JP7565738B2 - Corrosion inhibitor, anticorrosive paint, laminate, method for manufacturing laminate, and method for preventing corrosion of substrate - Google Patents

Description

The present invention relates to a corrosion inhibitor, an anticorrosive coating, a laminate, a method for producing a laminate, and a method for preventing corrosion of a substrate.
This application claims priority to U.S. Provisional Application No. US 62/912,655, filed October 9, 2019, the contents of which are incorporated herein by reference.

Steel structures such as ships, bridges, tanks, and plants are often installed in coastal areas, river mouths, underwater, etc., and are often placed in extremely severe corrosive environments. In order to withstand such corrosive environments, it is common for the surfaces of these structures to be coated with anti-corrosion paint.

Such anticorrosive paints may contain particles. For example, Patent Document 1 discloses a heavy-duty anticorrosive composition containing 2 to 80% by weight of titanium or titanium alloy flakes in a resin. Patent Document 2 discloses an anticorrosive paint composition in which flaky potassium titanate particles are blended into a resin composition.

Japanese Unexamined Patent Publication No. 1-170670 Japanese Patent Application Publication No. 6-329959

However, there continues to be a need for additional compositions to achieve corrosion protection.

The present invention has been made in consideration of the above circumstances, and provides an anticorrosive agent and an anticorrosive paint having anticorrosive effects, a laminate to which they are applied, a method for manufacturing the laminate, and a method for preventing corrosion of a substrate.

That is, the present invention includes the following aspects:

(1) A corrosion inhibitor comprising three or more sheets of tabular alumina, the sheets having a house-of-cards structure in which the tabular alumina are fixed together, and containing alumina particles having an average particle size of 3 μm or more and 1,000 μm or less.
(2) The corrosion inhibitor according to (1) above, wherein the alumina particles contain silicon and/or germanium.
(3) The corrosion inhibitor according to (1) or (2) above, wherein the alumina particles contain molybdenum.
(4) The corrosion inhibitor according to (3) above, wherein the molybdenum content relative to 100% by mass of the alumina particles is 10% by mass or less in terms of molybdenum trioxide.
(5) An anticorrosive paint containing the anticorrosive agent according to any one of (1) to (4) above.
(6) A laminate having a substrate and a layer containing the anticorrosive coating material described in (5) above.
(7) The laminate according to (6) above, wherein the substrate is a metal.
(8) A method for producing a laminate, comprising applying the anticorrosive coating material according to (5) above to a substrate.
(9) A method for protecting a substrate from corrosion, comprising applying the anticorrosive coating material according to (5) above to the substrate.

According to the anticorrosive agent, anticorrosive paint, and laminate of the present invention, it is possible to provide an anticorrosive agent, anticorrosive paint, and laminate that can exhibit a good anticorrosive effect.
Furthermore, according to the method for producing a laminate according to the present invention, a laminate capable of exhibiting a good corrosion prevention effect can be produced.
Furthermore, according to the method for preventing corrosion of a substrate of the present invention, it is possible to impart a good anticorrosive effect to the substrate.

FIG. 1 is a schematic diagram showing an example of the configuration of an alumina particle having a house-of-card structure according to an embodiment. FIG. 2 is a schematic diagram showing another example of the configuration of an alumina particle having a house-of-card structure according to the embodiment.

The following provides a detailed description of an embodiment of the present invention, including an anticorrosive agent, an anticorrosive coating, a laminate, a method for manufacturing a laminate, and a method for preventing corrosion of a substrate.

<Corrosion inhibitor>
The corrosion inhibitor according to one embodiment of the present invention (hereinafter, sometimes referred to as the "corrosion inhibitor of the embodiment" or simply the "corrosion inhibitor") contains alumina particles. The alumina particles are formed of three or more sheets of tabular alumina, have a card-house structure in which the tabular alumina are fixed together, and have an average particle size of 3 μm or more and 1000 μm or less.

The alumina particles contained in the corrosion inhibitor of this embodiment have the above-mentioned structure, and have a larger specific surface area than spherical alumina particles of the same shape, and have a house-of-cards structure with tabular alumina particles fixed thereto, so that the corrosion inhibitor of this embodiment has excellent corrosion prevention effect.
The anticorrosive effect of the anticorrosive agent can be confirmed by, for example, comparing a portion of an object to which the anticorrosive agent is applied with a portion to which the anticorrosive agent is not applied, and finding that the portion to which the anticorrosive agent is applied is more inhibited than the portion to which the anticorrosive agent is not applied. Examples of corrosion include metal corrosion (i.e., rust formation) and plastic corrosion (i.e., deterioration).
The corrosion resistance, which indicates that the corrosion inhibitor exerts an anticorrosive effect, may be rust prevention, salt resistance or seawater resistance (inhibition of corrosion caused by salt), and may also be alkali resistance (inhibition of corrosion caused by alkali), acid resistance (inhibition of corrosion caused by acid), etc.

Next, the alumina particles contained in the corrosion inhibitor of this embodiment will be described in detail below.

[Alumina particles having a house-of-card structure]
The alumina particles having a card house structure are formed by three or more sheets of flat alumina, and have a card house structure in which the flat alumina are fixed to each other. Hereinafter, the alumina particles having a card house structure may be simply abbreviated as alumina particles. The flat plate shape may be, for example, a three-dimensional hexahedral plate shape, and the shape of the two-dimensional projection plane may be a typical quadrangle with four corners (quadrangle plate shape), or the shape of the two-dimensional projection plane may be a polygon with five or more corners (hereinafter, the latter may be referred to as polygonal plate shape). The alumina particles of the embodiment may contain potassium. The alumina particles of the embodiment may contain mullite and/or a germanium compound. The alumina particles have a card house structure, and have an internal structure derived from the flat alumina constituting the card house structure, and have a high specific surface area compared to spherical alumina particles of the same particle diameter, so that the corrosion prevention effect is effectively exerted.

The morphology of the alumina particles can be confirmed by a scanning electron microscope (SEM). The card house structure refers to a structure in which, for example, plate-like particles are arranged in a complex manner without being oriented. In this specification, the card house structure refers to a structure formed by three or more flat alumina sheets, which are fixed to each other (see, for example, FIG. 1), and may refer to a structure in which, for example, three or more flat alumina sheets are intersected and assembled at two or more points, and the intersecting flat alumina sheets may be arranged in a disordered manner in the plane direction (see FIG. 2). The intersecting position may be any position on the flat alumina sheets. The disordered arrangement refers to a state in which the directions in which the planes intersect with each other are not limited in the direction of the X-axis, Y-axis, or Z-axis, and the angles at which the planes intersect with each other may be any angle. Details of the "flat alumina" will be described later.

Although it depends on the average particle size of the alumina particles required, when used as a corrosion inhibitor, the number of tabular alumina sheets contained in one alumina particle is, for example, 3 to 10,000 sheets, preferably 10 to 5,000 sheets, and especially 15 to 3,000 sheets, from the viewpoints of both performance and ease of production.

The intersection of the tabular alumina occurs when three or more sheets of tabular alumina adhere to each other through some kind of interaction, for example, by bonding together during the crystal formation process during the firing process. As a result, they may appear to be interpenetrating. The strength of the house-of-cards structure is increased when the tabular alumina adheres firmly to each other.

Intersection means that two or more faces meet at one point, and there are no restrictions on the position, diameter, area, etc. where the faces intersect. Also, the number of directions of the faces starting from the intersection point can be three, four or more.

The major axis, minor axis, and thickness of the surface of the flat alumina itself contained in this card house structure may be of any size. It may also contain flat alumina of multiple sizes.

As mentioned above, the tabular alumina may be either square or polygonal alumina. Within a single alumina particle, either square or polygonal alumina may be present, or both, and there is no restriction on the ratio.

In addition to the card house structure, the particles may contain particles of an approximately X-shape (sometimes called twin crystal alumina particles; see FIG. 1) in which two flat alumina sheets cross, an approximately T-shape, an approximately L-shape, or a single flat alumina sheet in any state. In order to obtain excellent fluidity, the content ratio of these is preferably low, and the content ratio of alumina particles having a card house structure formed by three or more flat alumina sheets and in which the flat alumina sheets are fixed to each other is preferably 80% or more by weight or number. More preferably, it is 90% or more, and even more preferably, it is 95% or more. The content ratio of twin crystals or a single flat alumina sheet can be easily adjusted by general classification operations such as sieve classification and air classification.

Due to their unique structure, alumina particles with a house-of-cards structure have extremely high crushing strength and do not easily collapse even when external stress is applied. As a result, when they are mixed into anti-corrosion paint, poor flow due to the anisotropy of the alumina particles themselves is unlikely to occur. Therefore, not only can the inherent functions of the alumina particles be fully utilized, but even if they are mixed with plate-like alumina particles, the plate-like alumina particles, which tend to be oriented in the longitudinal direction, can be made to exist in random directions. As a result, excellent mechanical strength and anti-corrosion effects can be achieved not only in the longitudinal direction but also in the thickness direction.

Due to their unique structure, alumina particles have excellent fluidity as a powder, making it possible to increase the discharge of feeders used for mechanical transport, such as hoppers and feeders, for use in industrial products. Alumina particles have internal voids due to their unique structure, so their bulk density is not significantly different from that of plate-like alumina particles, but compared to plate-like alumina particles, they have a higher degree of sphericity and, as mentioned above, have a higher crushing strength and are less likely to break, so it is assumed that the rolling of alumina particles has a significant effect on the ease of transport.

The alumina particles have a house-of-cards structure. The house-of-cards structure is as described above. The alumina particles preferably have flat alumina in a polygonal shape having four or more sides, with at least a portion of adjacent alumina particles in contact with each other, and more preferably have flat alumina in a polygonal shape having five or more sides, with at least a portion of adjacent alumina particles in contact with each other.

[Crystal form/α crystallization rate]
The alumina particles are aluminum oxide, and the crystal form is not particularly limited. For example, the alumina may be transition alumina having various crystal forms such as γ, δ, θ, κ, etc., or may contain alumina hydrate in transition alumina. However, it is preferable that the a-crystal form is basically the α-crystal form because it has superior mechanical strength.

The α-crystallization rate of the alumina particles can be determined by XRD measurement.
For example, using a wide-angle X-ray diffraction (XRD) device (Ultima IV, manufactured by Rigaku Corporation) described later, the prepared sample is placed on a holder for measurement samples, and measurement is performed under conditions of Cu/Kα radiation, 40 kV/40 mA, a scan speed of 2 degrees/min, and a scan range of 10 to 70°, and the α-crystallinity is determined from the intensity ratio of the α-alumina peak to the baseline. The α-crystallinity rate varies depending on the firing conditions and the raw materials used, and from the viewpoint of improving the crushing strength and fluidity of the alumina particles, the α-crystallinity rate is preferably 90% or more, and more preferably 95% or more. The sample to be measured may be alumina particles or tabular alumina obtained by breaking down into a card house structure by some mechanical processing.

[Average particle size]
The average particle size of the alumina particles having a house-of-card structure may be any size within the range in which the structure can be formed, but in terms of excellent fluidity and improved corrosion prevention effect, the average particle size is 3 μm or more, preferably 10 μm or more. Also, if the size is too large, the house-of-card structure may be exposed in the coating film of the corrosion-resistant paint, causing poor appearance, so the average particle size is 1000 μm or less, preferably 300 μm or less, and more preferably 100 μm or less.
An example of the range of the above numerical value is 3 μm or more and 300 μm or less, and may be 10 μm or more and 100 μm or less.
In this specification, the "average particle size of alumina particles" refers to a value calculated as a volume-based median diameter _D50 from a volume-based cumulative particle size distribution measured by a laser diffraction dry particle size distribution analyzer.

[Maximum particle size]
In addition, the maximum particle size of the alumina particles on a volume basis (hereinafter, in this specification, may be simply referred to as "maximum particle size") is not particularly limited, but is usually 3000 μm or less, preferably 1000 μm or less, and more preferably 500 μm or less.

If the maximum particle size of the alumina particles is larger than the above upper limit, the alumina particles may protrude from the surface of the anticorrosive paint coating, which may cause poor appearance, which is undesirable.

The average particle size and maximum particle size of the alumina particles referred to here are values determined by a dry method in which the alumina particles themselves, which are formed from three or more sheets of flat alumina and have a card-house structure in which the flat alumina sheets are adhered to each other, are measured using a laser diffraction particle size distribution meter.
The average particle size and maximum particle size can also be estimated by a wet method, for example, by dispersing the alumina particles in an appropriate solvent, specifically, by measuring a sample prepared by dispersing the alumina particles in a pure water medium containing sodium hexametaphosphate or the like as a dispersion stabilizer using a laser diffraction/scattering type particle size distribution measuring device.

[Aspect ratio of plate-like alumina]
The tabular alumina is preferably polygonal and has an aspect ratio, which is the ratio of particle diameter to thickness, of 2 to 500. An aspect ratio of 2 or more is advantageous for forming a house-of-cards structure while maintaining the performance specific to tabular alumina, and an aspect ratio of 500 or less is preferable because it is easy to adjust the average particle diameter of the alumina particles and can suppress the occurrence of poor appearance and reduction in mechanical strength due to the exposure of the house-of-cards structure in the coating film of the anticorrosive paint. More preferably, the aspect ratio is 5 to 300, even more preferably 7 to 100, and particularly preferably 10 to 50. An aspect ratio of 10 to 100 provides alumina particles having a house-of-cards structure that is excellent in optical properties such as thermal properties and brightness of tabular alumina, and has high fluidity and anticorrosive effect, which is preferable in terms of practicality.

In this specification, the thickness of the alumina plates is determined by measuring the thickness of 10 plates using a scanning electron microscope (SEM) and taking the average value.

The particle size of tabular alumina refers to the arithmetic mean value of the longest distance between two points on the contour line of the plate, and this value is measured using a scanning electron microscope (SEM).

The value of the long diameter of the tabular alumina means a value calculated by measuring the long diameter of any 100 tabular alumina particles from an image obtained by a scanning electron microscope (SEM).
The major axis of the tabular alumina can be measured, for example, by observing the alumina particles with an SEM and measuring the maximum length of the tabular alumina located at the center of the alumina particle. Alternatively, the maximum length of a single piece obtained by performing air classification on the alumina particles can be measured with an SEM. Alternatively, a method can be used in which, under conditions that do not destroy the tabular alumina itself, a card house structure is broken down by some mechanical processing to obtain a single piece, and the maximum length is measured with an SEM.

In addition, since the alumina particles having a house-of-cards structure preferably have an average particle diameter of, for example, 3 μm to 1000 μm, the tabular alumina constituting them preferably has a thickness of, for example, 0.01 μm to 5 μm, a major axis of 0.1 μm to 500 μm, and an aspect ratio, which is the ratio of the major axis to the thickness, of 2 to 500. In particular, when using these alumina particles as a corrosion inhibitor, it is more preferable that the thickness of the tabular alumina is 0.03 μm to 3 μm, the major axis of 0.5 μm to 100 μm, and the aspect ratio of 5 to 300, because of their ease of use. Even more preferably, the thickness of the tabular alumina is 0.1 μm to 3 μm, the major axis of 1 μm to 30 μm, and the aspect ratio of 10 to 100.

