CN114423619B - Manufacturing method for security element and security element - Google Patents

Abstract

The invention relates to a method for producing a security element (4) for producing a document of value, such as a banknote (2) or a check, wherein the method comprises the following steps: providing a polymer film (12) and forming a laterally structured microfibre structure (13) in the polymer film (12) by action of a standing light wave (16, 16 b) formed by interference of reference radiation with modulated object radiation, so as to form a position-dependent cross-link in the polymer film (12), and developing with a solvent, such that the polymer film (12) produces a coloured optically variable pattern in a top view of the security element (4).

Description

Manufacturing method for security element and security element

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

It is known in the prior art that photosensitive layers change their physical and chemical properties when they are illuminated by light. They are mainly used to fabricate micro-nano structures. The structures are produced by means of UV varnishes or so-called photoresists, ie ultimately by microlithographic methods.

One possibility for producing layers with iridescent colors is the so-called color-shifting layer structure. Although they can be applied completely on the substrate by vapor deposition methods, multicolored structures require multiple work steps and therefore require complex production. In particular, register stability must be taken into account in the individual work steps.

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

From the publication "Structural color using organized microfibrillation in glassy polymer films" by M. Ito et al., "Nature", June 20, 2019, Vol. 570, pp. 363–367, it is known that polymers are crosslinked by standing light waves (or light standing waves) and thus form structures consisting of microfibers which produce colored patterns.

The technical problem addressed by the present invention is to provide a production method for the simple production of a security element producing a colored optically variable pattern, as well as such a security element.

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

An optically variable security element for producing documents of value, such as banknotes or checks, etc., has a polymer film and optionally a reflector layer arranged beneath the polymer film. Transversely structured microfibers are formed in the polymer film, which give the polymer film a color effect that appears as a colored pattern. Here, microfibrils are produced according to the principle as described in the said "Nature" article. Thus, the microfibers are organized according to standing light waves. The standing light waves originate, for example, from the interference of coherent light beams, which lead to constructively interfering localization in the polymer film and thus generate (usually locally altered) crosslinks in the polymer, which are then exposed by means of suitable solvents.

This microfibrillation method described in "Nature" is designed in variant 1 with a reflector layer which is laterally structured in terms of reflectance and/or profile, so that in the exposure above the reflector layer with reflected light there is an interference between incident and reflected radiation and finally a microfibrous structure for the color pattern is formed. The reflector layer is crosslinked in such a way that the microfibers have a transverse structure forming 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 the two incident rays in the polymer film. A reflector layer is thus not required and, for example, volume holograms can be written directly into the polymer film.

The polymer film is thus designed by microfibrillation in particular as a volume hologram with regard to the color effect and has a sponge-like structure in which individual planes and/or holes are arranged periodically. Structural colors are thus produced.

The polymer film is "exposed" to a certain 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 areas and is subsequently developed by solvent-based removal of the non-crosslinked components. Patterning can be produced very simply by using a laterally structured reflector layer as an exposure step.

It is particularly advantageous and environmentally friendly to use water-soluble polymers, such as PVA, PA, PVP, PAA or PAM. In these polymers, the non-crosslinked regions can be removed only by means of water, 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 pixel structures. Additionally/alternatively, the distance of the reflector layer from the polymer layer can also be modulated laterally. The reflectivity can be varied, for example, by the thickness of the metal layer, which can fluctuate between a maximum reflective layer thickness and zero. Metals such as Al, Cu, Cr, Ni, Au, Ag etc. and their alloys 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 said materials are also possible.

The reflector layer produces an intensity modulation of the pattern shown in the polymer upon microfibrillation formation, which in turn leads to the formation of transversely structured microfibrils during the development process. The microfibers can be regarded as transversely structured Bragg planes, the action of which produces the color components. In particular, laterally changing structuring of the polymer film can thereby be dispensed with. These embodiments have particular advantages in terms of simple manufacturability, since the reflector layer can be produced and laterally structured relatively easily. However, they are difficult to imitate or counterfeit.

Of course, as an alternative or as an additional refinement, it is also possible to structure the polymer film transversely with regard to the color effect. This can be achieved, for example, by corresponding exposure with standing light waves which 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 other embodiments, the reflector layer is designed as a relief structure with corresponding reliefs for displaying the pattern, so that during exposure, the light reflected by this transversely structured surface in the form of an object beam superimposes and thus interferes with the incident beam serving as a reference in the polymer layer. The surface relief is used to achieve a phase modulation of the light reflected by the reflective layer, so this relief structure influences the color effect.

