JP7829280B2 - Inorganic filler powder, thermally conductive polymer composition, method for producing inorganic filler powder - Google Patents

Description

This invention relates to an inorganic filler powder used as a thermally conductive material, a thermally conductive polymer composition, and a method for producing the inorganic filler powder.

In recent years, with the advancement of electric vehicles and fuel cell vehicles, electrical components have been handling higher currents, and consequently, the amount of heat generated by these components is also increasing. For example, lithium-ion batteries for automobiles generate a large amount of heat due to their ability to continuously output high-current power for extended periods, and therefore require efficient heat dissipation. For this reason, thermally conductive polymer compositions with excellent thermal conductivity are sometimes used as heat dissipation materials in areas requiring insulation in electrical components that output high currents, such as lithium-ion batteries.

Conventionally, thermally conductive polymer compositions include those in which inorganic filler powders made of inorganic materials with excellent thermal conductivity are dispersed in a matrix material such as a resin with excellent insulating and moldable properties. Commonly used inorganic filler powders include aluminum oxide (alumina: _Al₂O₃ ), aluminum nitride (AlN), silicon dioxide ( _SiO₂ ), silicon nitride (SiN), and magnesium oxide (MgO) _, due to their thermal conductivity and specific gravity.

The thermal conductivity of a thermally conductive polymer composition can be improved by increasing the content of inorganic filler powder. For example, to obtain a thermally conductive polymer composition with a high thermal conductivity of 5 W/mK or higher, it is necessary to knead 1100 parts by mass or more of inorganic filler powder with 100 parts by mass of matrix material.

However, increasing the inorganic filler powder content in the matrix material also increases the hardness of the resulting thermally conductive polymer composition, leading to problems with reduced fluidity and moldability. Therefore, for example, Patent Documents 1 and 2 disclose polymer compositions that use spherical alumina particles as inorganic filler powder, thereby keeping the hardness low even with a high inorganic filler powder content and enabling easy kneading. Patent Document 3 discloses an alumina filler in which γ-alumina particles are formed on the surface of α-alumina particles. Furthermore, Patent Document 4 discloses the addition of low-hardness inorganic powder to a resin for the purpose of reducing wear on resin molding dies.

Patent No. 4361997 Japanese Patent Publication No. 2012-121742 Special Publication No. 6-51778 Japanese Patent Publication No. 2011-16962

However, the spherical alumina particles used in the resin compositions disclosed in Patent Documents 1 and 2 have the drawback of having a complex manufacturing process and high manufacturing costs.
Furthermore, the method of forming γ-alumina particles on the surface of α-alumina particles, as disclosed in Patent Document 3, also has the drawback of high manufacturing costs because it requires a heating process at high temperatures.
Furthermore, the method disclosed in Patent Document 4 had the problem that adding a large amount of low-hardness inorganic powder to high-hardness inorganic powder reduced the fluidity of the resin.

This invention was made in consideration of these circumstances, and aims to provide an inorganic filler powder that enables the low-cost acquisition of a thermally conductive polymer composition with excellent thermal conductivity and low hardness, a thermally conductive polymer composition using the same, and a method for producing the inorganic filler powder.

To solve the above problems, the present invention proposes the following means.
In other words, the inorganic filler powder of the present invention has a structure (excluding sintered) in which at least a portion of the surface of inorganic particles (excluding those with a metal coating) with a particle size of 1 μm or more is coated by fixing inorganic fine particles (excluding those with a metal coating) with a particle size of 10 nm or more and less than 0.1 μm, wherein the coverage rate of the surface of the inorganic particles by the inorganic fine particles is 30% or more, and the inorganic particles and inorganic fine particles are any of aluminum oxide, silicon carbide, aluminum nitride, silicon nitride, silicon dioxide, or magnesium oxide.
The present invention relates to the production of an inorganic filler powder having a structure (excluding sintered) in which at least a portion of the surface of an inorganic particle (excluding those with a metal coating) is coated by fixing inorganic fine particles (excluding those with a metal coating) having a particle size of 10 nm or more and less than 0.1 μm.

