METHOD FOR PRODUCING A SECURITY DOCUMENT

Description

DOMAINE DE L'INVENTION

[0001] The invention relates to a method for manufacturing a security document, such as a banknote, and a security document obtained by this method. ETAT DE LA TECHNIQUE

[0002] To protect security documents, such as banknotes, from counterfeiting, these documents are generally equipped with security features that make them difficult to reproduce. There are different types of security features.

[0003] Among the safety signs, Diffractive Optically Variable Image Devices (DOVIDs) are safety signs that include diffractive optical structures that give the safety sign an appearance that changes reversibly, predictably, and reproducibly depending on the viewing or lighting conditions of the safety sign.

[0004] For example, an observer examining the security mark sees movement or color changes in the security mark when they rotate or tilt the security document. This optical effect cannot be reproduced by photocopying the security document. Thus, the optical effect produced by the security mark allows the observer to quickly and easily verify the authenticity of the security document.

[0005] DOVIDs include, in particular, security holograms, which are the most widely used.

[0006] These security features are generally manufactured as a separate component, such as a strip, film, patch, and / or thread. The separate component is then applied or affixed to the substrate of the security document during the manufacturing process of the security document, for example, by a thermal transfer process such as hot stamping.

[0007] The security sign, in the form of a separate element, is generally made up of several layers. These layers typically include an adhesive layer to fix the element to the substrate, a metallic layer to reflect light, an embossed transparent layer with diffractive nanostructures or microstructures, a protective layer, a separation layer, and a support layer.

[0008] Once the separated element is fixed to the substrate of the security document, the support layer can be removed, thanks to the presence of the separation layer.

[0009] When the surface of the substrate bearing the safety sign is illuminated, the light travels successively through the protective layer, then through the embossed transparent layer, and is then reflected by the underlying metallic layer. The reflected light travels in the opposite direction through the embossed transparent layer and then through the protective layer towards the observer's eye.

[0010] The nanostructures or microstructures of the embossed layer have the effect of diffracting the light radiation, creating an optical effect that varies depending on the angle of observation and / or the angle of illumination.

[0011] One drawback of these security markings is that they generally have limited adhesion to the substrate. For example, physicochemical tests have shown that the tape, film, patch, or wire incorporating the security marking can detach after being exposed to hot water. The tape, film, patch, or wire can then be reattached to another substrate.

[0012] Furthermore, safety markings in the form of separate elements must be able to be applied using an automated machine that allows continuous application onto a moving substrate strip. In particular, safety markings such as DOVIDs are generally applied parallel to the direction of travel of the substrate strip. Consequently, these safety markings necessarily have limited geometric shapes and spatial arrangements on the substrate.

[0013] Furthermore, even though it is possible to deposit multiple security markings as separate elements in various locations on the same substrate, this requires passing the substrate successively through several automated machines, each dedicated to depositing a single security marking. Therefore, in practice, the number of security markings as separate elements is limited, and they are deposited in predefined, localized areas of the substrate.

[0014] Furthermore, we know of high-durability banknotes comprising a substrate, a first protective film covering one side of the substrate and a second protective film covering a second side of the substrate.

[0015] Document FR 2 991 627, for example, describes a method for manufacturing a security document comprising a paper substrate, a first protective film made of polymer material covering one side of the substrate, and a second protective film made of polymer material covering the other side of the substrate. The method may include a step of printing the substrate with graphic designs followed by a lamination step consisting of applying the protective films to the substrate surfaces.

[0016] Protective films both enhance the mechanical strength of the banknote and protect the printed surfaces of the substrate from abrasion and soiling. These protective films thus increase the lifespan of the banknote, particularly under harsh circulation conditions (high humidity, high temperature, abrasion, soiling).

[0017] It would be possible to integrate a security feature, such as a DOVID, into this type of banknote, for example by attaching it to one of the protective films using the adhesive layer.

[0018] However, in this case, attaching the safety sign to the protective film using heat transfer may deform the film. Furthermore, there is a risk that the safety sign could easily peel off the protective film.

[0019] For the reasons explained above, it would be preferable to attach the separate element (strip, film, patch, or thread) directly to the substrate before applying the protective film. This way, the security sign would be positioned between the substrate and the protective film, preventing it from being easily removed. Furthermore, this method would also protect the security sign from abrasion and soiling by the protective film, as well as from folding, tearing, and mechanical stress during the handling of the security document.

[0020] However, even in this case, it would still be possible to remove the protective film and then peel off the security symbol.

[0021] GB 2 566 975 A describes the preamble to claim 1. RESUME DE L'INVENTION

[0022] One aim of the invention is to propose a method for manufacturing a security document bearing a security mark including diffractive optical structures, which cannot be easily peeled off and which is resistant to abrasion and soiling, as well as to folding, tearing and mechanical stresses during the circulation of the security document.

[0023] This objective is achieved within the framework of the present invention through a method for manufacturing a safety document, comprising the following steps: a - forming microstructures in a substrate by pressing a matrix having microreliefs against a surface of the substrate, the microstructures having alternating protrusions and depressions, b - coating the microstructures with a step-index layer so that the coated microstructures can reflect incident light radiation, the step-index layer having a thickness less than a depth of the microreliefs, and c - covering the surface of the substrate, including the microstructures coated with the step-index layer, with a protective film of polymer material, the protective film comprising a layer of polymer material and a layer of adhesive, such that the adhesive layer at least partially fills the depressions and conforms to the protrusions of the microstructures coated with the step-index layer, in which the step-index layer has a refractive index different from a refractive index of the polymer material of the protective film.

