Heat-conductive sheet

Abstract

The purpose of the present invention is to provide a heat-conductive sheet that has excellent flame resistance and adhesiveness with respect to metal. The heat-conductive sheet according to the present invention contains a fluorine resin, an acrylic resin having a weight average molecular weight equal to or less than a predetermined value, and a heat-conductive filler.

Description

thermal conductive sheet

[0001] The present invention relates to a thermally conductive sheet.

[0002] In recent years, the amount of heat generated by electronic components such as integrated circuit (IC) chips, power semiconductors (e.g., IGBT modules), and plasma display panels (PDPs) has increased as their performance has improved. As a result, electronic component packages and electronic equipment that use electronic components need to take measures to prevent functional failures caused by temperature increases in the electronic components.

[0003] To prevent malfunctions of electronic components due to temperature rise, a common method is to promote heat dissipation by attaching a metal heat dissipator such as a heat sink, heat sink plate, or heat dissipation fin to the heat-generating body of the electronic component. When using a heat dissipator, the heat-generating body and the heat dissipator are usually attached in close contact with each other through a heat dissipation member such as a sheet-like member with high thermal conductivity (thermal conduction sheet) in between in order to efficiently transfer heat from the heat-generating body to the heat dissipator.

[0004] Such thermally conductive sheets are made of a composite material of a resin and a thermally conductive filler. Thermally conductive sheets are required to have excellent adhesion to heat-generating and heat-dissipating bodies and flame retardancy. For example, a thermally conductive sheet using a fluororesin as the resin, as described in Patent Document 1, is believed to have high flame retardancy.

[0005] International Publication No. 2017 / 145954

[0006] However, high temperatures during package assembly can cause package deformation, which can lead to the thermal conductive sheet peeling off from the heat sink, resulting in poor contact between the thermal conductive sheet and the heat sink, potentially resulting in poor heat dissipation. For example, if a thermal conductive sheet is used in an IC package consisting of an IC chip supported on a substrate, a metal heat sink, and a thermal conductive sheet interposed between the IC chip and the metal heat sink, high temperatures can cause the substrate to warp and other package deformations, which can distort the spacing between the IC chip and the metal heat sink, causing the thermal conductive sheet to peel off from the metal heat sink and resulting in poor contact between the thermal conductive sheet and the metal heat sink, potentially resulting in poor heat dissipation. Therefore, there is room for improvement in achieving sufficient adhesion to ensure contact between the thermal conductive sheet and the metal heat sink even during such deformations.

[0007] Therefore, an object of the present invention is to provide a thermally conductive sheet that has excellent adhesion to metals and flame retardancy.

[0008] The present inventors have conducted extensive research to achieve the above object, and have found that a thermally conductive sheet formed from a composite material containing a fluororesin, a thermally conductive filler, and a low-molecular-weight acrylic resin can provide excellent adhesion to metals and flame retardancy, leading to the completion of the present invention.

[0009] That is, the present invention aims to advantageously solve the above-mentioned problems, and the thermally conductive sheet of the present invention is characterized by comprising a fluororesin, an acrylic resin having a weight-average molecular weight of 20,000 or less, and a thermally conductive filler. The thermally conductive sheet of the present invention contains a fluororesin, thereby imparting excellent flame retardancy. The thermally conductive sheet of the present invention contains an acrylic resin having a weight-average molecular weight of 20,000 or less, thereby imparting excellent adhesion to metals. The thermally conductive sheet of the present invention contains a thermally conductive filler, thereby imparting excellent thermal conductivity. In the present invention, the weight-average molecular weight can be measured using gel permeation chromatography (GPC). Furthermore, the flame retardancy can be evaluated according to the method described in the Examples of this specification. Furthermore, excellent adhesion to metals can be measured according to the method described in the Examples of this specification, using tack to the metal as an adherend as an indicator. Furthermore, thermal conductivity can be measured according to the method described in the Examples of this specification, using thermal conductivity as an indicator.

