Hexagonal boron nitride powder and cosmetics for cosmetic use

Abstract

Provided are a hexagonal boron nitride powder for a cosmetic having low hiding power and excellent slidability and skin compatibility, and a cosmetic using this hexagonal boron nitride powder.　This hexagonal boron nitride powder for a cosmetic is made of scale-like primary particles and secondary particles formed by laminating the primary particles, wherein the average particle diameter is 4-15 μm, the aspect ratio is 30 or lower, the average thickness of the primary particles is 0.3-0.7 μm, and the number of secondary particles is 40% or less with respect to the number of primary particles and secondary particles. The oxygen concentration may be 0.5% by mass or lower. The cosmetic contains this hexagonal boron nitride powder.

Description

【Technical Field】 【0001】 The present invention relates to hexagonal boron nitride powder for cosmetics and cosmetics using the hexagonal boron nitride powder. 【Background Art】 【0002】 Hexagonal boron nitride (hereinafter also simply abbreviated as boron nitride or BN) is widely used as a pigment for cosmetics. 【0003】 For example, in Patent Document 1, a hexagonal boron nitride powder is disclosed in which the side surface of a powder having a flat shape has six crystal planes equivalent to the (100) plane and six crystal planes equivalent to the (110) plane in the crystal display of the hexagonal crystal system as the crystal habit plane. Further, in Patent Document 2, it is composed of plate-like aggregates in which primary particles having a flat shape with an average major diameter of 2 to 20 μm and a thickness of 0.05 to 0.5 μm are laminated, and the specific surface area is 1 to 10 m 2 / g, and the content rate of aggregates passing through a 45 μm sieve is 50% by mass or more, and further the soluble boron amount is 100 ppm or less, and a boron nitride powder for cosmetics is disclosed. Further, in Patent Document 3, it is composed of aggregates of primary particles having a scaly shape with an average major diameter of 4 to 15 μm, a thickness of 0.2 to 0.7 μm, and an aspect ratio of more than 20, and the specific surface area is 2 to 8 m 2 / g, and further a boron nitride powder for cosmetics is disclosed, characterized in that the soluble boron amount is 100 ppm or less. 【Prior Art Documents】 【Patent Documents】 【0004】 【Patent Document 1】 Japanese Patent Laid-Open No. 5-186205 【Patent Document 2】 Japanese Patent Laid-Open No. 2012-176910 【Patent Document 3】 Japanese Patent Laid-Open No. 2015-140337 【Summary of the Invention】 [Problems that the invention aims to solve] 【0005】 In recent years, cosmetics have prioritized a natural, bare-skin look over coverage, and lower concealment power is preferred. Furthermore, a light, easily spreadable texture is desired when applied to the skin, and high slipperiness is preferred. Additionally, there is a demand for products that adhere well to the skin without being too thick or heavy; in other words, products with a moderate level of skin compatibility are preferred. However, the hexagonal boron nitride powders disclosed in the aforementioned Patent Documents 1 to 3 did not satisfy all requirements regarding low opacity, slipperiness, and skin compatibility. 【0006】 This invention has been made in view of the above problems, and aims to provide a hexagonal boron nitride powder for cosmetics that has low opacity, excellent slipperiness and skin compatibility when used as a cosmetic, and a cosmetic using this hexagonal boron nitride powder. 【0007】 Here, low opacity means that when hexagonal boron nitride powder is applied to a 2cm x 2cm area of ​​black artificial leather using a flat nylon brush (bristle length 15mm, tip width 1.1cm), the whiteness is measured, and the measurement and surface adhesion removal with the brush are repeated until the change in the whiteness value becomes 0, the endpoint is set, and the opacity value obtained by subtracting the whiteness of the artificial leather (18.6) from the whiteness at the endpoint is 16 or less. Furthermore, "excellent slipperiness" refers to a measurement using the Trilabo Handy Tribomaster TL201Ts (manufactured by Trinity Lab Co., Ltd.), with a load of 30g, a travel speed of 10mm / s, a reciprocating distance of 50mm, and 20 reciprocating cycles. 0.05g of hexagonal boron nitride powder is placed on artificial leather, and the coefficient of dynamic friction after 20 reciprocating cycles is measured to be 0.31 or less. Furthermore, "excellent skin compatibility" refers to a skin compatibility index of 0.04 to 0.08, which is the difference between the dynamic friction coefficients of the first and second passes, measured using the Trilabo Handy Tribomaster TL201Ts (manufactured by Trinity Lab Co., Ltd.), with a load of 30g, a movement speed of 10mm / s, a reciprocating distance of 50mm, and two reciprocating passes. [Means for solving the problem] 【0008】 As a result of diligent research into the above-mentioned problems, the inventors have found that by reducing the number of secondary particles (aggregates) formed by the overlapping of primary particles (crystals, substrate particles) of hexagonal boron nitride powder to a predetermined ratio or less, it is possible to achieve a state with low opacity and excellent slipperiness and skin compatibility. 