Security device and security document or token with the security device

Abstract

A safety device designed to be provided in or on a substrate or in or on one or more layers applied to the substrate, comprising: a. a surface relief structure and b. a coating, wherein i. the surface relief structure is filled with the coating to form a flat surface, and wherein the coating has a refractive index greater than the refractive index of the surface relief structure, or ii. the coating forms all or part of the surface relief structure, wherein the surface relief structure of the coating is filled with one or more transparent lacquer layers to form a flat surface, wherein, for each alternative, the coating comprises particles of at least one metal oxide dispersed in a transparent lacquer, wherein the coating comprises 1 to 60 wt.-% metal oxide comprises, wherein the particles have a primary crystallite size of 1 to 100 nm, and wherein the concentration of the particles in the coating is high enough to induce a refractive index shift, so that the optical effect of the surface relief structure remains essentially unaffected or is even enhanced.

Description

AREA OF INVENTION The present invention relates to security devices as well as security documents or tokens containing security devices. GENERAL STATE OF THE ART Relief structures such as printed or embossed diffractive optical surface structures and similar security features can be easily counterfeited by mechanical lifting if the structure is uncoated and therefore exposed. The coating required to protect such structures must have a sufficiently high refractive index compared to the substrate onto which the feature is applied to preserve the feature's visibility. Such high refractive index (HRI) coatings used for relief structures applied to various substrates should fully or largely preserve the original, intended color, clarity, and visibility of the uncoated feature.Therefore, high transparency and low (or even no) color are essential requirements when achieving a diffractive effect visible in both reflection and transmission, particularly for relief structures applied to clear substrates. Until now, these requirements have been difficult to meet for a printed coating that demands good adhesion, chemical resistance, and high durability while simultaneously making the relief structure resistant to mechanical lifting. Traditionally, high-refractive index (HRI) coatings based on titanium dioxide (titanium(IV) oxide), zinc sulfide, or zinc selenide were deposited onto a surface using vacuum deposition, a technique that requires high temperatures and is expensive. Other sputtering techniques for metal layer deposition are not readily adaptable to high-speed printing. Furthermore, some surface structures require a layer of a high refractive index coating with a thickness on the micrometer scale. Metallized coatings are unsuitable because thicker coatings are expensive and can impart a highly reflective surface with prominent coloration within the coating. Thicker metallized coatings are also less robust.High refractive index coatings made from metal-containing polymers are known, but their use requires coating and subsequent curing at a high temperature to provide a metallic coating on the substrate (Wang et al., Proceedings of SPIE, Vol. 5724, 2005). This technique requires a processing temperature of over 100 °C and often up to 200 °C, which is too high for use with a polymer substrate and unsuitable for roll-to-roll printing processes. Metal dioxide particles of various sizes, used in suspension or other liquid formulations, are primarily known in HRI coating technology for applications in electronic displays. The coating clarity can be improved by reducing the coating weight. However, this can diminish the effectiveness of protecting the structure on certain relief patterns. Metal dioxide nanopowders, in which the particles are coated with various functional groups, can be dispersed in a suitable solvent and used as a coating formulation. However, these materials may exhibit poor adhesion to polymer substrates in the absence of suitable additives. The conventional method for using high-refractive-index metal oxides involves an additive mixed with a resin or carrier. This can improve adhesion, but depending on the ratio of the specific metal oxide to the resin used, it also leads to a reduction in the refractive index. On embossed substrates, such formulations based solely on titanium dioxide produce coatings with very poor adhesion and low transparency. Adhesion can be improved by increasing the amount of resin, but this reduces the refractive index. Although halogenated polymers with a high refractive index are available, they are not desirable from a cost and environmental point of view. Therefore, there is a need for a coating with a high refractive index that provides a durable coating which is resistant to mechanical lifting, transparent, has high adhesion, and is also easy to use. State of the art is revealed in US 2008 / 0191463A1, US 5944356A and US 2009 / 0141355A1. BRIEF SUMMARY OF THE INVENTION According to the present invention, a security device with the features of claim 1 and a security document or token with the features of claim 8 are provided. According to a first aspect, a coating is provided for use in protecting a surface relief structure, wherein the coating has a refractive index greater than that of the surface relief structure, and wherein the coating comprises particles of at least one metal oxide. In some embodiments, the coating may also comprise