Hexagonal boron nitride powder, resin composition, and method for producing hexagonal boron nitride powder

Abstract

The present disclosure aims to provide hexagonal boron nitride powder, etc., for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength. A hexagonal boron nitride powder according to one aspect of the present disclosure is a hexagonal boron nitride powder containing hexagonal boron nitride aggregates (101), characterized in that the average particle size is 25 μm or more and 70 μm or less, the cumulative frequency of particle size of 40 μm or more in the volume-based particle size distribution is 65% or more, the tap bulk density is 0.84 g / mL or more and 1.05 g / mL or less, and the oil absorption is 50 mL / 100 g or more and 84 mL / 100 g or less.

Description

Technical Field

[0001] The present disclosure relates to hexagonal boron nitride powder, a resin composition, and a method for producing hexagonal boron nitride powder.

Background Art

[0002] In recent years, with the miniaturization and high-powerization of electronic components, an increase in the amount of heat generated by electronic components has become a problem. Therefore, in order to efficiently dissipate heat from electronic components, development of materials having excellent thermal conductivity has been carried out.

[0003] A resin composition obtained by blending hexagonal boron nitride powder with a resin exhibits improved thermal conductivity compared to the resin itself. Therefore, such a resin composition is suitably used as a material for electronic components. It is known that the hexagonal boron nitride powder blended with the resin further improves the thermal conductivity of the resin composition by including aggregates in which single particles of hexagonal boron nitride are aggregated. As a technique related to such aggregates, Patent Document 1 discloses boron nitride powder that is useful for producing a heat transfer sheet having excellent filling properties in resin and excellent heat dissipation properties because it has a relatively small average particle size of 20 μm or less and a large tapped bulk density.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] For a resin composition as a material for heat dissipation, high thermal conductivity and insulation are required.

[0006] In the boron nitride powder described in Patent Document 1, due to its small average particle size, increasing the packing density tends to dramatically increase the number of interfaces between aggregates. As a result, both an improvement in thermal conductivity due to increased packing density and a decrease in thermal conductivity due to heat conduction loss at the aforementioned interfaces occur simultaneously. Therefore, the boron nitride powder described in Patent Document 1 has the problem of not being able to impart to the resin composition the thermal conductivity expected from the increased packing density.

[0007] In the case of the boron nitride powder described in Patent Document 1, increasing the packing density gradually improves the thermal conductivity, but it also increases the viscosity of the varnish containing the powder, resin composition, and solvent, thus worsening the varnish's coatability. Increasing the amount of solvent added improves the varnish's coatability, but as the amount of solvent added increases, the amount of solvent that must be dried off during the drying of the coated object also increases, resulting in an increase in voids in the coated object and consequently a decrease in the dielectric strength of the resin composition.

[0008] As described above, even with the boron nitride powder described in Patent Document 1, it is difficult to achieve both excellent thermal conductivity and dielectric strength.

[0009] One aspect of this disclosure aims to provide hexagonal boron nitride powder, etc., for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength. [Means for solving the problem]

[0010] To solve the above problems, a hexagonal boron nitride powder according to one aspect of the present disclosure is a hexagonal boron nitride powder containing hexagonal boron nitride aggregates, characterized in that it has an average particle size of 25 μm or more and 70 μm or less, a cumulative frequency of particle size of 40 μm or more in a volume-based particle size distribution of 65% or more, a tap bulk density of 0.84 g / mL or more and 1.05 g / mL or less, and an oil absorption capacity of 50 mL / 100 g or more and 84 mL / 100 g or less.

[0011] Furthermore, a manufacturing method according to one aspect of this disclosure is a method for producing hexagonal boron nitride powder containing hexagonal boron nitride aggregates, comprising a main heating step of heating a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and boron carbide under a nitrogen atmosphere at a temperature of 1500°C to 1900°C to obtain crude boron nitride powder, wherein in the raw material mixture, the ratio of the mass of the oxygen-containing boron compound converted to B and the mass of the carbon source converted to C (mass converted to B / mass converted to C) is 0.73 to 0.85, and the ratio of the mass of the oxygen-containing calcium compound converted to B2O3 on a B basis to 100 parts by mass of the total of the mass of the oxygen-containing boron compound converted to B2O3 on a B basis and the mass of the carbon source converted to C is based on the Ca basis of the oxygen-containing calcium compound. The invention is characterized by comprising: a main heating step in which the content converted to CaO is 4 parts by mass or more and 15 parts by mass or less, and the content of boron carbide is 10 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the total of the mass converted to B2O3 on the basis of B of the oxygenated boron compound, the mass converted to C of the carbon source, and the mass converted to CaO on the basis of Ca of the oxygenated calcium compound; a classification step in which the crude boron nitride powder is classified using a sieve with a mesh size of 15 μm or more and 40 μm or less; a reheating step in which the residue obtained from the classification step is heated at a temperature of 1850°C or more and 2000°C or less under a nitrogen atmosphere; and a crushing step in which the boron nitride powder obtained from the reheating step is ground to a clearance of 100 μm or more and 500 μm or less. [Effects of the Invention]

[0012] According to one aspect of this disclosure, hexagonal boron nitride powder and the like can be provided for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram showing an overview of hexagonal boron nitride powder, where reference numerals 1001 and 1002 indicate conventional powders, and reference numeral 1003 indicates an example of the powder according to the present disclosure. [Modes for carrying out the invention]

[0014] An embodiment of this disclosure will be described in detail below. In this specification, "A to B" representing a numerical range means "A or greater and B or less" unless otherwise specified.

[0015] <Hexagonal boron nitride powder> A hexagonal boron nitride powder according to one aspect of the present disclosure is a hexagonal boron nitride powder containing hexagonal boron nitride aggregates, characterized in that it has an average particle size of 25 μm or more and 70 μm or less, a cumulative frequency of particle size of 40 μm or more in a volume-based particle size distribution of 65% or more, a tap bulk density of 0.84 g / mL or more and 1.05 g / mL or less, and an oil absorption capacity of 50 mL / 100 g or more and 84 mL / 100 g or less.

[0016] As described above, in the boron nitride powder of Patent Document 1, as shown in the conventional example indicated by reference numeral 1001 in Figure 1, the average particle size is small, so as the packing density increases, the interfaces between aggregates 101 tend to increase dramatically. Therefore, the boron nitride powder of Patent Document 1 cannot impart to the resin composition the thermal conductivity expected from the increased packing density.

[0017] To solve this problem, one possible method is to increase the average particle size of the powder, as shown in the conventional example indicated by reference numeral 1002 in Figure 1. Increasing the average particle size reduces the number of interfaces between aggregates 101, thereby improving the thermal conductivity of the resin composition. However, when the average particle size is increased, during the powder manufacturing process, not only aggregates 101 but also bonded bodies 103, formed by the bonding of several single particles 102, are often formed. Although bonded bodies 103 contribute significantly to the formation of thermal pathways, their irregular shape tends to create large gaps between them and other particles. Therefore, increasing the average particle size reduces the tap bulk density of the powder, which lowers the filling rate of the powder into the resin. In addition, in the conventional example indicated by reference numeral 1002, the amount of oil absorbed also increases, which improves the viscosity of the varnish, worsens the varnish's coatability, and can also easily lead to a decrease in the dielectric strength of the resin composition.

