WO2016092734A1

WO2016092734A1 - Resin composition and preparation process thereof

Info

Publication number: WO2016092734A1
Application number: PCT/JP2015/005432
Authority: WO
Inventors: Youichiro SAKAGUCHI
Original assignee: Showa Denko K.K.
Priority date: 2014-12-08
Filing date: 2015-10-28
Publication date: 2016-06-16
Also published as: JP6538337B2; JP2016108457A; TW201634547A

Abstract

There is provided a resin composition having excellent electrical insulation properties and thickness-direction thermal conductivity. A hexagonal boron nitride powder (B) in the resin composition contains agglomerated particles having primary particles of hexagonal boron nitride agglomerated therein and has, in its particle size distribution curve, an initial maximum peak within a particle size range more than 1 mm or more but not more than 500 mm. It has such a characteristic that the particle size distribution curve changes after ultrasonic irradiation treatment and it becomes a particle size distribution curve satisfying the specific conditions.

Description

RESIN COMPOSITION AND PREPARATION PROCESS THEREOF

The present invention relates to a resin composition and a preparation process thereof.

Due to recent increase in calorific value of electrical parts or electronic parts due to downsizing or power increase of them, dissipation of heat generated in a narrow space from the electrical parts or electronic parts has become a problem. As a method of overcoming this problem, a means for placing an adhesive or adhesive sheet having insulating properties or thermal conductivity between a heating unit and a heat dissipation member that dissipates heat thereof, each constituting electrical parts or electronic parts, is known. Such an adhesive or adhesive sheet is made of a composition obtained by filling a thermosetting resin with an inorganic compound filler having high thermal conductivity. The calorific value from electrical parts or electronic parts tends to increase further so that adhesives or adhesive sheets used therefor are required to have further improved thermal conductivity.

In recent years, therefore, a technology of making use of hexagonal boron nitride (hBN) as a heat dissipating filler has attracted attentions for providing an adhesive or adhesive sheet having improved thickness-direction thermal conductivity. The primary particles of hexagonal boron nitride has, as a crystal structure thereof, a hexagonal mesh layered structure analogous to that of graphite and these particles are in scaly form. The plane-direction thermal conductivity of the scaly particles is about 20 times (from 60 to 80 W/m.K) of the thickness-direction thermal conductivity of them and thus, the particles have anisotropic thermal conductivity.

The thermal conductivity of agglomerated particles having primary hexagonal boron nitride particles agglomerated therein is, on the other hand, isotropic and they serve as an insulating and heat dissipating filler capable of improving the thickness-direction thermal conductivity of a molded or formed product containing these agglomerated particles.
For example, the following technology is proposed as a method of improving thermal conductivity by using the agglomerated particles of hexagonal boron nitride as a heat dissipation filler.

Patent Document 1 discloses an adhesive sheet prepared using, as a heat dissipation filler, a mixture of primary particles of hexagonal boron nitride and agglomerated particles of hexagonal boron nitride. Patent Document 2 discloses an adhesive sheet prepared using agglomerated particles of hexagonal boron nitride and alumina having a particle size smaller than that of the particles, deforming the agglomerated particles of hexagonal boron nitride, and thereby allowing close packing of the heat dissipation fillers to each other. Patent Document 3 discloses an adhesive sheet having both high thermal conductivity and electrical insulation properties, obtained by mixing two kinds of agglomerated particles of hexagonal boron nitride different in rupture strength and at the same time, different in average long diameter of the primary particles and carrying out cohesive failure of one of the two kinds of the agglomerated particles of hexagonal boron nitride in the adhesive sheet.

JP 2008-189818 A JP 2013-39834 A JP 2011-6586 A

High filling of only hexagonal boron nitride for improving thermal conductivity has caused a problem of deterioration in electrical insulation properties, in particular, deterioration in dielectric breakdown voltage performance (breakdown voltage performance) or adhesive properties of an adhesive or adhesive sheet.
An object of the present invention is to overcome such a problem of the prior art and provide a resin composition having excellent electrical insulation properties and thickness-direction thermal conductivity, and a preparation process of the resin composition.

With a view to overcoming the above-described problem, the present invention includes the following [1] to [5] as one embodiment.
[1] A resin composition having a resin component (A) and a hexagonal boron nitride powder (B) containing agglomerated particles having primary particles of hexagonal boron nitride agglomerated therein,
wherein the resin composition has a melt viscosity, as measured using a Koka type flow tester at 150°C, of 0.01 Pa.s or more but not more than 500 Pa.s,
wherein the hexagonal boron nitride powder (B) has a BET specific surface area more than 10 m²/g but less than 15 m²/g,
wherein the hexagonal boron nitride powder (B) has, in a particle size distribution curve thereof, one maximum peak within a particle size range more than 1 mm but not more than 500 mm and the maximum peak is regarded as an initial maximum peak, and
wherein the hexagonal boron nitride powder (B) has such a characteristic that when a dispersion obtained by dispersing the hexagonal boron nitride powder (B) in water is irradiated with ultrasonic waves having an oscillating frequency of 19.5 kHz and is thereby subjected to cohesive failure treatment for cohesive failure of the agglomerated particles, the particle size distribution curve changes and becomes a particle size distribution curve satisfying all of the following three conditions:

(Condition 1) the particle size distribution curve has a first maximum peak within a particle size range of 1 μm or more but not more than 20 μm;
(Condition 2) the particle size distribution curve has a second maximum peak within a particle size range more than 1 μm but not more than 350 μm and within a particle size range more than the particle size of the first maximum peak but not more than the particle size of the initial maximum peak; and
(Condition 3) a ratio of a height of the first maximum peak to a height of the second maximum peak [height of first maximum peak]/[height of second maximum peak] is 0.1 or more but not more than 8.0.

[2] The resin composition as described above in [1], further having a ceramic powder (C) other than the hexagonal boron nitride powder (B).
[3] The resin composition as descried above in [2], wherein a content of the resin component (A) is 5 mass% or more but not more than 40 mass%, a content of the hexagonal boron nitride powder (B) is 5 mass% or more but not more than 75 mass%, and a content of the ceramic powder (C) is 10 mass% or more but not more than 90 mass%.

[4] The resin composition as descried above in [2] or [3], wherein the ceramic powder (C) is at least one selected from alumina powder, aluminum nitride powder, glass beads, zinc oxide powder, magnesia powder, and silica powder.
[5] A process for preparing a resin composition by mixing a hexagonal boron nitride powder containing agglomerated particles having primary particles of hexagonal boron nitride agglomerated therein with a resin component,
wherein the hexagonal boron nitride powder has a BET specific surface area more than 10 m²/g but less than 15 m²/g and has, in a particle size distribution curve thereof, one maximum peak, as an initial maximum peak, within a particle size range more than 1 mm but not more than 500 mm, and
wherein when a particle size distribution curve obtained by irradiating a dispersion obtained by dispersing the hexagonal boron nitride powder in water with ultrasonic waves having an oscillating frequency of 19.5 kHz and measuring a particle size distribution of the hexagonal boron nitride powder irradiated with the ultrasonic waves satisfies all of the following three conditions, the hexagonal boron nitride powder not irradiated with ultrasonic waves is mixed with the resin component:

The resin composition of the present invention has excellent electrical insulation properties and thickness-direction thermal conductivity, and moreover, has a resin composition according to the present invention allows preparation of a resin composition having excellent electrical insulation properties and thickness-direction thermal conductivity, and moreover good porosity and fluidity.

FIG. 1 is a particle size distribution curve before and after ultrasonic irradiation treatment of a hexagonal boron nitride powder a of Preparation Example 1. FIG. 2 is a particle size distribution curve before and after ultrasonic irradiation treatment of a hexagonal boron nitride powder b of Preparation Example 2. FIG. 3 is a particle size distribution curve before and after ultrasonic irradiation treatment of a hexagonal boron nitride powder c of Comparative Preparation Example 1. FIG. 4 is a particle size distribution curve before and after ultrasonic irradiation treatment of a hexagonal boron nitride powder d of Comparative Preparation Example 2. FIG. 5 is a particle size distribution curve before and after ultrasonic irradiation treatment of a hexagonal boron nitride powder e of Comparative Preparation Example 3. FIG. 6 is a particle size distribution curve before and after ultrasonic irradiation treatment of a hexagonal boron nitride powder f of Comparative Preparation Example 4.

Embodiments of the resin composition and the preparation process thereof according to the present invention will hereinafter be described in detail.
Finding that a resin composition containing, as a filler, the hexagonal boron nitride powder as described below can have excellent electrical insulation properties and thickness-direction thermal conductivity and further, good porosity and fluidity, the present inventors have completed the present invention.

Described specifically, the hexagonal boron nitride powder of the present embodiment has a predetermined BET specific surface area and has, in its particle size distribution curve, one maximum peak at a predetermined position. Further, the hexagonal boron nitride powder of the present embodiment has such a characteristic that by cohesive failure treatment using ultrasonic irradiation treatment, the particle size distribution curve changes as a result of cohesive failure of agglomerated particles having primary particles of hexagonal boron nitride agglomerated therein and it becomes a particle size distribution curve having two maximum peaks of a predetermined height at a predetermined position. The hexagonal boron nitride powder of the present embodiment will hereinafter be described specifically. In the present invention, the term “hexagonal boron nitride powder” means a powder containing agglomerated particles having primary particles of hexagonal boron nitride agglomerated therein.

