JP6720014B2 - Hexagonal boron nitride primary particle aggregate, resin composition and use thereof - Google Patents

Description

The present invention relates to hexagonal boron nitride primary particle aggregates. The present invention also relates to a resin composition containing the hexagonal boron nitride primary particle aggregate and its use.

2. Description of the Related Art Conventionally, heat-generating electronic components represented by power devices, transistors, thyristors, CPUs, etc. have been increasing in power year by year, and on the other hand, along with the miniaturization of electronic devices, heat-generating electronic components have become The mounting density is also increasing, and the heat generation density inside electronic devices is also increasing year by year. Therefore, research and development on a method for efficiently radiating heat generated when using a heat-generating electronic component and a heat-dissipating material have been continuously advanced.

As a method of radiating the heat generated by the heat-generating electronic component, a heat-generating electronic component or a printed wiring board on which the heat-generating electronic component is mounted, through a flat interface or sheet-like thermal interface material having electrical insulation, A general method is to attach a heat dissipation member such as a heat sink. Further, a printed wiring board is usually provided with an electric insulating layer in the middle thereof, and this electric insulating layer is also required to be an excellent thermal interface material at the same time. As the thermal interface material, for example, a composition containing ceramic particles and a thermosetting resin is preferably used.

As the ceramic particles, hexagonal boron nitride, which basically has electrical insulation and is excellent in thermal conductivity, has attracted attention and is preferably used. However, due to its crystal structure, primary particles of hexagonal boron nitride are scaly, have a thermal conductivity of about 400 W/(m·K) in the in-plane direction (also referred to as a-axis direction), and a thickness direction (c The thermal conductivity (also referred to as the axial direction) is about 2 W/(m·K), and the anisotropy of the thermal conductivity is extremely large. Therefore, when the hexagonal boron nitride primary particles are directly filled in the resin and an attempt is made to manufacture a flat plate-shaped or sheet-shaped thermal interface material, the scale-shaped primary particles become flat along the surface direction of the thermal interface material. The direction in which heat is to be transferred (that is, the thickness direction of the thermal interface material) and the direction in which the hexagonal boron nitride primary particles have a high thermal conductivity do not necessarily coincide with each other, and are rather orthogonal. It was not possible to fully utilize the high in-plane thermal conductivity of crystalline boron nitride.

Therefore, the hexagonal boron nitride primary particles intended to relax the anisotropy of the thermal conductivity due to the shape of the hexagonal boron nitride primary particles and to have isotropic thermal conductivity. Aggregates have been developed.

For example, in Patent Documents 1 and 2, the hexagonal boron nitride primary particles are preliminarily isotropically aggregated so as not to be oriented in the same direction in a flat plate-shaped or sheet-shaped thermal interface material, and the thermal conductivity anisotropy is increased. A hexagonal boron nitride primary particle agglomerate in which γ is relaxed has been proposed. However, the shape of the hexagonal boron nitride primary particle aggregates disclosed herein has a distorted pine cone shape (for example, see Patent Document 1: Paragraph [0020] FIG. 6) or a lump shape (for example, Patent Document 2: Paragraph [ 3 to 5)), the resin filling rate is also limited, and the thermal conductivity when used as a thermal interface material does not always meet the current required level.

As an attempt to improve the shape of the aggregate, Patent Document 3 proposes the use of hexagonal boron nitride-coated particles having a high average sphericity, in which borate particles are coated with hexagonal boron nitride primary particles. The coated particles have a certain effect in suppressing the anisotropy of thermal conductivity and improving the filling property into the resin, but since they contain borate particles having a low thermal conductivity, they are hexagonal nitrided. There is a problem that the high thermal conductivity of boron cannot be fully utilized.

Furthermore, from the surface to the inside of the conventional hexagonal boron nitride primary particle aggregate, it is considered that there are still many voids, and the thermal conductivity of the aggregate alone cannot be sufficiently high. The particle strength of (1) was insufficient, and when the agglomerate and the resin were mixed to form a resin composition, for example, the primary particle agglomerate sometimes failed to withstand the force applied during kneading and was destroyed.

In Patent Document 4, by defining the porosity range of the agglomerated particles, the non-dielectric constant of the resin composition containing the agglomerated particles is kept low, and the calcium content of the agglomerated boron nitride powder is specified and kneaded with the resin. A boron nitride powder is disclosed which is intended to prevent the destruction of aggregated particles at the time. Since the agglomerated particles of Patent Document 4 have a slightly higher porosity, there is room for further improvement in terms of particle strength and thermal conductivity.

