Method for coating surface of composite tungsten oxide fine particles with silicon compound film

Abstract

To provide a coating method for forming a silicon compound film having uniform film thickness on the surface of composite tungsten oxide fine particles.SOLUTION: Provided is a method for coating the surface of composite tungsten oxide fine particles with a silicon compound film. The method comprises: preparing an alcohol dispersion comprising composite tungsten oxide fine particles and alcohol; adding a silane compound solution comprising an alkoxysilane compound and alcohol to the alcohol dispersion to prepare a silane compound-added alcohol dispersion; adding an alkaline aqueous solution to the silane compound-added alcohol dispersion to prepare a raw material mixed solution; and performing a stirring step of stirring the raw material mixed solution continuously for 12 hours or more to coat the surface of the composite tungsten oxide fine particles with the silicon compound film.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for coating the surfaces of composite tungsten oxide fine particles with a silicon compound film. [Background technology]

[0002] Hexaboride microparticles, tungsten oxide microparticles, and composite tungsten oxide microparticles are known as near-infrared-absorbing microparticles because they are transparent to visible light while absorbing near-infrared radiation. Near-infrared-absorbing microparticle dispersions, in which such near-infrared-absorbing microparticles are dispersed in a medium such as a resin that transmits visible light, have been widely used in fields such as near-infrared-absorbing (heat-shielding) window films, near-infrared-absorbing sheet materials, near-infrared-absorbing interlayer films for laminated glass, infrared-cut filters for electronic materials, and solar heat-shielding materials for agriculture.

[0003] These near-infrared absorbing window films, near-infrared absorbing sheet materials, agricultural near-infrared absorbing materials, and other materials are used outdoors, and are therefore exposed to sunlight and moisture. The near-infrared absorbing material particles contained in these materials can lose their near-infrared absorption function when exposed to high humidity and high temperature environments for long periods of time. This is thought to be because water and oxygen in the environment come into contact with the conductive compound particles, causing oxidation of the compound and desorption of the doping element.

[0004] Therefore, Patent Document 1 discloses infrared-shielding microparticles that are composite tungsten oxide microparticles represented by the general formula WyOz and / or the general formula MxWyOz, in which the average primary particle size of the microparticles is 1 nm or more and 800 nm or less, and the surfaces of the microparticles are coated with a silane compound and / or an organometallic compound. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-291109 Summary of the Invention [Problem to be solved by the invention]

[0006] Conventional techniques for coating near-infrared absorbing material particles have had problems with the uniformity of the coating film thickness. Specifically, if the coating film is too thick, the particle size increases significantly due to the coating process, increasing the amount of light scattering per particle (scattering cross section). On the other hand, if the coating film is too thin, it does not provide sufficient protection from oxidation due to heat or water. An object of the present invention is to provide a coating method for forming a silicon compound film of uniform thickness on the surface of composite tungsten oxide fine particles. [Means for solving the problem]

[0007] In order to solve the above problems, the inventors have investigated a method for coating a silicon compound film with a uniform thickness. an alcohol dispersion preparation step of preparing an alcohol dispersion containing composite tungsten oxide microparticles and alcohol; a silane compound adding step of adding a silane compound solution containing an alkoxysilane compound and an alcohol to the alcohol dispersion to prepare a silane compound-added alcohol dispersion; an alkaline aqueous solution adding step of adding an alkaline aqueous solution to the silane compound-added alcohol dispersion to prepare a raw material mixed solution; The method for coating the surfaces of composite tungsten oxide particles with a silicon compound film includes a stirring step in which the raw material mixed solution is continuously stirred for 12 hours or more, and the surfaces of the composite tungsten oxide particles are coated with a silicon compound film. [Effects of the Invention]

[0008] According to the present invention, a silicon compound film having a uniform thickness of 2 nm to 6 nm can be formed on the surface of composite tungsten oxide fine particles. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a flowchart showing a method for coating the surfaces of composite tungsten oxide fine particles with a silicon compound film according to Example 1. [Figure 2] 1 is a graph showing the results of evaluation of moist heat resistance (absorbance) according to Example 1 and Comparative Example 1. [Figure 3] 1 is a graph showing the results of evaluation of moist heat resistance (absorbance maintenance rate) according to Example 1 and Comparative Example 1. BEST MODE FOR CARRYING OUT THE INVENTION

[0010] This embodiment includes an alcohol dispersion preparation step of preparing an alcohol dispersion containing composite tungsten oxide microparticles and alcohol. a silane compound adding step of adding a silane compound solution containing an alkoxysilane compound and an alcohol to the alcohol dispersion to prepare a silane compound-added alcohol dispersion; an alkaline aqueous solution adding step of adding an alkaline aqueous solution to the silane compound-added alcohol dispersion to prepare a raw material mixed solution; The method for coating the surfaces of composite tungsten oxide particles with a silicon compound film includes a stirring step in which the raw material mixed solution is continuously stirred for 12 hours or more, and the surfaces of the composite tungsten oxide particles are coated with a silicon compound film.

