Method for producing modified hexagonal boron nitride powder

Abstract

To provide a method for producing a modified hexagonal boron nitride powder obtained by heat treatment of hexagonal boron nitride powder having negative triboelectrification property obtained by a synthesis reaction with the generation of ammonia odor effectively prevented in the modified hexagonal boron nitride powder and provide the modified hexagonal boron nitride powder obtained thereby.SOLUTION: A method comprises adjusting the moisture content of a hexagonal boron nitride powder having negative triboelectrification property as measured by a drying loss method to 50 ppm or less and thereafter heat-treating the hexagonal boron nitride powder at a temperature of 1300 to 2200°C under an atmosphere of inert gas having a dew point of -30°C or less. By this method, 200 g of the powder is allowed to be present in a closed container of 100 L internal volume with a voltage density of +1 V / g or higher as measured by triboelectrification test, a temperature of 25°C and a relative humidity of 70% in the gas phase, resulting in obtaining a modified hexagonal boron nitride powder in which the ammonia concentration in the gas phase of the sealed container after 60 minutes is 60 ppm or less.SELECTED DRAWING: None

Description

[Technical Field] 【0001】 This invention relates to a novel method for producing modified hexagonal boron nitride powder. More specifically, it provides a method for producing modified hexagonal boron nitride powder in which ammonia odor is effectively suppressed. [Background technology] 【0002】 Hexagonal boron nitride powder is a white powder with a hexagonal layered crystalline structure and is widely used as a powder base for cosmetics such as makeup. These cosmetics require properties such as covering power, transparency, spreadability, lubricity, and adhesion to human skin, and boron nitride powder is particularly effective in providing spreadability and adhesion. 【0003】 Conventionally, in order to further improve the above properties of hexagonal boron nitride powder, it has been considered to adjust the particle shape, such as the particle size (major axis), thickness, and aspect ratio of the primary particles constituting the powder (see Patent Document 1), but there is still room for improvement regarding ductility and adhesion. 【0004】 Furthermore, attempts have been made to obtain modified hexagonal boron nitride powder by calcining hexagonal boron nitride powder obtained through a synthesis reaction under a nitrogen atmosphere. 【0005】 For example, Patent Document 2 describes a material with a graphitization index (GI) of 2.0 or less and a specific surface area of ​​15 m². 2 A method for modifying hexagonal boron nitride powder with a density of less than / g is disclosed, which involves washing and drying the powder with an acidic aqueous solution, followed by heat treatment at 1800-1950°C for 1-5 hours under a nitrogen atmosphere. However, while this method improves the whiteness to, for example, 95.0 or higher, it is insufficient in improving the spreadability and adhesion required for cosmetic applications. 【0006】 Furthermore, based on the knowledge that hexagonal boron nitride powder obtained by synthesis reactions generally exhibits negative charge properties, which causes clumping (large aggregates visible to the naked eye), a modified hexagonal boron nitride powder has been proposed in which the negative triboelectric charge properties of the hexagonal boron nitride powder are modified to positive charge properties by heat treatment at a temperature of 1300 to 2200°C under the flow of an inert gas, thereby reducing the formation of clumps (see Patent Document 3). 【0007】 However, the modified hexagonal boron nitride powder obtained by the above modification treatment sometimes emits an ammonia odor, and such modified hexagonal boron nitride powder is unsuitable for use in cosmetics. [Prior art documents] [Patent Documents] 【0008】 [Patent Document 1] Patent No. 6109466 [Patent Document 2] Japanese Patent Publication No. 2004-35273 [Patent Document 3] WO2021 / 039586 publication [Overview of the Initiative] [Problems that the invention aims to solve] 【0009】 Accordingly, the object of the present invention is to provide a method for modifying hexagonal boron nitride powder having negative triboelectric properties obtained by a synthesis reaction, which effectively prevents the generation of ammonia odor from the resulting modified hexagonal boron nitride powder. [Means for solving the problem] 【0010】 The inventors of this invention conducted extensive research to solve the aforementioned problems and found that when the hexagonal boron nitride powder having negative triboelectric properties is subjected to heat treatment (modification treatment), the moisture contained in the hexagonal boron nitride powder having negative triboelectric properties, as well as the moisture in the inert gas supplied during the modification treatment, causes some kind of reaction in the heating region, including during the temperature rise, destabilizing the surface state of the resulting modified hexagonal boron nitride powder. Based on this finding, the inventors discovered that by adjusting the moisture content of the hexagonal boron nitride powder having negative triboelectric properties to a specific range, and by limiting the moisture content of the inert gas supplied as the atmospheric gas, the generation of ammonia odor from the modified hexagonal boron nitride powder can be almost completely prevented without adversely affecting the modification effect, thus completing the present invention. 