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

Abstract

The present invention provides hexagonal boron nitride powder and the like for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength. [Solution] A 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.

Description

Technical Field

[0001] The present invention 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, the 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 hexagonal boron nitride powder blended with a resin further improves the thermal conductivity of the resin composition by including agglomerated particles in which single particles of hexagonal boron nitride are aggregated. As such a powder containing agglomerated particles, Patent Document 1 discloses hexagonal boron nitride powder capable of achieving both excellent thermal conductivity and dielectric breakdown strength because the oil absorption amount is maintained low. Further, Patent Document 2 discloses a technique for achieving both excellent thermal conductivity and dielectric breakdown strength by controlling the particle size D90 of the agglomerated particles to a relatively small range of about 50 μm to 100 μm.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[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. [Means for solving the problem]

[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 and 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 total mass of the oxygen-containing boron compound converted to B2O3 based on B and the mass of the carbon source converted to C is 100 parts by mass, and the oxygen-containing The process includes: a main heating step in which a calcium compound is contained in a mass of 8 to 25 parts by mass converted to CaO on a Ca basis, and 12 to 45 parts by mass of boron carbide per 100 parts by mass of the total of the mass of the oxygenated boron compound converted to B2O3 on a B basis, the mass of the carbon source converted to C, and the mass of the oxygenated calcium compound converted to CaO on a Ca basis; 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. [Effects of the Invention]

[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. [Modes for carrying out the invention]

[0010] <Hexagonal boron nitride powder> 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, possesses sufficient strength to maintain its aggregated state without completely disintegrating during mixing with 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 high oil absorption is mixed with resin and solvent, the viscosity of the resulting varnish increases, and the varnish's coating properties decrease. Increasing the amount of solvent added improves the varnish's coating properties, but as the amount of solvent added increases, the amount of solvent that must be dried and removed during drying of the coated material also increases, resulting in an increase in voids in the coated material and consequently a decrease in the dielectric strength of the resin composition. Therefore, hexagonal boron nitride powder with high oil absorption 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"). These 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 which the cumulative volume frequency of the particle size distribution is 10% is an indicator of the content of fine particles with a relatively small particle size, and it is desirable to control it within a specific range. A larger particle size D10 indicates a smaller 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 greater the content of fine particles that can fill the gaps between single particles in the aggregate, thus reducing the oil absorption of this powder, 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 reaches 90% is preferably 85 μm or larger, and more preferably 95 μm or larger. The larger the particle size D90 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 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 ultrasonically treated at an output of 40W for dispersion. In this specification, the ultrasonic treatment before measuring particle size D70s is performed under the conditions of 250W 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] (Ratio 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 index of the ratio of aggregates that particularly contributes to the improvement of the thermal conductivity of this powder, and it is advisable to control it within a specific range. The higher the content, the higher the ratio 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, more preferably 25% by mass or more. The greater the content within these ranges, the larger the particle size to a sufficient extent for a large contribution to the improvement of thermal conductivity, and the larger the ratio of aggregates with a particle size small enough that voids are less likely to occur inside, so the effects of improving the thermal conductivity and dielectric strength of the resin composition by this powder are greater. There is no particular upper limit for the content of particles with a particle size of 75 μm or more and 150 μm or less, but as an example, this ratio may be 60% by mass or less.

[0020] Note that in this specification, as shown in the examples described later, 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 this powder that is classified under a 150-μm sieve and above a 75-μm sieve.

[0021] (Oil absorption) In this powder, the oil absorption is an index of the amount of gaps in the aggregates contained in this powder and the degree of development of the structure on the surface of the aggregates, and it is advisable to control it within a specific range. The lower the oil absorption, the smaller the gaps in the aggregates that take in the solvent as a whole for this powder. In this powder, the oil absorption is 60 mL / 100 g or more, preferably 65 mL / 100 g or more. The greater the oil absorption within these ranges, the greater the contribution to the improvement of thermal conductivity, so the effect of improving the thermal conductivity of the resin composition by this powder is 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 these ranges, the less the solvent uptake that causes void formation and dielectric strength reduction in the resin composition as a whole for this powder, so the effect of improving the dielectric strength of the resin composition by this powder is greater.

[0022] In addition, as the method for measuring the oil absorption amount in this specification, as shown in the examples described later, a method in accordance with JIS-K6217-4 is adopted.

[0023] (Specific surface area) The specific surface area of this powder is preferably 2.2 m 2 / g or more, more preferably 2.5 m 2 / g or more. The larger the specific surface area within these ranges, the smaller and denser the single particles constituting the aggregate, and the stronger the aggregation strength. Therefore, the effect of improving the thermal conductivity of the resin composition by this powder becomes greater. There is no particular upper limit to the specific surface area of this powder, but as an example, the specific surface area may be 4.0 m 2 / g or less.

