JP7748293B2 - Boron nitride powder, resin composition, and cured product of the resin composition - Google Patents

Description

This disclosure relates to boron nitride powder, a resin composition, a cured product of the resin composition, and a method for producing boron nitride powder.

Boron nitride powder possesses lubricity, high thermal conductivity, and insulating properties, and is widely used as a solid lubricant, thermally conductive filler, insulating filler, and other applications. In recent years, the increasing performance of electronic devices has created a demand for boron nitride with excellent thermal conductivity.

For example, Patent Document 1 proposes a hexagonal boron nitride powder and a method for producing the same, which, when used as a filler for insulating heat dissipation materials such as resins, can increase the thermal conductivity and withstand voltage (dielectric breakdown voltage) of the resins.

Hexagonal boron nitride powder is produced, for example, by a manufacturing method that includes a pressure nitridation process in which boron carbide is nitrided in a pressurized nitrogen-containing atmosphere to obtain a nitride, and a decarburization and crystallization process in which the nitride is mixed with a boron source and heat-treated to decarburize and promote crystallization, thereby obtaining hexagonal boron nitride (see, for example, Patent Document 2). The above-mentioned manufacturing method aims to improve decarburization performance in subsequent processes by nitriding boron carbide once.

Japanese Patent Application Laid-Open No. 2019-116401 International Publication No. 2019/073690

The present disclosure aims to provide boron nitride powder that can be used to prepare a resin composition that has excellent fillability in resin and can be used to produce a heat transfer sheet with excellent heat dissipation, a resin composition containing such boron nitride powder, a cured product of the resin composition, and a method for producing boron nitride powder.

One aspect of the present disclosure provides a boron nitride powder comprising agglomerated particles formed by agglomeration of boron nitride primary particles, the agglomerated particles having a tap density of more than 0.9 g/ ^cm3 , an average particle size of 20 μm or less, and a crushing strength of 10 MPa or more.

The boron nitride powder has a relatively small average particle size and a high tap density. This means that there are fewer voids within the aggregated particles, allowing for a larger loading when filling the same volume of resin than conventional boron nitride powders. The boron nitride powder also contains agglomerated particles with excellent crushing strength. This prevents the agglomerated particles from breaking down during kneading with resin, preventing the generation of primary particles or their pulverized form. As a result, the increase in viscosity associated with kneading with resin can be suppressed. In other words, the amount of boron nitride powder that can be blended with resin until the viscosity of the resin composition obtained by kneading with resin becomes excessive can be increased compared to conventional boron nitride powders. These effects enable the boron nitride powder to have excellent resin filling properties, and the increased loading rate of the boron nitride powder can improve the heat dissipation performance of heat transfer sheets produced.

The boron nitride powder may have an average particle size of 2 μm or more. By keeping the lower limit of the average particle size within the above range, it is possible to further reduce the increase in viscosity when kneaded with resin.

The above boron nitride powder may have an orientation index of 10 or less. An orientation index within this range means that there is a large proportion of agglomerated particles that can suppress the thermal conductivity anisotropy associated with the shape of the primary particles of boron nitride. This prevents the primary particles from orienting in the in-plane direction within the sheet in a heat transfer sheet or the like made using this boron nitride powder, making it possible to take advantage of the high thermal conductivity of the boron nitride powder and exhibiting higher heat dissipation properties.

One aspect of the present disclosure provides a resin composition containing a resin and the above-described boron nitride powder.

Because the resin composition contains the boron nitride powder described above, the content of boron nitride powder can be easily adjusted, making it useful for producing heat transfer sheets with excellent heat dissipation properties.

One aspect of the present disclosure provides a cured resin composition containing a resin and the above-described boron nitride powder.

Because the cured product is a cured product of a resin composition containing the above-mentioned boron nitride powder, the content of boron nitride powder can be easily adjusted, making it suitable for use as a heat transfer sheet with excellent heat dissipation properties.

One aspect of the present disclosure provides a method for producing boron nitride powder, comprising: a pressure nitriding step in which boron carbide powder having an average particle size of 15 μm or less is sintered in a pressurized nitrogen atmosphere to obtain a sintered product; an oxidation step in which the sintered product is heated in an atmosphere having an oxygen partial pressure of 20% or more to obtain a heat-treated product; and a crystallization step in which a mixture containing the heat-treated product and a boron source is heated to produce primary particles of boron nitride and the primary particles are agglomerated to obtain agglomerated particles, wherein the content of the boron source is less than 50 parts by mass per 100 parts by mass of the heat-treated product.

