JP6786047B2 - Method of manufacturing heat conductive sheet - Google Patents

Description

The present invention relates to a method for producing a heat conductive sheet.

In recent years, the heat generation density inside electronic devices has been increasing year by year with the increase in high speed and high integration of circuits of electric devices or electronic devices and the increase in the mounting density of heat-generating electronic components on printed wiring boards. Therefore, there is a demand for a member having high thermal conductivity and electrical insulation that efficiently dissipates heat generated by electronic parts and the like.

Boron nitride particles are insulating ceramics, which are relatively easy to synthesize and have excellent thermal conductivity, solid lubricity, chemical stability, and heat resistance. Therefore, they are highly conductive insulating sheets. Flexible thermal conductive rubber, heat-dissipating grease, heat-dissipating sealant, semiconductor encapsulant resin and other fillers, or molten metal and molten glass molding mold release agents, solid lubricants, cosmetic raw materials, etc. It is used for various purposes.

In recent years, in high-performance electronic devices, the amount of heat generated by various electronic components has increased along with high integration, high speed, miniaturization, and weight reduction, and the heat generation is even higher than before. There is a demand for a heat conductive sheet having conductivity. Therefore, various methods for producing a heat conductive sheet in which boron nitride, which is excellent in thermal conductivity and electrical insulation, is dispersed and blended in a resin material as a heat conductive filler have been proposed.

Boron nitride particles are generally scaly due to the hexagonal crystal form, and it is known that their thermal conductivity has directional anisotropy. Although ordinary scaly boron nitride particles exhibit high thermal conductivity in the plane direction (a-axis direction) (usually, the thermal conductivity is about 250 W / mK), they are sufficient in the thickness direction (c-axis direction). No thermal conductivity can be obtained (usually, the thermal conductivity is about 2 to 3 W / mK). This is mixed with a resin and applied to the surface of the substrate as a coating liquid for the composition to form a coating film, which is dried and semi-cured, and further press-molded to be cured. The interlayer filling layer is oriented in the film surface direction, and although the formed interlayer filling layer has excellent thermal conductivity in the layer surface direction, there is a problem that it exhibits only low thermal conductivity in the thickness direction.

Conventionally, in order to improve the thermal anisotropy of such boron nitride particles, a boron nitride particle aggregate in which primary boron nitride particles are randomly assembled and a method for producing the same have been proposed (see, for example, Patent Document 1).

Further, Patent Document 2 proposes a boron nitride particle agglomerate in which primary boron nitride particles are radially assembled.

Further, Patent Document 3 proposes a boron nitride particle aggregate in which primary boron nitride particles to which a silane coupling agent having a vinyl group is added are randomly oriented and aggregated.

In Patent Document 4, for the purpose of sufficiently orienting the scaly particles to the inside, the matrix composition containing the scaly particles is extruded into a rod shape with a small cross-sectional area, and a plurality of formed rod-shaped bodies are assembled and remolded. A method has been proposed in which a heat conductive sheet is obtained by slicing after curing and forming a sheet.

Patent Document 5 proposes a method of orienting a boron nitride powder in a certain direction by a magnetic field.

Japanese Unexamined Patent Publication No. 9-202663 JP 2015-6980 Japanese Unexamined Patent Publication No. 2012-56818 Japanese Unexamined Patent Publication No. 2000-108220 JP-A-2002-69392

Since the boron nitride agglomerated particles obtained by the methods described in Patent Documents 1 to 3 are randomly agglomerated, their structure collapses due to kneading with a resin or press molding, and when the sheet is formed, it is compared with the plane direction. Therefore, there is a problem that the thermal conductivity in the thickness direction is inferior.
Further, it is difficult to use the method described in Patent Document 4 in the process of manufacturing a semi-cured sheet by semiconductor and press molding.
Furthermore, the method described in Patent Document 5 is disadvantageous in terms of productivity and cost because it requires special equipment.

The present invention has been made in view of the above problems, and provides a method for producing a heat conductive sheet capable of producing a heat conductive sheet having excellent heat conductivity in the thickness direction with high productivity and low cost. With the goal.

The present inventors have diligently studied to solve the above problems. As a result, they have found that the above problems can be achieved by performing press molding when producing a heat conductive sheet containing boron nitride particles having a predetermined structure and a resin, and have completed the present invention.

That is, the present invention is as follows.
<1>
A step of preparing a resin composition containing boron nitride particles and a resin,
The step of forming the resin composition into a sheet and
Have,
The relationship between the longest diameter (r) in the width direction of the boron nitride particles and the longest diameter (R) in the direction perpendicular to the width direction of the boron nitride particles is represented by the following formula (1). A method for manufacturing a heat conductive sheet.
0.3r <R (1)
<2>
The sheet forming step is selected from the group consisting of melt extrusion method, solution flow method, rolling method, stretching method, coating method, bar coater method, doctor blade method, press molding method, calendering method and injection molding method. The method for producing a heat conductive sheet according to <1>, which is carried out by at least one method.
<3>
The method for producing a heat conductive sheet according to <1> or <2>, further comprising a press molding step performed after the sheet forming step.
<4>
The production of the heat conductive sheet according to any one of <1> to <3>, wherein the boron nitride particles contain an aggregate of boron nitride particles in which the (0001) planes of the hexagonal boron nitride primary particles overlap each other. Method.
<5>
The method for producing a heat conductive sheet according to <4>, wherein the boron nitride particle aggregate has a columnar shape.
<6>
The item according to any one of <1> to <5>, wherein the peak intensity ratio (<002> / <100>) of the <002> plane to the <100> plane of the X-ray diffraction of the heat conductive sheet is 30 or less. How to manufacture a heat conductive sheet.
<7>
The method for producing a heat conductive sheet according to any one of <3> to <6>, wherein vacuum press molding is performed in the press molding step.
<8>
The method for producing a heat conductive sheet according to <7>, wherein the pressure of the vacuum press molding is 0.1 to 1000 MPa.
<9>
The method for producing a heat conductive sheet according to any one of <3> to <8>, wherein the press temperature in the press molding step is 50 to 400 ° C.

