Aqueous silicic acid solution for undercoat for photocatalyst film, photocatalyst coating liquid, and member

Abstract

The present invention provides an aqueous silicic acid solution that can be uniformly applied to a base material and a coating method to which a conventional silicon compound-based binder is not applicable, and that is suitable as a binder for a photocatalyst which enables the achievement of a film having high water resistance and transparency and as an undercoat for a photocatalyst film.　The present invention specifically provides an aqueous silicic acid solution for an undercoat for a photocatalyst film, the aqueous silicic acid solution containing an alkali silicate and an aqueous solvent, wherein: the silicic acid concentration (in terms of SiO2) is 0.1% by mass to 3% by mass; the molar ratio (SiO2 / X) of silicon to an alkali X is 10,000 to 85; and the viscosity at 25°C is 1 mPa∙s to 50 mPa∙s. The present invention also specifically provides a photocatalyst coating liquid which comprises a photocatalyst and an aqueous silicic acid solution, wherein the aqueous silicic acid solution contains an alkali silicate and an aqueous solvent, and has a silicic acid concentration (in terms of SiO2) of 0.1% by mass to 3% by mass, a molar ratio (SiO2 / X) of silicon (in terms of SiO2) to an alkali X of 85 to 10,000, and a viscosity at 25°C of 1 mPa∙s to 50 mPa∙s.

Description

Silicic acid aqueous solution for undercoating of photocatalytic film, photocatalytic coating liquid and components

[0001] The present invention relates to an aqueous silicic acid solution for use as an undercoat for a photocatalytic film, a photocatalytic coating liquid containing the aqueous silicic acid solution, and a member having a surface coated with a coating film made from the liquid.

[0002] In recent years, consumers have increasingly sought hygienic living environments, and interest is growing in household products that have been treated to maintain cleanliness through antibacterial, antifungal, antiviral and anti-odor properties.

[0003] Photocatalytic materials have attracted attention because they are effective in a wide range of cleaning substrate surfaces, including antibacterial, antifungal, antiviral, deodorizing, and antifouling, due to the photocatalytic reaction that occurs when exposed to sunlight or artificial light. A photocatalytic reaction is a reaction caused by excited electrons and holes generated when a photocatalyst, such as titanium oxide, absorbs light. The excited electrons and holes generated on the titanium oxide surface by the photocatalytic reaction undergo a redox reaction with oxygen and water adsorbed on the titanium oxide surface, generating active species. These active species decompose organic microorganisms, viruses, odors, and dirt, which is thought to result in the cleaning effects described above on substrate surfaces.

[0004] The active species produced by photocatalytic reactions basically act on any type of organic matter, so photocatalysis has the advantage of being effective against a wide variety of microorganisms, viruses, odors and dirt, and there is almost no deterioration over time.

[0005] Recently, studies have been conducted to apply the above-mentioned photocatalytic action not only to outdoor use where ultraviolet light (wavelengths of 10 to 400 nm) can be used, but also to indoor spaces illuminated by light sources such as fluorescent lamps, most of which emit light in the visible region (wavelengths of 400 to 800 nm). For example, a tungsten oxide photocatalyst (JP 2009-148700 A: Patent Document 1) has been developed as a visible light responsive photocatalyst.

[0006] Generally, such photocatalysts are used by dispersing photocatalyst particles in a solvent, mixing with a binder (film-forming component) to form a coating liquid, and applying it to a substrate. The binder is selected from materials that fix the photocatalyst to the substrate and that do not or are resistant to degradation due to photocatalytic action.

[0007] Furthermore, when forming a photocatalytic film on the surface of a substrate that is expected to deteriorate due to photocatalytic action, such as an organic substrate or a substrate coated with an organic paint, an undercoat (primer) layer is often provided as an intermediate layer between the substrate surface and the photocatalytic film in order to prevent deterioration of the substrate surface and improve the adhesion of the photocatalytic film. Since the performance required of the materials used for the binder and undercoat is similar, the same materials are often used.

[0008] As binders and undercoats that are resistant to deterioration due to photocatalytic action, those whose main component is an inorganic material are preferred, and for example, a coating composition using an alkali metal silicate (JP 2001-098187 A: Patent Document 2) and an aqueous paint composition containing photocatalyst particles, an alkali metal silicate, and a weather-resistant resin emulsion (JP 2003-096394 A: Patent Document 3) have been disclosed; however, the former has problems such as poor water resistance and poor coatability and adhesion to organic substrates, and the dispersion stability of the photocatalyst particles is hindered due to the high alkaline component content, while the latter contains a resin emulsion, and therefore has problems such as inferior durability compared to inorganic materials alone, reduced photocatalytic functions such as hydrophilicity, and the dispersion stability of the photocatalyst particles is hindered due to the resin emulsion and alkaline component.

[0009] The present inventors have also discovered that by using a coating liquid in which a silicon compound binder is added to titanium oxide particles containing a solid solution of tin and a transition metal that enhances visible light activity and having iron and silicon attached to the surface, a highly transparent photocatalytic film that exhibits higher photocatalytic activity than conventional films can be obtained under visible light irradiation (JP 2021-175697 A: Patent Document 4). However, depending on the application method and substrate, the silicon compound binder can sometimes cause uneven application of the coating liquid, making it difficult to obtain a uniform coating film, or can have poor water resistance.

[0010] JP 2009-148700 A JP 2001-098187 A JP 2003-096394 A JP 2021-175697 A

[0011] Therefore, an object of the present invention is to provide an aqueous silicic acid solution that can be used as a binder or undercoat for a photocatalyst, which can be uniformly applied to substrates and by coating methods that have been difficult with conventional silicon compound binders, and which can produce a film with high water resistance and transparency.

[0012] In order to achieve the above object, the present inventors have conducted detailed studies on the raw materials and component ratios of silicon compound binders and undercoats. As a result, they have found that by using an alkali silicate in which the silicon to alkali molar ratio is set to a specific value, it is possible to form a highly uniform film even with coating methods and substrates that have been difficult to use in the past, and that the resulting coating film also has excellent water resistance and transparency, which led to the present invention.

[0013] Accordingly, the present invention provides the following silicic acid aqueous solution, a coating film formed using the silicic acid aqueous solution, a member having a coating film on its surface, and a method for producing the silicic acid aqueous solution. [1] A silicic acid aqueous solution for undercoating a photocatalytic film, comprising an alkali silicate and an aqueous solvent, and having a silicic acid concentration (SiO2 equivalent): 0.1 to 3 mass %, a molar ratio of silicon (SiO2 equivalent) to alkali X (SiO2 / X): 85 to 10,000, and a viscosity at 25°C of 1 to 50 mPa s. [2] The silicic acid aqueous solution according to [1], further containing a surfactant. [3] The silicic acid aqueous solution according to [1] or [2], wherein the aqueous solvent is a mixed solvent of water and a hydrophilic organic solvent. [4] A photocatalytic coating liquid comprising a photocatalyst and an aqueous silicic acid solution, wherein the aqueous silicic acid solution contains an alkali silicate and an aqueous solvent, and has a silicic acid concentration (SiO2 equivalent): 0.1 to 3 mass %, a molar ratio of silicon (SiO2 equivalent) to alkali X (SiO2 / X): 85 to 10,000, and a viscosity at 25°C of 1 to 50 mPa s. [5] The photocatalytic coating liquid according to [4], further comprising a surfactant. [6] The photocatalytic coating liquid according to [4] or [5], wherein the aqueous solvent is a mixed solvent of water and a hydrophilic organic solvent. [7] A member having a surface having a coating film made from the liquid according to any one of [1] to [6].

[0014] The aqueous silicic acid solution of the present invention has superior coating properties to conventional silicon compound binders, and can easily produce photocatalytic films and undercoats with high transparency and water resistance. Therefore, the aqueous silicic acid solution of the present invention is useful for forming photocatalytic films on various components used in actual environments where rapid cleaning of the substrate surface is required.