The relief structure can in particular act to generate a laterally varying distance between the polymer layer and the reflective layer. The relief structure can be produced by micro-nanolithography and transferred or replicated by various embossing methods. The relief structure can have, in particular, terraces at different height levels influencing the color effect at different distances from the polymer structure, micromirrors, blazed grating structures, Fresnel structures, moth-eye structures, subwavelength structures with sharp or rounded edges, sinusoidal grating structures, pillar structures or echelle grating structures with inclined or vertical sides. Different ones of these grating structures can also be used in side-by-side regions. The structures can also be superimposed mainly in the case of mesoperiods or quasi-periods or in the case of individual structures with different orders of magnitude in size. Particularly generally, the relief structure can be designed as a free-form surface. For example, patterns typical for banknotes can be displayed, wherein three-dimensional patterns are also possible. Under appropriate illumination, the optically variable effect can also be observed without a laterally structured reflective layer.

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

In another embodiment, the structuring of the microfibrous structure takes place perpendicularly to the surface of the polymer film. In this case, the sponge-like structure of the polymer film, in which the individual layers and/or pores are arranged, is produced such that a standing wave exposing the polymer film is applied such that when the polymer film comes into contact with the solvent, microfibrous structures form aperiodically. This can 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 embedded in the microfibrous structure in a targeted manner. For example, individual individual layers in the microfibrous structure are called defects which are selectively thicker or thinner than other layers of the microfibrous structure by irradiation with standing waves and subsequent solvent-based removal of uncrosslinked components in the polymer film. This results in minima/maximum values in the reflection spectrum in highly reflective regions (band gaps), and the color impression changes compared to the periodic design of the microfibrous structure.

In a second variant of this embodiment, the thickness of the layers of the microfibrous structure 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 get farther and farther from the substrate. This broadens the reflection spectrum. An increase in layer thickness can be achieved, for example, by using different photoinitiators as the distance between the layer and the substrate increases. Photoinitiators are compounds that decompose upon absorption of light and thereby form reactive particles that can initiate a reaction. The photoinitiators are exposed to light sources of different wavelengths and thus form different layer thicknesses in the different layers, for example photoinitiator A is used for the layer closer to the substrate, another photoinitiator B is used for the layer further away from the substrate, and another photoinitiator C is used for the layer furthest from the substrate. Photoinitiators are compounds which, upon absorption of light, decompose and thereby form reactive particles that can initiate a reaction. The photoinitiators are exposed to light sources of different wavelengths and thus form different layer thicknesses in the different layers.

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

In an embodiment, multiple polymer films of different polymers are applied in multiple passes, and exposure and development are performed in one pass. Development here means solvent-based removal of non-crosslinked components of the polymer. Different developer mixtures, eg 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 change in layer thickness is achieved by setting the development times differently for the layers, that is to say the time required for the solvent-based removal of the uncrosslinked components of the polymer by the individual individual layers, for example the upper layer has been completely developed while the lower layer has not yet been or only partially developed. Here, one polymer and one photoinitiator can always be used, and exposure and development can be performed in one process step.

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

In an embodiment, the developer mixture is laterally applied to the polymer film in a time-shifted manner. This can be done, for example, in two work steps. A developer mixture is first applied to a first location on a polymer film. The same developer mixture was 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 ahead of the first location, thereby producing different structural colors at laterally different locations.

According to another 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 varying its layer thickness, the thinner the layer, the more bluish the color impression. 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 region, the layer thickness is reduced and the color of the structure can thus for example change from red to blue via yellow and green.

The plastic and thus permanent deformation of the polymer layer takes place, for example, by embossing methods, preferably by engraving.

The sponge-like structure of the polymer membrane consists of microfibrils and cavities. If the cavities are at least partially open, they can be filled with a liquid or gaseous medium according to a further embodiment. For example, if one part of a cavity is filled with water and the other part is left with air, the area within the bounds of the cavity that is filled has a different color because air has a refractive index of 1 and water has a refractive index of 1.33. The polymer layer may also become transparent if the refractive index of the microfibers is of the same magnitude as that of the medium in the cavity.

