WO2021249609A1 - Retroreflective relief structure in embossing coating - Google Patents

Abstract

The invention relates to an embossing die for producing a flat retroreflector, which has a retroreflective corner cube relief structure formed in a curable embossing coating, in particular a UV coating, and retroreflects electromagnetic radiation incident from an incidence side. The embossing die has a number n of mutually adjacent depressions and/or elevations on one side, wherein each of the n depressions and/or elevations is formed from three faces arranged at a certain angle (α') to one another. The invention also relates to a method for producing an embossing die of this kind. According to the invention, the certain angle (α') is smaller than 90° by a certain value X, i.e., α' = 90° - X, so that the number n of mutually adjacent elevations and/or depressions moulded into the embossing coating by the embossing die each consist of three faces arranged at an angle of 90° to one another after the curable embossing coating has cured. Elevations of the embossing die correspond to depressions in the embossing coating, and depressions in the embossing die correspond to elevations of the embossing coating.

Description

Retroreflective relief structure in embossed lacquer

The invention relates to an embossing tool for producing a flat retroreflector which has a retroreflective corner cube relief structure formed in a curable embossing lacquer, in particular a UV lacquer, and which retroreflects electromagnetic radiation incident from an incidence side. The embossing tool has a number n of adjacent depressions and / or elevations on one side, each of the n depressions and / or elevations being formed from three flat surfaces arranged at a specific angle (α ') to one another. The invention also relates to a method for producing such an embossing tool. A retroreflective structure reflects incident electromagnetic radiation, for example light, largely independently of the direction of incidence and the orientation of the reflector in the direction from which the radiation is incident. This is known as retroreflection. In the case of plane mirrors, on the other hand, the reflection depends on the mirror orientation, which is perpendicular to the viewer only in exceptional cases. Flat retroreflectors have a large number of individual retroreflectors lying next to one another. Retroreflective foils are a common realization of flat retroreflectors. They are used in a variety of ways, especially in traffic engineering, but also in security technology, for example in accordance with US 2014/0226212 A1 as security elements for authenticating documents of value, such as banknotes, debit or ID cards. Most traffic signs and motor vehicles are also used - Identifiers equipped with retroreflective structures to increase visibility at night or to improve protection against forgery (cf. US 5656360). In general, two variants of retroreflectors are widespread: Embedded, high-refractive-index microspheres which are mirrored on the underside, as described, for example, in US Pat. No. 4,763,985 and US 2009/0300953 A1. The optical principle is based on so-called Lüneburg lenses, which reflect the incident light in the direction of the beam source. The other variant, in the field of which the invention is arranged, is based on relief structures made up of plane surfaces, so-called corner cube structures, which have the shape of cube corners. The incident light is deflected one after the other on three flat surfaces (so-called triple mirrors) arranged at an angle of exactly 90 ° to one another and reflected back to the beam source. This geometry has the highest efficiency of retroreflection with perpendicular incidence of light; the efficiency decreases for increasingly oblique incidence of light. The geometry of such a structure known from the prior art is shown in FIG. 1. FIG. 1 a schematically shows the relief structure of a flat retroreflector with the optical path of the retroreflection in a side view. The retroreflector is constructed as a film composite, which comprises a carrier film 1 and a layer 2 made of an embossing lacquer, with a reflection structure 3 being molded into the layer 2. The reflection structure 3 consists of plane surfaces which are arranged at an angle of 90 ° to one another, so that incident radiation is deflected several times and ultimately retroreflected. 1b illustrates in a top view the path of retroreflection as it occurs in the case of triangular surfaces of a cube-corner mirror, from the juxtaposition of which the retroreflective relief structure for the flat retroreflector can be built up. 1c and 1d show suitable cube corner surfaces for the relief structure in a top view or perspective.

Retroreflective sheeting for KLZ license plates and street signs is usually produced using the hot stamping process. Here, the structures of an embossing tool are converted into a thermoplastic almost true to shape. The production of an embossing tool with corner cube structures by means of rotating styluses is known, for example, from WO 2018/151959 A1, WO 2018/151960 A1 or WO 2018/151964 A1.

