Catalytically Accelerated Color Drying on a Valuable Document

Description

Technical field

[0001] The present invention relates to catalytically accelerated ink curing on a security document, such as a banknote or an identity document. Thus, the present invention relates to a security document comprising a substrate with optionally two opposing layers, at least one printed layer consisting of at least one printing ink, and optionally at least one further layer, wherein at least one of the at least one printing ink contains silver-ruthenium bimetallic particles. Furthermore, the present invention relates to a method for the catalytically accelerated curing of a printing ink on a security document, comprising the steps of providing a printing ink containing silver-ruthenium bimetallic particles and printing the printing ink onto a security document substrate, as well as the use of silver-ruthenium bimetallic particles for curing printing inks on a security document. Technical background

[0002] The production of securities, such as banknotes, typically involves several printing steps, employing a variety of sequential printing and finishing techniques. One of the first steps is usually a single-color or multi-color underprint using offset and / or letterset printing.

[0003] The printing equipment used for this can vary greatly and depends on the following parameters: the security inks or security pigments used; the type of (paper) substrate used (e.g., substrates based on cotton fibers; substrates based on mixed fibers, where mixed fibers are in particular cotton, flax, linen, cellulose and synthetic fibers; substrates based on plastic films; impregnated and pre-coated substrates); the provision of substrates to be printed that have an inhomogeneous structure (e.g., paper substrates provided with a film element; partially pre-printed substrates; substrates with partially applied, flat elements based on screen printing); the provision of substrates to be printed that have a different pH value and / or a different paper moisture content at different points, whereby differently rough and porous surfaces also have an influence.

[0004] While production errors that lead to a visible deterioration of the print result can be detected and corrected directly at the printing press (e.g., ink flow), production errors affecting substrate drying and / or substrate dimensions pose a risk of significantly delaying the subsequent production process and increasing waste. The various printing equipment required for the printing process also has a strong influence on the behavior of the printed sheets in subsequent printing steps. Printing steps that introduce heat lead to substrate shrinkage. In printed materials produced under high mechanical pressure, dimensional changes (especially enlargement) of the substrate become apparent. Intaglio printing, widely used in security and securities printing, places particular stress on the substrate and ink layers, making adequate drying essential.

[0005] If dimensional changes in the substrate exceed certain tolerance limits, often the only option is to reprint with modified printing equipment. Alternatively, the printing equipment for subsequent printing steps can be adapted, which is associated with considerable effort and high costs.

[0006] The insufficient drying of a printed substrate due to a problematic ink formulation or a substrate that does not absorb the ink can be partially accelerated by applying heat. However, heating printed materials often leads to uncontrollable changes in the dimensions of the printed substrate. This is particularly true for cellulose, cotton, and other paper substrates, where heat input causes moisture loss. As a result, the registration of subsequent print jobs often cannot be maintained. This effect also occurs when using plastic or hybrid substrates (e.g., multilayer film / paper / film substrates, as described in WO 2004 / 028825 A2). A further disadvantage is that this method does not significantly improve the settling of ink within a stack of documents.The term "ink transfer" refers to the phenomenon where the still-wet ink transfers to the next printed sheet, disrupting the printing result.

[0007] The drying of a printed substrate can be further accelerated by the addition of metal-containing drying agents. Oxidatively drying printing inks (i.e., printing inks that are not UV inks or UV varnishes) typically contain a drying agent or siccative. Drying agents are primarily organic salts of certain metals such as cobalt (e.g., cobalt octoate, cobalt naphthenate), manganese, calcium (e.g., calcium octoate), zirconium, or cerium. Metal-containing drying agents exert their effect in the presence of oxygen by accelerating the decomposition of on siteformed peroxides. Provided that storage and transport largely take place in the absence of air, a drying agent can be incorporated into the paint at the factory. However, as already mentioned, these types of drying agents only accelerate the decomposition of in-situ formed peroxides, but do not create an increased concentration of peroxides. Furthermore, the use of the aforementioned (heavy) metal-containing drying agents should be avoided as much as possible for reasons of occupational safety and environmental protection.

[0008] Furthermore, the drying of a printed substrate can be accelerated by adding peroxide-containing substances, such as inorganic or organic peroxides. These release oxygen radicals as they decompose, thus accelerating the oxidative drying of the ink layer. This also works in areas with a lack of air or oxygen, and consequently also with substrates that do not saturate and in tall or large stacks of valuable documents. However, a disadvantage is that peroxide-containing substances must only be added to the ink relatively shortly before printing, because the chemical reaction that begins with the addition of the peroxide prevents the ink from being stored for an extended period.

