JP2021066756A - Method for producing thermally conductive sheet - Google Patents

Abstract

To provide a technique for producing a thermally conductive sheet in which distortion, deformation, wrinkle, etc. generated on the surface thereof caused by directly irradiating the whole surface with an electron beam, and which is nonetheless excellent in sheet strength and thermal conductivity.MEANS FOR SOLVING THE PROBLEM: A method for producing a thermally conductive sheet including an electron beam-crosslinkable resin and a thermal conductive filler is provided that includes a step of irradiating at least one surface of a sheet including an electron beam-crosslinkable resin and a thermally conductive filler with an electron beam so that regions having different electron beam absorption doses are formed.SELECTED DRAWING: Figure 1

Description

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

In recent years, the amount of heat generated by electronic components such as power semiconductors (IGBT modules and the like) and integrated circuit (IC) chips has increased as the performance has improved. As a result, in electronic devices using electronic components, it is necessary to take measures against dysfunction due to the temperature rise of the electronic components.

Here, in general, as a countermeasure against functional failure due to temperature rise, a method of promoting heat dissipation by attaching a heat sink such as a metal heat sink, a heat sink, or a heat radiation fin to a heating element such as an electronic component is adopted. ing. Then, when using a heating element, in order to efficiently transfer heat from the heating element to the heating element, the heating element and the heating element are connected to each other via a sheet-like member (heat conductive sheet) having high thermal conductivity. Are in close contact with each other. Therefore, the heat conductive sheet used by being sandwiched between the heating element and the heat radiating element is required to have high thermal conductivity and high flexibility. Further, the heat conductive sheet is also required to exhibit high flame retardancy from the viewpoint of safety.

Therefore, for example, Patent Document 1 states that a thermally conductive resin composition containing an electron beam (EB) crosslinked olefin resin and a thermally conductive filler is compression-molded and then crosslinked by EB irradiation. A method for producing a heat conductive sheet (heat radiating sheet) having heat conductivity and excellent heat resistance is disclosed.

Japanese Unexamined Patent Publication No. 2012-54312

However, when the entire surface of the sheet is directly irradiated with an electron beam, a strong cross-linking reaction occurs on the sheet surface due to the influence of the electron beam irradiation, and the molecular chains on the sheet surface shrink, so that the surface is distorted, deformed, or wrinkled. Etc. (hereinafter collectively referred to as wrinkles, etc.) (the stronger the electron beam and the stronger the cross-linking reaction on the surface as the absorbed dose of the electron beam increases), the surface of the sheet becomes uneven, and a metal cover such as a heat sink There is a problem that a gap is generated between the body and the body, which causes deterioration of thermal resistance and deterioration of adhesion.

Therefore, an object of the present invention is to provide a technique for producing a heat conductive sheet having excellent sheet strength and heat conductivity while suppressing the occurrence of wrinkles on the surface.

The present inventor has conducted diligent studies in order to achieve the above object. Then, the present inventor irradiates at least one surface of the sheet containing the electron beam crosslinked resin and the heat conductive filler with an electron beam so that regions having different electron beam absorption doses are formed. The present invention has been completed by finding that a heat conductive sheet having excellent sheet strength and heat conductivity can be obtained while suppressing the occurrence of wrinkles on the sheet surface.

That is, the present invention aims to advantageously solve the above problems, and the method for producing a heat conductive sheet according to the present invention is a heat conduction including an electron beam crosslinked resin and a heat conductive filler. A method for producing a sheet, in which the electron beam is formed so that regions having different electron beam absorption doses are formed on at least one surface of the sheet containing the electron beam crosslinked resin and the heat conductive filler. It is characterized by including a step of irradiating. In this way, if the sheet surface is irradiated with an electron beam so that regions having different electron beam absorption doses are formed on at least one surface of the sheet containing the electron beam crosslinked resin and the heat conductive filler, the sheet surface is exposed. It is possible to provide a heat conductive sheet having excellent sheet strength and heat conductivity while suppressing the occurrence of wrinkles and the like.

Here, the step of irradiating the electron beam preferably includes irradiating the surface of the electron beam with the electron beam via a mask member in which portions having different transmittances of the electron beam are arranged in a pattern. According to the above method, it becomes easy to form regions having different electron beam absorbed doses on at least one surface of the heat conductive sheet.

The pattern of the mask member is preferably a mesh structure pattern having a plurality of rectangular openings or a punching structure pattern having a plurality of circular openings. With such a configuration, it becomes easy to form regions having different electron beam absorbed doses on at least one surface of the heat conductive sheet.

Further, the aperture ratio of the opening of the mask member is preferably 30% or more and 90% or less, more preferably 35% or more and 85% or less, and further preferably 40% or more and 80% or less. .. When the aperture ratio is within the above range, it is possible to achieve both the suppression of wrinkles of the obtained heat conductive sheet and the sheet strength at a high level while maintaining the adhesion between the sheet and the metal adherend. it can.

The step of irradiating the electron beam preferably includes irradiating the electron beam so that the irradiation dose per irradiation surface on the surface of the sheet is 50 kGy or more and 9000 kGy or less. If the irradiation dose of the electron beam per irradiation surface on the sheet surface is within the above range, it is possible to achieve both suppression of wrinkles and the like of the obtained heat conductive sheet and sheet strength at a high level.

Here, the irradiation dose in electron beam irradiation is expressed as an irradiation dose, and gray (Gy) is used as a unit. This is the "irradiation dose (1 Gy) when there is absorption of 1 joule of energy per 1 kg".
A film dosimeter is used for the irradiation dose based on the amount of electron beam transmitted. That is, in a film dosimeter, the film contains a substance that develops color with an electron beam, and when the film for dose measurement is irradiated with an electron beam to obtain absorbed energy, the spectral characteristics change and the film discolors. .. There is a correlation between the degree of discoloration and the irradiation dose, and the irradiation dose can be measured by measuring the absorbance at a specific wavelength. In the present invention, the "irradiation dose" of the electron beam refers to a value measured using a radiochromic film dosimeter (Far West Technology, FWT-60).

