JP7651902B2 - Thermally conductive sheet and its manufacturing method - Google Patents

Description

The present invention relates to a thermally conductive sheet and a method for manufacturing the same.

In recent years, the amount of heat generated by electronic components such as power semiconductors (e.g., IGBT modules) and integrated circuit (IC) chips has increased as their performance has improved. As a result, electronic devices that use electronic components need to take measures to prevent malfunctions caused by temperature increases in the electronic components.

To prevent functional failures caused by temperature rises in electronic components, a common method is to promote heat dissipation by attaching a heat sink, heat sink plate, heat dissipation fin or other heat sink made of metal to the heat generating body of the electronic component. When using the heat sink, in order to efficiently transfer heat from the heat generating body to the heat sink, a sheet-like member with high thermal conductivity is used, and a certain pressure is applied to this sheet-like member to bring the heat generating body and the heat sink (hereinafter sometimes collectively referred to as "adherend") into close contact.

Here, from the viewpoint of increasing the thermal conduction efficiency of the thermal conductive sheet, it is preferable that the sheet exhibits excellent thermal conductivity even under a relatively low pressure. Therefore, Patent Document 1 proposes a thermal conductive sheet that contains a thermoplastic resin that is liquid at room temperature and a particulate carbon material, and has a thermal resistance value of 0.20°C/W or less under a pressure of 0.05 MPa.

As in Patent Document 1, the flexibility of the thermally conductive sheet is increased by blending a thermoplastic resin that is liquid at room temperature, but when the sheet is sandwiched between adherends, the components of the thermally conductive sheet may drip (pump out) from the adherend due to the clamping pressure and heat from the heating element. Therefore, methods to deal with such problems have been considered in the past. For example, Patent Document 2 proposes a thermally conductive sheet that contains a resin that is liquid at room temperature and normal pressure, a resin that is solid at room temperature and normal pressure, and a particulate carbon material, and has a thermal resistance value of 0.30°C/W or less under a pressure of 0.05 MPa, and the content of the resin that is liquid at room temperature and normal pressure is 60% by mass or more and 75% by mass or less of the total content of the resin that is liquid at room temperature and normal pressure and the resin that is solid at room temperature and normal pressure. Patent Document 2 attempts to achieve both pump out suppression and thermal conductivity by controlling the content of the resin that is liquid at room temperature and normal pressure within a predetermined range.

International Publication No. WO 2017/145957 JP 2018-129377 A

The conventional composite sheets described above had room for improvement in terms of achieving a high level of both pump-out suppression and excellent thermal conductivity.

The present invention aims to provide a thermally conductive sheet and a method for manufacturing the same that can suppress pump-out and provide excellent thermal conductivity at a high level.

The present inventors have conducted extensive research to achieve the above object. The present inventors have newly discovered that a thermally conductive sheet containing a liquid resin and a particulate carbon material, in which the angle of the major axis direction of the particulate carbon material with respect to the sheet surface is 60° to 90°, and in which the proportion of components with a molecular weight of 1,000 or less is 1.0% or less when the molecular weight distribution of the thermally conductive sheet is measured, can suppress pump-out and have excellent thermal conductivity at a high level, and have completed the present invention.

That is, the present invention aims to advantageously solve the above problems, and the thermally conductive sheet of the present invention is a thermally conductive sheet containing a resin and a particulate carbon material, characterized in that the resin contains a liquid resin, the angle of the major axis direction of the particulate carbon material with respect to the surface of the thermally conductive sheet is 60° or more and 90° or less, and when the molecular weight distribution of the thermally conductive sheet is measured, the proportion of components having a molecular weight of 1,000 or less is 1.0% or less. A thermally conductive sheet containing a liquid resin and a particulate carbon material, in which the angle of the major axis direction of the particulate carbon material with respect to the surface of the thermally conductive sheet is 60° or more and 90° or less, and in which the proportion of components having a molecular weight of 1,000 or less is 1.0% or less when the molecular weight distribution of the thermally conductive sheet is measured, can suppress pump-out and have excellent thermal conductivity at a high level.
The "angle of the major axis direction of the particulate carbon material relative to the surface of the thermally conductive sheet" and the "molecular weight distribution of the thermally conductive sheet" can be measured according to the method described in the Examples of this specification.

Here, in the thermally conductive sheet of the present invention, it is preferable that the ratio of the liquid resin in the resin is 50% by mass or more and 100% by mass or less. If the ratio of the liquid resin in the resin components contained in the particulate carbon material is 50% by mass or more and 100% by mass or less, the thermal conductivity of the thermally conductive sheet can be further improved.

In the thermally conductive sheet of the present invention, the volume fraction of the particulate carbon material in the thermally conductive sheet is preferably 25% by volume or more and 55% by volume or less. If the volume fraction of the particulate carbon material in the thermally conductive sheet is 25% by volume or more and 55% by volume or less, the thermal conductivity and tensile strength of the thermally conductive sheet can be improved.
The volume fraction of the particulate carbon material in the thermally conductive sheet is a fraction when the total volume of the resin and the particulate carbon material is taken as 100 volume %.