[0009] Furthermore, the drying of a printed substrate can be accelerated by using UV-curing basecoats or hybrid UV systems instead of oxidatively curing systems. However, such inks are significantly more expensive, often possess sensitizing properties, and may exhibit poor overprintability. Moreover, as finished inks, they have a limited shelf life. Similar considerations apply to novel hybrid ink formulations that contain both conventional, oxidatively curing components and UV-curing components. Finally, such inks require irradiation with UV radiation, which represents an additional step in the printing process, requiring specialized equipment and a certain amount of (additional) energy, which can be disadvantageous from an economic standpoint.UV irradiation can also only be applied to individual sheets, not to stacks of sheets. A novel ink-drying process using UV radiation and photosensitizers to generate singlet oxygen, which overcomes some of the aforementioned disadvantages, is described, for example, in WO 2021 / 197663 A1, where photosensitizers such as those described in WO 2017 / 032892 A can be used. However, in this process, in addition to the inherently disadvantageous necessity of a UV irradiation step, the duration of UV irradiation required in practice has proven to delay the printing process.

[0010] Document WO 2011 / 197663 A1 discloses a security document substrate with two opposing main surfaces, wherein at least one of the two main surfaces has a photosensitizer for generating singlet oxygen.

[0011] The present invention is therefore based on the objective of providing security documents with improved ink drying properties in the printing process compared to the prior art. For example, it would be desirable if the security documents were to harden further after printing when stored in a stack, and if the stack height could be sufficiently high, for example at least 300 sheets or preferably at least 500 sheets. Furthermore, it would be desirable if additional steps in the printing process, such as irradiation with UV radiation, could be avoided or at least kept as short as possible. The present invention, as described herein and defined in the independent claims, solves this problem. The dependent claims describe preferred embodiments. Summary

[0012] According to a first aspect, the present invention relates to a security document comprising a substrate with optionally two opposing layers applied thereon, at least one printing layer made of at least one printing ink, and optionally at least one further layer, wherein at least one of the at least one printing ink contains silver-ruthenium bimetallic particles.

[0013] The present invention further relates, according to a second aspect, to a method for the catalytically accelerated curing of a printing ink on a security document, comprising the steps of providing a printing ink containing silver-ruthenium bimetallic particles and printing the printing ink onto a security document substrate.

[0014] According to a third aspect, the present invention further relates to the use of silver-ruthenium bimetallic particles for curing printing inks on a security document. Detailed description of the invention

[0015] The present invention is based on the finding that silver-ruthenium bimetallic particles contained in a printing ink, particularly one used in the field of security document printing, are able to catalytically accelerate ink drying. No UV radiation is required for ink drying; only the presence of oxygen is necessary, since the silver-ruthenium bimetallic particles catalytically convert oxygen to hydrogen peroxide in the presence of two protons. This occurs through a complex mechanism in which electrons generated by the catalytic oxidation of organic matter with silver are transferred to oxygen via a redox cascade involving ruthenium, forming hydrogen peroxide. This process is supported by ambient humidity. The inherently unstable hydrogen peroxide decomposes in a further step into two hydroxyl radicals, which accelerate the oxidative drying of the printing ink.This typically occurs through oxidative cross-linking of the polymer components contained in the printing ink. The reaction is particularly rapid at the surface of the ink, which is in contact with atmospheric oxygen and thus exposed to a high oxygen concentration. This is advantageous because rapid, or at least largely complete, surface drying allows for the timely stacking of printed sheets without the risk of ink smearing or transfer of still-wet ink from one sheet to another. Simultaneously, the silver-ruthenium bimetallic particles, utilizing oxygen diffused into the ink, also support the slower, deeper drying process.It is therefore possible to stack the printed sheets on top of each other to dry completely shortly after the printing process, especially in numbers of more than 500 or even more than 1000 printed sheets, as is commonly done in the printing of security documents.

[0016] Thus, according to a first aspect, the present invention relates to a security document comprising a substrate with optionally two opposing layers applied thereon, optionally at least one further layer, and at least one printing layer consisting of at least one printing ink, wherein at least one of the at least one printing ink contains silver-ruthenium bimetallic particles.

