JP7769372B2 - Scaly boehmite aggregate and method for producing the same - Google Patents

Description

The present invention relates to micro-sized scaly boehmite aggregates having a dense house-of-card structure formed by the aggregation of scaly boehmite crystals, and a method for producing the same.

Boehmite, alumina monohydrate (AlOOH), is highly versatile and is used as a filler, reinforcing material, flame retardant, luminescent material, fireproof material, thickener, etc. It is also used as a catalyst carrier, an electrically conductive filler base material, a refractory material, a raw material for high-purity alumina, a raw material for sinterable alumina, and a raw material for fluorescent materials. Boehmite can be produced with controlled morphology, and boehmite crystals come in a variety of shapes, including cubes, plates, hexagonal plates, discs, needles, and scales. Depending on the production method, boehmite can also be produced as agglomerates of boehmite crystals, which can be used for a variety of purposes, just like boehmite particles in which boehmite crystals are dispersed.

Previously, there have been reports on boehmite aggregates (Non-Patent Document 1). Specifically, Non-Patent Document 1 discloses that flower-like boehmite aggregates can be obtained by adding glucose surfactant to sodium aluminate solution (Bayer's solution) and stirring it at 70°C for 60 minutes (aging), then neutralizing the pH from 11.9 to 9.5 using 1M sulfuric acid. (Abstract, page 168, bottom right column)

Processing and Application of Ceramics 14〔2〕(2020)168-172

However, the boehmite flower-like aggregates disclosed in Non-Patent Document 1 are nano-sized aggregates of boehmite with a size of approximately 100 nm to 200 nm (left column on page 170), a specific surface area of 293.6 to 331.5 ^{m 2} /g, a pore diameter of 3.5 to 3.9 nm, and a pore volume of 0.258 to 0.308 ^{m 3} /g (Table 2 on page 171), and low crystallinity. Therefore, the boehmite flower-like aggregates have problems such as low thermal conductivity, high hygroscopicity, and a lower dehydration temperature than highly crystalline boehmite, making them unsuitable as a flame retardant. Furthermore, Non-Patent Document 1 neither describes nor suggests that the boehmite flower-like aggregates have a dense house-of-card structure.

The present invention has been developed in consideration of the above circumstances, and aims to provide micro-sized scaly boehmite aggregates having a dense house-of-card structure in which highly crystalline scaly boehmite crystals aggregate together, and a method for producing the same.

In order to solve the above problems, the inventors of the present invention have conducted extensive research and have come up with the present invention.
That is, the invention described in claim 1 relates to a scaly boehmite aggregate having a dense card-house structure in which scaly boehmite crystals are aggregated together, characterized in that the scaly boehmite aggregate has an oil absorption of refined linseed oil of 180 g/100 g or less as measured in accordance with the refined linseed oil method of JIS K5101-13-1 (2004), and has an on-plane thermal conductivity of an epoxy resin composition of 0.94 W/m K or more as measured by a hot wire method when filled into an epoxy resin up to the kneading limit.

The invention described in claim 2 may be the invention described in claim 1, wherein the coefficient of variation CV calculated using the following formula may be 40% or less.
Standard deviation SD = (D84-D16)/2 (Formula 1)
Coefficient of variation CV=(standard deviation SD/D50)×100 (Equation 2)

The invention described in claim 3 may be the invention described in claim 1 or claim 2, wherein the 1% weight loss temperature, which is the temperature at which a weight loss of 1 wt% is confirmed when heated at a heating rate of 30°C/min, with a weight loss rate of 0 wt% at 100°C in thermogravimetric analysis, is 420°C or higher.

The invention described in claim 4 relates to a method for producing the scaly boehmite aggregate described in any one of claims 1 to 3, characterized in that an aqueous suspension containing aluminum hydroxide having an average particle size (volume basis) of 4 to 20 μm as measured by a laser diffraction/scattering method and one additive selected from the group consisting of sodium hydroxide, a mixture of sodium hydroxide and sodium carbonate, and a mixture of sodium hydroxide and sodium phosphate is subjected to hydrothermal treatment while being stirred.