Corner cube structures for a retroreflective film are, due to their depth of usually 15 μm to 80 μm, demanding in terms of embossing. When embossing in a curable embossing lacquer, for example a UV lacquer, which cures after exposure to electromagnetic radiation in the ultraviolet (UV) wavelength range, a very flowable UV varnish required. Conversely, this UV varnish experiences a relatively high volume shrinkage during its curing, which is not isotropic, but is more pronounced in the vertical direction to the surface of the film than in the horizontal direction, since the UV varnish on the film does not change shape and form is firmly anchored. This "anisotropic" shrinkage leads to a flattening of the corner cube structures and an enlargement of the corner cube angle (depending on the paint viscosity, chemical composition, substrate, temperature ...).

Corner cube structures, in which the angle between the respective planar surfaces is no longer 90 °, but deviates from 90 ° by more than +/- 0.5 °, almost completely lose their retroreflective properties.

The invention is therefore based on the object of developing a generic embossing tool for corner cube structures in such a way that the disadvantages of the prior art are eliminated. The object of the invention is therefore to produce retroreflective sheeting with corner cube structures in embossed lacquer, in particular in UV lacquer. This object is achieved by the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

According to the invention, the certain angle (α ') is smaller than 90 ° by a certain value X, so α' = 90 ° -X, so that the number n of adjacent elevations and / or depressions molded into the embossing lacquer by the embossing tool after the curable embossing lacquer has hardened, each consist of three flat surfaces arranged at an angle of 90 ° to one another. Elevations in the embossing tool correspond to depressions in the embossing lacquer and depressions in the embossing tool correspond to elevations in the embossing lacquer.

The invention thus provides an embossing tool which retains or compensates for the shrinkage of the lacquer in that the embossed structures are made to be exaggerated. The embossing tool itself therefore shows no retroreflective properties. This results in a further special advantage of the invention that the embossing tool according to the invention is bent to form a cylinder or a cylinder segment and adjoining edges of the cylinder or an adjacent cylinder segment are welded to one another with an electromagnetic beam, for example a laser beam, to form an embossing cylinder can be. Because if a laser beam hits the non-retroreflective structures of the embossing tool, it is fundamentally not reflected back to the laser. In the case of an embossing tool with retroreflective structures, on the other hand, the laser beam is basically reflected back to the laser and can damage or destroy the laser in the process. A curable embossing lacquer in the context of this invention is an embossing lacquer which is applied in the plastically deformable state to a substrate as a carrier and in which an embossing structure is molded with an embossing tool. Such an embossing tool is, for example, an embossing die or an embossing cylinder. The embossing lacquer is then cured, for example by means of UV radiation or thermal radiation, so that the embossing lacquer can no longer be plastically deformed and the embossing structures can no longer be changed.

Radically or cationically curing lacquers, for example, are used as radiation-curing embossing lacquers, which can be used on a large number of substrates such as PET, PVC, PE, PP, PMMA or PC. An adhesive intermediate layer can be used for stable anchoring of the UV lacquer on the substrate.

The embossing tool according to the invention has a number n of adjoining depressions. The number n of adjacent elevations in the curable embossing lacquer, which is shaped with this embossing tool, corresponds in its geometry and dimensions to the corresponding recesses in the embossing tool before the embossing lacquer has hardened. After the embossing lacquer has hardened, the elevations in the embossing lacquer no longer correspond in terms of their geometry and their dimensions to the corresponding depressions in the embossing tool. Rather, the elevations flatten out due to the shrinkage during hardening, so that the angle between three flat surfaces, which are respectively arranged with respect to one another, increases. According to the invention, the elevations flatten out due to the shrinkage during curing by the specific value X in such a way that the angle between three flat surfaces arranged with respect to one another is 90 ° and the elevations show retroreflective properties. The value of X, by which the certain angle (α ') is smaller than 90 °, and by which the embossing lacquer flattens out during curing, depends on the curable embossing lacquer used, the substrate to which the embossing lacquer is applied, and the curing conditions and is different for different curable embossing lacquers. If the value X for a specific curable embossing lacquer in combination with the substrate to be used is not known, the value X must be determined through preliminary tests. Here, elevations are molded in the embossing lacquer with an experimental embossing tool which has depressions with a first angle α 'and, after the embossing lacquer has hardened, the angle α of the resulting flanks of the elevations is measured. The difference between the measured angle α and 90 ° gives the correction value by which the angle α 'must be corrected in the following pass. If necessary, this process is repeated until the angle α is equal to 90 °.