[0017] The valuable document can be any type of valuable document, for example a security document, a banknote or an identity document, and is preferably a banknote or an identity document, and especially preferably a banknote.

[0018] The security document comprises a substrate with optionally two opposing layers applied to it. Any substrate suitable for security documents can be used. These substrates are known to those skilled in the art and include, for example, paper substrates, substrates based on cotton fibers or mixed fibers, where mixed fibers may include, in particular, fibers of cotton, flax, linen, cellulose, and plastics, substrates based on plastic films, impregnated and pre-coated substrates. Hybrid substrates can also be used, such as paper-plastic substrates or multilayer substrates, for example, film / paper / film substrates, as known from WO 2004 / 028825 A2. The substrate can be impregnated with polymers of any kind and is preferably impregnated with polyvinyl alcohol.

[0019] The substrate optionally contains two layers applied opposite each other. In a preferred embodiment, the security document according to the invention comprises a substrate with two layers applied at least partially, and in particular completely, opposite each other.

[0020] The layers applied opposite the substrate are generally intended to protect the substrate, for example, by repelling dirt and / or moisture. However, they can also alternatively or additionally provide a substrate with improved adhesion for further layers, such as a printing layer. Accordingly, the layers applied opposite the substrate can be, for example, sizing layers and / or primer layers. Potential sizing layers and primer layers, as well as methods for applying them, are known to those skilled in the art. A sizing layer and / or primer layer can, for example, be formed by a physically drying coating. "Physically drying" means that drying occurs through evaporation and / or displacement of the solvents or dispersion agents into the substrate. The production of suitable coatings is described, for example, in EP 2634309 A1 and WO 2004 / 072378 A1.Water-based dispersion coatings are particularly preferred. For example, a sizing layer may contain polyvinyl alcohol and / or polyurethane. Examples of suitable primer compositions are those based on acrylates, polyester acrylates, urethane acrylates, polyester polyurethanes, and acrylonitrile styrene polyurethanes. Water-based dispersions, especially water-based dispersions of aliphatic components, are particularly preferred.

[0021] Furthermore, all coating compositions described herein may independently contain the usual additives, such as coalescing agents, leveling agents, wetting agents, defoaming agents, viscosity modifiers, dispersing agents, thinning agents, and crosslinking agents. Preferably, the compositions may also contain additives that are not visually detectable, such as luminescent agents.

[0022] The security document according to the invention further comprises at least one printed layer made of at least one printing ink, preferably several printing inks. A printed layer can be a layer applied to the entire surface or at least partially across the entire surface of the security document, made of a single printing ink or a mixture of printing inks, or alternatively, a printed pattern obtained by applying a specific printing ink. A printed layer can, of course, also contain several printed patterns made of different printing inks, applied side by side or (partially) on top of each other. Likewise, in addition to at least one printed pattern made of a single printing ink, a printed layer applied to the entire surface or at least partially across the entire surface of the security document can also be present. A printed layer made of more than one printing ink is preferred.The printing can be, for example, an intaglio print or a gravure print and can be applied, for example, directly to the substrate or to at least one, preferably both, of the two layers optionally applied opposite each other on the substrate or to any further layers of the security document.

[0023] According to the invention, it is further required that at least one of the printing inks on the security document contains silver-ruthenium bimetallic particles. Preferably, more than one printing ink contains silver-ruthenium bimetallic particles. For example, all or at least the majority of all printing inks used for printing the security document according to the invention can contain silver-ruthenium bimetallic particles.

[0024] In addition to at least one printing ink, the substrate and / or, if present, at least one of the two opposing layers, and / or, if present, at least one of the at least one further layer, may preferably also contain silver-ruthenium bimetallic particles. Since hydrogen peroxide and hydroxy radicals are also able to diffuse to a certain extent through the various layers and thus into the printing ink, hydrogen peroxide molecules and / or hydroxy radicals formed in the substrate and / or in at least one of the two opposing layers and / or in at least one of the at least one further layer can support the drying process of the printing ink. This is particularly helpful for supporting the deep drying process.

[0025] Silver-ruthenium bimetallic particles are characterized by their content of silver and ruthenium, with the silver and ruthenium preferentially in electrically conductive contact to achieve optimal catalytic activity. The bimetallic particles can consist entirely of silver and ruthenium, thus representing a compact or solid material. Nano- and micrometer-sized metal particles can be produced, for example, by milling, electrochemical, chemically reductive, capillary electrophoresis, hydrothermal synthesis, PVD, CVD, or sol-gel processes.