<(1) Hexagonal boron nitride powder (B)>
The BET specific surface area of the hexagonal boron nitride powder of the present embodiment is more than 10 m²/g but less than 15 m²/g, preferably more than 10 m²/g but not more than 13 m²/g from the standpoint of a balance between thermal conductivity and fluidity of a resin composition containing the hexagonal boron nitride powder. The BET specific surface area can be measured using a full-automatic BET specific surface area measuring apparatus “Multisorb 16”, product of Yuasa Ionics Co., Ltd.
The hexagonal boron nitride powder of the present embodiment has, in the particle size distribution curve thereof, one maximum peak (which will hereinafter be called “initial maximum peak”) within a particle size range more than 1 mm but not more than 500 mm. It has such a characteristic that the particle size distribution curve changes by the ultrasonic irradiation treatment and it becomes a particle size distribution curve satisfying all of the following three conditions:

It is to be noted that the particle size distribution curve in the present invention is a particle size frequency distribution curve. In the present invention, the maximum peak of the particle size distribution curve means a peak at which the sign of a first derivative available by differentiating a particle size distribution curve plotted with an abundance ratio on the ordinate and a particle size on the abscissa changes from positive to negative or negative to positive. This means that a so-called shoulder peak is not included in the maximum peak. When R2/R1, that is, a ratio of an abundance ratio (R2) at the maximum peak which is a lower one of two adjacent maximum peaks to an abundance ratio (R1) at a minimum peak present between the two adjacent maximum peaks is more than 1.0 but not more than 1.1, however, the lower maximum peak is not regarded as the maximum peak of the present invention.

Ultrasonic irradiation treatment may change the particle size distribution curve to that having a plurality of one or both of the first maximum peak and the second maximum peak. In this case, the first maximum peak having the smallest diameter among the plurality of maximum peaks and the second maximum peak having the largest diameter among the plurality of maximum peaks are used in calculation of the ratio in Condition 3.
Further, the ultrasonic irradiation treatment is, for example, the following treatment. A dispersion is prepared by dispersing 0.06 g of a hexagonal boron nitride powder in 50 g of water. The resulting dispersion is irradiated with ultrasonic waves for 3 minutes at an output of 150 W at an oscillating frequency of 19.5 kHz. Although an ultrasonic irradiator is not particularly limited, irradiation at an oscillating frequency of 19.5±1 kHz using an ultrasonic apparatus having a bolt-clamped electrostrictive oscillating element is preferred. For example, an ultrasonic homogenizer “US-150V”, product of NIHONSEIKI KAISHA LTD.can be used.

Further, the particle size distribution of a hexagonal boron nitride powder before and after ultrasonic irradiation treatment is measured, for example, by the following method. A hexagonal boron nitride powder is dispersed in pure water with the aid of a surface active agent and a particle size distribution is measured using laser diffraction×scattering method. As an apparatus for measuring a particle size distribution, for example, a particle size analyzer “Microtrac MT3300 EXII”, product of NIKKISO CO., LTD. can be used.

The first maximum peak can be supposed to be a peak attributable to small-diameter agglomerated particles formed by cohesive failure of agglomerated particles of a hexagonal boron nitride powder by ultrasonic irradiation treatment and primary particles. The second maximum peak can be supposed to be a peak attributable to original agglomerated particles (agglomerated particles before ultrasonic irradiation treatment) which have remained after ultrasonic irradiation treatment without cohesive failure. This means that the hexagonal boron nitride powder of the present embodiment contains both agglomerated particles susceptible to cohesive failure when specific external stimulation is applied thereto and agglomerated particles not susceptible to cohesive failure.

When a resin composition contains such hexagonal boron nitride powder (B) as a filler, cohesive failure of agglomerated particles of the hexagonal boron nitride powder (B) occurs by the external stimulation applied thereto. The hexagonal boron nitride powder (B) contained in the resin composition can therefore be supposed to have both the small-diameter agglomerated particles formed by cohesive failure of original agglomerated particles and primary particles; and the original agglomerated particles that have remained without cohesive failure.

The resin composition containing the hexagonal boron nitride powder (B) having the above-described characteristic can therefore have both excellent electrical insulation properties, particularly, excellent in dielectric breakdown voltage performance (breakdown voltage performance) and excellent thickness-direction thermal conductivity. The resin composition can be filled with the hexagonal boron nitride powder (B) of the present embodiment at a high filling ratio so that it can have decreased porosity and more stable electrical insulation properties and thermal conductivity.

The hexagonal boron nitride powder (B) has, in the particle size distribution curve thereof, only one initial maximum peak within a particle size range more than 1 μm but not more than 500 μm. It has preferably only one initial maximum peak within a particle size range of 10 μm or more but not more than 300 μm. When the hexagonal boron nitride powder (B) has only one maximum peak within the above range, the hexagonal boron nitride powder (B) has preferable dispersibility when the resin composition containing the hexagonal boron nitride powder (B) is processed into a sheet having a thickness of about 200 mm or less.

The first maximum peak falls within a particle size range of 1 μm or more but not more than 20 μm, more preferably within a particle size range of 3 μm or more but not more than 19 μm. When the first maximum peak falls within a particle size range of 1 mm or more, contact resistance between the agglomerated particles of the hexagonal boron nitride powder (B) decreases in the resin composition and the resin composition tends to have improved thermal conductivity. When the first maximum peak falls within a particle size range of 20 mm or less, on the other hand, the hexagonal boron nitride powder (B) in the resin composition is likely to have a closely packed structure so that due to a decrease in contact resistance between the agglomerated particles, the resin composition tends to have improved thermal conductivity.

Further, the second maximum peak falls within a particle size range more than 1 mm but not more than 350 mm, preferably within a particle size range of 5 mm or more but not more than 150 mm. When the second maximum peak falls within a particle size range more than 1 mm, contact resistance between the agglomerated particles of the hexagonal boron nitride powder (B) decreases in the resin composition and a sheet-like member or a plate-like member obtained by forming the resulting resin composition into a sheet or plate form tends to have improved thickness-direction thermal conductivity. When the second maximum peak falls within a particle size range of 350 μm or less, on the other hand, the hexagonal boron nitride powder (B) has a sufficiently small size relative to the thickness of a sheet-like member or plate-like member obtained by forming the resin composition into a sheet or plate form. Then, the sheet-like member or plate-like member has a smooth surface without large unevenness; the sheet-like member or plate-like member does not crack easily during formation thereof; and it can easily have excellent electrical insulation properties (particularly, breakdown voltage).

Further, a ratio of the height of the first maximum peak to the height of the second maximum peak [height of first maximum peak)/[height of second maximum peak] is 0.1 or more but not more than 8.0, more preferably 0.4 or more but not more than 4.0, still more preferably 0.5 or more but not more than 3.0. The ratios of 0.1 or more mean moderate cohesive failure of the agglomerated particles of the hexagonal boron nitride powder (B) so that the hexagonal boron nitride powder (B) in the resin composition is likely to have a closely packed structure. The ratios not more than 8.0, on the other hand, mean that the original agglomerated particles remain moderately without cohesive failure so that problems that occur when a resin composition contains primary particles of hexagonal boron nitride, more specifically, a problem of anisotropic thermal conductivity derived from the scaly primary particles of hexagonal boron nitride and a problem derived from a large specific surface area of the primary particles of hexagonal boron nitride hardly occur, leading to excellent thickness-depending thermal conductivity performance.

Further, the hexagonal boron nitride powder (B) contains agglomerated particles having primary particles of hexagonal boron nitride agglomerated therein. When a resin composition using, for example, the hexagonal boron nitride powder (B) as a filler is formed into a sheet, orientation of the primary particles of hexagonal boron nitride in a certain direction can be prevented or suppressed irrespective of an increase in filling ratio of the hexagonal boron nitride powder (B) in the sheet.
Further, the purity of the hexagonal boron nitride powder (B), that is, the content of hexagonal boron nitride in the hexagonal boron nitride powder (B) is preferably 96 mass% or more, more preferably 98 mass% or more, still more preferably 99 mass% or more, most preferably 99.5 mass% or more from the standpoint of improving thermal conductivity and insulation.

Further, the bulk density of the hexagonal boron nitride powder (B) is preferably 0.2 g/cm³or more, more preferably 0.3 g/cm³ or more, still more preferably 0.4 g/cm³ or more from the standpoint of improving the filling property of it when it is added to a resin composition as a filler. Although a measurement method of a bulk density is not particularly limited, the bulk density can be measured by charging a 300-mL graduated cylinder with 100 g of the hexagonal boron nitride powder (B) and vibrating the cylinder for 3 minutes by a shaker to find, as the bulk density, a vibrated bulk density.

<(2) Preparation process of hexagonal boron nitride powder (B)>
The hexagonal boron nitride powder (B) of the present embodiment having such characteristics that it has, in a particle size distribution curve thereof, one initial maximum peak within a particle size range more than 1 mm but not more than 500 mm and that when it is subjected to ultrasonic irradiation treatment, the particle size distribution curve thereof changes so as to satisfy all of the above-described three conditions can be prepared by the following process.

Described specifically, it is a process of forming a crude hexagonal boron nitride powder composed of 20 mass% or more but not more than 90 mass% of boron nitride and 10 mass% or more but not more than 80 mass of boron oxide (B₂O₃) into, for example, tablets and then baking the resulting tablets in a nitrogen gas-containing atmosphere and thereby obtaining a hexagonal boron nitride powder (B) containing agglomerated particles having primary particles of hexagonal boron nitride agglomerated therein.
In this preparation process of the hexagonal boron nitride powder (B), the hexagonal boron nitride powder (B) is preferably obtained by carrying out at least one of grinding and classification further after baking. The hexagonal boron nitride powder (B) is more preferably obtained by carrying out so-called “break down process”, that is, a process in which both grinding and classification are performed.

The hexagonal boron nitride powder (B) can also be prepared by a so-called “bottom up process” which is a process of dispersing scaly primary particles of hexagonal boron nitride or a low-crystalline crude hexagonal boron nitride powder in a liquid dispersion medium, spray drying the resulting slurry in the air, and spray drying further in the presence of a binder component. The bottom up process needs neither formation of a crude hexagonal boron nitride powder nor grinding after baking, each in the above-described break down process. In addition, this bottom up process allows formation of a powder composed of spherical granules having high strength and a sharp particle size distribution.
Preparation of the hexagonal boron nitride powder (B) by the break down process is most preferred in order to incorporate the hexagonal boron nitride powder (B) in the resin composition and thereby impart the resulting resin composition with high breakdown voltage, high thermal conductivity, low porosity, and melt fluidity (processability).

