Magnetic particles with a highly reflective protective membrane and a manufacturing process therefor

Abstract

Magnetic particles with a highly reflective protective membrane (300), comprising: a magnetic core (100), a shell (200) formed on the outer surface of the magnetic core (100), having a multi-layer structure and comprising a first shell (210) made of a dielectric material, and a second shell (220) made of metallic material, formed on the outer surface of the first shell (210) and being a multi-layer membrane in which two or more layers (221, 222, 223) of mutually different metallic materials are stacked on top of each other, and a highly reflective protective membrane (300) which is in contact with the second shell (220) and is formed on the outer surface of the shell (200), wherein the highly reflective protective membrane (300) comprises a multi-layer structure comprising a low refractive index membrane (310) formed on the outer surface of the shell (200) and having an optical thickness between 0.15 to 0.35 of the wavelength, and a high refractive index membrane (320) formed on the outer surface of the low refractive index membrane (310) and having an optical thickness between 0.15 to 0.35 of the wavelength, the dielectric material is one or more materials selected from a group comprising titanium dioxide, aluminum oxide, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide and zinc sulfide, the metallic material is one or more materials selected from a group comprising copper, nickel, gold, platinum, silver, aluminum and chromium, the low refractive index membrane (310) consists of one or more materials selected from a group comprising silicon dioxide, magnesium fluoride, polystyrene, polymethyl methacrylate, polystyrene-co-butadiene, vanadium pentoxide, cadmium oxide, polyisobutylene and polyethylene, and the high refractive index membrane (320) consists of one or more materials selected from a group comprising titanium dioxide, aluminum oxide, zinc oxide, zirconium oxide and chromium oxide.

Description

Technical area

[0001] The present invention relates to a magnetic particle having a highly reflective protective membrane and a manufacturing method thereof, particularly wherein a highly reflective protective membrane is formed outside shell layers formed on the outer surface of a magnetic core so that the physical properties are improved. State of the art

[0002] Magnetic powders are primarily used for permanent magnets, which are designed to provide a flow of magnetic force; for a variety of magnetic sensors for detecting changes in the properties of a magnetic substance caused by changes in an external magnetic field, thus estimating changes in external physical quantities; and for storage devices such as hard disk drives. They are also used in industry for counterfeit protection, generally detecting changes in the properties of a magnetic substance, and for integration into a product.However, conventional magnetic powders have limited use in industries requiring bright colors, as the powder exhibits a dark gray or brown color and also has the disadvantage that the magnetic property alone is insufficient to achieve anti-counterfeiting effects. Accordingly, a bright-colored magnetic substance was developed using core-shell technology to solve this problem. This allows magnetic particles to exhibit the characteristics of a bright body color, namely, luminous and very bright, and thus allows magnetic particles to be used in a wide variety of products, such as various colored inks, commercial paints, granular pigments for vehicles, pigments in cosmetics, catalyst paints, anti-counterfeiting inks, and so on.

[0003] Accordingly, the market requires a magnetic substance that can have different colors and can be used in different products, so a lot of research has been done on technologies to mask the dark color of a magnetic substance.

[0004] As a prior art related to masking the dark color of a magnetic material, Patent Document 1 "WHITE POWDER AND PRODUCTION METHOD THEREOF" is known, in which a titanium dioxide membrane and a silver membrane are used to produce a light-colored magnetic substance.

[0005] However, in the case of the state-of-the-art process, scratches can occur on the silver membrane during the ink manufacturing process, significantly reducing the brightness of the magnetic substance. Such scratches are caused by the unique property of the silver membrane, known as ductility. The light-colored magnetic substance must be stable during a dispersion process to be used in multi-colored inks, commercial paints, granular pigments for vehicles, pigments in cosmetics, catalyst paints, and anti-counterfeiting inks.

[0006] However, the brightness of the magnetic substance will inevitably be reduced when a conventional process is used, so the light-colored magnetic substances with such a limitation can no longer be used in a product. Fig. Figure 1 shows photographs of a silver membrane which was damaged when a conventional light-colored magnetic substance was used in an ink manufacturing process, where Fig. 1(a) shows a photograph of a light-colored magnetic substance before the ink manufacturing process, while Fig. Figure 1(b) shows a photograph of a silver membrane damaged by friction of a filler and pressure between rollers during a dispersion step in an ink manufacturing process. When the silver membrane of the magnetic substance is damaged, the reflectivity decreases, thus reducing the brightness. To solve this problem, an approach is provided in which a protective membrane is formed on the outermost surface of the light-colored magnetic substance.

[0007] In addition, in a case where a protective membrane is formed to prevent the generation of scratches on the silver membrane, although the number of scratches is significantly reduced, at the same time there arises another disadvantage that the brightness of the magnetic substance is significantly reduced depending on the refractive indices and the thickness of the protective membrane.