In the method for producing a heat conductive sheet of the present invention, the electron beam crosslinked resin preferably contains a liquid thermoplastic fluororesin under normal temperature and pressure. If the sheet irradiated with the electron beam contains a liquid thermoplastic fluororesin under normal temperature and pressure, it is possible to obtain a heat conductive sheet having sufficiently high heat conductivity by suppressing an increase in thermal resistance. Further, since the wrinkles on the surface caused by the irradiation of the electron beam can be suppressed, the sheet strength can be increased while maintaining the smoothness of the sheet.
In the present invention, "normal temperature" means 23 ° C., and "normal pressure" means 1 atm (absolute pressure).

Further, in the method for producing a heat conductive sheet of the present invention, the thickness of the obtained heat conductive sheet is preferably 50 μm or more and 600 μm or less. When the thickness of the sheet is within the above range, the obtained heat conductive sheet can be preferably used by being sandwiched between the heating element and the heat radiating element.
In the present invention, the "thickness" of the sheet refers to a value measured with an accuracy of 1/1000 mm using a digital indicator (manufactured by Mitutoyo Co., Ltd., ID-C112X).

According to the present invention, it is possible to provide a technique for producing a heat conductive sheet having excellent sheet strength and heat conductivity while suppressing the occurrence of wrinkles on the surface.

It is a schematic diagram which shows the mesh structure which is an example of the pattern of a mask member. It is a schematic diagram which shows the punching structure which is an example of the pattern of a mask member. It is a schematic diagram which shows the mesh structure which is an example of the pattern of a mask member. It is a schematic diagram which shows the elongated hole structure which is an example of the pattern of a mask member. It is a schematic diagram which shows the slit hole structure which is an example of the pattern of a mask member. It is a schematic diagram which shows the structure which provided circular opening and cross-shaped opening alternately which is an example of the pattern of a mask member. It is a schematic diagram of the heat conduction sheet for demonstrating the thermal resistance test in Examples 8 and 9.

Hereinafter, embodiments of the present invention will be described in detail. Here, the method for producing a heat conductive sheet of the present invention is used, for example, when manufacturing a heat conductive sheet containing an electron beam crosslinked resin and a heat conductive filler. Then, the heat conductive sheet manufactured by the manufacturing method of the present invention can be used, for example, by sandwiching it between the heating element and the heat radiating element when attaching the heat radiating element to the heating element. The present invention is not limited to the following embodiments.

(Manufacturing method of heat conductive sheet)
In the method for producing a heat conductive sheet of the present invention, an electron beam is formed so that a region having a different electron beam absorbed dose is formed on at least one surface of the sheet containing the electron beam crosslinked resin and the heat conductive filler. Includes a step of irradiating (electron beam irradiation step). Then, in the method for producing a heat conductive sheet of the present invention, an electron beam crosslinked resin containing a liquid thermoplastic fluororesin at normal temperature and pressure is used as the electron beam crosslinked resin of the sheet irradiated with electron beams in the irradiation step. It is preferable to use it.
In the method for producing a heat conductive sheet of the present invention, the sheet irradiated with the electron beam in the irradiation step may be used as it is as the heat conductive sheet, or the sheet irradiated with the electron beam is further subjected to post-treatment. May be used as a heat conductive sheet. That is, the method for producing a heat conductive sheet of the present invention may further include a post-treatment step of applying a post-treatment to the sheet irradiated with the electron beam after the irradiation step.

Then, in the method for producing a heat conductive sheet of the present invention, electrons are formed so that regions having different electron beam absorption doses are formed on at least one surface of the sheet containing the electron beam crosslinked resin and the heat conductive filler. Since the surface layer of the sheet is treated with an electron beam by irradiating it with rays, the sheet strength and heat conduction are suppressed while suppressing the occurrence of distortion, deformation, wrinkles, etc. (hereinafter collectively referred to as wrinkles, etc.) on the surface. A heat conductive sheet having excellent properties can be obtained.

<Irradiation process>
In the irradiation step, the electron beam is irradiated so that a region having a different electron beam absorbed dose is formed on at least one surface of the sheet containing the electron beam crosslinked resin and the heat conductive filler.

[Sheet]
Here, as the sheet for irradiating the electron beam, a sheet containing an electron beam crosslinked resin and a heat conductive filler and optionally further containing an additive can be used.

-Electron beam cross-linked resin-
As the electron beam crosslinked resin constituting the sheet, it is preferable to use at least a liquid thermoplastic fluororesin under normal temperature and pressure, a liquid thermoplastic fluororesin under normal temperature and pressure, and a solid thermoplastic fluororesin under normal temperature and pressure. It is more preferable to use it in combination with a resin. When a liquid thermoplastic fluororesin is not used as the electron beam crosslinked resin under normal temperature and pressure, the thermal resistance of the obtained heat conductive sheet increases, and a heat conductive sheet having excellent heat conductivity cannot be obtained. Further, if a liquid thermoplastic fluororesin under normal temperature and pressure and a solid thermoplastic fluororesin under normal temperature and pressure are used in combination as the electron beam crosslinked resin, the sheet strength of the obtained heat conductive sheet can be increased.
In the present invention, rubber and elastomer are included in "resin".