[0015] The present invention will be described in detail below.

[0016] The present invention relates to an aqueous silicic acid solution for undercoating a photocatalyst film (undercoat solution) or a photocatalyst coating solution, which contains an aqueous silicic acid solution suitable as an undercoat of a photocatalyst film and as a binder for the photocatalyst. First, the aqueous silicic acid solution for undercoating a photocatalyst film (undercoat solution), which is one embodiment of the present invention, will be described.

[0017] <Aqueous silicic acid solution (undercoat liquid) for undercoating photocatalytic films> The aqueous silicic acid solution for undercoating photocatalytic films of the present invention contains an alkali silicate and an aqueous solvent, and has a silicic acid concentration (SiO2 equivalent): 0.1 to 3 mass %; a molar ratio of silicon (SiO2 equivalent) to alkali X (SiO2 / X): 85 to 10,000; and a viscosity at 25°C of 1 to 50 mPa·s. The aqueous silicic acid solution of the present invention can be used as an undercoat liquid for photocatalytic films. The undercoat liquid in the present invention is applied to the surface of a substrate that may be deteriorated by photocatalytic action to form an undercoat, thereby preventing deterioration of the substrate surface due to the photocatalytic film.

[0018] <Alkali Silicate> The aqueous silicic acid solution of the present invention contains an alkali silicate in an aqueous solvent such that silicon and alkali have a specific molar ratio.

[0019] As the alkali silicate in the present invention, various known alkali silicates can be used. Examples include alkali metal silicates such as sodium silicate, potassium silicate, and lithium silicate; and alkaline earth metal silicates such as calcium silicate and magnesium silicate. These may be used alone or in combination of two or more. Among these, it is preferable to use sodium silicate, which is easily available and easy to handle.

[0020] The molar ratio (SiO2 / X) of silicon (SiO2 equivalent) in the alkali silicate to alkali X in the aqueous silicic acid solution is 85 to 10,000, preferably 100 to 5,000, more preferably 100 to 2,500, even more preferably 350 to 2,500, and particularly preferably 400 to 2,000. A molar ratio exceeding 10,000 can result in reduced stability of the solution and the need for significant effort to remove the alkali, while a molar ratio of less than 85 can result in reduced transparency and water resistance of the coating film. In particular, to obtain a coating film that is both water-resistant and highly uniform, the lower limit of the molar ratio (SiO2 / X) is preferably 100 or more, more preferably 100 or more, even more preferably 350 or more, and particularly preferably 400 or more. The upper limit of the molar ratio (SiO2 / X) is preferably 5,000 or less, more preferably 2,500 or less, and even more preferably 2,000 or less. The alkali X is preferably an alkali metal or an alkaline earth metal. The molar ratio (SiO / X) of silicon in the alkali silicate to alkali X in the aqueous silicic acid solution can be adjusted to a range of 85 to 10,000 by adjusting the parameters of the method for removing the alkali component from the alkali silicate, such as the amount of ion exchange resin added in the ion exchange method, the current density and treatment time in the electrodialysis method, or the amount of aqueous solvent added during filtration in the membrane filtration method.

[0021] The viscosity of the aqueous silicic acid solution at 25°C is 1 to 50 mPa·s, preferably 1.5 to 40 mPa·s, and more preferably 3 to 20 mPa·s. If the viscosity of the aqueous silicic acid solution at 25°C is less than 1 mPa·s, the coatability may be insufficient, and if it exceeds 50 mPa·s, the stability of the aqueous silicic acid solution may be reduced. The viscosity of the aqueous silicic acid solution at 25°C may be measured using any device that can measure the viscosity of Newtonian fluids and non-Newtonian fluids, and for example, a rotational viscometer or a vibration viscometer can be used. In the present invention, the value measured using a tuning fork vibration viscometer is used. To make the viscosity of the aqueous silicic acid solution at 25°C within the range of 1 to 50 mPa·s, the aqueous silicic acid solution in which the silicic acid concentration and the molar ratio of silicon to alkali X have been adjusted may be heat-treated.

[0022] The silicic acid concentration in the silicic acid aqueous solution, calculated as SiO2, is 0.1 to 3 mass%, preferably 0.2 to 2.5 mass%, and more preferably 0.3 to 2 mass%. If the silicic acid concentration in the silicic acid aqueous solution, calculated as SiO2, is less than 0.1 mass%, the coatability and thickness of the coating film may be insufficient, while if it exceeds 3 mass%, the stability of the silicic acid aqueous solution may be reduced. To adjust the silicic acid concentration in the silicic acid aqueous solution to 0.1 to 3 mass% calculated as SiO2, an alkali silicate with a known SiO2 concentration can be diluted with an aqueous solvent.

[0023] <Aqueous Solvent> As the aqueous solvent in the present invention, water is preferably used, but a mixed solvent of water and a hydrophilic organic solvent mixed with water in any ratio may also be used. Examples of water that may be used include purified water such as filtered water, deionized water, distilled water, and pure water. Examples of hydrophilic organic solvents include alcohols such as methanol, ethanol, and isopropanol; glycols such as ethylene glycol; and glycol ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, and propylene glycol-n-propyl ether. When a mixed solvent is used, the proportion of the hydrophilic organic solvent in the mixed solvent is preferably more than 0% by mass and 80% by mass or less, more preferably 60% by mass or less, and even more preferably 50% by mass or less.

[0024] <Surfactant> The surfactant in the present invention is an optional component that improves coatability by adjusting the interfacial tension of the liquid to improve wettability to the substrate. Examples include anionic surfactants such as fatty acid sodium salts, alkylbenzene sulfonates, higher alcohol sulfate salts, and polyoxyethylene alkyl ether sulfate salts; cationic surfactants such as alkyltrimethylammonium salts, dialkyldimethylammonium salts, alkyldimethylbenzylammonium salts, and quaternary ammonium salts; amphoteric surfactants such as alkylamino fatty acid salts, alkyl betaines, and alkylamine oxides; modified silicone oils such as polyether-modified silicones, aralkyl-modified silicones, alkyl-modified silicones, and higher fatty acid ester-modified silicones; and nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenol ethers, alkyl glucosides, polyoxyethylene fatty acid esters, sucrose fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, fatty acid alkanolamides, and acetylene-based surfactants. Among these, nonionic surfactants are preferred from the viewpoint of coating liquid stability. Furthermore, surfactants with an HLB (Griffin method) of 7 to 18 are preferred, and those with an HLB of 10 to 15 are even more preferred. Of these, acetylene-based surfactants and polyether-modified silicone-based surfactants are preferred because they exhibit good properties as aqueous surfactants.

[0025] The acetylene surfactant is preferably acetylene glycol or an alkoxylated acetylene glycol, and is more preferably one represented by the following general formula: R 1 -C≡C-R 2 R 1 and R 2 are each independently a monovalent organic group, preferably a hydrocarbon group having 1 to 20 carbon atoms in which some of the hydrogen atoms of the hydrocarbon group have been substituted with OH groups, or R 1 and R 2may have an ether bond in which a part of the hydrocarbon group having 1 to 20 carbon atoms contains an alkoxy group having 1 to 5 carbon atoms. Examples of acetylene-based surfactants include Olfine EXP4001, EXP4200, EXP4123, and EXP4300 (manufactured by Nissin Chemical Industry Co., Ltd.).

[0026] The polyether-modified silicone surfactant is preferably a side chain type in which a polyoxyalkylene group is introduced into the side chain of a silicone chain that serves as the main chain, and examples of side chain type polyether-modified silicones include KF-351A, KF-353, KF-355A, KF-615A, KF-640, KF-642, KF-643, KF-644, KF-6204, and KF-6011 (manufactured by Shin-Etsu Chemical Co., Ltd.).

