FR3012367A1 - SECURE DOCUMENT AND PIGMENT. - Google Patents

Abstract

The present invention relates to an article having a surface coated with a plurality of magnetically steerable particles (1) each having first (14) and second (15) regions, preferably opposite, with distinct optical properties, at least a portion of said particles. (1) being, in an area of greater than or equal to 1 mm of the article, substantially all oriented with their first region (14) facing said surface.

Description

The present invention relates to the field of magnetically orientable particles and that of their use within secure documents. It has already been proposed to use magnetic pigments in secure documents so as to produce optical effects easily recognizable by the man in the street and thus able to contribute to the authentication of the document. European Patent Application EP 1 819 525 A1 and International Application WO 2008/046702 A1 disclose security elements comprising magnetic platelet pigments fixed in a non-random orientation. EP 1819 525 A1 provides the possibility for the pigment to be luminescent.

Patent Application EP 1 950 256 A1 describes a platelet pigment. WO 01/60924 A2 discloses an interference pigment comprising a luminescent material. The application WO 03/000801 A2 describes a multilayer magnetic pigment whose faces may have distinct colors, so that a mixture of the particles of the pigment produces, after application, a new color corresponding to the combination of the colors of the faces. There is a need to further improve secure documents, security features for such documents and magnetically-orientable particles that can be used for this purpose. The invention responds to this need by means of an article having a surface coated with a set of magnetically steerable particles each having first and second regions, preferably opposite, with distinct optical properties, at least a part of said particles being, in an area of area greater than or equal to 1 mm 2 of the article, substantially all oriented with their first region facing said surface. The article may comprise a substrate of a plastic material, in particular a thermoplastic material or a fibrous substrate, in particular a paper-making material. The article may be a security element, such as a planchette, a flat fibrette, or a continuous element to be integrated into a substrate, such as a security thread, or to report on a substrate, such as a film of 30 laminated or glued safety. The article may be at least partially opaque, especially completely opaque, and / or be at least partially transparent and / or at least partially translucent.

Owing to the fact that the particles have in the invention two optical regions with distinct properties, preferably opposite each other, which are directed in a predefined manner towards the article, it is possible to benefit from new optical effects, making it possible to produce appearance changes according to the region observed by the observer. The invention also relates to a secure document, comprising an article, in particular a substrate or a security element, according to the invention. The invention further relates to a method for orienting magnetically orientable particles having first and second regions, preferably opposite, with distinct optical properties, in which: the particles are deposited on a surface, the particles thus deposited are oriented under the effect of a magnetic field and fixed with a predefined orientation of the first and second regions relative to said surface.

The invention further relates to a method for orienting magnetically orientable particles having a heterogeneous density and having first and second regions, preferably opposite, with distinct optical properties, in which: the particles are deposited on a surface; the particles thus deposited are mainly oriented in the same way relative to said surface under the effect of their heterogeneous density, and preferentially have their densest face facing said surface. The invention further relates to a magnetically steerable particle, having first and second regions, preferably opposite, with distinct optical properties, the regions having fluorescence or phosphorescence colors, under UV or IR light, or printing colors. distinct. The invention further relates to a magnetically steerable particle, having first and second regions, preferably opposite, with distinct optical properties, being formed by coextrusion or lamination of at least two materials defining said regions, preferably with densities for said materials, more preferably with a density difference of at least 0.2 g / cm 3 between said materials.

According to the invention, the particles are, in an area of greater than or equal to 1 mm2 of the article, substantially all oriented with their first region facing the surface of the article they are coated. By "substantially all", it is necessary to understand more than 50% of the particles present in said zone, better more than 60%, even better 80%. By "first region facing said surface", it should be understood that at least a portion of the first region is facing said surface.

Said surface of the article may be a face of a substrate, in particular a sheet substrate. Preferably, the article comprises particles oriented parallel to each other. This gives the observation a parallax effect, since when the direction of observation is parallel to the direction of orientation of the particles, and they are platelet, the underlying surface of the The article is visible between the particles, while for an observation direction substantially perpendicular to the direction of orientation of the particles, the underlying surface is masked. At least a portion of the particles may have been deposited on the surface of the article by printing, coating or coating.

