Patent Description:
Optically variable devices are optical devices whose optical performance depends on angle of incidence of illuminating light or angle of observation. A common example of an optically variable device is an iridescent security feature used as an anti-counterfeiting measure on banknotes, credit cards, stock certificates, government-issued identification documents, etc. An optically variable device may provide a visually varying image, for example an illusory three-dimensional (3D) image, a color-shifting image, or both. Such an image is difficult to counterfeit without knowledge of a specific recipe used to manufacture the optical variable device providing the image.

Optically variable devices may be made by coating a surface with an ink or paint including flat platelet-like reflective and, or color-shifting particles. Such surfaces show higher reflectance and brighter colors than surfaces coated with a paint or ink containing conventional pigments. Substrates painted or printed with color-shifting flaked pigments may show change of color when viewed at different angles.

Flaked pigments may contain a material that is magnetically sensitive, so as to be alignable or orientable in an applied magnetic field. Such flakes may be manufactured from a combination of magnetic and non-magnetic materials and mixed with a paint or ink vehicle in the production of magnetic paints or inks. A feature of these products is the ability of the flakes to become oriented along the lines of an applied field inside of a layer of liquid paint or ink, while substantially remaining in this position after drying or curing of the paint or ink vehicle. Relative orientation of the flake and its major dimension with respect to the coated surface determines the level of reflectance or its direction and, or may determine angle-dependent color or brightness of the paint or ink.

By way of example, <CIT> disclose methods and devices for producing color-shifting images on coated articles using magnetically alignable flakes including color-shifting coatings. The color-shifting images are defined by the magnetic field applied to the coatings as the coatings are dried or cured. For example, a sheet magnet shaped as a symbol, a letter, or another indicia may be brought in close proximity to the coating during cure. After the coating is cured, the sheet magnet is removed, and the indicia may be seen as a color-shifting image on the coating. The magnetic field application may be adapted for modern printing environments; for example, <CIT> disclose a method and apparatus for orienting magnetic flakes in high-speed, linear printing operation.

<CIT> and <CIT> are also useful in understanding the present invention.

A 3D illusive image may also be formed on the painted product by applying a spatially varying magnetic field to the surface of the product while the paint still is in the liquid state. When the paint is cured and the magnetic field is removed, the 3D illusive image remains visible on the surface of the painted product. The 3D illusive image appears because light rays incident on the paint layer are influenced differently by differently oriented magnetic particles. Raksha et al. in <CIT> disclose a method and apparatus to orient magnetic flakes in desired 3D patterns in a high-speed linear printing apparatus.

Despite interesting and often intriguing optical effects produced by solidified suspensions of magnetic flakes, their application in optical security devices has been somewhat limited, in particular for banknotes. The application of magnetically alignable flake suspensions in banknotes and other valuable documents may be hindered by a poor compatibility of two main printing processes mostly used in manufacturing of banknotes - offset printing and Intaglio printing - with magnetically alignable particle suspensions. An offset printing process typically produces a very thin ink film thickness, and as such, cannot transfer large magnetic particles, for example particles that are <NUM> micrometers in size. An Intaglio printing process typically uses a highly viscous ink, which does not allow efficient alignment of magnetic particles suspended in the ink, at least without taking special measures to lessen the viscosity of the ink while applying a magnetic field, as is disclosed by Raksha et al.

In accordance with an aspect of the disclosure, a thickness of a layer including oriented magnetic flakes is reduced by applying magnetic flakes absent any liquid binder or carrier to an adhesive surface in the presence of magnetic field, which orients the magnetic flakes. For example, magnetic particles may be dusted or blown onto an adhesive surface in the presence of the magnetic field, causing the magnetic flakes to adhere to the adhesive surface in an oriented manner. Then, a thin coating layer is be applied to the oriented magnetic particles adhered to the adhesive surface. The coating layer is cured to maintain the orientation of the magnetic flakes.

In accordance with an aspect of the disclosure, there is provided a method of manufacturing an optically variable device, the method comprising the steps set out in Claim <NUM>.