According to the present invention, by creating a structure in which a portion of the surface of inorganic particles with a particle size of 1 μm or more is coated with inorganic fine particles with a particle size of 10 nm or more and less than 0.1 μm, it is possible to realize an inorganic filler powder with high thermal conductivity when mixed with a matrix material at a lower cost compared to conventional spherical alumina particles. Furthermore, by mixing the inorganic filler powder of the present invention with a matrix material, a flexible, shape-conforming, and highly thermally conductive polymer composition can be obtained.

Furthermore, in this invention, the inorganic particles and inorganic fine particles may contain aluminum oxide.

The thermally conductive polymer composition of the present invention is characterized by being obtained by mixing the inorganic filler powder described in each of the above sections with a matrix material containing at least one of a resin material, an elastomer material, and a rubber material.

Furthermore, in the present invention, the thermally conductive polymer composition may contain 1200 parts by mass or more of the inorganic filler powder per 100 parts by mass of the matrix material.

The present invention provides a method for producing inorganic filler powder, comprising: a fluid polishing step of obtaining a fluid in which inorganic particles (excluding those with metal coatings) and inorganic fine particles (excluding those with metal coatings) are generated in the solvent by swirling a raw material slurry, which is a mixture of inorganic raw material powder and a solvent, at a peripheral speed of 10 m/s or more to polish the inorganic raw material powder ; and a drying step of removing the solvent from the fluid to deposit the inorganic fine particles on the surface of the inorganic particles, wherein the inorganic particles and inorganic fine particles are any of aluminum oxide, silicon carbide, aluminum nitride, silicon nitride, silicon dioxide, or magnesium oxide , and the inorganic filler powder is produced having a structure in which at least a portion of the surface of the inorganic particles with a particle size of 1 μm or more is coated with the inorganic fine particles with a particle size of 10 nm or more and less than 0.1 μm .

Furthermore, in this invention, the inorganic raw material powder may be fused alumina powder.

According to the present invention, it is possible to provide an inorganic filler powder that enables the low-cost acquisition of a thermally conductive polymer composition with excellent thermal conductivity and low hardness, a thermally conductive polymer composition using the same, and a method for producing the inorganic filler powder.

This is an electron microscope image (10,000x magnification) showing the inorganic filler powder of this embodiment. This is a magnified schematic diagram showing a single particle of inorganic filler powder. This is a binarized image obtained by cropping an arbitrary rectangular region from an electron microscope photograph of inorganic filler powder. This is an enlarged schematic diagram showing the thermally conductive polymer composition of this embodiment. This is an electron microscope image of the inorganic filler powder used in Verification Example 1 (polishing time: 3 minutes). This is an electron microscope image of the inorganic filler powder used in Verification Example 1 (polishing time 30 minutes). This is an electron microscope image of the inorganic filler powder used in Verification Example 1 (polishing time 45 minutes). This is an electron microscope image of the inorganic filler powder used in Verification Example 1 (polishing time 60 minutes). This graph shows the results of Verification Example 2.

The following describes an inorganic filler powder according to one embodiment of the present invention, a thermally conductive polymer composition using the same, and a method for producing the inorganic filler powder, with reference to the drawings. The embodiments described below are provided specifically to better illustrate the spirit of the invention and do not limit the present invention unless otherwise specified.

(Inorganic filler powder)
Inorganic filler powder is a thermally conductive material used to obtain a thermally conductive polymer composition by mixing it with a matrix material. _The inorganic filler powder of one embodiment of the present invention is aluminum oxide (alumina: _Al₂O₃ ) in the form of a fine powder.

Alumina was used as a filler in the thermally conductive polymer composition because its thermal conductivity is relatively high, at approximately 30 W/m·K.
In addition to the alumina used in this embodiment, other thermally conductive inorganic material powders such as silicon carbide (SiC), aluminum nitride (AlN), silicon nitride (SiN), silicon dioxide ( _SiO₂ ), and magnesium oxide (MgO) can be used as inorganic filler powders.