Patent Document 5 describes the structure of primary particles or aggregates of agglomerated boron nitride, but since the average diameter of the primary particles is small and the bulk density of the powder is low, it is highly filled in the resin composition. Had challenges.

As shown above, it is particularly important to improve the thermal conductivity within the practically allowable range of the non-dielectric constant that satisfies the heat dissipation requirement level of electronic devices using recent heat-generating electronic components. That is, by further increasing the sphericity of the hexagonal boron nitride primary particle agglomerates, further decreasing the porosity, and further increasing the particle strength, it is possible to further fill the resin composition with a higher content than before. No aggregate was obtained in the first place, and therefore a resin composition or a thermal interface material using the same was not developed.

JP, 9-202663, A JP, 2011-98882, A JP, 2001-122615, A JP, 2014-40341, A International publication pamphlet of WO2015/119198

In view of the problems of the prior art, the present invention has a shape that provides good filling characteristics, and as a result, it is possible to pack the boron nitride primary particle aggregates at a high density, and is also envisioned during secondary processing. An object is to provide a hexagonal boron nitride primary particle aggregate having a high particle strength so that it is not broken even when subjected to an external force. A resin containing the hexagonal boron nitride primary particle agglomerates, which is used as an electrical insulating layer of a printed wiring board on which heat-generating electronic components such as power devices are mounted and a thermal interface material that transfers heat to a heat dissipation member. It is intended to provide a composition and a thermal interface material using the resin composition.

The present inventors diligently studied high-strength hexagonal boron nitride primary particle aggregates that can be highly filled in a resin composition and are densely aggregated, and the requirements that the primary particle aggregates must satisfy The present invention has been achieved through intensive studies.

Means for Solving the Problems The present invention, which solves the above-mentioned problems, provides a hexagonal boron nitride primary particle aggregate having an average sphericity of 0.80 or more, a porosity of 40% or less, and a tap density (also referred to as tap bulk density) of 1.0 g. /Cm ³ or more and the particle strength is 5.0 MPa or more, and the hexagonal boron nitride primary particles are assumed to be aggregates. In the present invention, a hexagonal boron nitride primary particle aggregate in which primary particles in the form of flakes having an aspect ratio of major axis and thickness of the primary particles of more than 10 and 20 or less are aggregated is preferable. Further, in the present invention, the hexagonal boron nitride primary particle aggregate having an average particle diameter of 20 to 100 μm is preferable. Further, the present invention is a resin composition containing the hexagonal boron nitride primary particle aggregate, and a thermal interface material using the resin composition.

According to the practice of the present invention, industrially useful, a hexagonal boron nitride primary particle aggregate that exhibits high thermal conductivity, and a resin composition containing the hexagonal boron nitride primary particle aggregate, the resin composition was used. An electrically insulating member can be provided.

The hexagonal boron nitride primary particle aggregate of the present invention is an aggregate in which scaly primary particles of hexagonal boron nitride are in a spherical shape and are sintered with high strength, and are kneaded together with a resin. It is a hexagonal boron nitride primary particle aggregate having a high particle strength that is not easily broken by an external force of a certain degree.

The hexagonal boron nitride primary particle agglomerate of the present invention has an average sphericity, a tap density, and a particle strength that are unachievable by conventional techniques, thereby achieving a high thermal conductivity, and a boron nitride powder, and boron nitride. A resin composition containing powder can be obtained.

The hexagonal boron nitride primary particle aggregate of the present invention is not particularly limited to the production method, for example, a precursor preparation step of preparing a precursor of the hexagonal boron nitride primary particle aggregate, and the precursor Slurrying step for obtaining a slurry containing, spray drying step for spraying the slurry to obtain a precursor dry powder (hereinafter referred to as precursor dry powder), and firing the precursor dry powder for hexagonal crystal A manufacturing method that includes various steps including a firing step for obtaining a boron nitride primary particle aggregate can be preferably employed.