[0024] In the proposed process, the pressing carried out in step a allows both the formation of the microstructures and the reduction of the roughness of the substrate, which makes it possible to obtain in step b a good quality step index layer (i.e. a smooth, thin and uniform step index layer).

[0025] In this way, the microstructures are formed directly in the substrate.

[0026] Consequently, the safety sign cannot be separated from the substrate.

[0027] Furthermore, the microstructures and the step-index layer are protected by a polymer film, thus preserving the safety mark from abrasion and dirt. The safety mark is also protected from mechanical stress, giving it greater durability in traffic, particularly in harsh environments.

[0028] Furthermore, if the protective film made of polymer material is peeled off, the safety sign is altered or even destroyed.

[0029] Finally, since step b is carried out after step a, the pressing is unlikely to damage the step index layer, and the step index layer is unlikely to soil the matrix used to form the microstructures.

[0030] The proposed process may also have the following characteristics: In step a, the substrate is subjected to a pressure of between 10 and 100 MPa, preferably between 20 and 50 MPa; the pressing in step a is carried out by embossing, debossing, or stamping; the pressing in step a is carried out by embossing at a temperature of between 23 and 100 degrees Celsius, preferably around 90 degrees Celsius; the microstructures have a symmetrical or asymmetrical profile with respect to a direction normal to the surface of the substrate; the microstructures have a depth of between 1 micrometer and 100 micrometers, preferably between 5 and 50 micrometers, for example between 10 and 35 micrometers; the microstructures have a constant spatial period or a variable spatial period that varies along the surface of the substrate; the microstructures have a spatial period of between 1 and 500 micrometers, preferably between 20 and 150 micrometers.The microstructures are designed to reflect incident light radiation at one or more preferred diffraction angles, such that when the security document is observed by an observer at one of these preferred diffraction angles, the observer perceives a given first pattern, and when the security document is observed by the observer at another of these preferred diffraction angles, the observer perceives a second pattern, different from the first pattern. The substrate comprises a first face and a second face, opposite to the first face, in which the microstructures are formed in a surface of the first face of the substrate. The process includes an additional step of printing a pattern printed on the second face of the substrate in registration with the microstructures, or on the first face of the substrate, between the substrate and the step-index layer.The step-index layer has a refractive index lower or higher than the refractive index of a substrate layer in which the microstructures are formed. The step-index layer has a refractive index between 0.9 and 1.2 at a wavelength of 532 nanometers, or a refractive index between 1.7 and 2.4 at a wavelength of 532 nanometers. The step-index layer has a gloss index greater than 70 gloss units, measured at 60 degrees according to standard NF EN ISO 2813. During step b, the step-index layer is deposited onto the substrate by flexographic printing, screen printing, offset printing, or digital printing, for example, inkjet printing. The substrate surface has a first zone containing the microstructures and a second zone devoid of microstructures, with the step-index layer being deposited. in register with the first zone,or covers both the first zone and part of the second zone, or partially covers the first zone; the step-index layer is continuous or discontinuous in the first zone; the step-index layer comprises reflective particles, wherein the particles are metallic particles, such as silver or aluminum particles, or particles made of a material with a high refractive index; the step-index layer comprises colored pigments; the process includes an additional step of depositing an additional layer comprising colored pigments between the step-index layer and the protective polymer film; the step-index layer is deposited on the substrate in an amount of between 0.2 and 8 grams per square meter, preferably between 1.5 and 3 grams per square meter; in step c, the protective polymer film covers the entirety of a first face of the substrate.In step c, another protective film made of polymer material is applied to the substrate so that the other protective film covers the entirety of a second face of the substrate, opposite the first face. After step c, the substrate covered with the protective film(s) made of polymer material has a basis weight of between 50 and 130 grams per square meter. The substrate includes a paper layer, and in step a, microstructures are formed in the paper layer by deformation of the paper layer. The paper layer comprises plant fibers, such as cellulose fibers or fibers from annual plants, such as cotton fibers, to which synthetic fibers, such as polyamide or polyester fibers, may be added. In step a, pressing the matrix against the surface of the substrate creates differences in fiber density within the paper layer.The density differences forming, possibly in combination with the step-index layer, a pseudo-watermark, the paper layer has a basis weight of between 50 and 90 grams per square meter, the process includes a preliminary step, before step a, of impregnating the paper layer with an aqueous dispersion comprising a polymer or a prepolymer, such as an acrylic or acrylate polymer or a polyurethane or a polyvinyl alcohol (PVOH) or a polyvinylidene chloride (PVDC), the paper layer is impregnated on the surface or in the mass with an amount of aqueous dispersion of between 4 and 15 grams per square meter by dry weight.

[0031] The invention also relates to a security document obtained by a process as defined above.