The invention described in claim 5 is the invention described in claim 4, wherein the concentration of aluminum hydroxide in water is 1 to 20 wt %, the concentration of the additive in water is 0.05 to 2.00 mol/L, and the constant temperature of the hydrothermal treatment is 150 to 250°C.

The scaly boehmite aggregates of the present invention have a dense card-house structure, making them excellent for filling into fillers such as resins, and can impart isotropic thermal conductivity to the filler, thereby making it easier to utilize the properties of fillers such as resins and contributing to weight reduction of fillers such as resins.

The scaly boehmite aggregates of the present invention have little variation in particle size and are uniform in particle size, which helps reduce variations in the properties of the final product caused by variations in particle size. For example, when used as a filler, it is possible to reduce variations in the properties of filled materials such as resins. When used as a paint, it is possible to reduce variations in the properties of coating fluids. Furthermore, when used as a raw material for ceramics, it is possible to prevent variations in properties caused by poor sintering due to uneven particle size.

The method for producing scaly boehmite aggregates of the present invention directly synthesizes scaly boehmite aggregates without using an organic binder, so no organic binder remains in the aggregates. Common organic binders have the problem of not being highly weather-resistant and deteriorating, which can make it difficult to maintain the aggregates over the long term. However, the present invention makes it possible to produce highly weather-resistant aggregates made from scaly boehmite.

1 is an SEM photograph of the scaly boehmite aggregate of Example 1. 1 is an SEM photograph of scaly boehmite particles of Comparative Example 1. 1 is an SEM photograph of the scaly boehmite aggregate of Example 2. 1 is an SEM photograph of a scaly boehmite aggregate of Example 3. 1 is an SEM photograph of the scaly boehmite aggregate of Example 4. 1 is an SEM photograph of scaly boehmite particles of Comparative Example 2. 1 is an SEM photograph of the scaly boehmite aggregate of Example 5. 1 is an SEM photograph of scaly boehmite particles of Comparative Example 3. 1 is an SEM photograph of a scaly boehmite aggregate of Comparative Example 4. 1 is an SEM photograph of a cross section of a scaly boehmite aggregate of Example 2. 1 is an SEM photograph of a cross section of a scaly boehmite aggregate of Comparative Example 4.

The scaly boehmite aggregate of the present invention has a dense card-house structure in which scaly boehmite crystals are aggregated together. The oil absorption of the refined linseed oil, measured in accordance with the refined linseed oil method of JIS K5101-13-1 (2004), which reflects the volume of the voids in the card-house structure, is preferably 180 g/100 g or less, and more preferably 170 g/100 g or less. The lower limit of the oil absorption of the refined linseed oil may be preferably 80 g/100 g, and more preferably 90 g/100 g.

The scaly boehmite aggregate of the present invention has a dense card-house structure with a small void volume, but the oil absorption is higher than that of scaly boehmite particles with few voids in which scaly boehmite crystals are dispersed. On the other hand, the oil absorption of the scaly boehmite aggregate of the present invention is lower than that of scaly boehmite aggregates with a bulky card-house structure in which scaly boehmite crystals are aggregated together.

The scaly boehmite aggregate of the present invention preferably has a thermal conductivity of 0.94 W/m·K or more, and more preferably 0.96 W/m·K or more, on the surface of an epoxy resin composition when filled into an epoxy resin to the kneading limit, as measured by the hot wire method. Because the scaly boehmite aggregate of the present invention has a dense card-house structure, it is easy to fill into a filling material and can impart high thermal conductivity to the filling material.

The scaly boehmite aggregate of the present invention preferably has a coefficient of variation CV calculated using the following formula of 40% or less, more preferably 38% or less.
Standard deviation SD = (D84-D16)/2 (Formula 1)
Coefficient of variation CV=(standard deviation SD/D50)×100 (Equation 2)
The scaly boehmite aggregates of the present invention have little variation in particle size, that is, particles with uniform size. Furthermore, the scaly boehmite aggregates of the present invention have little variation in particle size compared to scaly boehmite particles in which boehmite crystals are dispersed.