[0008] DE 696 28 570 T2 describes a powder with multilayer films on its surface and a method for producing it. According to this method, a powder with metal oxide films on its surface is obtained, in which the films have an increased refractive index and therefore exhibit high reflectivity and a bright color, and which has a reduced number of metal oxide films on the surface and therefore has a small particle diameter. In particular, a powder with a multilayer film on its surface is described, comprising a base particle having on its surface a multilayer film comprising at least one metal oxide film formed by the hydrolysis of a metal alkoxide, and in which the multilayer film has been subjected to a heat treatment.Also described is a process for producing such a powder, comprising dispersing a base particle in a solution of a metal alkoxide, hydrolyzing the metal alkoxide to obtain a metal oxide, and depositing a film of the metal oxide on the surface of the base particle, performing the above steps two or more times to form a multilayer film, and performing a heat treatment at least in the last step, and in which the multilayer film is controlled to have an appropriate combination of components and appropriate film thicknesses in order to change the interference colors of the multilayer film and thus impart a light color to the powder.

[0009] DE 694 13 083 T2 describes a powder with a layer and a manufacturing method therefor. The powder comprises a metal or metal compound core on which at least one metal or metal oxide layer with a uniform thickness of 0.01 µm to 20 µm is located, wherein the metal of the metal or metal oxide layer is different from the metal forming the metal or metal compound core. Cited ReferencesPatent Document

[0010] Patent Document 1: KR 10 2006 0 028 393 A (published on March 29, 2006) RevelationTechnical Problem

[0011] The present invention is used to remedy the aforementioned deficiencies, wherein the object of the present invention is to protect a silver membrane from damage during an ink manufacturing process. Specifically, the object of the present invention is to counteract the friction with a filler and the pressure between the rollers during a dispersion step in the ink manufacturing process and to form a highly reflective protective membrane, so that the problem of attenuation of the brightness of a color can be solved when a low-reflective protective membrane is formed.Accordingly, a light-colored magnetic substance according to the present invention has a highly reflective protective membrane of a certain thickness formed as a thin membrane structure on the outermost surface of the substance, whereby the problem that the brightness of light colors is reduced can be solved. Technical solution

[0012] The object is achieved according to the invention by the magnetic particles according to claim 1. The highly reflective protective membrane with a multilayer structure is characterized in that it includes a low-refractive-index membrane formed on the outer surface of the shell, and a high-refractive-index membrane formed on the outer surface of the low-refractive-index membrane. Furthermore, it is characterized in that the low-refractive-index membrane is formed from one or more materials selected from a group consisting of silicon dioxide, magnesium fluoride, polystyrene, polymethyl methacrylate, polystyrene-co-butadiene, vanadium pentoxide, cadmium oxides, polyisobutylene, and polyethylene, while the high-refractive-index membrane is formed from one or more materials selected from a group consisting of titanium dioxide, aluminum oxide, zinc oxide, zirconium oxide, and chromium oxide.Furthermore, it is characteristic that the membrane with a low refractive index and the membrane with a high refractive index each have an optical thickness in the range between 0.15 and 0.35 of the wavelength.

[0013] Furthermore, the invention provides the manufacturing method for magnetic particles with a highly reflective protective membrane according to claim 3.

[0014] According to the present invention, the magnetic particles can be incorporated into various types of inks used in the anti-counterfeiting of common securities. Accordingly, the invention also provides the anti-counterfeiting ink for securities according to claim 4. Beneficial effects

[0015] According to the present invention, a magnetic particle has a protective membrane, which protects a silver membrane from damage during an ink manufacturing process, and also has a highly reflective structure, which prevents attenuation of brightness for a magnetic substance. Furthermore, the magnetic particle is protected by the protective membrane, thereby improving abrasion resistance, chemical resistance, and light resistance. Description of the drawings Fig. Figure 1 shows photos of an existing light-colored magnetic substance and a case study in which a silver membrane of a magnetic substance was damaged during an ink manufacturing process. Fig. Figure 2 shows a graph comparing the reflectivity between a light-colored magnetic substance and another light-colored magnetic substance containing a single layer of a low-reflective protective membrane. Fig. 3 shows a graph of the reflectivity of the protective membrane with a single-layer structure not according to the invention as a function of the optical thickness of the protective membrane. Fig. Figure 4 shows a transmission electron microscope (TEM) image of a protective membrane with a multilayer structure. Fig. Figure 5 shows a graph of the theoretical result for the reflectivity of a light-colored magnetic substance without a protective membrane, another light-colored magnetic substance with a low-reflective protective membrane, and yet another light-colored magnetic substance with a multilayer structure with a highly reflective protective membrane. Fig. 6 shows a magnetic particle not according to the invention in cross section. Fig. 7 shows a further magnetic particle not according to the invention in cross section. Fig. 8 shows yet another magnetic particle not according to the invention in cross section. Fig. 9 shows an example of a magnetic particle according to the invention in cross section. Fig. 10 shows yet another magnetic particle not according to the invention in cross section. Fig. 11 is a flowchart for a manufacturing method of magnetic particles according to the present invention. Fig. Figure 12 shows a graph of the reflectivity at 555 nm before and after a dispersion step for an ink. Fig. Figure 13 shows a graph of reflectivity at 555 nm before and after a chemical resistance test. <Liste der Bezugszeichen in den Zeichnungen> 100: magnetic core 200: Cover 300: highly reflective protective membrane 310: Low refractive index membrane 320: Membrane with high refractive index 210: Shell made of a dielectric material (first shell) 211 to 213: first to third dielectric membranes 220: Shell made of a metallic material (second shell) 221 to 223: first to third metallic membranes Embodiments of the invention