In the thermally conductive sheet of the present invention, it is preferable that the proportion of components with a molecular weight of 100,000 or more in the molecular weight distribution of the liquid resin is 7.0% or less. If the proportion of components with a molecular weight of 100,000 or more in the molecular weight distribution of the liquid resin is 7.0% or less, the entire sheet is soft, so it is easily deformed under pressure, has good compatibility with the adherend, and can reduce thermal resistance. The molecular weight distribution of the liquid resin can be measured according to the method described in the examples.

The present invention also aims to advantageously solve the above problems, and the method for producing a thermally conductive sheet of the present invention includes a primary sheet forming step of pressurizing a composition containing the resin and the particulate carbon material to form it into a sheet to obtain a primary sheet, a laminate forming step of stacking a plurality of the primary sheets in the thickness direction or folding or rolling the primary sheets to obtain a laminate, a slicing step of slicing the laminate at an angle of 45° or less in the stacking direction to obtain a secondary sheet, and a drying step of drying the secondary sheet under reduced pressure conditions of 120°C or higher for 3 hours or more. According to the method for producing a thermally conductive sheet of the present invention, it is possible to efficiently produce a thermally conductive sheet that can suppress pump-out and have excellent thermal conductivity at a high level.

Here, in the method for producing a thermally conductive sheet of the present invention, it is preferable that the drying time in the drying step is 12 hours or less. By setting the drying time of the secondary sheet to 12 hours or less, the thermal conductivity of the obtained thermally conductive sheet can be further improved.

The present invention provides a thermally conductive sheet and a method for manufacturing the same that can achieve high levels of both pump-out suppression and excellent thermal conductivity.

Hereinafter, an embodiment of the present invention will be described in detail.
The thermally conductive sheet of the present invention can be used by being sandwiched between a heat generating body and a heat dissipating body. That is, the thermally conductive sheet of the present invention can function as a heat dissipating member, and can constitute a heat dissipating device together with a heat dissipating body such as a heat sink, a heat dissipating plate, or a heat dissipating fin. In addition, the thermally conductive sheet of the present invention can be efficiently manufactured using the manufacturing method of the thermally conductive sheet of the present invention.

(Thermal Conduction Sheet)
The thermally conductive sheet of the present invention is a thermally conductive sheet containing a resin and a particulate carbon material. The resin constituting the thermally conductive sheet contains a liquid resin. Furthermore, the angle of the major axis direction of the particulate carbon material constituting the thermally conductive sheet with respect to the surface of the thermally conductive sheet is 60° or more and 90° or less. Furthermore, when the molecular weight distribution of the thermally conductive sheet is measured, the proportion of components having a molecular weight of 1,000 or less is 1.0% or less. Since the thermally conductive sheet of the present invention contains a liquid resin and a particulate carbon material oriented in a predetermined direction, it has high adhesion to the adherend and can efficiently exhibit thermal conductivity. Furthermore, since the proportion of components having a molecular weight of 1,000 or less is 1.0% or less, it is considered that the occurrence of pump-out can be suppressed.

<Molecular weight distribution of thermal conductive sheet>
In the molecular weight distribution of the thermal conductive sheet, the proportion of components having a molecular weight of 1,000 or less must be 1.0% or less, and the proportion of components having a molecular weight of 1,000 or less is preferably 0.7% or less, more preferably 0.5% or less, and even more preferably 0.3% or less, and it is particularly preferable that the thermal conductive sheet does not contain components having a molecular weight of 1,000 or less. If the proportion of components having a molecular weight of 1,000 or less in the molecular weight distribution of the thermal conductive sheet is equal to or less than the upper limit, the occurrence of pump-out can be suppressed well. As a method for controlling the content of components having a molecular weight of 1,000 or less contained in the thermal conductive sheet, for example, as described below, reduced pressure drying according to predetermined conditions can be performed at a predetermined timing in the manufacturing process of the thermal conductive sheet.

<Resin>
The resin constituting the thermal conductive sheet needs to contain a liquid resin. If the thermal conductive sheet contains a liquid resin as the resin, the thermal conductivity is increased due to the reasons such as an increased filling rate of the particulate carbon material in the thermal conductive sheet and an increased adhesion to the adherend.

<<Liquid resin>>
The liquid resin is not particularly limited as long as it is liquid at room temperature and normal pressure, and for example, a thermoplastic resin that is liquid at room temperature and normal pressure can be used.
In the present invention, "normal temperature" refers to 23° C., and "normal pressure" refers to 1 atm (absolute pressure).

Examples of liquid resins include fluororesins, silicone resins, acrylic resins, epoxy resins, and acrylonitrile-butadiene copolymers (nitrile rubber). These may be used alone or in combination of two or more types.

When the molecular weight distribution of the liquid resin is measured, it is preferable that the proportion of components with a molecular weight of 100,000 or more is 7.0% or less. If the proportion of components with a molecular weight of 100,000 or more in the liquid resin blended into the thermal conductive sheet is 7.0% or less, the entire sheet is soft, so it is easily deformed under pressure and fits well with the adherend, thereby reducing the thermal resistance. The lower limit of the proportion of components with a molecular weight of 100,000 or more in the liquid resin is not particularly limited, and may be, for example, 0.0%.

Furthermore, the resin constituting the thermally conductive sheet may contain other resins in addition to the liquid resin. Examples of other resins include solid resins.