[0011] The method for coating the surfaces of composite tungsten oxide microparticles with a silicon compound film according to this embodiment will be described in the following order: 1. composite tungsten oxide microparticles, 2. method for producing composite tungsten oxide microparticles, 3. formation of a silicon compound film on the surfaces of composite tungsten oxide microparticles, and 4. method for using an aqueous dispersion of composite tungsten oxide microparticles having a silicon compound film formed on the surface thereof.

[0012] 1. Composite tungsten oxide particles The composite tungsten oxide microparticles according to this embodiment exhibit a transmittance of light having a maximum value in the wavelength range of 350 nm to 600 nm and a minimum value in the wavelength range of 800 to 2100 nm, in terms of the light transmittance of a dispersion of near-infrared absorbing material microparticles dispersed in a medium such as a resin that transmits visible light. More specifically, the wavelength ranges where the maximum and minimum values ​​of transmittance occur are explained. The maximum value occurs in the wavelength range of 440 to 600 nm, and the minimum value occurs in the wavelength range of 1150 to 2100 nm. That is, the maximum value of transmittance occurs in the visible light range, and the minimum value of transmittance occurs in the near-infrared range.

[0013] Generally, tungsten trioxide (WO3) does not have effective free electrons, and therefore has little absorption / reflection property in the near-infrared region, making it ineffective as an infrared-shielding material. It is known that free electrons are generated in the tungsten oxide by reducing the ratio of oxygen to tungsten in tungsten trioxide to less than 3, but the present inventors have discovered that there is a range within a specific composition range of tungsten and oxygen in the tungsten oxide that is particularly effective as a near-infrared absorbing material.

[0014] The composition range of tungsten and oxygen is preferably such that the composition ratio of oxygen to tungsten is 3 or less, and further, when the tungsten oxide is expressed as WyOz, it is preferable that z / y is 2.2≦z / y≦2.999. If the z / y value is 2.2 or more, it is possible to prevent the appearance of an unintended WO2 crystalline phase in the tungsten oxide and to obtain chemical stability as a material, so that the tungsten oxide can be used as an effective near-infrared absorbing material. On the other hand, if the z / y value is 2.999 or less, the required amount of free electrons is generated in the tungsten oxide, making it an efficient near-infrared absorbing material.

[0015] Furthermore, in the tungsten oxide microparticles obtained by microparticulating the tungsten oxide, when expressed by the general formula WyOz, the so-called "Magnéli phase" having a composition ratio expressed by 2.45≦z / y≦2.999 is chemically stable and has good absorption properties in the near-infrared region, making it preferable as a near-infrared absorbing material.

[0016] Furthermore, it is also preferable to add an element M to the tungsten oxide to form a composite tungsten oxide, because by adopting this configuration, free electrons are generated in the composite tungsten oxide, and absorption characteristics derived from the free electrons are exhibited in the near-infrared region, making it effective as a near-infrared absorbing material for wavelengths around 1000 nm.

[0017] From the viewpoint of stability of the composite tungsten oxide to which the M element has been added, the M element is more preferably one or more elements selected from H, He, alkali metals, alkaline earth metals, rare earth elements, Mg, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Tl, Si, Ge, Sn, Pb, Sb, B, F, P, S, Se, Br, Te, Ti, Nb, V, Mo, Ta, Re, Be, Hf, Os, Bi, and I.

[0018] By combining the above-mentioned control of the oxygen content with the addition of an element that generates free electrons in the composite tungsten oxide, a more efficient near-infrared absorbing material can be obtained. When the general formula of the near-infrared absorbing material that combines the control of the oxygen content and the addition of an element that generates free electrons is written as MxWyOz (where M is the element M, W is tungsten, and O is oxygen), the relationship of 0.001≦x / y≦1, preferably 0.20≦x / y≦0.37 is satisfied.

[0019] Here, from the viewpoint of stability in MxWyOz to which the M element is added, the M element is more preferably one or more elements selected from alkali metals, alkaline earth metals, rare earth elements, Mg, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Tl, Si, Ge, Sn, Pb, Sb, B, F, P, S, Se, Br, Te, Ti, Nb, V, Mo, Ta, and Re, and from the viewpoint of improving the optical properties and weather resistance as a near-infrared absorbing material, the M element is still more preferably an alkali metal, alkaline earth metal, transition metal element, Group 4B element, or Group 5B element.

[0020] Next, the z / y value, which indicates the control of the oxygen content, will be described. Regarding the z / y value, in the infrared absorbing material expressed as MxWyOz, the same mechanism as in the near-infrared absorbing material expressed as WyOz described above works, and even when z / y=3.0, there is a supply of free electrons due to the amount of added M element described above, so 2.2≦z / y≦3.0 is preferable.

