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EP-4741172-A1 - SECURITY ELEMENT AND PIGMENTS FOR PREPARING AN OPTICALLY VARIABLE INK

EP4741172A1EP 4741172 A1EP4741172 A1EP 4741172A1EP-4741172-A1

Abstract

It is disclosed a security element for protecting valuables. The security element comprises a substantially flat substrate (2) having a surface defining a basic plane, and a printed layer (4) which is applied to the surface of the substrate (2) and comprises pigments (6, 8, 28) for generating a color-shift under white light illumination. The pigments (6, 8; 28) comprise at least one first dielectric layer (18) and at least one second dielectric layer (22) each as a spacer between two layers (14, 16), wherein the first (18) and second dielectric layers (22) have different layer thicknesses, the layer thickness of the first dielectric layer (18) is selected such that the pigments (6, 8, 28) are configured to generate a short wavelength color in reflection at a normal viewing angle (24) normal to the basic plane and under white light illumination, and the layer thickness of the second dielectric layer (22) is selected such that the pigments (6, 8, 28) are configured to generate a long wavelength color in reflection at an oblique viewing angle (26) of 60° relative to the normal of the basic plane and under the white light illumination.

Inventors

  • SCHERER, MAIK RUDOLF JOHANN
  • MAAK, Christian
  • Rudisch, Tobias
  • SAUER, AXEL
  • SCHERER, Kai Hermann

Assignees

  • Giesecke+Devrient Currency Technology GmbH

Dates

Publication Date
20260513
Application Date
20250331

Claims (15)

  1. A security element for protecting security papers, valuable documents or other valuables, the security element comprising: - a substantially flat substrate (2) having a surface defining a basic plane, and - a printed layer (4) which is applied to the surface of the substrate (2) and comprises pigments (6, 8; 28) for generating a color-shift under white light illumination, wherein - the pigments (6, 8; 28) comprise at least one first dielectric layer (18) and at least one second dielectric layer (22) each as a spacer between two further layers (14, 16), wherein the first (18) and second dielectric layers (22) have different layer thicknesses, - the layer thickness of the first dielectric layer (18) is selected such that the pigments (6, 8; 28) are configured to generate a short wavelength color in reflection at a normal viewing angle (24) normal to the basic plane and under white light illumination, and - the layer thickness of the second dielectric layer (22) is selected such that the pigments (6, 8; 28) are configured to generate a long wavelength color in reflection at an oblique viewing angle (26) of 60° relative to the normal of the basic plane and under the white light illumination.
  2. The security element as claimed in claim 1, wherein the printed layer (4) comprises a pigment blend, wherein the pigment blend includes first pigments (6) comprising the first dielectric layer (18) and includes second pigments (8) comprising the second dielectric layer (22).
  3. The security element as claimed in claim 4, wherein each of the pigments (6, 8) of the pigment blend comprise a five layer interference stack (12, 20) comprising a sequence of lower absorber layer, lower dielectric layer, reflector layer, upper dielectric layer and upper absorber layer, wherein the first dielectric layer (18) forms the upper dielectric layer and is repeated also as the lower dielectric layer in the first pigments (6) and wherein the second dielectric layer (22) forms the upper dielectric layer and is repeated also as the lower dielectric layer in the second pigments (8).
  4. The security element as claimed in claim 1, wherein at least some of the pigments comprise the first dielectric layer (18) and the second dielectric layer (22), such that these pigments each are an asymmetric pigment (28) generating the short wavelength color in reflection from a first pigment side and the long wavelength color in reflection from a second pigment side opposite to the first pigment side.
  5. The security element as claimed in claim 4, wherein the at least some of the pigments comprise a five layer interference stack comprising a sequence of lower absorber layer, lower dielectric layer, reflector layer, upper dielectric layer and upper absorber layer, wherein the first dielectric layer (18) forms the upper dielectric layer and the second dielectric layer (22) forms the lower dielectric layer.
  6. The security element as claimed in any of the above claims, wherein the short wavelength color comprises a peak of reflected light at a wavelength of 460 nm +/- 25 nm and at the normal viewing angle (24) and the long wavelength color comprises a peak of reflected light at a wavelength of 660 nm +/- 25 nm and at the oblique viewing angle (26).
  7. The security element as claimed in any of the above claims, wherein the first dielectric layer (18) has a thickness of 280 nm +/- 25 nm and the first pigments (6) or the asymmetric pigments (28) from the first pigment side generate a peak of reflected light at a wavelength of 436 nm +/- 25 nm and at the normal viewing angle (24).
  8. The security element as claimed in any of the above claims, wherein the second dielectric layer (22) has a thickness of 490 nm +/- 20 nm and the second pigments (8) or the asymmetric pigments (28) from the second pigment side generate a peak of reflected light at a wavelength of 485 nm +/- 25 nm and at the normal viewing angle (24).
  9. The security element as claimed in any of the above claims, wherein the first dielectric layer (18) and/or the second dielectric layer (22) comprise SiO2.
  10. The security element as claimed in any of the above claims, wherein the pigments (6, 8; 28) have a platelet shape.
  11. Pigments for producing a security element, which pigments (6, 8; 28) are configured for generating a color-shift under white light illumination, wherein - the pigments (6, 8; 28) being platelet shaped extending along a plane, - the pigments (6, 8; 28) comprise at least one first dielectric layer (18) and at least one second dielectric layer (22) each as a spacer between two further layers (14, 16), wherein the first (18) and second dielectric layers (22) have different layer thicknesses, - the layer thickness of the first dielectric layer (18) is selected such that the pigments (6, 8, 28) are configured to generate a short wavelength color in reflection at a normal viewing angle (24) to the plane and under white light illumination, and - the layer thickness of the second dielectric layer (22) is selected such that the pigments (6, 8, 28) are configured to generate a long wavelength color in reflection at an oblique viewing angle (26) of 60° relative to the normal of the plane and under white light illumination.
  12. The pigments as claimed in claim 11, wherein the pigments comprise a pigment blend, wherein the pigment blend includes first pigments (6) comprising the first dielectric layer (18) and includes second pigments (8) comprising the second dielectric layer (22).
  13. The pigments as claimed in claim 12, wherein the first pigments dominate in the pigment blend in numbers over the second pigments, preferably by making up 60 % to 80 % of the number of pigments in the pigment blend.
  14. The pigments as claimed in claim 13, wherein at least some of the pigments comprise the first dielectric layer (18) and the second dielectric layer (22), such that these pigments each are asymmetric pigments (28) generating the short wavelength color in reflection from a first pigment side and the long wavelength color in reflection from a second pigment side opposite to the first pigment side.
  15. An optically variable ink for producing a security element, the ink comprising pigments according to at least one of the above claims 11 to 14.