The article may include particles whose orientation relative to the article changes gradually as one goes through the article. Preferably, the article has at least two zones in which the particles have respective different orientations, in particular a first zone where the particles have their first region facing said receiving surface and a second zone, preferably of greater extent or equal to 1 mm2, where the particles have their second region facing said surface. This can benefit from the optical effects provided by both regions. Preferably, the article has at least two areas where the particles have different respective inclinations with their first region facing said surface. Preferably, the article has at least two zones in which the orientable particles have respective different inclinations, each of these zones being greater than or equal to 1 mm 2.

Preferably, the article has at least three zones in which the particles have different respective inclinations, each of these zones preferably being greater than or equal to 1 trn2. Preferably, the article has an area where the particles have an orientation between 0 ° and 90 ° relative to the normal to the article, better between 30 ° and 60 ° relative to the notinal, and another area where the particles have an orientation between -0 ° and -90 ° relative to normal, better -30 ° and -60 ° relative to normal. Preferably, the article has another zone in which the particles have an inclination of between -90 ° and + 90 ° relative to the normal, better still between -30 ° and + 30 ° relative to the normal, and even better being inclined perpendicularly to Article. This third zone can be located between the first two, when the article is observed from above. The first, second and / or third zones may define patterns, in particular alphanumeric, and / or that are otherwise found on the article, especially when it is a security element or a secure document. Preferably, the difference in angular inclination between the inclinations of the particles within each zone is less than or equal to 30 °, preferably less than 10 °. This difference may be substantially zero when the particles are parallel.

The article may comprise two adjacent zones in which the particles have substantially the same inclination, the inclination difference between said two adjacent zones being at least 10 °, preferably at least 20 °. Preferably, the particles are arranged at least partially on a colored and / or luminescent zone of the article, in particular of absorption or luminescence color different from those of the first and second regions, or of absorption or luminescence color. identical to that of one of said regions. This creates new optical effects. The colored area may be a solid color or, alternatively, form a pattern. The particles may be disposed at least partially on an at least partially transparent or at least partially translucent area of the article. The non-opacity of this zone may result from a demetallization or the presence of a window in the article.