[0016] Magnetic particles with a highly reflective protective membrane and a manufacturing method for the membrane according to the present invention will be described in detail below with reference to the accompanying drawings. The drawings are provided as examples to help those skilled in the art understand the inventive concept. Furthermore, the drawings may contain exaggerated representations for the purpose of clarity of description. In addition, the same reference numerals refer to the same component, feature, or structure throughout the drawings and the detailed description.

[0017] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Furthermore, in the following description and the accompanying drawings, descriptions of well-known functions and arrangements that would unnecessarily obscure the essence of the invention are omitted.

[0018] Although a magnetic particle is shown as spherical in the attached drawings, its shape is not limited to a sphere and can also take a plate shape.

[0019] Fig. Figure 2 shows a graph comparing the reflectivity between a light-colored magnetic substance and another light-colored magnetic substance with a single layer of a low-reflective protective membrane. Referring to Fig. 2, the reflectivity of the light-colored magnetic substance with a low-reflective protective membrane made of TiO2 or SiO2 is lower than that of the light-colored magnetic substance without a protective membrane. As previously described, a low-reflective protective membrane prevents damage to the silver membrane and increases the service life of the magnetic substance. However, the reflectivity of the silver membrane is simultaneously reduced, making it necessary to form a highly reflective protective membrane, which can be a single-layer or multi-layer structure. The highly reflective protective membrane is designed such that a thin membrane forms an interference layer structure, thereby preventing the reflectivity of the silver membrane from decreasing. Fig. Figure 3 shows a graph showing the reflectivity of a protective membrane, which is designed in the form of a single-layer structure not according to the invention, as a function of the optical thickness of the protective membrane. Fig. 3, the reflectivity of the protective membrane with a single-layer structure drops sharply when the optical thickness is less than a quarter of the wavelength, whereas the reflectivity of the protective membrane is equal to or slightly higher than that of silver as soon as the optical thickness is less than half the wavelength. Thus, the protective membrane acts as a highly reflective protective membrane for an optical thickness in the range between 0.3 and 0.6 of the wavelength. The protective membrane with a single-layer structure not according to the invention acts as a highly reflective protective membrane for optical thicknesses between (0.42·N)-0.17 and (0.42·N)+0.17 of the wavelength (N is an integer), whereby the protective membrane becomes physically thicker when N is equal to or greater than 2, but with significantly less practical benefit.Accordingly, the protective membrane with a single-layer structure acts as a highly reflective protective membrane when N is 1, so that the optical thickness is between 0.3 and 0.6 of the wavelength.

[0020] Fig. Figure 4 shows a TEM image of a protective membrane with a multilayer structure and Fig. Figure 5 shows a graph of the theoretical reflectivity results for a light-colored magnetic substance without a protective membrane, another light-colored magnetic substance with a low-reflective protective membrane, and yet another light-colored magnetic substance with a multilayer structure of a highly reflective protective membrane. According to Fig. 5, the light-colored magnetic substance with a multilayer structure of a highly reflective protective membrane theoretically has a higher reflectivity than the light-colored magnetic substance without a protective membrane and the other light-colored magnetic substance with a low-reflective protective membrane. However, since the membrane has an irregular surface and the magnetic substance does not actually have a flat contour, the light-colored magnetic substance with a multilayer structure of a highly reflective protective membrane may have a lower reflectivity than the light-colored magnetic substance without a protective membrane.However, if the protective membrane has multiple layers of a highly reflective structure, the reflectivity is significantly improved compared to a protective membrane with a low-reflective structure, and the durability, chemical resistance and light resistance are also improved compared to a case study without a protective membrane.

[0021] Fig. 6 shows a non-inventive magnetic particle in cross section. With reference to Fig. 6, the magnetic particle comprises a magnetic core 100, a shell 200 on the outer surface of the magnetic core 100, and a highly reflective protective membrane 300 formed on the outer surface of the shell 200, wherein the highly reflective protective membrane 300 may be present as a single layer or in multiple layers.

[0022] The magnetic core 100 is responsible for the magnetic property of a magnetic substance according to the invention.

[0023] Furthermore, the shell 200 is formed of a metallic or dielectric material and may comprise a single layer or multiple layers, wherein the layers may comprise the same material or different materials from one another, wherein only a structure defined in claim 1 is according to the invention.