The preparation process of the hexagonal boron nitride powder (B) will next be described in further detail. First, a crude hexagonal boron nitride powder to be added as a raw material will be described, followed by a description on each of molding or forming, baking, grinding, and classifying steps for the preparation of the hexagonal boron nitride powder (B).
(I) Crude hexagonal boron nitride powder as a raw material
The crude hexagonal boron nitride powder contains 20 mass% or more but not more than 90 mass% of boron nitride and 10 mass% or more but not more than 80 mass% of boron oxide. This crude hexagonal boron nitride powder can be prepared easily as described below.

The hexagonal boron nitride powder (B) can be prepared at high efficiency using the crude hexagonal boron nitride powder containing 20 mass% or more of boron nitride as a raw material. When the content of boron nitride is 90 mass% or less, the crude hexagonal boron nitride powder serving as a raw material can be prepared at high efficiency. From these standpoints, a boron nitride content in the crude hexagonal boron nitride powder is preferably 25 mass% or more, more preferably 30 mass% or more, still more preferably 35 mass% or more. It is preferably 85 mass% or less, more preferably 80 mass% or less, still more preferably 75 mass% or less. Further, it is preferably 25 mass% or more but not more than 85 mass%, more preferably 30 mass% or more but not more than 80 mass%, still more preferably 35 mass% or more but not more than 75 mass% or less.

A total content of boron nitride and boron oxide in the crude hexagonal boron nitride powder is preferably 90 mass% or more, more preferably 95 mass% or more, still more preferably 99 mass% or more, most preferably 100 mass%. The crude hexagonal boron nitride powder may contain other components in an amount not impairing the advantageous effect of the present invention, but the content of the other components in the crude hexagonal boron nitride powder is preferably 10 mass% or less, more preferably 5 mass% or less, still more preferably 1 mass% or less. The crude hexagonal boron nitride powder most preferably does not contain the other components.

(II)　Preparation process of crude hexagonal boron nitride powder
The crude hexagonal boron nitride powder can be obtained preferably by mixing boron oxide with an amino-containing compound, molding or forming the resulting mixture, and heating and then grinding the molded or formed product. First, boron oxide and the amino-containing compound used as raw materials for the preparation of the crude hexagonal boron nitride powder will be described and then mixing, molding or forming, heating, and grinding steps will each be described.

(II-1) Boron oxide
Boron oxide is a compound containing boron and oxygen and examples of it include a boric acid, boron oxide, and borax.
As the boric acid, one or more compounds represented by the following formula: (B₂O₃)×X(H₂O)[wherein X represents a number of 0 or more but not more than 3] such as orthoboric acid (H₃BO₃), metaboric acid (HBO₂), tetraboric acid (H₂B₄O₇), and boric anhydride (B₂O₃) can be used. Of these, orthoboric acid is suited for use because it is easily available and has good miscibility with an amino-containing compound such as melamine.

(II-2) Amino-containing compound
Examples of the amino-containing compound include aminotriazine compounds, guanidine compounds, and urea. Examples of the aminotriazine compounds include melamine, guanamine, and benzoguanamine, and condensates thereof such as melam, melem, and melon.
Of these compounds, compounds such as melamine and guanidine, having, at the amino group thereof and a group other than the amino group, a nitrogen atom are preferred.

(II-3) Mixing of boron oxide with amino-containing compound
The mixing method of boron oxide with the amino-containing compound is not particularly limited and either wet mixing or dry mixing may be used. Wet mixing is preferred. By wet mixing, a precursor is formed. For example, addition of water to a mixture of boric acid or boric anhydride and melamine produces a precursor represented by the following molecular formula: C₃N₃(NH₂×H₃BO₃)₃. A commonly used mixer such as Henschel mixer, ball mill, or ribbon blender can be used for wet mixing.

Boron oxide and the amino-containing compound are mixed preferably so as to give an atomic ratio (B/N) of from 1/3 to 2/1. The atomic ratio is a ratio of a boron atom (B) in boron oxide to a nitrogen atom (N) in the amino-containing compound. At the atomic ratio (B/N) of 1/3 or more, remaining of the amino-containing compound that does not become a precursor in the presence of water can be prevented or suppressed and therefore, the amino-containing compound can be prevented or suppressed from being carbonized during baking and thereby blackening or browning boron nitride. At the atomic ratio (B/N) up to 2/1, as the number of boron atoms is greater, hexagonal boron nitride having better crystallinity can be obtained.
When melamine is used as the amino-containing compound, melamine is added in an amount of preferably 30 parts by mass or more but not more than 65 parts by mass based on 100 parts by mass of boron oxide from the above-described standpoint.

(II-4) Molding or forming of mixture of boron oxide with amino-containing compound
Next, the precursor-containing mixture obtained as described above by mixing boron oxide and the amino-containing compound is molded or formed into, for example, tablets. Molding or forming of the precursor-containing mixture into a molded or formed product increases bulk density of the mixture. Then, a heating apparatus having a predetermined capacity can be charged with a large amount of the mixture, leading to improvement in the productivity of the hexagonal boron nitride powder (B).

(II-5) Heating of molded or formed product
Next, the molded or formed product obtained by molding or forming is heated. The boron oxide reacts and the amino-containing compound in the molded or formed product react with each other by this heating to produce hexagonal boron nitride.
The heating is performed preferably in an ammonia or non-oxidizing gas atmosphere. The non-oxidizing gas atmosphere is preferably a nitrogen gas atmosphere or an inert gas atmosphere such as argon gas atmosphere. Of these, an ammonia atmosphere is more preferred.
The heating temperature is preferably 400°C or more but not more than 1500°C, more preferably 600°C or more but not more than 1300°C, still more preferably 800°C or more but not more than 1200°C from the standpoint of improving reactivity between boron oxide and the amino-containing compound and grinding ease.
(II-6) Grinding of molded or formed product
Next, the product obtained by heating the molded or formed product is ground into a crude hexagonal boron nitride powder. The grinding method is not particularly limited and jaw grinding, crude roll grinding, and the like can be employed.

(III) Molding or forming of crude hexagonal boron nitride powder
For preparing the hexagonal boron nitride powder (B), the crude hexagonal boron nitride powder is molded or formed into, for example, tablets. The molding or forming is preferred from the standpoint of improving the strength of agglomerated particles having primary particles of hexagonal boron nitride agglomerated therein, productivity of hexagonal boron nitride powder (B), handling ease, and reactivity.
In the molding or forming, a binder may be added to the crude hexagonal boron nitride powder. Although the binder is not particularly limited, examples include resins such as polyvinyl alcohol (PVA), varnish, cellulose, and polyvinylidene fluoride (PVDF). Polyvinyl alcohol is preferred.

(IV) Baking of molded or formed product
Next, the molded or formed product obtained by molding or forming is baked. Agglomerated particles having primary particles of hexagonal boron nitride agglomerated therein are produced by pressure molding or forming the crude hexagonal boron nitride powder. When baking is performed without molding or forming, there is a fear that the hexagonal boron nitride powder (B) having high compression rupture strength cannot be prepared sufficiently.
The atmosphere during baking is preferably an atmosphere containing a nitrogen gas. A nitrogen gas concentration in the nitrogen-gas-containing atmosphere is preferably 60 vol% or more, more preferably 80 vol% or more, still more preferably 90 vol% or more, most preferably 99 vol% or more. An oxygen gas concentration in the atmosphere is preferably smaller.

The baking temperature can be set at 1000°C or more but not more than 2200°C. At the baking temperature of 1000°C or more, adequate increase in purity of boron nitride proceeds. At the baking temperature not more than 2200°C, degradation of hexagonal boron nitride hardly occurs. From such viewpoints, the baking temperature is preferably 1500°C or more but not more than 2200°C, more preferably 1600°C or more but not more than 2200°C, still more preferably 1700°C or more but not more than 2200°C.

The baking time can be set at 1 hour or more but not more than 20 hours. Baking time for 1 hour or more easily accelerates an adequate purity increase of boron nitride. Baking time for not more than 20 hours allows baking at a low cost. From such viewpoints, the baking time is preferably 2 hours or more but not more than 15 hours, more preferably 3 hours or more but not more than 10 hours.
The molded or formed product may be dried before baking. The drying temperature is preferably 150°C or more but not more than 400°C, more preferably 200°C or more but not more than 400°C. The drying time is preferably 2 hours or more but not more than 10 hours.

(V) Grinding of molded or formed product
Next, the product obtained by baking of the molded or formed product is ground into a hexagonal boron nitride powder (B). The grinding method is not particularly limited and jaw grinding, crude roll grinding, or the like can be employed.

(VI) Classification
Next, the ground product obtained by grinding is preferably classified. The classifying method is not particularly limited and the ground product can be classified by a vibratory sieve, air separation, water sieve, centrifugal separation, or the like. Of these, classification with a vibratory sieve is preferred. It is also possible to increase the content of agglomerated particles in the hexagonal boron nitride powder (B) by classifying and thereby removing scaly primary particles of hexagonal boron nitride other than the hexagonal boron nitride powder (B).

The above-described preparation process of the hexagonal boron nitride powder (B) is preferable for obtaining the hexagonal boron nitride powder (B) having, in the particle size distribution curve thereof, one initial maximum peak within a particle size range more than 1 mm but not more than 500 mm, but it is needless to say that the preparation process is not limited thereto. Another preparation process may be used insofar as it can prepare the hexagonal boron nitride powder (B) having such characteristics that it has, in a particle size distribution curve thereof, one initial maximum peak within a particle size range more than 1 mm but not more than 500 mm and that when it is subjected to ultrasonic irradiation treatment, the particle size distribution curve thereof changes so as to satisfy all of the above-described three conditions.