Examples of the resin that is liquid under normal temperature and pressure include a thermoplastic resin that is liquid under normal temperature and pressure and a thermosetting resin that is liquid under normal temperature and pressure.
Examples of the thermoplastic resin that is liquid under normal temperature and pressure include acrylic resin, epoxy resin, silicon resin, and fluororesin.
Examples of the thermosetting resin that is liquid under normal temperature and pressure include natural rubber; butadiene rubber; isoprene rubber; nitrile rubber; hydride nitrile rubber; chloroprene rubber; ethylene propylene rubber; chlorinated polyethylene; chlorosulfonated polyethylene; Butyl rubber; butyl halogenated rubber; polyisobutylene rubber; epoxy resin; polyimide resin; bismaleimide resin; benzocyclobutene resin; phenol resin; unsaturated polyester; diallyl phthalate resin; polyimide silicone resin; polyurethane; thermosetting polyphenylene ether; thermosetting Type-modified polyphenylene ether; and the like.

Examples of the resin that is solid under normal temperature and pressure include a thermoplastic resin that is solid under normal temperature and pressure and a thermosetting resin that is solid under normal temperature and pressure.
Examples of the thermoplastic resin that is solid under normal temperature and pressure include poly (2-ethylhexyl acrylate), a copolymer of acrylic acid and 2-ethylhexyl acrylate, polymethacrylic acid or an ester thereof, polyacrylic acid or Acrylic resin such as the ester; Silicon resin; Fluorine resin; Polyethylene; Polypropylene; Ethylene-propylene copolymer; Polymethylpentene; Polyvinyl chloride; Polyvinylidene chloride; Polyvinyl acetate; Ethylene-vinyl acetate copolymer; Polyvinyl alcohol Polyacetal; polyethylene terephthalate; polybutylene terephthalate; polyethylene naphthalate; polystyrene; polyacrylonitrile; styrene-acrylonitrile copolymer; acrylonitrile-butadiene-styrene copolymer (ABS resin); styrene-butadiene block copolymer or hydrogenation thereof Stuff; styrene-isoprene block copolymer or hydrogenated product thereof; polyphenylene ether; modified polyphenylene ether; aliphatic polyamides; aromatic polyamides; polyamideimide; polycarbonate; polyphenylene sulfide; polysulfone; polyether sulfone; polyether nitrile ; Polyether ketone; Polyketone; Polyurethane; Liquid polymer; Ionomer;
Examples of the thermosetting resin that is solid under normal temperature and pressure include natural rubber; butadiene rubber; isoprene rubber; nitrile rubber; hydride nitrile rubber; chloroprene rubber; ethylene propylene rubber; chlorinated polyethylene; chlorosulfonated polyethylene; Butyl rubber; butyl halogenated rubber; polyisobutylene rubber; epoxy resin; polyimide resin; bismaleimide resin; benzocyclobutene resin; phenol resin; unsaturated polyester; diallyl phthalate resin; polyimide silicone resin; polyurethane; thermosetting polyphenylene ether; thermosetting Type-modified polyphenylene ether; and the like.

~ Liquid thermoplastic fluororesin under normal temperature and pressure ~
Here, examples of the thermoplastic fluororesin that is liquid under normal temperature and pressure include vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, and vinylidene fluoride-hexafluoropentene-tetrafluoro. Examples thereof include an ethylene ternary copolymer, a perfluoropropene oxide copolymer, and a tetrafluoroethylene-propylene-vinylidene fluoride copolymer. These may be used alone or in combination of two or more.

~ Solid thermoplastic fluororesin under normal temperature and pressure ~
Further, as the solid thermoplastic fluororesin under normal temperature and pressure, a fluorine-containing monomer such as vinylidene fluoride-based fluororesin, tetrafluoroethylene-propylene-based fluororesin, tetrafluoroethylene-purple olovinyl ether-based fluororesin, etc. is polymerized. Examples thereof include an elastomer obtained in the above. More specifically, polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride, vinylidene fluoride. Ride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, polychlorotrifluoroethylene, ethylene-chlorofluoroethylene copolymer, tetrafluoroethylene-perfluorodioxol copolymer Combined, polyvinyl fluoride, tetrafluoroethylene-propylene copolymer, acrylic modified product of polytetrafluoroethylene, ester modified product of polytetrafluoroethylene, epoxy modified product of polytetrafluoroethylene and silane modified product of polytetrafluoroethylene And so on. These may be used alone or in combination of two or more.

When a liquid thermoplastic fluororesin under normal temperature and pressure and a solid thermoplastic fluororesin under normal temperature and pressure are used in combination, the liquid thermoplastic fluororesin under normal temperature and pressure in 100% by mass of the electron beam crosslinked resin. Is preferably 5% by mass or more, more preferably 10% by mass or more, preferably 50% by mass or less, and more preferably 30% by mass or less. When the proportion of the liquid thermoplastic fluororesin under normal temperature and pressure is at least the above lower limit value, the thermal conductivity of the obtained heat conductive sheet can be further enhanced. Further, when the ratio of the liquid thermoplastic fluororesin under normal temperature and pressure is not more than the above upper limit value, the sheet strength of the obtained heat conductive sheet can be increased.

-Thermal conductive filler-
The thermally conductive filler constituting the sheet is not particularly limited, and is, for example, a particulate material such as alumina particles, zinc oxide particles, inorganic nitride particles, silicon carbide particles, magnesium oxide particles, and particulate carbon material. , And fibrous materials such as carbon nanotubes, vapor-grown carbon fibers, carbon fibers obtained by carbonizing organic fibers, and cut products thereof can be used. Among them, as the heat conductive filler, at least one selected from the group consisting of a particulate carbon material, a particulate material such as inorganic nitride particles, and a fibrous carbon nanomaterial such as carbon nanotube (CNT) is used. Is preferable.
As the heat conductive filler, one type may be used alone, or two or more types may be used in combination.

The amount of the heat conductive filler contained in the sheet is not particularly limited, and can be, for example, 5 parts by mass or more and 200 parts by mass or less per 100 parts by mass of the above-mentioned electron beam crosslinked resin.