The scaly boehmite constituting the scaly boehmite aggregate of the present invention is micro-sized boehmite having a specific surface area of several m ² /g and high crystallinity.

The 1% weight loss temperature of the scaly boehmite aggregate of the present invention is preferably 420° C. or higher. Here, the 1% weight loss temperature is defined as the temperature at which a weight loss of 1 wt % is confirmed when heated at a temperature increase rate of 30° C./min in thermogravimetric analysis, with the weight loss rate at 100° C. being 0 wt %.
When heated, boehmite undergoes a dehydration reaction, transforming its crystalline phase into γ-alumina. When using boehmite as a flame retardant, the higher the onset temperature of the dehydration reaction, the better. This tends to decrease with finer boehmite particles or lower crystallinity, resulting in a lower 1% weight loss temperature. Nano-sized boehmite has a 1% weight loss temperature of approximately 300°C, which is too low for boehmite to be used in plastic materials, which require high temperatures for molding, a major application of boehmite as a flame retardant. Furthermore, the method for producing scaly boehmite aggregates described below does not involve forming aggregates using an organic binder such as polyvinyl alcohol, but rather directly synthesizes the scaly boehmite aggregates. Using an organic binder, the organic binder remaining in the product decomposes during thermal analysis, resulting in a weight loss and a lower 1% weight loss temperature.
That is, the scaly boehmite aggregates of the present invention are micro-sized and highly crystalline, and since no organic binder is used in their production, a 1% weight loss temperature of 420°C or higher can be ensured.

The tap density of the scaly boehmite aggregate of the present invention is preferably 0.22 g/cm ^or more, and more preferably 0.24 g/cm or ^more . The tap density of the scaly boehmite aggregate of the present invention is higher than the tap density of scaly boehmite particles in which scaly boehmite crystals are dispersed. Therefore, while scaly boehmite particles in which scaly boehmite crystals are dispersed and have a low tap density, tend to scatter, the scaly boehmite aggregate of the present invention is less likely to scatter and has excellent handleability.

Next, a method for producing the scaly boehmite aggregate of the present invention will be described.
The scaly boehmite aggregate of the present invention can be obtained by subjecting an aqueous suspension containing aluminum hydroxide as a raw material and an additive to hydrothermal treatment while stirring.

The aluminum hydroxide raw material preferably has an average particle size (volume basis) of 4 to 20 μm, more preferably 5 to 15 μm, as measured by laser diffraction/scattering. This is because if the average particle size of the aluminum hydroxide is less than 4 μm, the bulky card-house structure in which the scaly boehmite crystals aggregate cannot be formed. On the other hand, if the average particle size exceeds 20 μm, sedimentation is likely to occur during the reaction, which may result in damage to the hydrothermal treatment device, such as clogging of piping with reaction products.
The concentration of aluminum hydroxide in water is preferably 1 to 20 wt %, more preferably 2 to 17 wt %. If the concentration is less than 1 wt %, the amount of boehmite produced is small, which is uneconomical, and if the concentration is more than 20 wt %, the viscosity increases during synthesis, which makes poor stirring more likely to occur.

Furthermore, it is preferable that the particle size of the aluminum hydroxide used as the raw material is highly uniform. That is, it is preferable that the particle size distribution of the aluminum hydroxide exhibits a normal distribution or a unimodal distribution close to the normal distribution. This is because the more uniform the particle size of the aluminum hydroxide, the more reliably it is possible to obtain scaly boehmite aggregates having a dense house-of-card structure.
Furthermore, the raw material aluminum hydroxide preferably has primary particles having an average length of 2 to 8 μm as measured from an SEM image, and is in the form of secondary particles formed by aggregation of these particles.
If the average size of the primary particles is smaller than 2 μm, it is difficult to form a card-house structure in which the scaly boehmite crystals aggregate together, and if it is larger than 8 μm, the center of the card-house structure becomes hollow, making the aggregate brittle.

