EP4031380B1 - Method for producing a security element, and security element - Google Patents

Description

The invention relates to a manufacturing method for a security element that produces a colorful, optically variable motif. The invention further relates to such a security element.

Photosensitive layers which change their physical and chemical properties when exposed to light irradiation are known in the art. They are mainly used for the production of micro and nanostructures. The structures are created with the help of UV lacquers or so-called photoresists, i.e. ultimately with methods of microlithography.

One way of producing layers with iridescent colors are so-called color-shift layer structures. Although they can be applied to the entire surface of a substrate using a vapor deposition process, multicolored structures require many work steps and are therefore expensive to produce. In particular, registration problems in individual work steps must be taken into account.

Volume holograms are also known, which produce a three-dimensional image for the viewer through light diffraction and interference. Volume holograms store both the intensity and phases of incident light rays in a light-sensitive medium. The large-scale production of volume holograms is complicated and expensive, since complex equipment is required for exposure and, as a rule, very expensive photoresists have to be used for production.

From the publication M. Ito et al., "Structural color using organized microfibrillation in glassy polymer films", Nature, 20 June 2019, Vol. 570, pages 363-367 , it is known to crosslink a polymer by standing light waves, thus forming a structure of microfibrils that create a colorful motif.

• Bereitstellen eines Polymerfilms
• Bilden einer Struktur in dem Polymerfilm durch Einwirken von stehenden Lichtwellen durch Interferenz einer Referenzstrahlung mit einer modulierten Objektstrahlung zu einer ortsabhängigen Vernetzung im Polymerfilm und Entwickeln, so dass der Polymerfilm in Draufsicht auf das Sicherheitselement ein optisch variables Motiv erzeugt.

The document GB 2452066A reveals a:
Production method for a security element for the production of documents of value, the production method having the following steps

• providing a polymer film
• Forming a structure in the polymer film by the action of standing light waves by interference of a reference radiation with a modulated object radiation to a location-dependent crosslinking in the polymer film and development, so that the polymer film produces an optically variable motif in plan view of the security element.

The invention is based on the object of specifying a production method for the simple production of a security element that produces a colorful, optically variable motif, and such a security element.

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

The optically variable security element for producing documents of value, such as banknotes, checks or the like, has a polymer film and optionally a reflector layer arranged under the polymer film. Laterally structured microfibrils are formed in the polymer film, which give the polymer film a color effect that appears as a colorful motif. The microfibrils are produced according to the principle described in the Nature article mentioned. They are consequently organized according to standing light waves. These originate, for example, from the interference of coherent light beams, which lead to places of constructive interference in the polymer film and thus produce a (usually locally varying) cross-linking in the polymer, which is then exposed using suitable solvents.

This microfibrillation method described in Nature is carried out in a variant 1 with a reflector layer which is structured laterally with regard to the degree of reflection and/or profiling, so that when exposed to light in incident light above the reflector layer, the interference between incident and reflected radiation and ultimately the microfibril structure for the colorful motif is created. The reflector layer ensures the crosslinking in such a way that the microfibrils have the lateral structure that leads to the colorful motif. The reflector layer can, for. B. be formed as a metallic mirror layer and also be removed after the microfibrillation process. In a variant 2, interference between two incident beams is generated in the polymer film. Then you don't need a reflector layer and you can write a volume hologram directly into the polymer film, for example.

The polymer film is thus designed by microfibrillation with regard to a color effect, in particular as a volume hologram, and has a spongy structure in which individual planes and/or holes are arranged periodically. This creates structural colors.

The polymer film is quasi "exposed" by light that is applied in the form of standing waves, with variant 1 also structuring the lateral structuring of the reflector layer and the exposure, and subsequently developed by solvent-based removal of the non-crosslinked components. Because the laterally structured reflector layer is effective for the exposure step, the motif is very easy to produce.

The use of water-soluble polymers such as PVA, PA, PVP, PAA or PAM is particularly advantageous and environmentally friendly. With these, the non-crosslinked areas can only be removed with water and thus without a solvent.

In embodiments, the reflector layer is laterally modulated in terms of the degree of reflection, in particular pixelated. This can be done, for example, in the form of a pixel structure. In addition/alternatively, the distance between the reflector layer and the polymer layer can also be laterally modulated. For example, the degree of reflection can be varied over the thickness of a metallic layer, which can vary between a maximum reflective layer thickness and zero. Metals such as Al, Cu, Cr, Ni, Au, Ag etc. and their alloys are preferred as materials for the reflector layer, as well as ZnS, SiO2, MgF2 or thin-film interference layer systems known from the prior art. Special dielectrics or layer systems made from the materials mentioned are also possible.