For commercially available curable embossing lacquers, the specific value X usually has a value of less than or equal to 3.5 °, preferably from 0.3 ° to 3.5 °, so that the specific angle (α ') is greater than or equal to 86 .5 °, preferably 86.5 ° to 89.7 °. Embossing lacquers known from the prior art are, for example, UV-curable hybrid polymers for micro-optical systems from the company "micro resist technology GmbH" of the Ormocer type. For example (as of July 2015) the embossing lacquer "OrmoComp®" or "In- kOrmo "has a relative volume shrinkage of 5% to 7%," OrmoStamp® "from 4% to 6%," OrmoCore "and" OrmoClad "from 2% to 5%," OrmoClear® "from 3% to 5%, "OrmoClear®10" of <2% and "OrmoClear®30" of << 2%.

X is therefore the value by which the embossing lacquer shrinks after curing, usually by 0.3 ° to 3.5 °. For example, structures show in one certain UV lacquer after UV embossing by means of an embossing tool with recesses with an opening angle α 'of 90 ° due to the lacquer shrinkage an opening angle α of 92.5 °, so X = 2.5 °. In order to keep this shrinkage or this angle difference X in the next tool, the structures in the embossing tool are manufactured with a corner cube angle of 90 ° - 2.5 ° = 87.5 °.

The design of the embossing tool is determined depending on the structure of the retroreflective film, in particular whether the viewer is looking through the UV varnish at the cube-corner structures or through the layer that is complementary to the UV varnish. In the first case, the tool will have depressions in the form of cube corners, the opening angles of which are modified according to the invention with regard to the anticipated paint shrinkage. In the second case, the embossing tool has elevations in the form of modified cube corners, which leave corresponding depressions in the UV varnish.

The invention also relates to a method for producing an embossing tool according to the invention. The embossing tool here consists of a substrate, depressions being made in one side of the substrate with a stylus, the stylus having an angle of 70.52 ° -X at its tip. Such styluses consist, for example, of diamond, solid carbide, HSS-Co, SiC, TiC, tungsten or TiCN. In the state of the art, corresponding embossing tools are manufactured using diamond cutting. A diamond stylus with an angle at the tip of 70.52 ° is used to create a grid of parallel furrows in the substrate of an embossing tool. Then two more sets of grids are drawn, which are rotated by 60 ° or 120 ° to the first grid are. When using a diamond stylus with 70.52 °, corner cubes with an opening angle α 'of 90 ° remain in the embossing tool.

The substrate of the embossing tool preferably consists of brass (an alloy of copper and zinc), copper, nickel, nickel phosphorus, nickel vanadium, nickel silver (an alloy of nickel, copper and zinc) or cobalt. Softer substrates are also often used are used, on the surface of which a hard coating is deposited, e.g. Cr, CrN, Ti, TiN, carbides, W, DLC (diamond like carbon).

In order to produce an embossing tool according to the invention with a more acute opening angle of 90 ° -X, a correspondingly more pointed diamond stylus with an angle at the tip of 70.52 ° -X is used. For example, in the case of paint shrinkage with the value X = 2.5 °, a diamond stylus with an opening angle of 70.52 ° - 2.5 ° = 68.02 ° is required.

It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the specified combinations, but also in other combinations without departing from the scope of the present invention, insofar as this is covered by the scope of protection of the claims .

The advantages of the invention are explained on the basis of the following exemplary embodiments and the supplementary figures. The exemplary embodiments represent preferred embodiments to which, however, the invention is not intended to be restricted in any way. Furthermore, the representations in the figures are highly schematic for better understanding and do not reflect the real conditions. In particular, the proportions shown in the figures do not correspond to the real-world conditions and are used exclusively to improve the appearance vividness. Furthermore, the embodiments described in the following exemplary embodiments are reduced to the essential core information for better understanding. In the practical implementation, much more complex patterns or images can be used.

In detail show schematically:

1 shows a planar retroreflector known from the prior art and in this case in FIG. 1a the retroreflector in side view, in FIG 1d suitable cube corner surfaces for the relief structure in plan or perspective,

2 shows a first exemplary embodiment of a retroreflector manufactured with an embossing tool according to the invention in a side view,

3 shows a first exemplary embodiment of an embossing tool according to the invention in a side view,

4 shows a curable embossing lacquer embossed with the embossing tool from FIG. 3 and in this case in FIG. 4a before the curing of the embossing lacquer and in FIG. 4b after the curing of the embossing lacquer,

5 shows a second exemplary embodiment of a retroreflector manufactured with an embossing tool according to the invention in a side view, 6 shows a second exemplary embodiment of an embossing tool according to the invention in a side view,

7 shows a curable embossing lacquer embossed with the embossing tool from FIG. 6 and in this case in FIG. 7 a before the curing of the embossing lacquer and in FIG. 7 b after the curing of the embossing lacquer.

