Plate-like alumina powder, method for producing same, and coating material or cosmetic

Abstract

Provided are: a plate-like alumina powder of which, compared to conventional plate-like alumina powders, the brightness (glare) of the glossy appearance is suppressed even when the particle diameter is the same; and a method for producing the same. This plate-like alumina powder is constituted by a plurality of plate-like α-alumina particles, has an average particle diameter of 2 to 100 μm as determined by SEM observation, and has an average thickness of 0.2 to 3.0 μm as determined by SEM observation. The plate-like α-alumina particles are polycrystalline bodies, and, on the plate surfaces of the plate-like α-alumina particles, the ratio (L2 / L1) of the total length (L2) of grain boundaries to the peripheral length (L1) of the plate-like α-alumina particles (L2) is 0.30 to 2.00.

Description

Platy alumina powder, method for producing the same, and paint or cosmetic

[0001] The present invention relates to platy alumina powder, a method for producing the same, and paint or cosmetic.

[0002] Alumina (Al ２ O ３ ) is chemically stable, excellent in heat resistance, corrosion resistance, abrasion resistance, and insulation properties, and further has high strength and hardness. Taking advantage of this characteristic, alumina powder is widely used in various applications such as structural members, tools, abrasives, fillers, spark plugs, insulators, electronic substrates, and refractories.

[0003] In particular, platy alumina powder has a high light reflectance and high brightness on the surface of its particles. Therefore, taking advantage of this characteristic, it is used as a pigment added to paints and cosmetics. Note that platy alumina powder is a powder composed of a plurality of platy alumina particles, and is also called flaky alumina powder, lamellar alumina powder, flattened alumina powder, or alumina flakes.

[0004] As a document disclosing the use of platy alumina powder as a pigment, Patent Document 1 discloses Al ５０ flakes having a thickness of 500 nm or more, a D ９０ value of 15 to 30 μm and a D ２ O ３ flakes having a thickness of 500 nm or more, a D ２ O ３ value of 30 to 45 μm, and it is described that the Al ２ O ３ flakes have high chemical stability, a smooth surface, and high whiteness at the same time, and are used as a pigment base material (Claims 1,

[0010] and

[0015] of Patent Document 1). Further, Patent Document 1 describes that the Al ２ flakes are coated with a high refractive index layer such as TiO ２ or a low refractive index layer such as SiO

[0005] Patent Document 2 discloses a paint composition containing alumina flakes as a lustrous pigment, which can be suitably used as an automotive topcoat, and which forms a composite coating film with unprecedentedly strong lustrousness and novel, highly aesthetic design without degrading the finished appearance (Claim 1,

[0001] and

[0108] of Patent Document 2). Patent Document 2 also discloses that alumina flakes are aluminum oxide (Al ２ O ３ It is described that the material is coated with a metal oxide such as titanium dioxide, with a particle size of 10 to 30 μm and a thickness of 0.3 to 0.4 μm (Patent Document 2,

[0007] ).

[0006] Patent Document 3 discloses a pearlescent pigment containing a flake-like alumina crystal coated with metal or metal precursor particles, with aluminum oxide and zinc oxide as the main components in a mass ratio of 100:0.1 to 5 (Claim 1 of Patent Document 3). Patent Document 3 also states that the crystal has excellent gloss because its average particle thickness is 0.5 μm or less, its average particle size is 15 μm or more, and its aspect ratio is 50 or more (

[0001] of Patent Document 3).

[0007] Patent Document 4 discloses a method for producing plate-shaped alumina-based powder characterized by having a crystalline structure of α, β, γ-alumina alone, or two or more crystalline structures, by firing plate-shaped boehmite at 400°C to 1500°C (Claim 2 of Patent Document 4). Patent Document 4 also describes that by surface coating plate-shaped boehmite or plate-shaped alumina powder with a hydrophobic compound such as polysiloxane, and incorporating this coated powder into cosmetics, cosmetics with a good feel can be obtained (

[0013] of Patent Document 4).

[0008] Patent Document 5 discloses hexagonal plate-shaped alumina obtained by firing hexagonal plate-shaped boehmite at a temperature of 450 to 1500°C, characterized by being in the shape of a substantially hexagonal plate, having a ratio of major axis to minor axis of 1 to 1.3, and an aspect ratio of 40 to 100, and states that it has high orientation and low diffuse reflection, resulting in increased luster, and can be suitably used as a filler for the purpose of luster in paints and cosmetics (Claim 4 and

[0045] of Patent Document 5).

[0009] Patent Document 6 discloses plate-shaped alumina particles characterized by having a thickness of 0.01 to 5 μm, an average particle diameter of 0.1 to 500 μm, an aspect ratio of 2 to 500 (ratio of particle diameter to thickness), a polygonal plate shape, and containing molybdenum within the particles (Claim 1 of Patent Document 6). Patent Document 6 also states that these plate-shaped alumina particles can be suitably used as a thermally conductive filler, cosmetic, abrasive, high-gloss pigment, lubricant, substrate for conductive powders, ceramic material, etc. (

[0101] of Patent Document 6).

[0010] Japanese Patent Publication No. 2014-218424, Japanese Patent Publication No. Hei 10-298458, Japanese Patent Publication No. Hei 2010-502774, Japanese Patent Publication No. 2012-071996, Japanese Patent Publication No. 2003-002642, Japanese Patent Publication No. 2019-123664

[0011] Although research has been conducted to improve the luster of plate-shaped alumina powder, there was still room for improvement in conventional plate-shaped alumina powder. Specifically, to take advantage of the high reflectivity of plate-shaped alumina powder, it is effective to increase the particle size of the powder.

[0012] However, if the particle size becomes too large, the scattered glitter (shimmer) becomes too strong. Here, scattered glitter refers to a state in which areas of high brightness (bright spots) exist locally within the powder. For example, if the brightness from the particles constituting plate-like alumina powder differs greatly depending on the orientation of the particles (the angle between the particle and the observation direction), the brightness from certain areas will be high. Such scattered glitter (shimmer) may be undesirable depending on the application. In particular, pigments with scattered glitter tend to be avoided in cosmetic applications.

