Method for manufacturing surface-treated inorganic powders

Abstract

The present invention provides a manufacturing method that enables the production of surface-treated inorganic powders that exhibit small variations in hydrophobicity regardless of the stirring time of the slurry after mixing inorganic powders and a silane coupling agent, and also have excellent dispersibility within the composition. [Solution] The method for producing surface-treated inorganic powder according to the present invention includes the steps of: preparing a surface-treated slurry by mixing a dispersion slurry containing inorganic powder with an average primary particle size of 0.01 to 2 μm and water, with a pH in the range of 3.0 to 5.5, and a silane coupling agent solution containing a hydrolyzed product of a silane coupling agent; spray-drying the surface-treated slurry to obtain a spray-dried product; and vacuum-drying the spray-dried product to obtain surface-treated inorganic powder, wherein the amount of silane coupling agent used is such that the surface area of ​​the inorganic powder is 1 × 10⁻⁶ 5 m 2 The volume is 0.7 to 1.5 mol per unit area, the spray drying temperature is 155 to 245°C, and the vacuum drying temperature is 75 to 125°C.

Description

[Technical Field]

[0001] This invention relates to a method for producing surface-treated inorganic powders. [Background technology]

[0002] Conventionally, in composite materials manufactured by polymerizing and curing a curable composition containing polymerizable monomers and inorganic powders, it is common practice to hydrophobize the surface of the inorganic powder by treating it with a silane coupling agent in order to improve the operability of the composition and the mechanical strength of the composite material.

[0003] One known method for obtaining surface-treated inorganic powders with a silane coupling agent involves adding the inorganic powder to an organic solvent such as alcohol or toluene, then adding the silane coupling agent and water, followed by distillation of the water and organic solvent using an evaporator. However, this method can result in discoloration or aggregation of the surface-treated inorganic powder, posing practical challenges.

[0004] To solve these problems, Patent Document 1 describes a method of mixing a dispersion slurry containing inorganic powder and water with a silane coupling agent solution containing a hydrolyzed product of a silane coupling agent, spray-drying the mixed slurry, and then vacuum-drying the spray-dried product, wherein the amount of silane coupling agent used is the surface area of ​​the inorganic powder (1 × 10⁻⁶). 5 m 2 A method has been proposed in which the amount per unit is 0.4 to 1.5 mol, the spray drying temperature is 150 to 270°C, and the vacuum drying temperature is 10 to 70°C. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 4986444 [Overview of the project] [Problems that the invention aims to solve]

[0006] According to the method described in Patent Document 1, it is possible to obtain a surface-treated inorganic powder that is colorless and has few aggregated particles.

[0007] However, when the inventors investigated the method described in Patent Document 1, they found that as the stirring time of the slurry after mixing the inorganic powder and the silane coupling agent increased (in other words, as the bench time until spray drying increased), the hydrophobicity of the final surface-treated inorganic powder decreased. This phenomenon leads to problems such as variations in the performance of the surface-treated inorganic powder and a decrease in productivity due to the constraint of stirring time.

[0008] Furthermore, further investigations by the inventors revealed that the dispersibility of the final surface-treated inorganic powder composition is also affected by the spray drying temperature and the vacuum drying temperature.

[0009] Therefore, the object of the present invention is to provide a manufacturing method that can produce a surface-treated inorganic powder that exhibits small variations in hydrophobicity regardless of the stirring time of the slurry after mixing the inorganic powder and the silane coupling agent, and also has excellent dispersibility in the composition. [Means for solving the problem]

[0010] The following embodiments are specific means for solving the above problems. <1> A step of preparing a surface treatment slurry by mixing a dispersion slurry containing inorganic powder with an average primary particle size of 0.01 to 2 μm and water, with a pH in the range of 3.0 to 5.5, with a silane coupling agent solution containing a hydrolyzed silane coupling agent, The process involves spray-drying the surface treatment slurry to obtain a spray-dried product, The process includes a step of vacuum-drying the spray-dried material to obtain a surface-treated inorganic powder, The amount of silane coupling agent used is equal to the surface area of ​​the inorganic powder, which is 1 × 10 5 m 2A method for producing a surface-treated inorganic powder, wherein the amount is 0.7 to 1.5 mol per hit, the temperature of the spray drying is 155 to 245 ° C, and the temperature of the vacuum drying is 75 to 125 ° C.

