Curable resin composition

Abstract

To provide a curable resin composition which enables production of a resin composition having high peel strength.SOLUTION: A curable resin composition contains an epoxy group-containing compound, a benzoxazine ring structure-containing compound and hexagonal boron nitride powder, wherein the hexagonal boron nitride powder has an average particle diameter D50 determined by particle size distribution of 3-30 μm, and a ratio (D50 / d) of an average particle diameter d determined by a BET specific surface area to D50 of 19 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a novel curable resin composition. [Background technology]

[0002] In fields such as high-performance computers and smartphones, power semiconductors are used to efficiently utilize electricity, but part of the electrical energy is converted into heat, which can cause failures in electronic devices. For this reason, resin compositions containing resin components and highly thermally conductive inorganic fillers (hereinafter sometimes referred to as "thermal dissipation fillers") are widely used as heat dissipation materials with excellent heat dissipation properties.

[0003] In recent years, there has been an increasing demand for improved thermal conductivity in heat dissipation materials, and hexagonal boron nitride, which has high thermal conductivity, is used as a heat dissipation filler to impart high thermal conductivity to resin compositions. Hexagonal boron nitride is a white powder that generally has a hexagonal layered structure similar to graphite, and has many properties such as high thermal conductivity, high electrical insulation, high temperature stability, and chemical stability.

[0004] The resin composition is usually obtained by curing a curable resin composition containing a filler. Compositions containing epoxy group-containing compounds are widely used as curable resin compositions, and curing agents such as amines or acid anhydrides are generally blended to cure the epoxy group-containing compounds. Therefore, hydrophilic impurities such as amines or acid anhydrides, which are residues of the curing agents, may be generated in the cured resin composition, resulting in a deterioration of the physicochemical properties. Benzoxazine ring structure-containing compounds are known as curing agents that generate less of the above-mentioned impurities, and curable resin compositions using such compounds are also known.

[0005] For example, Patent Document 1 discloses a curable resin composition containing a benzoxazine ring structure-containing compound, an epoxy group-containing compound, and an inorganic filler. The inorganic filler contains hexagonal boron nitride, specifically, pinecone-shaped boron nitride, which is an aggregate of scale-like primary particles. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2011-231196 Summary of the Invention [Problem to be solved by the invention]

[0007] As mentioned above, amine-based or acid anhydride-based curing agents commonly used to polymerize and cure epoxy-containing compounds produce hydrophilic impurities in the cured resin composition. It has been found that the presence of these impurities adversely affects the compatibility of the heat-dissipating filler with the resin, due to the hydrophobicity of hexagonal boron nitride, which is used as a heat-dissipating filler, and can result in the formation of bubbles in the resin composition. The presence of bubbles in a resin composition can lead to a deterioration in physical properties. For example, in a sheet laminated with a metal foil such as copper and a resin composition, unevenness or cracks can occur on the surface of the resin composition, reducing the peel strength between the resin composition and the metal foil, resulting in the problem of the metal foil peeling off from the resin composition.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a curable resin composition that can provide a resin composition having high peel strength. [Means for solving the problem]

[0009] The present inventors believed that the use of a compound containing a benzoxazine ring structure would suppress the generation of hydrophilic impurities that are poorly compatible with hydrophobic hexagonal boron nitride, improving compatibility and suppressing bubble formation. However, even when using the resin components described above, bubble formation still occurred in some cases. As a result of further intensive research, the inventors discovered that the characteristics of hexagonal boron nitride are also important for suppressing bubble formation, and that the use of hexagonal boron nitride with a specific particle size and properties close to single particles can highly suppress bubble formation, leading to the completion of the present invention. That is, the present invention includes the following:

[0010] 1. A curable resin composition comprising an epoxy group-containing compound, a benzoxazine ring structure-containing compound, and hexagonal boron nitride powder, characterized in that the hexagonal boron nitride powder has an average particle size D50 determined from particle size distribution of 3 μm to 30 μm, and the ratio (D50 / d) of the average particle size d determined from the BET specific surface area to the D50 is 19 or less.

