Method for producing a security element having a relief structure and security element having a relief structure

Abstract

The invention relates to a method for producing a security element (4) comprising the following steps: providing a UV-embossing lacquer layer (8) and embossing a relief structure (9) and hardening the UV-embossing lacquer layer (8); printing a lacquer as a primer on the relief structure (9) and drying the lacquer, wherein the dried primer has a layer thickness of a maximum of 2 µm; and depositing by PVD of a ZnS layer (12) of a maximum thickness of 1 µm to cover the dried primer layer (10).

Description

[0001] V erze hr enzum H herstellunge in es

[0002] Safety element with relief structure and Safety element with relief structure

[0003] The invention relates to a method for producing a security element comprising providing a UV embossing lacquer layer and embossing a relief structure and hardening the UV embossing lacquer layer and PVD deposition of a maximum 1 pm thick zinc sulfide (ZnS) layer for covering the relief structure.

[0004] The invention further relates to a security element comprising a UV embossing lacquer layer into which a relief structure is embossed, and a maximum 1 pm thick, PVD-deposited ZnS layer for covering the relief structure.

[0005] Such a security element is, for example, from the

[0006] DE 102020000732 Al is known. It provides an optically variable security element for protecting valuables, etc., which has a ZnS layer on a relief structure. Similar structures are known from the following publications: EP 2953798 B1, DE 10201805447 A1, DE 102018005454 A1, and DE 10201005474 A1.

[0007] Security elements with micromirrors or relief structures are also disclosed in the following publications: EP 1966769 B2, EP 1879154 Bl, EP 2507068 Bl, EP 2632739 Bl, EP 2633344 Bl, EP 2655080 A2, EP 2909660 A2, EP 3294566 Bl, EP 3302998 Bl, EP 3339048 Bl, EP 3621819 A1, EP 2539158 Bl, EP 3427968 Bl, EP 3372420 Bl, EP 3194178 Bl, EP 3415964 Bl, EP 3634770 Bl and EP 3752371 A2. WO 2022 / 096833 A1 describes such a security element with a multilayer structure on which the relief structure forms the micromirrors. WO 2019 / 158625 A2 discloses a list for two superimposed, flat layers, which includes, among other things, an unspecified adhesion promoter layer and a high-index layer. A layer thickness range of a few nanometers to a few micrometers is specified.

[0008] Thin ZnS layers, i.e. layers with a maximum layer thickness of 1 pm, which are applied to UV embossing lacquer using PVD, exhibit problems with layer adhesion. To improve adhesion, it is known to perform a plasma treatment with various gases, such as nitrogen, oxygen, or argon, during the PVD process and to use excitation, for example with direct current or medium-frequency voltage. While this improves adhesion, it is not sufficient to prevent the ZnS layer from being removed using adhesive tape. Accordingly, the durability of such a security element is insufficient. Counterfeiting is also possible, particularly if, as is usually the case, the ZnS layer is covered with an equalizing protective layer. In this case, a counterfeiter can peel off the equalizing protective layer using adhesive tape, since the ZnS layer detaches from the embossing lacquer layer along with the equalizing protective layer.In this way, the embossed structure can be molded and used for a counterfeit.

[0009] The invention is therefore based on the object of further developing the manufacturing method and the security element mentioned above in such a way that their durability is increased and their susceptibility to counterfeiting attacks is reduced. The invention is defined in the independent claims. The dependent claims relate to preferred developments.

[0010] The process for producing a security element involves, in a first step, providing a UV embossing lacquer layer, and embossing and curing a relief structure thereon. Before applying the ZnS layer, a primer in the form of a lacquer is printed onto the relief structure as a layer, which acts as an adhesion promoter. The primer layer is dried. The dried primer layer has a maximum layer thickness of 2 μm. Subsequently, a maximum 1 μm thick ZnS layer is deposited using PVD to cover the primer layer.

[0011] The thickness of the primer layer ensures that the embossed structures of the relief structure are not undesirably rounded. For a relief structure formed as a micromirror structure, the dried primer layer is preferably formed with a layer thickness of 100 nm to 300 nm, preferably 150 nm to 250 nm. If the relief structure has a hologram structure, thinner layer thicknesses are preferred, namely a range of 30 nm to 200 nm, preferably 50 nm to 100 nm.

