of the polymer layer 12 and provides it with a color effect. In this way, multiple exposures are made, the polymer layer 12 is developed by using a solvent to form the subsequently laterally structured microfibrous structure 13.

According to a preferred embodiment, a mask is retained over the security feature to form, for example, a hologram or other optically variable feature in a defined area. For this purpose, a metal layer which is present regionally is preferably used as a mask. The mask is preferably removed from the polymer layer regionally after exposure.

The mask remaining on the substrate is particularly preferably transparent in the visible spectral range (i.e. not visible or at least not noticeable in the end product) and opaque or at least translucent in the UV range. The mask is made of 50nm thick TiO, for example₂And (3) layer composition. The layer is substantially transparent in the visible range, but is transparent inThe ultraviolet range with a wavelength of about or below 300nm shows only a very low transmittance.

Alternatively, the mask may be spaced apart from the film web (Folienbahn).

It should be noted that in the

finished security element

4, 6, the color effect associated with the angle of inclination is likewise formed by interference during illumination, possibly also without the reflector layer 10. The microfibers are formed upon development of the polymer layer, wherein the microfibers may be at a different distance from each other than the original distance of the antinodes upon exposure due to the development process. In this way, in the case where the exposure wavelength is in the ultraviolet range, the bragg maximum upon observation may be in the visible spectral range after the polymer layer is developed.

An alternative to far field exposure is exposure by a gridded beam 20 (e.g. from a laser or LED) having the required coherence length and deflected by the polymer layer 12 according to a scan pattern 22. This is shown in fig. 3D. The wavelength of the laser radiation can be designed differently at individual locations in order to produce a lateral structuring of the polymer layer 12 with regard to the color effect.

These additional structuring options allow, for example, to provide the reflector layer 10 with a pixel structure for the pattern that is dependent on the brightness, and to assign to each pixel, by means of an additional structuring (mask or grid), a sub-pixel whose color is adapted to be formed.

The laterally structured reflector layer 10 below the polymer layer 12 can be present either over the entire surface, as shown in fig. 2 and 3, or over parts of the surface. Fig. 4 shows a pixelated portion of the reflector layer 10, which is composed of reflector pixels 24 with high reflection and reflector pixels 26 with low or no reflection. The standing wave is thus only formed at pixels where the reflector layer 10 has sufficient reflection, or the intensity depends on the pixel reflectivity and/or area coverage. In this way a colored, gridded pixel image can be produced.

As already explained with reference to fig. 3B, 3C and 3D, an additional meshing of the polymer layer 12 can also be produced by exposure, which thus has

polymer layer pixels

28 and 30, as shown in fig. 5, in which the color impression after development with a solvent differs. Thereby realizing a color pixel image. Of course, more than two different pixel types are possible.

The principles of fig. 4 and 5 can of course also be combined, as shown in fig. 6. Here, the pixel grid of the reflector pixels and the polymer layer pixels does not necessarily have to be the same, even though this may be advantageous. The brightness of the individual colors within the color pixels formed by the polymer layer pixels can be adjusted in particular in a position-dependent manner by means of a very high pixel density in the reflector layer. The brightness for each color point can thus be selected freely ultimately to produce a pattern.

In a particular embodiment, it is therefore provided that both the polymer layer 12 and the reflector layer 10 have a pixel structure, wherein the pixel density in the reflector layer 10 is at least twice the pixel density of the polymer layer 12. The fact that the reflector layer 10 is responsible for the intensity at one location and the polymer layer 12 for the color can thus be used particularly advantageously.

The mentioned effects are embodied in the microfibrous structure and are retained after development even without the reflector layer 10.

Fig. 7 shows an embodiment in which the reflector layer 10 has

platforms

32, 34, 36 at different heights. This makes use of the fact that the path length of the reflected radiation is related to the nodes and antinodes of the standing wave being arranged inside the polymer layer. Due to the different height levels, i.e. different distances of the

platforms

32, 34, 36 relative to the polymer layer 12, each

platform

32, 34, 36 produces a different color intensity, while the polymer layer 12 remains otherwise unchanged. This method can be considered in particular for unstructured polymer layers 12, for example polymer layers obtained by far-field exposure 16 according to fig. 3A, in order to distribute different color intensities in a laterally structured manner.

Fig. 8 shows an embodiment in which the reflector layer 10 has a relief structure on its side facing the polymer layer 12. In this way, as already explained in the background section of the description, a security element similar to a volume hologram can be produced particularly simply by designing the relief structure such that it reproduces, for example, a three-dimensional optical impression. Due to the exposure through the reflector layer 10, corresponding object rays are thereupon formed and overall a polymer containing microfibers is formed, wherein the microfibers act like bragg planes in volume holograms, which also provide three-dimensional views from various different viewing angles.