at least one transparent varnish. The term varnish refers to a material that results in a durable protective layer on and / or in the coating. It has been advantageously found that when a coating is applied to a substrate and the solvent has been removed, the presence of a transparent varnish provides improved adhesion of the coating to the substrate. The use of a transparent varnish also improves the transparency of the coating.It was surprisingly discovered that in cases where the coating exhibits a slight tint, the use of a transparent varnish reduces this tint. Examples of transparent varnishes include, but are not limited to, nitrocellulose and cellulose acetyl butyrate. Examples of surface relief structures include, but are not limited to, printed or embossed surface relief structures. According to another aspect, a coating is provided for use in protecting a surface relief structure, wherein the coating forms all or part of the surface relief structure, and the coating comprises particles of at least one metal oxide. According to a further aspect, a dispersion is provided comprising particles of one or more metal oxides, at least one organic solvent, and at least one transparent lacquer. In one embodiment of this aspect of the invention, the dispersion provides a coating for use in protecting a surface relief structure. In another embodiment, the coating itself can form all or part of the surface relief structure. Exemplary transparent lacquers include, but are not limited to, nitrocellulose and cellulose acetyl butyrate. In a preferred embodiment, the surface relief structure is a diffractive optical surface structure. The surface structure can be printed or embossed, or it can be formed by any other method known in the prior art. Advantageously, the refractive index of the coatings is sufficiently higher than that of the surface structure so that the diffractive effect of the diffractive optical surface structure is preserved. Examples of diffractive optical surface structures that can benefit from the coating compositions of the present invention include, but are not limited to, diffraction gratings, holograms, and diffractive optical elements. In another embodiment, the surface relief structure is a non-diffractive structure that includes, for example, a sub-wavelength structure, a lens, a microlens arrangement, or a waveguide. It will be appreciated that a person skilled in the art could select a specific coating that meets the requirements of the surface relief structure in question. Preferably, the concentration of metal particles is high enough to induce a refractive index shift, so that the optical effect of the surface relief structure remains essentially unchanged or is even enhanced. In yet another embodiment, the coating of the present invention has a refractive index greater than 1.0, preferably greater than 1.4, more preferably greater than 1.7, and even more preferably greater than 1.8. Preferably, the primary crystallite size of the metal oxide particles is between 1 and 100 nm. More preferably, the primary crystallite size of the metal oxide particles is between 5 and 25 nm. Advantageously, when used as a printing ink, the coatings of the present invention can be printed in their present form without the need for a resin or carrier material. When used as a protective layer over a relief structure, the coatings of the present invention offer high transparency and low coloration and are resistant to mechanical lifting. The optical effect of the surface relief structure is maintained even when the relief is removed, thus preventing contact printing. The visibility and gloss of the relief structure are preserved. The coatings also provide high adhesion. In one embodiment, the metal oxide is titanium oxide or zirconium oxide, or mixtures thereof. Titanium oxide is a preferred metal oxide. In another embodiment, the metal oxide particles are coated with an inorganic and / or organic coating. A preferred inorganic coating is an oxide, non-limiting examples of which are aluminum oxide, silicon dioxide, and zirconium oxide, or mixtures thereof. Non-restrictive examples of preferred organic coatings are polyols, esters, siloxanes, silanes, silicon-containing organic compounds, and carboxylic acids and mixtures thereof. A particularly preferred organic coating is a fatty carboxylic acid. In yet another embodiment, the coating or dispersion further comprises at least one dispersing agent. Preferably, the dispersing agent is a neutral, anionic, or cationic polymer or copolymer. Examples of dispersing agents include alkylolammonium salts of a polymer or copolymer, polyhydroxystearic acids, and polycaprolactone polyols. In one embodiment, the dispersing agent may comprise a hyperdispersing agent. In yet another embodiment, the coating or dispersion further comprises at least one binder. Preferred binders are organic binders. Further preferred binders are organic polymers such as ketone- or aldehyde-based polymers. In yet another embodiment, the metal oxide particles are doped with another metal oxide. Non-restrictive examples of suitable organic solvents are ketones, esters, glycols and glycol ethers and mixtures thereof. In an alternative embodiment, the coating or dispersion may also comprise a radiation-curing resin, for example a resin that can be cured by actinic radiation such as UV radiation, X-rays or electron beams. The radiation-curing resin is preferably a transparent or translucent printing ink formed from a clear resin material. Such a transparent or translucent printing ink is particularly