[0024] For example, Fig. 7 shows a case not according to the invention in which the shell comprises a metallic material, while in Fig. 8 shows a case example not according to the invention in which the shell is made of metallic and dielectric materials.

[0025] The shell 220 made of a metallic material in Fig. 7 can consist of a single layer (reference numeral 200 in Fig. 7(a)), consisting of two layers (reference numerals 221 and 222 in Fig. 7(b)), consisting of three layers (reference numerals 221, 222, and 223 in Fig. 7(c)) or more.

[0026] In the following, the non-inventive magnetic particle I is described with reference to Fig. 7 for the case where its casing 200 is a casing 220 made of a metallic material.

[0027] According to Fig. 7, the magnetic particle I comprises a shell 220 made of a metallic material, a magnetic core 100 and a highly reflective protective membrane 300 on the outer surface of the shell 220 made of the metallic material, wherein the highly reflective protective membrane 300 may have a single-layer or multi-layer structure.

[0028] The magnetic core 100 is responsible for the magnetic property of the magnetic particle I according to the invention.

[0029] Specifically, the magnetic core 100 of the magnetic particle I is a strong magnetic substance, wherein the magnetic core 100 preferably includes one or more materials selected from the group comprising iron, nickel, cobalt, iron oxide, nickel oxide, cobalt oxide, and a multi-component material of two or more of the elements iron, nickel, and cobalt.

[0030] The size of the magnetic core 100 can be adjusted depending on the industry in which the magnetic particle I is used. When the magnetic particle I is used in an anti-counterfeiting ink, the magnetic core 100 preferably has a size between 0.4 and 40 µm and a flat shape to improve reflectivity.

[0031] The shell 220 made of a metallic material increases the brightness of the magnetic particle I. The shell 220 preferably includes one or more of the following metallic materials: copper, nickel, gold, platinum, silver, aluminum and chromium, and more preferably one or more of the following materials: copper, nickel, silver and chromium.

[0032] The shell 220 made of a metallic material can be a metallic spherical membrane made of a single layer or a multi-layer membrane in which different metallic membranes are present in a layered structure.

[0033] In particular, in the case example, the single-layer membrane consists of a single membrane layer, preferably of one or more materials selected from copper, nickel, silver and chromium, wherein the shell 220 of a metallic material has a thickness between 10 and 500 nm in order to increase the brightness of the magnetic particle I.

[0034] Furthermore, the shell 220 made of the metallic material comprises a layered membrane formed by stacking two or more layers of different metallic materials, the layered membrane being formed by stacking a first metallic membrane 221 and a second metallic membrane 222 as shown in Fig. 7(b) and Fig. 7(c), or the first metallic membrane 221, the second metallic membrane and a third metallic membrane 223 are stacked as shown in Fig. 7(c).

[0035] Preferably, the first to third metallic membranes 221 to 223 comprise one or more materials selected from copper, nickel, gold, platinum, silver, aluminum, and chromium, wherein the materials for producing the individual membranes are different from each other.

[0036] The highly reflective protective membrane 300 protects the shell 220 made of the metallic material and prevents the light reflection from decreasing due to the interference effect of the light. In particular, the protective membrane 300 prevents damage to the shell 200 caused by the friction of a filler and the pressure between the rollers during a dispersion step in the ink manufacturing process, thus improving the abrasion resistance, chemical resistance, and light resistance of the magnetic particles I. The highly reflective protective membrane 300 comprises a single layer or multiple layers. The protective membrane with a single-layer structure not according to the invention, as described above, functions as a highly reflective protective membrane when the optical thickness is between (0.42·N)-0.17 and (0.42·N)+0.17 of the wavelength (N is an integer). Preferably, N is equal to 1, i.e., the membrane has an optical thickness between 0.3 and 0.6 of the wavelength.Furthermore, the protective membrane with a single-layer structure preferably includes one or more materials selected from a group comprising titanium dioxide, aluminum oxide, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide, and zinc sulfide. The protective membrane with a multi-layer structure comprises a low-refractive-index membrane 310 and a high-refractive-index membrane 320, wherein the low-refractive-index membrane 310 has a refractive index between 1.2 and 1.8 and the high-refractive-index membrane 320 has a refractive index between 1.6 and 2.7.The low-refractive-index membrane preferably comprises one or more materials selected from a group comprising silicon dioxide, magnesium fluoride, polystyrene, polymethyl methacrylate, polystyrene-co-butadiene, vanadium pentoxide, cadmium oxide, polyisobutylene, and polyethylene, while the high-refractive-index membrane preferably comprises one or more materials selected from a group comprising titanium dioxide, aluminum oxide, zinc oxide, zirconium oxide, and chromium oxide. Preferably, the optical thicknesses of the low-refractive-index membrane and the high-refractive-index membrane are each between 0.15 and 0.35 of the wavelength. In the case of an optically isotropic unit, a value calculated by multiplying a physical thickness and a refractive index is equal to another value calculated by multiplying a reference wavelength by an optical thickness.However, the range of physical thickness (nm) covering an optimal optical thickness for an interference effect can vary considerably, as the materials used for the protective membrane have different refractive indices. Accordingly, the range of physical thickness is determined based on the optical thickness.