The method for producing the boron nitride powder described above includes an oxidation step in which the material is heated in an atmosphere with an oxygen partial pressure of 20% or higher (e.g., air) after the pressure nitriding step, thereby reducing the carbon content in the sintered product after nitriding. This allows for a reduction in the amount of boron source used in the subsequent crystallization step. During the crystallization step, the boron source melts and forms a liquid phase, promoting the dissolution and recrystallization of boron nitride in the sintered product. However, this liquid phase can become trapped within the agglomerated particles formed by the aggregation of boron nitride primary particles. Removal of this liquid phase within the agglomerated particles results in voids within the agglomerated particles. Therefore, an increase in the liquid phase can result in the formation of agglomerated particles with a high void ratio, resulting in a powder with a low tap density. In this case, when kneaded with resin, resin impregnation into the agglomerated particles can occur, raising concerns about reduced handleability due to the resulting increased viscosity, making it difficult to increase the filling rate to an expected level. In contrast, the manufacturing method disclosed herein makes it possible to reduce the amount of boron source added, and produces boron nitride powder with excellent tap density and agglomerated particle crushing strength.

Furthermore, in the above manufacturing method, by using a boron carbide powder with a relatively small average particle size, the particle size of the fired product obtained in the pressure nitriding process can be kept small, and the particle surface area can be maintained large. This increases the surface area that comes into contact with oxygen in the oxidation process, improving decarburization efficiency and allowing the content of the boron source used in the subsequent crystallization process to be relatively small. As a result, agglomerated particles with an appropriate average particle size can be obtained, and boron nitride powder can be prepared that suppresses viscosity increases when kneaded with resin.

The oxygen content of the heat-treated product may be 4% by mass or less, based on the total amount of the heat-treated product. By keeping the oxygen content of the heat-treated product within this range, the fillability of the resulting boron nitride powder into resin can be further improved.

The present disclosure provides boron nitride powder that can be used to prepare a resin composition that can be used to produce a heat transfer sheet with excellent fillability into resin and excellent heat dissipation, a resin composition containing such boron nitride powder, a cured product of the resin composition, and a method for producing boron nitride powder.

Embodiments of the present disclosure are described below. However, the following embodiments are merely examples for explaining the present disclosure and are not intended to limit the present disclosure to the following content.

Unless otherwise specified, the materials exemplified in this specification can be used alone or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition refers to the total amount of those multiple substances present in the composition, unless otherwise specified.

One embodiment of the boron nitride powder according to the present disclosure includes agglomerated particles formed by agglomeration of primary particles of boron nitride. The boron nitride powder has a tap density of more than 0.9 g/ ^cm3 and an average particle size of 20 μm or less. In the boron nitride powder, the agglomerated particles have a crushing strength of 10 MPa or more. The primary particles of the boron nitride may be primary particles of hexagonal boron nitride. The hexagonal boron nitride may have a small variation in particle shape. The shape of the primary particles of hexagonal boron nitride may be, for example, scale-like, disk-like, or the like.

The above boron nitride powder has sufficiently suppressed voids in the agglomerated particles, resulting in a high tap density. However, measurements of the void ratio of agglomerated particles vary depending on the measurement, and the correlation with the tap density of the phosphor powder is not necessarily good.

The lower limit of the tap density of the boron nitride powder is greater than 0.9 g/cm ³ , and may be, for example, 0.91 g/cm ³ or more, 0.92 g/cm ³ or more, 0.93 g/cm ³ or more, 0.94 g/cm ³ or more, or 0.95 g/cm ³ or more. When the lower limit of the tap density is within the above range, the fillability of the boron nitride powder into resin can be further improved. The upper limit of the tap density of the boron nitride powder is not particularly limited, but may be, for example, about 1.5 g/cm ³ , 1.3 g/cm ³ or less, or 1 g/cm ³ or less, based on the theoretical density of boron nitride (2.26 g/cm ³ ). The tap density of the boron nitride powder may be adjusted within the above range, and may be, for example, greater than 0.9 g/cm ³ and 1.5 g/cm ³ or less, or 0.91 to 1 g/cm ³ .

The term "tap density" as used herein refers to a value determined in accordance with the method described in JIS R 1628:1997, "Method for measuring bulk density of fine ceramic powders." Specifically, boron nitride powder is filled into a 100 ^cm3 dedicated container, and tapped under the conditions of a tapping time of 180 seconds, 180 tappings, and a tap lift of 18 mm. The bulk density is measured, and the resulting value is taken as the tap density. A commercially available device can be used for the measurement, such as a "Powder Tester" (product name) manufactured by Hosokawa Micron Corporation.

The upper limit of the average particle size of the boron nitride powder is 20 μm or less, but may be, for example, 19 μm or less, or 18 μm or less. By ensuring that the upper limit of the average particle size is within the above range, for example, when the boron nitride powder is filled into a resin and molded into a sheet-like compact having a thickness of several tens of μm, the difference between the thickness of the compact and the particle size of the boron nitride particles can be sufficiently large, allowing for the production of a defect-free compact. The lower limit of the average particle size of the boron nitride powder may be, for example, 2 μm or more, 3 μm or more, 5 μm or more, 7 μm or more, or 10 μm or more. By ensuring that the lower limit of the average particle size is within the above range, the boron nitride powder can be obtained with excellent packing properties. This further suppresses the increase in viscosity when the boron nitride powder is mixed with a resin. The average particle size of the boron nitride powder may be adjusted within the above range, for example, 2 to 20 μm, 5 to 19 μm, or 7 to 18 μm.