【0011】 The present invention provides a method for modifying hexagonal boron nitride powder having negative triboelectric properties (hereinafter sometimes simply referred to as negatively charged hexagonal boron nitride powder), characterized by adjusting the moisture content, measured by the drying loss method, to 50 ppm or less, and then heat-treating it at a temperature of 1300 to 2200°C under the flow of an inert gas with a dew point of -30°C or lower. 【0012】 In the modification method of the present invention, it is preferable to perform the heat treatment under the flow of an inert gas, as this can convert the negatively charged hexagonal boron nitride powder obtained by the synthesis reaction to positive charge and suppress the formation of clumps. In this case, it is effective to perform the heat treatment until the voltage density of the hexagonal boron nitride powder becomes +1V / g or higher. 【0013】 Furthermore, according to the modification method of the present invention, it is possible to obtain modified hexagonal boron nitride powder characterized by having a voltage density of +1 V / g or higher as measured by a triboelectric test, and by placing 200 g of the powder in a sealed container with an internal volume of 100 L in which the gas phase is adjusted to a temperature of 25 °C and a relative humidity of 70% RH, and having an ammonia concentration of 60 ppm or less in the gas phase of the sealed container after 60 minutes. 【0014】 In the present invention, the triboelectric charging test is performed using an anodized aluminum rotating drum (drum capacity: 332 cc) equipped with a window made of borosilicate glass. As described in the examples below, the powder of the sample is placed in the rotating drum, and the drum is rotated at a speed of 10 rpm for 300 seconds to triboelectrically charge the powder of the sample (detailed conditions are shown in the examples). 【0015】 The above-mentioned modified hexagonal boron nitride powder preferably has an average major axis of primary particles in the range of 2 to 20 μm, an average thickness of 0.2 to 2.0 μm, and an average aspect ratio of 5 to 30. 【0016】 In addition, the above-mentioned modified hexagonal boron nitride powder is suitably used as a cosmetic formulation agent. 【Effects of the Invention】 【0017】 The modified boron nitride powder obtained by the method for modifying hexagonal boron nitride powder of the present invention has positive chargeability, hardly generates an ammonia odor, can be suitably used as a raw material for cosmetics, suppresses the generation of lumps due to aggregation of boron nitride particles, and can greatly improve the spreadability and adhesiveness of cosmetics when used as a cosmetic raw material. 【Modes for Carrying Out the Invention】 【0018】 <Negative triboelectric charging boron nitride powder> In the present invention, as the negative triboelectric charging boron nitride powder to be subjected to the modification treatment, those obtained by known production methods by various synthesis reactions are used without particular limitation. For example, the melamine method and the reduction nitridation method can be mentioned as typical production methods. In any of these methods of production, the obtained powder has functional groups such as OH and NH2 and impurities such as B2O3 on the surface of the particles constituting the powder, and these cannot be completely removed by acid washing or the like. The hexagonal boron nitride powder obtained through such washing and drying becomes negatively charged when triboelectrically charged. 【0019】 The negatively charged hexagonal boron nitride powder exhibits a negative voltage density in triboelectricity tests, generally having a voltage density of -0.5 to -50 V / g. Preferred characteristics of the negatively charged hexagonal boron nitride powder include an average major diameter of primary particles of 2 to 20 μm, particularly 3 to 12 μm; an average thickness of 0.2 to 2.0 μm, particularly 0.3 to 1.2 μm; and an average aspect ratio (average major diameter / average thickness) of 5 to 30, particularly 6 to 20. Furthermore, the median diameter (D1: midpoint diameter D1 based on volume) measured by laser diffraction / scattering is also important. 50 The diameter is preferably 4 to 40 μm. Furthermore, when the powder is ultrasonically dispersed in ethanol, it is preferable that the median diameter (D2) measured by laser diffraction-scattering method is about 2 to 20 μm. 【0020】 In the method for modifying hexagonal boron nitride powder of the present invention, it is important that the negatively charged hexagonal boron nitride powder is adjusted so that the moisture content measured by the drying loss method is 50 ppm or less before being subjected to the modification treatment described later. 