~ Particle material ~
The particulate material is not particularly limited, and for example, graphite such as artificial graphite, scaly graphite, flaky graphite, natural graphite, acid-treated graphite, expansive graphite, and expanded graphite; carbon black; inorganic nitride. Material particles, alumina particles, zinc oxide particles, silicon carbide particles, magnesium oxide particles and the like can be used. One of these may be used alone, or two or more thereof may be used in combination at an arbitrary ratio.
Among these, it is preferable to use at least one from the group consisting of expanded graphite and inorganic nitride particles. By using expanded graphite or inorganic nitride particles, the thermal conductivity of the heat conductive sheet can be further improved.

Expanded graphite can be obtained, for example, by chemically treating graphite such as scaly graphite with sulfuric acid or the like, heat-treating it to expand it, and then refining it. Examples of the expanded graphite include EC1500, EC1000, EC500, EC300, EC100, EC50 (trade names, all manufactured by Ito Graphite Industry Co., Ltd.) and the like.

Examples of the inorganic nitride particles include boron nitride particles, aluminum nitride particles, and silicon nitride particles. These may be used alone or in combination of two or more at any ratio.
Among these, boron nitride particles are preferable in terms of imparting electrical insulation and thermal conductivity to the heat conductive sheet.
Here, as a specific example of a commercially available product of boron nitride particles, for example, "PT" series manufactured by Momentive Performance Materials Japan (for example, "PT-110");"ShowaDenko" manufactured by Showa Denko Corporation. N UHP series (for example, "Showa N UHP-1"); Dangdong Chemical Engineering Institute Co., Ltd. , Ltd. Examples thereof include "HSL" and "HS" manufactured by the company.

Here, the content ratio of the particulate material in the heat conductive filler is preferably 40% by mass, more preferably 70% by mass, and 130% by mass or less in 100% by mass of the electron beam crosslinked resin. It is preferably present, and more preferably 100% by mass or less. This is because when the content ratio of the particulate material is within the above range, the thermal conductivity and the adhesion to the adherend are excellent.

The volume average particle diameter of the particulate thermal conductive filler is preferably 0.1 μm or more, more preferably 1 μm or more, preferably 300 μm or less, and more preferably 250 μm or less. preferable. When the volume average particle diameter of the particulate heat conductive filler is within the above range, the heat conductivity of the heat conductive sheet can be improved.
The volume average particle size can be measured using a laser diffraction / scattering type particle size measuring device (LA-960 series manufactured by Horiba Seisakusho Co., Ltd.).

~ Fibrous material ~
The fibrous carbon nanomaterial containing CNT, which can be suitably used as a fibrous material, may consist only of CNT, or may be a mixture of CNT and a fibrous carbon nanomaterial other than CNT. May be good.
The CNTs in the fibrous carbon nanomaterial are not particularly limited, and single-walled carbon nanotubes and / or multi-walled carbon nanotubes can be used, but the CNTs are carbon nanotubes having one to five layers. It is preferably a single-walled carbon nanotube, and more preferably a single-walled carbon nanotube.

Further, the fibrous carbon nanomaterial described above is not particularly limited, and a raw material compound and a carrier gas are supplied on a substrate having a catalyst layer for CNT production on the surface, and a chemical vapor phase growth method is performed. When CNTs are synthesized by the (CVD method), a method of dramatically improving the catalytic activity of the catalyst layer by allowing a trace amount of oxidizing agent (catalyst activator) to be present in the system (super growth method; international release). It is preferable to use a fibrous carbon nanomaterial containing CNTs produced according to No. 2006/011655). In the following, the carbon nanotubes obtained by the super growth method may be referred to as "SGCNT".
Here, the fibrous carbon nanomaterial containing SGCNT produced by the super growth method may be composed of only SGCNT, or in addition to SGCNT, for example, other carbon nanostructures such as non-cylindrical carbon nanostructures. Structures may be included.

When the heat conductive filler contains a fibrous material, the content ratio of the fibrous material in the heat conductive filler is preferably 0.001% by mass or more, preferably 0.005% by mass or more. It is more preferably 20% by mass or less, and more preferably 10% by mass or less. When the content ratio of the fibrous material is at least the above lower limit value, the heat transfer path can be satisfactorily formed, so that the heat conductivity of the heat conductive sheet can be further enhanced. Further, when the content ratio of the fibrous material is not more than the above upper limit value, it is possible to suppress the decrease in the flexibility of the heat conductive sheet due to the blending of the fibrous material, and the heat conductive sheet and the adherend (heating element, heat radiator). ), And the heat conductive sheet can exhibit excellent heat conductivity.

-Additive-
The additive that can be arbitrarily blended in the sheet is not particularly limited, and is, for example, a flame retardant, a plasticizer, an acid receiving agent such as MgO, a toughness improving agent, a hygroscopic agent, an adhesive strength improving agent, and a wettability improving agent. , Ion trapping agent and the like.
Then, the blending amount of the additive can be appropriately adjusted within the range in which the desired effect can be obtained.

-Sheet preparation method-
The sheet containing the electron beam crosslinked resin and the heat conductive filler is not particularly limited, and can be prepared by using any sheet forming method.
Among them, the sheet containing the electron beam crosslinked resin and the heat conductive filler contains the above-mentioned electron beam crosslinked resin and the heat conductive filler, and optionally further contains a composite material containing an additive. A step of pressing and molding into a sheet to obtain a composite material sheet, and a step of laminating a plurality of composite material sheets in the thickness direction, or folding or winding the composite material sheet to obtain a laminate, and laminating. It is preferable to manufacture the body through a step of slicing the body at an angle of 45 ° or less with respect to the laminating direction to obtain a sheet. The sheet manufactured through the above-mentioned steps has a structure in which slice pieces of the composite material sheet constituting the laminated body are joined in parallel, and the heat conductive filler is oriented in the thickness direction. This is because it has excellent thermal conductivity in the thickness direction.