[0021] Furthermore, when the composite tungsten oxide microparticles described above have a hexagonal crystal structure, the microparticles have improved transmittance in the visible light region and improved absorption in the near-infrared region. In this hexagonal crystal structure, six octahedra formed by WO6 units are assembled to form hexagonal voids (tunnels), and the M element is arranged in the voids to form one unit, and many of these units are assembled to form the hexagonal crystal structure.

[0022] In order to obtain the effects of improving the transmittance in the visible light region and improving the absorption in the near-infrared region according to this embodiment, it is sufficient that the composite tungsten oxide microparticles contain a unit structure (a structure in which six octahedrons formed from WO6 units are assembled to form hexagonal voids, and an M element is arranged in the voids).

[0023] When cations of the element M are added to these hexagonal voids, absorption in the near-infrared region is improved. Generally, the hexagonal crystal is formed when an element M having a large ionic radius is added, and specifically, the hexagonal crystal is easily formed when one or more elements selected from Cs, Rb, K, Tl, In, Ba, Li, Ca, Sr, Fe, and Sn are added, which is preferable. Furthermore, among these elements M having a large ionic radius, composite tungsten oxide particles doped with one or more elements selected from Cs and Rb can achieve both absorption in the near-infrared region and transmission in the visible light region.

[0024] When the composite tungsten oxide microparticles having a hexagonal crystal structure have a uniform crystal structure, the amount of the added M element is 0.001≦x / y≦1, preferably 0.2≦x / y≦0.5, more preferably 0.20≦x / y≦0.37, and most preferably x / y=0.33. This is because, theoretically, when z / y=3, x / y=0.33, and the added M element is thought to be arranged in all of the hexagonal voids.

[0025] The near-infrared absorbing material particles according to this embodiment have a particle diameter of 200 nm or less. From the viewpoint of exhibiting better infrared shielding properties, the particle diameter is preferably 10 nm or more and 100 nm or less, more preferably 10 nm or more and 80 nm or less, even more preferably 10 nm or more and 60 nm or less, and most preferably 10 nm or more and 40 nm or less. When the particle diameter is in the range of 10 nm or more and 40 nm or less, the best infrared shielding properties are exhibited. Here, the particle size refers to the average diameter of individual non-aggregated near-infrared absorbing material particles, and is the average particle size of the near-infrared absorbing material particles contained in the near-infrared absorbing material particle dispersion described below. On the other hand, the particle size does not include the size of aggregates of composite tungsten oxide fine particles, and is different from the dispersed particle size.

[0026] The average particle size is calculated from an electron microscope image of the near-infrared absorbing material fine particles. The average particle size of the composite tungsten oxide microparticles contained in the near-infrared absorbing material microparticle dispersion can be determined by measuring the particle sizes of 100 composite tungsten oxide microparticles using an image processing device from a transmission electron microscope image of a thin-sectioned sample of the composite tungsten oxide microparticle dispersion extracted by cross-section processing, and calculating the average value. A microtome, cross-section polisher, focused ion beam (FIB) device, etc. can be used for cross-section processing to extract the thin-sectioned sample. The average particle size of the composite tungsten oxide microparticles contained in the near-infrared absorbing material microparticle dispersion refers to the average particle size of the composite tungsten oxide microparticles dispersed in the solid medium matrix.

[0027] Furthermore, near-infrared absorbing material microparticle dispersions containing composite tungsten oxide microparticles according to this embodiment largely absorb light in the near-infrared region, particularly in the wavelength region around 1000 nm, and therefore often exhibit a transmission color tone ranging from blue to green. The dispersed particle diameter of the near-infrared absorbing material microparticles can be selected depending on the intended use. First, when used in applications requiring transparency, it is more preferable for the dispersed particle diameter to be 800 nm or less. This is because particles with a dispersed particle diameter smaller than 800 nm do not completely block light due to scattering, and can maintain visibility in the visible light region while simultaneously efficiently maintaining transparency. In particular, when transparency in the visible light region is important, it is preferable to further consider scattering by the particles. The dispersed particle diameter of the near-infrared absorbing material microparticles described above is a concept that includes the diameter of the aggregate of composite tungsten oxide microparticles, and is a different concept from the particle diameter of the near-infrared absorbing material microparticles according to the present embodiment described above.

[0028] When the reduction of scattering by particles is important, the dispersed particle diameter is preferably 200 nm or less, more preferably 10 nm to 200 nm, and even more preferably 10 nm to 100 nm. The reason for this is that a small dispersed particle diameter reduces the scattering of light in the visible light region with wavelengths of 400 nm to 780 nm due to geometric scattering or Mie scattering, which can prevent the infrared-shielding film from becoming like frosted glass and losing its clear transparency. That is, when the dispersed particle diameter is 200 nm or less, the geometric scattering or Mie scattering is reduced, resulting in a Rayleigh scattering region. In the Rayleigh scattering region, scattered light is proportional to the sixth power of the dispersed particle diameter, so scattering is reduced as the dispersed particle diameter decreases, improving transparency. Furthermore, a dispersed particle diameter of 100 nm or less is preferable because scattered light is significantly reduced. From the viewpoint of preventing light scattering, a small dispersed particle diameter is preferable, and a dispersed particle diameter of 10 nm or more facilitates industrial production.