The inorganic filler powder of this embodiment is obtained by polishing electrofused alumina powder (inorganic raw material powder), and has a structure in which at least a portion of the surface of inorganic particles with a particle size of 1 μm or more is coated with inorganic fine particles with a particle size of 10 nm or more and less than 0.1 μm.

Figure 1 is an electron microscope image (10,000x magnification) of the inorganic filler powder of this embodiment. Figure 2 is a magnified schematic diagram showing one particle of the inorganic filler powder. As shown in Figures 1 and 2, the inorganic filler powder of this embodiment has a structure in which a portion of the surface of inorganic particles with a particle size of 1 μm or more is covered with inorganic fine particles with a particle size of 10 nm or more and less than 0.1 μm. The inorganic fine particles constituting the inorganic filler powder are attached (adhered) to the surface of the inorganic particles.
In the following explanation, "inorganic particles" refers to alumina particles with a particle size of 1 μm or larger, and "inorganic fine particles" refers to alumina particles with a particle size of 10 nm or larger and less than 0.1 μm.

The surface coverage rate of inorganic particles by inorganic fine particles constituting such inorganic filler powder is said to be 30% or more.
The coverage rate referred to here is the ratio (%) of the surface area (plane) of inorganic microparticles to the surface area (plane) of inorganic particles when an arbitrary range of inorganic filler powder is viewed in plan view. As an example of measuring this coverage rate, as shown in Figure 3, an image is prepared by cropping an arbitrary rectangular region from an electron microscope image of inorganic filler powder (for example, at a magnification of about 10,000 to 100,000 times) and binarizing it. In such an image, the parts where inorganic particles are exposed are shown in black, and the parts covered by inorganic microparticles are shown in white. Then, by calculating the area occupied by the white region (area covered by inorganic microparticles) relative to the area of the cropped rectangular region (surface area of inorganic particles) using image processing, the coverage rate (%) can be obtained.

The inorganic filler powder of this embodiment, as described above, has a structure in which a portion of the surface of inorganic particles with a particle size of 1 μm or more is coated with inorganic fine particles with a particle size of 10 nm or more and less than 0.1 μm. Compared to conventional methods using spherical alumina particles, this allows for the creation of an inorganic filler powder with high thermal conductivity when mixed with a matrix material at a lower cost. Furthermore, by mixing the inorganic filler powder of this embodiment with a matrix material, a flexible, shape-following, and highly thermally conductive polymer composition can be obtained.

(Thermally conductive polymer composition)
Figure 4 is an enlarged schematic diagram showing the thermally conductive polymer composition of this embodiment.
The thermally conductive polymer composition of this embodiment consists of the inorganic filler powder of this embodiment dispersed in a matrix material. For example, it may be a paste-like mixture in which 1200 parts by mass or more of the inorganic filler powder of this embodiment is mixed with 100 parts by mass of the matrix material. For example, the thermally conductive polymer composition of this embodiment can be obtained by mixing 1200 to 7000 parts by mass of the inorganic filler powder of this embodiment with 100 parts by mass of resin.

The matrix material into which the inorganic filler powder is mixed may contain at least one of the following: a resin material, an elastomer material, and a rubber material.
The matrix material is not particularly limited to resin materials, and known resin materials can be used. Specifically, examples include hydrocarbon resins, unsaturated polyester resins, acrylic resins, vinyl ester resins, epoxy resins, xyleneformaldehyde resins, guanamine resins, diallyl phthalate resins, phenolic resins, furan resins, polyimide resins, melamine resins, urea resins, and the like.

The matrix material is not particularly limited to elastomer materials; known elastomer materials can be used. Specifically, examples include polystyrene elastomers, polyvinyl chloride elastomers, polyurethane elastomers, polyester elastomers, and polyamide elastomers.

The rubber material used in the matrix is not particularly limited; any known rubber material can be used. Specifically, it can be either natural rubber or synthetic rubber; examples include urethane rubber, silicone rubber, and fluororubber.

The thermally conductive polymer composition of this embodiment exhibits a 40% or greater reduction in hardness compared to conventional thermally conductive polymer compositions obtained by mixing raw material alumina particles (electrofused alumina particles) and resin, at the same blending ratio. This reduction in hardness, i.e., the increased softness, enhances the fluidity of the thermally conductive polymer composition of this embodiment.