By subjecting the resulting hexagonal boron nitride powder (B) to ultrasonic irradiation treatment to measure its particle size distribution and confirming that the particle size distribution curve changes so as to satisfy all of the above-described three conditions after the ultrasonic irradiation treatment, the characteristics of the resin composition containing the hexagonal boron nitride powder (B) can be expected in the stage before using the hexagonal boron nitride powder (B) (for example, before incorporation of it in the resin composition as a filler).

<(3) Resin composition>
The resin composition can be imparted with heat dissipation properties by containing, as a filler, the hexagonal boron nitride powder (B) obtained as described above. This means that the resin composition of the present embodiment contains a resin component (A) and the hexagonal boron nitride powder (B).
In the resin composition obtained by mixing such a hexagonal boron nitride powder (B) in the resin component (A), partial cohesive failure of agglomerated particles of the hexagonal boron nitride powder (B) occurs by the external stimulation given to the resin composition at the time of preparing the resin composition, at the time of molding or forming the resin composition, or at the time of using the molded or formed product of the resin composition so that the resin composition or molded or formed product (for example, sheet) thereof is presumed to contain small-diameter agglomerated particles derived from cohesive failure of original agglomerated particles and primary particles, and the original particles which have remained without being subjected to cohesive failure.
In the resin composition or molded or formed product thereof, the contact frequency among the agglomerated particles of the hexagonal boron nitride powder (B) is presumed to be markedly enhanced.

The resin composition or molded or formed product according to the present embodiment has excellent electrical insulation properties, particularly, excellent dielectric breakdown voltage performance (breakdown voltage performance) and excellent thickness-direction thermal conductivity. The resin composition can be filled at a high filling ratio with the hexagonal boron nitride powder (B) so that the resin composition or molded or formed product according to the present embodiment has decreased porosity and more stable electrical insulation properties and thermal conductivity.
Since the resin composition or molded formed product according to the present embodiment has excellent dielectric breakdown voltage performance (breakdown voltage performance) and excellent thickness-direction thermal conductivity, it can be used as a heat dissipation member for exothermic electronic parts such as power devices, transistors, thyristors, CPUs (central processing units), and the like. In addition, it can be used as a heat dissipation adhesive or heat dissipation sheet for fixing and insulating electronic parts such as power semiconductors, semiconductor elements including optical semiconductors, semiconductor devices, metal plates for circuit, circuits made of the metal plate, circuit substrates, and hybrid integrated circuits.

(I) Resin component (A)
The kind of the resin component (A) is not particularly limited and thermosetting resins or thermoplastic resins may be used either singly or in combination of two or more. Specific examples of the thermosetting resin include epoxy resins, urethane resins, phenol resins, (meth)acryloyl-containing resins, vinyl ester resins, and silicone resins. Of these, resins containing an epoxy resin are preferred from the standpoint of adhesion with a base material. The term “(meth)acryloyl” as used herein means acryloyl and/or methacryloyl. The term “(meth)acryl” also means acryl and/or methacryl.

When the molded or formed product of the resin composition of the present embodiment is used, for example, as a thermoconductive adhesive sheet of a power semiconductor module, not only adhesion with a base material but also heat resistance and breakdown voltage become necessary. A resin component satisfying these required characteristics should be selected. First, the thermosetting resin will be described.
As the resin component (A), a first thermosetting resin (A-1) as described below can be used. Described specifically, the first thermosetting resin (A-1) has at least one kind of a reactive group selected from an epoxy group and a (meth)acryloyl group and the number of the reactive groups per molecule is three or more. It is a resin having a molecular weight, per reactive group, of 80 or more but less than 200 and has a number average molecular weight of 300 or more but less than 1000.

The first thermosetting resin (A-1) is added in order to increase crosslink density of the resin composition of the present embodiment after curing and imparting the cured product with heat resistance and breakdown voltage. The thermosetting resin having, in one molecule thereof, three or more reactive groups and having a molecular weight, per reactive group, less than 200 can improve the crosslink density and improve the heat resistance. The resin composition containing the thermosetting resin having a number-average molecular weight less than 1000 has moderate fluidity and excellent moldability or formability. The molded or formed product of such a resin composition therefore does not easily have minute cracks or voids and has excellent voltage resistance.

Examples of the epoxy-containing resin as the first thermosetting resin (A-1) include glycidylamine epoxy resins, heterocyclic epoxy resins, and trifunctional or higher functional aromatic epoxy resins.
Specific examples of the glycidylamine epoxy resins include N,N,N’,N’-tetraglycidyl-4,4’-diaminodiphenylmethane (trade name: “Epotote YH-434L”, product of NIPPON STEEL & SUMIKIN CHEMICAL CO., LTD.), N,N,N’,N’-tetraglycidyl-1,3-benzenedi (methanamine) (trade name: “TETRAD-X”, product of MITSUBISHI GAS CHEMICAL COMPANY, INC.), 4-(glycidyloxy)-N,N-diglycidylaniline, and 3-(glycidyloxy)-N,N-diglycidylaniline.

Specific examples of the heterocyclic epoxy resins include triglycidyl isocyanurate (“TEPIC-S”, trade name; product of Nissan Chemical Industries, Ltd.). Specific examples of the trifunctional or higher functional aromatic epoxy resins include tetrafunctional naphthalene epoxy resin (“EPICLON HP-4700”, trade name; product of DIC Corporation), and triphenylmethane epoxy resin (“1032H60”, trade name; product of Mitsubishi Chemical Corporation).

Examples of the (meth)acryloyl-containing resin as the first thermosetting resin (A-1) include (meth)acrylic acid esters of a polyol having, in one molecule thereof, three or more hydroxyl groups and heterocyclic (meth)acrylates.
Specific examples of the (meth)acrylic acid esters of a polyol having, in one molecule thereof, three or more hydroxyl groups include trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol pentaacrylate, and dipentaerythritol hexaacrylate.

Specific examples of the heterocyclic (meth)acrylates include resins such as tris(2-acryloyloxyethyl) isocyanurate and tris(2-methacryloyloxyethyl) isocyanurate.
The intended performance can be achieved, for example, by adjusting the amount of the first thermosetting resin (A-1) to 25 mass% or more but not more than 60 mass% of the resin component (A). More preferably, the amount is 30 mass% or more but not more than 50 mass%. When the amount of the first thermosetting resin is 25 mass% or more, preferable heat resistance and breakdown voltage characteristic can be achieved, while when the amount is 60 mass% or less, the cured product has excellent flexibility.

As the resin component (A), a second thermosetting resin (A-2) as described below can also be used. Described specifically, the second thermosetting resin (A-2) is, for example, an epoxy resin other than the first thermosetting resin (A-1) or a (meth)acryloyl-containing resin. As described above, an epoxy resin is particularly preferred from the standpoint of adhesive properties. The second thermosetting resin (A-2) is incorporated in order to control the fluidity of the resin composition of the present embodiment or adhesive properties and flexibility of the cured product.

Examples of the epoxy resin as the second thermosetting resin (A-2) include bifunctional glycidyl ether epoxy resins, glycidyl ester epoxy resins, polyfunctional epoxy resins not included in the first thermosetting resins (A-1), and linear aliphatic epoxy resins.
Specific examples of the bifunctional glycidyl ether epoxy resins include bisphenol A epoxy resins, bisphenol F epoxy resins, bisphenol S epoxy resins, hydrogenated bisphenol A epoxy resins, and biphenyl epoxy resins.

Specific examples of the glycidyl ester epoxy resins include glycidyl hexahydrophthalate and glycidyl esters of a dimer acid.
Specific example of the polyfunctional epoxy resin not included in the first thermosetting resins (A-1) include phenol novolac epoxy resins, cresol novolac epoxy resins, biphenylaralkyl epoxy resins, and glycidyl ether epoxy resins such as naphthalene aralkyl epoxy resins.

Specific examples of the linear aliphatic epoxy resins include linear aliphatic epoxy resins such as epoxydated polybutadiene and epoxydated soybean oil. The above-described epoxy resins may be used either singly or in combination of two or more.
Examples of the (meth)acryloyl-containing resins include (meth)acrylic acid esters of a diol compound and (meth)acrylic acid esters of an adduct of a polyol with caprolactone.

Specific examples of the (meth)acrylic acid esters of a diol compound include ethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(methacrylate), neopentyl glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, and tripropylene glycol di(meth)acrylate.
Specific examples of the (meth)acrylic acid esters of an adduct of a polyol with caprolactone include (meth)acrylic acid ester of pentaerythritol×caprolactone and (meth)acrylic acid ester of dipentaerythritol×caprolactone.

Further, when the epoxy resin is used as the first thermosetting resin (A-1) and the second thermosetting resin (A-2), a curing agent or a curing accelerator (curing catalyst) may be added. Examples of the curing agent include alicyclic acid anhydrides such as methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, and himic anhydride, aliphatic acid anhydrides such as dodecenylsuccinic anhydride, and aromatic acid anhydrides such as phthalic anhydride and trimellitic anhydride.

Additional examples include bispheols such as 2,2-bis(4-hydroxyphenyl)propane (another name: bisphenol A), 2-(3-hydroxyphenyl)-2-(4’-hydroxyphenyl)propane, bis(4-hydroxyphenyl)methane (another name: bisphenol F), and bis(4-hydroxyphenyl)sulfone (another name: bisphenol S), phenol resins such as phenol×formaldehyde resins, phenol×aralkyl resins, naphthol×aralkyl resins, and phenol-dicyclopentadiene copolymers, and organic dihydrazides such as dicyandiamide and adipic acid dihydrazide.

Examples of the curing catalyst include tris(dimethylaminomethyl)phenol, dimethylbenzylamine, and 1,8-diazabicyclo(5,4,0)undecene, and derivatives thereof. Additional examples of the curing catalyst include imidazoles such as 2-methylimidazole, 2-ethyl-4-methylimidazole, and 2-phenylimidazole. They may be used either singly or in combination of two or more.