[0013] In view of these problems, the inventors conducted thorough research. As a result, they found that plate-shaped alumina powder having a specific average particle size and average thickness, as well as a specific polycrystalline state, suppresses the scattered glare (shimmer) compared to conventional plate-shaped alumina powder, even with the same particle size.

[0014] This invention was completed based on the above findings, and aims to provide plate-shaped alumina powder and a method for producing the same, which suppress the scattered glare (shininess) compared to conventional plate-shaped alumina powder, even with the same particle size.

[0015] The present invention encompasses the following embodiments (1) to (8). In this specification, the expression "~" includes the numerical values ​​at both ends. That is, "X~Y" is synonymous with "X or more and Y or less".

[0016] (1) Plate-shaped alumina powder comprising a plurality of plate-shaped α-alumina particles, wherein the average particle diameter determined by SEM observation is 2 μm or more and 100 μm or less, and the average thickness determined by SEM observation is 0.2 μm or more and 3.0 μm or less, wherein the plate-shaped α-alumina particles are polycrystalline, and the average value of the ratio (L2 / L1) of the total length of grain boundaries to the perimeter length (L1) of the plate-shaped α-alumina particles to the plate surface (L1) is 0.30 or more and 2.00 or less.

[0017] (2) The plate-shaped alumina powder of the above (1), wherein the average value of L2 / L1 is 0.50 or more and 1.50 or less.

[0018] (3) The plate-shaped alumina powder according to (1) or (2) above, wherein the average particle diameter is 5 μm or more and 50 μm or less, and the average thickness is 0.3 μm or more and 1.0 μm or less.

[0019] (4) A method for producing plate-shaped alumina powder according to any of (1) to (3) above, comprising the steps of: preparing a raw material mixture containing aluminum hydroxide powder and additives; and firing the raw material mixture by holding it at a temperature in the range of 1000°C to 1300°C for a period of less than 20 hours, wherein the raw material mixture contains alkali metals (AM) ２Silicon (Si) is present in an amount of 0.01% to 0.5% by mass, in terms of oxygen equivalent. ２ A method comprising a raw material mixture containing 0.1% by mass or more and 0.3% by mass or less, and fluorine (F) in an amount of 0.1% by mass or more and 5.0% by mass or less, calculated as F equivalent, wherein the heating rate in the calcination step is 50°C / hour or more and 150°C / hour or less.

[0020] (5) The additive is an alkali silicofluoride (AM ２ SiF ６ ), or aluminum fluoride (AlF ３ ) and silicon dioxide (SiO ２ The method described in (4) above, including ).

[0021] (6) The additive is an alkali metal (AM) oxide (AM ２ O) and carbonate (AM ２ CO ３ The method of (5) above, further comprising one or both of the above.

[0022] (7) Any of the methods (4) to (6) above, wherein the alkali metal (AM) is sodium (Na) and potassium (K) or both.

[0023] (8) A paint or cosmetic containing any of the plate-shaped alumina powders described in (1) to (3) above.

[0024] According to the present invention, a plate-shaped alumina powder and a method for producing the same are provided, which suppress the scattered glare (shininess) compared to conventional plate-shaped alumina powder, even when the particle size is the same.

[0025] This illustrates the mechanism of glossiness produced by plate-shaped alumina powder. It schematically shows the scattering of light passing through a polycrystalline material. It also schematically shows the surface of plate-shaped particles.

[0026] Specific embodiments of the present invention (hereinafter referred to as "these embodiments") are described below. However, the present invention is not limited to the following embodiments, and various modifications are possible as long as they do not alter the essence of the invention. Furthermore, in this specification, any combination of preferred embodiments can be adopted as long as technical consistency can be maintained. For example, one of the preferred numerical ranges can be arbitrarily combined with the other.

[0027] <<1. Plate-shaped alumina powder>> The plate-shaped alumina powder of this embodiment (hereinafter sometimes simply referred to as "alumina powder") is composed of multiple plate-shaped α-alumina (Al ２ O ３ It is composed of particles (hereinafter sometimes simply referred to as "alumina particles," "plate-like particles," or "particles"). In other words, it mainly contains plate-like α-alumina particles. Here, plate-like α-alumina particles are particles made of α-alumina (corundum). α-alumina has a trigonal corundum crystal system and is excellent in chemical stability, heat resistance, corrosion resistance, wear resistance, and insulation, as well as high strength and hardness. Furthermore, it has high light reflectivity, and high brightness and whiteness. Because the alumina powder of this embodiment mainly contains plate-like α-alumina particles, it is possible to fully utilize these characteristics.

[0028] In this specification, "powder" refers to an aggregate of many independent particles. That is, many particles come together to form powder. Because each particle constituting the powder is independent, the powder as a whole exhibits fluidity in its standalone state. It is sufficient that the majority of the particles constituting the powder are independent. Some particles may combine to form aggregates, as long as the powder as a whole exhibits fluidity. Furthermore, an aggregate of many particles dispersed in a medium such as a liquid or resin is also referred to as powder.

[0029] The plate-shaped alumina powder of this embodiment has a plate-like (scaly, flaky, flattened, or flake-like) shape for the alumina particles that constitute it. That is, each particle has a large plate surface and a small plate thickness. Specifically, the average particle diameter of the plate-shaped alumina powder is 2 μm or more and 100 μm or less, and the average thickness is 0.2 μm or more and 3.0 μm or less. Here, the average particle diameter and average thickness are the average values ​​of the particle diameter and thickness of each particle constituting the alumina powder, based on the number of particles. The particle diameter is the major axis of the particle. That is, the diameter of the major axis of the particle plate surface is defined as the particle size. The thickness is the thickness of the plate surface. The particle diameter and thickness can be determined by observing the particles constituting the alumina powder with a scanning electron microscope (SEM). That is, the average particle diameter and average thickness are values ​​obtained by SEM observation. Also, usually, the average particle diameter is larger than the average thickness.