According to the present invention, a heat conductive sheet having excellent heat conductivity in the thickness direction can be manufactured with high productivity and low cost.

FIG. 1 is an explanatory diagram for explaining R and r in the present embodiment. FIG. 2A is an SEM image of the boron nitride particle aggregate obtained in Example 1 at a magnification of 120,000 times. FIG. 2B is an SEM image of the boron nitride particle aggregate having a magnification of 50,000 times. FIG. 3A is an SEM image in which the hexagonal boron nitride primary particles are not aggregated, that is, the conventional hexagonal boron nitride primary particles have a magnification of 25,000 times. FIG. 3B shows a portion where the end face side is observed in the SEM image of the hexagonal boron nitride primary particles in which the hexagonal boron nitride primary particles are not aggregated. FIG. 4 is an SEM image of a conventional boron nitride particle agglomerate that is randomly agglomerated at a magnification of 1000 times. FIG. 5 is a diagram for showing the scale of the boron nitride particle aggregate observed in FIG. 2 (a). FIG. 6 is a diagram for showing the scales of the three boron nitride particle aggregates observed in FIG. 2 (b).

Hereinafter, embodiments of the present invention (hereinafter, referred to as “the present embodiment”) will be described. It should be noted that the present embodiment is an example for explaining the present invention, and the present invention is not limited to the present embodiment.

The method for producing a heat conductive sheet according to the present embodiment includes a step of preparing a resin composition containing boron nitride particles and a resin, and a step of forming the resin composition into a sheet, and the boron nitride is described. The relationship between the longest diameter (r) in the width direction of the particles and the longest diameter (R) in the direction perpendicular to the width direction of the boron nitride particles is represented by the following formula (1).
0.3r <R (1)
Since it is configured as described above, according to the method for manufacturing a heat conductive sheet according to the present embodiment, a heat conductive sheet having excellent heat conductivity in the thickness direction can be manufactured with high productivity and low cost.

(Boron nitride particles)
In the present embodiment, the relationship between the longest diameter (r) of the boron nitride particles in the width direction and the longest diameter (R) in the direction perpendicular to the width direction of the boron nitride particles is as described above in the formula (1). ). The boron nitride particles in the present embodiment are not particularly limited as long as they satisfy the relationship of the formula (1), and are composed of one hexagonal boron nitride primary particle (hereinafter, also referred to as “boron nitride primary particle”). It may be an agglomerate composed of two or more hexagonal boron nitride primary particles (hereinafter, also referred to as “boron nitride secondary particles”). Further, the shape of the boron nitride particles is not particularly limited as long as it satisfies the formula (1), and can take various shapes such as columnar, flat, granular, lumpy, spherical, and fibrous.

In the present embodiment, the "width direction of the boron nitride particles" is a direction substantially parallel to the plane direction (a-axis direction) of the (0001) plane of the hexagonal boron nitride primary particles, and the boron nitride particles are boron nitride. In the case of primary particles, it can also be said to be the longest diameter of the (0001) plane of hexagonal boron nitride primary particles. The "direction perpendicular to the width direction of the boron nitride particles" is a direction substantially parallel to the c-axis direction of the hexagonal boron nitride primary particles, and the boron nitride particles are boron nitride secondary particles. In this case, it can also be said to be the longest diameter in the stacking direction of the hexagonal boron nitride primary particles.
It can be easily confirmed by scanning electron microscope (SEM) observation that the boron nitride particles in the present embodiment satisfy the formula (1). Specific examples include, but are not limited to, measuring the length of the side corresponding to R and the length of the side corresponding to r by approximating the boron nitride particles to a rectangle in the captured SEM image. Be done.
In the present embodiment, substantially parallel means a state within ± 10 ° from the parallel direction, and substantially vertical means a state within ± 10 ° from the vertical direction.
R and r will be specifically described with reference to FIG. The hexagonal boron nitride primary particles shown in FIG. 1 (a) are shown as a two-dimensional rectangle as a typical example. In the hexagonal boron nitride primary particles shown as "an example satisfying 0.3r <R" in FIG. 1A, the long side (r) corresponds to the (0001) plane, and the short side (R) Corresponds to the end face. On the other hand, in the hexagonal boron nitride primary particles shown as “an example satisfying r <R” in FIG. 1A, the short side (r) corresponds to the (0001) plane, and the long side (R) Corresponds to the end face.
Further, in the boron nitride particle agglomerate shown as “an example satisfying 0.3r <R” in FIG. 1 (b), the long side (r) corresponds to the (0001) plane, and the short side (R) ) Corresponds to the end face. On the other hand, in the boron nitride particle aggregate shown as "an example satisfying r <R" in FIG. 1B, the short side (r) corresponds to the (0001) plane, and the long side (R) corresponds to the (0001) plane. Corresponds to the end face.