[0011] <2> The amount of the silane coupling agent used is 0.8 to 1.0 mol per 1 × 10 5 m 2 per surface area of the inorganic powder. The method for producing a surface-treated inorganic powder according to <1>.

[0012] <3> For adjusting the pH of the dispersion slurry, at least one acid selected from the group consisting of carboxylic acids having 1 to 3 carbon atoms, nitric acid, hydrochloric acid, hydrobromic acid, and hydroiodic acid is used. The method for producing a surface-treated inorganic powder according to <1> or <2>.

[0013] <4> The content of the inorganic powder in the dispersion slurry is 30 to 50% by mass. The method for producing a surface-treated inorganic powder according to any one of <1> to <3>.

[0014] <5> The inorganic powder is silica-based composite oxide particles. The method for producing a surface-treated inorganic powder according to any one of <1> to <4>.

[0015] <6> The inorganic powder is composed of 100 parts by mass of silica-based composite oxide particles (A) having an average primary particle diameter of 0.05 μm or more and less than 0.1 μm and 100 to 400 parts by mass of silica-based composite oxide particles (B) having an average primary particle diameter of 0.1 to 1.5 μm. The method for producing a surface-treated inorganic powder according to any one of <1> to <5>.

[0016] <7> The silica-based composite oxide particles are at least one selected from silica-zirconia particles and silica-titania particles. The method for producing a surface-treated inorganic powder according to <5> or <6>.

Advantages of the Invention

[0017] According to the present invention, it is possible to provide a manufacturing method that makes it possible to obtain a surface-treated inorganic powder that exhibits small variations in hydrophobicity regardless of the stirring time of the slurry after mixing the inorganic powder and the silane coupling agent, and also has excellent dispersibility in the composition. [Modes for carrying out the invention]

[0018] The following describes specific embodiments to which the present invention is applied. In this specification, unless otherwise specified, the notation "x~y" using numerical values ​​x and y means "x or greater and y or less". If a unit is attached only to the numerical value y in such notation, that unit shall also apply to the numerical value x.

[0019] <Method for manufacturing surface-treated inorganic powder> The method for producing surface-treated inorganic powder according to this embodiment (hereinafter also referred to as "the production method according to this embodiment") includes the steps of: preparing a surface treatment slurry by mixing a dispersion slurry containing inorganic powder with an average primary particle size of 0.01 to 2 μm and water, with a pH in the range of 3.0 to 5.5, with a silane coupling agent solution containing a hydrolyzed product of a silane coupling agent (hereinafter also referred to as "the surface treatment slurry preparation step"); spray-drying the surface treatment slurry to obtain a spray-dried product (hereinafter also referred to as "the spray-drying step"); and vacuum-drying the spray-dried product to obtain surface-treated inorganic powder (hereinafter also referred to as "the vacuum-drying step"). The amount of silane coupling agent used is such that the surface area of ​​the inorganic powder is 1 × 10⁻⁶. 5 m 2 The volume is 0.7 to 1.5 mol per unit area, the spray drying temperature is 155 to 245°C, and the vacuum drying temperature is 75 to 125°C.

[0020] According to the manufacturing method of this embodiment, a surface-treated inorganic powder can be obtained that exhibits small variations in hydrophobicity regardless of the stirring time of the slurry after mixing the inorganic powder and the silane coupling agent, and also has excellent dispersibility in the composition. The reason for this is not entirely clear, but it is presumed to be as follows.

[0021] Generally, the pH of a dispersion slurry containing inorganic powder and water is neutral to weakly basic. When such a neutral to weakly basic dispersion slurry is mixed with a silane coupling agent solution, the silane coupling agents condense and oligomerize as the stirring time of the slurry increases (in other words, as the bench time until spray drying increases). When the silane coupling agents oligomerize, their reactivity with the inorganic powder surface decreases, and the inorganic powder is not sufficiently hydrophobic. In this regard, the manufacturing method according to this embodiment adjusts the pH of the dispersion slurry to a range of 3.0 to 5.5, thereby suppressing the condensation reaction between the silane coupling agents. Furthermore, in the manufacturing method according to this embodiment, the temperatures for spray drying and vacuum drying are appropriately set. As a result, it is believed that surface-treated inorganic powder with little variation in hydrophobicity can be obtained regardless of the stirring time of the slurry. In addition, because the temperatures for spray drying and vacuum drying are appropriately set in the manufacturing method according to this embodiment, the resulting surface-treated inorganic powder also has excellent dispersibility in the composition.