During microfibrillation formation, the reflector layer generates an intensity modulation of a displayed motif in the polymer, which in turn leads to the formation of laterally structured microfibrils during the development process. These can be viewed as laterally structured Bragg planes that cause a color component. In particular, a laterally varying structuring of the polymer film can then be dispensed with. Since a reflector layer can be produced relatively easily and structured laterally, these embodiments have particular advantages in terms of simple manufacturability. Nevertheless, they are difficult to imitate or even counterfeit.

Of course, as an alternative or as an additional development, it is equally possible to structure the polymer film laterally with regard to the color effect. This can e.g. B. be achieved by appropriate exposure to standing light waves that are lateral, spectral and / or different in intensity.

The microfibril structure provides a type of volume hologram in embodiments. For this purpose, in embodiments, the polymer film is exposed, as is known for reflection, transmission or Denisjuk holograms. In other embodiments, the reflector layer is designed as a relief structure, which has a corresponding relief for representing a motif, so that during exposure, the light reflected from this laterally structured surface in the sense of an object beam is superimposed in the polymer layer and thus interferes with the incident beam, which serves as a reference. The surface relief ensures a phase modulation of the light reflected by the reflective layer, so that this relief structure affects the color effect.

The relief structure can in particular bring about a laterally varying distance between the polymer layer and the reflector layer. It can be produced using micro- and nanolithography methods and transferred or duplicated using various embossing processes. In particular, the relief structure can have: plateaus in different height levels that affect the color effect and are at different distances from the polymer structure, micromirrors, a blazed grating structure, a Fresnel structure, a moth's eye structure, a sub-wavelength grating with sharp or rounded edges, a sine grating structure, a columnar structure or a stepped grating structure with sloping or vertical flanks. Various of these lattice structures can also be used in adjacent areas. The structures mentioned can also overlap, especially if the mean periods or quasi-periods or the individual structure sizes have different orders of magnitude. In very general terms, the relief structure can be designed as a free-form surface. For example, motifs typical of banknotes can be displayed, with three-dimensional motifs also being possible. The optically variable effect is at suitable illumination can also be observed without a laterally structured reflection layer.

In other embodiments, the microfibril structure creates a multicolored pixilated image, wherein a pixel structure that causes the pixelated image is formed in the reflector layer (if not removed), the polymer layer, or both.

In a further embodiment, the microfibril structure is structured perpendicularly to the surface of the polymer film. The spongy structure of the polymer film, in which individual layers and/or holes are arranged, is created in such a way that the standing waves with which the polymer film is exposed are applied in such a way that the microfibril structure does not form periodically when the polymer film comes into contact with the solvent. This can be realized, for example, by irradiating the polymer layer with standing waves of different wavelengths.

In a first variant of this embodiment, imperfections are built into the microfibril structure in a targeted manner. Individual layers in the microfibril structure are referred to as defects, for example, which are made thicker or thinner in a targeted manner than the other layers of the microfibril structure by irradiation with the standing waves and the subsequent solvent-based removal of non-crosslinked components in the polymer film. This creates a minimum/maximum in the reflection spectrum within the highly reflective range (band gap) and the color impression changes compared to a periodic development of the microfibril structure.

In a second variant of this embodiment, the thickness of the layers of the microfibril structure is formed by irradiating the polymer structure with the standing waves in such a way that the thickness of the layers increases continuously or gradually the further away the layers are from the substrate. This creates a broadening of the reflectance spectrum. The increase in the thickness of the layers can be achieved, for example, by using different photoinitiators the further away the layers are from the substrate. Photoinitiators are chemical compounds that break down after absorbing light, forming reactive particles that can start a reaction. The photoinitiators are exposed to light sources of different wavelengths, which results in different layer thicknesses in the different layers, e.g. one photoinitiator A can be used for the layers that are closer to the substrate, another photoinitiator B for the layers that are further away from the substrate, and another photoinitiator C for the layers that are furthest away from the substrate. Photoinitiators are chemical compounds that break down after absorbing light, forming reactive particles that can start a reaction. The photoinitiators are exposed to light sources of different wavelengths, resulting in different layer thicknesses in the different layers.