[0032] The security document can be a banknote. PRESENTATION DES DESSINS

[0033] Other features and advantages will become clear from the following description, which is purely illustrative and not exhaustive and should be read in conjunction with the attached drawings, including: there figure 1 schematically represents a banknote conforming to a first embodiment of the invention, the figure 2 schematically represents a banknote conforming to a second embodiment of the invention, the figure 3 is a cross-sectional view, schematically representing a first example of microstructures forming part of the safety sign, the figure 4 is a cross-sectional view, schematically representing a second example of microstructures forming part of the safety sign, the figure 5 schematically represents a first safety sign conforming to a first embodiment of the invention, the figures 5A et 5B schematically illustrate two examples of microstructures that can be part of the first safety sign of the figure 4 , there figure 6 schematically represents a second safety sign conforming to a second embodiment of the invention, the figure 7 schematically represents a third safety sign conforming to a third embodiment of the invention, the figures 7A à 7D schematically represent microstructures of the third security sign of the figure 7 , THE figures 8A And 8B schematically represent a fourth safety sign conforming to a fourth embodiment of the invention, the figures 9A et 9B schematically represent a sixth safety sign conforming to a sixth embodiment of the invention, the figure 10 schematically represents a fifth safety sign conforming to a seventh embodiment of the invention, the figure 11 schematically represents a fifth safety sign conforming to an eighth embodiment of the invention, the figure 12 schematically represents the steps in a process for manufacturing a banknote according to an implementation method of the invention, the figure 13 represents the substrate in which the microstructures were formed, the figures 14 à 16 represent the substrate after the step of depositing a step-index layer, the figures 17 et 18 represent the substrate after the application of a protective film. DESCRIPTION DETAILLEE D'UN MODE DE REALISATION

[0034] On the figure 1 The banknote 1 shown comprises a substrate 2, a first protective film 3 and a second protective film 4.

[0035] Substrate 2 may consist of one or more layers of material. The materials of the different layers may be identical or different.

[0036] Substrate 2 can be a "standard" (or "mono-material") substrate or a composite (or "multi-material") substrate.

[0037] Among standard substrates, "paper substrate" is a single-jet or multi-jet substrate whose primary raw material is paper. The paper can be enhanced with various components known to those skilled in the art to improve its physical, optical, and / or mechanical properties. These components are added during the pulp preparation stage or through coating techniques once the paper sheet has been formed.

[0038] Standard substrates also include the "polymer substrate," which is a single-layer or multi-layer substrate whose main raw material is polymer. The polymer can be enhanced with various components to improve its physical, optical, and / or mechanical properties. These components are added either in bulk or after the polymer sheet has been formed, such as a transparent, semi-transparent, or opaque adhesion primer.

[0039] Unlike standard substrates, "composite substrates" are multilayer and multi-material substrates. These substrates generally comprise one or more layers of paper and one or more layers of polymer.

[0040] In the example shown on the figure 1 , substrate 2 comprises a single layer of paper 11. The paper layer 11 has a first face 5 and a second face 6, opposite to the first face 5.

[0041] The first face 5 is surface-impregnated with a first impregnation composition 12. The second face 6 is surface-impregnated with a second impregnation composition 13. Both the first impregnation composition 12 and the second impregnation composition 13 comprise a polymer, such as, for example, an acrylic or acrylate polymer, a polyurethane, a polyvinyl alcohol (PVOH), or a polyvinylidene chloride (PVDC), which improves the surface cohesion of the paper layer 11. These polymers can be used alone or in mixtures, possibly with the addition of a crosslinking agent. These impregnation compositions 12 and 13 can reduce surface roughness and increase the gloss of the substrate.

[0042] Alternatively, substrate 2 can be mass-impregnated with an impregnation composition: that is, the impregnation composition penetrates the entire volume of substrate 2, and not just the surface.

[0043] The paper layer 11 may include plant fibers, such as cellulose fibers or fibers from annual plants, such as cotton fibers, to which may be added synthetic fibers, such as, for example, polyamide, polyester, (PVA), (PE) or (PP) fibers or multi-component fibers (formed by an assembly of several of these distinct components).

[0044] The paper layer 11 preferably has a weight between 50 and 90 grams per square meter.

[0045] The paper layer 11 can be single-jet or multi-jet.

[0046] The paper layer 11 may further include one or more additives intended to improve the physical, optical and / or mechanical properties of the paper. These additives may be added in bulk to the paper pulp before the formation of the paper layer, or applied to the faces of the paper layer after its formation.

[0047] In particular, the paper layer 11 may include a wet strength additive, fillers, a bulk sizing agent and / or pigments.

[0048] Alternatively, in the case of a composite substrate 2, substrate 2 can be obtained by layering several layers made of different materials. Substrate 2 comprises, for example, one or more polymer layers bonded to one or more paper layers.

[0049] Substrate 2 can be printed with a plurality of graphic designs. Each graphic design corresponds to a banknote.

[0050] Graphic designs may include identifying features, for example a printed design, such as a serial number associated with the banknote, indications of the banknote's face value and / or an indication of a currency.

[0051] In addition, banknote 1 may include security features fixed to or embedded in substrate 2 such as a security thread, a watermark, fluorescent ink, iridescent ink, optically variable ink, etc.

[0052] In the example shown on the figure 1 The first protective film 3 covers the entire first side 5 of the paper layer 11. Similarly, the second protective film 4 covers the entire second side 6 of the paper layer 11. Thus, protective films 3 and 4 protect the graphic designs and security features from abrasion and soiling. Protective films 3 and 4 also enhance the mechanical strength of the banknote 1.

[0053] Each protective film 3 and 4 includes a layer of polymer material 14 and an adhesive layer 15 enabling the protective film to be bonded to the substrate 2.

[0054] The polymer material layer 14 is, for example, made of polypropylene. The polymer material layer 14 has a thickness of between 1 and 100 µm, preferably between 5 and 50 µm, preferably between 5 and 30 µm.

