JP7584225B2 - Alumina filler for thermally conductive resin composition, thermally conductive resin composition, and method for producing alumina filler for thermally conductive resin composition - Google Patents

Description

The present invention relates to an alumina filler for a thermally conductive resin composition, a thermally conductive resin composition, and a method for producing an alumina filler for a thermally conductive resin composition.

In recent years, with the development of electric vehicles and fuel cell vehicles, electrical components are becoming larger in current and the amount of heat generated by electrical components is also increasing. For example, lithium ion batteries for automobiles generate a lot of heat because they output a large current of power continuously for a long period of time, and it is necessary to efficiently dissipate the large amount of heat generated to the outside. For this reason, it is common to use resin materials with excellent thermal conductivity (thermally conductive resin compositions) as packages for electrical components that output a large current, such as lithium ion batteries.

Conventionally, examples of thermally conductive resin compositions include those in which an inorganic filler having excellent thermal conductivity is dispersed in a resin material having excellent insulating properties and moldability. As the inorganic filler, an alumina filler made of fused alumina (Al ₂ O ₃ ) crystal particles is generally used from the viewpoints of thermal conductivity and specific gravity.

The thermal conductivity of a thermally conductive resin composition can be improved by increasing the content of alumina filler. As an example, to obtain a thermally conductive resin composition with a high thermal conductivity of 5 W/mk or more, it is necessary to knead 1,100 parts by mass or more of alumina filler with 100 parts by mass of resin material.

However, on the other hand, increasing the alumina filler content increases the hardness of the resulting thermally conductive resin composition, reducing its ability to be filled into the area of use (shape conformity), and there are also issues with making it difficult to uniformly mix the alumina filler into the resin material during production.

For this reason, for example, Patent Document 1 discloses a resin composition that uses spherical alumina particles as an alumina filler, thereby keeping the hardness low even at a high content and allowing easy kneading.
Furthermore, Patent Document 2 discloses rounded fused alumina particles having an average particle size of 5 to 4000 μm and a circularity of 0.85 or more.

Patent No. 4361997 Patent No. 4817683

However, the spherical alumina particles used in the resin composition disclosed in Patent Document 1 have a problem in that the manufacturing process is complicated and the manufacturing cost is high.
In addition, although the rounded fused alumina particles disclosed in Patent Document 2 use inexpensive fused alumina as a manufacturing raw material, there is a problem in that the yield of the rounded fused alumina particles obtained after classification is low, resulting in high costs.

The present invention has been made in consideration of these circumstances, and aims to provide an alumina filler for a thermally conductive resin composition that can produce a thermally conductive resin composition that has excellent thermal conductivity and low hardness at low cost, a thermally conductive resin composition that uses the alumina filler, and a method for producing the alumina filler for a thermally conductive resin composition.

In order to solve the above problems, the present invention proposes the following means.
That is, the alumina filler for thermally conductive resin compositions of the present invention is an alumina filler for thermally conductive resin compositions containing fused alumina particles which are polished raw material alumina particles, characterized in that the fused alumina particles contain 1 mass% or more of fine fused alumina particles which are the supernatant solid content in a 2-hour sedimentation test in water, and the specific surface area of the fused alumina particles is 25% or more larger than the specific surface area of the raw material alumina particles.

According to the present invention, by containing at least 1 mass % or more of fine fused alumina particles of 1 μm or less, it is possible to realize an alumina filler for a thermally conductive resin composition that has excellent moldability when kneaded with a resin because the hardness is not too high, and is capable of forming a thermally conductive resin composition containing a large amount of filler. In addition, since expensive rounded alumina particles are not used, it is possible to realize a low-cost alumina filler for a thermally conductive resin composition.

The thermally conductive resin composition of the present invention is characterized in that it contains the alumina filler for thermally conductive resin compositions described in each of the above paragraphs in the resin.

In addition, in the present invention, the alumina filler for the thermally conductive resin composition may be contained in an amount of 1,200 parts by mass or more per 100 parts by mass of the resin.

The method for producing an alumina filler for a thermally conductive resin composition of the present invention is a method for producing an alumina filler for a thermally conductive resin composition as described in each of the above paragraphs, and is characterized by having a particle polishing step in which a slurry containing the fused alumina particles and a solvent is swirled at a peripheral speed of 10 m/sec or more, and the fused alumina particles are polished by colliding with each other.