Here, from the viewpoint of reducing the thermal resistance of the obtained heat conductive sheet and improving the handleability, the thickness of the sheet containing the electron beam crosslinked resin and the heat conductive filler is preferably 50 μm or more and 300 μm or less. .. The heat conductive sheet obtained by irradiating the sheet with an electron beam usually has the same thickness as the sheet. Then, the heat conductive sheet can be suitably used by being sandwiched between the heating element and the heat radiating element.

[Electron beam irradiation]
Irradiation of the above-mentioned sheet with an electron beam is not particularly limited, and is preferably performed using an electron beam irradiator in a low oxygen concentration environment having an oxygen concentration of 1000 mass ppm or less, for example. A low oxygen concentration environment can be prepared by a method of depressurizing the inside of the chamber or a method of introducing nitrogen into the chamber, but a method of introducing nitrogen into the chamber is preferable in terms of production efficiency.

Here, the electron beam may irradiate at least one surface of the sheet. Further, the irradiation distance of the potential line is preferably 5 mm or more and 20 mm or less. Further, the transport speed is preferably 5 mm / s or more and 100 mm / s or less. The faster the transport speed, the better the production efficiency, but it is necessary to increase the beam current when the electron beam irradiation dose is the same. However, simply increasing the beam current may cause wrinkles and the like, which may not be preferable. Therefore, by carrying out the transfer a plurality of times, wrinkling can be suppressed with a relatively weak acceleration voltage and beam current. However, from the viewpoint of improving production efficiency, it is usually preferable that the number of times of transportation is small. Therefore, it is more preferable to adjust the beam current and the transport speed to suppress the occurrence of wrinkles and the like.

Further, in the present invention, the step of irradiating the electron beam includes a step of irradiating at least one surface of the sheet with the electron beam so that regions having different electron beam absorbed doses are formed. Here, the "absorbed dose" of the electron beam does not mean the amount irradiated on the mask, but the amount actually hitting the sheet surface.
The method of irradiating the electron beam so that regions having different electron beam absorbed doses are formed on at least one surface of the sheet is not particularly limited, but for example, portions having different electron beam transmittances are arranged in a pattern. A method may be adopted in which a mask member is placed (laminated) on a sheet and an electron beam is irradiated through the mask member. Regions with different electron beam absorbed doses can be formed by changing the electron beam irradiation dose for each location, but if a mask member is used as described above, the electron beam can be formed at a constant irradiation dose. Regions with different electron beam absorbed doses can be easily formed while irradiating.
By irradiating the electron beam so that regions with different electron beam absorbed doses are formed, wrinkles on the surface caused by the irradiation of the electron beam can be suppressed, so that the sheet can be kept smooth while maintaining the smoothness of the sheet. Can increase strength

The irradiation dose of the electron beam is preferably 50 kGy or more, more preferably 100 kGy or more, further preferably 1000 kGy or more, particularly preferably 2000 kGy or more, and 9000 kGy or less per irradiation surface. It is preferably 8000 kGy or less, more preferably 6000 kGy or less. When the irradiation dose of the electron beam is at least the above lower limit value, the strength of the obtained heat conductive sheet can be sufficiently increased. Further, when the irradiation dose of the electron beam is not more than the above upper limit value, wrinkles on the surface of the obtained heat conductive sheet can be suppressed.
Further, since the electron beam is irradiated linearly, the sheet is conveyed at a constant speed in order to make the irradiation dose uniform. The irradiation dose of the electron beam is determined by the acceleration voltage and the beam current.
Here, the acceleration voltage is preferably 50 kV or more, more preferably 70 kV or more, more preferably 250 kV or less, still more preferably 200 kV or less. The beam current is preferably 1.0 mA or more, more preferably 1.7 mA, more preferably 15 mA or less, and even more preferably 12 mA or less.
When irradiating both sides of the sheet with electron beams, if the irradiation dose to at least one surface (irradiation surface) is within the above range, the irradiation dose to the other surface (irradiation surface) is outside the above range. However, it is preferable that the irradiation dose to both surfaces (irradiation surfaces) of the sheet is within the above range.

[Mask member]
The mask member can be a plate-shaped or sheet-shaped member in which portions having different electron beam transmittances are arranged in a pattern. Here, the portions having different electron beam transmittances are, for example, more than the “highly transparent portion” having high electron beam transmittance and the “non-transmissive portion” that does not transmit the electron beam or the “highly transparent portion” having the electron beam transmittance. Can be composed of low "low transparency"

The pattern of the mask member can be, for example, a mesh structure pattern having a plurality of rectangular openings or a punching structure pattern having a plurality of circular openings using a metal that does not transmit an electron beam. Here, the openings can be provided in a pattern as, for example, a "highly transparent portion", and the other portions can be formed as a "non-transparent portion".
Alternatively, a pattern may be formed with a mask member using a non-metal that allows an electron beam to pass through.
Alternatively, a pattern may be formed by combining two or more kinds of materials having different electron beam transmittances as a "highly transparent portion" and a "non-transmissive portion" or a "lowly transmitted portion", respectively. Alternatively, the pattern may have an uneven shape composed of a thin portion as a "high transmission portion" and a thick portion as a "low transmission portion".

Here, when a non-metal is used as the mask member, the electron beam transmittance in the highly transmissive portion of the mask member is preferably 50% or more and less than 100%. Further, the electron beam transmittance in the low transmittance portion of the mask member is preferably 10% or more and less than 50%.
In the present invention, the electron beam transmittance refers to a value obtained by setting the electron beam absorbed dose when the mask member is not installed to 100% and dividing by the electron beam absorbed dose when the mask member is installed. Refers to the value measured by an electron beam absorbed dose meter.