According to another preferred embodiment, the reflector layer can remain on the polymer layer. Alternatively, the polymer layer may be painted dark or black on the surface facing away from the viewing (direction). Alternatively, the polymer layer can be modified by adding soot or carbon black to optimize the effect of the interference colors. In order to further increase the color intensity, transparent high-refractive particles, for example made of TiO2, are added to the polymer in order to further increase the difference in refractive index between the fibrous layer and the continuous layer. Underprints with diffusely scattered colors can also lead to different colors, especially complementary colors, when viewed specularly or non-specularly.

Of course, the aspects mentioned for the security element also apply to the production method, which follows the principle mentioned in the mentioned "Nature" article with regard to the production of the microfibres. In particular, a security element is provided which is produced or obtainable by one of the production methods described.

According to a further preferred embodiment, the polymer layer according to the invention is hardened or solidified in a subsequent process step. This has the particular advantage of increasing the stability of the polymer layer, for example against mechanical abrasion, subsequent mechanical stress, solvents or other environmental influences. The hardening is particularly preferably achieved by irradiating the polymer layer with UV radiation. The stability of the polymer layer and of the reflector layer remaining on the polymer layer is further increased 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 type described above. In one refinement, the document of value is designed, for example, as a banknote or a cheque.

The security element according to the invention can be combined with any other security elements of the value document, such as holograms, eg micromirrors, dimple mirrors or subwavelength structures with operating effects or 3D surfaces (Fresnel). This is each preferably achieved in such a way that the reflective layer produces reflections for generating standing waves and produces the other features described in other lateral subregions. Optionally, the portion generating the standing wave is removed after exposure, which can be done, for example, by etching. Furthermore, combinations with the following features are possible: magnet, conductivity, fluorescence, phosphorescence. These any further security elements are here particularly preferably arranged laterally beside the microfibres.

The invention is explained in more detail below on the basis of an embodiment with reference to the drawings which likewise disclose the features essential to the invention. These examples are for illustration only and are not to be construed as limiting. For example, a description of an embodiment having a plurality of elements or components should not be construed as that all of those elements or components are necessary for the practice. Conversely, other embodiments may also contain alternative elements or components, fewer elements or components, or additional elements or components. Unless otherwise stated, elements or parts of different embodiments can be combined with each other. Modifications and variations described for one of the embodiments can also be applied to the other embodiments. In order to avoid repetitions, identical or corresponding elements are provided with the same reference symbols in the different figures and are not explained again. In the attached picture:

Figure 1 shows a schematic view of a banknote with several security elements,

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

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

3A to 3D show different possibilities for exposing polymer layers to form structures according to FIG. 3 ,

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

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

Figure 6 shows a view similar to that of Figures 4 and 5, wherein both the reflector layer and the polymer layer are pixel-like structured,

FIG. 7 shows an embodiment of a security element having a reflector layer with different platforms,

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

Figure 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 manufacture.

FIG. 1 schematically shows a plan view of a bank note 2 with a plurality of security elements. One anti-counterfeiting element 4 is designed in the form of a patch, and the other anti-counterfeiting element is designed in the form of an anti-counterfeiting strip or anti-counterfeiting thread 6 . The specific surface design of the security element can be selected according to the application. The following description refers purely to the security element 4 by way of example.

FIG. 2 shows a sectional view obtained by cutting 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 substrate 8, which is then applied to the banknote paper of the banknote 2, whereby the security element 4 is thus designed as a so-called transfer element.

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

The microfibrous structure 13 includes microfibers 13a and cavities 13b (see FIG. 3 ). In variant 1, the microfibrous structure is produced by irradiating commercially available planar polymers, such as polystyrene or polycarbonate films or corresponding membranes (preferably parallel to their surface normal) with radiation having a coherence length greater than the thickness of the polymer layer 12. Preference is given to using UV radiation. During this illumination, the reflector layer is located below the polymer layer 12 (see FIGS. 3-8 ), forming object rays in retroreflection. 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 smaller than the coherence length of the radiation used, whereby standing waves are formed in the polymer layer 12 . The polymer is cross-linked at the antinodes where the standing wave intensity is the largest, and a periodic mechanical stress field is formed between the cross-linked areas and the uncross-linked areas, where the uncross-linked areas are located at the nodes of the standing waves. Preferably additional photoinitiators are added to the polymer.

By using a suitable solvent, a microfibrous structure 13 exposed in this way is then formed according to FIG. 3 , wherein the individual individual planes of the microfibers 13 a and cavities 13 b are automatically and periodically aligned according to the standing wave structure. For details of fabrication, refer to the Nature article and the associated supplementary material published in Nature. The contents of these publications are hereby fully incorporated herein.