Description

The invention relates to a security element for protecting security papers, valuable documents or other valuables. The security element comprises a substantially flat substrate having a surface, and a printed layer which is applied to the surface of the substrate and comprises pigments having a color-shift generating structure. The pigments are configured to generate a colourshift effect. The invention further relates to pigments for preparing an optically variable ink. For security purposes, valuable documents are often provided with security elements comprising a viewing-angle-dependent effect. These security elements allow for authentication of the valuable document to be verified while simultaneously increasing security against counterfeiting, as they cannot be reproduced even with the most modern copiers. Effect pigments, especially color-shift pigments, are used to create this viewing-angle-dependent effect. These optically variable color-shift pigments have a multilayer structure, conveying a different image impression to a viewer at different viewing angles, for example, displaying a different color or brightness impression depending on the viewing angle. The effect pigments can, for example, be integrated into the substrate of the valuable document or applied to a surface of the substrate, for example as an ink. In the ink, the effect pigments are contained in a binder and can therefore be easily printed. If blue-to-red color-shift pigments are used in a security thread or in optically variable (magnetic or non-magnetic) inks, the color change, which depends on the viewing angle, appears relatively dark and is therefore difficult to detect. This results from the fact that the red shift color is usually very dark, as it is clearly overlaid by the blue color, which is visible when viewed from above in straight view normal to the surface of the substrate - this results in a lack of color purity of the blue-to-red color-shift effect. It is, therefore, an object of the invention to provide a security element and pigments for preparing an optically variable ink, enhancing the optical effect of a valuable. The invention is defined by the independent claims. The dependent claims relate to preferred embodiments. The invention provides a security element for protecting security papers, valuable documents and other valuables. The security element comprises a substantially flat substrate having a surface. A plane surface defines a basic plane for the purposes of this application. The substrate can be optical transparent or semi-transparent and can be a polymer substrate or a polymer composite substrate. The polymer substrate is preferably made out of polyethylene terephthalate (PET), or out of polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polyester like polycarbonate (PC), cellophane, polyactide (PLA), celluloseacetat or other common polymer types. Polymer substrates are preferred because they are resistant against external (for example physical or chemical) factors. Onto the surface of the substrate, a printed layer is applied, which comprises pigments for generating a color-shift effect under white light illumination. Hence, the pigments exhibit an optically variable effect. The pigments have a platelet shape extending along a plane and are arranged in the printed layer with the plane generally parallel to the basic plane. The color-shift effect provided by the pigments is a color-shift from short wavelengths to long wavelengths and preferably a blue-to-red color-shift effect. In the context of this disclosure, the term "blue-to-red color-shift" does only indicate the direction of the color-shift effect from short wavelengths to long wavelengths and does not necessarily mean that there is a starting point exactly at a blue wavelength and an ending point exactly at a red wavelength. Hence, a blue-to-red color-shift does for example although comprise a color-shift from greenish/cyan to reddish. The pigments comprise five layers including at least one first dielectric layer and at least one second dielectric layer each as a spacer between two further layers. Therefore, the pigments comprise thin film structures which comprise individual layers having a thickness in micro- or nanometre range. The layers are preferably applied by means of physical vapour deposition (PVD) or chemical vapour deposition (CVD). The pigments including the at least five layers have a maximum thickness of 20 µm, preferably 10 µm, highly preferably 2 µm, and have preferably a symmetrical structure in its stack of layers. The pigments preferably comprise an interference layer stack comprising following layer sequence: absorber/dielectric material/reflector/dielectric material/absorber; absorber/dielectric material/magnetic reflector/dielectric material/absorber; absorber/dielectric material/reflector/magnet/reflector/dielectric material/absorber. The absorber, the reflector and/or the magnetic reflecto