In the present embodiment, from the viewpoint of increasing the possibility that the a-axis direction of the boron nitride primary particles coincides with the thickness direction of the heat conductive sheet (hereinafter, also simply referred to as “orientation”) when the heat conductive sheet is used. The relationship between the longest diameter (r) in the width direction of the boron nitride particles and the longest diameter (R) in the direction perpendicular to the width direction of the boron nitride particles further satisfies the formula (2). That is, it is preferable that the longest diameter in the direction perpendicular to the width direction of the boron nitride particles is larger than the longest diameter in the width direction of the boron nitride particles.
r <R (2)

The particle shape of the hexagonal boron nitride primary particles themselves in the present embodiment is not particularly limited as long as the boron nitride particles satisfy the formula (1). For example, columnar, scaly, flat, granular, spherical, fibrous, whiskers and the like can be mentioned. In the present embodiment, the shape of the boron nitride primary particles constituting the boron nitride secondary particles is preferably scaly. When the boron nitride particles are composed of one boron nitride primary particle, the shape of the boron nitride primary particle is preferably spherical or columnar, and more preferably columnar. It can be easily confirmed by the above-mentioned SEM observation that the boron nitride primary particles have a columnar shape.

The average particle size of the hexagonal boron nitride primary particles is not particularly limited, but the median diameter is preferably 0.1 to 50 μm, more preferably 0.1 to 5 μm, and particularly preferably 0.1 to 1 μm. When the average particle size is within the above range, the (0001) planes of the hexagonal boron nitride primary particles are likely to overlap and aggregate, and as a result, the orientation tends to increase, which is preferable.
Here, the average particle size of the hexagonal boron nitride primary particles can be measured by, for example, a wet laser diffraction / scattering method.

The agglutination form of the hexagonal boron nitride primary particles is not particularly limited, and examples thereof include natural agglutination, agglutination by a coagulant, and physical agglutination. The natural agglutination is not limited to the following, and examples thereof include agglutination caused by van der Waals force, electrostatic force, adsorbed moisture and the like. Aggregation by a flocculant is not limited to the following, and examples thereof include agglomeration by a flocculant such as a coupling agent, an inorganic salt, and a polymer substance. For example, when a coupling agent is used as a coagulant, it is preferable to change the surface state of the hexagonal boron nitride primary particles to a state having high surface energy, that is, a state in which coagulation is easy to occur. Examples of the physical agglutination include a method of agglutination by operations such as mixed granulation, extrusion granulation, and spray drying. Above all, from the viewpoint of cohesive force, aggregation with a coupling agent is preferable.

In the present embodiment, from the viewpoint of orientation, it is preferable that the boron nitride particles include a boron nitride particle aggregate in which the (0001) planes of the hexagonal boron nitride primary particles overlap each other. In the present specification, the "boron nitride particle agglomerate" is an aggregate unit of secondary particles formed by agglomeration of inorganic fillers containing hexagonal boron nitride primary particles, and can take various shapes. That is, the shape of the boron nitride particle aggregate is not particularly limited as long as the (0001) planes of the hexagonal boron nitride primary particles are overlapped and aggregated, and are, for example, columnar, flat, granular, or massive. It may be spherical, fibrous, or the like. From the viewpoint of further enhancing the orientation, it is particularly preferable that the boron nitride particle aggregate has a columnar shape.

In the present embodiment, when the boron nitride particles contain the boron nitride secondary particles, typical examples of the method for confirming the particle shape are not limited to the following, but the longest diameter R in the stacking direction of the boron nitride secondary particles. 'And the longest diameter r'in the width direction of the boron nitride secondary particles are specified by SEM observation, and the magnitude relationship between them is confirmed, that is, a method of confirming that R'> 0.3r' is used. Can be mentioned. In the present embodiment, from the viewpoint of orientation, it is preferable to specify by a method for confirming that R'> r'is satisfied. The "stacking direction" referred to here is a direction that is substantially parallel to the c-axis direction of the hexagonal boron nitride primary particles. Further, the "width direction" referred to here is a direction substantially perpendicular to the stacking direction. In other words, the width direction is a direction substantially parallel to the plane direction (a-axis direction) of the (0001) plane of at least one hexagonal boron nitride primary particle. In the present embodiment, the longest diameter in the width direction is, for example, the longest diameter in the plane direction of the (0001) plane of the hexagonal boron nitride primary particles when the boron nitride particle aggregate is a columnar shape extending straight in the vertical direction. However, the longest diameter in the width direction is not limited to this relationship, and can take a value larger than the longest diameter in the plane direction of the (0001) plane of the hexagonal boron nitride primary particles.

In the present specification, the term "columnar" represents the shape of the hexagonal boron nitride primary particles themselves or the shape derived from the aggregation mode of the hexagonal boron nitride primary particles, and includes prismatic, columnar, rod-shaped, and the like. It means a shape and is different from a spherical shape. Further, the columnar shape includes a columnar shape that extends straight in the vertical direction, a columnar shape that extends in an inclined manner, a columnar shape that extends while bending, a columnar shape that extends in a branch shape, and the like. It can be easily confirmed that the boron nitride particle aggregate has a columnar shape by SEM observation or the like as described above.

As described above, the boron nitride particle aggregate in the present embodiment has a structure in which hexagonal boron nitride primary particles are laminated, and the laminated structure is not particularly limited, but from the viewpoint of orientation, each layer. The number of particles of the hexagonal boron nitride primary particles is preferably 2 or less, and more preferably 1. Such a laminated structure can be said to be a typical structure observed when the boron nitride particle aggregate has a columnar shape.