[0022] The following describes in detail each element of the manufacturing method according to this embodiment.

[0023] [Dispersed slurry] The dispersion slurry contains inorganic powder with an average primary particle size of 0.01 to 2 μm and water, and has a pH in the range of 3.0 to 5.5 at 25°C.

[0024] Examples of inorganic powders include spherical or irregularly shaped particles of amorphous silica, silica-based composite oxides (silica-zirconia, silica-titania, silica-titania-barium oxide, silica-titania-zirconia, etc.), quartz, alumina, and glass. When surface-treated inorganic powders are used in applications such as dental filling and restorative materials, silica-based composite oxide particles are preferred as the inorganic powder because they provide radiopaqueness and excellent abrasion resistance and surface smoothness. It is more preferable to use at least one selected from silica-zirconia particles and silica-titania particles. Silica-zirconia particles or silica-titania particles may contain alkali metal oxides such as sodium oxide and lithium oxide.

[0025] The average primary particle size of inorganic powder is 0.01 to 2 μm. When the average primary particle size of inorganic powder is 0.01 μm or larger, the formation of aggregated particles tends to be suppressed. On the other hand, when the average primary particle size of inorganic powder is 2 μm or smaller, dispersibility improves, and the inorganic powder tends to settle less easily in the surface treatment slurry.

[0026] The average primary particle size of the inorganic powder is preferably 0.03 to 0.6 μm. While it is not necessary for all particles constituting the inorganic powder to have a particle size within the 0.03 to 0.6 μm range, it is preferable that the proportion of particles with a particle size within this range be 60% by mass. Furthermore, it is preferable that the proportion of particles with a particle size smaller than 0.03 μm be 30% by mass or less, and the proportion of particles with a particle size larger than 0.6 μm be 20% by mass or less.

[0027] In one embodiment, the inorganic powder consists of 100 parts by mass of silica-based composite oxide particles (A) with an average primary particle diameter of 0.05 μm or more and less than 0.1 μm, and 100 to 400 parts by mass of silica-based composite oxide particles (B) with an average primary particle diameter of 0.1 to 1.5 μm.

[0028] The average primary particle diameter of inorganic powders is defined as the arithmetic mean obtained by taking a photograph of the particles using a scanning electron microscope, selecting 30 or more particles observed within the unit field of view of the photograph, and determining the particle diameter (maximum diameter) of each particle. In this specification, when "average primary particle diameter" is mentioned, it refers to the average primary particle diameter measured by the same method.

[0029] The dispersion slurry contains water as a dispersion medium. In addition to water, the dispersion slurry may further contain an organic solvent that is miscible with water. Examples of organic solvents that are miscible with water include methanol, ethanol, propanol, butanol, and acetone.

[0030] The inorganic powder content in the dispersed slurry is preferably 30 to 50% by mass, and more preferably 35 to 45% by mass.

[0031] The pH of the dispersed slurry at 25°C is in the range of 3.0 to 5.5, preferably in the range of 3.5 to 5.0. By setting the pH of the dispersed slurry in the range of 3.0 to 5.5, the variation in the hydrophobicity of the resulting surface-treated inorganic powder tends to be reduced, regardless of the stirring time of the slurry after mixing the inorganic powder and the silane coupling agent.

[0032] Generally, the pH of a dispersion slurry containing inorganic powder and water is neutral to weakly basic. Therefore, to adjust the pH of the dispersion slurry to the range of 3.0 to 5.5, it is necessary to add an acid to the dispersion slurry. The acid is not particularly limited as long as it can adjust the pH of the dispersion slurry to the range of 3.0 to 5.5, but it is preferable that it does not remain on the surface-treated inorganic powder. Examples of such acids include at least one acid selected from the group consisting of carboxylic acids having 1 to 3 carbon atoms (formic acid, acetic acid, propionic acid), nitric acid, hydrochloric acid, hydrobromic acid, and hydroiodic acid.