[Silicon and Germanium]
In addition, the alumina particles having a house of cards structure preferably contain silicon (silicon atoms and/or inorganic silicon compounds) and/or germanium (germanium atoms and/or inorganic germanium compounds), and in particular, those containing silicon and/or germanium on the surface of the tabular alumina are preferred. In particular, containing silicon and/or germanium locally on the surface is preferred in smaller amounts than containing it inside, for example, in order to effectively improve affinity with a binder.

The silicon and germanium may be derived from the silicon, silicon compound, and germanium compound used as shape control agents in the method for producing alumina particles described below.

The silicon contained in the alumina particles may be silicon alone or silicon in a silicon compound. The tabular alumina particles according to the embodiment may contain at least one selected from the group consisting of mullite, Si, SiO ₂ , SiO, and aluminum silicate produced by reacting with alumina as silicon or a silicon compound, and may contain the above-mentioned substance in the surface layer. Mullite will be described later.

The amount of silicon and/or germanium unevenly distributed on the surface of the tabular alumina containing silicon and/or germanium can be measured, for example, by analysis using an X-ray fluorescence analyzer (XRF) and analysis using X-ray photoelectron spectroscopy (XPS).

In general, X-ray fluorescence spectroscopy (XRF) is a method for quantitatively analyzing the bulk composition of a material by detecting fluorescent X-rays generated by X-ray irradiation and measuring the wavelength and intensity. In general, X-ray photoelectron spectroscopy (XPS) is a method for analyzing the elemental composition of a sample surface by irradiating the sample surface with X-rays and measuring the kinetic energy of photoelectrons emitted from the sample surface. The presence of silicon and/or germanium biased on the surface of the tabular alumina and its vicinity can be estimated from whether the [Si]/[Al]% (surface) or [Ge]/[Al]% (surface) obtained from the XPS analysis result is larger than the [Si]/[Al]% (bulk, molar ratio) or [Ge]/[Al]% (bulk, molar ratio) obtained from the XRF analysis result of the product. This is because the surface of the plate-like alumina obtained by blending silicon and/or germanium has a higher amount of silicon and/or germanium than the innermost part of the plate-like alumina. The above-mentioned XRF analysis can be performed using Primus IV manufactured by Rigaku Corporation. XPS analysis can be performed using Quantera SXM manufactured by ULVAC-PHI, Inc.

The alumina particles preferably contain silicon atoms and/or inorganic silicon compounds locally on the surface of the tabular alumina constituting the alumina particles. In XPS analysis, the molar ratio of Si to Al, [Si]/[Al], is preferably 0.001 or more, more preferably 0.01 or more, even more preferably 0.02 or more, and particularly preferably 0.1 or more.
The upper limit of the molar ratio [Si]/[Al] in the XPS analysis is not particularly limited, but may be 0.5 or less, 0.4 or less, or 0.3 or less.

The alumina particles preferably have a molar ratio of Si to Al [Si]/[Al] obtained by XPS analysis of 0.001 to 0.5, more preferably 0.01 to 0.4, even more preferably 0.02 to 0.3, and particularly preferably 0.1 to 0.3. When the molar ratio of Si to Al obtained by XPS analysis is within the above range, alumina particles having a card house structure formed by tabular alumina can be easily obtained, and the obtained alumina particles exhibit excellent fluidity and crushing strength, and are preferable because they can improve the corrosion prevention effect. In addition, the affinity with the binder can be improved, for example.

The presence of a large amount of silicon atoms and/or inorganic silicon compounds on the surface of the tabular alumina not only makes the surface properties of the alumina particles made of tabular alumina more hydrophobic than when they are not present, but also improves the affinity with organic compounds and various binders and matrices when used as a corrosion inhibitor. Furthermore, the silicon atoms and/or silicon compounds present on the surface of the alumina particles act as reaction points to contribute to reactions with various coupling agents, including organic silane compounds, making it possible to easily adjust the surface condition of the alumina surface.

When the amount of Si on the surface of alumina particles is analyzed using the above-mentioned X-ray photoelectron spectroscopy (XPS) apparatus, a sample is pressed and fixed onto double-sided tape, and composition analysis can be performed under the following conditions.
X-ray source: Monochromatic AlKα, beam diameter 100 μmφ, output 25 W
Measurement: Area measurement (1000 μm square), n = 3
Charge correction: C1s = 284.8 eV

When the alumina particles further contain silicon, Si is detected by XRF analysis. In the alumina particles according to the embodiment, the molar ratio of Si to Al [Si]/[Al] obtained by XRF analysis is preferably 0.0003 or more and 0.1 or less, more preferably 0.0005 or more and 0.08 or less, even more preferably 0.005 or more and 0.05 or less, and even more preferably 0.005 or more and 0.01 or less.

When the molar ratio [Si]/[Al] obtained by the XRF analysis is within the above range, alumina particles having a house-of-cards structure formed from tabular alumina can be easily obtained, and the obtained alumina particles exhibit excellent fluidity and crushing strength, and are preferable because they can improve the corrosion prevention effect.

The alumina particles contain silicon corresponding to the silicon or silicon compound used in the manufacturing method thereof. The silicon content relative to 100 mass% of the alumina particles obtained by XRF analysis is preferably 0.01 mass% to 8 mass% in terms of silicon dioxide (SiO ₂ ), more preferably 0.1 mass% to 5 mass% inclusive, even more preferably 0.5 mass% to 4 mass% inclusive, and particularly preferably 0.5 mass% to 2 mass% inclusive.
By having the silicon content within the above range, alumina particles having a house-of-cards structure formed by tabular alumina can be easily obtained, and the obtained alumina particles exhibit excellent fluidity and crushing strength, and can improve the corrosion prevention effect, which is preferable.

The XRF analysis shall be carried out under the same measurement conditions as those described in the examples below, or under compatible conditions that give the same measurement results.

(germanium)
The alumina particles may contain germanium. Also, the alumina particles may contain germanium in a surface layer.
Although it depends on the raw material used, the alumina particles may contain at least one germanium or germanium compound selected from the group consisting of compounds such as Ge, _GeO2 , GeO, _GeCl2 , _GeBr4 , _{GeI4, GeS2 ,} AlGe, GeTe, _{GeTe3 , As2} , GeSe, GeS3As, SiGe, _Li2Ge , FeGe, SrGe, GaGe, and oxides thereof, and may contain the above-mentioned substances in the surface layer.
In addition, the "germanium or germanium compound" contained in the alumina particles according to the embodiment and the "raw germanium compound" used as a shape control agent of the raw material may be the same type of germanium compound.

The alumina particles according to the embodiment may contain germanium or a germanium compound in the surface layer of the tabular alumina constituting the particles. By containing germanium or a germanium compound in the surface layer, for example, when mixed with a resin to produce an anticorrosive paint, it is possible to provide an anticorrosive paint with good compatibility with the resin, and increased adhesion due to the anchor effect, thereby providing an anticorrosive paint with even better anticorrosive effect. In addition, by containing germanium or a germanium compound with a low Mohs hardness in the surface layer of the alumina particles, it is possible to make the equipment used to produce the anticorrosive paint less susceptible to wear.

Germanium or a germanium compound is contained in the surface layer of the tabular alumina, and thus a remarkable anticorrosive effect and wear-reducing effect of the equipment are exhibited. Here, the "surface layer" refers to within 10 nm from the surface of the tabular alumina particle according to the embodiment. This distance corresponds to the detection depth of XPS. The surface layer containing this germanium is a very thin layer of within 10 nm, and in the case of germanium dioxide, for example, if the germanium dioxide structure on the surface and interface has many defects, it will have better compatibility with the resin, and will be able to exhibit a more remarkable anticorrosive effect than germanium dioxide with no or few structural defects.
The alumina particles preferably have germanium or germanium compounds unevenly distributed on the surface layer of the tabular alumina. Here, "distributed unevenly on the surface layer" refers to a state in which the mass of germanium or germanium compounds per unit volume in the surface layer is greater than the mass of germanium or germanium compounds per unit volume in the other layers. The uneven distribution of germanium or germanium compounds on the surface layer can be determined by comparing the results of surface analysis by XPS and overall analysis by XRF. By distributing germanium or germanium compounds unevenly on the surface layer, it is possible to exert excellent corrosion prevention effects and equipment wear reduction effects based on germanium or germanium compounds at a similar level with a smaller amount than when germanium or germanium compounds are present not only on the surface layer but also on the other layers (inner layers).

The germanium content relative to 100 mass% of the alumina particles obtained by XRF analysis, calculated as germanium dioxide ( _GeO2 ), is preferably 0.01 mass% or more and 8 mass% or less, more preferably 0.1 mass% or more and 5 mass% or less, and even more preferably 0.5 mass% or more and 4 mass% or less.

(Mullite)
The alumina particles according to the embodiment may contain mullite in the surface layer of the tabular alumina constituting the alumina particles. By containing mullite in the surface layer, for example, when mixed with a resin to produce an anticorrosive coating, it is possible to provide an anticorrosive coating having a good compatibility with the resin, and an anchor effect to increase adhesion, thereby providing an anticorrosive coating having a better anticorrosive effect. In addition, by containing mullite with a low Mohs hardness in the surface layer of the alumina particles, it is possible to make the equipment for producing the anticorrosive coating less likely to wear out.

Mullite, when contained in the surface layer of tabular alumina, exhibits a remarkable anticorrosive effect and wear-reducing effect on equipment. The "mullite" that may be contained in the surface layer of alumina particles is a _{composite oxide} of _{Al and Si and is represented by AlXSiYOz , but the} values of x, _y , and _z are not particularly limited. _A more _{preferred range} is _Al2Si1O5 to _{Al6Si2O13 ,} and _includes , for example, _{Al2.85Si1O6.3 , Al3Si1O6.5 , Al3.67Si1O7.5} , _{Al4Si1O8 , or Al6Si2O13} . _The tabular alumina particles _may contain at least _one compound selected from the group consisting of _{Al2.85Si1O6.3 , Al3Si1O6.5 , Al3.67Si1O7.5 , Al4Si1O8} , and _Al6Si2O13 in the surface layer _. Here, the "surface layer" refers to within _{10 nm} from the surface of the _tabular alumina. This distance corresponds to the detection depth of XPS. In addition, this mullite surface layer is a very thin layer of 10 nm or less, and if the defects of the mullite crystal on the surface and interface increase, the compatibility with the resin becomes even better, and the anticorrosive effect can be more significantly exhibited compared to mullite with no or few crystal defects.
The alumina particles preferably have mullite unevenly distributed in the surface layer of the tabular alumina. Here, "distributed unevenly in the surface layer" refers to a state in which the mass of mullite per unit volume in the surface layer is greater than the mass of mullite per unit volume in the other parts of the surface layer. The uneven distribution of mullite in the surface layer can be determined by comparing the results of the surface analysis by XPS and the overall analysis by XRF. By distributing mullite unevenly in the surface layer, it is possible to exert excellent corrosion prevention effects and equipment wear reduction effects based on mullite at the same level with a smaller amount than when mullite is present not only in the surface layer but also in the other parts of the surface layer (inner layer).

Moreover, the mullite of the surface layer may form a mullite layer, or may be in a state where mullite and alumina are mixed. The interface between the mullite and alumina of the surface layer may be in a state where the mullite and alumina are in physical contact with each other, or the mullite and alumina may form chemical bonds such as Si-O-Al. In comparison with the combination of alumina and SiO ₂ , the combination of alumina and mullite as essential components can be more firmly bonded to the alumina and mullite and is less likely to peel off, from the viewpoint of the high similarity of the constituent atomic composition and the ease of forming chemical bonds such as Si-O-Al based on the flux method when the flux method is adopted. For this reason, if the amount of Si is at the same level, the combination of alumina and mullite as essential components is more preferable because it can exert the corrosion prevention effect and wear reduction effect of the equipment for a longer period of time. The technical effects of a combination containing alumina and mullite as essential components can be expected from either alumina and mullite alone or alumina, mullite and silica, but if anything, the combination of the former two will provide a higher level of technical effect.

The presence or absence of mullite on the surface of alumina particles can be analyzed using a wide-angle X-ray diffraction (XRD) device such as Ultima IV manufactured by Rigaku Corporation.
For example, a sample is placed on a measurement sample holder having a depth of 0.5 mm, and is filled flat with a constant load. The sample is then set in the wide-angle X-ray diffraction (XRD) apparatus and measured under the conditions of Cu/Kα radiation, 40 kV/40 mA, a scan speed of 2 degrees/min, and a scan range of 10 to 70 degrees.
The presence or absence of mullite can be determined by the following formula, where the peak height of mullite observed at 2θ=26.2±0.2 degrees is A, the peak height of the (104) plane α-alumina observed at 2θ=35.1±0.2 degrees is B, and the baseline value at 2θ=30±0.2 degrees is C. The value of R may be, for example, 0.02 or more, 0.02 or more and 0.3 or less, 0.05 or more and 0.2 or less, or 0.1 or more and less than 0.12.
R=(A-C)/(B-C)
(R: ratio of mullite peak height A to α-alumina (104) plane peak height B)

[molybdenum]
The alumina particles having a house-of-card structure may contain molybdenum.

The molybdenum may be derived from a molybdenum compound used as a fluxing agent in the method for producing alumina particles described below.

Molybdenum has catalytic and optical functions. In addition, by utilizing molybdenum, it is possible to produce alumina particles with excellent fluidity in the manufacturing method described below.

The molybdenum is not particularly limited, and includes, in addition to molybdenum metal, molybdenum oxide, partially reduced molybdenum compounds, molybdates, etc. The molybdenum compound may be contained in the tabular alumina particles in any of its possible polymorphic forms or in combination, and may be contained in the tabular alumina particles as α-MoO ₃ , β-MoO ₃ , MoO ₂ , MoO, a molybdenum cluster structure, etc.

The form in which molybdenum is contained is not particularly limited, and it may be contained in a form in which it is attached to the surface of the tabular alumina of alumina particles having a house-of-cards structure, or it may be contained in a form in which it is substituted for part of the aluminum in the crystal structure of the alumina, or it may be a combination of these.

The molybdenum content relative to 100% by mass of the alumina particles obtained by XRF analysis is preferably 10% by mass or less in terms of molybdenum trioxide (MoO ₃ ), and by adjusting the firing temperature, firing time, and flux conditions, it is preferably 0.001% by mass or more and 8% by mass or less, more preferably 0.01% by mass or more and 8% by mass or less, and even more preferably 0.1% by mass or more and 5% by mass or less. A molybdenum content of 10% by mass or less is preferable because it improves the α single crystal quality of alumina.

The XRF analysis shall be carried out under the same measurement conditions as those described in the examples below, or under compatible conditions that give the same measurement results.