[0029] By setting the dispersed particle diameter to 800 nm or less, the haze value of the near-infrared absorbing material microparticle dispersion in which the near-infrared absorbing material microparticles are dispersed in a medium can be set to 10% or less at a visible light transmittance of 85% or less. In particular, by setting the dispersed particle diameter to 100 nm or less, the haze can be set to 1% or less. Incidentally, the light scattering of the near-infrared absorbing material fine particle dispersion must be considered in terms of the dispersion particle diameter, since it is necessary to take into account the aggregation of the near-infrared absorbing material fine particles.

[0030] 2. Manufacturing method of composite tungsten oxide microparticles The composite tungsten oxide microparticles of the present embodiment, represented by the general formula MxWyOz, can be produced by a solid-state reaction method, in which the tungsten compound, which is the starting material of the tungsten oxide microparticles, is heat-treated in a reducing gas atmosphere, or in a mixed gas atmosphere of reducing gas and inert gas, or in an inert gas atmosphere.The composite tungsten oxide microparticles obtained by being pulverized or the like to obtain a predetermined particle size after the heat treatment have sufficient near-infrared absorbing power, and have the properties preferred as near-infrared absorbing microparticles.

[0031] The starting material for obtaining the composite tungsten oxide microparticles represented by the general formula MxWyOz according to this embodiment may be one or more powders selected from tungsten trioxide powder, tungsten dioxide powder, tungsten oxide hydrate, tungsten hexachloride powder, ammonium tungstate powder, tungsten oxide hydrate powder obtained by dissolving tungsten hexachloride in alcohol and then drying, tungsten oxide hydrate powder obtained by dissolving tungsten hexachloride in alcohol and then adding water to precipitate and then drying, tungsten compound powder obtained by drying an aqueous solution of ammonium tungstate, and metallic tungsten powder, and a powder of a simple substance or compound containing the M element may be mixed in a ratio of 0.20≦x / y≦0.37.

[0032] Furthermore, when the tungsten compound, which is the starting material for obtaining the composite tungsten oxide microparticles, is in the form of a solution or dispersion, the elements can be easily mixed uniformly. From this viewpoint, it is more preferable that the starting material for the composite tungsten oxide microparticles is a powder obtained by mixing an alcohol solution of tungsten hexachloride or an aqueous solution of ammonium tungstate with a solution of the compound containing the M element, and then drying the mixture. From a similar viewpoint, it is also preferable that the starting material for the composite tungsten oxide microparticles is a powder obtained by mixing a dispersion obtained by dissolving tungsten hexachloride in alcohol, adding water to form a precipitate, with a powder of a simple substance or compound containing the M element, or a solution of a compound containing the M element, and then drying the resulting mixture.

[0033] Examples of compounds containing the element M include, but are not limited to, tungstates, chlorides, nitrates, sulfates, oxalates, oxides, carbonates, and hydroxides of the element M, as long as they are in a solution state. Furthermore, when the composite tungsten oxide microparticles are produced industrially, using a tungsten oxide hydrate powder or tungsten trioxide and a carbonate or hydroxide of the element M is a preferred production method, since no harmful gases are generated during the heat treatment stage or the like.

[0034] Here, the conditions for heat treatment of the composite tungsten oxide particles in a reducing atmosphere or in a mixed gas atmosphere of a reducing gas and an inert gas will be described. First, the starting material is heat-treated in a reducing gas atmosphere or a mixed gas atmosphere of a reducing gas and an inert gas. The heat-treatment temperature is preferably higher than the temperature at which the composite tungsten oxide microparticles crystallize. Specifically, the heat-treatment temperature is preferably 500°C or higher and 1000°C or lower, and more preferably 500°C or higher and 800°C or lower. If desired, the material may be further heat-treated in an inert gas atmosphere at a temperature of 500°C or higher and 1200°C or lower.

[0035] The reducing gas is not particularly limited, but H2 is preferred. When H2 is used as the reducing gas, its concentration can be appropriately selected depending on the firing temperature and the amount of the starting materials, and is not particularly limited. For example, it is 20 vol% or less, preferably 10 vol% or less, and more preferably 7 vol% or less. This is because a reducing gas concentration of 20 vol% or less can avoid the generation of WO2, which does not have solar radiation shielding function, due to rapid reduction. By this heat treatment, the z / y ratio in the composite tungsten oxide is 2.2≦z / y≦3.0.