In this specification, the average particle size of agglomerated particles refers to the 50% cumulative diameter (median diameter) in the volume-based cumulative particle size distribution. More specifically, it refers to the particle size (D50) at which the cumulative value reaches 50% in the volume-based cumulative particle size distribution obtained by laser diffraction scattering for boron nitride powder. Laser diffraction scattering is measured in accordance with the method described in JIS Z 8825:2013, "Particle Size Analysis - Laser Diffraction and Scattering Method." Measurements can be performed using a laser diffraction scattering particle size distribution analyzer, such as the "LS-13 320" (product name) manufactured by Beckman Coulter. Note that measurements are performed without homogenization, in the presence of agglomerated particles.

The lower limit of the crushing strength of the agglomerated particles is 10 MPa or more, but may be, for example, 11 MPa or more, 12 MPa or more, or 13 MPa or more. Having a lower limit of the crushing strength within this range can further improve the heat dissipation properties of the resin composition and molded article obtained by kneading with the resin. The upper limit of the crushing strength of the agglomerated particles may be, for example, 30 MPa or less, 28 MPa or less, 25 MPa or less, or 20 MPa or less. Having an upper limit of the crushing strength within this range allows at least a portion of the agglomerated particles to disintegrate appropriately when kneaded with the resin, suppressing the generation of voids and further improving the insulating properties of the resin composition and molded article obtained. The crushing strength of the agglomerated particles may be adjusted within the above range, for example, 10 to 30 MPa, 11 to 25 MPa, or 13 to 20 MPa.

The crushing strength in this specification refers to a value measured in accordance with JIS R 1639-5:2007 "Fine Ceramics - Measurement of Granule Properties - Part 5: Single Granule Crushing Strength." The crushing strength σ (unit: MPa) of a single agglomerate particle is calculated using the equation σ = α × P / (π × d ² ) from the dimensionless number α (α = 2.48), which varies depending on the position within the agglomerate, the crushing test force P (unit: N), and the particle diameter d (unit: μm). Measurements were performed on 20 or more agglomerate particles, and the value at a cumulative failure rate of 63.2% was calculated. A microcompression tester can be used for the measurement. An example of a microcompression tester that can be used is the "MCT-W500" (product name) manufactured by Shimadzu Corporation.

The orientation of primary particles in the above boron nitride powder is sufficiently suppressed. The upper limit of the orientation index of the above boron nitride powder may be, for example, 10 or less, 9.8 or less, 9.6 or less, 9.5 or less, or 9 or less. By ensuring that the upper limit of the orientation index is within the above range, the resin composition and molded article can exhibit more than sufficient heat dissipation properties for practical use. The lower limit of the orientation index of the boron nitride powder is not limited, but it is difficult to produce a boron nitride powder with an index of less than 6, and may be, for example, 6 or more, 6.2 or more, 6.5 or more, or 6.7 or more. The orientation index of the boron nitride powder may be adjusted within the above range, and may be, for example, 6.0 to 10, 6.2 to 9.5, or 6.5 to 9.

In this specification, the orientation index refers to a value measured according to the following method. An X-ray diffraction spectrum of boron nitride powder is obtained by performing X-ray diffraction measurements on the boron nitride powder, and the peak intensities I(002) and I(100) corresponding to the (002) and (100) planes are obtained from the X-ray diffraction spectrum. The obtained peak intensities are used to calculate the orientation index [I(002)/I(100)] of the boron nitride powder. An X-ray diffraction device such as the "ULTIMA-IV" (product name) manufactured by Rigaku Corporation can be used.

The upper limit of the specific surface area of the boron nitride powder may be, for example, 7 m ² /g or less, 6.5 ^{m 2} /g or less, 6 m ² /g or less, or 5.5 m ² /g or less. When the upper limit of the specific surface area is within the above range, the primary particle diameter of boron nitride is sufficiently large, and when formed into a resin composition and a molded article, better heat dissipation properties can be exhibited. The lower limit of the specific surface area of the boron nitride powder may be, for example, 1.5 m ² /g or more, 2 m ² /g or more, 2.5 m ² /g or more, or 3 m ² /g or more. When the lower limit of the specific surface area is within the above range, the agglomeration of the boron nitride primary particles is dense, and the agglomerated particles tend to have sufficiently high crushing strength, which makes it possible to further suppress the collapse of the agglomerated particles when the boron nitride powder is kneaded with a resin or molded. The specific surface area of the boron nitride powder may be adjusted within the above range, for example, 1.5 to 7 ^{m 2} /g, 2 to 6.5 ^{m 2} /g, or 2.5 to 6 m ² /g.

The specific surface area in this specification refers to a value measured using a specific surface area measurement device in accordance with JIS Z 8830:2013, "Method for measuring the specific surface area of powders (solids) by gas adsorption," and is a value calculated using the BET single-point method using nitrogen gas. An example of a specific surface area measurement device that can be used is the "MONOSORB MS-22" (product name) manufactured by QUANTACHROME.

