DE102015207268A1 - Security element and method for producing a security element - Google Patents

Abstract

Security element (10) with an introduced by laser lithography optical security feature, comprising a transparent polymeric film (11) and a directly or indirectly applied to the film ink layer (14), characterized in that the ink layer contains metal pigments (12).

Description

The present invention relates to a security element with an introduced by laser lithography optical security feature comprising a transparent polymeric film and a directly or indirectly applied to the film ink layer.

Security elements with a transparent film as a carrier material are well known. In the form of adhesive labels with embossed holograms, kinegrams or other optically variable elements, they are often used for counterfeiting or counterfeit protection.

In many cases, for this purpose, a film is provided with the aid of a finely structured embossing tool having an embossed structure (relief structure), or a lacquer applied thinly to a film is embossed. With the aid of a subsequent metallization step, the structure can be observed in reflection as a phase-modulating structure. Likewise, a film, or a film with a suitable lacquer layer, first metallized and then embossed. Here as well, a phase-modulating structure observable in reflection is produced as a result.

The metallization step takes place in many cases by vacuum evaporation or by sputtering processes. The resulting metallization is thus full-surface. However, partial metallizations can also be carried out by, for example, first making a partial release-layer printing on the film. After a subsequent evaporation, the release layer and thus the metallization can be partially removed.

In addition to embossing processes for producing optical security elements, or optically variable elements, laser lithography methods are also known. With the aid of a focused, pulsed laser beam, in this case a film which is provided with a metallization is processed in such a way that the laser radiation focused in the metallization layer leads to a local, microscopic demetallization. Placed in precalculated distribution, many such demetallization points yield the desired optically variable structure. For example, these demetallization points may be arranged on an orthogonal grid in the X and Y directions. The diameter of the demetallization points resulting from the lithography step is essentially dependent on the intensity distribution of the laser light in the focus area and the introduced energy of the laser pulse, but also on the layer thickness of the metallization. It is desirable to have a diameter of the demetallization points in the size of the grid spacing of the orthogonal grid.

Investigations in Tobias Kresse, "Realization of a Cylindrical Volumetric Data Memory", Dissertation University of Heidelberg, 2005, show a dependence of the layer thickness of a metallization and the resulting optical absorption for light in the visible range. With increasing layer thickness, the absorption also increases, which then passes through a maximum at the so-called percolation point and then drops again as the layer thickness increases. For aluminum, for example, a maximum absorption of about 20% to 25% is achieved. This absorption is used during the lithography process to briefly heat the metal layer during the laser pulse. The result is a "demetallization point". If such a layer thickness is selected with maximum absorption, there is the possibility of a stable lithography process with desirably constant large diameters of the demetallization points, since fluctuations in the metallization layer thickness result in only slight absorption fluctuations. It also follows that a structure with a low laser power and therefore can be generated efficiently, since there is a high absorption. However, it turns out to be disadvantageous that the layer thickness of maximum absorption leads to a reflection of only about 50%. Such fumes do not appear typically reflective, but slightly greyish and reduce the visibility of the introduced security feature.

EP 2 005 425 B1 discloses a holographic memory material having a polymeric film, a metallic first layer having a relatively low thickness deposited thereon, which is in the region of an absorption maximum to achieve high absorption, followed by a non-metallic second layer followed by a metallic third layer having a higher thickness as the first layer. Thus, a structure is produced which exhibits both high absorption (through the first layer) and high reflection and optical density (through the third layer). Decisive for the property of this three-layer structure is that the two metallic layers are electrically separated from each other by the second layer. The conductivity properties of the first and third layers essentially determine their respective optical properties. In this way, it is possible to produce a lithographically exposable material which has a high absorption which is stable with respect to layer thickness fluctuations and thus enables a stable and efficient lithography process and also has high reflection, which makes an introduced structure clearly visible on later use. The disadvantage of this process is the complicated and thus costly manufacturing process of three thin films by sputtering or sputtering.

