Hexagonal boron nitride aggregated particles, hexagonal boron nitride powder, resin composition, and resin sheet

Abstract

Provided are hexagonal boron nitride aggregated particles and hexagonal boron nitride powder, each of which can be filled into a resin to produce a resin composition with an extremely high dielectric strength and thermal conductivity, and to reduce the density of the resin composition. Provided are hexagonal boron nitride aggregated particles, in which aggregated particles of hexagonal boron nitride primary particles have a longer diameter ranging from 5 to 10 µm, a longer diameter / shorter diameter ranging from 1.0 to 1.3, and a circularity within a range from 0.3 to 0.8, and a maximum diameter of primary particles which can be confirmed on the surface of the aggregated particles on an SEM observation image at 10,000 magnification is 4 µm or less. Provided is a hexagonal boron nitride powder including aggregated particles of hexagonal boron nitride primary particles, in which a particle size (Dso) at a cumulative volume frequency of 50% in a particle size distribution as measured by a wet laser diffraction particle size distribution analysis is from 5 to 150 µm, a volume-based median diameter of pores as measured by a mercury porosimetry is 3.0 µm or less, and a content of impurity elements is 500 ppm or less

Description

Hexagonal boron nitride agglomerated particles and hexagonal boron nitride powder, resin composition, and resin sheet

[0001] The present invention relates to novel hexagonal boron nitride agglomerated particles and hexagonal boron nitride powder, and uses thereof. More specifically, the present invention provides hexagonal boron nitride agglomerated particles and hexagonal boron nitride powder that can impart extremely high dielectric strength and thermal conductivity to a resin composition obtained by filling the particles into a resin, and that can reduce the density and weight of the resin composition.

[0002] Hexagonal boron nitride powder is generally a white powder having a hexagonal layer structure similar to that of graphite, and has many properties such as high thermal conductivity, high electrical insulation, high lubricity, corrosion resistance, mold releasability, high temperature stability, chemical stability, etc. Therefore, resin compositions filled with hexagonal boron nitride powder are suitably used as thermally conductive and insulating sheets after molding.

[0003] In recent years, there has been an increasing demand for insulating sheets with low dielectric constants, low dielectric dissipation factors, and high thermal conductivity for use in high-frequency devices, and this has led to an increasing demand for hexagonal boron nitride powder, which has a lower dielectric constant than other highly thermally conductive fillers such as aluminum nitride and aluminum oxide.

[0004] In particular, hexagonal boron nitride powder, which has a lower specific gravity than other highly thermally conductive fillers such as aluminum nitride and aluminum oxide, is attracting attention for use as a low-density, thermally conductive insulating sheet for automotive applications.

[0005] Hexagonal boron nitride powder and the hexagonal boron nitride agglomerated particles that make up this powder contain primary particles consisting of scaly particles derived from the crystalline structure, and the scaly particles have thermal anisotropy. Therefore, in the case of a thermally conductive insulating sheet that uses boron nitride powder containing the above-mentioned scaly particles as single particles as a filler, the scaly particles are oriented in the plane direction of the thermally conductive insulating sheet, so that heat is conducted in the c-axis direction of the scaly particles, which has low thermal conductivity, and the thermal conductivity of the thermally conductive insulating sheet in the thickness direction is low.

[0006] In order to improve the thermal anisotropy of hexagonal boron nitride particles having such a scale-like structure, a hexagonal boron nitride powder has been proposed that contains agglomerated particles in which the scale-like particles of hexagonal boron nitride are aggregated in random directions.

[0007] For example, there have been proposed agglomerated particles in which scale-like particles are randomly oriented by spray-drying or the like and then reheating amorphous boron nitride powder, or a mixture of amorphous boron nitride powder and an auxiliary agent such as an oxide, which is press-molded and sintered, and then crushed into a sintered body in which scale-like particles are randomly oriented (see Patent Document 1). These agglomerated particles have large diameters of the scale-like particles, which are the primary particles that make up the agglomerates, with most of the primary particles exceeding 10 μm in size. It is believed that when the boron nitride powder composed of these agglomerated particles is filled in a resin, the resin penetrates into the voids between the scale-like primary particles, filling the voids, thereby preventing the presence of large bubbles in the resin composition and thereby exhibiting high insulation resistance.

[0008] However, there is still room for improvement in the dielectric strength. That is, when filled into resin, the resin easily penetrates into the large voids. However, if the voids become smaller toward the center of the agglomerated particles or if continuous long voids are formed, there is a concern that voids of a size that reduces the dielectric strength may remain in the resin composition. This tendency is particularly pronounced when the viscosity of the resin is high. In addition to the above problems, there is also a concern that the inclusion of impurity elements as sintering aids may cause a decrease in dielectric strength in the sintered body.

[0009] Furthermore, in the case of the agglomerated particles in which the voids are reduced by allowing resin to penetrate into the voids within the agglomerated particles, the specific gravity of the boron nitride powder in the resin composition is almost the same as that of the bulk body, and it is therefore not possible to reduce the weight of the resin composition.

[0010] The applicant also developed a cellulose ester having a maximum torque of 0.20 to 0.50 Nm, a DBP absorption of 50 to 100 ml / 100 g, and a tapped bulk density of 0.66 to 0.95 g / cm 3(See Patent Document 2.) The invention described in the above Patent Document aims to provide a hexagonal boron nitride powder that is easy to fill into resins, requires a small amount of solvent to form a varnish, exhibits high thermal conductivity when filled into resins, and is substantially free of relatively large voids that adversely affect insulation resistance, thereby imparting high insulation resistance to resin compositions. However, the hexagonal boron nitride particles that make up the powder form agglomerates that are so dense that the particle size of the primary particles cannot be detected, and further improvement in weight reduction due to the formation of voids within the particles has been desired, along with improvement in insulation resistance.

[0011] JP 2017-165609 A International Publication No. 2018-123571

[0012] Therefore, an object of the present invention is to provide hexagonal boron nitride agglomerated particles and hexagonal boron nitride powder which contain agglomerated particles formed by agglomerating scaly primary particles of hexagonal boron nitride, which can eliminate the anisotropy of thermal conduction through agglomeration to exhibit high thermal conductivity when filled into a resin, and which can impart extremely high dielectric strength to a resin composition obtained when filled into a resin, and which can also reduce the weight of such a resin composition.

[0013] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that hexagonal boron nitride agglomerated particles and hexagonal boron nitride powder obtained by employing a specific production method in which a reduction-nitridation reaction is carried out using an oxygen-containing boron compound, a carbon source, and an oxygen-containing calcium compound, which are known as raw materials for obtaining boron nitride by reduction-nitridation, eliminate the anisotropy of thermal conduction due to the agglomeration of scaly primary particles, and that the generation of agglomerated particles during the reaction results in small scaly primary particles constituting the agglomerated particles. Therefore, when the agglomerated particles are filled into a resin, even if voids that are difficult for the resin to penetrate due to such voids are formed, the dielectric strength of the resin composition is not reduced and voids that the resin cannot penetrate can be secured within the agglomerated particles, thereby enabling the resin composition to be made lighter. This finding has led to the completion of the present invention.

[0014] That is, according to the present invention, there is provided hexagonal boron nitride agglomerated particles (hereinafter also referred to as specific hexagonal boron nitride agglomerated particles), which are agglomerated particles of hexagonal boron nitride primary particles, having a major axis of 5 to 10 μm, a major axis / minor axis ratio of 1.0 to 1.3, and a circularity of 0.3 to 0.8, and in which the maximum diameter of the primary particles that can be confirmed on the surface of the agglomerated particles in an SEM image at a magnification of 10,000 times is 4 μm or less.

[0015] According to the present invention, the hexagonal boron nitride particles are composed of aggregated particles of hexagonal boron nitride primary particles, and have a particle size (D 50 The present invention provides a hexagonal boron nitride powder having a pore size of 5 to 150 μm, a volume-based median pore diameter of 3.0 μm or less as measured by mercury intrusion porosimetry, and an impurity element content of 500 ppm or less.

