Security feature, printing ink, value document and value document system

Abstract

The invention relates to a security feature for securing a value document, wherein the safety feature is based on an ESR-active substance and a luminescent substance, wherein the ESR-active substance and the luminescent substance are either the same substance or different substances, wherein - the luminescent substance emits in the IR wavelength range, is based on a host lattice with garnet structure or perovskite structure and has rare earth metals with a rare earth metal content of ≥2.0 %; - the ESR-active substance has an ESR-active transition metal; - the ESR-active substance has an ESR signal with a g value of <1.9 or >2.1, is based on a transition-metal-containing host lattice with garnet structure or perovskite structure, has a transition metal content of the ESR-active transition metal of ≥5 % and contains rare earth metals, wherein the rare earth metal content is ≥2.0 % and <13.0 % in the event of a garnet structure, and ≥2.0 % and <18.0 % in the event of a perovskite structure.

Description

[0001]Security feature, printing ink, valuable document, and valuable document system The invention relates to a security feature for securing a valuable document, wherein the security feature contains an electron spin resonance (ESR)-active substance and a luminescent substance. The invention further relates to a valuable document provided with the security feature, in particular a banknote, and to a valuable document system. The invention also relates to a printing ink having the security feature for printing a valuable document. To secure and authenticate valuable documents, such as banknotes, bank cards, ID cards, or passports, product packaging, and other products to be secured, such as medications, these are usually provided with machine-detectable security features, such as materials with specific magnetic properties. For this purpose, ESR-active materials can be used, for example.ESR-active substances are substances that contain one or more unpaired electrons. In ESR-active substances, increased microwave absorption can be measured when a suitable magnetic field is applied at suitable wavelengths. In the following, the abbreviations ESR and EPR ("electron paramagnetic resonance") are used synonymously. In the following, ESR-active substances are also referred to as ESR substances. In an ESR measurement, the so-called g-value (or effective g-value) of a signal is typically determined, where g is also known as the gyromagnetic factor or Landé factor and has a value of approximately 2.0023 for an unaffected free electron. The acquisition and evaluation of g-values ​​of ESR signals is described in the state of the art (see: “Quantitative EPR”, Gareth R. Eaton, Sandra S. Eaton, David P. Barr, Ralph T.Weber; Springer-Verlag Wien New York, 2010; ISBN 978-3-211-92947-6). Depending on the electron's environment and other influencing factors (e.g., interaction with local or neighboring electron spins and nuclear spins, spin-orbit coupling, crystal field, etc.), the measured g-value can exhibit significant deviations from g = 2, allowing conclusions to be drawn about the nature of the spin-bearing center and its environment. In the context of this invention, "spin-bearing center" refers in particular to ESR-active transition metal cations with unpaired electrons, which generate the detected ESR signal; therefore, the synonymous term "spin-bearing transition metal center" is also used. While ESR-active substances in the prior art generally have complex ESR spectra with multiplets or several non-overlapping signals with several g-values, substances according to the invention show only a single broad main signal with a single g-value.This is advantageous because additional, smaller secondary signals or multiplets are not suitable for high-speed detection, as explained below. In particular, when measured on a commercially available X-band spectrometer, the broad main signal exhibits an ESR linewidth—i.e., the distance between the maximum and minimum of the signal—of at least 40 Gauss, preferably at least 100 Gauss. This makes detection less susceptible to interference from shifts in the measurement signal. The g-values ​​mentioned below also refer to a measurement in a commercially available X-band spectrometer. DE 102013016121 A1 describes agglomerates with a luminescent and a non-luminescent, but spectroscopically detectable, substance. Garnets and perovskites are mentioned as a matrix for a luminescent substance, and SrTiO3:Mn is mentioned as an example of an ESR-active substance.CN 102321403 A describes an ink containing magnetic resonance materials and inorganic luminescent materials. The described luminescent materials emit in the VIS range and are excited either in the UV or NIR range. Garnets, among others, are also mentioned as magnetic resonance materials. WO 99 / 52708 A1 describes a marking of liquids, gases, or solids with at least one set of particles that differ in physical, chemical, biological, magnetic (e.g., ESR), or spectral (e.g., fluorescence) properties. US 2005112768 A1 describes a method for authenticating a polymer substrate with a forensic marker that can be detected, for example, by ESR, and another marker that can be detected spectroscopically. The markers are incorporated directly into the polymer matrix.EP 1849915 A1 describes a polymer or paper document with two security features, one of which can have magnetic properties and the other fluorescent properties. The security features are applied in a complementary pattern in separate areas. WO 2011046749 A2 describes a rare-earth metal-containing luminescent substance based on garnets that has magnetic properties. The magnetic properties are changed by altering the composition, which influences the magnetic moment. Furthermore, luminescent substances for securing valuable documents with high contents of transition metals and a garnet or perovskite structure are known in the general state of the art, for example, based on doped yttrium-iron garnets or doped yttrium-chromium perovskites.However, such luminescent substances generally do not exhibit a single characteristic ESR signal with a g-value >2.1 or <1.9. The ESR-active substances described in the prior art are not suitable for high-speed detection, as will be explained below. However, it has surprisingly been found that, through a specific selection of the