【0009】 Figure 1 is a schematic diagram illustrating primary particles 10, 11 and secondary particles 12 in hexagonal boron nitride powder 1. Figure 2 is an electron microscope image of hexagonal boron nitride powder illustrating the primary and secondary particles. As shown in Figure 1, one or more small particles 10 are stacked on the surface of flattened primary particles (large particles) 11 to form stepped secondary particles 12. Figure 2 shows an electron microscope image of an example of stepped secondary particles. The inventors have also found that the proportion of such secondary particles 12 can be reduced to below a predetermined level by controlling the conditions during the heat treatment of crude BN to obtain hexagonal boron nitride powder 1. 【0010】 In this invention, secondary particles are formed by stacking primary particles (large particles) with a particle size of 3 to 20 μm on top of primary particles (small particles), and the particle size of the primary particles (small particles) is 10 to 80% of the particle size of the primary particles (large particles). If the particle size of a primary particle (small particle) is more than 80% of the particle size of a primary particle (large particle), it can be considered the same as a primary particle (large particle) and is not a primary particle (small particle). For example, if the particle size of the large particles is 3 μm, the particle size of the small particles will be 0.3 to 2.4 μm, and if the particle size of the large particles is 20 μm, the particle size of the small particles will be 2 to 16 μm. Figure 3 is a schematic diagram illustrating primary particles (large particles) and primary particles (small particles). Figure 3(a) shows an example of secondary particles 12A with a relatively large particle size, and Figure 3(b) shows an example of secondary particles 12B with a relatively small particle size. Secondary particle 12A is formed by stacking primary particle (small particle) 10A on primary particle (large particle) 11A, and secondary particle 12B is formed by stacking primary (small particle) 10B on primary particle (large particle) 11B. In this case, as described above, as long as the condition that the particle size of the primary particle (small particle) is 10-80% of the particle size of the primary particle (large particle) is met, the range of particle size of the primary particle (small particle) is not limited, nor is the range of particle size of the primary particle (large particle). That is, for example, primary particle (small particle) 10A and primary particle (large particle) 11B may both be particles of the same or equivalent size with a particle size of X, and in the case of secondary particle 12A, which has a relatively large particle size, primary particle 10A may be a small particle, and in the case of secondary particle 12B, which has a relatively small particle size, primary particle 11B may be a large particle. 【0011】 Furthermore, as mentioned above, a secondary particle refers to a structure in which primary particles (small particles) are stacked on top of primary particles (large particles), and a structure in which multiple primary particles (small particles) are stacked on top of one primary particle (large particle) is considered a single secondary particle. Furthermore, primary particles (large particles) stacked on top of other primary particles (large particles), or primary particles (small particles) stacked on top of other primary particles (small particles), are not included in secondary particles. A stack of one primary particle (large particle) on top of one primary particle (large particle) is considered two primary particles, and a stack of one primary particle (small particle) on top of one primary particle (small particle) is also considered two primary particles. 【0012】 The diameter of the primary particles is determined by observation using a scanning electron microscope (SEM, magnification: 4000x). For the secondary particles, hexagonal boron nitride powder is applied with a brush, the flattened surface of the hexagonal boron nitride powder is pressed into contact with the artificial leather, and an air dryer is used to remove excess powder, ensuring that the powder particles do not overlap. As a result, the layered state can be observed, and only the number of particles that can be clearly determined to be layered is measured. 【0013】 The present invention has been made based on the above findings and is as follows. [1] A hexagonal boron nitride powder composed of scaly primary particles and secondary particles formed by laminating the primary particles, having an average particle size of 4 to 15 μm, having an aspect ratio of 30 or less, the average thickness of the primary particles being 0.3 to 0.7 μm, and the number of the secondary particles being 40% or less among the total number of the primary particles and the secondary particles, a hexagonal boron nitride powder for cosmetics. [2] The hexagonal boron nitride powder for cosmetics according to [1], having an oxygen concentration of 1.0% by mass or less. [3] A cosmetic containing the hexagonal boron nitride powder according to [1] or [2]. 