The invention has for its object to provide a security element that combines high absorption, high reflection, high process stability and ease of production.

The invention solves this problem with the features of the independent claims. A security element according to the invention comprises a transparent film and a metal pigment-containing color layer applied thereto. The present invention has recognized that the optical absorption of such a color layer remains constant with a large variation of the ink layer thickness. The decisive factor for the absorption is in fact the single metallic pigment, which is present in the color layer in an electrically isolated manner from neighboring pigments.

Thus, the optical property is determined via the layer thickness of a single pigment.

With the metal pigment paint according to the invention, it is possible to achieve optical properties which, with a significantly varying ink layer thickness, have nearly constant absorptions for a lithographic process. Furthermore, colors with metal pigments can have a high absorption. There is therefore a high process stability of the lithographic process, for example with regard to fluctuations in the laser power or fluctuations in the focus position relative to the absorbing layer and the associated fluctuations in the intensity distribution in the absorbing layer. The required laser power is also comparatively low. The energy introduced by absorption into the pigments or the color layer leads to a displacement of metal and thus to a "demetallization point". It turns out that films with a metal pigment coating with an optical density of approx. OD 0.5 up to an optical density of approx. OD 1.5 can be successfully exposed with identical laser pulse powers.

The color layer contains metallic pigments preferably having a relatively large mean diameter of at least 1 .mu.m, in certain embodiments preferably at least 2 .mu.m or at least 5 .mu.m or at least 10 .mu.m. For certain printing processes, for example inkjet printing, a mean diameter of at most 50 μm, more preferably at most 30 μm, even more preferably at most 20 μm or 15 μm may be advantageous. The metal pigments are preferably disk-shaped with a small thickness relative to the diameter. The average thickness is between 10 nm and 200 nm, preferably between 10 nm and 150 nm. The diameter of the metallic pigments is preferably in a narrow range around the average diameter. Advantageously, less than 10% of the metallic pigments have a mean diameter less than half the average diameter of all pigments, and / or less than 10% of the metallic pigments have a mean diameter that is more than twice as large is like the mean diameter of all pigments.

The color layer can be directly, i. immediately, be arranged on the slide. Alternatively, one or more further layers may be present between the film and the color layer, for example an adhesion promoter and / or a lacquer layer.

An advantageous printing of the film leads to an arrangement of the pigments with predominant or even almost complete parallelism of the pigments to the film surface. The result is a mirror layer of high optical quality with little scattering. By means of a printing step, color layers can be formed which contain a plurality of pigments arranged one above the other and predominantly or nearly parallel to one another. However, the pigments do not advantageously touch each other and are therefore advantageously arranged electrically insulated from one another.

It is advantageous for the introduction of an optical security feature that the metallization in a layer system according to the invention can be designed in a simple manner in a batch-wise or, alternatively, individually at the unit level. Printing in non-full-surface form by conventional printing methods, such as screen printing or gravure printing, are possible. However, inks suitable for inkjet processes with metallic pigments are also possible and allow the production of lithograph-compatible coating systems. Advantageously, a correlation is possible between information visible to a human eye of an imprint and an optical security feature incorporated therein at least in part.

The layer structure of film and ink layer can be lithographically processed in one embodiment without further intermediate steps and then processed further. In an alternative embodiment, the layer structure may initially be provided with one or more further layers, for example a colored ink layer or an adhesive layer optionally with liners or hot-melt adhesives or other layers, before a lithographic process is carried out. Furthermore, it is possible that the metal-pigment ink layer is colored by adding suitable colors and thus results in a colored metallic shiny security feature. Although during the lithography process mostly a microscopic demetallization Nevertheless, process parameters are possible which produce a refractive index change of the film or a relief formation in such a layer structure and thus do not constitute an optical amplitude-modulating structure but an optical phase-modulating structure.

If the ink layer is in one embodiment in air, so is not covered by another layer, such as an adhesive layer, it can also be ablated microscopically.