The metal used for the mask member is not particularly limited, and for example, stainless steel (SUS), iron, aluminum, and copper can be used.
Further, examples of the non-metal used for the mask member include any one that appropriately transmits an electron beam, such as a resin film having a thickness of about 1 to 20 μm, an adhesive, an adhesive, and a resin non-woven fabric.

Although suitable configuration examples of the mask member pattern are shown as examples in FIGS. 1 to 6, the mask member pattern of the present invention is not limited thereto.

As shown in FIGS. 1 to 6, the opening (highly transparent portion) of the mask member can be, for example, a slit (long hole) shape, a circular shape (including an ellipse), a rectangular shape, or the like. Further, the low transmission portion can be, for example, a mesh shape or a grid shape.
Further, when the mask members having a circular, rectangular, or diamond pattern are transferred onto the sheet, irradiated with an electron beam, and then removed, regions having different absorbed doses are formed in the pattern opposite to that in FIGS. 1 to 6. be able to.

For example, the pattern of the mask member can be a punching structure in which a plurality of circular openings are arranged and provided as shown in FIG. The diameter of each opening is preferably 3φ or more and 20φ or less, and the distance between the openings is preferably 1 mm or more and 5 mm or less. It is preferable that the pattern of the actual mask member is sequentially designed according to the thermal characteristics, adhesion, and sheet strength required by the size of the adherend used as the heat conductive sheet of the electronic material.

For example, the pattern of the mask member may have a structure in which a plurality of rectangular openings are provided as shown in FIGS. 1 and 3. The rectangle includes, for example, a square, a rectangle, a rhombus, and the like.

For example, the pattern of the mask member may have a structure in which a plurality of slit (long hole) shaped openings are provided as shown in FIGS. 4 and 5. In FIG. 4, each opening is staggered in its lateral direction, and in FIG. 5, each opening is staggered in its longitudinal direction. The slit shape can be preferably used in that a pattern can be formed by matching the direction in which the sheet strength is desired and the longitudinal direction of the opening.

Further, the pattern of the mask member may have a structure in which circular openings and cross-shaped openings are alternately provided, as shown in FIG. 6, for example.

The mask member can be formed as a mesh structure in which a wire rod is woven. The mesh structure can be constructed by using, for example, a plain weave wire mesh, a twill weave wire mesh, or the like.

In the mask member, the aperture ratio of the opening (highly transparent portion) is preferably 50% or more and 90% or less. When the aperture ratio of the mask member is within the above range, it is possible to achieve both suppression of wrinkles and the like in the obtained heat conductive sheet and sheet strength at a high level.

By providing the mask member with a highly permeable portion and a non-transmissive portion or a low permeable portion, the electron beam absorbed dose is high in the highly permeable portion of the mask member, and the electron absorbed dose is highly permeable in the non-transmissive portion or the low permeable portion. Since it is lower than the portion, it is possible to make a difference in the absorbed dose to the electron beam on the surface of the heat conductive sheet. Since a pattern-like difference can be provided in the electron beam absorbed dose depending on the shape of the highly permeable portion and the non-transmissive portion or the low permeable portion of the mask member, the region having a pattern similar to the pattern of the mask member and having a different electron beam absorbed dose. Is transferred and formed on the sheet surface. Here, the region on the sheet corresponding to the high transmission portion of the mask member is called an electron beam high absorbed dose region, and the region on the sheet corresponding to the non-transmissive portion or the low transmission portion of the mask member is the electron beam low absorbed dose region. Called.

Further, the electron beam may be irradiated with the mask member separated from the sheet surface. In that case, the distance between the mask member and the sheet is preferably 0.1 mm or more and 1 mm or less. By setting it to the above lower limit value or more, contact between the mask member and the sheet can be prevented, and by setting it to the above upper limit value or less, the pattern of the mask member can be transferred more accurately.

By irradiating the sheet surface with the electron beam through the mask member as described above, it is possible to easily form a region having a different electron beam absorbed dose on the surface. That is, regions having different electron beam absorbed doses to which the pattern of the mask member is substantially transferred can be easily formed on the sheet surface. Then, by irradiating the sheet with electron beams so that regions having different electron beam absorbed doses are formed in the above pattern, it is possible to increase the sheet strength while suppressing wrinkles and the like in the obtained heat conductive sheet. it can.

Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to these examples. In the following description, "%" and "part" representing quantities are based on mass unless otherwise specified. Further, in Examples and Comparative Examples, the thickness of the sheet, the thermal resistance value of the heat conductive sheet, the wrinkle resistance, and the tensile strength were measured and evaluated by the following methods.

<Thickness of heat conductive sheet>
The thickness (total film thickness) of the heat conductive sheet was measured with an accuracy of 1/1000 mm using a digital indicator (manufactured by Mitutoyo Co., Ltd., ID-C112X).

<Thermal resistance value>
The thermal resistance value of the heat conductive sheet was measured using a resin material thermal resistance tester (manufactured by Hitachi Technology and Service Co., Ltd.). Here, a heat conductive sheet cut into a substantially square 1.0 cm square is used as a sample, and the thermal resistance value (° C./W) when a relatively low pressure of 0.30 MPa is applied at a sample temperature of 50 ° C. and the sample. The thermal resistance value (° C./W) when a relatively high pressure of 0.80 MPa was applied at a temperature of 50 ° C. was measured and evaluated according to the following criteria.
A: 0.21 or less B: 0.21 or more and less than 0.30 C: 0.30 or more and less than 0.40 D: 0.40 or more