[0016] Furthermore, according to the present invention, there is provided a hexagonal boron nitride powder containing the specific hexagonal boron nitride agglomerated particles as the agglomerated particles of the hexagonal boron nitride primary particles. The hexagonal boron nitride powder has a BET specific surface area of ​​1 to 15 m. 2 / g, and the oil absorption is preferably 50 to 190 cc / 100 g.

[0017] Furthermore, by using the specific hexagonal boron nitride agglomerated particles of the present invention together with pore-free hexagonal boron nitride single particles, it is possible to form a hexagonal boron nitride powder that compensates for the thermal anisotropy of single particles without reducing dielectric strength. That is, according to the present invention, a hexagonal boron nitride powder containing specific hexagonal boron nitride agglomerated particles and hexagonal boron nitride single particles can be obtained, in which the content of the specific hexagonal boron nitride agglomerated particles is 5% or more, and the particle size at 50% cumulative volume frequency of particle size distribution measured by a wet laser diffraction particle size distribution method (D 50 ) is 5 to 150 μm.

[0018] Furthermore, the present invention also provides a resin composition filled with the above boron nitride powder as a filler, which has excellent thermal conductivity and high dielectric strength, and a resin sheet made of the above resin composition.

[0019] The specific hexagonal boron nitride agglomerated particles of the present invention are composed of agglomerated particles of fine, scale-like primary particles of hexagonal boron nitride, and exhibit the effect of eliminating the anisotropy of thermal conductivity due to agglomeration. In addition to the above properties, the agglomerated particles have fine pores on their surfaces, thereby exhibiting excellent properties not found in conventional agglomerates. That is, because the primary particles constituting the specific hexagonal boron nitride agglomerated particles of the present invention are fine, the gaps between the particles are extremely small, and, unlike the gaps between conventional plate-like agglomerated particles, the voids that exist in such gaps when the particles are filled into a resin have almost no effect on the dielectric strength.

[0020] The hexagonal boron nitride powder of the present invention is composed of agglomerated particles of fine, scale-like primary particles of hexagonal boron nitride, and therefore exhibits the effect of eliminating the anisotropy of thermal conductivity due to agglomeration. Furthermore, in addition to the above-mentioned properties, the fine primary particles of hexagonal boron nitride have a densely agglomerated structure, which forms fine pores on the surface of the agglomerated particles, thereby exhibiting excellent properties not found in conventional agglomerates. That is, as mentioned above, when the agglomerated particles of the primary particles that make up the hexagonal boron nitride powder of the present invention are filled into a resin, the voids that exist in the gaps have almost no effect on the dielectric strength.

[0021] Therefore, a resin composition filled with the hexagonal boron nitride powder or the hexagonal boron nitride powder containing the specific hexagonal boron nitride agglomerated particles can exhibit extremely high dielectric strength. Furthermore, the gaps present within the agglomerated particles can reduce the weight of the hexagonal boron nitride powder, and ultimately the weight of the resin composition obtained by filling it with the hexagonal boron nitride powder.

[0022] Furthermore, both the specific hexagonal boron nitride agglomerated particles and hexagonal boron nitride powder of the present invention can be produced without employing a sintering method that requires a sintering aid, and have a low content of impurity elements, allowing for high purification, thereby making it possible to minimize the decrease in the dielectric strength of the resin composition.

[0023] SEM photograph of specific agglomerated particles in the hexagonal boron nitride powder obtained in Example A1 SEM photograph of the hexagonal boron nitride powder obtained in Example A1 SEM photograph of the surface of a coarse agglomerated particle in the hexagonal boron nitride powder obtained in Example A1

[0024] (Specific hexagonal boron nitride agglomerated particles) The specific hexagonal boron nitride agglomerated particles of the present invention are agglomerated particles of hexagonal boron nitride primary particles, characterized in that the particles have a major axis of 5 to 10 μm, a major axis / minor axis ratio of 1.0 to 1.3, and a circularity of 0.3 to 0.8, and the maximum diameter of the primary particles that can be confirmed on the surface of the agglomerated particles in an SEM image at a magnification of 10,000 times is 4 μm or less.

[0025] The hexagonal boron nitride powder was identified by X-ray diffraction measurement of the sample powder, which confirmed the absence of peaks attributable to hexagonal boron nitride other than those of hexagonal boron nitride. The X-ray diffraction measurement was performed using a fully automated horizontal multipurpose X-ray diffractometer, SmartLab (trade name), manufactured by Rigaku Corporation. The measurement conditions were a scan speed of 20 degrees / min, a step width of 0.02 degrees, and a scan range of 10 to 90 degrees. The GI value, which represents the crystallinity of the hexagonal boron nitride powder, was calculated from the integrated intensity ratio (area ratio) of the (100), (101), and (102) diffraction lines in the X-ray diffraction spectrum of the hexagonal boron nitride powder using the formula: GI = [{(100) + (101)} / [(102)]. The GI value of hexagonal boron nitride powder is 2.5 or less, and this has also been confirmed for the specific hexagonal boron nitride agglomerated particles obtained in the examples.

[0026] The specific hexagonal boron nitride agglomerated particles of the present invention are composed of agglomerates of scaly primary particles of hexagonal boron nitride, but such agglomerates can be obtained by employing specific reaction conditions for synthesizing hexagonal boron nitride and generating agglomerated particles during the reaction, rather than by a manufacturing method such as conventional granulation or sintering methods in which previously produced scaly primary particles of hexagonal boron nitride are agglomerated.

[0027] Therefore, the specific hexagonal boron nitride agglomerated particles of the present invention are composed of fine, scale-like primary particles of hexagonal boron nitride, and in SEM images at 10,000x magnification, the maximum diameter of the primary particles visible on the surface of the agglomerated particles is 4 μm or less, particularly 3 μm or less. This results in extremely small gaps within the agglomerated particles, and when filled into a resin, the voids present in these gaps have almost no effect on the dielectric strength, allowing the resulting resin composition to exhibit extremely high dielectric strength. Furthermore, the presence of pores (gaps) within and on the surface of the agglomerated particles also makes it possible to reduce the weight of the resin composition obtained by filling the hexagonal boron nitride powder.

[0028] The specific hexagonal boron nitride agglomerated particles of the present invention, which have a major axis of 5 to 10 μm, a major axis / minor axis ratio of 1.0 to 1.3, and a circularity of 0.3 to 0.8, have excellent fluidity when used in combination with other hexagonal boron nitride particles in a resin composition, and are effective in improving the resin filling property and the thermal conductivity of the resin composition.

[0029] The major and minor diameters were calculated from an SEM image at a magnification of 10,000. The circularity was calculated by calculating 4π×(area) / (perimeter) from an SEM image at a magnification of 10,000 using image analysis software. 2 was calculated using the formula:

[0030] According to the production method described below, it is difficult to produce a pure powder consisting only of the specific hexagonal boron nitride agglomerated particles of the present invention, and agglomerated particles of various shapes and sizes are generated. Therefore, it is preferable to remove coarse particles and fine particles by classification or the like to use the powder with a higher content of the specific hexagonal boron nitride agglomerated particles.

[0031] As described above, the median diameter of the pores of the specific hexagonal boron nitride agglomerated particles of the present invention cannot be selectively measured, but it has been confirmed that the agglomerated particles constituting the hexagonal boron nitride powder obtained by the manufacturing method described below have a similar pore structure due to the dense agglomeration of fine primary particles. Therefore, the pore diameter of the hexagonal boron nitride powder can be measured, and the median diameter of the pores of the specific hexagonal boron nitride agglomerated particles can be specified. In the present invention, the median diameter of the pores of such specific hexagonal boron nitride agglomerated particles is preferably 3.0 μm or less, preferably 2.8 μm, and more preferably 2.5 μm or less. Similarly, for the specific hexagonal boron nitride agglomerated particles of the present invention, the pore volume of pores having a pore diameter of 3 μm or less can be determined to be 0.5 cm 3 / g or more, preferably 0.5 to 2.0 cm 3 / g.