In modifications, different polymer materials are used for the individual polymer layers and the same photoinitiator. Different polymers produce different band gaps because layer thicknesses and refractive indices vary by polymer.

In embodiments, multiple polymer films of different polymers are applied in multiple passes, and exposure and development is performed in one pass. Development here means the solvent-based removal of the non-crosslinked components of the polymers. Different developer mixtures, such as concentrations of acetic acid in water or mixtures of acetic acid in different solvents, develop the different layers of the microfibril structure in the polymer film at different rates. In modifications, the layer thickness variation described is achieved by setting the development time, i.e. the time that individual layers require for the solvent-based removal of the non-crosslinked components of the polymers, differently for the layers, so that e.g. upper layers are already fully developed and lower ones are not yet, or only partially. One type of polymer and photoinitiator can be used consistently, and exposure and development are performed in one operation.

This use of different developer mixtures can be used not only for structuring the microfibril structure perpendicular to the surface of the polymer film, but also for lateral structuring. Different developer mixtures can be used laterally next to each other on the polymer film in order to achieve different degrees of development laterally and thus different structural colors.

In embodiments, the developer mixtures are applied to the polymer film laterally with a time offset. This can be done in two steps, for example. First, a developer mixture is applied to the polymeric film at a first location. Then the same developer mixture is also applied to a second location on the polymer film. At the In the first location on the polymeric film, this gives the developer mixture more time to interact with the polymeric film than in the second location, and development is more advanced at the first location, producing different structural colors at laterally different locations.

According to a further embodiment, the polymer layer is designed to be elastically or plastically deformable. As a result, a reversible or irreversible change in the color of the polymer layer can be produced by mechanical pressure on a part or the entire surface of the polymer layer by changing 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 shifts towards the bluish. For example, if the polymer layer is red and mechanical pressure is applied to it with a finger, the polymer layer is compressed in this area, the layer thickness decreases and the structural color can change from red to yellow and green to blue, for example.

A plastic and thus permanent deformation of the polymer layer takes place, for example, with an embossing process, preferably by means of intaglio printing.

The spongy structure of the polymer film consists of microfibrils and cavities. If these cavities are at least partially open-pored, they can be filled with a liquid or gaseous medium according to a further embodiment. For example, if part of the cavities is filled with water and air remains in another part, regions in the area of the filled cavities show different colors, since air has an index of refraction of 1 and water of 1.33. It is also possible for the polymer layer to become transparent if the refractive index of the microfibrils and the medium in the cavities are the same.

According to a further preferred embodiment, the reflector layer can remain on the polymer layer. Alternatively, the polymer layer can be coated with a dark or black color on the surface that is not viewed. Alternatively, the polymer layer can be modified by adding carbon black or carbon black to bring out the interference colors to their best advantage. To further increase the color intensity, transparent, high-refractive particles, for example made of TiO2, are added to the polymer so that the difference in refractive index between the fibrillated layers and continuous layers is further increased. Suppression with a diffusely scattering color also leads to different, in particular complementary, colors when viewed specularly or non-specularly.

Of course, the aspects mentioned for the security element also apply equally to the production process, which follows the principle mentioned in the Nature article with regard to the production of the microfibrils. In particular, a security element is provided which is produced or obtainable using one of the production methods mentioned.

According to a further preferred embodiment, the polymer layer according to the invention is cured in a subsequent process step. This has the particular advantage that the resistance of the polymer layer to, for example, mechanical abrasion, subsequent mechanical pressure, solvents or other environmental influences is increased. The hardening is particularly preferably carried out by irradiating the polymer layer with ultraviolet radiation. A further increase in the durability of the polymer layer and the reflector layer remaining on the polymer layer results from embedding the polymer layer between protective layers and/or films.

The invention also relates to a document of value with a security element of the type mentioned. In one embodiment, the document of value is embodied as a bank note or check, for example.

The security element according to the invention can be combined with any other security elements of the document of value, for example holograms, micro-mirrors, e.g. with running effects or 3D surfaces (Fresnel-like), micro-concave mirrors or sub-wavelength structures. In each case, this preferably takes place in such a way that the reflective layer generates the reflection for generating the standing wave and generates the other features described in other lateral partial areas. Optionally, the part that generates the standing wave is removed after exposure, which can be done, for example, by etching. Furthermore, the combination with the following features is possible: magnet, conductivity, fluorescence, phosphorescence. Any of these other security elements are particularly preferably arranged laterally next to the microfibrils.