[0018] Therefore, the inventors of the present invention have repeatedly studied the above problems and found the powder exemplified by reference numeral 1003 in FIG. 1. In the example shown by reference numeral 1003, the bonded body 103 is removed as compared with reference numeral 1002, and the hexagonal boron nitride powder is mainly composed of aggregates 101 of a certain size and single particles 102 of fine powder. In such a powder, the aggregates 101 contribute to the improvement of the thermal conductivity of the resin composition, and the single particles 102 fill the gaps between the aggregates 101, so that the powder exhibits a high tapped bulk density. Further, due to the high tapped bulk density, it can be expected that the hexagonal boron nitride powder can be filled into the resin at a high filling rate. Further, since the single particles 102 are present on the surface of the aggregates 101, the powder exhibits a low oil absorption amount. Due to the low oil absorption amount, the increase in the viscosity of the resin composition when the hexagonal boron nitride powder is filled into the resin is reduced, and the resin composition is imparted with high dielectric strength.

[0019] Hereinafter, the hexagonal boron nitride powder according to one aspect of the present disclosure will be described in detail. In the description, the hexagonal boron nitride powder according to one aspect of the present disclosure may be abbreviated as "this powder".

[0020] This powder contains hexagonal boron nitride aggregates (hereinafter, may be simply referred to as "aggregates"). An aggregate is a secondary particle formed by aggregation of a plurality of primary particles (single particles) of hexagonal boron nitride. The aggregates contained in this powder are observed using a known microscopic observation method such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0021] Generally, a single particle of hexagonal boron nitride has a flake shape due to its hexagonal crystal structure, and exhibits a high thermal conductivity in the tangential direction of the main surface of the flake, but a lower thermal conductivity in the normal direction of the main surface. Compared with such single particles exhibiting thermal anisotropy, aggregates formed by randomly orienting a plurality of single particles have reduced thermal anisotropy, which is advantageous for use as a filler in a resin composition.

[0022] (Average particle size, particle size distribution) The average particle size of this powder is an indicator of heat path formation and void generation in a resin composition obtained by filling a resin with this powder, and it is desirable to control it within a specific range. The average particle size of this powder is 25 μm or larger, preferably 50 μm or larger. The larger the average particle size within this range, the fewer the interfaces between aggregates that cause heat conduction loss in the resin composition, and the better the heat path formation, thus the greater the effect of this powder on improving the thermal conductivity of the resin composition. Alternatively, the average particle size of this powder is 70 μm or smaller, preferably 65 μm or smaller. An average particle size within this range tends to mean that there are a certain amount of fine particles with a relatively small particle size in this powder. Such fine particles fill the gaps between larger particles in the resin composition, and are useful for improving heat path formation and avoiding void generation in the gaps between particles.

[0023] The cumulative frequency of particles with a particle size of 40 μm or larger in the volume-based particle size distribution of this powder is an indicator of the proportion of particles with a particle size of 40 μm or larger, and it is desirable to control it to a certain value or higher. Larger particles have superior thermal conductivity per particle, ignoring the gaps between particles, compared to smaller particles. In this powder, the cumulative frequency of particles with a particle size of 40 μm or larger in the volume-based particle size distribution is 65% or higher, preferably 70% or higher. The larger the cumulative frequency of particles with a particle size of 40 μm or larger within these ranges, the greater the effect of this powder on improving the thermal conductivity of the resin composition. The cumulative frequency of particles with a particle size of 40 μm or larger is, for example, 85% or less, but this disclosure is not limited thereto.

[0024] The cumulative frequency of particle sizes between 20 μm and 40 μm in the volume-based particle size distribution of this powder is preferably 10% or less, and more preferably 7% or less. The smaller the cumulative frequency of particle sizes between 20 μm and 40 μm within this range, the more effectively the decrease in the tap bulk density of this powder can be suppressed, resulting in improved fillability in resins.

[0025] The cumulative frequency of particles with a particle size of less than 20 μm in the volume-based particle size distribution of this powder is an indicator of the proportion of fine particles, and it is desirable to control it to a certain value or higher. Fine particles play a role in filling the gaps between large aggregates, contributing to a decrease in oil absorption and an increase in tap bulk density. In this powder, the cumulative frequency of particles with a particle size of less than 20 μm in the volume-based particle size distribution is preferably 15% or more, and more preferably 20% or more. The larger the cumulative frequency of particles with a particle size of less than 20 μm within these ranges, the greater the effect of this powder on improving the dielectric strength of the resin composition. The cumulative frequency of particles with a particle size of less than 20 μm is, for example, 30% or less, but this disclosure is not limited thereto.

[0026] The particle size exhibiting the mode in the volume-based particle size distribution range of 40 μm or more is preferably 65 μm or more, and more preferably 80 μm or more. The larger the particle size exhibiting the mode within these ranges, the greater the effect of this powder on improving the thermal conductivity of the resin composition. Conventionally, when the particle size exhibiting the mode becomes as large as described above, there is a problem that a decrease in tap bulk density and an increase in oil absorption tend to occur. However, with this powder, it is possible to mitigate these problems by increasing the cumulative frequency of particle sizes of 40 μm or more and the particle size exhibiting the mode, while keeping the average particle size within the above range. The particle size exhibiting the mode in the range of 40 μm or more is, for example, 105 μm or less, but this disclosure is not limited thereto.

[0027] The particle size exhibiting the mode in the volume-based particle size distribution range of less than 40 μm is preferably 15 μm or less, and more preferably 12 μm or less. The smaller the particle size exhibiting the mode within this range, the greater the effect of this powder on improving the dielectric strength of the resin composition. The particle size exhibiting the mode in the range of less than 40 μm is, for example, 5 μm or more, but this disclosure is not limited thereto.

[0028] The value obtained by dividing the average particle size (in μm) of this powder by the cumulative frequency (in %) of particles with a particle size of 40 μm or more in the volume-based particle size distribution is preferably 0.50 μm / % to 0.90 μm / % and more preferably 0.70 μm / % to 0.88 μm / %. When this powder satisfies such a value, the effect of improving the thermal conductivity and dielectric strength of the resin composition by this powder tends to be greater.

[0029] In this specification, the particle size distribution is measured by the wet laser diffraction particle size distribution method, as shown in the examples described later, and is volume-based. The particle size distribution is measured while the sample is subjected to ultrasonic treatment at an output of 40W for dispersion. The average particle size refers to the median diameter (D50) in the particle size distribution measured in this way.

[0030] (Tap volume density) In this powder, tap bulk density is an indicator of the amount of space between the particles constituting the powder, and it is preferable to control it within a specific range. The tap bulk density of this powder is 0.84 g / mL or higher, preferably 0.87 g / mL or higher, and more preferably 0.91 g / mL or higher. The higher the tap bulk density within this range, the more the amount of space between particles is reduced, so this powder forms more heat paths in the resin composition and further improves the thermal conductivity of the resin composition. The tap bulk density of this powder is 1.05 g / mL or lower, preferably 1.01 g / mL or lower. The lower the tap bulk density within this range, the more it is possible to reduce the weight of this powder required to form sufficient heat paths in the resin composition.

[0031] In this specification, the tap bulk density of this powder is measured using, for example, a Tap Denser KYT-5000 manufactured by Seishin Corporation, under the measurement conditions described in the examples below.