suitable for printing on translucent security elements such as numerical diffractive optical elements (DOEs) and lens structures. In a particularly preferred embodiment, the transparent or translucent printing ink preferably comprises an acrylic-based, UV-curing, clear, embossable varnish or a coating of this type. Such UV-curing lacquers are available from various manufacturers, including Kingfisher Ink Limited, product type Ultraviolet UVF-203, or similar. Alternatively, radiation-curing embossable coatings can be based on other compounds, for example, nitrocellulose. According to a non-inventive aspect, a method for protecting a surface relief structure is provided, comprising the following steps: (a) providing a dispersion comprising metal oxide particles in an organic solvent according to one of the aspects mentioned above, (b) applying the dispersion to a surface relief structure, and (c) removing the solvent to form a coating, wherein the coating has a refractive index greater than that of the surface relief structure. The surface relief structure may be printed or embossed or formed by another method according to the prior art. In one embodiment of this aspect, the method further comprises the following step: (d) applying one or more transparent lacquer layers to the coating formed in this way. The transparent varnish may contain one or more UV-curing components. Furthermore, or alternatively, the dispersion itself may also comprise at least one transparent varnish. Furthermore, or alternatively, the dispersion may also comprise one or more binders. In a further embodiment, the dispersion can also comprise one or more radiation-curing resins, for example, a resin that can be cured by actinic radiation such as UV radiation, X-rays, or electron beams. The radiation-curing resin is preferably a transparent or translucent printing ink formed from a clear resin material. In a particularly preferred embodiment, the transparent or translucent printing ink preferably comprises an acrylic-based, UV-curing, clear varnish or a similar coating. In one embodiment, the surface relief structure is a diffractive optical surface structure. In another aspect, the application of the dispersion to a surface or substrate can itself form all or part of the surface relief structure. In yet another aspect, the coatings or dispersions can be used as a printing ink, with any state-of-the-art printing process being applicable. Advantageously, such coatings or dispersions can be used in roll-to-roll printing processes, eliminating the need for high-temperature sputtering. In yet another aspect of the present invention, the use of a coating or dispersion according to one of the aspects and embodiments mentioned above is provided for the protection of surface relief structures. In a preferred embodiment, the surface relief structure is a diffractive optical surface structure. In another aspect, a safety device is provided which has a surface relief structure and a coating according to one of the embodiments mentioned above. In another aspect, a safety device is provided which includes a surface relief structure that is protected by a coating according to one of the above-mentioned methods. Throughout this specification, the terms "includes" or "comprehensive" or grammatical variations thereof are to be understood as specifying the presence of specified features, integers, steps or constituents, but not excluding the presence or addition of one or more other features, integers, steps, constituents or groups thereof not specifically mentioned. DETAILED DESCRIPTION Definitions Security document As used herein, the term "security document" includes all types of documents of value or tokens, as well as identification documents, including but not limited to the following: items of currency such as banknotes and coins, credit cards, checks, passports, identity cards, securities and share certificates, driving licences, deeds of ownership, travel documents such as airline and train tickets, admission tickets and passes, birth, death and marriage certificates and academic transcripts. The invention is applicable in particular, but not exclusively, to security documents such as banknotes or identification documents such as identity cards or passports, which are formed from a substrate onto which one or more printing layers are applied. substrate As used here, the term "substrate" refers to the base material from which the security document or token is formed. The base material may be paper or another fibrous material such as cellulose; a plastic or polymer material, including but not limited to polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET); or a composite material of two or more materials, such as a paper laminate and at least one plastic material, or of two or more polymer materials. Safety device or feature As used here, the term "security device or feature" refers to any large number of security devices, elements or features designed to protect the security document or token from forgery, copying, alteration or manipulation.Security devices or features can be provided in or on the substrate of the security document or in or on one or more layers applied to the base substrate, and can take many different forms, such as security threads embedded in layers of the security document; security inks such as fluorescent, luminescent and phosphorescent inks, metallic inks, iridescent inks, photochromic, thermochromic, hydrochromic or piezochromic inks; printed and embossed inks, including relief structures; interference layers; liquid crystal