[0037] For example, the optical thickness of titanium dioxide and silicon dioxide, with an optical thickness of one-quarter the wavelength, is calculated as follows. Titanium dioxide has a refractive index of 2.35 at a reference wavelength of 510 nm, so the physical thickness is calculated using the following mathematical expression: 2.35⋅physical thickness = 0.25⋅510 nm

[0038] Thus, titanium dioxide has a physical thickness of 54 nm and an optical thickness of one-quarter the wavelength. In contrast, silicon dioxide has a refractive index of 1.46 at the same reference wavelength, so the physical thickness is calculated using the following mathematical expression: 1.46 x physical thickness = 0.25 x 510 nm. Thus, silicon dioxide has a physical thickness of 87 nm and an optical thickness of one-quarter the wavelength.

[0039] The reflectivity, which is enhanced by an interference effect, increases slightly outside the above-mentioned optical thickness range. The reflectivity R of a highly reflective multi-layer protective membrane can be calculated using the following mathematical expression. R = tan h2 (SlnnHnL+12lnnH2nS) (In this expression, n H , n L , n S(respectively, for the refractive indices of a high-refractive-index membrane, a low-refractive-index membrane, and a substrate, and S indicates how often the high-refractive-index membrane and the low-refractive-index membrane occur repeatedly.)

[0040] According to the expression, the reflectivity is closely related to the refractive indices of the light-refracting membranes and the number of repetitions of the high-refractive-index membrane and the low-refractive-index membrane. The more often the high-refractive-index membrane and the low-refractive-index membrane are used, the more the reflectivity increases. However, in reality, the reflectivity only increases slightly even when the number of repetitions is greatly increased. Accordingly, for economical applicability, the protective membrane preferably comprises a layer of a high-refractive-index membrane and a layer of a low-refractive-index membrane.

[0041] In the following, the non-inventive magnetic particle II is described with reference to Fig. 8 is described for the case where its shell 200 has a shell 210 made of a dielectric material and another shell 220 made of a metallic material.

[0042] According to Fig. 8, the magnetic particle II comprises a magnetic core 100, a shell 210 made of a dielectric material formed on the outer surface of the magnetic core 100, another shell 220 made of a metallic material formed on the outer surface of the shell 210 made of the dielectric material, and a highly reflective protective membrane 300 formed on the outer surface of the shell 220 made of the metallic material, wherein the highly reflective protective membrane 300 may have a single-layer or multi-layer structure.

[0043] The magnetic core 100 is located in the center of the magnetic particle II to provide a magnetic property to the magnetic particle II.

[0044] Specifically, the magnetic core 100 of the magnetic particle II is a strong magnetic substance and thus similar to the magnetic core 100 of the magnetic particle I, wherein the magnetic core 100 preferably includes one or more materials selected from a group comprising iron, nickel, cobalt, iron oxide, nickel oxide, cobalt oxide and a multi-component material of two or more of the elements iron, nickel and cobalt.

[0045] The size of the magnetic core 100 can be adjusted depending on the industry in which the magnetic particle II is used. When the magnetic particle II is used in an anti-counterfeiting ink, the magnetic core 100 preferably has a size between 0.4 and 40 µm and a flat shape to improve reflectivity.

[0046] The second shell 220 made of a metallic material is located on the outer surface of the shell 210 (hereinafter referred to as the first shell), and the highly reflective protective membrane 300 includes a low refractive index membrane 310 and a high refractive index membrane 320 and is formed on the outer surface of the second shell 220 made of a metallic material, thereby improving the reflectivity of the magnetic particle II.

[0047] Similar to the shell made of a metallic material for the magnetic particle I, the second shell 220 preferably includes one or more of the following materials: copper, nickel, gold, platinum, silver, aluminum and chromium, and more preferably one or more of the following materials: copper, nickel, silver and chromium.

[0048] The second shell 220 preferably has a thickness between 40 and 150 nm. The second shell 220 increases the brightness of the magnetic particle II.

[0049] As in Fig. 9, the second shell 220 made of metallic material in a magnetic particle according to the invention is a multilayer membrane in which two or more mutually different metallic membranes 221 to 223 are present in a layered structure.

[0050] Specifically, the second metallic material shell 220 comprises a layered membrane formed by stacking two or more layers of different metallic materials. Although in Fig. 9 shows an example of the case where the second shell 220 is constructed in such a way that two metallic layers (a first metallic membrane 221 and a second metallic membrane 222) are stacked on top of each other, the number of the laminated metallic membranes is not limited in the present invention. Here, the laminated membrane, which according to Fig. 9 preferably comprises a first metallic membrane 221 and a second metallic membrane 222, has a thickness between 40 and 150 nm (this corresponds to the total thickness of all layered metallic membranes).