In the present embodiment, the longest diameter in the width direction is, for example, the longest diameter in the plane direction of the (0001) plane of the hexagonal boron nitride primary particles when the secondary boron nitride particles are columnar extending straight in the vertical direction. However, the longest diameter in the width direction is not limited to this relationship, and can take a value larger than the longest diameter in the plane direction of the (0001) plane of the hexagonal boron nitride primary particles.

In the present embodiment, the values of R and r are not particularly limited. When the boron nitride particles in the present embodiment contain boron nitride particle aggregates, R = 0.3 × A to 10 × A (μm), where A (μm) is the average particle size of the hexagonal boron nitride primary particles. Moreover, it is preferable that r = 0.3 × A to 3 × A (μm) is satisfied, and more specifically, when the average particle size of the hexagonal boron nitride primary particles is 0.5 μm, R = It is typical to take a value of about 0.15 to 5 μm, and it is typical to take a value of about r = 0.15 to 1.5 μm. Specific examples of the method for measuring the longest diameter are not limited to the following, but if the boron nitride particles are columnar in the captured SEM image, the shape is approximated to a rectangle, and the long and short sides of the rectangle are obtained. For example, measuring the length of the particle.

In the preferred embodiment of the present embodiment described above, as long as the hexagonal boron nitride primary particles are laminated so as to form a columnar shape as the boron nitride secondary particles, the "(0001) plane of the hexagonal boron nitride primary particles". The state of "overlapping" is not limited to the mode of completely overlapping, but also includes the mode of overlapping in the width direction.

Further, the longest diameter r in the width direction of the boron nitride particles and the longest diameter r'in the width direction of the boron nitride secondary particles are not particularly limited, but are less than the thickness T of the heat conductive sheet (r <. T, r'<T) is preferable, r <0.9 × T, r ′ <0.9 × T, more preferably r <0.5 × T, r ′ <0.5. × T. When the above range is satisfied, there is a tendency that deterioration of smoothness due to unevenness on the surface of the heat conductive sheet can be prevented, and deterioration of orientation due to crushing of boron nitride particles also tends to be prevented.

The boron nitride particle aggregate in the present embodiment may contain an inorganic filler other than hexagonal boron nitride primary particles, and such an inorganic filler is not limited to the following, but for example, aluminum nitride, aluminum oxide, and the like. Metal oxides such as zinc oxide, silicon carbide and aluminum hydroxide, metals and alloys such as metal nitrides, metal carbides and metal hydroxides, spherical, powdery, fibrous, needle-like, composed of carbon, graphite and diamond. Examples thereof include scaly and whisker-like fillers. These may be contained alone or may contain a plurality of types.

The boron nitride particle aggregate may contain a binder from the viewpoint of increasing the robustness of the boron nitride particle aggregate. The term "robustness" as used herein is a property indicating the strength of bonding when hexagonal boron primary particles in the present embodiment are bonded to each other. The binder originally acts to firmly bind the primary particles together and stabilize the shape of the agglomerates.

As such a binder, a metal oxide is preferable, and specifically, aluminum oxide, magnesium oxide, yttrium oxide, calcium oxide, silicon oxide, boron oxide, cerium oxide, zirconium oxide, titanium oxide and the like are preferably used. Among these, aluminum oxide and yttrium oxide are preferable from the viewpoints of thermal conductivity and heat resistance as oxides and a binding force for binding hexagonal boron nitride primary particles to each other. The binder may be a liquid binder such as alumina sol, or a binder that is converted into a metal oxide by firing, such as an organometallic compound, may be used. One of these binders may be used alone, or two or more of these binders may be mixed.

(Manufacturing method of boron nitride particle aggregate)
The boron nitride particle aggregate preferably used in the present embodiment is not particularly limited in terms of production method as long as the above-mentioned constitution can be obtained, but is preferably produced by the following method. That is, in the preferred method for producing the boron nitride particle aggregate in the present embodiment, the step (A) of adding the coupling agent to the hexagonal boron nitride primary particles and the coupling agent obtained in the step (A) are added. This includes a step (B) of dispersing the boron nitride primary particles in a solvent. Further, it is more preferable to further include the step (C) of separating the boron nitride particle aggregate from the dispersion obtained in the step (B).
When the step (A) and the step (B) are included, the hexagonal boron nitride primary particles to which the coupling agent is added agglomerate between the (0001) planes due to the surface charge in the dispersion liquid, and the bonding phase between the coupling agents. It is preferable because the boron nitride particle agglomerates having the desired constitution of the present embodiment can be easily obtained.

In the step (A), the method of adding the coupling agent to the hexagonal boron nitride primary particles is not particularly limited, but a dry method in which the coupling agent stock solution is uniformly dispersed in the filler being stirred at high speed by a stirrer. A wet method or the like in which the filler is immersed in a dilute solution of the coupling agent and stirred is suitable.
In the step (A), the hexagonal boron nitride primary particles to which a coupling agent is added may be bonded to each other to form an aggregate of boron nitride particles. The stirring temperature is not particularly limited, but no special temperature control such as heating or cooling is required. Usually, the temperature rises by stirring, but it is preferably 200 ° C. or lower. The stirring speed is not particularly limited, but it is usually preferably stirred at 10 to 10000 rpm, and more preferably 100 to 3000 rpm. The stirring time is preferably 5 minutes to 180 minutes, more preferably 10 minutes to 60 minutes from the viewpoint of effective stirring time and productivity.