[0032] (Oil absorption amount) In this powder, the oil absorption amount is an indicator of the number of voids in the aggregates contained in the powder, the degree of development of the structure on the aggregate surface, and the wettability to the resin, and it is preferable to control it within a specific range. The lower the oil absorption amount, the less voids there are in the aggregates that take in solvent when preparing the varnish, overall in this powder. The oil absorption amount of this powder is 50 mL / 100g or more, preferably 63 mL / 100g or more. The higher the oil absorption amount within this range, the more aggregates that contribute greatly to improving thermal conductivity are contained in this powder, and therefore the effect of this powder on improving the thermal conductivity of the resin composition is greater. Alternatively, the oil absorption amount of this powder is 84 mL / 100g or less, preferably 80 mL / 100g or less, and more preferably 75 mL / 100g or less. The lower the oil absorption amount within this range, the less solvent is taken in overall in this powder, which causes void formation and a decrease in dielectric strength in the resin composition, and therefore the effect of this powder on improving the dielectric strength of the resin composition is greater.

[0033] Furthermore, the method for measuring the amount of oil absorbed in this specification is the method compliant with JIS-K6217-4, as shown in the examples described later.

[0034] (specific surface area) In this powder, the specific surface area is an indicator of the size of the individual particles contained in the powder, and it is desirable to control it within a specific range.

[0035] A higher specific surface area indicates that the individual particles constituting the aggregate are smaller and the aggregate is denser. The specific surface area of ​​this powder is 1.8 m². 2 Preferably 2.3m / g or more. 2 A value of 1 / g or higher is more preferable. Within this range, the higher the value, the greater the amount of dense aggregates that easily form thermal pathways, which contribute significantly to improving thermal conductivity. Therefore, the effect of this powder on improving the thermal conductivity of the resin composition tends to be greater.

[0036] Furthermore, the specific surface area of ​​this powder is 4.0 m². 2 Preferably less than / g, and 3.5m 2 More preferably less than / g, and 3.0m 2A value of less than / g is even more preferable. The lower the value within this range, the easier it is for the resin to penetrate into the aggregates when the powder is mixed with the resin, and the suppression of void formation, which causes a decrease in dielectric strength. Therefore, the effect of improving the dielectric strength of the resin composition by this powder tends to be greater.

[0037] In this specification, the specific surface area of ​​this powder is measured by the BET 1-point method, as shown in the examples described later. The specific surface area of ​​this powder can be measured, for example, using a Flowsorb III 2310 (manufactured by Micromeritics).

[0038] (Compressive fracture strength) In this powder, the compressive fracture strength is an indicator of the hardness of the aggregates contained in the powder, and it is desirable to control it within a specific range.

[0039] A higher compressive strength indicates that the aggregates are less likely to break down when mixed with the resin. The compressive strength of this powder is preferably 1.0 MPa or higher, and more preferably 2.0 MPa or higher. The higher the strength within this range, the more likely the aggregates are to remain intact in the resin composition, thus suppressing orientation within the resin composition and increasing the effect of this powder on improving the thermal conductivity of the resin composition.

[0040] Furthermore, the compressive fracture strength of this powder is preferably 5.0 MPa or less, and more preferably 4.0 MPa or less. The lower the value within this range, the easier it is for aggregates containing closed pores to break down when the powder is mixed with the resin. This suppresses the formation of voids in the resin composition, and the effect of improving the dielectric strength of the resin composition by this powder tends to be greater.

[0041] In this specification, the compressive fracture strength of this powder is measured by the method shown in the examples described later. The compressive fracture strength of this powder can be measured, for example, using the MCT-510 (product name) manufactured by Shimadzu Corporation.

[0042] (composition) The particles constituting this powder contain boron nitride as the main component, and preferably consist substantially of boron nitride. The boron nitride content in this powder is, for example, 90, 95, or 99% by mass or more, preferably 99.95% by mass or more, and more preferably 99.97% by mass or more. The higher the boron nitride content within these ranges, the less curing inhibition of the resin composition, which is caused by impurity elements and leads to a decrease in thermal conductivity and dielectric strength, can be reduced.

[0043] The boron nitride content in this powder is the value obtained by subtracting the mass percentage (in mass%) of elements other than B and N in this powder from 100. The boron nitride content is measured by X-ray fluorescence analysis, and can be confirmed using, for example, the Rigaku ZSX Primus2 (product name) X-ray fluorescence analyzer.

[0044] <Resin composition> Applications of this powder include its use as a filler in resins to improve dielectric strength and thermal conductivity. That is, resin compositions containing this powder and resins also fall within the scope of this disclosure. A resin composition according to one aspect of this disclosure has excellent thermal conductivity and dielectric strength due to this powder. Hereinafter, a resin composition according to one aspect of this disclosure may be abbreviated as "this resin composition." This resin composition can be used in a variety of applications, for example, as a thermally conductive resin composition or as a material for thermally conductive molded articles.

[0045] In this resin composition, the content of this powder is preferably 30 to 90% by volume, and more preferably 40 to 80% by volume, relative to the resin composition. A powder content of 30% by volume or more facilitates the formation of thermal pathways, making it easier to improve the thermal conductivity of the resin composition. A powder content of 90% by volume or less facilitates the uniform mixing of the resin and the powder, preventing the formation of gaps due to insufficient resin and making it easier to improve the dielectric strength of the resin composition.

[0046] Examples of resins included in this resin composition include: thermoplastic resins such as polyolefins, vinyl chloride resins, methyl methacrylate resins, nylon and fluororesins; thermosetting resins such as epoxy resins, phenolic resins, urea resins, melamine resins, unsaturated polyester resins, silicon resins and bismaleimidotriazine resins; synthetic rubber; and the like.

[0047] This resin composition may further contain other components in addition to the powder and resin. The other components are appropriately selected from known components depending on the application of the resin composition. Examples of other components include thermally conductive fillers such as aluminum nitride and aluminum oxide; known additives such as polymerization initiators, curing agents, polymerization inhibitors, polymerization retarders, coupling agents, plasticizers, ultraviolet absorbers, pigments, dyes, antibacterial agents, organic fillers, and organic-inorganic composite fillers.

[0048] This resin composition can be used as a material for heat-dissipating molded products such as heat dissipation sheets, phase change sheets, heat dissipation tapes, heat dissipation resin substrates in printed circuit board (PWB) base resin substrates or copper-clad laminate (CCL) base resin substrates, and insulating layers in metal base substrates such as aluminum base substrates or copper base substrates; and as a heat dissipation material such as heat dissipation grease, heat dissipation adhesive, gap filler, heat dissipation paint, heat dissipation coating, and encapsulant for power devices.

[0049] Furthermore, the uses of this powder are not limited to fillers for resins. Examples of other uses include boron nitride processed products such as boron nitride molded products or raw materials for cubic boron nitride, nucleating agents for engineering plastics, phase change materials, solid or liquid thermal interface materials, release agents for molds of molten metal or molten glass, cosmetics, and raw materials for composite ceramics.

[0050] <Method for producing hexagonal boron nitride powder> A method for producing hexagonal boron nitride powder according to one aspect of the present disclosure is a method for producing hexagonal boron nitride powder containing hexagonal boron nitride aggregates, comprising a main heating step of heating a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and boron carbide under a nitrogen atmosphere at a temperature of 1500°C to 1900°C to obtain crude boron nitride powder, wherein in the raw material mixture, the ratio of the mass of the oxygen-containing boron compound converted to B to the mass of the carbon source converted to C (mass converted to B / mass converted to C) is 0.73 to 0.85, and the oxygen-containing calcium compound is present in proportion to 100 parts by mass of the total of the mass of the oxygen-containing boron compound converted to B2O3 on a B basis and the mass of the carbon source converted to C. The invention is characterized by comprising: a main heating step in which the content converted to CaO on a Ca basis is 4 parts by mass or more and 15 parts by mass or less, and the content of boron carbide is 10 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the total of the mass converted to B2O3 on a B basis for oxygenated boron compounds, the mass converted to C for carbon sources, and the mass converted to CaO on a Ca basis for oxygenated calcium compounds; a classification step in which the crude boron nitride powder is classified using a sieve with a mesh size of 15 μm or more and 40 μm or less; a reheating step in which the residue obtained from the classification step is heated at a temperature of 1850°C or more and 2000°C or less under a nitrogen atmosphere; and a crushing step in which the boron nitride powder obtained from the reheating step is ground to a clearance of 100 μm or more and 500 μm or less.