[0036] On the other hand, the method for producing composite tungsten oxide is not limited to the solid-phase reaction method.By setting appropriate production conditions, it can also be produced by a thermal plasma method.The production conditions that should be set appropriately include, for example, the feed rate when feeding raw materials into the thermal plasma, the flow rate of the carrier gas used to feed raw materials, the flow rate of the plasma gas that maintains the plasma region, and the flow rate of the sheath gas that flows just outside the plasma region.

[0037] The composite tungsten oxide bulk or particles may be microparticulated via a near-infrared absorbing material microparticle dispersion liquid, which will be described later. To obtain composite tungsten oxide microparticles from the near-infrared absorbing material microparticle dispersion liquid, the solvent may be removed by a known method. Furthermore, the composite tungsten oxide bulk or particles can be micronized by dry micronization using a jet mill or the like. However, even in dry micronization, it is necessary to determine the milling conditions (micronization conditions) that can impart the particle size, crystallite size, and a-axis length and c-axis length of the lattice constant of the resulting composite tungsten oxide. For example, if a jet mill is used, it is sufficient to select a jet mill that provides the appropriate milling conditions, such as air volume and processing time.

[0038] 3. Formation of a silicon compound film on the surface of composite tungsten oxide particles Patent Document 1 describes a technique for coating the surface of composite tungsten oxide particles with a silane compound and an organometallic compound, but the thickness of the coating film in Patent Document 1 is unknown. The method of coating the surface of a composite tungsten oxide with a silicon compound film according to this embodiment controls the temperature during the formation of the silicon compound film, thereby controlling the film thickness of the silicon compound film.

[0039] The method for coating the surface of a composite tungsten oxide with a silicon compound film according to this embodiment will be described in the following order: <1. alcohol dispersion preparation step>, <2. silane compound introduction step>, <3. alkaline aqueous solution addition step>, <4. stirring step>, and <5. cleaning step>.

[0040] 1. Alcohol dispersion preparation process In the alcohol dispersion preparation step, an alcohol dispersion containing composite tungsten oxide microparticles and alcohol is prepared. The alcohol dispersion contains 0.01% by mass or more and 20% by mass or less of composite tungsten oxide microparticles. The content of the composite tungsten oxide microparticles in the alcohol dispersion is not a problem as long as the composite tungsten oxide microparticles can be dispersed, and industrially dispersible if the content is 0.01% by mass or more and 20% by mass or less. The particle diameter of the composite tungsten oxide microparticles in the alcohol dispersion is preferably 200 nm or less.

[0041] The alcohol in the alcohol dispersion may be mixed with water to be added subsequently in order to hydrolyze the alkoxysilane compound contained in the silane compound solution, and alcohols with a carbon number of 3 or less, such as methanol, ethanol, and propanol, are preferred.

[0042] The alcohol dispersion may be produced by mixing composite tungsten oxide microparticles having a particle size of 200 nm or less with alcohol, and dispersing the composite tungsten oxide microparticles in the liquid by irradiating them with ultrasound using an ultrasonic homogenizer or the like. Alternatively, the alcohol dispersion may be obtained by placing coarse composite tungsten oxide particles, alcohol, and beads in a paint shaker, pulverizing the coarse particles with the paint shaker, and dispersing them in the liquid.

[0043] Alternatively, an aqueous dispersion containing composite tungsten oxide microparticles and water can be prepared in advance, and then the aqueous dispersion can be added to alcohol to form an alcohol dispersion. The amount of the aqueous dispersion added to the alcohol is determined according to the content of composite tungsten oxide microparticles contained in the alcohol dispersion. Furthermore, when the aqueous dispersion is added to alcohol to prepare an alcohol dispersion, water is included in the alcohol dispersion. The inclusion of a large amount of water in the alcohol dispersion leads to decomposition of the alkoxysilane compound added in the silane compound addition step, as described below. Therefore, the amount of water that can be contained in the alcohol dispersion is limited to 2% by mass or less.

[0044] 2. Silane Compound Addition Process In the silane compound adding step, a silane compound solution containing an alkoxysilane compound and an alcohol is added to the alcohol dispersion to prepare a silane compound-added alcohol dispersion. Examples of alkoxysilane compounds that can be used in the silane compound solution include tetramethoxysilane, tetraethoxysilane, trimethoxysilane, triethoxysilane, trimethoxymethylsilane, diethoxydimethylsilane, and dimethoxydimethylsilane. Of these, orthosilicate compounds of tetramethoxysilane or tetraethoxysilane are preferred, with tetraethoxysilane being even more preferred. Orthosilicate compounds are preferred because they are easy to form into a silicon oxide film, which is a silicon compound film, and tetraethoxysilane is even more preferred because its reactivity makes it easy to control the thickness of the silicon compound film.

