Method for producing hexagonal boron nitride powder, hexagonal boron nitride powder and resin composition

Abstract

Provided is a production method by which the filling property of a hexagonal boron nitride powder, which contains hexagonal boron nitride particles having a low aspect ratio, with respect to a resin can be improved while maintaining the characteristic of having small thermal conductivity anisotropy. This method for producing a hexagonal boron nitride powder comprises crushing, by using a means that does not substantially involve grinding primary particles, a hexagonal boron nitride raw material powder, which contains hexagonal boron nitride particles having an aspect ratio of 1.5-5.0 and an aggregate including hexagonal boron nitride particles having an aspect ratio of greater than 5.0.

Description

[Technical Field]

[0001] The present invention relates to a novel method for producing hexagonal boron nitride powder, a novel hexagonal boron nitride powder obtainable by the method, and a novel resin composition containing the same. More specifically, the present invention provides a method for improving the fillability of hexagonal boron nitride powder containing hexagonal boron nitride particles with a low aspect ratio and aggregates containing hexagonal boron nitride particles with a high aspect ratio that tend to form aggregates, while maintaining its small thermal conductivity anisotropy. The present invention also provides a hexagonal boron nitride powder obtainable by the method that has high fillability in resin and can reduce thermal conductivity anisotropy, and a resin composition containing the hexagonal boron nitride powder. [Background technology]

[0002] Hexagonal boron nitride is a material with dielectric strength and thermal conductivity that is used as a thermally conductive filler for heat dissipation components. Hexagonal boron nitride has a lower thermal conductivity in the thickness direction (c-axis direction) than in the width direction (ab-axis direction), resulting in large thermal conductivity anisotropy. Therefore, when a resin composition containing hexagonal boron nitride powder is used as a heat dissipation component, sufficient thermal conductivity may not be obtained in certain directions.

[0003] As a means of suppressing the above-mentioned thermal conductivity anisotropy, it is necessary to reduce the anisotropy, that is, to reduce the length (L) of the particle in the width direction (the ab-axis direction) and the thickness direction (c Axis Square Attempts have been made to produce particles with a small aspect ratio, which is the ratio (L / t) of the length (t) of the particle in the direction of the particle diameter to the length (t) of the particle. For example, Patent Document 1 proposes a method for obtaining hexagonal boron nitride particles with a small aspect ratio by mixing boron nitride powder with a lithium salt and heating the mixture. By using a hexagonal boron nitride powder containing hexagonal boron nitride particles with such a small aspect ratio, it is said that the use of a hexagonal boron nitride powder containing hexagonal boron nitride particles with a small aspect ratio makes it easy to fill a resin composition with the hexagonal boron nitride powder without causing orientation, thereby making it possible to reduce the thermal conductivity anisotropy of the resin composition. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication 2016-141600 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when a resin composition is formed by filling the hexagonal boron nitride powder having a small aspect ratio into a resin, it has been found that the filling property of the hexagonal boron nitride powder is insufficient, and the handling property of the resin composition is poor.

[0006] Therefore, an object of the present invention is to provide a manufacturing method that can improve the fillability of the hexagonal boron nitride powder with a small aspect ratio into resin while maintaining the characteristic of small thermal conductivity anisotropy of the powder. [Means for solving the problem]

[0007] As a result of extensive research, the present inventors have found that methods for obtaining hexagonal boron nitride powder with a small aspect ratio, including the above-mentioned method, do not necessarily produce a high proportion of particles with the desired low aspect ratio, and that the resulting hexagonal boron nitride powder also contains hexagonal boron nitride particles with a large aspect ratio, which form relatively large aggregates and reduce the fillability of the resin. That is, scaly hexagonal boron nitride particles with a large aspect ratio tend to form aggregates, and due to their shape, spaces between the particles tend to form within the aggregates. Therefore, when these aggregates are filled into a resin, the resin has difficulty penetrating the spaces between the particles within the aggregates, making it difficult to improve the fillability.

[0008] Based on the above findings, the inventors conducted further research and found that the above object can be achieved by disintegrating the hexagonal boron nitride powder using a method that does not substantially involve pulverization of the primary particles having a small aspect ratio (i.e., does not involve cleavage fracture), thereby selectively reducing agglomerates that reduce the fillability into resin, which led to the completion of the present invention.

[0009] That is, the present invention provides a method for producing hexagonal boron nitride powder, which comprises disintegrating a hexagonal boron nitride raw material powder containing hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0 and aggregates containing hexagonal boron nitride particles having an aspect ratio of more than 5.0, by means that does not substantially involve pulverization of primary particles.

[0010] The hexagonal boron nitride raw material powder preferably contains hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0 at a ratio of 20% to 90% by number, and preferably has a D50 of 0.2 to 10.0 μm and a D90 / D10 ratio of more than 3.0.

[0011] When disintegrating by a method that does not substantially involve pulverization of the primary particles, it is preferable to disintegrate the hexagonal boron nitride powder so that the D90 / D10 ratio is 3.0 or less.

[0012] The present invention also provides a hexagonal boron nitride powder that can be produced by the above-mentioned method and that exhibits high packing properties even when it contains high-aspect-ratio hexagonal boron nitride particles that, together with low-aspect-ratio hexagonal boron nitride particles, form relatively large aggregates that reduce packing properties in resins.

[0013] That is, according to the present invention, there is provided a hexagonal boron nitride powder containing 20% ​​or more by number of hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0, having a D50 of 0.2 to 10.0 μm, and a D90 / D10 of 3.0 or less.

[0014] Furthermore, the present invention also provides a resin composition containing the above-mentioned hexagonal boron nitride powder, which has small thermal conductivity anisotropy and is easy to handle. [Effects of the Invention]

[0015] According to the manufacturing method of the present invention, it is possible to obtain a hexagonal boron nitride powder containing low-aspect-ratio hexagonal boron nitride particles and having few agglomerates that cause poor packing. When the hexagonal boron nitride powder thus obtained is filled into a resin, the low-aspect-ratio hexagonal boron nitride particles reduce the thermal conductivity anisotropy. Furthermore, the high packing property improves the handleability of the resin composition, making it easy to produce, for example, a resin sheet with sufficiently high thermal conductivity. DETAILED DESCRIPTION OF THE INVENTION

[0016] <Method for producing hexagonal boron nitride powder> The method for producing hexagonal boron nitride powder of the present invention involves disintegrating a hexagonal boron nitride raw material powder containing hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0 and agglomerates containing hexagonal boron nitride particles having an aspect ratio of more than 5.0, by means that does not substantially involve pulverization of primary particles.

[0017] By disintegrating in this manner using a method that does not substantially involve pulverization of primary particles (hereinafter sometimes referred to as the "disintegration process"), cleavage fracture of hexagonal boron nitride particles with an aspect ratio of 1.5 to 5.0 can be prevented, and the packing ability can be improved by reducing agglomerates containing hexagonal boron nitride particles with an aspect ratio exceeding 5.0, which are a cause of reduced packing ability, while maintaining the property of reducing thermal conductivity anisotropy.

