EP4240595B1 - Method for producing a security document - Google Patents

Description

FIELD OF THE INVENTION

The invention relates to a method of manufacturing a security document, such as a banknote, and to a security document obtained by this method.

STATE OF THE ART

In order to protect security documents, such as banknotes, from forgery, these security documents are usually provided with security features that make the security document difficult to reproduce. There are different types of security features.

Among security features, Diffractive Optically Variable Image Devices (DOVIDs) are security features that include diffractive optical structures that provide the security feature with an appearance that changes in a reversible, predictable, and reproducible manner depending on the viewing or lighting conditions of the security feature.

For example, an observer examining the security feature sees movements or changes in colour of the security feature when rotating or tilting the security document. This optical effect cannot be reproduced by photocopying the security document. Thus, the optical effect produced by the security feature allows the observer to ensure quickly and easily that the security document is authentic.

DOVIDs include security holograms, which are the most widely used.

These security features are typically manufactured as a separate element, such as a strip, film, patch and/or thread. The separate element is then attached or fixed to the security document substrate during the manufacture of the security document, for example by a heat transfer process, such as hot stamping.

The security sign in the form of a separate element is usually made up of a superposition of several layers. The layers typically include an adhesive layer for fixing the element to the substrate, a metal layer for reflecting light, an embossed transparent layer with diffractive nanostructures or microstructures, a protective layer, a separation layer and a support layer.

Once the separate element is fixed on the security document substrate, the support layer can be removed, thanks to the presence of the separation layer.

When the surface of the substrate on which the security sign is fixed is illuminated, the light radiation propagates successively in the protective layer, then in the embossed transparent layer, then it is reflected by the underlying metal layer. The reflected light radiation propagates in the opposite direction in the embossed transparent layer then in the protective layer towards the eye of the observer.

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

A disadvantage of these security signs is that they generally have limited adhesion strength to the substrate. For example, physicochemical tests have shown that the tape, film, patch or wire incorporating the security sign can be peeled off after being brought into contact with hot water. The tape, film, patch or wire can then be reattached to another substrate.

In addition, security features in the form of separate elements must be capable of being deposited by means of an automatic machine enabling continuous deposition on a moving substrate web. In particular, security features such as DOVIDs are generally deposited parallel to the direction of travel of the substrate web. As a result, these Security signs necessarily have geometric shapes and spatial arrangements on the substrate that are limited.

Furthermore, even if it is possible to deposit several security features in the form of separate elements in several places on the same substrate, this requires passing the substrate successively through several automatic machines, each machine being dedicated to the deposit of a single security feature. Thus, in practice, the number of security features in the form of separate elements is limited, and they are deposited in predefined localized areas of the substrate.

Furthermore, high durability banknotes are known comprising a substrate, a first protective film covering a first face of the substrate and a second protective film covering a second face of the substrate.

The document FR 2 991 627 describes for example a method of manufacturing a security document comprising a paper substrate, a first protective film of polymer material extending on a first face of the substrate, and a second protective film of polymer material extending on a second face of the substrate. The method may comprise a step of printing the substrate with graphic patterns and then a laminating step consisting of applying the protective films to the faces of the substrate.

Protective films allow both to reinforce the mechanical resistance of the banknote and to protect the faces of the substrate that are printed from abrasion and dirt. These protective films thus allow to increase the life of the banknote, in particular in difficult circulation conditions (high humidity, high temperature, abrasion, dirt).

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.

However, in this case, attaching the security sign to the protective film by thermal transfer may deform the protective film. In addition, there is a risk that the security sign could be easily peeled off the protective film.

For the reasons set out above, it would be rather desirable to fix the separate element (tape, film, patch or wire) directly to the substrate, before the application of the protective film. In this way, the security sign would be arranged between the substrate and the protective film, in order to prevent the security sign from being easily peeled off. Furthermore, in this way, the security sign would also be protected from abrasion and soiling by the protective film, as well as from folding, tearing and mechanical stresses during the circulation of the security document.

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

GB 2 566 975 A describes the preamble of claim 1.

SUMMARY OF THE INVENTION

An aim of the invention is to provide a method for manufacturing a security document provided with a security sign including diffractive optical structures, which cannot be easily peeled off and which is resistant to abrasion and dirt, as well as to folding, tearing and mechanical stresses during circulation of the security document.