[0051] To solve the problems of the conventional technology described above, the inventors conducted extensive research. As a result, the inventors found that in the manufacturing process of hexagonal boron nitride powder, it is useful to perform classification after heat treatment for reductive nitriding to collect particles with larger particle sizes, and then reheat and grind these particles. Through this series of processes, in addition to fine powder, single-particle aggregates are also removed by classification, and a powder consisting mainly of aggregates is obtained. When the obtained powder is reheated, the aggregates grow, becoming larger aggregates, and the strength of the aggregates also improves. Next, when the grown aggregates are ground, the aggregates are partially broken down, and a certain amount of fine powder particles are generated from the aggregates. The hexagonal boron nitride powder finally produced contains almost no single-particle aggregates and is mainly composed of somewhat large aggregates and fine single particles, making it useful for imparting excellent thermal conductivity and dielectric strength to resin compositions.

[0052] The following describes in detail a method for producing hexagonal boron nitride powder according to one aspect of this disclosure. In this description, the method for producing hexagonal boron nitride powder according to one aspect of this disclosure may be abbreviated as "this manufacturing method." This manufacturing method may be the method for producing this powder described above, but is not limited thereto. Furthermore, the hexagonal boron nitride powder produced by this manufacturing method is also within the scope of this disclosure.

[0053] This manufacturing method includes a main heating step, a first classification step, a reheating step, and a crushing step in that order. Furthermore, this manufacturing method may further include a mixing step before the main heating step. Also, after the main heating step and before the first classification step, this manufacturing method may further include at least one of a grinding step and an acid washing step in that order. Furthermore, this manufacturing method may further include a second classification step after the crushing step.

[0054] (Mixing process) The mixing process involves mixing hexagonal boron nitride powder as raw materials to obtain a raw material mixture. The types and ratios of raw materials to be mixed will be clear to those skilled in the art from the description of the raw material mixture, which will be discussed later in relation to the main heating process. The mixing of raw materials can be carried out using known methods, such as using a vibratory mill, bead mill, ball mill, Henschel mixer, drum mixer, vibratory stirrer, or V-type mixer. Even when using three or more types of raw materials, there are no particular restrictions on the order in which the raw materials are mixed; all raw materials may be mixed simultaneously, or they may be mixed in any order. Furthermore, to mix the raw materials more uniformly, they may be mixed while being crushed.

[0055] (Main heating process) The main heating process involves heating the raw material mixture under a nitrogen atmosphere. During this main heating process, the reduction nitridation reaction of the boron element contained in the raw material mixture proceeds, yielding hexagonal crude boron nitride powder.

[0056] (raw material mixture) The raw material mixture contains an oxygenated boron compound, a carbon source, an oxygenated calcium compound, and boron carbide.

[0057] (oxygen-containing boron compound) Oxygenated boron compounds are any compounds containing boron and oxygen, preferably compounds comprising at least boron and oxygen, and optionally hydrogen, or inorganic salts of such compounds. Examples of oxygenated boron compounds include boric acid, boric anhydride, metaboric acid, perboric acid, subboric acid, sodium tetraborate, and sodium perborate. Among these, readily available boric acid or boron oxide are preferably used as oxygenated boron compounds.

[0058] When the oxygenated boron compound is granular, its average particle size is preferably 30 μm or more, more preferably 50 μm or more, and even more preferably 100 μm or more. The larger the average particle size within this range, the easier the oxygenated boron compound is to handle. Furthermore, the average particle size of the oxygenated boron compound is preferably 800 μm or less, more preferably 700 μm or less, and even more preferably 500 μm or less. The smaller the average particle size within this range, the easier the reduction nitridation reaction of the oxygenated boron compound tends to proceed. In this specification, the average particle size of each component contained in the raw material mixture refers to the volume average particle size.

[0059] (Carbon source) The carbon source can be a known carbon material that acts as a reducing agent for oxygen-containing boron compounds. Examples of carbon sources include: amorphous carbon such as carbon black, activated carbon, and carbon fiber; crystalline carbon such as diamond, graphite, and nanocarbon; and pyrolytic carbon obtained by thermal decomposition of monomers or polymers. Among these, amorphous carbon is preferred as the carbon source from the viewpoint of high reactivity, and carbon black is even more preferred from the viewpoint of industrially controlled quality. Examples of carbon black include acetylene black, furnace black, and thermal black.

[0060] When the carbon source is granular, its average particle size is preferably 0.01 μm or larger, and more preferably 0.05 μm or larger. Within this range, the larger the average particle size, the easier the carbon source is to handle. Alternatively, the average particle size of the carbon source is preferably 5 μm or smaller, more preferably 4 μm or smaller, and even more preferably 3 μm or smaller. Within this range, the smaller the average particle size, the higher the reactivity of the carbon source.

[0061] (Calcium oxygen-containing compounds) An oxygen-containing calcium compound is any compound containing calcium and oxygen. In the main heating process, the oxygen-containing calcium compound prevents the volatilization of the oxygen-containing boron compound by forming a high-melting-point composite oxide with it, and also functions as a catalyst in the reaction of directly nitriding boron carbide. Preferably, the oxygen-containing calcium compound is a salt of calcium with an organic or inorganic acid. Examples of oxygen-containing calcium compounds include calcium carbonate, calcium bicarbonate, calcium hydroxide, calcium oxide, calcium nitrate, calcium sulfate, calcium phosphate, and calcium oxalate. Among these, calcium oxide and calcium carbonate are preferred as oxygen-containing calcium compounds. The oxygen-containing calcium compound may be a single compound or a combination of two or more types.

[0062] When the oxygenated calcium compound is in granular form, its average particle size is preferably 0.01 μm or more, more preferably 0.05 μm or more, and even more preferably 0.1 μm or more. Furthermore, the average particle size of the oxygenated calcium compound is preferably 200 μm or less, more preferably 120 μm or less, and even more preferably 80 μm or less.

[0063] (Boron carbide) Boron carbide may be of known type. Boron carbide promotes the formation of aggregates in the main heating process. When boron carbide is granular, its average particle size is preferably 30 μm or more, more preferably 50 μm or more, and even more preferably 70 μm or more. Within this range, the larger the average particle size, the more likely it is that larger aggregates, which contribute more to improving thermal conductivity, will be formed. Alternatively, the average particle size of boron carbide is preferably 250 μm or less, more preferably 180 μm or less, and even more preferably 150 μm or less. Within this range, the smaller the average particle size, the less likely it is that coarse aggregates will be formed.

[0064] (Ratio of ingredients) In the raw material mixture, the ratio of the mass of the oxygenated boron compound converted to B to the mass of the carbon source converted to C (mass converted to B / mass converted to C) is 0.73 or higher, preferably 0.74 or higher, and more preferably 0.78 or higher. The higher the ratio within this range, the less carbon remains after the reductive nitriding reaction, thus increasing the effect of improving the dielectric strength of the resin composition produced by the hexagonal boron nitride powder. Alternatively, the ratio is 0.85 or lower, preferably 0.81 or lower. The lower the ratio within this range, the less excessive growth of the particles constituting the aggregates due to the reductive nitriding reaction can be avoided, thus reducing the number of aggregates in the produced hexagonal boron nitride powder and suppressing the softening of the aggregates.