The additive is preferably any one selected from sodium hydroxide, a mixture of sodium hydroxide and sodium carbonate, and a mixture of sodium hydroxide and sodium phosphate. The concentration of the additive in water is preferably 0.05 to 2.00 mol/L, and more preferably 0.10 to 1.00 mol/L. If the concentration is below 0.05 mol/L, it is difficult to form scaly boehmite, and if the concentration is above 2.00 mol/L, the high pH will cause the produced boehmite to dissolve, reducing the amount that can be recovered.

The water used to prepare the suspension may be hard or soft water, but soft water, which is less affected by magnesium ions and calcium ions, is preferred.

The constant temperature for the hydrothermal treatment is preferably 150 to 250°C, more preferably 160 to 230°C.
If the temperature is below 150°C, the reaction from aluminum hydroxide to boehmite does not proceed easily, and if the temperature is above 250°C, expensive equipment that can withstand high pressures is required.
The reaction time is preferably in the range of 3 to 24 hours. If it is less than 3 hours, scaly boehmite aggregates may not be obtained. If it exceeds 24 hours, no particular effect is obtained and it is uneconomical in terms of energy.
The rate of temperature rise to a constant temperature is preferably 100° C./hour or less. If the rate is not 100° C./hour or less, the temperature inside the reaction vessel is likely to vary, making it difficult for the reaction to proceed uniformly.
The pressure for the hydrothermal treatment is preferably the pressure that occurs naturally at a constant temperature, and no special pressure is required.

The stirring blade tip speed (peripheral speed) in the hydrothermal treatment is preferably 0.4 to 4.0 m/sec, more preferably 0.5 to 3.0 m/sec. This is because if the speed is less than 0.4 m/sec, the raw materials are likely to settle and it becomes difficult for a uniform reaction to proceed, and if the speed is more than 4.0 m/sec, an expensive motor capable of high-speed stirring is required.
The blade tip speed (circumferential speed) can be calculated using the following formula:
V = π × D × N/60 (Formula 3)
(V: blade tip speed (m/s), π: Pi, D: blade diameter (m), N: rotation speed (rpm))

Next, the present invention will be explained using examples, but the present invention is not limited to the following examples.

Example 1
59 g of sodium hydroxide (Kanto Denka Kogyo Co., Ltd.) was added to 7000 g of soft water and stirred until a transparent solution was obtained. 700 g of aluminum hydroxide (grade name: BF083, average particle size (laser diffraction/scattering method): 10 μm, average primary particle size (30-point length measurement from SEM image): 6.5 μm, Nippon Light Metal Co., Ltd.) was added and stirred thoroughly to prepare an aqueous suspension. This aqueous suspension was placed in a stirred autoclave (volume: 10 L) and hydrothermally treated at 200 °C for 12 hours while stirring at 1.65 m/sec (impeller diameter: 0.195 m, 162 rpm). The temperature was raised from room temperature (25 °C) to 186 °C over 2 hours, and then from 186 °C to 200 °C over 0.5 hours. The hydrothermally treated slurry was dehydrated, washed with water, and dried to obtain a sample.

Example 2
25 g of sodium hydroxide (Kanto Denka Kogyo Co., Ltd.) was added to 3,000 g of soft water and stirred until a transparent solution was obtained. 300 g of aluminum hydroxide (grade: BF083, average particle size (laser diffraction/scattering method): 10 μm, average primary particle size (30-point measurement from SEM image): 6.5 μm, Nippon Light Metal Co., Ltd.) was added and stirred thoroughly to prepare an aqueous suspension. This aqueous suspension was placed in a stirred autoclave (volume: 5 L) and hydrothermally treated at 180 °C for 8 hours while stirring at 1.49 m/sec (impeller diameter: 0.142 m, 200 rpm). The temperature was raised from room temperature (25 °C) to 180 °C over 2 hours. The hydrothermally treated slurry was dehydrated, washed with water, and dried to obtain a sample.