[0055] The adhesive layer 15 may include a hot melt adhesive, such as an ethylene-vinyl acetate copolymer, or a crosslinkable adhesive, for example an adhesive containing a crosslinking agent by reaction with ambient air, such as a crosslinkable polyurethane, or a photocurable adhesive, the crosslinking of which is caused by exposure to UV radiation, or an aqueous-based adhesive or an adhesive based on a non-aqueous solvent.

[0056] Banknote 1 illustrated on the figure 1 further includes an optically variable security sign 7. The security sign 7 includes diffractive microstructures enabling the security sign 7 to change its appearance to the observer depending on the tilt or orientation of the security document 1 relative to the observer's eye and / or relative to a light source.

[0057] On the figure 2 , substrate 2 is identical to substrate 1 figure 1 .

[0058] In the example shown on the figure 2 , the first protective film 3 covers only part of the first side 5 of the paper layer 11.

[0059] Furthermore, the banknote does not include a second protective film 4 covering the second side 6 of the paper layer 11.

[0060] The protective film 3 protects the optically variable safety sign 7 from abrasion and soiling.

[0061] As in the example of the figure 1 , on the figure 2 , the protective film 3 comprises a layer of polymer material 14 and an adhesive layer 15 enabling the protective film to be bonded to the substrate 2.

[0062] THE figures 3 et 4 schematically represent two examples of microstructures 21 that are part of the safety sign 7. The microstructures 21 are patterns formed by raised and / or recessed reliefs obtained by deformation of the surface of the substrate 2. More precisely, the microstructures 21 have alternating raised and recessed areas.

[0063] In the first example illustrated on the figure 3 , microstructures 21 are binary microstructures, that is to say that the relief patterns only have two levels of depth, namely: a lower level and an upper level.

[0064] In the first example of the figure 3 The relief patterns have a crenellated profile. In other words, the microstructures 21 have facets 22 that are oriented parallel to a face of the substrate 2.

[0065] In the second example illustrated on the figure 4 , the 21 microstructures are multilevel microstructures, meaning that the relief patterns comprise more than two levels of depth. More specifically, in the example illustrated on the figure 4 The relief patterns present a plurality of levels.

[0066] In the second example of the figure 4 The relief patterns have a sawtooth profile. In other words, the microstructures 21 have inclined facets 23 which are oriented at a non-zero, non-right angle α with a face of the substrate 2.

[0067] Each raised motif has a depth h between 1 micrometer and 100 micrometers, preferably between 5 and 50 micrometers, for example between 10 and 35 micrometers.

[0068] The relief patterns are periodic and have a spatial period (or step) A of between 1 and 500 micrometers, preferably between 20 and 150 micrometers.

[0069] In addition, the microstructures 21 are coated with a layer with a step index 16.

[0070] The 16 index step coating may include an ink formed from a binder and reflective particles dispersed in the binder. The reflective particles may be metallic particles, such as silver or aluminum particles, for example.

[0071] The 16 index step layer may also include colored pigments. The colored pigments may be dispersed in the binder.

[0072] Alternatively, the microstructures 21 can be coated with an additional layer comprising colored pigments. The additional layer covers the step-index layer 16.

[0073] The step-index layer 16 has a thickness less than the dimensions of the raised patterns. More precisely, the step-index layer 16 has a thickness less than the depth h of the microstructures 21, for example, less than 0.25 times the depth of the microstructures. Furthermore, the thickness of the step-index layer 16 is less than the spatial period A of the microstructures 21. For this purpose, the ink can be deposited in an amount between 0.1 and 5 grams per square meter, for example, approximately 2.5 grams per square meter, which corresponds to an ink thickness of approximately 2.5 micrometers.

[0074] As illustrated on the figures 3 et 4 The raised patterns are embedded in the adhesive layer 15; that is, the adhesive layer 15 completely fills the recesses and conforms to the protrusions of the reliefs. The adhesive layer 15 preferably has a thickness greater than the depth h of the microstructures 21. For example, the thickness of the adhesive layer 15 can be approximately 15 µm.

[0075] The paper layer 11 has a first refractive index n1, the coating layer 12 has a second refractive index n2, the step-index layer 16 has a third refractive index n3, and the protective film 14 and the adhesive layer 15 have a fourth refractive index n4, such that: n1 ≈ n2 ≈ n4 > n3

[0076] For example, for a wavelength of 532 nanometers: n2 can be equal to 1.4, n3 can be between 0.9 and 1.2 for a 16 index step layer made of a composition including metallic particles, and n4 can be between 1.49 and 1.65.

[0077] Metallic particles, such as gold, silver, copper, and aluminum, generally have a refractive index of less than 1.2 (for example, aluminum particles have a refractive index of 0.93 for a wavelength of 532 nanometers).

[0078] Alternatively, the paper layer 11 has a first refractive index n1, the coating layer 12 has a second refractive index n2, the step-index layer 16 has a third refractive index n3, and the protective film 14 and the adhesive layer 15 have a fourth refractive index n4, such as: n1 ≈ n2 ≈ n4 < n3

[0079] For example, for a wavelength of 532 nanometers: n2 can be equal to 1.4, n3 can be greater than 1.5, for example between 1.7 and 2.4 for a 16 index step layer made of a composition including particles in a high refractive index material, and n4 can be between 1.49 and 1.65.

[0080] As illustrated on the figures 3 et 4 , when the safety sign 7 is illuminated by a light source, the incident radiation R1 emitted by the light source propagates successively through the protective film 14 and through the adhesive layer 15, then it is reflected by the step-index layer 16 which covers the microstructures

[0081] The reflected radiation R2 propagates successively through the adhesive layer 15 and the protective film 14.