[0065] In the raw material mixture, the content of the oxygenated calcium compound, converted to CaO based on Ca, is 4 parts by mass or more, preferably 6 parts by mass or more, relative to 100 parts by mass of the total mass of the oxygenated boron compound converted to B2O3 based on B and the mass of the carbon source converted to C. Within this range, the higher the content of the oxygenated calcium compound, the more oxygenated boron compound forms a complex oxide with the oxygenated calcium compound, making it less likely for the oxygenated boron compound to volatilize, thus allowing for favorable control of the particle size of the hexagonal boron nitride powder produced. Furthermore, the content is 15 parts by mass or less, preferably 12 parts by mass or less. Within this range, the lower the content of the oxygenated calcium compound, the lower the melting point of the complex oxide of the oxygenated boron compound and the oxygenated calcium compound, thus further promoting the reductive nitridation reaction in the main heating process.

[0066] In the raw material mixture, the content of boron carbide relative to 100 parts by mass of the total mass of the oxygenated boron compound converted to B2O3 based on B, the mass of the carbon source converted to C, and the mass of the oxygenated calcium compound converted to CaO based on Ca is 10 parts by mass or more, preferably 14 parts by mass or more. Within this range, the higher the boron carbide content, the more the formation of a sufficient amount of aggregates is promoted. Alternatively, the content is 30 parts by mass or less, preferably 24 parts by mass or less. Within this range, the lower the boron carbide content, the less boron carbide, which is less susceptible to reductive nitridation compared to the oxygenated boron compound, and the amount of carbon remaining after the reductive nitridation reaction is reduced, thus increasing the effect of improving the dielectric strength of the resin composition produced by the hexagonal boron nitride powder.

[0067] Furthermore, by using a raw material mixture having the above-mentioned component ratios, hexagonal boron nitride powder with relatively large particle sizes can be obtained. Conventionally, there has been a problem that as the particle size increases, the tap bulk density tends to decrease and the oil absorption amount tends to increase. However, in this manufacturing method, by carrying out the first classification step described later and the subsequent crushing step, it is possible to remove the bonding bodies that widen the gaps between particles while incorporating fine particles that fill the gaps into the powder, thereby mitigating these problems.

[0068] (Heating conditions) In the main heating process, the raw material mixture is heated under a nitrogen atmosphere. Nitrogen is supplied to the reaction system by known methods. The nitrogen atmosphere mainly consists of nitrogen gas, for example, containing 90% or more by volume of nitrogen gas. The nitrogen atmosphere may also contain a non-oxidizing gas such as argon gas or helium gas as the remainder.

[0069] In the main heating step, the heating temperature is 1500°C or higher, preferably 1600°C or higher. The higher the heating temperature within this range, the easier the reaction proceeds, and the more efficiently boron nitride can be obtained. Alternatively, the heating temperature can be 1900°C or lower, preferably 1800°C or lower. The lower the heating temperature within this range, the more the grain growth of the single particles constituting the aggregate can be suppressed, resulting in a denser aggregate, which makes it easier to increase the strength of the aggregate.

[0070] In the main heating process, the heating time can be adjusted according to the composition of the raw material mixture and the heating temperature, but as an example, it may be between 5 and 20 hours. A heating time of 5 hours or more is advantageous from the viewpoint of allowing the reduction-nitridation reaction to proceed sufficiently. A heating time of 20 hours or less is advantageous from the viewpoint of carrying out the main heating process at a low cost.

[0071] The main heating process can be carried out using a known reactor capable of controlling the reaction atmosphere. Examples of reactors include atmosphere-controlled high-temperature furnaces that perform heat treatment by high-frequency induction heating or heater heating. The heating method may be batch, pusher, or continuous.

[0072] (Grinding process) The grinding process involves grinding the crude boron nitride powder obtained in the main heating process. By performing the grinding process, by-products that remained inside the particles constituting the crude boron nitride powder before grinding are exposed on the particle surface, making it possible to suitably wash away the by-products in the subsequent acid washing process. The grinding process can be carried out using known grinding equipment, and examples of such equipment include stone mills, ball mills, hammer mills, roll crushers, pin mills, jet mills, and mortars.

[0073] (Acid cleaning process) The acid washing step involves washing the crude boron nitride powder obtained by the main heating step, or by the grinding step if performed, with acid. The crude boron nitride powder obtained by the reductive nitridation reaction in the main heating step may contain impurities such as composite oxides consisting of boron oxide and calcium oxide, in addition to hexagonal boron nitride particles. Therefore, it is preferable to wash the crude boron nitride powder with acid to remove impurities. Examples of acids include hydrochloric acid, nitric acid, sulfuric acid, and acetic acid. The method of acid washing is not particularly limited, and known methods may be used. As an example, the crude boron nitride powder is placed in a container and the powder is brought into contact with 5 to 10 times the amount of dilute hydrochloric acid (5 to 20% by mass HCl) for 6 hours or more. In the acid washing step, stirring may be performed with a stirring blade or the like to ensure efficient washing.

[0074] After acid washing, the powder may be washed with water to remove any remaining acid. One method of washing with water is to filter the used acid, then disperse the powder in an equal amount of pure water, and filter again.

[0075] The drying conditions for the powder obtained by acid washing or water washing are preferably a drying temperature of 50°C to 250°C, in air or under reduced pressure. The drying time is not particularly limited, but it is preferable to dry it for a time such that the moisture content approaches 0% as closely as possible under the aforementioned drying conditions.

[0076] (1st classification process) The first classification step involves classifying the crude boron nitride powder obtained by the main heating step, or, if performed, by the grinding step or acid washing step, in order to remove smaller particles and recover larger particles as residue. According to the first classification step, aggregates formed by the bonding of several single particles are removed from the crude boron nitride powder, and a powder mainly composed of aggregates with a certain degree of particle size is obtained.

[0077] The mesh size of the sieve used in the first classification step is 15 μm or larger, preferably 20 μm or larger. The larger the mesh size within this range, the more sufficiently large particles that did not pass through the sieve can be recovered as residue. The mesh size of the sieve used in the first classification step is 40 μm or smaller, preferably 30 μm or smaller. The smaller the mesh size within this range, the more particles can be recovered as residue, improving the yield of this manufacturing method.

[0078] The classification in the first classification step may be either wet classification or dry classification, but wet classification is preferred. Normally, crude boron nitride powder is prone to becoming charged due to friction, but in wet classification, charging is prevented, thus preventing particles from being attracted to the sieve by static electricity and causing clogging, and allowing for suitable classification.

[0079] Wet classification may be carried out using methods known in the art. For example, a slurry containing dispersed coarse boron nitride powder may be introduced into a slurry cleaner equipped with a screen having a predetermined mesh size, and the discharged material that does not pass through the screen may be collected from the outlet. As the dispersion medium in the slurry, for example, water or an aqueous ethanol solution may be used. Alternatively, wet classification and acid washing may be performed simultaneously by adding an acid such as hydrochloric acid to the slurry.

[0080] (Reheating process) The reheating step involves heating the residue obtained from the first classification. By reheating the crude boron nitride powder that has undergone heating in the main heating step, the strength of the aggregates contained in the residue is improved, resulting in boron nitride powder containing aggregates that are less likely to break down during mixing with resin.