When the (meth)acryloyl-containing resin is used as the first thermosetting resin (A-1) and the second thermosetting resin (A-2), an organic peroxide may be added as the curing agent. Specific examples of the organic peroxide include diisopropyl peroxydicarbonate, t-butylperoxy-2-ethylhexanoate, t-hexylperoxy-2-ethylhexanoate, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate, t-amylperoxy-2-ethylhexanoate, lauryl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexanone, cyclohexanone peroxide, methyl ethyl ketone peroxide, dicumyl peroxide, t-butylcumyl peroxide, and cumene hydroperoxide.

Further, a thermoplastic resin (A-3) may be used as the resin component (A). The thermoplastic resin (A-3) imparts an uncured or cured resin composition or a molded or formed product (for example, sheet) thereof with moderate flexibility and plays an important role, for example, in improving workability during handling of the sheet or as a stress relaxing agent of the cured composition.
Specific examples of the thermoplastic resin (A-3) include polyvinyl butyral resins, polyester resins, phenoxy resins, and acrylic copolymers. Polyvinyl butyral resins and polyester resins are preferred particularly in order to impart the resin composition with flexibility. These thermoplastic resins (A-3) are added in an amount of preferably 5 mass% or more but not more than 30 mass%, more preferably 10 mass% or more but not more than 25 mass%, each based on the resin component (A). The amounts of 5 mass% or more can provide a resin composition or a molded or formed product having sufficient flexibility, while the amounts not more than 30 mass% provide a resin composition having improved moldability or formability.

(II) Ceramic powder (C) other than hexagonal boron nitride powder (B)
The resin composition of the present embodiment can further contain the ceramic powder (C) other than the hexagonal boron nitride powder (B). The ceramic powder (C) is an important component for enhancing the fluidity of a resin composition and at the same time, enhancing the thickness-direction thermal conductivity of a cured resin composition. Although the kind of the ceramic powder (C) is not particularly limited, one or more ceramic powder selected from, for example, alumina powder, aluminum nitride powder, glass beads, zinc oxide powder, magnesia powder, silica powder (for example, fused silica), and cubic boron nitride (cBN) can be used. Of these, aluminum nitride powder is particularly preferred because it has high thermal conductivity (200 W/m×K).

As the ceramic powder (C), however, that having compression rupture strength of 100 MPa or more but not more than 1500 MPa is preferably used. When the compression rupture strength of the hexagonal boron nitride powder (B) falls within a range of 1.0 MPa or more but not more than 20 MPa, it is presumed that partial cohesive failure or deformation of agglomerated particles of the hexagonal boron nitride powder (B) occurs during heating or pressuring and they are brought into surface contact with the particles of the ceramic powder (C) having large compression rupture strength. As a result, a heat-transfer path route efficient in thickness direction is formed in the resin composition or molded or formed product thereof and it has improved thickness-direction thermal conductivity. From the standpoint of obtaining a molded or formed product with high thermal conductivity, the compression rupture strength of the hexagonal boron nitride powder (B) is preferably from 1.0 to 3.3 MPa.

Specific examples of the ceramic powders (C) having large compression rupture strength include “FAN-f50-J” (volume-average particle size: 50 μm) and “FAN-f30” (volume-average particle size: 30 μm), each aluminum nitride of Furukawa Denshi Co., Ltd. Additional examples include “CB-A50S” (volume-average particle size: 50 μm), “CB-A30S” (volume-average particle size: 28 μm), “CB-A20S” (volume-average particle size: 21 μm), “AS-10” (volume-average particle size: 39 μm), “AS-20” (volume-average particle size: 22 μm), “AL-17-1” (volume-average particle size: 60 μm), “AL-17-2” (volume-average particle size: 60 μm), and “AL-13KT” (volume-average particle size: 97 μm), each alumina of Showa Denko K.K. Further examples include “J-320” (volume-average particle size: 50 μm), “GB301S” (volume-average particle size: 50 μm), “GB301SA-PN” (volume-average particle size: 50 μm), “GB301SB-PN” (volume-average particle size: 50 μm), and “GB-301SC-PN” (volume-average particle size: 50 μm), each glass beads of Potters-Ballotini Co., Ltd. Still further examples include “FB-20D” (volume-average particle size: 23 μm) and “FB-950” (volume-average particle size: 24 μm), each fused silica of DENKI KAGAKU KOGYO KABUSHIKI KAISHA.

The compression rupture strength of the hexagonal boron nitride powder (B) or ceramic powder (C) can be measured using, for example, a micro compression tester (for example, “MCT-510”) of SHIMADZU CORPORATION. This micro compression tester can measure compression rupture strength by applying a test force to powder particles fixed between an upper pressure terminal and a lower pressure plate while increasing the force at a certain increasing rate and then measures the deformation amount of the powder particles at this time. The compression rupture strength can be calculated based on the following formula (1) shown in JIS R 1639-5 (2007):
Cs=2.48×(P/πd²) ... (1)
In the formula (1), Cs represents compression rupture strength (MPa), P represents test force (N), d represents the particle size (mm) of powder particles, and π represents a circular constant.

The volume-average particle size of the ceramic particle (C) is preferably 20 μm or more but not more than 100 μm, more preferably 40 μm or more but not more than 80 μm. When the ceramic particle (C) has a volume-average particle size of 20 μm or more, deformation and cohesive failure of agglomerated particles of the hexagonal boron nitride powder (B) occur efficiently. When they have a volume-average particle size not more than 100 μm, the resulting resin composition can be applied smoothly to a base material.
In the resin composition, the resin component (A), hexagonal boron nitride powder (B), and the ceramic powder (C) are mixed so as to give a content of the resin component (A) of 5 mass% or more but not more than 40 mass%, a content of the hexagonal boron nitride powder (B) of 5 mass% or more but not more than 75 mass%, and a content of the ceramic powder (C) of 10 mass% or more but not more than 90 mass%.

When the content of the resin component (A) in the resin composition falls within a range of 5 mass% or more but not more than 40 mass%, the resin composition thus obtained has the hexagonal boron nitride powder (B) and ceramic powder (C) dispersed well therein. When the content of the hexagonal boron nitride powder (B) in the resin composition falls within a range of 5 mass% or more but not more than 75 mass% and the content of the ceramic powder (C) falls within a range of 10 mass% or more but not more than 90 mass%, the resin composition thus obtained succeeds in having good thermal conductivity, porosity, and breakdown voltage.

In the resin composition, a total content of the hexagonal boron nitride powder (B), the ceramic powder (C), and the other inorganic particles except for a volatile component is preferably 50 mass% or more but not more than 95 mass% or less, more preferably 60 mass% or more but not more than 90 mass% or less. When the total content is 95 mass% or less, the resulting resin composition has improved adhesive properties and strength. When the total content is 50 mass% or more, on the other hand, the resulting resin composition can have sufficient heat dissipation properties.

A preferable mass ratio [ceramic powder (C)]/[hexagonal boron nitride powder (B)] of the ceramic powder (C) having large compression rupture strength to the hexagonal boron nitride powder (B) is preferably 0.1 or more but not more than 20, more preferably 0.2 or more but not more than 10. When the mass ratio is 0.1 or more, deformation or cohesive failure of agglomerated particles of the hexagonal boron nitride powder (B) occurs sufficiently. When the mass ratio is not more than 20, on the other hand, the resulting resin composition is excellent in heat dissipation properties because shortage of the agglomerated particles of the hexagonal boron nitride powder (B) to fill voids therewith does not occur due to deformation and cohesive failure.

(III) Coupling agent
Further, the resin composition of the present embodiment may contain a coupling agent for the purpose of improving the dispersibility of inorganic fillers such as hexagonal boron nitride powder (B) and ceramic powder (C) in the resin component (A), for the purpose of improving processability of the resin composition, for the purpose of improving adhesive properties to a base material, or the like. For example, the hexagonal boron nitride powder (B) may be reacted with a coupling agent for surface treatment.

Examples of the coupling agent include silane series coupling agents, titanate series coupling agents, and aluminum series coupling agents. Of these, silane series coupling agents are most preferred because they can improve the above-described dispersibility, processability, adhesive properties, and the like.
Of the silane coupling agents, aminosilane compounds such as g-aminopropyltrimethoxysilane, g-aminopropyltriethoxysilane, g-(2-aminoethyl)aminopropyltrimethoxysilane, g-(2-aminoethyl)aminopropyltriethoxysilane, g-anilinopropyltrimethoxysilane, g-anilinopropyltriethoxysilane, N-β-(N-vinylbenzylaminoethyl)-g-aminopropyltrimethoxysilane, and N-β-(N-vinylbenzylaminoethyl)-g-aminopropyltriethoxysilane are particularly preferred.

(IV) Other additives
The resin composition of the present embodiment may contain other additives such as inorganic filler for the purpose of controlling the properties other than heat dissipation properties insofar as their amount does not inhibit the heat dissipation properties. Examples of such an inorganic filler include aluminum hydroxide for imparting the resin composition with flame retardancy, fumed silica for controlling the fluidity of the resin composition, and an inorganic pigment such as titanium oxide used for coloring.

(V) Fluidity of resin composition
Further, the resin composition of the present invention is preferably equipped with fluidity necessary for molding or forming processing because it is sometimes used after molded or formed into a sheet or the like. To obtain good molding or forming processability while achieving high thermal conductivity in thickness direction, the resin composition has a melt viscosity, as measured at 150°C using a Koka type flow tester, of 0.01 Pa×s or more but not more than 500 Pa×s, 0.1 Pa×s or more but not more than 200 Pa×s. When the melt viscosity is 0.01 Pa×s or more but not more than 500 Pa×s, a molded or formed product such as sheet can be prepared easily from the resin composition having preferable fluidity.