[0027] When a surfactant is added to the aqueous silicic acid solution of the present invention, the amount added is preferably 0.001 to 1.0% by mass, more preferably 0.005 to 0.2% by mass, and even more preferably 0.01 to 0.1% by mass, based on the aqueous silicic acid solution. If the amount of surfactant added to the aqueous silicic acid solution is less than 0.001% by mass, the coatability may not be sufficiently improved, and if it exceeds 1.0% by mass, the durability of the film obtained from the aqueous silicic acid solution may decrease.

[0028] In order to improve adhesion and durability to the substrate, the aqueous silicic acid solution (undercoat liquid) for undercoating the photocatalytic film may further contain metal compounds containing aluminum, titanium, zirconium, etc.; organic resins such as fluorine-based resins, acrylic resins, urethane resins, etc. However, these optional components are added within the range in which the aqueous silicic acid solution satisfies the above-mentioned silicic acid concentration, molar ratio of silicon to alkali X, and viscosity. The blending amount of components other than the alkali silicate, aqueous solvent, and surfactant in the aqueous silicic acid solution (undercoat liquid) for undercoating the photocatalytic film is not particularly limited, but is preferably 0.5 or less, more preferably 0.1 or less, even more preferably 0.05 or less, and particularly preferably 0.01 or less, as a mass ratio based on the alkali silicate silicic acid (SiO2 equivalent).

[0029] Next, a photocatalytic coating liquid, which is another embodiment of the present invention, will be described.

[0030] <Photocatalytic Coating Liquid> The photocatalytic coating liquid of the present invention contains a photocatalyst and a silicic acid aqueous solution. The silicic acid aqueous solution contained in the photocatalytic coating liquid, like the silicic acid aqueous solution for the undercoat of the photocatalytic film, contains an alkali silicate and an aqueous solvent, and has a silicic acid concentration (SiO2 equivalent): 0.1 to 3 mass %; a molar ratio of silicon (SiO2 equivalent) to alkali X (SiO2 / X): 85 to 10,000; and a viscosity at 25°C of 1 to 50 mPa·s. In other words, the silicic acid aqueous solution for the undercoat of the photocatalytic film of the present invention can be used as a binder for the photocatalytic coating liquid. The photocatalytic coating liquid of the present invention is applied to a substrate to form a photocatalytic film, and contains at least a photocatalyst and the silicic acid aqueous solution of the present invention. <Photocatalyst> The photocatalyst is a general term for a substance that exhibits photocatalytic activity when irradiated with light having an energy equal to or greater than a predetermined band gap. As such a substance, particles of known metal oxide semiconductors such as titanium oxide, tungsten oxide, zinc oxide, tin oxide, iron oxide, bismuth oxide, bismuth vanadate, strontium titanate, etc. can be used alone or in combination of two or more. Among these, it is preferable to use titanium oxide particles, which have a particularly high photocatalytic activity under irradiation with light including ultraviolet light of 400 nm or less, are chemically stable, and are relatively easy to synthesize into nano-sized particles and disperse in a solvent.

[0031] <Titanium Oxide Particles> The titanium oxide particles themselves are not particularly limited, and titanium oxide particles used as a photocatalyst can be used.

[0032] Although three types of crystalline phases are generally known for titanium oxide particles: rutile, anatase, and brookite, the titanium oxide particles of the present invention are preferably primarily anatase or rutile. Here, "primarily" means that titanium oxide particles of this crystalline phase account for 50% by mass or more of the total titanium oxide particles, preferably 70% by mass or more, more preferably 90% by mass or more, and may even be 100% by mass.

[0033] To enhance the photocatalytic activity of titanium oxide particles, those that support metal compounds such as platinum, gold, palladium, iron, copper, and nickel, or those that form solid solutions with non-metallic components such as nitrogen, carbon, and sulfur, or metal components such as tin, vanadium, chromium, manganese, niobium, molybdenum, rhodium, antimony, tungsten, and cerium can also be used.

[0034] In this specification, the term "solid solution" refers to a phase in which atoms at the lattice points of a certain crystal phase are replaced with other atoms, or other atoms enter the lattice gaps, i.e., a mixed phase in which another substance is considered to have dissolved into a certain crystal phase, and refers to a homogeneous crystalline phase. A solid solution in which solvent atoms at the lattice points are replaced with solute atoms is called a substitutional solid solution, and a solid solution in which solute atoms enter the lattice gaps is called an interstitial solid solution, and in this specification, both of these are referred to.

[0035] The method for dissolving a different metal in a metal oxide crystal is not particularly limited, but examples thereof include a gas phase method (such as a CVD method or a PVD method), a liquid phase method (such as a hydrothermal method or a sol-gel method), and a solid phase method (such as a high-temperature calcination method).

[0036] <Titanium Oxide Particle Dispersion> The titanium oxide particles described above are preferably dispersed in an aqueous dispersion medium. Water is preferably used as the aqueous dispersion medium, but a mixed solvent of water and a hydrophilic organic solvent, mixed with water at any ratio, may also be used. Examples of water that are preferred include purified water such as filtered water, deionized water, distilled water, and pure water. Examples of hydrophilic organic solvents that are preferred include alcohols such as methanol, ethanol, and isopropanol; glycols such as ethylene glycol; and glycol ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, and propylene glycol-n-propyl ether. When a mixed solvent is used, the proportion of the hydrophilic organic solvent in the mixed solvent is preferably more than 0% by mass and 80% by mass or less, more preferably 60% by mass or less, and even more preferably 50% by mass or less.

[0037] The dispersed particle diameter of titanium oxide particles in the titanium oxide particle dispersion is the 50% cumulative distribution diameter (hereinafter referred to as D 50 The average particle size (sometimes expressed as "average particle size") is preferably 5 to 30 nm. This is because if the average particle size is less than 5 nm, the photocatalytic activity may be insufficient, and if it exceeds 30 nm, the dispersion and the photocatalytic film obtained using the dispersion may become opaque.

[0038] In addition, the 90% cumulative distribution diameter (hereinafter referred to as D 90 The thickness (D) is preferably 5 to 100 nm, more preferably 5 to 80 nm. 90 However, if the diameter is less than 5 nm, the photocatalytic activity may be insufficient, and if the diameter is more than 100 nm, the dispersion and the photocatalytic film obtained using the dispersion may become opaque. 50 and D 90 As an apparatus for measuring the amount of carbon dioxide, for example, ELSZ-2000ZS (manufactured by Otsuka Electronics Co., Ltd.), Nanotrac UPA-EX150 (manufactured by Nikkiso Co., Ltd.), LA-910 (manufactured by Horiba Ltd.), etc. can be used.

[0039] Titanium oxide particles are D 50 and D90 It is preferable that the particle has a molecular weight within the above range, since it is possible to obtain a dispersion having high photocatalytic activity and a highly transparent photocatalytic film from the dispersion.

[0040] Examples of such titanium oxide photocatalysts include anatase titanium oxide particles whose surfaces are modified with peroxo groups (Japanese Patent Laid-Open No. 10-67516), and titanium oxide particles in which tin and a transition metal that enhances visible light activity are dissolved in a solid solution and in which iron and silicon are attached to the surface (Japanese Patent Laid-Open No. 2021-175697).

[0041] The concentration of titanium oxide particles in the titanium oxide particle dispersion is preferably 0.01 to 20% by mass, particularly preferably 0.5 to 10% by mass, from the viewpoint of ease of producing a photocatalytic film of a required thickness.

[0042] <Aqueous silicic acid solution> The aqueous silicic acid solution contained in the photocatalytic coating liquid contains an alkali silicate in an aqueous solvent such that silicon and alkali have a specific molar ratio.

[0043] <Alkali silicate> As the alkali silicate, various known alkali silicates can be used. Examples include alkali metal silicates such as sodium silicate, potassium silicate, and lithium silicate; and alkaline earth metal silicates such as calcium silicate and magnesium silicate. These may be used alone or in combination of two or more. Among these, it is preferable to use sodium silicate, which is easy to obtain and handle.