<Tensile strength>
The tensile strength (seat strength) was measured as follows. First, the heat conductive sheet was punched and molded with dumbbell No. 2 in accordance with JIS K6251 to prepare a sample piece. Using a tensile tester (manufactured by Shimadzu Corporation, trade name AG-IS20kN), pinch 1 cm from both ends of the sample piece, and at a temperature of 25 ° C, the direction perpendicular to the normal from the surface of the sample piece ( Tensile strength in the MD direction (MPa) and tensile strength in the TD direction (MPa) were measured at a tensile speed of 500 mm / min in the MD direction or the TD direction, respectively, and evaluated according to the following criteria.
A: 1.5 or more B: 1.0 or more and less than 1.5 C: 0.5 or more and less than 1.0 D: less than 0.5

<Gender than wrinkles>
The wrinkle property of the sheet after the electron beam irradiation was inspected by visual observation to check the degree of wrinkle on the entire surface of the heat conductive sheet one day after the electron beam irradiation. Based on the obtained test results, evaluation was made according to the following criteria.
A: No wrinkles B: Weak wrinkles on the edge of the sheet C: Many strong wrinkles on the edge of the sheet, gentle wrinkles on the whole D: Many strong wrinkles on the whole sheet, wrinkles on the whole and flat Can not

(Example 1)
<Preparation of sheet>
400 mg of carbon nanotubes (manufactured by Nippon Zeon Co., Ltd., single-walled carbon nanotubes, specific surface area: 600 m ² / g) were weighed, mixed in 2 L of methyl ethyl ketone as a solvent, and stirred with a homogenizer for 2 minutes to obtain a crude dispersion. Next, using a wet jet mill (manufactured by Joko Co., Ltd., product name: "JN-20"), the obtained crude dispersion liquid was applied to a 0.5 mm flow path of the wet jet mill for two cycles at a pressure of 100 MPa. It was passed and the carbon nanotubes were dispersed in methyl ethyl ketone. Then, a dispersion liquid having a solid content concentration of 0.20% by mass was obtained.
Then, the obtained dispersion was filtered under reduced pressure using Kiriyama filter paper (No. 5A) to obtain a sheet-shaped easily dispersible carbon nanotube aggregate.
Next, 70 parts of a liquid thermoplastic fluororesin (manufactured by Daikin Kogyo Co., Ltd., trade name: "Daiel G-101") under normal temperature and pressure and a solid thermoplastic fluororesin under normal temperature and pressure (3M Japan Co., Ltd.) Manufactured by, trade name: "Dynion FC-2211", Mooney viscosity: 27ML _{1 + 4} , 100 ° C) and 30 parts, and expanded fluoropolymer (manufactured by Ito Graphite Industry Co., Ltd., trade name: " EC300 ”, volume average particle diameter: 50 μm, average particle diameter in the minor axis direction: 10 to 20 μm) 90 parts and 0.5 part of the easily dispersible carbon nanotube aggregate obtained above are pressed with a pressurizing kneader (Nippon Spindle). The mixture was stirred and mixed at a temperature of 150 ° C. for 20 minutes. Next, the obtained mixture was put into a crusher and crushed for 10 seconds to obtain a composite material.
Next, 50 g of the obtained composite material was sandwiched between PET films (protective films) having a thickness of 50 μm subjected to sandblasting, and the conditions were that the roll gap was 550 μm, the roll temperature was 50 ° C., the roll linear pressure was 50 kg / cm, and the roll speed was 1 m / min. Rolling and molding (primary pressurization) was performed at
Subsequently, the obtained composite material sheet was cut into a length of 150 mm × width of 150 mm × thickness of 0.5 mm, 300 sheets were laminated in the thickness direction of the composite material sheet, and further, the temperature was 120 ° C. and the pressure was 0.1 MPa for 3 minutes. By pressing (secondary pressurization) in the stacking direction, a laminated body having a height of about 150 mm was obtained.
After that, while pressing the side surface of the second-pressurized laminate with a pressure of 0.3 MPa, a woodworking slicer (manufactured by Marunaka Iron Works Co., Ltd., trade name: "Super Finishing Planer Super Mecha S") was used. By slicing at an angle of 0 ° with respect to the stacking direction (in other words, in the normal direction of the main surface of the laminated composite material sheet), a sheet having a length of 150 mm, a width of 150 mm, and a thickness of 0.10 mm can be obtained. Obtained.
<Manufacturing of heat conductive sheet>
A wire mesh mesh having a pattern as shown in FIG. 1 (manufactured by Hasegawa Wire Mesh Co., Ltd., 20 mesh, wire diameter 0.20 mm, opening 1.39 mm, opening ratio 71.0%) is placed (laminated) on the above sheet. Using a low-energy electron beam irradiator (manufactured by Hamamatsu Photonics Co., Ltd., model number: L12978), the sheet is moved at a tube voltage of 70 kV, a tube current of 5.19 mA, and a transfer speed of 15 mm / s. The sheet was uniformly irradiated with the electron beam, and electron beam irradiation (irradiation dose of each irradiation surface: 1000 kGy) was carried out under the conditions of an irradiation distance of 10 mm and an oxygen concentration of 1000 ppm. Further, the back side was also irradiated with electron beams on both sides in the same manner.
Then, the thermal resistance value, tensile strength, and wrinkle wrinkle property of the obtained heat conductive sheet were measured and evaluated as described above. The results are shown in Table 1.