【Effects of the Invention】 【0014】 According to the present invention, there are provided a hexagonal boron nitride powder for cosmetics having low hiding power and excellent slipperiness and skin affinity, and a cosmetic using this hexagonal boron nitride powder. 【Brief Description of the Drawings】 【0015】 [Figure 1] It is a schematic diagram for explaining primary particles and secondary particles in a hexagonal boron nitride powder. [Figure 2] It is an electron micrograph of a hexagonal boron nitride powder for explaining primary particles and secondary particles. [Figure 3] It is a schematic diagram for explaining primary particles (large particles) and primary particles (small particles). 【Modes for Carrying Out the Invention】 【0016】 Hereinafter, the hexagonal boron nitride powder of the present invention, its production method, and a cosmetic using the hexagonal boron nitride powder will be described in order. 【0017】 <Hexagonal boron nitride powder> The hexagonal boron nitride powder of the present invention is a hexagonal boron nitride powder for cosmetic use, and is composed of flaky primary particles and secondary particles, with an average particle size of 4 to 15 μm, an aspect ratio of 30 or less, an average thickness of 0.3 to 0.7 μm for the primary particles, and the number of secondary particles being 40% or less of the total number of primary and secondary particles. 【0018】 (Average particle size: 4~15μm) If the average particle size of hexagonal boron nitride powder is less than 4 μm, it will turn white and lose transparency when used as a cosmetic. On the other hand, if the average particle size of the powder exceeds 15 μm, it may not feel smooth to the touch. Therefore, the average particle size of hexagonal boron nitride powder should be between 4 and 15 μm. Preferably, the average particle size is 5 μm or more. Also preferably, the average particle size is 12 μm or less. In this invention, the average particle size of hexagonal boron nitride powder refers to the average particle size of the powder as a mixture of secondary and primary particles. The average particle size can be measured by the method described in the examples below. 【0019】 (Aspect ratio: 30 or less) If the aspect ratio is less than 7, the concealing power may become too high when used as a cosmetic, so it is preferable that it be 7 or higher. On the other hand, if the aspect ratio exceeds 30, the texture may not feel smooth. Therefore, the aspect ratio should be 30 or less. Preferably, the aspect ratio is 7 or higher. More preferably, the aspect ratio is 10 or higher. Even more preferably, the aspect ratio is 14 or higher. Also preferably, the aspect ratio is 20 or less. Note that the aspect ratio refers to the value obtained by dividing the average particle size (μm) of the powder mentioned above by the average thickness (μm) of the primary particles described later (average particle size / average thickness), and can be measured by the method described in the examples below. 【0020】 (Average thickness of primary particles: 0.3~0.7μm) As described above, since the average particle size of the powder is to be 4 to 15 μm, it is difficult to manufacture hexagonal boron nitride powder with an average primary particle thickness of less than 0.3 μm. On the other hand, if the average primary particle thickness exceeds 0.7 μm, the skin may not feel smooth when used as a cosmetic. Therefore, the average primary particle thickness should be 0.3 to 0.7 μm. Preferably, the average thickness is 0.3 to 0.6 μm, and more preferably, 0.3 to 0.5 μm. The average primary particle thickness can be measured by the method described in the examples below. 【0021】 (Secondary particle ratio: 40% or less) In this invention, secondary particles are, as mentioned above, primary particles (large particles) with a particle size of 3 to 20 μm on which primary particles (small particles) are stacked, and the particle size of the primary particles (small particles) is 10 to 80% of the particle size of the primary particles (large particles). In this invention, the ratio of the number of secondary particles to the total number of primary and secondary particles (hereinafter also simply referred to as the secondary particle ratio) is set to 40% or less. If the secondary particle ratio exceeds 40%, when used as a cosmetic, it will not be possible to achieve low opacity and excellent slipperiness and skin compatibility. Therefore, in this invention, the secondary particle ratio is set to 40% or less. Preferably, the secondary particle ratio should be as low as possible, preferably 30% or less, and more preferably 20% or less. The secondary particle ratio may be 0%, 3% or more, or 5% or more. The proportion of secondary particles can be measured by the method described in the examples below. 【0022】 (Oxygen concentration: 1.0% by mass or less) While not particularly limited in this invention, the oxygen concentration is preferably 1.0% by mass or less. An oxygen concentration of 1.0% by mass or less can further suppress the adhesion between particles, allowing particles to adhere uniformly oriented on the skin surface. More preferably, the oxygen concentration is 0.8% by mass or less, even more preferably 0.6% by mass or less, and even more preferably 0.5% by mass or less. More preferably, the oxygen concentration is 0.4% by mass or less, and even more preferably 0.3% by mass or less. The oxygen concentration may be 0% by mass or 0.1% by mass or higher. 