[0024] In this specification, the specific surface area of this powder is measured by the BET one-point method, as shown in the examples described later. The specific surface area of this powder can be measured using, for example, Macsorb HM model-1201 (trade name) manufactured by Mountech Co., Ltd.

[0025] (Composition) The particles constituting this powder contain boron nitride as the main component and are preferably substantially composed of boron nitride. The content of boron nitride in this powder is, as an example, 90, 95 or 99 mass% or more, preferably 99.95 mass% or more, more preferably 99.97 mass% or more. The higher the content of boron nitride within these ranges, the more the curing inhibition of the resin composition, which is also caused by impurity elements and leads to a decrease in thermal conductivity and dielectric breakdown voltage, can be reduced.

[0026] The content of boron nitride in this powder is the value obtained by subtracting the content mass ratio (unit: mass%) of elements other than B and N in this powder from 100. The content of boron nitride is measured by the fluorescent X-ray analysis method, and can be confirmed, for example, by ZSX Primus2 (trade name) manufactured by Rigaku Corporation as a fluorescent X-ray analyzer.

[0027] (Resin composition) Applications of this powder include its use as a filler in resins to improve dielectric strength and thermal conductivity. In other words, resin compositions containing this powder and resin are 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 rubber; 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 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.

[0032] 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.

[0033] <Method for producing hexagonal boron nitride powder> To solve the problems of the conventional technology described above, the inventors conducted extensive research. As a result, the inventors found that when a reheating treatment is performed after a heat treatment for reductive nitriding in the manufacturing process of hexagonal boron nitride powder, the aggregates contained in the resulting powder have sufficient strength to maintain their aggregated state without completely collapsing during mixing with resin. However, simply performing such a reheating treatment may increase the oil absorption of the resulting powder, which leads to a decrease in the dielectric strength of the resin composition. This is thought to be because single particles that did not constitute aggregates before the reheating treatment become fixed together after the reheating treatment, and gaps are created between the fixed single particles. The inventors further investigated the above problems and found that when the reheated hexagonal boron nitride powder is ground, the aggregates do not collapse, but the fixed single particles can be broken down, resulting in a certain amount of fine particles with a relatively small particle size being generated in the powder, thus avoiding an increase in the oil absorption of the powder, and thus completing the present invention.

[0034] The following describes in detail a method for producing hexagonal boron nitride powder according to one aspect of the present invention. In this description, the method for producing hexagonal boron nitride powder according to one aspect of the present invention may be abbreviated as "this production method." This production method may be the method for producing this powder described above, although this is not limited to this method. Furthermore, the hexagonal boron nitride powder produced by this production 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 that order. Furthermore, this manufacturing method may include a mixing step before the main heating step. Also, this manufacturing method may include a grinding step after the main heating step and before the acid washing step. Furthermore, this manufacturing method may include a classification step after the crushing step.

[0036] (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.

[0037] (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.

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

[0039] (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.

[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] (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.

[0044] 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.

[0045] (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.

[0046] (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 0.75 or higher, 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 1.04 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 reducing the softening of the aggregates.

[0047] In the raw material mixture, the content of the oxygenated calcium compound, converted to CaO based on Ca, is 8 parts by mass or more, preferably 12.7 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 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 content of boron carbide per 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 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, which is less susceptible to reductive nitridation compared to the oxygenated boron compound, is present, 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.

[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 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 mill grinders, ball mills, hammer mills, roll crusher pin mills, jet mills, and mortars.

[0054] (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.

[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 step involves heating the acid-washed coarse boron nitride powder. By reheating the coarse boron nitride powder obtained in the main heating step, 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. 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 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 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] (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 particles with a relatively small particle size in the powder, which fills 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.

[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 involves classifying the hexagonal boron nitride powder obtained from the crushing process. The classification process removes unwanted particles of a certain particle size from 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 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 distribution at the cumulative volume frequency of 70% in the wet laser diffraction particle size distribution method to the particle size D70 at the cumulative volume frequency of 70% in the wet laser diffraction particle size distribution method, after ultrasonic treatment with ethanol as the dispersion medium at 250 W for 20 minutes, 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 oxygenated boron compound, a carbon source, an oxygenated 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 oxygenated boron compound converted to B and 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 total mass of the oxygenated boron compound converted to B2O3 on a B basis and the mass of the carbon source converted to C is 100 parts by mass, and the oxygenated calcium compound is converted to CaO on a Ca basis. A manufacturing method comprising: a main heating 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.