[0018] In the present invention, the aspect ratio is the ratio of the length (L) in the width direction (the ab-axis direction) of the particle to the thickness direction (c Axis Square It is expressed as the ratio (L / t) of the length (t) of the beam (direction).

[0019] In the present invention, the aspect ratio is measured by measuring the width (L) and thickness (t) of hexagonal boron nitride particles from an SEM image of the hexagonal boron nitride powder and calculating the aspect ratio (L / t) of the particles. In this way, 300 different particles are randomly selected, and the aspect ratio of each is calculated. From the results, the proportion of hexagonal boron nitride particles whose aspect ratio falls within a predetermined range is calculated.

[0020] In the hexagonal boron nitride raw material powder used in the method for producing hexagonal boron nitride powder of the present invention, the proportion of hexagonal boron nitride particles with a low aspect ratio of 1.5 to 5.0 is preferably 20% by number or more, more preferably 40% by number or more, and even more preferably 50% by number or more, since the higher the proportion, the smaller the thermal conductivity anisotropy of the resulting hexagonal boron nitride powder. There is no particular upper limit, but it is generally 95% by number or less, particularly 90% by number or less, and even more preferably 80% by number or less.

[0021] The hexagonal boron nitride raw material powder used in the method for producing a hexagonal boron nitride powder of the present invention contains agglomerates containing hexagonal boron nitride particles with an aspect ratio exceeding 5.0. As described above, such hexagonal boron nitride agglomerates cause a decrease in packing ability, so by using the production method of the present invention on a hexagonal boron nitride raw material powder containing such agglomerates, packing ability can be improved. Such agglomerates may be composed solely of hexagonal boron nitride particles with an aspect ratio exceeding 5.0, or may be composed of both hexagonal boron nitride particles with an aspect ratio exceeding 5.0 and hexagonal boron nitride particles with an aspect ratio of 5.0 or less.

[0022] The presence of such agglomerates in a hexagonal boron nitride raw material powder can be confirmed by an SEM image. Specifically, if agglomerates of a plurality of hexagonal boron nitride particles are observed in an SEM image at 5000x magnification of a hexagonal boron nitride raw material powder in which at least 300 hexagonal boron nitride particles are observed, and if at least one hexagonal boron nitride particle has an aspect ratio exceeding 5.0, the hexagonal boron nitride raw material powder can be said to contain agglomerates of hexagonal boron nitride particles having an aspect ratio exceeding 5.0.

[0023] The hexagonal boron nitride raw material powder used in the manufacturing method of the present invention preferably contains hexagonal boron nitride particles with an aspect ratio of more than 5.0 in an amount of at least 5% by number, more preferably at least 8% by number, and even more preferably at least 10% by number, which makes it easier to form aggregates that cause reduced packing, thereby enhancing the effects of the present invention.

[0024] The hexagonal boron nitride raw material powder used in the manufacturing method of the present invention preferably has a D90 / D10 ratio of more than 3.0. BET ) and the ratio of D50 (D50 / D BET ) is preferably greater than 4.5. BET If the value exceeds 4.5, it means that a large amount of agglomerates is contained, and it can be said that the effect of improving the packing property by the crushing step of the present invention is high.

[0025] In the present invention, D10, D50, and D90 refer to the cumulative 10% value (D10), cumulative 50% value (D50), and cumulative 90% value (D90) in a particle size distribution curve, respectively. The particle size distribution curve is calculated from the volumetric particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer after dispersing the powder to be measured in ethanol and subjecting it to ultrasonic treatment. The ultrasonic treatment was carried out by adding 20 g of ethanol as a dispersion medium to a 50 mL screw cap vial, dispersing 1 g of the powder to be measured in the ethanol, and then sonicating the mixture using an ultrasonic homogenizer with the tip positioned 10 mm from the bottom of the screw cap vial at an amplitude of 40% for 20 minutes.

[0026] The specific surface area in the present invention is the BET specific surface area measured by the gas adsorption method. BET is an index of the particle size of the primary particles, and D50 / D BET The smaller the value, the fewer the aggregates. BET is the following formula D BET = 6 / (specific surface area x density of hexagonal boron nitride) The density of hexagonal boron nitride is 2.1 g / cm 3 is.

[0027] The method for preparing the hexagonal boron nitride raw material powder used in the crushing step of the present invention is not particularly limited, and any method capable of preparing a hexagonal boron nitride raw material powder containing hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0 and aggregates containing hexagonal boron nitride particles having an aspect ratio of more than 5.0 can be used without any particular limitation. For example, there can be mentioned a method having a step of mixing boron nitride powder with a lithium salt and a step of heating the mixture, as described in JP 2016-141600 A.

[0028] In the method comprising the steps of mixing boron nitride powder with a lithium salt and heating the mixture, the use of lithium salt as a flux makes it possible to obtain hexagonal boron nitride particles grown in the c-axis direction, i.e., hexagonal boron nitride particles with a low aspect ratio. Since it is difficult to avoid the by-production of hexagonal boron nitride particles with a high aspect ratio in this method, the hexagonal boron nitride raw material powder prepared by this method usually contains hexagonal boron nitride particles with an aspect ratio of more than 5.0 and contains agglomerates containing these particles.

[0029] In this method, the mixing ratio of boron nitride powder to lithium salt is not particularly limited, but since increasing the concentration of lithium salt relative to boron nitride facilitates the production of hexagonal boron nitride particles with a low aspect ratio, the lithium salt is preferably 0.15 mol or more, more preferably 0.20 mol or more, per mol of boron nitride. On the other hand, considering the industrial use of this method, the lithium salt is preferably 0.50 mol or less, more preferably 0.40 mol or less, per mol of boron nitride, from the standpoints of handleability and cost. If the lithium salt is reduced, high-aspect-ratio hexagonal boron nitride particles are more likely to be by-produced, and the problem of the present invention, namely, reduced packing due to aggregates containing high-aspect-ratio hexagonal boron nitride particles, becomes more pronounced. Therefore, applying the method of the present invention to a hexagonal boron nitride raw material powder containing hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0 and aggregates containing hexagonal boron nitride particles having an aspect ratio of more than 5.0 prepared in this manner is preferable, as it significantly improves packing. From the above, a preferred embodiment of the method for producing a hexagonal boron nitride raw material powder of the present invention is one in which the method of the present invention is applied to a hexagonal boron nitride raw material powder obtained by a method comprising at least the steps of mixing boron nitride powder and a lithium salt so that the lithium salt is 0.15 mol to 0.50 mol per mol of boron nitride, and heating the mixture, i.e., a method for producing a hexagonal boron nitride powder comprising a crushing step of crushing the hexagonal boron nitride raw material powder by means that does not substantially involve pulverization of primary particles.

[0030] The above manufacturing method preferably includes a lithium salt removal step after the heating step. The lithium salt removal step is a step of washing the hexagonal boron nitride with an acid to remove raw material residues such as lithium salts attached to the hexagonal boron nitride, as well as by-products such as boric acid and lithium-boron composite oxide. In the lithium salt removal step, a dilute acid such as hydrochloric acid is preferably used. The washing method is not particularly limited, and may be acid washing by showering, acid washing by immersion, or acid washing by stirring.