[0044] The molar ratio (SiO2 / X) of silicon (SiO2 equivalent) in the alkali silicate to alkali X in the aqueous silicic acid solution is 85 to 10,000, preferably 100 to 5,000, more preferably 100 to 2,500, even more preferably 350 to 2,500, and particularly preferably 400 to 2,000. A molar ratio exceeding 10,000 can result in reduced stability of the solution and the need for significant effort to remove the alkali, while a molar ratio of less than 85 can result in reduced transparency and water resistance of the coating film. In particular, to obtain a coating film that is both water-resistant and highly uniform, the lower limit of the molar ratio (SiO2 / X) is preferably 100 or more, more preferably 100 or more, even more preferably 350 or more, and particularly preferably 400 or more. The upper limit of the molar ratio (SiO2 / X) is preferably 5,000 or less, more preferably 2,500 or less, and even more preferably 2,000 or less. The alkali X is preferably an alkali metal or an alkaline earth metal. The molar ratio (SiO / X) of silicon in the alkali silicate to the alkali X in the aqueous silicic acid solution can be adjusted to a range of 85 to 10,000 by an ion exchange method for adsorbing and removing the alkali X, electrodialysis using an ion exchange membrane, an ultrafiltration membrane, a reverse osmosis membrane, or the like.

[0045] The viscosity of the aqueous silicic acid solution at 25°C is 1 to 50 mPa·s, preferably 1.5 to 40 mPa·s, and more preferably 3 to 20 mPa·s. If the viscosity of the aqueous silicic acid solution at 25°C is less than 1 mPa·s, the coatability may be insufficient, and if it exceeds 50 mPa·s, the stability of the aqueous silicic acid solution may be reduced. In order to make the viscosity of the aqueous silicic acid solution at 25°C within the range of 1 to 50 mPa·s, it is sufficient to heat-treat the aqueous silicic acid solution in which the silicic acid concentration and the molar ratio of silicon to alkali X have been adjusted.

[0046] The silicic acid concentration in the silicic acid aqueous solution, calculated as SiO2, is 0.1 to 3 mass%, preferably 0.2 to 2.5 mass%, and more preferably 0.3 to 2 mass%. If the silicic acid concentration in the silicic acid aqueous solution, calculated as SiO2, is less than 0.1 mass%, the coatability and thickness of the coating film may be insufficient, while if it exceeds 3 mass%, the stability of the silicic acid aqueous solution may be reduced. To adjust the silicic acid concentration in the silicic acid aqueous solution to 0.1 to 3 mass% calculated as SiO2, an alkali silicate with a known SiO2 concentration may be diluted with an aqueous solvent.

[0047] <Aqueous Solvent> Water is preferably used as the aqueous solvent, but a mixed solvent of water and a hydrophilic organic solvent mixed with water in any ratio may also be used. Examples of water that may be used include purified water such as filtered water, deionized water, distilled water, and pure water. Examples of hydrophilic organic solvents include alcohols such as methanol, ethanol, and isopropanol; glycols such as ethylene glycol; and glycol ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, and propylene glycol-n-propyl ether. When a mixed solvent is used, the proportion of the hydrophilic organic solvent in the mixed solvent is preferably more than 0% by mass and 80% by mass or less, more preferably 60% by mass or less, and even more preferably 50% by mass or less.

[0048] <Surfactant> The surfactant is an optional component that improves coatability by adjusting the interfacial tension of the liquid to improve wettability to the substrate. Examples include anionic surfactants such as fatty acid sodium salts, alkylbenzene sulfonates, higher alcohol sulfate salts, and polyoxyethylene alkyl ether sulfate salts; cationic surfactants such as alkyltrimethylammonium salts, dialkyldimethylammonium salts, alkyldimethylbenzylammonium salts, and quaternary ammonium salts; amphoteric surfactants such as alkylamino fatty acid salts, alkyl betaines, and alkylamine oxides; modified silicone oils such as polyether-modified silicones, aralkyl-modified silicones, alkyl-modified silicones, and higher fatty acid ester-modified silicones; and nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenol ethers, alkyl glucosides, polyoxyethylene fatty acid esters, sucrose fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, fatty acid alkanolamides, and acetylene-based surfactants. Among these, nonionic surfactants are preferred from the viewpoint of coating liquid stability. Furthermore, surfactants with an HLB (Griffin method) of 7 to 18 are preferred, and those with an HLB of 10 to 15 are even more preferred. Of these, acetylene-based surfactants and polyether-modified silicone-based surfactants are preferred because they exhibit good properties as aqueous surfactants.

[0049] The acetylene surfactant is preferably acetylene glycol or an alkoxylated acetylene glycol, and is more preferably one represented by the following general formula: R 1 -C≡C-R 2 R 1 and R 2 are each independently a monovalent organic group, preferably a hydrocarbon group having 1 to 20 carbon atoms in which some of the hydrogen atoms of the hydrocarbon group have been substituted with OH groups, or R 1 and R 2may have an ether bond in which a part of the hydrocarbon group having 1 to 20 carbon atoms contains an alkoxy group having 1 to 5 carbon atoms. Examples of acetylene-based surfactants include Olfine EXP4001, EXP4200, EXP4123, and EXP4300 (manufactured by Nissin Chemical Industry Co., Ltd.).

[0050] The polyether-modified silicone surfactant is preferably a side chain type in which a polyoxyalkylene group is introduced into the side chain of a silicone chain that serves as the main chain, and examples of side chain type polyether-modified silicones include KF-351A, KF-353, KF-355A, KF-615A, KF-640, KF-642, KF-643, KF-644, KF-6204, and KF-6011 (manufactured by Shin-Etsu Chemical Co., Ltd.).

[0051] When a surfactant is added to the aqueous silicic acid solution, the amount added is preferably 0.001 to 1.0% by mass, more preferably 0.005 to 0.2% by mass, and even more preferably 0.01 to 0.1% by mass, relative to the aqueous silicic acid solution. If the amount of surfactant added to the aqueous silicic acid solution is less than 0.001% by mass, the coatability may not be sufficiently improved, and if it exceeds 1.0% by mass, the durability of the film obtained from the aqueous silicic acid solution may decrease.

[0052] To improve adhesion to the substrate and durability, the photocatalytic coating liquid may further contain metal compounds containing aluminum, titanium, zirconium, etc.; organic resins such as fluorine-based resins, acrylic resins, urethane resins, etc. However, these optional components are added within the range in which the aqueous silicic acid solution satisfies the above-mentioned silicic acid concentration, molar ratio of silicon to alkali X, and viscosity. The blending amounts of components other than the alkali silicate, aqueous solvent, and surfactant in the aqueous silicic acid solution (undercoat liquid) for undercoating the photocatalytic film are not particularly limited, but the silicic acid (SiO of the alkali silicate) is preferably 0.05 to 0.05. ２The mass ratio based on the total mass of the photocatalyst particles (calculated as a percentage of the total mass of the photocatalyst particles) is preferably 0.5 or less, more preferably 0.1 or less, even more preferably 0.05 or less, and particularly preferably 0.01 or less. In particular, in the case of organic resins, especially resin emulsions, it is desirable to consider not impairing photocatalytic functions such as durability and hydrophilicity, dispersion stability of the photocatalyst particles, etc.

[0053] The dispersed particle diameter of titanium oxide particles in the photocatalytic coating solution is the 50% cumulative distribution diameter (hereinafter referred to as D) on a volume basis measured by dynamic light scattering using laser light. 50 The average particle size (sometimes expressed as "average particle size") is preferably 5 to 50 nm, more preferably 5 to 30 nm. This is because if the average particle size is less than 5 nm, the photocatalytic activity may be insufficient, and if it exceeds 50 nm, the coating liquid and the photocatalytic film obtained using the same may become opaque.