[0045] The content of the alkoxysilane compound in the silane compound solution can be determined by the amount of composite tungsten oxide microparticles that coat the surface. To form a silicon compound film on the surface of composite tungsten oxide microparticles with a particle diameter of 10 nm to 200 nm, an excess of 5 parts by mass or more, and more preferably 10 parts by mass or more, of the alkoxysilane compound is added per 1 part by mass of the composite tungsten oxide microparticles. Adding an excess of the alkoxysilane compound over the composite tungsten oxide microparticles is necessary to promote the hydrolysis of unreacted alkoxysilane compound in the system during the stirring step described below, thereby growing a silicon compound film on the surface of the composite tungsten oxide microparticles. It is also necessary to form a silicon compound film on the surface of all of the composite tungsten oxide microparticles contained.

[0046] Alcohols that can be used in the silane compound solution are those that are miscible with water, such as methanol, ethanol, and propanol.

[0047] The silane compound solution is added to the alcohol dispersion to prepare the silane compound-added alcohol dispersion. When the silane compound solution is added to the alcohol dispersion, it is preferable to add the silane compound solution while stirring the alcohol dispersion. In the silane compound addition step, when the silane compound solution is added to the alcohol dispersion, it is necessary to maintain the temperature at a constant temperature selected from the range of 35° C. to 80° C. In the present invention, "maintaining a constant temperature" means maintaining the temperature within ±0.1° C. of the selected temperature.

[0048] 3. Addition of alkaline aqueous solution In the alkaline aqueous solution adding step, an alkaline aqueous solution is added to the silane compound-added alcohol dispersion to prepare a raw material mixed solution. The alkaline aqueous solution is a mixture of raw materials with a concentration of 5 x 10 -5 mol / L or more 5 x 10 -4 An aqueous solution of hydroxides such as sodium hydroxide or potassium hydroxide, or aqueous ammonia, can be used, with a concentration of 0.1 mole / L or less. The addition of an alkaline aqueous solution can promote the hydrolysis of the alkoxysilane compound. Furthermore, the addition of an alkaline aqueous solution can also supply the water necessary for the hydrolysis of the alkoxysilane compound.

[0049] In order to precisely control the coating of the surface of the composite tungsten oxide microparticles by the hydrolysis reaction of the alkoxysilane compound, it is desirable to add water to the silane compound-added alcohol dispersion while stirring it before adding the alkaline aqueous solution to the silane compound-added alcohol dispersion, and then add the alkaline aqueous solution. The reason why it is preferable to add water before adding the alkaline aqueous solution is to uniformly mix water into the silane compound-added alcohol dispersion in advance. This is because uniformly mixing water into the silane compound-added alcohol dispersion in advance allows the added alkaline aqueous solution to react uniformly and quickly with the silane compound, which is preferable.

[0050] When adding an alkaline aqueous solution to a silane compound-added alcohol dispersion, or when adding water to a silane compound-added alcohol dispersion and then adding an alkaline aqueous solution, it is necessary to provide an interval of 5 minutes or more, preferably 10 minutes or more, between adding each solution or water to the alcohol dispersion, continue stirring the alcohol dispersion, and maintain the temperature at a constant temperature selected from the range of 35° C. to 80° C. By providing such a time interval, continuing stirring, and controlling the temperature, the alcohol dispersion, the solutions, and water are mixed together to form a homogeneous raw material mixture solution.

[0051] <4. Mixing process> In the stirring process, the raw material mixture solution is kept at a constant temperature selected from the range of 35°C to 80°C and stirred for 12 hours or more. Even during the stirring process, unreacted alkoxysilane compounds remain in the raw material mixture solution, and the unreacted alkoxysilane compounds undergo a hydrolysis reaction, causing a film of silicon compound to grow on the surface of the composite tungsten oxide particles.

[0052] If it is desired to increase the relative thickness of the silicon compound film formed on the surface of the composite tungsten oxide microparticles, the temperature may be increased by increasing the temperature selected from the range of 35°C to 80°C. The raw material mixture solution is kept at a constant temperature selected from the range of 35°C to 80°C inclusive in order to promote the reaction of the alkoxysilane compound. If the temperature is below 35°C, the reaction of the alkoxysilane compound does not proceed, and a silicon compound film with a thickness of 2 nm or more cannot be formed on the composite tungsten oxide. On the other hand, if the temperature exceeds 80°C, it becomes difficult to control the reaction of the alkoxysilane compound, and there is a risk that the silicon compound film will not be formed uniformly on the surface of the composite tungsten oxide microparticles.

[0053] In order to form a silicon compound film of uniform thickness on the surface of the composite tungsten oxide microparticles through the reaction of the alkoxysilane compound, the content of the composite tungsten oxide microparticles in the raw material mixture solution is preferably 5% by mass or less. If the content of the composite tungsten oxide microparticles in the raw material mixture solution exceeds 5% by mass, the thickness of the silicon compound film may vary from particle to particle. Furthermore, the content of the composite tungsten oxide microparticles in the raw material mixture solution may be 0.01% by mass or more.