[0051] The highly reflective protective membrane 300 on the outer surface of the second shell 220 protects the shell 220 from damage caused by the friction of a filler and the pressure between the rollers during a dispersion step in an ink manufacturing process, thus improving the abrasion resistance, chemical resistance, and light resistance of the magnetic particle II. A dielectric material of the first shell 210 is selected from a group comprising titanium dioxide, aluminum oxide, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide, and zinc sulfide, wherein the first shell 210 preferably has a thickness between 10 and 500 nm.

[0052] The first shell 210 according to Fig. 10, which shows a magnetic particle not according to the invention, has a layered membrane constructed by stacking two or more inorganic layers with different refractive indices. Although Fig. 10 illustrates an example of the case where the first shell 210 is configured such that three dielectric membranes having different refractive indices (a first dielectric membrane 211, a second dielectric membrane 212, and a third dielectric membrane 213) are stacked on top of each other, the present invention is not limited in the number of stacked dielectric membranes.

[0053] The highly reflective protective membrane 300 protects the shell 220 made of a metallic material and prevents the reflectivity of the light from decreasing due to an interference effect of the light. In particular, the protective membrane 300 prevents the shell 220 from being damaged by the friction of a filler and the pressure between the rollers during a dispersion step in the ink manufacturing process, thus improving the abrasion resistance, chemical resistance, and light resistance of the magnetic particles II. The highly reflective protective membrane 300 can be constructed of a single layer or multiple layers. The protective membrane with a single-layer structure not according to the invention, as described above, functions as a highly reflective protective membrane when the optical thickness is between (0.42·N) - 0.17 and (0.42·N) + 0.17 of the wavelength (N is an integer).Preferably, N is equal to 1, meaning the membrane has an optical thickness between 0.3 and 0.6 of the wavelength. Furthermore, the protective membrane with a single-layer structure is preferably made of one or more materials selected from a group comprising titanium dioxide, aluminum oxide, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide, and zinc sulfide. The protective membrane with a multi-layer structure comprises a low-refractive-index membrane 310 and a high-refractive-index membrane 320, wherein the low-refractive-index membrane 310 has a refractive index between 1.2 and 1.8, while the high-refractive-index membrane 320 has a refractive index between 1.6 and 2.7.The low-refractive-index membrane is preferably made of one or more materials selected from a group comprising silicon dioxide, magnesium fluoride, polystyrene, polymethyl methacrylate, polystyrene-co-butadiene, vanadium pentoxide, cadmium oxide, polyisobutylene, and polyethylene, while the high-refractive-index membrane is preferably made of one or more materials selected from a group comprising titanium dioxide, aluminum oxide, zinc oxide, zirconium oxide, and chromium oxide. Preferably, the low-refractive-index membrane and the high-refractive-index membrane each have an optical thickness between 0.15 and 0.35 of the wavelength. For an optically isotropic unit, a value for the optical thickness is calculated by multiplying a physical thickness by a refractive index, i.e., the refractive index of a medium is multiplied by its thickness (nm).However, the range of physical thickness (nm) corresponding to the optimal optical thickness for an interference effect can vary considerably, as the materials used for the protective membrane have different refractive indices. Accordingly, the range of physical thickness is determined based on the optical thickness. The reflectivity enhanced by an interference effect increases slightly outside the aforementioned optical thickness range.

[0054] A method for producing the magnetic particles II with a highly reflective protective membrane is described with reference to Fig. 11 described in detail.

[0055] According to Fig. 11, the manufacturing method for the magnetic particles II comprises the formation of a first shell made of a dielectric material on the outer surface of a magnetic core (S21), the formation of a second shell made of metallic material on the outer surface of the first shell (S22), the formation of a low refractive index membrane on the outer surface of the second shell (S23), and the formation of a high refractive index membrane on the outer surface of the low refractive index membrane, wherein only a method having the features according to claim 3 is in accordance with the invention.

[0056] Preferably, the formation of the first shell of a dielectric material and the second shell of metallic material around the magnetic core is carried out by soaking a metal precursor solution with the magnetic core and subsequently adding a reducing agent to the soaked solution to form a metallic membrane around the magnetic core, or by mixing the magnetic core with metallic particles and stirring the mixture until a metallic membrane is physically formed around the magnetic core, and subsequently oxidizing the metallic membrane thus formed to produce a complex of an inorganic substance with a magnetic core.

[0057] In one embodiment, a shell made of a dielectric material may have different thicknesses depending on the reaction temperature during the formation of the shell, wherein the reaction temperature in the case where the dielectric material is titanium dioxide is preferably between 20 and 85 °C.