[0054] In addition, the 90% cumulative distribution diameter (hereinafter referred to as D 90 The thickness (D) is preferably 5 to 100 nm, more preferably 5 to 80 nm. 90 However, if the diameter is less than 5 nm, the photocatalytic activity may be insufficient, and if the diameter is more than 100 nm, the coating solution and the photocatalytic film obtained using the same may become opaque. 50 and D 90 The above-mentioned device can be used to measure the above.

[0055] The mass ratio of titanium oxide particles to the aqueous silicic acid solution serving as a binder [titanium oxide particles / silicic acid (SiO2 equivalent)] is preferably in the range of 0.01 to 99, more preferably 0.01 to 9, and even more preferably 0.01 to 2.5. This is because if the mass ratio exceeds 99, the adhesion of the titanium oxide particles to the surface of various substrates may be insufficient, and if it is less than 0.01, the photocatalytic activity may be insufficient.

[0056] Next, a method for producing the aqueous silicic acid solution for use as an undercoat for the photocatalytic film of the present invention or the aqueous silicic acid solution contained as a binder in the photocatalytic coating liquid of the present invention will be described.

[0057] <Method for Producing an Aqueous Silica Solution> The method for producing an aqueous silicic acid solution of the present invention comprises the following steps (1) to (3): (1) a step of producing an aqueous alkali silicate solution from an alkali silicate and water, (2) a step of removing the alkali from the aqueous alkali silicate solution, and (3) a step of heating and then cooling the aqueous silicic acid solution from which the alkali has been removed. Each step will be described in detail below.

[0058] Step (1) In step (1), an aqueous alkali silicate solution is produced by mixing an alkali silicate with water. Various known alkali silicates can be used as the alkali silicate. Examples include alkali metal silicates such as sodium silicate, potassium silicate, and lithium silicate; and alkaline earth metal silicates such as calcium silicate and magnesium silicate. These may be used alone or in combination. Among these, sodium silicate is preferred because it is easy to obtain and handle. The aqueous alkali silicate solution prepared in step (1) preferably has a SiO concentration of 0.1 to 3.0% by mass, more preferably 0.2 to 2.5% by mass, and even more preferably 0.3 to 2% by mass. The SiO concentration can be determined by using sodium silicate with a known concentration, but it can also be measured using other common methods for quantifying metal components. For example, the Si concentration can be quantified using a calibration curve method using an ICP emission spectrometer or an X-ray fluorescence spectrometer, and then converted to SiO.

[0059] Step (2) In step (2), the alkaline component is removed from the aqueous alkali silicate solution. The method for removing the alkaline component is not limited as long as it can remove the alkaline component from the alkali silicate solution. Examples of suitable methods include ion exchange using an ion exchange resin, electrodialysis using an ion exchange membrane, and membrane filtration using an ultrafiltration membrane or reverse osmosis membrane. While any method can be selected from the methods described above, the ion exchange method is particularly preferred, and it is preferable to use an acrylic or styrene resin as the ion exchange resin. Furthermore, it is preferable that the ion exchange resin be an acidic resin with a sulfo group or a carboxyl group introduced therein. The method for removing the alkaline component will be described in more detail below, taking the ion exchange method as an example. To the aqueous alkali silicate solution prepared in step (1), an amount of ion exchange resin sufficient to remove the alkaline component is added. Subsequently, the aqueous alkali silicate solution to which the ion exchange resin has been added is stirred, for example, for 15 minutes to 1 hour, and the ion exchange resin is filtered off, thereby removing the alkaline ions.

[0060] Step (3): In step (3), the silicic acid aqueous solution from which the alkaline components have been removed is heated to increase its viscosity, and then cooled to stop the increase in viscosity. Increasing the viscosity of the silicic acid aqueous solution can improve its coatability. When heated, the silicic acid aqueous solution quickly gels and colloidizes, making it unusable as an undercoat or photocatalyst binder. Therefore, after increasing the viscosity to a predetermined level, the solution is cooled to stabilize it.

[0061] From the viewpoint of viscosity control, the heating temperature is 30 to 80°C, preferably 40 to 60°C. This is because a heating temperature below 30°C may require a long time to increase the viscosity, while a heating temperature above 80°C may increase the viscosity in a short time, making it difficult to control. The heating time is adjusted so that the viscosity falls within the aforementioned range (viscosity at 25°C: 1 to 50 mPa·s), and is preferably 1 minute to 8 hours. A cooling temperature of 30°C or less is appropriate. This is because a cooling temperature above 30°C may reduce the effect of inhibiting the progression of gelation and colloidal formation of the silicic acid aqueous solution. Furthermore, the solution can be further stabilized by cooling and diluting the silicic acid aqueous solution with water and / or a hydrophilic organic solvent to reduce the silicic acid concentration. The water and / or hydrophilic organic solvents described above can be used. The silicic acid aqueous solution obtained by the above process can be used as the silicic acid aqueous solution for undercoating the photocatalyst film of the present invention.

[0062] <Method for Producing Photocatalytic Coating Liquid> The method for producing the photocatalytic coating liquid of the present invention includes, for example, a method of mixing the aqueous silicic acid solution obtained by the above steps (1) to (3) with the titanium oxide particle dispersion obtained by the following steps (4) to (6).

[0063] (4) A step of producing a peroxotitanic acid solution from a raw material titanium compound, a basic substance, hydrogen peroxide, and an aqueous dispersion medium; (5) A step of heating the peroxotitanic acid solution produced in the above step (4) at 80 to 250°C under pressure control to obtain a titanium oxide particle dispersion; and (6) A step of mixing the titanium oxide particle dispersion obtained in step (5) with the aqueous silicic acid solution obtained in steps (1) to (3). Each step will be described in detail below.

[0064] Step (4): In step (4), a peroxotitanic acid solution is produced by reacting a starting titanium compound, a basic substance, and hydrogen peroxide in an aqueous dispersion medium. More specifically, a basic substance is added to the starting titanium compound in the aqueous dispersion medium to form titanium hydroxide, and impurity ions other than the contained metal ions are removed. Hydrogen peroxide is then added to obtain a peroxotitanic acid solution.

[0065] Examples of the raw titanium compound include inorganic acid salts such as titanium chloride, nitrate, and sulfate, organic acid salts such as formic acid, citric acid, oxalic acid, lactic acid, and glycolic acid, and titanium hydroxide precipitated by adding an alkali to an aqueous solution of these to hydrolyze the salt. One or more of these may be used in combination. Among these, titanium chlorides (TiCl3, TiCl4) are preferably used.

[0066] In the present invention, the aqueous dispersion medium may be any of those described above, and is used in such a manner as to obtain the above-mentioned composition. The concentration of the aqueous solution of the starting titanium compound formed from the starting titanium compound and the aqueous dispersion medium is preferably 60% by mass or less, particularly 30% by mass or less. The lower limit of the concentration may be appropriately selected, but is usually preferably 1% by mass or more.

[0067] The basic substance is used to smoothly convert the starting titanium compound into titanium hydroxide. Examples include alkali metal or alkaline earth metal hydroxides such as sodium hydroxide and potassium hydroxide, ammonia, and amine compounds such as alkanolamines and alkylamines. Among these, ammonia is particularly preferred and is added in an amount that will adjust the pH of the starting titanium compound aqueous solution to 7 or higher, particularly 7 to 10. The basic substance may also be used in the form of an aqueous solution of an appropriate concentration together with the aqueous dispersion medium. Hydrogen peroxide is used to convert the starting titanium compound or titanium hydroxide into peroxotitanium, i.e., a titanium oxide compound containing Ti—O—O—Ti bonds, and is typically used in the form of aqueous hydrogen peroxide. The amount of hydrogen peroxide added is preferably 1.5 to 20 times the molar amount of Ti or the combined molar amount of Ti, transition metal, and Sn. In the reaction to convert the starting titanium compound or titanium hydroxide into peroxotitanium acid by adding hydrogen peroxide, the reaction temperature is preferably 5 to 80°C, and the reaction time is preferably 30 minutes to 24 hours.

