Phosphorescent pigment material, methods for its production and its use

Abstract

The invention relates to an inorganic, phosphorescent pigment material comprising a compound of the general formula EA 1-x Ln x CuSi4O 10 , where EA means one or more elements selected from the group consisting of calcium (Ca), strontium (Sr) and barium (Ba) and Ln means one or more elements selected from the group consisting of erbium (Er), thulium (Tm) and holmium (Ho), for 0 < x ≤ 0.9.

Description

[0001] The invention relates to an inorganic, phosphorescent pigment material comprising a compound of the general formula EA 1-x Ln x CuSi4O 10 , where EA means one or more elements selected from the group consisting of calcium (Ca), strontium (Sr) and barium (Ba) and Ln means one or more elements selected from the group consisting of erbium (Er), thulium (Tm) and holmium (Ho), for 0 < x ≤ 0.9.

[0002] It is common practice to mark securities, banknotes, and other security-relevant documents with resistant substances to guarantee their authenticity and ensure high resistance to external influences. Besides marking banknotes, it is also known to mark, for example, high-quality synthetic or natural fibers (cotton) to verify the authenticity of garments made from them. Generally, there are numerous product markings using such substances to establish product authenticity, where not only trademark law but also product liability aspects play a role. A very wide variety of products, from canned beverages to brake pads or safety-relevant components in machinery, are marked for authenticity.Substances used for authenticity marking to protect products must have a long lifespan and remain effective even under demanding environmental conditions. In particular, they must withstand chemical and mechanical stresses such as those encountered during washing in a standard household washing machine. Furthermore, high chemical resistance is required, for example, against biodegradation products from human skin secretions, tanning agents from leather, and reducing agents and plasticizers that may be present in plastic materials.

[0003] A large number of marking substances with such properties are already known. These are often characterized by specific optical signatures that include typical combinations of extinction behavior, fluorescence, and phosphorescence. The detectability and uniqueness of these signatures contribute significantly to counterfeit protection.

[0004] In HUANG, Y.; TANABE, S.: Forward and back energy transfer between Cu 2+ and Yb 3+ in Ca 1-x CuSi4O 10 :Y bx crystals. In: J. Appl.Phys., Vol. 112, 2012, Art. No. 093521, a calcium-copper silicate of the composition Ca 1-x CuSi4O 10 investigated, which partially involved Yb 3+ It is doped and exhibits near-infrared emission under visible excitation. Additional narrowband emission lines around 1000 nm are revealed, which correspond to 4f-4f transitions of Yb. 3+This is due to the fact that the focus is on the physical analysis of energy transfer between Cu. 2+ - and Yb 3+ -centers as well as temperature-dependent forward and reverse transfer processes. This work serves to provide a fundamental understanding of the luminescence mechanisms and the temporal decay behavior. A targeted design of the doping to generate diverse, distinguishable optical signatures for security applications is not described.

[0005] European patent application EP 3 431 569 B1 describes a phosphorescent material based on CaCuSi4O. 10The technology disclosed reveals a compound that is broadbandly excitable by visible light and exhibits intense, broadband emission in the near-infrared range between approximately 800 and 1200 nm. The teaching focuses on optimizing the emission intensity through defined Ca / Si and Cu / Si ratios, as well as controlled crystal phases and XRD intensity ratios. The primary goal is broadband NIR emission as an alternative to narrowband rare-earth phosphors. Targeted lanthanide doping for spectral fine-tuning or for generating multiple distinguishable security signatures is not disclosed; rather, the aim is to achieve the most uniform and broad emission characteristic possible.

[0006] In JING, J. [et al.]: A comparative study on different RE-doped (RE=Pr, Nd, Sm) SrCuSi4O 10 Blue pigments with high near-infrared reflectance. In: Dyes Pigments, Vol. 150, 2018, pp. 9-15, strontium copper silicates of the general formula Sr 1-xRE x CuSi4O 10 The materials described in this publication are those in which strontium is partially replaced by rare-earth ions such as praseodymium, neodymium, or samarium. The materials disclosed therein exhibit a cuprorivaite-like crystal structure and are used as blue pigments with high near-infrared reflectance. The rare-earth doping leads to additional, narrowband absorption structures in the NIR range, which influence solar NIR reflection. Visible-excited emission or phosphorescence in the NIR range is not described.