Fig. 1

eine schematische Darstellung einer Banknote mit mehreren Sicherheitselementen,

Fig. 2

eine Schnittdarstellung durch eines des Sicherheitselemente der Fig. 1,

Fig. 3

eine schematische Schnittdarstellung einer Polymerschicht im Sicherheitselement der Fig. 2,

Fig. 3A bis 3D

verschiedene Möglichkeiten zur Belichtung der Polymerschicht, um die Struktur gemäß Fig. 3 auszubilden,

Fig. 4

ein Sicherheitselement ähnlich dem der Fig. 2, jedoch mit Pixel-artig strukturierter Reflektorschicht,

Fig. 5

eine Schnittdarstellung ähnlich der Fig. 2, jedoch mit Pixelartig strukturierter Polymerschicht,

Fig. 6

eine Darstellung ähnlich der Fig. 4 und 5, wobei sowohl Reflektorschicht als auch Polymerschicht Pixel-artig strukturiert sind,

Fig. 7

eine Ausführungsform des Sicherheitselementes mit einer Reflektorschicht, die verschiedene Plateaus aufweist,

Fig. 8

eine Darstellung einer Ausführungsform ähnlich der Fig. 2 zur Bereitstellung eines Sicherheitsmerkmals in Art eines Volumenhologramms und

Fig. 9

eine Darstellung einer Ausführungsform zur Bereitstellung eines Sicherheitsmerkmals in Art eines Volumenhologramms wobei bei der Herstellung zwei interferierende Strahlen verwendet werden.

The invention is explained in more detail below using exemplary embodiments with reference to the accompanying drawings, which also disclose features that are essential to the invention. These embodiments are provided for illustration only and are not to be construed as limiting. For example, is a description of an embodiment having a plurality of elements or components should not be construed as implying that all such elements or components are necessary 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 exemplary embodiments can be combined with one another unless otherwise stated. Modifications and variations that are described for one of the embodiments can also be applicable to other embodiments. To avoid repetition, the same or corresponding elements in different figures are denoted by the same reference symbols and are not explained more than once. In the figures show:

1

a schematic representation of a bank note with several security elements,

2

a sectional view through one of the security elements 1 ,

3

a schematic sectional view of a polymer layer in the security element 2 ,

Figures 3A to 3D

different ways of exposing the polymer layer according to the structure 3 to train

4

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

figure 5

a sectional view similar to that 2 , but with a pixel-like structured polymer layer,

6

a representation similar to that Figures 4 and 5 , whereby both the reflector layer and the polymer layer have a pixel-like structure,

7

an embodiment of the security element with a reflector layer that has different plateaus,

8

a representation of an embodiment similar to that of FIG 2 to provide a security feature in the form of a volume hologram and

9

a representation of an embodiment for providing a security feature in the form of a volume hologram, two interfering beams being used in the production.

1 FIG. 1 shows a schematic plan view of a bank note 2 which has a number of security elements. A security element 4 is designed in the form of a patch, another security element in the form of a security strip or security thread 6. The specific two-dimensional design of the security element can be selected depending on the application. The description below refers to the security element 4 purely by way of example.

2 shows a sectional view through the security element 4. It is applied to a substrate 8, for example banknote paper of the banknote 2, with an intermediate carrier also being used as the substrate 8 can, which is then applied to a banknote paper of the banknote 2, so that the security element 4 is then designed as a so-called transfer element.

On the substrate 8 there is a polymer layer 12 which has been provided with a microfibril structure 13 from its upper side 14 using the process described in the cited Nature article. This is shown schematically in 3 are shown and organized according to standing waves resulting from interference of an object beam with a reference beam. There are two variants for providing these beams: Variant 1 works with a reflector layer under the polymer layer. Then an incident ray is the reference ray, the ray reflected at the reflector layer is the object ray. In an alternative variant 2, the object and reference beams are irradiated independently.