<Precursor preparation step>
The precursor preparation step is a step of preparing a precursor of hexagonal boron nitride primary particle aggregates. The precursor is not particularly limited as long as it is a substance that can obtain a boron nitride primary particle aggregate by using it as a raw material, for example, boron nitride obtained by firing a mixture containing melamine and boric acid, A hexagonal boron nitride primary particle aggregate in which primary particles having an average sphericity of 0.80 or more and a high sphericity and an aspect ratio of average diameter and average thickness of more than 10 and 20 or less are aggregated are easily obtained. It is preferably prepared as a precursor. When the average sphericity is 0.8 or more, the filling property into the resin is increased, and a thermal interface material having high thermal conductivity can be obtained. Incidentally, depending on the conditions for firing a mixture containing melamine and boric acid, the crystallinity of the obtained boron nitride may change from an amorphous state to a state where crystallization has advanced, but especially of boron nitride as the precursor. There is no limitation on the crystallinity, and a mixture of a plurality of boron nitrides having different crystallinities prepared and mixed together is also a precursor that easily obtains a high hexagonal boron nitride primary particle aggregate having a particle strength of 5.0 MPa or more. It can be preferably used.

<Slurry process>
The slurry forming step is a step of obtaining a slurry containing the precursor. The type of liquid component of the slurry is not particularly limited, but water or a water-soluble organic solvent such as alcohol is generally used as the liquid component. As a preferable composition ratio of the slurry, for example, the concentration of the liquid component is preferably 55% by mass or less, and more preferably 50% by mass or less. Further, 0.5 to 5.0% by mass of a sintering aid for promoting sintering of the precursor can be preferably added to the slurry, and the remainder is a precursor of hexagonal boron nitride primary particle aggregates. It becomes the mass ratio of. As the type of sintering aid, alkali metal, alkaline earth metal carbonates, nitrates, oxides, and nitrides are preferably used, and calcium carbonate is particularly preferable. Further, the binder of the precursors and the slurry stabilizer can be preferably added in a ratio of 0.1 to 2 parts by mass with respect to 100 parts by mass of the slurry. As the binder, polyvinyl alcohol (also referred to as PVA) and carboxymethyl cellulose (also referred to as CMC) are preferably used. Further, the slurry stabilizer is a sulfate ester type, phosphate ester type, carboxylic acid type, sulfonic acid type anionic surfactant, such as polyoxyethylene tridecyl ether phosphate ester, polyoxyethylene alkyl (C8). Ether phosphate ester, polyoxyethylene alkyl (C10) ether phosphate ester, polyoxyethylene alkyl (C12, 13) ether phosphate ester, polyoxyethylene lauryl ether phosphate ester, polyoxyethylene styrenated phenyl ether phosphate ester , Polyoxyalkylene branched decyl ether type, polyoxyalkylene tridecyl ether type, polyoxyalkylene alkyl ether type, polyoxyethylene styrenated phenyl ether type, polyoxyethylene naphthyl ether type, polyoxyethylene phenyl ether type, polyoxy A slurry stabilizer containing at least one surfactant selected from ethylene lauryl ether type nonionic surfactants and polymer type surfactants such as sodium naphthalenesulfonate formalin condensate is preferably used. Among the listed surfactants, those containing a phosphoric acid ester type surfactant are preferable, and among the phosphoric acid ester type surfactants, polyoxyethylene alkyl (C8) ether phosphoric acid ester and polyoxyethylene are particularly preferable. Alkyl (C10) ether phosphate, polyoxyethylene alkyl (C12, 13) ether phosphate, and polyoxyethylene lauryl ether phosphate are preferably used.

Further, in the slurry forming step, a solid component in the slurry, that is, a precursor of hexagonal boron nitride primary particle aggregates and a sintering aid may be finely pulverized by using a ball mill or the like to form a more uniform and stable slurry. It is preferably carried out.

<Spray drying process>
In the spray drying step, for example, using a device generally called a rotary atomizer, a slurry containing a precursor of the hexagonal boron nitride primary particle aggregates, for example, by spraying using a atomizer in a high temperature container, The precursor dry powder from which the liquid component is removed can be preferably obtained. The temperature in the container is not particularly limited, but is usually preferably in the range of 150 to 250°C. The atomizing method of the atomizer is not particularly limited.

<Firing process>
In the firing step, the precursor dry powder is usually fired in a furnace at 1600 to 2300°C, preferably 1700 to 1900°C. It is a step of finally obtaining the hexagonal boron nitride primary particle aggregate of the present invention. The type of the furnace is not particularly limited, and either a known resistance heating element heating furnace, a known type of high frequency furnace, or a batch type furnace or a continuous type furnace may be selected.

Next, obtained in the firing step, the average sphericity of the hexagonal boron nitride primary particle aggregate of the present invention, tap density, particle strength, average particle diameter, and the aspect ratio of the major axis and thickness of the primary particles, respectively, The definition and measurement method are described below.