【0023】 <Method for producing hexagonal boron nitride powder> Next, preferred conditions for the method of producing hexagonal boron nitride powder according to the present invention will be described. The present invention relates to the method for producing hexagonal boron nitride powder described above, and comprises a primary heating step of heating crude boron nitride powder to a first holding temperature of 1500 to 1900°C under an inert gas atmosphere and holding it at the first holding temperature for 1 to 5 hours, and a secondary heating step of subsequently heating the crude boron nitride powder from the first holding temperature to a second holding temperature of 1950 to 2300°C under an inert gas atmosphere after the primary heating step and holding it at the second holding temperature for 3 to 20 hours. Note that the temperatures described below, including the temperatures mentioned above, are furnace temperatures. 【0024】 First, prepare a crude boron nitride powder containing high-purity boron nitride powder with a chaotic structure as the raw material. Such boron nitride powder can be obtained by uniformly mixing boric acid and / or its dehydrated product with urea and / or its compounds (such as dicyandiamide and melamine), and heating the mixture in an inert gas atmosphere from 500°C to 1200°C. 【0025】 When mixing as described above, the nitrogen (N) / boron (B) molar ratio must be between 1 and 5. If the N / B molar ratio is less than 1, the amount of impurities increases, leading to excessive grain growth of primary particles. On the other hand, if it exceeds 5, the amount of N becomes too high, and sufficient grain growth cannot be expected. Therefore, when mixing as described above, the molar ratio of nitrogen (N) to boron (B) should be between 1 and 5. 【0026】 The above heat treatment can be used to obtain boron nitride powder with a random layer structure containing appropriate amounts of oxygen and carbon. Here, the appropriate amounts of oxygen and carbon are defined as an oxygen (O) content of 10 to 40 mass%, a carbon (C) content of 0.01 to 10 mass%, and an oxygen (O) / carbon (C) ratio of 2.0 or higher in molar ratio. Furthermore, in this invention, a random layered structure refers to a BN structure that does not take sharp hexagonal peaks when X-ray diffracted, but rather a broad, not completely crystallized structure. 【0027】 (Two-stage heating) Next, a two-stage heating process is performed to obtain hexagonal boron nitride powder from the coarse boron nitride powder, in which the proportion of secondary particles according to the present invention is 40% or less. The inventors focused on the fact that the boiling point of the boric acid compound present in the impurities, which make up about 40% by mass of the above material, is around 1880°C, that the sublimation temperature of tBN (randomly layered boron nitride powder) is 2500°C or higher, and that a structural change occurs in the liquid phase of hexagonal boron nitride at about 1400°C. Furthermore, by controlling the holding temperature in stages, we were able to suppress the formation of secondary particles contained in the hexagonal boron nitride powder, although the reason for this is unknown. Based on these findings, we discovered that by applying a two-stage heating process to coarse boron nitride powder, in which a first holding treatment and a second holding treatment are performed sequentially under specific heat treatment conditions, hexagonal boron nitride powder with a secondary particle ratio of 40% or less can be obtained. 【0028】 Primary heating process: First heating treatment and first holding treatment In the primary heating step, the crude boron nitride powder is heated to a first holding temperature of 1500-1900°C under an inert gas atmosphere and held at the first holding temperature for 1-5 hours. Specifically, for example, the crude boron nitride powder obtained as described above is first subjected to a heat treatment (first heat treatment) in an inert gas atmosphere under normal or pressurized pressure, starting from room temperature (20-25°C) and increasing at a heating rate of 2-10°C / min to an intermediate holding temperature (first holding temperature) of 1500-1900°C. If the holding temperature is too low, crystal growth will not proceed, and the proportion of secondary particles will increase. On the other hand, if the first holding temperature is too high, the first holding temperature will become the same as the second holding temperature, making it difficult to observe the effect of two-stage heating, and the proportion of secondary particles in the resulting hexagonal boron nitride powder will be the same as when the first holding treatment is not performed. Therefore, the first holding temperature is set to 1500 to 1900°C. Preferably, the first holding temperature is 1700 to 1900°C. Subsequently, the material is held at the first holding temperature described above for 1 to 5 hours (first holding treatment). If the holding time is too short or too long, the proportion of secondary particles will increase. The holding time at the first holding temperature is preferably 3 to 5 hours. 