1. a - former des microstructures dans un substrat par pressage d'une matrice présentant des microreliefs contre une surface du substrat, les microstructures présentant en alternance des saillies et des creux,
2. b - revêtir les microstructures d'une couche à saut d'indice afin que les microstructures revêtues puissent renvoyer un rayonnement lumineux incident, la couche à saut d'indice présentant une épaisseur inférieure à une profondeur des microreliefs, et
3. c - recouvrir la surface du substrat, y compris les microstructures revêtues de la couche à saut d'indice, par un film de protection en matériau polymère, le film de protection comprenant une couche en matériau polymère et une couche d'adhésif, de sorte que la couche d'adhésif remplit au moins partiellement les creux et épouse les saillies des microstructures revêtues de la couche à saut d'indice,

dans lequel la couche à saut d'indice présente un indice de réfraction différent d'un indice de réfraction du matériau polymère du film de protection.This aim is achieved within the framework of the present invention by means of a method of manufacturing a security document, comprising the steps of:

1. a - forming microstructures in a substrate by pressing a matrix having microreliefs against a surface of the substrate, the microstructures having alternating projections and depressions,
2. 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
3. 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 an adhesive layer, such that the adhesive layer at least partially fills the recesses and fits the projections of the microstructures coated with the step-index layer,

wherein the step-index layer has a refractive index different from a refractive index of the polymer material of the protective film.

In the proposed method, the pressing performed in step a allows both to form the microstructures and to reduce the roughness of the substrate, which allows to obtain in step b a good quality step index layer (i.e. a smooth, thin and uniform step index layer).

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

As a result, the security sign cannot be separated from the substrate.

In addition, the microstructures and the step-index layer are protected by the protective film made of polymer material, so as to preserve the safety sign from abrasion and dirt. The safety sign is also protected from mechanical stress, giving it better durability in traffic, and more particularly in harsh environments.

In addition, if the protective film made of polymer material is peeled off, the security sign is altered or even destroyed.

Finally, since step b is performed after step a, pressing is not likely to damage the step-index layer, and the step-index layer is not likely to contaminate the matrix used to form the microstructures.

• lors de l'étape a, le substrat est soumis à une pression comprise entre 10 et 100 MPa, de préférence entre 20 et 50 MPa,
• le pressage de l'étape a est réalisé par embossage, gaufrage ou estampage,
• le pressage de l'étape a est réalisé par embossage à une température comprise entre 23 et 100 degrés Celsius, de préférence égale à environ 90 degrés Celsius,
• les microstructures présentent un profil symétrique ou un profil asymétrique par rapport à une direction normale à la surface du substrat,
• les microstructures présentent une profondeur comprise entre 1 micromètre et 100 micromètres, de préférence entre 5 et 50 micromètres, par exemple entre 10 et 35 micromètres,
• les microstructures présentent une période spatiale constante ou une période spatiale variable qui varie le long de la surface du substrat,
• les microstructures présentent une période spatiale comprise entre 1 et 500 micromètres, de préférence entre 20 et 150 micromètres,
• les microstructures sont propre à renvoyer le rayonnement lumineux incident selon un ou plusieurs angles privilégiés de diffraction de sorte que lorsque le document de sécurité est observé par un observateur selon l'un de ces angles privilégiés de diffraction, l'observateur perçoit un premier motif donné, et lorsque le document de sécurité est observé par l'observateur selon un autre de ces angles privilégiés de diffraction, l'observateur perçoit un deuxième motif, différent du premier motif,
• le substrat comprend une première face et une deuxième face, opposée à la première face, et dans lequel les microstructures sont formées dans une surface de la première face du substrat, le procédé comprenant une étape additionnelle d'imprimer un motif imprimé sur la deuxième face du substrat en registre avec les microstructures, ou sur la première face du substrat, entre le substrat et la couche à saut d'indice,
• la couche à saut d'indice présente un indice de réfraction inférieur ou supérieur à un indice de réfraction d'une couche du substrat dans laquelle sont formées les microstructures,
• la couche à saut d'indice présente un indice de réfraction compris entre 0,9 et 1,2 à une longueur d'onde de 532 nanomètres, ou un indice de réfraction compris entre 1,7 et 2,4 à une longueur d'onde de 532 nanomètres,
• la couche à saut d'indice présente un indice de brillance supérieur à 70 unités de brillances, mesuré à 60 degrés selon la norme NF EN ISO 2813,
• lors de l'étape b, la couche à saut d'indice est déposée sur le substrat par impression par flexographie, par impression par sérigraphie, par impression Offset ou par impression numérique, par exemple par impression jet d'encre,
• la surface du substrat présente une première zone dans laquelle sont présentes les microstructures et une deuxième zone dépourvue de microstructures, la couche à saut d'indice étant déposée en registre avec la première zone, ou recouvre à la fois la première zone et une partie de la deuxième zone, ou recouvre partiellement la première zone,
• la couche à saut d'indice est continue ou discontinue dans la première zone,
• la couche à saut d'indice comprend des particules réfléchissantes, et dans lequel les particules sont des particules métalliques, tels que des particules d'argent ou d'aluminium, ou des particules constituées d'un matériau à haut indice de réfraction,
• la couche à saut d'indice comprend des pigments colorés,
• le procédé comprend une étape supplémentaire de déposer une couche additionnelle comprenant des pigments colorés entre la couche à saut d'indice et le film de protection en matériau polymère,
• la couche à saut d'indice est déposée sur le substrat en une quantité comprise entre 0,2 et 8 grammes par mètre carré, de préférence entre 1,5 et 3 grammes par mètre carré,
• lors de l'étape c, le film de protection en matériau polymère recouvre la totalité d'une première face du substrat,
• lors de l'étape c, un autre film de protection en matériau polymère est appliqué sur le substrat de sorte que l'autre film de protection recouvre la totalité d'une deuxième face du substrat, opposée à la première face,
• après l'étape c, le substrat recouvert du ou des film(s) de protection en matériau polymère présente un grammage compris entre 50 et 130 grammes par mètre carré,
• le substrat comprend une couche de papier et, lors de l'étape a, les microstructures sont formées dans la couche de papier par déformation de la couche de papier,
• la couche de papier comprend des fibres végétales, telles que des fibres de cellulose ou des fibres de plantes annuelles, telles que des fibres de coton, auxquelles peuvent être ajoutées des fibres en matière synthétique, telles que des fibres en polyamide ou en polyester,
• lors l'étape a, le pressage de la matrice contre la surface du substrat crée des différences de densité des fibres à l'intérieur de la couche de papier, les différences de densités formant, éventuellement en combinaison avec la couche à saut d'indice, un pseudo-filigrane,
• la couche de papier présente un grammage compris entre 50 et 90 grammes par mètre carré,
• le procédé comprend une étape préalable, avant l'étape a, d'imprégner la couche de papier avec une dispersion aqueuse comprenant un polymère ou un prépolymère, tel qu'un polymère acrylique ou acrylate ou un polyuréthane ou un alcool polyvinylique (PVOH) ou un polychlorure de vinylidène (PVDC),
• la couche de papier est imprégnée en surface ou en masse d'une quantité de dispersion aqueuse comprise entre 4 et 15 grammes par mètre carré en poids de matière sèche.