[0030] Thus, by using particles with a large surface area and a small thickness, the luster of the alumina powder can be enhanced. This point will be explained using Figure 1. As shown in Figure 1, when a powder containing particles with a large surface area and a small thickness is formed on a substrate by a coating or other method, the larger particle surface areas tend to be oriented so that they are parallel to the substrate surface. The larger the particle diameter, the larger the surface area of ​​the particle surface areas that are parallel to the substrate surface. In addition, light incident from the outside is reflected from the particle surface and radiated outwards as reflected light. Therefore, the larger the particle diameter, the easier it is for incident light to be reflected from the particle surface, and as a result, the perceived luster is higher.

[0031] If the average particle diameter is less than 2 μm or the average thickness is greater than 3.0 μm, the powder may not have sufficient luster. On the other hand, powders with an average particle diameter greater than 100 μm or an average thickness less than 0.2 μm are difficult to manufacture. They also have low strength and are difficult to handle. For plate-shaped alumina powder, the average particle diameter is preferably between 5 μm and 50 μm, and the average thickness is preferably between 0.3 μm and 1.0 μm.

[0032] Preferably, the average aspect ratio of the alumina powder is 20 or more and 50 or less. Here, the average aspect ratio is the ratio of the average particle diameter to the average thickness (average particle diameter / average thickness). By setting the average aspect ratio to 20 or more, it is possible to further enhance the luster of the powder. Also, by setting the average aspect ratio to 50 or less, the strength of the alumina powder is increased and handling becomes easier.

[0033] It should be noted that alumina powder does not necessarily have to consist entirely of plate-shaped α-alumina particles. As long as the alumina powder as a whole satisfies the above-mentioned requirements for average particle diameter and average thickness, it may contain particles other than plate-shaped α-alumina particles. However, in order to take advantage of the excellent effects based on plate-shaped α-alumina particles, a higher proportion of plate-shaped α-alumina particles is preferable. The proportion of plate-shaped α-alumina particles is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more.

[0034] In this embodiment, the alumina powder is composed of polycrystalline plate-like α-alumina particles. That is, each particle is not made up of a single crystal, but rather of multiple crystallites (or crystal grains). In each particle, adjacent crystallites are in contact via grain boundaries. Furthermore, on the plate surface of the plate-like α-alumina particles, the average value of the ratio (L2 / L1) of the total length of grain boundaries to the perimeter (L1) of the plate-like α-alumina particle is 0.30 or more and 2.00 or less. In this way, by making the alumina particles polycrystalline and limiting the ratio of their perimeter to grain boundary length (L2 / L1) to within a predetermined range, the scattered glare (glare) can be suppressed.

[0035] This point will be explained using Figure 2, which schematically shows the scattering of light passing through a polycrystalline material. When the crystalline form of alumina particles is polycrystalline, the crystal structure is uniform within each crystallite contained in the polycrystal, and the refractive index is uniform. Therefore, light passing through the crystallite travels in a straight line without scattering. On the other hand, the crystal structure is disordered at the grain boundaries, and in some cases, it is amorphous. Therefore, the refractive index changes at the grain boundaries. As a result, light does not travel in a straight line at the grain boundaries but is scattered in all directions. As a result, reflected light is weakened, and the glare can be reduced. In contrast, when the alumina particles are single crystals, scattered light is suppressed and reflected light is strengthened, resulting in an excessively strong glare.

[0036] The scattered glitter (sparkle) can be controlled by adjusting the proportion of the grain boundary area within each particle. The proportion of the grain boundary area can be estimated by the ratio (L2 / L1) of the total grain boundary length (L2) to the particle's perimeter (L1). Here, as shown in Figure 3, the particle's perimeter (L1) is the perimeter of the plate surface of a plate-like particle. The total grain boundary length (L2) is the sum of the grain boundary lengths exposed on the plate surface of the plate-like particle. Figure 3 schematically represents the plate surface of a plate-like particle. In Figure 3, L1 is shown as a solid line and L2 as a dashed line. L2 / L1 is the ratio of the perimeter (L1) of each particle to the total grain boundary length (L2), and the average value of L2 / L1 is the average value of the ratio (L2 / L1) across multiple particles.

[0037] If the average value of L2 / L1 is between 0.30 and 2.00, the proportion of grain boundaries becomes appropriate, making it possible to achieve both high brilliance and subdued glare. Conversely, if the average value of L2 / L1 is less than 0.30, the proportion of grain boundaries is insufficient, resulting in inadequate reduction of glare due to light scattering. On the other hand, if the average value of L2 / L1 is greater than 2.00, the proportion of grain boundaries becomes excessively large, leading to increased scattered light and weakened reflected light, resulting in a loss of brilliance. From the viewpoint of achieving an even better balance between high brilliance and subdued glare, the average value of L2 / L1 is preferably between 0.40 and 1.80, and more preferably between 0.50 and 1.50.

[0038] Furthermore, whether or not plate-shaped α-alumina particles are polycrystalline can be determined by observing the particles with a scanning electron microscope (SEM). The average value of L2 / L1 can also be determined by observing plate-shaped alumina powder with an SEM. Specifically, by observing the surface or cross-section of the particles with an SEM, the grain boundaries of the crystals can be observed, confirming that the plate-shaped α-alumina particles are polycrystalline. The ratio of L1 to L2 allows for the determination of the ratio of the grain boundary length on the particle surface to the outer circumference of the particle, which can then be used to evaluate the degree of polycrystallineity.