<<Solid resin>>
The solid resin is not particularly limited as long as it is not liquid at room temperature and normal pressure. For example, a thermoplastic resin that is solid at room temperature and normal pressure, or a thermosetting resin that is solid at room temperature and normal pressure can be used.

Examples of thermoplastic resins that are solid at room temperature and normal pressure include acrylic resins such as poly(2-ethylhexyl acrylate), a copolymer of acrylic acid and 2-ethylhexyl acrylate, polymethacrylic acid or its ester, and polyacrylic acid or its ester; silicone resins; fluororesins; polyethylene; polypropylene; ethylene-propylene copolymers; polymethylpentene; polyvinyl chloride; polyvinylidene chloride; polyvinyl acetate; ethylene-vinyl acetate copolymers; polyvinyl alcohol; polyacetal; polyethylene terephthalate; polybutylene terephthalate; polyethylene naphthalate; polystyrene; and polyacrylonitrile. Examples of such copolymers include styrene-acrylonitrile copolymers, acrylonitrile-butadiene copolymers (nitrile rubbers), acrylonitrile-butadiene-styrene copolymers (ABS resins), styrene-butadiene block copolymers or hydrogenated products thereof, styrene-isoprene block copolymers or hydrogenated products thereof, polyphenylene ethers, modified polyphenylene ethers, aliphatic polyamides, aromatic polyamides, polyamideimides, polycarbonates, polyphenylene sulfides, polysulfones, polyethersulfones, polyethernitriles, polyetherketones, polyketones, polyurethanes, liquid crystal polymers, and ionomers. These may be used alone or in combination of two or more.
In the present invention, rubber is included in the "resin".

Examples of thermosetting resins that are solid at room temperature and pressure include natural rubber, butadiene rubber, isoprene rubber, nitrile rubber, hydrogenated nitrile rubber, chloroprene rubber, ethylene propylene rubber, chlorinated polyethylene, chlorosulfonated polyethylene, butyl rubber, halogenated butyl rubber, polyisobutylene rubber, epoxy resin, polyimide resin, bismaleimide resin, benzocyclobutene resin, phenolic resin, unsaturated polyester, diallyl phthalate resin, polyimide silicone resin, polyurethane, thermosetting polyphenylene ether, and thermosetting modified polyphenylene ether. These may be used alone or in combination of two or more.

<<Liquid resin content in resin>>
The resin constituting the thermal conductive sheet may contain only liquid resin, or may contain other resins such as the solid resin described above in addition to the liquid resin. When the resin contains a resin other than the liquid resin, the proportion of the liquid resin in the resin is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. If the content of the liquid resin is equal to or more than the above lower limit, the compressibility of the obtained thermal conductive sheet can be increased. If the compressibility of the thermal conductive sheet is high, it is easy to be crushed when pressurized, and it adheres well to the adherend, and when it is crushed, the density of the particulate carbon material in the sheet increases, and the thermal conductivity can also be increased.

<Resin content>
The content of the resin in the thermal conductive sheet is preferably 45% by volume or more, more preferably 50% by volume or more, and preferably 75% by volume or less, more preferably 70% by volume or less, and even more preferably 60% by volume or less. If the content of the resin in the thermal conductive sheet is equal to or greater than the lower limit, the tensile strength and compressibility of the thermal conductive sheet can be increased. If the content of the resin in the thermal conductive sheet is equal to or less than the upper limit, the thermal conductivity of the thermal conductive sheet can be further increased.

<Carbon material particles>
Here, the particulate carbon material is not particularly limited, and examples thereof include artificial graphite and natural graphite. Artificial graphite includes carbon black and pyrolytic graphite. Natural graphite includes flake graphite such as expanded graphite and spheroidal graphite, as well as flake graphite. These may be used alone or in combination of two or more. The term "particulate carbon material" refers to a carbon material having an aspect ratio of 20 or less.

Among the above, it is preferable to use flake graphite as the particulate carbon material, and among flake graphite, it is more preferable to use expanded graphite. If flake graphite is used as the particulate carbon material, the particulate carbon material can be appropriately concentrated with each other, achieving both resistance to tearing and compressibility under pressure at a higher level. Furthermore, if expanded graphite is used among the flake graphite, the thermal conductivity of the heat conductive sheet can be further increased.

<Orientation of particulate carbon material>
The orientation of the particulate carbon material in the thermally conductive sheet must satisfy the requirement that the angle of the major axis direction of the particulate carbon material with respect to the surface of the thermally conductive sheet (hereinafter sometimes referred to as the "orientation angle of the particulate carbon material") is 60° or more and 90° or less. Furthermore, the orientation angle of the particulate carbon material is more preferably 65° or more, even more preferably 70° or more, and preferably 90° or less. If the orientation angle of the particulate carbon material is within the above specified range, the thermal resistance of the thermally conductive sheet can be reduced and thermal conductivity can be increased.