The above-mentioned boron nitride powder can be produced, for example, by the following method. One embodiment of a method for producing boron nitride powder includes a pressure nitriding step in which boron carbide powder having an average particle size of 15 μm or less is sintered in a pressurized nitrogen atmosphere to obtain a sintered product; an oxidation step in which the sintered product is heated in an atmosphere having an oxygen partial pressure of 20% or more to obtain a heat-treated product; and a crystallization step in which a mixture containing the heat-treated product and a boron source is heated to produce primary particles of boron nitride, and the primary particles are agglomerated to obtain agglomerated particles. In the mixture, the content of the boron source is less than 50 parts by mass per 100 parts by mass of the heat-treated product.

The boron carbide powder ( _B4C powder) used in the pressure nitriding step has an average particle size of 15 μm or less. The upper limit of the average particle size of the boron carbide powder may be, for example, 14 μm or less, 13 μm or less, 12 μm or less, or 11 μm or less. When the upper limit of the average particle size of the boron carbide powder is within the above range, the particle size of the fired product obtained in the pressure nitriding step can be kept small. This allows the particle surface area to be maintained large, increasing the contact surface with oxygen in the oxidation step, and thereby improving decarburization efficiency. The lower limit of the average particle size of the boron carbide powder may be, for example, 1 μm or more, 2 μm or more, 4 μm or more, or 5 μm or more. When the lower limit of the average particle size of the boron carbide powder is within the above range, the particle size of the obtained agglomerated particles can be appropriately increased, thereby further suppressing an increase in viscosity when the boron nitride powder is kneaded into a resin to form a resin composition. The average particle size of the boron carbide powder means a value measured by the same method as the method for measuring the average particle size of the agglomerated particles in the boron nitride powder described above.

In the pressure nitriding process, boron carbide powder is fired under a pressurized nitrogen atmosphere to obtain a fired product containing _boron carbonitride powder ( _B4CN4 powder). The lower limit of the firing temperature in the pressure nitriding process may be 2000°C or higher, or 2100°C or higher. By setting the lower limit of the firing temperature within the above range, the crystallinity of the obtained boron carbonitride can be improved and the proportion of hexagonal boron carbonitride can be increased. Increasing the proportion of hexagonal boron carbonitride in the pressure nitriding process can further improve the thermal conductivity of the boron nitride powder obtained later. The upper limit of the firing temperature in the pressure nitriding process may be 2300°C or lower, or 2250°C or lower. The firing temperature may be adjusted within the above range, for example, 2000 to 2300°C.

The lower limit of the pressure (ambient pressure) in the pressure nitriding process may be, for example, 0.6 MPa or more, 0.7 MPa or more, or 0.8 MPa or more. By setting the lower limit of the pressure in the pressure nitriding process within this range, the nitriding of boron carbide can be sufficiently promoted. The upper limit of the pressure (ambient pressure) in the pressure nitriding process may be, for example, 1 MPa or less, or 0.9 MPa or less. By setting the upper limit of the pressure in the pressure nitriding process within this range, increases in the production costs of boron nitride powder can be suppressed. The pressure may be adjusted within the above range, for example, 0.6 to 1 MPa, or 0.8 to 0.9 MPa.

The nitrogen gas concentration in the pressurized nitrogen atmosphere in the pressure nitriding process may be, for example, 95% by volume or more, 98% by volume or more, or 99.9% by volume or more. The firing time in the pressure nitriding process is not particularly limited as long as it is within a range that allows sufficient nitriding to proceed, and may be, for example, 6 to 30 hours, 8 to 25 hours, or 10 to 20 hours.

In the oxidation process, a calcined product containing boron carbonitride powder is calcined in an oxygen-containing atmosphere to decarburize some of the carbon contained in the calcined product. This decarburization reduces the amount of boron source used in the subsequent crystallization process. As a result, the proportion of liquid phase in the crystallization process can be reduced, resulting in boron nitride powder with a high tap density and excellent agglomerated particle crushing strength.

The oxygen-containing atmosphere used in the oxidation process may be an atmosphere with an oxygen partial pressure of 20% or more, such as atmospheric air. The pressure (atmospheric pressure) used in the oxidation process may be, for example, 0.1 to 0.5 MPa, 0.1 to 0.3 MPa, or 0.1 to 0.2 MPa, or may be atmospheric pressure (0.1 MPa).

The lower limit of the firing temperature in the oxidation step may be, for example, 600°C or higher, 650°C or higher, or 700°C or higher. Keeping the lower limit of the firing temperature within this range allows for a further reduction in the carbon content in the boron carbonitride. The upper limit of the firing temperature in the oxidation step may be, for example, 1000°C or lower, or 950°C or lower. Keeping the upper limit of the firing temperature within this range allows for suppression of oxidation of the boron carbonitride after firing.