[0012] The primer is a varnish. It preferably contains a volatile solvent and a film-forming agent as a non-volatile component. The solvent evaporates during the physical drying process, and the non-volatile components adhere to the relief structure as a smooth film.

[0013] The varnish can be a clear varnish, i.e. pigment-free. Alternatively, a comparatively small proportion of pigments can be present. They serve as spacers to improve the winding behavior of a web with the security elements and the flow. The pigments do not significantly influence the chemical composition or drying behavior of the varnish. Particle materials, e.g. carbon black or TiO2, are therefore generally suitable as solid materials for the pigments. They have a minimum dimension that does not exceed the achievable dry film thickness of the primer, and are therefore usually nanoscale particles. Since, as will be explained, the pigments are present in a very small proportion at most, they do not noticeably interfere with the visual impression of the security element. Their visual effect is therefore irrelevant.

[0014] To achieve the desired thin layer thickness homogeneously with the printing process, the varnish is adjusted to a comparatively high viscosity using a film former and, if present, contains only a small amount of solids in the form of pigments, which does not adversely increase the viscosity. This can be determined through a simple series of experiments. Commercially available primers are designed in an inverted manner for this purpose and contain a high solids content in the comparatively low-viscosity film former (also known as the binder).

[0015] In order to achieve the desired, lowest possible layer thickness of a maximum of 2 pm, preferably less than 2 pm, for the dried primer layer and at the same time use a technically advantageous printing process to produce the primer layer, it is preferable to set a certain viscosity for the film former (also referred to as binder). The viscosity is preferably in the range of 10 mPa s to 400 mPa s, preferably 80 mPa s. The varnish is a clear varnish or, as a pigment, contains only a low solids content of a maximum of 8 wt.%, preferably a maximum of 5 wt.%. This lack of or low solids content makes it easier to achieve the desired thin layer thickness during printing. Commercially available primers, on the other hand, are formulated to have the highest possible solids content.

[0016] A varnish made of nitrocellulose dissolved in esters and / or ketones, which has a viscosity in the above-mentioned range without the pigment components, makes it possible to work without or with the above-mentioned very small addition of pigments, ie solid materials, and still provide a viscosity suitable for printing.

[0017] For the further manufacturing process, it is preferable if the primer is tack-free after drying. "Tack-free" in the technical sense means that the surface is not sticky. A surface is tack-free when coated film pieces of approximately 100 cm 2 which were stacked, loaded with a weight of 10 kg and stored at 40 °C for 72 hours, can then be easily separated from each other without damaging the coatings.

[0018] Tests have shown that a solvent-based 2K PU primer is particularly preferred. Such primers typically contain, and here, also preferably, esters and / or ketones as solvents. This polyurethane coating consists of a polyol component and an isocyanate component as a crosslinker. These can be aliphatic or aromatic. The polyol component is preferably a polyester polyol. Examples of solvents that can be used include ethyl acetate, butyl acetate, propyl acetate, MEK, acetone, and dimethyl carbonate.

[0019] Optionally, the ZnS layer is covered with a leveling top layer, usually transparent in the visible spectral range. This allows the resistance of the security element to be easily improved without the top layer being removed and used to replicate the relief layer in a counterfeit attack.

[0020] The following commonly used UV-curing embossing lacquers are suitable for the UV embossing lacquer layer. Possible materials include: prepolymers, e.g., high-molecular-weight acrylic acid esters, urethane acrylates, epoxy acrylates, polyester acrylates, amine-modified polyester acrylates, reactive diluents, low-molecular-weight compounds for adjusting viscosity, e.g., TPGDA, TMPTA, DPGDA, HDDA, and stabilizers that prevent premature reaction or gelling. Possible photoinitiators include phosphine oxide derivatives, benzophenone derivatives, thioxanthones, and alpha-hydroxy ketones. The primers mentioned achieve the desired adhesion for all of these ingredients of the UV embossing lacquer layer, which can be used individually or in any combination.

[0021] The relief structure preferably has depressions and intermediate sections, and the lacquer as primer and the ZnS layer are applied both in the depressions and in the intermediate sections. For this purpose, the relief structure can be embossed in one area of ​​the UV embossing lacquer layer, and the primer and the ZnS layer are applied over the entire area. They are preferably not located only in the depressions of the relief structure. It is particularly advantageous for use in optically effective relief structures such as holograms or micromirrors if the lacquer conforms to the surface structure.