[0031] The lithium salt removal step may be performed before the disintegration step of the present invention, after the disintegration step, or both before and after the disintegration step. If the lithium salt removal step is not performed before the disintegration step, performing the disintegration step in the presence of relatively hard lithium salts promotes disintegration of the aggregates, allowing the disintegration step to be performed efficiently. In addition, the acid washing step after disintegration (described later) and the lithium salt removal step for removing raw material residues such as lithium salts adhering to the hexagonal boron nitride and by-products such as boric acid and lithium-boron composite oxide can be performed as a common operation, allowing for more efficient production of hexagonal boron nitride powder. For these reasons, performing the disintegration step before the lithium salt removal step is a preferred method.

[0032] When the crushing step is carried out before the lithium salt removal step, it is difficult to accurately measure the various physical properties of the hexagonal boron nitride raw material powder, such as the aspect ratio, specific surface area, particle size distribution, etc. Therefore, in this case, a portion of the hexagonal boron nitride raw material powder before the crushing step is separated, and after the lithium salt removal step is carried out, the various physical properties are measured and used as the physical properties of the hexagonal boron nitride raw material powder before the crushing step.

[0033] In the above production method, a water washing step and a drying step may be carried out before the crushing step.

[0034] The water washing step is a step of washing the hexagonal boron nitride raw material powder with water, with the aim of removing the acid that has adhered to the hexagonal boron nitride raw material powder in the lithium salt removal step. The water washing method is not particularly limited, and after filtering the hexagonal boron nitride raw material powder, the powder may be washed with water by showering or by soaking.

[0035] The drying step is a step of drying the produced hexagonal boron nitride raw material powder. The drying method is not particularly limited and may be high-temperature drying or reduced-pressure drying.

[0036] In the crushing step of the present invention, it can be confirmed that the crushing of primary particles is not substantially accompanied by the specific surface area (S B ) and the specific surface area (S A ) can be compared. That is, S B and S A The ratio (S B / S A If S is 0.75 or more, the crushing of primary particles hardly occurs in the crushing step, and it can be said that the crushing is substantially not accompanied by the crushing of primary particles. B / S A is more preferably 0.80 or more, and even more preferably 0.90 or more.

[0037] In the present invention, specific embodiments of disintegration without substantially pulverizing primary particles include, for example, a method using a mill such as a jet mill, a planetary mill, or a millstone-type mill, and a method of disintegrating agglomerates by collisions between particles or between particles and the wall of the device during pneumatic powder transport while passing through an ejector or the like. When using these mills or mills, the operating conditions can be adjusted appropriately so as not to pulverize primary particles. Note that millstone-type mills are less likely to pulverize primary particles than other devices, and operating conditions can be easily adjusted, making them a preferred method that can easily perform disintegration without substantially pulverizing primary particles.

[0038] In a millstone-type grinder, powder particles are ground primarily by collisions between rotating grinding stones arranged above and below. When using a millstone-type grinder, operating conditions such as the material, size, distance between grinding stones, and rotation speed can be adjusted appropriately. Examples of grinding stone materials that can be used include silicon carbide, silicon nitride, zirconia, alumina, diamond, and stainless steel. However, alumina is particularly preferred because it is inexpensive and does not easily cause pulverization of primary particles of hexagonal boron nitride particles. If the grinding stone distance is too wide, agglomerated particles may not be sufficiently broken down, while if it is too narrow, primary particles may be easily pulverized or the equipment may be easily damaged. Therefore, the grinding stone distance is preferably set to 3 to 20 times, and preferably 4 to 15 times, the D50 of the hexagonal boron nitride raw material powder. The size and rotation speed of the grinding stone can be adjusted as appropriate; for example, the grinding stone size can be 150 to 500 mm in diameter, and the rotation speed can be 1000 to 3000 rpm. The grinding process using a millstone-type grinder can be carried out in a dry manner, i.e., by supplying the hexagonal boron nitride raw material powder to the device as a powder without dispersing it in a liquid such as water, or in a wet manner, i.e., by dispersing the hexagonal boron nitride raw material powder in a liquid such as water and supplying it to the device as a slurry. When carried out in a dry manner, the raw material is easy to handle. When carried out in a wet manner, thermal expansion of the grinding stone can be suppressed, making it easy to prevent the grinding stone spacing from becoming too narrow even during long, continuous operation. Both the dry and wet methods have the advantages described above, and the method can be selected appropriately depending on the situation.

[0039] In the present invention, after the crushing step, steps such as an acid washing step, a water washing step, a drying step, and a classification step may be carried out.

[0040] The acid washing step is a step for the purpose of removing impurities generated in the crushing step using an acid. The operation may be performed in the same manner as the lithium salt removal step. The acid washing step and the lithium salt removal step may be performed simultaneously, i.e., the acid washing step may also serve as the lithium salt removal step.

[0041] The water washing step is a step of washing the hexagonal boron nitride powder with water for the purpose of removing the acid that has adhered to the hexagonal boron nitride powder in the acid washing step. The water washing method is not particularly limited, and after filtering the hexagonal boron nitride powder, water washing by showering or water washing by immersion may be performed.

[0042] Furthermore, the drying step is a step of drying the produced hexagonal boron nitride powder. The drying method is not particularly limited, and may be high-temperature drying or reduced-pressure drying.

[0043] Furthermore, the classification step is a step of separating the hexagonal boron nitride powder according to particle size and / or particle shape, etc. The classification operation may be, for example, sieving, or a method of passing a slurry in which hexagonal boron nitride particles are dispersed through a filter, or may be wet classification or air classification.

[0044] <Hexagonal boron nitride powder> The present invention provides a hexagonal boron nitride powder that can be produced by the above-mentioned method for producing hexagonal boron nitride powder, and that exhibits high packing properties even when it contains hexagonal boron nitride particles with a large aspect ratio that, together with hexagonal boron nitride particles with a low aspect ratio, form relatively large aggregates that reduce packing properties in resin.

[0045] That is, the present invention provides a hexagonal boron nitride powder containing 20% ​​or more by number of hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0, D50 of 0.2 to 10.0 μm, and D90 / D10 of 3.0 or less.

[0046] Such hexagonal boron nitride powder contains hexagonal boron nitride particles with a low aspect ratio and has few large aggregates that hinder its fillability into resin, so it has high fillability into resin and can also reduce the thermal conductivity anisotropy of resin compositions filled with it.

[0047] The hexagonal boron nitride powder of the present invention contains 20% or more by number of hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0. By containing 20% ​​or more by number of low-aspect-ratio hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0, it is possible to reduce the thermal conduction anisotropy and, for example, to sufficiently increase the thermal conductivity in the thickness direction of a resin sheet. The greater the amount of the low-aspect-ratio hexagonal boron nitride particles blended, the easier it is to reduce the thermal conduction anisotropy; therefore, the amount is preferably 40% or more by number, and more preferably 50% or more by number.

[0048] In the hexagonal boron nitride powder of the present invention, the upper limit of the proportion of hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0 is not particularly limited, but is generally 95% by number or less, and particularly 80% by number or less.