[0054] The stirring time in the stirring step is 12 hours or more continuously, preferably 18 hours or more, and more preferably 24 hours or more. In the method for coating the surface of composite tungsten oxide microparticles with a silicon compound film, a silicon compound film is grown on the surface of the composite tungsten oxide microparticles during continuous stirring for 12 hours or more. If an attempt is made to form a silicon compound film with a thickness of 2 nm or more with a stirring time of less than 12 hours, the silicon compound film may not be formed on the surface of the composite tungsten oxide microparticles, or the film may have a thickness of 2 nm or less.

[0055] The silicon compound film formed by the method for coating the surface of the composite tungsten oxide microparticles with a silicon compound film in this embodiment has a thickness of 2 nm to 7 nm, preferably 2 nm to 5 nm, and more preferably 2 nm to 4.95 nm. If the thickness of the silicon compound film formed on the surface of the composite tungsten oxide microparticles is less than 2 nm, the composite tungsten oxide microparticles cannot be protected from water or other external intrusion. On the other hand, if the thickness of the silicon compound film formed on the surface of the composite tungsten oxide microparticles exceeds 7 nm, the particle size of the near-infrared absorbing microparticles (composite tungsten oxide microparticles coated with a silicon compound) becomes too large, which may deteriorate the optical properties, such as haze, of the near-infrared absorbing material microparticle dispersion. The silicon compound formed on the surface of the composite tungsten oxide particles is considered to be silicon oxide. Then, through the stirring step, the particles become coated composite tungsten oxide particles with a silicon compound film formed on the surface.

[0056] 5. Cleaning process In the washing step, the reacted raw material mixed solution in which the stirring step is completed and coated composite tungsten oxide microparticles with a silicon compound film formed on the surface are dispersed is subjected to centrifugal dehydration washing using alcohol or water to remove unreacted alkoxysilane compound and alkali.

[0057] First, the reacted raw material mixed solution in which the coated composite tungsten oxide microparticles are dispersed is washed with an alcohol such as methanol, ethanol, propanol, or butanol, and solid-liquid separation is performed by centrifugal dehydration. After that, washing with water is performed multiple times, and solid-liquid separation is performed by centrifugal dehydration. Finally, the unreacted alkoxysilane compound and alkali are removed to obtain the final aqueous dispersion of coated composite tungsten oxide microparticles with a silicon compound film formed on the surface, in which coated composite tungsten oxide microparticles with a silicon compound film formed on the surface are dispersed in water.

[0058] 4. Method of using an aqueous dispersion of coated composite tungsten oxide microparticles having a silicon compound film formed on the surface The aqueous dispersion of coated composite tungsten oxide microparticles having a silicon compound film formed on the surface thereof obtained in the washing step can be used as it is as an aqueous dispersion of coated composite tungsten oxide microparticles having a silicon compound film formed on the surface thereof. Alternatively, the aqueous dispersion of coated composite tungsten oxide microparticles can be used after solvent substitution with a water-immiscible organic solvent such as toluene.

[0059] A water-soluble resin (e.g., acrylic resin) is added to an aqueous dispersion of coated composite tungsten oxide microparticles having a silicon compound film formed on the surface thereof to form a coating liquid. The coating liquid is then applied to a transparent substrate such as glass, and the resin is cured to form a near-infrared absorbing material microparticle dispersion in which coated composite tungsten oxide microparticles having a silicon compound film formed on the surface thereof are dispersed in the resin. [Example]

[0060] [Example 1] 1 shows a flow chart illustrating a method for forming a silicon compound film on the surface of composite tungsten oxide fine particles according to the present invention, but the present invention is not limited to these examples.

[0061] 1. Alcohol dispersion preparation process An aqueous dispersion A consisting of a cesium composite tungsten oxide having an average particle size of 35 nm and a concentration of 19.1 mass %, with the remainder being water, was prepared using a paint shaker. 26.26 mL of ethanol and 44 μL of the aqueous dispersion A were added to a screw tube, and ultrasonic waves were applied for 3 minutes to obtain an alcohol dispersion sample. Eight similar samples were prepared to conduct tests at eight different temperatures after the silane compound addition process.

[0062] 2. Silane Compound Addition Process Eight screw tubes containing the alcohol dispersion were placed in thermostatic baths at temperatures of 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 70°C, and 80°C, respectively, and stirred at 300 rpm using a stirrer. On the other hand, 0.1 mol % of tetraethoxysilane was added to ethanol and mixed to prepare a silane compound solution. To the alcohol dispersion in each screw tube, 6.65 mL of the silane compound solution was added to prepare an alcohol dispersion containing the silane compound.