The amount of Mo on the surface of the alumina particles can be analyzed using the above-mentioned X-ray photoelectron spectroscopy (XPS) device.
For example, the composition analysis can be performed by using the above-mentioned X-ray photoelectron spectroscopy (XPS) device, pressing and fixing a sample onto a double-sided tape, under the following conditions: When the [Mo]/[Al] (molar ratio) determined from the XPS analysis result is taken as the amount of Mo on the surface of the house-of-card-type alumina particle, the amount of Mo is preferably 0.0005 or more.
X-ray source: Monochromatic AlKα, beam diameter 100 μmφ, output 25 W
Measurement: Area measurement (1000 μm square), n = 3
Charge correction: C1s = 284.8 eV

[potassium]
The alumina particles having a house-of-card structure may contain potassium.

The potassium may be derived from potassium that can be used as a fluxing agent in the method for producing alumina particles described below.
By utilizing potassium, alumina particles having excellent fluidity can be produced with high efficiency in the method for producing alumina particles described below.

The potassium is not particularly limited, but includes potassium metal, potassium oxide, partially reduced potassium compounds, etc.

The form in which potassium is contained is not particularly limited, and it may be contained in a form in which it is attached to the surface of the tabular alumina of alumina particles having a house-of-cards structure, or it may be contained in a form in which it is substituted for part of the aluminum in the crystal structure of the alumina, or it may be a combination of these.

The potassium content relative to 100% by mass of the alumina particles, as calculated by XRF analysis, is preferably 0.05% by mass or more, more preferably 0.05 to 5% by mass, even more preferably 0.1 to 3% by mass, and particularly preferably 0.1 to 1% by mass. Alumina particles having a potassium content within the _above range are preferred because they have a house-of-cards structure and have suitable values for the average particle size, etc. Furthermore, they are preferred because they have excellent fluidity and can improve the anticorrosive effect.

The XRF analysis shall be carried out under the same measurement conditions as those described in the examples below, or under compatible conditions that give the same measurement results.

(Inevitable impurities)
The alumina particles may contain unavoidable impurities.

Inevitable impurities are impurities that originate from metal compounds used in production, are present in raw materials, or are inevitably mixed into alumina particles during the manufacturing process. Although they are not actually necessary, they are present in trace amounts and do not affect the properties of the alumina particles.

Examples of unavoidable impurities include, but are not limited to, magnesium, calcium, strontium, barium, scandium, yttrium, lanthanum, cerium, sodium, etc. These unavoidable impurities may be contained alone or in combination of two or more kinds.

The content of unavoidable impurities in the alumina particles is preferably 10,000 ppm or less, more preferably 1,000 ppm or less, and even more preferably 10 to 500 ppm, relative to the mass of the alumina particles.

(other atoms)
The other atoms refer to those that are intentionally added to the alumina particles for the purpose of imparting anticorrosive effect, mechanical strength, and electrical or magnetic functions, within the scope of not impairing the effects of the present invention.

Other atoms include, but are not limited to, zinc, manganese, calcium, strontium, yttrium, etc. These other atoms may be used alone or in combination of two or more.

The content of other atoms in the alumina particles is preferably 5% by mass or less, and more preferably 2% by mass or less, based on the mass of the alumina particles.

[Organic Compounds]
In one embodiment, the alumina particles may contain an organic compound. The organic compound is present on the surface of the alumina particles and has a function of adjusting the surface properties of the alumina particles. For example, alumina particles containing an organic compound on the surface improve affinity with resin, and therefore the function of the alumina particles as a corrosion inhibitor can be maximized.

Organic compounds include, but are not limited to, organosilanes, alkylphosphonic acids, and polymers.

The organic silanes include alkyltrimethoxysilanes or alkyltrichlorosilanes having an alkyl group with 1 to 22 carbon atoms, such as methyltrimethoxysilane, dimethyldimethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, iso-propyltrimethoxysilane, iso-propyltriethoxysilane, pentyltrimethoxysilane, and hexyltrimethoxysilane, 3,3,3-trifluoropropyltrimethoxysilane, tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilanes, phenyltrimethoxysilane, phenyltriethoxysilane, p-chloromethylphenyltrimethoxysilane, and p-chloromethylphenyltriethoxysilanes.

Examples of the phosphonic acid include methylphosphonic acid, ethylphosphonic acid, propylphosphonic acid, butylphosphonic acid, pentylphosphonic acid, hexylphosphonic acid, heptylphosphonic acid, octylphosphonic acid, decylphosphonic acid, dodecylphosphonic acid, octadecylphosphonic acid, 2_ethylhexylphosphonic acid, cyclohexylmethylphosphonic acid, cyclohexylethylphosphonic acid, benzylphosphonic acid, phenylphosphonic acid, and dodecylbenzenephosphonic acid.

As the polymer, for example, poly(meth)acrylates can be suitably used. Specific examples include polymethyl(meth)acrylate, polyethyl(meth)acrylate, polybutyl(meth)acrylate, polybenzyl(meth)acrylate, polycyclohexyl(meth)acrylate, polyt-butyl(meth)acrylate, polyglycidyl(meth)acrylate, polypentafluoropropyl(meth)acrylate, etc., and also include general-purpose polymers such as polystyrene, polyvinyl chloride, polyvinyl acetate, epoxy resin, polyester, polyimide, polycarbonate, etc.

The organic compounds may be contained alone or in combination of two or more.

The form in which the organic compound is contained is not particularly limited, and the organic compound may be covalently bonded to the alumina or may coat the alumina.

The content of the organic compound is preferably 20% by mass or less, and more preferably 10 to 0.01% by mass, relative to the mass of the alumina particles. A content of the organic compound of 20% by mass or less is preferable because it makes it easier to express the physical properties derived from the alumina particles.

[Crushing strength]
Alumina particles preferably have a higher crushing strength because their inherent fluidity is lost if the card house structure is destroyed by mechanical dispersion by compression, shear, etc. The crushing strength varies depending on the intersection position, number, area, thickness, aspect ratio, etc. of the tabular alumina, and the crushing strength required for various applications varies, and from the viewpoint of practicality, the crushing strength is preferably 1 MPa to 100 MPa, more preferably 20 MPa to 100 MPa, and even more preferably 50 MPa to 100 MPa.
The crushing strength of alumina particles can be measured using, for example, a microparticle crushing force measuring device NS-A100 manufactured by Nanoseeds Co., Ltd., or an MCT-510 manufactured by Shimadzu Corporation. The difference between the peak value at the time of crushing and the baseline (a state in which no force is applied) is defined as the crushing force F [N], and the crushing strength S [Pa] is defined as the average value of 10 values calculated by the following formula.

S=2.8F/(π・D ² )
In the above formula, F is the crushing force [N] and D is the particle diameter [m].

As described above, the alumina particles are formed of three or more flat alumina sheets, and have a card house structure that is fixed together. The inventors have found that alumina particles that contain a moderate amount of silicon atoms and/or inorganic silicon compounds have a higher crushing strength than those that do not. The crushing strength varies depending on the content of silicon atoms and/or inorganic silicon compounds, and the more the content is appropriately high, the higher the fluidity and crushing strength of the particles can be. In addition, for example, the crushing strength can be increased by adopting specific manufacturing conditions as a manufacturing method. The crushing strength can also be adjusted arbitrarily depending on the manufacturing conditions. For example, the crushing strength of the alumina particles can be increased by increasing the firing temperature.

[Powder fluidity]
The powder of alumina particles according to the embodiment has excellent flowability as a powder compared with plate-like alumina particles and twin crystal alumina particles, because the alumina itself constituting the powder has a unique structure and preferably has a specific average particle diameter, but in order to further improve the flowability, it is preferable that the shape of the maximum surrounding surface on a volume basis when the alumina particles forming a card house structure of one unit are surrounded so as to include all the plate-like alumina constituting the particle is spherical or approximately spherical. Furthermore, if necessary, a lubricant, fine silica particles, etc. may be added arbitrarily to further improve the flowability.

The fluidity of the powder of card-house structured alumina particles can be determined, for example, by measuring the angle of repose according to JIS R9301-2-2. The value of the angle of repose is preferably 50° or less, since problems such as hopper bridges, feed necks, uneven supply, and reduced discharge volume are unlikely to occur during mechanical transport using feeders, hoppers, etc. A value of 40° or less is more preferable.

The alumina particles may be formed of three or more flat alumina sheets, have a card-house structure in which the flat alumina sheets are bonded together, and have an average particle size of 1 to 1000 μm. More preferably, the alumina particles have an internal structure in which the three or more flat alumina sheets are intersected and assembled at two or more points as a bonded card-house structure, and the intersecting flat plates are arranged in a disorderly manner in the plane direction.

Conventionally known twin crystal alumina particles have a structure with noticeable corners due to their shape, and are less likely to roll than the alumina particles according to the embodiment, so they do not have sufficient fluidity as a filler. In addition, even if alumina particles had the same house-of-cards structure as the alumina particles according to the embodiment, a particle with a moderately large average particle size would have better fluidity. The alumina particles according to the embodiment exhibit particularly excellent fluidity due to the synergistic effect of the house-of-cards structure and the preferred average particle size.

[pH of isoelectric point]
The pH of the isoelectric point of the alumina particles is, for example, in the range of 2 to 8, preferably in the range of 2.5 to 7, and more preferably in the range of 3 to 6. Alumina particles having an isoelectric point within the above range have a high electrostatic repulsive force, and can enhance the dispersion stability when blended into the above-mentioned dispersion medium, and can be easily modified by surface treatment with a coupling treatment agent or the like for the purpose of further improving performance.

The pH value of the isoelectric point is obtained by measuring the zeta potential with a zeta potential measuring device (Malvern, Zetasizer Nano ZSP), stirring 20 mg of sample and 10 mL of 10 mM KCl aqueous solution for 3 minutes with a foam remover (Thinky, ARE-310) in stirring/defoaming mode, leaving the supernatant to stand for 5 minutes as the measurement sample, adding 0.1 N HCl to the sample with an automatic titrator, and measuring the zeta potential in the range up to pH = 2 (applied voltage 100 V, Monomodl mode), and evaluating the pH of the isoelectric point where the potential becomes zero.

[Specific surface area]
The specific surface area of the alumina particle powder is usually in the range of 50 to 0.001 ^{m 2} /g, preferably in the range of 10 m ² /g to 0.01 ^{m 2} /g, and more preferably in the range of 5.0 m ² /g to 1 m ² /g. When it is in the above range, the number of tabular alumina particles forming the card house structure is appropriate, and the corrosion prevention effect is excellent, and there is no significant increase in viscosity when made into a slurry, resulting in excellent processability.

This specific surface area can be measured using JIS Z 8830: BET single point method (adsorption gas: nitrogen), etc.

[Porosity]
The alumina particles are formed of three or more flat alumina sheets, and have a card house structure in which the flat alumina sheets are fixed to each other, so that the alumina particles have voids. When the proportion of voids is large, the shape tends to be uniform and the fluidity tends to be improved, so the void ratio is preferably 10% by volume or more. More preferably, it is 30% by volume or more. In addition, when the proportion of voids is large, the crushing strength of the powder decreases, so the void ratio is preferably 90% by volume or less. More preferably, it is 70% by volume or less. When the void ratio is within this range, the bulk density is appropriate, the fluidity is not impaired, and the handleability is also good. This void ratio can be determined by measurements such as gas adsorption method and mercury porosimetry, such as JIS Z 8831.

The porosity can be estimated simply by mixing alumina particles with a liquid curable compound such as an epoxy compound or a (meth)acrylic monomer, curing the mixture, and then cutting and polishing the cross section, followed by observing the cross section with a SEM.

<Method of manufacturing alumina particles>
Next, the method for producing alumina particles according to the embodiment will be described in detail, but the method for producing alumina particles according to the embodiment is not limited to the method for producing alumina particles described below.

The average particle size, fluidity, specific surface area, mechanical strength, porosity, thickness and aspect ratio of the tabular alumina, etc., of the alumina particles can be adjusted in the manufacturing method described in detail later. For example, when a flux method is adopted as the manufacturing method, the average particle size of the aluminum compound, the purity of the aluminum compound, the proportion of at least one shape control agent selected from silicon, silicon compounds and germanium compounds, the type of other shape control agent, the proportion of other shape control agents, the state of existence of at least one shape control agent selected from silicon, silicon compounds and germanium compounds and the aluminum compound, the state of existence of other shape control agents and the aluminum compound, etc.

Alumina particles may be obtained based on any manufacturing method as long as they have a house of cards structure. However, using alumina with an existing structure and obtaining alumina with a unique structure such as a house of cards structure through post-treatment is not preferable because it requires multiple manufacturing steps and reduces productivity. For example, it is preferable from the viewpoint of productivity to adopt a manufacturing method for alumina particles that can selectively form a house of cards structure from existing alumina raw materials, can easily contain molybdenum, and can also easily contain potassium, silicon, germanium, etc., and can satisfy all of these requirements at once.

That is, in obtaining alumina particles, it is preferable to obtain them by calcining an aluminum compound in the presence of a molybdenum compound, at least one shape control agent selected from silicon, a silicon compound, and a germanium compound, and, if necessary, other shape control agents, in terms of superior fluidity, dispersibility, and productivity.
In addition, since almost all of the alumina particles produced can have a card house structure and thus have superior productivity, it is preferable to obtain the alumina particles by calcining an aluminum compound in the presence of a molybdenum compound, a potassium compound, at least one shape control agent selected from silicon, a silicon compound and a germanium compound, and, if necessary, other shape control agents.

More specifically, a preferred method for producing alumina particles includes a step of calcining an aluminum compound in the presence of a molybdenum compound and at least one shape control agent selected from silicon, a silicon compound, and a germanium compound (calcination step). The calcination step may be a step of calcining a mixture obtained in a step of obtaining a mixture to be calcined (mixing step). The mixture preferably further contains a potassium compound. The mixture preferably further contains a metal compound described below. The metal compound is preferably an yttrium compound.

When organic compounds are used as the molybdenum compound or silicon compound, the organic components are burned off by firing. In other words, alumina particles can be obtained more easily by incorporating molybdenum into the alumina particles when the molybdenum compound reacts with the aluminum compound at high temperature to form aluminum molybdate, which then decomposes into alumina and molybdenum oxide at a higher temperature. The molybdenum oxide sublimes, but can be recovered and reused. Hereinafter, this manufacturing method is called the flux method. This flux method will be described in detail later.

The shape control agent plays an important role in the growth of plate-like crystals. In the commonly used flux method using a molybdenum compound, molybdenum oxide reacts with an aluminum compound to form aluminum molybdate, and then the change in chemical potential during the decomposition of this aluminum molybdate serves as the driving force for crystallization, forming hexagonal bipyramidal polyhedral particles with well-developed idiomorphic faces (113). In the manufacturing method of the embodiment, the shape control agent is localized near the particle surface during the α-alumina growth process, significantly inhibiting the growth of the idiomorphic faces (113), resulting in relatively fast growth of the crystal orientation in the face direction, and the (001) or (006) face growing, which is believed to form a plate-like shape. By using a molybdenum compound as a flux agent, it is possible to more easily form alumina particles made of tabular alumina containing molybdenum, with a high α-crystallization rate, particularly an α-crystallization rate of 90% or more.