The lower limit of the calcination time in the oxidation step may be, for example, 6 hours or more, or 7 hours or more. By keeping the lower limit of the calcination time within this range, it is possible to sufficiently reduce the carbon content in the boron carbonitride. The upper limit of the calcination time in the oxidation step is not particularly limited, but may be, for example, 20 hours or less, or 15 hours or less. When the upper limit of the calcination time is within this range, a decrease in the processing efficiency of the step can be more sufficiently suppressed. If the calcination time is too short, the carbon content will not be sufficiently reduced. By keeping the upper limit of the calcination time within this range, it is possible to more sufficiently suppress the oxidation of some of the boron atoms in the heat-treated product to produce boric acid, thereby more effectively suppressing a decrease in the tap density of the boron nitride powder that occurs with an increase in the amount of boric acid.

The heat-treated product obtained in the oxidation step has a reduced oxygen content compared to the oxygen content of the fired product before heat treatment. The upper limit of the oxygen content of the heat-treated product may be, for example, 4% by mass or less, 3% by mass or less, 2% by mass or less, or 1.5% by mass or less, based on the total amount of the heat-treated product. By keeping the oxygen content of the heat-treated product within this range, it is possible to suppress an increase in the amount of liquid phase during firing in the subsequent crystallization step, and to further increase the tap density of the resulting boron nitride powder. The lower limit of the oxygen content of the heat-treated product is not particularly limited, but may be, for example, 0.1% by mass or more, or 0.5% by mass or more, based on the total amount of the heat-treated product.

The oxygen content in this specification refers to the value measured using a simultaneous oxygen/nitrogen analyzer. For example, the "EMGA-910" (product name) manufactured by Horiba, Ltd. can be used as a simultaneous oxygen/nitrogen analyzer.

The crystallization process generates primary particles of boron nitride by heating a mixture containing the heat-treated product obtained in the oxidation process and a boron source, and then obtains agglomerated particles composed of the primary particles agglomerated. That is, the crystallization process decarbonizes the boron carbonitride and increases the crystallinity of the boron nitride, resulting in boron nitride powder containing agglomerated particles composed of predetermined agglomerated primary particles of boron nitride.

The boron source may include, for example, at least one selected from the group consisting of boric acid and boron oxide. More specifically, the boron source may be boric acid, boron oxide, or a mixture thereof. The mixture heated in the crystallization step may contain, in addition to boron carbonitride and the boron source, known additives used in the _B4C method.

The upper limit of the amount of boron source in the mixture is less than 50 parts by mass per 100 parts by mass of the heat-treated product, but may be, for example, 45 parts by mass or less, 43 parts by mass or less, or 40 parts by mass or less. By setting the upper limit of the amount of boron source within this range, the tap density of the resulting boron nitride powder can be further increased, and the crushing strength of the agglomerated particles can be further improved. The amount of boron source in the mixture may be 30 parts by mass or more, 32 parts by mass or more, 34 parts by mass or more, or 36 parts by mass or more per 100 parts by mass of the heat-treated product. Setting the lower limit of the amount of boron source in this range can promote particle growth of boron nitride primary particles. The amount of boron source in the mixture may be adjusted within the above range, and may be, for example, 30 parts by mass or more but less than 50 parts by mass, 34 to 45 parts by mass, or 36 to 40 parts by mass per 100 parts by mass of the heat-treated product.

The lower limit of the firing temperature to which the mixture is heated in the crystallization process may be, for example, 1900°C or higher, or 2000°C or higher. By setting the lower limit of the firing temperature within the above range, particle growth can be sufficiently promoted. The upper limit of the firing temperature to which the mixture is heated in the crystallization process may be, for example, 2200°C or lower, or 2150°C or lower. By setting the upper limit of the firing temperature within the above range, yellowing of the boron nitride powder can be suppressed. The firing temperature in the crystallization process may be adjusted within the above range, and may be, for example, 1900 to 2200°C or 2000 to 2150°C. The firing temperature to which the mixture is heated in the crystallization process is preferably lower than the firing temperature of the boron carbide powder in the pressure nitriding process.

The crystallization process may be carried out under atmospheric pressure or under a pressure equal to or higher than atmospheric pressure. Note that all atmospheric pressures in this specification refer to gauge pressure. The lower limit of the pressure (atmospheric pressure) in the crystallization process may be, for example, 10 kPa or more, 15 kPa or more, or 20 kPa or more. Setting the lower limit of the pressure within the above range prevents auxiliary agents such as boric acid from being removed from the system, making the reaction field of the boron nitride particles more uniform. The upper limit of the pressure (atmospheric pressure) in the crystallization process may be, for example, 80 kPa or less, 60 kPa or less, or 40 kPa or less. Setting the upper limit of the pressure within the above range more sufficiently prevents the collapse of aggregated particles during the crystallization process. The pressure (atmospheric pressure) in the crystallization process may be adjusted within the above range, for example, 10 to 80 kPa, 10 to 60 kPa, or 20 to 40 kPa.

The lower limit of the firing time in the crystallization step may be, for example, 0.5 hours or more, 1 hour or more, or 3 hours or more. By setting the lower limit of the firing time within the above range, particle growth can be sufficiently promoted. The upper limit of the firing time in the crystallization step may be, for example, 40 hours or less, 30 hours or less, 20 hours or less, or 10 hours or less. By setting the upper limit of the firing time within the above range, increases in manufacturing costs can be suppressed. The firing time may be adjusted within the above range, for example, 0.5 to 40 hours, or 1.0 to 30 hours.