stoichiometry, suitable ESR-active substances according to the invention can be produced which, in combination with the luminescent substances according to the invention, result in a novel, extended security feature. Such security features, which have both ESR and luminescent properties, have new, advantageous properties. Based on the prior art, the present invention is based on the object of providing a security feature for protecting a valuable document that ensures increased security against forgery and, at the same time, has good incorporation into a valuable document.The invention is further based on the object of providing a valuable document provided with the above security feature, as well as a printing ink having the above security feature. This object is achieved by the feature combinations defined in the independent claims. Further developments of the invention are the subject of the dependent claims. Detailed description of the preferred embodiments The subject of the present invention is a security feature for securing valuable documents, wherein the security feature combines luminescent properties with detectability by means of electron spin resonance (ESR). The security feature according to the invention is preferably in powder form and is advantageously incorporated into a valuable document substrate, e.g., into the paper substrate or polymer substrate of a banknote. ESR-active substances are known.Luminescent substances and luminescent features based thereon are also known. According to the present invention, a combined ESR / luminescence security feature is used to secure valuable documents, whereby the combination results in the exercise of advantageous synergistic effects. The functionality of detectability by means of ESR, on the one hand, and the functionality of the luminescent property, on the other hand, complement each other in many advantageous ways when using substances according to the invention. Furthermore, the simultaneous use of ESR and luminescence properties of the substances according to the invention combines two different technologies, creating an expanded authentication space. This increases forgery security and also creates new security options.For example, an ESR-active security feature is not impaired by optical contamination on the value document and is thus resistant to interference from IR-absorbing elements, overprints, and dirt. Even in the event of fire damage to the value document, an ESR-active security feature offers advantages over a luminescent security feature, as the signal is still measurable even if the value document is completely or partially blackened. In combination with the luminescence of the security feature, this allows the extent of the impairment or damage to be determined. Furthermore, a so-called fitness assessment of the banknote can be carried out by means of a relative comparison of the intensities of the ESR signals with the intensities of the luminescence signals. This means that, for example, the degree of contamination of the banknote caused by aging and wear can be quantified.The fitness assessment is an example of a new, expanded functionality that is only created by the inventive combination of ESR and luminescence properties. Due to their outstanding performance, optical security features are often used to secure valuable documents. Optical detectors and sensors for their evaluation are well known. ESR security features, on the other hand, are rarely used due to their complex detection. In this way, increased security is achieved with the inventive combined security features because the ESR component would not be detected during the analysis of a banknote using known optical methods, e.g., using a luminescence spectrometer. Optical security features have the advantage that they can be used to very easily generate a large number of securely separable codes. Even minor differences between individual codes, such as, for example,A spectral shift or a change in the intensity of individual emission bands can be reliably separated from one another in very short periods of time. Therefore, luminescent substances are still very easily detectable even during high-speed processing of banknotes, where only very short periods in the millisecond range are available for measurement. Recording a high-resolution ESR spectrum is not possible during these periods. The ESR codes must therefore differ very significantly from one another to ensure reliable separation. It is therefore technically challenging to produce suitable ESR-active substances that can be reliably separated in high-speed detection. Therefore, significantly fewer different, separable high-speed ESR components are usually available. By combining ESR and luminescence properties, a variety of codes is advantageously enabled.Furthermore, both properties, namely detectability via ESR on the one hand and luminescence on the other, can thus be read at different security levels. For example, the central bank may be able to evaluate both properties, while commercial banks may only know and evaluate the ESR security feature or only the luminescent security feature. The security feature according to the invention comprises a luminescent substance and an ESR-active substance, whereby both substances independently have a garnet structure or a perovskite structure. This results in particular advantages, namely increased forgery security and the identical behavior of both substances when incorporated into a security document. In certain cases, the ESR-active substance and the luminescent substance can even be identical, i.e.It is a single substance that can act both as an ESR-active substance and as a luminescent substance. The security feature according to the invention comprises a luminescent substance and an ESR-active substance, both substances being based on an inorganic host lattice, wherein - the luminescent substance emits in the IR wavelength range, has a perovskite structure or a garnet structure, and contains a rare earth metal content of ≥ 2.0%; - the ESR-active substance has a garnet structure and contains a rare earth metal content of ≥ 2.0% and < 13.0%, or has a perovskite structure and contains a rare earth metal content of ≥ 2.0% and < 18.0%; - the ESR-active substance has a transition metal content of the ESR-active transition metal of ≥ 5%; and - the ESR-active substance has an ESR signal with a g-value <1.9 or >2.1.It is preferred that