A preparation process of a melt viscosity measurement sample and a measuring method of melt viscosity will hereinafter be described in detail.
(V-1) Preparation process of melt viscosity measurement sample
A cresol novolac epoxy resin “N-680”, product of DIC Corporation and a novolac phenol resin “Shonol BRN-5384Y”, product of Showa Denko K.K. are weighed and mixed to give a mass ratio of 2:1. To the resulting resin composition are added 1.0 mass%, in terms of a resin total, of a silane coupling agent “Z-6040”, product of Toray Industries, Inc. and an adequate amount of an organic solvent to obtain a resin solution. The resin solution contains neither a curing agent nor a curing catalyst in order to prevent curing during measurement of the melt viscosity.
The resin solution thus obtained and the hexagonal boron nitride powder (B) are weighed so as to give a resin content, in the resin solution, of 43.2 mass% and a content of the hexagonal boron nitride powder (B) of 55.8 mass% and a planetary centrifugal mixer (“Awatori Rentaro ARE-310”, product of THINKY CORPORATION) is charged with them. They are mixed until the hexagonal boron nitride powder (B) is dispersed uniformly. The resulting mixture is used as a sample mixture.

Next, the sample mixture is applied onto a release polyethylene terephthalate (PET) sheet (“PET 100SG2S”, product of PANAC Co., Ltd.) with a thickness of 250 mm to obtain a sample sheet. The resulting sample sheet is dried at 70°C for 30 minutes under normal pressure and then, dried at 70°C for 30 minutes under reduced pressure to remove the organic solvent from the sample mixture. The dried sample mixture is released from the release PET sheet and then ground into a melt viscosity measurement sample.

(V-2) Measurement method of melt viscosity
1.6 g of the melt viscosity measurement sample is weighed and melt viscosity of it is measured using a Koka type flow tester (“CFT-500A”, product of SHIMADZU CORPORATION). The melt viscosity is measured at 150°C at each measuring load of 6 MPa, 4 MPa, 2 MPa, and 1 MPa and the lowest melt viscosity among the measurement results at these measuring loads is designated as a melt viscosity of the melt viscosity measurement sample.

Further, in order to obtain good melt fluidity while achieving high thickness-direction thermal conductivity, the melt viscosity of the melt viscosity measurement sample having a content of the hexagonal boron nitride powder (B) of 55.8 mass% is preferably 0.01Pa×s or more but not more than 500Pa×s, more preferably 0.1Pa×s or more but not more than 200Pa×s when the sample is measured at a temperature of 150°C under a measuring load of 6 MPa.

(VI) Preparation process of resin composition
The preparation process of the resin composition of the present embodiment is not particularly limited and it can be prepared in various processes. The following is one example of it.
First, for example, a thermosetting resin as the resin component (A) is mixed with a curing agent or curing accelerator in an amount necessary for curing the thermosetting resin. Next, after addition of a solvent to the resulting mixture if necessary, the hexagonal boron nitride powder (B) and the ceramic powder (C) having large compression rupture strength are added and they are preliminarily mixed. The resulting preliminary mixture is kneaded in a planetary mixer or the like to obtain a resin composition. When a resin composition containing a coupling agent is prepared, the coupling agent may be added at any stage before kneading.

The resin composition thus obtained is placed on a base material and cured while applying a predetermined pressing pressure to obtain a cured product excellent in heat dissipation properties. In general, high filling of an inorganic filler to improve the heat dissipation properties generates voids in the cured product so that a pressing pressure should be increased in the pressing step. As described above, however, since the resin composition of the present embodiment contains the hexagonal boron nitride powder (B), the agglomerated particles of the hexagonal boron nitride powder (B) which has undergone deformation or cohesive failure is presumed to enter void portions to decrease the porosity. As a result, the resin composition of the present embodiment has excellent thermal conductivity and heat dissipation properties.

In order to achieve excellent thermal conductivity and heat dissipation properties, pressing pressure for deformation or cohesive failure of agglomerated particles of the hexagonal boron nitride powder (B), fluidity of the resin composition, and temperature for controlling curing are important. The pressure falls within a range of preferably 1 MPa or more but not more than 100 MP, more preferably 2MPa or more but not more than 50 MPa. It is presumed that at the pressure not more than 100 MPa or less, cohesive failure of the filler other than the hexagonal boron nitride powder (B) is not likely to occur, while at the pressure of 1 MPa or more, deformation or cohesive failure of the agglomerated particles of the hexagonal boron nitride powder (B) occurs.

The temperature range is preferably 70°C or more but not more than 200°C, more preferably 90°C or more but not more than 180°C. At temperatures not more than 200°C, the resin component (A) is unlikely to be degraded by oxidation or the like and at temperatures of 70°C or more, the resin composition has sufficient fluidity. The cured product obtained from it can therefore keep flatness and at the same time, curing proceeds smoothly. When the resin composition of the present embodiment is cured under such conditions, the cured product has a porosity as low as 5% or less.

Next, processing of the resin composition of the present embodiment into a sheet will be described. When the resin composition is processed into a sheet, a resin composition dispersion or solution obtained by dispersing or dissolving the resin composition in an organic solvent is used in consideration of application properties. The resin composition dispersion or solution is applied to a support film by using an application apparatus such as applicator or knife coater and then heating to dry the organic solvent. The drying temperature is preferably 40°C or more but not more than 150°C, more preferably 50°C or more but not more than 120°C. At the drying temperature of 40°C or more, the organic solvent hardly remains, while at the drying temperature not more than 150°C, the reaction of the curable resin component (A) does not proceed excessively. The film thickness after solvent drying falls within a range of preferably 30 mm or more but not more than 500 mm, more preferably 50 mm or more but not more than 300 mm. When the film thickness is 30 mm or more, the film thus formed is not influenced by the particle size of the filler added and does not lose flatness. When the film thickness is not more than 500 μm, the organic solvent is unlikely to remain and therefore, does not adversely affect the thermal conductivity or physical properties of the cured product.

Although the preparation process of the sheet is not particularly limited, it can be prepared by applying the resin composition solution to a support film, covering a portion or entirety of the solution-applied surface of the sheet with a covering film, and then heating and pressurizing the resulting stacked body under the above-described conditions. For obtaining a thick resin composition layer, it is recommended to apply the resin composition solution to two support films, stack a solution-applied surface of one of the support films over a solution-applied surface of the other support film, and then heating and pressurizing the resulting stacked body under the above-described conditions.

The heating temperature condition to obtain a sheet in which adequate cohesive failure or deformation of agglomerated particles of the hexagonal boron nitride powder (B) is presumed to occur is preferably a softening point of the resin component (A) to be used or more. More specifically, it is preferably 50°C or more but not more than 150°C, more preferably 70°C or more but not more than 120°C. At the heating temperature condition of 50°C or more, it is presumed that the resin softens and adequate cohesive failure or deformation of agglomerated particles of the hexagonal boron nitride powder (B) occurs. This leads to improvement in thermal conductivity. At the heating temperature condition not more than 150°C, on the other hand, the curing reaction of the resin component (A) does not proceed excessively, leading to improvement in adhesive properties during mounting of electronic parts or the like.

The pressure condition is preferably 1 MPa or more but not more than 100 MPa, more preferably from 2 to 50 MPa. At the pressure condition of 1 MPa or more, cohesive failure or deformation of agglomerated particles of the hexagonal boron nitride powder (B) occurs, leading to improvement in thermal conductivity. At the pressure condition not more than 100 MPa, on the other hand, there does not occur decrease in thickness-direction thermal conductivity due to cohesive failure of most of agglomerated particle of the hexagonal boron nitride powder (B) or orientation of scaly primary particles of hexagonal boron nitride in the in-plane direction.

As a means of heating×pressurizing during sheet formation, a batch-system pressing machine can be used. A roll press capable of carrying out continuous heating×pressurizing is preferable in consideration of productivity. A line speed when the roll press is used is preferably 0.1 m/min or more but not more than 5 m/min, more preferably 0.3 m/min or more but not more than 3 m/min. The line speed of 0.1 m/min or more provides good productivity, while at the line speed not more than 5 m/min, sufficient cohesive failure or deformation of agglomerated particles of the hexagonal boron nitride powder (B) occurs, leading to improvement in thickness-direction thermal conductivity.

The support film and covering film used in preparation of the sheet can be selected depending on the using purpose of the sheet. Examples of these films include metal foils such as copper and aluminum foils, and polymer films such as polypropylene, polycarbonate, polyethylene naphthalate, polyethylene terephthalate, polytetrafluoroethylene, polyphenylene sulfide, polyvinylidene fluoride, and polyimide films. When the polymer film is used, it may be subjected to release treatment for improving the release properties from the resin composition. The support film or covering film has a thickness of preferably 10 mm or more but not more than 200 mm.

The sheet thus obtained is placed on the base material and thermally cured while applying a pressure to it using a predetermined press. A cured product excellent in heat dissipation properties can then be obtained. When an electronic part or the like is bonded to the sheet, it can be bonded by releasing at least one of support films from the sheet, attaching the electronic part or the like to the resin composition surface, and then curing under heat×pressure. For bonding of an electronic part or the like, an excessively large pressure may damage the electronic part or the like so that a pressure should be adjusted to fall within a pressure range permitting bonding without damaging the electronic part or the like.

When the sheet is used for bonding of an electronic part or the like, pressurization×heating may be performed under conditions at which only bonding occurs. The pressure range is preferably 0.1 MPa or more but not more than 10 MPa, more preferably 0.5 MPa or more but not more than 8 MPa. The pressure of 0.1 MPa or more allows bonding, while the pressure not more than 10 MPa is less likely to damage an electronic part or the like. The temperature range is preferably 70°C or more but not more than 200°C, more preferably 90°C or more but not more than 180°C. At the temperature not more than 200°C, the resin component (A) is less likely to be degraded due to oxidation or the like, while at the temperature of 70°C or more, the electronic part or the like is likely to bond to the sheet because the resin composition has sufficient fluidity.

Example

The present invention will hereinafter be described in further detail by Examples and Comparative Examples.
(1) Preparation of hexagonal boron nitride powder (B)
(1-1) Preparation of crude hexagonal boron nitride powder
Boric acid (4 g), 2 g of melamine, and 1 g of water were mixed by stirring. The reaction mixture was poured in a mold and pressurized to obtain a molded product in tablet form. The resulting molded product was dried at 300°C for one hour in a drier and then preliminarily baked at 1000°C under an ammonia atmosphere. The resulting preliminary baked product was ground into a crude hexagonal nitride powder.