As a result, the thermally conductive polymer composition of this embodiment can improve shape conformability in the filled portion, allowing it to adhere closely to the heat transfer target object without gaps and efficiently transfer heat. Furthermore, compared to conventional thermally conductive polymer compositions that mix raw material alumina particles (electrofused alumina particles) with resin, a larger amount of inorganic filler powder can be mixed in while maintaining the same level of hardness. Therefore, the thermally conductive polymer composition of this embodiment can improve thermal conductivity compared to conventional thermally conductive polymer compositions using raw material alumina particles.

(Method for producing inorganic filler powder)
In manufacturing the inorganic filler powder of this embodiment, first, an inorganic raw material powder is prepared. In this embodiment, particulate electrofused alumina was used as the inorganic raw material powder. Particulate electrofused alumina, produced by the reduction and melting of bauxite in an electric arc furnace, was used as the inorganic raw material powder because it has a large particle size and a broad particle size distribution, and it can be mixed with matrix materials such as resins at a high packing rate, thereby improving the thermal conductivity of the thermally conductive polymer composition.
Commercially available electrofused alumina powder can be used as the raw material. For example, electrofused alumina powder that has passed through a sieve with a mesh size of 100 μm can be used.

Next, the raw material slurry, which is a mixture of this electrofused alumina powder (inorganic raw material powder) and a solvent, is swirled and flowed at a peripheral speed of 10 m/s or more, causing the electrofused alumina powders to collide with each other and be polished (flow polishing process). This generates a fluid in which inorganic particles and inorganic fine particles are produced in the solvent.

As a solvent for forming a slurry from this inorganic raw material powder, fused alumina powder, a stable liquid that does not dissolve alumina, such as water, is used. In this embodiment, ion-exchanged water is used as the solvent. When water is used as the solvent, the concentration of fused alumina powder should be, for example, about 70% to 80% by mass.

One method for polishing by slurrying electrofused alumina powder and water (raw material slurry) is, for example, an emulsification and dispersion device (Aspec Disperser ZERO, manufactured by Hiroshima Metal & Machinery Co., Ltd.). In this emulsification and dispersion device, a stirring rotor rotates at high speed within a water-cooled stator. When the aforementioned electrofused alumina powder and water are introduced into the gap between the stator and the stirring rotor, the rotation of the stirring rotor causes the electrofused alumina powder to become homogeneously dispersed in the water, forming a raw material slurry (dispersion). Within this raw material slurry, the particles of the electrofused alumina powder collide with each other, resulting in self-polishing.

The rotor's rotation speed is set to a peripheral speed of 10 m/s or more. This causes the raw material slurry to swirl and flow at a peripheral speed of 10 m/s or more, efficiently polishing the electrofused alumina powder and generating inorganic particles with a particle size of 1 μm or more and inorganic fine particles with a particle size of 10 nm or more and less than 0.1 μm in the solvent.

This is thought to be because the collision polishing of the electrofused alumina powder particles removes the sharp edges of the particles, generating inorganic particles, while the removed edges become inorganic microparticles.

Furthermore, the polishing time for the electrofused alumina powder (inorganic raw material powder) in this particle polishing process should be between 3 minutes and 60 minutes. If the polishing time is less than 3 minutes, there is a concern that sufficient inorganic fine particles will not be generated. Conversely, if polishing is performed for a long time, there is a concern that the alumina particles will be crushed, becoming too fine, resulting in too few inorganic particles. If the particles become too fine, the viscosity may become too high during mixing with the resin, leading to poor moldability, or a sufficient amount of inorganic filler powder may not be mixed into the matrix material, resulting in insufficient improvement in thermal conductivity.

The emulsification and dispersion apparatus may be supplied with either electrofused alumina powder and water separately as two liquids, or as a pre-mixed slurry.

While other methods such as bead mills and ball mills can be used to create a slurry of electrofused alumina powder and water for polishing, these methods have drawbacks, including excessive grinding effect and potential quality degradation due to the inclusion of media such as beads or balls.