【0029】 Secondary heating process: Second heating treatment and second holding treatment In the secondary heating step, after the primary heating step described above, the crude boron nitride powder is heated in an inert gas atmosphere from the first holding temperature to a second holding temperature of 1950 to 2300°C, and held at the second holding temperature for 3 to 20 hours. Specifically, for example, after the first holding (intermediate holding) described above, a heat treatment (second heat treatment) is performed in an inert gas atmosphere under normal pressure or pressurized conditions, raising the temperature to a second holding temperature of 1950 to 2300°C at a heating rate of 1 to 5°C / min. After that, the material is held at the second holding temperature for 3 to 20 hours (second holding treatment) and then cooled to room temperature (15 to 25°C). The holding time at the second holding temperature is preferably 6 hours or more. Furthermore, the holding time at the second holding temperature is preferably 15 hours or less. Furthermore, heating (increasing temperature) to the second holding temperature in the secondary heating process should be started immediately after the primary heating process without cooling to room temperature (15-25°C). It is preferable to start heating to the second holding temperature immediately after the completion of the primary heating process. 【0030】 Next, the obtained boron nitride powder may be pulverized and classified. In this way, the hexagonal boron nitride powder for cosmetic use of the present invention can be obtained. 【0031】 <Cosmetics using hexagonal boron nitride powder> The hexagonal boron nitride powder of the present invention can be used as a cosmetic pigment for various applications as described below. In other words, it can be suitably used in makeup cosmetics such as foundation, face powder, primer, face color, and blush, as well as skincare cosmetics such as sunscreen, cream, lotion, and toner. There are no particular restrictions on the basic components of the cosmetic composition as described above; conventionally known components can be used. In addition, the hexagonal boron nitride powder of the present invention can be used in place of conventional boron nitride powder or inorganic powders (for example, anhydrous silicic acid, aluminum oxide, titanium dioxide, zinc oxide, zirconium oxide, mica, talc) in the conventional components. [Examples] 【0032】 The following describes embodiments of the present invention. 【0033】 <Example 1> Melamine and boric acid were mixed in a 1:1 mass ratio, heated to 700°C under an N2 atmosphere and 1 atm, and held at 700°C for 10 hours. The resulting crude boron nitride (crude BN) was pulverized, and the average particle size was confirmed to be 4 μm by laser diffraction (dry, 3.0 bar). Next, 5 kg of the crude boron nitride (crude BN) described above was packed into a lidded graphite crucible and first heated to 1880°C at a heating rate of 4°C / min (first heat treatment). Then, it was held at 1880°C for 3 hours (first holding treatment). After that, the temperature was raised to 2000°C at a heating rate of 4°C / min (second heat treatment), and it was held at 2000°C in an N2 atmosphere for 10 hours, and then cooled to room temperature (20°C) in the same N2 atmosphere to obtain hexagonal boron nitride powder (BN powder). After cooling, the hexagonal boron nitride powder was pulverized and classified. The obtained products were identified using an X-ray diffractometer and confirmed to be highly crystalline boron nitride powder. 【0034】 <Example 2> Hexagonal boron nitride powder was obtained under the same conditions as in Example 1, except that the holding time in the first holding treatment was 5 hours instead of 3 hours. 【0035】 <Example 3> Hexagonal boron nitride powder was obtained under the same conditions as in Example 1, except that the heating temperature (first holding temperature) in the first heat treatment was 1860°C instead of 1880°C. 【0036】 <Example 4> Hexagonal boron nitride powder was obtained under the same conditions as in Example 2, except that the heating temperature (first holding temperature) in the first heat treatment was 1860°C instead of 1880°C. 【0037】 <Example 5> Hexagonal boron nitride powder was obtained under the same conditions as in Example 2, except that the holding time in the second holding treatment was 8 hours instead of 10 hours. 【0038】 <Example 6> Hexagonal boron nitride powder was obtained under the same conditions as in Example 2, except that the heating temperature (second holding temperature) in the second heat treatment was set to 1970°C instead of 2000°C, and the holding time in the second holding treatment was set to 6 hours instead of 10 hours. 【0039】 <Example 7> Compared to Example 2 described above, hexagonal boron nitride powder was obtained under the same conditions as Example 2, except that the heating temperature (second holding temperature) in the second heat treatment was set to 1950°C instead of 2000°C, and the holding time in the second holding treatment was set to 4 hours instead of 10 hours. 【0040】 <Example 8> Hexagonal boron nitride powder was obtained under the same conditions as in Example 2, except that the heating temperature (second holding temperature) in the second heat treatment was set to 1950°C instead of 2000°C, and the holding time in the second holding treatment was set to 3 hours instead of 10 hours. 