Powder of hexagonal boron nitride, resin composition, and method for producing powder of hexagonal boron nitride

Abstract

A purpose of the present invention is to provide, for example, a powder of hexagonal boron nitride which is for obtaining resin compositions having both excellent thermal conductivity and dielectric strength. This powder of hexagonal boron nitride includes aggregates of hexagonal boron nitride, and has an oil absorption of 60-90 mL / 100 g and a content of particles having particle diameters of 75-150 μm of 20 mass% or more. The powder has a particle size distribution, determined by a wet laser diffraction particle size distribution method, in which the particle diameter D10 at 10% volume-cumulative frequency is 5-15 μm. After having undergone a 20-minute ultrasonic treatment at 250 W using ethanol as a dispersion medium, the powder has a particle size distribution, determined by the wet laser diffraction particle size distribution method, in which the particle diameter D70s at 70% volume-cumulative frequency is 28 μm or larger.

Description

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

[0001] This invention relates to hexagonal boron nitride powder, resin compositions, and methods for producing hexagonal boron nitride powder.

[0002] In recent years, the miniaturization and increased power of electronic components have led to a problem of increased heat generation. Therefore, research is underway to develop materials with excellent thermal conductivity to efficiently dissipate heat from electronic components.

[0003] Resin compositions obtained by blending hexagonal boron nitride powder with a resin exhibit improved thermal conductivity compared to the resin itself, making such resin compositions suitable for use as materials for electronic components. It is known that hexagonal boron nitride powder blended with a resin contains aggregated particles formed by the aggregation of single particles of hexagonal boron nitride, further improving the thermal conductivity of the resin composition. As an example of such a powder containing aggregated particles, Patent Document 1 discloses hexagonal boron nitride powder that maintains low oil absorption, thereby achieving both excellent thermal conductivity and dielectric strength. Furthermore, Patent Document 2 discloses a technology that achieves both excellent thermal conductivity and dielectric strength by controlling the particle size D90 of the aggregated particles to a relatively small range of about 50 μm to 100 μm.

[0004] International Publication No. 2018 / 123571, Japanese Patent Publication No. 2023-147855

[0005] However, the aggregated particles described in Patent Document 1 have a problem in that some of the aggregated particles break down during kneading with the resin, resulting in the loss of the excellent thermal conductivity inherent in the aggregated particles. Furthermore, the aggregated particles described in Patent Document 2 have a problem in that many of them are relatively small in size, resulting in many interfaces between the aggregated particles, making it difficult to further improve the thermal conductivity of the resin composition.

[0006] One aspect of the present invention aims to provide hexagonal boron nitride powder, etc., for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength.

[0007] To solve the above problems, a hexagonal boron nitride powder according to one aspect of the present invention is a hexagonal boron nitride powder containing hexagonal boron nitride aggregates, wherein the oil absorption is 60 mL / 100 g or more and 90 mL / 100 g or less, the content of particles with a particle size of 75 μm or more and 150 μm or less is 20% by mass or more, the particle size D10 at the cumulative volume frequency of 10% of the particle size distribution in the wet laser diffraction particle size distribution method is 5 μm or more and 15 μm or less, and the particle size D70s at the cumulative volume frequency of 70% of the particle size distribution in the wet laser diffraction particle size distribution method after ultrasonic treatment with ethanol as a dispersion medium at 250 W for 20 minutes is 28 μm or more.

[0008] To solve the above problems, a method for producing hexagonal boron nitride powder according to one aspect of the present invention 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 at a temperature of 1500°C to 1800°C under a nitrogen atmosphere, wherein 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) of the raw material mixture is 0.75 to 1.04, and the B of the oxygen-containing boron compound is based on B ２ O ３ For every 100 parts by mass of the total of the mass converted to and the mass converted to C of the carbon source, the oxygenated calcium compound is contained in an amount of 8 to 25 parts by mass, converted to CaO on a Ca basis, and the oxygenated boron compound is B on a B basis. ２ O ３ The process includes: a main heating step in which, per 100 parts by mass of the total of the mass converted to C of the carbon source and the mass converted to CaO on a Ca basis of the oxygen-containing calcium compound, 12 to 45 parts by mass of the boron carbide; an acid washing step in which the crude boron nitride powder obtained in the main heating step is acid washed; a reheating step in which the acid washed crude boron nitride powder 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 120 μm to 300 μm.

[0009] According to one aspect of the present invention, it is possible to provide hexagonal boron nitride powder and the like for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength.