Example 3
25 g of sodium hydroxide (Kanto Denka Kogyo Co., Ltd.) was added to 3,000 g of soft water and stirred until a transparent solution was obtained. 300 g of aluminum hydroxide (grade: B103, average particle size (laser diffraction/scattering method): 7 μm, average primary particle size (30-point measurement from SEM image): 4 μm, Nippon Light Metal Co., Ltd.) was added and stirred thoroughly to prepare an aqueous suspension. This aqueous suspension was placed in a stirred autoclave (volume: 5 L) and hydrothermally treated at 200 °C for 12 hours while stirring at 1.49 m/sec (impeller diameter: 0.142 m, 200 rpm). The temperature was raised from room temperature (25 °C) to 186 °C over 2 hours, and then from 186 °C to 200 °C over 0.5 hours. The hydrothermally treated slurry was dehydrated, washed with water, and dried to obtain a sample.

Example 4
86 g of sodium carbonate (Tokuyama Corporation) and 10 g of sodium hydroxide (Kanto Denka Kogyo Co., Ltd.) were added to 3,000 g of soft water and stirred until a transparent solution was obtained. 300 g of aluminum hydroxide (grade: BF083, average particle size (laser diffraction/scattering method): 10 μm, average primary particle size (30-point measurement from SEM image): 6.5 μm, Nippon Light Metal Co., Ltd.) was added and stirred thoroughly to prepare an aqueous suspension. This aqueous suspension was placed in a stirred autoclave (volume: 5 L) and hydrothermally treated at 180 °C for 8 hours while stirring at 1.49 m/sec (impeller diameter: 0.142 m, 200 rpm). The temperature was raised from room temperature (25 °C) to 180 °C over 2 hours. The hydrothermally treated slurry was dehydrated, washed with water, and dried to obtain a sample.

Example 5
88 g of sodium hydroxide (Kanto Denka Kogyo Co., Ltd.) and 13 g of sodium phosphate dodecahydrate (Kanto Chemical Co., Ltd.) were added to 7000 g of soft water and stirred until a clear solution was obtained. Then, 700 g of aluminum hydroxide (grade name: BF083, average particle size (laser diffraction/scattering method): 10 μm, average primary particle size (30-point measurement from SEM image): 6.5 μm, Nippon Light Metal Co., Ltd.) was added and stirred thoroughly to prepare an aqueous suspension. This aqueous suspension was placed in a stirred autoclave (volume: 10 L) and hydrothermally treated at 205 °C for 12 hours while stirring at 1.65 m/sec (blade diameter: 0.195 m, 162 rpm). The temperature was raised from room temperature (25 °C) to 186 °C over 2 hours, and then from 186 °C to 205 °C over 0.5 hours. After the heat treatment, the slurry was dehydrated, washed with water, and dried to obtain a sample.

Comparative Example 1
A sample was produced in the same manner as in Example 1, except that the aluminum hydroxide was changed to aluminum hydroxide (grade name: BF013, average particle diameter (laser diffraction/scattering method): 1 μm, average value of primary particles (measured at 30 points in an SEM image): 1 μm, manufactured by Nippon Light Metal Co., Ltd.).

Comparative Example 2
A sample was produced in the same manner as in Example 4, except that the aluminum hydroxide was changed to aluminum hydroxide (grade name: BF013, average particle diameter (laser diffraction/scattering method): 1 μm, average value of primary particles (measured at 30 points in an SEM image): 1 μm, manufactured by Nippon Light Metal Co., Ltd.).

Comparative Example 3
A sample was produced in the same manner as in Example 5, except that the aluminum hydroxide was changed to aluminum hydroxide (grade name: BF013, average particle diameter (laser diffraction/scattering method): 1 μm, average value of primary particles (measured at 30 points in an SEM image): 1 μm, manufactured by Nippon Light Metal Co., Ltd.).

Comparative Example 4
86 g of sodium carbonate (Tokuyama Corporation) was added to 3,000 g of soft water and stirred until a transparent solution was obtained. 300 g of aluminum hydroxide (grade: BF083, average particle size (laser diffraction/scattering method): 10 μm, average primary particle size (30-point measurement from SEM image): 6.5 μm, Nippon Light Metal Co., Ltd.) was added and stirred thoroughly to prepare an aqueous suspension. This aqueous suspension was placed in a stirring autoclave (volume: 5 L) and hydrothermally treated at 180°C for 10 hours while stirring at 1.71 m/sec (impeller diameter: 0.142 m, rotation speed: 230 rpm). The temperature was raised from room temperature (25°C) to 180°C over 2 hours. The hydrothermally treated slurry was dehydrated, washed with water, and dried to obtain a sample.