【0021】 In other words, if the moisture content of the negatively charged hexagonal boron nitride powder subjected to the modification treatment exceeds 50 ppm, the resulting modified hexagonal boron nitride powder is prone to hydrolysis upon contact with moisture in the ambient air, generating ammonia. It is presumed that this is because, during the heat treatment of the negatively charged hexagonal boron nitride powder, a portion of the hexagonal boron nitride undergoes hydrolysis, generating active sites in the hydrolyzed portion. After cooling, these active sites in the modified hexagonal boron nitride powder react with moisture in the ambient air to generate ammonia. Therefore, it is extremely important to keep the moisture content of the negatively charged hexagonal boron nitride powder below 50 ppm, and it is particularly preferable to reduce it to below 30 ppm, or even to an undetectable level. 【0022】 Furthermore, the moisture content of typical negatively charged hexagonal boron nitride powder is approximately 0.01 to 1% by mass. 【0023】 <Pre-drying> In the present invention, pre-drying is preferred as a method for adjusting the moisture content of the negatively charged hexagonal boron nitride powder to the above range. The method of pre-drying is not particularly limited, but it is most preferably carried out by vacuum heating. As an example of specific conditions for vacuum heating, it is preferable to heat at a pressure of 1000 Pa or less, preferably 50 Pa or less, and at a temperature of 50 to 500°C, preferably 100 to 250°C. Furthermore, under conditions where a vacuum is not formed, it is preferable to have an inert gas such as nitrogen as the atmospheric gas. The reason for setting the upper temperature limit at 300°C is to effectively prevent hydrolysis of the hexagonal boron nitride when removing moisture. 【0024】 Furthermore, the above pre-drying is preferably carried out in the heating furnace used for the modification treatment, but it may also be carried out in a separate heating furnace. If a separate heating furnace is used, care must be taken to ensure that the negatively charged hexagonal boron nitride powder does not adsorb moisture from the outside air after pre-drying before transferring the powder to the heating furnace for the modification treatment. 【0025】 <Modification treatment> In the method for modifying hexagonal boron nitride powder of the present invention, the modification treatment is carried out by heating the negatively charged hexagonal boron nitride powder at a temperature of 1300 to 2200°C, preferably 1700 to 2000°C, in the presence of an inert gas with a dew point of -30°C or lower. 【0026】 In the above method, it is important to use an inert gas with a dew point of -30°C or lower, preferably -50°C or lower, as the inert gas used in the reforming process, along with the configuration for adjusting the moisture content of the negatively charged hexagonal boron nitride powder. That is, if the dew point of the inert gas is higher than the above range, the moisture in the inert gas will also be a factor in the generation of ammonia odor in the resulting modified hexagonal boron nitride powder. Examples of the above inert gas include helium, neon, argon and other noble gases, and nitrogen. 【0027】 The dew point of an inert gas can be measured by known methods, but for example, it can be measured by attaching a dew point meter (e.g., Techne Measuring Instruments Co., Ltd.: Online Dew Point Meter TK-100) to the supply piping of the inert gas. 【0028】 Furthermore, during the modification process, some heating furnace materials may absorb and contain moisture. Therefore, it is preferable to perform a preheating at a temperature higher than the treatment temperature before the modification process to remove moisture from inside the heating furnace. Alternatively, if a heating furnace that has already undergone modification treatment is to be used again, the preheating step can be omitted. 【0029】 In the present invention, the modification treatment can be carried out using a known apparatus capable of controlling the atmosphere, such as an atmosphere-controlled high-temperature furnace that performs heat treatment by high-frequency induction heating or heater heating. In addition to batch furnaces, continuous furnaces such as pusher-type tunnel furnaces and vertical reactors can also be used. 【0030】 Furthermore, in the modification treatment, the heating temperature is set to 1300 to 2200°C, preferably 1700 to 2000°C, but the time for holding the heating temperature is appropriately determined to achieve the desired modification effect. Generally, it is preferably 0.2 hours or more, and particularly preferably 0.5 hours or more. Furthermore, in the above modification treatment, if the heating time is too long, hydrolysis by moisture in the inert gas may progress, which is undesirable. Therefore, the upper limit of the heating time is preferably 20 hours, and particularly preferably around 15 hours. Also, the higher the heating temperature, the more likely hydrolysis of hexagonal boron nitride by the present moisture will progress. In particular, hydrolysis becomes significant above 500°C, so the upper limit of the above heating time is preferably the total heating time above 500°C. 