The first adhesive layer may be only partially cured during depositing the magnetic flakes thereon. The substrate includes a release layer, in which case the coating layer may be adhered to a second substrate, and the release layer may be removed, to obtain a "flipped" orientation pattern of the magnetic flakes. The method may be adaptable to high printing speeds.

In one embodiment, a second adhesive layer may be provided on top of the first adhesive layer or beside the first adhesive layer. A second magnetic field may be applied to the second adhesive layer, and second magnetic flakes absent a liquid carrier or binder may be provided onto the second adhesive layer in the presence of the second magnetic field, so that the second magnetic flakes oriented by the second magnetic field adhere to the second adhesive layer. After this, the second adhesive layer may be cured.

In accordance with the disclosure, there is further provided an optically variable device according to claim <NUM>, obtained from the method described herein, and comprising a substrate that includes a release layer; an adhesive layer over the substrate; a plurality of oriented magnetic flakes supported by the adhesive layer; and a separately applied coating layer over the substrate adjacent the adhesive layer. The coating layer encapsulates the magnetic flakes extending from the adhesive layer, so that a portion of each one of the plurality of oriented magnetic flakes is adhesively attached to the adhesive layer, and a remaining portion of the same magnetic flake extends out of the adhesive layer into the coating layer. A second substrate is adhered to the coating layer.

In one embodiment, the adhesive layer is disposed on the substrate, and the coating layer is disposed on the adhesive layer. The coating covering the flakes on the adhesive layer may be of the same material as the adhesive layer, or may be a different material. The magnetic flakes may be partially disposed in the adhesive layer. In one embodiment, the magnetic flakes are reflective, and may include color-shifting multilayer coatings. By carefully selecting magnets to generate the magnetic fields, the magnetic flakes may be oriented so as to create a visual appearance of a 3D object such as a hemisphere, a cone, a funnel, a combination of different images obtained at separated stations, etc. The magnetic alignment may be repeated to create other images on top or aside a first image.

Exemplary embodiments will now be described m conjunction with the drawings, in which:.

While the present teachings are described in conjunction with vanous embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art.

Referring to <FIG> with further reference to <FIG>, <FIG>, a method <NUM> (<FIG>) of manufacturing an optically variable device <NUM> (the manufactured device is shown in <FIG>) may include a step <NUM> of providing a substrate <NUM> with an adhesive layer <NUM> (<FIG>) on the substrate <NUM> (<FIG>), which may be deposited, for example, by coating or printing. The substrate <NUM> may also be provided with the adhesive layer <NUM> already present on the substrate <NUM>, and the adhesive <NUM> may require only activation, for example by heating. In a magnetic field application step <NUM>, a magnetic field <NUM> (<FIG>) is applied, for example by providing a permanent magnet <NUM> (<FIG> and <FIG>) under the substrate <NUM> (<FIG> and <FIG>). An electromagnet may also be used. The magnetic field <NUM> generated by the magnet <NUM> extends through and over the adhesive layer <NUM> (<FIG>).

In a flake application step <NUM>, magnetic flakes <NUM> are applied to the adhesive layer <NUM>, for example, by blowing the magnetic flakes <NUM> onto the adhesive layer <NUM> using a stream <NUM> of gas e.g. air, argon, or nitrogen, having the magnetic flakes <NUM> suspended in the stream <NUM> of gas and carried by the stream <NUM> of gas, as shown schematically in <FIG>. Alternatively, the magnetic flakes <NUM> may be provided by dusting, or spreading the magnetic flakes <NUM> with the help of mechanical means, such as a blade, for example. Upon reaching the adhesive layer <NUM>, the magnetic flakes <NUM> adhere to the adhesive layer <NUM> (<FIG>). The magnetic field <NUM> causes the magnetic flakes <NUM> to become oriented or aligned along field lines <NUM> of the magnetic field <NUM> (<FIG>).