[0010] <Hexagonal Boron Nitride Powder> In the hexagonal boron nitride powder of the present invention, when the cumulative volume frequency of the particle size distribution after ultrasonic treatment satisfies specific conditions, the aggregates contained in the powder have sufficient strength to maintain their aggregated state without completely collapsing during mixing with the resin. However, under such cumulative volume frequency conditions alone, the oil absorption of the hexagonal boron nitride powder may increase. For example, when hexagonal boron nitride powder with a high oil absorption rate is mixed with a resin and a solvent, the viscosity of the resulting varnish increases, and the coatability of the varnish decreases. Increasing the amount of solvent added improves the coatability of the varnish, but as the amount of solvent added increases, the amount of solvent that must be dried and removed during 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. Therefore, hexagonal boron nitride powder with a high oil absorption rate has limitations in its use, such as difficulties in combining it with solvents.

[0011] The inventors have further investigated the above-mentioned problems and have found that by including a specific amount of fine particles with a certain particle size in hexagonal boron nitride powder, an increase in the amount of oil absorbed by the powder can be avoided, and therefore, a powder can be provided for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength. The inventors speculate that this effect is due to the following principle: In essence, the aggregated particles contained in hexagonal boron nitride powder have gaps between the individual particles that make up the aggregated particles, and a phenomenon occurs in which these gaps accept resin and solvent, i.e., the powder absorbs oil. The amount of oil absorbed is measured using dibutyl phthalate as a reference, for example. The more gaps there are in the hexagonal boron nitride powder, the more oil the powder absorbs. However, since the fine particles present in the hexagonal boron nitride powder can fill these gaps, an increase in the amount of oil absorbed by the powder can be avoided.

[0012] The hexagonal boron nitride powder according to one aspect of the present invention will be described in detail below. In this description, the hexagonal boron nitride powder according to one aspect of the present invention may be abbreviated as "this powder".

[0013] This powder contains hexagonal boron nitride aggregates (hereinafter sometimes simply referred to as "aggregates"). The aggregates are secondary particles composed of multiple primary particles (single particles) of hexagonal boron nitride. The aggregates contained in this powder can be observed using known microscopic observation methods such as scanning electron microscopy (SEM) or transmission electron microscopy (TEM).

[0014] (Particle Size Distribution) In this powder, the particle size D10 at the cumulative volume frequency of 10% of the particle size distribution is an indicator of the content of fine particles with a relatively small particle size in this powder, and it is preferable to control it within a specific range. The larger the particle size D10, the smaller the content of fine particles. In this powder, the particle size D10 is 5 μm or larger, preferably 8 μm or larger. The larger the particle size D10 is within this range, the easier it is to reduce the fine particles that contribute little to improving thermal conductivity. Also, the particle size D10 is 15 μm or smaller, preferably 14 μm or smaller. The smaller the particle size D10 is within this range, the larger the content of fine particles that can fill the gaps between single particles in the aggregate, so the oil absorption amount of this powder decreases, and therefore the effect of this powder on improving the dielectric strength of the resin composition becomes greater.

[0015] In this powder, the particle size D90 at which the cumulative volume frequency of the particle size distribution is 90% is preferably 85 μm or larger, and more preferably 95 μm or larger. The larger the particle size D90 is within this range, the easier it is to form heat paths, and the greater the effect of improving the thermal conductivity of the resin composition. Furthermore, the particle size D90 is preferably 150 μm or smaller, and more preferably 135 μm or smaller. The smaller the particle size D90 is within this range, the more effectively the generation of gaps within the aggregated particles can be suppressed.

[0016] In this powder, the particle size D70 at which the cumulative volume frequency of the particle size distribution reaches 70% after ultrasonic treatment (hereinafter, this particle size D70 may be referred to as "particle size D70s" to distinguish it from the particle size D70 at which the cumulative volume frequency of the particle size distribution reaches 70% before ultrasonic treatment) is an indicator of the strength of aggregates in this powder, and it is desirable to control it within a specific range. The larger the particle size D70s, the higher the strength of the aggregates, indicating that the aggregates are less likely to collapse even when subjected to stress during kneading of this powder with resin. In this powder, the particle size D70s is 28 μm or larger, preferably 40 μm or larger. The larger the particle size D70s is within this range, the more aggregates there are that maintain an aggregated state without completely collapsing even during kneading of this powder with resin, thus increasing the effect of this powder on improving the thermal conductivity of the resin composition. There is no particular upper limit for the particle size D70s, but as an example, the particle size D70s may be 90 μm or less.

[0017] In this powder, the ratio D70s / D70 of the particle size D70s at the cumulative volume frequency of the particle size distribution after ultrasonic treatment to the particle size D70s at the cumulative volume frequency of the particle size distribution before ultrasonic treatment is preferably 0.35 or higher, and more preferably 0.45 or higher. The higher the ratio D70s / D70 is within this range, the more aggregates remain in an aggregated state without completely collapsing during mixing of this powder with the resin, thus increasing the effect of this powder on improving the thermal conductivity of the resin composition. There is no particular upper limit to the ratio D70s / D70, but as an example, the ratio D70s / D70 may be 1 or less.