[0081] In the reheating process, to prevent reactions other than the reductive nitriding reaction between the residue and the atmosphere, the residue is heated under a nitrogen atmosphere. The explanation of the nitrogen atmosphere is the same as that described above for the nitrogen atmosphere in the main heating process. If the reductive nitriding reaction has progressed sufficiently in the main heating process, the atmosphere in the reheating process may be a non-oxidizing gas atmosphere or a mixed atmosphere of nitrogen gas and a non-oxidizing gas.

[0082] In the reheating step, the heating temperature is higher than the heating temperature in the main heating step, preferably 1850°C or higher, and more preferably 1900°C or higher. Within this range, the higher the heating temperature, the more likely the single particles constituting the aggregates are to stick together, making the aggregates stronger and improving the thermal conductivity. Furthermore, the heating temperature is preferably 2000°C or lower, and more preferably 1950°C or lower. Within this range, the lower the heating temperature, the more the yellowing of the powder due to reheating can be suppressed.

[0083] In the reheating step, the heating time can be adjusted according to the heating temperature, but for example, it may be between 2 and 10 hours. The reheating step can be carried out using a known reactor capable of controlling the reaction atmosphere. Examples of the reactor and heating method are the same as those described above for the main heating step.

[0084] (Crushing process) The crushing step is a process of grinding the boron nitride powder obtained in the reheating step. The crushing step generates a certain amount of fine particles with a relatively small particle size in the powder, which fill the gaps between aggregates, thus preventing an increase in the oil absorption of the resulting hexagonal boron nitride powder. In this specification, "grinding" refers to crushing the particles constituting the powder by friction using shear stress.

[0085] Grinding can typically be performed by sliding two members with powder sandwiched between them. During sliding, the clearance between the two members is 100 μm or more, preferably 150 μm or more. The larger the clearance within this range, the more the collapse of aggregates during sliding can be reduced, and the more the generation of excessive fine particles can be avoided, thus increasing the effect of improving the thermal conductivity of the resin composition obtained with hexagonal boron nitride powder. Alternatively, the clearance between the two members can be 500 μm or less, preferably 300 μm or less. The smaller the clearance within this range, the more fine particles can be generated to fill the gaps between aggregates, thus increasing the effect of improving the dielectric strength of the resin composition obtained with hexagonal boron nitride powder.

[0086] Grinding can be carried out using a known grinding machine. An example of a grinding machine is a millstone-type grinding machine in which a rotating grinding wheel slides against a fixed grinding wheel.

[0087] (2nd classification process) The second classification step is a step in which the hexagonal boron nitride powder obtained in the crushing step is classified. The second classification step allows for the removal of unwanted particles of a certain particle size contained in the powder. The classification in the second classification step can be carried out by known methods. Examples of classification include the removal of coarse particles using a sieve and the removal of ultrafine particles using airflow classification.

[0088] <Additional Notes> This disclosure is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of this disclosure.

[0089] <Summary> As can be understood from the above description, this disclosure encompasses the following aspects:

[0090] Embodiment 1: Hexagonal boron nitride powder containing hexagonal boron nitride aggregates, characterized in that the average particle size is 25 μm or more and 70 μm or less, the cumulative frequency of particle size 40 μm or more in the volume-based particle size distribution is 65% or more, the tap bulk density is 0.84 g / mL or more and 1.05 g / mL or less, and the oil absorption capacity is 50 mL / 100 g or more and 84 mL / 100 g or less. According to this embodiment, hexagonal boron nitride powder is provided for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength.

[0091] Embodiment 2: Hexagonal boron nitride powder according to Embodiment 1, wherein the oil absorption amount is 63 mL / 100g or more and 75 mL / 100g or less. According to this embodiment, the effect of improving the thermal conductivity and dielectric strength of the resin composition by the hexagonal boron nitride powder is further enhanced.

[0092] Embodiment 3: Hexagonal boron nitride powder according to Embodiment 1 or 2, wherein the tap bulk density is 0.91 g / mL or more and 1.01 g / mL or less. According to this embodiment, the effect of improving the thermal conductivity of the resin composition by the hexagonal boron nitride powder is further enhanced, and it is possible to reduce the weight of the powder required to form sufficient heat paths in the resin composition.

[0093] Embodiment 4: A resin composition comprising hexagonal boron nitride powder according to any one of Embodiments 1 to 3 and a resin. According to this embodiment, a resin composition is provided that achieves both excellent thermal conductivity and dielectric strength.

[0094] Embodiment 5: A method for producing hexagonal boron nitride powder containing hexagonal boron nitride aggregates, comprising a main heating step of heating a raw material mixture containing an oxygenated boron compound, a carbon source, an oxygenated calcium compound, and boron carbide under a nitrogen atmosphere at a temperature of 1500°C to 1900°C to obtain crude boron nitride powder, wherein in the raw material mixture, the ratio of the mass of the oxygenated boron compound converted to B to the mass of the carbon source converted to C (mass converted to B / mass converted to C) is 0.73 to 0.85, and the content of the oxygenated calcium compound converted to CaO based on Ca is 4 mass per 100 parts by mass of the total mass of the oxygenated boron compound converted to B2O3 based on B and the mass of the carbon source converted to C A manufacturing method characterized by comprising: a main heating step in which the content of boron carbide is 10 to 30 parts by mass, with respect to 100 parts by mass of the total mass of the oxygenated boron compound converted to B2O3 based on B, the mass of the carbon source converted to C, and the mass of the oxygenated calcium compound converted to CaO based on Ca; a classification step (first classification step) in which the crude boron nitride powder is classified using a sieve with a mesh size of 15 μm to 40 μm; a reheating step in which the residue obtained in the classification step is heated in a nitrogen atmosphere at a temperature of 1850°C to 2000°C; and a crushing step in which the boron nitride powder obtained in the reheating step is ground to a clearance of 100 μm to 500 μm. According to this embodiment, hexagonal boron nitride powder can be produced to obtain a resin composition that has both excellent thermal conductivity and dielectric strength.

[0095] Embodiment 6: The manufacturing method according to Embodiment 5, wherein the classification in the classification step is wet classification. According to this embodiment, clogging caused by particles being adsorbed onto the sieve due to static electricity is prevented, and classification can be performed effectively. [Examples]

[0096] An embodiment of the present disclosure is described below. In each of the examples and comparative examples, hexagonal boron nitride powder was produced under various conditions, the properties of the obtained powder were measured, and the properties of the resin composition containing the powder as a filler were evaluated.

[0097] [Example 1] 705 g of boron oxide, 275 g of carbon black, 84 g of calcium oxide, and 190 g of boron carbide were mixed using a ball mill to obtain a raw material mixture. The ratio B / C of the raw material mixture, which is the mass of boron oxide converted to B and the mass of carbon black converted to C, was 0.80. The content of calcium oxide converted to CaO, based on Ca, was 8.6 parts by mass per 100 parts by mass of the total mass of boron oxide converted to B2O3 based on B and the mass of carbon black converted to C. The content of boron carbide, based on Ca, was 17.9 parts by mass per 100 parts by mass of the total mass of boron oxide converted to B2O3 based on B, the mass of carbon black converted to C and the mass of calcium oxide converted to CaO, based on Ca. 1000 g of the raw material mixture was subjected to a reduction nitridation reaction by heating in a graphite Tamman furnace at 1700°C for 8 hours under a nitrogen gas atmosphere.