[0032] The pore volume-based median diameter and pore volume were calculated from an integrated pore distribution obtained by measuring pores of 0.0036 μm to 200 μm by mercury intrusion porosimetry using an Autopore IV9520 manufactured by Micromeritics, with the pore diameter plotted on the horizontal axis and the integrated pore volume plotted on the vertical axis.

[0033] The specific hexagonal boron nitride agglomerated particles of the present invention are obtained by a manufacturing method described below that does not use a sintering aid, so there is no decrease in purity due to the sintering aid and they are highly pure. For example, when used as a filler in a resin composition that constitutes an insulating sheet, this contributes to improving voltage resistance in conjunction with the effect of the small pores. That is, the specific hexagonal boron nitride agglomerated particles of the present invention preferably have an impurity element content of 500 ppm or less, particularly 400 ppm or less.

[0034] The impurity elements referred to here are calcium, magnesium, sodium, aluminum, lithium, strontium, iron, sulfur, nickel, chromium, manganese, silicon, phosphorus, titanium, barium, and cobalt. The content of the impurity elements refers to the total content of the impurity elements.

[0035] The impurity element content of the hexagonal boron nitride agglomerated particles is a value measured by X-ray fluorescence analysis of the hexagonal boron nitride powder obtained by the production method described below. In the examples, a ZSX Primus2 (trade name) manufactured by Rigaku Corporation was used as the X-ray fluorescence analyzer.

[0036] The specific hexagonal boron nitride agglomerated particles of the present invention have a C% of 0.04% or less and an O% of 0.4% or less, which, together with the low amounts of the impurity elements, are more effective in improving thermal conductivity and dielectric strength.

[0037] The O% of the specific hexagonal boron nitride agglomerated particles was measured using an oxygen / nitrogen analyzer EMGA-620 manufactured by Horiba, Ltd. The C% of the hexagonal boron nitride powder was measured using an EMIA-110 manufactured by Horiba, Ltd.

[0038] (Hexagonal boron nitride powder) The hexagonal boron nitride powder of the present invention is characterized by being composed of agglomerated particles of hexagonal boron nitride primary particles, having a volume-based median pore diameter of 3 μm or less, and having an impurity element content of 500 ppm or less.

[0039] The hexagonal boron nitride was identified as a hexagonal boron nitride powder by confirming in X-ray diffraction measurement that the sample powder had no peaks attributable to anything other than hexagonal boron nitride. The X-ray diffraction measurement was the same as for the specific hexagonal boron nitride agglomerated particles, and the hexagonal boron nitride powder of the present invention had a GI value of 2.5 or less, which was also confirmed in the examples.

[0040] The hexagonal boron nitride powder of the present invention is composed of aggregates of scaly primary particles of hexagonal boron nitride, but these aggregates can be obtained by employing specific reaction conditions for synthesizing hexagonal boron nitride to produce aggregated particles, rather than by a manufacturing method such as conventional granulation or sintering, which involves agglomerating previously produced scaly primary particles of hexagonal boron nitride.

[0041] Therefore, the agglomerated particles constituting the hexagonal boron nitride powder of the present invention are composed of fine, scale-like primary particles of hexagonal boron nitride, and the gaps between the agglomerated particles are extremely small, exhibiting a characteristic volume-based median pore diameter of 3.0 μm or less. This volume-based median diameter is preferably 2.8 μm, more preferably 2.5 μm or less. If the pore volume-based median diameter exceeds 3.0 μm, air bubbles that affect insulation resistance tend to remain when the particles are filled into a resin, making it difficult to exhibit high dielectric strength. That is, by having the volume-based median pore diameter of the agglomerated particles constituting the hexagonal boron nitride powder of the present invention exhibit the above-mentioned value, when the particles are filled into a resin, the voids present in the gaps have almost no effect on dielectric strength, and the resulting resin composition can exhibit extremely high dielectric strength. Furthermore, the presence of pores (gaps) within and on the surfaces of the agglomerated particles also makes it possible to reduce the weight of the resin composition obtained by filling the hexagonal boron nitride powder. The pores can be confirmed from the SEM photograph of FIG.

[0042] The hexagonal boron nitride powder of the present invention has a volume-based median diameter of the pores of the agglomerated particles, and the pore volume of the pores having a pore diameter of 3 μm or less is 0.2 cm 3 / g or more, preferably 0.5 to 2.0 cm 3 / g is preferable in order to further enhance the effect of reducing the weight of the resin composition due to the remaining pores.

[0043] The pore volume-based median diameter and pore volume were calculated from an integrated pore distribution obtained by measuring pores of 0.0036 μm to 200 μm by mercury intrusion porosimetry using an Autopore IV9520 manufactured by Shimadzu Corporation, with pore diameters plotted on the horizontal axis and integrated pore volumes plotted on the vertical axis.

[0044] The hexagonal boron nitride powder of the present invention is obtained by a manufacturing method described below that does not use sintering aids, so there is no reduction in purity due to sintering aids and it is highly pure, and when used as a filler in a resin composition that constitutes an insulating sheet, for example, it contributes to improving voltage resistance in conjunction with the effect of the small pores. That is, the hexagonal boron nitride powder of the present invention is characterized by its high purity, with an impurity content of 500 ppm or less, particularly 400 ppm or less.

[0045] The impurity elements referred to here include the same as those in the specific hexagonal boron nitride agglomerated particles, and the content of the impurity elements refers to the total content of the impurity elements. The content ratio of the impurity elements in the hexagonal boron nitride powder is measured in the same manner as in the specific hexagonal boron nitride agglomerated particles.

[0046] The hexagonal boron nitride powder of the present invention has a C% of 0.04% or less and an O% of 0.4% or less, which, together with the low amounts of the impurity elements, is more effective in improving thermal conductivity and dielectric strength.

[0047] The O% of the hexagonal boron nitride powder is measured in the same manner as the specific hexagonal boron nitride agglomerated particles. The particle size of the hexagonal boron nitride powder of the present invention, including the case of a mixed hexagonal boron nitride powder described later, is determined by taking into consideration the fillability into a resin, and is determined by the particle size at 50% cumulative volume frequency (D 50 ) is 5 to 150 μm, preferably 10 to 100 μm, and particularly preferably 20 to 80 μm. In order to adjust the particle size to the above range, the hexagonal boron nitride powder of the present invention can be produced by separating coarse agglomerated particles, specifically agglomerated particles having a size of more than 200 μm, particularly more than 150 μm, from the coarse hexagonal boron nitride powder obtained by the production method described below using a classification means such as a sieve. Alternatively, the hexagonal boron nitride powder can be produced by separating the coarse particles, crushing them to an appropriate size, and then returning them to the hexagonal boron nitride powder.

[0048] In the examples, the particle size measurement by the wet laser diffraction particle size distribution method was carried out using an LA-950V2 (trade name) manufactured by HORIBA Co., Ltd. The hexagonal boron nitride powder of the present invention has a specific surface area of ​​1 to 15 m 2 / g, and 1.5 to 14.0 m 2 / g is more preferable, and 2.0 to 13.0 m 2 / g. That is, 15.0 m 2 Hexagonal boron nitride powder exceeding 1 m / g contains a large amount of fine particles, and such hexagonal boron nitride powder contains a large amount of boron nitride powder with low crystallinity, which not only causes thermal resistance in the insulating and heat-dissipating sheet but is also undesirable from the viewpoint of handling, as the powder tends to fly around. 2 Hexagonal boron nitride powder with a particle size of less than 1 / g is not preferred because it increases the proportion of flat hexagonal boron nitride single particles that have grown into grains, and the anisotropy of their thermal conductivity may lead to a decrease in the thermal conductivity of the resin composition when filled into a resin.

[0049] The specific surface area of ​​the hexagonal boron nitride powder is a value measured by the BET single-point method, and in the examples, the measurement is carried out using a Macsorb HM model-1201 (trade name) manufactured by Mountech Co., Ltd.