[0018] 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. The particle size distribution is measured while the sample is subjected to ultrasonic treatment at an output of 40 W for dispersion. In this specification, the ultrasonic treatment before measuring the particle size D70s is performed under the conditions of 250 W for 20 minutes using ethanol as the dispersion medium, as shown in the examples described later. For the measurement of particle sizes D10, D70, and D90, powder without the dispersion medium is used as the sample, while for the measurement of particle size D70s, a slurry containing the dispersion medium and powder is used as the sample.

[0019] (Proportion of aggregates) In this powder, the content of particles with a particle size of 75 μm or more and 150 μm or less is an indicator of the proportion of aggregates that particularly contribute to improving the thermal conductivity of this powder, and it is preferable to control it within a specific range. The higher the content, the greater the proportion of aggregates. The content of particles with a particle size of 75 μm or more and 150 μm or less is 20% by mass or more, and more preferably 25% by mass or more. The higher the content within these ranges, the greater the proportion of aggregates that have a particle size large enough to contribute sufficiently to improving thermal conductivity, and a particle size small enough that voids are less likely to form inside, and thus the effect of improving the thermal conductivity and dielectric strength of the resin composition by this powder becomes greater. There is no particular upper limit to the content of particles with a particle size of 75 μm or more and 150 μm or less, but as an example, the content may be 60% by mass or less.

[0020] In this specification, the content of particles with a particle size of 75 μm or more and 150 μm or less is measured by weighing the portion of the powder that is classified below a sieve with a mesh size of 150 μm and above a sieve with a mesh size of 75 μm, as shown in the examples described later.

[0021] (Oil Absorption) In this powder, the oil absorption is an indicator of the number of voids in the aggregates contained in the powder and the degree of development of the structure on the surface of the aggregates, and it is preferable to control it within a specific range. The lower the oil absorption, the less voids there are in the aggregates that take in solvents in the powder as a whole. In this powder, the oil absorption is 60 mL / 100 g or more, preferably 65 mL / 100 g or more. The higher the oil absorption within this range, the more aggregates that contribute greatly to improving thermal conductivity are contained in the powder, and therefore the effect of this powder on improving the thermal conductivity of the resin composition becomes greater. Also, in this powder, the oil absorption is 90 mL / 100 g or less, preferably 85 mL / 100 g or less. The lower the oil absorption within this range, the less solvent is taken in by the powder as a whole, 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 becomes greater.

[0022] 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.

[0023] (Specific surface area) The specific surface area of ​​this powder is preferably 2.2 m². ２ It is 2.5 m or more per gram, and more preferably 2.5 m ２ The specific surface area is greater than or equal to 1 / g. The larger the specific surface area within these ranges, the smaller and denser the individual particles constituting the aggregate become, and the stronger the aggregation becomes, thus the greater the effect of this powder on improving the thermal conductivity of the resin composition. There is no particular upper limit to the specific surface area of ​​this powder, but as an example, the specific surface area is 4.0 m². ２ It may be less than / g.

[0024] 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 using, for example, Mountec's Macsorb HM model-1201 (trade name).

[0025] (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 caused by impurity elements, which leads to a decrease in thermal conductivity and dielectric strength, can be reduced.

[0026] The boron nitride content in this powder is the value obtained by subtracting the mass percentage (unit: 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.

[0027] <Resin Composition> One application of this powder is as a filler for resins, aimed at improving dielectric strength and thermal conductivity. That is, a resin composition containing this powder and a resin is also within the scope of the present invention. A resin composition according to one aspect of the present invention has excellent thermal conductivity and dielectric strength due to this powder. Hereinafter, a resin composition according to one aspect of the present invention may be abbreviated as "this resin composition". This resin composition can be used for various purposes, for example, as a thermally conductive resin composition or as a material for thermally conductive molded articles.

[0028] 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.

[0029] 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 rubbers; and the like.

[0030] 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.

[0031] This resin composition can be used as a material for heat-dissipating molded products such as heat-dissipating sheets, phase change sheets, heat-dissipating tapes, heat-dissipating resin substrates in printed wiring boards (PWBs) or copper-clad laminate (CCL)-based resin substrates, and insulating layers in metal-based substrates such as aluminum-based substrates or copper-based substrates; and as heat-dissipating materials such as heat-dissipating greases, heat-dissipating adhesives, gap fillers, heat-dissipating paints, heat-dissipating coats, and encapsulating materials for power devices.