The magnetic flakes <NUM> are applied to the adhesive layer <NUM> in presence of the magnetic field <NUM>. In case of dusting of deposition with gaseous stream, the magnetic field <NUM> facilitates orientation of the magnetic flakes <NUM> during their flight towards the adhesive layer <NUM>, so that the magnetic flakes <NUM> may land onto the adhesive layer <NUM> already oriented along the magnetic field <NUM> lines. If the magnetic field <NUM> is not applied in the flake application step <NUM>, some of the magnetic flakes <NUM> may land flat on and adhere flat to the adhesive layer <NUM>, which may make hinder their further orientation of the magnetic flakes <NUM> by the magnetic field <NUM>.

In an optional adhesive layer curing step <NUM> of the method <NUM> (<FIG>), the adhesive layer <NUM> may be fully cured e.g. by applying heat <NUM> (<FIG>), ultraviolet (UV) light, etc., after application of the magnetic flakes <NUM> in the flake application step <NUM>. The adhesive layer <NUM> may be already partially cured (partially uncured) prior to application of the magnetic flakes <NUM>.

In a coating step <NUM> of the method <NUM> (<FIG>), the adhesive layer <NUM> having the magnetic flakes <NUM> adhered to the adhesive layer <NUM>, or anchored in the adhesive layer <NUM>, is coated with a coating layer <NUM> (<FIG>), for example a transparent adhesive layer or a varnish layer. The coating layer <NUM> may also include a semitransparent colored layer in combination with the magnetic flakes <NUM>, which may be colored or non-colored. In a curing step <NUM>, the coating layer <NUM> is cured e.g. by applying heat <NUM>, UV light, or both (<FIG>), so as to substantially preserve the orientation of the magnetic flakes <NUM> after the magnetic field <NUM> is removed. In this step, the adhesive layer <NUM> may also be fully cured, from a partially or fully uncured state.

A second adhesive layer, not shown, may be provided on top of the adhesive layer <NUM> or beside the adhesive layer <NUM>. A second magnetic field, not shown, may be applied to the second adhesive layer, and second magnetic flakes may be provided onto the second adhesive layer in the presence of the second magnetic field so that the second magnetic flakes oriented by the second magnetic field adhere to the second adhesive layer. The second magnetic flakes may also be absent a liquid carrier or binder. The second magnetic field may be different from the magnetic field <NUM>, for example the second magnetic field may have a different orientation or strength, or field lines pattern. The second magnetic flakes may also be different from the magnetic flakes <NUM>, for example the second magnetic flakes may have different color, size, material composition, etc. Magnetic fields and different flake types may be applied consecutively to obtain multi-color 3D indicia.

The manufactured optically variable device <NUM> is shown in <FIG>. The optically variable device <NUM> includes the substrate <NUM>, the adhesive layer <NUM> over the substrate <NUM>, and the magnetic flakes <NUM> supported by the adhesive layer <NUM>. The magnetic flakes <NUM> are adhered to the substrate <NUM>, and may appear extending from the substrate <NUM>. The magnetic flakes <NUM> are oriented by the magnetic field <NUM> (<FIG>). Herein, the term "oriented" means that the magnetic flakes <NUM> are aligned, that is, disposed in a non-random, coordinated fashion. The coating layer <NUM> extends over the substrate <NUM> adjacent the adhesive layer <NUM>, encapsulating the magnetic flakes <NUM>. As seen in <FIG>, a portion 23A of the magnetic flake <NUM> is adhesively attached to the adhesive layer <NUM>, and another portion 23B of the same magnetic flake <NUM> extends out of the adhesive layer <NUM> into the coating layer <NUM>. In one embodiment, the magnetic field <NUM> may be configured to have the field lines parallel to the surface of the substrate <NUM>. Most of the flakes <NUM> planarized by the magnetic field <NUM> would have one major side in contact with the adhesive layer <NUM>, and another major side in contact with the coating layer <NUM>.