[0050] The hexagonal boron nitride powder of the present invention preferably has an oil absorption of 50 to 190 ml / 100 g, particularly 55 to 170 ml / 100 g, and even more preferably 60 to 150 ml / 100 g. The DBP absorption indicates the characteristics of the hexagonal boron nitride powder, such as the amount of open pores within the agglomerated particles, the presence or absence of a structure on the particle surface, and the wettability with resin. The more open pores there are and the more structure there is, the higher the DBP absorption. That is, an oil absorption that is too low is undesirable because the open pores are large and the structure is not sufficiently developed. On the other hand, an oil absorption that is too high leads to the opposite tendency, and is undesirable because it tends to trap large bubbles when made into a resin composition. Furthermore, in the hexagonal boron nitride powder of the present invention, the agglomerated particles are composed of extremely fine primary particles, and therefore, even if the structure is well-developed, the open pores formed by this are small, and do not pose a problem when made into a resin composition.

[0051] The oil absorption of the hexagonal boron nitride powder is measured based on the procedure specified in JIS K5101-13-1:2004 ("Testing methods for pigments" - Part 13: Oil absorption - Section 1: Refined linseed oil method).

[0052] Furthermore, the hexagonal boron nitride powder of the present invention has a tapped bulk density of 0.40 g / cm 3 It is preferable that the tapped bulk density is equal to or greater than 0.40 g / cm. That is, the tapped bulk density is an index showing the particle shape and particle size distribution width among the properties of hexagonal boron nitride powder, and a high value indicates a state in which there are few large open pores, many nearly spherical agglomerated particles, and the particle size distribution is close to close-packed. The hexagonal boron nitride powder of the present invention can be made to have a high tapped bulk density by increasing the content of the specific hexagonal boron nitride agglomerated particles described below, which makes it easier to fill gaps within the powder. The tapped bulk density of all hexagonal boron nitride powders produced in the examples was 0.40 g / cm. 3 That was all.

[0053] The hexagonal boron nitride powder of the present invention preferably contains 5% or more, preferably 10% or more, of agglomerated particles of hexagonal boron nitride primary particles having a major axis of 5 to 10 μm, a major axis / minor axis ratio of 1.0 to 1.3, and a circularity of 0.3 to 0.8 (hereinafter also referred to as spherical hexagonal boron nitride agglomerated particles). These spherical hexagonal boron nitride agglomerated particles are most preferably the specific hexagonal boron nitride agglomerated particles. That is, as will be understood from the production method described below, the specific hexagonal boron nitride agglomerated particles are obtained as a hexagonal boron nitride powder containing hexagonal boron nitride agglomerated particles that are inevitably produced in the reaction, and the hexagonal boron nitride agglomerated particles that make up this powder commonly have a structure in which fine primary particles are densely agglomerated. Therefore, it can be said that it is most preferable to contain specific hexagonal boron nitride particles as spherical hexagonal boron nitride agglomerated particles in order to satisfy the properties of the hexagonal boron nitride powder of the present invention.

[0054] It should be noted that spherical hexagonal boron nitride agglomerated particles other than the specific hexagonal boron nitride agglomerated particles described above can also be used as long as they satisfy the above-described characteristics of the hexagonal boron nitride powder of the present invention and also satisfy the above-described major axis, ratio, and circularity.

[0055] The major axis, major axis / minor axis, and circularity are determined by checking an SEM image at 10,000x magnification, as with the specific hexagonal boron nitride agglomerated particles. The hexagonal boron nitride powder of the present invention can be mixed with hexagonal boron nitride powders containing other hexagonal boron nitride particles within a range that satisfies the above-mentioned properties. Powders mixed with the above-mentioned other hexagonal boron nitride particles are also called "mixed hexagonal boron nitride powders."

[0056] The mixed hexagonal boron nitride powder contains specific hexagonal boron nitride agglomerated particles and other hexagonal boron nitride particles, and the content of the specific hexagonal boron nitride agglomerated particles is preferably 5% or more, more preferably 10% or more, and even more preferably 20% or more.

[0057] In the present invention, the other hexagonal boron nitride powder to be mixed is not particularly limited as long as it does not affect the properties of the hexagonal boron nitride of the present invention, but a powder consisting of hexagonal boron nitride single particles that suppresses the generation of voids after filling into a resin is preferred. The hexagonal boron nitride single particles generally have a flat shape and have anisotropic thermal conductivity, but when filled into a resin, they come into contact with agglomerated particles of hexagonal boron nitride primary particles, particularly specific hexagonal boron nitride agglomerated particles, thereby eliminating the anisotropy and allowing the powder to exhibit excellent thermal conductivity.

[0058] The hexagonal boron nitride single particles preferably have an aspect ratio (ratio of the particle's major axis to its thickness (major axis / thickness)) of 3 to 20 and a major axis of 1 to 30 μm. It is also possible to use hexagonal boron nitride agglomerated particles other than the specific hexagonal boron nitride agglomerated particles in combination, as long as the effects of the present invention provided by the specific hexagonal boron nitride agglomerated particles are not significantly impaired.

[0059] In the hexagonal boron nitride powder of the present invention, including the mixed hexagonal boron nitride powder, the spherical hexagonal boron nitride agglomerated particles, typified by the specific hexagonal boron nitride agglomerated particles, have an appropriate size suitable for filling the gaps between other agglomerated particles, and are approximately spherical. Therefore, by having them present in the hexagonal boron nitride powder in the above ratio, the fillability of the hexagonal boron nitride powder in resin is improved, and the remaining of relatively large bubbles can be effectively suppressed, which is presumed to contribute to improving the dielectric strength of the resin composition.

[0060] The proportion of the spherical hexagonal boron nitride agglomerated particles in the hexagonal boron nitride powder is preferably higher than the above-mentioned lower limit value in order to promote the above-mentioned action, and in the case of specific hexagonal boron nitride agglomerated particles, it is preferable to increase the proportion by separating particles having a relatively large particle size by classifying the hexagonal boron nitride powder obtained by the production method described below using a sieve or the like.

[0061] The specific hexagonal boron nitride agglomerated particles are produced as part of a hexagonal boron nitride powder by a production method described below, and are characterized in that the primary particles of hexagonal boron nitride constituting the agglomerated particles are smaller and more uniform in size than other agglomerated particles, with most of the primary particles having a major axis of 4 μm or less, which also contributes to improving the dielectric strength of the resin composition.

[0062] In the present invention, the content (%) of spherical hexagonal boron nitride agglomerated particles in a hexagonal boron nitride powder, including the specific hexagonal boron nitride agglomerated particles, can be determined using the following method. First, using a dry vibrating sieve KFC-500-1D manufactured by KOWA Corporation and a SUS mesh with a mesh size of 45 μm, the hexagonal boron nitride powder was sieved over and under the 45 μm sieve for 30 minutes, and the weight percentage X% of particles that passed the 45 μm sieve was determined. Subsequently, spherical hexagonal boron nitride agglomerated particles such as the specific hexagonal boron nitride agglomerated particles were separated from other particles from the SEM observation image at 500x magnification of the recovered 45 μm sieve, and the area percentage Y% of the SEM observation image of the spherical hexagonal boron nitride agglomerated particles that passed the 45 μm sieve was determined. Finally, the percentage (X × Y × 0.01) obtained by multiplying the weight percentage X% of particles passing through a 45 μm sieve by the area percentage Y% of the SEM image of the hexagonal boron nitride agglomerated particles passing through a 45 μm sieve was determined to be the percentage of spherical hexagonal boron nitride agglomerated particles in the present invention. For example, if the weight percentage X of the hexagonal boron nitride powder passing through a 45 μm sieve is 20 wt %, and the area percentage Y of the SEM image of the spherical hexagonal hexagonal boron nitride agglomerated particles passing through a 45 μm sieve is 60%, the percentage Z of spherical hexagonal boron nitride agglomerated particles is calculated as 20 × 60 × 0.01 = 12%.

[0063] In the hexagonal boron nitride powder of the present invention, the mixed hexagonal boron nitride powder containing the specific hexagonal boron nitride agglomerated particles and other hexagonal boron nitride particles preferably has a median pore size of 3.0 μm or less, preferably 2.8 μm or less, and more preferably 2.5 μm or less, similar to the specific hexagonal boron nitride agglomerated particles. Furthermore, in the hexagonal boron nitride powder, the pore volume of pores having a pore size of 3 μm or less is preferably 0.5 cm 3 / g or more, preferably 0.5 to 2.0 cm 3As described above, in order to maintain the properties of the specific hexagonal boron nitride agglomerated particles, the other hexagonal boron nitride particles in the hexagonal boron nitride powder are preferably single particles.