[0098] The obtained crude hexagonal boron nitride powder was pulverized and placed in a container. For acid washing, five times the volume of the powder was added as dilute hydrochloric acid (7% by mass HCl), and the mixture was stirred for 24 hours at 300 rpm using a stirring blade. After acid washing, the dilute hydrochloric acid was filtered, and the filtered boron nitride powder was dispersed in the same volume of pure water as the used dilute hydrochloric acid. The dispersion was introduced into a slurry cleaner SS95-250K with a mesh size of 25 μm, the residue was collected, and the mixture was dried.

[0099] The powder obtained after drying was reheated in a graphite Tamman furnace under a nitrogen gas atmosphere at 1940°C for 2 hours. The obtained powder was ground using a stone mill type grinder MKZA10-15 JMIV under conditions of a clearance of 160 μm and a rotation speed of 1200 rpm. The obtained powder was sieved through a 120 μm mesh to obtain the hexagonal boron nitride powder of Example 1. The volume-based particle size distribution of the obtained hexagonal boron nitride powder was measured using the method described later, and the average particle size and the cumulative frequency of particles with a particle size of 40 μm or larger were calculated. In addition, the oil absorption amount and tap bulk density of the powder were measured using the method described later.

[0100] [Example 2] The same procedure as in Example 1 was followed, except that the mesh size of the slurry cleaner was changed to 40 μm, to obtain the hexagonal boron nitride powder of Example 2.

[0101] [Example 3] Except for changing the mesh size of the slurry cleaner to 15 μm, the same procedure as in Example 1 was performed to obtain the hexagonal boron nitride powder of Example 3.

[0102] [Example 4] Except for changing the clearance of the millstone grinder to 300 μm, the same procedure as in Example 1 was performed to obtain the hexagonal boron nitride powder of Example 4.

[0103] [Example 5] Except for changing the clearance of the millstone grinder to 100 μm, the same procedure as in Example 1 was performed to obtain the hexagonal boron nitride powder of Example 5.

[0104] [Example 6] Except for the following points, the same procedure as in Example 1 was performed to obtain the hexagonal boron nitride powder of Example 6. The ratio B / C of the mass of boron oxide (converted to B) to the mass of carbon black (converted to C) in the raw material mixture was set to 0.73. The amount of calcium oxide (calculated as CaO, based on Ca) was set at 8.4 parts by mass per 100 parts by mass of the total mass of boron oxide (calculated as B2O3 based on B) and carbon black (calculated as C). The boron carbide content was set at 17.4 parts by mass per 100 parts by mass, which is the sum of the mass of boron oxide converted to B2O3 based on the B standard, the mass of carbon black converted to C, and the mass of calcium oxide converted to CaO based on the Ca standard. The temperature for the reduction-nitridation reaction was set to 1750°C. The clearance of the millstone grinder was changed to 120 μm.

[0105] [Comparative Example 1] The same procedure as in Example 1 was followed to obtain the hexagonal boron nitride powder of Comparative Example 1, except that classification using a slurry cleaner and grinding were not performed.

[0106] [Comparative Example 2] The same procedure as in Example 1 was followed, except that classification using a slurry cleaner was not performed, to obtain the hexagonal boron nitride powder of Comparative Example 2.

[0107] [Comparative Example 3] The same procedure as in Example 1 was followed, except that grinding was not performed, to obtain the hexagonal boron nitride powder of Comparative Example 3.

[0108] [Comparative Example 4] The same procedure as in Example 6 was followed, except that the temperature of the reduction-nitridation reaction was set to 1700°C, and classification and grinding using a slurry cleaner were not performed, to obtain the hexagonal boron nitride powder of Comparative Example 4.

[0109] [Measurement of particle size distribution] Hexagonal boron nitride powder from both the examples and comparative examples was placed in a Microtrac-Bell MT3000 ultrasonic analyzer, and the particle size distribution was measured while ultrasonic treatment was performed at an output of 40W for dispersion. The median diameter (D50) in the obtained particle size distribution was determined as the average particle size. In addition, the cumulative frequency of particles with a particle size of 40 μm or larger in the obtained particle size distribution was calculated.

[0110] [Measurement of oil absorption] For both the examples and comparative examples, measurements were performed in accordance with JIS-K6217-4 using hexagonal boron nitride powder and dibutyl phthalate (DBP) as the solvent. A DBP drop-torque curve was obtained with the x-axis representing DBP drop volume (mL / 100g powder) and the y-axis representing torque (Nm). Specifically, 20g of powder was placed in a mixing chamber, and DBP was added dropwise at a rate of 4.0 mL / min while stirring with a rotor at 125 rpm, and the torque was measured over time. The DBP drop-torque curve was created from these measurement results. An oil absorption meter S-500 (manufactured by Asahi Research Institute Co., Ltd.) was used as the measuring device. DBP used was special grade reagent (distributor code 021-06936) manufactured by Wako Pure Chemical Industries, Ltd.

[0111] As described above, the maximum torque value of the DBP drip rate-torque curve was defined as T1, and the DBP drip rate at the 70% torque value of T1 was defined as the oil absorption amount. If there were two or more DBP drip rates corresponding to the 70% torque value, the largest DBP drip rate corresponding to a 70% torque value smaller than the DBP drip rate representing the maximum torque value T1 was defined as the oil absorption amount. Here, if there were two or more data points showing the same maximum torque value, the data point with the largest DBP drip rate among those data points was defined as T1.

[0112] [Measurement of tap bulk density] The tap bulk density of hexagonal boron nitride powder in both the examples and comparative examples was measured using a Tapdenser KYT-5000 manufactured by Seishin Corporation. The hexagonal boron nitride powder was roughly packed into a 100 mL sample cell, and tapping was performed at a tapping speed of 120 times / min, a tapping height of 5 cm, and 500 taps. The tap bulk density was calculated from the mass and volume after tapping. The sample cell consisted of a lidded cylinder with a diameter of 28 mm and a height of 163 mm.

[0113] [Measurement of specific surface area] The nitrogen adsorption isotherms of hexagonal boron nitride powder in both the examples and comparative examples were measured by gas adsorption tests using a FlowSorb III 2310 (Micromeritics) with nitrogen gas as the adsorbent species. Specifically, crude hexagonal boron nitride powder that had been vacuum dried and degassed at 200°C for 10 minutes as a pretreatment was subjected to a gas flow rate of 15 cm³. 3 The adsorption and desorption isotherms of nitrogen gas were measured using the continuous flow method under the condition of / min, and the specific surface area was calculated using the BET method.

[0114] [Measurement of compressive failure strength] Compression tests were conducted using a microcompression testing machine MCT-510 manufactured by Shimadzu Corporation to measure the compressive fracture strength of hexagonal boron nitride powder. The compression test was performed by applying a load to the sample with an indenter, showing a graph of test force (mN)-displacement (μm) at which the sample fractured, defining the fracture point P as the test force (mN) at which the displacement became constant, and calculating the fracture strength (MPa) using the following formula.

[0115] Breaking strength (MPa) = (2.48P) / (3.14d) 2 ) (d: average particle size) The measurement conditions used were: indenter = FLAT200 (200 μm flat plate indenter), test force = 50 (mN), loading speed = 4.8420 (mN / sec), and load holding time = 5 (sec). Approximately 100 aggregates were selected from the obtained hexagonal boron nitride powder, and their compressive fracture strengths were measured. The average value and standard deviation were then determined.