<Average sphericity>
The average sphericity of the hexagonal boron nitride primary particle agglomerates referred to in the present invention is calculated based on a two-dimensional image obtained by actually photographing a large number of the primary particle agglomerates, and the average sphericity is calculated by an image analysis method. It is an average value, and the sphericity of one primary particle aggregate is measured by the following procedure, for example. That is, any one hexagonal boron nitride primary particle aggregate (referred to as P ₁ ) fixed to the conductive double-sided tape on the sample table was photographed using a scanning electron microscope, and the obtained two-dimensional From the image, the projected area of P1 (denoted by S ₁ ) and the perimeter (denoted by L ₁ ) are measured using image analysis software. The sphericity of P ₁ (denoted by R ₁ ) of the present invention is based on the area of a perfect circle (that is, πr ₁ ² ) having a radius (denoted by r ₁ ) having the same perimeter as S1. Assuming that R ₁ =S ₁ /πr ₁ ² is satisfied, the relationship of L ₁ =2πr ₁ is also established. Therefore, in the formula of R ₁ =4πS ₁ /L ₁ ² , the above S ₁ , L It can be calculated by substituting each of the measured values of ₁ . In the present invention, the sphericity of 100 arbitrary hexagonal boron nitride primary particle agglomerates was determined in this manner, and the average value was used as the average sphericity of the present invention. Since the sphericity of a true sphere is 1, a value closer to this value is closer to a true sphere, and the filling property into the resin composition is good. The average sphericity of the hexagonal boron nitride primary particle aggregate of the present invention is 0.80 or more. When the average sphericity is less than 0.8, the filling property into the resin composition is poor and the thermal conductivity is low. The average sphericity is more preferably 0.85 or more.

<Porosity>
The porosity referred to in the present invention is the ratio of the voids present in the hexagonal boron nitride primary particle aggregate of the present invention, and the porosity needs to be 40% or less. The porosity can be measured by a mercury porosimeter using a mercury porosimeter. From the viewpoint of increasing the packing density, it is preferable that the porosity is low, but the value obtained during the study of the present invention was 29% or more. When the porosity is small, the volume occupied by air having a small relative dielectric constant tends to be small, and the overall relative dielectric constant tends to be high.

<Tap density>
The tap density of the hexagonal boron nitride primary particle agglomerate of the present invention is a value measured according to JIS R1628 and can be measured using a commercially available device, and the hexagonal boron nitride primary particle agglomerate is 100 cm ³ The bulk density after the tapping was performed in a dedicated container of No. 1 and tapping was performed under the conditions of a tapping time of 180 seconds, a tapping frequency of 180 times, and a tap lift of 18 mm, and the tap density was obtained. The tap density of the hexagonal boron nitride primary particle aggregate of the present invention is 1.0 g/cm ³ or more. If the tap density is less than 1.0 g/cm ³ , the viscosity of the resin when it is filled increases, which is not suitable for actual production. Incidentally, the lower the porosity, the higher the tap density tends to be, but in the case where the mixing ratio of the sintering aid is increased at a low slurry concentration, the tap density is not high even if the porosity is relatively low. Therefore, it is necessary that at least both the porosity and the tap density are defined and both are satisfied.

<Particle strength>
The particle strength of the hexagonal boron nitride primary particle aggregate referred to in the present invention is a value actually based on the measured value of the compressive strength of individual particles. The particle strength can be measured according to JIS R1639-5 using a commercially available compression tester capable of measuring the compression strength of fine particles. At this time, assuming that the particle size of the hexagonal boron nitride primary particle aggregate is d (unit is mm) and the breaking test force is P (unit is N), the breaking strength Cs (unit MPa) is Cs=2.48P. It is calculated from the formula of /πd ² . In the present invention, the average value of the breaking strengths of 10 particles is defined as the particle strength. In the hexagonal boron nitride primary particle aggregate of the present invention, the particle strength needs to be 5.0 MPa or more. When the particle strength is less than 5.0 MPa, for example, when kneading with a resin to form a composition, the aggregate particles are broken, which is not preferable.