[0064] The pore volume-based median diameter and pore volume can be measured in the same manner as for the specific hexagonal boron nitride agglomerated particles. (Method for Producing Boron Nitride Powder) The method for producing the specific hexagonal boron nitride agglomerated particles and hexagonal boron nitride powder of the present invention is not particularly limited, but a typical example of a production method includes mixing an oxygen-containing boron compound having a residue on a 425 μm sieve of 3 mass% or less, a carbon source, and an oxygen-containing calcium compound in such a ratio that the B / C (element ratio) ratio, which is the ratio of the B source contained in the oxygen-containing boron compound to the C source contained in the carbon source, is 0.63 to 0.73, and the oxygen-containing calcium compound is 4 to 12 mass parts, calculated as CaO, per 100 mass parts of the total amount of the oxygen-containing boron compound and the carbon source (calculated as BO and C), and heating the mixture to a maximum temperature of 1910 to 2000°C in a nitrogen atmosphere to perform reduction-nitridation, followed by removing by-products other than boron nitride present in the reaction product by acid washing.

[0065] (Raw Materials) The greatest feature of the manufacturing method of the present invention is that raw materials, an oxygen-containing boron compound with controlled particle size, a carbon source, and an oxygen-containing calcium compound, are mixed in a predetermined ratio as described below, and then reduced and nitrided at a high temperature of 1910° C. or higher. The role of each raw material is as follows:

[0066] (Oxygen-Containing Boron Compound) In the production method of the present invention, any compound containing a boron atom can be used as the oxygen-containing boron compound as a raw material without any limitations. For example, boric acid, boric anhydride, metaboric acid, perboric acid, hypoboric acid, sodium tetraborate, sodium perborate, etc. can be used. In general, boric acid and boron oxide, which are readily available, are preferably used. Furthermore, it is preferable that the average particle size of the oxygen-containing boron compound used is such that the residue on a 425 μm sieve is 3% by mass or less, preferably 1.5% by mass or less. In particular, it is more preferable that the residue on a 300 μm sieve is 50% or less, and even more preferable that the residue on a 250 μm sieve is 70% or less. That is, if the residue on the 425 μm sieve of the oxygen-containing boron compound is 3 mass% or more, coarse boron nitride agglomerated particles of 150 μm or more are likely to remain, and the remaining voids within the coarse agglomerated particles increase, causing the median diameter of the pores in the resulting hexagonal boron nitride powder to exceed 3 μm, resulting in a decrease in dielectric strength.

[0067] (Oxygen-containing calcium compound) The oxygen-containing calcium compound forms a composite oxide with the oxygen-containing boron compound to form a composite oxide with a high melting point, and has the role of preventing the oxygen-containing boron compound from volatilizing.

[0068] In the production method of the present invention, the oxygen-containing calcium compound used as the catalyst and the volatilization inhibitor for the oxygen-containing boron compound may be any known compound without any particular limitation, but oxygen-containing calcium compounds containing oxygen and calcium are particularly preferred. 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, oxygen-containing calcium compounds are preferred. Examples of oxygen-containing calcium compounds that can be used include calcium carbonate, calcium bicarbonate, calcium hydroxide, calcium oxide, calcium nitrate, calcium sulfate, calcium phosphate, and calcium oxalate, and two or more of these can also be used in combination. Among these, calcium oxide and calcium carbonate are preferred.

[0069] The oxygen-containing calcium compound may be used in combination of two or more kinds. The average particle size of the oxygen-containing calcium compound is preferably 0.01 to 200 μm, more preferably 0.05 to 120 μm, and particularly preferably 0.1 to 80 μm.

[0070] (Carbon Source) In the production method of the present invention, known carbon materials that act as reducing agents can be used as the carbon source without any particular limitation. Examples 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 pyrolyzing a monomer or polymer. Among these, highly reactive amorphous carbon is preferred, and carbon black is particularly preferred because of its industrial quality control. Examples of carbon black that can be used include acetylene black, furnace black, and thermal black. The average particle size of the carbon source is preferably 0.01 to 5 μm, more preferably 0.02 to 4 μm, and particularly preferably 0.05 to 3 μm. By setting the average particle size of the carbon source to 5 μm or less, the reactivity of the carbon source is enhanced, and by setting the average particle size to 0.01 μm or more, the carbon source is easily handled.

[0071] In the production method of the present invention, the form of the mixture containing the above-mentioned raw materials to be supplied to the reaction is not particularly limited, and the raw materials may be supplied in powder form or may be formed into granules. In the production method of the present invention, the method of mixing the raw materials is not particularly limited, and a general mixer such as a vibration mill, a bead mill, a ball mill, a Henschel mixer, a drum mixer, a vibration agitator, or a V-shaped mixer can be used.

[0072] The granulation method when granulation is carried out can be carried out by a known method such as extrusion granulation, rolling granulation, granulation using a compactor, etc., using a binder as needed. In this case, the size of the granules is preferably about 5 to 10 mm.

[0073] (Preparation of Raw Materials) In the present invention, the reduction-nitridation reaction is carried out by supplying a carbon source and nitrogen. To effectively obtain the desired hexagonal boron nitride agglomerated particles, the ratio of the B source to the carbon source contained in the oxygen-containing boron compound, calculated as B / C (element ratio), must be 0.63 to 0.73, preferably 0.65 to 0.72. That is, if the molar ratio exceeds 0.73, the proportion of boron compound that volatilizes without being reduced increases, making it more likely to form a composite oxide with the Ca auxiliary. In addition, the melting point of the composite oxide is low, making it easier for plate-like particles to grow, making it difficult to obtain the desired agglomerated particles with small primary particles and pores. Furthermore, if the molar ratio is less than 0.63, the carbon content is high, and there is a risk of carbon-derived impurities remaining.

[0074] In the present invention, to effectively obtain the desired hexagonal boron nitride agglomerated particles, it is necessary to mix the oxygen-containing calcium compound in a ratio of 4 to 12 parts by mass, calculated as CaO, per 100 parts by mass of the combined oxygen-containing boron compound and carbon source (calculated as BO and C). If the CaO-calculated amount is 4 parts by mass or less, the proportion of boron compound that volatilizes without being reduced increases, resulting in a lower yield. Furthermore, the melting point of the CaO-BO composite oxide formed with the remaining boron compound decreases, promoting particle growth of the hexagonal boron nitride particles and making it difficult to form the desired agglomerated particles with pores. If the CaO-calculated amount is 12 parts by mass or more, calcium-derived impurities may remain, which is undesirable.

[0075] In the present invention, by adjusting the particle size and composition ratio of the raw materials, the desired excessive particle growth is prevented, and agglomerated particles with a small primary particle size can be produced with high selectivity. (Reduction Nitridation) In the method for producing boron nitride of the present invention, the nitrogen source can be supplied to the reaction system by known means. For example, the most common method is to flow nitrogen gas through the reaction system of the reactor exemplified below. Furthermore, the nitrogen source to be used is not limited to the above-mentioned nitrogen gas, and is not particularly limited as long as it is a gas that can be nitrided in the reduction-nitridation reaction. Specifically, in addition to the above-mentioned nitrogen gas, ammonia gas can also be used. Furthermore, a gas obtained by mixing nitrogen gas or ammonia gas with a non-oxidizing gas such as hydrogen, argon, or helium can also be used.

[0076] In the above production method, in order to obtain a hexagonal boron nitride powder that is highly crystalline and has fine pores, it is necessary to adopt a heating temperature in the reduction-nitridation reaction of typically 1910°C to 2000°C, preferably 1920°C to 1980°C. By setting the heating temperature to 1910°C to 2000°C, the crystallinity of the hexagonal boron nitride particles is improved, the outer periphery of the primary particle shape becomes sharper, and the number of fine pores between the particles increases. Furthermore, if the temperature is less than 1910°C, the reduction-nitridation reaction does not proceed well, and it is difficult to obtain white hexagonal boron nitride with high crystallinity. At temperatures exceeding 2000°C, the effect plateaus, which is not only economically disadvantageous but also undesirably increases damage to the heating furnace.