[0049] Moreover, the content of hexagonal boron nitride particles with a high aspect ratio exceeding 5.0 is generally 5% by number or more, particularly 8% by number or more, and even 20% by number or more. Such high aspect ratio boron nitride particles tend to form aggregates, and the aggregates formed containing these particles are the main cause of reduced packing properties.

[0050] The hexagonal boron nitride powder of the present invention has a D90 / D10 ratio of 3.0 or less. A smaller D90 / D10 value indicates a sharper particle size distribution, while a larger D90 / D10 value indicates a broader particle size distribution. In other words, the hexagonal boron nitride powder of the present invention has a sharp particle size distribution.

[0051] Hexagonal boron nitride powders with a broad particle size distribution can be said to contain aggregated coarse particles. As mentioned above, due to their shape, hexagonal boron nitride particles form aggregates with gaps between the particles, and resins have difficulty penetrating these gaps. Therefore, hexagonal boron nitride powders with a large amount of aggregates have difficulty achieving high packing properties. The hexagonal boron nitride powder of the present invention has a sharp particle size distribution and few aggregates as mentioned above, so it exhibits high packing properties. As mentioned above, D10, D50, and D90 are measured by dispersing the powder in ethanol and subjecting it to ultrasonic treatment. Some aggregates are destroyed during measurement. However, by measuring D90 / D10 for hexagonal boron nitride powder subjected to ultrasonic treatment under specified conditions, it is possible to evaluate the state of aggregates that affect packing properties.

[0052] The smaller the D90 / D10 of the hexagonal boron nitride powder, the fewer agglomerates there are and the higher the packing ability, so D90 / D10 is preferably 2.7 or less, and even more preferably 2.5 or less. There are no particular restrictions on the lower limit of D90 / D10 as long as it is 1.0 or more, but since a smaller D90 / D10 makes production difficult due to the need to precisely control the particle size and the occurrence of agglomerations, D90 / D10 is preferably 1.2 or more, and even more preferably 1.5 or more.

[0053] The D50 of the hexagonal boron nitride powder of the present invention is 0.2 to 10.0 μm, and more preferably 1.0 to 5.0 μm. This facilitates its use as a thermally conductive filler for heat dissipation components. That is, if the D50 is small, the powder tends to float, making it difficult to handle, and the viscosity tends to increase when filling the resin, reducing the efficiency of the filling process. If the D50 is large, the resin composition may have a tendency to have unevenness on its surface, making it difficult to use in thin resin sheets.

[0054] The D10 of the hexagonal boron nitride powder of the present invention is preferably 1.5 μm or more, more preferably 1.8 μm or more, because if the D10 is too small, the presence of fine powder tends to increase the viscosity when filling a resin. If the D90 is too large, irregularities tend to occur on the surface of the resin composition, making it difficult to use in thin resin sheets. Therefore, the D90 is preferably 10.0 μm or less, more preferably 7.5 μm or less.

[0055] The hexagonal boron nitride powder of the present invention has a specific surface area of ​​1.5 m 2 / g~4.0m 2 / g. When the specific surface area is in the above range, it is easy to obtain high packing properties while reducing the thermal conductivity anisotropy. In addition, the particle diameter (D BET ) and the ratio of D50 (D50 / D BET ) is preferably 4.5 or less, more preferably 4.0 or less.

[0056] The hexagonal boron nitride powder of the present invention, possessing the above-described properties, enables the viscosity of the resin (resin filling viscosity) when filled into a resin to be reduced. For example, when 20% by volume of the hexagonal boron nitride powder is filled into a silicone resin (CY52-276A manufactured by Dow-Toray Industries, Inc.), the resin filling viscosity, which is the viscosity at a measurement temperature of 25°C and a shear rate of 1 / s, can be reduced to 130 Pa·s or less, and even 125 Pa·s or less. A resin filling viscosity of 130 Pa·s or less means that the hexagonal boron nitride powder exhibits high filling properties, allowing it to be filled into a resin at high density. Furthermore, the resin has good fluidity, resulting in good handleability and facilitating the production of, for example, a resin sheet.

[0057] Furthermore, the hexagonal boron nitride powder of the present invention exhibits a DBP absorption (mL / 100g) calculated from a curve measured in accordance with JIS-K-6217-4:2017, where the horizontal axis is the DBP drop amount (mL) and the vertical axis is torque (N m), of 80mL / 100g or less, particularly 75mL / 100g or less, and even 70mL / 100g or less. Such low DBP absorption means high fillability into resin.

[0058] <Resin composition> A resin composition according to one embodiment of the present invention contains the hexagonal boron nitride powder and a resin. Because this resin composition contains a certain amount or more of low-aspect-ratio hexagonal boron nitride particles as described above, it has low thermal conductivity anisotropy. Furthermore, because it is filled with hexagonal boron nitride powder, which has high packing properties, the viscosity does not increase easily, making it easy to handle and, for example, to prepare a resin sheet. There are no particular limitations on the method for preparing the resin composition, and the resin composition can be prepared by a known preparation method.

[0059] The resin is not particularly limited and may be, for example, a silicone-based resin or an epoxy-based resin. Examples of epoxy resins include bisphenol A type epoxy resins, bisphenol S type epoxy resins, bisphenol F type epoxy resins, hydrogenated bisphenol A type epoxy resins, polypropylene glycol type epoxy resins, polytetramethylene glycol type epoxy resins, naphthalene type epoxy resins, phenylmethane type epoxy resins, tetrakisphenolmethane type epoxy resins, biphenyl type epoxy resins, epoxy resins having a triazine nucleus in the skeleton, bisphenol A alkylene oxide adduct type epoxy resins, bisphenol AF type epoxy resins, phenol novolac type epoxy resins, tert-butyl-catechol type epoxy resins, naphthol type epoxy resins, naphthylene ether type epoxy resins, glycidylamine type epoxy resins, glycidyl ester type epoxy resins, cresol novolac type epoxy resins, anthracene type epoxy resins, linear aliphatic epoxy resins, epoxy resins having a butadiene structure, alicyclic epoxy resins, heterocyclic epoxy resins, spiro ring-containing epoxy resins, cyclohexanedimethanol type epoxy resins, trimethylol type epoxy resins, and halogenated epoxy resins. These epoxy resins may be used alone or in combination of two or more.

[0060] As a curing agent for epoxy resins, an active ester compound containing a naphthalene structure may be used. The active ester compound containing a naphthalene structure is not particularly limited as long as it has a naphthalene structure and an arylcarbonyloxy group, but an active ester compound having a polynaphthylene oxide structure and an arylcarbonyloxy group is preferred, and an active ester compound in which an arylcarbonyloxy group is bonded to the naphthalene nucleus of a polynaphthylene oxide structure is more preferred. The polynaphthylene oxide structure may be a polynaphthylene oxide structure substituted with an alkyl group having 1 to 4 carbon atoms, or may further have a polyphenylene oxide structure.