<Average particle size>
The average particle size of the hexagonal boron nitride primary particle agglomerates of the present invention is 20 to 100 μm in accordance with JIS Z8825, in a particle size distribution measurement by a laser diffraction light scattering method, with a particle size at a cumulative value of 50% of the cumulative particle size distribution. Is preferred. If it is less than 20 μm, the thermal conductivity decreases due to an increase in contact thermal resistance with an increase in the total number of boron nitride particles and resin interfaces. If it is larger than 100 μm, the particle strength of the boron nitride particles tends to decrease, and therefore, due to the shear stress received during the kneading with the resin, a part of the hexagonal boron nitride primary particle agglomerates is converted into particles having a shape close to the primary particles. Since the particles are broken and the broken particles are oriented in the same direction, the thermal conductivity tends to decrease. Further, when compounded in a thick resin, the influence of the average particle size is small, but in the case of molding into a sheet shape such as a heat dissipation sheet, the sheet thickness is limited, and a size up to 100 μm is preferable. is there.

For the average particle diameter of the present invention, a commercially available particle size distribution measuring device can be used. In the measurement, water was used as the solvent and hexametaphosphoric acid was used as the dispersant. Incidentally, at the time of measuring the average particle diameter, hexagonal boron nitride primary particle aggregates are crushed, in order to avoid changing the particle diameter of the actual aggregate, the slurry is well dispersed Although it was confirmed, the dispersion treatment by ultrasonic treatment was not intentionally carried out. The refractive index of water was 1.33, and the refractive index of the boron nitride powder was 1.80.

<Aspect ratio of major axis and thickness>
The aspect ratio of the major axis and the thickness of the primary particles in the present invention is a value calculated by actually measuring a cross-sectional photographic image of the primary particles forming the hexagonal boron nitride primary particle aggregate. That is, a SEM image of a particle image of a cross section of a hexagonal boron nitride primary particle aggregate which is embedded in a resin and precisely cut is taken by a scanning electron microscope, and the scaly primary particles reflected in strips are imaged. It is a value calculated by importing it into an analyzer and analyzing it with software. Since the hexagonal boron nitride primary particle aggregates embedded in the resin are cut in any cross section, the aspect ratio is not an average value obtained from the measurement values of arbitrarily selected particles, but a strip shape. After observing 100 visible particles, 10 particles that fall within the 10th place from the longer major axis are selected, the major axis and thickness of the 10 particles are measured, and the aspect ratio = major axis/thickness The aspect ratio of each of the 10 particles was calculated by the following calculation formula, and the average value thereof was used as the aspect ratio.

<Resin Composition Containing Hexagonal Boron Nitride Primary Particle Aggregate>
Next, a resin composition containing a hexagonal boron nitride primary particle aggregate, which is a second embodiment of the present invention, will be described. The proportion of the primary particle aggregates contained in the resin composition is preferably 20% by volume or more and 80% by volume or less. If it is less than 20% by volume, the thermal conductivity of the resin composition is low, and if it exceeds 80% by volume, molding or molding of the resin composition becomes difficult, which is not preferable.

At this time, various ceramic mix powders having an average particle size smaller than that of the hexagonal boron nitride primary particle aggregate of the present invention (hereinafter referred to as various ceramic powders), for example, aluminum nitride, hexagonal boron nitride, boron nitride, silicon nitride Powders of aluminum oxide, zinc oxide, magnesium oxide, magnesium hydroxide, silicon dioxide, and silicon carbide may be appropriately added in an amount not less than the range of the object of the present invention. The appropriate average particle size of various ceramic powders varies depending on the average particle size of the aggregate of the boron nitride powder of the present invention, but is 40% or less with respect to the average particle size of the aggregate of the boron nitride powder of the present invention. It is preferably 20% or less, more preferably 20% or less. For example, when the average particle diameter of the aggregate of the boron nitride powder of the present invention is 50 μm, it is preferably 20 μm or less, more preferably 10 μm or less. Since the packing structure of the particles can be made denser, the packing property is improved, and as a result, the thermal conductivity of the resin composition can be improved. The average particle size of various ceramic powders is measured by the same procedure as that for the hexagonal boron nitride primary particle aggregate of the present invention.