The above mechanism is merely speculative, and even if the effects of the present invention are achieved through a mechanism different from the above mechanism, it is still within the technical scope of the present invention.

The alumina particles utilize a molybdenum compound, which gives the alumina a high alpha crystal rate and an idiomorphic shape, allowing for excellent dispersibility in the matrix, mechanical strength, and high thermal conductivity.

In addition, the alumina particles obtained by the above manufacturing method have excellent dispersibility because the particles contain molybdenum, and therefore the isoelectric point of the zeta potential is shifted to the acidic side compared to normal alumina. Furthermore, by utilizing the properties of the molybdenum contained in the alumina particles, it may be possible to apply them to applications such as oxidation reaction catalysts and optical materials.

[Method of manufacturing alumina particles using the flux method]
The method for producing the alumina particles is not particularly limited, but from the viewpoint of being able to suitably control alumina having a high α crystallization rate at a relatively low temperature, a production method using a flux method utilizing a molybdenum compound can be preferably applied.

More specifically, a preferred method for producing alumina particles includes a step of calcining an aluminum compound in the presence of a molybdenum compound, at least one shape control agent selected from the group consisting of silicon, silicon compounds, and germanium compounds, and, if necessary, other shape control agents.

The present inventors have newly discovered that when using a flux method in which a molybdenum compound is used as a fluxing agent in combination with a shape control agent, and these are mixed with an aluminum compound and fired, the size of the raw aluminum compound, the amount of molybdenum compound used (and the amount of potassium compound used when a potassium compound is used as a fluxing agent), and the amount of shape control agent used are important factors for selectively producing alumina particles.

In the flux method, it is also preferable to use a molybdenum compound and a potassium compound as fluxing agents.
The compound containing molybdenum and potassium as a fluxing agent can be produced, for example, in the firing process using a molybdenum compound and a potassium compound, which are cheaper and easier to obtain, as raw materials. Here, the case where a molybdenum compound and a potassium compound are used as fluxing agents will be described as an example, combining the case where a molybdenum compound and a potassium compound are used as fluxing agents and the case where a compound containing molybdenum and potassium is used as fluxing agents.

In a method for producing alumina particles, in which a molybdenum compound is used as an essential fluxing agent and a shape control agent are used in combination, and these are mixed with an aluminum compound and fired, compared to a case in which only a molybdenum compound such as molybdenum trioxide is used, when a molybdenum compound and a potassium compound are used as fluxing agents, or when a compound containing molybdenum and potassium is used as a fluxing agent, the firing step is carried out in the presence of a compound containing molybdenum and potassium that is difficult to vaporize, so that the fluxing agent is not released outside the system and deterioration of the firing work environment is reduced, and further, since the compound containing molybdenum and potassium contained in the mixture of alumina particles and fluxing agent particles generated in the cooling step is often highly water-soluble, it becomes possible to remove more molybdenum from alumina more easily.
By using a molybdenum compound and a potassium compound as a fluxing agent, or by using a compound containing molybdenum and potassium as a fluxing agent, and by including the cooling step, it is possible to obtain alumina particles having a house-of-cards structure without the need for strong crushing, and furthermore, to very high yield of alumina particles having a house-of-cards structure. This is believed to be because, with such a configuration, the fluxing agent occupies the spaces between the alumina particles having a house-of-cards structure, so to speak, the fluxing agent acts as a spacer, preventing the particles from fusing together, and the fluxing agent can be easily removed in a post-treatment step.
From the viewpoint of preventing fusion between particles, the amount of the flux agent (the amount of the molybdenum compound and the potassium compound when the total amount of the raw materials converted into oxides is taken as 100 _mass %) is preferably 2 mass % or _more in terms of _Mo2K2O7 .

[Mixing process]
The mixing step is a step of mixing raw materials such as an aluminum compound, a molybdenum compound, a shape control agent, etc. to obtain a mixture. The mixture may further contain a potassium compound. The contents of the mixture will be described below.

(Aluminum Compounds)
The raw aluminum compound is a raw material for the alumina particles, and is not particularly limited as long as it becomes alumina by heat treatment. For example, aluminum chloride, aluminum sulfate, basic aluminum acetate, aluminum hydroxide, boehmite, pseudo-boehmite, transition alumina (γ-alumina, δ-alumina, θ-alumina, etc.), α-alumina, mixed alumina having two or more crystal phases, etc. can be used, and aluminum hydroxide and/or transition alumina are preferred.

The aluminum compound may be composed of only the aluminum compound, or may be a complex of the aluminum compound and an organic compound. For example, an organic/inorganic complex obtained by modifying an aluminum compound with an organic silane compound, an aluminum compound complex adsorbing a polymer, etc. can be suitably used. When using such complexes, the organic compound content is not particularly limited, since the organic components of the organic compound are burned off by firing. However, from the viewpoint of efficiently producing alumina particles having a house-of-cards structure, the content is preferably 60% by mass or less, and more preferably 30% by mass or less.

The specific surface area of the aluminum compound is not particularly limited. Since the molybdenum compound in the fluxing agent acts more effectively, a larger specific surface area is preferable, but by adjusting the firing conditions and the amount of molybdenum compound used, any specific surface area can be used as a raw material.

According to the flux method described in detail below, the shape of the alumina particles reflects the shape of the raw material aluminum compound. Any shape, such as spheres, amorphous, aspected structures (wires, fibers, ribbons, tubes, etc.), or sheets, can be used, but in terms of improving the fluidity of the powder, it is preferable to use spherical aluminum compounds, as the resulting alumina particles will be closer to spherical.

In addition, in the method for producing alumina particles from an aluminum compound, the average particle size of the alumina particles also basically reflects the particle size of the raw aluminum compound.

According to the flux method described below, it is assumed that during the firing process, the formation of plate-like alumina crystals mainly occurs within the raw aluminum compound particles, and three or more adjacent plate-like alumina crystals cross and adhere to each other, resulting in a house-of-cards structure. It is assumed that the average particle size of the resulting alumina particles having a house-of-cards structure mainly reflects the average particle size of the raw aluminum particles.

Therefore, when an aluminum compound with a smaller average particle size is used as a raw material, alumina particles with a smaller average particle size are more likely to be obtained, and when an aluminum compound with a larger average particle size is used, alumina particles with a larger average particle size are more likely to be obtained.

Since the alumina particles preferably have an average particle size of 3 μm or more and 1000 μm or less, it is advisable to use an aluminum compound having an average particle size that is the same or nearly the same as the alumina particles of the specific average particle size that is desired to be generated, within the above range.

Alumina particles having a house-of-cards structure can be obtained, for example, by forming tabular alumina in a method for producing alumina particles including a step of calcining an aluminum compound in the presence of a molybdenum compound, at least one shape control agent selected from silicon, silicon compounds, and germanium compounds, and, if necessary, other shape control agents, and by bringing three or more sheets of tabular alumina into contact with each other's crystal faces at multiple points at the same time as they are formed, intersecting and bonding them together. This bonding makes the house-of-cards structure not easily broken (unraveled) by external stress such as pressure, and provides a fixed state. For example, the flux conditions when the tabular alumina is formed affect the crushing strength of the resulting alumina particles having a house-of-cards structure.

The smaller the amount of molybdenum compound, the faster and more frequently three or more flat alumina plates will adhere to an aluminum compound particle, resulting in a strong house-of-cards structure with high crushing strength.

According to the findings of the present inventors focusing on the flux method, specifically, for example, when 1) an aluminum compound having an average particle size of 2 μm or more, particularly 4 μm or more, that corresponds to the particle size of the alumina particles to be obtained is used as the raw aluminum compound, 2) the amount of the molybdenum compound as the fluxing agent is 0.005 to 0.236 moles of molybdenum metal in the molybdenum compound per mole of aluminum metal in the aluminum compound, and 3) the amount of the silicon compound as the shape control agent is 0.003 to 0.09 moles of silicon metal in the silicon compound per mole of aluminum metal in the aluminum compound, it is preferable because alumina particles having a card-house structure with higher fluidity and higher crushing strength can be obtained.

In the flux method, a molybdenum compound and a potassium compound are used as fluxing agents, and silicon or a silicon compound is used in combination as a shape control agent. These are mixed with an aluminum compound and then fired. This is preferable because it allows selective production of alumina particles that are formed of three or more alumina plates within a specific average particle size range and have a card house structure in which the alumina plates are bonded together, by 1) using a raw aluminum compound with a specific average particle size, 2) limiting the amount of molybdenum compound and potassium compound used to a specific range, and 3) limiting the amount of silicon or a silicon compound used to a specific range.

In addition, the average particle size and shape of the alumina particles having a house-of-cards structure can be adjusted as desired through the crushing and classification processes described below.

(Molybdenum Compounds)
As described below, the molybdenum compound functions as a fluxing agent in the growth of α-crystals of alumina. The molybdenum compound is not particularly limited, but examples thereof include molybdenum oxide and compounds containing an acid radical anion (MoO _x ⁿ⁻ ) formed by bonding molybdenum metal with oxygen.

The compound containing the acid radical anion (MoO _x ⁿ⁻ ) is not particularly limited, but examples thereof include molybdic acid, sodium molybdate, potassium molybdate, lithium molybdate, H ₃ PMo ₁₂ O ₄₀ , H ₃ SiMo ₁₂ O ₄₀ , NH ₄ Mo ₇ O ₁₂ , and molybdenum disulfide.

The molybdenum compound may also contain sodium or silicon, in which case the molybdenum compound containing sodium or silicon acts as both a flux agent and a shape control agent.

Of the above molybdenum compounds, it is preferable to use molybdenum oxide from the viewpoint of cost. The above molybdenum compounds may be used alone or in combination of two or more kinds.

In addition, potassium molybdate (K _{2 Mon} O _3n+1 , n=1 to 3) contains potassium and can therefore function as a potassium compound, which will be described later. In the manufacturing method of the embodiment, the use of potassium molybdate as a fluxing agent is synonymous with the use of a molybdenum compound and a potassium compound as fluxing agents.

The amount of the molybdenum compound used is not particularly limited, but is preferably 0.005 to 0.236 moles of molybdenum metal in the molybdenum compound per mole of aluminum metal in the aluminum compound, more preferably 0.007 to 0.09 moles, and even more preferably 0.01 to 0.04 moles. When the amount of the molybdenum compound used is within the above range, it is preferable because alumina particles having a house-of-cards structure made of tabular alumina with a high aspect ratio and excellent dispersibility are easily obtained. In addition, when a molybdenum compound is used as a fluxing agent when the flux method is employed, the alumina particles contain molybdenum, and this evidence can be used to identify the manufacturing method by which the unknown alumina particles were manufactured.

When a molybdenum compound and a potassium compound are used as fluxing agents, the amount of the molybdenum compound used is not particularly limited, but the molar ratio of the molybdenum element of the molybdenum compound to the aluminum element of the aluminum compound (molybdenum element/aluminum element) is preferably 0.01 to 3.0, more preferably 0.1 to 1.0, and even more preferably 0.30 to 0.70 for good productivity and favorable crystal growth. When the amount of the molybdenum compound used is within the above range, it is preferable because alumina particles having a house-of-cards structure made of tabular alumina with a high aspect ratio and excellent dispersibility are easily obtained.

(Potassium compounds)
When a molybdenum compound and a potassium compound are used as a fluxing agent, the potassium compound is not particularly limited, but includes potassium chloride, potassium chlorite, potassium chlorate, potassium sulfate, potassium hydrogen sulfate, potassium sulfite, potassium hydrogen sulfite, potassium nitrate, potassium carbonate, potassium hydrogen carbonate, potassium acetate, potassium oxide, potassium bromide, potassium bromate, potassium hydroxide, potassium silicate, potassium phosphate, potassium hydrogen phosphate, potassium sulfide, potassium hydrogen sulfide, potassium molybdate, potassium tungstate, etc. In this case, the potassium compound includes isomers, as in the case of the molybdenum compound. Among these, it is preferable to use potassium carbonate, potassium hydrogen carbonate, potassium oxide, potassium hydroxide, potassium chloride, potassium sulfate, or potassium molybdate, and it is more preferable to use potassium carbonate, potassium hydrogen carbonate, potassium chloride, potassium sulfate, or potassium molybdate.

The above potassium compounds may be used alone or in combination of two or more.

As described above, potassium molybdate contains molybdenum and can therefore function as the molybdenum compound described above. In the manufacturing method of the embodiment, the use of potassium molybdate as a fluxing agent is synonymous with the use of a molybdenum compound and a potassium compound as fluxing agents.

As potassium compounds used when raw materials are charged or generated in a reaction during the heating process during calcination, water-soluble potassium compounds such as potassium molybdate do not vaporize even at the calcination temperature range and can be easily recovered by washing after calcination, which reduces the amount of molybdenum compounds released outside the calcination furnace and allows for a significant reduction in production costs.

When a molybdenum compound and a potassium compound are used as fluxing agents, the molar ratio of the molybdenum element of the molybdenum compound to the potassium element of the potassium compound (molybdenum element/potassium element) is preferably 5 or less, more preferably 0.01 to 3, and even more preferably 0.5 to 1.5, since this further reduces production costs. When the molar ratio (molybdenum element/potassium element) is within the above range, alumina particles of a preferred particle size can be obtained.

(Silicon or silicon compounds)
In the method for producing alumina particles, it is preferable to use silicon or a silicon compound as a shape control agent, since the resulting alumina particles have better fluidity, etc. Silicon or a silicon compound plays an important role in the growth of tabular crystals of alumina by firing an alumina compound in the presence of a molybdenum compound.

The silicon in the silicon compound selectively adsorbs to the [113] plane of the alumina α crystal, suppressing the selective adsorption of the fluxing agent molybdenum oxide to the [113] plane, forming a plate-like shape with a well-developed (001) or (006) plane and a close-packed hexagonal lattice crystal structure that is the most thermodynamically stable. It is presumed that the greater the amount of silicon, the more favorable the formation of crystals in the (001) or (006) plane will be, resulting in a thinner plate-like alumina.

In addition, silicon, when present in a sufficient amount to selectively adsorb to the [113] plane of the alumina α crystal, can suppress selective adsorption of molybdenum oxide to the [113] plane, forming a plate-like shape with a developed (001) or (006) plane and a thermodynamically most stable close-packed hexagonal lattice crystal structure. It is presumed that the greater the amount of silicon, the more likely it is that the intersections between the plate-like alumina will have the thermodynamically most stable close-packed hexagonal lattice crystal structure, as with other points, and can be firmly fixed. In other words, the more appropriately the amount of silicon is increased, the greater the crushing strength of the resulting alumina particles with a house-of-cards structure will be.

The type of silicon or silicon compound is not particularly limited, and any known silicon compound can be used as long as it is not only silicon atom.Specific examples of these include metal silicon (silicon atom), organic silane compounds, silicone resins, silica (SiO ₂ ) fine particles, artificially synthesized silicon compounds such as silica gel, mesoporous silica, SiC, and mullite; natural silicon compounds such as biosilica.Among these, it is preferable to use organic silane compounds, silicone resins, and silica fine particles from the viewpoint of forming a more uniform composite or mixture with aluminum compounds.The above-mentioned may be used alone or in combination of two or more kinds.