[0061] Other curing agents for epoxy resins may also be used, such as amine-based resins, acid anhydride-based resins, phenol-based resins, imidazoles, benzoxazine-based resins, and cyanate ester-based resins. These curing agents may be used alone or in combination of two or more. The amount of these curing agents added to the epoxy resin is, in terms of equivalent ratio to the epoxy resin, 0.5 to 1.5, preferably 0.7 to 1.3. In this specification, these curing agents are also included in the term "resin."

[0062] Furthermore, as the silicone-based resin, known curable silicone resins that are mixtures of addition reaction type silicone resins and silicone-based crosslinking agents can be used without limitation. Examples of addition reaction type silicone resins include polyorganosiloxanes such as polydimethylsiloxanes that have alkenyl groups such as vinyl groups or hexenyl groups as functional groups in the molecule. Examples of silicone-based crosslinking agents include polyorganosiloxanes having silicon-bonded hydrogen atoms, such as dimethylhydrogensiloxy group-endblocked dimethylsiloxane-methylhydrogensiloxane copolymers, trimethylsiloxy group-endblocked dimethylsiloxane-methylhydrogensiloxane copolymers, trimethylsiloxane group-endblocked poly(methylhydrogensiloxane), and poly(hydrogensilsesquioxane). Furthermore, as the curing catalyst, known platinum-based catalysts used for curing silicone resins can be used without limitation. Examples include fine particle platinum, fine particle platinum supported on carbon powder, chloroplatinic acid, alcohol-modified chloroplatinic acid, olefin complexes of chloroplatinic acid, palladium, and rhodium catalysts.

[0063] Furthermore, the resin may contain a thermoplastic resin. The thermoplastic resin can improve the mechanical strength of the cured resin and can also improve the film formability when used in the form of an adhesive film. Examples of thermoplastic resins include phenoxy resins, polyvinyl acetal resins, polyimide resins, polyamideimide resins, polyetherimide resins, polysulfone resins, polyethersulfone resins, polyphenylene ether resins, polycarbonate resins, polyetheretherketone resins, and polyester resins, with phenoxy resins and polyvinyl acetal resins being particularly preferred. In this specification, these thermoplastic resins are also included in the term "resin."

[0064] The blending ratio of the resin to the hexagonal boron nitride powder in the resin composition according to one embodiment of the present invention may be determined appropriately depending on the application. For example, the hexagonal boron nitride powder may be blended in an amount of preferably 30 to 90% by volume, more preferably 40 to 80% by volume, and even more preferably 50 to 70% by volume of the total resin composition.

[0065] The resin composition may contain components other than hexagonal boron nitride and resin, such as inorganic fillers, curing accelerators, discoloration inhibitors, surfactants, dispersants, coupling agents, colorants, plasticizers, viscosity modifiers, and antibacterial agents, as appropriate, within limits that do not affect the effects of the present invention.

[0066] Examples of the curing accelerator include organic phosphine compounds, imidazole compounds, amine adduct compounds, amine compounds, organometallic complexes, and organometallic salts. The curing accelerators may be used alone or in combination of two or more.

[0067] Examples of uses of the resin composition according to one embodiment of the present invention include sheet-like laminate materials (resin sheets) such as adhesive films and prepregs, circuit boards (for laminate boards and multilayer printed wiring boards), solder resists, underfill materials, thermal adhesives, die bonding materials, semiconductor encapsulation materials, hole filling resins, component embedding resins, thermal interface materials (sheets, gels, greases, etc.), power module substrates, and heat dissipation members for electronic components.

[0068] The sheet-like laminate material (resin sheet) can also be used as an insulating layer for a multilayer printed wiring board. To increase the number of layers and density of the build-up layers of a multilayer printed wiring board, it is common to form a conductor layer on the surface of the insulating layer of the multilayer printed wiring board by roughening treatment followed by electroless plating, or by electroless plating and electrolytic plating. A preferred example of such a sheet-like laminate material is a resin composition containing at least (i) an epoxy resin and (ii) an active ester compound containing a naphthalene structure as the resin.

[0069] The active ester compound containing a naphthalene structure not only functions as a curing agent for epoxy resins, but is also thought to contribute to the formation of fine irregularities on the surface of a cured resin that have good adhesion to a plating layer when the surface of the cured resin is roughened. The cured resin is a cured resin composition. Therefore, this resin composition exhibits a low dielectric loss tangent and forms a roughened surface that has excellent adhesion to a plating layer, resulting in an interlayer insulating sheet that forms an insulating layer with well-balanced properties that can accommodate the increased layering and density of build-up layers in multilayer printed wiring boards. Furthermore, by including hexagonal boron nitride powder in this resin composition, an interlayer insulating sheet with sufficiently high thermal conductivity can be realized. [Example]

[0070] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Test methods are as follows.

[0071] <Crushing by Mortar Mill> The hexagonal boron nitride raw material powder was crushed dry using a mortar mill (manufactured by Masuko Sangyo Co., Ltd.: Super Mass Colloid Mill MKCA6-5J, grinding wheel material: alumina, grinding wheel diameter: 150 mm) at a predetermined rotational speed and grinding wheel interval. Thereafter, washing with dilute hydrochloric acid was performed as an acid washing step to obtain hexagonal boron nitride powder.

[0072] <Measurement of Aspect Ratio> Scanning electron microscope images of the hexagonal boron nitride raw material powder or the hexagonal boron nitride powder at a magnification of 5000 times were obtained using a FE-SEM (manufactured by Hitachi High-Technologies Corporation: S5500). From the obtained scanning electron microscope images, the length (L) in the width direction and the length (t) in the thickness direction of the hexagonal boron nitride particles were measured, and the aspect ratio (L / t) of the particles was calculated. The above aspect ratio was calculated for 300 randomly extracted particles, and from the results, the content ratio of the hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0 was calculated. In the measurement of the hexagonal boron nitride raw material powder used for crushing by the mortar mill, when the lithium salt removal step was not performed in the preparation process, a part of the sample was collected, washed with dilute hydrochloric acid, and then the above aspect ratio was measured. In addition, the images used for the measurement of the aspect ratio were observed, and the presence or absence of aggregates containing hexagonal boron nitride particles having an aspect ratio exceeding 5.0 in the hexagonal boron nitride raw material powder was also evaluated.

[0073] <Measurement of D10, D50, and D90> The particle size distributions of the hexagonal boron nitride raw powder and the hexagonal boron nitride powder were measured using a laser diffraction / scattering particle size distribution analyzer (MT3000, manufactured by Nikkiso Co., Ltd.), and the D10, D50, and D90 values ​​were calculated from the results. The measurement samples were prepared as follows. First, 20 g of ethanol was added to a 50 mL screw-top vial as a dispersion medium, and 1 g of the powder to be measured was dispersed in the ethanol. Next, using an ultrasonic homogenizer (SONIFIER SFX250, manufactured by Branson) with the tip positioned 10 mm from the bottom of the screw-top vial, the sample was sonicated at 40% amplitude for 20 minutes. The particle size distribution of the sonicated sample was then measured. When measuring the hexagonal boron nitride raw powder to be crushed using a stone mill, if the preparation process did not include a lithium salt removal step, a portion of the sample was taken and washed with dilute hydrochloric acid before the D10, D50, and D90 measurements were performed.