【0041】 <Example 9> Hexagonal boron nitride powder was obtained under the same conditions as in Example 2, except that the heating temperature (first holding temperature) in the first heat treatment was set to 1600°C instead of 1880°C, the heating temperature (second holding temperature) in the second heat treatment was set to 2050°C instead of 2000°C, and the holding time in the second holding treatment was set to 5 hours instead of 10 hours. 【0042】 <Comparative Example 1> Melamine and boric acid were mixed in a mass ratio of 1:1, heated to 700°C under an N2 atmosphere and 1 atm, and held at 700°C for 10 hours. The resulting crude boron nitride (crude BN) was pulverized, and the average particle size was confirmed to be 4 μm in the same manner as in Example 1. Next, 5 kg of the crude boron nitride (crude BN) described above was packed into a lidded graphite crucible and first heated to 1880°C in an N2 atmosphere at a heating rate of 4°C / min. After that, it was held at 1880°C in an N2 atmosphere for 3 hours, and then cooled to room temperature in an N2 atmosphere over a period of 5 hours or more. The boron nitride powder was then crushed, classified, and washed, and subsequently held at 2000°C for 10 hours in an N2 atmosphere, before being cooled to room temperature. The resulting boron nitride powder was then crushed and classified to obtain hexagonal boron nitride powder. 【0043】 <Comparative Example 2> Melamine and boric acid were mixed in a mass ratio of 1:1, heated to 700°C under an N2 atmosphere and 1 atm, and held at 700°C for 10 hours. The resulting crude boron nitride (crude BN) was pulverized, and the average particle size was confirmed to be 4 μm in the same manner as in Example 1. Next, 5 kg of the crude boron nitride (crude BN) was packed into a lidded graphite crucible and first heated to 2000°C at a heating rate of 4°C / min in an N2 atmosphere. After that, it was held at 2000°C in an N2 atmosphere for 10 hours, and then cooled to room temperature under the same N2 atmosphere. The resulting boron nitride powder was further purified by grinding, classification, and washing to remove impurities and obtain hexagonal boron nitride powder. 【0044】 <Comparative Example 3> Hexagonal boron nitride powder was obtained under the same conditions as in Comparative Example 2, except that the heating temperature in the heat treatment of crude boron nitride (crude BN) was set to 1750°C instead of 2000°C, and the holding time in the holding treatment was set to 5 hours instead of 10 hours. 【0045】 <Comparative Example 4> Hexagonal boron nitride powder was obtained under the same conditions as in Comparative Example 2, except that the heating temperature in the heat treatment of crude boron nitride (crude BN) was set to 2040°C instead of 2000°C, and the holding time in the holding treatment was set to 9 hours instead of 10 hours. 【0046】 <Comparative Example 5> Melamine and boric acid were mixed in a 1:1 mass ratio, heated to 700°C under an N2 atmosphere and 1 atm, and held at 700°C for 10 hours. The resulting crude boron nitride (crude BN) was pulverized, and the average particle size was confirmed to be 4 μm by laser diffraction (dry, 3.0 bar). Next, 5 kg of the crude boron nitride (crude BN) was packed into a lidded graphite crucible and first heated to 1450°C at a heating rate of 4°C / min (first heat treatment). Then, it was held at 1450°C for 10 hours (first holding treatment). After that, the temperature was raised to 2050°C at a heating rate of 4°C / min (second heat treatment), and it was held at 2000°C for 2 hours in an N2 atmosphere, and then cooled to room temperature (20°C) under the same N2 atmosphere. The resulting boron nitride powder was further purified by grinding, classification, and washing to remove impurities and obtain hexagonal boron nitride powder. 【0047】 <Comparative Example 6> Hexagonal boron nitride powder was obtained under the same conditions as in Comparative Example 2, except that the heating temperature in the heat treatment of crude boron nitride (crude BN) was set to 2100°C instead of 2000°C, and the holding time in the holding treatment was set to 9 hours instead of 10 hours. 【0048】 Table 1 shows the measurement results for the average thickness, average particle size, oxygen concentration, aspect ratio, and secondary particle ratio of the hexagonal boron nitride powder obtained under each of the above manufacturing conditions. The methods for measuring average thickness, average particle size, oxygen concentration, aspect ratio, and secondary particle ratio are as follows. 【0049】 (Average thickness of primary particles) The obtained hexagonal boron nitride powder (BN powder) was observed using a scanning electron microscope (SEM) (magnification: 20,000x, acceleration voltage: 10kV). Among the flaky primary particles, those whose planar portions formed an angle of 75 to 90 degrees with respect to the observation surface were measured for thickness. Thirty thicknesses were arbitrarily measured, and the average thickness (sum of the 30 thicknesses / 30) was taken as the average thickness. In addition, when measuring the average thickness, both primary particles (large particles) and primary particles (small particles) were included in the primary particle measurement. Furthermore, even when primary particles were stacked, each individual primary particle was treated as the target for measuring the average thickness. 