If the silicon compound is an organic silicon compound, the organic components are burned off by firing, becoming silicon atoms or inorganic silicon compounds, which are then contained in the alumina particles. If the silicon compound is an inorganic silicon compound, the silicon atoms or inorganic silicon compounds that do not decompose at the high temperatures of firing are left as they are and are contained locally on the surface of the tabular alumina by firing. From the above viewpoint, it is preferable to use silicon atoms and/or inorganic silicon compounds, which can increase the silicon atom content with a smaller amount at the same molecular weight.

The shape of the silicon or silicon compound is not particularly limited, and for example, spherical, amorphous, aspected structures (wires, fibers, ribbons, tubes, etc.), sheets, etc. can be suitably used.

The amount of silicon or the silicon compound used is not particularly limited, but it is preferable to use a sufficient amount that can be selectively adsorbed onto the [113] face of the α-crystal of alumina, and the amount of silicon metal in the silicon compound is preferably 0.003 to 0.09 mol, more preferably 0.005 to 0.04 mol, and even more preferably 0.007 to 0.03 mol, per mol of aluminum metal in the aluminum compound used as the raw material.
When a molybdenum compound and a potassium compound are used as a fluxing agent, the addition ratio of the silicon compound to the aluminum compound is preferably 0.01 to 10 mass%, more preferably 0.03 to 7 mass%, and even more preferably 0.03 to 3 mass%.
When the amount of silicon compound is within the above range, the aspect ratio of the tabular alumina is high, and it is preferable that the alumina particles have excellent dispersibility. When the amount of silicon compound is insufficient, the adsorption of the molybdenum oxide, which is the fluxing agent, to the [113] face is often not sufficiently suppressed, and the aspect ratio of the tabular alumina tends to be small and the tabular alumina tends to be non-uniform. Furthermore, when the amount of silicon compound is insufficient, the alumina particles produced tend to be polyhedral alumina, not a house of cards structure, which is not preferable. Also, when the amount of silicon compound is too large, the excess silicon becomes an oxide by itself, and other crystals other than alumina such as 3Al ₂ O _{3.2SiO 2} are included, which is not preferable.

As described above, silicon or a silicon compound may be added to the aluminum compound as desired, but may also be contained in the aluminum compound as an impurity.

In the above manufacturing method, there is no particular restriction on the method of adding silicon or silicon compounds, and it is possible to use a dry blend method in which silicon or silicon compounds are directly added and mixed as powder, mixing using a mixer, or a method in which silicon or silicon compounds are dispersed in a solvent or monomer beforehand and then added.

By undergoing a process of calcining an aluminum compound in the presence of a molybdenum compound and a silicon compound, alumina particles having a house-of-cards structure in which silicon atoms and/or inorganic silicon compounds are unevenly distributed on the surface of the tabular alumina and in its vicinity can be easily obtained. According to the findings of the present inventors, the use of a silicon compound during preparation is an important factor for easily obtaining a house-of-cards structure, while the presence of silicon atoms and/or inorganic silicon compounds unevenly distributed on the surface of the alumina particles produced by calcination and in its vicinity brings about a significant change in the surface condition of alumina, which originally has few active sites, and is an important factor that not only maximizes the excellent properties of alumina itself, but also makes it possible to impart an even better surface condition by integrating with a surface treatment agent through a reaction starting from the active sites.

(Germanium compounds)
A germanium compound may be used as a shape control agent in combination with silicon or a silicon compound, or in place of silicon or a silicon compound. The germanium compound plays an important role in the growth of tabular crystals of alumina by firing an alumina compound in the presence of a molybdenum compound.

The raw germanium compound used as the shape control agent is not particularly limited, and known compounds can be used. Specific examples of raw germanium compounds include germanium metal, germanium dioxide, germanium monoxide, germanium tetrachloride, and organic germanium compounds having a Ge-C bond. The raw germanium compounds may be used alone or in combination of two or more kinds. They may also be used in combination with other shape control agents as long as the effect of the present invention is not impaired.

The shape of the raw germanium compound is not particularly limited, and for example, spherical, amorphous, aspected structures (wires, fibers, ribbons, tubes, etc.), sheets, etc. can be suitably used.

The amount of germanium compound used is not particularly limited, but it is preferable that the amount of germanium metal in the germanium compound is 0.002 to 0.09 mol per 1 mol of aluminum metal in the aluminum compound used as the raw material, more preferably 0.004 to 0.04 mol, and even more preferably 0.005 to 0.03 mol.

(Other shape control agents)
In the alumina particles, other shape control agents than those mentioned above may be used as necessary, so long as they do not inhibit the formation of tabular alumina by at least one shape control agent selected from silicon, silicon compounds, and germanium compounds, in order to adjust the fluidity, dispersibility, mechanical strength, average particle size, aspect ratio of the tabular alumina, etc. The other shape control agents, like these, contribute to the growth of plate-like crystals of alumina by firing the alumina compound in the presence of a molybdenum compound.

The state in which the other shape control agent is present is not particularly limited as long as it can come into contact with the aluminum compound. For example, a physical mixture of the shape control agent and the aluminum compound, or a complex in which the shape control agent is present uniformly or locally on the surface or inside of the aluminum compound, etc. can be suitably used.

In addition, other shape control agents may be added to the aluminum compound as desired, but may also be contained in the aluminum compound as impurities.

There are no particular limitations on the method of adding other shape control agents, and they may be added by dry blending, directly adding the powder, mixing using a mixer, or dispersing the powder in a solvent or monomer beforehand.

As for the type of other shape control agent, similarly to at least one shape control agent selected from silicon, silicon compounds, and germanium compounds, there is no particular restriction as long as molybdenum oxide can suppress selective adsorption to the [113] face of α-alumina during high-temperature firing in the presence of a molybdenum compound and form a plate-like shape. It is preferable to use a metal compound other than a molybdenum compound and an aluminum compound, in that the aspect ratio of the plate-like alumina is higher, the alumina particles have better fluidity and dispersibility, and are more productive. Alternatively, it is more preferable to use sodium atoms and/or sodium compounds.

There are no particular limitations on the sodium atom and/or sodium compound, and known compounds may be used. Specific examples include sodium carbonate, sodium molybdenum, sodium oxide, sodium sulfate, sodium hydroxide, sodium nitrate, sodium chloride, metallic sodium, and the like. Of these, it is preferable to use sodium carbonate, sodium molybdate, sodium oxide, and sodium sulfate from the viewpoint of industrial ease of availability and ease of handling. Sodium or compounds containing sodium atoms may be used alone or in combination of two or more.

The shape of the sodium atom and/or sodium compound is not particularly limited, and for example, spherical, amorphous, aspected structures (wires, fibers, ribbons, tubes, etc.), sheets, etc. can be suitably used.

The amount of sodium atoms and/or sodium compounds used is not particularly limited, but is preferably 0.0001 to 2 moles, and more preferably 0.001 to 1 mole, of sodium metal per mole of aluminum metal in the aluminum compound. When the amount of sodium atoms and/or sodium compounds used is within the above range, it is preferable because alumina particles having a high aspect ratio and excellent dispersibility are easily obtained.

(Metal Compounds)
The metal compound can have a function of promoting the crystal growth of alumina, as described below. The metal compound can be used during firing as desired. Note that the metal compound has a function of promoting the crystal growth of α-alumina, and is therefore not essential for the production of alumina particles.

The metal compound is not particularly limited, but preferably contains at least one selected from the group consisting of Group II metal compounds and Group III metal compounds.

Examples of the Group II metal compounds include magnesium compounds, calcium compounds, strontium compounds, barium compounds, etc.

Examples of the Group III metal compounds include scandium compounds, yttrium compounds, lanthanum compounds, and cerium compounds.

The above-mentioned metal compounds refer to oxides, hydroxides, carbonates, and chlorides of metal elements. For example, yttrium compounds include yttrium oxide (Y ₂ O ₃ ), yttrium hydroxide, and yttrium carbonate. Of these, the metal compounds are preferably oxides of metal elements. These metal compounds include isomers.

Among these, metal compounds of period 3, period 4, period 5, and period 6 are preferred, metal compounds of period 4 and period 5 are more preferred, and metal compounds of period 5 are even more preferred. Specifically, magnesium compounds, calcium compounds, yttrium compounds, and lanthanum compounds are preferred, magnesium compounds, calcium compounds, and yttrium compounds are more preferred, and yttrium compounds are particularly preferred.

The addition rate of the metal compound is preferably 0.02 to 20 mass% relative to the mass equivalent value of aluminum atoms in the aluminum compound, and more preferably 0.1 to 20 mass%. When the addition rate of the metal compound is 0.02 mass% or more, the crystal growth of α-alumina containing molybdenum can proceed favorably, which is preferable. On the other hand, when the addition rate of the metal compound is 20 mass% or less, alumina particles with a low content of impurities derived from the metal compound can be obtained, which is preferable.

(yttrium)
When an aluminum compound is calcined in the presence of an yttrium compound as a metal compound, crystal growth proceeds more favorably in the calcination step, and α-alumina and a water-soluble yttrium compound are produced. At this time, since the water-soluble yttrium compound is likely to be localized on the surface of the α-alumina, which is an alumina particle, the yttrium compound can be removed from the alumina particles, if necessary, by washing with water, alkaline water, or a liquid obtained by warming these.

When a molybdenum compound is used as a fluxing agent, the amounts of the aluminum compound, molybdenum compound, and shape control agent used are not particularly limited. Examples of the amount of the compound containing molybdenum element converted into an oxide as molybdenum trioxide ( _MoO3 ) and the total amount of the raw material converted into an oxide is taken as 100 mass %, include firing the following mixture 1) or 2):
1-1) an aluminum compound containing 80 mass % or more of aluminum element calculated as Al ₂ O ₃ ;
A molybdenum compound having a content of 1.0% by mass or more in terms of _MoO3 ;
A silicon compound containing 0.4% by mass or more of silicon or silicon element in terms of _SiO2 ;
A mixture of the above.
1-2) an aluminum compound containing 80 mass % or more of aluminum element calculated as Al ₂ O ₃ ;
A molybdenum compound having a content of 1.0% by mass or more in terms of _MoO3 ;
A germanium compound having a content of 0.4% by mass or more in terms of _GeO2 ;
A mixture of the above.

By using the mixture of 1-1) or 1-2) above, alumina particles having a house-of-cards structure can be produced more efficiently.
A common phenomenon that occurs when the mixture of 1-1) or 1-2) above is fired is that the crystal growth proceeds while at least a part of the original shape of the aluminum compound used as the raw material is retained at the beginning of the crystal growth. As a result, it is considered that plate-like alumina is formed from each part of the raw aluminum compound as a starting point, and a card house structure is formed by three or more plate-like alumina sheets that are fixed to each other.

In the above 1-1), a silicon compound containing 0.4 mass% or more of silicon or silicon element calculated as _SiO2 is used, and it is considered that by using a relatively large proportion of this compound, deformation of the raw material aluminum compound is suppressed and the shape of the aluminum compound used as the raw material can be maintained.
In the above 2), a germanium compound of 0.4 mass% or less calculated as _GeO2 is used. By using a relatively large proportion of this germanium compound, it is believed that deformation of the raw material aluminum compound is suppressed and the shape of the raw material aluminum compound can be maintained.

In the above 1-1), in order to more easily produce alumina particles having a house-of-cards structure and excellent fluidity, it is preferable that the blending amounts of each raw material in the mixture are as follows, when the total amount of raw materials calculated as oxide is 100 mass %.
In the above 1-1), when the total amount of the raw materials calculated as oxides is taken as 100 mass%, the blending amount of the aluminum compound _calculated as _Al2O3 is preferably 80 mass% or more, more preferably 85 mass% or more and 99 mass% or less, and even more preferably 85 mass% or more and 95 mass% or less.
In the above 1-1), when the total amount of the raw materials calculated as oxides is taken as 100 mass%, the molybdenum compound is preferably 1.0 mass% or more, more preferably 2.0 mass% or more and 15 mass% or less, and even more preferably 4.0 mass% or more and 10 mass% or less, calculated as _MoO3 .
In the above 1-1), when the total amount of the raw materials calculated as oxides is taken as 100 mass%, the blending amount of _silicon or a silicon compound containing elemental silicon is preferably 0.4 mass% or more, more preferably 0.4 mass% or more and 5.0 mass% or less, and even more preferably 0.5 mass% or more and 2.0 mass% or less, calculated as SiO2.

In the above 1-2), in order to more easily produce alumina particles having a house-of-cards structure and excellent fluidity, it is preferable that the blending amounts of each raw material in the mixture are as follows, when the total amount of raw materials calculated as oxide is 100 mass %.
In the above 1-2), when the total amount of the raw materials calculated as oxides is taken as 100 mass%, the blending amount of the aluminum compound _calculated as _Al2O3 is preferably 80 mass% or more, more preferably 85 mass% or more and 99 mass% or less, and even more preferably 85 mass% or more and 95 mass% or less.
In the above 1-2), when the total amount of the raw materials calculated as oxides is taken as 100 mass%, the blending amount of the molybdenum compound, calculated as _MoO3 , is preferably 1.0 mass% or more, more preferably 2.0 mass% or more and 15 mass% or less, and even more preferably 4.0 mass% or more and 10 mass% or less.
In the above 1-2), when the total amount of the raw materials calculated as oxides is taken as 100 mass%, the amount of the germanium compound is preferably 0.4 mass% or more, more preferably 0.4 mass% or more and 5.0 mass% or less, and even more preferably 0.5 mass% or more and 2.0 mass% or less, calculated as _GeO2 .

When a molybdenum compound and a potassium compound are used as fluxing agents, the amounts of the above-mentioned aluminum compound, molybdenum compound, potassium compound, and shape control agent used are not particularly limited. Examples of the _{amount of the compound containing molybdenum and potassium, or the molybdenum compound containing molybdenum and the potassium compound containing potassium, converted into oxide form as potassium molybdate (Mo2K2O7 ) and} the total amount of the raw materials converted into oxide form is taken as 100 mass%, include firing the following mixture 1) or 2):
2-1) an aluminum compound containing 10% by mass or more of aluminum element calculated as Al ₂ O ₃ ;
50% by mass or more of the molybdenum compound and the _potassium compound calculated as _{Mo2K2O7 ;}
A silicon compound containing 0.3% by mass or more of silicon or silicon element in terms of _SiO2 ;
A mixture of the above.
2-2) an aluminum compound containing 50% by mass or more of aluminum element calculated as Al ₂ O ₃ ;
The molybdenum compound and the potassium compound _are present in an amount of 30 mass% or less in terms of _{Mo2K2O7 ;}
A silicon compound containing 0.01% by mass or more of silicon or silicon element in terms of _SiO2 ;
A mixture of the above.