<Resin>
The type of resin that can be used in the resin composition containing the hexagonal boron nitride primary particle aggregates, which is the second embodiment of the present invention, is not particularly limited, and examples thereof include epoxy resin, silicone resin, and silicone rubber. , Acrylic resin, phenol resin, melamine resin, urea resin, unsaturated polyester, fluororesin, polyimide, polyamideimide, polyimide such as polyetherimide, polyester such as polybutylene terephthalate and polyethylene terephthalate, polyphenylene ether, polyphenylene sulfide, wholly aromatic Group polyester, polysulfone, liquid crystal polymer, polyether sulfone, polycarbonate, maleimide modified resin, ABS resin, AAS (acrylonitrile-acrylic rubber/styrene) resin, AES (acrylonitrile/ethylene/propylene/diene rubber-styrene) resin, polyglycolic acid resin , Polyphthalamide, polyacetal, nylon resin and the like can be preferably mentioned. For these resins, particularly thermosetting resins, a curing agent, an inorganic filler, a silane coupling agent, and further improvement of wettability and leveling property and promotion of viscosity reduction are promoted to reduce the occurrence of defects during heat and pressure molding. Additives can be included. Examples of this additive include a defoaming agent, a surface conditioner, and a wetting and dispersing agent. Further, the epoxy resin is suitable as an insulating layer of a printed wiring board because it has excellent heat resistance and adhesive strength to a copper foil circuit. Further, silicone resin and silicone rubber are suitable as a thermal interface material because they have excellent heat resistance, flexibility and adhesion to a heat sink and the like. Regarding the thermal interface material using the thermosetting resin, the thermosetting resin may be B-staged in advance before actually using it.

When the hexagonal boron nitride primary particle agglomerate of the present invention is mixed with a resin to form a resin composition, an organic solvent may be added if necessary in order to facilitate mixing of the two. Examples of the organic solvent include alcohols such as ethanol and isopropanol, 2-methoxyethanol, 1-methoxyethanol, 2-ethoxyethanol, 1-ethoxy-2-propanol, 2-butoxyethanol, 2-(2-methoxyethoxy). ) Ethanol, ether alcohols such as 2-(2-ethoxyethoxy)ethanol and 2-(2-butoxyethoxy)ethanol, glycol ethers such as ethylene glycol monomethyl ether and ethylene glycol monobutyl ether, acetone, methyl ethyl ketone, methyl isobutyl ketone And ketones such as diisobutyl ketone ketone, and hydrocarbons such as toluene and xylene. These diluents may be used alone or in combination of two or more.

The electrical insulating member according to the third embodiment of the present invention is formed by molding or molding the resin composition containing the hexagonal boron nitride primary particle aggregates according to the second embodiment, and if necessary, other It is a thermal interface material made by combining it with materials.

<Evaluation of thermal conductivity of thermal interface material>
The thermal conductivity H (unit: W/(m·K)) of the thermal interface material of the present invention is the thermal diffusivity A (unit: m ² /sec), density B (unit: kg/) of the thermal interface material. It can be calculated as H=A×B×C from m ³ ) and the specific heat capacity C (unit is J/(kg·K)). The thermal diffusivity A is a value measured according to JIS R1611, according to the laser flash method using a commercially available device. The density B is a value obtained by using the Archimedes method according to JIS K6268. Further, the specific heat capacity C is a value obtained by using a DSC measuring device in accordance with JIS K7123.

Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.
<Example 1>
<Preparation of hexagonal boron nitride primary particle aggregate>
52 kg of boric acid (manufactured by Nitto Denko) and 50 kg of melamine (manufactured by DSM) were mixed and baked in a batch type high-frequency furnace (manufactured by Fuji Denpa) under nitrogen atmosphere at 1000°C for 4 hours, and then at 1600°C. After firing for 4 hours, a precursor (a) of hexagonal boron nitride primary particle aggregates in an amorphous state, 52 kg of boric acid and 50 kg of melamine were mixed, and the mixture was mixed in a batch type high-frequency furnace at 1000° C. in a nitrogen atmosphere at 4° C. for 4 hours. After the firing for 4 hours, it was further fired at 2000° C. for 4 hours to obtain a precursor (b) of a hexagonal boron nitride primary particle aggregate in a crystalline state.

15.72 kg of the powder of the precursor (a), 5.24 kg of the powder of the precursor (b), 0.54 kg of calcium carbonate (PC-700 manufactured by Shiraishi Industry Co., Ltd.) as a sintering aid, and 78. 5 kg was mixed using a Henschel mixer, and then pulverized for 5 hours in a ball mill having a container inner diameter of 50 cm and a ball diameter of 10 cm to obtain a water slurry. Further, with respect to 100 parts by mass of the water slurry, 0.5 part by mass of polyvinyl alcohol resin (product name Gohsenol, manufactured by Nippon Gosei Kagaku Co., Ltd.) and an anionic surfactant (product name: PRYSURF A219B, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.). 0.5 parts by mass was added, and the mixture was heated and stirred at 50° C. until dissolved, and then was passed through a rotary atomizer (PK-05, Pris) operated at a rotation speed of 7000 rpm in a spray dryer at a temperature of 230° C. Squirted out. The recovered water-slurry dried product was further calcined in a batch type high-frequency furnace under a nitrogen atmosphere at 1850 to 1950° C. for 4 hours, and then crushed to obtain hexagonal boron nitride primary particle aggregates of Example 1. Got The conditions at this time are summarized in Table 1.