(1-2) Preparation of hexagonal boron nitride powder (B)
To 100 parts by mass of the crude hexagonal boron nitride powder was added 10 parts by mass of an aqueous polyvinyl alcohol solution having a concentration of 2.5 mass%. After mixing under stirring in a mixer, the reaction mixture was poured in a mold and then pressurized to obtain a molded product in tablet form. The resulting molded product was dried at 200°C for one hour in a drier to obtain a dried product. The resulting dried product was baked at 1800°C for 4 hours in a nitrogen gas atmosphere and a baked product was obtained. The baked product thus obtained was ground and classified by a dry vibratory sieve into powders above and below a 106-mm sieve. After classification, the powder below the 106-mm sieve was collected as the hexagonal boron nitride powder (B).

By designating a sample collected before the preparation conditions reached a stable range as a hexagonal boron nitride powder of Comparative Preparation Example and a sample collected after the preparation conditions reached a stable range as a hexagonal boron nitride powder of Preparation Example, hexagonal boron nitride powders a and b of Preparation Examples 1 and 2 and hexagonal boron nitride powders c, d, and e of Comparative Preparation Examples 1 to 4 were obtained. The hexagonal boron nitride powder f of Comparative Preparation Example 4 is a mixture obtained by mixing the classified product (classified into a range of from 45 to 106 mm) of the hexagonal boron nitride powder c of Comparative Preparation Example 1 and the hexagonal boron nitride powder d of Comparative Preparation Example 2 at a mass ratio of 1:1.

The hexagonal boron nitride powders a and b of Preparation Examples 1 and 2 satisfy all of the above-described three conditions of the present invention; the hexagonal boron nitride powders c, d, and e of Comparative Preparation Examples 1 to 3 do not satisfy at least one of the above-described three conditions of the present invention; and the hexagonal boron nitride powder f of Comparative Preparation Example 4 has two maximum peaks before ultrasonic irradiation.
The BET specific surface area and pressure rupture strength of each of the hexagonal boron nitride powders a to f were measured. The BET specific surface area was measured using a full-automatic BET specific surface area measuring apparatus “Multisorb 16”, product of Yuasa Ionics Co., Ltd. The compression rupture strength was measured by the above-described method by using a micro compression tester “MCT-510” of SHIMADZU CORPORATION.

The hexagonal boron nitride powders a to f were each subjected to ultrasonic irradiation treatment (rupture treatment). The ultrasonic rupture treatment was given to a dispersion obtained by dispersing 0.06 g of each of the hexagonal boron nitride powders a to f in 50 g of water by using an ultrasonic irradiation apparatus having a bolt-clamped electrostrictive oscillating element. The ultrasonic irradiation treatment was performed under the following conditions: output of ultrasonic irradiation apparatus of 150 W, oscillatory frequency of 19.5 kHz, and irradiation time of 3 minutes.
The particle size distribution of each of the hexagonal boron nitride powders a to f before and after the ultrasonic irradiation treatment was then measured. The particle size distribution curve thus obtained is shown in FIGS. 1 to 6. From the particle size distribution curve before and after the ultrasonic irradiation treatment, a first maximum peak height and a second maximum peak height were read and at the same time, a ratio of the first maximum peak height to the second maximum peak height [first maximum peak height/second maximum peak height] was calculated. The measurement results of them are collectively shown in Table 1.

(2) Preparation 1 of resin composition
A resin composition was prepared by mixing 73.9 parts by mass of any one of the hexagonal boron nitride powder a to f, 26.1 parts by mass of a bisphenol A epoxy resin (“Epotote YD-128”, trade name; product of NIPPON STEEL & SUMIKIN CHEMICAL CO., LTD., epoxy equivalent: 190 g/eq), and 0.8 part by mass of an imidazole compound (1-(cyanoethyl)-2-undecylimidazole) as a curing catalyst and kneading the resulting mixture using a planetary centrifugal mixer (“Awatori Rentaro ARE-310”, product of THINKY CORPORATION).

The resulting resin composition was formed into a sheet having a thickness of from 200 to 500 mm and cured by heating and pressurizing using a hot press for 30 minutes at a pressure of 6 MPa and a temperature of 130°C and thus, pressed cured sheets of Examples 11 and 12 and Comparative Examples 11 to 14 were prepared. The thickness-direction thermal conductivity and porosity of the resulting pressed cured sheets were measured. The results are shown in Table 2.

Respective measurement methods of thermal conductivity and porosity will next be described. First, a measurement method of specific gravity necessary for calculation of thermal conductivity and porosity will be described. The specific gravity ρ of a test sample is calculated by measuring each of the mass W(a) of the test sample in the air and the mass W(f1) of it in water using an electronic balance (“CP224S”) and a specific gravity/density measurement kit (“YDK01”/”YDK01-OD”/”YDK01LP”), each product of Sartorius Mechatronics Japan K.K., and a specific gravity of the test sample is calculated based on the following formula (2):
ρ=W(a)×ρ(fl)/{W(a)-W(fl)} ...(2)
wherein in the formula (2), ρ(fl) represents a density of liquid (water).

The measurement method of thermal conductivity will next be described. Thermal diffusivity at 25°C of a square test sheet with a side length of 10 mm obtained by cutting a pressed cured sheet having such a thickness as described above in the preparation of the resin composition is measured using a thermal conductivity measurement apparatus (“LFA 447 Nano Flash”), product of NETZSCH. Further, based on the specific gravity of the pressed cured sheet measured by the above method and a specific heat of the pressed cured sheet determined separately, thickness-direction thermal conductivity of the pressed cured sheet is calculated using the following formula (3):
Thermal conductivity (W/m×K) = (thermal diffusivity) × (specific heat) × (specific gravity) ... (3)

Next, the measurement method of porosity will be described. First, the specific gravity of a resin component which is a raw material of the resin composition and a filler such as hexagonal boron nitride powder are measured using the above method. From the resulting specific gravities and mixed amounts (mass%), a theoretical specific gravity of the pressed cured sheet is then derived. Next, the specific gravity (actual specific gravity) of the pressed cured sheet is measured using the above method and porosity is calculated using the following formula (4):
Porosity (%) = 100-((actual specific gravity/theoretical specific gravity) × 100) ... (4)

It is apparent from the results shown in Table 2 that the pressed cured sheets of Examples 11 and 12 using the hexagonal boron nitride powders a and b of Preparation Examples 1 and 2 have porosity smaller than that of the pressed cured sheets of Comparative Examples 11 to 14 and are well balanced between thickness-direction thermal conductivity and porosity.
The hexagonal boron nitride powder f used in Comparative Example 14 is a 1:1 (mass ratio) mixture of the classified product (classified into a range of from 45 to 106 mm) of the hexagonal boron nitride powder c of Comparative Preparation Example 1 and the hexagonal boron nitride powder d of Comparative Preparation Example 2. The pressed cured sheet of Comparative Example 14, similar to Comparative Examples 11 to 13, has large porosity.

(3) Preparation 2 of resin composition
A resin composition was prepared by mixing any one of the hexagonal boron nitride powders a to f, a bisphenol A epoxy resin (“Epotote YD-128”, trade name; product of NIPPON STEEL & SUMIKIN CHEMICAL CO., LTD., epoxy equivalent: 190 g/eq) as the resin component (A), the ceramic powder (C) other than the hexagonal boron nitride powder (B), and an imidazole compound (1-(cyanoethyl)-2-undecylimidazole, “CURESOL C11Z-CN”, product of SHIKOKU CHEMICALS CORPORATION) as a curing catalyst and then treating the resulting mixture in a manner similar to that of “Preparation 1 of resin composition”. Amounts of each raw material added are as shown in Table 3 (unit: parts by mass).

Pressed cured sheets of Examples 21, 22, 32, and 42 and Comparative Examples 21 to 24 were prepared from the resulting resin composition in a manner similar to that of “Preparation 1 of resin composition”. The thickness-direction thermal conductivity and porosity of each of the pressed cured sheets were measured. The results are shown in Table 3.
An inorganic filler used as the ceramic powder (C) is spherical alumina (“CB-A50S”, product of Showa Denko K.K., volume-average particle size: 50 μm), aluminum nitride (“FAN-f50-J”, product of Furukawa Denshi Co., Ltd., volume-average particle size: 50 μm), or spherical glass beads (“GB301S”, product of Potters-Ballotini Co., Ltd., volume-average particle size: 50 μm).

It is apparent from the results of Table 3 that in particular, the pressed cured sheets containing aluminum nitride as the ceramic powders (C) have higher thermal conductivity and are better balanced between the thickness-direction thermal conductivity and porosity, compared with the pressed cured sheets not containing the ceramic powders (C) (refer to Table 2).
(4) Preparation 3 of resin composition
A cresol novolac epoxy resin (“EPICLON N-680”, trade name; product of DIC Corporation, number average molecular weight: 1280, epoxy equivalent: 218 g/eq) and a novolac phenol resin (“Shonol BRN-5834Y”, trade name; product of Showa Denko K.K.) were weighed and mixed to give a mass ratio of 2:1. To the resulting resin mixture were added 1 mass%, based on a total amount of the resin composition (total amount of the resin component (A), the hexagonal boron nitride powder (B), and the silane coupling agent) of 3-glycidoxypropyltrimethoxysilane (silane coupling agent “Z-6040”, product of Toray Industries, Inc.) and an organic solvent to obtain a resin solution.

The resin solution thus obtained and any one of the hexagonal boron nitride powders a to f were weighed so as to give a resin content, in the resin solution, of 43.2 mass% and an amount of the hexagonal boron nitride powder (B) of 55.8 mass% and they were poured in a planetary centrifugal mixer (“Awatori Rentaro ARE-310”, product of THINKY CORPORATION). They were mixed until the hexagonal boron nitride powder (B) was dispersed uniformly and the resulting mixture was used as a sample mixture.