[0007] In ZHANG, Y. [et al.]: Sol-gel synthesis and properties of europium:strontium copper silicates blue pigments with high near-infrared reflectance. In: Dyes Pigm., Vol. 131, 2016, pp. 154-159, comparable strontium-copper silicates are disclosed in which strontium is partially substituted by europium. Here, too, the focus is on increasing NIR reflection to provide so-called "cool pigments." The investigated materials exhibit high chemical and structural stability as well as an intense blue intrinsic color. The optical characterization is limited to absorption and reflection properties in the visible and near-infrared spectral ranges; active luminescence or the targeted generation of characteristic emission signatures is not disclosed.

[0008] The aim of the present invention is to provide further, previously unknown signatures for security marking. For this purpose, a special layered silicate is proposed which is characterized by excellent chemical and physical resistance as well as by characteristic optical properties and is therefore suitable for the permanent and reliable marking of security documents.

[0009] Egyptian blue is an inorganic pigment known since antiquity, used particularly in ancient Egypt for the coloring of art and religious objects. Its use can be traced back to the third millennium BC.

[0010] Chemically, Egyptian Blue is a crystalline calcium-copper silicate with the idealized molecular formula CaCuSi4O. 10The crystal structure corresponds to that of a layered silicate (phyllosilicate) and is composed of pseudo-hexagonal layers made up of linked SiO₄ tetrahedra. Within these layers, the silicon atoms are tetrahedrally surrounded by oxygen atoms. Copper(II) ions are coordinated in interlayers and contribute to the stabilization of the structure by forming bonds with the free oxygen atoms of the silicate layers. Calcium ions occupy additional positions between the layers and also contribute to structural integrity.

[0011] The natural mineral with this structure is called cuprorivaite. The specific arrangement of the silicate tetrahedra and the coordination of the copper ions are essential for the characteristic properties of Egyptian Blue, especially its intense blue color and its high thermal and chemical resistance.

[0012] More recently, it has been discovered that Egyptian Blue exhibits pronounced near-infrared (NIR) emission, particularly when excited in the visible spectrum. This NIR emission opens up new application possibilities for the material, such as in security marking and optical sensors. A report on near-infrared light emission is published as Ajo, (D.), Pozza, (G.), Chiari, (G.), Dc Zuane, (F.) and Favaro, (M.) - Photoluminescence of the Inorganic Pigments Egyptian Blue, Han Blue and Han Purple, Journal of Cultural Heritage, 1, 2000, 393-398.

[0013] Besides Egyptian blue, another related material is known as Han blue, which was used particularly during the Han dynasty in ancient China. Both materials, Egyptian blue and Han blue, are similar in their general structure as crystalline phyllosilicates with embedded copper(II) ions, but differ in essential chemical and structural details.

[0014] While Egyptian Blue has the ideal composition CaCuSi4O 10 Han blue exhibits the calcium ion being replaced by a barium ion, resulting in the molecular formula BaCuSi4O 10The larger ionic radius of the barium ion compared to the calcium ion results in structural differences: The silicate tetrahedral layers in Han blue are more distorted, and the distance between the layers is increased. These changes in the crystal structure affect both the stability and the optical properties of the material.

[0015] Optically, Han blue shows a slightly shifted absorption in the visible range compared to Egyptian blue, as well as differences in emission in the near-infrared range. While both materials are known for their NIR emission, the intensity and emission wavelength differ due to the different crystal field environments of the copper ions.

[0016] The targeted selection between calcium- and barium-based layered silicates thus opens up the possibility of generating specific signatures in the visible and infrared range by substituting structurally related systems and making these usable for security applications.

[0017] European patent EP 3 266 849 B1 discloses phosphorescent materials based on barium, strontium, or calcium copper silicates, which are excitable by visible light and exhibit intense emission in the near-infrared (NIR) range. The patent discloses phosphors comprising a defined mixture of an MCuSi₂O₆ crystal phase and an MCuSi₄O₆ crystal phase. 10The invention contains a crystal phase, the ratio of which is specifically adjusted to achieve a broad excitation bandwidth and high emission intensity. It is further described that the optional addition of SiO2 can influence light scattering and improve luminescence. The invention is aimed at applications in NIR spectroscopy, optical coherence tomography, solar cells, and security printing. A key focus is on exploiting the visible excitability and broad emission characteristics through targeted phase control and potential doping.