<Measurement of average sphericity>
The average sphericity of the produced hexagonal boron nitride primary particle aggregate of Example 1 was measured by the following procedure. Each roughening-treated aggregate fixed to the conductive double-sided tape on the sample table was photographed with a scanning electron microscope (JSM-6010LA, manufactured by JEOL Ltd.), and the obtained two-dimensional image of the aggregate was subjected to image analysis. The projected area and the perimeter of the aggregate two-dimensional image were measured by the procedure described above from the photograph by importing into software (Mac-View, manufactured by Mountech Co., Ltd.), and the sphericity of each aggregate was calculated. The magnification of the image at this time was 100 times, and the number of pixels for image analysis was 15.1 million pixels. By repeating this operation, the sphericity of 100 arbitrary aggregates was determined, and the average value was used as the average sphericity of Example 1. This value is shown in Table 1.

<Porosity>
The porosity of the produced hexagonal boron nitride primary particle aggregate of Example 1 is a value obtained by measuring the pore volume using a mercury porosimeter (PASCAL 140-440, manufactured by FISONS INSTRUMENTS). If the porosity is εg (%), it can be calculated from the equation εg=Vg/(Vg+1/ρt)×100. Vg is a value (cm ³ /g) obtained by subtracting the interparticle void volume from the total pore volume measured by a mercury porosimeter. The interparticle void volume was the cumulative volume at the inflection point in the cumulative distribution curve of the pore volume measured by the mercury porosimeter. Further, ρt is the true specific gravity of the hexagonal boron nitride primary particles and is set to 2.34 (g/cm ³ ).

<Tap density>
The tap density of the produced hexagonal boron nitride primary particle aggregate of Example 1 was measured by using a powder property evaluation device (trade name: Powder Tester PT-E type, manufactured by Hosokawa Micron Co., Ltd.), and the primary particle aggregate of 100 cm 3 The bulk density after filling in a container and performing tapping under the conditions of a tapping time of 180 seconds, a tapping frequency of 180 times, and a tap lift of 18 mm was determined as the tap density of Example 1. This value is shown in Table 1.

<Particle strength>
The measurement of the particle strength of the produced hexagonal boron primary particle aggregate of Example 1 was carried out using a micro compression tester (MCT-510, manufactured by Shimadzu Corporation) using a plane indenter of φ200 μm, and the particle diameter d(mm The fracture test force P(N) of the hexagonal boron nitride primary particle agglomerates in () was determined. The breaking strength Cs (MPa) was obtained from the equation of Cs=2.48P/πd ² . The average value obtained by repeating the same measurement 10 times with different particles was used as the strength of Example 1. This value is shown in Table 1.

<Measurement of average particle size>
The average particle size of the produced hexagonal boron nitride primary particle aggregates of Example 1 was measured using a particle size distribution analyzer (MT3300EX, manufactured by Nikkiso Co., Ltd.). The measurement time for each measurement is 30 seconds. In measuring the particle size distribution, water was used as the solvent for dispersing the aggregate and hexametaphosphoric acid was used as the dispersant. At this time, the refractive index of water was 1.33, and the refractive index of the boron nitride powder was 1.80. The measured values of the average particle diameter of Example 1 are shown in Table 1.

<Measurement of major axis of primary particles and aspect ratio of average thickness>
For the measurement of the aspect ratio, as a pretreatment for observation, the hexagonal boron nitride primary particle aggregates were embedded in a resin, processed by a CP (cross section polisher) method, fixed on a sample stage, and then subjected to osmium coating. Then, a scanning electron microscope, for example, "JSM-6010LA" (manufactured by JEOL Ltd.) was used to take an SEM image, and the obtained cross-sectional particle image was analyzed by image analysis software, for example, "A image-kun" (manufactured by Asahi Kasei Engineering Co., Ltd.). ) And can measure. The magnification of the image at this time was 100 times, and the number of pixels for image analysis was 15.1 million pixels. In manual measurement, after observing 100 particles that look like strips, select 10 particles that fall within the 10th place from the longer major axis, measure the major axis and the length of the 10 particles, The aspect ratio of each 10 particles was calculated from the calculation formula of aspect ratio=major axis/thickness, and the average value thereof was used as the aspect ratio. The aspect ratio of the primary particles constituting the hexagonal boron nitride primary particle aggregate of Example 1 measured by this method was 13.8.