[0010] In the thermally conductive sheet of the present invention, the fluororesin preferably contains a fluororesin that is liquid at room temperature and normal pressure. By using a fluororesin that is liquid at room temperature and normal pressure, the adhesion of the thermally conductive sheet to metal is further improved, making it possible to ensure sufficient contact between the thermally conductive sheet and metal members such as heat sinks even when the electronic component package or electronic device is deformed. In the present invention, "room temperature" refers to 23°C, and "normal pressure" refers to 1 atm (absolute pressure).

[0011] In the thermal conductive sheet of the present invention, the fluororesin preferably contains both a fluororesin that is liquid at room temperature and normal pressure and a fluororesin that is solid at room temperature and normal pressure. If the fluororesin contains a fluororesin that is solid at room temperature and normal pressure in addition to a fluororesin that is liquid at room temperature and normal pressure, the thermal conductive sheet can have both excellent tack and tensile strength, improving handleability.

[0012] In the thermal conductive sheet of the present invention, the weight average molecular weight of the acrylic resin is preferably 10,000 or less. As the weight average molecular weight of the acrylic resin decreases, the adhesion to metal becomes better.

[0013] In the thermally conductive sheet of the present invention, the acrylic resin preferably contains an acrylic resin that is liquid at room temperature and normal pressure. By using an acrylic resin that is liquid at room temperature and normal pressure as the acrylic resin, the thermally conductive sheet can have a higher adhesion to metal, and can ensure sufficient contact between the thermally conductive sheet and metal members such as heat sinks even when the electronic component package or electronic device is deformed.

[0014] In the thermal conductive sheet of the present invention, the acrylic resin preferably contains a hydroxyl group, which provides better adhesion to metals.

[0015] In the thermal conductive sheet of the present invention, the content of the acrylic resin is preferably 20% by mass or more and 70% by mass or less of the total resin. By having the content of the acrylic resin be 20% by mass or more of the total resin, better adhesion to metals can be achieved. By having the content of the acrylic resin be 70% by mass or less of the total resin, the content of the fluororesin can be ensured and sufficient flame retardancy can be obtained.

[0016] The thermally conductive sheet of the present invention preferably has a configuration in which strips made of a composition containing the fluororesin, the acrylic resin, and the thermally conductive filler are joined in parallel to each other, and has excellent thermal conductivity and can be manufactured with high production efficiency.

[0017] According to the present invention, it is possible to provide a thermally conductive sheet that has excellent adhesion to metals and flame retardancy.

[0018] Embodiments of the present invention are described in detail below. The thermally conductive sheet of the present invention can be used, for example, by sandwiching it between a heat-generating body (electronic component) and a heat sink when attaching the heat sink to the heat-generating body. That is, the thermally conductive sheet of the present invention can be used as a heat dissipation member to form a heat dissipation device together with a heat sink, heat sink plate, heat spreader, heat dissipation fin, or other heat sink (especially a metal heat sink). The thermally conductive sheet of the present invention can also form an electronic component package together with a heat-generating body (electronic component) in contact with the thermally conductive sheet and a heat sink in contact with the heat-generating body via the thermally conductive sheet. The thermally conductive sheet of the present invention is particularly suitable for use as a thermally conductive sheet interposed between an integrated circuit (IC) chip and a metal heat sink in an integrated circuit (IC) package. Examples of metals for the metal heat sink include copper, iron, and aluminum. The thermally conductive sheet of the present invention can be manufactured by any method as long as it has the specified components and properties described below.