From the viewpoint of cohesiveness and robustness, the above-mentioned coupling agent preferably has at least one selected from the group consisting of an aryl group, an amino group, an epoxy group, a cyanate group, a mercapto group and a halogen. Examples of such a coupling agent include, but are not limited to, silane-based, titanate-based, zirconate-based, zirconium aluminate-based, and aluminate-based coupling agents. Of these, a silane-based coupling agent (hereinafter, also referred to as “silane coupling agent”) is preferable from the viewpoint of cohesive force.

The above-mentioned silane coupling agent preferably has any one or more selected from the group consisting of an aryl group, an amino group, an epoxy group, a cyanate group, a mercapto group and a halogen. Among them, it is more preferable to have an aryl group because it has a π-π bond and the bond between the silane coupling agents becomes stronger. Such coupling agents are not limited to the following, and include, for example, phenyltrimethoxysilane, phenyltriethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, p-styryltrimethoxysilane, and naphthyltrimethoxysilane. , Naftiltriethoxysilane, (trimethoxysilyl) anthracene, (triethoxysilyl) anthracene and the like.

It is also preferable to use a silane coupling agent having a crosslinked structure as the above-mentioned coupling agent. Specific examples of such a silane coupling agent are not limited to the following, but are limited to 1,6-bis (trimethoxysilyl) hexane, tris- (trimethoxysilylpropyl) isocyanurate, and bis (triethoxysilylpropyl) tetra. Examples thereof include sulfide and hexamethyldisilazane.

The silane coupling agent having the above-mentioned crosslinked structure can also be obtained by reacting and crosslinking the organic functional groups. The combination of organic functional groups in this case is not limited to the following, but for example, amino group-epoxy group, epoxy group-cyanate group, amino group-cyanate group, amino group-sulfonic acid group, amino group-halogen, mercapto. Examples thereof include a combination of organic functional groups of a coupling agent such as a group-cyanate group.

The various coupling agents described above may be used alone or in combination of two or more.

The amount of the coupling agent added in the step (A) depends on the surface area of the inorganic filler, that is, the surface area of the hexagonal boron nitride primary particles, but is 0.01 to 10 with respect to the hexagonal boron nitride primary particles. It is preferable to add by mass%, and it is more preferable to add 1 to 2% by mass.

In the above manufacturing method, the stirring method that can be used when adding the coupling agent is not particularly limited, and for example, a vibration mill, a bead mill, a ball mill, a Henschel mixer, a drum mixer, a vibration stirrer, a V-shaped mixer, or the like. Stirring can be performed using a general stirrer.

In the step (B), the hexagonal boron nitride primary particles to which the coupling agent obtained in the step (A) is added are dispersed in a solvent. The method of dispersing the hexagonal boron nitride primary particles to which the coupling agent is added in the solvent is not particularly limited. Hexagonal boron nitride primary particles to which a coupling agent having an aromatic skeleton is added tend to agglomerate between (0001) planes, and thus are preferably ultrasonically dispersed in a solvent.

The solvent used in the step (B) is not particularly limited, and water and / or various organic solvents can be used. When using hexagonal boron nitride primary particles having an aromatic ring skeleton, that is, to which a coupling agent having an aryl group is added, a highly polar solvent is preferable.
The amount of the solvent used is 0.5 to 20 times by mass with respect to the hexagonal boron nitride primary particles from the viewpoint of reducing the load at the time of separation in the step (C) and from the viewpoint of uniformly dispersing in the step (B). It is preferable to do so.

In the step (B), various surfactants may be added from the viewpoint of adjusting the degree of cohesion of the boron nitride particle aggregates. The surfactant is not particularly limited, and for example, an anionic surfactant, a cationic surfactant, a nonionic surfactant and the like can be used, and one of these may be used alone. Two or more kinds may be mixed and used. The concentration of the surfactant in the dispersion obtained in the step (B) is not particularly limited, but is usually 0.1% by mass or more and 10% by mass or less, and 0.5% by mass or more and 5% by mass or less. It is preferable to do so.

In the step (C), the boron nitride particle aggregate is separated from the dispersion liquid obtained in the step (B). The method for separating the boron nitride particle aggregate from the dispersion obtained in the step (B) is not particularly limited, and for example, static separation, filtration, a centrifuge, heat treatment and the like can be used.

Another treatment may be performed on the boron nitride particle aggregates obtained from these. For example, surface oxidation that is temperature-treated under oxygen, steam treatment, surface modification with an organometallic compound or polymer using a carrier or reaction gas at room temperature or at warm, sol-gel method using boehmite or SiO ₂ can be used. Is. In these treatments, one kind may be used alone, or two or more kinds may be mixed.

Further, the boron nitride particle agglomerates of the present embodiment prepared as described above may be subjected to typical post-steps such as scrutiny, pulverization, classification, purification, washing and drying, if necessary. If fine particles are included, they may be removed first. As an alternative to sieving, the grinding of the boron nitride particle agglomerates may be performed using a sieving net, a sorting mill, a structured roller crusher or a cutting wheel. For example, it is also possible to perform a dry milling process in a ball mill.