[0032] Note that the use of this powder is not limited to a filler for resins. Examples of other uses include raw materials for boron nitride processed products such as boron nitride molded products or cubic boron nitride, nucleating agents for engineering plastics, phase change materials, solid or liquid thermal interface materials, release agents in molds for molten metals or molten glass, cosmetics, and composite ceramic raw materials, etc.

[0033] <Manufacturing method of hexagonal boron nitride powder> To solve the problems of the above-mentioned prior art, the inventors of the present invention have conducted intensive studies. As a result, the inventors of the present invention have found that in the manufacturing process of hexagonal boron nitride powder, when a reheating treatment is performed after a heat treatment for the purpose of reductive nitridation, the aggregates contained in the obtained powder have sufficient strength to maintain an aggregated state without completely collapsing during kneading with a resin. However, simply performing such a reheating treatment may increase the oil absorption amount of the obtained powder, which leads to a decrease in the insulation breakdown voltage of the resin composition. This is considered to be due to the fact that single particles that did not constitute aggregates before the reheating treatment adhere to each other by the reheating treatment, and gaps are generated between the adhered single particles. The inventors of the present invention have further studied the above problems, and when the reheated hexagonal boron nitride powder is ground, the aggregates can be crushed without collapsing the adhered single particles, and as a result, a certain amount of fine powder particles with a relatively small particle size are generated in the powder, and it has been found that an increase in the oil absorption amount of the powder can be avoided, thus completing the present invention.

[0034] Hereinafter, the manufacturing method of hexagonal boron nitride powder according to one aspect of the present invention will be described in detail. In the description, the manufacturing method of hexagonal boron nitride powder according to one aspect of the present invention may be abbreviated as "this manufacturing method". Although not limited, this manufacturing method may be the manufacturing method of the above-mentioned powder. Also, the hexagonal boron nitride powder manufactured by this manufacturing method is also within the scope of the present invention.

[0035] This manufacturing method includes a main heating step, an acid washing step, a reheating step, and a crushing step in this order. Also, this manufacturing method may further include a mixing step before the main heating step. Also, this manufacturing method may further include a grinding step after the main heating step and before the acid washing step. Also, this manufacturing method may further include a classification step after the crushing step.

[0036] (Mixing step) The mixing step is a step of mixing raw materials of hexagonal boron nitride powder to obtain a raw material mixture. The types and ratios of the raw materials to be mixed will be apparent to those skilled in the art from the description of the raw material mixture described later for the main heating step. The mixing of the raw materials can be carried out using known methods. For example, it can be carried out using mixers such as vibration mills, bead mills, ball mills, Henschel mixers, drum mixers, vibration stirrers, V-shaped mixers, etc. Even when using three or more types of raw materials, the order of mixing the raw materials is not particularly limited, and all the raw materials may be mixed simultaneously, or they may be mixed in any order sequentially. Also, in order to mix the raw materials more uniformly, the raw materials may be mixed while being crushed.

[0037] (Main heating step) The main heating step is a step of heating the raw material mixture in a nitrogen atmosphere. According to the main heating step, the reduction nitridation reaction of the boron element contained in the raw material mixture proceeds, and a hexagonal rough boron nitride powder is obtained.

[0038] (Raw material mixture) The raw material mixture contains an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and boron carbide.

[0039] (Oxygen-containing boron compounds) Oxygen-containing boron compounds are any compounds containing boron and oxygen, preferably compounds consisting of at least boron and oxygen, and optionally hydrogen, or inorganic salts of such compounds. Examples of oxygen-containing boron compounds include boric acid, boric anhydride, metaboric acid, perboric acid, subboric acid, sodium tetraborate, and sodium perborate. Among these, boric acid or boron oxide, which are readily available, are preferably used as oxygen-containing boron compounds.

[0040] 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. Alternatively, 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, average particle size refers to the volume average particle size.

[0041] (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.

[0042] 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.

[0043] (Oxygen-containing calcium compounds) Oxygen-containing calcium compounds are any compounds containing calcium and oxygen. In the main heating step, oxygen-containing calcium compounds prevent the volatilization of oxygen-containing boron compounds by forming a high-melting-point composite oxide together with oxygen-containing boron compounds, and also function as a catalyst in the reaction of directly nitriding boron carbide. Oxygen-containing calcium compounds are preferably salts of organic or inorganic acids with calcium. 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. Oxygen-containing calcium compounds may be one type or a combination of two or more types.

[0044] When the oxygen-containing 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 oxygen-containing calcium compound is preferably 200 μm or less, more preferably 120 μm or less, and even more preferably 80 μm or less.