【0031】 In the method for modifying hexagonal boron nitride powder of the present invention, the modification treatment is preferably carried out under the flow of an inert gas having the dew point. Specifically, it is preferable to place the boron nitride powder under the supply of an inert gas heated to a predetermined temperature and perform the heat treatment by bringing the flowing inert gas into contact with the boron nitride powder. 【0032】 The conditions for the flow of such inert gas are not particularly limited as long as they allow for the diffusion of gas present near the surface of the boron nitride powder and the exhaust of a portion of it. For example, methods such as supplying and exhausting the inert gas onto a layer of boron nitride powder to facilitate flow, or flowing the boron nitride powder by replacing (supplying and exhausting) a portion of the inert gas while the boron nitride powder is fluidized by the inert gas (a so-called fluidized bed method) can be employed. Among these, fluidizing the boron nitride powder is preferable for efficient modification. 【0033】 Furthermore, the flow rate of the inert gas varies depending on the type of heat treatment, the treatment temperature, the internal structure of the apparatus, etc., and cannot be determined in general terms, but 0.02 to 5 L (volume at 25°C) / min per liter of boron nitride powder to be treated is appropriate. In addition, in the embodiment where boron nitride powder is treated by fluidizing a fluidized bed, sufficient treatment is possible even with a reduced flow rate of the inert gas. Specifically, treatment is possible with 0.02 to 0.5 L (volume at 25°C) / min per liter of boron nitride powder to be treated, and in particular, 0.05 to 0.3 L (volume at 25°C) / min. 【0034】 According to the heat treatment under specific conditions described above, the functional groups and impurities present in trace amounts on the particle surface constituting the hexagonal boron nitride powder raw material are thermally decomposed, and the thermal decomposition products are removed, changing the triboelectric properties. The voltage density in the triboelectric test shows +1V / g or higher, and a modified hexagonal boron nitride powder with fewer clumps and a fluffy texture is obtained. Furthermore, this modified hexagonal boron nitride powder has the characteristic of generating very little ammonia despite undergoing heat treatment. 【0035】 The modification method for hexagonal boron nitride powder of the present invention also allows for the appropriate adjustment of whiteness. Specifically, the degree of coloration can be adjusted by the type of inert gas used in the modification method. For example, by using argon gas to generate many nitrogen vacancies in the boron nitride, the whiteness of the boron nitride decreases and the yellowness increases, resulting in modified hexagonal boron nitride powder suitable for cosmetics that is close to the color of human skin. Furthermore, when nitrogen gas is used, modified hexagonal boron nitride powder with high whiteness can be obtained. 【0036】 <Modified hexagonal boron nitride powder> According to the present invention, a modified hexagonal boron nitride powder is provided in which 200 g of the powder is placed in a sealed container where the voltage density measured by a triboelectric test is +1 V / g or higher, the gas phase is adjusted to a temperature of 25°C and a humidity of 70%, and after 60 minutes the ammonia concentration in the gas phase is 60 ppm or less. 【0037】 The modified hexagonal boron nitride powder of the present invention can reduce the ammonia concentration in the gas phase, which exhibits the ammonia odor suppression effect, to 60 ppm or less, particularly 40 ppm or less, and even further to 20 ppm or less, making it suitable for use in cosmetic applications. 【0038】 In the modified hexagonal boron nitride powder of the present invention, when the voltage density is less than +1V / g (or when negatively charged), the particles tend to aggregate and form clumps. A voltage density of +10V / g or higher is preferred, and +30V / g or higher is more preferred. There is no particular upper limit to the voltage density, but if it becomes excessively positively charged, it becomes difficult to handle, so it is desirable that this voltage density be +200V / g or less, and more preferably +100V / g or less. 【0039】 The modified hexagonal boron nitride powder of the present invention, due to its positive triboelectric properties, has fewer clumps and a fluffy texture, resulting in good spreadability and adhesion when incorporated into cosmetics. 【0040】 Furthermore, due to its positive frictional charging properties, it possesses excellent fluidity. This fluidity can be expressed by the avalanche energy (change in potential energy before and after avalanche), which indicates powder fluidity. That is, the avalanche energy is measured using a rotating drum type powder fluidity measuring device, similar to the voltage density mentioned above (see Examples for detailed conditions), and the lower this value, the better the fluidity, and the better the spreadability and adhesion. For example, in the modified hexagonal boron nitride powder of the present invention, this avalanche energy is in the range of 40 mJ / kg or less, preferably 35 mJ / kg or less, and more preferably 30 mJ / kg or less. There is no lower limit to this preferred range of avalanche energy, but it is usually 10 mJ / kg or more. 