[0019] (Thermal Conductive Sheet) The thermal conductive sheet of the present invention comprises a fluororesin, an acrylic resin having a weight-average molecular weight of a predetermined value or less, and a thermally conductive filler. The thermal conductive sheet of the present invention may further comprise other components, such as a resin other than the fluororesin and acrylic resin, an acrylic resin having a weight-average molecular weight exceeding a predetermined value, and additives. The thermal conductive sheet of the present invention comprises a fluororesin, thereby imparting excellent flame retardancy. The thermal conductive sheet of the present invention comprises an acrylic resin having a weight-average molecular weight of a predetermined value or less, thereby imparting excellent adhesion to metals. The thermal conductive sheet of the present invention comprises a thermally conductive filler, thereby imparting excellent thermal conductivity. In the present invention, flame retardancy can be evaluated according to the method described in the Examples of this specification. Excellent adhesion to metals can be measured using tack to the metal as an adherend as an index according to the method described in the Examples of this specification. Thermal conductivity can be measured using thermal conductivity as an index according to the method described in the Examples of this specification. In the present invention, rubber and elastomers are considered to be included in the term "resin."

[0020] <Fluororesin> Fluororesin is an effective component for imparting excellent flame retardancy to a thermally conductive sheet. Furthermore, fluororesin constitutes the matrix resin of the thermally conductive sheet and also functions as a binder that binds thermally conductive fillers and the like within the thermally conductive sheet. Fluororesin is preferred as the matrix resin for a thermally conductive sheet because it is resistant to decomposition at high temperatures (i.e., has excellent heat resistance), has sufficient shape retention (i.e., the property of suppressing liquefaction during molding of the thermally conductive sheet and maintaining a certain solid shape), and has flexibility. It should be noted that one type of fluororesin may be used alone, or two or more types may be used in combination.

[0021] Here, fluororesins can be classified according to their state at room temperature and normal pressure. Specifically, fluororesins can be classified into those that are liquid at room temperature and normal pressure and those that are solid at room temperature and normal pressure. From the viewpoint of obtaining better adhesion of the thermally conductive sheet to metal, it is preferable that the fluororesin contains at least a fluororesin that is liquid at room temperature and normal pressure. However, if the fluororesin is composed solely of a fluororesin that is liquid at room temperature and normal pressure, the strength of the thermally conductive sheet tends to decrease. On the other hand, if the fluororesin contains a fluororesin that is solid at room temperature and normal pressure in addition to a fluororesin that is liquid at room temperature and normal pressure, the thermally conductive sheet can achieve both excellent tack and tensile strength, improving handleability. Therefore, it is more preferable that the fluororesin contains both a fluororesin that is liquid at room temperature and normal pressure and a fluororesin that is solid at room temperature and normal pressure.

[0022] Furthermore, fluororesins can be classified into thermoplastic fluororesins and thermosetting fluororesins, and among these, it is preferable to use thermoplastic fluororesins. The use of thermoplastic fluororesins further improves the flexibility of the thermally conductive sheet in a high-temperature environment during use (heat dissipation), and enables good adhesion between the heat-generating body and the heat-dissipating body via the thermally conductive sheet. Furthermore, thermosetting fluororesins may be used in combination with the fluororesin, provided that the properties and effects of the thermally conductive sheet of the present invention are not impaired.

[0023] <<Fluororesin that is liquid at room temperature and normal pressure>> Using a fluororesin that is liquid at room temperature and normal pressure as the fluororesin further increases the adhesion of the thermally conductive sheet to metal, thereby ensuring sufficient contact between the thermally conductive sheet and metal members such as a heat sink even when the electronic component package or electronic device is deformed. Here, the fluororesin that is liquid at room temperature and normal pressure is not particularly limited as long as it is a fluororesin that is liquid at room temperature and normal pressure, and may be thermoplastic or thermosetting. Among these, from the viewpoint of increasing the adhesion between the thermally conductive sheet and the adherend when the thermally conductive sheet is used under normal conditions (when the electronic component package or electronic device is not deformed) and thereby allowing for good heat dissipation from the heat-generating element, thermoplastic fluororesins that are liquid at room temperature and normal pressure are preferred.