Comparative Example 5
A sample was produced in the same manner as in Example 3, except that the sodium hydroxide was changed to aluminum hydroxide (grade name: BF083, average particle diameter (laser diffraction/scattering method): 10 μm, average value of primary particles (measured at 30 points on an SEM image): 6.5 μm, manufactured by Nippon Light Metal Co., Ltd.), and that the aqueous suspension of sodium hydroxide and aluminum hydroxide was allowed to stand without stirring and mixing, and then the hydrothermal treatment was carried out.

The following various tests were conducted on the above Examples 1 to 5 and Comparative Examples 1 to 5.

1. Whether or not boehmite is formed
The crystalline phase was measured using an X-ray diffractometer (D2 Phaser, manufactured by Bluker Corporation). When no unreacted raw materials remained and boehmite crystals were observed, the sample was evaluated as ◯, and when unreacted raw materials remained, the sample was evaluated as ×.
2. Presence or absence of card house structure The sample was attached to carbon tape, and the particle surface and shape were observed using a scanning electron microscope (JEOL Ltd. JSM-7500FA). Samples in which a card house structure was observed were rated as ○, and samples in which no card house structure was observed or where almost no card house structure was observed were rated as ×.
3. Particle size (D50, D16, D84 (all by volume))
The sample was dispersed in a 0.2% aqueous solution of sodium hexametaphosphate, and the particle size distribution (volume basis) was measured using a laser diffraction/scattering particle size distribution analyzer (MT3000, manufactured by Microtrac Bell Co., Ltd.), and the values of D50, D16, and D84 were read.
4. Standard deviation SD, coefficient of variation CV
The standard deviation SD and coefficient of variation CV were calculated using the following formula:
Standard deviation SD = (D84-D16)/2 (Formula 1)
Coefficient of variation CV = (standard deviation SD/D50) x 100 (Equation 2)
5. Oil absorption: Using linseed oil reagent (manufactured by Kanto Chemical Co., Ltd.), oil absorption was measured in accordance with the refined linseed oil method of JIS K5101-13-1 (2004). The measurement procedure was as follows.
(1) 2 g of sample was weighed and placed on a glass measuring plate.
(2) Linseed oil was gradually added from the dropper, 4 to 5 drops at a time, and the sample was kneaded into the linseed oil using a palette knife.
(3) The above procedure (2) was repeated, and the dropping was continued until lumps of linseed oil and sample were formed.
(4) After that, linseed oil was added drop by drop and the process was repeated until the paste became soft and hard.
(5) The oil absorption value was calculated using the following formula:
Oil absorption (g/100g) = (weight of linseed oil used until the end point (g) / weight of sample (g)) x 100 (Equation 4) 6. Tap density
0.3 g, 0.5 g, or 1 g of sample was placed in a 10 mL measuring cylinder and dropped from a certain height at a constant speed until the volume stopped changing. The volume after filling was read, and the tap density was calculated using the following formula.
Tap density (g/cm ³ )=sample weight (g)/volume after filling (cm ³ ) (Equation 5)
7. Specific Surface Area Using a fully automatic specific surface area measuring device (Macsorb (registered trademark) HM model-1200 manufactured by Mountech Co., Ltd.), the sample was pretreated by heating and evacuating under vacuum at 150°C for 30 minutes before measuring the BET surface area, and then measured by the BET flow method (single-point method) at a temperature close to the liquid nitrogen temperature (77K).
8.1% Weight Loss Temperature: This was measured using a differential thermal and thermogravimetric simultaneous analyzer (TG-DTA 2000SA, manufactured by Bruker AXS Co., Ltd.). The temperature was increased at a rate of 30°C/min, and the temperature at which the weight loss reached 1.00% was read relative to 100°C.
9. Thermal Conductivity A sample was kneaded into a resin of the following specified size to the limit of kneading, and the thermal conductivity of the resin composition was measured.
(1) Resin type: Epoxy resin (bisphenol A type)
(Product name: R140P, Manufacturer: Mitsui Chemicals, Inc., Viscosity at 25°C: 12,000-15,000 cps)
(2) Preparation method: 30 g of epoxy resin was placed in a 205 mL paper cup, and the sample was gradually mixed until the kneading limit was reached. This process was repeated using a planetary centrifugal mixer (ARE-310, manufactured by Thinky Corporation). After the sample was mixed to the kneading limit, 0.6 g of 2-ethyl-4-methylimidazole (manufactured by Wako Pure Chemical Industries, Ltd.) was added, thoroughly mixed, degassed, and then heat-cured at 120°C for 2 hours. The resulting cured product was processed into the desired shape to obtain test pieces of the resin composition.
The volume filling rate of the sample at the kneading limit was calculated using the following formula.
Volume filling rate of sample (vol%) = (volume of sample (cm ³ ) / (volume of sample (cm ³ ) + volume of epoxy resin (cm ³ ))) × 100 (Equation 6)
Sample volume (cm ³ ) = Sample weight (g) / Sample density (g/cm ³ ) (Equation 7)
Volume of epoxy resin (cm ³ )=Weight of epoxy resin (g)/Density of epoxy resin (g/cm ³ ) (Equation 8)
Density of boehmite: 3.0 g/cm ³ , density of epoxy resin: 1.16 g/cm ³
(3) Measurement method: Hot ray method (product name: QTM-500, manufactured by Kyoto Denshi Co., Ltd.)
(4) Shape of resin composition test piece: disc with a diameter of 5 cm and a thickness of 1.5 cm