the luminescent substance and the ESR-active substance have the same structure, ie both substances each have a garnet structure or both substances each have a perovskite structure. Furthermore, it is preferred that the luminescent substance and the ESR-active substance are the same substance. The ESR-active substance preferably has an alkaline earth metal content ≥2.0%. Furthermore, it is preferred that both the ESR-active substance and the luminescent substance have a grain size D99 <20 µm. The designation “D99” refers to the grain size below which 99% of the particles of a grain size distribution fall. Preferably, the ESR-active substance and the luminescent substance have essentially the same grain size D99. According to a particularly preferred variant, the grain size D99 of the ESR-active substance and the grain size D99 of the luminescent substance differ from each other by less than 20%.The ESR-active substance preferably contains Fe, Cr or Mn as the ESR-active transition metal or ESR-active cation, with Fe being particularly preferred. Mixtures of these elements are also possible. The ESR-active substance preferably contains rare earth metals. These are the elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. They are preferably trivalent rare earth metal cations. The rare earth metal cations are particularly preferably selected from Y, La, Nd, Gd, Er, Ho, Tm, Yb and further preferably from Y, Gd, Yb and Er. The rare earth metal in the ESR-active substance is particularly preferably in the +3 oxidation state. Furthermore, it is preferred that the luminescent substance contains Nd, Yb, Er, Ho or Tm as the luminescence-emitting rare earth metal. Mixtures of these elements are also possible.According to a particularly preferred variant, the luminescent substance and the ESR-active substance are the same substance, whereby upon suitable optical excitation of an absorption band of the ESR-active transition metal or ESR-active cation, an energy transfer takes place between the transition metal and the luminescence-emitting rare earth metal, so that the luminescence-emitting rare earth metal is excited to luminescence emission. Furthermore, more complex or multi-stage energy transfers are possible, for example, from the transition metal to a first rare earth metal and from the first rare earth metal to a second rare earth metal, etc. Camouflage systems are known; see, for example, WO 2013 / 091860 A1, WO 2013 / 091842 A1, and WO 2013 / 091859 A1. Here, the exact composition of the substances of the security feature is concealed by the addition of further substances.Surprisingly, it has been found that the ESR and luminescent substances according to the invention have advantageous properties with regard to mutual camouflage due to their inventive stoichiometries and comparable structures. The ESR-active substance and the luminescent substance can be chemically similar, i.e., they can, for example, contain the same element or several identical elements. For example, the ESR-active substance can contain yttrium and the luminescent substance can also contain yttrium. According to an alternative, the ESR-active substance and the luminescent substance can contain one element or several elements that can be incorporated into the structure of the respective other substance in the form of a homologous mixed series. For example, the ESR-active substance contains gadolinium and the luminescent substance contains yttrium. In this way, the respective element ratio, i.e.the respective elemental composition of the ESR-active substance and the luminescent substance is concealed in the case of an elemental analysis. Using the just described consistent structure based on the same chemical elements or based on different chemical elements that can be incorporated into the crystal structure in the same way, this results in the advantage that both the ESR-active substance and the luminescent substance can be camouflaged by the same camouflage system or camouflage each other. Furthermore, the targeted selection of rare earth garnet structures and perovskite structures results in the ESR-active substance and the luminescent substance behaving in a comparable manner when, for example, they are incorporated into a security document substrate, so that no or at most a slight separation of the individual components occurs (e.g.However, it is challenging to find systems that are, on the one hand, similar enough to each other and, on the other hand, exhibit suitable properties to act as efficient, ESR-active substances or luminescent substances. This means that these are substances that, even in small quantities, exhibit a sufficiently strong and characteristic ESR signal or luminescence signal to be suitable as a security feature. It has been shown that garnet structures and perovskite structures containing rare earth metals can solve this technical problem.Both structures can incorporate a high proportion of various rare earth metals and can therefore be specifically tailored in terms of their chemical structure and physical properties to utilize the advantages of mutual camouflage and prevent local separation into individual substances. At the same time, perovskite structures containing rare earth metals and garnet structures can be used to produce efficient ESR-active substances and luminescent substances. This eliminates the need to combine the ESR-active substances and the luminescent substances, for example, through agglomeration; instead, they can be mixed and used as separate pigment particles. This is technically less complex and allows for a smaller grain size compared to agglomerates.The luminescent substance used according to the invention is characterized in that it emits in the IR wavelength range. The luminescence emission is preferably in a range from 700 nm to 2500 nm, particularly preferably in a range from 800 nm to 2200 nm. It is preferred that the luminescent substance has essentially no luminescence emission in the visible wavelength range, i.e. < 5% of the total emission. This results in the advantage that the luminescent substance on the value document is not visible to the naked eye during excitation and detection. The luminescent substance preferably has an excitation band in a range from 700 nm to 1000 nm, so that the excitation of the luminescence can also be carried out invisibly. Furthermore, it is preferred that the luminescent substance shows no visible