The microfibril structure 13 includes microfibrils 13a and cavities 13b (cf. 3 ). In variant 1, it is produced by irradiating a commercially available, flat polymer, such as a polystyrene or polycarbonate sheet or a corresponding film (preferably parallel to its surface normal) with radiation whose coherence length is greater than the thickness of the polymer layer 12. UV radiation is preferably used. During this irradiation, there is a reflector layer under the polymer layer 12 (cf. Figures 3-8 ), so that the object beam is created in back reflection. In variant 2 ( 9 ) the object and reference beam are irradiated independently. In both cases, the thickness of the polymer layer 12 is less than the coherence length of the radiation(s) used, as a result of which standing waves form within the polymer layer 12. At the antinodes, where the intensity of the standing waves is at its maximum, the polymer becomes cross-linked, and a periodic mechanical stress field is formed between cross-linked and non-cross-linked areas, with the latter lying at the nodal points of the standing waves. An additional photoinitiator is preferably added to the polymer.

By using a suitable solvent, the microfibril structure 13 exposed in this way is then exposed according to FIG 3 formed, wherein individual levels of the micro-glasses 13a and the cavities 13b automatically arrange themselves periodically according to the standing wave structure. With regard to the details of the preparation, reference is made to the Nature article together with the associated supplementary material published in Nature. The content of these publications is hereby fully integrated.

When it 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 a lateral structuring of the reflector layer 10 lying underneath during the exposure - even if the polymer layer 12 is used in the security element without a reflector layer 10 lying underneath.

Structuring can take place during exposure, ie the microfibril structure 13 consisting of microfibrils 13a and cavities 13b can be structured laterally, ie transversely to the surface 14. Figure 3A 12 shows an embodiment in which a wide field exposure 16 exposes the entire polymer layer 12 uniformly. An additionally laterally structured reflector layer 10 creates a laterally structured microfibril structure 13 and thus the colorful motif.

The substrate 8 on which the polymer layer 12 is located is not shown here or in the following. It can be arranged between the polymer layer 12 and the reflector layer 10 or on the upper side or the side of the polymer layer 12 facing the exposure 16 . In both cases, the substrate 10 must be transparent to the illumination wavelengths required to structure the polymer layer 12 .

Figure 3B shows that by using a mask 18, a structured exposure can also take place. The mask 18 blocks the wide-field exposure 16 at individual points, so that light can only strike the polymer layer 12 at the gaps in the mask 18 . Correspondingly, light can also only be reflected on the reflector layer in these areas and interfere with the incident light if there is a reflection effect in these illuminated areas. The color effect only occurs at the points where light falls through the mask and is also reflected at the reflector layer. In this way, it is particularly possible, in addition to or as an alternative to the structuring caused by the reflector layer 10, a structuring, z. B. pixelation in the polymer layer 12, this pixelation affects the color. The exposure can also take place with different wavelengths, so that the color tone generated by the polymer layer 12 differs laterally, e.g. with tri-color pixels formed from sub-pixels in primary colors (e.g. red, green, blue). For this purpose, several wide-field exposures 16a, 16b are used one after the other at different wavelengths or in different wavelength ranges. The additional lateral structuring is carried out by different masks 18a, 18b, which each act on the corresponding wide-field exposure 16a, 16b. Figure 3C illustrates these sequential exposures in a common representation. The multicolor is not limited to two colors, of course; Equally, three, four or more different exposure steps can also take place, with each exposure step exposing a different partial area of the polymer layer 12 and providing it with a color effect. After being exposed multiple times in this way, the polymer layer 12 is developed by using the solvent in order to form the then laterally structured microfibril structure 13 .

According to a preferred embodiment, the mask remains on the security feature, for example in order to form holograms or other optically variable features in certain areas. A metal layer that exists in certain areas is preferably used as a mask for this purpose. Areas of the mask are preferably removed from the polymer layer after exposure.

The mask remaining on the substrate is particularly preferably transparent in the visible spectral range (ie not visible in the end product or at least inconspicuous) and opaque or at least semitransparent in the UV range. In this case, the mask consists, for example, of a 50 nm thick layer of TiO ₂ . This is largely transparent in the visible range, but shows only very low transmission in the UV range at wavelengths around or below 300 nm.

Alternatively, the mask can be separate from the film web.

It should be noted that in the finished security element 4, 6 the tilt angle-dependent color effect is also caused by interference when illuminated - possibly even without a reflector layer 10. During the development of the polymer layer, the microfibrils are formed, with their distance from one another may deviate from the original distance of the antinodes during exposure due to the development process. In this way it is possible that with exposure wavelengths in the UV range after development of the polymer layer the Bragg maxima are in the visible range of the spectrum when observed.