[0082] In the case of binary microstructures such as those illustrated on the figure 3 , each pattern reflects light symmetrically with respect to an axis orthogonal to the face of the substrate 2.

[0083] In the case of multilevel microstructures such as those illustrated on the figure 4 , each pattern reflects light asymmetrically with respect to an axis orthogonal to the face of the substrate 2.

[0084] However, due to their dimensions (particularly their depth and / or period), the microstructures form a diffraction grating. The rays are reflected in several directions by the various raised patterns, so that the reflected rays create interference patterns that give the safety sign a different appearance depending on the angle or orientation from which it is viewed.

[0085] By playing with the period, depth, density and orientation of the microstructures, it is possible to generate reversible, predictable and reproducible diffractive optical effects.

[0086] There figure 5 schematically represents a first safety sign 7 in accordance with a first embodiment of the invention.

[0087] In this first embodiment, the safety sign 7 has several zones. Within each zone, the substrate exhibits microstructures. The microstructures of a first zone extend along a first longitudinal direction, and the microstructures of a second zone extend along a second longitudinal direction different from the first. In other words, the microstructures extend along longitudinal directions that differ from one zone to another.

[0088] In the example shown on the figure 5A , microstructures are multilevel microstructures.

[0089] In the example shown on the figure 5B , the microstructures are binary microstructures.

[0090] The first zone closest to the center of the safety sign 7 has microstructures having a longitudinal direction oriented with a first angle β1 (e.g. 30°) relative to a reference direction.

[0091] The second zone which immediately surrounds the first zone has microstructures having a longitudinal direction oriented with a second angle β2 (for example 20°) relative to the reference direction.

[0092] The third zone, which immediately surrounds the first zone, has microstructures with a longitudinal direction oriented with a third angle β3 (for example 10°) relative to the reference direction.

[0093] And so on so that the nth zone which immediately surrounds the n-1st zone has microstructures having a longitudinal direction oriented with an nth angle βn with respect to the reference direction, less than the angle βn-1.

[0094] Because of their orientations along different directions, the microstructures exhibit preferred diffraction angles along different directions.

[0095] In the embodiment illustrated on the figure 5 The different zones appear to have different levels of brightness, ranging from brightest to least bright when moving through successive zones from the center of the safety sign to the periphery of the safety sign.

[0096] However, when banknote 1 is rotated in the first direction (arrow I) within the substrate plane (around the Z-axis), the brightness levels vary, with the maximum brightness shifting from the center to the periphery. Conversely, when banknote 1 is rotated in the second direction (arrow II) within the substrate plane, opposite to the first direction, the brightness levels vary, with the maximum brightness shifting from the periphery to the center.

[0097] There figure 6 schematically represents a second safety sign 7 conforming to a second embodiment of the invention.

[0098] In this second embodiment, the safety sign 7 has several zones. In each zone, the substrate has microstructures.

[0099] The first zone Z1 of the safety sign 7 presents first microstructures having facets exhibiting a first angle of inclination α1 with respect to a direction orthogonal to the faces of the substrate.

[0100] The second zone Z2 of the safety sign 7 exhibits second microstructures with facets oriented symmetrically with respect to the facets of the first microstructures. In other words, the second microstructures have facets exhibiting a second inclination angle α2 with respect to the direction orthogonal to the substrate faces, such that α2 = - α1.

[0101] In the embodiment illustrated on the figure 6 The first zone and the second zone appear to have different levels of brightness to an observer examining the safety sign.

[0102] However, by tilting the banknote in a first direction (arrow III) around an axis parallel to the faces of the substrate (for example a longitudinal axis X of the banknote), the first area appears brighter than the second area to the stationary observer, while by tilting the banknote in a second direction (arrow IV), opposite to the first direction, the second area appears brighter than the first area.

[0103] There figure 7 schematically represents a third safety sign conforming to a third embodiment of the invention.

[0104] In this third embodiment, the safety sign 7 comprises circular microstructures 21. The microstructures 21 are arranged concentrically around each other.

[0105] Furthermore, the microstructures are oriented with angles that vary monotonically away from the center of the safety sign.

[0106] Furthermore, the microstructures 21 are spaced with a period, measured along a radial direction, which varies monotonically with distance from the center of the safety sign. In this case, the period decreases with distance from the center of the safety sign.

[0107] There figure 7A schematically shows the microstructures 21 of the safety sign 7 of the figure 7 .

[0108] There figure 7B shows the profile of the microstructures closest to the center of the safety sign 7, that is, the microstructures extending along a ring with the smallest diameter.

[0109] There figure 7C shows the profile of the microstructures furthest from the center of the safety sign 7, that is, the microstructures extending along a ring with the largest diameter.

[0110] In the example of figures 7A à 7C The inclination of the reliefs forming the microstructures increases at the same time as the diameter of the rings formed by the microstructures.

[0111] In the implementation of the figure 7 The safety sign appears brighter in an area extending along a diameter of the safety sign.

[0112] By tilting the banknote in a first direction (arrow V) around an axis parallel to the faces of the substrate (for example a transverse axis Y of the banknote), the brightest area moves by rotating around the center in a first direction of rotation, for the stationary observer.

[0113] By tilting the banknote in a second direction (arrow VI) around the Y axis, the brightest area moves by rotating around the center in a second direction of rotation, opposite to the first direction of rotation, for the stationary observer.