[0045] (Boron carbide) Boron carbide may be a known material. Boron carbide promotes the formation of aggregates in the main heating step. When boron carbide is granular, its average particle size is preferably 30 μm or more, more preferably 50 μm or more, and still more preferably 70 μm or more. The larger the average particle size within these ranges, the greater the contribution to improving the thermal conductivity, and the tendency to form larger aggregates is higher. Also, the average particle size of boron carbide is preferably 250 μm or less, more preferably 180 μm or less, and still more preferably 150 μm or less. The smaller the average particle size within these ranges, the smaller the amount of coarse aggregates formed.

[0046] (Ratio of components) In the raw material mixture, the ratio of the mass converted to B of the oxygen-containing boron compound to the mass converted to C of the carbon source (mass converted to B / mass converted to C) is 0.75 or more, preferably 0.78 or more. The higher this ratio within these ranges, the lower the amount of carbon remaining after the reduction nitridation reaction, so the effect of improving the dielectric breakdown strength of the resin composition with the produced hexagonal boron nitride powder is greater. Also, the ratio is 1.04 or less, preferably 0.81 or less. The lower this ratio within these ranges, the excessive growth of the particles constituting the aggregates due to the reduction nitridation reaction can be avoided, so the number of aggregates in the produced hexagonal boron nitride powder can be reduced, and the softening of the aggregates can be reduced.

[0047] In the raw material mixture, B based on the B of the oxygen-containing boron compound ２ O ３The content of the oxygenated calcium compound, converted to CaO based on Ca, per 100 parts by mass of the total mass converted to C of the carbon source, is 8 parts by mass or more, preferably 12.7 parts by mass or more. Within this range, the higher the content of the oxygenated calcium compound, the more oxygenated boron compound forms a complex oxide together 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 25 parts by mass or less, preferably 17.8 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 facilitating the reductive nitridation reaction in the main heating process.

[0048] In the raw material mixture, the oxygenated boron compound meets the B standard. ２ O ３ The content of boron carbide per 100 parts by mass of the total of the mass converted to C in the carbon source and the mass converted to CaO on a Ca basis of the oxygenated calcium compound is 12 parts by mass or more, preferably 15 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 45 parts by mass or less, preferably 26 parts by mass or less. Within this range, the lower the boron carbide content, the less boron carbide is difficult to reduce and nitride compared to the oxygenated boron compound, and the amount of carbon remaining after the reduction and nitriding reaction is reduced, thus increasing the effect of improving the dielectric strength of the resin composition produced by the hexagonal boron nitride powder.

[0049] (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.

[0050] In the main heating step, the heating temperature is 1500°C or higher, preferably 1650°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 1800°C or lower, preferably 1750°C or lower. Within this range, the lower the heating temperature, the more the grain growth of the single particles constituting the aggregated particles is suppressed, resulting in denser aggregated particles, and thus making it easier to increase the strength of the aggregated particles.

[0051] 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.

[0052] 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.

[0053] (Grinding Process) The grinding process is a process of 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 grinding equipment include stone mill grinders, ball mills, hammer mills, roll crusher pin mills, jet mills, mortars, and the like.

[0054] (Acid Washing Process) The acid washing process is a process of washing the crude boron nitride powder obtained by the main heating process, or by the grinding process if performed, with acid. The crude boron nitride powder obtained by the reductive nitridation reaction in the main heating process 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, a method may be used in which 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 process, stirring may be performed with a stirring blade or the like in order to wash efficiently.

[0055] 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.

[0056] 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.

[0057] (Reheating process) The reheating process involves heating the acid-washed coarse boron nitride powder. By reheating the coarse boron nitride powder obtained in the main heating process, the strength of the aggregates contained in the coarse boron nitride powder is improved, resulting in aggregates that are less likely to break down during mixing with resin.

[0058] In the reheating process, to prevent reactions other than the reductive nitriding reaction between the crude boron nitride powder and the atmosphere, it is preferable to heat the crude boron nitride powder 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.

[0059] 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 easily the single particles constituting the aggregated particles adhere to each other, making the aggregated particles stronger and improving the thermal conductivity. Alternatively, 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 effectively the yellowing of the powder due to reheating can be suppressed.

[0060] In the reheating step, the heating time can be adjusted according to the heating temperature, but for example, it may be between 2 hours 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.

[0061] (Disintegration Process) The disintegration process is a process of grinding the boron nitride powder obtained in the reheating process. The disintegration process generates a certain amount of particles with a relatively small particle size in the powder, filling 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 the pulverization of the particles constituting the powder by friction using shear stress.

[0062] Grinding can typically be performed by sliding two members with powder sandwiched between them. During sliding, the clearance between the two members is 120 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 is 300 μm or less, preferably 250 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.

[0063] 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.