In the case where the article comprises a fibrous substrate, it is preferably a paper, and may comprise natural and / or synthetic fibers, especially cellulose fibers. The substrate may comprise a single paper jet or be multijet. The particles may be deposited on one side of this substrate, which may be a sheet substrate. The thickness of the article, including the particle coating according to the invention, is preferably between 10 and 900 better between 10 and 300 μm, more preferably between 10 and 150 μm. The substrate, when fibrous, may comprise a watermark, which may be at least partially superimposed or not with the particles. Security item and secure document The article according to the invention may form all or part of a security element intended to secure a document, or of the document itself. The security element may be in the form of a security thread intended to be introduced into a document, for example into a window (s), a foil or a patch. The security element may extend from one edge to the other of the document. The article of the invention may further be a particulate security element, such as a planchette or a small plane fiber. The security element and / or the secure document may comprise the particles according to the invention as sole security means, or preferably in combination with other security means. Among the other means of securing, some are detectable in daylight or in artificial light, without the use of a particular apparatus. These additional securing means comprise, for example, colored fibers or boards, fully or partially printed or metallized wires. These security means are said to be first level. Other types of securing means are detectable only by a relatively simple apparatus, such as an ultraviolet (UV) or infrared (IR) emitting lamp. These securing means comprise, for example, fibers, boards, strips, wires or particles. These securing means may be visible to the naked eye or not, being for example luminescent under lighting of a Wood lamp emitting at a wavelength of 365 nm. These security means are called second level. Other types of securing means require for their detection a more sophisticated detection device. These securing means are for example capable of generating a specific signal when they are subjected, simultaneously or not, to one or more external excitation sources. The automatic detection of the signal makes it possible to authenticate, if necessary, the document. These securing means comprise, for example, tracers in the form of active materials, particles or fibers capable of generating a specific signal when these tracers are subjected to optronic, electrical, magnetic or electromagnetic excitation. These security means are said to be third level. The secure document comprising or constituted by an article according to the invention may be a means of identifying a person, in particular a passport, an identity card, a driving license, or a means of payment, notably a payment card. , a bank note, a voucher, a voucher, a specific payment method such as a token or a wafer used in particular in casinos, or a secure label, a transport card, a loyalty card, a service card or a membership card, card or ticket, an interactive playing card or collectible card. The additional security means or means present in the document or security element that it comprises may have first, second and / or third level security features. Method 25 In order to implement the invention and to produce the article according to the invention, a step of orienting the particles making it possible to arrange their first region in a predefined manner relative to a surface of the article may be useful. One solution may be to use particles that will naturally move in a predefined manner when introduced into a fluid medium for example. In particular, the particles may be made to orient themselves in a particular manner under the effect of their own weight in the fluid medium in which they are dispersed. These particles may in particular have their center of gravity which is not located mid-thickness, so that the particles tend to turn with their heavier side turned down. Another solution may be to have magnetic particles all having the same remanent magnetization of predefined orientation relative to the first region, when they are deposited on the surface of the article. The application of the magnetic field to orient the particles on the article causes their first region to orient in a predefined manner relative to the article. If the particles are made by extrusion, the remanent magnetization prior to the deposition on the article can for example be obtained by magnetizing the magnetic particles at their exit from the extrusion die when the plastic matrix is still fluid and allows the movement of the particles. The orientation of the particles deposited on the article by means of a magnetic field can be carried out by any known means. To freeze particles with a predefined orientation, given by the magnetic field, one can use a binder that hardens after the particles have been oriented. It may be a fluid medium at the time of the step of orientation of the particles under the effect of the magnetic field, which then freezes due for example to a crosslinking reaction and / or polymerization, following eg exposure to a stimulus including UV, thermal or chemical, and / or evaporation, and / or solidification by cooling. The particles can be deposited by printing on the surface of the article, especially when the article is fibrous and constituted for example by paper. In an exemplary implementation of the invention, after removal of the particles, they are oriented under the effect of a magnetic field by for example scrolling under a magnetic head surface thus coated. If necessary, several magnetic heads may be used to orient the particles differently according to the zones of the article. It is also possible, or additionally, to pass the article several times with different orientations under the same magnetic head, so as to change the orientation of the particles. The particles once deposited and oriented, may be at least partially covered by at least one other layer of the article, for example a varnish or a protective film, or by another security element.

Particles The particles according to the invention are magnetically steerable, that is to say that they have the property of being able to orient themselves along the lines of a magnetic field to which they are exposed, which makes it possible to direct them from a non-random way on the article, by exposing the particles present on the article to a magnetic field. The latter can be created by one or more permanent magnets and / or electromagnets. The particles preferably have a non-zero magnetic susceptibility. The magnetic particles according to the invention may in particular be ferromagnetic or ferrimagnetic particles. Each particle may or may not have residual magnetism after extinction of the magnetic field which brings it into the desired orientation. The particles may have an orientation of their predefined remanent magnetic field relative to the first region prior to removal of the particles on the surface of the article as indicated above. The particles are for example with their magnetic axis transverse to said regions or parallel thereto. The magnetic properties of the particles according to the invention can be provided by the incorporation within the particles of a magnetic material, this magnetic material being for example dispersed within a matrix in each particle or in the form of a layer of a magnetic material. The magnetic material may be selected from cobalt, nickel, iron, their alloys and their oxides such as Fe3O4, gadolinium, terbium, dysprosium, erbium, their alloys and oxides. As described in EP 1 759 610, this may be multilayer materials of which at least one of the layers is magnetic, in the case of a composite the magnetic material is present as particles (grains) in the polymer matrix. It can also be a magnetic core coated with an organic or mineral matrix or vice versa. Optical properties of each region, first or second, may be homogeneous. Preferably, the first and second regions each have a homogeneous color in the visible and / or a homogeneous luminescence color in the UV or IR luminescence excitation radiation.