The method for producing boron nitride powder may include other processes. Examples of such processes include a pulverization process and a classification process. In the method for producing boron nitride powder, for example, a pulverization process may be carried out after the crystallization process. In the pulverization process, a general pulverizer or crusher can be used. For example, a ball mill, vibration mill, jet mill, etc. can be used. Note that in this specification, "pulverization" also includes "crushing."

The boron nitride powder according to the present disclosure has a small average particle size, a relatively large tap density, and high crushing strength of the agglomerated particles contained in the powder, making it suitable for use as a filler to be mixed with resins and the like. Furthermore, since the boron nitride powder can suppress the increase in viscosity when mixed with resin, the filling amount of the boron nitride powder can be increased compared to conventional methods, and molded articles (e.g., heat transfer sheets) formed using this resin composition can be expected to have excellent heat dissipation properties. One embodiment of the resin composition contains a resin and the boron nitride powder described above.

The resin composition can be used after molding, curing, etc. One embodiment of the cured product is a cured product of a resin composition containing a resin and the above-mentioned boron nitride powder. The shape of the cured product is not particularly limited, and may be a block, sheet, or film. In the case of a sheet-shaped cured product (sheet), the thickness of the sheet may be, for example, 0.5 mm or less, 0.2 mm or less, or 0.1 mm or less. Because the above-mentioned sheet contains the above-mentioned boron nitride, which has a relatively small particle size, a relatively thin sheet can be produced. Because the above-mentioned cured product contains the above-mentioned boron nitride, it is useful, for example, for heat dissipation components, etc.

Examples of resins include liquid crystal polymers, fluororesins, silicone resins, silicone rubber, acrylic resins, polyolefins (such as polyethylene), epoxy resins, phenolic resins, melamine resins, urea resins, unsaturated polyesters, polyimides, polyamideimides, polyetherimides, polybutylene terephthalate, polyethylene terephthalate, polyphenylene ether, polyphenylene sulfide, wholly aromatic polyesters, polysulfones, polyethersulfones, polycarbonates, maleimide-modified resins, ABS (acrylonitrile-butadiene-styrene) resins, AAS (acrylonitrile-acrylic rubber-styrene) resins, and AES (acrylonitrile-ethylene-propylene-diene rubber-styrene) resins.

The lower limit of the resin content may be, for example, 15% by volume or more, 20% by volume or more, or 30% by volume or more, based on the total volume of the resin composition. Having the lower limit of the resin content within this range more sufficiently prevents a decrease in the moldability of the resin composition. The upper limit of the resin content may be, for example, 60% by volume or less, 50% by volume or less, or 40% by volume or less, based on the total volume of the resin composition. Having the lower limit of the resin content within this range further improves the thermal conductivity of the resin composition.

The resin composition of the present disclosure uses the boron nitride powder described above, allowing for a relatively large amount of boron nitride powder to be contained. The lower limit of the boron nitride powder content may be, for example, 40 vol.% or more, 50 vol.% or more, 60 vol.% or more, or 65 vol.% or more, based on the total volume of the resin composition. Having the lower limit of the boron nitride powder content within the above range improves the thermal conductivity of the resin composition, making it possible to obtain a molded product with excellent heat dissipation properties. The upper limit of the boron nitride powder content may be, for example, 85 vol.% or less, 80 vol.% or less, or 70 vol.% or less, based on the total volume of the resin composition. Having the upper limit of the boron nitride powder content within the above range prevents a decrease in the moldability of the resin composition.

In addition to the resin and boron nitride powder, the resin composition may further contain a curing agent to cure the resin. The curing agent can be selected appropriately depending on the type of resin. When the resin is an epoxy resin, examples of the curing agent include phenol novolac compounds, acid anhydrides, amino compounds, and imidazole compounds. The lower limit of the curing agent content may be, for example, 0.5 parts by mass or more, or 1 part by mass or more, per 100 parts by mass of resin. The upper limit of the curing agent content may be, for example, 15 parts by mass or less, or 10 parts by mass or less, per 100 parts by mass of resin.

Although several embodiments of the present disclosure have been described above, the present disclosure is in no way limited to the above-described embodiments. Furthermore, the descriptions of the above-described embodiments can be mutually applied.

The present disclosure will be described in more detail below using examples and comparative examples. Note that the present disclosure is not limited to the following examples.

Example 1
[Preparation of boron carbide powder]
100 parts by mass of orthoboric acid (manufactured by Nippon Denko Corporation, hereinafter simply referred to as "boric acid") and 35 parts by mass of acetylene black (HS100, manufactured by Denka Co., Ltd.) were mixed using a Henschel mixer and then loaded into a graphite crucible. The graphite crucible was placed in an arc furnace and heated at 2200°C for 5 hours in an argon atmosphere to synthesize a boron carbide ( _B4C ) powder. The synthesized boron carbide powder was pulverized in a ball mill for 1 hour and classified using a sieve with 63 μm mesh. The boron carbide powder remaining under the sieve was further washed with an aqueous nitric acid solution to remove impurities such as iron, filtered, and dried to produce a boron carbide powder ( _B4C powder) with an average particle size of 15 μm.