[0058] The following “Examples” are not according to the invention, but illustrate certain aspects of the production of coated magnetic particles. (Example 1)<Bildung einer Schicht aus Titandioxid an der Außenfläche eines magnetischen Kerns>

[0059] Particles of a magnetic core (120 g) and distilled water (24 mL) are added to ethanol (3.6 L). The mixture is then dispersed using ultrasonics. Tetrabutoxytitanium (TBOT) (38 mL) and ethanol (300 mL) are mixed together and then slowly added to the dispersed mixture over half an hour. At room temperature, the mixture is stirred with TBOT at 300 rpm for 3 hours. The powdered magnetic particles are then separated from the mixture using a magnet, washed twice with ethanol, and dried. (Example 2) <Bildung einer Silbermembran auf einem magnetischen Kern, der mit einer Titandioxid-Schicht beschichtet ist>

[0060] Glucose (85 g) and potassium tartrate (5 g) are dissolved in distilled water (800 mL) to form a reducing solution. Sodium hydroxide (NaOH, 12 g), ammonium hydroxide (NH4OH, 100 mL), and silver nitrate (AgNO3, 55 g) are dissolved in distilled water (800 mL) to form a colorless and transparent solution of the silver amine complex.

[0061] The magnetic core particles (80 g) with a titanium dioxide layer, prepared in a previous process, are placed in distilled water (2.4 L). A colorless and transparent silver amine complex solution is then mixed with the mixture in distilled water using ultrasonication. At room temperature, the mixture is stirred with the silver amine complex solution at 300 rpm for 3 hours and then mixed with the reducing solution. The mixture is then stirred using ultrasonication for 20 minutes to form a silver membrane. The core particles are separated from the mixture using a magnet, washed twice with ethanol, and dried. (Example 3) <Bildung einer Schutzmembran aus Siliciumdioxid an der Außenfläche der Silbermembran um den magnetischen Kern>

[0062] The magnetic core particles with a silver membrane (120 g), distilled water (30 mL), and NH4OH (150 mL) are placed in ethanol (3.6 L). The mixture is then dispersed using ultrasonication for 3 minutes. Tetraethyl orthosilicate (TEOS, 40 mL) and ethanol (300 mL) are mixed together and then slowly added to the dispersed mixture over a period of half an hour. At room temperature, the mixture with TEOS is stirred at 300 rpm for 3 hours. The powdered magnetic particles are separated from the mixture using a magnet, washed twice with ethanol, and dried. (Example 4) <Bildung einer Schutzmembran aus Titandioxid an der zuäußerst liegenden Außenfläche des magnetischen Kerns mit einer Schutzmembran aus Siliciumdioxid>

[0063] The magnetic core particles with a protective silica membrane (120 g) and distilled water (24 mL) are placed in ethanol (3.6 L). The mixture is then dispersed using ultrasonics for 3 minutes. TBOT (38 mL) and ethanol (300 mL) are mixed together and then slowly added to the dispersed mixture over a period of half an hour. The mixture is stirred at 300 rpm at 85 °C for 3 hours. The powdered magnetic particles are separated from the mixture using a magnet, washed twice with ethanol, and dried. (Experiments)[Abrasion resistance test]

[0064] The reflectivity of an ink was measured before and after a dispersion process to demonstrate that the resulting magnetic particles possess improved abrasion resistance. Ink produced using magnetic particles was printed onto paper using an applicator, and the printed paper was then dried for 48 hours. The reflectivity of the dried paper was then measured using a reflectivity meter (Varian, Cary 5000). Fig. Figure 12 shows a graph of reflectivity at 555 nm before and after a dispersion step for an ink. Referring to Fig. 12, the produced magnetic particles exhibit a slightly lower reflectivity than those without a protective membrane. After the dispersion step, the reflectivity of a magnetic particle with a silver membrane visibly decreases, as the silver membrane was damaged by the dispersion process, whereas the reflectivity of the magnetic particles with a highly reflective protective membrane decreases only slowly. [Chemical resistance test]

[0065] The reflectivity was measured to demonstrate that the manufactured magnetic particles possess improved chemical resistance. Ink produced using magnetic particles was printed onto paper using an applicator, after which the printed paper was dried for 48 hours. The reflectivity of the dried paper was then measured using a reflectivity meter (Varian, Cary 5000). A chemical resistance test was conducted on the dried paper, and then the reflectivity of the dried paper was measured again. The chemical resistance test was performed by applying a 5% sodium hypochlorite solution at 23°C for half an hour.Magnetic particles with a silver membrane showed reduced reflectivity because the silver membrane was damaged in the sodium hypochlorite solution, whereas the reflectivity of the fabricated magnetic particles decreased less than for the version with a silver membrane, which is due to the fact that a protective membrane is formed on the outermost surface of the magnetic particles. Fig. Figure 13 shows a graph of reflectivity at 555 nm before and after a chemical resistance test. According to Fig. 13, the magnetic particles with a protective membrane exhibited lower reflectivity before the chemical resistance test than those without, whereas the magnetic particles with a protective membrane exhibited higher reflectivity after the chemical resistance test than those without. As a result, the brightness was improved. [Lightfastness test]