Each particle according to the invention advantageously has a multilayer structure, the optical regions with distinct optical properties consisting respectively of two layers of different materials. These two layers can be materials having colors, in the visible, different, with for example an AE gap in the space CIE Lab 1976 of at least 2. The particles can be of varied shape, in particular of thickness between 0.1 and 100 nm and / or width between 1 and 200 gm and / or length between 1 and 2500 um The particles may be non-platelet shape, in particular ovoid, spherical or at least partially cylindrical in shape.

The particles may be fo-orated by at least three layers and preferably comprise at least one magnetically orientable inner layer, in particular magnetic, in particular ferromagnetic. The outer layers may be opaque enough to mask the color of the magnetically orientable layer. The outer layers may alternatively be insufficiently opaque to do this.

At least one of the layers defining the optical regions may be non-magnetic, better each of said layers being non-magnetic. The particles may be coated with a coating material, preferably at least partially transparent or at least partially translucent. The particles may be formed by eoextrusion of at least two materials defining said optical regions. Preferably, the particles have a center of gravity which is not located mid-thickness of said particles, one of the regions being preferably defined by a denser material than that defining the other region. The first and second optical regions may have distinct luminescence properties, including distinct fluorescence or phosphorescence colors. These colors can be obtained after application to the particle substrate or after magnetic orientation of the particles, for example by irradiation with a laser when the particles comprise a laser-sensitive layer. The first and second regions may have distinct colors, and in particular have different shades, the color of each region being provided by printing different materials, in particular different metallic materials, by metallization, or by pigmentation of a polymer matrix. in the case of an article comprising such a matrix. The particle may have an iridescent effect on at least a portion of its surface. For example, the first and second regions are foed by extrusion or even coextrusion, and the masterbatch used to make each of the layers has different respective pigments and / or dyes. When the particles comprise multiple layers, the latter may each have a thickness of between 10 and 5000 nm. The particles may be included in a matrix lying on said surface of the article, in particular an ink.

When the particles are disposed in a fluid medium, in particular a medium capable of allowing the particles to congeal in the orientation imparted following the evaporation of a solvent constituting all or part of said medium and / or to a crosslinking and / or solidification of said medium, the particles may have mainly, especially substantially all, a denser face downwardly, in particular to a surface of the substrate that receives them. The invention can be better understood on reading the following detailed description, non-limiting examples of implementation thereof, and on examining the appended drawing, in which - FIGS. 1 to 3 and FIG. 4a to 4c represent, in section, various examples of particles according to the invention, FIG. 5 represents an example of a secure document according to the invention; FIG. 6 illustrates the orientation of the particles in different areas of the secure document of FIG. 5, FIG. 7 shows an example of a pattern obtainable by means of particles according to the invention, and FIG. 8 illustrates another example of the arrangement of particles on an article. In the example of FIG. 1, the magnetically orientable particles 1 according to the invention comprise three layers, including a magnetic inner layer 10 covered on its two opposite sides by respective outer layers 11 and 12, having distinct optical properties, and defining respectively the first and second regions 14 and 15.

The layer 10 is for example a layer of a thermoplastic material loaded with magnetic particles, for example ferrite or soft iron, and the layers 11 and 12 are for example layers of a thermoplastic material loaded with a dye and / or a luminescent substance.

The outer layers 11 and 12 may further consist of at least one dielectric material comprising a dye and / or a phosphor. The outer layers 11 and 12 may themselves further consist of the phosphor. The thickness of the layers 11 and 12 may be the same, or alternatively the thicknesses are different. The layers 11 and 12 may be in different materials such that the center of gravity of the particle is not equidistant from its outer faces. This can help the particles orient themselves with a predefined orientation under the effect of gravity, which can facilitate their removal with a predefined initial orientation of the first region relative to the article.