In the production method according to variant 2, according to fig. 9, the object beam and the reference beam are incident as

separate beams

42, 44 that can interfere. The object ray 44 is not a back reflection from the reference ray as is the case in variant 1. Therefore, no reflector layer is provided. In contrast, as in holography, the object beam 44 is modulated by the object 46. Alternatively, the modulation is generated by means of an optical beam shaping element (for example a DMD or the like). The two (or more) rays may be incident from the same side or opposite sides of the polymer layer 12.

In particular, the following embodiments and embodiments are possible:

1. light source

a. Exposure of the entire surface/part of the surface:

the light source may expose the polymer layer designed as a polymer film on the whole or part of the surface. For the exposure of the entire surface, for example an LED is sufficient (see fig. 3A), the coherence length of which is sufficient. If the film should be exposed pixel by pixel or area by area, a deflectable strongly focused light beam (e.g. laser) can be used (see fig. 3D), or a directly modulatable light source (micro-LED) or optical element such as an SLM (spatial light modulator), a DMD (digital micro-mirror device) or a DOE (diffractive optical element). Another possibility for producing an exposure of a part of the surface is to use a mask (see fig. 3B, 3C).

Illumination of the entire surface or at least a part of the surface may also be achieved by a self-luminous display or a self-luminous screen. Here, the display or screen illuminates the entire surface or illuminates the polymer in a pattern.

In industrial production methods based on the roll-to-roll (R2R) principle, the exposure can alternatively also be realized by LEDs arranged in rows, wherein the rows are oriented parallel to the axis of rotation of the roll. The polymer is illuminated directly by the LED or by imaging optics between the LED and the polymer.

In all cases, a coherent superposition of the object ray and the reference ray in the polymer is required.

The advantage of the microfibrillation method is that the light can be modulated on the micrometer length scale by using DOE/SLM/DMD in combination with LEDs or lasers as light source. Thus, resolutions up to 25000DPI can be achieved by the microfibrillation method. At the same time, a high degree of individualization possibilities is achieved by the flexibility of the optical elements. Since the microfibers exhibiting bragg planes are embedded in the polymer film, it is impossible to make a duplicate or a mold for counterfeiting purposes, which achieves high security against forgery.

b. Change of wavelength:

illumination with different wavelengths of light will produce different structural colors. Thus, a larger color space can be covered by additive color mixing of the RGB pixels. The exposure may be generated sequentially or simultaneously by different colored display contents of the display, a monochromatic laser, or LEDs with different emission wavelengths. For this purpose, for example, the polymer film can be covered successively by different masks 18 and exposed with monochromatic radiation through said masks (see fig. 3D).

2. Surface coverage of reflective layer

The reflective layer under the polymer may be present on the entire surface or on a part of the surface. If a reflective layer is present over the entire surface, pixel-by-pixel gridding of the colors can be achieved by modulating the light source only. If the reflective layer is gridded, standing waves are only formed in pixels where the reflective layer is present underneath. Therefore, a gridding part (Rasternung) can be generated.

3. Relief structure of reflective layer

An embossed reflective relief structure may be placed under the polymer film. The embossed structure can consist of all possible relief structures, such as micromirrors, fresnel micromirrors, blazed gratings, moth-eye structures, sub-wavelength gratings, sinusoidal gratings, manhattan gratings or azz-tak structures. Structuring can also be achieved by applying, for example, a darker color, wherein the color is applied to the side of the relief structure facing the lighting device. These and other structures may also be arranged alongside or overlapping each other. Thus, for example, reflection can be suppressed regionally by means of moth-eye structures arranged regionally or other light-absorbing structures, and microfibers are not generated in the polymer layer in these regions.

The relief pattern may be

a) Directly on the membrane and remaining there,

b) directly on the film and stripped or at least partially removed in a transfer step after exposure (for example by completely or partially etching away the metal coating, while retaining the relief structure),

c) co-operating in register beneath the film,

d) statically under the film (not co-operating), exposure registration is achieved, for example, by a synchronized flash light source.

The following embodiments are preferred:

I. monochrome pixel image with structural color:

by using one of the techniques described in 1a. and 2, monochromatic, iridescent pixel images can be produced. By varying the area coverage of the colored regions, different hue saturations can also be produced. The patterns produced have a resolution of up to 25000 DPI.

The use of a pixilated image is also advantageous for security features having micro-imaging elements such as microlenses due to the high resolution, wherein the pixilated image can be used as a microstructure image in the focal plane of the micro-imaging element.