<Properties of particulate carbon material>
The volume average particle diameter of the particulate carbon material is preferably 3 μm or more, more preferably 5 μm or more, even more preferably 8 μm or more, even more preferably 12 μm or more, even more preferably 16 μm or more, preferably 200 μm or less, more preferably 150 μm or less, preferably 100 μm or less, and even more preferably 60 μm or less. If the volume average particle diameter of the particulate carbon material is equal to or greater than the lower limit, the contact resistance between the particulate carbon material can be reduced, and the thermal conductivity of the thermal conductive sheet is improved. On the other hand, if the volume average particle diameter of the particulate carbon material is equal to or less than the upper limit, the particulate carbon material can be appropriately filled in the thermal conductive sheet, and the thermal conductivity of the thermal conductive sheet can be further improved.
In the present invention, the "volume average particle size" can be measured in accordance with JIS Z8825, and represents a particle size at which the cumulative volume calculated from the small diameter side is 50% in a particle size distribution (volume basis) measured by a laser diffraction method.

The particulate carbon material preferably has an aspect ratio (major axis/minor axis) of more than 1.2, more preferably more than 2, even more preferably more than 4, even more preferably more than 6, and preferably 20 or less, more preferably 15 or less, and even more preferably 10 or less. If the aspect ratio of the particulate carbon material is within the above-mentioned range, the orientation angle of the particulate carbon material with respect to the surface of the thermally conductive sheet can easily fall within the desired range described below, thereby further improving the thermal conductivity.

<Content of particulate carbon material>
The content of the particulate carbon material in the thermal conductive sheet is preferably 25% by volume or more, more preferably 30% by volume or more, even more preferably 40% by volume or more, and preferably 55% by volume or less, and more preferably 50% by volume or less, based on 100% by volume of the total volume of the resin and the particulate carbon material. If the content of the particulate carbon material in the thermal conductive sheet is equal to or more than the lower limit, the thermal conductivity of the thermal conductive sheet can be further improved. If the content of the particulate carbon material in the thermal conductive sheet is equal to or less than the upper limit, the tensile strength and compressibility of the thermal conductive sheet can be improved.

<Other ingredients>
The thermally conductive sheet of the present invention may further contain components other than the above-mentioned components. For example, fibrous carbon materials can be used as the other components. The fibrous carbon materials are not particularly limited, and carbon materials having an aspect ratio of 20 or more can be used. More specifically, the fibrous carbon materials can be carbon nanotubes, vapor-grown carbon fibers, carbon fibers obtained by carbonizing organic fibers, and cut products thereof. These may be used alone or in combination of two or more.

<<Thermal resistance of thermal conductive sheet>>
The thermal resistance of the thermal conductive sheet is preferably 0.100°C/W or less, more preferably 0.080°C/W or less, even more preferably 0.060°C/W or less, and particularly preferably 0.050 or less. If the thermal resistance is equal to or less than the upper limit, the thermal conductive sheet has excellent thermal conductivity. The thermal resistance value of the thermal conductive sheet is, for example, the thermal resistance value when a pressure of 0.9 MPa is applied, and can be measured according to the method described in the examples of this specification.

<Thickness>
The thickness of the thermal conductive sheet is preferably 50 μm or more, more preferably 80 μm or more, more preferably 90 μm or more, even more preferably 100 μm or more, and preferably 500 μm or less, more preferably 300 μm or less, and even more preferably 200 μm or less. If the thickness of the thermal conductive sheet is equal to or greater than the lower limit, the strength and thermal conductivity of the thermal conductive sheet can be increased. On the other hand, if the thickness of the thermal conductive sheet is equal to or less than the upper limit, the thermal resistance value can be appropriately reduced.

(Method of manufacturing thermally conductive sheet)
The method for producing a thermally conductive sheet of the present invention includes: (A) a primary sheet forming step of pressurizing a composition containing a resin and a particulate carbon material to form it into a sheet to obtain a primary sheet; (B) a laminate forming step of stacking a plurality of primary sheets in the thickness direction or folding or rolling the primary sheet to obtain a laminate; (C) a slicing step of slicing the laminate at an angle of 45° or less to the stacking direction to obtain a secondary sheet; and (D) a drying step of drying the secondary sheet under reduced pressure conditions at 120° C. or higher for 3 hours or more.
The method for producing a thermally conductive sheet of the present invention may optionally include further steps other than the above steps (A) to (D).

The method for manufacturing a thermally conductive sheet of the present invention makes it possible to efficiently manufacture a thermally conductive sheet that exhibits excellent thermal conductivity while suppressing pump-out.

<(A) Primary sheet molding step>
In the primary sheet forming step, a composition containing a resin and a carbon material is pressed into a sheet to obtain a primary sheet.

<<Composition>>
The composition includes a resin and a carbon material particle. The composition may further include other components in addition to the resin and the carbon material particle.

As the particulate carbon material, the particulate carbon material described above in the "Thermal Conductive Sheet" section can be used in the ratio described above.

-Other ingredients-
The composition may further contain other components in addition to the above-mentioned resin and particulate carbon material. As the other components, for example, the fibrous carbon material and dispersant described in the section "Thermal Conductive Sheet" can be used. The dispersant is not particularly limited, and any known dispersant can be used. The content of the dispersant in the composition can be adjusted within a range in which the desired effect of the present invention can be obtained.

-Preparation of composition-
The composition is not particularly limited and can be prepared by mixing the above-mentioned components.
The mixing of the above-mentioned components is not particularly limited, and can be performed using a known mixing device such as a kneader; a mixer such as a Henschel mixer, a Hobart mixer, or a high-speed mixer; a twin-screw kneader; or a roll. The mixing may be performed in the presence of a solvent such as ethyl acetate. The resin may be dissolved or dispersed in advance in a solvent to form a resin solution, which may then be mixed with the particulate carbon material and other components that are optionally added. The mixing time may be, for example, 5 minutes or more and 60 minutes or less. The mixing temperature may be, for example, 5° C. or more and 160° C. or less.