[0022] Analogous to the manufacturing process, a security element is also provided, featuring a UV embossing lacquer layer into which a relief structure is embossed. A primer layer with a maximum thickness of 2 μm is printed directly on the UV embossing lacquer layer. A maximum thickness of 1 μm of ZnS is deposited by PVD on this primer layer, which acts as an adhesion promoter. The primer layer is thus located directly between the UV embossing lacquer layer and the ZnS layer.

[0023] The properties of the security element achieved above in connection with the manufacturing process naturally also apply to the security element itself.

[0024] The invention is explained in more detail below using exemplary embodiments with reference to the accompanying drawings, which also disclose features essential to the invention. These exemplary embodiments are for illustrative purposes only and are not to be interpreted as restrictive. For example, a description of an embodiment with a large number of elements or components should not be interpreted to mean that all of these elements or components are necessary for implementation. Rather, other embodiments may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different exemplary embodiments may be combined with one another unless otherwise stated. Modifications and variations described for one of the exemplary embodiments may also be applicable to other exemplary embodiments.To avoid repetition, identical or corresponding elements in different figures are designated by the same reference numerals and are not explained more than once. The figures show:

[0025] Fig. 1 shows a top view of a valuable document with a security element,

[0026] Fig. 2 is a schematic sectional view through the security element, Fig. 3 is a block diagram for a method for producing the security element of Fig. 2.

[0027] Fig. 1 shows a top view of a valuable document 2 with a security element 4. This is preferably an optically variable security element. A sectional view through the security element 4 along line AA is shown in Fig. 2.

[0028] With regard to the optically variable properties, the security element 4 can, in particular, have all possible configurations described in DE 102020000732 A1, which is explicitly cited and incorporated herein. Where a simplified structure is explained below, this is equally possible and clarifies the decisive configurations, including those with regard to the security element(s), described in the aforementioned DE 102020000732 A1.

[0029] The security element 4 has a substrate 6 on which a UV embossing lacquer layer 8 is arranged. The materials mentioned above are suitable for the UV embossing lacquer 8. A relief structure 9 is formed in the UV embossing lacquer 8, which is designed, for example, as a micromirror structure and / or a hologram structure. A primer layer 10 made of the materials mentioned above, in particular a solvent-based 2K PU primer, is located on the UV embossing lacquer 8. The solvents are, for example, esters or ketones.

[0030] The primer layer 10 has a layer thickness of preferably less than 2 pm; for a relief structure with micromirrors, a layer thickness in the range of 200 nm is preferred; for a hologram structure, the primer layer 10 is preferably thinner, namely with a layer thickness preferably between 50 nm and 100 nm, particularly preferably 75 nm. As will be explained with reference to Fig. 3, the primer layer 10 is applied to the embossed embossing lacquer layer 8 by means of a printing process. It follows the surface structure. The carrier substrate 6 comprises, for example, a PET film.

[0031] After the primer layer 10 has physically dried, a thin ZnS layer 12 is applied using PVD. The primer layer 10 now achieves a level of adhesion that prevents the ZnS layer 12 from being removed using adhesive tape.

[0032] A leveling cover layer 14 covers the thus coated relief structure 9. Due to the adhesion provided by the primer layer 10, it is not possible to peel off the cover layer 14 with the ZnS layer 12 in order to remove the shape of the relief structure 9 in a counterfeiting attack.

[0033] Fig. 3 shows the multi-step manufacturing process in a block diagram.

[0034] In a step S1, a UV embossing varnish 8 is applied, for example, printed, to the substrate 6, which is in the form of a PET film. Particularly preferred is the material based on a mixture of urethane acrylate and amine-modified polyester acrylate with TPGDA as a reactive diluent. Subsequently, the relief structure 9 is embossed into the UV embossing varnish 8 (step S2). The UV embossing varnish 8 is then cured using UV radiation (step S3), so that the embossed relief structure 9 is stabilized for the subsequent process steps. Subsequently, in a step S4, the primer, preferably a 2K PU primer, is printed on using a printing process. Particularly preferred is the film former containing nitrocellulose and no pigments, thus being a clear varnish. Alternatively, a nanoscale particle material, e.g. carbon black or TiO2, is added as pigment in a proportion of max. 8 wt.% or less, e.g. 1, 2, 3, 4, 5, 6 or 7 wt.%.The viscosity of the film former, ie the nitrocellulose in the solvent, is in a range of 10 mPa s to 400 mPa s and particularly preferably has a value of 80 mPa s.