Multicolor pixel images with structural colors

By using a combination of the techniques described in 1a. and 2. and the use of light sources with different wavelengths as described in 1b, a multicoloured, iridescent image of pixels can be produced. By varying the area coverage of the colored regions, different hue saturations can also be produced. The patterns produced have a resolution of up to 25000 DPI.

The use of a pixilated image is also of interest for security features having micro-imaging elements such as microlenses due to high resolution. This would enable the upgrading of already existing microlens features in the banknote market, since these microlens features were hitherto only monochrome.

Production of microfibrillated holograms

Volume holograms produced by using a microfibrillation process are referred to as microfibrillated holograms. Using the microfibrillation process, the photoresist can be replaced by a polymer film, such as a polymer commonly found on the market with the addition of a small amount of photoinitiator. The manufacturing process is otherwise identical to the usual manufacture of volume holograms by exposure to interfering radiation. The hologram can thus be recorded in all the representations already described, for example as a reflection, transmission or Denisjuk hologram.

Alternatively, the hologram can also be produced without using a substantially existing object. It is therefore possible to generate object rays using SLMs or DMDs (using only reflection holograms).

List of reference numerals

2 banknote

4 security element

6 security thread

8 base

10 reflector layer

12 Polymer layer

13 microfiber structure

13a microfiber

13b cavity

14 upper side

16. 16a, 16b far field exposure

18. 18a, 18b mask

20 laser

22 scan pattern

24. 26 reflector pixel

28. 30 polymer layer pixel

32. 34, 36 platform

38 embedding medium

40 relief layer

42 reference radiation

44 object radiation

46 object

Claims

1. A production method for a security element (4) for producing documents of value, such as banknotes (2) or checks or the like, wherein the production method comprises the following steps:

-providing a polymer film (12) and

-forming a laterally structured microfibrous structure (13) in the polymer film (12) by the action of a standing light wave (16, 16b) formed by interference of the reference radiation with the modulated object radiation, in order to form position-dependent cross-links in the polymer film (12), and developing with a solvent, so that the polymer film (12) produces a colored optically variable pattern in a top view onto the security element (4).

2. The method of manufacture of claim 1, further comprising the steps of:

-producing a reflector layer (10) under the polymer film (12), which has a lateral structuring in terms of reflectivity and/or profiling, and

-impinging radiation having a coherence length larger than the thickness of the polymer film onto the polymer film to form a laterally structured microfiber structure (13), wherein the reference radiation is formed by incident radiation and the object radiation is formed by reflected radiation on the reflector layer.

3. The production method as claimed in claim 2, wherein the lateral structuring of the reflector layer (10) is designed to act as a pixel structure for generating a pixel image.

4. A production method as claimed in claim 2 or 3, wherein the reflector layer (10) is provided laterally modulated in such a way that it has segments of different reflectivity.

5. The production method as claimed in claim 2 or 3, wherein the microfibrous structure (13) is laterally structured with respect to the color produced thereby, in particular in the form of a pixel structure.

6. The production method as claimed in one of claims 2 to 5, wherein the reflector layer (10) is designed with a relief structure for lateral structuring in the molding contour.

7. The method of manufacture according to claim 6, wherein said relief structure has at least one of the following structures:

-platforms (32, 34, 36) in different height levels,

-a micro-mirror, which is,

-a blazed grating structure,

-a Fresnel structure having a plurality of Fresnel zones,

-a sinusoidal grating structure having a sinusoidal grating structure,

-a column of different height,

echelle gratings with straight or slanted sides,

moth eye structure and

subwavelength structures with sharp or rounded edges.

8. The production method as claimed in one of claims 1 to 7, wherein the polymer film (12) is irradiated such that the transversely structured microfibrous structures (13) provide a multicolored image, in particular a pixellated image.

9. Method of manufacturing according to one of claims 1 to 8, wherein the reflector layer (10) is removed after the formation of the laterally structured microfibre structure (13).

10. The production method according to one of claims 1 to 9, wherein, for forming the transversely structured microfibrous structure (13), the polymer film (12) is irradiated with two mutually interfering radiation rays, one of which forms the reference radiation and the other is modulated and forms the object radiation, so that the transversely structured microfibrous structure (13) provides a volume hologram.

11. The production method according to one of claims 1 to 10, wherein the microfibrous structures (13) are structured in the polymer film (10) perpendicularly to the surface of the polymer film (10) with respect to the layer thickness of the individual layers of the microfibrous structures (13).

12. A security element for providing security to documents of value, such as banknotes (2) or checks, having a structured microfibrous structure (13) produced by a method according to one of the preceding claims.

13. Security element according to claim 12, wherein the laterally structured microfibrous structure (13) provides a volume hologram.