[0024] Examples of thermoplastic fluororesins that are liquid at room temperature and normal pressure include vinylidene fluoride-hexafluoropropylene copolymers, vinylidene fluoride-hexafluoropentene-tetrafluoroethylene terpolymers, perfluoropropene oxide polymers, and tetrafluoroethylene-propylene-vinylidene fluoride copolymers. Commercially available thermoplastic fluororesins that are liquid at room temperature and normal pressure include Viton (registered trademark) LM manufactured by DuPont Corporation, Dai-El (registered trademark) G-101 (vinylidene fluoride-hexafluoropropylene copolymer) manufactured by Daikin Industries, Ltd., Dyneon FC2210 manufactured by 3M Limited, and the SIFEL series manufactured by Shin-Etsu Chemical Co., Ltd.

[0025] The viscosity of the fluororesin that is liquid at room temperature and normal pressure is not particularly limited, but from the viewpoint of good kneadability, fluidity and excellent moldability, the viscosity (viscosity coefficient) at a temperature of 80°C is preferably from 500 P to 20,000 P, and more preferably from 1,000 P to 10,000 P. In the present invention, the "viscosity (viscosity coefficient)" of the fluororesin that is liquid at room temperature and normal pressure can be measured at a temperature of 80°C using an E-type viscometer according to the method described in the examples of this specification.

[0026] Incidentally, the molecular weight of a fluororesin that is liquid at room temperature and atmospheric pressure is generally smaller than the molecular weight of a fluororesin that is solid at room temperature and atmospheric pressure, as described below. Therefore, for example, when a thermal conductive sheet contains a fluororesin that is liquid at room temperature and atmospheric pressure and a fluororesin that is solid at room temperature and atmospheric pressure, of the two different peaks obtained using gel permeation chromatography (GPC), the peak on the lower molecular weight side usually represents the fluororesin that is liquid at room temperature and atmospheric pressure, and the peak on the higher molecular weight side usually represents the fluororesin that is solid at room temperature and atmospheric pressure.

[0027] <<Fluororesin that is solid at room temperature and normal pressure>> The fluororesin that is solid at room temperature and normal pressure is not particularly limited as long as it is a fluororesin that is solid at room temperature and normal pressure, and may be thermoplastic or thermosetting. Among these, from the viewpoint of ensuring good adhesion between the thermal conductive sheet and the adherend even under normal conditions (when the electronic component package or electronic device is not deformed), a thermoplastic fluororesin that is solid at room temperature and normal pressure is preferred.

[0028] Examples of thermoplastic fluororesins that are solid at room temperature and normal pressure include elastomers obtained by polymerizing fluorine-containing monomers, such as vinylidene fluoride-based fluororesins, tetrafluoroethylene-propylene-based fluororesins, and tetrafluoroethylene-perfluorovinyl ether-based fluororesins. More specific examples include polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, tetrafluoroethylene-ethylene copolymers, polyvinylidene fluoride, polychlorotrifluoroethylene, ethylene-chlorofluoroethylene copolymers, tetrafluoroethylene-perfluorodioxole copolymers, polyvinyl fluoride, tetrafluoroethylene-propylene copolymers, vinylidene fluoride-hexafluoropropylene copolymers, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymers, acrylic-modified polytetrafluoroethylene, ester-modified polytetrafluoroethylene, epoxy-modified polytetrafluoroethylene, and silane-modified polytetrafluoroethylene. Among these, vinylidene fluoride-hexafluoropropylene copolymer is preferred from the viewpoint of processability.

[0029] Furthermore, commercially available thermoplastic fluororesins that are solid at room temperature and normal pressure include, for example, Dai-el (registered trademark) G-912, G-700 series, Dai-el G-550 series / G-600 series, and Dai-el G-310 manufactured by Daikin Industries, Ltd.; KYNAR (registered trademark) series and KYNAR FLEX (registered trademark) series manufactured by ALKEMA; Dyneon FC2211 and FPO3600ULV manufactured by 3M; and Viton (registered trademark) A-100, A-200, and A-500 series manufactured by Chemours.