【0041】 In the above-mentioned modified hexagonal boron nitride powder, other properties are not particularly limited, but it is desirable that it has properties that have been conventionally required for cosmetic applications. 【0042】 For example, this modified hexagonal boron nitride powder follows the properties of the negatively charged hexagonal boron nitride powder, but preferably has an average major axis of primary particles in the range of 2 to 20 μm, particularly 3 to 12 μm, an average thickness of 0.2 to 2.0 μm, particularly 0.3 to 1.2 μm, and an average aspect ratio (average major axis / average thickness) in the range of 5 to 30, particularly 6 to 20. Also, the median diameter (D1: midpoint diameter D based on volume) measured by laser diffraction / scattering method is also important. 50 The median diameter (D2) is preferably 4 to 40 μm. Furthermore, when the powder is ultrasonically dispersed in ethanol, the median diameter (D2) measured by laser diffraction / scattering is preferably 2 to 20 μm. 【0043】 Furthermore, the amount of eluted boron in the modified hexagonal boron nitride powder of the present invention is preferably 20 ppm or less. This amount of eluted boron is measured by the method specified in the Quasi-Drug Raw Materials Standards 2021. The Quasi-Drug Raw Materials Standards 2021 stipulate that the amount of eluted boron should be 20 ppm or less from the viewpoint of safety and hygiene. 【0044】 Furthermore, when expressed in the Lab color system, the modified hexagonal boron nitride powder of the present invention has a whiteness (L * value) of 90.0 or more, a redness (a * value) of -3.0 to 0, and a yellowness (b * value) in the range of 0 to 15.0. 【0045】 The modified hexagonal boron nitride powder of the present invention has almost no lumps, and thus a low bulk density is also one of its characteristics. The bulk density varies greatly depending on the average major diameter and average thickness. For example, if the average major diameter is about 4 μm and the average thickness is about 0.6 μm, the loose bulk density is 0.11 g / cm 3 or less, and the tapped bulk density is 0.33 g / cm 3 or less. If the average major diameter is about 9 μm and the average thickness is about 1 μm, the loose bulk density is 0.2 g / cm 3 or less, and the tapped bulk density is 0.6 g / cm 3 or less. 【0046】 Also, through a fluidization test using a rotary drum type powder fluidity measuring device used for measuring charge density and avalanche energy, it can be shown that the modified hexagonal boron nitride powder of the present invention is a powder with few lumps and a fluffy feeling. That is, in the fluidization test performed by rotating a standard rotary drum containing 100 cc of sample powder at high speed, when measuring the height of the lower part of the powder layer in the drum, for the modified hexagonal boron nitride powder of the present invention, the height of this powder layer is 2.2 cm or more at a drum rotation speed of 20 rpm and 2.8 cm or more at 50 rpm. The larger this value is, the more it indicates that the powder has few lumps and a fluffy feeling. 【0047】 Furthermore, the basic fluidity energy in dynamic fluidity testing is known as a parameter that indicates that a powder contains almost no aggregates and has a fluffy texture. This basic fluidity energy is indicated by the torque applied to a rotor when a rotor is inserted into a 160 mL split container filled with the sample powder, using a powder rheometer (for example, Malvern FT-4), as shown in the examples described later. The lower this basic fluidity energy (torque applied to the rotor), the less clumpy and fluffier the powder is. The boron nitride powder of the present invention has a basic fluidity energy of 100 mJ or less, and this value also indicates that it is a powder with few clumps (aggregates) and a fluffy texture. [Examples] 【0048】 The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. 【0049】 In the following examples and comparative examples, various tests and measurements were performed by the following methods. 【0050】 (1) Average major axis, average thickness, aspect ratio Boron nitride powder was subjected to ultrasonic homogenization to break down aggregated particles and obtain monodisperse, and then dried before being used as the measurement sample. Specifically, 2 g of boron nitride powder was mixed with 25 mL of deionized water to form a slurry, which was then ultrasonically treated using a BRANSON SONIFIRE ultrasonic homogenizer (model SFX250, tip: 20 kHz microchip 1 / 4 inch) at an amplitude of 40% for a processing time of 20 minutes, followed by filtration and drying. 【0051】 Next, 1 g of boron nitride powder was added to 9 g of epoxy resin (Henkel EA E-30CL), stirred and degassed using a Kurabo Mazelstar, and then poured into a 10 mm square, 1 mm thick mold. The mixture was then heated at 70°C to cure. After cooling, the cured resin composition was removed from the mold, and one side of it was milled in cross-section. Multiple SEM images were then taken at a magnification of 1000x. 