[0068] The peroxotitanic acid solution thus obtained may contain an alkaline substance or an acidic substance for the purpose of adjusting the pH, etc. Examples of alkaline substances include ammonia, sodium hydroxide, calcium hydroxide, and alkylamines, while examples of acidic substances include inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, carbonic acid, phosphoric acid, and hydrogen peroxide, and organic acids such as formic acid, citric acid, oxalic acid, lactic acid, and glycolic acid. In this case, the pH of the resulting peroxotitanic acid solution is preferably 1 to 9, particularly 4 to 7, from the standpoint of safety in handling.

[0069] Step (5): In step (5), the peroxotitanic acid solution obtained in step (4) is subjected to a hydrothermal reaction under pressure control at a temperature of 80 to 250°C, preferably 100 to 250°C, for 0.01 to 24 hours. From the viewpoints of reaction efficiency and reaction controllability, a reaction temperature of 80 to 250°C is appropriate, resulting in the conversion of peroxotitanic acid to titanium oxide particles. Note that "under pressure control" here refers to applying appropriate pressure to maintain the reaction temperature when the reaction temperature exceeds the boiling point of the dispersion medium. This also includes controlling the reaction temperature at atmospheric pressure when the temperature is below the boiling point of the dispersion medium. The pressure used here is typically about 0.12 to 4.5 MPa, preferably about 0.15 to 4.5 MPa, and more preferably about 0.20 to 4.5 MPa. The reaction time is preferably 1 minute to 24 hours. Step (5) results in a titanium oxide particle dispersion.

[0070] The pH of the titanium oxide particle dispersion obtained in step (5) is preferably 7 to 14, and more preferably 9 to 14. The titanium oxide particle dispersion obtained in step (2) may contain an alkaline substance or an acidic substance for adjusting the pH to the aforementioned pH, and the alkaline substance, acidic substance, and pH adjustment method are the same as those for the peroxotitanic acid solution obtained in step (4).

[0071] The dispersed particle diameter (D 50 and D 90) is preferably in the range as already described, and the particle diameter can be controlled by adjusting the reaction conditions. For example, the particle diameter can be reduced by shortening the reaction time or the temperature rise time.

[0072] The concentration of titanium oxide particles in the titanium oxide particle dispersion liquid thus produced can be calculated from the mass of the nonvolatile content (titanium oxide particles) after sampling a portion of the titanium oxide particle dispersion liquid and heating it at 105°C for 1 hour to volatilize the solvent, and the mass of the sampled titanium oxide particle dispersion liquid, according to the following formula: concentration of titanium oxide particle dispersion liquid (%) = [mass of nonvolatile content (g) / mass of titanium oxide particle dispersion liquid (g)] × 100

[0073] Step (6): In step (6), the titanium oxide particle dispersion liquid obtained in step (5) and the silicic acid aqueous solution obtained by steps (1) to (3) are mixed in the ratio described above (i.e., the [titanium oxide particles / silicic acid (SiO2 equivalent)] ratio is preferably 0.01 to 99, more preferably 0.01 to 9, and even more preferably 0.01 to 2.5). The mixing method is not particularly limited as long as it is a method that uniformly mixes the two types of liquid, and for example, mixing can be performed by stirring using a commonly available stirrer. The temperature during mixing is preferably 5 to 50°C, preferably 10 to 30°C, and the time is preferably 1 minute to 1 hour.

[0074] The total concentration of titanium oxide particles and silicic acid (SiO2 equivalent) in the photocatalytic coating liquid thus prepared is preferably 0.01 to 20 mass%, particularly preferably 0.5 to 10 mass%, from the viewpoint of ease of producing a photocatalytic film of the required thickness. Regarding concentration adjustment, if the concentration is higher than the desired concentration, it can be reduced by diluting it with an aqueous solvent, and if it is lower than the desired concentration, it can be increased by volatilizing or filtering off the aqueous solvent.

[0075] <Components having a coating film formed from a silicic acid aqueous solution or a photocatalytic coating liquid on their surface> The silicic acid aqueous solution for undercoating a photocatalytic film and the photocatalytic coating liquid of the present invention can be used to form an undercoat or a photocatalytic film on the surface of various components, respectively. Here, the various components are not particularly limited, and examples of the materials for the components include organic materials and inorganic materials. These can have various shapes according to their respective purposes and uses.

[0076] Examples of organic materials include synthetic resin materials such as polyvinyl chloride resin (PVC), polyethylene (PE), polypropylene (PP), polycarbonate (PC), acrylic resin, polyacetal, fluororesin, silicone resin, ethylene-vinyl acetate copolymer (EVA), acrylonitrile-butadiene rubber (NBR), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyvinyl butyral (PVB), ethylene-vinyl alcohol copolymer (EVOH), polyimide resin, polyphenylene sulfide (PPS), polyetherimide (PEI), polyetheretherimide (PEEI), polyetheretherketone (PEEK), melamine resin, phenolic resin, and acrylonitrile-butadiene-styrene (ABS) resin; natural materials such as natural rubber; and semi-synthetic materials made from the above synthetic resin materials and natural materials. These may be commercialized into desired shapes and configurations such as films, sheets, textile materials, textile products, other molded products, and laminates.

[0077] Inorganic materials include, for example, non-metallic inorganic materials and metallic inorganic materials. Examples of non-metallic inorganic materials include glass, ceramics, and stone. These may be commercialized into various forms such as tiles, glass, mirrors, walls, and decorative materials. Examples of metallic inorganic materials include cast iron, steel, iron, iron alloys, aluminum, aluminum alloys, nickel, nickel alloys, and zinc die-cast. These may be plated with the metallic inorganic material, coated with the organic material, or plated on the surface of the organic material or non-metallic inorganic material.

[0078] The aqueous silicic acid solution or photocatalytic coating liquid for undercoating a photocatalytic film of the present invention is particularly useful for producing a photocatalytic film by applying it to various members made of inorganic substances such as glass and metal, and organic substances such as resin, and is particularly useful for producing a transparent photocatalytic film on various substrates.

[0079] The method for forming an undercoat or photocatalyst film on the surface of various substrates may involve applying an aqueous silicic acid solution or a photocatalyst coating liquid to the surface of the substrate by a known application method such as spray coating or dip coating, and then drying by a known drying method such as far-infrared drying, IH drying, or hot air drying. The thickness of the undercoat or photocatalyst film may be selected from various options, but is usually preferably in the range of 10 nm to 10 μm. This results in the formation of the undercoat or photocatalyst coating described above.

[0080] The undercoat formed in this manner is transparent, and can suppress deterioration of the substrate due to the photocatalytic action of the above-mentioned photocatalytic film, and can increase the adhesion between the substrate and the photocatalytic film.

[0081] The photocatalytic film formed in this manner is transparent and exhibits good photocatalytic action, particularly in the ultraviolet region (wavelength 10 to 400 nm). Various substrates on which the photocatalytic film is formed can exhibit cleaning, deodorizing, antibacterial, and other effects on the surface of the component due to the photocatalytic action of titanium oxide.

[0082] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to the following examples. Various evaluations in the present invention were carried out as follows.

[0083] (1) 50% and 90% cumulative distribution diameters of titanium oxide particles in the liquid (D 50 and D 90 ) D of titanium oxide particles in the liquid 50 and D 90 was calculated as the 50% and 90% cumulative distribution diameters on a volume basis measured by dynamic light scattering using a laser beam. Specifically, the photocatalytic coating liquid was measured using a particle size distribution analyzer (ELSZ-2000ZS, manufactured by Otsuka Electronics Co., Ltd.).

[0084] (2) Determination of alkali metal ion and alkaline earth metal ion concentrations The concentrations of alkali metal ions and alkaline earth metal ions in the solution were evaluated by ion chromatography. Specifically, the silicic acid aqueous solution was appropriately diluted and filtered through a 0.2 μm sieve, and then quantified using an ion chromatograph (930 Compact IC Flex, manufactured by Metrohm) equipped with a Metrosep C3 250 / 4.0 cation analysis column and an electric conductivity detector.