【0050】 (Average particle size) Hexagonal boron nitride powder was added to a surfactant (nonionic surfactant) diluted to 0.25% by mass with water, dispersed in an ultrasonic cleaner, and then the BN sample was irradiated with a laser beam. The resulting light scattering pattern was analyzed, and the particle size distribution was measured (wet mode of a particle size analyzer (Mastersizer 3000 (manufactured by Spectris Co., Ltd.))) to obtain the average particle size. 【0051】 (Aspect ratio) The aspect ratio was defined as the value obtained by dividing the average particle size (μm) obtained by the measurement method described above by the average thickness (μm) obtained by the measurement method described above. 【0052】 (Oxygen concentration) A 10 mg sample was placed in an oxygen / nitrogen analyzer and measured using non-dispersive infrared absorption by inert gas-impulse heating and melting. 【0053】 (Secondary particle ratio) Hexagonal boron nitride powder was applied to artificial leather (Sapure®) with a brush, ensuring that the flat surfaces of the hexagonal boron nitride powder adhered closely to the artificial leather. An air dryer was then used to remove excess powder, ensuring that the powder particles did not overlap. The particle surface of the powder was observed using a SEM (magnification: 4000x, acceleration voltage: 10kV, secondary electrons). A total of 100 primary and secondary particles were arbitrarily counted, and the proportion of secondary particles was calculated. 【0054】 [evaluation] The hexagonal boron nitride powders obtained under the above manufacturing conditions were evaluated as follows. 【0055】 <Concealing ability> Hexagonal boron nitride powder was applied to a 2cm x 2cm area of ​​artificial leather (Sapure®) (black) using a flat nylon brush (bristle length 15mm, tip width 1.1cm). The whiteness was measured using a Konica Minolta colorimeter CR-14 in "HUNTER" mode. The process of "measurement → removal of surface adhesion with brush → measurement" was repeated until the whiteness value no longer fluctuated. The endpoint was defined as the whiteness value at the endpoint minus the whiteness value of the artificial leather (18.6). A value of 16 or less was considered acceptable, as it indicated that the concealment ability was sufficiently low. 【0056】 <Coefficient of Dynamic Friction> Using a Trilabo Handy Tribomaster TL201Ts (manufactured by Trinity Lab Co., Ltd.), the coefficient of dynamic friction was measured by placing 0.05g of hexagonal boron nitride powder on artificial leather under a load of 30g, a travel speed of 10mm / s, a reciprocating distance of 50mm (25mm each way), and 20 reciprocating cycles. The fingerprint tactile contact and substrate were made of artificial leather (Sapure®). The measurement taken during the 20th round trip was used as the coefficient of dynamic friction for evaluation. A coefficient of kinetic friction of 0.31 or less was considered acceptable, indicating excellent sliding properties. 【0057】 <Skin compatibility> Using the Trilabo Handy Tribomaster TL201Ts (manufactured by Trinity Lab Co., Ltd.), the coefficient of dynamic friction was measured by placing 0.05g of hexagonal boron nitride powder on artificial leather with a load of 30g, a travel speed of 10mm / s, a reciprocating distance of 50mm, and 2 reciprocating passes. The fingerprint tactile contactor uses artificial leather (Sapler®) as its base. The difference in the coefficient of kinetic friction between the first and second measurements was defined as the skin-comfort index. If the skin compatibility index exceeds 0.08, it is judged to have too much adhesion and feel thick, and if it is less than 0.04, it is judged to have no fit to the skin. A score of 0.04 to 0.08 is considered to be excellent in terms of skin compatibility and is deemed acceptable. 【0058】 <Texture> Hexagonal boron nitride powder was applied to human skin, and its texture was evaluated by six evaluators according to the following criteria. Evaluation Criteria A: Between 5 and 6 people responded that they felt the texture was smooth. B: Three to four people responded that they felt the texture was smooth. C: Two or fewer people responded that they felt the texture was smooth. 【0059】 [Table 1] 【0060】 As shown in Table 1, the hexagonal boron nitride powders of Examples 1-9, which had a secondary particle ratio of 40% or less, had a dynamic friction coefficient of 0.31 or less, indicating excellent lubricity. Furthermore, the hexagonal boron nitride powders of Examples 1-9 had a skin compatibility index of 0.04-0.08, indicating excellent skin compatibility. Furthermore, the hexagonal boron nitride powders of Examples 1 to 9 had an opacity of 16 or less, demonstrating that the opacity could be sufficiently reduced. Based on the above, it was found that Examples 1 to 9 all exhibited excellent concealment, slipperiness, and skin compatibility. 