[0033] The dispersion slurry is preferably in a dispersed state such that the inorganic powder does not substantially settle even after standing for one hour. By achieving such a dispersed state, uniform surface treatment is easily performed in the subsequent spray drying process, and variations in the hydrophobicity of the resulting surface-treated inorganic powder tend to be less likely. Generally, known homogenizers are used to disperse the inorganic powder. Examples of homogenizers include devices with a structure in which spherical ceramic balls are crushed and dispersed while being stirred with a stirring rod in a cylindrical container; devices with a structure in which spherical ceramic balls are crushed and dispersed while rotating a cylindrical rotor at high speed; ultra-high pressure impact type emulsification dispersion devices; devices with a structure in which crushing and dispersion are performed with a chopper; and so on.

[0034] The temperature at which the dispersion slurry is prepared is preferably around room temperature, for example, in the range of 10 to 60°C.

[0035] [Silane coupling agent solution] The silane coupling agent solution contains hydrolysates of the silane coupling agent.

[0036] Examples of silane coupling agents include silane coupling agents having radical polymerizable groups such as vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, 3-acryloyloxypropyltrimethoxysilane, 3-methacryloyloxypropyltrimethoxysilane, and 12-methacryloyloxidedecyltrimethoxysilane; silane coupling agents having epoxy groups such as 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and 3-glycidoxypropyltrimethoxysilane; silane coupling agents having amino groups such as N-2-(aminoethyl)-3-aminopropyltrimethoxysilane and 3-aminopropyltrimethoxysilane; silane coupling agents having ureido groups such as 3-ureidopropyltriethoxysilane; silane coupling agents having halogen atoms such as 3-chloropropyltrimethoxysilane; and lower alkoxysilanes such as methyltrimethoxysilane, ethyltrimethoxysilane, and methyltriethoxysilane. Among these, silane coupling agents having radical polymerizable groups and lower alkoxysilanes are preferred.

[0037] A silane coupling agent solution is prepared by adding the silane coupling agent to water or a mixed solvent of water and an organic solvent. Examples of organic solvents include methanol, ethanol, propanol, butanol, and acetone.

[0038] The amount of silane coupling agent added is preferably 1 to 10 parts by mass, and more preferably 3 to 8 parts by mass, per 100 parts by mass of solvent.

[0039] The silane coupling agent solution is preferably prepared by adding a hydrolysis aid to adjust the pH to a range of 3.0 to 5.5 at 25°C. Examples of hydrolysis aids include carboxylic acids such as acetic acid, citric acid, malic acid, and lactic acid; and mineral acids such as hydrochloric acid, sulfuric acid, and nitric acid. Among these, carboxylic acids are preferred because they do not remain in the surface-treated inorganic powder.

[0040] When preparing the silane coupling agent solution, the temperature is preferably around room temperature, for example, in the range of 10 to 60 °C.

[0041] [Surface treatment slurry preparation step] In the surface treatment slurry preparation step, a dispersion slurry and a silane coupling agent solution are mixed to prepare a surface treatment slurry. At that time, it is preferable to mix under stirring so as to obtain a uniform slurry. The mixing method of the dispersion slurry and the silane coupling agent solution is not particularly limited, and the dispersion slurry may be added to the silane coupling agent solution, or the silane coupling agent solution may be added to the dispersion slurry. By this mixing, the hydrolyzate of the silane coupling agent is adsorbed on the surface of the inorganic powder.

[0042] The amount of the silane coupling agent used in the surface treatment slurry preparation step is 0.7 to 1.5 mol per 1×10 5 m 2 of the surface area of the inorganic powder. By setting the amount of the silane coupling agent used to be 0.7 mol or more per 1×10 5 m 2 of the surface area of the inorganic powder, the inorganic powder tends to be sufficiently hydrophobized. On the other hand, by setting the amount of the silane coupling agent used to be 1.5 mol or less per 1×10 5 m 2 of the surface area of the inorganic powder, the generation of agglomerated particles in the subsequent spray drying step and vacuum drying step tends to be suppressed. The amount of the silane coupling agent used is preferably 0.75 to 1.45 mol, more preferably 0.8 to 1.4 mol, still more preferably 0.8 to 1.3 mol, and particularly preferably 0.8 to 1.0 mol per 1×10 5 m 2 of the surface area of the inorganic powder.

[0043] The surface area of the inorganic powder can be calculated from the specific surface area (m 2 / g) and the amount used (g) of the inorganic powder used. The specific surface area of the inorganic powder is determined by performing nitrogen adsorption and desorption at liquid nitrogen temperature (-196 °C) and analyzing by the BET method.