In addition, in the present invention, the particle polishing process may be carried out for a period of time ranging from 3 minutes to 60 minutes.

In addition, in the present invention, the particle polishing process may be a process for increasing the specific surface area of the fused alumina particles by 25% or more compared to the specific surface area of the fused alumina particles before the particle polishing process is performed.

In addition, in the present invention, the particle polishing process may be a process for increasing the average particle diameter D50 of the fused alumina particles by -15% or more compared to the average particle diameter D50 of the fused alumina particles before the particle polishing process is performed.

The present invention provides an alumina filler for a thermally conductive resin composition that can produce a thermally conductive resin composition having excellent thermal conductivity and low hardness at low cost, a thermally conductive resin composition using the alumina filler, and a method for producing the alumina filler for a thermally conductive resin composition.

1 is a micrograph showing an electrically fused alumina particle according to an embodiment of the present invention. 1 is a micrograph showing conventional raw alumina particles. 1 is a graph showing the relationship between the number of parts by mass of alumina particles and the thermal conductivity of the obtained thermally conductive resin composition. Photographs showing a two-hour sedimentation test.

The following describes, with reference to the drawings, an alumina filler for a thermally conductive resin composition according to one embodiment of the present invention, a thermally conductive resin composition using the alumina filler, and a method for producing the alumina filler for a thermally conductive resin composition. Note that the following embodiments are specifically described to provide a better understanding of the gist of the invention, and do not limit the present invention unless otherwise specified.

(Alumina filler for thermally conductive resin composition)
An alumina filler for a thermally conductive resin composition (hereinafter referred to as alumina filler) is a thermally conductive material to be mixed with a resin to obtain a thermally conductive resin composition. The alumina filler according to one embodiment of the present invention is alumina (Al ₂ O ₃ ) in the form of a fine powder.

Alumina was used as a filler in the thermally conductive resin composition because it has a relatively high thermal conductivity of about 30 W/m·K.

The alumina filler of the present embodiment contains fused alumina particles obtained by polishing raw material alumina particles (fused alumina particles), and specifically contains at least 1 mass % or more of fine fused alumina particles of 1 μm or less.
In the following description, the term "fine fused alumina particles" refers to fused alumina particles having a size of 1 μm or less, and the term "fused alumina particles" simply refers to fused alumina particles with no particular particle size restriction.

The reason why fused alumina particles produced by the reduction and melting of bauxite in an electric arc furnace are used as the raw alumina particles used to produce the polished fused alumina particles is that they have a large particle diameter and a broad particle size distribution, and can be mixed with resins, etc. at a high filling rate, thereby enhancing the thermal conductivity of the thermally conductive resin composition.

In addition, in the alumina filler of this embodiment, the specific surface area of the fused alumina particles is 25% or more larger than the specific surface area of the raw alumina particles. In other words, in the manufacturing method of the alumina filler described later, fused alumina particles are used in which the specific surface area of the raw alumina particles is increased by 25% or more by polishing the raw alumina particles.

The specific surface area in this embodiment is the surface area per unit mass ( ^m2 /g). The specific surface area in this embodiment was measured by the BET method (molecule with a known adsorption area is adsorbed to the powder particle surface with liquid nitrogen, and the specific surface area of the sample is calculated from the amount of adsorption).
The specific surface area was measured using a fully automatic gas adsorption measurement device (Autosorb-iQ: Quantachrome Instruments Japan).

Furthermore, in a 2-hour sedimentation test in water, the fused alumina particles contained in the alumina filler of this embodiment have a total solids mass relative to the supernatant solids mass of at least 0.3 mass% or more, preferably 2.0 mass% or more, and more preferably 5.0 mass% or more.

In the sedimentation test in this embodiment, the sample fused alumina particles are suspended in water to a concentration of 2.4% by mass to form a slurry, which is then poured into a container so that the liquid level is 20 mm high and allowed to stand for 2 hours. The supernatant is then separated and the water is evaporated at 90°C, the mass of the remaining solids is measured, and the ratio (percentage) of the mass of the remaining solids to the total solids mass of the slurry is calculated.