Next, the solvent is removed from the fluid obtained in the particle polishing process, which consists of inorganic particles and inorganic microparticles generated in the solvent. This process generates inorganic filler powder with inorganic microparticles attached to the surface of the inorganic particles (drying process).

In this drying process, for example, a heated dryer is used to heat the fluid to about 80°C to 100°C, thereby evaporating the solvent from the fluid and causing inorganic fine particles to adhere (fix) to the surface of the inorganic particles during this solvent evaporation process.
This results in an inorganic filler powder having a structure in which at least a portion of the surface of inorganic particles with a particle size of 1 μm or more is coated with inorganic fine particles with a particle size of 10 nm or more and less than 0.1 μm, and the surface coverage rate of the inorganic particles by inorganic fine particles is 30% or more.

In this embodiment, the particle size is the median diameter (central diameter), i.e., the average particle size D50 at which the cumulative frequency reaches 50%. This average particle size D50 was measured using a laser diffraction scattering particle size distribution analyzer (MT3300EXII: Microtrac-Bell Co., Ltd.).

(Method for producing a thermally conductive polymer composition)
The method for producing the thermally conductive polymer composition of this embodiment involves kneading the inorganic filler powder of this embodiment described above into a matrix material. To knead the inorganic filler powder into the matrix material, such as a resin, a rotary/revolving type mixer (e.g., Rentaro: manufactured by Shinky Co., Ltd.) can be used.

The thermally conductive polymer composition of this embodiment comprises a matrix material containing the inorganic filler powder of this embodiment. For example, the thermally conductive polymer composition of this embodiment can be produced by adding 1200 to 7000 parts by mass of the inorganic filler powder of this embodiment to 100 parts by mass of the matrix material and kneading it in a mixer.

In this case, the inorganic filler powder has a structure in which at least a portion of the surface of inorganic particles with a particle size of 1 μm or more is coated with inorganic fine particles with a particle size of 10 nm or more and less than 0.1 μm. Therefore, compared to the case where raw alumina particles are used directly as alumina filler, the amount of inorganic filler powder can be increased while maintaining a similar hardness or viscosity in the resulting thermally conductive polymer composition. This makes it possible to obtain a thermally conductive polymer 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 spirit of the invention. This embodiment and its variations are included within the scope and spirit of the invention, as well as within the scope of the claims and its equivalents.

The following shows the verification results that confirmed the effects of the present invention.
As the inorganic raw material powder, fused alumina powder V325F (average particle size D50 = 11.1 μm: manufactured by Nippon Light Metal Co., Ltd.) was used. The fused alumina powder was mixed with deionized water to make a slurry with a concentration of 72% by mass. Using an emulsification and dispersion device (Aspec Disperser ZERO, manufactured by Hiroshima Metal & Machinery Co., Ltd.), the peripheral speed of the stirring rotor was set to 32 m/s or 35 m/s, and the time was set to 3 minutes, 30 minutes, 45 minutes, and 60 minutes, respectively, to polish the fused alumina powder and obtain a fluid.
Next, the fluid was heated to 90°C using a heating furnace to evaporate the water solvent, and the residue was ground in a mortar to obtain inorganic filler powder.

Then, the obtained inorganic filler powder and a butadiene-based polymer (R-45HT: manufactured by Idemitsu Kosan Co., Ltd.) as a matrix material were kneaded using a rotational and revolutionary mixer (Awatori Rentaro: manufactured by Shinky Co., Ltd.) to obtain a thermally conductive polymer composition.
At this time, 1400 parts by mass of inorganic filler powder were added to 100 parts by mass of matrix material.

(Verification Example 1)
Electron microscope images were taken of the inorganic filler powders obtained for each of the polishing times described above. Figure 5 shows the result for a polishing time of 3 minutes, Figure 6 for a polishing time of 30 minutes, Figure 7 for a polishing time of 45 minutes, and Figure 8 for a polishing time of 60 minutes.
In the photographs shown in Figures 5 to 8, the black background area is a magnified view of a portion of the surface of a single inorganic particle, and the numerous particles smaller than 0.1 μm displayed on top of this background are inorganic microparticles.