[0039] In this embodiment, it is preferable that the plate-shaped alumina powder, in the powder X-ray diffraction (XRD) pattern obtained in an oriented state, has a diffraction peak intensity ratio based on α-alumina within a predetermined range. Specifically, the diffraction peak intensity I based on the (006) plane of α-alumina is within a predetermined range. Ａ And the intensity of the diffraction peak I based on the (113) plane of α-alumina Ｂ Ratio to (I Ａ / I Ｂ ) is preferably 0.3 or more and 10 or less. The intensity of the diffraction peak based on the (104) plane of α-alumina I Ｃ And the intensity of the diffraction peak I based on the (113) plane of α-alumina Ｂ Ratio to (I Ｃ / I Ｂ ) is preferably 4.0 or more and 10 or less. Also, the intensity of the diffraction peak based on the (116) plane of α-alumina I Ｄ And the intensity of the diffraction peak I based on the (113) plane of α-alumina ＢRatio to (I Ｄ / I Ｂ ) is preferably 3.0 or more and 7.5 or less. The intensity of the diffraction peak based on the (018) plane of α-alumina I Ｅ And the intensity of the diffraction peak I based on the (113) plane of α-alumina Ｂ Ratio to (I Ｅ / I Ｂ ) is preferably 1.0 to 7.5. The intensity of the diffraction peak based on the (1010) plane of α-alumina I Ｆ And the intensity of the diffraction peak I based on the (113) plane of α-alumina Ｂ Ratio to (I Ｆ / I Ｂ Preferably, the value is between 5.0 and 20.0.

[0040] The (006) plane of α-alumina corresponds to the crystal plane (c plane) perpendicular to the c axis of the α-alumina crystal. Furthermore, the (104), (116), (018), and (1010) planes of α-alumina are all crystal planes with a small angle from the c plane, i.e., crystal planes similar to the c plane. The plate-like alumina powder of this embodiment is composed of plate-like α-alumina particles with a large plate surface and small plate thickness. Therefore, when the plate-like alumina powder is filled into a measuring holder to obtain a powder XRD pattern, the particles are oriented so that their plate surfaces align with the measuring surface. Consequently, in the XRD pattern of the plate-like alumina powder, the diffraction peak intensity (I) is based on the c plane (plate surface) and similar planes such as the (006), (104), (116), (018), and / or (1010) planes. Ａ , I Ｃ , I Ｄ , I Ｅ and I Ｆ ) is strongly observed.

[0041] The XRD pattern in the oriented state can be determined as follows: The sample is placed on a glass sample plate, pressed down so that the sample surface is flat, and then measured using an XRD measuring device. Because the sample is in the form of plate-shaped particles, when filling the powder sample, it is easy to align it with a specific crystal axis relative to the surface of the sample plate, resulting in a selective orientation effect. If the plate-shaped alumina powder is pulverized when preparing the sample for measurement, the shape of the powder changes, which may impair the selective orientation effect. Therefore, the measurement is performed without adjusting the particle size. In this specification, peak intensity refers to the peak area (integrated intensity). When CuKα is used as the X-ray source, the diffraction peak based on the (006) plane of α-alumina appears at 2θ = 41.7 ± 0.5°. Similarly, the diffraction peak based on the (113) plane appears at 2θ = 43.3 ± 0.5°. The diffraction peak based on the (104) plane appears at 2θ = 35.1 ± 0.5°. The diffraction peak based on the (116) plane appears at 2θ = 57.5 ± 0.5°. The diffraction peak based on the (018) plane appears at 2θ = 61.3 ± 0.5°. The diffraction peak based on the (1010) plane appears at 2θ = 76.9 ± 0.5°.

[0042] The alumina powder of this embodiment preferably contains alkali metals (AM) ２ It contains an amount of 0.01% by mass or more and 0.5% by mass or less in terms of oxygen, and silicon (Si) is SiO ２ It contains an amount of 0.01% by mass or more and 0.3% by mass or less. The alkali metal (AM) content is AM ２ A silicon (Si) content of 0.01% to 0.3% by mass is more preferable in terms of oxygen. ２ A conversion of 0.01% to 0.2% by mass is more preferable. Preferably, the alkali metal (AM) is either sodium (Na) or potassium (K), or both. Including these components makes it possible to further enhance the luster of the alumina powder. Particularly preferable is sodium (Na) instead of potassium (K). ２ It contains an amount of 0.01% to 0.5% by mass, calculated as oxygen. This makes it possible to more effectively suppress the glare of the alumina powder.

[0043] The alumina powder of this embodiment preferably contains at least aluminum (Al), alkali metal (AM), silicon (Si), and oxygen (O). However, other components (elements other than Al, AM, Si, and O) may be included as long as the above requirements are satisfied. The presence of impurities that are inevitably introduced during the manufacturing process is also acceptable. One such other component is fluorine (F). Fluorine (F) is a component derived from additives added during the production of alumina powder. That is, as will be described later, the additives added during the production of alumina powder contain fluorine (F). Most of the fluorine volatilizes during the calcination process, but some may remain. The amount of fluorine in the alumina powder is typically 0.5% by mass or less, 0.3% by mass or less, or 0.1% by mass or less.

[0044] If other components (elements other than Al, AM, Si, and O) are present in large quantities, the luster of the alumina powder may be impaired. Therefore, the content of other components is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 1% by mass or less, particularly preferably 0.5% by mass or less, and most preferably 0.1% by mass or less. In particular, since iron (Fe) has the effect of coloring the alumina powder, it is preferable to reduce its content. Preferably, the content of iron (Fe) is such that iron oxide (Fe) ２ O ３ This is 0.05% by mass or less when converted to a different value.

[0045] The specific surface area (S) of the alumina powder in this embodiment ＢＥＴ ) is 0.1m ２ / g or more 5.0m ２ Preferably less than or equal to 0.3 m ２ / g or more 2.0m ２ A value of less than / g is even more preferable. If the specific surface area is excessively large, the particle size of the alumina powder will become smaller, which may reduce its luster. On the other hand, alumina powder with a small specific surface area is difficult to manufacture.