Then, the sample mixture was applied onto a release PET sheet (“PET 100SG2S”, product of PANAC Co., Ltd.) with a thickness of 250 mm to obtain a sample sheet. The resulting sample sheet was dried at 70°C for 30 minutes under normal pressure and then, dried at 70°C for 30 minutes under reduced pressure to remove the organic solvent from the sample mixture. The dried sample mixture was released from the release PET sheet and then ground into melt viscosity measurement samples of Examples 51 and 52 and Comparative Examples 51 to 54.

The melt viscosity measurement sample (1.6 g) was weighed and the melt viscosity of it was measured using a Koka type flow tester (“CFT-500A”, product of SHIMADZU CORPORATION). The melt viscosity was measured at 150°C at each of the following loads of 6 MPa, 4 MPa, 2 MPa, and 1 MPa and the lowest melt viscosity among the measurement results at the above measuring loads was designated as a melt viscosity of the melt viscosity measurement sample. The results are shown in Table 4.

The melt viscosity is an indicator of fluidity which is an important characteristic when a molded or formed product such as sheet is obtained from a resin composition containing a resin component. It is apparent that the respective samples of Examples 51 and 52 using the hexagonal boron nitride powders a and b of Preparation Examples 1 and 2 each have low melt viscosity and are well balanced between the fluidity and the above-described thermal conductivity or porosity.

(5)　Preparation 4 of resin composition
A resin solution was prepared by using, as the resin component (A), 35 parts by mass of N,N,N’,N’-tetraglycidyl-4,4’-diaminodiphenylmethane (“Epotote YH-434L”, trade name; product of NIPPON STEEL & SUMIKIN CHEMICAL CO., LTD.), 10 parts by mass of a bisphenol A epoxy resin (“Epotote YD-128”, trade name; product of NIPPON STEEL & SUMIKIN CHEMICAL CO., LTD.), 25 parts by mass of a polyvinyl butyral resin (“S-LEC SV-02”, trade name; product of SEKISUI CHEMICAL CO.,LTD.) which was a thermosetting resin, 10 parts by mass of a phenol novolac resin (“Shonol BRN-5384Y”, trade name; product of Showa Denko K.K.), and 20 parts by mass of a polyfunctional phenol resin (“SN-395”, trade name; product of NIPPON STEEL & SUMIKIN CHEMICAL CO., LTD.); and adding 150 parts by mass of propylene glycol monomethyl ether (product of Wako Pure Chemical Industries, Ltd.) as a solvent to dissolve the resin component therein.

To the resulting resin solution was added 0.3 part by mass of 1-(cyanoethyl)-2-undecylimidazole (“CURESOL C11Z-CN”, trade name; product of SHIKOKU　CHEMICALS　CORPORATION) as a curing catalyst. To the resulting resin solution were added further 125 parts by mass of the hexagonal boron nitride powder a as the hexagonal boron nitride powder (B), 452 parts by mass of aluminum nitride (“FAN-f50”, trade name; product of Furukawa Denshi Co., Ltd.) as the ceramic powder (C) other than the hexagonal boron nitride powder (B), and 25 parts by mass of propylene glycol monomethyl ether as a solvent. The resulting resin solution was kneaded in a planetary centrifugal mixer (“Awatori Rentaro ARE-310”, product of THINKY CORPORATION) to obtain a resin composition.

The resin composition thus prepared was applied to a 100-mm thick electrolytic copper foil by means of an auto bar coater (“PI-1210”, product of TESTER SANGYO CO,.LTD.) to give a film thickness of about 300 mm and a width of 10 cm after solvent drying. After drying at 70°C for 20 minutes under normal pressure, the resulting film was dried at 70°C for 20 minutes under reduced pressure to remove the solvent. A sheet having a film of the resin composition formed on the electrolytic copper foil was obtained.

A PET film was laminated to the surface of the resulting sheet on which the resin composition was formed. The laminate thus obtained was heated×pressurized three times using a tabletop roll press (“SA-601”, name of apparatus, product of TESTER SANGYO CO,.LTD.) under the conditions of temperature of 120°C, applied pressure of 6 MPa, and rolling rate of 0.3 m/min to obtain a sheet of Example 61 with a thickness of about 200 mm.

In a manner similar to that used in Example 61 except that the kinds and amounts of the hexagonal boron nitride powder (B) and the ceramic powder (C) were changed as shown in Table 5, sheets of Examples 62, 72, and 82, and Comparative Examples 61 to 64 were obtained. The breakdown voltage, thermal conductivity, and porosity of the sheets were measured. The measurement results are shown in Table 5 collectively. The measurement method of the thermal conductivity and porosity are similar to those of the pressed cured sheet. The following is the measurement method of breakdown voltage.

The sheets were each cut into a square with a side length of 50 mm and a PET film was released from them. The resulting sheet made of the resin composition was then press cured at a temperature of 180°C and pressure of 3 MPa while being sandwiched between 35 mm-thick electrolytic copper foils with a side length of 70 mm. From the resulting double-sided copper-clad sheet, the copper foils on both sides were released and five single-layer cured sheets were obtained. A dielectric breakdown voltage test was performed using the sheets under the following conditions.

A voltage was applied to the cured sheets by using an AC power supply with a frequency of 50 Hz. The voltage application was performed by repeating a cycle of increasing the pressure to 5 kV at a rate of 5 kV/min, keeping the pressure for one minute, and then decreasing the pressure to 0 kV at a rate of 5 kV/min. Dielectric breakdown of the cure sheets was judged when energization of 1 mA or more was found during the above voltage application. For the test, a hipot and insulation resistance tester “TOS9201”, product of KIKUSUI ELECTRONICS CORPORATION was used and as an electrode, a cylindrical one with a diameter of 25 mm and a cylindrical one with a diameter of 75 mm were used.
Five cured sheets were each subjected to the above-described dielectric breakdown voltage test and their breakdown voltage was evaluated based on the percentage (pass percentage) of the sheets that passed the test without causing dielectric breakdown.

It is apparent from Table 5 that the cured sheets of Examples 61, 62, 72, and 82 prepared using the resin compositions containing the hexagonal boron nitride powder a of Preparation Example 1 or b of Preparation Example 2 showed good breakdown voltage and are well balanced between breakdown voltage and thermal conductivity, porosity, or fluidity.

Claims

A resin composition comprising a resin component (A) and a hexagonal boron nitride powder (B) comprising agglomerated particles having primary particles of hexagonal boron nitride agglomerated therein,
wherein the resin composition has a melt viscosity, as measured using a Koka type flow tester at 150°C, of 0.01 Pa.s or more but not more than 500 Pa.s,
wherein the hexagonal boron nitride powder (B) has a BET specific surface area more than 10 m²/g but less than 15 m²/g,
wherein the hexagonal boron nitride powder (B) has, in a particle size distribution curve of the hexagonal boron nitride powder (B), one maximum peak within a particle size range more than 1 mm but not more than 500 mm and the maximum peak is regarded as an initial maximum peak, and
wherein the hexagonal boron nitride powder (B) has such a characteristic that when a dispersion obtained by dispersing the hexagonal boron nitride powder (B) in water is irradiated with ultrasonic waves having an oscillating frequency of 19.5 kHz and is thereby subjected to cohesive failure treatment for cohesive failure of the agglomerated particles, the particle size distribution curve changes and becomes a particle size distribution curve satisfying all of the following three conditions:
(Condition 1) the particle size distribution curve has a first maximum peak within a particle size range of 1 μm or more but not more than 20 μm;
(Condition 2) the particle size distribution curve has a second maximum peak within a particle size range more than 1 μm but not more than 350 μm and within a particle size range more than the particle size of the first maximum peak but not more than the particle size of the initial maximum peak; and
(Condition 3) a ratio of a height of the first maximum peak to a height of the second maximum peak [height of first maximum peak]/[height of second maximum peak] is 0.1 or more but not more than 8.0.
The resin composition according to Claim 1, further comprising a ceramic powder (C) other than the hexagonal boron nitride powder (B).
The resin composition according to Claim 2,
wherein a content of the resin component (A) is 5 mass% or more but not more than 40 mass%, a content of the hexagonal boron nitride powder (B) is 5 mass% or more but not more than 75 mass%, and a content of the ceramic powder (C) is 10 mass% or more but not more than 90 mass%.
The resin composition according to Claim 2 or 3,
wherein the ceramic powder (C) is at least one selected from alumina powder, aluminum nitride powder, glass beads, zinc oxide powder, magnesia powder, and silica powder.
A process for preparing a resin composition by mixing a hexagonal boron nitride powder comprising agglomerated particles having primary particles of hexagonal boron nitride agglomerated therein with a resin component,
wherein the hexagonal boron nitride powder has a BET specific surface area more than 10 m²/g but less than 15 m²/g and has, in a particle size distribution curve of the hexagonal boron nitride powder, one maximum peak, as an initial maximum peak, within a particle size range more than 1 mm but not more than 500 mm, and
wherein when a particle size distribution curve obtained by irradiating a dispersion prepared by dispersing the hexagonal boron nitride powder in water with ultrasonic waves having an oscillating frequency of 19.5 kHz and measuring a particle size distribution of the hexagonal boron nitride powder irradiated with the ultrasonic waves satisfies all of the following three conditions, the hexagonal boron nitride powder not irradiated with ultrasonic waves is mixed with the resin component:
(Condition 1) the particle size distribution curve has a first maximum peak within a particle size range of 1 μm or more but not more than 20 μm;
(Condition 2) the particle size distribution curve has a second maximum peak within a particle size range more than 1 μm but not more than 350 μm and within a particle size range more than the particle size of the first maximum peak but not more than the particle size of the initial maximum peak; and
(Condition 3) a ratio of a height of the first maximum peak to a height of the second maximum peak [height of first maximum peak]/[height of second maximum peak] is 0.1 or more but not more than 8.0.