[0116] [Fabrication of resin sheets, evaluation of thermal conductivity and dielectric strength] Hexagonal boron nitride powder was packed into epoxy resin to prepare resin sheets for both the examples and comparative examples, and their thermal conductivity and dielectric strength were evaluated. Specifically, a base resin mixture was prepared with 100 parts by mass of epoxy resin (JER828, manufactured by Mitsubishi Chemical Corporation), 5 parts by mass of a curing agent (imidazole-based curing agent, Curesol 2E4MZ, manufactured by Shikoku Chemicals Co., Ltd.), and 210 parts by mass of a solvent, methyl ethyl ketone. Next, the base resin mixture and hexagonal boron nitride powder were mixed so that the ratio of epoxy resin to hexagonal boron nitride powder was 30% by volume of epoxy resin and 70% by volume of hexagonal boron nitride powder, and the mixture was stirred in a rotation / revolution mixer (MAZERUSTAR, manufactured by Kurabo Industries Ltd.) to obtain varnish. This varnish was applied to a PET film to a thickness of approximately 250-300 μm using a PI-1210 automatic coating machine manufactured by Tester Industries Co., Ltd., and after drying, it was cured under reduced pressure, at a temperature of 200°C, a pressure of 5 MPa, and a holding time of 30 minutes to produce a resin sheet with a thickness of 200 μm.

[0117] The resin sheet was analyzed using a thermal wave thermal analyzer, and its thermal conductivity was calculated. The calculated thermal conductivity was evaluated according to the following criteria. A thermal conductivity evaluation of "4" or "3" indicates that the resin sheet has excellent thermal conductivity. 4: 19.5 W / m·K < Thermal conductivity 3: 19.0 W / m·K < Thermal conductivity ≤ 19.5 W / m·K 2: 18.0 W / m·K < Thermal conductivity ≤ 19.0 W / m·K 1: Thermal conductivity ≤ 18.0 W / m·K

[0118] Furthermore, the dielectric strength of the resin sheet was measured using a dielectric strength tester (manufactured by Tama Densoku Co., Ltd.). The measured dielectric strength was evaluated according to the following criteria. A dielectric strength evaluation of "4" or "3" indicates that the dielectric strength of the resin sheet is excellent. 4:45KV / mm < Dielectric Strength 3: 43KV / mm < Dielectric strength ≤ 45KV / mm 2: 41KV / mm < Dielectric strength ≤ 43KV / mm 1: Dielectric strength ≤ 41KV / mm

[0119] 〔result〕 Tables 1 and 2 show the manufacturing conditions, powder properties, and resin sheet evaluation results for the examples and comparative examples.

[0120] [Table 1]

[0121] [Table 2]

[0122] In Tables 1 and 2, *1: The B / C ratio refers to the ratio of the mass of boron oxide converted to B to the mass of carbon black converted to C in the raw material mixture (mass converted to B / mass converted to C). *2: The CaO content refers to the amount of calcium oxide converted to CaO on a Ca basis, relative to 100 parts by mass of the total of the mass of boron oxide converted to B2O3 on a B basis and the mass of carbon black converted to C on a C basis in the raw material mixture. *3: Boron carbide content refers to the amount of boron carbide contained in the raw material mixture, calculated per 100 parts by mass of the total mass of boron oxide converted to B2O3 (based on B), carbon black converted to C, and calcium oxide converted to CaO (based on Ca).

[0123] As can be seen from the comparison between Comparative Example 1 and Comparative Example 2, in Comparative Example 2, which underwent grinding, the dielectric strength improved compared to Comparative Example 1 because the fine particles generated by grinding filled the gaps between the aggregates. However, since the contribution of the fine particles to the improvement of thermal conductivity was small, the thermal conductivity did not improve in Comparative Example 2. Also, as can be seen from the comparison between Comparative Example 1 and Comparative Example 3, in Comparative Example 3, which underwent classification without grinding, the thermal conductivity improved compared to Comparative Example 1 due to an increase in the proportion of aggregates. However, in Comparative Example 3, the amount of oil absorbed increased, and the dielectric strength did not improve.

[0124] As can be seen from the comparison between Examples 1-6 and Comparative Examples 1-4, in Examples 1-6, by performing classification after the main heating process and grinding the residue, the average particle size and the cumulative frequency of particles with a particle size of 40 μm or more fell within a suitable range. In addition, in Examples 1-6, the oil absorption amount and tap bulk density also fell within a suitable range, probably because the ratio of aggregates to fine particles in the powder was suitably controlled. As a result, both the thermal conductivity and dielectric strength of the resin sheets in Examples 1-6 were improved compared to Comparative Example 1. [Industrial applicability]

[0125] The hexagonal boron nitride powder relating to this disclosure can, for example, be used as a filler in resins used in electronic components. [Explanation of Symbols]

[0126] 101 Aggregates 102 Single Particle 103 Zygote 1001, 1002 Powders relating to conventional examples 1003 Powder relating to this disclosure

Claims

1. A hexagonal boron nitride powder containing hexagonal boron nitride aggregates, The average particle size is between 25 μm and 70 μm. In the volume-based particle size distribution, the cumulative frequency of particle sizes 40 μm or larger is 65% or more. The tap bulk density is 0.84 g / mL or more and 1.05 g / mL or less. The oil absorption capacity is 50 mL / 100 g or more and 84 mL / 100 g or less. A hexagonal boron nitride powder characterized by the following features.

2. The oil absorption amount is 63 mL / 100 g or more and 75 mL / 100 g or less. The hexagonal boron nitride powder according to claim 1.

3. The bulk density of the tap is 0.91 g / mL or more and 1.01 g / mL or less. The hexagonal boron nitride powder according to claim 1.

4. A resin composition comprising hexagonal boron nitride powder according to any one of claims 1 to 3 and a resin.

5. A method for producing hexagonal boron nitride powder containing hexagonal boron nitride aggregates, The main heating step involves heating a raw material mixture containing an oxygenated boron compound, a carbon source, an oxygenated calcium compound, and boron carbide under a nitrogen atmosphere at a temperature of 1500°C to 1900°C to obtain crude boron nitride powder. In the aforementioned raw material mixture, The ratio of the mass of the oxygenated boron compound converted to B to the mass of the carbon source converted to C (mass converted to B / mass converted to C) is 0.73 or more and 0.85 or less. Based on the B standard of the aforementioned oxygen-containing boron compound, B ２ O ３ The content of the oxygenated calcium compound, converted to CaO based on Ca, is 4 parts by mass or more and 15 parts by mass or less, relative to 100 parts by mass of the total of the mass converted to C of the carbon source and the mass converted to C of the carbon source. Based on the B standard of the aforementioned oxygen-containing boron compound, B ２ O ３ The main heating step is characterized in that, with respect to 100 parts by mass of the total of the mass converted to C of the carbon source, the mass converted to CaO on a Ca basis of the oxygenated calcium compound, the content of boron carbide is 10 parts by mass or more and 30 parts by mass or less, A classification step is performed in which the aforementioned coarse boron nitride powder is classified using a sieve with a mesh size of 15 μm or more and 40 μm or less. The residue obtained in the classification step is reheated in a nitrogen atmosphere at a temperature of 1850°C to 2000°C. A crushing step is performed in which the boron nitride powder obtained by the reheating step is ground to a clearance of 100 μm or more and 500 μm or less. A method for producing hexagonal boron nitride powder according to any one of claims 1 to 3, characterized by containing the following:

6. The classification in the aforementioned classification process is a wet classification. The manufacturing method according to claim 5.

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