【0052】 Next, the obtained SEM images were analyzed using an image analysis device (A-Image-kun: manufactured by Asahi Kasei Engineering Co., Ltd.) to measure the major axis and thickness of the boron nitride particles. The analysis mode of A-Image-kun was set to needle-shaped particle analysis. Particles that were difficult to measure accurately due to overlapping of multiple particles were ignored, and all particles contained in one field of view of the SEM image were measured. This was repeated for multiple fields of view, thereby measuring the major axis and thickness for more than 1000 particles. The average values ​​were taken as the average major axis and average thickness, and the aspect ratio was calculated by dividing the average major axis by the average thickness (aspect ratio = average major axis / average thickness). 【0053】 (2) Median diameter (D1: μm) 0.5 g of boron nitride powder was placed in 25 cc of ethanol, and after light shaking, the particle size distribution of the boron nitride suspension was measured using a laser diffraction / scattering particle size distribution analyzer (HORIBA LA-950V2), and the average particle size (D) based on volume was determined. 50 The median diameter (D1) was defined as ). 【0054】 (3) Median diameter (D2: μm) 20 g of ethanol was added to a 50 mL screw-cap vial as a dispersion medium, and 1 g of boron nitride powder was dispersed in the ethanol. Next, using a BRANSON ultrasonic homogenizer (SONIFIER SFX250), the tip (20 kHz microchip 1 / 4 inch) was placed 1-5 mm from the bottom of the screw-cap vial, and ultrasonic treatment was performed at an amplitude of 40% for 20 minutes. The boron nitride suspension treated with ultrasonic treatment was then used as a measurement sample, and the particle size distribution was measured in the same manner as the median diameter (D1), and the average particle size (D1) based on volume was determined. 50 The median diameter (D2) was defined as ). 【0055】 (4) Whiteness, redness, and yellowness according to the Lab color system Using ZE6000 manufactured by Nippon Denshoku Industries Co., Ltd., the whiteness (L * Value), redness (a * Value), yellowness (b * The value was measured. 【0056】 The measurements were performed by filling a quartz glass cell with a diameter of 30 mm and a height of 13 mm with boron nitride powder. 【0057】 (5) Light bulk density, tap bulk density Using the Seishin Corporation Tap Denser KYT-5000, the bulk density of the lightly loaded material (g / cm³) was measured. 3 ) and tap bulk density (g / cm³) 3 ) was measured. 【0058】 The sample cell was 100 ml, the tapping speed was 120 times / min, the tapping height was 5 cm, and the number of taps was 500. 【0059】 (6) Amount of eluted boron The eluted boron was extracted using a method compliant with the 2021 Quasi-Drug Raw Material Standards, and the amount of boron (ppm) was measured using an ICP emission spectrometer. 【0060】 Specifically, 2.5 g of boron nitride powder was placed in a Teflon® beaker, 10 mL of ethanol was added and stirred well, then 40 mL of water was added and stirred well again, after which a Teflon® watch glass was placed on top and heated at 50°C for 1 hour. 【0061】 After cooling, the solution was filtered, the residue was washed with a small amount of water, and the washings were mixed with the filtrate. This solution was further filtered through a membrane filter (0.22 μm). The entire filtrate was placed in a Teflon® beaker, 1 mL of sulfuric acid was added, and it was boiled on a hot plate for 10 minutes. 【0062】 After cooling, the liquid was placed in a polyethylene volumetric flask. A Teflon® beaker was washed with a small amount of water and placed in the polyethylene volumetric flask. Water was then added to make exactly 50 mL, which was used as the sample solution. The amount of boron in the sample solution was measured using an ICP emission spectrometer. 【0063】 (7) Voltage density (V / g) A triboelectric charging test was performed using the REVOLUTION rotary drum type powder flowability measuring device manufactured by Mercury Scientific, and the voltage density (V / g) was measured. 【0064】 Specifically, 100cc of boron nitride powder was placed in a standard rotating drum, static electricity was removed using an ionizer, and then the amount of charge (V) was measured while the drum rotated at a speed of 10 rpm for 300 seconds. 【0065】 Immediately after the rotation of the rotating drum begins, the charge amount (V) fluctuates significantly and is unstable. Therefore, the average charge amount (V) during the stable period of 200-300 seconds was calculated, and the value obtained by dividing this by the weight (g) of boron nitride powder placed in the rotating drum was defined as the voltage density (V / g). 【0066】 The standard rotating drum described above has a cylindrical shape with an inner surface made of anodized aluminum, and each side of the cylinder is fitted with a window made of boron silicate glass, with a drum capacity of 332cc. 【0067】 (8) Avalanche energy A fluidity test was performed using the same powder fluidity measuring device that was used for measuring the electrostatic charge density, and the avalanche energy (mJ / kg) was measured. 【0068】 Specifically, 100cc of boron nitride powder was placed in the standard rotating drum described above, static electricity was removed using an ionizer, and then the drum was rotated at a rotation speed of 0.3 rpm. The avalanche energy (change in potential energy before and after the avalanche) (mJ / kg) of the resulting avalanche was measured, and the average value of 150 avalanches was taken as the avalanche energy (mJ / kg). 