[0077] The time for the reduction-nitridation reaction is determined as appropriate, but is generally about 6 to 30 hours. The above production method can be carried out using a known reaction apparatus capable of controlling the reaction atmosphere. For example, an atmosphere-controlled high-temperature furnace that performs heat treatment by high-frequency induction heating or heater heating can be used. In addition to batch furnaces, continuous furnaces such as pusher-type tunnel furnaces and vertical reactors can also be used.

[0078] (Acid Washing) In the production method of the present invention, the reaction product obtained by the above-mentioned reduction-nitridation contains, in addition to hexagonal boron nitride, impurities such as a composite oxide composed of boron oxide and calcium oxide, and therefore is preferably washed with an acid. There are no particular limitations on the method for acid washing, and any known method can be used without limitation. For example, a method can be used in which the by-product-containing boron nitride obtained after the nitriding treatment is crushed and placed in a container, and dilute hydrochloric acid (5 to 20% by mass HCl) is added in an amount 5 to 10 times the amount of hexagonal boron nitride powder containing the impurities, and the mixture is allowed to contact for 4 to 8 hours.

[0079] The acid used in the acid washing may be other than hydrochloric acid, such as nitric acid, sulfuric acid, or acetic acid. After the acid washing, the boron nitride is washed with pure water to remove any remaining acid. The washing method involves filtering the acid used in the acid washing, dispersing the acid-washed boron nitride in pure water in the same amount as the acid used, and filtering again.

[0080] (Drying) The conditions for drying the hydrous aggregates after the acid washing and water washing are preferably drying in the atmosphere or under reduced pressure at 50 to 250° C. The drying time is not particularly specified, but it is preferable to dry until the moisture content approaches 0%.

[0081] (Classification) After drying, the boron nitride powder may be crushed, if necessary, and then subjected to removal of coarse particles using a sieve or the like, and removal of fine particles using air classification or the like.

[0082] The hexagonal boron nitride powder of the present invention containing specific hexagonal boron nitride can be obtained by the above method. Furthermore, the obtained hexagonal boron nitride powder is preferably subjected to classification treatments such as removal of coarse particles, removal of fine powder by air classification or wet classification, as necessary, to obtain a hexagonal boron nitride powder with an increased content of specific hexagonal boron nitride agglomerated particles. The classification treatment is particularly preferably dry sieve classification, with a mesh size of 30 to 90 μm.

[0083] (Uses of Boron Nitride Powder) The use of the boron nitride powder of the present invention is not particularly limited, and it can be applied to known uses without particular restriction. An example of a suitable use is use as a filler in resins for the purposes of improving electrical insulation and imparting thermal conductivity. In the use of the boron nitride powder, the resulting resin composition has high electrical insulation and thermal conductivity. In particular, in the use of a powder containing the specific boron nitride agglomerated particles, the resulting resin composition has high thermal conductivity and high electrical insulation with a dielectric strength of 80 kV / mm or more.

[0084] The resin composition of the present invention can be used for known applications without any restrictions, but by mixing it with a resin described below to form a thermally conductive resin composition or a thermally conductive molded body, it can be preferably used for applications such as thermal interface materials such as polymer-based heat-dissipating sheets and phase change sheets, organic heat-dissipating sheets such as heat-dissipating tapes, heat-dissipating greases, heat-dissipating adhesives and gap fillers, heat-dissipating paints such as heat-dissipating paints and heat-dissipating coats, heat-dissipating resin substrates such as PWB-based resin substrates and CCL-based resin substrates, insulating layers for metal-based substrates such as aluminum-based substrates and copper-based substrates, encapsulants for power devices, and interlayer insulating films.

[0085] Furthermore, when the boron nitride powder of the present invention is prepared into a resin composition, it can be mixed with a thermally conductive filler such as aluminum nitride or aluminum oxide, which is a common highly thermally conductive insulating filler.

[0086] Examples of the resin include thermoplastic resins such as polyolefin, vinyl chloride resin, methyl methacrylate resin, nylon, and fluororesin; thermosetting resins such as epoxy resin, phenol resin, urea resin, melamine resin, unsaturated polyester resin, silicon resin, and bismaleimide triazine resin; fluororesin, liquid crystal polymer, and synthetic rubber.

[0087] The resin composition may also contain, as needed, known additives as compounding agents for resin compositions, 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. Other inorganic fillers may also be included as long as the effects of the present invention are not impaired.

[0088] The boron nitride powder of the present invention can also be used as a raw material for boron nitride processed products such as cubic boron nitride and boron nitride molded products, a nucleating agent for engineering plastics, a phase change material, a solid or liquid thermal interface material, a release agent for molten metal or molten glass molds, cosmetics, a composite ceramic raw material, and the like.

[0089] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the examples, the values ​​measured were determined by the following methods.

[0090] [Pore volume-based median diameter (μm)] and [Volume of pores of 3 μm or less (cm 3 / g)] The pore volume-based median diameter (D1) of the obtained specific hexagonal boron nitride agglomerated particles and hexagonal boron nitride powder was measured by mercury intrusion porosimetry. The pore distribution from 0.0036 μm to 200 μm was determined, and the median diameter was calculated from the cumulative pore distribution plotted with pore diameter on the horizontal axis and cumulative pore volume on the vertical axis. The calculation was performed using an Autopore IV9520 manufactured by Micromeritics.

[0091] [Particle size D2 (μm) at 50% cumulative volume frequency in particle size distribution of hexagonal boron nitride powder] For the obtained hexagonal boron nitride powder, the particle size (D2) at 50% cumulative volume frequency in the particle size distribution was measured using an LA-950V2 manufactured by HORIBA. The particle size distribution of the hexagonal boron nitride powder was measured using a particle size distribution measuring device MT3000 manufactured by Nikkiso Co., Ltd. The measurement sample was prepared by the following method. First, 20 g of ethanol was added as a dispersion medium to a 50 mL screw tube bottle, and 1 g of hexagonal boron nitride powder was dispersed in the ethanol. The particle size at 50% cumulative volume frequency in the obtained particle size distribution was taken as (D2).

[0092] [Oil absorption of hexagonal boron nitride powder in accordance with JIS K5101-13-1:2004] The oil absorption of the obtained hexagonal boron nitride powder was determined according to the procedure specified in JIS K5101-13-1:2004 ("Testing methods for pigments" - Part 13: Oil absorption - Section 1: Refined linseed oil method).

[0093] [Specific surface area of ​​hexagonal boron nitride powder (m 2 The specific surface area of ​​the resulting hexagonal boron nitride powder was measured using a Macsorb HM model-1201 manufactured by Mountech Co., Ltd.

[0094] [Impurity Element Concentration (mass %) in Hexagonal Boron Nitride Powder] The concentrations of impurity elements (calcium, magnesium, sodium, aluminum, lithium, strontium, iron, sulfur, nickel, chromium, manganese, silicon, phosphorus, titanium, barium, and cobalt) in the obtained hexagonal boron nitride powder were measured using a ZSX Primus2 (trade name) manufactured by Rigaku Corporation.

[0095] Example A1: 195 g of boron oxide (2.1% by mass) remaining on a 425 μm sieve, 99 g of carbon black, and 55.2 g of calcium carbonate were mixed using a Spartan mixer. The resulting mixture had a (B / C) element ratio of 0.68, and the CaO-equivalent mass content of the oxygen-containing calcium compound per 100 parts by mass of the oxygen-containing boron compound and carbon source combined (calculated as BO and C) was 10.5 parts by mass. 1500 g of the resulting mixture was nitrided in a graphite Tammann furnace under a nitrogen gas atmosphere by holding the furnace at 1500°C for 6 hours and then at 1940°C for 4 hours.