[0046] The alumina powder of this embodiment has high chemical stability because it is mainly composed of α-alumina. It also has the characteristics of a large average particle size and excellent dispersibility. Alumina powder with these characteristics is suitably used in the fields of paints and cosmetics. However, the alumina powder of this embodiment is not limited to use in paints and cosmetics. It can be applied to known applications such as reinforcing materials and gas barrier materials mixed into plastics and resin films, as well as structural members, tools, abrasives, fillers, spark plugs, insulators, electronic circuit boards, and refractories.

[0047] <<2. Method for Manufacturing Plate-Shaped Alumina Powder>> The method for manufacturing the plate-shaped alumina powder of this embodiment is not limited as long as the above requirements are satisfied. However, it is preferably manufactured by the following procedure. A preferred manufacturing method comprises the steps of preparing a raw material mixture containing aluminum hydroxide powder and additives (raw material mixing step), and firing this raw material mixture by holding it at a temperature in the range of 1000°C to 1300°C for less than 20 hours (firing step). The raw material mixture contains alkali metals (AM) ２ Silicon (Si) is present in an amount of 0.01% to 0.5% by mass, in terms of oxygen equivalent. ２ The mixture contains 0.1% to 0.3% by mass (converted to F equivalent), and fluorine (F) in an amount of 0.1% to 5.0% by mass (converted to F equivalent). The heating rate during the calcination process is 50°C / hour to 150°C / hour. A post-treatment step may also be provided after the calcination process if necessary.

[0048] The manufacturing method of this embodiment is particularly characterized by the use of aluminum hydroxide powder as a raw material and the control of the composition of the raw material mixture and the calcination conditions within a specific range. This characteristic makes it possible to obtain alumina powder that has the desired size and polycrystalline state, and as a result, maintains a high level of brilliance while suppressing the scattered glare (glare). Each step will be described in detail below.

[0049] <Raw Material Mixing Process> In the raw material mixing process, a raw material mixture containing aluminum hydroxide powder and additives is prepared. Aluminum hydroxide has the chemical formula Al(OH) ３This compound is represented by [formula] and dehydrates upon heating, transforming into α-type aluminum oxide (α-alumina). Two types of aluminum hydroxide are known: gibbsite (γ-type aluminum hydroxide) and bayerite (α-type aluminum hydroxide). In this embodiment, either gibbsite or bayerite may be used. However, powder made from thermodynamically stable gibbsite is preferred.

[0050] As long as the alumina powder of this embodiment can be obtained, the size of the raw material aluminum hydroxide powder is not particularly limited. However, preferably the volume average particle size (D50) is 0.5 μm or more and 15 μm or less. Alternatively, the specific surface area (S ＢＥＴ ) is 0.5m ２ / g or more 20m ２ It is less than / g. Note that D50 is the cumulative 50% diameter in the volume-based particle size distribution determined by a laser diffraction / scattering particle size distribution analyzer. Also, S ＢＥＴ is N ２ These values ​​were obtained by measurement using the gas adsorption method.

[0051] The additives are components that become liquid phase in the subsequent calcination process and act as mineralizing agents. The additives contain at least silicon (Si) and fluorine (F), and may also contain alkali metals (AM) as needed. As described later, if the composition of the liquid phase components is appropriately controlled, the crystallization and grain growth of alumina particles proceed appropriately, and as a result, it becomes possible to obtain alumina powder having the desired size and polycrystalline state.

[0052] Therefore, in order to obtain the desired alumina powder, it is important to appropriately control the composition of the raw material mixture, including additives. Specifically, the raw material mixture contains alkali metals (AM). ２ Silicon (Si) is present in an amount of 0.01% to 0.5% by mass, in terms of oxygen equivalent. ２The composition of the additives is adjusted so that it contains 0.1% to 0.3% by mass of other substances and fluorine (F) in an amount of 0.1% to 5.0% by mass in F equivalent. When the composition of the raw material mixture is within the above range, it is possible to obtain alumina powder with the desired size and polycrystalline state, resulting in a powder that maintains high luster while suppressing glare. Conversely, if the composition of the raw material mixture is inappropriate, it may adversely affect the particle size and crystalline state.

[0053] The amount of alkali metals (AM) in the raw material mixture is AM ２ A concentration of 0.01% by mass or more and 0.5% by mass or less in terms of oxygen is more preferable. Furthermore, the alkali metal is preferably sodium (Na) and potassium (K), or both, with sodium (Na) being particularly preferred. The amount of silicon (Si) in the raw material mixture is SiO ２ A conversion of 0.10% to 0.20% by mass is more preferable. The amount of fluorine (F) in the raw material mixture is more preferably 0.1% to 1.0% by mass in terms of F conversion. By limiting the composition of the raw material mixture to the above range, the polycrystalline state of the alumina powder can be controlled to a more preferable range.

[0054] The additive contains at least silicon (Si) and fluorine (F). The additive may also contain alkali metals (AM), or it may not. The raw material, aluminum hydroxide powder, may contain alkali metals (AM) as impurities. If the amount of alkali metal in the aluminum hydroxide powder is sufficient, the additive does not need to contain alkali metals. On the other hand, if the amount of alkali metal in the aluminum hydroxide powder is insufficient, it is preferable to add alkali metals as an additive. Furthermore, the additive may also contain aluminum (Al). If the additive contains aluminum, this aluminum is incorporated into the alumina powder during the calcination process.

[0055] As long as alumina powder having the desired size and polycrystalline state is obtained, the additive may contain other components besides alkali metals (AM), silicon (Si), fluorine (F), aluminum (Al), and oxygen (O). However, if the amount of other components is excessively high, it may adversely affect the particle size and polycrystalline state. The content of other components is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and particularly preferably 1% by mass or less. The additive does not need to contain other components besides alkali metals (AM), silicon (Si), fluorine (F), aluminum (Al), and oxygen (O) in amounts exceeding the amount of impurities.