[0044] The stirring time of the surface treatment slurry after mixing the dispersed slurry and the silane coupling agent solution is, for example, in the range of 1 to 24 hours. In the manufacturing method according to this embodiment, as described above, the pH of the dispersed slurry is adjusted to the range of 3.0 to 5.5, so the variation in the hydrophobicity of the surface-treated inorganic powder tends to be small regardless of the stirring time of the surface treatment slurry.

[0045] [Spray drying process] In the spray drying process, the surface treatment slurry obtained in the surface treatment slurry preparation process is spray-dried to obtain a spray-dried product.

[0046] The spray drying temperature is in the range of 155 to 245°C. Setting the spray drying temperature above 155°C promotes the condensation reaction of hydrolyzed silane coupling agents adsorbed on the surface of the inorganic powder, resulting in a sufficiently hydrophobic surface-treated inorganic powder. On the other hand, setting the spray drying temperature below 245°C tends to suppress the formation of aggregated particles. A spray drying temperature in the range of 160 to 240°C is preferable.

[0047] The spray drying temperature refers to the temperature of the hot air outlet inside the spray dryer. The type of spray dryer is not particularly limited; examples include a system that sprays surface treatment slurry from a nozzle with fine pores (fluid nozzle system); and a system that drops surface treatment slurry onto a high-speed rotating body for spraying (atomizer system).

[0048] [Vacuum drying process] In the vacuum drying process, the spray-dried material obtained in the spray drying process is vacuum-dried to obtain a surface-treated inorganic powder.

[0049] The vacuum drying temperature is in the range of 75 to 125°C. Setting the vacuum drying temperature above 75°C tends to further promote the condensation reaction of hydrolyzed silane coupling agents adsorbed on the surface of the inorganic powder, resulting in a sufficiently hydrophobic surface-treated inorganic powder. On the other hand, setting the vacuum drying temperature below 125°C tends to suppress the formation of aggregated particles. The vacuum drying temperature is preferably in the range of 80 to 120°C. The vacuum drying time is preferably in the range of 1 to 72 hours, and more preferably in the range of 2 to 24 hours.

[0050] Vacuum drying refers to drying under a pressure of 10 hPa or less, preferably 5 hPa or less. Vacuum drying temperature refers to the temperature of the atmosphere surrounding the inorganic powder inside the vacuum dryer.

[0051] The surface-treated inorganic powder obtained through the vacuum drying process described above is highly hydrophobic. The contact angle of the surface-treated inorganic powder with water is, for example, 90 degrees or more, and preferably 100 to 120 degrees. Furthermore, the surface-treated inorganic powder obtained through the vacuum drying process described above is characterized by minimal discoloration. [Examples]

[0052] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0053] The types and abbreviations of the inorganic powders used in the examples and comparative examples are shown in Table 1 below.

[0054] [Table 1]

[0055] Furthermore, the types and abbreviations of the silane coupling agents and polymerizable monomers used in the examples and comparative examples are as follows. [Silane coupling agent] • SC1:3-Methacryloyloxypropyltrimethoxysilane [polymerizable monomers] UDMA: 1,6-bis(methacryloyloxyethoxycarbonylamino)trimethylhexane • 3G: Triethylene glycol dimethacrylate

[0056] <Example 1> A slurry was obtained by adding deionized water to a mixture of inorganic powders F1 (100 parts by mass) and F2 (150 parts by mass) so that the inorganic powder content was 40% by mass, and then dispersing the mixture using a circulating pulverizer SC Mill (manufactured by Nippon Coke Industries Co., Ltd.). Acetic acid was added to the obtained slurry to adjust the pH to 4.0 (25°C) to obtain a dispersed slurry.

[0057] Meanwhile, SC1 (12.7 parts by mass) as a silane coupling agent and acetic acid (0.008 parts by mass) as a hydrolysis aid were added to water (200 parts by mass), and the mixture was stirred for 1 hour and 30 minutes to obtain a silane coupling agent solution with a pH of 4.0.