The proposed method may further have the following particularities:

• in step a, the substrate is subjected to a pressure of between 10 and 100 MPa, preferably between 20 and 50 MPa,
• the pressing of step a is carried out by embossing, embossing or stamping,
• the pressing of step a is carried out by embossing at a temperature between 23 and 100 degrees Celsius, preferably equal to approximately 90 degrees Celsius,
• the microstructures have a symmetrical profile or an asymmetrical profile with respect to a direction normal to the substrate surface,
• 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,
• microstructures exhibit a constant spatial period or a variable spatial period that varies along the substrate surface,
• the microstructures have a spatial period of between 1 and 500 micrometers, preferably between 20 and 150 micrometers,
• the microstructures are capable of reflecting the incident light radiation according to one or more preferred diffraction angles so that when the security document is observed by an observer according to one of these preferred diffraction angles, the observer perceives a first given pattern, and when the security document is observed by the observer according to 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 the first face, and in which the microstructures are formed in a surface of the first face of the substrate, the method comprising an additional step of printing a printed pattern on the second face of the substrate in register 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 a refractive index of a layer of the substrate in which the microstructures are formed,
• The step-index layer has a refractive index of between 0.9 and 1.2 at a wavelength of 532 nanometers, or an index of refraction 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,
• in step b, the step-index layer is deposited on the substrate by flexographic printing, screen printing, offset printing or digital printing, for example by inkjet printing,
• the surface of the substrate has a first zone in which the microstructures are present and a second zone devoid of microstructures, the step-index layer being deposited in register with the first zone, or covers both the first zone and a 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, and wherein the particles are metallic particles, such as silver or aluminum particles, or particles made of a high refractive index material,
• the step-index layer includes colored pigments,
• the method comprises an additional step of depositing an additional layer comprising colored pigments between the step-index layer and the protective film made of polymer material,
• 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 film made of polymer material covers the entirety of a first face of the substrate,
• in step c, another protective film 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 weight of between 50 and 130 grams per square meter,
• the substrate comprises a paper layer and, in step a, the microstructures are formed in the paper layer by deformation of the paper layer,
• the paper layer comprises plant fibres, such as cellulose fibres or annual plant fibres, such as cotton fibres, to which synthetic fibres, such as polyamide or polyester fibres, may be added,
• in step a, pressing the matrix against the substrate surface 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 weight of between 50 and 90 grams per square meter,
• the method comprises 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 mass with a quantity of aqueous dispersion of between 4 and 15 grams per square meter by weight of dry matter.