By using the mixture of 2-1) or 2-2) above, alumina particles having a house-of-cards structure can be produced more efficiently.
A common phenomenon that occurs when the mixture of 2-1) or 2-2) above is fired is that the crystal growth proceeds while at least a part of the original shape of the aluminum compound used as the raw material is retained at the beginning of the crystal growth. As a result, it is considered that a card house structure is formed by three or more sheets of plate-like alumina, with each plate-like alumina starting from a part of the raw aluminum compound, and the plate-like alumina is fixed to each other.

In the above 2-1), a silicon compound containing 0.3 mass% or more of silicon or silicon element calculated as _SiO2 is used, and it is considered that by using a relatively large proportion of this compound, deformation of the raw material aluminum compound is suppressed and the shape of the aluminum compound used as the raw material can be maintained.
In the above-mentioned 2), the molybdenum compound and the _{potassium compound are used in an amount of 30 mass% or less calculated as Mo2K2O7 . By} using the molybdenum compound and the potassium compound in a relatively small proportion, it is considered that the deformation of the raw material aluminum compound is suppressed and the shape of the aluminum compound used as the raw material can be maintained.

In the above 2-1), in order to more easily produce alumina particles having a house of card structure and excellent fluidity, it is preferable that the blending amounts of each raw material in the mixture are as follows, when the total amount of raw materials calculated as oxide is 100 mass %.
In the above 2-1), when the total amount of the raw materials calculated as oxides is taken as 100 mass%, the blending amount of the aluminum compound, calculated as Al ₂ O ₃ , is preferably 10 mass% or more, more preferably 10 mass% or more and 70 mass% or less, even more preferably 20 mass% or more and 45 mass% or less, and particularly preferably 25 mass% or more and 40 mass% or less.
In the above 2-1), when the total amount of the raw materials calculated as oxides is taken as 100 mass%, the blending amount of the molybdenum compound and the _{potassium compound, calculated as Mo2K2O7 , is} preferably 50 mass% or more, more preferably 50 mass% or more and 80 mass% or less, even more preferably 55 mass% or more and 75 mass% or less, and even more preferably 60 mass% or more and 70 mass% or less.
In the above 2-1), when the total amount of the raw materials calculated as oxides is taken as 100 mass%, the blending amount of _silicon or a silicon compound containing elemental silicon is preferably 0.3 mass% or more, more preferably 0.3 mass% or more and 5 mass% or less, and even more preferably 0.4 mass% or more and 3 mass% or less, calculated as SiO2.

In the above 2-2), in order to more easily produce alumina particles having a house of card structure and excellent fluidity, it is preferable that the blending amounts of each raw material in the mixture are as follows, when the total amount of raw materials calculated as oxide is 100 mass %.
In the above 2-2), when the total amount of the raw materials calculated as oxides is taken as 100 mass%, the blending amount of the aluminum compound, calculated as Al ₂ O ₃ , is preferably 50 mass% or more, more preferably 50 mass% or more and 96 mass% or less, even more preferably 60 mass% or more and 95 mass% or less, and particularly preferably 70 mass% or more and 90 mass% or less.
In the above 2-2), when the total amount of the raw materials calculated as oxides is taken as 100 mass%, the blending amount of the molybdenum compound and the _{potassium compound, calculated as Mo2K2O7 , is} preferably 30 mass% or less, more preferably 2 mass% or more and 30 mass% or less, even more preferably 3 mass% or more and 25 mass% or less, and particularly preferably 4 mass% or more and 10 mass% or less.
In the above 2-2), when the total amount of the raw materials calculated as oxides is taken as 100 mass%, the blending amount of _silicon or a silicon compound containing elemental silicon is preferably 0.01 mass% or more, more preferably 0.01 mass% or more and 5 mass% or less, even more preferably 0.05 mass% or more and 3 mass% or less, and particularly preferably 0.15 mass% or more and 3 mass% or less, calculated as SiO2.

When the mixture further contains the above-mentioned yttrium compound, the amount of the yttrium compound used is not particularly limited, but preferably, when the total amount of the raw material calculated as oxide is 100 mass%, 5 mass% or less of the yttrium compound calculated as Y ₂ O ₃ can be mixed. More preferably, when the total amount of the raw material calculated as oxide is 100 mass%, 0.01 mass% or more and 3 mass% or less of the yttrium compound calculated as Y ₂ O ₃ can be mixed. In order to more suitably proceed with the crystal growth, more preferably, when the total amount of the raw material calculated as oxide is 100 mass%, 0.1 mass% or more and 1 mass% or less of the yttrium compound calculated as Y ₂ O ₃ can be mixed.

When a molybdenum compound and a potassium compound are used as fluxing agents, alumina particles having a house-of-cards structure in which silicon and/or germanium are unevenly distributed on the surface of tabular alumina and in its vicinity can be easily obtained by carrying out a process of calcining an aluminum compound in the presence of a molybdenum compound and a potassium compound, and at least one shape control agent selected from silicon, a silicon compound, and a germanium compound. According to the findings of the present inventors, the use of at least one shape control agent selected from silicon, a silicon compound, and a germanium compound during preparation is an important factor for easily obtaining a house-of-cards structure, while the presence of silicon and/or germanium unevenly distributed on the surface of alumina particles and in their vicinity produced by calcination brings about a significant change in the surface state of alumina, which is originally poor in active sites, and is an important factor that not only maximizes the excellent properties of alumina itself, but also enables the integration of the surface treatment agent through a reaction starting from the active sites to provide an even better surface state.

[Firing process]
The calcination step is preferably a step of calcining an aluminum compound in the presence of a molybdenum compound, at least one shape control agent selected from silicon, a silicon compound, and a germanium compound, and optionally other shape control agents. The calcination step may be a step of calcining the mixture obtained in the mixing step.

Alumina particles can be obtained, for example, by firing an aluminum compound in the presence of a molybdenum compound and a shape control agent. As mentioned above, this manufacturing method is called the flux method. Based on the above-mentioned flux method, it is presumed that the formation of tabular alumina and the formation of a house-of-cards structure involving the adhesion of three or more sheets of the above-mentioned tabular alumina proceed in parallel.

The flux method is classified as a solution method. More specifically, the flux method is a crystal growth method that utilizes the fact that the crystal-flux two-component phase diagram shows a eutectic type. The mechanism of the flux method is assumed to be as follows. That is, when a mixture of solute and flux is heated, the solute and flux become liquid phase. In this case, since the flux is a flux, in other words, since the solute-flux two-component phase diagram shows a eutectic type, the solute melts at a temperature lower than its melting point and forms a liquid phase. If the flux is evaporated in this state, the concentration of the flux decreases, in other words, the effect of the flux in lowering the melting point of the solute is reduced, and the evaporation of the flux becomes the driving force for the crystal growth of the solute (flux evaporation method). It is also a preferable method to grow crystals in a flux agent in the liquid phase, and the solute and flux can also cause crystal growth of the solute by cooling the liquid phase (slow cooling method).

The flux method has the advantages of allowing crystal growth at temperatures much lower than the melting point, allowing precise control of the crystal structure, and allowing the formation of idiomorphic crystals.

The mechanism of alumina particle production by the flux method using a molybdenum compound as a flux is not entirely clear, but it is presumed to be due to the following mechanism, for example. That is, when an aluminum compound is fired in the presence of a molybdenum compound, aluminum molybdate is first formed. At this time, as can be understood from the above explanation, the aluminum molybdate grows alumina crystals at a temperature lower than the melting point of alumina. Then, for example, by evaporating the flux, the aluminum molybdate decomposes, and the crystals grow, resulting in alumina particles. That is, the molybdenum compound functions as a flux, and alumina particles are produced via an intermediate called aluminum molybdate.

Here, when a potassium compound and a shape control agent are used in combination in the flux method, it becomes possible to produce alumina particles having a card house structure formed by three or more plate-like alumina sheets with high efficiency. More specifically, when a molybdenum compound and a potassium compound are used in combination, the molybdenum compound and the potassium compound react to form potassium molybdate. At the same time, the molybdenum compound reacts with an aluminum compound to form aluminum molybdate. Then, for example, aluminum molybdate decomposes in the presence of potassium molybdate, and crystals grow in the presence of a shape control agent, thereby obtaining alumina particles having a card house structure formed by three or more plate-like alumina sheets. That is, when alumina particles are produced via an intermediate called aluminum molybdate, if potassium molybdate is present, alumina particles having a card house structure formed by three or more plate-like alumina sheets are obtained.

As mentioned above, potassium or potassium compounds, such as potassium molybdate, act as fluxing agents.

The above mechanism is merely speculative, and even if the effects of the present invention are achieved through a mechanism different from the above mechanism, it is still within the technical scope of the present invention.

The above-mentioned potassium molybdate is not particularly limited in its constitution, but usually contains a molybdenum atom, a potassium atom, and an oxygen atom. The structural formula is preferably represented by K _{2 Mon} O _3n+1 . In this case, n is not particularly limited, but is preferably in the range of 1 to 3, since the alumina particle growth promotion functions effectively. In addition, potassium molybdate may contain other atoms, and examples of the other atoms include sodium, magnesium, silicon, etc.

In one embodiment of the present invention, the above-mentioned calcination may be performed in the presence of a metal compound. That is, the above-mentioned metal compound may be used in combination with a molybdenum compound and a potassium compound in the calcination. This allows alumina particles with better fluidity to be produced. Although the mechanism is not entirely clear, it is presumed to be due to, for example, the following mechanism. That is, the presence of a metal compound during the crystal growth of alumina particles prevents or suppresses the excessive formation of alumina crystal nuclei and/or promotes the diffusion of aluminum compounds necessary for the crystal growth of alumina, in other words, prevents the excessive generation of crystal nuclei and/or increases the diffusion rate of aluminum compounds, making it possible to more precisely control the direction of alumina crystal growth, making it easier to control the shape, such as by reflecting the shape of the precursor, and it is believed that alumina particles with higher fluidity can be obtained. Note that the above mechanism is merely presumed, and even if the effects of the present invention are obtained by a mechanism different from the above mechanism, it is within the technical scope of the present invention.

The firing method is not particularly limited, and can be any known, commonly used method. When the firing temperature exceeds 700°C, the aluminum compound reacts with the molybdenum compound to form aluminum molybdate. Furthermore, when the firing temperature reaches 900°C or higher, the aluminum molybdate decomposes and the shape control agent acts to form tabular alumina. Furthermore, tabular alumina can be obtained by incorporating molybdenum into aluminum oxide particles as aluminum molybdate decomposes to form alumina and molybdenum oxide.

In addition, the state of the aluminum compound, shape control agent, molybdenum compound, and potassium compound during firing is not particularly limited, and it is sufficient that the molybdenum compound, potassium compound, and shape control agent are present in such close proximity that they can act on the aluminum compound. Specifically, simple mixing in which powders of the molybdenum compound, shape control agent, and aluminum compound are mixed together, mechanical mixing using a grinder, or mixing using a mortar or the like may be used, and mixing may be performed in a dry or wet state.

There is no particular limitation on the firing temperature conditions, and the firing temperature is appropriately determined depending on the target average particle size, fluidity, dispersibility, aspect ratio of the tabular alumina, etc. Usually, the firing temperature is sufficient as long as the maximum temperature is 900° C. or higher, which is the decomposition temperature of aluminum molybdate (Al ₂ (MoO ₄ ) ₃ ).

In general, to control the shape of α-alumina obtained after firing, it is necessary to perform high-temperature firing at 2000°C or higher, which is close to the melting point of α-alumina. However, there are major issues with industrial use in terms of the burden on the firing furnace and fuel costs.

The above-mentioned preferred method for producing alumina particles can be carried out at high temperatures exceeding 2000°C, but even at temperatures below 1600°C, which is considerably lower than the melting point of α-alumina, it is possible to form alumina particles made of tabular alumina with a high α-crystallization rate and a high aspect ratio.

According to the preferred manufacturing method described above, even if the maximum firing temperature is 900°C to 1600°C, it is possible to easily, efficiently, and at low cost form alumina particles having a high aspect ratio of tabular alumina and an alpha crystallization rate of 90% or more. Firing at a maximum temperature of 920 to 1500°C is more preferable, and firing at a maximum temperature in the range of 950 to 1400°C is most preferable.

The higher the firing temperature, the more the alpha crystallization at the intersections of the tabular alumina improves, just like at other locations, resulting in alumina particles with a card-house structure that has excellent mechanical strength.

As for the firing time, it is preferable to raise the temperature to the specified maximum temperature in a range of 15 minutes to 10 hours, and to hold the temperature at the maximum firing temperature for a range of 5 minutes to 30 hours. To efficiently form tabular alumina, it is more preferable to hold the temperature at the firing time for about 10 minutes to 15 hours.

The longer the holding time at the maximum firing temperature, the more the alpha crystallization at the intersections of the tabular alumina improves, just like at other locations, resulting in alumina particles with a card-house structure that has excellent crushing strength.

The firing atmosphere is not particularly limited as long as the effects of the present invention can be obtained. For example, an oxygen-containing atmosphere such as air or oxygen, or an inert atmosphere such as nitrogen or argon is preferable, and an air atmosphere is more preferable when cost is taken into consideration.

The calcination device is not necessarily limited, and a so-called calcination furnace can be used. The calcination furnace is preferably made of a material that does not react with the sublimated molybdenum oxide, and it is preferable to use a highly airtight calcination furnace so that the molybdenum oxide is efficiently utilized. Examples of calcination furnaces that can be used in this case include tunnel furnaces, roller hearth furnaces, rotary kilns, and muffle furnaces.

In the preferred manufacturing method described above, alumina particles having a house-of-cards structure are selectively obtained, and a powder containing the alumina particles at a ratio of 60% or more of the total on a number basis can be easily obtained. By selecting more suitable conditions within the manufacturing method, it is possible to more easily obtain a powder containing alumina particles having a house-of-cards structure in which the three or more flat alumina particles are intersected and assembled at two or more points, and the intersecting flat plates are arranged in a disorderly manner in the surface direction, at a ratio of 80% or more of the total on a number basis.

[Cooling process]
When a molybdenum compound and a potassium compound are used as fluxing agents, the method for producing alumina particles may include a cooling step. The cooling step is a step of cooling the alumina crystal-grown in the firing step. More specifically, the cooling step may be a step of cooling a composition containing the alumina obtained by the firing step and the fluxing agent in a liquid phase.

The cooling rate is not particularly limited, but is preferably 1 to 1000°C/hour, more preferably 5 to 500°C/hour, and even more preferably 50 to 100°C/hour. A cooling rate of 1°C/hour or more is preferable because it can shorten the manufacturing time. On the other hand, a cooling rate of 1000°C/hour or less is preferable because the firing container is less likely to crack due to heat shock and can be used for a long time.

There are no particular limitations on the cooling method, and the material may be cooled naturally or using a cooling device.

[Post-processing process]
The method for producing alumina particles according to the embodiment may include a post-treatment step. The post-treatment step is a post-treatment step for alumina particles having a house-of-cards structure, and is a step for removing a fluxing agent. The post-treatment step may be performed after the above-mentioned firing step, after the above-mentioned cooling step, or after the firing step and the cooling step. In addition, the post-treatment step may be repeated two or more times as necessary.