upconversion.To promote the homogeneous incorporation of the ESR-active substance and the luminescent substance into the value document substrate, other parameters such as grain size, material density, or surface charge can be adjusted. Preferably, the ESR-active substance and the luminescent substance have the same structure, i.e., they are isostructural, which allows for a greater degree of agreement in processing properties. According to a special preferred variant, the ESR-active substance and the luminescent substance are identical. This greatly increases counterfeit security because any change in the stoichiometry of the host lattice or matrix influences both aspects, i.e., the luminescence properties and the ESR properties.If the exact composition of the substance is unknown, any attempt to adjust the ESR properties would change the luminescence properties, and any attempt to adjust the luminescence properties would change the ESR properties. Furthermore, such a security feature is harder to find and thus better protected because, due to its multifunctionality, less material can be used overall. Furthermore, no technical difficulties arise that are due to different substance behavior. According to a further preferred variant, the ESR-active substance and the luminescent substance are different. This allows for a higher number of codings, as there are fewer limitations in the selection and modification of the respective ESR-active substances and luminescent substances. Furthermore, the properties of the substances (e.g.Signal intensity) can be optimized more easily, since there are no mutual dependencies between ESR properties and luminescence properties. In the case of a single luminescent, ESR-active substance, it is even more challenging to find a substance that can generate luminescence properties and ESR properties simultaneously and in high quality. This is because, for example, the transition metal content, which is of crucial importance for the ESR properties, can have a negative effect on the luminescence properties, particularly through luminescence quenching. Here, too, the solution to the technical problem is provided by a suitable selection of the inventive stoichiometries of rare-earth metal-containing perovskite structures and garnet structures. It is particularly preferred that an energy transfer takes place between the ESR-active transition metal and the luminescence-emitting rare-earth metal cation.This means that to detect the luminescence of the security feature, the rare earth metal cation is not directly excited, but rather the ESR-active transition metal is excited, and the energy is subsequently transferred to the luminescent rare earth metal cation. In this way, both system components are strongly coupled. Consequently, a change in the ESR properties without generating a change in the luminescence properties is made more difficult because both systems interact with each other. The same applies in reverse, i.e., a change in the luminescence properties without generating a change in the ESR properties is made more difficult because both systems interact with each other. All content information cited herein is in atomic percentages, not weight percentages, i.e., the fictitious substance A7XO. 12has a content of 5% X based on the molecular formula. It is expedient that the transition metal content of the ESR-active element is at least 5% to ensure sufficient efficiency for high-speed detection, e.g., with processing speeds of 10 m / s. The transition metal content is preferably at least 10%, particularly preferably at least 15%. The transition metal content and its limits mentioned here refer only to the ESR-active transition metal or ESR-active cation. Other, non-ESR-active transition metals, which may also be present in the substance, are only taken into account in the calculation for the total number of atoms, but not for the transition metal content. ESR-active transition metals are characterized by the fact that they have unpaired electrons and are therefore measurable by ESR spectroscopy.The ESR-active transition metals are preferably Fe, Cr, Mn, and Cu, particularly preferably Fe and Cr, because Fe and Cr can be incorporated particularly well in high concentrations into garnet and perovskite structures and are then particularly well suited for energy transfer from the transition metal to the rare earth metal emitter. Fe and Cr preferably have an oxidation state of +3, and Mn and Cu have an oxidation state of +2. The ESR-active transition metal is particularly preferably Fe. 3+ , which due to its d 5-electron configuration can generate particularly high spin densities. Garnet structures containing rare earth metals and perovskite structures with the high contents of ESR-active transition metals according to the invention exhibit only a single, strongly broadened ESR signal. In particular, they do not exhibit a pattern of several separate narrowband ESR signals, as is typical for analogous materials with a low content of the ESR-active transition metal below 5%. Narrowband ESR signals and multiband ESR signals (i.e., multiplets) are disadvantageous for high-speed detection. However, the strongly broadened individual signals at a content above 5% are advantageous because they can still be measured stably even at high processing speeds. For garnet structures A3B5O 12and perovskite structures ABO3, which have exclusively rare earth metals in the A positions, this greatly broadened ESR signal is typically at g = 2.0. This is disadvantageous because, for example, luminescent and magnetic security features based on YIG (the abbreviation YIG stands for yttrium iron garnet or yttrium iron garnet) are known, which would interfere with the detection of an ESR-active substance with an ESR signal at g = 2.0. For the generation of codes based on the ESR signals, it is advantageous if ESR-active substances with different g values ​​can be produced. In particular, g values ​​that differ significantly from g = 2 (i.e., g > 2.1 or g < 1.9) are particularly suitable for use as security features. Preferred values ​​are g > 2.2 or g < 1.8.This can be achieved if the rare earth metals in the A-positions in the garnet and perovskite structures of the ESR-active material are partially replaced by other elements with different charge numbers. If only rare earth metals are used, the maximum rare earth metal content is 3 / 20 = 15% for garnet structures A3B5O12 and 