[0030] The Mooney viscosity (ML) of solid fluororesin at room temperature and pressure １＋４, 100°C) is preferably 3.5 or more, more preferably 10 or more, even more preferably 20 or more, and is preferably 120 or less, more preferably 100 or less, even more preferably 70 or less, even more preferably 50 or less, and particularly preferably 30 or less. １＋４ , 100°C) can be measured at a temperature of 100°C in accordance with JIS K6300 according to the method described in the examples of this specification.

[0031] <<Proportion of Fluorine Resin in All Resins>> The proportion of fluororesin in all resins contained in the thermal conductive sheet is not particularly limited, but from the viewpoint of imparting excellent flame retardancy to the thermal conductive sheet and ensuring shape retention when molded, it is preferably 30% by mass or more, and more preferably 60% by mass or more. Furthermore, from the viewpoint of ensuring the content of acrylic resin and imparting excellent adhesion to metals to the thermal conductive sheet, the proportion of fluororesin in all resins contained in the thermal conductive sheet is preferably 80% by mass or less, and more preferably 75% by mass or less.

[0032] <<Proportion of fluororesin that is liquid at room temperature and normal pressure in the fluororesin>> The proportion of fluororesin that is liquid at room temperature and normal pressure in the fluororesin (i.e., the proportion of fluororesin that is liquid at room temperature and normal pressure in the total amount of fluororesin that is liquid at room temperature and normal pressure and fluororesin that is solid at room temperature and normal pressure) is not particularly limited, but from the viewpoint of imparting excellent adhesion to metal to the thermal conductive sheet, it is preferably 30% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more. Furthermore, from the viewpoint that the fluororesin contains a fluororesin that is solid at room temperature and normal pressure, the tack and tensile strength of the thermal conductive sheet can be compatible and handling can be improved, so the proportion of fluororesin that is liquid at room temperature and normal pressure in the fluororesin is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 75% by mass or less.

[0033] <Acrylic resin with a weight-average molecular weight of a predetermined value or less> An acrylic resin with a weight-average molecular weight of a predetermined value or less (hereinafter sometimes referred to as a "low-molecular-weight acrylic resin") is an effective component for imparting excellent adhesion to metals to a thermally conductive sheet. By using a low-molecular-weight acrylic resin, in particular, adhesiveness is imparted to the thermally conductive sheet, allowing excellent adhesion to metals to be achieved.

[0034] The term "acrylic resin" refers to a polymer containing, as repeating units, (meth)acrylic acid ester monomer units and, optionally, repeating units other than (meth)acrylic acid ester monomer units (hereinafter referred to as "other repeating units"). In the present invention, "(meth)acrylic" refers to acrylic and / or methacrylic.

[0035] In addition, from the viewpoint of further enhancing the adhesion of the thermally conductive sheet to metal, the low-molecular-weight acrylic resin preferably contains an acrylic resin that is liquid at room temperature and normal pressure. Examples of commercially available acrylic resins that are liquid at room temperature and pressure include "ARUFON UP-1000," "ARUFON UP-1010," "ARUFON UP-1020," "ARUFON UP-1021," "ARUFON UP-1061," "ARUFON UP-1080," "ARUFON UP-1110," "ARUFON UP-1170," "ARUFON UP-1190," "ARUFON UP-1500," "ARUFON UH-2000," "ARUFON UH-2041," "ARUFON UH-2190," "ARUFON UC-3510," and "ARUFON UC-3510," all manufactured by Toagosei Co., Ltd. UG-4010," "ARUFON US-6100," "ARUFON US-6170," and "Actflow UT-1001, UMM-1001" manufactured by Soken Chemical & Engineering Co., Ltd.