Post-treatment methods include washing and high-temperature treatment, which may be combined.

The cleaning method is not particularly limited, but it can be removed by washing with water, an aqueous ammonia solution, an aqueous sodium hydroxide solution, or an acidic aqueous solution.

In this case, the molybdenum content can be controlled by appropriately changing the concentration and amount of water, ammonia water solution, sodium hydroxide water solution, and acidic water solution used, as well as the cleaning area and cleaning time.

Another high-temperature treatment method is to raise the temperature above the sublimation or boiling point of the flux.

[Crushing process]
In the fired product, the alumina particles may aggregate and may not satisfy the particle size range suitable for the embodiment, so the alumina particles may be pulverized as necessary to satisfy the particle size range suitable for the embodiment.

The method for grinding the fired product is not particularly limited, and any conventional grinding method such as a ball mill, jaw crusher, jet mill, disk mill, spectromill, grinder, or mixer mill can be used.

[Classification process]
The alumina particles are preferably subjected to classification treatment in order to adjust the average particle size and improve the flowability of the powder, or to suppress an increase in viscosity when mixed with a binder for forming a matrix.

Classification can be either wet or dry, but from the viewpoint of productivity, dry classification is preferred. Dry classification includes classification using a sieve, as well as wind classification, which classifies based on the difference between centrifugal force and fluid drag, but from the viewpoint of classification accuracy, wind classification is preferred, and can be performed using a classifier such as an air classifier that utilizes the Coanda effect, a swirling air classifier, a forced vortex centrifugal classifier, or a semi-free vortex centrifugal classifier.

The above-mentioned grinding and classification processes can be carried out at any stage required, including before or after the organic compound layer formation process described below. Depending on whether or not grinding and classification are carried out and the conditions for carrying out the grinding and classification, for example, the average particle size of the resulting alumina particles can be adjusted. The average particle size of the alumina particles is closely related to the angle of repose, and even if the manufacturing method and manufacturing conditions for the alumina particles themselves as described above are not sufficient for adjustment, the fluidity of the alumina particles can be adjusted by changing the average particle size of the alumina particles (indirectly changing the angle of repose) by selecting the conditions for classification, etc.

Specifically, for example, when alumina particles having a house-of-cards structure and a desired average particle size are not available, alumina particles having a larger average particle size can be classified, etc., to obtain alumina particles having a smaller average particle size and a house-of-cards structure that has better fluidity than known alumina particles having the same average particle size.

[Organic compound layer formation process]
In one embodiment, the above-mentioned method for producing alumina particles may further include an organic compound layer forming step of forming an organic compound layer on the surface of the tabular alumina. For example, the treatment is carried out at a temperature at which the organic compound does not decompose, usually after a baking step or after a post-treatment step.

The method for forming an organic compound layer on the surface of the tabular alumina of the alumina particles is not particularly limited, and any known method can be appropriately adopted. For example, a method in which a solution or dispersion containing an organic compound is brought into contact with alumina particles containing molybdenum and then dried can be mentioned.

An example of an organic compound that can be used to form this organic compound layer is an organic silane compound.

(Organosilane Compound)
When the alumina particles having a card house structure contain silicon atoms and/or inorganic silicon compounds, the above-mentioned surface modification effect can be expected compared to when they do not contain them, but they can also be used after being reacted with alumina particles containing silicon atoms and/or inorganic silicon compounds and organic silane compounds.Compared to the alumina particles containing silicon atoms and/or inorganic silicon compounds and having a card house structure, the alumina particles having a card house structure, which are the reaction product of the silicon atoms and/or inorganic silicon compounds and organic silane compounds, can have better affinity with the matrix based on the reaction between the silicon atoms and/or inorganic silicon compounds localized on the surface of the tabular alumina particles constituting the alumina particles and the organic silane compounds, and are preferable.

Examples of the organic silane compounds include alkyltrimethoxysilanes or alkyltrichlorosilanes having an alkyl group with 1 to 22 carbon atoms, such as methyltrimethoxysilane, dimethyldimethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, isopropyltrimethoxysilane, isopropyltriethoxysilane, pentyltrimethoxysilane, and hexyltrimethoxysilane, 3,3,3-trifluoropropyltrimethoxysilane, tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilanes, phenyltrimethoxysilane, phenyltriethoxysilane, p-chloromethylphenyltrimethoxysilane, p-chloromethylphenyltriethoxysilanes, and γ-glycidoxypropyltrimethoxysilane. Examples of the organic silane include epoxy silanes such as silane, γ-glycidoxypropyltriethoxysilane, and β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; amino silanes such as γ-aminopropyltriethoxysilane, N-β(aminoethyl)γ-aminopropyltrimethoxysilane, N-β(aminoethyl)γ-aminopropylmethyldimethoxysilane, γ-aminopropyltrimethoxysilane, and γ-ureidopropyltriethoxysilane; mercapto silanes such as 3-mercaptopropyltrimethoxysilane; p-styryltrimethoxysilane, vinyltrichlorosilane, vinyltris(β-methoxyethoxy)silane, vinyltrimethoxysilane, vinyltriethoxysilane, and γ-methacryloxypropyltrimethoxysilane; and polymeric epoxy, amino, and vinyl silanes. The organic silane compounds may be used alone or in combination of two or more.

The organic silane compound may be covalently bonded to at least some or all of the silicon atoms and/or inorganic silicon compounds on the surface of the tabular alumina of the alumina particles, and the alumina may be entirely or partially covered with the reactant. The method of providing the alumina surface may be attachment by immersion or chemical vapor deposition (CVD).

The amount of the organosilane compound used is preferably 20% by mass or less, and more preferably 10 to 0.01% by mass, based on silicon atoms, relative to the mass of silicon atoms or inorganic silicon compounds contained on the surface of the tabular alumina of the alumina particles. When the amount of the organosilane compound used is 20% by mass or less, it is preferable because it is easy to express the physical properties derived from the alumina particles.

The reaction between the alumina particles containing silicon atoms and/or inorganic silicon compounds and the organic silane compound can be carried out by a known and commonly used method for modifying the surface of fillers, such as a spray method using a fluid nozzle, a dry method using stirring with shear force, a ball mill, a mixer, or a wet method using a water-based or organic solvent-based system. It is desirable to carry out the treatment using shear force to the extent that the alumina particles used in the embodiment are not destroyed.

The system temperature in the dry method or the drying temperature after treatment in the wet method is appropriately determined according to the type of organosilane compound within the range where it does not thermally decompose. For example, when treating with the organosilane compounds described above, a temperature of 80 to 150°C is desirable.

[Post-processing process]
In the method for producing alumina particles, as long as the effect is not impaired, any step may be added during the production of the alumina particles, or a post-treatment step may be added to arbitrarily adjust the particle size, shape, etc. For example, a granulation step such as rolling granulation or compression granulation, or granulation by a spray-drying method using a binding agent as a binder may be mentioned, and the alumina particles can be easily obtained using commercially available equipment.

<Anti-corrosion paint>
The corrosion inhibitor of the present embodiment is preferably blended in the corrosion inhibitor. As one embodiment of the present invention, an anticorrosive paint containing the corrosion inhibitor of the embodiment (hereinafter, sometimes referred to as "anticorrosive paint of the embodiment" or simply "anticorrosive paint") can be provided. The anticorrosive paint is preferably used as an anticorrosive paint for steel structures such as vehicles, bridges, and plant facilities, an iron rust prevention paint, and particularly as an iron rust prevention paint for undercoat.

The anticorrosive coating of this embodiment may contain various optional components, if necessary, in addition to the alumina particles. Examples of the optional components include solvents, resins, drying accelerators, hardeners, hardening accelerators, thickeners, anti-settling agents, anti-cracking agents, anti-sagging agents, flow-extending agents, anti-foaming agents, UV absorbers, light stabilizers, and other pigments that do not fall under the category of the alumina particles.

As one embodiment of the present invention, there can be provided an anticorrosive coating material containing the alumina particles of the embodiment, a solvent, and a resin.
As one embodiment of the present invention, there can be provided an anticorrosive coating material containing the alumina particles of the embodiment, a solvent, a resin, and a drying accelerator.

From the viewpoint of reducing the viscosity of the anticorrosive coating material and facilitating application, it is preferable that the anticorrosive coating material contains a solvent. The solvent is capable of dispersing the alumina particles. The solvent is capable of dispersing or dissolving the optional components.
Examples of the solvent include alcohol-based solvents such as ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, and isobutyl alcohol; ester-based solvents such as ethyl acetate, propyl acetate, and butyl acetate, and ketone-based solvents such as methyl ethyl ketone, methyl butyl ketone, and cyclohexanone; ether ester-based solvents such as methyl cellosolve acetate and butyl cellosolve acetate; amide-based solvents such as dimethylformamide and dimethylacetamide; aromatic hydrocarbon-based solvents such as toluene and xylene; aliphatic hydrocarbon-based solvents such as hexane, heptane, and white spirits; mixed solvents of aromatic hydrocarbons and aliphatic hydrocarbons such as mineral spirits; water, mixtures thereof, and the like, but are not limited to these.

The solvent content relative to the total mass of the anticorrosive paint (100 mass%) may be, for example, 3 to 90 mass%, or 5 to 85 mass%.

The content of solids (components excluding volatile components such as solvents) relative to the total mass of the anticorrosive paint (100% by mass) may be, for example, 10 to 97% by mass, or 15 to 95% by mass.

From the viewpoint of bonding the alumina particles contained in the anticorrosive coating material together and increasing the adhesion between the substrate and the anticorrosive coating material, it is preferable that the anticorrosive coating material contains a resin.
As the resin, any resin that can be blended in a paint can be used, and a conventionally known resin may be used. Examples of the resin include acrylic resin, polyester resin, alkyd resin, urethane resin, epoxy resin, vinyl chloride-vinyl acetate copolymer resin, polyamide resin, melamine resin, etc. One type of resin may be used alone, or two or more types may be used in combination.
The content of the resin (solid content) in the anticorrosive coating may be 5 mass % or more, 10 to 97 mass %, or 30 to 80 mass % relative to 100 mass % of the total mass of the solid content of the anticorrosive coating (components excluding volatile components such as solvents).

From the viewpoint of effectively exerting the anticorrosion effect, the content of alumina particles in the anticorrosion paint may be 3% by mass or more, 3 to 95% by mass, 10 to 70% by mass, or 20 to 60% by mass relative to 100% by mass of the total mass of the solids content of the anticorrosion paint (components excluding volatile components such as solvents).

The pigment volume concentration (PVC) represents the ratio of the binder component to the pigment, and is a value expressed as pigment volume/pigment volume+binder component volume.
The critical pigment volume concentration (CPVC) is the point at which all the interstices between the pigment particles are filled with binder components.
The binder component includes the above-mentioned resins.

From the viewpoint of effectively exerting the anticorrosive effect and appropriately improving the strength of the coating film, the content of alumina particles in the anticorrosive paint may be, for example, 30 to 90 mass % or 50 to 80 mass % relative to 100 mass % of the alumina particle content at the critical pigment volume concentration (CPVC) when the alumina particles are used as a pigment.

Furthermore, from the viewpoint of effectively exerting the anticorrosive effect and appropriately improving the strength of the coating film, the mass ratio of p/b (pigment/binder component) may be, for example, 0.1 to 10, 0.5 to 2, or 0.7 to 1.5.

Examples of drying accelerators include cobalt desiccants, such as cobalt naphthenate, cobalt octenate, and cobalt stearate.

Examples of other pigments that do not fall under the category of alumina particles include various pigments such as color pigments and extender pigments.
As the coloring pigment, conventionally known pigments can be appropriately used depending on the desired hue and performance. Examples of the coloring pigment include organic pigments such as azo pigments, benzimidazolone pigments, isoindolinone-isoindoline pigments, phthalocyanine pigments, quinacridone pigments, dioxazine pigments, diketopyrrolopyrrole pigments, quinophthalone pigments, perylene-perinone pigments, thioindigo pigments, anthraquinone pigments, and threne pigments, and inorganic pigments such as iron oxide, carbon black, titanium oxide, and zinc sulfide. Examples of the extender pigments include barium sulfate, calcium carbonate, talc, and kaolin.
Furthermore, various metals such as zinc powder and aluminum powder or metal salts which can be used as anticorrosive pigments may be used in combination.

The anticorrosive coating of this embodiment can be produced, for example, by a method including mixing the above-mentioned alumina particles and, if necessary, the above-mentioned optional components. Examples of mixers that can be used in this process include a sand mill, homomixer, homogenizer, colloid mill, microfluidizer, sonolator, and cavitron.

<Laminate>
The substrate coated with the anticorrosive coating of the embodiment can be provided as a laminate having the substrate and a layer containing the anticorrosive coating of the embodiment. Such a laminate is useful in that anticorrosive properties are imparted to the substrate. An example of the layer containing the anticorrosive coating is a cured product of the anticorrosive coating of the embodiment. The cured product of the anticorrosive coating can be formed, for example, by drying at room temperature or curing by heating the coating of the anticorrosive coating applied to the substrate.

The dry film thickness of the layer containing the above-mentioned anticorrosive coating is, for example, preferably 50 to 700 μm, more preferably 60 to 300 μm. When the dry film thickness is within the above range, the anticorrosive effect is well exhibited.

The substrate is not particularly limited, but is preferably a metal or a metal-containing material, and more specifically, examples of the substrate include the above-mentioned vehicles, bridges, plant facilities, and other steel structures or parts thereof.

Methods
The anticorrosive coating material of the present embodiment can be used by applying it to a substrate. That is, in one embodiment, the present invention provides a method for producing a laminate, which includes applying the anticorrosive coating material to a substrate. According to the method for producing a laminate of the present embodiment, the laminate having anticorrosive properties can be produced.
The method of applying the anticorrosive coating is not particularly limited, and can be roll coating, spraying, brushing, spatula, bar coater, or dip coating. As a post-treatment method after coating, room temperature drying to heat curing can be performed. When heat curing is performed, the heating temperature is preferably in the range of 50 to 250°C, and more preferably in the range of 60 to 230°C. The heating time is preferably in the range of 2 to 30 minutes, and more preferably in the range of 5 to 20 minutes.

As described above, the anticorrosive coating of the present embodiment contains alumina particles, which improves the chemical stability and strength of the coating film, and the anticorrosive coating has excellent anticorrosive effect. The alumina particles of the embodiment having a card house structure have a larger specific surface area than spherical alumina particles of the same shape, and exhibit excellent anticorrosive effect.
In addition, since the anticorrosive paint of this embodiment contains alumina particles, the alumina particles dispersed in the coating film are expected to efficiently prevent the diffusion of corrosive components into the metal, thereby suppressing the corrosion of metals such as iron.
Furthermore, when the anticorrosive paint contains the above-mentioned resin, the resin provides a barrier effect to prevent corrosion.

The present invention will be described in detail below with examples and comparative examples, but the present invention is not limited to these examples.