The alumina filler of this embodiment contains at least 1 mass % of fine fused alumina particles of 1 μm or less, and thus can realize an alumina filler that has high thermal conductivity when kneaded with resin at a low cost compared to when spherical alumina particles are used. Furthermore, by mixing the alumina filler of this embodiment with resin, a thermally conductive resin composition with low hardness and excellent shape conformability can be obtained.

(Thermal conductive resin composition)
The thermally conductive resin composition of the present embodiment is made of a resin containing the alumina filler of the present embodiment. For example, it may be a paste-like composition in which 1200 parts by mass or more of the alumina filler of the present embodiment is mixed with 100 parts by mass of resin. For example, the thermally conductive resin composition of the present embodiment can be obtained by mixing 1200 parts by mass to 1500 parts by mass of the alumina filler of the present embodiment with 100 parts by mass of resin.

The resin to be mixed with the alumina filler is not particularly limited, and any known resin can be used. Specific examples include hydrocarbon resins, unsaturated polyester resins, acrylic resins, vinyl ester resins, epoxy resins, xylene formaldehyde resins, guanamine resins, diallyl phthalate resins, phenol resins, furan resins, polyimide resins, melamine resins, and urea resins.

Compared to a conventional thermally conductive resin composition in which raw material alumina particles (electrically fused alumina particles) and resin are mixed, the thermally conductive resin composition of this embodiment has a hardness reduced by 9% to 57% at the same blending ratio. This reduction in hardness, i.e., softening, increases the fluidity of the thermally conductive resin composition of this embodiment. As a result, the thermally conductive resin composition of this embodiment can improve the shape conformability in the filled portion, and can efficiently transfer heat by adhering closely to the heat transfer object without gaps. In addition, compared to a conventional thermally conductive resin composition in which raw material alumina particles (electrically fused alumina particles) and resin are mixed, a large amount of filler can be mixed when the same hardness is achieved, so the thermally conductive resin composition of this embodiment can improve thermal conductivity.

(Method of producing alumina filler for thermally conductive resin composition)
The alumina filler for a thermally conductive resin composition of this embodiment is produced by carrying out a particle polishing step of polishing raw material alumina particles (electrically fused alumina particles).
In the particle polishing step, a slurry in which raw alumina particles and a solvent are mixed is swirled at a peripheral speed of 10 m/sec or more, and the raw alumina particles are polished by colliding with each other.

In the particle polishing step, commercially available fused alumina powder can be used as raw alumina particles (fused alumina particles). Since it is preferable that the raw fused alumina powder does not contain particles with a diameter exceeding 100 μm in consideration of the surface roughness of the composition, fused alumina powder that has passed through a sieve with a mesh size of 100 μm is used.
The raw material alumina particles (fused alumina particles) may be, for example, fused alumina particles containing less than 1% fine fused alumina particles.

The solvent used to turn the raw fused alumina powder into a slurry is a stable liquid that does not dissolve alumina, such as water. In this embodiment, ion-exchanged water is used as the solvent. When water is used as the solvent, the concentration of raw alumina particles may be, for example, about 70% to 80% by mass.

One example of a means for turning the raw alumina particles and water into a slurry and polishing it is an emulsification/dispersion device (Aspec Disperser ZERO, manufactured by Hiroshima Metal & Machinery Co., Ltd.). In this emulsification/dispersion device, an agitation rotor rotates at high speed inside a water-cooled stator. When the raw alumina particles and water described above are introduced into the gap between the stator and the agitation rotor, the rotation of the agitation rotor creates a slurry (dispersion liquid) in which the raw alumina particles are homogeneously dispersed in the water, and the raw alumina particles collide with each other in this slurry, causing them to polish themselves.

The rotation speed of the rotor may be a peripheral speed of 10 m/sec or more. This allows the slurry to rotate and flow at a peripheral speed of 10 m/sec or more, and the raw alumina particles can be polished efficiently. The polishing time for the raw alumina particles in this particle polishing process may be in the range of 3 minutes to 60 minutes. If the polishing time is less than 3 minutes, there is a concern that the specific surface area will not be large enough compared to the raw alumina particles. Furthermore, if polishing is performed for a long period of time, the alumina particles may be crushed and the particles may become too fine. If the particles become too fine, the viscosity may become too high when kneaded with the resin, resulting in poor moldability, or a sufficient amount of filler may not be mixed, resulting in insufficient improvement in thermal conductivity.