[0018] European patent EP 3 431 569 B1 discloses phosphorescent materials based on calcium copper silicates, in particular CaCuSi4O 10as the main crystal phase. Phosphorescent materials with specific molar ratios of calcium, copper, and silicon (0.15 ≤ Ca / Si < 0.25 and 0.13 ≤ Cu / Si < 0.25) are disclosed, wherein the chemical composition and phase purity are described by defined intensity ratios of characteristic XRD peaks. The phosphorescent material is excited by visible light over a broad wavelength range and emits broadband fluorescence in the near-infrared range (800–1200 nm) with high intensity. The invention described herein is directed in particular to applications in NIR spectroscopy, optical coherence tomography (OCT), photovoltaic systems, and security and authentication printing. The aim is to provide an alternative to conventional rare-earth-based NIR phosphors that enables broader excitation and emission.

[0019] One limitation of the materials described so far is that their spectroscopic properties are relatively rigid, which can make clear differentiation difficult or impossible, especially under higher security requirements. However, the material remains well-suited for applications where only simple detection of near-infrared emission is required, such as excitation with visible light and detection with an IR camera. For higher security requirements, however, a clearly distinguishable measurement property is needed to differentiate it from other materials.

[0020] The object of the invention is to provide an inorganic pigment material that exhibits high chemical and mechanical resistance, shows intense and characteristic emission in the near-infrared range under visible light excitation, and is particularly suitable for applications as security features due to a large number of distinguishable variants.

[0021] The problem according to the invention is solved by providing a phosphorescent pigment material comprising a compound of the general formula: EA1−xLnxCuSi4O10 where EA represents one or more elements selected from the group consisting of calcium (Ca), strontium (Sr), and barium (Ba), Ln represents one or more elements selected from the group consisting of erbium (Er), thulium (Tm), and holmium (Ho), and 0 < x ≤ 0.9. The material has a layered structure and, when excited with visible light in the range of 400 to 700 nm, exhibits near-infrared emission between 800 and 1100 nm, whereby both the spectral range and the spectral composition of the emissions change depending on the doping, thus enabling a variety of security signatures.

[0022] Further advantageous embodiments are specified in the dependent claims to claim 1. MANUFACTURING EXAMPLE

[0023] Generally, EACO3 (with EA = Ca, Sr, Ba individually or mixtures thereof) and Ln2O3 (with Ln = Yb, Er, Tm, Ho, individually or several) are mixed with CuO and SiO2 in a stoichiometric ratio (1:1:4) with the addition of lithium-containing fluxing agents such as LiCl or LiF as a dry powder. The rare earth elements replace some of the alkaline earth ions. The powder mixture is then annealed in a high-temperature step (800–1000 °C) and converted to the product.

[0024] Non-inventive example of an implementation: 0.9 CaCO3 + 0.05 Yb2O3 + 1 CuO + 4 SiO2 + 0.25 mass% LiF → Approx 0,9 Yb 0,1 CuSi4O 10 + 0.9 CO2 + 0.25 mass% LiF

[0025] The reaction equation shown represents the desired reaction. Under real-world conditions, however, the reaction proceeds partially incompletely and / or undesirable side phases are formed, such as various rare-earth silicates like Yb₂Si₂O₇ (or homologous compounds depending on the elements used). These side phases are neither the cause nor beneficial for the properties described in this patent. In terms of material properties, the side phases represent inactive material. The desired properties can also be achieved with very high doping levels; however, the increased presence of side phases progressively degrades the quality of the materials. Due to the presence of side phases, the actual doping level x cannot be precisely determined, which is why all specifications refer to the stoichiometry of the materials used.

[0026] After the reaction, the material is removed from the crucible and dry-ground into a powder. This is followed by wet milling to adjust the particle size to suit the specific application. Optionally, wet sieving can be performed at this stage to further refine the particle size distribution. The milled product is washed with water to remove the flux. It is then filtered and dried at 80 °C. Finally, a coarse dry sieving (100 µm) is performed to loosen the product. The particle size can be adjusted from 50 nm to 20 µm by milling and sieving.

[0027] The powder materials can then be used in various applications as security pigments for marking and authenticating high-security documents (banknotes, identity cards, etc.) or in the field of product protection.

[0028] For example, the powder can be dispersed in water, alcohols, or organic solvents, with known stabilizing agents being added. The dispersion is suitable as a coating or as an additive in the production of specialty papers for security documents, such as banknotes, checks, and securities.

[0029] Another example is adding the powder to a plastic masterbatch, which can then be used to mark larger batches of plastic.

[0030] It is also possible to incorporate the powder particles into cotton or plastic fibers in order to mark them and the textiles made from them. Exemplary embodiments: Example [1] (not according to the invention):

[0031] According to the stoichiometric ratio, 23.14 g CaCO3, 5.06 g Yb2O3, 61.74 g SiO2, and 20.44 g CuO are weighed out together with 0.25 g LiF as a melting agent and thoroughly mixed.