[0011] 2. The curable resin composition according to item 1 above, wherein the content of the benzoxazine ring structure-containing compound is 1.0 mol to 5.0 mol per 1 mol of epoxy groups in the curable resin composition, and the amount of the hexagonal boron nitride powder blended is 55 to 75% by volume of the entire curable resin composition. [Effects of the Invention]

[0012] The curable resin composition of the present invention provides a resin composition that can exhibit high peel strength, which makes it easy to obtain, for example, highly reliable integrated circuit boards and printed circuit boards for heat dissipation of electronic components. DETAILED DESCRIPTION OF THE INVENTION

[0013] The curable resin composition of the present invention contains an epoxy group-containing compound, a benzoxazine ring structure-containing compound, and hexagonal boron nitride powder. The materials constituting this embodiment will be described in detail below.

[0014] [Epoxy group-containing compound] The epoxy group-containing compound used in the present invention is not particularly limited as long as it has one or more epoxy groups in the molecule. Examples include bisphenol A epoxy resins, bisphenol S epoxy resins, bisphenol F epoxy resins, hydrogenated bisphenol A epoxy resins, polypropylene glycol epoxy resins, polytetramethylene glycol epoxy resins, naphthalene epoxy resins, phenylmethane epoxy resins, tetrakisphenolmethane epoxy resins, biphenyl epoxy resins, epoxy resins having a triazine nucleus in the skeleton, and bisphenol A alkylene oxide adduct epoxy resins. These epoxy resins may be used alone or in combination of two or more.

[0015] [Benzoxazine ring structure-containing compounds] The benzoxazine ring structure-containing compound used in the present invention is not particularly limited as long as it has one or more benzoxazine ring structures in the molecule, but compounds containing two or more benzoxazine ring structures in the molecule are preferred because they form a crosslinked structure after curing, allowing a resin composition with excellent chemical stability to be obtained.

[0016] A benzoxazine ring is a molecular structure in which two carbon atoms of oxazine are shared with a benzene ring, as shown in the following formula 1. Note that the hydrogen atoms of the benzene ring or oxazine ring may be substituted with other functional groups.

[0017] [ka]

[0018] Typical compounds containing two benzoxazine ring structures in the molecule include a Pd-type benzoxazine ring structure-containing compound represented by the following formula 2 and a Fa-type benzoxazine ring structure-containing compound represented by the following formula 3. In particular, a Pd-type benzoxazine ring structure-containing compound is preferred because, after curing, it is crosslinked by a rigid structure, resulting in a structure with a small free volume, and it is easy to obtain excellent thermal stability and mechanical properties.

[0019] [ka]

[0020] In the formula, R1 and R2 are each independently -H, -CH3, or -C(CH3)3, preferably -H, and X is -CH2-, -C(CH3)2-, -SO2-, -O-, or -O-Ph-C(CH3)2-Ph-O-, preferably -CH2-.

[0021] [ka]

[0022] In the formula, R1 and R2 are each independently -H, -CH3, or -C(CH3)3, preferably -H, and X is -CH2-, -C(CH3)2-, -SO2-, -O-, or -O-Ph-C(CH3)2-Ph-O-, preferably -CH2-.

[0023] In the curable resin composition of the present invention, the content of the benzoxazine ring structure-containing compound is preferably 1.0 mol to 5.0 mol, more preferably 2.0 mol to 3.5 mol, per mol of epoxy groups in the epoxy group-containing compound in the curable resin composition. When the content of the benzoxazine ring structure-containing compound is greater than 5.0 mol per mol of epoxy groups in the epoxy group-containing compound, the resin composition tends to soften at temperatures above the glass transition point of the benzoxazine ring structure-containing compound. Since the glass transition point of the benzoxazine ring structure-containing compound is usually lower than the heating temperature used to cure the curable resin composition, the resin composition may soften during the curing step described below, making sheet formation difficult. Furthermore, when the content of the benzoxazine ring structure-containing compound is less than 1.0 mol per mol of epoxy groups, curing may be insufficient.

[0024] [Hexagonal boron nitride powder] The hexagonal boron nitride powder of the present invention is used as a heat dissipating filler. In the present invention, the hexagonal boron nitride powder has a particle size D50 determined from particle size distribution of 3 μm to 30 μm, and a ratio (D50 / d) of the particle size d determined from the BET specific surface area to the D50 is 19 or less.