The raw alumina particles and water can be supplied to the emulsification/dispersion device as two separate liquids, or as a premixed slurry.

Bead mills and ball mills are also used as a method of making a slurry of raw alumina particles and water for polishing, but there are concerns that the grinding effect is too great and that the quality may be reduced due to the inclusion of media such as beads and balls.

After carrying out the above-described particle polishing process, polished fused alumina particles are obtained by solid-liquid separation through filtration and moisture removal through drying. The alumina filler for thermally conductive resin compositions of this embodiment contains 1 mass % or more of fine fused alumina particles of 1 μm or less obtained by the above-described method.

In this way, the average particle diameter D50 of the fused alumina particles obtained by colliding and polishing raw alumina particles does not change significantly from that of the raw alumina particles. For example, the average particle diameter D50 of the fused alumina particles is greater than the average particle diameter D50 of the raw alumina particles by -15% or more, and is at most about 15%.

In this embodiment, the average particle diameter D50 is the median diameter, that is, the particle diameter at which the cumulative frequency is 50%. The average particle diameter D50 was measured using a laser diffraction scattering type particle size distribution measuring device (MT3300EXII: Microtrack Bell Co., Ltd.).

When raw alumina particles are polished, the average particle diameter D50 of the fused alumina particles is always smaller than the average particle diameter D50 of the raw alumina particles. In other words, in principle, the average particle diameter D50 of the fused alumina particles will never be greater than the average particle diameter D50 of the raw alumina particles. However, if the fine particles are not sufficiently recovered after polishing, the apparent average particle diameter D50 of the fused alumina particles may be larger.

On the other hand, the specific surface area of polished fused alumina particles is significantly increased compared to the raw alumina particles. For example, the rate of change in the specific surface area of polished fused alumina particles relative to the raw alumina particles is at least 25% and at most about 150%.

This is believed to be the result of the raw alumina particles colliding with each other to slightly round off the sharp corners of the raw alumina particles, resulting in an increase in the number of fine fused alumina particles. FIG. 1 shows a micrograph of the fused alumina particles of this embodiment, and FIG. 2 shows a micrograph of conventional raw alumina particles. The micrographs show that the corners of the fused alumina particles after polishing are rounded off on particles of several μm or more, and that the number of fine fused alumina particles of 1 μm or less has increased. In this way, the increase in fine fused alumina particles makes it possible to increase the filler content in the resin composition when used as an alumina filler, thereby improving thermal conductivity.

The alumina filler of this embodiment can be one in which the average particle diameter D50 of the fused alumina particles is greater than the average particle diameter D50 of the raw alumina particles by -15% or more, and more preferably in the range of -15% or more and +15% or less.

(Method for producing thermally conductive resin composition)
In the method for producing the thermally conductive resin composition of the present embodiment, the alumina filler containing the polished fused alumina particles of the present embodiment described above is kneaded into a resin. To knead the alumina filler into the resin, for example, a rotation/revolution type mixer (Mixertaro, manufactured by Thinky Corporation) can be used.

The thermally conductive resin composition of this embodiment is made of a resin containing the alumina filler of this embodiment. For example, the thermally conductive resin composition of this embodiment can be produced by adding 1200 to 1500 parts by mass of the alumina filler of this embodiment to 100 parts by mass of resin and kneading with a mixer. At this time, since the alumina filler contains polished fused alumina particles, the amount of alumina filler filled can be increased while maintaining the hardness or viscosity of the resulting thermally conductive resin composition at the same level, compared to when the raw alumina particles are used as the alumina filler as is. This results in a thermally conductive resin composition with high thermal conductivity.

Although one embodiment of the present invention has been described above, this embodiment is presented as an example and is not intended to limit the scope of the invention. This embodiment can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. This embodiment and its variations are within the scope of the invention and its equivalents as described in the claims, as well as the scope and gist of the invention.