[0114] Furthermore, varying the period creates an optical relief effect, meaning the safety sign is perceived by the observer as three-dimensional. If the period is constant, the effect remains the same, however, the safety sign is not perceived by the observer as three-dimensional.

[0115] There figure 7D illustrates a variant of the safety sign of the figures 7A à 7C in which the microstructures are binary microstructures.

[0116] THE figures 8A And 8B schematically represent a fourth safety sign conforming to a fourth embodiment of the invention.

[0117] In this fourth embodiment, the safety sign 7 has several zones. In each zone, the substrate has microstructures.

[0118] In the example of the figure 8A , the first zone Z1 of the safety sign 7 presents initial microstructures having facets whose angles of inclination vary increasing from a central axis (X axis) away from the central axis.

[0119] Furthermore, the initial microstructures exhibit a period that varies monotonically with distance from the central axis. More precisely, the period of the initial microstructures decreases with increasing distance from the central axis.

[0120] The second zone Z2 of the safety sign 7 presents second microstructures having facets whose angles of inclination vary decreasingly from the central axis X away from the central axis.

[0121] Furthermore, the second microstructures exhibit a period that varies monotonically with distance from the central axis. More precisely, the period of the second microstructures decreases with increasing distance from the central axis.

[0122] Thus, the first zone Z1 appears darker near the X-axis and lighter farther from the X-axis, to an observer. Conversely, the second zone Z2 appears lighter near the X-axis and darker farther from the X-axis.

[0123] By tilting the banknote in a first direction around the X axis, the reflection in the first zone Z1 moves and travels through the first zone in a first direction, approaching the X axis, while the reflection in the second zone Z2 travels through the second zone in a second direction, opposite to the first direction, moving away from the X axis.

[0124] In the example of the figure 8B , the first zone Z1 of the safety sign 7 and the second zone Z2 of the safety sign each have microstructures with facets whose angles of inclination vary decreasingly from a central axis (X axis) away from the central axis.

[0125] Furthermore, the microstructures exhibit a period that varies monotonically with distance from the central axis. More precisely, the period of the microstructures decreases with distance from the central axis.

[0126] By tilting the banknote in the first direction around the X axis, the reflection in the first zone Z1 and the reflection in the second zone Z2 move by traversing the first zone and the second zone respectively, moving away from the X axis.

[0127] There figure 9 schematically represents a fifth safety sign 7 conforming to a fifth embodiment of the invention.

[0128] In this fifth embodiment, banknote 1 includes the security feature 7 and a printed design 8.

[0129] The security mark 7 comprises microstructures formed in a surface of the first face 5 of the substrate 2 (or front). The printed pattern 8 is printed on the second face 6 of the substrate 2 (or back), opposite the first face 5.

[0130] Alternatively, the printed pattern 8 can be printed on the first face 5 of the substrate 2 (or recto), between the substrate 2 and the index step layer 16.

[0131] Thus, when an observer O observes the safety sign 7 from a first angle, the safety sign 7 reflects the light back to the observer O, so that the safety sign 7 masks the presence of the printed pattern 8.

[0132] When the observer views the safety sign 7 from a second angle, different from the first angle, the reflective effect of the safety sign is less intense, so that the safety sign 7 allows the printed pattern 8 to show through the substrate 2 (if the pattern is printed on the reverse) or through the index-stepped layer 16 (if the pattern is printed on the front).

[0133] There figure 10 schematically represents a sixth safety sign conforming to a sixth embodiment of the invention.

[0134] In this sixth embodiment, the banknote 1 includes the security feature 7. The security feature 7 includes microstructures formed in a surface of the first face 5 of the substrate 2.

[0135] Furthermore, the security feature 7 forms a pseudo-watermark 9 when viewed in transmission. This is because, during the banknote manufacturing process, the pressing of the paper layer 11 creates differences in fiber density within the paper layer 11. These density differences, combined with the presence of the step-index layer 16, create a visual effect similar to that of a watermark.

[0136] Thus, when an observer O observes the security sign 7 from a first angle, the security sign 7 masks the light transmitted towards the observer O, so that the security sign 7 masks the presence of the pseudo-watermark 9.

[0137] When the observer views the security sign 7 from a second angle, different from the first angle, the intensity of the light transmitted through the security sign is greater, so that the security sign 7 reveals the pseudo-watermark 9 transparently through the substrate 2.

[0138] There figure 11 schematically represents a fifth safety sign 7 conforming to a seventh embodiment of the invention.

[0139] In this seventh embodiment, the security sign 7 has several zones.

[0140] In the example of the figure 11 Some Z1 zones of safety mark 7 exhibit microstructures with facets displaying a first surface roughness. Other Z2 zones of safety mark 7 do not display microstructures. Unlike the Z1 zones, the Z2 zones have not been subjected to pressure, so they exhibit a second surface roughness, different from the first roughness.

[0141] For example, the second roughness is greater than the first roughness.

[0142] Thus, the security sign 7 appears more matte in the second zones Z2 and more glossy in the first zones Z1.

[0143] There figure 12 schematically represents the steps of a process for manufacturing 100 banknotes, in accordance with an implementation method of the invention.

[0144] According to step 101, the substrate is manufactured. In the case of a substrate 2 formed from a single layer of paper 11, the substrate is manufactured in the form of a continuous paper strip 17 on a paper machine. The continuous paper strip 11 is impregnated on each of its faces with an impregnation composition 12, 13.

[0145] According to step 102, the continuous strip of substrate is cut into a plurality of 18 separate substrate sheets.