[0058] The obtained silane coupling agent solution was added to the dispersion slurry and stirred. At that time, the surface area of ​​the inorganic powder was 1 × 10⁻⁶. 5 m 2 The amounts of silane coupling agent solution and dispersion slurry used were adjusted so that the amount of silane coupling agent used per unit was 1.0 mol. Then, after stirring time of 1 hour, 2 hours, or 6 hours, the mixture was spray-dried using a spray dryer (Sakamoto Giken Co., Ltd., spray dryer TSR-2W) at an air temperature of 230°C. Subsequently, the spray-dried material was vacuum-dried for 18 hours at 90°C and 1 hPa to obtain surface-treated inorganic powder.

[0059] <Examples 2-16, Comparative Examples 1-8> Type and amount of inorganic powder used, pH of the dispersion slurry, amount of silane coupling agent used, surface area of ​​inorganic powder (1 × 10⁻⁶) 5 m 2Surface-treated inorganic powders were obtained in the same manner as in Example 1, except that the amount of silane coupling agent used per unit, the spray drying temperature, and the vacuum drying temperature were changed as shown in Table 2 below. However, in Example 5, formic acid was used instead of acetic acid to adjust the pH of the dispersion slurry, and in Comparative Example 2, a 1% hydrochloric acid aqueous solution was used instead of acetic acid to adjust the pH of the dispersion slurry. In Comparative Example 1, no pH adjustment was performed on the dispersion slurry.

[0060] [Table 2]

[0061] <Rating> [Contact angle of surface-treated inorganic powder with respect to water] Surface-treated inorganic powder was packed into a metal ring with a hole 12 mm in diameter and 1 mm thick, and pellets were prepared by applying a load of 50 MPa for 5 minutes. 2.0 μL of deionized water was dropped onto the top surface of these pellets, and the contact angle was measured using a contact angle meter. The results are shown in Table 3 below.

[0062] [Percentage of aggregated particles in the composition] A paste-like composition was obtained by mixing 80 parts by mass of surface-treated inorganic powder with 20 parts by mass of polymerizable monomer (a mixture of UDMA / 3G = 7 / 3 (mass ratio)) and kneading for 10 minutes. Next, 10 mL of ethanol was added to 0.01 g of the composition and shaken by hand to form a slurry. The particle size distribution of the obtained slurry was evaluated using a laser diffraction particle size analyzer (Beckman Coulter, LS13320), and the volume % of particles with a particle size of 10 μm or larger was defined as the proportion of agglomerated particles. The results are shown in Table 3 below. If the proportion of agglomerated particles in the composition exceeds 10 volume %, the handling properties of the composition deteriorate, which is undesirable. In Table 3 below, "-" means that the hydrophobicity of the surface-treated inorganic powder was unsuitable, and a paste-like composition was not prepared.

[0063] [comprehensive evaluation] The surface-treated inorganic powder was evaluated in four stages based on its contact angle with water and the proportion of aggregated particles in the composition. The results are shown in Table 3 below. A: Has suitable hydrophobicity and dispersibility. B: Possesses suitable hydrophobicity. Dispersibility is within acceptable limits. C: While possessing suitable hydrophobicity, its dispersibility is outside the acceptable range. D: Unsuitable due to hydrophobicity.

[0064] [Table 3]

[0065] As shown in Table 3, the pH of the dispersed slurry was adjusted to a range of 3.0 to 5.5, and the amount of silane coupling agent used was equal to the surface area of ​​the inorganic powder (1 × 10⁻⁶). 5 m 2 In Examples 1 to 16, where the amount per unit was 0.7 to 1.5 mol, the spray drying temperature was 155 to 245°C, and the vacuum drying temperature was 75 to 125°C, surface-treated inorganic powders were obtained that exhibited sufficiently high hydrophobicity regardless of the stirring time of the slurry after mixing the inorganic powder and the silane coupling agent, had little variation in hydrophobicity, and also exhibited excellent dispersibility in the composition.

[0066] In contrast, in Comparative Examples 1 and 2, where the pH of the dispersed slurry was not adjusted or was adjusted to less than 3.0, the hydrophobicity of the surface-treated inorganic powder was insufficient. The amount of silane coupling agent used was calculated based on the surface area of ​​the inorganic powder (1 × 10⁻⁶). 5 m 2 In Comparative Example 3, where the amount used per unit was less than 0.7 mol, the hydrophobicity of the surface-treated inorganic powder was insufficient. On the other hand, the amount of silane coupling agent used was calculated based on the surface area of ​​the inorganic powder (1 × 10⁻⁶). 5 m 2 In Comparative Example 4, where the amount per unit was greater than 1.5 mol, the hydrophobicity of the surface-treated inorganic powder was sufficient, but its dispersibility in the composition was poor. Furthermore, in Comparative Example 5, where the spray drying temperature was less than 155°C, and Comparative Example 7, where the vacuum drying temperature was less than 75°C, the hydrophobicity of the surface-treated inorganic powder was insufficient. On the other hand, in Comparative Example 6, where the spray drying temperature was greater than 245°C, and Comparative Example 8, where the vacuum drying temperature was greater than 125°C, the hydrophobicity of the surface-treated inorganic powder was sufficient, but its dispersibility in the composition was poor.