The invention also relates to a security document obtained by a method as defined above.

The security document can be a banknote.

PRESENTATION OF THE DRAWINGS

• la figure 1 représente de manière schématique un billet de banque conforme à un premier mode de réalisation de l'invention,
• la figure 2 représente de manière schématique un billet de banque conforme à un deuxième mode de réalisation de l'invention,
• la figure 3 est une vue en coupe, représentant de manière schématique un premier exemple de microstructures faisant partie du signe de sécurité,
• la figure 4 est une vue en coupe, représentant de manière schématique un deuxième exemple microstructures faisant partie du signe de sécurité,
• la figure 5 représente de manière schématique un premier signe de sécurité conforme à un premier mode de réalisation de l'invention,
• les figures 5A et 5B illustrent de manière schématique deux exemples de microstructures pouvant faire partie du premier signe de sécurité de la figure 4,
• la figure 6 représente de manière schématique un deuxième signe de sécurité conforme à un deuxième mode de réalisation de l'invention,
• la figure 7 représente de manière schématique un troisième signe de sécurité conforme à un troisième mode de réalisation de l'invention,
• les figures 7A à 7D représentent de manière schématique des microstructures du troisième signe de sécurité de la figure 7,
• les figures 8A et 8B représentent de manière schématique un quatrième signe de sécurité conforme à un quatrième mode de réalisation de l'invention,
• les figures 9A et 9B représentent de manière schématique un sixième signe de sécurité conforme à un sixième mode de réalisation de l'invention,
• la figure 10 représente de manière schématique un cinquième signe de sécurité conforme à un septième mode de réalisation de l'invention,
• la figure 11 représente de manière schématique un cinquième signe de sécurité conforme à un huitième mode de réalisation de l'invention,
• la figure 12 représente de manière schématique des étapes d'un procédé de fabrication d'un billet de banque conforme à un mode de mise en oeuvre de l'invention,
• la figure 13 représente le substrat dans lequel ont été formées les microstructures,
• les figures 14 à 16 représentent le substrat après l'étape de dépôt d'une couche à saut d'indice,
• les figures 17 et 18 représentent le substrat après l'étape d'application d'un film de protection.

Other features and advantages will emerge from the following description, which is purely illustrative and not limiting and must be read in conjunction with the attached drawings, among which:

• there figure 1 schematically represents a banknote conforming to a first embodiment of the invention,
• there figure 2 schematically represents a banknote conforming to a second embodiment of the invention,
• there figure 3 is a sectional view, schematically representing a first example of microstructures forming part of the security sign,
• there figure 4 is a sectional view, schematically representing a second example of microstructures forming part of the security sign,
• there figure 5 schematically represents a first security sign in accordance with a first embodiment of the invention,
• THE Figures 5A and 5B schematically illustrate two examples of microstructures that may be part of the first security feature of the figure 4 ,
• there figure 6 schematically represents a second security sign in accordance with a second embodiment of the invention,
• there figure 7 schematically represents a third security sign in accordance with a third embodiment of the invention,
• THE Figures 7A to 7D schematically represent microstructures of the third security feature of the figure 7 ,
• THE Figures 8A And 8B schematically represent a fourth security sign in accordance with a fourth embodiment of the invention,
• THE Figures 9A and 9B schematically represent a sixth security sign in accordance with a sixth embodiment of the invention,
• there figure 10 schematically represents a fifth security sign in accordance with a seventh embodiment of the invention,
• there figure 11 schematically represents a fifth security sign in accordance with an eighth embodiment of the invention,
• there figure 12 schematically represents steps of a method of manufacturing a banknote in accordance with an embodiment of the invention,
• there figure 13 represents the substrate in which the microstructures were formed,
• THE Figures 14 to 16 represent the substrate after the step of depositing a step-index layer,
• THE Figures 17 and 18 represent the substrate after the step of applying a protective film.

DETAILED DESCRIPTION OF AN EMBODIMENT

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

The substrate 2 may consist of one or more layers of materials. The materials of the different layers may be the same or different.

Substrate 2 may be a “standard” (or “single-material”) substrate or a composite (or “multi-material”) substrate.

Among the standard substrates, the "paper substrate" is a single-ply or multi-ply substrate, comprising only paper as its main raw material. The paper may be added with various components known to those skilled in the art to improve its physical, optical and/or mechanical properties. These components are added at the stage of preparing the paper pulp or by coating techniques once the sheet of paper has been formed.