<<Molding of the composition>>
The composition prepared as described above can be optionally degassed and crushed, and then pressed to form into a sheet. The sheet-like product obtained by pressing the composition in this manner can be used as a primary sheet. If a solvent is used during mixing, it is preferable to remove the solvent before forming into a sheet. For example, if degassing is performed using vacuum degassing, the solvent can be removed at the same time as degassing.

Here, the composition can be molded into a sheet using a known molding method such as press molding, rolling molding, or extrusion molding, without any particular limitation, as long as the molding method is one in which pressure is applied. Among them, the composition is preferably molded into a sheet by rolling molding (primary processing), and more preferably molded into a sheet by passing the composition between rolls while sandwiched between protective films. The protective film is not particularly limited, and may be a polyethylene terephthalate (PET) film that has been subjected to sandblasting treatment. The roll temperature may be 5°C or higher and 150°C or lower, the roll gap may be 50 μm or higher and 2500 μm or lower, the roll linear pressure may be 1 kg/cm or higher and 3000 kg/cm or lower, and the roll speed may be 0.1 m/min or higher and 20 m/min or lower.

<(B) Laminate formation process>
In the laminate formation step, a laminate is obtained in which a plurality of primary sheets containing a resin and a particulate carbon material are formed in the thickness direction by stacking a plurality of primary sheets obtained in the primary sheet molding step in the thickness direction, or by folding or rolling the primary sheets. Here, the formation of the laminate by folding the primary sheets is not particularly limited, and can be performed by folding the primary sheets at a constant width using a folding machine. In addition, the formation of the laminate by rolling the primary sheets is not particularly limited, and can be performed by rolling the primary sheets around an axis parallel to the short side direction or long side direction of the primary sheets. In addition, the formation of the laminate by stacking the primary sheets is not particularly limited, and can be performed using a lamination device. For example, if a sheet lamination device (manufactured by Nikkiso Co., Ltd., product name "Hi-Stacker") is used, it is possible to suppress the intrusion of air between the layers, and therefore a good laminate can be efficiently obtained.

In the lamination process, it is preferable to apply pressure (secondary pressure) to the obtained laminate in the lamination direction while heating it. By applying secondary pressure to the laminate in the lamination direction while heating it, it is possible to promote fusion between the laminated primary sheets.

Here, the pressure applied to the laminate in the lamination direction may be 0.05 MPa or more and 0.50 MPa or less.
The heating temperature of the laminate is not particularly limited, but is preferably 50° C. or higher and 170° C. or lower.
Furthermore, the heating time for the laminate can be, for example, from 10 seconds to 30 minutes.

In the laminate obtained by stacking, folding, or rolling the primary sheet, the particulate carbon material is presumably oriented in a direction approximately perpendicular to the stacking direction. For example, if the particulate carbon material has a scale-like shape, the direction of the long axis of the main surface of the scale-like shape is presumably approximately perpendicular to the stacking direction.

<(C) Slicing step>
In the slicing step, the laminate is sliced at an angle of 45° or less with respect to the lamination direction to obtain a secondary sheet consisting of slices of the laminate. Here, the method for slicing the laminate is not particularly limited, and examples thereof include a multi-blade method, a laser processing method, a water jet method, and a knife processing method. Among them, the knife processing method is preferred because it is easy to make the thickness of the secondary sheet uniform. In addition, the cutting tool used for slicing the laminate is not particularly limited, and a slicing member having a smooth plate surface with a slit and a blade portion protruding from the slit portion (for example, a plane or slicer with a sharp blade) can be used.

The angle at which the laminate is sliced is preferably 30° or less with respect to the stacking direction, more preferably 15° or less with respect to the stacking direction, and preferably approximately 0° with respect to the stacking direction (i.e., in the direction along the stacking direction).
In the secondary sheet thus obtained, the particulate carbon material is well oriented in the thickness direction, and the angle of the major axis direction of the particulate carbon material with respect to the sheet surface is 60° to 90°. More specifically, when the particulate carbon material is scaly, the angle of the major axis direction of the main surface of the scaly shape with respect to the sheet surface is 60° to 90°.

<(D) Drying step>
In the drying step, the secondary sheet is dried under reduced pressure at 120° C. or higher for 3 hours or more to obtain a thermally conductive sheet. In the drying step, the proportion of components having a molecular weight of 1,000 or less in the obtained thermally conductive sheet can be efficiently reduced to 1.0% or less by performing the drying step under the above conditions. The drying step is not particularly limited, and can be suitably performed, for example, using a vacuum oven or the like.

The drying temperature in the drying process must be 120°C or higher, preferably 130°C or higher, more preferably 140°C or higher, and preferably 200°C or lower, more preferably 180°C or lower. If the drying temperature in the drying process is equal to or higher than the lower limit, the proportion of components with a molecular weight of 1,000 or less in the obtained thermal conductive sheet can be efficiently reduced. Furthermore, if the drying temperature in the drying process is equal to or lower than the upper limit, it is possible to effectively prevent components with a molecular weight of more than 1,000 from being removed from the thermal conductive sheet.