Table 1 shows the manufacturing methods for the Examples and Comparative Examples, and Tables 2 and 3 show the results of various tests on the Examples and Comparative Examples.

Figures 1 to 9 are SEM photographs of Examples 1 to 5 and Comparative Examples 1 to 4. Figure 10 is an SEM photograph showing a cross section of the scaly boehmite aggregate of Example 2, and Figure 11 is an SEM photograph showing a cross section of the scaly boehmite aggregate of Comparative Example 4. The lower rows of Figures 1 to 9 are SEM photographs that are enlarged portions of the upper rows. By referring to these SEM photographs, the following can be analyzed based on the above results.

As shown in the SEM photographs in the lower rows of each example, the scaly boehmite aggregates form a dense house-of-cards structure. Furthermore, as shown in the SEM photographs in the upper rows of each example, the scaly boehmite aggregates have small particle size variations and are composed of particles of uniform size. From the SEM photograph of the cross section of the scaly boehmite aggregate of Example 2 in Figure 10, it can be seen that the scaly boehmite aggregate of Example 2 forms a dense house-of-cards structure. Furthermore, from the SEM photograph of the cross section of the scaly boehmite aggregate of Comparative Example 4 in Figure 11, it can be seen that the scaly boehmite aggregate of Comparative Example 4 forms a bulky house-of-cards structure.
On the other hand, it is clear that Comparative Examples 1 to 3 are scaly boehmite particles in which scaly boehmite crystals that do not form agglomerations of a house-of-card structure are dispersed.
Example 1 and Comparative Example 1 were produced by the same method, except for the difference in the average particle size of the aluminum hydroxide raw material. Because Example 1 has a card-house structure, its oil absorption is higher than that of Comparative Example 1, which has fewer voids. On the other hand, because the card-house structure is dense, the oil absorption of Example 1 is approximately 50% of that of Comparative Example 4, which is a scaly boehmite aggregate with a bulky card-house structure. Furthermore, the difference in the coefficient of variation (CV) indicates that Example 1 has smaller particle size variation than Comparative Example 1, resulting in particles with a more uniform particle size. Furthermore, because Example 1 has a dense card-house structure, it is easier to fill into resin. The volume filling rate, when filled to the kneading limit in the resin composition, is higher than that of Comparative Example 1, and the thermal conductivity of the resin composition is also higher. Furthermore, the tap density of Example 1 is higher than that of Comparative Example 1, and Example 1 is less likely to scatter than Comparative Example 1, resulting in superior handleability.
Example 4 and Comparative Example 2, and Example 5 and Comparative Example 3, which were produced by the same method except for the difference in the average particle size of the raw material aluminum hydroxide, can also be analyzed in a similar manner as above. In particular, the tap density of Example 4 is about four times that of Comparative Example 2, and the tap density of Example 5 is ten times that of Comparative Example 3. Therefore, Example 4 is less prone to scattering than Comparative Example 2, and Example 5 is less prone to scattering than Comparative Example 3, and Examples 4 and 5 have excellent handleability.