Standard substrates also include "polymer substrate", which is a single-layer or multi-layer substrate, comprising only polymer as its main raw material. The polymer may be adjuvanted with various components to improve its physical, optical and/or mechanical properties. These components are added in bulk or after formation of the polymer sheet, as a transparent, semi-transparent or opaque adhesion primer.

Unlike standard substrates, “composite substrates” are multi-layer, multi-material substrates. These substrates typically include one or more layers of paper and one or more layers of polymer.

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

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. The first impregnation composition 12 and the second impregnation composition 13 comprise a polymer, such as for example an acrylic or acrylate polymer or a polyurethane or a polyvinyl alcohol (PVOH) or a polyvinylidene chloride (PVDC), making it possible to improve the surface cohesion of the paper layer 11. These polymers can be used alone or in a mixture, optionally with the addition of a crosslinking agent. These impregnation compositions 12 and 13 can make it possible to reduce the surface roughness and increase the gloss of the substrate.

Alternatively, the substrate 2 may be mass impregnated with an impregnation composition: that is to say, the impregnation composition penetrates into the entire volume of the substrate 2, and not only on the surface.

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

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

Paper layer 11 can be single-ply or multi-ply.

The paper layer 11 may further comprise one or more additive(s) 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 be applied to the faces of the paper layer, after the formation of the paper layer.

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

Alternatively, in the case of a composite substrate 2, the substrate 2 may be obtained by superimposing several layers formed in different materials. The substrate 2 comprises for example one or more polymer layer(s) associated with one or more paper layer(s).

The substrate 2 can be printed with a plurality of graphic patterns. Each graphic pattern corresponds to a banknote.

The graphic designs may include identifying features, for example a printed design, such as a serial number associated with the banknote, indications of the face value of the banknote and/or an indication of a currency.

In addition, the banknote 1 may include security features fixed on or integrated into the substrate 2 such as a security thread, a watermark, a fluorescent ink, an iridescent ink, an optically variable ink, etc.

In the example illustrated on the figure 1 , the first protective film 3 covers the entire first face 5 of the paper layer 11. Similarly, the second protective film 4 covers the entire second face 6 of the paper layer 11. Thus, the protective films 3 and 4 protect the graphic patterns and the security features from abrasion and dirt. The protective films 3 and 4 also enhance the mechanical strength of the banknote 1.

Each protective film 3 and 4 comprises a layer of polymer material 14 and a layer of adhesive 15 making it possible to secure the protective film to the substrate 2.

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.

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

The banknote 1 illustrated on the figure 1 further comprises an optically variable security sign 7. The security sign 7 comprises diffractive microstructures allowing the security sign 7 to change 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.

On the figure 2 , substrate 2 is identical to the substrate of the figure 1 .

In the example illustrated on the figure 2 , the first protective film 3 covers only part of the first face 5 of the paper layer 11.

Furthermore, the banknote does not include a second protective film 4 covering the second face 6 of the paper layer 11.

The protective film 3 protects the optically variable safety sign 7 from abrasion and dirt.

As in the example of the figure 1 , on the figure 2 , the protective film 3 comprises a layer of polymer material 14 and a layer of adhesive 15 making it possible to secure the protective film to the substrate 2.

THE Figures 3 and 4 schematically represent two examples of microstructures 21 forming part of the security sign 7. The microstructures 21 are patterns formed by protruding and/or recessed reliefs having been obtained by deformation of the surface of the substrate 2. More precisely, the microstructures 21 have alternating protrusions and recesses.

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

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

In the second example illustrated on the figure 4 , the microstructures 21 are multilevel microstructures, that is to say that the relief patterns comprise a number of depth levels greater than 2. More precisely, in the example illustrated in the figure 4 , the relief patterns have a plurality of levels.

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 to form a non-zero and non-right angle α with a face of the substrate 2.

Each relief pattern has a depth h of between 1 micrometer and 100 micrometers, preferably between 5 and 50 micrometers, for example between 10 and 35 micrometers.

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

In addition, the microstructures 21 are coated with a 16-step index layer.

The step index layer 16 may comprise an ink formed from a binder and reflective particles dispersed in the binder. reflective particles can be metallic particles, such as silver or aluminum particles for example.

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

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

The step-index layer 16 has a thickness less than the dimensions of the relief patterns. More specifically, 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. In addition, 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 of between 0.1 and 5 grams per square meter, for example approximately 2.5 grams per square meter, or approximately 2.5 micrometers of ink thickness.