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

The structuring can take place during the exposure, ie the microfibrous structure 13 composed of microfibers 13 a and cavities 13 b can be structured transversely, ie transversely to the surface 14 . FIG. 3A shows an embodiment in which the far-field exposure 16 exposes the entire polymer layer 12 uniformly. A transversely structured microfibrous structure 13 and thus a colored 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 or below. The substrate can 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 wavelengths 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 can only be incident on the polymer layer 12 at the gaps of the mask 18 . Correspondingly, light is also reflected at the reflector layer only in these regions and the incident light interferes as long as there is reflection in these illuminated regions. The color effect is also only formed at the positions where light shines through the mask and is simultaneously reflected on the reflector layer. In this way it is possible in particular to form structuring, for example pixelation, in polymer layer 12 in addition to or alternatively to the structuring produced by the action of reflector layer 10 , wherein such pixilation influences the colour. Exposure can also be done with different wavelengths, so that the hue produced by the polymer layer 12 differs laterally, for example with three-color pixels formed by sub-pixels of primary colors (eg red, green, blue). To this end, a plurality of far-field exposures 16 a , 16 b are used sequentially in time at different wavelengths or in different wavelength ranges. The additional lateral structuring is realized here by means of different masks 18a, 18b which each act on the corresponding far-field exposure 16a, 16b. FIG. 3C shows these successive exposures in a common view. Of course, pleochroism is not limited to two colors; three, four or more different exposure steps can likewise be carried out, wherein each exposure step exposes a different partial area region of the polymer layer 12 and provides it with a color effect. With multiple exposures in this way, the polymer layer 12 is developed by using a solvent to form a microfibrous structure 13 which is subsequently transversely structured.

According to a preferred embodiment, a mask remains on the security feature in order to form, for example, a hologram or another optically variable feature in defined areas. For this purpose, a regionally present metal layer is preferably used as a mask. The mask is preferably removed regionally from the polymer layer after exposure.

The mask remaining on the substrate is particularly preferably transparent in the visible spectral range (ie invisible or at least inconspicuous in the final product) and opaque or at least translucent in the UV range. Here, the mask consists, for example, of a 50 nm thick layer of TiO ₂ . The layer is substantially transparent in the visible range, but exhibits only a very low transmission in the ultraviolet range at a wavelength of about or below 300 nm.

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

It should be noted that in the produced security elements 4 , 6 , the tilt angle-dependent color effects also form by interference during illumination, possibly also without reflector layer 10 . Microfibrils are formed when the polymer layer is developed, wherein, due to the development process, the distances of the microfibers to each other may differ from the original distance of the antinodes upon exposure. In this way, with an exposure wavelength in the ultraviolet range, it is possible, after development of the polymer layer, to have a Bragg maximum when observed in the visible spectral range.

An alternative to far-field exposure is exposure by means of a gridded beam 20 (eg from a laser or LED) having the desired coherence length and deflected through the polymer layer 12 according to a scan pattern 22 . This is shown in Figure 3D. In this case, the wavelength of the laser radiation can be configured differently at individual locations in order to produce a lateral structuring of the polymer layer 12 with regard to color effects.

These additional structuring options allow, for example, to provide the reflector layer 10 with a brightness-dependent pixel structure for patterning, and to assign each pixel a sub-pixel that adjusts its color by means of an additional structuring (mask or grid).

The laterally structured reflector layer 10 below the polymer layer 12 can be present on the entire surface, as shown in FIGS. 2 and 3 , or on parts of the surface. FIG. 4 shows a pixilated portion of the reflector layer 10 consisting of reflector pixels 24 with high reflection and reflector pixels 26 with low or no reflection. Standing waves are thus formed only at pixels where the reflector layer 10 has sufficient reflection, or whose 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 FIGS. 3B , 3C and 3D , an additional gridding of the polymer layer 12 can also be produced by exposure, so that this polymer layer has polymer layer pixels 28 and 30 in which, as shown in FIG. 5 , the color impression after development with a solvent is different. A colored pixel image is thereby achieved. Of course, more than two different pixel types are also possible.

The principles of FIGS. 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 identical, even though this may be advantageous. In particular, due to the very high pixel density in the reflector layer, the brightness of the individual individual colors within a color pixel formed from polymer layer pixels can be adjusted in a position-dependent manner. Ultimately, the brightness for each color point can thus be freely selected to generate the 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 manifested in the microfibrous structure and are maintained after development even without reflector layer 10 .