(Preparation process)
In the method for producing a heat conductive sheet according to the present embodiment, first, boron nitride primary particles and boron nitride particle aggregates obtained as illustrated above are used as fillers and added to a resin to prepare a resin composition. .. The filler in the present embodiment may contain an inorganic filler in addition to the boron nitride particle aggregate, and examples of such an inorganic filler are not limited to the following, but aluminum nitride, aluminum oxide, zinc oxide, and the like. Metal oxides such as silicon carbide and aluminum hydroxide, metal nitrides, metal carbides, metals and alloys such as metal hydroxides, spherical, powdery, fibrous, needle-like, scaly, consisting of carbon, graphite and diamond. Examples include whisker-like fillers. These may be contained alone or may contain a plurality of types.
The above-mentioned resins are not limited to the following, but for example, thermoplastic resins (polyolefin, vinyl chloride resin, methyl methacrylate resin, nylon, fluororesin), thermosetting resins (epoxy resin, phenol resin, urea resin, melamine). Resin, unsaturated polyester resin, silicon resin), synthetic rubber, liquid gel and the like. Among the above, a thermosetting resin is preferable.
The method for producing the resin composition in the present embodiment is not particularly limited, and examples thereof include a method in which each component is sequentially mixed with a solvent and sufficiently stirred. At this time, in order to uniformly dissolve or disperse each component, known treatments such as stirring, mixing, and kneading can be performed. Specifically, the dispersibility of a filler such as a boron nitride particle agglomerate in the composition can be improved by performing the stirring and dispersing treatment using a stirring tank equipped with a stirring machine having an appropriate stirring ability. The above-mentioned stirring, mixing, and kneading treatment can be appropriately performed using, for example, an apparatus for mixing such as a ball mill or a bead mill, or a known apparatus such as a revolving or rotating type mixing apparatus.
Further, when preparing the resin composition in the present embodiment, an organic solvent can be used if necessary. The type of the organic solvent is not particularly limited as long as it can dissolve a part or all of the resin components in the composition, but for example, ketones such as acetone, methyl ethyl ketone, cyclohexanone and methyl cellsolve; toluene and xylene. Aromatic hydrocarbons such as; amides such as dimethylformamide; propylene glycol monomethyl ether and its acetate and the like. As the solvent, one type may be used alone, or two or more types may be used in combination.
The resin composition obtained as described above contains 10 to 90 vol% of boron nitride particles, with the volume of the resin composition as 100%, from the viewpoint of improving the thermal conductivity in the thickness direction of the heat conductive sheet. Is preferable, more preferably 30 to 80 vol%, still more preferably 40 to 75 vol%.

(Sheet process)
In the method for producing a heat conductive sheet according to the present embodiment, the resin composition prepared as described above is made into a sheet. When the sheet is formed, the orientation of the boron nitride particles of 0.3r <R can be enhanced by the flow in the thickness direction of the sheet or the pressurization in the thickness direction. Such sheet forming methods are not limited to the following, but include, for example, melt extrusion method, solution salivation method, rolling method, stretching method, coating method, bar coater method, doctor blade method, press molding method, injection molding method, and the like. Various known sheet forming methods such as a calendar processing method can be mentioned. As these sheet forming methods, one type alone may be adopted, or two or more types may be combined. When the sheet forming step is carried out by the above press molding method, various conditions at the time of press molding in the press molding step described later can be preferably adopted.

(Press molding process)
In the method for producing a heat conductive sheet according to the present embodiment, it is preferable that the resin composition sheet prepared as described above is further subjected to a press molding step. That is, the press molding step in this embodiment is carried out after the sheet forming step described above. By being subjected to the press molding step, the orientation tends to be further enhanced, and a heat conductive sheet without gaps such as an air layer tends to be obtained. The press forming step in the present embodiment is not particularly limited as long as it can be formed into a sheet by applying pressure, and examples thereof include a vacuum press, a heating press, a vacuum heating press, and the like to further enhance the orientation. From the viewpoint of producing a sheet without gaps, a vacuum press and a vacuum heating press are preferable. Further, the press forming method may be a continuous press or a multi-stage press.

In the press molding step, the pressure when performing press molding is not particularly limited, but from the viewpoint of further enhancing the orientation, the absolute pressure is preferably 0.1 to 1000 MPa, more preferably 0.11 to 500 MPa. It is more preferably 1 to 100 MPa, and even more preferably 5 to 30 MPa.

In the press molding step, the temperature at which press molding is performed, that is, the press temperature, when a thermoplastic resin is used as the resin, is preferably higher than the melt viscosity of the resin and lower than the decomposition temperature of the resin. When a thermosetting resin is used as the resin, it is preferably set to a temperature higher than the temperature at which the curing reaction proceeds and lower than the decomposition temperature of the resin. Therefore, the press temperature is not particularly limited, but is preferably 50 to 400 ° C, more preferably 60 to 350 ° C, and even more preferably 100 to 250 ° C.

The heat conductive sheet obtained as described above has an intensity ratio (I <002> / I <100>) of <002> X-ray diffraction intensity to <100> X-ray diffraction of 30 or less. Is preferable. Here, the intensity ratio of <002> X-ray diffraction to <100> X-ray diffraction (I <002> / I <100>) is the X-ray in the thickness direction of the sheet of the resin composition. That is, the intensity of the <002> diffraction line obtained by irradiating the sheet at an angle of 90 ° with respect to the length direction of the sheet and the peak intensity ratio of the <100> diffraction line (I <002> / I < 100>). In the present embodiment, the strength ratio of 30 or less suggests that the orientation is sufficiently high, and the thermal conductivity in the sheet thickness direction tends to be higher.

Next, the present embodiment will be described in more detail by way of examples, but the present embodiment is not limited to these examples.