[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. [Examples]

[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] A raw material mixture was obtained by mixing 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 using a ball mill. The ratio B / C of the raw material mixture, which is the ratio of the mass of boron oxide converted to B (based on B) to the mass of carbon black converted to C (based on C), was 0.78. The content of calcium oxide converted to CaO (based on Ca) was 17.8 parts by mass per 100 parts by mass of the total mass of boron oxide converted to B2O3 (based on B) and carbon black converted to C (based on C). The content of boron carbide converted to CaO (based on Ca) was 16.2 parts by mass per 100 parts by mass of the total mass of boron oxide converted to B2O3 (based on B) and carbon black converted to C (based on C) and calcium oxide converted to CaO (based on Ca). 1000g 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 content of 75-150 μm, oil absorption, and specific surface area of ​​the obtained hexagonal boron nitride powder were measured by the method described later.

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

[0077] [Example 3] Except for changing the temperature of the main heating step for the reduction-nitridation reaction to 1730°C and changing the clearance of the grinder to 280 μm, the same procedure as in Example 1 was performed 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 followed to obtain the hexagonal boron nitride powder of Comparative Example 2, with the following modifications. 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 and the mass of carbon black converted to C was set to 0.73; the content of calcium oxide converted to CaO based on Ca was set to 9.1 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; and the content of boron carbide was set to 16.0 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. 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] Hexagonal boron nitride powder from both the examples and comparative examples was 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%, D70 at a cumulative volume frequency of 70%, and D90 at a cumulative volume frequency of 90% were determined.

[0081] [Measurement of particle size D70s after ultrasonic treatment] The hexagonal boron nitride powders for both the examples and comparative examples were dispersed in ethanol and subjected to ultrasonic treatment at 250W for 20 minutes using an ultrasonic homogenizer. The resulting slurry was placed in a HORIBA LA-950V2 and subjected to ultrasonic treatment at 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 a particle size of 75-150 μm] The hexagonal boron nitride powder for both the examples and comparative examples was sieved through a 150 μm mesh sieve. The portion that fell through the sieve was then sieved through a 75 μm mesh sieve, 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] 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 analyzer 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 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.

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

[0086] [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.

[0087] 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 "◎" or "〇" indicates that the resin sheet has excellent thermal conductivity. ◎: 18W / m·K<Thermal conductivity> 〇:17W / m·K<thermal conductivity≦18W / m·K △: 15 W / m·K < Thermal conductivity ≤ 17 W / m·K ×: 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. If the dielectric strength evaluation is "◎" or "〇", it can be said that the dielectric strength of the resin sheet is excellent. ◎: 45KV / mm < Dielectric strength ○: 43KV / mm < Dielectric strength ≤ 45KV / mm △: 41KV / mm < Dielectric strength ≤ 43KV / mm ×: Dielectric strength ≤ 41KV / mm²

[0089] 〔result〕 Table 1 shows the manufacturing conditions, powder property measurement results, and resin sheet evaluation results for the examples and comparative examples.

[0090] [Table 1]

[0091] In Table 1, *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).

[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 both 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 both 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. [Industrial applicability]

[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. A hexagonal boron nitride powder containing hexagonal boron nitride aggregates, 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. In wet laser diffraction particle size distribution analysis, the particle size D10 at the cumulative volume frequency of 10% of the particle size distribution is between 5 μm and 15 μm. In wet laser diffraction particle size distribution analysis, after ultrasonic treatment with ethanol as the dispersion medium at 250 W for 20 minutes, the particle size D70s at the cumulative volume frequency of 70% of the particle size distribution is 28 μm or larger. Hexagonal boron nitride powder.

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 is 0.35 or greater, after ultrasonic treatment with ethanol as the dispersion medium for 20 minutes at 250 W, to the particle size D70 at which the cumulative volume frequency of the particle size distribution is 70%. The hexagonal boron nitride powder 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, 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 1800°C. The aforementioned raw material mixture is 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 0.75 or more and 1.04 or less. Based on the B standard of the aforementioned oxygen-containing boron compound, B ２ O ３ The oxygen-containing calcium compound is contained in an amount of 8 to 25 parts by mass, calculated based on Ca, with respect to 100 parts by mass of the total mass of the carbon source and the mass of the carbon source converted to C. Based on the B standard of the aforementioned oxygen-containing boron compound, B ２ O ３ A main heating step comprising: a total of 100 parts by mass of the mass converted to C, the mass of the carbon source converted to C, and the mass of the oxygenated calcium compound converted to CaO based on Ca, with 12 parts by mass or more and 45 parts by mass or less of the boron carbide; An acid washing step is performed to acid wash the crude boron nitride powder obtained in the main heating step, 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, A crushing step is performed in which the boron nitride powder obtained by the reheating step is ground to a clearance of 120 μm or more and 300 μm or less. A manufacturing method that includes this.

5. After the main heating step and before the acid washing step, the process further includes a grinding step in which the coarse boron nitride powder obtained in the main heating step is ground. The manufacturing method according to claim 4.

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