As illustrated in the Figures 3 and 4 , the relief patterns are embedded in the adhesive layer 15, that is to say that the adhesive layer 15 completely fills the recesses and matches the projections 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 may be equal to approximately 15 µm.

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: $$\mathrm{n1}\approx \mathrm{n2}\approx \mathrm{n4}\mathrm{>}\mathrm{n3}$$

• n2 peut être égal à 1,4,
• n3 peut être compris entre 0,9 et 1,2 pour une couche à saut d'indice 16 formée d'une composition comprenant des particules métalliques, et
• n4 peut être compris entre 1,49 et 1,65.

For example, for a wavelength of 532 nanometers:

• n2 can be equal to 1.4,
• n3 may be between 0.9 and 1.2 for a 16-step index layer formed from a composition comprising metal particles, and
• n4 can be between 1.49 and 1.65.

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

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 that: $$\mathrm{n1}\approx \mathrm{n2}\approx \mathrm{n4}\mathrm{<}\mathrm{n3}$$

• n2 peut être égal à 1,4,
• n3 peut être supérieur à 1,5, par exemple compris entre 1,7 et 2,4 pour une couche à saut d'indice 16 formée d'une composition comprenant des particules en un matériau à haut indice de réfraction, et
• n4 peut être compris entre 1,49 et 1,65.

For example, for a wavelength of 532 nanometers:

• n2 can be equal to 1.4,
• n3 may be greater than 1.5, for example between 1.7 and 2.4 for a layer with an index step 16 formed from a composition comprising particles of a material with a high refractive index, and
• n4 can be between 1.49 and 1.65.

As illustrated in the Figures 3 and 4 , when the security 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 returned by the step-index layer 16 which covers the microstructures

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

In the case of binary microstructures such as those illustrated in the figure 3 , each pattern reflects light symmetrically about an axis orthogonal to the face of substrate 2.

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

However, due to their dimensions (in particular their depth and/or their period), the microstructures form a diffraction grating. The rays are returned in several directions by the different patterns in relief so that the reflected rays create interferences between them which give the security sign a different appearance depending on the angle or orientation from which it is observed.

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

There figure 5 schematically represents a first security sign 7 in accordance with a first embodiment of the invention.

In this first embodiment, the security sign 7 has several zones. In each zone, the substrate has microstructures. The microstructures of a first zone extend in a first longitudinal direction and the microstructures of a second zone extend in a second longitudinal direction different from the first longitudinal direction. In other words, the microstructures extend in longitudinal directions that differ from one zone to another.

In the example illustrated on the Figure 5A , microstructures are multilevel microstructures.

In the example illustrated on the Figure 5B , the microstructures are the binary microstructures.

The first, central-most area of the security sign 7 has microstructures having a longitudinal direction oriented with a first angle β1 (e.g. 30°) relative to a reference direction.

The second zone which immediately surrounds the first zone has microstructures having a longitudinal direction oriented with a second angle β2 (e.g. 20°) relative to the reference direction.

The third zone which immediately surrounds the first zone has microstructures having a longitudinal direction oriented with a third angle β3 (e.g. 10°) relative to the reference direction.

And so on so that the n-th zone immediately surrounding the n-1th zone has microstructures having a longitudinal direction oriented with an n-th angle βn relative to the reference direction, less than the angle βn-1.

Due to their orientations in different directions, the microstructures present preferred diffraction angles in different directions.

In the embodiment illustrated in the figure 5 , the different areas appear to have different levels of gloss, ranging from the most gloss to the least gloss when traversing the successive areas from the centre of the security feature towards the periphery of the security feature.

However, by rotating banknote 1 in a first direction (arrow I) in the plane of the substrate (around the Z axis), the gloss levels vary and the maximum gloss level moves from the center to the periphery. Conversely, by rotating banknote 1 in a second direction (arrow II) in the plane of the substrate, opposite to the first direction, the gloss levels vary and the maximum gloss level moves from the periphery to the center.

There figure 6 schematically represents a second security sign 7 in accordance with a second embodiment of the invention.

In this second embodiment, the security sign 7 has several zones. In each zone, the substrate has microstructures.

The first zone Z1 of the security sign 7 has first microstructures having facets having a first inclination angle α1 relative to a direction orthogonal to the faces of the substrate.

The second zone Z2 of the security sign 7 has second microstructures having facets oriented symmetrically with respect to the facets of the first microstructures. In other words, the second microstructures have facets having a second angle of inclination α2 relative to the direction orthogonal to the faces of the substrate, such that α2 = - α1.

In the embodiment illustrated in the figure 6 , the first zone and the second zone appear to have different brightness levels, to an observer examining the security feature.