The results of verifying the effects of the present invention are shown below.
(Verification Example 1)
As raw alumina particles (electrically fused alumina particles), electrically fused alumina powder V325F (average particle diameter D50 = 11.1 μm: manufactured by Nippon Light Metal Co., Ltd.) was used. The raw alumina particles were made into a slurry with a concentration of 72 mass% with ion-exchanged water, and the raw alumina particles were polished using an emulsifier/disperser (Aspec Disperser ZERO manufactured by Hiroshima Metal & Machinery Co., Ltd.) with the peripheral speed of the stirring rotor set to 35 m/s and the time set to 60 minutes. As a result, electrically fused alumina particles of Verification Example 1 (hereinafter referred to as polished alumina particles 1) were obtained.

Then, the electrically fused alumina particles 1 and a butadiene-based polymer (R-45HT: manufactured by Idemitsu Kosan Co., Ltd. (hereinafter referred to as resin 1)) were kneaded using a rotation/revolution type mixer (Thinky Mixer: manufactured by Thinky Corporation) to obtain a thermally conductive resin composition of Verification Example 1.
At this time, a graph showing the relationship between the number of parts by mass (PHR) of raw alumina particles and polished alumina particles 1 added per 100 parts by mass of resin 1 and the thermal conductivity of the obtained thermally conductive resin composition is shown in Figure 3.
The thermal conductivity was measured by a disk heat flow meter method (made by our company, based on JIS A 1412-1 or ASTM D5470, with an aluminum rod).

3, the measured value of the thermal conductivity of the thermally conductive resin composition does not change significantly between the case where raw alumina particles are added to Resin 1 and the case where polished fused alumina particles 1 are added. Therefore, it was confirmed that there is no difference in the filling state between the raw alumina particles and the polished fused alumina particles 1.
On the other hand, in the case of raw alumina particles, a maximum of 1200 parts by mass of raw alumina particles could be added per 100 parts by mass of resin, whereas in the case of polished fused alumina particles, 1400 parts by mass could be added. Therefore, the thermally conductive resin composition using polished fused alumina particles as a filler was able to increase the thermal conductivity because a larger amount of filler could be added.

(Verification Example 2)
As raw alumina particles (electrically fused alumina particles), electrically fused alumina powder V325F (average particle size D50 = 11.1 μm: manufactured by Nippon Light Metal Co., Ltd.) was used (hereinafter referred to as raw material 1). 1270 g of this raw material 1 was mixed with 680 g of ion-exchanged water to form a slurry, and raw material 1 was subjected to a polishing process using an emulsifying/dispersing device (Aspec Disperser ZERO manufactured by Hiroshima Metal & Machinery Co., Ltd.) with a peripheral speed of 35 m/s and a time of 3 minutes (sample 01), 30 minutes (sample 02), 45 minutes (sample 03), and 60 minutes (sample 04).
Then, 100 parts by mass of Resin 1 and 1,400 parts by mass each of Raw Material 1 and Samples 01 to 04 were kneaded together to prepare a thermally conductive resin composition of Verification Example 2.

Table 1 shows the specific surface area and average particle size D50 of Raw material 1 and Samples 01 to 04, and the rate of change for each of them relative to Raw material 1. Table 1 also shows the hardness of each of the thermally conductive resin compositions using Raw material 1 and Samples 01 to 04, and the rate of change for each of the thermally conductive resin compositions using Raw material 1. The instruments used for each measurement are as follows.
Measurement of specific surface area: BET method, fully automatic gas adsorption measurement device (Autosorb-iQ: Quantachrome Instruments Japan)
Measurement of average particle diameter D50: Laser diffraction scattering method, particle size distribution measuring device (MT3300EXII: Microtrack Bell Co., Ltd.)
Hardness measurement: Durometer (Asker Rubber Hardness Meter Type A: Kobunshi Keiki Co., Ltd.)

(Verification Example 3)
As raw material alumina particles (electrically fused alumina particles), a uniform mixture of 75 mass% of electrically fused alumina powder V325F (average particle size D50 = 11.1 μm: manufactured by Nippon Light Metal Co., Ltd.) and 25 mass% of electrically fused alumina powder F220 (average particle size D50 = 60 μm: manufactured by Nippon Light Metal Co., Ltd.) was used (hereinafter referred to as raw material 2). 1270 g of this raw material 2 was mixed with 680 g of ion-exchanged water to form a slurry, and raw material 2 was subjected to a polishing process using an emulsifier/disperser (Aspec Disperser ZERO manufactured by Hiroshima Metal & Machinery Co., Ltd.) at a peripheral speed of 32 m/s and a time of 15 minutes (sample 11), 30 minutes (sample 12), and 45 minutes (sample 13).
Then, 100 parts by mass of Resin 1 and 1,400 parts by mass each of Raw Material 2 and Samples 11 to 13 were kneaded together to prepare a thermally conductive resin composition of Verification Example 3.