The drying time in the drying step must be 3 hours or more, preferably 5 hours or more, preferably 12 hours or less, and more preferably 10 hours or less. If the drying time in the drying step is equal to or greater than the above lower limit, the proportion of components with a molecular weight of 1,000 or less in the obtained thermal conductive sheet can be effectively reduced. If the drying time in the drying step is equal to or less than the above upper limit, low molecular weight components are not removed more than necessary, and the obtained thermal conductive sheet can be given appropriate flexibility, thereby further increasing the thermal conductivity of the thermal conductive sheet.

The atmosphere in the drying process is not particularly limited as long as it is a reduced pressure condition. Here, the reduced pressure condition is preferably a reduced pressure of -0.1 MPa based on atmospheric pressure (1 atm).

The mass reduction rate of the secondary sheet before and after the drying process is preferably 0.5% or more, more preferably 1.0% or more, more preferably 1.5% or more, and preferably 3.0% or less, and more preferably 2.0% or less. By selecting the conditions (combination of vacuum pressure, drying temperature, and drying time) that result in a mass reduction rate within the above range and carrying out the drying conditions, it is possible to efficiently reduce the proportion of components with a molecular weight of 1,000 or less in the obtained thermal conductive sheet while effectively suppressing the volatilization of components with a molecular weight of more than 1,000.

Then, following the drying process according to the above conditions, a cooling process can be carried out. The conditions for the cooling process are not particularly limited, but for example, the thermally conductive sheet can be cooled for 3 hours or more under normal temperature and pressure conditions.

In the thermally conductive sheet obtained through the various processes described above, the particulate carbon material is well oriented in the thickness direction. For example, when the particulate carbon material has a scale shape, the direction of the long axis of the main surface of the scale shape is approximately the same as the thickness direction of the secondary sheet.

The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" expressing amounts are based on mass unless otherwise specified. In addition, when calculating the volume fraction, the volume of each blended component was calculated by dividing the mass of each blended component by its theoretical specific gravity.
In the examples, various measurements and evaluations were carried out according to the following methods.

<Orientation angle of particulate carbon material>
The orientation angle of the particulate carbon material in the thermally conductive sheet was determined by observing a cross section of a thermally conductive sheet cut into a regular octagon with a scanning electron microscope (SEM, Hitachi High-Technologies Corporation's "SU-3500") at a magnification that covered the entire sheet from top to bottom. The magnification was 700x. Fifty lines were drawn along the major axis of the particulate carbon material in this cross section, and the average angle of the major axis with respect to the surface of the thermally conductive sheet was calculated. If the angle was 90° or more, the supplementary angle was used. This was performed for eight surfaces, and the largest value among the eight surfaces was taken as the orientation angle of the particulate carbon material in the thermally conductive sheet.

<Mass reduction rate>
The mass α1 of the secondary sheet immediately after the slicing process and the mass α2 of the thermally conductive sheet obtained after the drying process and the cooling process were each weighed, and the mass reduction rate was calculated according to the following formula.
(Mass reduction rate) = 1 - (mass of thermal conductive sheet α2) ÷ (mass of secondary sheet α1)

<Molecular weight distribution>
The molecular weight distribution of the liquid resin as a raw material was measured by gel permeation chromatography (GPC) according to the following conditions. The GPC measurement was performed using a GPC system (Tosoh Corporation, HLC-8220) with two H-type columns (Tosoh Corporation, HZ-M) connected in series, tetrahydrofuran as a solvent, and a column temperature of 40° C. A differential refractometer (Tosoh Corporation, RI-8320) was used as a detector.
When measuring the molecular weight distribution of the thermally conductive sheet, the thermally conductive sheet was dissolved in an organic solvent or the like, and the solid components were removed by filtration, and the molecular weight of the extracted resin component was measured in the manner described above.

The pump-out resistance of the thermally conductive sheets obtained in the examples and comparative examples was measured as follows.
That is, two copper plates each of 50 mm square and copper foil (coarse copper foil) with one side roughened were prepared. The coarse copper foil was placed on one of the copper plates with the rough surface facing up, and a thermally conductive sheet cut to a size of 10 mm x 10 mm square was placed in the approximate center of the rough surface side of the coarse copper foil. Next, the other coarse copper foil was placed on the placed thermally conductive sheet with the rough surface facing down, and the other copper plate was placed on top of the coarse copper foil, thereby obtaining a laminate consisting of copper plate/coarse copper foil/thermal conductive sheet/coarse copper foil/copper plate, in which the thermally conductive sheet was sandwiched between the rough surface side of the coarse copper foil and the copper plate, as a test piece. Next, a weight of 1500 g was placed on the obtained test piece, and the test piece was stored in a thermostatic chamber at a temperature of 150 ° C. for 72 hours. At this time, the pressure applied to the thermally conductive sheet sandwiched between the copper plate and the coarse copper foil was 0.15 MPa. After 72 hours of storage, the copper plate and the rough copper foil of the test piece were peeled off from the thermal conductive sheet, and the "stains" spreading on the rough surfaces of the two rough copper foils were visually observed, and the average value (mm) of the maximum diameter of the outline of the "stains" was measured. The "stains" were formed spreading in an approximately concentric circular shape, and could be approximated to a circular or elliptical shape. Evaluation was performed according to the following criteria.
A smaller average value of the maximum diameter indicates that the heat conductive sheet has better pump-out resistance. If a heat conductive sheet is rated A or B below, it can be said that the pump-out resistance is relatively good.
A: The average maximum diameter is less than 15 mm. B: The average maximum diameter is 15 mm or more and less than 20 mm. C: The average maximum diameter is 20 mm or more.