[0067] [Percentage of silane coupling agent not adsorbed on the surface of inorganic powder] The surface-treated inorganic powders (1 g) obtained in Examples 1, 10, and Comparative Example 1 were mixed with acetonitrile (10 g) and ultrasonically dispersed for 20 minutes to form a slurry. The obtained slurry was filtered, and the filtrate (sample solution) was analyzed by HPLC to determine the peak area. In addition, the acetonitrile solution of the silane coupling agent SC1 (0.1% by mass, 0.01% by mass, or 0.001% by mass) was analyzed by HPLC to determine the peak area, and a calibration curve was created. Next, the amount of unreacted SC1 in the sample solution was determined using the created calibration curve. Then, the percentage (by mass) of silane coupling agent not adsorbed on the surface of the inorganic powder was determined from the amount of silane coupling agent used per 1 g of inorganic powder and the amount of unreacted SC1 mentioned above. The results are shown in Table 4 below.

[0068] [Table 4]

[0069] As shown in Table 4, in Examples 1 and 10, where the pH of the dispersed slurry was adjusted to the range of 3.0 to 5.5, the proportion of silane coupling agent SC1 not adsorbed on the surface of the inorganic powder was relatively low. In contrast, in Comparative Example 1, where the pH of the dispersed slurry was not adjusted, the proportion of silane coupling agent SC1 not adsorbed on the surface of the inorganic powder was significantly higher than in Examples 1 and 10, and this proportion increased as the stirring time of the slurry after mixing the inorganic powder and the silane coupling agent increased.

Claims

1. A step of preparing a surface treatment slurry by mixing a dispersion slurry containing inorganic powder with an average primary particle size of 0.01 to 2 μm and water, with a pH in the range of 3.0 to 5.5, with a silane coupling agent solution containing a hydrolyzed product of a silane coupling agent, The process involves spray-drying the surface treatment slurry to obtain a spray-dried product, The process includes a step of vacuum-drying the spray-dried material to obtain a surface-treated inorganic powder, The amount of silane coupling agent used is the surface area of ​​the inorganic powder, 1 × 10 ５ I understand ２ A method for producing a surface-treated inorganic powder, wherein the volume is 0.7 to 1.5 mol per unit area, the spray drying temperature is 155 to 245°C, and the vacuum drying temperature is 75 to 125°C.

2. The amount of silane coupling agent used is the surface area of ​​the inorganic powder, 1 × 10 ５ I understand ２ A method for producing a surface-treated inorganic powder according to claim 1, wherein the amount is 0.8 to 1.0 mol per unit.

3. A method for producing a surface-treated inorganic powder according to claim 1, wherein at least one acid selected from the group consisting of carboxylic acids having 1 to 3 carbon atoms, nitric acid, hydrochloric acid, hydrobromic acid, and hydroiodic acid is used to adjust the pH of the dispersed slurry.

4. The method for producing a surface-treated inorganic powder according to claim 1, wherein the content of the inorganic powder in the dispersed slurry is 30 to 50% by mass.

5. The method for producing a surface-treated inorganic powder according to claim 1, wherein the inorganic powder is silica-based composite oxide particles.

6. A method for producing a surface-treated inorganic powder according to claim 1, wherein the inorganic powder comprises 100 parts by mass of silica-based composite oxide particles (A) having an average primary particle diameter of 0.05 μm or more and less than 0.1 μm, and 100 to 400 parts by mass of silica-based composite oxide particles (B) having an average primary particle diameter of 0.1 to 1.5 μm.

7. The method for producing a surface-treated inorganic powder according to claim 5 or 6, wherein the silica-based composite oxide particles are at least one selected from silica-zirconia particles and silica-titania particles.

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