[0085] (3) Measurement of Liquid Viscosity The liquid viscosity was measured using a tuning fork vibro viscometer (SV-10, manufactured by A&D Co., Ltd.) after adjusting the liquid temperature to 25°C.

[0086] (4) Evaluation of the coatability of the undercoat liquid or photocatalytic coating liquid on various substrates 2 g of the liquid was spread on one side of a substrate cut to A4 size (210 mm x 297 mm) using a #7 wire bar coater, and the state of the liquid film when the liquid film was spread and dried for 30 minutes in an oven set at 80°C was evaluated visually according to the following criteria. Polyethylene terephthalate (PET) film and polyvinyl chloride (PVC) sheet were used as substrates for checking the coatability. Condition of the liquid film and the coating film Very good (indicated as ◎): The liquid film spreads over the entire surface, and no cissing occurs on the coating film Good (indicated as ○): The liquid film spreads over the entire surface at first, but cissing occurs at the edges of the coating film Poor (indicated as ×): The liquid film does not spread over the entire surface, and random cissing occurs on the coating film

[0087] (5) Evaluation of uniformity of undercoat or photocatalytic film The uniformity of the coating film prepared on the PET film in (4) was evaluated by appearance (visual inspection) and X-ray fluorescence analysis. Specifically, the X-ray fluorescence intensity of Si was measured at five points randomly selected from the coating film, and the ratio of the maximum value to the minimum value (maximum value / minimum value) was used to evaluate the uniformity according to the following criteria. The X-ray fluorescence intensity of Si was measured under vacuum conditions using an energy dispersive X-ray fluorescence analyzer (EDX-7000, manufactured by Shimadzu Corporation). Very good (indicated as ◎): The appearance was transparent, and the ratio of the maximum value to the minimum value of the X-ray fluorescence intensity of Si was 1.5 or less. Good (indicated as ○): The appearance was transparent, and the ratio of the maximum value to the minimum value of the X-ray fluorescence intensity of Si was more than 1.5 and less than 2.0. Poor (indicated as ×): The appearance was cloudy, or the ratio of the maximum value to the minimum value of the X-ray fluorescence intensity of Si was more than 2.0.

[0088] (6) Water Resistance Evaluation of Undercoat or Photocatalytic Film The water resistance of the coating film prepared on the PET film in (4) was evaluated by the change in the Si fluorescent X-ray intensity before and after immersion in pure water. Specifically, the Si fluorescent X-ray intensity was measured before and after immersion in pure water (room temperature, 16 hours) at five points randomly selected from the coating film, and the Si residual rate at each point (Si fluorescent X-ray intensity after immersion in pure water / Si fluorescent X-ray intensity before immersion in pure water x 100) was used to measure the arithmetic mean value of the five points and evaluated according to the following criteria. The Si fluorescent X-ray intensity was measured using the same device as in (5). Very good (indicated as ◎): the arithmetic mean value of the Si residual rate is 98% or more Good (indicated as ○): the arithmetic mean value of the Si residual rate is 95% or more but less than 98% Poor (indicated as ×): the arithmetic mean value of the Si residual rate is less than 95%

[0089] (7) Identification of the Crystalline Phase of Titanium Oxide Particles The crystalline phase of the titanium oxide particles was evaluated by powder X-ray diffraction. Specifically, the obtained titanium oxide particle dispersion was dried at 105°C for 3 hours, and the recovered titanium oxide particles were subjected to powder X-ray diffraction (D2 PHASER, Bruker AXS K.K.).

[0090] (8) Preparation of Silicic Acid Aqueous Solution / Comparative Sample [Preparation Example 1-1] 13.8 g of sodium silicate (JIS No. 3 sodium silicate, SiO concentration: 29% by mass, NaO concentration: 9.5% by mass) was diluted with 186.2 g of pure water to obtain a sodium silicate aqueous solution with a SiO concentration of 2.0% by mass. 20 mL of a strongly acidic cation exchange resin (Amberlite HPR1024H, manufactured by Organo Corporation) was added to 200 g of this sodium silicate aqueous solution and stirred for 15 minutes to obtain a Na + After removing the ion exchange resin, the ion exchange resin was filtered off. The liquid was then heated to 40°C and stirred for 60 minutes, and then cooled to 25°C. To 99.95 g of the liquid thus obtained was added 0.05 g of an acetylene-based surfactant (Olfine EXP. 4200, manufactured by Nissin Chemical Industry Co., Ltd., HLB = 10 to 13), to obtain an aqueous silicic acid solution (A) of the present invention.

[0091] Preparation Example 1-2: 13.8 g of sodium silicate (JIS No. 3 sodium silicate, SiO concentration: 29% by mass, NaO concentration: 9.5% by mass) was diluted with 186.2 g of pure water to obtain a sodium silicate aqueous solution with a SiO concentration of 2.0% by mass. 20 mL of a strongly acidic cation exchange resin (Amberlite HPR1024H, manufactured by Organo Corporation) was added to 200 g of this sodium silicate aqueous solution and stirred for 15 minutes to obtain a Na + After removing the ion exchange resin, the ion exchange resin was filtered off. Subsequently, this liquid was heated to 40°C and stirred for 60 minutes, and then cooled to 25°C. To 24.99 g of the liquid thus obtained were added 74.96 g of pure water and 0.05 g of a polyether-modified silicone surfactant (KF-643, manufactured by Shin-Etsu Chemical Co., Ltd., HLB = 14), to obtain an aqueous silicic acid solution (B) of the present invention.

[0092] Preparation Example 1-3 An aqueous silicic acid solution (C) of the present invention was obtained in the same manner as in Preparation Example 1-1, except that the amount of the strongly acidic cation exchange resin added was 30 mL.

[0093] Preparation Example 1-4 An aqueous silicic acid solution (D) of the present invention was obtained in the same manner as in Preparation Example 1-1, except that 19 mL of strongly acidic cation exchange resin was added, the heating time was 80 minutes, and no surfactant was added.

[0094] Preparation Example 1-5 An aqueous silicic acid solution (E) of the present invention was obtained in the same manner as in Preparation Example 1-1, except that the amount of the strongly acidic cation exchange resin added was 40 mL.

[0095] Preparation Example 1-6 An aqueous silicic acid solution (F) of the present invention was obtained in the same manner as in Preparation Example 1-1, except that the heating time was changed to 10 minutes.

[0096] Preparation Example 1-7: 13.8 g of sodium silicate (JIS No. 3 sodium silicate, SiO concentration: 29% by mass, NaO concentration: 9.5% by mass) was diluted with 186.2 g of pure water to obtain a sodium silicate aqueous solution with a SiO concentration of 2.0% by mass. 20 mL of a strongly acidic cation exchange resin (Amberlite HPR1024H, manufactured by Organo Corporation) was added to 200 g of this sodium silicate aqueous solution and stirred for 15 minutes to obtain a Na + The ion exchange resin was removed, and then the ion exchange resin was filtered off. The liquid was then heated to 40°C and stirred for 60 minutes, and then cooled to 25°C. 24.98 g of pure water, 49.98 g of ethanol, and 0.05 g of a surfactant (Olfine EXP. 4200, manufactured by Nissin Chemical Industry Co., Ltd.) were added to 24.99 g of the liquid thus obtained, to obtain an aqueous silicic acid solution (G) of the present invention.

[0097] Preparation Example 1-8 A comparative sample (H) was obtained in the same manner as in Preparation Example 1-1, except that the amount of the strongly acidic cation exchange resin added was 16 mL.

[0098] Preparation Example 1-9 Comparative sample (I) was obtained in the same manner as in Preparation Example 1-1, except that the amount of strongly acidic cation exchange resin added was 18 mL.