[0025] <Particle size D50 calculated from particle size distribution> The hexagonal boron nitride powder used in the present invention has a particle size D50 determined from particle size distribution of 3 μm to 30 μm. By setting D50 to 30 μm or less, high dispersibility is achieved during mixing, allowing the hexagonal boron nitride powder to be uniformly present in the resin composition, resulting in a resin composition with high peel strength. D50 is preferably 22 μm or less. Furthermore, by setting D50 to 3 μm or more, the thermal conduction distance between particles becomes appropriate, tending to improve the thermal conductivity of the resin composition. The average particle size is preferably 10 μm or more, and more preferably 15 μm or more. Note that D50 is the particle size value at which the cumulative volume frequency is 50% in the volume frequency distribution obtained by particle size distribution measurement. The particle size distribution of the hexagonal boron nitride powder can be measured by the method described in the Examples.

[0026] <d50 d> The hexagonal boron nitride powder of the present invention has an average particle size d determined from the BET specific surface area and a ratio of D50 to d (D50 / d) of 19 or less. D50 is measured as the particle size including agglomerated particles. On the other hand, the average particle size d determined from the BET specific surface area is not affected by the influence of agglomerated particles and depends on the particle size of the hexagonal boron nitride primary particles. Therefore, a small D50 / d means that there are few agglomerated particles, and hexagonal boron nitride exists as single particles in the powder, indicating properties close to single particles.

[0027] In hexagonal boron nitride agglomerated particles, there are many voids between particles. When blended into a resin composition, bubbles derived from the voids between particles will occur. However, when it exists as single particles and there are few agglomerated particles, it is presumed that the generation of bubbles derived from the voids between particles will be reduced, and high peel strength can be obtained. The lower limit value of D50 / d is not particularly limited, but generally it is 1 or more, especially 5 or more.

[0028] The particle size d determined from the BET specific surface area is obtained from the following formula.

[0029] d = 6 / (ρ·SA) Here, ρ represents the density of hexagonal boron nitride, which is 2.27 g / m in the present invention 3 is used. SA is the BET specific surface area (m 2 / g) of the hexagonal boron nitride powder, and it can be measured by the method described in the examples.

[0030] <BET specific surface area> The BET specific surface area of the hexagonal boron nitride powder of the present invention is preferably 1.0 m 2 / g to 2.5 m 2 / g, more preferably 1.3 m 2 / g to 2.0 m 2 / g. By setting the BET specific surface area within the above range, it becomes easier to control D50 to 3 μm to 30 μm. Also, by setting the BET specific surface area to 1.0 m 2 / g or more, it is easy to obtain high peel strength. By setting the BET specific surface area to 2.5 m 2 By making the SiO2 content less than 1 / g, high thermal conductivity is easily obtained.

[0031] <Aspect ratio> The hexagonal boron nitride powder of the present invention preferably has an average aspect ratio in the range of 1 to 10. Since the thermal conductivity of hexagonal boron nitride differs greatly between the ab-axis direction and the c-axis direction, it may become oriented in a resin composition, increasing the thermal conductivity anisotropy of the resin composition, but if the average aspect ratio is within the above range, it becomes easy to reduce the thermal conductivity anisotropy of the resin composition.

[0032] <Amount of hexagonal boron nitride powder> In the curable resin composition of the present invention, the content of the hexagonal boron nitride powder is preferably 55% by volume to 75% by volume, and more preferably 60% by volume to 70% by volume, of the entire curable resin composition. By setting the content of the hexagonal boron nitride powder to 55% by volume or more, it becomes easy to increase the thermal conductivity of the resin composition after curing. Furthermore, by setting the content of the hexagonal boron nitride powder to 75% by volume or less, bubbles are less likely to be generated during sheet molding, making it easier to obtain a resin composition with higher peel strength.

[0033] <Method for producing hexagonal boron nitride powder> The method for producing the hexagonal boron nitride powder to be blended in the resin composition of the present invention is not particularly limited. Hexagonal boron nitride is generally produced by a reduction-nitridation method in which an oxygen-containing boron compound is reacted with nitrogen at high temperature, or a melamine method in which melamine borate is thermally decomposed.