[0066] The occurrence of yellowing was tested and the degree of yellowing was monitored to demonstrate that the produced magnetic particles exhibited improved lightfastness. Ink produced using magnetic particles was printed onto paper using an applicator, after which the printed paper was dried for 48 hours. The reflectivity of the dried paper was then measured using a reflectivity meter (Varian, Cary 5000). A lightfastness test was conducted on the dried paper, and then the reflectivity of the dried paper was measured again. The lightfastness test was carried out using a lightfastness tester (Atlas, Ci4000 Xenon Weather-Ometer) for 100 hours.After the light resistance test, yellowing was observed for the ink made of magnetic particles with a silver membrane instead of a protective membrane, while the yellowing was less severe for the ink made of the magnetic particles with a protective membrane.

[0067] Although some exemplary embodiments of the present invention have been shown and described by way of limited embodiments and drawings, the present invention is not intended to be limited to the described exemplary embodiments. Rather, one skilled in the art will recognize that modifications and variations to these exemplary embodiments are possible, the scope of which is defined by the claims.

Claims

[1] Magnetic particles with a highly reflective protective membrane (300), comprising: a magnetic core (100), a shell (200) formed on the outer surface of the magnetic core (100), having a multi-layer structure and comprising a first shell (210) made of a dielectric material, and a second shell (220) made of metallic material, formed on the outer surface of the first shell (210) and being a multi-layer membrane in which two or more layers (221, 222, 223) of mutually different metallic materials are stacked on top of each other, and a highly reflective protective membrane (300) which is in contact with the second shell (220) and is formed on the outer surface of the shell (200), wherein the highly reflective protective membrane (300) comprises a multi-layer structure comprising a low refractive index membrane (310) formed on the outer surface of the shell (200) and having an optical thickness between 0.15 to 0.35 of the wavelength, and a high refractive index membrane (320) formed on the outer surface of the low refractive index membrane (310) and having an optical thickness between 0.15 to 0.35 of the wavelength, the dielectric material is one or more materials selected from a group comprising titanium dioxide, aluminum oxide, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide and zinc sulfide, the metallic material is one or more materials selected from a group comprising copper, nickel, gold, platinum, silver, aluminum and chromium, the low refractive index membrane (310) consists of one or more materials selected from a group comprising silicon dioxide, magnesium fluoride, polystyrene, polymethyl methacrylate, polystyrene-co-butadiene, vanadium pentoxide, cadmium oxide, polyisobutylene and polyethylene, and the high refractive index membrane (320) consists of one or more materials selected from a group comprising titanium dioxide, aluminum oxide, zinc oxide, zirconium oxide and chromium oxide. [2] Magnetic particles with a highly reflective protective membrane (300) according to claim 1, characterized by in that the highly reflective protective membrane (300) is designed in a multi-layer structure such that the low refractive index membrane (310) and the high refractive index membrane (320) formed on the outer surface of the low refractive index membrane (310) are repeatedly layered on top of one another. [3] A manufacturing method for magnetic particles having a highly reflective protective membrane (300) according to claim 1 or 2, comprising: the formation of a first shell (210) consisting of a dielectric material on the outer surface of a magnetic core (100), the formation of a second shell (220) consisting of metallic material on the outer surface of the first shell (210), wherein the second shell (220) is formed by stacking two or more layers (221, 222, 223) of different metallic materials on top of each other, and the formation of a highly reflective protective membrane (300) on the outer surface of the second shell (220) so that it is in contact with the second shell (220), wherein the highly reflective protective membrane (300) comprises a multi-layer structure comprising a low refractive index membrane (310) having an optical thickness between 0.15 and 0.35 of the wavelength and formed on the outer surface of the second shell (220), and a high refractive index membrane (320) formed on the outer surface of the low refractive index membrane (310) and having an optical thickness between 0.15 and 0.35 of the wavelength, wherein a dielectric material for forming the first shell (210) is one or more materials selected from a group comprising titanium dioxide, aluminum oxide, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide and zinc sulfide, a metallic material for forming the second shell (220) is one or more materials selected from a group comprising copper, nickel, gold, platinum, silver, aluminum and chromium, the low refractive index membrane (310) consists of one or more materials selected from a group comprising silicon dioxide, magnesium fluoride, polystyrene, polymethyl methacrylate, polystyrene-co-butadiene, vanadium pentoxide, cadmium oxide, polyisobutylene and polyethylene, and the high refractive index membrane (320) consists of one or more materials selected from a group comprising titanium dioxide, aluminum oxide, zinc oxide, zirconium oxide and chromium oxide. [4] Anti-counterfeiting ink for securities, comprising the magnetic particles with a highly reflective protective membrane (300) according to one of claims 1 or 2.

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