[0026] However, for cost reasons, it is preferred that the bimetallic particles have a layer of silver and ruthenium only on their surface, applied to a support material inside the bimetallic particles. This is entirely sufficient for catalytic activity and significantly reduces the amount of silver and ruthenium required for the bimetallic particles. Potential support materials include all conceivable materials, such as other metals; their oxides or other derivatives, metal alloys, metalloids and their compounds, nonmetals and their compounds, plastics, ceramics, or glasses. Preferably, the support material is an aluminum oxide, for example, boehmite, silicon dioxide, zirconium dioxide, titanium dioxide, steel, carbon, or cellulose. Depending on the size of the bimetallic particles and the type of support material, the support material accounts for the majority of the total weight of the bimetallic particles.For example, the proportion of carrier material in the bimetallic particles can be more than 50 wt.%, more than 70 wt.%, more than 80 wt.%, more than 90 wt.%, more than 95 wt.%, more than 98 wt.%, or even more than 99 wt.%. Preferably, the proportion of silver and ruthenium in the bimetallic particle is 1 to 20 wt.%, such as 2 to 18 wt.%, 3 to 17 wt.%, or 5 to 15 wt.%. All values ​​given in wt.% refer to the total weight of the bimetallic particles.

[0027] Preferably, a thin layer of silver or silver alloy is first applied to the bimetallic particles on the substrate material. This layer is preferably applied electroplated or deposited. Other coating processes, such as PVD, CVD, sputtering, sol, gel, and reduction processes, are also suitable plating methods. A ruthenium layer is then applied to the silver layer, the amount of ruthenium preferably being kept low relative to the amount of silver, for example, in an Ag / Ru ratio of 1 / 0.001 to 1 / 0.2 or from 1 / 0.005 to 1 / 0.1.The application of the ruthenium layer is preferably controlled such that the silver-containing surface is in contact with the environment, or can come into contact with the environment, through continuous, preferably finely formed, free areas, openings, pores, cracks, gaps, or the like in the ruthenium layer, thereby ensuring contact between the silver and the ruthenium. If the silver surface is covered with ruthenium clusters, the catalytic effect of the silver can be advantageously enhanced. Preferably, ruthenium is applied in a thickness in the nanometer range, with a maximum thickness of approximately 500 nm, particularly approximately 50 nm, and a minimum thickness of approximately 5-10 nm proving especially suitable.

[0028] The size of silver-ruthenium bimetallic particles is not inherently limited. However, in printing ink applications, particle sizes of no more than 5 µm (D90 value) have proven particularly suitable for ensuring a uniform appearance of the printed product. Particles with a D90 value greater than 5 µm, when used in the substrate or in subsequent layers, tend to cause a potentially undesirable graying of the substrate or subsequent layer. This may be acceptable in some applications, for example, if additional colors are present in the substrate or subsequent layer that mask the graying. At the same time, particle sizes should preferably not fall below 100 nm (D50 value), as otherwise the desired catalytic effect may not be sufficiently pronounced.The particle sizes of the bimetallic particles are therefore preferably in the range of 0.1 to 5 µm, preferably from 0.2 µm to 4 µm, such as from 0.3 to 3 µm, from 0.5 to 3 µm, or from 1 to 3 µm, where the lower values ​​represent D50 values ​​and the upper values ​​represent D90 values. In a particle size distribution, the D50 and D90 values ​​indicate the size below which 50% (D50 value) and 90% (D90 value) of the particles in the distribution are found. The D50 and D90 values ​​are known to those skilled in the art and can be determined by them without difficulty, for example, using appropriate instruments from Microtrac, Inc.

[0029] The surface area of ​​the silver-ruthenium bimetallic particles can range from 0.1 to 1000 m² / g, for example from 0.5 to 500 m² / g, from 1 to 100 m² / g, from 2 to 50 m² / g, from 5 to 30 m² / g, or from 10 to 20 m² / g.

[0030] Suitable silver-ruthenium bimetallic particles are commercially available, for example under the trade names AGXX or AGXX hybrid from Heraeus GmbH.

[0031] The printing ink used for printing the security document according to the invention is not limited as long as it contains the silver-ruthenium bimetallic particles described above.