An alternative to wide-field exposure is exposure with a rasterized light beam 20 (e.g., from a laser or LED) having the required coherence length and swept across the polymer layer 12 according to a scan pattern 22 . this shows 3D . The wavelength of the laser radiation can be designed differently at the 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 e.g. B. to provide the reflector layer 10 with a pixel structure for the motif, which is related to the brightness, and by the additional structuring (masks or grid) to provide each pixel with its color-adjusting sub-pixels.

The laterally structured reflector layer 10 below the polymer layer 12 can be present both over the entire surface, as in the Figures 2 and 3 is shown, as well as partially. 4 shows a pixelation of the reflector layer 10 consisting of reflector pixels 24 with high reflection and reflector pixels 26 with low reflection or without reflection. The standing wave then only forms at the pixels at which the reflector layer 10 has sufficient reflection, or the intensity depends on the pixel reflectance and/or area coverage. In this way, a colored, rasterized pixel image can be generated.

As already mentioned with reference to the Figures 3B, 3C and 3D explained, an additional screening of the polymer layer 12 can also result from the exposure, so that this polymer layer has pixels 28 and 30, such as figure 5 shows in which the color impression differs after developing with the solvent. This makes a colorful pixel image possible. Of course, more than two different pixel types are also possible.

The principle of Figures 4 and 5 can of course also be combined, like 6 shows. In this case, the pixel grid of reflector pixels and polymer layer pixels does not necessarily have to be identical, even if this can be advantageous. In particular, a very high pixel density in the reflector layer allows the brightness of an individual color within a color pixel, which is formed by a polymer layer pixel, to be adjusted in a location-dependent manner. Ultimately, the brightness for each color point can be freely selected to create a motif.

It is therefore provided in particular embodiments that both the polymer layer 12 and the reflector layer 10 have a pixel structure, with the pixel density in the reflector layer 10 being at least twice the pixel density of the polymer layer 12 . In this way, the fact that the reflector layer 10 is responsible for the intensity at one point and the polymer layer 12 for the color can be used particularly favorably.

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

7 shows an embodiment in which the reflector layer 10 has plateaus 32, 34, 36 at different heights. This exploits the fact that the path length of the reflected radiation is relevant to the placement of the nodes and antinodes of the standing waves within the polymer layer. Due to the different height levels, i.e. the distances between the plateaus 32, 34, 36 and the polymer layer 12, each plateau 32, 34, 36 produces a different color intensity with an otherwise unchanged polymer layer 12. This approach is used in particular with a non-structured polymer layer 12, as is the case, for example, with the wide-field exposure 16 according to Figure 3A is obtained, in order to distribute different color intensities laterally structured.

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 general part of the description, a security element similar to a volume hologram can be produced in a particularly simple manner, in that the relief structure is designed in such a way that it reflects a three-dimensional optical impression, for example. Due to the exposure to this reflector layer 10, a corresponding object beam and overall a polymer containing microfibrils then arises, with the microfibrils functioning similarly to the Bragg planes in a volume hologram which also offers a three-dimensional representation from a wide variety of viewing angles.

In the manufacturing process in variant 2 according to 9 Object beam and reference beam are irradiated as separate beams 42, 44 capable of interference. The object beam 44 does not come from the back reflection of the reference beam, as was the case with variant 1. It is therefore not a reflector layer intended. Rather, the object beam 44 is modulated by an object 46 as in the case of a holographic recording. Alternatively, the modulation is generated by means of optical beam shaping elements (eg DMD or the like). The two (or more) beams can be irradiated from the same or from opposite sides of the polymer layer 12 .

1. 1. Lichtquellen
   1. a. Voll-/ teilflächige Belichtung:
      Die Lichtquelle kann die als Polymerfilm ausgebildete Polymerschicht voll- und teilflächig belichten. Für eine vollflächige Belichtung sind z. B. LEDs ausreichend (Fig. 3A), deren Kohärenzlänge genügt. Wenn der Film pixelbeziehungsweise bereichsweise belichtet werden soll, kann ein ablenkbarer, stark fokussierter Lichtstrahl (z. B. Laser) benutzt werden (Fig. 3D), oder aber direkt modulierbare Lichtquellen (Micro-LED) bzw. optische Elemente, wie SLM (spatial light modulator), DMD (digital micromirror device) oder DOE (diffractive optical element). Eine weitere Möglichkeit, eine teilflächige Belichtung zu erzeugen, ist die Benutzung einer Maske (Fig. 3B, 3C).