2 shows schematically a first exemplary embodiment of a retroreflector produced with an embossing tool according to the invention in a side view for viewing from above, i.e. on the top of the embossing lacquer 2 according to FIG. 1a. The embossing lacquer 2 is shown in dashed lines before it has hardened, and with a solid line after it has hardened. Before hardening, the flanks 3 'of the embossed structures have the angle α' to one another in their "valley". During hardening, the height of the embossed structures shrinks, so that their flanks 3 in their "valley" have the angle α = after hardening 90 ° to each other.

3 shows a schematic side view of a first exemplary embodiment of an embossing tool 4, in one side of which two depressions 5 are made. The depressions 5 have flanks which are aligned at an angle α 'to one another. 4 shows a curable embossing lacquer 2 embossed with this embossing tool 4. Before the embossing lacquer 2 cures, according to FIG Structures according to FIG. 4b have shrunk in height, so that their flanks 3 after hardening in their "valley" have the angle α = 90 ° to one another.

Fig. 5 shows schematically a second embodiment of a retroreflector manufactured with an embossing tool according to the invention in side view for viewing from below, ie through the embossing lacquer 2 through to the opposite side of the embossing lacquer 2. The substrate 6 and the embossing lacquer 2 must in this case be transparent so that they allow incident light to pass through with almost no attenuation and almost no scattering. In this case, incident light rays are not reflected directly on the reflective upper side of the embossing lacquer as in FIG. 1 a or FIG. 2 or FIG. 4b, but only after passing through the transparent substrate 6 and the transparent embossing lacquer 2. The reflective surface is in both The cases are the same, but the opposite side of this surface is reflective.

In accordance with FIG. 2a, the embossing lacquer 2 is shown with broken lines before it has hardened, and with a solid line after it has hardened. Before hardening, the flanks 3 ′ of the embossed structures have the angle α ′ to one another at their apex. During the hardening, the height of the embossed structures shrinks, so that their flanks 3 after hardening have the angle α = 90 ° to one another at their apex.

6 shows a schematic side view of a second exemplary embodiment of an embossing tool 4, on one side of which three elevations 8 are applied. The elevations 8 have flanks which are aligned at an angle α 'to one another. 7 shows a curable embossing lacquer 2 which is embossed with this embossing tool. Before the embossing lacquer 2 cures, according to FIG. lacquer 2, the embossed structures according to FIG. 7b have shrunk in height, so that their flanks 3 after hardening have an angle α = 90 ° to one another at their apex. The representations in FIGS. 2 to 7 are not true to scale. In particular, the depth of the depressions 5 or the height of the elevations 8 of the respective embossing tool 4 or 7 and the height of the elevations before the curing of the embossing lacquer 2 are shown greatly exaggerated for better clarity. With this, of course, the angle α 'is also made more acute than in

Reality. The angle α, on the other hand, is shown almost true to scale as a 90 ° angle.

Claims

P a te n claims 1. Embossing tool for producing a flat retroreflector, which has a retroreflective corner cube relief structure formed in a curable embossing lacquer, in particular a UV lacquer, and retroreflecting electromagnetic radiation incident from an incidence side Number n of adjoining depressions and / or elevations, each of the n depressions and / or elevations being formed from three plane surfaces arranged at a specific angle (α ') to one another, characterized in that the specific angle (α') is smaller than 90 ° by a certain value X, so that the n depressions / or elevations formed by the embossing tool in the curable embossing lacquer each consist of three at an angle of 90 after the embossing lacquer has hardened ° there are planar surfaces arranged in relation to one another. 2. Embossing tool according to claim 1, characterized in that the value of X depends on the curable embossing lacquer used, the curing conditions and the substrate to which the embossing lacquer is applied, and is different for different curable embossing lacquers. 3. Embossing tool according to claim 2, characterized in that the specific value X has a value of less than or equal to 3.5 °, preferably from 0.3 ° to 3.5 °, so that the specific angle (α ') is greater than or equal to is equal to 86.5 °, preferably 86.5 ° to 89.7 °. 4. A method for producing an embossing tool according to one of the preceding claims, characterized in that the embossing tool consists of a substrate, with depressions in one side of the substrate and / or elevations are introduced with a stylus, the stylus having an angle of 70.52 ° -X at its tip.