Table 2 shows the specific surface area and average particle size D50 of raw material 2 and samples 11 to 13, as well as the rate of change for each of them relative to raw material 2. Table 2 also shows the hardness of each of the thermally conductive resin compositions using raw material 2 and samples 11 to 13, as well as the rate of change for the thermally conductive resin composition using raw material 2. The equipment used for each measurement was the same as in Verification Example 1.

(Verification Example 4)
As raw alumina particles (electrically fused alumina particles), a uniform mixture of 50% by mass of fused alumina powder V325F (average particle size D50 = 11.1 μm: manufactured by Nippon Light Metal Co., Ltd.) and 50% by mass of fused alumina powder F220 (average particle size D50 = 60 μm: manufactured by Nippon Light Metal Co., Ltd.) was used (hereinafter referred to as raw material 3). 1270 g of this raw material 3 was mixed with 680 g of ion-exchanged water to form a slurry, and raw material 3 was subjected to a polishing process using an emulsifier/disperser (Aspec Disperser ZERO manufactured by Hiroshima Metal & Machinery Co., Ltd.) at a peripheral speed of 32 m/s and a time of 15 minutes (sample 21), 30 minutes (sample 22), and 45 minutes (sample 23).
Then, 100 parts by mass of Resin 1 and 1,400 parts by mass each of Raw Material 3 and Samples 21 to 23 were kneaded together to prepare a thermally conductive resin composition of Verification Example 4.

Table 3 shows the specific surface area and average particle size D50 of raw material 3 and samples 21 to 23, as well as the rate of change for each of these relative to raw material 3. Table 3 also shows the hardness of each of the thermally conductive resin compositions using raw material 3 and samples 21 to 23, as well as the rate of change for the thermally conductive resin composition using raw material 3. The equipment used for each measurement was the same as in Verification Example 1.

(Verification Example 5)
As raw alumina particles (electrically fused alumina particles), electrically fused alumina powder V325F (average particle size D50 = 11.1 μm: manufactured by Nippon Light Metal Co., Ltd.) was pulverized and classified by a jet mill (model number STJ-200: manufactured by Seishin Enterprise Co., Ltd.) to obtain an electrically fused alumina powder having an average particle size D50 = 3.9 μm (hereinafter referred to as raw material 4). 720 g of this raw material 4 was mixed with 680 g of ion-exchanged water to form a slurry, and raw material 4 was subjected to a polishing treatment using an emulsifier/disperser (Aspec Disperser ZERO manufactured by Hiroshima Metal & Machinery Co., Ltd.) with a peripheral speed of 32 m/s and a time of 30 minutes (sample 31) and 60 minutes (sample 32).
Then, 100 parts by mass of Resin 1 and 1,100 parts by mass each of Raw Material 4, Samples 31, and 32 were kneaded together to prepare a thermally conductive resin composition of Verification Example 5.

The specific surface area, average particle diameter D50, and rate of change of each of raw material 4, samples 31, and 32 are shown in Table 4. The hardness of each of the thermally conductive resin compositions using raw material 4, samples 31, and 32, and the rate of change of each of the thermally conductive resin compositions using raw material 4 are shown in Table 4. The equipment used for each measurement was the same as in Verification Example 1.

The results shown in Tables 1 to 4 confirm that by polishing commercially available fused alumina powders, raw materials 1 to 4, respectively, it is possible to increase only the specific surface area (+25% to +146%) without significantly changing the average particle diameter D50 (within the range of -15% to +10%).

The thermally conductive resin composition of this embodiment, which was made using samples 01-04, 11-13, 21-23, 31, and 32 obtained by polishing raw materials 1-4, was confirmed to have reduced hardness (-9% to -57%) and improved shape conformability compared to the conventional thermally conductive resin composition made using raw materials 1-4.