[0032] The initial mixture is placed in a corundum crucible and annealed in air at 1000 °C for 5 hours. The resulting annealed cake is then crushed in a mortar and sieved over a 1 mm sieve to loosen it. The resulting powder is wet-milled with 2 mm zirconium oxide beads and then washed with water to remove any remaining flux.

[0033] The powder is filtered and dried at 80 °C in a drying oven. Finally, it is sieved at 100 µm to loosen it.

[0034] Further embodiments were synthesized using an analogous procedure. The starting materials used and their stoichiometry, as well as the observed effects, can be found in the following table: EA 1-x Ln x CuSi4O 10 : [Example No.] Figure No. . EA Ln x Observed effect [1] Fig. 2 Approx Yb 0,1 Additional emission lines around 1,000 nm [2] Fig. 3 Approx Er 0,1 Additional emission lines around 1,550 nm [3] Fig. 4 Approx Tm 0,2 Additional emission lines at 1,700 nm - 1,900 nm [4] Fig. 4 Approx Ho 0,2 Additional emission lines at 1,900 nm - 2,050 nm [5] Fig. 3 Sr Er 0,1 Additional emission lines around 1,550 nm [6] Fig. 5 Fig. 6 Approx He, Tm,Ho Each 0.07 Additional emission lines at 1,550 nm - 2,050 nm [7] Fig. 5 Fig. 6 Approx Yb, He 0.1 each Additional emission lines at 1,550 nm ( Fig. 5) Additional emission lines at 1,000 nm ( Fig. 6) [8] Fig. 2 Approx Yb 0,4 Additional emission lines around 1,000 nm [9] Fig. 3 Ba Er 0,1 Additional emission lines around 1,550 nm

[10] Fig. 2 Sr Yb 0,1 Absorption lines around 1000 nm

[11] Fig. 2 Sr Yb 0,5 Absorption lines around 1000 nm

[0035] For comparison of the additional effects listed above, in Fig.1 the emission spectra of Egyptian Blue (calcium copper silicate, CaCuSi4O) 10 ) and Han Blue (barium copper silicate, CaCuSi4O 10 ) represented from the state of the art. REFERENCE MARK LIST

[0036] - no reference marks -

Claims

[1] Inorganic phosphorescent pigment material comprising a compound of the general formula EA 1-x Ln x CuSi4O 10 where EA means one or more elements selected from the group consisting of calcium (Ca), strontium (Sr) and barium (Ba), Ln means one or more elements selected from the group consisting of erbium (Er), thulium (Tm) and holmium (Ho), For 0 < x ≤ 0.9, the following applies: the material has a layered structure and When excited with visible light in the range of 400 to 700 nm, it shows emission in the near-infrared (NIR) range at a wavelength between 800 nm and 1100 nm, and in some cases in the range of 1550 nm to 2050 nm. [2] Pigment material according to claim 1, characterized by , that EA is exclusively calcium. [3] Pigment material according to one of claims 1 or 2, characterized by, that the material has an average particle size in the range of 50 nm to 20 µm. [4] Pigment material according to any one of claims 1 to 3, characterized by , that the material is produced by dry mixing of carbonates of the elements EA, oxides of the elements Ln, copper oxide and silicon dioxide, optionally with the addition of a lithium-containing melting agent, and is annealed at a temperature between 800 °C and 1000 °C. [5] Method for producing a pigment material according to any one of claims 1 to 4, comprising the steps: a) Mixing carbonates of the elements EA, oxides of the elements Ln, copper oxide and silicon dioxide in a stoichiometric ratio according to the formula EA 1-x Ln x CuSi4O 10 , b) Addition of a lithium-containing melting agent, c) Heating the powder at a temperature between 800 °C and 1000 °C, d) Grinding, e) Washing to remove the melting agent, and f) Drying and sieving the obtained product. [6] Inorganic phosphorescent pigment material obtainable by the method of claim 5. [7] Use of a pigment material according to one of claims 1 to 4 and 6 as a security feature in printing inks, inks, plastics, paper, films, fibers or coatings, wherein the pigment material produces a NIR emission when illuminated with visible light, the specific properties of which can be measured by NIR detectors. [8] Use according to claim 7, wherein the following properties are used to authenticate the feature: - Spectral composition of the emission, - Onset and decay behavior and / or - Variations in the excitation characteristics.

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