(Synthesis example)
1.5% by mass of phenyltrimethoxylane (manufactured by Tokyo Kasei) was added dropwise as a coupling agent to the boron nitride primary particles ("UHP-S2" manufactured by Showa Denko) having an average particle size of 0.5 μm, and the mixture was stirred with a mixer. Then, boron nitride primary particles to which a coupling agent was added were obtained. Subsequently, the boron nitride primary particles to which the above-mentioned coupling agent was added were added to methyl ethyl ketone, and the particles were sufficiently dispersed by an ultrasonic disperser to obtain a dispersion liquid. The dispersion was allowed to stand for 6 hours to remove the fine particles to obtain boron nitride particles having (0001) planes laminated. The boron nitride particles were further ultrasonically dispersed in ethanol, sprinkled on aluminum foil, and after the solvent was evaporated, they were pressed against a carbon tape to observe an SEM image of the boron nitride particle aggregates. The result is shown in FIG.

In the SEM images shown in FIGS. 2 (a) and 2 (b), the end faces of the hexagonal boron nitride primary particles were observed more than the (0001) plane of the hexagonal boron nitride primary particles, and most of the boron nitride was observed. It was found that the (0001) planes of the primary particles overlapped and aggregated. That is, many boron nitride particles (boron nitride particle aggregates) in which the (0001) planes of the boron nitride primary particles overlap and aggregated were observed. Regarding the boron nitride particle aggregate shown in FIG. 2A, the number of hexagonal boron nitride primary particles per layer is 1, and the longest diameter R of the boron nitride particle aggregate in the stacking direction and the boron nitride particles When the longest diameter r in the width direction of the agglomerates was measured, it was R = 0.66 μm, r = 0.49 μm, and R / r = 1.35. R was specified as shown in FIG. Further, from FIG. 2B, three boron nitride particle aggregates (boron nitride particle aggregates 1 to 3) having a particularly typical columnar shape were selected, and the particle size was measured by the method described later. did. In each of the boron nitride particle aggregates 1 to 3, the number of hexagonal boron nitride primary particles per layer was 1. The boron nitride particle aggregate was charged and the SEM image was distorted. Table 1 shows the results of determining r and R in the boron nitride particle aggregates 1 to 3. In addition, these R were specified as shown in FIG.

(Measurement of particle size)
The morphology of the boron nitride particles was analyzed by analyzing a plurality of SEM observation images of 2.5 μm × 1.8 μm square with an electron microscope (FE-SEM-EDX (SU8220): manufactured by Hitachi High-Technology Co., Ltd.). The R and r of the boron nitride particles (boron nitride particle aggregates) in which the (0001) planes of the boron nitride primary particles were overlapped and aggregated in the above observation range were measured. At that time, the boron nitride particles observed in the SEM image were approximated to a rectangle as shown in FIG. 1 (b), and the length of the side corresponding to R and the length of the side corresponding to r were measured. The R / r of the boron nitride particle aggregates existing in the observation range shown in FIG. 2B were determined and found to be 1.0 or more. Therefore, the boron nitride particle aggregates obtained in the synthetic example were obtained. The representative value of R / r was specified to be 1.0 or more.

(Example 1)
As the resin (base resin), 100 parts by mass of a triphenylmethane type epoxy resin (“EPPN-501H” manufactured by Nippon Kayaku Co., Ltd.) and a curing agent (phenonovolac curing agent, “DL-92” manufactured by Meiwa Kasei Co., Ltd. ) 63 parts by mass and 0.1 parts by mass of a curing catalyst (2phenylimidazole, manufactured by Shikoku Kasei) were used. The compounding ratio of the epoxy resin and the curing agent was such that the functional group of the epoxy resin / curing agent was 1.0 in the equivalent ratio.
Next, 70% by volume of the above-mentioned base resin and 30% by volume of boron nitride particles prepared in the synthetic example are mixed, an organic solvent methyl ethyl ketone is further added, and each component is dissolved or dispersed by stirring at room temperature to dissolve or disperse the resin composition. The varnish was made. This resin composition varnish is applied to a copper foil having a thickness of 50 μm whose surface has been mold-released so that the resin thickness after drying is 200 to 300 μm, and dried at 120 ° C. for 10 minutes. A resin composition (sheeted resin composition) in which the components were uniformly dispersed was obtained. Further, this resin composition was filled in a 1 mm thick press die and cured with a vacuum press at 10 MPa for 30 minutes at 200 ° C. to obtain a heat conductive sheet (test piece) of Example 1.

(Example 2)
The test piece of Example 2 was obtained in the same manner as in Example 1 except that the blending amount of the base resin was changed to 40% by volume and the blending amount of the boron nitride particles produced in the synthetic example was changed to 60% by volume. Got

(Comparative Example 1)
The test piece of Comparative Example 1 was prepared in the same manner as in Example 1 except that the blending amount of the base resin was changed to 70% by volume and 30% by volume of UHP-S2 not treated with the silane coupling agent was blended. Obtained. When the shape of the boron nitride particles according to UHP-S2 before compounding was observed by SEM, the boron nitride particles were not aggregated. Further, as can be seen from FIGS. 3 (a) and 3 (b), no structure in which the hexagonal boron nitride primary particles (0001) overlap each other was observed. The boron nitride particles observed in these SEM images were approximated to a rectangle as shown in FIG. 1 (a), and the length of the side corresponding to R and the length of the side corresponding to r were measured. The longest diameter r'in the width direction of the hexagonal boron nitride primary particles is 0.3 to 1.0 μm, and the longest diameter R'in the direction perpendicular to the width direction of the hexagonal boron nitride primary particles is 0.01. It was ~ 0.05 μm. That is, the observed boron nitride particles were all R'<0.3r'. When the R / r of the boron nitride particles existing in the observation range shown in FIG. 3 (b) were determined, they were all sufficiently smaller than those of Synthesis Examples (that is, Examples 1 and 2), and 0. Since it was 2 or less, the representative value of R / r in Comparative Example 1 was specified as 0.2 or less.