[Preparation of boron carbonitride powder]
The boron carbide powder thus prepared was filled into a boron nitride crucible and heated in a resistance heating furnace at 2100°C for 25 hours under a nitrogen gas atmosphere of 0.85 MPa to obtain boron carbonitride (B ₄ CN ₄ ) powder (pressure nitriding step).

The resulting boron carbonitride powder was placed in a muffle furnace and heated in air at 700°C for 5 hours (oxidation step) to obtain a heat-treated product. The oxygen content of the heat-treated product was 1.8% by mass. The oxygen content was measured using a simultaneous oxygen/nitrogen analyzer (Horiba, Ltd., EMGA-910 model).

[Preparation of boron nitride powder]
Boric acid, the boron source, was added to 100 parts by mass of the heat-treated product so that the content of boric acid was 40 parts by mass, and the mixture was mixed using a Henschel mixer to obtain a mixture. The mixture was filled into a boron nitride crucible and decarbonized by heating using a resistance heating furnace to synthesize a boron nitride powder containing aggregated particles formed by aggregation of primary particles (crystallization process). The crystallization process was carried out under conditions of a nitrogen gas atmosphere at a pressure of 13 kPa, heating from room temperature to 2000°C, and maintaining the temperature at 2000°C for 5 hours. The synthesized boron nitride powder was crushed in a mortar for 10 minutes and then classified using a nylon sieve with 75 μm mesh.

Example 2
Boron nitride powder was prepared in the same manner as in Example 1, except that the boron carbide powder used was changed to one with an average particle size of 10 μm, and the oxygen content of the heat-treated product was 2.3.

Example 3
Boron nitride powder was prepared in the same manner as in Example 1, except that the boron carbide powder used was changed to one with an average particle size of 3 μm, and the oxygen content of the heat-treated product was 2.5%.

(Comparative Example 1)
A boron nitride powder was prepared in the same manner as in Example 1, except that a powder having an average particle size of 16 μm was used as the boron carbide powder, the oxygen content of the heat-treated product was 2.2, and the content of boric acid was changed to 55 parts by mass per 100 parts by mass of the heat-treated product.

(Comparative Example 2)
A boron nitride powder was prepared in the same manner as in Example 1, except that a powder having an average particle size of 10 μm was used as the boron carbide powder, the oxygen content of the heat-treated product was 1.9, and the content of boric acid was changed to 55 parts by mass per 100 parts by mass of the heat-treated product.

(Comparative Example 3)
A boron nitride powder was prepared in the same manner as in Example 1, except that a powder having an average particle size of 26 μm was used as the boron carbide powder, the oxygen content of the heat-treated product was 2.2, and the content of boric acid was changed to 40 parts by mass per 100 parts by mass of the heat-treated product.

(Comparative Example 4)
A boron nitride powder was prepared in the same manner as in Example 1, except that a powder having an average particle size of 16 μm was used as the boron carbide powder, the oxygen content of the heat-treated product was 2.0, and the content of boric acid was changed to 75 parts by mass per 100 parts by mass of the heat-treated product.

(Comparative Example 5)
A boron nitride powder was prepared in the same manner as in Example 1, except that a powder having an average particle size of 16 μm was used as the boron carbide powder and the content of boric acid was changed to 55 parts by mass per 100 parts by mass of the heat-treated product.

(Comparative Example 5)
For comparison, boron nitride powder containing agglomerated particles was prepared by the so-called melamine borate method, in which boron nitride was prepared and granulated. Specifically, 100 parts by mass of boric acid powder (purity 99.8% by mass or higher, manufactured by Kanto Chemical Co., Inc.) and 90 parts by mass of melamine powder (purity 99.0% by mass or higher, manufactured by Wako Pure Chemical Industries, Ltd.) were mixed in an alumina mortar for 10 minutes to obtain a mixed raw material. The dried mixed raw material was placed in a hexagonal boron nitride container and placed in an electric furnace. While flowing nitrogen gas through the electric furnace, the temperature was increased from room temperature to 1000°C at a rate of 10°C/min. After holding at 1000°C for 4 hours, heating was stopped and the material was allowed to cool naturally. The electric furnace was opened when the temperature reached 100°C or below. In this manner, a calcined product containing low-crystalline boron nitride was obtained.

Next, 20 parts by weight of boric acid and 3 parts by weight of sodium carbonate (purity 99.5% or higher) as an auxiliary agent were added to 100 parts by weight of the above-mentioned calcined product, and mixed for 10 minutes using an alumina mortar. The mixture was placed in the above-mentioned electric furnace. While circulating nitrogen gas through the electric furnace, the temperature was raised from room temperature to 2000°C at a rate of 10°C/min. After maintaining the firing temperature of 2000°C for 4 hours, heating was stopped and the mixture was allowed to cool naturally. When the temperature reached 100°C or below, the electric furnace was opened. The resulting fired product was collected and pulverized in an alumina mortar for 3 minutes to obtain coarse boron nitride powder.