In particular, the following versions and configurations are possible:

1. 1. Light sources
   1. a. Full/partial exposure:
      The light source can expose the polymer layer, which is designed as a polymer film, over the whole or part of the surface. For a full-surface exposure z. B. LEDs sufficient ( Figure 3A ) whose coherence length suffices. If the film is to be exposed pixel or area by area, a deflectable, strongly focused light beam (e.g. laser) can be used ( 3D ), or directly modulated light sources (micro-LED) or optical elements such as SLM (spatial light modulator), DMD (digital micromirror device) or DOE (diffractive optical element). Another way to create a partial exposure is to use a mask ( Figures 3B, 3C ).

Full or at least partial illumination can also be provided by a self-illuminating display or a self-illuminating screen. The display or screen illuminates the entire surface or the polymer in a pattern.

In an industrial manufacturing process based on the roll-to-roll (R2R) principle, the exposure can alternatively also be carried out by lines arranged in a row LEDs are done with the line aligned parallel to the axis of rotation of a roller. The polymer is illuminated directly by the LEDs or by imaging optics between the LEDs and the polymer.

In all cases, a coherent superimposition of an object beam and a reference beam in the polymer is required.

The microfibrillation method has the advantage that light can be modulated on micrometer length scales by using DOE/SLM/DMD in combination with LEDs or lasers as the light source. Thus, resolutions of up to 25,000 DPI can be achieved with the microfibrillation process. At the same time, the flexibility of the optical elements allows a high degree of customization. Since the microfibrils that represent the Bragg planes are embedded in the polymer film, no imprints or molds can be made for counterfeiting purposes, which leads to a high level of protection against counterfeiting.

b. Wavelength variation:

Irradiation with light of different wavelengths produces different structural colors. It is thus possible to cover a large color space through additive color mixing of RGB pixels. The exposures could be generated sequentially or simultaneously by a different colored display of the display, monochromatic lasers or LEDs with different emission wavelengths. For this purpose, the polymer film could, for example, be successively covered with different masks 18 and exposed through them with monochromatic radiation ( 3D ).

2. Area coverage of the reflective layer

The reflective layer below the polymer can be present both over the entire surface and over part of the surface. If the reflective layer covers the entire surface, the color could be screened pixel by pixel simply by modulating the light source. If the reflective layer is gridded, standing waves only form in the pixels below which there is a reflective layer. It is thus possible to create a grid.

3. Relief structure of the reflective layer

It is possible to place an embossed, reflective relief structure under the polymer film. The embossed structure can consist of all possible relief structures such as micromirrors, Fresnel-like micromirrors, blazed gratings, moth-eye structures, sub-wavelength gratings, sinusoidal gratings, Manhattan gratings or Aztec structures. Structuring by applying a dark color, for example, is also possible, with the color being applied to the side of the relief structure that faces the illumination. These and other structures can also be arranged next to one another or superimposed. It is thus possible, for example, to suppress the reflection in certain areas by means of moth-eye structures arranged in certain areas or other light-absorbing structures and not to produce any microfibrils in the polymer layer in these areas.

The Reliefmaster can either

1. a) are located directly on the film and remain there,
2. b) are located directly on the film and are peeled off in a transfer step after exposure or are at least partially removed (e.g. by completely or partially etching away the metallic coating while the relief structure remains),
3. c) run below the foil in the register,
4. d) are stationary under the foil (do not move), the exposure takes place in register, e.g. B. by a synchronized flash light source.

1. I. Einfarbiges Pixelbild mit Strukturfarben:
   Unter Verwendung einer der in 1a. und 2. beschriebenen Techniken kann ein einfarbiges, irisierendes Pixelbild hergestellt werden. Durch Variation der Flächendeckung von farbigen Bereichen können zudem verschiedene Sättigungen des Farbtons hergestellt werden. Die hergestellten Motive weisen eine Auflösung von bis zu 25.000 DPI auf.

The following embodiments are preferred:

1. I. Monochrome pixel image with structural colors:
   Using one of the in 1a. and 2. described techniques, a monochromatic, iridescent pixel image can be produced. By varying the area coverage of colored areas, different hue saturations can also be produced. The motifs produced have a resolution of up to 25,000 DPI.