[0032] Thus, in principle, any printing ink suitable for printing security documents can be used as a base, and silver-ruthenium bimetallic particles can be incorporated into this base to produce a printing ink suitable for the present invention. Suitable printing inks are known to those skilled in the art and are described, for example, in WO 2013178325 A2, EP 2888112 B1, DE 102012010534 A1, WO 2013178325 A2, EP 2888112 B1, WO 2018197039 A1, and EP 3660110 B1. It is understood that the printing ink should preferably be oxidatively drying or curing so that the catalytic effect of the silver-ruthenium particles can be fully realized. For example, the ink can even more preferably be an oxidatively drying offset ink or an oxidatively drying intaglio ink. Printing inks, especially oxidatively drying offset inks or oxidatively drying intaglio inks, based on vegetable oils (e.g.,Linseed oil, wood oil) or mineral oils, dissolved hydrocarbon resins and / or modified rosin resins, and / or alkyd resins, especially high molecular weight alkyd resins.

[0033] Silver-ruthenium bimetallic particles can be incorporated directly into the production of a printing ink or added to a previously manufactured ink at a later stage. Particularly in the latter case, the user must ensure thorough mixing of the printing ink and bimetallic particles. The silver-ruthenium bimetallic particles are typically present in the printing ink in amounts ranging from 0.1 to 10 wt.%, such as 0.2 to 8 wt.%, 0.3 to 7 wt.%, 0.5 to 5 wt.%, 0.7 to 4 wt.%, 0.8 to 3.5 wt.%, or 1 to 3 wt.%, for example, 2 wt.%, based on the dry weight of the bimetallic printing ink.

[0034] The security may further comprise at least one additional layer, which may, for example, be applied over the printing. It is particularly preferred that the at least one additional layer is a top coating. A top coating is the outermost layer of a security, i.e., the layer that comes into direct contact with the environment. A top coating typically provides protection against external influences such as moisture, grease, oil, solvents, and dirt, as well as against physical impacts. A top coating is preferably applied to both sides of the security and usually covers the entire surface.

[0035] In principle, the at least one additional layer can be any other layer commonly used in security document printing. In the case of a top coating, it is preferred that it be a cationic or radical UV-curing layer.

[0036] According to a second aspect, the present invention relates to a method for the catalytically accelerated curing of a printing ink on a security document, comprising the steps of providing a printing ink containing silver-ruthenium bimetallic particles and printing the printing ink onto a security document substrate. The security document, as well as all essential and optional components thereof, such as the substrate, the printing ink, and the silver-ruthenium bimetallic particles, can be configured as described above in connection with the security document according to the first aspect of the present invention. The printing of the printing ink onto a security document substrate can be carried out using any conventional printing method, for example, offset printing or intaglio printing.

[0037] According to a third aspect, the present invention relates to the use of silver-ruthenium bimetallic particles for curing printing inks on a security document. The security document, as well as all essential and optional components thereof, such as the substrate, the printing ink, and the silver-ruthenium bimetallic particles, can be configured as described above in connection with the security document according to the first aspect of the present invention.

Claims

1. Value document comprising - a substrate with optionally two layers applied opposite each other thereon, - at least one print from at least one printing ink, and - optionally at least one further layer, characterized in that at least one of the at least one printing ink contains silver-ruthenium bimetallic particles.

2. Value document according to claim 1, wherein in addition the substrate and / or, if present, at least one of the two layers applied opposite each other, and / or, if present, at least one of the at least one further layer contains silver-ruthenium bimetallic particles.

3. Value document according to claim 1 or 2, wherein the silver-ruthenium bimetallic particles have a D90 value for the particle size of at most 5 µm.

4. Value document according to one of claims 1-3, wherein the substrate is impregnated with polyvinyl alcohol.

5. Value document according to one of claims 1-4, wherein at least one further layer is present, and at least one of the at least one further layer is a cationic or radical UV-curing top coating.

6. Value document according to one of claims 1-5, wherein the valuable document is a banknote or an identity document.

7. Method for catalytically accelerated curing of a printing ink on a value document, comprising the steps of providing a printing ink containing silver-ruthenium bimetallic particles and printing the printing ink onto a value document substrate.

8. Method according to claim 7, wherein the printing ink is an oxidative drying ink, preferably an oxidative drying offset ink or oxidative drying intaglio ink.

9. Method according to claim 7 or 8, wherein the printing ink is curable by hydroxyl radicals.

10. Use of silver-ruthenium bimetallic particles for curing printing inks on a value document.