[0034] For example, in the reduction nitridation method, a raw material mixture containing an oxygen-containing boron compound, a carbon source, and an oxygen-containing alkaline earth metal is heated to a temperature of 1600 to 2300°C under a nitrogen atmosphere to obtain crude hexagonal boron nitride powder, which is then crushed as necessary, washed with an acidic aqueous solution, and then washed with pure water, and then dried under reduced pressure to remove moisture.

[0035] Examples of the oxygen-containing boron compounds include boron trioxide, diboron dioxide, tetraboron trioxide, metaboric acid, perboric acid, orthoboric acid, sodium tetraborate, and sodium perborate. Examples of carbon sources include carbon black, activated carbon, nanocarbon, and boron carbide. Examples of the oxygen-containing alkaline earth metals include magnesium oxide, calcium oxide, magnesium carbonate, and calcium carbonate. The acidic aqueous solution is not particularly limited, and examples thereof include hydrochloric acid, sulfuric acid, nitric acid, and acetic acid.

[0036] [Other ingredients] The curable resin composition of the present invention may contain other components within the range that does not affect the effects of the present invention, such as inorganic fillers, curing agents, discoloration inhibitors, surfactants, dispersants, coupling agents, colorants, plasticizers, viscosity modifiers, and antibacterial agents.

[0037] In addition to the benzoxazine ring structure-containing compound, the curable resin composition of the present invention may contain a known curing agent for curing epoxy group-containing compounds. Examples of curing agents include amine-based curing agents such as boron trifluoride monoethylamine complex, boron trifluoride diethylamine complex, boron trifluoride isopropylamine complex, boron trifluoride chlorophenylamine complex, boron trifluoride benzylamine complex, and boron trifluoride aniline complex. However, in the present invention, the presence of amine-based curing agent residue in the curable resin composition can reduce compatibility between the resin and the hexagonal boron nitride powder, potentially generating bubbles. Therefore, it is preferable to use a small amount of amine-based curing agent. An amount of amine-based curing agent of 0.5 parts by mass or less, particularly 0.2 parts by mass or less, per 100 parts by mass of the epoxy-containing polymerizable monomer can easily improve curability while suppressing bubble formation. The curable resin composition of the present invention can be cured without the addition of an amine-based curing agent, so the amine-based curing agent may be substantially absent.

[0038] In addition to amine-based curing agents, examples of curing agents include acid anhydride-based curing agents such as methyltetrahydrophthalic anhydride, hexahydrophthalic anhydride, and benzophenonetetracarboxylic anhydride. However, in the present invention, as with amino-based curing agents, the presence of acid anhydride-based curing agent residues in the curable resin composition can lead to poor compatibility between the resin and the hexagonal boron nitride powder, potentially generating bubbles. Therefore, it is preferable to use a small amount of acid anhydride-based curing agent. An amount of acid anhydride-based curing agent of 0.5 parts by mass or less, particularly 0.2 parts by mass or less, per 100 parts by mass of epoxy-containing polymerizable monomer can improve curability while suppressing the generation of bubbles. The curable resin composition of the present invention can be cured without the addition of an acid anhydride-based curing agent, so the acid anhydride-based curing agent may not be substantially added.

[0039] [Method for producing curable resin composition] The method for producing the curable resin composition of the present invention is not particularly limited, and the composition can be prepared by a known method. For example, the curable resin composition can be obtained by weighing each component to be blended in the curable resin composition and mixing them using a mixer.

[0040] [Uses of the curable composition] The curable resin composition can be obtained by molding into a desired shape and then curing. The curable resin composition can be cured by heating to polymerize the epoxy group-containing compound. The heating conditions can be adjusted depending on the composition of the resin composition, and examples include a method of heating at 160 to 240°C for 2 to 5 hours.

[0041] The resin composition of the present invention can be suitably used as a heat dissipation material, and is particularly suitable for a sheet-shaped heat dissipation material in which a metal foil and a resin composition are laminated, which require high peel strength and thermal conductivity. Examples of such sheets include highly integrated circuit boards and printed circuit boards for heat dissipation of electronic components.

[0042] When a sheet-shaped heat dissipation material is obtained, in the method for producing the curable resin composition, a mixing step in which a solvent is used to mix the components to be mixed into the curable resin composition to obtain a low-viscosity mixture is followed by a coating step in which the mixture is formed into a sheet, and a solvent removal step in which the solvent is removed from the sheet-shaped molded product obtained in the coating step, thereby obtaining a curable resin composition molded into a desired shape.The curable resin composition molded into a sheet in this way can be polymerized and cured to obtain a sheet-shaped heat dissipation material.