<Examples 2 to 11>
The hexagonal boron nitride primary particle agglomerates of Examples 2 to 11 were different from those of Example 1 except that the mixing conditions of the slurry in the slurry forming process, the rotation speed condition of the atomizer in the spray drying process, and the baking temperature condition in the baking process were changed. It was made. Table 1 shows these conditions and the measured values of the obtained hexagonal boron nitride primary particle aggregates.

<Comparative Examples 1 to 12>
The hexagonal boron nitride primary particle agglomerates of Comparative Examples 1 to 12 were different from those of Example 1 except that the slurry compounding conditions in the slurry forming process, the atomizer rotation speed conditions in the spray drying process, and the baking temperature conditions in the baking process were changed. It was made. However, under the conditions of Comparative Example 3, the spraying step could not be performed, and a hexagonal boron nitride primary particle aggregate could not be obtained. Table 2 shows these conditions and the measured values of the obtained hexagonal boron nitride primary particle aggregates.

<Comparative Example 13>
Boron carbide powder (#1200, manufactured by Denka Co., Ltd.) was treated at 2250° C. in an atmosphere of nitrogen gas at a pressure of 0.1 MPa to obtain a boron nitride precursor. A hexagonal boron nitride primary particle aggregate of Example 13 was prepared. Table 2 shows the measured values of the obtained hexagonal boron nitride primary particle aggregates.

<Production of resin composition and thermal interface material>
In order to evaluate the practical properties as a thermal interface material, the hexagonal boron nitride primary particle aggregates of Examples 1 to 11 and Comparative Examples 1 to 13 (excluding Comparative Example 3) were used in an amount of 60% by volume, an epoxy resin (trade name). 36% by volume of Epicoat 807, manufactured by Mitsubishi Chemical Co., Ltd., and 4% by volume of a curing agent (trade name Acmex H-84B, manufactured by Nippon Gohsei Co., Ltd.) to form a resin composition, and further PET. After being applied on the sheet to have a thickness of 1.0 mm, vacuum degassing at 500 PaA was performed for 10 minutes. After that, press heating and pressurization under conditions of a temperature of 150° C. and a pressure of 160 kg/cm ² for 60 minutes to form a sheet having a thickness of 0.5 mm, and Examples 1 to 11 and Comparative Examples 1 to 13 (excluding Comparative Example 3). ) Was prepared.

<Measurement of thermal conductivity>
The thermal conductivity H (unit: W/(m·K)) of the thermal interface materials of Examples 1 to 11 and Comparative Examples 1 to 13 (excluding Comparative Example 3) was 10 mm width×10 mm×thickness 0. Using a measurement sample cut into a size of 5 mm, a thermal diffusivity A (unit: m ² /sec) by a laser flash method using a xenon flash analyzer (LFA447 NanoFlash, manufactured by NETZSCH), a specific gravity measurement kit (A&D) Density B (unit: kg/m ³ ) measured using a DSC measuring device (ThermoPlusEvo DSC8230, manufactured by Rigaku) of specific heat capacity C (unit: J/(kg·K)) From each measured value, H=A×B×C was obtained. These results are also shown in Tables 1 and 2. If the thermal conductivity of the thermal interface material was 10 W/(m·K) or more, it was judged that the thermal interface material was improved from the conventional level.

As is clear from the comparison between the examples and the comparative examples, it is understood that the thermal interface material using the resin composition containing the hexagonal boron nitride primary particle aggregate of the present invention has excellent thermal conductivity.

The hexagonal boron nitride primary particle agglomerate of the present invention exhibits high thermal conductivity, and an electrical insulating member using a resin composition containing the same can be preferably used as a thermal interface material for a printed wiring board.

Claims (5)

Average sphericity of 0.80 or more, a porosity of 40% or less, a tap density of 1.0 g / cm ³ or more state, and are more particle strength is 5.0 MPa, the primary particles are flake is you are aggregated, Hexagonal boron nitride primary particle aggregates. The hexagonal boron nitride primary particle aggregate according to claim 1, wherein the aspect ratio of the major axis and the thickness of the primary particles is more than 10 and 20 or less. The hexagonal boron nitride primary particle aggregate according to claim 1 or 2, having an average particle diameter of 20 to 100 µm. A resin composition comprising the hexagonal boron nitride primary particle aggregate according to claim 1. A thermal interface material using the resin composition according to claim 4.