<Tensile strength>
The thermally conductive sheets manufactured in the examples and comparative examples were punched out in the X and Y directions defined as follows to obtain test pieces of 20 mm x 50 mm. The obtained test pieces were subjected to a tensile test at a tensile speed of 20 mm/min using a small tabletop testing machine (manufactured by Nidec-Shimpo Corporation, "FGS-500TV", using FGP-50 as a digital force gauge). The distance between the chucks was 30 mm. The sheet strength (N/mm) of the thermally conductive sheet was calculated by dividing the maximum strength (N) during the tensile test by the thickness (mm) of the test piece.
The "X direction" means "the direction in which the thermal conductivity is highest when measured on the main surface of the thermal conductive sheet (the direction perpendicular to the stacking direction of the laminate)" and the "Y direction" means "the direction perpendicular to the X direction (the direction coinciding with the stacking direction of the laminate)."
Here, the thermal conductivity can be measured by the following method.
<<Thermal Conductivity>>
For each of the main surfaces of the thermally conductive sheets, the thermal diffusivity α (m ² /s), specific heat at constant pressure Cp (J/g·K), and specific gravity ρ (g/m ³ ) were measured by the following methods.
[Thermal diffusivity α (m ² /s)]
The thermal diffusivity was measured using a thermal property measuring device (manufactured by Bethel Corporation, product name "Thermowave Analyzer TA35").
[Constant pressure specific heat Cp (J/g K)]
The specific heat was measured using a differential scanning calorimeter (manufactured by Rigaku, product name "DSC8230") under a temperature increase condition of 10°C/min.
[Specific gravity ρ (g/m ³ )]
The specific gravity (density) (g/m ³ ) was measured using an automatic specific gravity meter (manufactured by Toyo Seiki Seisakusho, product name "DENSIMETER-H").
The obtained measured values are then used to calculate the following formula (I):
λ=α×Cp×ρ...(I)
The thermal conductivity λ (W/m·K) of the thermal conductive sheet was calculated by substituting the above.

<Thermal resistance and compressibility>
The thermal resistance value of the thermal conductive sheet was measured using a thermal resistance tester (Hitachi Technology & Services Co., Ltd., product name "Resin Material Thermal Resistance Measuring Device"). Here, the thermal conductive sheet cut into a roughly 1 cm square was used as a sample, and the initial film thickness was measured. Then, the thermal resistance value (°C/W) and the sheet thickness (unit: mm) were measured when a pressure of 0.9 MPa was applied at a sample temperature of 50°C. The smaller the thermal resistance value, the better the thermal conductivity of the thermal conductive sheet, and for example, the better the heat dissipation characteristics when interposed between a heat generating body and a heat sink.
The compression ratio was calculated by dividing the sheet thickness when pressed at 0.9 MPa by the initial sheet thickness and subtracting the result from 1. The results are shown in Table 1. The larger the compression ratio, the easier it is to compress when pressed.

Example 1
<Preparation of Composition>
70 parts of a thermoplastic fluororesin (manufactured by Daikin Industries, Ltd., trade name "Daiel G-101") as a liquid resin, 30 parts of a thermoplastic fluororesin (manufactured by 3M Japan Ltd., trade name "Dyneon FC2211") as a solid resin, and 90 parts of expanded graphite (manufactured by Ito Graphite Industries Co., Ltd., trade name "EC300", volume average particle size: 50 μm) as a particulate carbon material (41 volume % with respect to the total volume of the resin and particulate carbon material) were mixed and stirred at a temperature of 150 ° C. for 20 minutes using a pressure kneader (manufactured by Nippon Spindle Co., Ltd.). Next, the resulting mixture was put into a crusher (manufactured by Osaka Chemical Co., Ltd., trade name "Wonder Crush Mill D3V-10") and crushed for 10 seconds to obtain a composition.
<(A) Primary sheet molding step>
Next, 50 g of the obtained composition was sandwiched between 50 μm-thick PET films (protective films) that had been sandblasted, and roll-molded (primary pressing) under conditions of a roll gap of 550 μm, a roll temperature of 50° C., a roll linear pressure of 50 kg/cm, and a roll speed of 1 m/min to obtain a primary sheet with a thickness of 0.5 mm.
<(B) Laminate formation process>
Next, the obtained primary sheet was cut to a size of 150 mm length x 150 mm width x 0.5 mm thickness, and 120 sheets were stacked in the thickness direction of the primary sheet. Further, the sheets were pressed (secondary pressurization) in the stacking direction at a temperature of 120°C and a pressure of 0.1 MPa for 3 minutes to obtain a laminate with a height of approximately 60 mm.
<(C) Slicing step>
Thereafter, the entire upper surface of the obtained laminate was pressed with a metal plate, leaving a length required for slicing, and a pressure of 0.1 MPa was applied in the lamination direction (i.e., from above) to fix the laminate. Note that the side and back of the laminate were not fixed. At this time, the temperature of the laminate was 25°C.
Next, a cutting blade (double-edged, blade angle: 20°, maximum thickness of blade: 3.5 mm, material: carbide, Rockwell hardness: 91.5, silicon processing on blade surface: none, total length: 200 mm) was attached to the press portion of a servo press (manufactured by Electric Discharge Precision Machining Laboratory), and the laminate was sliced in the stacking direction (in other words, in the direction coinciding with the normal to the main surface of the stacked pre-thermal conductive sheets) at a slicing speed of 200 mm/sec and a slice width of 100 μm to obtain a secondary sheet of length 150 mm x width 60 mm x thickness 0.10 mm.
<(D) Drying process ~ cooling process>
The obtained secondary sheet was dried for 5 hours in a vacuum oven at a reduced pressure of -0.1 MPa and an atmosphere of 150°C. The dried product was then placed at room temperature and normal pressure for 3 hours to cool, thereby obtaining a thermally conductive sheet.
The thermally conductive sheet thus obtained was subjected to various evaluations as described above. The results are shown in Table 1.