1 / 5 = 20% for perovskite structures ABO3. It has been shown that for a sufficient change in the ESR signal, a maximum rare earth metal content of 13% for garnets and 18% for perovskites, or less, is advantageous. At the same time, however, a minimum amount of rare earth metal is also advantageous for the generation of significantly modified ESR signals. Furthermore, a minimum content of rare earth metals is necessary to ensure the compatibility of the ESR-active material and the luminescent material, e.g. B. to create the camouflage effects mentioned above or to adjust the physical properties.In particular, a minimum amount of rare earth metals is necessary if the ESR-active substance and the luminescent substance are the same substance. The rare earth metal content is therefore at least 2%, preferably at least 5%, particularly preferably at least 7.5%. To compensate for the occupancy of the missing rare earth metal sites, alkaline earth metals are preferably used, particularly preferably Ca and Sr. The alkaline earth metal content is preferably 2.0% or more, particularly preferably 5% or more. The alkaline earth metal content is preferably less than 18%. To balance the charge difference between the divalent alkaline earth metals and the trivalent rare earth metals, the elements Ti, Sn, Zr, Nb, and Ta are preferably used. The content of at least one of these elements is preferably at least 1.25%, particularly preferably at least 2.5%. The total content of the elements Ti, Sn, Zr, Nb, and Ta is preferably less than 15%.Further elements, preferably Al, can also be incorporated in order to modify, for example, parameters such as the position and shape of the ESR signal or the body color of the resulting material. According to a preferred embodiment, the ESR-active material contains at least 2.5% Al and less than 10% Al. This makes it possible to produce ESR-active materials that have advantageous properties, e.g., characteristic ESR signals combined with low body color. If the ESR-active material and the luminescent material are separate materials, then the luminescent material can be a pure rare earth metal garnet or rare earth metal perovskite. However, if the ESR-active material and the luminescent material are the same material, then the rare earth metal content, as described for ESR-active materials, must be a maximum of 13% for garnets and a maximum of 18% for perovskites. This allows, on the one hand, the adjustment of certain luminescence parameters (e.g.,Spectrum, decay time, intensity) is made more difficult, while on the other hand, as described, counterfeit security is increased. The luminescent substance preferably comprises the rare earth metals La, Y, or Gd as non-emitting rare earth metals. The luminescent substance preferably comprises the rare earth metals Nd, Yb, Er, Ho, or Tm as emitting rare earth metals. Simultaneous incorporation of mixtures of several different rare earth metals is possible. In addition to the rare earth metals mentioned, further rare earth metals may be present, e.g., as codopants for adjusting specific luminescence parameters such as excitability or decay time of the luminescent substance. The luminescent substance preferably has a decay time of more than 10 µs and less than 10 ms, particularly preferably 80 µs to 3 ms. Longer decay times have a detrimental effect on high-speed detection and shorter decay times are more difficult to detect fluorescent interference.to separate impurities. The mixture of the ESR-active substance and the luminescent substance on the one hand, as well as the individual luminescent ESR-active substance, which fulfills both functions simultaneously, on the other hand, are referred to as feature substances in the further description of application possibilities. For use as a security feature, the feature substance is preferably incorporated into the substrate of the value document, so that a fully detectable security feature is obtained. According to a first preferred embodiment, the substrate of the value document is paper, i.e., the feature substance is added to the paper stock, for example, during the production of the banknote substrate in a paper mill. According to a further preferred embodiment, the substrate of the value document is a polymer, i.e.The feature substance is added, for example, during film extrusion in order to incorporate it into a polymer body. However, the feature substance does not necessarily have to be contained in the value document substrate, but can also be applied to the value document as a coating, e.g., as a varnish, or be located in a partial or intermediate layer of the value document, for example, within the ink-accepting layer of polymer banknotes or within one of the adhesive layers in multi-layer hybrid banknotes (e.g., with a paper / film / paper multi-layer arrangement) or card bodies. Furthermore, the feature substance does not necessarily have to be applied over the entire surface of the value document, but can also be present only within certain partial areas, e.g., in the form of a barcode.According to a further preferred embodiment, the feature substance is added to a printing ink in order to generate full-surface or local areas with an ESR signal and a luminescence signal, as required. Furthermore, the feature substance can be integrated into any security elements, such as security threads, security strips, hologram foils, and security patches or security labels. In this form, it is possible to equip only certain partial areas of a value document with an ESR signal and a luminescence signal and to form an ESR luminescence security feature. According to a first preferred embodiment, the ESR luminescence security feature is applied over the entire surface. This advantageously secures the value document as a whole, and manipulation or forgery of partial areas is ruled out. According to a second preferred embodiment, the ESR luminescence security feature is present only on partial areas.Here, holistic security is dispensed with in order to advantageously obtain increased variability or additional coding options (example: barcodes suitable for denomination separation, in particular for distinguishing between different banknote values) or to advantageously secure only certain, particularly critical areas (example: the precise detectability of the removal or tampering with a security patch or security label arranged to secure a banknote). According to a