[0043] Examples of the solvent include ketones such as cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone, and diisobutyl ketone; glycol ethers such as ethylene glycol monomethyl ether and ethylene glycol monobutyl ether; and ether alcohols such as 2-methoxyethanol, 1-methoxyethanol, 2-ethoxyethanol, 1-ethoxy-2-propanol, 2-butoxyethanol, 2-(2-methoxyethoxy)ethanol, 2-(2-ethoxyethoxy)ethanol, and 2-(2-butoxyethoxy)ethanol. These solvents may be used alone or in combination of two or more. Addition of a solvent can improve the dispersibility of the hexagonal boron nitride powder or reduce the viscosity of the mixture, improving coatability.

[0044] The solvent may be mixed simultaneously with the components constituting the curable resin composition, or the solvent may be added after the components constituting the curable resin composition are mixed. The stirring speed and viscosity during mixing are not particularly limited, but it is preferable to adjust them by adding an appropriate solvent so that a uniform coating thickness is obtained without coating unevenness in the subsequent coating step.

[0045] In the coating step, the mixture obtained in the mixing step is coated onto a substrate such as a film or metal foil to a uniform thickness and formed into a sheet, and the coating conditions may be appropriately adjusted to obtain a molded product of the desired shape. In this case, by using metal foil as the substrate, a sheet in which the metal foil and the resin composition are laminated can be obtained.

[0046] The solvent removal step is a step of removing the solvent from the molded body obtained in the coating step, thereby obtaining a curable resin composition molded into a sheet. The method for removing the solvent is not particularly limited; for example, it is preferable to perform ventilation drying at room temperature followed by vacuum drying at a high temperature. The drying temperature and time during vacuum drying may be appropriately selected depending on the solvent used and the types of epoxy group-containing compound and benzoxazine ring-containing compound. If the solvent is not sufficiently removed, bubbles are likely to be generated due to solvent evaporation when the curable resin composition is cured, which may result in a decrease in peel strength. Furthermore, if the drying temperature is too high or the drying time is too long, a portion of the epoxy group-containing compound may react during the solvent removal step, making the heat dissipation material more susceptible to defects such as cracks.

[0047] The sheet-shaped curable resin composition can be cured to obtain a sheet-shaped heat dissipating material. The curing method is not particularly limited, but curing can be performed, for example, by heating and pressurizing at high temperatures. In particular, curing by heating and pressurizing under reduced pressure in a vacuum is preferred to remove gas components from the film formed by coating and obtain a dense resin molded product with high thermal conductivity and dielectric strength. The heating conditions can be, for example, 160°C to 240°C for 2 to 5 hours. In particular, when a curing agent is contained as an additional component, the heating temperature can be 160°C to 200°C. The pressure conditions can be, for example, 10 MPa or higher, particularly 15 to 30 MPa. [Example]

[0048] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples. In the examples and comparative examples, the various values ​​were measured by the following methods.

[0049] (1) Average particle size (D50) of hexagonal boron nitride powder This was determined by a laser diffraction scattering method using an LA-950V2 manufactured by HORIBA Corp. Specifically, 0.3 g of hexagonal boron nitride powder was dispersed in 50 cc of ethanol, and measurements were taken to calculate the volume-based average particle size (D50).

[0050] (2) BET specific surface area (SA) of hexagonal boron nitride powder A gas adsorption test was conducted using a Micromeritics FlowSorb III2310 with nitrogen gas as the adsorbed species, and the nitrogen adsorption isotherm was measured. Specifically, the hexagonal boron nitride powder was pretreated by vacuum drying and degassing at 200°C for 10 minutes, and the nitrogen adsorption isotherm was measured at a gas flow rate of 15 cm. 3 The adsorption / desorption isotherm of nitrogen gas was measured using a continuous flow method under the condition of 1 / min, and the specific surface area was calculated by the BET method.