[0064] (Classification Process) The classification process is a process of classifying the hexagonal boron nitride powder obtained by the crushing process. The classification process allows for the removal of unwanted particles of a certain particle size contained in the powder. Classification 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.

[0065] <Additional Notes> The present invention 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 the present invention.

[0066] <Summary> As can be understood from the above description, the present invention encompasses the following aspects.

[0067] Embodiment 1: Hexagonal boron nitride powder containing hexagonal boron nitride aggregates, wherein the oil absorption capacity is 60 mL / 100 g or more and 90 mL / 100 g or less, the content of particles with a particle size of 75 μm or more and 150 μm or less is 20% by mass or more, the particle size D10 at the cumulative volume frequency of 10% of the particle size distribution in the wet laser diffraction particle size distribution method is 5 μm or more and 15 μm or less, and the particle size D70s at the cumulative volume frequency of 70% of the particle size distribution in the wet laser diffraction particle size distribution method after ultrasonic treatment with ethanol as a dispersion medium at 250 W for 20 minutes is 28 μm or more.

[0068] Embodiment 2: The hexagonal boron nitride powder of Embodiment 1, wherein the ratio D70s / D70 of the particle size D70s at which the cumulative volume frequency of the particle size distribution in the wet laser diffraction particle size distribution method is 70% after ultrasonic treatment with ethanol as the dispersion medium at 250 W for 20 minutes, to the particle size D70s at which the cumulative volume frequency of the particle size distribution in the wet laser diffraction particle size distribution method is 70%, is 0.35 or more.

[0069] Embodiment 3: A resin composition comprising hexagonal boron nitride powder according to Embodiment 1 or 2 and a resin.

[0070] Embodiment 4: 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 1800°C, wherein 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) of the raw material mixture is 0.75 to 1.04, and the B of the oxygen-containing boron compound is based on B ２ O ３ For every 100 parts by mass of the total of the mass converted to and the mass converted to C of the carbon source, the oxygenated calcium compound is contained in an amount of 8 to 25 parts by mass, converted to CaO on a Ca basis, and the oxygenated boron compound is B on a B basis. ２ O ３ A manufacturing method comprising: a main heating step in which, per 100 parts by mass of the total of the mass converted to C of the carbon source and the mass converted to CaO on a Ca basis of the oxygen-containing calcium compound, 12 parts by mass or more and 45 parts by mass of the boron carbide; an acid washing step in which the crude boron nitride powder obtained in the main heating step is acid washed; a reheating step in which the acid washed crude boron nitride powder is heated in a nitrogen atmosphere at a temperature of 1850°C or more and 2000°C or less; and a crushing step in which the boron nitride powder obtained in the reheating step is ground to a clearance of 120 μm or more and 300 μm or less.

[0071] Embodiment 5: The manufacturing method of Embodiment 4, further comprising a grinding step of grinding the crude boron nitride powder obtained by the main heating step, after the main heating step and before the acid washing step.

[0072] An embodiment of the present invention 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.

[0073] [Example 1] 695 g of boron oxide, 275 g of carbon black, 173 g of calcium oxide, and 185 g of boron carbide with an average particle size of 170 μm were mixed using a ball mill to obtain a raw material mixture. 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 0.78. ２ O ３ The amount of calcium oxide converted to CaO on a Ca basis was 17.8 parts by mass per 100 parts by mass, which is the sum of the mass converted to C and the mass converted to C of carbon black. ２ O ３ The boron carbide content was 16.2 parts by mass per 100 parts by mass, which is the sum of the mass converted to carbon, the mass converted to carbon (C) of carbon black, and the mass converted to CaO based on the Ca of calcium oxide. 1000 g of the raw material mixture was subjected to a reduction nitridation reaction by heating it at 1700°C for 8 hours in a graphite Tamman furnace under a nitrogen gas atmosphere.

[0074] The obtained crude hexagonal boron nitride powder was pulverized and placed in a container. For acid washing, five times the volume of dilute hydrochloric acid (7% by mass HCl) was added to the powder, and the mixture was stirred with a stirring blade at 300 rpm for 24 hours. 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 pure water was then filtered. This operation was repeated five times, and the powder was then vacuum dried at 200°C for 6 hours.

[0075] 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 millstone 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 particle sizes D10, D70, and D90, particle size D70s after ultrasonic treatment, particle size content of 75 to 150 μm, oil absorption, and specific surface area of ​​the obtained hexagonal boron nitride powder were measured by the method described later.

[0076] [Example 2] The same procedure as in Example 1 was performed, except that the temperature of the main heating step for the reduction-nitridation reaction was changed to 1730°C and the clearance of the grinder was changed to 120 μm, to obtain the hexagonal boron nitride powder of Example 2.