Because of the high resolution, the use of the pixel image is also advantageous for security features with micro-imaging elements such as micro-lenses, where the pixel image can function as a microstructure image in the focal plane of the micro-imaging elements.

II. Multicolor pixel image with structural colors

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

Because of the high resolution, the use of the pixel image also makes sense for security features with micro-imaging elements such as micro-lenses. This would enable an upgrading of microlens features that already exist in the banknote market, since these have only been monochromatic to date.

III. Production of microfibrillation holograms

Volume holograms that are produced using the microfibrillation process are referred to as microfibrillation holograms. With the microfibrillation process, a photoresist can be replaced by the polymer film, e.g. commercially available polymers to which small amounts of photoinitiators have been added. The production process is otherwise identical to the usual production of volume holograms by exposure to interfering rays. Thus, the hologram can be recorded in all the forms already described, such as B. as a reflection, transmission or Denisjuk hologram.

Alternatively, the hologram can also be produced without using a physical object. So the object beam can be generated by using an SLM or DMD (reflection hologram only).

Reference List

2

bank note

4

security element

6

security thread

8th

substrate

10

reflector layer

12

polymer layer

13

microfibril structure

13a

microfibril

13b

cavity

14

top

16, 16a, 16b

wide field exposure

18, 18a, 18b

mask

20

Laser

22

scan pattern

24, 26

reflector pixels

28.30

polymer layer pixels

32, 34, 36

plateau

38

embedding medium

40

relief layer

42

reference radiation

44

object radiation

46

object

Claims (13)

1. Production method for a security element (4) for producing documents of value, such as banknotes (2), cheques or the like, the production method having steps as follows:

   - providing a polymer film (12) and

   - forming a laterally structured microfibril structure (13) in the polymer film (12) by exposure to standing light waves (16, 16, 16b) by interference of a reference radiation with a modulated object radiation for location-dependent crosslinking in the polymer film (12) and development with a solvent, so that the polymer film (12) generates a chromatic, optically variable motif in plan view onto the security element (4).
2. Production method according to Claim 1, further comprising the steps of:

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

   - irradiating the polymer film with radiation having a coherence length greater than one polymer film thickness, to form the laterally structured microfibril structure (13), the reference radiation being formed by the incident radiation, and the object radiation by the radiation reflected at the reflector layer.
3. Production method according to Claim 2, the lateral structuring of the reflector layer (10) being configured as a pixel structure which brings about the pixel image.
4. Production method according to Claim 2 or 3, the reflector layer (10) being provided with lateral modulation such that it has portions differing in reflectivity.
5. Production method according to Claim 2 or 3, the microfibril structure (13) being structured laterally in terms of a colour generated by said structure, more particularly in the form of a pixel structure.
6. Production method according to any of Claims 2 to 5, the reflector layer (10) being configured with a relief structure for the lateral structuring in terms of profiling.
7. Production method according to Claim 6, the relief structure having at least one of the following structures:

   - plateaus (32, 34, 36) at different height levels,

   - micro mirrors,

   - blazed grating structure,

   - Fresnel structure

   - sinusoidal grating structure,

   - columns differing in height,

   - echelon gratings with straight or slanted flanks,

   - moth-eye structures and

   - sub-wavelength structures with sharp or rounded edges.
8. Production method according to any of Claims 1 to 7, the polymer film (12) being irradiated such that laterally structured microfibril structure (13) provides a multi-colour image, preferably a pixel image.
9. Production method according to any of Claims 1 to 8, the reflector layer (10) being removed after the formation of the laterally structured microfibril structure (13).
10. Production method according to any of Claims 1 to 9, the polymer film (12), for the formation of the laterally structured microfibril structure (13) being irradiated with two beams capable of mutual interference, of which one forms the reference beam and the other is modulated and forms the object beam, so that the laterally structured microfibril structure (13) provides a volume hologram.
11. Production method according to any of Claims 1 to 10, the microfibril structure (13) in the polymer film (10) being structured perpendicularly to the surface of the polymer film (10) in terms of layer thicknesses of individual layers of the microfibril structure (13).
12. Security element for securing documents of value, such as bank notes (2), cheques or the like, having a structured microfibril structure (13) generated by a method according to any of the above claims.
13. Security element according to Claim 12, the laterally structured microfibril structure (13) providing a volume hologram.

Images