However, by tilting the banknote in a first direction (arrow III) around an axis parallel to the faces of the substrate (e.g. a longitudinal axis X of the banknote), the first area appears brighter than the second area for 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.

There figure 7 schematically represents a third security sign in accordance with a third embodiment of the invention.

In this third embodiment, the security sign 7 comprises microstructures 21 of circular shape. The microstructures 21 are arranged around each other in a concentric manner.

Additionally, the microstructures are oriented with angles that vary monotonically away from the center of the security sign.

Furthermore, the microstructures 21 are spaced with a period, measured in a radial direction, which varies monotonically away from the center of the security sign. In this case, the period decreases as one moves away from the center of the security sign.

There Figure 7A schematically shows the microstructures 21 of the safety sign 7 of the figure 7 .

There Figure 7B shows the profile of the microstructures closest to the centre of the safety feature 7, i.e. the microstructures extending in a ring with the smallest diameter.

There Figure 7C shows the profile of the microstructures furthest from the centre of the safety feature 7, i.e. the microstructures extending in a ring with the largest diameter.

In the example of the Figures 7A to 7C , the inclination of the reliefs forming the microstructures increases at the same time as the diameter of the rings formed by the microstructures.

In the embodiment of the figure 7 , the security sign appears brighter in an area extending along a diameter of the security sign.

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.

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.

In addition, the variation of the period provides an optical relief effect, i.e. the security sign is perceived by the observer as being three-dimensional. If the period is constant, the described effect remains the same, however the security sign is not perceived by the observer as being three-dimensional.

There Figure 7D illustrates a variation of the safety sign of Figures 7A to 7C , in which the microstructures are binary microstructures.

THE Figures 8A And 8B schematically represent a fourth security sign in accordance with a fourth embodiment of the invention.

In this fourth embodiment, the security sign 7 has several zones. In each zone, the substrate has microstructures.

In the example of the Figure 8A , the first zone Z1 of the security sign 7 has first microstructures having facets whose inclination angles vary increasingly from a central axis (X axis) away from the central axis.

Moreover, the early microstructures exhibit a period that varies monotonically away from the central axis. More precisely, the period of the early microstructures decreases with distance from the central axis.

The second zone Z2 of the security sign 7 has second microstructures having facets whose inclination angles vary in a decreasing manner from the central axis X away from the central axis.

Moreover, the second microstructures exhibit a period that varies monotonically away from the central axis. More precisely, the period of the second microstructures decreases with distance from the central axis.

Thus, the first zone Z1 appears darker near the X-axis while it appears lighter far from the X-axis, for an observer. Conversely, the second zone Z2 appears lighter near the X-axis while it appears darker far from the X-axis.

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.

In the example of the Figure 8B , the first zone Z1 of the security sign 7 and the second zone Z2 of the security sign each have microstructures having facets whose inclination angles vary in a decreasing manner from a central axis (X axis) away from the central axis.

Moreover, the microstructures exhibit a period that varies monotonically away from the central axis. More precisely, the period of the microstructures decreases with distance from the central axis.

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 through the first zone and the second zone respectively, moving away from the X axis.

There figure 9 schematically represents a fifth security sign 7 in accordance with a fifth embodiment of the invention.

In this fifth embodiment, the banknote 1 comprises the security feature 7 and a printed pattern 8.

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

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

Thus, when an observer O observes the security sign 7 from a first angle, the security sign 7 reflects light towards the observer O, so that the security sign 7 masks the presence of the printed pattern 8.

When the observer observes the security sign 7 from a second angle, different from the first angle, the reflective effect of the security sign is less intense, so that the security sign 7 allows the printed pattern 8 to appear transparently through the substrate 2 (if the pattern is printed on the back) or through the step-index layer 16 (if the pattern is printed on the front).

There figure 10 schematically represents a sixth security sign in accordance with a sixth embodiment of the invention.

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

In addition, the security feature 7 forms a pseudo-watermark 9 when viewed in transmission. Indeed, during the manufacture of the banknote, the step of pressing the paper layer 11 creates differences in the density of the fibers within the paper layer 11. These differences in density, associated with the presence of the step-index layer 16, create a visual effect similar to that of a watermark.

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, such that the security sign 7 masks the presence of the pseudo-watermark 9.

When the observer observes 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.

There figure 11 schematically represents a fifth security sign 7 in accordance with a seventh embodiment of the invention.

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

In the example of the figure 11 , some areas Z1 of the security sign 7 have microstructures having facets having a first surface roughness. Other areas Z2 of the security sign 7 do not have microstructures. Unlike the areas Z1, the areas Z2 have not been subjected to pressure, so that they have a second surface roughness, different from the first roughness.