[0051] (3) Aspect ratio of hexagonal boron nitride powder The particle shapes of hexagonal boron nitride powder were photographed at 10,000x magnification using a scanning electron microscope (SEM) SU3500 manufactured by Hitachi High-Technologies Corporation. The length and thickness of each hexagonal boron nitride particle were read from the resulting image, and the length / thickness ratio was calculated as the aspect ratio. The aspect ratio in this example was calculated as the arithmetic mean of the observation results for 50 particles.

[0052] (4) Peel strength of the curable resin composition The curable resin composition was cured using a heat press under conditions of a temperature of 220°C, a pressure of 20 MPa, and a holding time of 4 hours. Copper foil was placed on one side of the cured resin composition, and the sheet was attached by heating and pressing at 200°C and a pressure of 20 MPa under vacuum pressure for 10 minutes. The same procedure was then performed on the other side to produce a sheet laminated with metal foil and resin composition. The peel strength of the resulting sheet laminated with metal foil and resin composition was measured and evaluated using an autograph AG-Xpus manufactured by Shimadzu Corporation according to a method conforming to JIS C6481:1996.

[0053] Example 1 1950 g of boron oxide, 830 g of carbon black, and 552 g of calcium carbonate were mixed in a mixer / stirrer. This mixture was heated to 1500 °C at a rate of 10 °C / min in a graphite Tammann furnace under a nitrogen atmosphere. After holding for 6 hours, the temperature was increased to 1800 °C at a rate of 5 °C / min and reduced and nitrided for 2 hours to obtain crude hexagonal boron nitride powder. The resulting crude hexagonal boron nitride powder was then crushed in a stone mill and acid-washed by adding an equal amount of 35 wt% hydrochloric acid to the crude hexagonal boron nitride powder and stirring for 15 hours. After filtering the hydrochloric acid, the powder was washed with pure water and then dehydrated by suction filtration until the moisture content in the powder was 50 wt% or less. The crude hexagonal boron nitride powder washed in the above manner was dried under reduced pressure at 200 °C for 12 hours and cooled to obtain the desired hexagonal boron nitride powder BN-A. The physical properties of the obtained hexagonal boron nitride powder are shown in Table 1.

[0054] An epoxy group-containing compound (JER828 (trade name), manufactured by Mitsubishi Chemical Corporation) and a benzoxazine ring structure-containing compound (Pd-type benzoxazine, manufactured by Shikoku Kasei Co., Ltd.) represented by the following formula (4) were weighed out at 2.2 mol per mol of epoxy group in the epoxy group-containing compound, and the BN-A was weighed out so that the total volume was 60% of the curable resin composition. 55 parts by mass of cyclohexanone as a solvent was added per 100 parts by mass of the curable resin composition, and the mixture was mixed using a planetary centrifugal mixer (Mazerustar KK-250S, manufactured by Kurabo Industries Co., Ltd.) to produce a mixture. The mixture was applied to a release polyethylene film using an automatic coating device (PI-1210, manufactured by Tester Sangyo Co., Ltd.) with a Bird applicator. The mixture coated on the film was dried for 10 minutes and then vacuum-dried at 100°C for 30 minutes using a vacuum dryer to obtain a sheet-like curable resin composition. Copper foil was placed on the sheet-like curable resin composition, and the composition was cured by applying heat and pressure using a heating press to prepare a test specimen. The peel strength of the obtained test specimen was measured. The composition of the curable resin composition and the evaluation results are shown in Table 2.

[0055] [ka]

[0056] [Examples 2 to 5, Comparative Example 1] Curable resin compositions and test specimens were obtained and peel strength was evaluated in the same manner as in Example 1, except that the composition of the curable resin composition was changed as shown in Table 2. The composition of the curable resin composition and the evaluation results are shown in Table 2.

[0057] Example 6 An epoxy group-containing compound (manufactured by Mitsubishi Chemical Corporation, JER828 (trade name)), a benzoxazine ring structure-containing compound represented by the formula (4) (Pd-type benzoxazine, manufactured by Shikoku Kasei Co., Ltd.) at 2.2 mol per 1 mol of epoxy group in the epoxy group-containing compound, 0.1 parts by mass of an amine-based curing agent per 100 parts by mass of the epoxy-containing compound, and the BN-A at 60% by volume of the total curable resin composition were weighed out. Cyclohexanone was further added as a solvent in an amount of 55 parts by mass per 100 parts by mass of the curable resin composition, and the mixture was mixed using a planetary centrifugal mixer (Kurashiki Boseki Co., Ltd., Mazerustar KK-250S) to prepare a mixture. Using the mixture, a test specimen was prepared in the same manner as in Example 1, and peel strength was measured. The composition of the curable resin composition and the evaluation results are shown in Table 2.