Fig. 7 shows an embodiment in which the reflector layer 10 has mesas 32, 34, 36 at different heights. This takes advantage of the fact that the path length of the reflected radiation is related to the nodes and antinodes of the standing waves that are set inside the polymer layer. Due to the different height levels, ie the 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 is especially conceivable for unstructured polymer layers 12 , for example 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 in such a way that it reproduces, for example, a three-dimensional optical impression. Due to the exposure through this reflector layer 10, corresponding object rays are then formed and overall a polymer comprising microfibers is formed, wherein the microfibers act like Bragg planes in volume holograms which also provide three-dimensional views from various viewing angles.

In the production method in 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 back-reflected from the reference ray, as is the case in variant 1 . Therefore, no reflector layer is provided. Instead, object beam 44 is modulated by object 46 as in holography. Alternatively, the modulation is produced by means of an optical beam shaping element (for example a DMD or similar). Two (or more) rays may be incident from the same side of the polymer layer 12 or from opposite sides.

In particular the following implementations and designs are possible:

1. Light source

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

The light source can expose the polymer layer designed as a polymer film over the entire or part of the surface. For exposure of the entire surface, for example LEDs are sufficient (see Fig. 3A), the coherence length of which is sufficient. If the film should be exposed pixel-by-pixel or area-by-area, deflectable, intensely focused light beams (e.g. lasers) can be used (see Figure 3D), or light sources capable of direct modulation (micro-LEDs) or optical elements such as SLMs (Spatial Light Modulators), DMDs (Digital Micromirror Devices) or DOEs (Diffractive Optical Elements). Another possibility to produce exposure of parts of the surface is to use a mask (see Fig. 3B, 3C).

Illumination of the entire surface or at least parts of the surface can also be achieved by self-luminous displays or self-luminous screens. In this case, the display or screen illuminates the entire surface or illuminates the polymer pattern-wise.

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

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

The advantage of the microfibrillation method is that by using DOE/SLM/DMD combined with LED or laser as the light source, the light can be modulated on the micrometer length scale. Therefore, a resolution up to 25000DPI can be achieved by the microfibrillation method. At the same time, a high degree of individualization is possible through the flexibility of the optics. Since the microfibers exhibiting the Bragg plane are embedded in the polymer film, no replicas or molds can be produced for counterfeiting purposes, which achieves high security against counterfeiting.

b. Change of wavelength:

Irradiation with light of different wavelengths produces different structural colors. Thus, a larger color space can be covered by additive color mixing of RGB pixels. The exposure can be produced sequentially or simultaneously by display contents of different colors of the display, monochromatic lasers or LEDs with different emission wavelengths. For this purpose, for example polymer films can be successively covered with different masks 18 and exposed with monochromatic radiation through said masks (see FIG. 3D ).

2. Surface coverage of reflective layer

The reflective layer beneath the polymer can be present on the entire surface or on a portion of the surface. If the reflective layer is present on the entire surface, pixel-by-pixel gridding of colors can be achieved only by modulating the light source. If the reflective layer is gridded, standing waves will only form in pixels with a reflective layer underneath. Rasterizations can thus be generated.

3. Relief structure of reflective layer

An embossed reflective relief structure may be placed beneath the polymer film. Embossed structures can consist of all possible relief structures, such as micromirrors, Fresnel micromirrors, blazed gratings, moth-eye structures, subwavelength gratings, sinusoidal gratings, Manhattan gratings or Aztec structures. Structuring can also be achieved by applying, for example, a darker color, the color being applied to the side of the relief structure facing the lighting device. These and other structures may also be arranged alongside each other or overlapped. Thus, reflection can be suppressed regionally, for example by means of regionally arranged moth-eye structures or other light-absorbing structures, without producing microfibers in the polymer layer in these regions.

Embossed patterns can

a) lie directly on the film and remain there,

b) lie directly on the film and are lifted off or at least partially removed in a transfer step after exposure (for example by completely or partially etching away the metal coating, leaving the relief structure),

c) run together in register under the film,

d) Stationary under the film (no co-operation), exposure in register, for example by means of a synchronized flash light source.

The following implementations are preferred:

I. Monochrome pixel image with structured color:

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

The use of pixel images is also advantageous for security features with micro imaging elements such as microlenses due to the high resolution, wherein the pixel images can be used as microstructure images in the focal plane of the micro imaging elements.