[0056] According to one preferred embodiment, the additive is aluminum fluoride (AlF ３ ) and silicon dioxide (SiO ２ ) contains. Also, if the amount of alkali metal in the aluminum hydroxide powder is insufficient, the additive contains alkali metal (AM) oxides (AM ２ O) and carbonate (AM ２ CO ３ Preferably, the additive further includes one or both of the following. In another further preferred embodiment, the additive is alkali silicofluoride (AM ２ SiF ６ ) contains these compounds (AlF ３ SiO ２ AM ２ O, AM ２ CO ３ AM ２ SiF ６ By using an appropriate amount of these compounds as additives, it becomes possible to reliably produce alumina powder with the desired size and polycrystalline state. Furthermore, these compounds are inexpensive and readily available. Therefore, plate-shaped alumina powder can be obtained at a lower cost.

[0057] The preparation of the raw material mixture may be carried out by mixing aluminum hydroxide powder and additives. The mixing may be performed by known methods. It may be carried out dry or wet. Dry mixing may be performed, for example, using a dry mixer such as an air blender, a V-type blender, a rocking blender, a Henschel mixer, a Nauta mixer, etc. In the case of wet mixing, a solvent such as water is added to the aluminum hydroxide powder and additives to form a slurry, and the obtained slurry may be mixed using a wet mixer such as a ball mill, an attritor, a bead mill, etc.

[0058] <Firing Step> In the firing step, the obtained raw material mixture is fired by holding it at a temperature within the range of 1000 °C or higher and 1300 °C or lower for a time less than 20 hours. Thereby, a fired product is obtained. During firing, aluminum hydroxide (Al(OH) ３ ) dehydrates and transforms into α-alumina (Al ２ O ３ ). Also, the alkali metal (AM), silicon (Si), and fluorine (F) in the raw material mixture become a liquid phase in an oxide state and act as a mineralizer that promotes the crystallization of α-alumina. That is, the crystal structure changes during the transition from Al(OH) ３ to α-alumina. By adding a mineralizer, the crystallization of α-alumina (corundum) is promoted, so that the transition can be carried out at a lower temperature.

[0059] During firing, the mineralizing agent components (AM, Si, F) that become liquid phase cover the surface of the alumina particles, and some penetrate into the interior of the particles. These liquid phase components promote particle diffusion and growth. Specifically, some aluminum atoms (Al) dissolve from the surface of the alumina particles into the liquid phase, and the dissolved aluminum precipitates in other locations on the particle surface. This causes the alumina particles to grow. At this time, differences in the growth direction of the alumina particles and changes in the growth rate occur depending on the composition of the liquid phase components. Therefore, if the composition of the liquid phase components is appropriately adjusted, the growth of the aluminum particle surface is controlled, and as a result, particles with high plate-like properties and a desired polycrystalline state can be obtained. The liquid phase components also have the effect of smoothing the surface of the alumina particles. If there are irregularities on the surface of the alumina particles, these irregularities preferentially diffuse into the liquid phase, causing the irregularities to disappear. Therefore, if the composition of the liquid phase components is appropriately controlled, it becomes possible to obtain alumina powder with the desired size and polycrystalline state, as well as a smooth surface.

[0060] If the firing temperature is below 1000°C, the transition to α-alumina and grain growth may be insufficient. This may make it difficult to obtain alumina powder with excellent dispersibility. On the other hand, if the firing temperature exceeds 1300°C, the particles may sinter together, forming a strong sintered mass. Furthermore, the energy consumption for firing may become excessive, leading to increased manufacturing costs. The firing temperature is preferably between 1000°C and 1200°C, and more preferably between 1100°C and 1200°C. The firing time is preferably between 10 hours and less than 20 hours. This ensures that the transition to α-alumina and grain growth are reliably secured while preventing excessive sintering of the particles. The firing furnace is not limited as long as the desired alumina powder can be obtained. However, from the viewpoint of effectively exerting the effect of additives during firing, a stationary furnace that can fire using a sealed firing container is preferred.

[0061] In the manufacturing method of this embodiment, the heating rate during the firing process is set to 50°C / hour or more and 150°C / hour or less. In controlling the polycrystalline state of alumina powder, the heating rate during firing is important, along with the composition of the liquid phase component and the firing temperature. The faster the heating rate, the larger the grain boundary length. Therefore, the ratio (L2 / L1) of the total length of grain boundaries (L2) to the perimeter length (L1) of the alumina powder becomes larger. Conversely, the slower the heating rate, the smaller the ratio (L2 / L1). By adjusting the composition of the liquid phase component and the firing temperature within a predetermined range, and by adjusting the heating rate within a predetermined range, the ratio (L2 / L1) can be controlled within a desired range, and as a result, it becomes possible to obtain alumina powder with suppressed scattered glitter (glare). Note that the heating rate mentioned above is the heating rate in the temperature range of at least 25°C (room temperature) to 1000°C.

[0062] <Post-processing> If necessary, post-processing such as desodium filtration, crushing, and / or classification may be performed on the calcined material obtained through the calcination process. Desodium filtration removes excess alkali metal components adhering to the surface of the calcined material. This allows for adjustment of the alkali metal content in the final alumina powder. Alkali metal components on the particle surface can be removed, for example, by washing and filtering the calcined material. Crushing involves applying a small amount of mechanical energy to the calcined material to break the bonds of aggregated particles formed during calcination. Crushing can be performed dry or wet using crushers such as pot mills, pin mills, and / or jaw crushers. Classification involves separating particles according to their size to obtain alumina powder of the desired particle size. Classification can be performed by methods such as sieving, airflow classification, elutriation, and / or centrifugal separation. Post-processing may be performed as necessary. If the desired alumina powder can be obtained after calcination, post-processing may be omitted.

[0063] In this way, the plate-shaped alumina powder of this embodiment can be produced. The obtained alumina powder has the desired size and polycrystalline state, and its surface is smooth. Therefore, while maintaining a high level of brilliance, the scattered glare (glare) is suppressed. Such alumina powder can be suitably used in a variety of applications, including paints and cosmetics.