【0061】 Furthermore, regarding the evaluation results for skin texture, Comparative Examples 1-6 received a C, while Examples 1-9, with an average particle size of 4-15 μm and an average primary particle thickness of 0.3-0.7 μm, received an A or B. In particular, Examples 1-8, with an average primary particle thickness in the range of 0.3-0.5 μm, received an A for skin texture evaluation. 【0062】 Furthermore, in Examples 1-8, where the aspect ratio was 10 or higher, the concealment strength could be reduced to 15.8 or lower, demonstrating a further reduction in concealment strength. 【0063】 Furthermore, considering the low opacity, excellent slipperiness, and skin compatibility required in this invention, among Examples 1 to 9, Examples 2 and 4 had particularly low opacity and superior slipperiness and skin compatibility. Specifically, Examples 2 and 4, in which the secondary particle ratio was 20% or less, had a dynamic friction coefficient of 0.29 and 0.27, respectively, exhibiting particularly excellent slipperiness among the examples. They also had a concealing power of 14.8 and 14.5, indicating a particularly low concealing power, and a skin compatibility index of 0.047 and 0.045, demonstrating particularly excellent skin compatibility. Furthermore, comparing Example 2 and Example 4, Example 4, which had a lower proportion of secondary particles, exhibited lower opacity, but superior slipperiness and skin compatibility. 【0064】 Furthermore, focusing on oxygen concentration, it was found that in Examples 2, 5-8, and 5-8, where the only difference in manufacturing was the conditions in the secondary heating process, Example 2 had an oxygen concentration of 0.2 mass%, which allowed for a lower dynamic friction coefficient than Examples 5, 6, 7, and 8, where the oxygen concentrations were 0.4 mass%, 0.6 mass%, 0.8 mass%, and 1.2 mass%, respectively. This resulted in superior lubricity and particularly low concealment. Additionally, Example 2 had a skin compatibility index of 0.047, which was found to be superior to Examples 5, 6, 7, and 8, where the skin compatibility indices were 0.052, 0.059, 0.060, and 0.078. 【0065】 On the other hand, the hexagonal boron nitride powder of Comparative Example 1, obtained by cooling to room temperature between the primary and secondary heating steps in the present invention, had a secondary particle ratio of 89%, and failed to achieve the desired opacity, slipperiness, and skin compatibility. 【0066】 Furthermore, the hexagonal boron nitride powder of Comparative Example 2, obtained without the treatment in the primary heating step of the present invention, had a secondary particle ratio of 61%, and failed to achieve the desired opacity, slipperiness, and skin compatibility. 【0067】 Furthermore, the hexagonal boron nitride powder of Comparative Example 3, obtained without the treatment in the primary heating step of the present invention and with a heating temperature (holding temperature) of 1750°C in the secondary heating step of the present invention, had a secondary particle ratio of 85% and an average particle size of 3.3 μm, and failed to obtain the desired opacity, slipperiness, and skin compatibility. 【0068】 Furthermore, the hexagonal boron nitride powder of Comparative Example 4, obtained without the treatment in the primary heating step of the present invention, had a secondary particle ratio of 52%, an average particle size of 22.5 μm, and an average primary particle thickness of 0.87 μm, and failed to achieve the desired slipperiness and skin compatibility. 【0069】 Furthermore, the hexagonal boron nitride powder of Comparative Example 5, obtained by setting the heating temperature (first holding temperature) in the primary heating step of the present invention to 1450°C, the holding time to 10 hours, and the holding time in the secondary heating step of the present invention to 2 hours, had a secondary particle ratio of 55%, an average particle size of 2 μm, and an average primary particle thickness of 0.89 μm, and failed to obtain the desired opacity, slipperiness, and skin compatibility. 【0070】 Furthermore, the hexagonal boron nitride powder of Comparative Example 6, obtained without the treatment in the primary heating step of the present invention, had a secondary particle ratio of 51%, an average particle size of 29.6 μm, an aspect ratio of 33, and an average primary particle thickness of 0.89 μm, and failed to achieve the desired slipperiness and skin compatibility. [Explanation of symbols] 【0071】 1. Hexagonal boron nitride powder 10 Primary particles (small particles) 11 Primary particles (large particles) 12 Secondary particles

Claims

[Claim 1] A hexagonal boron nitride powder comprising flaky primary particles and secondary particles formed by stacking the primary particles, The average particle size is 4 to 15 μm. The aspect ratio is between 14 and 30. The average thickness of the primary particles is 0.3 to 0.7 μm. A hexagonal boron nitride powder for cosmetic use, wherein the number of secondary particles is 3% or more and 40% or less of the total number of primary and secondary particles. [Claim 2] The hexagonal boron nitride powder for cosmetic use according to claim 1, wherein the oxygen concentration is 1.0% by mass or less. [Claim 3] A cosmetic composition containing hexagonal boron nitride powder according to claim 1 or 2.

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