Comparative Example 4 was produced in a manner substantially similar to that of Example 2, except that sodium carbonate was used instead of sodium hydroxide as a raw material additive, and is a scaly boehmite aggregate having a bulky card-house structure in which scaly boehmite crystals aggregate together, as shown in Figure 9. Furthermore, since Comparative Example 4 has a bulky card-house structure, it is difficult to fill into the resin, and the volume filling rate, when filled into the resin composition up to the kneading limit, is lower than those of the Examples, and the thermal conductivity is also low.
In this way, whether the house-of-card structure is bulky or dense can be determined from the difference in the volume filling rate, in which the scaly boehmite aggregates are filled into the resin composition up to the kneading limit, and the difference in thermal conductivity.
Comparative Example 5 was produced in a manner similar to that of Example 3, except that the aqueous suspension was allowed to stand. In Comparative Example 5, a molded product was obtained, and boehmite formation was confirmed, but other tests could not be carried out satisfactorily.
The specific surface area of highly crystallized boehmite is several m ² /g to several tens of m ² /g. The specific surface areas of the scaly boehmite in the examples and comparative examples are several m ² /g to several tens of m ² /g, and it can be seen that both the examples and comparative examples are micro-sized boehmite with high crystallinity.
The reason why the 1% weight loss temperatures of both the Examples and Comparative Examples exceed 420°C is that neither the Examples nor the Comparative Examples used an organic binder in their production and that the boehmite has high crystallinity.

The scaly boehmite aggregates of the present invention are suitable as thermally conductive fillers for resins and the like.

Claims (5)

A scaly boehmite aggregate having a dense card-house structure in which scaly boehmite crystals are aggregated together, characterized in that the refined linseed oil absorption, measured in accordance with the refined linseed oil method of JIS K5101-13-1 (2004), is 180 g/100 g or less, and the on-plane thermal conductivity of an epoxy resin composition, measured by the hot wire method when filled into an epoxy resin to the kneading limit, is 0.94 W/m·K or more. 2. The scaly boehmite aggregate according to claim 1, wherein the coefficient of variation CV calculated using the following formula is 40% or less.
Standard deviation SD = (D84-D16)/2 (Formula 1)
Coefficient of variation CV=(standard deviation SD/D50)×100 (Equation 2) The scaly boehmite aggregate according to claim 1 or 2, characterized in that in thermogravimetric analysis, the weight loss rate at 100°C is set to 0 wt% and the 1% weight loss temperature, at which a weight loss of 1 wt% is confirmed when heated at a heating rate of 30°C/min, is 420°C or higher. A method for producing the scaly boehmite aggregates described in any one of claims 1 to 3, characterized in that an aqueous suspension containing aluminum hydroxide having an average particle size (volume basis) of 4 to 20 μm as measured by a laser diffraction/scattering method and one additive selected from sodium hydroxide, a mixture of sodium hydroxide and sodium carbonate, and a mixture of sodium hydroxide and sodium phosphate is subjected to hydrothermal treatment while stirring. The method for producing scaly boehmite aggregates described in claim 4, characterized in that the concentration of aluminum hydroxide in water is 1 to 20 wt %, the concentration of additives in water is 0.05 to 2.00 mol/L, and the constant temperature for hydrothermal treatment is 150 to 250°C.