[0096] Next, the by-product-containing boron nitride was crushed and placed in a container, and hydrochloric acid (7% by mass HCl) was added in an amount five times the amount of the by-product-containing boron nitride, followed by stirring for 24 hours at 350 rpm using a Three-One motor. After the acid washing, the acid was filtered, and the boron nitride obtained by filtration was dispersed in pure water in the same amount as the acid used, and then filtered again. This operation was repeated until the aqueous solution after filtration became neutral, and then the solution was vacuum dried at 200°C for 12 hours.

[0097] After drying, the resulting powder was passed through a sieve with 90 μm openings to obtain a white hexagonal boron nitride powder. The physical properties of the resulting hexagonal boron nitride powder were measured by the methods described above and are shown in Tables 1 and 2.

[0098] Example A2 The same procedures as in Example A1 were carried out except that the boron oxide residue on the 425 μm sieve was 2.9 mass%, the (B / C) element ratio was 0.72, and the mass content of the oxygen-containing calcium compound in terms of CaO was 8 parts by mass per 100 parts by mass of the total mass of the oxygen-containing boron compound and carbon source in terms of BO and C. The respective conditions and measured values ​​are shown in Tables 1 and 2.

[0099] Example A3 The same procedures as in Example A1 were carried out except that the boron oxide residue on the 425 μm sieve was 1.0 mass %, the (B / C) element ratio was 0.65, and the mass content of the oxygen-containing calcium compound, calculated as CaO, was 6 parts by mass per 100 parts by mass of the total mass of the oxygen-containing boron compound and carbon source, calculated as BO and C. The respective conditions and measured values ​​are shown in Tables 1 and 2.

[0100] Example A4 The same procedure was followed as in Example A1, except that the boron oxide residue on a 425 μm sieve was 0.1 mass % and the boron oxide residue on a 300 μm sieve was 15.0 mass %. The conditions and measured values ​​are shown in Tables 1 and 2.

[0101] Example A5: The same procedure as in Example A1 was repeated, except that the boron oxide residue on the 425 μm sieve was 0.5 mass%, the (B / C) element ratio was 0.70, and the mass content of the oxygen-containing calcium compound, calculated as CaO, per 100 mass parts of the total mass of the oxygen-containing boron compound and carbon source, calculated as BO and C, was 8 mass parts. The resulting boron nitride powder was also subjected to the 45 μm sieve treatment described above to produce a sample in which the proportion of specific hexagonal boron nitride agglomerated particles was increased to 52%. The measured values ​​are shown in Tables 1 and 2.

[0102] Comparative Examples A1 to A6 were the same as Example A1, except for the changes in the conditions and the parts shown in Table 1. The conditions and measured values ​​are shown in Tables 1 and 2. The hexagonal boron nitride powder produced in Comparative Example A2 did not contain hexagonal boron nitride agglomerated particles that fall within the scope of the present invention. The hexagonal boron nitride powder produced in Comparative Example A6 was not a white powder, but rather contained some residual carbon and black areas, and analysis of the powder properties was not performed.

[0103] [Resin Composition] The boron nitride powder obtained in Examples A1 to A6 was filled into an epoxy resin to prepare a resin composition, and its thermal conductivity was evaluated. The epoxy resin was a mixture of 100 parts by weight of JER828 manufactured by Mitsubishi Chemical Corporation, 5 parts by weight of a curing agent (imidazole-based curing agent, Curesol 2E4MZ manufactured by Shikoku Kasei Co., Ltd.), and 210 parts by weight of methyl ethyl ketone as a solvent. Next, the varnish-like mixture and the hexagonal boron nitride powder were mixed in a planetary centrifugal mixer (MAZERUSTAR manufactured by Kurabo Industries, Ltd.) to obtain a resin composition containing 35% by volume of the base resin and 65% by volume of the boron nitride powder obtained in Examples A1 to A6.

[0104] The above resin composition was coated onto a PET film to a thickness of approximately 250 to 300 μm using a Tester Sangyo automatic coating machine PI-1210, dried, and 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 220 μm thick sheet. The sheet was analyzed using a temperature wave thermal analyzer, and the results of calculating the thermal conductivity are shown in Table 3. The thermal conductivity of the sheets filled with the hexagonal boron nitride powder produced in Examples 1 to 5 was 8.5 W / m·K or higher, indicating high thermal conductivity. Furthermore, the dielectric strength was measured using a voltage resistance tester (manufactured by Tama Densoku Co., Ltd.), resulting in a high dielectric strength of 80 kV / mm or higher, suggesting that the resin composition has few large voids that could cause a decrease in dielectric strength.

[0105] The actual density was calculated from the volume and weight of the produced sheet, and the theoretical density of the sheet, calculated from the filling rate of the resin and hexagonal boron nitride particles, was taken as 100%, and the ratio of the actual density to the theoretical density of the sheet was calculated to obtain the relative density of the resin sheet. The resin sheet produced using the hexagonal boron nitride powder of the present invention has an actual density / theoretical density of 89-97%, indicating that the resin sheet contains minute voids that do not reduce thermal conductivity or dielectric strength. Therefore, by using the hexagonal boron nitride powder of the present invention, it was possible to achieve a lightweight inorganic-filled resin sheet while maintaining high thermal conductivity and dielectric strength.

[0106] Furthermore, the resin compositions were obtained in the same manner except that the boron nitride powders obtained in Comparative Examples A1 to A5 were used, and the thermal conductivity, dielectric strength, and resin sheet relative density were measured and are shown in Table 3. As is clear from the table, none of the sheets filled with the boron nitride powder obtained in Comparative Examples 1 to 5 were able to simultaneously achieve a thermal conductivity of 8.5 W / m K, a dielectric strength of 80 kV / mm or more, and a resin sheet relative density of 97.0% or less.

[0107]

[0108]

[0109] Example B1: Following the same procedures as in Production Example A1, reaction, acid washing, washing with pure water, and drying were carried out. The dried powder was then sieved through a 45 μm mesh sieve to obtain a white hexagonal boron nitride powder containing specific hexagonal boron nitride agglomerated particles. From the SEM image at 10,000x magnification, the presence of specific hexagonal boron nitride agglomerated particles was confirmed. The major axis of the resulting hexagonal boron nitride powder was 5-10 μm, the major axis / minor axis ratio was 1.0-1.3, the circularity was 0.3-0.8, and the maximum diameter of the primary particles visible on the agglomerated particle surface was 4 μm or less. The SEM image of the specific hexagonal boron nitride agglomerated particles is shown in FIG. 1 above. Furthermore, the proportion of specific hexagonal boron nitride agglomerated particles calculated from the area ratio of the 500x SEM image was 53%. For a particle having a particle size close to the average particle size of the obtained specific hexagonal boron nitride agglomerated particles, the specific major axis, major axis / minor axis, circularity, and maximum diameter of the primary particle are shown in Table 2. Furthermore, the content of impurity elements of the specific hexagonal boron nitride agglomerated particles, which are specified from the measurements of the obtained hexagonal boron nitride powder, the pore volume-based median diameter D1 (μm), and the pore volume (cm) of pores having a pore diameter of 3 μm or less are also shown. 3 The values ​​of the impurity element concentration (g / g) are also shown in Table 4. The impurity element concentration was 40 ppm, the oxygen concentration was 0.2%, and the carbon concentration was 0.01%.

[0110] [Mixed hexagonal boron nitride powder and resin composition] 100 parts by mass of the boron nitride powder containing the specific hexagonal boron nitride agglomerated particles produced in this manner was mixed with 5 parts by mass of commercially available hexagonal boron nitride powder consisting of single hexagonal boron nitride particles (wet particle size distribution D2 = 10 μm, average aspect ratio 10, impurity element concentration 40 ppm, oxygen concentration 0.2%, carbon concentration 0.01%) to obtain a mixed hexagonal boron nitride powder. The pore volume-based median diameter D1 (μm) and the pore volume (cm3) of pores with a pore diameter of 3 μm or less were measured for the obtained mixed hexagonal boron nitride powder. 3 / g), and particle size D2 (μm) at 50% cumulative volume frequency in the wet particle size distribution.