(Comparative Example 2)
A test piece of Comparative Example 2 was obtained in the same manner as in Comparative Example 1 except that the blending amount of the base resin was changed to 40% by volume and the blending amount of UHP-S2 was changed to 60% by volume. The boron nitride particles are the same as in Comparative Example 1, r'is 0.3 to 1.0 μm, R'is 0.01 to 0.05 μm, and the observed boron nitride particles are all R'. It was <0.3r'. The representative value of R / r in Comparative Example 2 was also specified to be 0.2 or less.

(Comparative Example 3)
Comparative Example 3 in the same manner as in Example 2 except that 60% by volume of a randomly oriented boron nitride particle aggregate SGPS (average particle size: 12 μm, manufactured by Denki Kagaku Kogyo Co., Ltd.) was blended in place of UHP-S2. Test piece was obtained. As a result of observing the SGPS before compounding by SEM, as shown in FIG. 4, the structure in which (0001) of the hexagonal boron nitride primary particles overlapped with each other was not observed. The number of hexagonal boron nitride primary particles per layer was 3 or more, and the shape of the random aggregate was spherical.

Table 2 summarizes the thermal conductivity of the test pieces of Examples 1 and 2 and Comparative Examples 1 to 3 and the orientation intensity ratio of the boron nitride primary particles in the heat conductive sheet.

The evaluation method of each characteristic of the heat conductive sheet is as follows.

(Thermal conductivity)
A test piece (10 mm × 10 mm × thickness 1 mm) was cut out from the above-mentioned press-molded heat conductive sheet. For this test piece, the thermal conductivity in the sheet thickness direction was measured by the laser flash method using a xenon flash analyzer LFA447 type thermal conductivity meter manufactured by NETZSCH.

(Orientation strength ratio)
The orientation of the boron nitride primary particles in the heat conductive sheet is the intensity of the I (002) diffraction line (2θ = 26.5 °) and the I (100) diffraction line (2θ = 41.5) by the X-ray diffraction method. °) To intensity (I (002) / I (100)).
The thickness direction of the hexagonal boron nitride primary particles coincides with the crystallographic I (002) diffraction line, that is, the c-axis direction, and the in-plane direction coincides with the I (100) diffraction line, that is, the a-axis direction. There is. When the boron nitride primary particles constituting the boron nitride particle aggregate have a completely random orientation (non-orientation), (I (002) / I (100)) ≈6.7 (“JCPDS [powder X] Linear diffraction database] "No. 34-0421 [BN] crystal density value [Dx]). When (I (002) / I (100)) is small, the a-axis direction of the hexagonal boron nitride particles is oriented in the thickness direction of the heat conductive sheet, that is, the orientation is increased, and as a result, the thickness direction is increased. Thermal conductivity increases.
In the above-mentioned measurement, a test piece having a size of 5 mm × 5 mm × thickness 0.2 mm was cut out from the above-mentioned press-molded heat conductive sheet and used as a measurement sample. Further, using a "fully automatic horizontal multipurpose X-ray diffractometer SmartLab" (manufactured by Rigaku Co., Ltd.), an X-ray was applied to a 5 mm × 5 mm flat surface of the test piece in the thickness direction of the test piece. At the time of measurement, CuKα ray was used as the X-ray source, the tube voltage was 45 kV, and the tube current was 360 mA.

Claims (8)

A step of preparing a resin composition containing boron nitride particles and a resin,
The step of forming the resin composition into a sheet and
Have,
The relationship between the longest diameter (r) in the width direction of the boron nitride particles and the longest diameter (R) in the direction perpendicular to the width direction of the boron nitride particles is expressed by the following formula (1) .
The boron nitride particles contain an aggregate of boron nitride particles in which the (0001) planes of the hexagonal boron nitride primary particles overlap each other.
The per layer of boron particles aggregate nitride, the number of particles of the hexagonal boron nitride primary particles, 2 Ru der Hereinafter, a method for fabricating the heat conductive sheet.
0.3r <R (1) The sheet forming step is selected from the group consisting of melt extrusion method, solution flow method, rolling method, stretching method, coating method, bar coater method, doctor blade method, press molding method, calendering method and injection molding method. The method for producing a heat conductive sheet according to claim 1, which is carried out by at least one method. The method for producing a heat conductive sheet according to claim 1 or 2, further comprising a press molding step performed after the sheet forming step. The method for producing a heat conductive sheet according to any one of claims 1 to 3 , wherein the boron nitride particle aggregate has a columnar shape. The heat according to any one of claims 1 to 4 , wherein the peak intensity ratio (<002> / <100>) of the <002> plane to the <100> plane of the X-ray diffraction of the heat conductive sheet is 30 or less. Method of manufacturing a conductive sheet. The method for producing a heat conductive sheet according to any one of claims 3 to 5 , wherein vacuum press molding is performed in the press molding step. The method for producing a heat conductive sheet according to claim 6 , wherein the pressure of the vacuum press molding is 0.1 to 1000 MPa. The method for producing a heat conductive sheet according to any one of claims 3 to 7 , wherein the press temperature in the press molding step is 50 to 400 ° C.