[0058] Example 7 1950 g of boron oxide, 830 g of carbon black, and 276 g of calcium carbonate were mixed in a mixer. This mixture was heated to 1500 °C at a rate of 10 °C / min in a graphite Tammann furnace under a nitrogen atmosphere. After holding for 6 hours, the temperature was increased to 1800 °C at a rate of 5 °C / min and reduced and nitrided for 2 hours to obtain crude hexagonal boron nitride powder. The resulting crude hexagonal boron nitride powder was then crushed in a stone mill. An equal amount of 35 wt% hydrochloric acid was added to the resulting crude hexagonal boron nitride powder and stirred for 15 hours for acid washing. After filtering the hydrochloric acid, the powder was washed with pure water and then dehydrated by suction filtration until the moisture content in the powder was 50 wt% or less. The crude hexagonal boron nitride powder washed in the above manner was dried under reduced pressure at 200 °C for 12 hours and cooled to obtain the desired hexagonal boron nitride powder (BN-B). The physical properties of the obtained hexagonal boron nitride powder are shown in Table 1.

[0059] Using the hexagonal boron nitride powder BN-B, curable resin compositions and test specimens were prepared and the peel strength was measured in the same manner as in Example 1. The compositions of the curable resin compositions and the evaluation results are shown in Table 2.

[0060] Comparative Example 2 1950 g of boron oxide, 830 g of carbon black, 552 g of calcium carbonate, and 180 g of boron carbide were mixed in a mixer / stirrer. This mixture was heated to 1500 °C at a rate of 10 °C / min in a graphite Tammann furnace under a nitrogen atmosphere, held there for 6 hours, then heated to 1900 °C at a rate of 5 °C / min, and reduced and nitrided for 2 hours to obtain crude hexagonal boron nitride powder. The resulting crude hexagonal boron nitride powder was then crushed in a stone mill. An equal amount of 35 wt% hydrochloric acid was then added to the resulting crude hexagonal boron nitride powder, and the mixture was stirred for 15 hours for acid washing. The hydrochloric acid was filtered, followed by washing with pure water and dehydration by suction filtration until the moisture content of the powder was 50 wt% or less. The crude hexagonal boron nitride powder washed as described above was dried under reduced pressure at 200°C for 12 hours, and then heated to 1750°C at a rate of 5°C / min in a nitrogen gas atmosphere using a graphite Tammann furnace, and held at that temperature for 6 hours to obtain the desired hexagonal boron nitride powder (BN-C). The physical properties of the obtained hexagonal boron nitride powder are shown in Table 1.

[0061] Using the hexagonal boron nitride powder BN-C, curable resin compositions and test specimens were prepared and the peel strength was measured in the same manner as in Example 1. The compositions of the curable resin compositions and the evaluation results are shown in Table 2.

[0062] [Table 1]

[0063] [Table 2]

[0064] Examples 1 to 7 achieved high peel strengths of 4.0 N / 10 mm or more. On the other hand, Comparative Example 1, which used the same BN-A as Example 1 but did not contain a benzoxazine ring structure-containing compound, had low peel strength. This is presumably due to insufficient curing. Furthermore, Comparative Example 2, which used BN-C with a D50 / d of more than 19, had low peel strength.

Claims

[Claim 1] A curable resin composition comprising an epoxy group-containing compound, a benzoxazine ring structure-containing compound, and hexagonal boron nitride powder, wherein the content of the benzoxazine ring structure-containing compound is 1.0 mol to 5.0 mol per 1 mol of epoxy groups in the curable resin composition, the amount of the hexagonal boron nitride powder mixed is 55 to 75% by volume of the entire curable resin composition, and the hexagonal boron nitride powder has an average particle size D50 determined from particle size distribution of 3 μm to 30 μm, and the ratio (D50 / d) of the average particle size d determined from the BET specific surface area to the D50 is 19 or less.

Images