particularly preferred embodiment, the ESR-active substance is a garnet structure with iron as the ESR-active transition metal, wherein the garnet structure simultaneously contains erbium and at least one alkaline earth metal. Other elements such as rare earth metals, transition metals and aluminum can also be included.In this way, ESR-active substances can be produced which have a characteristic ESR signal and are simultaneously efficient luminescent substances, and are therefore particularly suitable as a combined ESR and luminescent substance. Possible embodiments of security features according to the invention are described below by way of example: In a preferred embodiment, the luminescent substance has a garnet structure and the ESR-active substance has a garnet structure. In a further preferred embodiment, the luminescent substance has a perovskite structure and the ESR-active substance has a perovskite structure. In a further preferred embodiment, the luminescent substance has a perovskite structure and the ESR-active substance has a garnet structure. In a further preferred embodiment, the luminescent substance has a garnet structure and the ESR-active substance has a perovskite structure.In a preferred embodiment, the ESR and luminescent substances are the same substance, which has a garnet structure. In a preferred embodiment, the ESR and luminescent substances are the same substance, which has a perovskite structure. The ESR-active substances according to the invention are also particularly well suited for securing value document systems. For example, one value document class (e.g., a first denomination of a banknote currency) contains a first ESR-active substance and a first luminescent substance. A second value document class (e.g., a second denomination of a banknote currency) contains the same first ESR-active substance and a second luminescent substance, so that the denominations can be differentiated from one another based on their luminescence. This can be continued analogously for value document systems with further value document classes.Furthermore, the security of the value document system can also be achieved by having a first value document class contain a first ESR-active substance and a first luminescent substance, and a second value document class have a second ESR-active substance and the first luminescent substance, whereby the different value document classes can be authenticated based on the different ESR-active substances. Embodiments Embodiment 1: The luminescent substance used is an Nd-doped yttrium aluminum garnet with the formula Y. 2,95 Nd 0,05 Al5O 12with a grain size D99 in a range of 9 µm to 10 µm. The rare earth metal content of the luminescent material is 15%. To prepare the luminescent material, 5.585 g of Y2O3, 0.141 g of Nd2O3, and 4.274 g of AlO2 are thoroughly mixed in an agate mortar. 10 g of Na2SO4 are added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and calcined at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. Wet grinding in a ball mill produces a grain size D99 in a range of 9 µm to 10 µm. Alternatively, the grain size can be adjusted by dry grinding with a jet mill. When the neodymium is excited with excitation light of 488 nm, the luminescent substance emits a characteristic emission spectrum in the range 850–1100 nm with an emission maximum at 1064 nm.The ESR-active substance is a garnet structure with the formula Ca. 0,5 Y 2,5 Fe 4,5 Sn 0,5 O 12with a grain size of D99 in a range of 9 µm to 10 µm. The rare earth metal content of the ESR-active substance is 12.5%. The transition metal content of the ESR-active transition metal Fe in the ESR-active substance is 22.5%. The alkaline earth metal content of the ESR-active substance is 2.5%. To prepare the ESR-active substance, 0.652 g of CaCO3, 3.680 g of Y2O3, 4.684 g of Fe2O3, and 0.982 g of SnO2 are thoroughly mixed in an agate mortar. 10 g of Na2SO4 are added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and annealed at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. Wet grinding in a ball mill produces a D99 grain size in a range of 9 µm to 10 µm. Alternatively, the grain size can be adjusted by dry grinding with a jet mill.The ESR-active substance exhibits a characteristic ESR spectrum with an asymmetric shape and a g-value of 2.22. A mixture of the luminescent substance and the ESR-active substance, e.g., in a weight ratio of 1:1, exhibits advantages according to the invention, such as minimal separation during metering in a paper machine and mutual camouflage by increasing the analytically detected plausible element combinations. Embodiment 2: A Yb-doped lutetium aluminum perovskite (Lu) is used as the luminescent substance. 0,95 Yb 0,05AlO3 with a grain size D99 in a range of 5 µm to 6 µm is used. The rare earth metal content of the luminescent material is 20%. To prepare the luminescent material, 7.565 g of Lu2O3, 0.394 g of Yb2O3, and 2.040 g of AlO2 are thoroughly mixed in an agate mortar and pestle. 10 g of Na2SO4 are added as a flux and mixed in the agate mortar and pestle. The mixture is transferred to a corundum crucible and annealed at 1000 °C for 24 hours. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. Wet grinding in a ball mill produces a grain size D99 in a range of 5 µm to 6 µm. Alternatively, the grain size can be adjusted by dry grinding with a jet mill. When the ytterbium is excited with excitation light of 940 nm, the luminescent substance emits in the range of 960–1040 nm.A perovskite structure with the formula CaEr3Fe3.5Nb0.5O12 with a grain size D99 in a range of 5 µm to 6 µm is used as the ESR-active material. The rare earth metal content of the ESR-active material is 15%. The transition metal content of the ESR-active transition metal Fe in the ESR-active material is 17.5%. The alkaline earth metal content of the ESR-active material is 5%. To prepare the ESR-active material, 1.315 g of CaCO3, 7.539 g of Er2O3, 3.672 g of Fe2O3, and 0.873 g of Nb2O5 are thoroughly mixed in an agate mortar. 