Then, in each of the electron microscope images shown in Figures 5 to 8, three arbitrary rectangular regions (0.5 μm × 0.5 μm) were set (fields 1 to 3), and after performing the binarization process shown in Figure 3, the coverage rate (%) of inorganic particles by inorganic microparticles was calculated. The results of this coverage rate are shown in Table 1.

Table 1 shows that the coating rate increased with increasing polishing time. It was confirmed that by polishing the inorganic raw material powder by diffusing electrofused alumina powder, an inorganic raw material powder, into a solvent and then swirling the slurry at a peripheral speed of 10 m/s or more, it is possible to produce inorganic filler powder in which the surface of inorganic particles with a particle size of 1 μm or more is coated with inorganic fine particles with a particle size of 10 nm or more and less than 0.1 μm.

(Verification Example 2)
Next, the hardness of each thermally conductive polymer composition was measured using inorganic filler powder polished for 3 minutes (average coverage 21.4%), inorganic filler powder polished for 30 minutes (average coverage 30.1%), inorganic filler powder polished for 45 minutes (average coverage 56.2%), inorganic filler powder polished for 60 minutes (average coverage 90.9%), and electrofused alumina powder, which is a raw material that is not polished (average coverage 14.5%).
Hardness measurement: Durometer (Asker Rubber Hardness Tester Type A: Polymer Instruments Co., Ltd.)

Then, the improvement rate (%) in hardness of the thermally conductive polymer composition using each inorganic filler powder, with respect to the hardness of the thermally conductive polymer composition using electrofused alumina powder as the raw material, was measured.
Hardness improvement rate (%) = Hardness of the thermally conductive polymer composition using inorganic filler powder / Hardness of the thermally conductive polymer composition using electrofused alumina powder × 100
These results are shown in Figure 9 as a graph.

As shown in Figure 9, it was found that by manufacturing a thermally conductive polymer composition using inorganic filler powder with an average coverage rate of 30% or more of inorganic fine particles over inorganic particles, the hardness was reduced by more than 40% compared to conventional thermally conductive polymer compositions using electrofused alumina powder without polishing treatment, significantly increasing flexibility.

Claims (5)

A structure (excluding sintered structures) in which at least a portion of the surface of inorganic particles (excluding those with a metal coating) with a particle size of 1 μm or more is coated by fixing inorganic fine particles (excluding those with a metal coating) with a particle size of 10 nm or more and less than 0.1 μm, wherein the coverage rate of the surface of the inorganic particles by the inorganic fine particles is 30% or more.
The inorganic filler powder is characterized in that the inorganic particles and inorganic fine particles are any of aluminum oxide, silicon carbide, aluminum nitride, silicon nitride, silicon dioxide, or magnesium oxide. A thermally conductive polymer composition characterized by mixing the inorganic filler powder described in claim 1 with a matrix material containing at least one of a resin material, an elastomer material, and a rubber material. The thermally conductive polymer composition according to claim 2, characterized in that it contains 1200 parts by mass or more of the inorganic filler powder per 100 parts by mass of the matrix material. The process comprises a fluid polishing step in which a raw material slurry, obtained by mixing inorganic raw material powder and a solvent, is flowed in a swirling manner at a peripheral speed of 10 m/s or more to polish the inorganic raw material powder, thereby obtaining a fluid in which inorganic particles (excluding those coated with metal) and inorganic fine particles (excluding those coated with metal) are generated in the solvent; and a drying step in which the solvent is removed from the fluid and the inorganic fine particles are attached to the surface of the inorganic particles.
The inorganic particles and inorganic fine particles are any of aluminum oxide, silicon carbide, aluminum nitride, silicon nitride, silicon dioxide, or magnesium oxide.
A method for producing inorganic filler powder, characterized by producing an inorganic filler powder having a structure in which at least a portion of the surface of inorganic particles having a particle size of 1 μm or more is coated with inorganic fine particles having a particle size of 10 nm or more and less than 0.1 μm. The method for producing inorganic filler powder according to claim 4, characterized in that the inorganic raw material powder is electrofused alumina powder.