FIG. 2 represents a variant of the invention, in which the particles have a bilayer structure, with a layer 11 which defines the first optical region 14 and a layer 12 which defines the second optical region 15. The layers 11 and 12 are of preferably made of different materials, so that the regions 14 and 15 have different optical properties. The layers 11 and 12 are, for example, layers of a dielectric or thermoplastic material, charged with respective different phosphors, so that the luminescence properties of the regions 14 and 15 are different. The magnetic properties of the particles are preferably imparted to them by a diffuse magnetic charge, within layers 11 and 12.

In one variant, the optical regions 14 and 15 are defined by interferential multilayer structures. Each particle may comprise a coating which may extend all around the core of the particle, as illustrated in FIG. 3. This coating is for example intended to protect the particle from its environment, in particular mechanically and / or chemically. The coating material is for example a transparent material which does not affect the optical properties of the regions 14 and 15. It can be a transparent varnish.

The coating may also make it possible to optimize the interactions of the particles with their medium, and in particular to prevent agglomeration of the particles. The particles may also have a non-platelet shape, for example a form of non-polygonal section, in particular spherical as illustrated in FIG. 4A.

The particles may be formed with two layers 11 and 12, of which for example only one is magnetic, these two layers 11 and 12 defining optical regions 14 and 15, for example each substantially hemispherical. The interface between the layers 11 and 12 may be in the equatorial plane or not. FIG. 4B illustrates the possibility of having a partially cylindrical envelope surface of revolution, with two opposite planar faces. FIG. 4C illustrates the possibility of having an oval cross-section, with an interface between the layers 11 and 12, for example oblique with respect to the major axis. FIG. 5 shows an example of a secure document 20 comprising particles according to the invention. More specifically, these particles are present in an area 21 of the document 20, which extends from one edge to the opposite edge of the document but which could have any outline and be located at any location of the secure document allowing observation. FIG. 6 illustrates the possibility of orienting the particles differently according to the location on the document 20. In the example under consideration, the particles 1 are oriented differently along three zones Z1 to Z3 which extend over the width of the document 20, within the coated zone 21. All the particles 1 have substantially the same orientation in the first zone Zr, with the first region 14 facing outwards and the second region 15 facing the core of the article 22, in this case the substrate of the document 20. The angular orientation has particles 1 in the zone Z1 with respect to the normal is for example positive and between 0 and 90 °, for example order of 45 ° in the illustrated example. The angular orientation of particles 1 in zone Z3 is for example negative and between 0 and -90 °, for example of the order of -45 ° as illustrated. In the intermediate zone Z2, the particles are for example oriented substantially perpendicular to the surface of the substrate 22. The substrate 22 is for example at least partially transparent. In the case where the substrate is transparent at least in the zone 21 coated with the particles, there is, for example, for particles whose first optical region 14 produces a red fluorescence and the second region produces a blue fluorescence when the particles are subjected to UV illumination, more or less intense perceptions of the fluorescences of each zone according to the angle of observation adopted. The table above summarizes the effects obtained for six observation angles 01 to 06. Angle Z1 Z2 Z3 observation 01 Fluorescence red Fluorescence red intense Fluorescence zero intense 02 Fluorescence red Fluorescence zero Fluorescence blue of average intensity average intensity 03 Fluorescence none Intense blue fluorescence Intense blue fluorescence 04 Blue fluorescence Zero fluorescence Medium intensity average intensity red fluorescence 05 Nil fluorescence Intense red fluorescence Intense red fluorescence 06 Intense blue fluorescence Intense blue fluorescence Null fluorescence An example of alphanumeric pattern 70 formed by several zones 71 and 72 comprising particles with distinct respective orientations. The orientation of the particles of the bottom zone 71 is for example 45 ° relative to the normal and the orientation of the particles of the zones 72 defining the characters is for example -45 ° relative to the normal. The alphanumeric pattern may be integrated, as illustrated, with a security thread 7, which may itself be integrated into a secure document 20 as illustrated in FIG. 5.