[0063] 3. Addition of alkaline aqueous solution Eight samples of the silane compound-added alcohol dispersion solution were maintained at each temperature. After 10 minutes, 10.03 mL of distilled water was added, and after another 10 minutes, an alkaline aqueous solution was added to prepare a raw material mixture solution. The alkaline aqueous solution was a pre-prepared 1 mol% sodium hydroxide solution. The amount of composite tungsten oxide microparticles contained in the raw material mixture solution at each temperature was estimated to be approximately 0.024 mass%.

[0064] <4. Mixing process> The raw material mixture solution maintained at each temperature was stirred at 300 rpm for 24 hours.

[0065] 5. Cleaning process After the stirring step, each raw material mixture solution was centrifuged (10,000 rpm, 30 minutes) once with ethanol and twice with distilled water, and then redispersed in distilled water to prepare aqueous dispersions of coated composite tungsten oxide microparticles at each temperature. The content of the composite tungsten oxide microparticles in each aqueous dispersion of coated composite tungsten oxide microparticles was set to 0.01% by mass.

[0066] 6. Evaluation of Silicon Compound Film Thickness and Uniformity The composite tungsten oxide microparticles contained in the eight aqueous dispersions of coated composite tungsten oxide microparticles according to Example 1 prepared at each temperature were observed under a transmission electron microscope, and the thickness of the silicon compound film was measured. The results are shown in Table 1.

[0067] The results in Table 1 demonstrate that by controlling the reaction temperature during silicon compound film formation, the film thickness of the silicon compound film can be controlled to the single nanometer order, and a film with a uniform thickness can be formed.

[0068] [Table 1]

[0069] [Comparative Example 1] The aqueous dispersion A prepared in Example 1 and water were added to the screw tube prepared in <1. Alcohol dispersion preparation step> of Example 1, and diluted to a content of composite tungsten oxide microparticles of 0.01 mass %, thereby preparing an aqueous dispersion according to Comparative Example 1.

[0070] 7. Evaluation of humidity and heat resistance Of the aqueous dispersions of coated composite tungsten oxide microparticles prepared in Example 1, the aqueous dispersion of coated composite tungsten oxide microparticles in which a silicon compound film was prepared at a reaction temperature of 70°C and the aqueous dispersion of Comparative Example 1 were left for 3 days in a dryer set to 80°C. The dryer was operated at 80°C for only 7 hours per day. Then, from the beginning, the absorbance at a wavelength of 1000 nm was measured every day using a spectrophotometer, the optical path length of which was set to 1 mm.

[0071] The measurement results are shown in Figures 2 and 3. Figure 2 is a graph showing the results of evaluating the moist heat resistance (absorbance) of the aqueous dispersion of coated composite tungsten oxide microparticles according to Example 1 and the aqueous dispersion of Comparative Example 1, with the vertical axis representing absorbance and the horizontal axis representing elapsed time (days), with the measurement results of Example 1 shown by a solid line and the measurement results of Comparative Example 1 shown by a dashed line. Figure 3 is a graph showing the results of evaluating the moist heat resistance (absorbance retention rate) of the aqueous dispersion of coated composite tungsten oxide microparticles according to Example 1 and the aqueous dispersion of Comparative Example 1, with the vertical axis representing absorbance retention rate and the horizontal axis representing elapsed time (days), with the measurement results of Example 1 shown by a solid line and the measurement results of Comparative Example 1 shown by a dashed line.

[0072] 2 and 3, especially the results in Fig. 3, it was found that the decrease in absorbance retention rate in the moist heat resistance evaluation was more gradual in Example 1 than in Comparative Example 1. In other words, it was found that the silicon compound film coating reduced contact between the composite tungsten oxide microparticles and water molecules, improving the moist heat resistance of the composite tungsten oxide microparticles.

Claims

1. an alcohol dispersion preparation step of preparing an alcohol dispersion containing composite tungsten oxide microparticles and alcohol; a silane compound adding step of adding a silane compound solution containing an alkoxysilane compound and an alcohol to the alcohol dispersion to prepare a silane compound-added alcohol dispersion; an alkaline aqueous solution adding step of adding an alkaline aqueous solution to the silane compound-added alcohol dispersion to prepare a raw material mixed solution; The method for coating the surface of composite tungsten oxide particles with a silicon compound film includes a stirring step of continuously stirring the raw material mixed solution for 12 hours or more to coat the surface of the composite tungsten oxide particles with a silicon compound film.

2. 2. The method for coating the surfaces of composite tungsten oxide particles with a silicon compound film according to claim 1, wherein the silicon compound film on the composite tungsten oxide particles has a film thickness of 2 nm to 6 nm.

3. 3. The method for coating the surfaces of composite tungsten oxide particles with a silicon compound film according to claim 1, wherein the alkoxysilane compound is tetraethoxysilane.

4. 3. The method for coating the surfaces of composite tungsten oxide particles with a silicon compound film according to claim 1, wherein the alkaline aqueous solution is an aqueous sodium hydroxide solution.

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