devices; lenses and lenticular structures; optically variable devices (OVDs) such as diffractive devices, including diffraction gratings, holograms and diffractive optical elements (DOEs). Diffractive optical elements (DOEs) As used here, the term "diffractive optical element" refers to a numerical-type diffractive optical element (DOE). Numerical-type DOEs rely on mapping complex data to reconstruct a two-dimensional intensity pattern in the far field (or reconstruction plane). Thus, when essentially collimated light, for example from a point light source or a laser, is incident on the DOE, an interference pattern is generated that produces a projected image in the reconstruction plane. This image is visible when a suitable viewing surface is positioned in the reconstruction plane or when the DOE is viewed during transmission onto the reconstruction plane. The transformation between the two planes can be approximated by a fast Fourier transform (FFT). Therefore, complex data containing amplitude and phase information must be physically encoded in the DOE's microstructure. This DOE data can be computed by performing an inverse FFT transformation of the desired reconstruction (i.e., the desired far-field intensity pattern). DOEs are sometimes referred to as computer-generated holograms, but they are different from other types of holograms such as rainbow holograms, Fresnel holograms, and volume reflection holograms. The invention will now be described expediently with regard to certain embodiments and examples. These embodiments and examples serve only for illustrative purposes and should not be understood as limiting the scope of protection of the invention. It will be understood that variations of the described invention that are apparent to a person skilled in the art are within the scope of protection of the invention. Similarly, the present invention may find application in areas not expressly mentioned in this document, and the fact that some applications are not specifically described should not be interpreted as limiting the applicability of the invention as a whole. The coating or dispersion can be produced by dispersing particles of a metal oxide in a suitable organic solvent. According to the invention, the coating or dispersion comprises 1 to 60 wt.% metal oxide, more preferably 2 to 40 wt.%, and most preferably 2 to 20 wt.%. A particularly preferred metal oxide is titanium oxide. The particle size of the metal oxide is important for the production of high refractive index (HRI) coatings. Preferably, the primary crystallite size of the metal oxide is between 1 and 100 nm, more preferably between 5 and 25 nm. It will be appreciated that the measured particle size does not necessarily reflect this primary crystallite size and that it may depend on the extent of particle surface treatment and on the presence or absence of additives that can influence primary crystallite agglomeration. Furthermore, those skilled in the art will appreciate that different particle size measurement techniques can produce different results depending on the nature of the sample being analyzed. Preferred metal oxides are titanium oxide, zirconium oxide, zinc oxide, tin oxide and cerium oxide. Any suitable organic solvent can be used to disperse the metal oxide. Preferred organic solvents are ketones, esters, glycols and glycol ethers, and mixtures thereof. The coating or dispersion may also comprise at least one binder material, preferably an organic binder material. Organic binder materials are materials that can act as viscosity modifiers, exhibit film-forming properties, impart mechanical strength to the films or coatings formed with them, or a combination thereof. Generally, the at least one organic binder exhibits minimal solubility in polar solvents and / or solvents with a high boiling point. Generally, the at least one organic binder material is compatible with other solvents used to form the coating or dispersion. Furthermore, the at least one organic binder is compatible with the dispersion, such that a homogeneous solution is created and maintained when combined with the dispersion. In one embodiment, the organic binder materials used include, among others, high-molecular-weight polymers. Examples of such materials include, but are not limited to, polyethylene oxide (PEO), polyvinyl alcohol (PVA), and polyacrylic acid (PAA). In another embodiment, the organic binder is an alkylcellulose ether. Examples of alkylcellulose ethers include, but are not limited to, methylcellulose, hydroxypropyl methylcellulose, and derivatives of hydroxyethylcellulose. In one embodiment, the at least one organic binder is present in the coating or dispersion at a concentration of 0.5 to 20% by weight. In another embodiment, the at least one organic binder is present in the coating or dispersion at a concentration of 0.5 to 15% by weight. In yet another embodiment, the at least one organic binder is present in the coating or dispersion at a concentration of 0.5 to 12% by weight. The coating or dispersion may also include at least one dispersing agent. In one embodiment, the at least one dispersing agent is present in the coating or dispersion at a concentration of 1 to 15% by weight. In another embodiment, the at least one dispersing agent is present in the coating or dispersion at a concentration of 2 to 10% by weight. In yet another embodiment, the at least one dispersing agent is present in the coating or dispersion at a concentration of 3 to 8% by weight. Preferably, the dispersing agent is an anionic or cationic polymer or copolymer. A particularly