[0074] <Measurement of specific surface area> Measurement was performed using a BET specific surface area meter (Macsorb HM model-1201 manufactured by Mountec Co., Ltd.) When measuring the hexagonal boron nitride raw material powder used for crushing with a stone mill, if the lithium salt removal step was not performed in the preparation process, a portion of the sample was taken and washed with dilute hydrochloric acid before measuring the specific surface area.

[0075] <Measurement of resin filling viscosity> A resin composition was prepared by filling 20% ​​by volume of hexagonal boron nitride powder into a silicone resin (CY52-276A manufactured by Dow Toray Industries, Inc.). The viscosity was measured at a measurement temperature of 25°C and a shear rate of 1 / s using a rheometer (AR2000ex manufactured by TA Instruments), and this viscosity was taken as the resin filling viscosity.

[0076] <Measurement of plane orientation index> A mixture of 100 parts by weight of epoxy resin (JER806 manufactured by Mitsubishi Chemical Corporation) and 28 parts by weight of curing agent (alicyclic polyamine-based curing agent, JER Cure 113 manufactured by Mitsubishi Chemical Corporation) was prepared as the base resin. Next, 40% by volume of the base resin and 60% by volume of hexagonal boron nitride powder were mixed using methyl ethyl ketone as a solvent, and the solvent was then dried to obtain a resin composition. The dried resin composition was then poured into a mold and cured using a heat press under conditions of a temperature of 150°C, a pressure of 5 MPa, and a holding time of 1 hour to produce a sheet with a diameter of 10 mm and a thickness of 0.15 mm. XRD measurement was performed on the obtained resin sheet using a fully automatic horizontal multipurpose X-ray diffractometer (SmartLab manufactured by Rigaku Corporation) by irradiating X-rays parallel to the thickness direction of the resin sheet. The peak area (I) of the 002 plane derived from the obtained hexagonal boron nitride primary particles was measured. 002 ) and the peak area of ​​the 100 plane (I 100 ) from the following equation Planar orientation index=log((I 002 / I 100 ) / 6.67) The planar orientation index of the resin composition was calculated using the above method. 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 closer the planar orientation index is to 0, the less oriented the hexagonal boron nitride particles are and the smaller the thermal conductivity anisotropy is.

[0077] Example 1 500 g (20.1 mol) of commercially available hexagonal boron nitride and 500 g (6.8 mol) of lithium carbonate powder were placed in a crucible and heat-treated at 1400 °C for 5 hours under a nitrogen flow. After cooling, the sample was washed with dilute hydrochloric acid to dissolve and remove the flux as a lithium salt removal process. The sample was then filtered, washed with water, and dried to obtain a hexagonal boron nitride raw powder. 150 g of the resulting hexagonal boron nitride raw powder, which contained hexagonal boron nitride particles with an aspect ratio of 1.5 to 5.0 and agglomerates containing hexagonal boron nitride particles with an aspect ratio greater than 5.0, was crushed in a stone mill at a rotation speed of 2200 rpm with a grinding wheel gap of 20 μm to obtain hexagonal boron nitride powder. The content of hexagonal boron nitride particles with an aspect ratio of 1.5 to 5.0, the specific surface area, D10, D50, D90, and resin filling viscosity of the obtained hexagonal boron nitride powder were measured. The obtained boron nitride powder was then used to prepare a resin composition, and the thermal conductivity and plane orientation index of the resin composition were measured. The physical properties of the hexagonal boron nitride raw material powder and the conditions for the crushing process are shown in Table 1, the evaluation results of the obtained hexagonal boron nitride powder are shown in Table 2, and the evaluation results of the resin composition are shown in Table 3.

[0078] <Examples 2 and 3, Comparative Example 2> Hexagonal boron nitride powder and a resin composition were obtained and their physical properties were evaluated in the same manner as in Example 1, except that the conditions for the crushing process using the stone mill were changed as shown in Table 1. The evaluation results are shown in Tables 2 and 3.

[0079] Example 4 500 g (20.1 mol) of commercially available hexagonal boron nitride and 500 g (6.8 mol) of lithium carbonate powder were placed in a crucible and heat-treated at 1400°C for 5 hours under a nitrogen flow. After cooling the sample, the lithium salt removal step was not performed. 150 g of the resulting hexagonal boron nitride raw powder containing the lithium salt was used and crushed in a stone mill at a rotation speed of 2200 rpm and a grinding wheel spacing of 20 μm to obtain hexagonal boron nitride powder. The physical properties of the resulting hexagonal boron nitride powder were evaluated in the same manner as in Example 1. Furthermore, hexagonal boron nitride powder and a resin composition were obtained in the same manner as in Example 1, and their physical properties were evaluated. The physical properties of the hexagonal boron nitride raw powder and the crushing process conditions are shown in Table 1, the evaluation results of the resulting hexagonal boron nitride powder are shown in Table 2, and the evaluation results of the resin composition are shown in Table 3.

[0080] <Comparative Example 1> A resin composition was produced using the hexagonal boron nitride raw material powder before the crushing step prepared in Example 1, and the thermal conductivity of the resin composition was evaluated. The physical properties of the hexagonal boron nitride powder are shown in Table 2, and the evaluation results of the resin composition are shown in Table 3. [Table 1] [Table 2]

[0081] [Table 3] In Tables 1 and 2, aggregates containing hexagonal boron nitride particles with an aspect ratio of more than 5.0 were observed in all of the hexagonal boron nitride raw material powders. In addition, in all of the hexagonal boron nitride raw material powders and hexagonal boron nitride powders, hexagonal boron nitride particles with an aspect ratio of less than 1.5 were not observed. In Table 1, the lithium salt removal step column indicates that the lithium salt removal step was performed before the crushing step, with a "Y" and that the crushing step was performed without the lithium salt removal step, with an "N."

[0082] The evaluation results showed that Examples 1 to 4, in which the crushing process of the present invention was carried out, had lower resin viscosity and DBP absorption amounts for all hexagonal boron nitride powders, and improved fillability in resin, compared to Comparative Example 1, in which the crushing process was not carried out, and Comparative Example 2, in which the crushing process involved pulverization of primary particles.

[0083] In addition, in Examples 1 to 4, the proportion of hexagonal boron nitride particles with an aspect ratio of 1.5 to 5.0 hardly changed before and after the crushing process, and the plane orientation index was close to 0. B / S A In Comparative Example 2, in which the crushing step involved pulverization of primary particles, the ratio of hexagonal boron nitride particles with an aspect ratio of 1.5 to 5.0 decreased, and the plane orientation index became farther from 0. This demonstrates that crushing without pulverization of primary particles can produce hexagonal boron nitride powder with low thermal conductivity anisotropy and high packing ability.

[0084] <Evaluation of Resin Composition Sheet Formation> The hexagonal boron nitride powders of Examples 1 to 4 were mixed with base resins to prepare sheet-shaped adhesive films as resin compositions, and the adhesive films were evaluated for their sheetability. Five types of resins A to E were used as base resins.