FIG. 8 shows a variant in which article 22 comprises a substrate coated with particles 1 on its two faces. In the case where the article 22 is at least transparent or translucent, this allows in particular to benefit from the superposition of the optical effects produced on each side. The coating of the first face 81 of the article may, as illustrated, be complementary to the coating of the second face 82 of the article, or alternatively may not be. The superposition of the effects can in particular produce a visible pattern only according to a certain angle of observation. The invention is not limited to the illustrated examples. The particularities of the examples illustrated can be combined within non-illustrated variants. The expression "10 having one" is synonymous with "having at least one" except when the opposite is specified.

Claims (44)

REVENDICATIONS1. An article having a surface coated with a plurality of magnetically steerable particles (1) each having first (14) and second (15) regions, preferably opposite, with distinct optical properties, at least a portion of said particles (1) being, in an area of greater than or equal to 1 m2 m2 of the article, substantially all oriented with their first region (14) facing said surface. 2. Article according to claim 1, comprising a fibrous substrate (22), in particular papermaker. 3. Article according to claim 1, comprising a substrate (22) of a plastic material, in particular thermoplastic. 4. An article according to claim 1, being a security element, in particular a planchette, a flat fibrette, or a continuous element to integrate with or report on a substrate (22). 5. Article according to claim 1, at least a portion of the particles being disposed on the surface of the article by printing, coating or coating. 6. Article according to any one of the preceding claims, being at least partially opaque, especially completely opaque. 20 7. Article according to any one of the preceding claims, being at least partially transparent or at least partially translucent. 8. Article according to any one of the preceding claims, at least a portion of said particles (1) being oriented parallel to each other. 9. Article according to any one of the preceding claims, at least part of said particles (1) having an inclination (a) relative to the article, which changes gradually as one traverses the article. 10. Article according to any one of claims I to 9, having at least two areas where the orientable particles (1) have different respective inclinations, with their first region (14) facing the article. 30 11. Article according to any one of the preceding claims, having at least two areas where the orientable particles (1) have respective different orientations, including a first zone where the particles have their first region oriented towards the article and a second zone, preferably of greater than or equal to 1 mm2, where the particles (1) have their second region oriented towards the article. 12. Article according to any one of the preceding claims, having at least two zones where the particles (1) orientable have different respective inclinations, each of these areas being greater than or equal to 1 min2. 13. Article according to any one of the preceding claims, having at least three zones where the particles (1) have different respective inclinations, each of these areas being preferably greater than or equal to 1 2 MM. 14. Article according to one of claims 10, 12 or 13, having an area where the particles (1) have an inclination of between 0 ° and 90 ° relative to the normal to the article, preferably between 30 ° and 60 ° relative to the normal, and another zone where the particles (1) have an inclination of between 0 ° and -90 ° relative to the normal, better between -30 ° and -60 ° relative to normal. 15. Article according to claims 13 and 14, having another area where the particles (1) have an inclination of between -90 ° and + 90 ° relative to the normal, better between -30 ° and + 30 ° relative to normal , even better being inclined perpendicularly to the article. 16. Article according to one of claims 14 and 15, the difference in angular inclination between the orientations of the particles (1) within each zone being less than or equal to 30 °, preferably 10 °. 17. Article according to any one of the preceding claims, comprising two adjacent zones in which the particles (1) have substantially the same inclination, the inclination difference between said two adjacent zones being at least 10 °, of preferably at least 20 °. 18. Article according to any one of the preceding claims, the particles (1) being arranged at least partially on a colored area and / or luminescent of the article, in particular of absorption or luminescence color different from those of the first ( 14) and second (15) regions, or of absorption or luminescence color identical to that of one of said regions (14, 15). 19. Article according to claim 18, the colored area being a solid color or forming a pattern. 20. Article according to any one of the preceding claims, the particles (1) being disposed at least partially on an area at least partially transparent and or at least partially translucent of the article. 21. Article according to any one of the preceding claims, the first (14) and second (15) regions having luminescent properties, in particular being of distinct fluorescence or phosphorescence colors. 