II. 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., polychromatic, iridescent pixel images can be produced. By varying the area coverage of the colored areas, different tonal saturations can also be produced. The patterns produced have a resolution of up to 25000DPI.

Due to the high resolution, the use of pixel images also makes sense for security features with micro imaging elements such as microlenses. This will make it possible to upgrade the microlens features already present in the banknote market, as these have hitherto only been monochromatic.

III. Fabrication of Microfibrillated Holograms

Volume holograms produced by using the microfibrillation method are called microfibrillated holograms. Using the microfibrillation method, the photoresist can be replaced by a polymer film, such as a commercially available polymer with a small amount of photoinitiator added. The production process is otherwise identical to the usual production of volume holograms by exposure to interference radiation. Holograms can thus be recorded in all representations already described, for example as reflection, transmission or Denisjuk holograms.

Alternatively, holograms can also be produced without using substantially existing objects. Object rays can thus be generated using SLMs or DMDs (only using reflection holograms).

list of reference signs

2 banknotes

4 Anti-counterfeiting elements

6 security thread

8 bases

10 reflector layers

12 polymer layers

13 Microfibril structure

13a microfiber

13b cavity

14 upper side

16, 16a, 16b Far Field Exposures

18, 18a, 18b masks

20 lasers

22 scan patterns

24, 26 reflector pixels

28, 30 polymer layer pixels

32, 34, 36 platforms

38 Embedded Media

40 relief layers

42 Reference Radiation

44 Object Radiation

46 objects

Claims (17)

1. A manufacturing method for a security element (4) for manufacturing a document of value, wherein the manufacturing method has the steps of:

-providing a polymer film (12) and

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

2. A method of manufacture according to claim 1, wherein said document of value is a banknote (2) or a check.

3. The method of manufacturing of claim 1, further comprising the step of:

-creating a reflector layer (10) under the polymer film (12), said reflector layer having a lateral structuring in terms of reflectivity and/or profiled contour, and

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

4. A method of manufacturing as claimed in claim 3, wherein the lateral structuring of the reflector layer (10) is designed as a pixel structure acting to produce a pixel image.

5. A method of manufacture as claimed in claim 3, wherein the reflector layer (10) is provided transversely so as to have sections of different reflectivity.

6. A method of manufacture as claimed in claim 3, wherein the microfibrous structure (13) is laterally structured in terms of the colour produced thereby.

7. The method of manufacturing as claimed in claim 6, wherein the microfibrous structure (13) is laterally structured in the form of a pixel structure with respect to the color produced thereby.

8. A production method according to claim 3, wherein the reflector layer (10) is designed with a relief structure for the purpose of being structured laterally with respect to the shaping contour.

9. The method of manufacturing as claimed in claim 8, wherein said relief structure has at least one of the following structures:

platforms (32, 34, 36) at different height levels,

the micro-mirrors are provided with a light-emitting device,

a blazed grating structure is provided,

a fresnel structure is used,

a sinusoidal grating structure is provided which is arranged in a plane,

the columns of different heights are chosen so that,

an echelle grating with straight or inclined sides,

-moth-eye structure

Sub-wavelength structures with sharp or rounded edges.

10. A method of manufacture as claimed in claim 1, wherein said polymer film (12) is irradiated such that the laterally structured microfibrous structure (13) provides a multicoloured image.

11. The method of manufacturing as defined in claim 10 wherein said multi-colored image is a pixel image.

12. A method of manufacture as claimed in claim 3, wherein the reflector layer (10) is removed after formation of the laterally structured microfibre structure (13).

13. A method of manufacture as claimed in claim 1, wherein, for forming the laterally structured microfibre structure (13), the polymer film (12) is irradiated with two mutually interfering radiation rays, one of which forms a reference radiation and the other radiation is modulated and forms an object radiation, whereby the laterally structured microfibre structure (13) provides a volume hologram.

14. The production method according to claim 1, wherein the microfibrous structure (13) is structured in the polymer film perpendicularly to the surface of the polymer film in terms of the layer thickness of the individual layers of the microfibrous structure (13).

15. Security element for providing security to documents of value, having a structured microfibrous structure (13) produced by a method according to one of the preceding claims.

16. A security element according to claim 15, wherein the document of value is a banknote (2) or a check.

17. A security element as claimed in claim 15 in which the transversely structured microfibrous structure (13) provides a volume hologram.