[0035] Subsequently, in step S5, a ZnS layer is deposited in a PVD process

[0036] 12. Standard manufacturing processes can be used.

[0037] Subsequently, in step S6, the security element 4 is covered with an equalizing cover layer 14.

[0038] List of reference symbols

[0039] 2 Value document 4 Security element

[0040] 6 (Carrier) substrate

[0041] 8 UV embossing varnish

[0042] 9 Relief structure

[0043] 10 Primer layer 12 ZnS layer

[0044] 14 leveling top layer

[0045] S1-S6 step

Claims

Patent claims 1. A method for producing a security element (4) comprising the following steps: Providing a UV embossing lacquer layer (8) and embossing a relief structure (9) and hardening the UV embossing lacquer layer (8), Printing a lacquer as a primer onto the relief structure (9) and drying the lacquer, wherein the dried primer has a layer thickness of at most 2 pm, preferably less than 2 pm, and PVD deposition of a maximum 1 pm thick ZnS layer (12) to cover the dried primer layer (10).

2. The method of claim 1, wherein the primer comprises a physically drying film former.

3. The process according to claim 2, wherein the film former has a viscosity of 10 mPa s to 400 mPa s.

4. The method according to any one of claims 1 to 3, wherein the lacquer is a clear lacquer or has a solids content of max. 8 wt.% of a particulate material as a pigment.

5. A method according to any one of claims 1 to 4, wherein the paint is tack-free after drying.

6. The method according to any one of claims 1 to 5, wherein the primer is a solvent-based 2K PU varnish.

7. A process according to any one of claims 1 to 6, wherein the film former comprises nitrocellulose dissolved in ester.

8. The method according to any one of claims 1 to 7, wherein the UV embossing lacquer layer (8) comprising a micromirror structure is provided as a relief structure (9) and the dried primer layer (10) has a layer thickness of 100 nm to 300 nm, preferably 150 nm to 250 nm.

9. The method according to any one of claims 1 to 8, wherein the UV embossing lacquer layer (8) having a hologram structure is provided as a relief structure (9) and the dried primer layer (10) has a layer thickness of 30 nm to 200 nm, preferably 50 nm to 100 nm.

10. Method according to one of claims 1 to 9, wherein the ZnS layer (12) is covered with a cover layer (14) leveling the relief structure (9).

11. Method according to one of claims 1 to 10, wherein the relief structure has depressions and the lacquer and optionally also the ZnS layer (12) are also applied outside the depressions, in particular the lacquer follows the surface shape of the relief structure in a surface-conforming manner.

12. Security element comprising: a UV embossing lacquer layer (8) into which a relief structure (9) is embossed, a primer layer (10) printed on the UV embossing lacquer layer (8) and having a layer thickness of maximum 2 pm, and a PVD-deposited ZnS layer (12) with a maximum thickness of 1 pm on the primer layer (10), wherein the primer layer (10) lies directly between the UV embossing lacquer layer (8) and the ZnS layer (12).

13. Security element according to claim 12, wherein the primer layer (10) is a nitrocellulose lacquer layer.

14. Security element according to one of claims 12 to 13, wherein the UV embossing lacquer layer (8) has a micromirror structure provided as a relief structure (9) and the primer layer (10) has a layer thickness of 100 nm to 300 nm, preferably 150 nm to 250 nm.

15. Security element according to one of claims 12 to 14, wherein the UV embossing lacquer layer (8) has a hologram structure as a relief structure (9) and the primer layer (10) has a layer thickness of 30 nm to 200 nm, preferably 50 nm to 100 nm.

16. Security element according to one of claims 12 to 15, wherein the ZnS layer (12) is covered with a cover layer (14) leveling the relief structure (9).

17. Security element according to one of claims 12 to 16, wherein the relief structure has depressions and the lacquer and optionally also the ZnS layer (12) are also applied outside the depressions, in particular the lacquer follows the surface shape of the relief structure in a surface-conforming manner.

18. A value document or a precursor of a value document not yet fit for circulation, comprising a security element (4) according to one of claims 12 to 17.

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