(Verification Example 6)
Using the raw material 1 and sample 04 of the above-mentioned verification example 1, 400 parts by mass and 600 parts by mass of resin 1 were kneaded with 100 parts by mass, respectively, to prepare thermally conductive resin compositions (compositions 1 to 4), and the viscosities were measured. The viscosities and the rate of change in viscosity are shown in Table 5.
The instruments used for the measurements are as follows:
Viscosity measurement: B-type viscometer (PM-2A manufactured by Malcom Co., Ltd.)

The results shown in Table 5 confirm that, compared to conventional thermally conductive resin compositions 1 and 2 using raw material 1, thermally conductive resin compositions 3 and 4 using sample 04, which is the polished fused alumina particles of this embodiment, have a reduced viscosity (-37% to -59%) and improved shape conformability.

(Verification Example 7)
A two-hour sedimentation test was conducted on raw material 1 of the above-mentioned verification example 1, sample 04, and a fused alumina powder (sample 05) obtained by grinding raw material 1 with a bead mill (manufactured by IMEX Co., Ltd.: peripheral speed 7.2 m/s) to identify their properties.

The test method was to mix 1 g of each of the fused alumina powders (raw material 1, sample 04, and sample 05) with 40 g of ion-exchanged water to create a slurry (each with a concentration of 2.4% by mass), which was then poured into a capped sample tube (inner diameter 10 mm) so that the liquid level was 20 mm high. After leaving it to stand at room temperature for 2 hours, the supernatant solids were separated and their mass was measured. The ratio of the supernatant solids mass to the total solids mass was then calculated. The results are shown in Table 6. The test conditions are shown in Figure 4.

According to the results shown in Table 6, it was found that the polished fused alumina powder of this embodiment (samples 04 and 05) had a total solid mass relative to the supernatant solid mass of 5.7% by mass and 0.3% by mass, respectively, compared to the fused alumina powder of raw material 1 (raw material 1). It was also found that the ratio of the total solid mass to the supernatant solid mass tends to increase as the polishing time of the fused alumina powder of raw material 1 becomes longer.

From these results, it was confirmed that fine fused alumina particles of 1 μm or less, which are small enough to float on water, are formed by polishing the fused alumina particles (raw material 1). By forming such fine polished fused alumina particles of 1 μm or less, the specific surface area of samples 04 and 05 is increased relative to raw material 1, and the thermal conductivity can be improved.

Claims (7)

An alumina filler for a thermally conductive resin composition, comprising fused alumina particles which are abraded raw alumina particles,
The fused alumina particles contain 1 mass % or more of fine fused alumina particles which are supernatant solids in a 2-hour sedimentation test in water,
An alumina filler for a thermally conductive resin composition, characterized in that the specific surface area of the electrically fused alumina particles is 25% or more larger than the specific surface area of the raw material alumina particles. A thermally conductive resin composition comprising the alumina filler for thermally conductive resin compositions according to claim 1 in a resin. The thermally conductive resin composition according to claim 2, characterized in that the alumina filler for the thermally conductive resin composition is contained in an amount of 1,200 parts by mass or more per 100 parts by mass of the resin. A method for producing the alumina filler for a thermally conductive resin composition according to claim 1, comprising the steps of :
A method for producing an alumina filler for a thermally conductive resin composition, comprising a particle polishing step of swirling a slurry of a mixture of fused alumina particles and a solvent at a peripheral speed of 10 m/sec or more to cause the fused alumina particles to collide with each other and be polished. The method for producing an alumina filler for a thermally conductive resin composition according to claim 4, characterized in that the particle polishing process is carried out for a period of 3 minutes or more and 60 minutes or less. The method for producing an alumina filler for a thermally conductive resin composition according to claim 4 or 5, characterized in that the particle polishing process is a process for increasing the specific surface area of the fused alumina particles by 25% or more compared to the specific surface area of the fused alumina particles before the particle polishing process is carried out. The method for producing an alumina filler for a thermally conductive resin composition according to any one of claims 4 to 6, characterized in that the particle polishing process is a process for increasing the average particle diameter D50 of the fused alumina particles by -15% or more compared to the average particle diameter D50 of the fused alumina particles before the particle polishing process is carried out.

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