[0085] The composition of Resin A is shown in Table 4. 15 parts by mass of liquid bisphenol A epoxy resin (epoxy equivalent 180, "828US" manufactured by Mitsubishi Chemical Corporation) and 15 parts by mass of biphenyl epoxy resin (epoxy equivalent 291, "NC3000H" manufactured by Nippon Kayaku Co., Ltd.) were added to a mixed solution of 15 parts by mass of methyl ethyl ketone (hereinafter referred to as "MEK") and 15 parts by mass of cyclohexanone, and the mixed solution was heated with stirring to dissolve, thereby obtaining mixed solution A. Then, the dissolved mixed solution A was mixed with 43 parts by mass of a naphthalene-type active ester compound ("EXB9411-65BK" manufactured by DIC Corporation, active ester equivalent 272, toluene solution with solid content of 65%), 0.15 parts by mass of a curing accelerator ("4-dimethylaminopyridine" manufactured by Koei Chemical Industry Co., Ltd.), and 15 parts by mass of a phenoxy resin (YL6954BH30, MEK solution with solid content of 30% by mass, weight-average molecular weight of 40,000), to obtain resin A.

[0086] [Table 4] The composition of Resin B is shown in Table 5. 15 parts by mass of liquid bisphenol A epoxy resin ("828US" manufactured by Mitsubishi Chemical Corporation, epoxy equivalent weight 190) and 15 parts by mass of biphenyl ル type 15 parts by mass of epoxy resin (epoxy equivalent weight 291, Nippon Kayaku Co., Ltd., "NC3000H") was added to a mixed solution of 5 parts by mass of MEK and 5 parts by mass of cyclohexanone, and the mixture was stirred and heated to dissolve, yielding mixed solution B. The dissolved mixed solution B was then mixed with 25 parts by mass of a solution in which a triazine structure-containing active ester compound (DIC Corporation, "EXB9510," active ester equivalent weight 214) was dissolved in MEK to a nonvolatile content of 60%, 0.1 parts by mass of a curing accelerator (Koei Chemical Co., Ltd., "4-dimethylaminopyridine"), and 15 parts by mass of a phenoxy resin (YX6954BH30, MEK and cyclohexanone solution with a solids content of 30% by mass, weight-average molecular weight 35,000), yielding resin B.

[0087] [Table 5] The composition of Resin C is shown in Table 6. 10 parts by mass of a bisphenol-type epoxy resin ("ZX1059" manufactured by Nippon Steel Chemical Co., Ltd., a 1:1 mixture of bisphenol A type and bisphenol F type, epoxy equivalent weight 169), 10 parts by mass of a crystalline bifunctional epoxy resin ("YX4000HK" manufactured by Mitsubishi Chemical Corporation, epoxy equivalent weight 185), and 20 parts by mass of a dicyclopentadiene-type epoxy resin ("HP-7200H" manufactured by DIC Corporation, epoxy equivalent weight 275) were dissolved in 40 parts by mass of solvent naphtha with stirring and heating to obtain mixed solution C.

[0088] Next, epoxy resin A was synthesized. The composition of epoxy resin A is shown in Table 7. 222 g of bixylenol-type epoxy resin (YX4000 manufactured by Mitsubishi Chemical Corporation, epoxy equivalent weight 185), 15 g of bisphenolacetophenone (phenolic hydroxyl group equivalent weight 145), 170 g of biscresolfluorene (manufactured by JFE Chemical Corporation, phenolic hydroxyl group equivalent weight 190), and 150 g of cyclohexanone were placed in a reaction vessel and stirred to dissolve. Subsequently, 0.5 g of tetramethylammonium chloride solution was added dropwise, and the mixture was allowed to react under a nitrogen atmosphere. Epoxy resin A was obtained by diluting with a solvent.

[0089] Then, mixed solution C cooled to room temperature (25°C) with 12 parts by weight of epoxy resin A, 12 parts by weight of triazine skeleton-containing phenolic resin (curing agent, DIC Corporation's "LA-7054" MEK solution with a hydroxyl group equivalent of 125 and a solids content of 60%), 15 parts by weight of naphthalene-type curing agent (Nippon Steel Chemical Co., Ltd.'s "SN-485" MEK solution with a hydroxyl group equivalent of 215 and a solids content of 60%), 3 parts by weight of curing accelerator (4-dimethylaminopyridine, MEK solution with a solids content of 2% by weight), and 2 parts by weight of flame retardant (Sanko Co., Ltd.'s "HCA-HQ" 10-(2,5-dihydroxyphenyl)-10-hydro-9-oxa-10-phosphaphenanthrene-10-oxide, average particle size of 2 μm) to obtain resin C. [Table 6]

[0090] [Table 7] The composition of Resin D is shown in Table 8. Five parts by mass of a liquid naphthalene-type epoxy resin (epoxy equivalent 144, "HP4032SS" manufactured by DIC Corporation), five parts by mass of a crystalline bifunctional epoxy resin ("YX4000HK" manufactured by Mitsubishi Chemical Corporation, epoxy equivalent approximately 185), and 12 parts by mass of a biphenyl-type epoxy resin ("NC3000L" manufactured by Nippon Kayaku Co., Ltd., epoxy equivalent 269) were heated and dissolved in 30 parts by mass of solvent naphtha with stirring to obtain mixed solution D.

[0091] Next, phenoxy resin A was synthesized. Table 9 shows the composition of phenoxy resin A. 191 g of tetramethylbiphenyl-type epoxy resin (YX4000 manufactured by Mitsubishi Chemical Corporation, epoxy equivalent 185), 210 g of 9,10-dihydroxyanthracene (phenolic hydroxyl group equivalent 210), and 150 g of cyclohexanone were placed in a reaction vessel and stirred to dissolve. Subsequently, 0.5 g of tetramethylammonium chloride solution was added dropwise to the solution, and the mixture was allowed to react under a nitrogen atmosphere. Phenoxy resin A was obtained by diluting with a solvent.

[0092] Then, mixed solution D cooled to room temperature (25°C), 5 parts by mass of phenoxy resin A, 20 parts by mass of bisphenol A dicyanate prepolymer ("BA230S75" manufactured by Lonza Japan Co., Ltd., cyanate equivalent of about 232, MEK solution with 75% by mass of non-volatile content), 6 parts by mass of phenol novolac type multifunctional cyanate ester resin ("PT30S" manufactured by Lonza Japan Co., Ltd., cyanate equivalent of about 133, MEK solution with 85% by mass of non-volatile content), and curing accelerator (4-dimethylaminopyridine, solid content 2 parts by mass). Resin D was prepared by mixing 1 part by mass of a curing accelerator (Tokyo Chemical Industry Co., Ltd., cobalt(III) acetylacetonate (Co(III)Ac)), MEK solution with a solids content of 1% by mass), 3 parts by mass of a curing accelerator (Tokyo Chemical Industry Co., Ltd., cobalt(III) acetylacetonate (Co(III)Ac)), MEK solution with a solids content of 1% by mass), 2 parts by mass of rubber particles (Ganz Chemical Industry Co., Ltd., Staphyloid AC3816N), and 2 parts by mass of a flame retardant (Sanko Co., Ltd., "HCA-HQ", 10-(2,5-dihydroxyphenyl)-10-hydro-9-oxa-10-phosphaphenanthrene-10-oxide, average particle size 2 μm). [Table 8]