[0064] <<3. Paints and Cosmetics>> The paint or cosmetic of this embodiment contains the plate-shaped alumina powder described above. The paint or cosmetic may contain the alumina powder alone or in a surface-treated form. Surface treatment can be performed by applying a surface treatment agent such as a silicon compound, an alkylsilane compound, and / or a fluorine compound to the particle surface of the plate-shaped alumina powder. In addition to the plate-shaped alumina powder, the paint and cosmetic may also contain a solvent or a resin. The solvent may be water-based or non-water-based. Furthermore, the paint and cosmetic may also contain known additives such as oils, pigments other than plate-shaped alumina powder, fillers, surfactants, viscosity modifiers, preservatives, fragrances, humectants, physiologically active ingredients, salts, chelating agents, neutralizing agents, and / or pH adjusters.

[0065] The present invention will be described in more detail using the following examples and comparative examples. However, the present invention is not limited to the following examples.

[0066] (1) Preparation of plate-shaped alumina powder [Examples 1-5 and Comparative Example 1] Aluminum hydroxide (gibbsite) powder (Nippon Light Metal Co., Ltd.) was prepared as the alumina source for Examples 1-3 and Comparative Example 1. In addition, a different aluminum hydroxide (gibbsite) powder (Nippon Light Metal Co., Ltd.) was prepared as the alumina source for Examples 4 and 5. The properties of the prepared aluminum hydroxide powders are shown in Table 1 below. Note that the Na shown in Table 1 ２ O is the total value of the eluted sodium content on and near the crystal surface of aluminum hydroxide and the non-eluted sodium content incorporated within the crystal lattice, and f-Na ２ O consists solely of dissolved sodium on and near the crystal surface of aluminum hydroxide.

[0067] Separately from this, aluminum fluoride (DO-FLUORIDE CHEMICALS CO., LTD.; AlF ３ ), silicon oxide (Maru-Kama Kamaido Ceramics Co., Ltd., Snow Brand Silica SP-3; SiO ２ ), sodium carbonate (Kanto Chemical Co., Inc.; Na ２ CO ３ ), and sodium fluorosilicate (Kanto Chemical Co., Inc., sodium hexafluorosilicate; Na ２ SiF ６ ) were prepared.

[0068] Next, the prepared aluminum hydroxide powder and the additives were mixed to prepare a raw material mixture. The blending amount of the additives was adjusted so that the content ratio of each additive in the raw material mixture became the value shown in Table 1 below. The mixing was performed by a method of putting the raw materials into a bag and shaking (manual method; Examples 1, 2, 4, and 5) or a method using a rocking mixer (mechanical method; Examples 3 and Comparative Example 1).

[0069] <Firing Step> The obtained raw material mixture was put into a firing container, and the firing container was closed with a dedicated lid. The covered firing container was placed in a firing furnace and fired in the air. The firing was performed using an electric furnace or a shuttle kiln. During firing, the temperature was raised to the firing temperature at a heating rate of 100 °C / hour, and then held at that temperature for 10 hours (Examples 1 to 5) or 20 hours (Comparative Example 1), and then naturally cooled. The firing temperature was 1100 °C (Examples 1 to 5) or 1050 °C (Comparative Example 1). After the temperature in the furnace had dropped completely, the firing container container was taken out of the furnace, and the fired product (alumina) was recovered.

[0070] <Post-treatment> Post-treatment was performed on the obtained fired product. Wet crushing was performed while adjusting the crushing strength so that the plate shape was not broken, and filtration treatment was performed to obtain a crushed product (alumina). Then, using a dryer, the obtained crushed product was dried at 105 °C overnight. And after the crushing treatment, coarse particles and fine particles were removed.

[0071] In this way, plate-shaped alumina powders of Examples 1 to 5 and Comparative Example 1 were produced. The manufacturing conditions of the plate-shaped alumina powder are summarized in Table 1 below.

[0072] [Comparative Example 2] In Comparative Example 2, commercially available plate-shaped alumina powder (Kinsei Matec Co., Ltd., YFA10030) was obtained and its properties were evaluated.

[0073]

[0074] (2) Evaluation The plate-shaped alumina powder obtained in Examples 1 to 5 and Comparative Examples 1 and 2 was used as a sample, and various properties were evaluated as follows.

[0075] <Particle Size Distribution> The particle size distribution of alumina powder was measured using a laser diffraction / scattering particle size distribution analyzer (Nikkiso Co., Ltd., Microtrac MT3300). Specifically, the alumina powder was directly placed into the input port of the analyzer, dispersed for 1 minute using the dispersion function installed in the analyzer, and then the particle size distribution was measured. The average particle diameter (D50) was determined as the particle diameter corresponding to a cumulative particle size distribution rate of 50% by volume.

[0076] <SEM Observation> Alumina powder was evaluated using a scanning electron microscope (JEOL Ltd., JSM-F100; SEM) and image analysis software (ImageJ). Specifically, SEM images of the obtained alumina powder were taken, and the particles (plate-shaped α-alumina particles) captured in the images were evaluated using ImageJ to measure particle diameter and thickness, and the average particle diameter and average thickness were calculated. The average particle diameter was the average value of 80 particles, and the average thickness was the average value of 30 particles. The ratio of the average particle diameter to the average thickness (average particle diameter / average thickness) was then calculated as the average aspect ratio.

[0077] Furthermore, the crystalline state of the particles constituting the alumina powder (plate-like α-alumina particles) was investigated using electron backscatter diffraction (EBSD) analysis. Specifically, EBSD measurements were performed using a scanning electron microscope (JEOL Ltd., JSM-6490A; SEM) equipped with a crystal orientation analyzer for electron microscopes and an analysis tool (TSL Solutions Co., Ltd., OIM EBSD system) to observe the crystalline morphology of alumina. During this process, the particle plate surface was observed, and the particle perimeter (L1), the total length of the grain boundaries on the plate surface (L2), and the major axis of the particle (L3) were measured, and L2 / L1 and L2 / L3 were calculated. The same procedure was performed for 30 particles, and the average values ​​of L2 / L1 and L2 / L3 were obtained.