[0111] The mixed hexagonal boron nitride powder was filled into an epoxy resin to prepare a resin composition, and its thermal conductivity was evaluated. The epoxy resin was a mixture of 100 parts by weight of JER828 (manufactured by Mitsubishi Chemical Corporation), 5 parts by weight of a curing agent (imidazole-based curing agent, Curesol 2E4MZ (manufactured by Shikoku Kasei Co., Ltd.)), and 210 parts by weight of methyl ethyl ketone as a solvent. The varnish-like mixture and the hexagonal boron nitride powder were then mixed in a rotation / revolution mixer (MAZERUSTAR (manufactured by Kurabo Industries Co., Ltd.)) to obtain a resin composition with a base resin content of 35% by volume and a hexagonal boron nitride powder content of 65% by volume.

[0112] The resin composition was coated onto a PET film to a thickness of approximately 250 to 300 μm using a Tester Sangyo automatic coater PI-1210, dried, and 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 220 μm thick sheet. The sheet was analyzed using a temperature wave thermal analyzer to calculate the thermal conductivity. Furthermore, the dielectric strength was measured using a voltage resistance tester (manufactured by Tama Densoku Co., Ltd.). The results are also shown in Table 5.

[0113] Example B2 The same procedure as in Example B1 was repeated, except that the boron oxide residue on the 425 μm sieve was 2.8 mass%, the (B / C) element ratio was 0.73, and the mass content of the oxygen-containing calcium compound (calculated as CaO) per 100 mass parts of the total mass of the oxygen-containing boron compound and carbon source (calculated as BO and C) was 8.2 mass parts. The conditions and measured values ​​are shown in Table 4. The impurity element concentration was 20 ppm, the oxygen concentration was 0.2%, and the carbon concentration was 0.01%.

[0114] [Mixed hexagonal boron nitride powder and resin composition] Using the boron nitride powder containing the specific hexagonal boron nitride agglomerated particles thus produced, a mixed hexagonal boron nitride powder was prepared in the same manner as in Example B1, and this was further mixed with a resin to prepare a resin composition, and the thermal conductivity and dielectric strength were measured. The results are also shown in Table 5.

[0115] Example B3 The same procedure as in Example B1 was repeated, except that the boron oxide residue on the 425 μm sieve was 1.0 mass%, the (B / C) element ratio was 0.66, and the mass content of the oxygen-containing calcium compound (calculated as CaO) was 7 parts by mass per 100 parts by mass of the total mass of the oxygen-containing boron compound and carbon source (calculated as BO and C). The conditions and measured values ​​are shown in Table 4. The impurity element concentration was 20 ppm, the oxygen concentration was 0.2%, and the carbon concentration was 0.01%.

[0116] [Mixed hexagonal boron nitride powder and resin composition] Using the boron nitride powder containing the specific hexagonal boron nitride agglomerated particles thus produced, a mixed hexagonal boron nitride powder was prepared in the same manner as in Example B1, and this was further mixed with a resin to prepare a resin composition, and the thermal conductivity and dielectric strength were measured. The results are also shown in Table 5.

[0117] Example B4 The same procedure as in Example B1 was repeated, except that the boron oxide residue on a 425 μm sieve was 0.1 mass% and the boron oxide residue on a 300 μm sieve was 15.0 mass%. The conditions and measured values ​​are shown in Table 4. The impurity element concentration was 20 ppm, the oxygen concentration was 0.2%, and the carbon concentration was 0.01%.

[0118] [Mixed hexagonal boron nitride powder and resin composition] Using the boron nitride powder containing the specific hexagonal boron nitride agglomerated particles thus produced, a mixed hexagonal boron nitride powder was prepared in the same manner as in Example B1, and this was further mixed with a resin to prepare a resin composition, and the thermal conductivity and dielectric strength were measured. The results are also shown in Table 5.

[0119] Example B5 The procedure was the same as in Example B1, except that the boron oxide residue on the 425 μm sieve was 0.5 mass%, the (B / C) element ratio was 0.70, and the mass content of the oxygen-containing calcium compound, calculated as CaO, per 100 mass parts of the total mass of the oxygen-containing boron compound and carbon source, calculated as BO and C, was 8 mass parts. The measured values ​​are shown in Table 4. The impurity element concentration was 20 ppm, the oxygen concentration was 0.2%, and the carbon concentration was 0.01%.

[0120] Example B6 100 mass parts of the boron nitride powder containing specific hexagonal boron nitride agglomerated particles produced in Example B1 are mixed with 15 mass parts of commercially available hexagonal boron nitride powder (wet particle size distribution D2 = 10 μm, average aspect ratio 10, impurity element concentration 40 ppm, oxygen concentration 0.2%, carbon concentration 0.01%) to prepare mixed hexagonal boron nitride powder, and then, in the same manner as in Example B1, this is mixed with resin to prepare a resin composition, and thermal conductivity and dielectric strength are measured.The results are also shown in Table 5.

[0121]

[0122] As described above, the thermal conductivity of the sheets filled with the hexagonal boron nitride powder prepared in Examples B1 to B6 was 9.0 W / m K or higher, indicating high thermal conductivity. In addition, the sheets had a high dielectric strength of 80 kV / mm or higher, suggesting that the resin composition had few large voids that could cause a decrease in dielectric strength.

[0123] For comparison, a sample containing agglomerated particles having a primary particle diameter of 8 μm was used instead of the specific hexagonal boron nitride agglomerated particles of Example B1. 50 A mixed hexagonal boron nitride powder was prepared in the same manner as in Example B1 using commercially available hexagonal boron nitride powder having a particle size of 25 μm. A resin composition was prepared using this and subjected to various tests. The resulting composition showed low values ​​of thermal conductivity of 8.8 W / m K and dielectric strength of 65 kV / mm.

Claims

1. Hexagonal boron nitride agglomerated particles, which are agglomerated particles of hexagonal boron nitride primary particles, characterized in that the major axis is 5 to 10 μm, the major axis / minor axis ratio is 1.0 to 1.3, the circularity is in the range of 0.3 to 0.8, and the maximum diameter of the primary particles that can be confirmed on the surface of the agglomerated particles in an SEM image at a magnification of 10,000 times is 4 μm or less.

2. The particle diameter (D) of the particle size distribution measured by a wet laser diffraction particle size distribution method is 50% of the cumulative volume frequency, and is composed of agglomerated particles of hexagonal boron nitride primary particles. 50 ) is 5 to 150 μm, the volume-based median diameter of pores measured by mercury intrusion porosimetry is 3.0 μm or less, and the content of impurity elements is 500 ppm or less.

3. The hexagonal boron nitride powder according to claim 2, which contains the hexagonal boron nitride agglomerated particles according to claim 1 as agglomerated particles of the hexagonal boron nitride primary particles.

4. BET specific surface area is 1 to 15 m 2 4. The hexagonal boron nitride powder according to claim 2 or 3, having a viscosity of 100 psig / 100 g and an oil absorption of 50 to 190 cc / 100 g.

5. A method for producing a hexagonal boron nitride agglomerated particle according to claim 1, comprising the hexagonal boron nitride agglomerated particle according to claim 1 and a single particle of hexagonal boron nitride, the content of the hexagonal boron nitride agglomerated particle being 5% or more, and the particle size at a cumulative volume frequency of 50% in a particle size distribution measured by a wet laser diffraction particle size distribution method (D 50 ) is 5 to 150 μm.

6. The hexagonal boron nitride powder according to claim 5, wherein the volume-based median diameter of the pores measured by mercury intrusion porosimetry is 3.0 μm or less.

7. A resin composition comprising the hexagonal boron nitride powder according to any one of claims 2 to 4.

8. A resin composition comprising the hexagonal boron nitride powder according to claim 5 or 6.

9. A resin sheet comprising the resin composition according to claim 7.

10. The resin sheet according to claim 9, wherein the sheet density is 89 to 97% of the theoretical density.

11. A resin sheet comprising the resin composition according to claim 8.

12. The resin sheet according to any one of claims 9 to 11, which has a dielectric strength of 80 kV / mm or more.