【0069】 (9) Height of the powder layer as determined by the fluidity test A fluidity test was conducted using the powder fluidity measuring device described above, and the height (cm) of the powder layer was measured. 【0070】 Specifically, 100cc of boron nitride powder was placed in the standard rotating drum described above, static electricity was removed using an ionizer, and then the height of the powder layer (height at the lowest point of the powder layer) (cm) was measured when the drum was rotated at predetermined speeds (20 and 50 rpm). 【0071】 The height (cm) of this powder layer is an indicator of how easily the powder can be fluidized, and the height tends to be greater for powders that are more easily fluidized. 【0072】 In the table summarizing the results of the examples, "-" indicates that the voltage density was high, causing the powder to become charged and adhere to the drum wall, resulting in inaccurate measurement. 【0073】 (10) Basic fluidity energy obtained by dynamic fluidity testing A dynamic fluidity test was performed using a Malvern FT-4 powder rheometer, and the fundamental fluidity energy (mJ) was measured. 【0074】 Specifically, a container consisting of a 160 mL split container with a height of 89 mm and a cylinder with a height of 51 mm placed on top was filled with boron nitride powder exceeding a height of 89 mm. After performing conditioning (rotor blade tip speed of 60 mm / sec, entry angle of 5°) four times, the cylinder placed on top of the split container was slid to level off the boron nitride powder. 【0075】 Next, the torque (mJ) applied to the rotor blade was measured while moving from a height of 100 mm to 10 mm from the bottom of the container, with the rotor blade tip speed at 100 mm / sec and an entry angle of -5°. This torque value was defined as the basic fluidity energy (mJ). Basic fluidity energy (mJ) is an indicator of the fluidity of the powder, and tends to be smaller for powders with good fluidity. 【0076】 (11) Measurement of ammonia odor 200g of boron nitride powder and an ammonia concentration meter (SENKO, model number: SGT-P-NH3) were placed in a 100L transparent sealed container. The container was then sealed after replacing the air inside with air at 25°C and 70% RH relative humidity, and the ammonia concentration (ppm) was measured after 60 minutes. The boron nitride powder was spread on a 25cm diameter dish before being placed in the sealed container, and the container was placed in a room adjusted to 25°C to maintain the temperature inside the container at 25°C during the test. 【0077】 (12) Condition where clumps form The hexagonal boron nitride powder samples were visually inspected and evaluated according to the following criteria. ◎: It's very fluffy with almost no lumps. ○: Some clumps are observed, but it is fluffy. ×: Many clumps are observed, giving it a heavy feel. 【0078】 (13) Moisture content The moisture content was measured using a heat-drying type moisture meter MS-70 (manufactured by A&D Company, Limited). Specifically, 5g of hexagonal boron nitride powder was weighed onto a balance, and the weight change was measured while heating it to 200°C. The measurement was stopped when the weight change was 0.001% / min or less. The difference between the weight before heating and the weight at the end of the measurement was taken as the moisture content, and the moisture content (mass %) was calculated. 【0079】 <Example 1> A mixture of the following formulations was prepared using a ball mill. 70 parts by mass of boron oxide Carbon black 30 parts by mass 10 parts by mass of calcium carbonate This mixture was placed in a graphite-type Tamman furnace and heated to 1500°C at a rate of 15°C / min under a nitrogen gas atmosphere. The mixture was then held at 1500°C for 6 hours for reductive nitriding treatment, followed by heating to 1800°C at a rate of 15°C / min for 2 hours for crystallization treatment to obtain crude hexagonal boron nitride powder. 【0080】 Next, the obtained crude hexagonal boron nitride powder was placed in a polyethylene container, and 10 times the amount of hydrochloric acid aqueous solution (10% by mass HCl) was added, and the mixture was stirred at a rotation speed of 300 rpm for 15 hours. 【0081】 After the acid washing described above, the crude hexagonal boron nitride obtained by filtration was washed with pure water having a resistivity of 1 MΩ·cm at 25°C in an amount equivalent to 300 times the original amount. Then, dehydration was carried out by suction filtration until the moisture content of the filtered powder was 50% by mass or less. 【0082】 After washing with pure water, the obtained powder was dried under reduced pressure at 200°C for 15 hours at a pressure of 1 kPaA, and then classified using a vibrating sieve with a mesh opening of 45 μm to obtain negatively charged hexagonal boron nitride powder. The voltage density of this hexagonal boron nitride powder was -36 V / g, and the evaluation of clumping was "×", indicating that it contained many clumps. 