[0077] [Example 3] The same procedure as in Example 1 was performed, except that the temperature of the main heating step for the reduction-nitridation reaction was changed to 1730°C and the clearance of the grinder was changed to 280 μm, to obtain the hexagonal boron nitride powder of Example 3.

[0078] [Comparative Example 1] The same procedure as in Example 1 was followed, except that classification was performed without grinding after reheating, to obtain the hexagonal boron nitride powder of Comparative Example 1.

[0079] [Comparative Example 2] The same procedure as in Example 1 was performed, except for the following changes, to obtain the hexagonal boron nitride powder of Comparative Example 2. - The amount of raw materials used was changed, and in the raw material mixture: the ratio B / C of the mass of boron oxide converted to B to the mass of carbon black converted to C was set to 0.73; B based on the B of boron oxide ２ O ３ The amount of calcium oxide converted to CaO on a Ca basis shall be 9.1 parts by mass per 100 parts by mass of the total of the mass converted to C of carbon black and the mass converted to C of carbon black; and the amount of boron oxide converted to B on a B basis. ２ O ３ The boron carbide content was set to 16.0 parts by mass per 100 parts by mass, which is the sum of the mass converted to carbon, the mass converted to carbon (C) of carbon black, and the mass converted to CaO based on the Ca of calcium oxide. The temperature of the main heating step for the reduction-nitridation reaction was changed to 1950°C. Classification was performed without reheating or grinding after acid washing.

[0080] [Measurement of particle sizes D10, D70, and D90] The hexagonal boron nitride powders from both the examples and comparative examples were placed in a HORIBA LA-950V2 ultrasonic device, and the particle size distribution was measured while ultrasonic treatment was performed at an output of 40W for dispersion. From the obtained particle size distribution, the particle size D10 at a cumulative volume frequency of 10%, the particle size D70 at a cumulative volume frequency of 70%, and the particle size D90 at a cumulative volume frequency of 90% were determined.

[0081] [Measurement of particle size D70s after ultrasonic treatment] Hexagonal boron nitride powder from both the example and comparative example was dispersed in ethanol and ultrasonically treated with an ultrasonic homogenizer at an output of 250W for 20 minutes. The resulting slurry was placed in a HORIBA LA-950V2 and ultrasonically treated at an output of 40W for dispersion while measuring the particle size distribution to determine the particle size D70s at a cumulative volume frequency of 70%.

[0082] [Measurement of particle content with particle size of 75-150 μm] The hexagonal boron nitride powder of each example and comparative example was passed through a sieve with a mesh size of 150 μm. The portion that fell through the sieve was then passed through a sieve with a mesh size of 75 μm, and the portion remaining on the sieve was weighed. The mass of the portion remaining on the sieve was calculated relative to the mass of the hexagonal boron nitride powder used.

[0083] [Measurement of Oil Absorption] Measurements were performed in accordance with JIS-K6217-4 using hexagonal boron nitride powder and dibutyl phthalate (DBP) as the solvent for both the examples and comparative examples. A DBP dropping volume-torque curve was obtained with the horizontal axis representing DBP dropping volume (mL / 100g powder) and the vertical axis representing torque (Nm). Specifically, 20g of powder was placed in the mixing chamber, and DBP was added dropwise at a rate of 4.0 mL / min while stirring at 125 rpm with a rotor, and the torque was measured over time. A DBP dropping volume-torque curve was created from the measurement results. An oil absorption measuring device S-500 (manufactured by Asahi Research Institute Co., Ltd.) was used as the measuring device. DBP was prepared using Wako Pure Chemical Industries, Ltd.'s special grade reagent (distributor code 021-06936).

[0084] 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 showing 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.

[0085] [Measurement of Specific Surface Area] The specific surface area of ​​the hexagonal boron nitride powders in the examples and comparative examples was measured using a Macsorb HM model-1201 manufactured by Mountec.

[0086] [Preparation 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.

[0087] The thermal conductivity of the resin sheet was calculated by analyzing it using a thermal wave analysis device. The calculated thermal conductivity was evaluated according to the following criteria. A thermal conductivity evaluation of "4" or "3" indicates that the thermal conductivity of the resin sheet is excellent. 4: 18 W / m·K < thermal conductivity 3: 17 W / m·K < thermal conductivity ≤ 18 W / m·K 2: 15 W / m·K < thermal conductivity ≤ 17 W / m·K 1: thermal conductivity ≤ 15 W / m·K

[0088] 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

[0089] [Results] Table 1 shows the manufacturing conditions, powder properties measurement results, and resin sheet evaluation results for the examples and comparative examples.