10 g of Na2SO4 are added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and calcined at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. Wet grinding in a ball mill produces a D99 grain size in a range of 5 µm to 6 µm.Alternatively, the grain size can be adjusted by dry grinding with a jet mill. The ESR-active substance exhibits a characteristic ESR spectrum with a g-value of 1.68. The ESR-active substance exhibits essentially no Er luminescence upon matrix excitation or upon excitation of the typical Er excitation lines. A mixture of the luminescent substance and the ESR-active substance, e.g., in a weight ratio of 1:1, exhibits inventive advantages such as low separation during metering in a paper machine and mutual camouflage by increasing the analytically detected plausible element combinations. Furthermore, the substance mixture can be added, for example, to a printing ink that is printed on valuable documents, e.g., banknotes, for security purposes. The substance mixture exhibits inventive advantages in the printing ink, such as low separation.Example 3: The luminescent substance used is an Nd-doped lutetium aluminum garnet of the formula Lu. 2,9 Nd 0,1 Al5O 12with a grain size D99 in a range of 5 µm to 6 µm. The rare earth metal content of the luminescent material is 15%. To prepare the luminescent material, 6.798 g of Lu2O3, 0.198 g of Nd2O3, and 3.003 g of AlO2 are thoroughly mixed in an agate mortar and pestle. 10 g of Na2SO4 are added as a flux and mixed in the agate mortar and pestle. The mixture is transferred to a corundum crucible and calcined at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. Wet grinding in a ball mill produces a grain size D99 in a range of 5 µm to 6 µm. Alternatively, the grain size can be adjusted by dry grinding with a jet mill. When the neodymium is excited with excitation light of 488 nm, the luminescent substance emits a characteristic emission spectrum in the range 850–1100 nm with an emission maximum at 1064 nm.The ESR-active material used is a perovskite structure with the formula CaEr3Fe. 3,5 Nb 0,5 O 12with a grain size of D99 in a range of 5 µm to 6 µm. The rare earth metal content of the ESR-active substance is 15%. The transition metal content of the ESR-active transition metal Fe in the ESR-active substance is 17.5%. The alkaline earth metal content of the ESR-active substance is 5%. To prepare the ESR-active substance, 1.315 g of CaCO3, 7.539 g of Er2O3, 3.672 g of Fe2O3, and 0.873 g of Nb2O5 are thoroughly mixed in an agate mortar. 10 g of Na2SO4 are added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and annealed at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. A D99 grain size in a range of 5 µm to 6 µm is produced by wet grinding in a ball mill. Alternatively, the grain size can be adjusted by dry grinding with a jet mill.The ESR-active substance exhibits a characteristic ESR spectrum with a g-value of 1.68. The ESR-active substance exhibits essentially no Er luminescence upon matrix excitation or upon excitation of the typical Er excitation lines. A mixture of the luminescent substance and the ESR-active substance, e.g., in a weight ratio of 1:1, exhibits inventive advantages such as low separation during metering in a paper machine and mutual camouflage by increasing the analytically detected plausible element combinations. Furthermore, the substance mixture can be added, for example, to a printing ink that is printed on valuable documents, e.g., banknotes, for security purposes. The substance mixture exhibits inventive advantages in the printing ink, such as low separation. Embodiment 4 A garnet structure of the formula Ca2ErSn2Fe3O12 with a grain size D99 in a range of 9 µm to 10 µm is used as luminescent substance and ESR-active substance.The rare earth metal content of the material is 5%. The transition metal content of the ESR-active transition metal Fe in the material is 15%. The alkaline earth metal content of the ESR-active material is 10%. To prepare the ESR-active material, 2.147 g of CaCO3, 2.051 g of Er2O3, 2.569 g of Fe2O3, and 3.233 g of SnO2 are thoroughly mixed in an agate mortar. 10 g of Na2SO4 are added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and annealed at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. Wet grinding in a ball mill produces a D99 grain size in a range of 9 µm to 10 µm. Alternatively, the grain size can be adjusted by dry grinding with a jet mill.Upon excitation of the Fe absorption band with visible light, for example, in the wavelength range from 450 nm to 500 nm, the substance emits a characteristic emission spectrum with several emission bands in the range from 1450 nm to 1600 nm after energy transfer to erbium. Furthermore, the substance exhibits a characteristic ESR spectrum with a g-value of 1.75. Since the luminescent substance and the ESR-active substance are the same substance, separation is not possible when incorporated into paper, polymers, printing inks, etc. Furthermore, safety is increased because deviations from the matrix stoichiometry used affect both the luminescence properties and the ESR properties, and these parameters therefore cannot be adjusted independently of each other.Comparative Example 1: Luminescent perovskite structures and garnet structures with ESR-active transition metals are known. For example, working examples 1 to 3 of document EP 0975468 B1 describe the materials Y2.8Tm0.2Cr1.2Al3.8O12, Y0.9Tm0.1CrO3, and Gd2.9Tm0.1Fe3.5Al1.5O12. However, these materials have an excessively high rare earth metal content of 15% in the case of the garnet structures and 20% in the case of the perovskite structure, and they do not contain any other elements such as alkaline earth metals. As a result, none of these materials is suitable as an ESR-active material according to the invention because no single characteristic ESR signal with a g-value >2.1 or <1.9 is generated, as required for advantageous high-speed detection of ESR-active materials.Comparative Example 2: Perovskite structures and garnet structures with alkaline earth metals and ESR-active transition metals are known. For example, US Pat. No. 4,376,264 A describes the use of SrTiO3 doped with 1000 ppm Cr or Mn as an ESR feature for banknotes. The transition metal content of ESR-active transition metals there, at 0.1%, is significantly lower than the >5% required for high-speed detection. Accordingly, such materials produce a multiplet of several fine lines of low intensity, which is unsuitable for high-speed detection. Even if the transition metal content were increased by a factor of more than 50, such a material would not be suitable as an ESR-active material according to the invention because no rare earth metals are present in the structure in suitable quantities.However, these are necessary to generate characteristic ESR signals with a g-value >2.1 or <1.9.