Application of the magnetic flakes <NUM> and the coating layer <NUM> in separate steps may enable resulting optically variable devices <NUM> to remain quite thin. Essentially, the minimal thickness of the coating layer <NUM> is limited by size of individual flakes <NUM>. For instance, for <<NUM> micrometer sized flakes, the coating layer <NUM> thickness may remain as small as <NUM>-<NUM> micrometers. In the flake application step <NUM>, the magnetic flakes <NUM> are applied to the adhesive layer <NUM> absent the coating layer <NUM>. The magnetic flakes <NUM> may extend from the adhesive layer <NUM> e.g. by <NUM>-<NUM> micrometers. Once the magnetic flakes <NUM> adhere to the adhesive layer <NUM>, being oriented along the field lines <NUM> of the magnetic field <NUM>, the coating layer <NUM> may be applied to the adhesive layer <NUM> in the coating step <NUM>, to encapsulate the magnetic flakes <NUM> within the coating layer <NUM>, which can remain as thin as <NUM> micrometers. It is preferred that the coating layer <NUM> be substantially transparent to visible light, being colorless or colored, depending on required optical performance of the optically variable device <NUM>. Smaller magnetic flakes <NUM>, for example having an average size of <NUM> to <NUM> micrometers, may be preferable, depending on a particular printing application.

The magnetic flakes <NUM> may be reflective, e.g. the magnetic flakes <NUM> may have an optical reflectivity at visible wavelengths between <NUM> and <NUM> of at least <NUM>%. Reflective magnetic flakes <NUM>, when oriented, for example by a spherical or conical permanent magnet, may create a visual appearance of a metallic 3D-looking object, due to apparent reflectivity varying with illumination angle and, or observation angle. The magnetic flakes <NUM> may also include pearlescent or multilayer color-shifting coatings, which change color upon a change of angle of observation or illumination. Flakes which include multilayer color-shifting coatings may create a visual appearance of color-shifting 3D-looking objects, and may be particularly attractive for optical security applications. The magnetic flakes <NUM> may also have low reflectivity, so as to appear dark or black on a light background.

The shape of 3D-looking objects depends on shape and magnetization direction of the magnet <NUM> placed under the substrate <NUM> (<FIG>). The magnet <NUM> may be shaped and oriented to create the magnetic field <NUM> of a particular configuration. Furthermore, the resulting 3D looking shape may be inverted by flipping over the structure of the optically variable device <NUM>.

Turning to <FIG> with further reference to <FIG>, an optically variable device <NUM> is manufactured using the method <NUM> of <FIG>. A substrate <NUM> of the optically variable device <NUM> includes a release layer 41A. The coating layer <NUM> is be adhered to a second substrate <NUM> as shown in <FIG>. The release layer 41A may be then removed as shown in <FIG>, resulting in the optically variable device <NUM> being supported upside down by the second substrate <NUM>, as shown in <FIG>.

Referring to <FIG>, the adhesive layer <NUM> may include voids 22A in the adhesive layer <NUM>, e.g. forming visible indicia such as the number "<NUM>", for example. The voids 22A in the adhesive layer <NUM> may be formed using any suitable method, such as silk screen printing or other printing methods, lithography, etc. Once the magnetic flakes <NUM> are applied to the adhesive layer <NUM> in the flake application step <NUM> of the method <NUM>, the magnetic flakes applied to the voids 22A may be removed, for example, by directing a flow of gas on the voids 22A or by shaking. Masking may be applied while printing the adhesive, and, or providing the magnetic flakes <NUM>, and, or providing further coating. For added security, the magnetic flakes <NUM> may optionally include a diffractive pattern and, or covert identification indicia discernible under magnification.

Several prototypes of the optically variable device <NUM> (<FIG>) have been manufactured, and optically variable performance of the prototypes has been evaluated. Referring to <FIG>, and <FIG>, with further reference to <FIG>, a plan-view photograph (<FIG>) of a prototype of the optically variable device <NUM> (<FIG>) is shown. The adhesive layer <NUM> of the prototype of <FIG> included an adhesive ink layer, the magnetic flakes <NUM> included a color-shifting magnetic pigment changing color from gold at normal angle of viewing to green color at oblique angles. The coating layer <NUM> included varnish. The adhesive ink was cured prior to application of the varnish.