22. Article according to any one of the preceding claims, the first (14) and second (15) regions having distinct colors, in particular having different hues. 23. Article according to the preceding claim, the color of each region (14, 15) being provided by printing of different materials, including different metallic materials. 24. Article according to claim 22, the color of each region (14, 15) being provided by metallization. 25. Article according to claim 22, the particles (1) comprising a polymer matrix, the color of each region being provided by pigmentation of the polymer matrix. 26. Article according to any one of the preceding claims, the particles (1) having an iridescent effect on at least a part of their surface. 27. Article according to any one of the preceding claims, the particles (1) being platelet-shaped, in particular having a thickness of between 0.1 and 100 μm and / or a width of between 1 and 200 μm and / or a length between 1 and 2500um. 28. Article according to any one of claims 1 to 26, the particles (1) 25 being of non-platelet shape, in particular being of ovoid shape, spherical or at least partially cylindrical in revolution. 29. Article according to any one of the preceding claims, the particles (1) having a multilayer structure. 30. Article according to the preceding claim, the first (14) and second 30 (15) regions being defined by two layers of different materials. 31. Article according to one of claims 29 and 30, the particles (1) being formed of at least three layers (10, 11, 12) and preferably comprising at least one inner layer (10) magnetically orientable, especially magnetic, in particular ferromagnetic. 32. Article according to claim 31, at least one of the layers (11, 12) defining said regions (14, 15) being non-magnetic, better each of said layers being non-magnetic. 33. Article according to any one of the preceding claims, the particles (1) being coated with a coating material, preferably at least partially transparent or at least partially translucent. 34. Article according to any one of the preceding claims, the particles (1) being formed by coextrusion of at least two materials defining said regions (14, 15). 35. Article according to any one of the preceding claims, the particles (1) having a center of gravity which is not situated at the mid-thickness of said particles, one of the regions (14, 15) being preferably defined by a denser material than that defining the other region (15, 14). 36. Article according to any one of the preceding claims, the particles (1) being remanent magnetization, in particular with a magnetic axis transverse to said regions (14, 15) or parallel thereto. 37. Article according to any one of the preceding claims, the particles (1) being included in a matrix lying on the article, in particular an ink. 38. A secure document (20) comprising an article according to any one of the preceding claims, the secure document preferably being a means of identification of a person, in particular a passport, an identity card, a driving license, or a means of payment, including a payment card, a bank note, a voucher, a voucher, a specific payment method such as a token or a wafer, in particular used in casinos or a secure label, a card of transportation, a loyalty card, a service card or a membership card, a card or an access ticket, or an interactive playing or collectible card. 39. A method of orienting magnetically orientable particles (1) having first (14) and second (15) regions, preferably opposite, with distinct optical properties, wherein i. the particles (1) are deposited on a surface, ii. the particles thus deposited are oriented under the effect of a magnetic field and fixed with a predefined orientation of the first (14) and second (15) regions relative to said surface. 40. A method of orienting magnetically orientable particles (1) having a heterogeneous density and having first (14) and second (15) regions, preferably opposite, with distinct optical properties, wherein: i. the particles (1) are deposited on a surface, ii. the particles thus deposited are mainly oriented in the same way relative to said surface under the effect of their heterogeneous density, and preferentially have their densest face facing said surface. 41. Method according to one of claims 39 or 40, the particles (1) being deposited in the dispersed state within a fluid medium, in particular a fluid medium capable of allowing the particles to freeze in the orientation conferred following the evaporation of a solvent constituting all or part of said medium and / or a crosslinking and / or solidification of said medium. 42. The method of claim 41, the particles (1) having, in particular, substantially all a denser face within the downwardly fluid medium, in particular to a surface of the substrate (22) which receives them. 43. Particle (1) magnetically steerable, having first (14) and second (15) regions, preferably opposite, with distinct optical properties, regions having fluorescence or phosphorescence colors, under UV or W illumination, or colors come from distinct print. 44. Magnetically-steerable particle (1) having first (14) and second (15) regions, preferably opposite, with distinct optical properties, being formed by coextrusion or lamination of at least two materials defining said regions, with preferably, different densities for said materials, more preferably with a density difference of at least 0.2 giern3 between said materials.