[0078] <Crystalline Phase> The crystalline phase of alumina powder was investigated using an X-ray diffractometer (Rigaku Corporation, RINT Ultima III). Specifically, the obtained alumina powder (sample) was placed on a glass sample plate, pressed down to flatten the sample surface and selectively oriented, and then measured using an XRD measuring device. CuKα was used as the X-ray source. Then, it was investigated whether or not diffraction peaks based on α-alumina (corundum phase) were detected in the obtained X-ray diffraction pattern.

[0079] Furthermore, for samples in which diffraction peaks based on α-alumina were detected, the intensity (height) of the diffraction peaks based on the (006), (113), (104), (116), (018), and (1010) planes of α-alumina was determined by I Ａ , I Ｂ , I Ｃ , I Ｄ , I Ｅ and I Ｆ The peak intensity ratio I is calculated as follows: Ａ / I Ｂ , I Ｃ / I Ｂ , I Ｄ / I Ｂ , I Ｅ / I Ｂ and I Ｆ / I Ｂ The result was calculated.

[0080] <Component Analysis> Component analysis was performed using a scanning X-ray fluorescence analyzer (Rigaku Corporation, ZSX Primus IV). Specifically, alumina powder was placed in a platinum crucible and set in a high-frequency melting apparatus. After preheating at 700°C for 60 seconds, a sloshing treatment was performed at 1200°C for 120 seconds to produce a glass bead sample. Next, the glass bead sample was evaluated using a scanning X-ray analyzer. The obtained data was compared with the data of a standard sample for calibration to obtain the evaluation results.

[0081] <Specific surface area> Specific surface area (S ＢＥＴ ) uses a manual specific surface area measuring device (Micromeritics Instrument Corp., FlowSorbIII 2305 model), N ２ Evaluation was performed using the gas adsorption method.

[0082] <Glitter (Spread of Shine)> The glitter of plate-shaped alumina powder was evaluated. Specifically, five healthy adults (3 men, 2 women) were selected as evaluators. An appropriate amount (approximately 1 g) of plate-shaped alumina powder was applied to the back of their hands, and the powder was visually observed after application. Scoring was then assigned according to the evaluation criteria below.

[0083] 3 points: The scattered glitter (glare) is suppressed. 2 points: The scattered glitter is somewhat strong. 1 point: The scattered glitter is strong.

[0084] Then, for each sample, the scores of the five evaluators mentioned above were averaged, and the resulting average value (rounded to two decimal places) was used to evaluate the sense of presence (glamorousness).

[0085] (3) Evaluation Results The evaluation results obtained for Examples 1 to 5 and Comparative Examples 1 and 2 are summarized in Table 2 below.

[0086] Examples 1 to 5 both had average particle diameter and average thickness within the ranges defined in this embodiment (average particle diameter 2 to 100 μm, average thickness 0.2 to 3.0 μm). Furthermore, the particles constituting the powder were polycrystalline, and the average value of the ratio of the total grain boundary length (L2) to the perimeter length (L1) of the particles (L2 / L1) was within the range defined in this embodiment (0.30 to 2.0). In addition, Examples 1 to 3 had relatively high detection scores of 2.4 to 3.0.

[0087] In contrast, Comparative Example 1 had a small average ratio (L2 / L1) and did not meet the range (0.30 to 2.0) specified in this embodiment. As a result, the evaluation result for the sense of presence was low at 1.2. Comparative Example 2 was composed of single-crystal particles.

[0088]

[0089] From the results above, it is understood that this embodiment provides plate-shaped alumina powder and a method for producing the same, which suppress the scattered glare (shininess) compared to conventional plate-shaped alumina powder, even with the same particle size.

Claims

1. Plate-shaped alumina powder comprising a plurality of plate-shaped α-alumina particles, wherein the average particle diameter determined by SEM observation is 2 μm or more and 100 μm or less, and the average thickness determined by SEM observation is 0.2 μm or more and 3.0 μm or less, wherein the plate-shaped α-alumina particles are polycrystalline, and the average value of the ratio (L2 / L1) of the total length of grain boundaries to the perimeter length (L1) of the plate-shaped α-alumina particles is 0.30 or more and 2.00 or less on the plate surface of the plate-shaped α-alumina particles.

2. The plate-shaped alumina powder according to claim 1, wherein the average value of L2 / L1 is 0.50 or more and 1.50 or less.

3. The plate-shaped alumina powder according to claim 1 or 2, wherein the average particle diameter is 5 μm or more and 50 μm or less, and the average thickness is 0.3 μm or more and 1.0 μm or less.

4. A method for producing plate-shaped alumina powder according to claim 1 or 2, comprising the steps of: preparing a raw material mixture containing aluminum hydroxide powder and additives; and firing the raw material mixture by holding it at a temperature in the range of 1000°C to 1300°C for less than 20 hours, wherein the raw material mixture contains alkali metals (AM) ２ Silicon (Si) is present in an amount of 0.01% to 0.5% by mass, in terms of oxygen equivalent. ２ A method comprising a raw material mixture containing 0.1% by mass or more and 0.3% by mass or less, and fluorine (F) in an amount of 0.1% by mass or more and 5.0% by mass or less, calculated as F equivalent, wherein the heating rate in the calcination step is 50°C / hour or more and 150°C / hour or less.

5. The additive is an alkali silica fluoride (AM ２ SiF ６ ), or aluminum fluoride (AlF ３ ) and silicon dioxide (SiO ２ The method according to claim 4, including ) 6. The method according to claim 5, wherein the additive further comprises one or both of an oxide (AM ２ O) and a carbonate (AM ２ CO ３ ) of an alkali metal (AM).

7. The method according to claim 4, wherein the alkali metal (AM) is sodium (Na) and potassium (K) or both.

8. A paint or cosmetic comprising the plate-shaped alumina powder described in claim 1 or 2.

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