[0090]

[0091] In Table 1, *1: The B / C ratio refers to the ratio of the mass of boron oxide converted to B in the raw material mixture to the mass of carbon black converted to C (mass converted to B / mass converted to C). *2: The amount of CaO refers to the amount of boron oxide in the raw material mixture, based on B. ２ O ３ This refers to the content of calcium oxide converted to CaO on a Ca basis, per 100 parts by mass, which is the sum of the mass converted to C and the mass converted to C of carbon black. *3: The amount of boron carbide refers to the amount of boron oxide in the raw material mixture, converted to B on a B basis. ２ O ３ This refers to the boron carbide content per 100 parts by mass, which is the sum of the mass converted to C in carbon black and the mass converted to CaO based on the Ca of calcium oxide.

[0092] As can be seen from the comparison between Comparative Example 1 and Comparative Example 2, in Comparative Example 1, the reheating process improved the aggregation strength of the aggregates contained in the hexagonal boron nitride powder, increasing the particle size D70s of the powder, and consequently improving the thermal conductivity of the resin sheet containing the powder. However, in Comparative Example 1, the amount of oil absorbed by the powder also increased, resulting in a decrease in the dielectric strength of the resin sheet. Therefore, while the powder obtained by the reheating process imparted excellent thermal conductivity to the resin composition, it was not possible to achieve both excellent thermal conductivity and dielectric strength simultaneously.

[0093] As can be seen from the comparison between Examples 1-3 and Comparative Example 1, in Examples 1-3, by performing a crushing step in addition to the reheating step, the particle size D10 decreased and the oil absorption amount decreased, resulting in a significant improvement in dielectric strength while maintaining the thermal conductivity of the resin sheet. From these results, it was found that a resin composition with excellent thermal conductivity and dielectric strength can be obtained using hexagonal boron nitride powder in which D70s and D10 are within a specific range. Furthermore, it was found that hexagonal boron nitride powder for obtaining a resin composition with excellent thermal conductivity and dielectric strength can be produced by a manufacturing method that includes a reheating step and a crushing step.

[0094] As can be seen from the comparison between Example 1 and Examples 2 and 3, in Example 1, by adjusting the crushing conditions to set the clearance in the crushing process to 160 μm, the particle size D70s was increased, and as a result, the thermal conductivity of the resin sheet was further improved.

[0095] One embodiment of the present invention, hexagonal boron nitride powder, can be used, for example, as a filler for resins used in electronic components.

Claims

1. Hexagonal boron nitride powder containing hexagonal boron nitride aggregates, wherein the oil absorption is 60 mL / 100 g or more and 90 mL / 100 g or less, the content of particles with a particle size of 75 μm or more and 150 μm or less is 20% by mass or more, the particle size D10 at the cumulative volume frequency of 10% of the particle size distribution in the wet laser diffraction particle size distribution method is 5 μm or more and 15 μm or less, and the particle size D70s at the cumulative volume frequency of 70% of the particle size distribution in the wet laser diffraction particle size distribution method after ultrasonic treatment with ethanol as a dispersion medium at 250 W for 20 minutes is 28 μm or more.

2. The ratio D70s / D70 of the particle size D70s at which the cumulative volume frequency of the particle size distribution in the wet laser diffraction particle size distribution method occurs after ultrasonic treatment with ethanol as the dispersion medium at 250 W for 20 minutes, to the particle size D70 at which the cumulative volume frequency of the particle size distribution in the wet laser diffraction particle size distribution method occurs at 70% of the particle size distribution, is 0.35 or more, according to claim 1.

3. A resin composition comprising the hexagonal boron nitride powder according to claim 1 or 2 and a resin.

4. 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 1800°C, wherein 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) of the raw material mixture is 0.75 to 1.04, and the B of the oxygen-containing boron compound is based on B ２ O ３ For every 100 parts by mass of the total of the mass converted to and the mass converted to C of the carbon source, the oxygenated calcium compound is contained in an amount of 8 to 25 parts by mass, converted to CaO on a Ca basis, and the oxygenated boron compound is B on a B basis. ２ O ３ A manufacturing method comprising: a main heating step in which, per 100 parts by mass of the total of the mass converted to C of the carbon source and the mass converted to CaO on a Ca basis of the oxygen-containing calcium compound, 12 parts by mass or more and 45 parts by mass of the boron carbide; an acid washing step in which the crude boron nitride powder obtained in the main heating step is acid washed; a reheating step in which the acid washed crude boron nitride powder is heated in a nitrogen atmosphere at a temperature of 1850°C or more and 2000°C or less; and a crushing step in which the boron nitride powder obtained in the reheating step is ground to a clearance of 120 μm or more and 300 μm or less.

5. The manufacturing method according to claim 4, further comprising a grinding step of grinding the crude boron nitride powder obtained by the main heating step, after the main heating step and before the acid washing step.

Images