Claims

P a t e n t a n s p r ü c h e 1.A security feature for securing a valuable document, wherein the security feature is based on an ESR-active substance and a luminescent substance, wherein the ESR-active substance and the luminescent substance are either the same substance or different substances, wherein - the luminescent substance emits in the IR wavelength range, is based on a host lattice with a garnet structure or perovskite structure and contains rare earth metals with a rare earth metal content ≥2.0%; - the ESR-active substance contains an ESR-active transition metal; - the ESR-active substance has an ESR signal with a g-value <1.9 or >2.1, is based on a transition metal-containing host lattice with a garnet structure or perovskite structure, has a transition metal content of the ESR-active transition metal ≥5%, and contains rare earth metals, where the rare earth metal content is ≥2.0% and <13.0% in the case of a garnet structure and ≥2.0% and <18.0% in the case of a perovskite structure. 2.The security feature according to claim 1, wherein the ESR-active substance and the luminescent substance are the same substance.

3. The security feature according to claim 2, wherein, upon suitable optical excitation of an absorption band of the ESR-active transition metal or ESR-active cation, an energy transfer takes place between the transition metal and the luminescence-emitting rare earth metal, so that the rare earth metal is excited to emit luminescence.

4. The security feature according to claim 1, wherein the ESR-active substance and the luminescent substance are different substances but have the same structure, namely either a garnet structure or a perovskite structure.

5. The security feature according to any one of claims 1 to 4, wherein the ESR-active substance has an alkaline earth metal content ≥2.0%.

6. The security feature according to any one of claims 1 to 5, wherein both the ESR-active substance and the luminescent substance have a grain size D99 <20 µm.

7. The security feature according to any one of claims 1 to 6, wherein the grain size D99 of the ESR-active substance and the grain size D99 of the luminescent substance differ from one another by less than 20%.

8. A security feature according to any one of claims 1 to 7, wherein the ESR-active substance contains Fe, Cr, Mn, or mixtures thereof as ESR-active transition metal or ESR-active cation, with Fe being preferred. 9.The security feature according to any one of claims 1 to 8, wherein the luminescent substance contains Nd, Yb, Er, Ho, Tm, or mixtures thereof as the luminescence-emitting rare earth metal.

10. The security feature according to any one of claims 1 to 9, wherein the ESR-active substance exhibits only a single broad main peak with a single g-value.

11. A value document with a security feature according to one of claims 1 to 10.

12. A value document according to claim 11, wherein the security feature is incorporated into the substrate of the value document.

13. A value document according to claim 11, wherein the security feature is printed on the value document in the form of a printing ink.

14. A value document according to one of claims 11 to 13, wherein the value document is a banknote.

15. A printing ink comprising the security feature according to one of claims 1 to 10. 16.A value document system comprising at least a first value document class, in particular a banknote type according to a first denomination, and a second value document class, in particular a banknote type according to a second denomination, wherein the first value document class consists of first value documents according to one of claims 11 to 14 and the second value document class consists of second value documents according to one of claims 11 to 14, wherein the first value documents each have a first ESR-active substance and a first luminescent substance and the second value documents each have the first ESR-active substance and a second luminescent substance, so that the different value document classes can be distinguished from one another on the basis of the luminescence.

17. A value document system comprising at least a first value document class, in particular a banknote type according to a first denomination, and a second value document class, in particular a banknote type according to a second denomination, wherein the first value document class consists of first value documents according to one of claims 11 to 14 and the second value document class consists of second value documents according to one of claims 11 to 14, wherein the first value documents each have a first ESR-active substance and a first luminescent substance and the second value documents each have a second ESR-active substance and the first luminescent substance, so that the different value document classes can be distinguished from one another based on the ESR signal.