【0083】 The negatively charged hexagonal boron nitride powder obtained by the above manufacturing method is placed in a carbon container (inner diameter 500 mm, inner height 50 mm) surface-coated with boron nitride at a density of 0.25 g / cm³. 3 The boron nitride powder was packed to a height of 45 mm, and these were stacked in 10 layers in a graphite Tamman furnace with an internal volume of 1,000 L. As a preliminary drying step, the furnace was heated to 200°C under a vacuum pressure of 30 Pa or less, and vacuum heating was performed for 3 hours. After cooling to room temperature, a portion of the boron nitride powder was extracted, and the moisture content of the negatively charged hexagonal boron nitride powder after vacuum heating was measured. The moisture content was found to be below the detection limit (10 ppm). 【0084】 Next, the negatively charged hexagonal boron nitride powder, with its moisture content adjusted as described above, was processed by replacing the furnace with nitrogen gas having a dew point of -90°C, an oxygen concentration of less than 0.1 ppm, and a carbon dioxide concentration of less than 0.03 ppm. Then, while supplying the nitrogen gas at a flow rate of 40 L (volume at 25°C) / min, the temperature was raised to a processing temperature of 1500°C at a rate of 15°C / min, and held at 1500°C for 4 hours to obtain modified hexagonal boron nitride powder. 【0085】 Furthermore, the carbon container used was one in which nitrogen channels were provided between each stage so that nitrogen could flow over the top of the powder layer. 【0086】 The modified hexagonal boron nitride powder obtained was subjected to the measurements described in (1) to (12) above, and the measurement results are shown in Table 1. 【0087】 <Examples 2-5, Comparative Examples 1-3> Modified hexagonal boron nitride powder was obtained in the same manner as in Example 1, except that the pre-drying conditions for the negatively charged hexagonal boron nitride powder, the dew point of the inert gas during the heat treatment, the treatment temperature, and the temperature holding time were changed as shown in Table 1. The measurement results for the obtained modified hexagonal boron nitride powder are also shown in Table 1. 【0088】 [Table 1] 【0089】 [Cosmetic Testing] Using the modified hexagonal boron nitride powder obtained in the above examples and comparative examples, powder foundations were prepared in the following proportions. 【0090】 Modified hexagonal boron nitride powder 20.0% by mass Mica 15.0% by mass Synthetic phlogopite 12.0% by mass Ethylhexyl methoxycinnamate 8.0% by mass (Vinyl dimethicone / methicone silsesquioxane) Crosspolymer 8.0% by mass (Diphenyl Dimethicone / Vinyl Diphenyl Dimethicone / Silsesquioxane crosspolymer 8.0% by mass Nylon 12 3.0% by mass Silica 3.0% by mass Talc 3.0% by mass Acrylates crosspolymer 3.0% by mass Perfluorooctyltriethoxysilane 3.0% by mass Zinc oxide 3.0% by mass Polymethyl methacrylate polymer 3.0% by mass Silicone-treated red iron oxide (red iron oxide) 1.0% by mass Silicone-treated yellow iron oxide 0.6% by mass Silicone-treated black iron oxide 0.4% by mass Silicone-treated titanium dioxide 6.0% by mass When the obtained powder foundation was taken onto a makeup sponge and applied to the skin, the visual inspection shown in Table 1 revealed that the products marked with ◎ provided a uniform finish with a single application, while the products marked with ○ and × resulted in uneven application with a single application and required 2-3 applications to achieve a uniform finish. 【0091】 Furthermore, when the modified hexagonal boron nitride powder of the example was used, there was almost no ammonia odor during the preparation and use of the powder foundation. In particular, when the ammonia concentration was 30 ppm or less, there was no ammonia odor at all. In contrast, when the hexagonal boron nitride powder of the comparative example, which had an ammonia concentration exceeding 60 ppm, was used, there was a clearly noticeable ammonia odor during the preparation and use of the powder foundation.

Claims

[Claim 1] A method for modifying hexagonal boron nitride powder, characterized by adjusting the negatively triboelectric hexagonal boron nitride powder so that the moisture content measured by the drying loss method is 50 ppm or less, and then heat-treating it at a temperature of 1300 to 2200°C under the flow of an inert gas with a dew point of -30°C or lower. [Claim 2] The modification method according to claim 1, wherein the heat treatment is carried out until the voltage density of the hexagonal boron nitride powder becomes +1 V / g or more. [Claim 3] Modified hexagonal boron nitride powder having a voltage density of +1 V / g or higher as measured by a triboelectric test, characterized in that 200 g of the modified hexagonal boron nitride powder is placed in a sealed container with an internal volume of 100 L, where the gas phase is adjusted to a temperature of 25°C and a relative humidity of 70% RH, and after 60 minutes the ammonia concentration in the gas phase of the sealed container is 60 ppm or less. [Claim 4] The modified hexagonal boron nitride powder according to claim 3, wherein the average major diameter of the primary particles is in the range of 2 to 20 μm, the average thickness is in the range of 0.2 to 2.0 μm, and the average aspect ratio is in the range of 5 to 30. [Claim 5] Modified hexagonal boron nitride powder according to claim 3 or 4, used as a compounding agent for cosmetics.

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