For example, the second roughness is greater than the first roughness.

Thus, the security sign 7 appears more matte in the second zones Z2 and more shiny in the first zones Z1.

There figure 12 schematically represents steps of a method of manufacturing 100 banknotes, in accordance with an embodiment of the invention.

According to a 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 strip of paper 17 on a paper machine. The continuous strip of paper 11 is impregnated on each of its faces with an impregnation composition 12, 13.

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

According to a 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.

According to a step 104, microstructures 21 are formed in each individual substrate sheet.

For this purpose, a matrix 33 having protruding and/or recessed microreliefs is pressed against a surface of the substrate sheet 18.

During this step, the substrate may be subjected to a pressure between 10 and 100 Megapascals (MPa), preferably between 20 and 50 MPa.

Pressing can be done by embossing, debossing or stamping.

For example, the pressing is carried out by embossing at a temperature between 20 and 100 degrees Celsius, preferably at a temperature greater than about 80 degrees, for example equal to about 90 degrees Celsius. The substrate sheet 18 is driven in scrolling between a first cylinder 31 and a second cylinder 32. The first cylinder 31 carries the matrix 33 having the protruding and/or hollow microreliefs. The second cylinder 32 is a counter-pressure cylinder. The first cylinder 31 and the second cylinder 32 are driven in rotation. At each revolution of the first cylinder 31, the matrix 33 having 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 has the effect of deforming the surface of the substrate sheet 18 to create microstructures 21. These microstructures 21 consist of patterns in the form of protruding and/or recessed reliefs which are complementary to the micropatterns of the matrix.

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 have been formed.

For example, the surface of the substrate sheet 18 may have 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) prior to the pressing step.

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

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.

Alternatively, during this step 104, microstructures 21 may be formed simultaneously on each of two opposite faces of the substrate. For this purpose, two matrices arranged on either side of the substrate and each having protruding and/or recessed microreliefs are pressed simultaneously against respective surfaces of the two opposite faces of the substrate.

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

The step index layer 16 may 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 solidifies the microstructures.

The reflective particles can be metallic particles, such as silver or aluminum particles for example.

Due to the reduced roughness of the substrate surface due to the pressing step, the obtained step-index layer is of good quality, that is, the step-index layer is smooth, thin and uniform. In particular, the step-index layer has a thickness much smaller than the dimensions of the microstructures 21, so that it completely matches the shape of the microstructures 21.

The 16-step index coat 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 the gloss index at 20°, 60° and 85°).

The step index 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.

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

The step index layer 16 may be deposited only on the microstructures 21. That is, the step index layer 16 may be discontinuous and be located in register with the relief patterns as illustrated in FIG. figure 14 .

The deposition of a discontinuous step-index layer 16 makes it possible to obtain adhesion of the protective film directly with the paper layer 11 at the locations 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 sign 7 is destroyed.

For example, the 16-step index layer may be deposited by covering only certain areas of the surface, in a repeating pattern or screen, such as a checkerboard, a grid, a succession of dots or lines, or a succession of repeating images.

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

Alternatively, the step index layer 16 may be continuous but not cover all of the microstructures 21. In other words, the step index layer 21 does not cover the entire surface occupied by the microstructures 21. The step index layer 21 may be continuous as illustrated in FIG. figure 16 , or discontinuous.

Optionally, the method may comprise a step 106 in which an additional layer comprising colored pigments is deposited over the index step layer 16.

According to a 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.

Preferably, the first protective film 3 is applied to the first face of the substrate sheet 18 such 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 such that the second protective film 4 covers the entire second face of the substrate sheet 18.

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

After hot lamination, the adhesive layer can be in a thermoplastic or elastomeric state.

The same method is used to apply the second protective film 4 to the second face of the substrate sheet 18.

During this step, the material of the adhesive layer flows and takes the shape of the relief patterns, that is to say, it fills the hollows between the projections.

In this way, the microstructures 21 are embedded in the material of the adhesive layer. The microstructures 21 are thus protected against abrasion, dirt and/or crushing. The Figures 17 and 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 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 face of the substrate.

Step 107 results in obtaining a multilayer (or laminate) assembly comprising the substrate sheet 18, the first film 3 and the second film 4.

According to a 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.

• un substrat 2 présentant une première face et une deuxième face, opposée à la première face,
• un premier film de protection 3 recouvrant la première face du substrat,
• un deuxième film de protection 4 recouvrant la deuxième face du substrat.

Each banknote obtained at the end of step 108 includes:

• a substrate 2 having a first face and a second face, opposite 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.

No printing step is performed after step 107 of applying protective films 3 and 4.

Claims (15)

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.

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