[0146] According to step 103, each individual substrate sheet 18 is printed on each of its faces with a plurality of graphic patterns. Each graphic pattern corresponds to a banknote.

[0147] According to step 104, 21 microstructures are formed in each individual substrate sheet.

[0148] For this purpose, a matrix 33 with raised and / or recessed microreliefs is pressed against a surface of the substrate sheet 18.

[0149] During this step, the substrate can be subjected to a pressure of between 10 and 100 Megapascals (MPa), preferably between 20 and 50 MPa.

[0150] Pressing can be done by embossing, debossing or stamping.

[0151] For example, the pressing is carried out by embossing at a temperature between 20 and 100 degrees Celsius, preferably above approximately 80 degrees, for example, approximately 90 degrees Celsius. The substrate sheet 18 is conveyed between a first cylinder 31 and a second cylinder 32. The first cylinder 31 carries the die 33 with the raised and / or recessed microreliefs. The second cylinder 32 is a counter-pressure cylinder. The first cylinder 31 and the second cylinder 32 are rotated. With each revolution of the first cylinder 31, the die 33 with the microreliefs is pressed against a surface of the substrate sheet 18. The pressing of the microreliefs against the surface of the substrate sheet 18 deforms the surface of the substrate sheet 18, creating microstructures 21.These microstructures 21 are made up of patterns in the form of raised and / or recessed reliefs which are complementary to the micropatterns of the matrix.

[0152] During this step, the pressing also has the effect of reducing the roughness of the surface of the substrate sheet 18 in which the microstructures 21 were formed.

[0153] For example, the surface of substrate sheet 18 may exhibit a Bendtsen roughness value greater than 300 mL per minute (measured according to ISO 8791-2:2013 Paper and board - Determination of roughness / smoothness (air flow methods) - Part 2: Bendtsen method) before the pressing stage.

[0154] Preferably, the surface of the substrate sheet 18 in which the microstructures 21 were formed has a Bendtsen roughness value of less than 100 mL per minute, preferably less than 60 mL per minute, after the pressing step.

[0155] There figure 13 schematically represents the substrate sheet after step 104. The substrate has microstructures 21 formed in a surface on one of its faces.

[0156] Alternatively, in this step 104, microstructures 21 can be formed simultaneously on each of two opposite faces of the substrate. For this purpose, two matrices, positioned on either side of the substrate and each exhibiting raised and / or recessed microreliefs, are pressed simultaneously against respective surfaces of the two opposite faces of the substrate.

[0157] According to step 105, the microstructures 21 are coated with a layer with a step index 16.

[0158] The step-index layer 16 can be deposited on the surface of the substrate sheet 18 as a composition comprising reflective particles, a binder, and a crosslinking agent. Crosslinking the composition hardens the step-index layer. Once hardened, the step-index layer consolidates the microstructures.

[0159] Reflective particles can be metallic particles, such as silver or aluminum particles, for example.

[0160] Due to the reduced surface roughness of the substrate resulting from the pressing step, the resulting step-index layer is of good quality; that is, it is smooth, thin, and uniform. In particular, the step-index layer is significantly thinner than the dimensions of the microstructures 21, allowing it to perfectly conform to their shape.

[0161] The 16 index jump coating preferably has a gloss index greater than 70 gloss units, measured at 60 degrees according to standard NF EN ISO 2813 November 2014 (Paints and varnishes - Determination of gloss index at 20°, 60° and 85°).

[0162] The index step layer 16 is deposited on the surface of the substrate sheet 18 in an amount of between 0.2 and 8 grams per square meter, preferably between 1.5 and 3 grams per square meter.

[0163] The 16 index step layer can be deposited by flexographic printing, screen printing, offset printing or digital printing, for example by inkjet printing.

[0164] The step-index layer 16 can only be deposited on microstructures 21. That is to say, the step-index layer 16 can be discontinuous and localized in register with the raised patterns as illustrated on the figure 14 .

[0165] The deposition of a discontinuous step-index layer 16 allows the protective film to adhere directly to the paper layer 11 in places where the paper layer 11 is not covered by the step-index layer 16. Thus, in the event of an attempt to peel off or tear off the protective film, the security mark 7 is destroyed.

[0166] For example, the 16 index step layer can be deposited by covering only certain areas of the surface, according to a repeating pattern or grid, such as a checkerboard, a grid, a succession of dots or lines, or a succession of repeating images.

[0167] Alternatively, the step-index layer 16 can be deposited both on and around the microstructures 21. That is, the step-index layer 16 is continuous and covers all the microstructures 21. It is arranged in a flat layer as illustrated in the figure 15 .

[0168] Alternatively, the step-index layer 16 can be continuous but not cover all the microstructures 21. In other words, the step-index layer 21 does not cover the entire surface area occupied by the microstructures 21. The step-index layer 21 can be continuous as illustrated in the figure 16 , or discontinuous.

[0169] Optionally, the process may include a step 106 in which an additional layer comprising colored pigments is deposited over the index-stepping layer 16.

[0170] According to step 107, each face of the substrate sheet 18 is covered with a protective film 3, 4 made of polymer material. The first face of the substrate sheet 18 is covered with a first protective film 3 and the second face of the substrate sheet 18, opposite the first face, is covered with a second protective film 4.

[0171] Preferably, the first protective film 3 is applied to the first face of the substrate sheet 18 so that the first protective film covers the entire first face of the substrate sheet 18. Similarly, the second protective film 4 is applied to the second face of the substrate sheet 18 so that the second protective film 4 covers the entire second face of the substrate sheet 18.