The invention will be explained below with reference to preferred embodiments with reference to the accompanying figures. It shows

1 a schematic representation of the layer structure of a security element according to the invention;

2 a schematic representation of a metal pigment in an advantageous embodiment; and

3 a schematic representation of the layer structure of a security element according to the invention in a further embodiment.

The security element 10 is, for example, a label for affixing to a product or packaging for purposes of counterfeit protection. For the production of the security element 10 First, a transparent polymer film 11 made of a plastic, which is the major part of the thickness of the security element 10 represents and serves as a substrate or carrier film. The polymer film 11 is then treated with a metal pigment 12 , in particular aluminum pigments, containing color 13 or a lacquer printed, for example by means of digital printing, to a metal pigments 12 containing paint or varnish layer 14 to create. The metal pigments 12 make up the largest proportion of additives in the paint layer 14 , The layer thicknesses are in 1 not to scale reproduced, but the thickness of the film 11 in relation to the thickness of the paint layer 14 considerably larger than in 1 shown. The metal pigments 12 lead to the formation of a visually perceptible reflective ink layer 14 ,

An aluminum pigment 12 is exemplary in 2 shown enlarged. The mean diameter D of the metal pigment 12 is the diameter of a circle of the same area as the area of the metal pigment 12 in a top view. The metal particles 12 have the shape of a flat disc.

The metal pigments 12 are in the color layer 14 advantageously predominantly arranged approximately parallel to each other and approximately parallel to the layer profile.

In a subsequent step, an optical security feature by means of laser lithography in the color layer 14 brought in. The security feature may include individualized information to make the security feature as counterfeit-proof as possible. This may include, for example, a hologram, in particular a phase hologram and / or an amplitude hologram. However, other optically recognizable structures can also be introduced. The laser lithographic processing makes diffractive and / or non-diffractive microscopic structures in the ink layer 14 educated. More specifically, with the aid of a focused, pulsed laser beam, the metallized film is processed in such a way that the metallization layer in the metallization 14 Focused laser radiation leads to a local, microscopic demetalization (laser-induced material displacement and / or ablation). Placed in precalculated distribution, many such demetallization points yield the desired optically variable structure. For example, these demetallization points may be arranged on an orthogonal grid in the X and Y directions.

In the embodiment according to 3 is in addition to the in 1 shown layer structure an adhesive layer 15 provided, in particular on the film 11 opposite side of the paint layer 14 is appropriate. The security element 10 is here for example designed as an adhesive label. The adhesive layer 15 may be associated with a substrate or release liner, not shown. In this embodiment, the lithographic processing may alternatively be performed either before or after the application of the adhesive layer 15 and / or further layers.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 2005425 B1 [0007]

Claims (8)

Security element ( 10 ) with an optical security feature introduced by means of laser lithography, comprising a transparent polymeric film ( 11 ) and a directly or indirectly applied to the film ink layer ( 14 ), characterized in that the color layer metal pigments ( 12 ) contains. Security element according to claim 1, characterized in that the metal pigments ( 12 ) Include aluminum pigments. Security element according to one of the preceding claims, characterized in that the metal pigments ( 12 ) have a mean diameter D of at least 1 micron. Security element according to one of the preceding claims, characterized in that the metal pigments ( 12 ) are disc-shaped with a small diameter d relative to the diameter D. A security element according to claim 4, characterized in that the average thickness d of the metallic pigments ( 12 ) is between 10 nm and 200 nm. Security element according to one of the preceding claims, characterized in that the metal pigments ( 12 ) in the paint layer ( 14 ) are arranged predominantly or substantially completely electrically insulated from one another. Method for producing a security element ( 10 ), comprising the steps of providing a transparent polymeric film ( 11 ), Applying a color layer ( 14 ) directly or indirectly on the film ( 11 ) and introducing an optical security feature by means of laser lithography, characterized in that for the color layer ( 14 ) a metal pigments ( 12 ) containing color is used. A method according to claim 7, characterized in that the metal pigment-containing paint on the film ( 11 ) is printed.

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