preferred dispersing agent is an alkylolammonium salt of a polymer or copolymer. Particularly preferred coatings comprise a metal oxide dispersion and a transparent lacquer. The transparent lacquer may include one or more UV-curing components. In one embodiment, the coating can be formed by printing as a printing ink, after which the coating thus formed is itself coated with one or more layers of a transparent lacquer. Again, the transparent lacquer may include one or more UV-curing components. In an alternative embodiment, mixtures of the dispersion with transparent lacquer can be applied to a surface, and the coating thus formed can optionally be followed by one or more layers of transparent lacquer. In some applications, it is advantageous to apply more than one coat of paint. Regarding the coating thickness, it is important to note that this can vary considerably depending on the nature of the surface relief structure in question. Effective coatings can be achieved using a metal oxide coating depth of approximately 10 to 1000 nm, preferably 50 to 700 nm. The lacquer coating can have a depth of up to 10 micrometers or even more. With regard to providing an effective coating that is resistant to counterfeiting by mechanical lifting, the entire surface relief structure is preferably filled with the coating, including any additional lacquer coatings, so that a flat surface is created. Excellent coatings can be obtained by mixing 1, 1.5, or 2 parts of a printing ink composition containing 10% by weight (10% metal oxide dispersed in an organic solvent) with 1 part clear varnish. The mixture can be printed as is, and the resulting coating can be treated with one or more further varnish topcoats. Similarly, it is advantageous to use 1 or 2 parts of a 10 wt. % printing ink composition in solvent with 1 or 2 parts varnish and 1 or 2 parts organic solvent. High-performance coatings were also produced by mixing 1 part of a 40 wt.% printing ink composition with 1 to 5 parts of transparent varnish or by mixing 1 part of a 40 wt.% printing ink with 4 parts varnish in 2 parts organic solvent. EXAMPLES Metal oxide dispersion Metal oxide dispersions were obtained from various sources, namely MK Impex, Ontario, Canada; NanoGram Corporation, California, USA; Chem-Well Tech Co., Ltd., South Korea; and Sumitomo Osaka Cement Co. Ltd., Japan. The coating or dispersion may also include a radiation-cured resin. In one example, titanium dioxide nanoparticles (40 to 45 wt%) were dispersed in an organic solvent, typically dipropylene glycol monomethyl ether. The dispersions further comprised 11 to 13 wt% binder and 6 to 8 wt% dispersing agent. Methyl ethyl ketone (MEK) was added to these dispersions to dilute them to a concentration of about 13 wt% (a dispersion of 1:2 in MEK has proven advantageous) by conventional mixing methods. Optionally, one or more transparent lacquers and / or UV-curing resins can be added to the metal oxide dispersion before coating. Coating process The coating can be applied using well-known commercial methods such as gravure printing, flexographic printing, screen printing, and the like. In one embodiment, a serial in-line process is carried out simultaneously with the process for producing the relief structures, i.e., the coating with a high refractive index itself forms the relief structure. Alternatively or additionally, the coating can be applied as a separate process for creating the relief structure. These processes enable an economically efficient method for applying the coating at printing speeds of up to 120 m / min or more. This can then be followed by further printing processes with one or more transparent varnish layers.

Claims

A safety device designed to be provided in or on a substrate or in or on one or more layers applied to the substrate, comprising: a. a surface relief structure and b. a coating, wherein i. the surface relief structure is filled with the coating to form a flat surface, and wherein the coating has a refractive index greater than the refractive index of the surface relief structure, or ii. the coating forms all or part of the surface relief structure, wherein the surface relief structure of the coating is filled with one or more transparent lacquer layers to form a flat surface, wherein, for each alternative, the coating comprises particles of at least one metal oxide dispersed in a transparent lacquer, wherein the coating comprises 1 to 60 wt.-% metal oxide comprises, wherein the particles have a primary crystallite size of 1 to 100 nm, and wherein the concentration of the particles in the coating is high enough to induce a refractive index shift, so that the optical effect of the surface relief structure remains essentially unaffected or is even enhanced. Safety device according to claim 1, wherein the coating further comprises one or more radiation-curing resins. Safety device according to claim 2, wherein the radiation-curing resin is a UV-curing resin. Safety device according to one of the preceding claims, wherein the surface relief structure is a diffractive optical surface structure. Safety device according to one of the preceding claims, wherein the metal oxide is titanium oxide or zirconium oxide or mixtures thereof. Safety device according to one of the preceding claims, wherein the metal oxide particles are coated with an inorganic and / or organic coating. Safety device according to one of the preceding claims, further comprising at least one dispersing aid. Security document or token comprising a substrate and a security device according to any one of claims 1 to 7.