[0099] Preparation Example 1-10 Comparative sample (J) was obtained in the same manner as in Preparation Example 1-1, except that the heating time was 100 minutes. During heating, the transparency of the liquid decreased and progress of gelation was observed.

[0100] Preparation Example 1-11 34.4 g of sodium silicate (JIS No. 3 sodium silicate, SiO concentration: 29% by mass, NaO concentration: 9.5% by mass) was diluted with 165.6 g of pure water to obtain a sodium silicate aqueous solution with a SiO concentration of 5.0% by mass. 50 mL of a strongly acidic cation exchange resin (Amberlite HPR1024H, manufactured by Organo Corporation) was added to 200 g of this sodium silicate aqueous solution and stirred for 15 minutes to obtain a Na + After removing the ion exchange resin, the liquid was filtered off. Then, the liquid was heated to 40°C and stirred for 60 minutes. Comparative sample (K) gelled and lost its fluidity, so it could not be used as an undercoat liquid or a photocatalyst coating liquid.

[0101] Preparation Example 1-12 13.3 g of colloidal silica (Snowtex 30, manufactured by Nissan Chemical Industries, Ltd., SiO concentration: 30% by mass) was diluted with 186.7 g of pure water to obtain a silica dispersion (L) with a SiO concentration of 2.0% by mass.

[0102] (9) Measurement results of aqueous silicic acid solution / comparison sample Table 1 shows the SiO2 concentration, Na concentration, and + The concentration, SiO2 / Na molar ratio, and viscosity are shown together.

[0103]

[0104] (10) Evaluation of Coatability of Silicic Acid Aqueous Solution / Comparative Sample and Coating Film (Undercoat) Thereof Table 2 shows the evaluation results of the PET film coatability, PVC sheet coatability, and appearance, uniformity, and water resistance of the coating film when coated on the PET film of the liquid prepared in each Preparation Example.

[0105]

[0106] (11) Preparation of Titanium Oxide Particle Dispersion [Preparation Example 2-1] A 36% by weight titanium(IV) chloride aqueous solution was diluted 10-fold with pure water, and then 10% by weight ammonia water was gradually added to neutralize and hydrolyze the solution, yielding a titanium hydroxide precipitate. The pH was 8.5. The resulting precipitate was deionized by repeatedly adding pure water and decanting. After this deionization, 35% by weight hydrogen peroxide was added to the titanium hydroxide precipitate so that the HO / Ti (molar ratio) was 8.0. The mixture was then stirred at 60°C for 2 hours to allow the mixture to react thoroughly, yielding a transparent orange peroxotitanic acid solution. 400 mL of the peroxotitanic acid solution was placed in a 500 mL autoclave and hydrothermally treated at 130°C for 90 minutes. Pure water was then added to adjust the concentration, yielding a titanium oxide particle dispersion (a) (titanium oxide concentration: 1.0% by weight). Dispersed particle diameter D of titanium oxide particles in titanium oxide particle dispersion liquid 50 is 18 nm, D 90 The crystal phase was anatase type titanium oxide.

[0107] (12) Preparation of Photocatalytic Coating Liquid / Comparative Sample Preparation Example 3-1 100 g of pure water and 100 g of titanium oxide particle dispersion liquid (a) were added to 100 g of silicic acid aqueous solution (A) to obtain a photocatalytic coating liquid (α).

[0108] Preparation Example 3-2 A photocatalytic coating liquid (β) was obtained in the same manner as in Preparation Example 3-1, except that the aqueous silicic acid solution (C) was used.

[0109] Preparation Example 3-3 A photocatalytic coating liquid (γ) was obtained in the same manner as in Preparation Example 3-1, except that the aqueous silicic acid solution (D) was used.

[0110] Preparation Example 3-4 A photocatalytic coating liquid (δ) was obtained in the same manner as in Preparation Example 3-1, except that the aqueous silicic acid solution (E) was used.

[0111] Preparation Example 3-5 A photocatalytic coating liquid (ε) was obtained in the same manner as in Preparation Example 3-1, except that the aqueous silicic acid solution (F) was used.

[0112] Preparation Example 3-6 25 g of the titanium oxide particle dispersion liquid (a) was added to 100 g of the silicic acid aqueous solution (G) to obtain a photocatalytic coating liquid (ζ).

[0113] Preparation Example 3-7 A comparative sample (η) was obtained in the same manner as in Preparation Example 3-1, except that the comparative sample (H) was used.

[0114] Preparation Example 3-8 A comparative sample (θ) was obtained in the same manner as in Preparation Example 3-1, except that the comparative sample (I) was used.

[0115] Preparation Example 3-9 Comparative sample (ι) was obtained in the same manner as in Preparation Example 3-1, except that comparative sample (J) was used.

[0116] Preparation Example 3-10 A comparative sample (κ) was obtained in the same manner as in Preparation Example 3-1, except that the silica dispersion (L) was used.

[0117] (13) Measurement Results of Photocatalytic Coating Liquid / Comparative Sample Table 3 shows the SiO concentration, TiO concentration, TiO / SiO (binder) mass ratio, and dispersed particle diameter of titanium oxide particles (D 50 , D 90 ) are shown below.

[0118]

[0119] (14) Evaluation of Coatability of Photocatalytic Coating Liquid / Comparative Sample and Coating Film Thereof Table 4 shows the evaluation results of the PET film coatability, PVC sheet coatability, and appearance, uniformity, and water resistance of the coating film when coated on the PET film of the liquid prepared in each Preparation Example.

[0120]

[0121] (15) Summary of Evaluation Results From the results of Examples and Comparative Examples 1, 2, 6, and 7, it was found that the undercoat made from the silicic acid aqueous solution with a small SiO / Na molar ratio was poor in water resistance, and that the photocatalytic coating solution prepared by mixing with the titanium oxide particle dispersion caused particle aggregation, resulting in the photocatalytic film becoming cloudy.

[0122] From the results of the examples and comparative examples 3 and 8, it was found that the undercoat made from the highly viscous silicic acid aqueous solution that had undergone gelation had small defects and was low in uniformity, and that the photocatalytic coating solution prepared by mixing it with a titanium oxide particle dispersion also had small defects in the photocatalytic film.

[0123] The results of Example and Comparative Example 4 show that aqueous silicic acid solutions with high SiO2 concentrations have low liquid stability and rapidly gel when heated, making them unsuitable for use as undercoats or photocatalytic coating solutions.

[0124] The results of Example and Comparative Examples 5 and 9 show that the undercoat liquid made of silica dispersion and the photocatalytic coating liquid have poor coatability and coating film uniformity, and the undercoat is also poor in water resistance.

Claims

1. An aqueous silicic acid solution for use as an undercoat for a photocatalyst film, comprising an alkali silicate and an aqueous solvent, and having a silicic acid concentration (SiO2 equivalent): 0.1 to 3 mass %, a molar ratio of silicon (SiO2 equivalent) to alkali X (SiO2 / X): 85 to 10,000, and a viscosity at 25°C of 1 to 50 mPa·s.

2. The aqueous silicic acid solution according to claim 1, further comprising a surfactant.

3. The aqueous silicic acid solution according to claim 1 or 2, wherein the aqueous solvent is a mixed solvent of water and a hydrophilic organic solvent.

4. A photocatalytic coating liquid comprising a photocatalyst and an aqueous silicic acid solution, the aqueous silicic acid solution comprising an alkali silicate and an aqueous solvent, and having a silicic acid concentration (SiO2 equivalent): 0.1 to 3 mass %, a molar ratio of silicon (SiO2 equivalent) to alkali X (SiO2 / X): 85 to 10,000, and a viscosity at 25°C of 1 to 50 mPa·s.

5. The photocatalytic coating liquid according to claim 4, further comprising a surfactant.

6. The photocatalytic coating liquid according to claim 4 or 5, wherein the aqueous solvent is a mixed solvent of water and a hydrophilic organic solvent.

7. A member having a coating film formed on its surface from the liquid according to claim 1 or 4.

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