[0172] In the case where the first protective film 3 includes a layer of hot melt adhesive, the first protective film 3 is hot laminated onto the substrate sheet 18 at a temperature between 50 and 200°C, preferably between 80 and 150°C, so as to cause the adhesive layer to melt.

[0173] After hot lamination, the adhesive layer may exhibit a thermoplastic or elastomeric state.

[0174] The same process is used to apply the second protective film 4 to the second side of the substrate sheet 18.

[0175] During this step, the adhesive layer material flows and conforms to the shape of the embossed patterns, that is to say, it fills the hollows between the protrusions.

[0176] In this way, the microstructures 21 are embedded in the adhesive layer material. The microstructures 21 are thus protected against abrasion, soiling, and / or crushing. figures 17 et 18 schematically represent the protective film covering the microstructures 21. On the figure 17 The protective film 14 covers a surface of the substrate which is the total surface area of ​​one face of the substrate. On the figure 18 , the protective film 14 covers a surface of the substrate which is only a part of the total surface of the substrate face.

[0177] Step 107 leads to obtaining a multilayer (or laminate) assembly comprising the substrate sheet 18, the first film 3 and the second film 4.

[0178] According to step 108, each multilayer assembly comprising the substrate sheet 18, the first protective film 3 and the second protective film 4 is cut into a plurality of individual banknotes.

[0179] Each banknote obtained at the end of step 108 includes: a substrate 2 having a first face and a second face, opposite to the first face, a first protective film 3 covering the first face of the substrate, a second protective film 4 covering the second face of the substrate.

[0180] No printing steps are performed after step 107, which consists of applying protective films 3 and 4.

Claims

1. A method for manufacturing a security document (1), comprising steps of: a- forming microstructures (21) in a substrate (2) by pressing a die (33) having microreliefs against a surface of the substrate (2), the microstructures having alternating protrusions and recesses, b- coating the microstructures (21) with a step-index layer (16) so that the microstructures can reflect incident light radiation, and c- covering the surface of the substrate (2), including the microstructures (21) and the step-index layer, with a protective film (3) made of polymeric material, wherein the step-index layer (16) has a refractive index different from a refractive index of the protective film (3) made of polymeric material, the method being characterized in that the step-index layer (16) has a thickness smaller than a depth (h) of the microreliefs, and in that the protective film (3) comprises a layer made of polymeric material (14) and an adhesive layer (15), so that the adhesive layer (15) at least partially fills the recesses and conforms to the protrusions of the microstructures (21) coated with the step-index layer (16).

2. The method according to claim 1, wherein, during step a, the substrate (2) is subjected to a pressure comprised between 10 and 100 MPa, preferably between 20 and 50 MPa.

3. The method according to any of claims 1 and 2, wherein the pressing of step a is carried out by embossing at a temperature comprised between 23 and 100 degrees Celsius, preferably equal to about 90 degrees Celsius.

4. The method according to any of claims 1 to 3, wherein the microstructures (21) have a depth comprised between 1 micrometer and 100 micrometers, preferably between 5 and 50 micrometers, for example between 10 and 35 micrometers.

5. The method according to any of claims 1 to 4, wherein the substrate (2) comprises a first face (5) and a second face (6), opposite to the first face, and wherein the microstructures are formed in a surface of the first face (5) of the substrate (2), the method comprising an additional step of printing a printed pattern (8) on the second face (6) of the substrate (2) in register with the microstructures, or on the first face (5) of the substrate (2), between the substrate (2) and the step-index layer (16).

6. The method according to any of claims 1 to 5, wherein the step-index layer (16) has a gloss index greater than 70 gloss units, measured at 60 degrees according to standard NF EN ISO 2813.

7. The method according to any of claims 1 to 6, wherein the step-index layer (16) is deposited on the substrate (2) in an amount comprised between 0.2 and 8 grams per square meter, preferably between 1.5 and 3 grams per square meter.

8. The method according to any of claims 1 to 7, wherein, during step c, the protective film (3) made of polymeric material covers the entire first face (5) of the substrate (2).

9. The method according to any of claims 1 to 8, wherein after step c, the substrate (2) covered with the protective film(s) (3, 4) made of polymeric material has a weight comprised between 50 and 130 grams per square meter.

10. The method according to any of claims 1 to 9, wherein the substrate (2) comprises a paper layer (11), the paper layer (11) comprising vegetable fibers, such as cellulose fibers or fibers of annual plants such as cotton fibers, to which fibers made of synthetic material can be added, such as polyamide or polyester fibers, and, during step a, the microstructures (21) are formed in the paper layer (11) by deformation of the paper layer (11).

11. The method according to claim 10, wherein during step a, the pressing of the die (33) against the surface of the substrate (2) creates differences in the density of the fibers inside the paper layer (11), the differences in densities forming, possibly in combination with the step-index layer (16), a pseudo-watermark (9).

12. The method according to any of claims 10 and 11, wherein the paper layer (11) has a weight comprised between 50 and 90 grams per square meter.

13. The method according to any of claims 10 to 12, comprising a preliminary step, before step a, of impregnating the paper layer (11) with an aqueous dispersion comprising a polymer or a prepolymer, such as an acrylic or acrylate polymer or a polyurethane or a polyvinyl alcohol (PVOH) or a polyvinylidene chloride (PVDC).

14. A security document (1) obtained by one method in accordance with any of claims 1 to 13.

15. The security document (1) according to claim 14, the security document being a banknote.