To 100 parts by weight of the boron nitride coarse powder obtained as described above, 3 parts by weight of calcium carbonate as an auxiliary agent and 140 parts by weight of water were added, mixed using a Henschel mixer, and then pulverized in a ball mill for 5 hours to obtain a slurry. To 100 parts by weight of the obtained slurry, 0.5 parts by weight of polyvinyl alcohol resin and 0.5 parts by weight of anionic surfactant were added, and the mixture was stirred at 50°C until dissolved, obtaining a solution. The resulting solution was sprayed using a rotary atomizer into a spray dryer set at 17,000 rpm and 230°C, yielding a powder of loosely agglomerated boron nitride primary particles. The resulting powder was further heat-treated in a batch-type high-frequency furnace under a nitrogen atmosphere at 1,850°C for 4 hours to produce boron nitride powder.

<Evaluation of boron nitride powder properties>
The tap density, average particle size, crushing strength, and orientation index were evaluated for each of the boron nitride powders obtained in Examples 1 and 2 and Comparative Examples 1 to 5. The results are shown in Table 1.

<Evaluation of boron nitride powder as a filler>
Resin compositions were prepared using the boron nitride powders obtained in Examples 1 and 2 and Comparative Examples 1 to 5, and the boron nitride powders were evaluated as fillers for resins by the methods described below. In Table 1, the viscosity and thermal conductivity are shown as relative values based on the measured values for the boron nitride powder of Comparative Example 1.

[Film forming property]
A resin composition was obtained by mixing boron nitride powder with a mixture of 100 parts by mass of a naphthalene-type epoxy resin (HP4032, manufactured by DIC Corporation) and 10 parts by mass of an imidazole compound (2E4MZ-CN, manufactured by Shikoku Chemicals Corporation) as a curing agent, so that the boron nitride powder was 60% by volume. A Thinky Mixer (product name) was used to knead the resin. The kneading conditions were 1600 rpm for 3 minutes. The resulting resin composition was applied to a PET film to a thickness of 500 μm. The resin was then cured by heating and pressurizing under relatively mild conditions of 150°C and 50 kgf/ ^cm² for 60 minutes, producing a 50 μm resin sheet (evaluation sheet). The film obtained as described above was visually observed and evaluated according to the following criteria.
A: No defects were observed in the film.
B: Defects were observed in the film.

[Viscosity of Resin Composition]
A composition was obtained by mixing boron nitride powder with silicone oil (KF-96L, manufactured by Shin-Etsu Chemical Co., Ltd.). The amount of boron nitride powder in the resin composition was adjusted to 25% by volume. A Thinky Mixer (product name: Awatori Rentaro) manufactured by Thinky Corporation was used to knead the resin. The kneading conditions were 1600 rpm for 3 minutes. The viscosity of the resulting composition was measured using a rotational viscometer (MCR302, manufactured by Anton Paar Japan) to confirm the shear rate dependency of the viscosity. The measurements were performed at 25°C over a shear rate range of 0.01 to 100 sec ^-1 . The results are shown in Table 1. In Table 1, the viscosity values are relative to the viscosity at a shear rate of 1 sec ^-1 for the resin composition using the boron nitride powder prepared in Comparative Example 1.

[Thermal conductivity of resin sheet]
The thermal conductivity was evaluated using a resin sheet (evaluation sheet) manufactured in the same manner as the resin sheet used to evaluate the film formability. The thermal conductivity H (unit: W / (m K)) was calculated from the values of thermal diffusivity A (unit: m ² / sec), density B (unit: kg / m ³ ), and specific heat capacity C (unit: J / (kg K)) based on the formula H = A × B × C. The thermal diffusivity A was measured by processing the evaluation sheet into a length of 10 mm, a width of 10 mm, and a thickness of 0.3 mm using a laser flash method. The measurement device used was a xenon flash analyzer (manufactured by NETZSCH, product name: LFA447NanoFlash). The density B was determined using the Archimedes method. The specific heat capacity C was determined using a DSC (manufactured by Rigaku Corporation, product name: ThermoPlusEvoDSC8230).

The present disclosure provides boron nitride powder that can be used to prepare a resin composition that can be used to produce a heat transfer sheet with excellent fillability into resin and excellent heat dissipation, a resin composition containing such boron nitride powder, a cured product of the resin composition, and a method for producing boron nitride powder.

Claims (5)

The boron nitride powder contains agglomerated particles formed by agglomerating primary particles of boron nitride,
The tap density is more than 0.9 g/ ^cm3 and the average particle size is 20 μm or less,
The boron nitride powder has a crushing strength of 10 MPa or more. The boron nitride powder according to claim 1, having an average particle size of 2 μm or more. The boron nitride powder according to claim 1 or 2, having an orientation index of 10 or less. A resin composition containing a resin and the boron nitride powder described in any one of claims 1 to 3. A cured resin composition containing a resin and the boron nitride powder described in any one of claims 1 to 3.