To provide a 3D appearance of a metal ball image <NUM> seen in the photograph of <FIG>, a spherical-cylindrical magnet pair including a spherical magnet <NUM> atop a cylindrical magnet <NUM> (<FIG>) has been placed under the optically variable device <NUM>. The direction of viewing of <FIG> is shown in <FIG> at 74A. The direction of viewing 74A is shown in <FIG> superimposed with the spherical <NUM> and cylindrical <NUM> magnets only to illustrate the geometry of the magnets in relation to the geometry of observation. For an actual observation, the spherical <NUM>-cylindrical <NUM> magnet pair was removed. In <FIG>, the same prototype is viewed at an oblique angle shown in <FIG> at 74B. <FIG> shows a plan view of the spherical <NUM>-cylindrical <NUM> magnet pair.

Referring to <FIG>, and <FIG>, with further reference to <FIG> and <FIG>, a prototype of <FIG> has a similar layer structure as the prototype of <FIG>, the only difference being the position of the spherical magnet <NUM> (<FIG>) in the magnet pair used to orient the magnetic flakes <NUM> (<FIG>). In <FIG>, the direction of viewing is shown at 74A. In <FIG>, the spherical magnet <NUM> is positioned close to an edge of the cylindrical magnet <NUM>, resulting in a shifted position of a metal ball image <NUM> in <FIG>. In <FIG>, the prototype of <FIG> is viewed at an oblique angle, as shown in <FIG> at 74B. <FIG> shows a plan view of the spherical <NUM>-cylindrical <NUM> magnet pair.

Turning to FIGs. 1OA, 1OB, and <FIG>, with further reference to <FIG> and <FIG>, a prototype of FIG. 1OA has a similar layer structure as the prototype of <FIG>, the only difference being that instead of the spherical <NUM>-cylindrical <NUM> magnet pair, a cylindrical <NUM> - rectangular <NUM> magnet pair (<FIG>) is used to orient the magnetic flakes <NUM> (<FIG>) to form an image of a 3D cone <NUM> within around-cornered rectangle <NUM> (FIGs. The direction of viewing corresponding to FIG. 1OA is shown at in <FIG> at 74A. 1OB, the same prototype is viewed at an oblique viewing angle shown in <FIG> at 74B. <FIG> shows a plan view of the cylindrical <NUM> - rectangular <NUM> magnet pair.

The cylindrical <NUM> - rectangular <NUM> magnet pair shown in <FIG> has been used to orient the magnetic flakes <NUM> in prototypes of <FIG>, <FIG>, <FIG> described below. These prototypes have been manufactured with different layer materials, using varying layer curing schedules.

In a prototype shown in <FIG>, the adhesive layer <NUM> (<FIG>) included not adhesive ink but a same varnish material as the coating layer <NUM>. The varnish of the adhesive layer <NUM> was cured after application of the Go/Gr color-shifting magnetic pigment flakes. The 3D effect was present, as can be seen by comparing <FIG>, when the varnish was used in the adhesive layer <NUM>.

Claim 1:
A method comprising:
providing an optically variable device (<NUM>) that includes a first substrate (<NUM>) and release layer (41A),
depositing an adhesive layer (<NUM>) on the first substrate (<NUM>);
applying a magnetic field (<NUM>) to the adhesive layer, and providing magnetic flakes (<NUM>) absent a liquid carrier or binder onto the adhesive layer in the presence of the magnetic field so that the magnetic flakes oriented by the magnetic field adhere to the adhesive layer;
applying a coating layer (<NUM>) over the magnetic flakes;
adhering the coating layer to a second substrate (<NUM>);
curing the coating layer; and
removing the release layer after adhering the coating layer to the second substrate.