(Example 2)
At the time of manufacturing the heat conductive sheet, the sheet and the heat conductive sheet were manufactured in the same manner as in Example 1 except that the number of times of transport was 6 times in total and the irradiation dose of each irradiation surface was changed to 3000 kGy. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 3)
At the time of manufacturing the heat conductive sheet, the sheet and the heat conductive sheet were manufactured in the same manner as in Example 1 except that the number of times of transport was 12 times in total and the irradiation dose of each irradiation surface was changed to 6000 kGy. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 4)
At the time of manufacturing the heat conductive sheet, the sheet and the heat conductive sheet were manufactured in the same manner as in Example 1 except that the number of times of transport was 18 times in total and the irradiation dose of each irradiation surface was changed to 9000 kGy. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 5)
When preparing the sheet, instead of 90 parts of the expanded graphite "EC300", the expanded graphite "EC100" (manufactured by Ito Graphite Industry Co., Ltd., volume average particle diameter: 200 μm, average particle diameter in the minor axis direction: 10 to 20 μm) Using 50 parts, the slice thickness was adjusted to 150 μm to manufacture a heat conductive sheet, and when manufacturing the heat conductive sheet, the number of transports was set to 4 times in total, and the irradiation dose on each irradiation surface was changed to 2000 kGy. Manufactured a heat conductive sheet in the same manner as in Example 1. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 6)
Other than using 100 parts of the liquid thermoplastic fluororesin "Daiel G-101" under normal temperature and pressure without using the solid thermoplastic fluororesin "Dynion FC-2211" under normal temperature and pressure when preparing the sheet, A sheet and a heat conductive sheet were produced in the same manner as in Example 2. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 7)
When preparing the sheet, instead of 90 parts of expanded graphite "EC300", boron nitride particles (Dangdong Chemical Engineering Institute Co., Ltd.) as a heat conductive filler having electrical insulation, trade name: "HSL" , Volume average particle diameter: 36 μm, major axis average particle diameter in electron microscope observation: 30 μm, minor axis average particle diameter: 0.5 to 3 μm, hexagonal boron nitride particles) 90 parts , A sheet and a heat conductive sheet were produced in the same manner as in Example 2. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 8)
When manufacturing the heat conductive sheet, a punching metal (manufactured by Ishikawa Wire Mesh Co., Ltd., φ10, interval 0.5 mm, aperture ratio 64%) having a shape pattern in which circular openings as shown in FIG. 2 are arranged in a staggered pattern was used. Other than that, a heat conductive sheet was produced in the same manner as in Example 2. Further, in the thermal resistance test, as shown in FIG. 7, the heat conductive sheet was cut out into a substantially square of 10 mm square so that the electron beam irradiation portion (electron beam high absorbed dose region) on the surface of the heat conductive sheet came to the left end. (Fig. 7), the thermal resistance was measured. The total area of the electron beam high absorbed dose region at the time of thermal resistance measurement was 50% of the total area.

(Example 9)
In Example 8, the heat conductive sheet is formed into a substantially square of 10 mm square so that the electron beam irradiation portion (electron beam high absorbed dose region region) on the surface of the heat conductive sheet straddles three circles as shown in FIG. (Fig. 7), and the thermal resistance was measured. The total area of the electron beam high absorbed dose region at the time of thermal resistance measurement was 30% of the total area.

(Comparative Example 1)
The sheet and the heat conductive sheet were manufactured in the same manner as in Example 1 except that the wire mesh was not used when the heat conductive sheet was manufactured. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 2)
When the heat conductive sheet was manufactured, the sheet and the heat conductive sheet were manufactured in the same manner as in Example 8 except that the electron beam was irradiated only on one side without using the punching metal. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 3)
The sheet and the heat conductive sheet were manufactured in the same manner as in Example 3 except that the wire mesh was not used when the heat conductive sheet was manufactured. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 4)
The sheet and the heat conductive sheet were manufactured in the same manner as in Example 4 except that the wire mesh was not used when the heat conductive sheet was manufactured. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 5)
A sheet was produced in the same manner as in Example 1, and a heat conductive sheet was used as it was without irradiation with an electron beam using a wire mesh. Then, various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1.

As shown in Table 1, Examples 1 to 9 in which the electron beam was irradiated through the metal mesh as the mask member and the punching metal were compared with Comparative Examples 1 to 4 in which the electron beam was irradiated without using the mask member. It can be seen that the occurrence of wrinkles and the like is well suppressed, and the sheet strength and thermal resistance value are hardly reduced. Further, in Comparative Example 5 in which the electron beam was not irradiated, the sheet strength was lower than that in Examples 1 to 9 in which the electron beam was irradiated. That is, it can be seen that in Examples 1 to 9 of the present invention, a heat conductive sheet having excellent sheet strength and thermal conductivity is obtained while suppressing the occurrence of wrinkles on the sheet surface due to electron beam irradiation. ..

According to the present invention, it is possible to obtain a heat conductive sheet having excellent sheet strength and heat conductivity while suppressing the occurrence of wrinkles on the surface.

Claims (8)

A method for manufacturing a heat conductive sheet containing an electron beam crosslinked resin and a heat conductive filler.
Manufacture of a heat conductive sheet including a step of irradiating at least one surface of a sheet containing an electron beam crosslinked resin and a heat conductive filler with an electron beam so that regions having different electron beam absorbed doses are formed. Method. The method according to claim 1, wherein the step of irradiating the electron beam includes irradiating the surface of the electron beam through a mask member in which portions having different transmittances of the electron beam are arranged in a pattern. The method according to claim 2, wherein the pattern of the mask member is a mesh structure-like pattern having a plurality of rectangular openings. The method according to claim 2, wherein the pattern of the mask member is a punching structure-like pattern having a plurality of circular openings. The method according to claim 3 or 4, wherein the mask member has an aperture ratio of 30% or more and 90% or less. 2. The step of irradiating the surface with an electron beam through the mask member includes irradiating the surface with the electron beam so that the irradiation dose per irradiation surface on the surface of the sheet is 50 kGy or more and 9000 kGy or less. The method for producing a heat conductive sheet according to any one of ~ 5. The method according to any one of claims 1 to 6, wherein the electron beam crosslinked resin contains a liquid thermoplastic fluororesin under normal temperature and pressure. The method according to any one of claims 1 to 7, wherein the thickness of the heat conductive sheet is 50 μm or more and 600 μm or less.

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