<Synthesis of alumina particles>
[Synthesis Example 1] Synthesis of Alumina Particles 144.8 g of aluminum hydroxide (manufactured by Nippon Light Metals Co., Ltd., average particle size 40 μm), 0.95 g of silicon dioxide (manufactured by Kanto Chemical Co., Ltd., special grade), and 5 g of molybdenum trioxide (manufactured by Taiyo Koko Co., Ltd.) were mixed in a mortar to obtain a mixture. The obtained mixture was placed in a crucible and fired at 1100° C. for 10 hours in a ceramic electric furnace. After cooling, the crucible was removed to obtain 97.0 g of a light blue powder. The obtained powder was crushed in a mortar until it passed through a 106 μm sieve.
Next, 100 g of the obtained light blue powder was dispersed in 150 mL of 0.5% ammonia water, and the dispersion solution was stirred at room temperature (approximately 25°C to 30°C) for 0.5 hours, after which the ammonia water was removed by filtration, and the molybdenum remaining on the particle surface was removed by washing with water and drying, to obtain 94.0 g of powder. Then, fine particles were classified and removed using an air classifier utilizing the Coanda effect (Hyplec Classifier HPC-ZERO type manufactured by Powder Systems Co., Ltd.), to obtain 66 g of alumina particle powder. In addition, when the zeta potential was measured, it was found that the isoelectric point of the obtained alumina particles was pH 5.3.
The obtained powder was confirmed by SEM observation to be an alumina particle having a card house structure formed by three or more flat alumina sheets and fixed together. The average particle size of the obtained powder was measured to be 35 μm. It was also confirmed that the flat alumina itself constituting the card house structure had a polygonal plate shape, a thickness of 400 nm, an average particle size of 8 μm, and an aspect ratio of 20. Furthermore, when XRD measurement was performed, a sharp scattering peak derived from α-alumina appeared, and no alumina crystal system peak other than the α crystal structure was observed. The α conversion rate was 99% or more (almost 100%). Furthermore, the result of X-ray fluorescence quantitative analysis (XRF) showed that the obtained particles contained 0.71 mass% silicon in terms of silicon dioxide, 0.76 mass% molybdenum in terms of molybdenum trioxide, and the molar ratio of Si to Al [Si]/[Al] was 0.007.
In addition, the powder obtained was analyzed by X-ray photoelectron spectroscopy (XPS), and the surface composition [Si]/[Al] of the tabular alumina of the alumina particles was found to be 0.24, which was significantly higher than the [Si]/[Al]% value of the bulk composition determined by X-ray fluorescence quantitative analysis, confirming that silicon atoms and/or inorganic silicon compounds were unevenly distributed on the surface of the tabular alumina.
Furthermore, the formation of a mullite layer on the surface of the tabular alumina was confirmed by XRD measurement.
Furthermore, the specific surface area of the resulting powder was measured and found to be 1.4 (m ² /g), and the angle of repose was found to be 33°.
Furthermore, the crushing strength of the obtained powder was calculated using a microparticle crushing strength measuring device NS-A100 manufactured by Nano Seeds Corporation, and was found to be 28 MPa.

[Synthesis Example 2] Synthesis of Alumina Particles 144.8 g of aluminum hydroxide (manufactured by Nippon Light Metals Co., Ltd., average particle size 60 μm), 0.90 g of silicon dioxide (manufactured by Kanto Chemical Co., Ltd., special grade), and 5 g of molybdenum trioxide (manufactured by Taiyo Koko Co., Ltd.) were mixed in a mortar to obtain a mixture. The obtained mixture was placed in a crucible and fired at 1100° C. for 10 hours in a ceramic electric furnace. After cooling, the crucible was removed to obtain 96.0 g of a light blue powder. The obtained powder was crushed in a mortar until it passed through a 106 μm sieve.
Next, 100 g of the obtained light blue powder was dispersed in 150 mL of 0.5% ammonia water, and the dispersion solution was stirred at room temperature (25°C to 30°C) for 0.5 hours, after which the ammonia water was removed by filtration, and the molybdenum remaining on the particle surface was removed by washing with water and drying, to obtain 94.0 g of powder. Then, fine particles were classified and removed using an air classifier utilizing the Coanda effect (Hyplec Classifier HPC-ZERO type, manufactured by Powder Systems Co., Ltd.), to obtain 65 g of alumina particle powder. In addition, when the zeta potential was measured, it was found that the isoelectric point of the obtained alumina particles was pH 5.2.
The obtained powder was confirmed by SEM observation to be an alumina particle having a card house structure formed by three or more flat alumina sheets and fixed together. The average particle size of the obtained powder was measured to be 55 μm. It was also confirmed that the flat alumina itself constituting the card house structure had a polygonal plate shape, a thickness of 400 nm, a major axis of 9 μm, and an aspect ratio of 23. Furthermore, when XRD measurement was performed, a sharp scattering peak derived from α-alumina appeared, and no alumina crystal system peak other than the α crystal structure was observed. The α conversion rate was 99% or more (almost 100%). Furthermore, the result of X-ray fluorescence quantitative analysis (XRF) showed that the obtained particles contained 0.70 mass% silicon in terms of silicon dioxide, 0.73 mass% molybdenum in terms of molybdenum trioxide, and the molar ratio of Si to Al [Si]/[Al] was 0.007.
In addition, the powder obtained was analyzed by X-ray photoelectron spectroscopy (XPS), and the surface composition [Si]/[Al] of the tabular alumina of the alumina particles was found to be 0.24, which was significantly higher than the [Si]/[Al]% value of the bulk composition determined by X-ray fluorescence quantitative analysis, confirming that silicon atoms and/or inorganic silicon compounds were unevenly distributed on the surface of the tabular alumina.
Furthermore, the formation of a mullite layer on the surface of the tabular alumina was confirmed by XRD measurement.
Furthermore, the specific surface area of the resulting powder was measured and found to be 1.4 (m ² /g), and the angle of repose was 30°.
Furthermore, the crushing strength of the obtained powder was calculated using a microparticle crushing strength measuring device NS-A100 manufactured by Nano Seeds Corporation, and was found to be 29 MPa.

<Evaluation of alumina particles obtained in synthesis examples and commercially available spherical alumina particles>
[Analysis of the shape of alumina particles using a scanning electron microscope]
The alumina particles obtained in the above synthesis examples and commercially available spherical alumina particles (DAS-30, manufactured by Denka Co., Ltd.) were used as samples, which were fixed to a sample support stand with double-sided tape and observed with a surface observation device (VE-9800, manufactured by Keyence Corporation) to confirm the presence or absence of the card house structure of the alumina particles.

[Measurement of the major axis L of the plate-like alumina]
The long diameter L of the tabular alumina was calculated as the arithmetic average value of the maximum length between two points on the outline of the plate for any 100 tabular alumina particles located at the center of the alumina particle, measured using a scanning electron microscope (SEM).

[Measurement of thickness D of plate-like alumina]
The thickness of 10 pieces was measured using a scanning electron microscope (SEM), and the average value was used as the thickness D (μm).

[Aspect ratio L/D of plate-like alumina]
The aspect ratio was calculated using the following formula.
(Aspect ratio)=(long diameter L of plate-like alumina / thickness D of plate-like alumina)

[Measurement of average particle size of alumina particles by particle size distribution measurement]
The prepared sample was measured using a laser diffraction type dry particle size distribution analyzer (HELOS (H3355) & RODOS, manufactured by Japan Laser Corporation) under conditions of a dispersion pressure of 0.3 MPa and a suction pressure of 90 hPa, and _D50 (μm) was calculated from the volume-based cumulative particle size distribution, which was defined as the average particle size of the alumina particles.

[Measurement of angle of repose]
A 300 g sample was prepared, and the angle of repose of the sample was measured according to the method of JIS R9301-2-2.

[Measurement of specific surface area]
The prepared samples were pretreated at 300° C. for 3 hours, and the specific surface area of the pretreated samples was measured using a TriStar 3000 manufactured by Micromeritics.

[XRD peak intensity ratio and analysis of the presence or absence of mullite]
The prepared sample was placed on a measurement sample holder with a depth of 0.5 mm, and packed flat with a constant load. The sample was then set in a wide-angle X-ray diffraction (XRD) device (Ultima IV, manufactured by Rigaku Corporation) and measured under the conditions of Cu/Kα radiation, 40 kV/40 mA, a scan speed of 2 degrees/min, and a scan range of 10 to 70 degrees.
The peak height of mullite observed at 2θ = 26.2 ± 0.2 degrees was designated as A, the peak height of the (104) plane α-alumina observed at 2θ = 35.1 ± 0.2 degrees was designated as B, and the baseline value at 2θ = 30 ± 0.2 degrees was designated as C. The presence or absence of mullite was determined by the following formula.
A value of 0.02 or more was judged as "present" for mullite, and a value of less than 0.02 was judged as "absent" for mullite.

[Si content in plate-like alumina surface layer]
The prepared sample was press-fixed onto double-sided tape and subjected to composition analysis under the following conditions using an X-ray photoelectron spectroscopy (XPS) apparatus Quantera SXM (ULVAC-PHI, Inc.).

(Measurement conditions)
X-ray source: Monochromatic AlKα, beam diameter 100 μmφ, output 25 W Measurement: Area measurement (1000 μm square), n = 3
Charge correction: C1s = 284.8 eV

The amount of Si in the surface layer of the tabular alumina particles was determined as [Si]/[Al] based on the results of XPS analysis.

[Analysis of Alpha Conversion Rate]
The prepared sample was placed on a measurement sample holder with a depth of 0.5 mm, packed flat under a constant load, and set in a wide-angle X-ray diffractometer (Ultima IV, manufactured by Rigaku Corporation) and measured under conditions of Cu/Kα radiation, 40 kV/40 mA, scan speed of 2 degrees/min, and scan range of 10 to 70 degrees. The α conversion rate was calculated from the ratio of the strongest peak heights of α-alumina and transition alumina.

[Analysis of Si content in alumina particles]
Using a Primus IV X-ray fluorescence (XRF) analyzer (manufactured by Rigaku Corporation), about 70 mg of the prepared sample was placed on filter paper, covered with a PP film, and subjected to composition analysis.
The [Si]/[Al] ratio determined from the results of the XRF analysis was taken as the amount of Si in the alumina particles.
The amount of silicon determined from the results of XRF analysis was calculated in terms of silicon dioxide (mass %) relative to 100 mass % of the alumina particles.

[Analysis of Mo content in alumina particles]
About 70 mg of the prepared sample was placed on filter paper and covered with a PP film, and a composition analysis was performed using a Primus IV fluorescent X-ray analyzer (manufactured by Rigaku Corporation).
The amount of molybdenum determined from the results of the XRF analysis was calculated in terms of molybdenum trioxide (mass %) relative to 100 mass % of the alumina particles.

[Measurement of crushing strength]
The crushing strength was measured using a microparticle crushing force measuring device NS-A100 manufactured by Nanoseeds Co., Ltd. The difference between the peak value at the time of crushing and the baseline (when no force is applied) was taken as the crushing force F [N], and the crushing strength S [Pa] was taken as the average value of 10 values calculated using the following formula.
S=2.8F/(π・D ² )
In the above formula, F is the crushing force [N] and D is the particle diameter [m].

[Measurement of isoelectric point]
Zeta potential measurements were performed using a zeta potential measuring device (Malvern Instruments, Zetasizer Nano ZSP). 20 mg of sample and 10 mL of 10 mM KCl aqueous solution were stirred for 3 minutes in a stirring/defoaming mode using a foam remover (Thinky Corporation, ARE-310), and the supernatant was left to stand for 5 minutes to be used as the measurement sample. Using an automatic titration device, 0.1 N HCl was added to the sample, and zeta potential measurements were performed in the range up to pH = 2 (applied voltage 100 V, Monomodl mode), and the pH of the isoelectric point where the potential becomes zero was evaluated.

The oxide equivalent composition of the raw material compounds (total is 100 mass%) and the evaluation results of each alumina particle obtained in the synthesis example are shown in Table 1 below. In Table 1, "N.D." stands for "not detected" and indicates that it was not detected.

<Preparation of anti-corrosion paint>
[Examples 1-2, Comparative Examples 1-2]
An alumina pigment shown in Table 2 was added to a modified alkyd resin (product name "Beckosol (registered trademark) 11-035", solid content 50 mass %) so that the CPVC (critical pigment volume concentration) was 75%. Next, a cobalt drier was added in an amount of 2 mass % of the paint before dilution described below, and then the paint was diluted with xylene to a concentration suitable for spraying to obtain an anticorrosion paint.

<Evaluation of rust prevention>
The corrosion-resistant coatings of Examples 1 and 2 and Comparative Examples 1 and 2 were evaluated for rust prevention as follows. A cold-rolled steel plate was prepared as a test panel. The test panel was washed with acetone and then left to stand for 1 hour. Prior to application of the corrosion-resistant coating, the back surface of the test panel was coated with a clear coat of automotive grade (trade name "2K Topcaots"). Each of the corrosion-resistant coatings of Examples 1 and 2 and Comparative Examples 1 and 2 was then spray-coated onto the test panel to form a dry film having a film thickness of 0.1524 mm. The film was cured at room temperature for 72 hours.

A water bath containing deionized water containing 0.5% sodium chloride was prepared and kept at 30°C. A portion of the panel sprayed with the above anticorrosive paint was submerged in the water bath for 9 days and observed. Photographs of the panel were taken on each of the 1st to 9th days (except for the 4th and 5th days).

The evaluation results are shown in Table 2.
◯: No rust was observed on the test panel. ×: Rust was observed on 50% or more of the immersed area of the test panel.

From Table 2, it was confirmed that the anticorrosive paints of Examples 1 and 2 had better rust prevention properties than the anticorrosive paints of Comparative Examples 1 and 2.

The configurations and combinations thereof in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications of the configurations are possible without departing from the spirit of the present invention. Furthermore, the present invention is not limited to each embodiment, but is limited only by the scope of the claims.

The corrosion inhibitor of this embodiment can provide an inhibitor with good corrosion prevention effect.

Claims (8)

The alumina powder is formed of three or more sheets of tabular alumina, has a card house structure in which the tabular alumina particles are fixed together, and contains alumina particles having an average particle size of 3 μm or more and 1000 μm or less.
The alumina particles comprise molybdenum . The corrosion inhibitor according to claim 1, wherein the alumina particles contain silicon and/or germanium. 2. The corrosion inhibitor according to claim 1 , wherein the content of molybdenum is 10% by mass or less, calculated as molybdenum trioxide, relative to 100% by mass of the alumina particles. An anticorrosive paint comprising the anticorrosive agent according to any one of claims 1 to 3 . A laminate comprising a substrate and a layer containing the anticorrosive coating according to claim 4 . The laminate of claim 5 , wherein the substrate is a metal. A method for producing a laminate, comprising applying the anticorrosive coating according to claim 4 to a substrate. A method for protecting a substrate from corrosion, comprising applying the anticorrosive coating material according to claim 4 to the substrate.

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