[0093] [Table 9] The composition of Resin E is shown in Table 10. 15 parts by mass of liquid bisphenol A epoxy resin (epoxy equivalent 180, "jER828EL" manufactured by Japan Epoxy Resins Co., Ltd.) and 15 parts by mass of biphenyl epoxy resin (epoxy equivalent 291, "NC3000H" manufactured by Nippon Kayaku Co., Ltd.) were heated and dissolved in 15 parts by mass of MEK and 15 parts by mass of cyclohexanone with stirring to obtain mixed solution E. Then, the dissolved mixed solution E was mixed with 20 parts by mass of a naphthalene-type active ester compound ("EXB9460S-65T" manufactured by DIC Corporation, active ester equivalent 223, toluene solution with a solid content of 65%), 6 parts by mass of a triazine-containing cresol novolak resin ("LA3018-50P" manufactured by DIC Corporation, 2-methoxypropanol solution with a phenol equivalent of 151 and a solid content of 50%), 0.05 parts by mass of a curing accelerator ("4-dimethylaminopyridine" manufactured by Koei Chemical Industry Co., Ltd.), and 7 parts by mass of a phenoxy resin (YL6954BH30, a 1:1 solution of MEK and cyclohexanone with a nonvolatile content of 30% by weight, a weight-average molecular weight of 40,000), to obtain resin E.

[0094] [Table 10] The hexagonal boron nitride powders of Examples 1 to 4 were mixed with Resin A or Resin B in amounts of 50 parts by mass, 80 parts by mass, or 115 parts by mass, respectively, and uniformly dispersed in a high-speed rotary mixer to produce each resin varnish. Each resin varnish was then applied to polyethylene terephthalate (38 μm thick) using a die coater so that the resin thickness after drying was 40 μm, and the resulting material was dried at 80 to 120°C (average 100°C) for 6 minutes to obtain sheet-like adhesive films (Examples 5 to 28). Similarly, the hexagonal boron nitride powders of Comparative Examples 1 and 2 were mixed with Resin A or Resin B to obtain sheet-like adhesive films (Comparative Examples 3 to 14).

[0095] In addition, 50 parts by mass, 80 parts by mass, and 115 parts by mass of the hexagonal boron nitride powder of Examples 1 to 4, which had been surface-treated with a phenylaminosilane coupling agent (manufactured by Shin-Etsu Chemical Co., Ltd., "KBM573"), were mixed with Resin C or Resin D, respectively, and uniformly dispersed in a high-speed rotary mixer to prepare each resin varnish. Next, each resin varnish was uniformly applied to the release surface of a polyethylene terephthalate film ("AL5" manufactured by Lintec Corporation, thickness 38 μm) so that the thickness of the resin composition layer after drying would be 30 μm, and the film was dried at 80 to 120°C (average 100°C) for 4 minutes to obtain sheet-shaped adhesive films (Examples 29 to 52). Similarly, the hexagonal boron nitride powder of Comparative Examples 1 and 2 was mixed with Resin C or Resin D to obtain sheet-shaped adhesive films (Comparative Examples 15 to 26).

[0096] Furthermore, 50 parts by mass, 80 parts by mass, and 115 parts by mass of the hexagonal boron nitride powders of Examples 1 to 4 were mixed with Resin E, respectively, and uniformly dispersed using a high-speed rotary mixer to prepare each resin varnish. Next, each resin varnish was applied to polyethylene terephthalate (38 μm thick) using a die coater so that the resin thickness after drying was 40 μm, and then dried at 80 to 120°C (average 100°C) for 6 minutes (residual solvent content: approximately 2 wt%). A 15 μm thick polypropylene film was then laminated to the surface of the resin composition, and the resulting film was wound into a roll. The rolled adhesive film was slit to a width of 507 mm, and sheet-shaped adhesive films measuring 507 × 336 mm (Examples 53 to 64) were obtained from this. Similarly, Resin E was mixed with the hexagonal boron nitride powders of Comparative Examples 1 and 2 to obtain sheet-shaped adhesive films (Comparative Examples 27 to 32).

[0097] The sheet-like adhesive film obtained was visually evaluated for sheet quality. When a smooth and uniform surface was formed in appearance, it was judged to be good, and when there were irregularities, liquid repelling, streaks, or other defects in appearance, it was judged to be bad.

[0098] <Result> Tables 11 to 13 show the results of sheet formation for the sheet-like adhesive films in Examples 5 to 64 and Comparative Examples 3 to 32. It was demonstrated that sheet formation was possible for all resin compositions when the hexagonal boron nitride powders of Examples 1 to 4 were used. When the hexagonal boron nitride powder of Comparative Example 1 was used, sheet formation was possible when the blending amount of boron nitride powder was 50 parts by mass or 80 parts by mass, but at 115 parts by mass, liquid repellency and streaks occurred during application with a die coater, resulting in poor appearance and making it impossible to form a sheet. When the hexagonal boron nitride powder of Comparative Example 2 was used, sheet formation was possible when the blending amount of boron nitride powder was 50 parts by mass, but at 80 parts by mass or 115 parts by mass, liquid repellency and streaks occurred during application with a die coater, resulting in poor appearance and making it impossible to form a sheet. Therefore, it was demonstrated that sheet-like adhesive films with no problems in appearance can be produced using the hexagonal boron nitride powder of the present invention. [Table 11] [Table 12] [Table 13]

Claims

1. A method for producing hexagonal boron nitride powder, comprising crushing a hexagonal boron nitride raw material powder containing hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0 and aggregates containing hexagonal boron nitride particles having an aspect ratio of more than 5.0, using a millstone grinder in a manner that does not substantially involve crushing of primary particles.

2. 2. The method for producing hexagonal boron nitride powder according to claim 1, wherein the hexagonal boron nitride raw material powder contains hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0 in a proportion of 20% by number or more and 90% by number or less.

3. 3. The method for producing a hexagonal boron nitride powder according to claim 1, wherein the hexagonal boron nitride raw material powder has a D50 of 0.2 to 10.0 μm and a D90 / D10 ratio of more than 3.

0.

4. 4. The method for producing hexagonal boron nitride powder according to claim 1, wherein the crushing is performed so that the D90 / D10 of the hexagonal boron nitride powder is 3.0 or less.

5. A hexagonal boron nitride powder containing hexagonal boron nitride particles having an aspect ratio of 1.5 to 5.0 at a rate of 20% or more by number, having a D50 of 0.2 to 3.6 μm, a D10 of 1.5 μm or more, a D90 / D10 of 3.0 or less, a DBP absorption of 80 mL / 100 g or less as calculated from a curve measured in accordance with JIS-K-6217-4:2017, with the horizontal axis representing the DBP drop amount (mL) and the vertical axis representing the torque (N·m), and a ratio D50 / D BET of the particle diameter D BET calculated from the specific surface area to the D50, which is 4.0 or less.

6. A resin composition comprising the hexagonal boron nitride powder according to claim 5 and a resin.

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