Example 2
In preparing the composition, the liquid resin was changed to thermoplastic NBR (manufactured by Nippon Zeon Co., Ltd., product name "Nipol 1312"), the solid resin was changed to NBR (manufactured by Nippon Zeon Co., Ltd., product name "Nipol 3350"), and the number of parts of the particulate carbon material was changed to 150 parts (40% by volume equivalent to Example 1). Other than these, various operations, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

Example 3
The drying temperature in the drying step was changed to 130° C., and the drying time was changed to 3 hours. Except for this, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

Example 4
The drying time in the drying step was changed to 12 hours. Except for this, various operations, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

Example 5
In preparing the composition, the amount of the liquid resin was changed to 100 parts, and no solid resin was added, but the same operations, measurements and evaluations were carried out as in Example 1. The results are shown in Table 1.

Example 6
In preparing the composition, the amount of the particulate carbon material was changed to 130 parts (51% by volume), but the various operations, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

(Example 7)
In preparing the composition, the amount of the particulate carbon material was changed to 50 parts (28% by volume), but the various operations, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 1)
Except for not carrying out the drying step to the cooling step (D), various operations, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 2)
In preparing the composition, no liquid resin was used, and the amount of solid resin was changed to 100 parts. Except for this, various operations, measurements, and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 3)
In preparing the composition, the particulate carbon material was changed to one having a volume average particle diameter of 250 μm (manufactured by Ito Graphite Industries Co., Ltd., product name "EC50", volume average particle diameter: 250 μm) and the blending amount was changed to 50 parts (28 volume % in volume conversion). Except for this, various operations, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

From Table 1, it can be seen that the thermal conductive sheets of Examples 1 to 7, which contain a liquid resin and a particulate carbon material, in which the angle of the long axis direction of the particulate carbon material with respect to the surface of the thermal conductive sheet is 60° or more and 90° or less, and in which the proportion of components with a molecular weight of 1,000 or less is 1.0% or less when the molecular weight distribution of the thermal conductive sheet is measured, are able to achieve a high level of both suppressing pump-out and having excellent thermal conductivity.
On the other hand, in Comparative Examples 1 and 3, in which the proportion of components with a molecular weight of 1,000 or less exceeded 1.0%, and in Comparative Example 2, in which no liquid resin was blended, it was found that it was not possible to achieve both a high level of pump-out suppression effect and high thermal conductivity.

The present invention provides a thermally conductive sheet and a method for manufacturing the same that can achieve high levels of both pump-out suppression and excellent thermal conductivity.

Claims (5)

A thermally conductive sheet comprising a resin and a particulate carbon material,
the resin comprises a liquid resin,
the angle of the major axis direction of the particulate carbon material with respect to the surface of the thermal conductive sheet is 60° or more and 90° or less;
When the molecular weight distribution of the thermal conductive sheet is measured, the proportion of components having a molecular weight of 1,000 or less is 1.0% or less,
A thermally conductive sheet , wherein in the molecular weight distribution of the liquid resin, the proportion of components having a molecular weight of 100,000 or more is 7.0% or less . The thermal conductive sheet according to claim 1, wherein the ratio of the liquid resin in the resin is 50% by mass or more and 100% by mass or less. The thermal conductive sheet according to claim 1 or 2, wherein the volume fraction of the particulate carbon material in the thermal conductive sheet is 25 volume % or more and 55 volume % or less. A method for producing a thermal conductive sheet according to any one of claims 1 to 3 , comprising the steps of:
a primary sheet forming step of pressing a composition containing the resin and the particulate carbon material into a sheet shape to obtain a primary sheet;
a laminate forming step of laminating a plurality of the primary sheets in a thickness direction or folding or rolling the primary sheet to obtain a laminate;
a slicing step of slicing the laminate at an angle of 45° or less in the lamination direction to obtain a secondary sheet;
A drying step of drying the secondary sheet under reduced pressure at 120° C. or higher for 3 hours or more;
A method for producing a thermal conductive sheet comprising the steps of: The method for producing a thermal conductive sheet according to claim 4 , wherein the drying time in the drying step is 12 hours or less.