[0036] The low-molecular-weight acrylic resin may also optionally contain a functional group. Examples of functional groups include a hydroxyl group, a carboxyl group, an epoxy group, a long-chain alkyl group, and an alkoxysilyl group. When the low-molecular-weight acrylic resin contains a functional group, the low-molecular-weight acrylic resin may contain one type of functional group or multiple types of functional groups. Such functional groups are preferably functional groups that further improve the effects of the present invention, and for example, functional groups that impart high viscosity to the low-molecular-weight acrylic resin are preferred. Thus, from the perspective of imparting high viscosity to the low-molecular-weight acrylic resin, the low-molecular-weight acrylic resin preferably contains a hydroxyl group. When the low-molecular-weight acrylic resin contains a hydroxyl group, the hydroxyl value of the low-molecular-weight acrylic resin is preferably 20 mg KOH / g resin or more, more preferably 30 mg KOH / g resin or more, and preferably 120 mg KOH / g resin or less, and more preferably 80 mg KOH / g resin or less. Low molecular weight acrylic resins containing hydroxyl groups have high viscosity, which gives the thermal conductive sheet greater adhesiveness, thereby enabling it to have better adhesion to metals.Furthermore, it can achieve both excellent tack and tensile strength, improving handleability.

[0037] <<(Meth)acrylic Acid Ester Monomer Unit>> Examples of (meth)acrylic acid ester monomers that can form (meth)acrylic acid ester monomer units include alkyl acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, isobutyl acrylate, n-pentyl acrylate, isopentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, and stearyl acrylate; acrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, isobutyl methacrylate, n-pentyl methacrylate, isopentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, stearyl methacrylate, and other methacrylic acid alkyl esters; and derivatives of these monomers substituted so as to contain the above-mentioned functional groups. Among these, monomers selected from methyl acrylate, ethyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, and 2-ethylhexyl methacrylate, or derivatives of such monomers substituted to contain a hydroxyl group, are preferred, and monomers selected from n-butyl acrylate, ethyl methacrylate, and 2-ethylhexyl acrylate, or derivatives of such monomers substituted to contain a hydroxyl group, are more preferred. These can be used alone or in combination of two or more.

[0038] <<Other Repeating Units>> Furthermore, examples of the "other repeating units" that the low molecular weight acrylic resin may contain include, but are not particularly limited to, the above-mentioned monomer units having a functional group (e.g., a hydroxyl group), aromatic vinyl monomer units, nitrile group-containing monomer units, conjugated diene monomer units, and alkylene structural units.

[0039] - Monomer Unit Having a Hydroxy Group - Examples of the monomer unit having the functional group described above include a monomer unit having a hydroxy group. Examples of the monomer having a hydroxy group that can form the monomer unit having a hydroxy group include ethylenically unsaturated alcohols such as (meth)allyl alcohol, 3-butene-1-ol, and 5-hexene-1-ol; alkanol esters of ethylenically unsaturated carboxylic acids such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, di-2-hydroxyethyl maleate, di-4-hydroxybutyl maleate, and di-2-hydroxypropyl itaconate; ２ =CR Ｚ -COO-(C ｎ H ２ｎ O) ｍ -H (wherein m is an integer from 2 to 9, n is an integer from 2 to 4, R Ｚrepresents hydrogen or a methyl group) and (meth)acrylic acid esters; mono(meth)acrylic acid esters of dihydroxy esters of dicarboxylic acids such as 2-hydroxyethyl-2'-(meth)acryloyloxyphthalate and 2-hydroxyethyl-2'-(meth)acryloyloxysuccinate; vinyl ethers such as 2-hydroxyethyl vinyl ether and 2-hydroxypropyl vinyl ether; alkyl ethers such as (meth)allyl-2-hydroxyethyl ether, (meth)allyl-2-hydroxypropyl ether, (meth)allyl-3-hydroxypropyl ether, (meth)allyl-2-hydroxybutyl ether, (meth)allyl-3-hydroxybutyl ether, (meth)allyl-4-hydroxybutyl ether, and (meth)allyl-6-hydroxyhexyl ether; mono(meth)allyl ethers of ethylene glycol; polyoxyalkylene glycol mono(meth)allyl ethers such as diethylene glycol mono(meth)allyl ether and dipropylene glycol mono(meth)allyl ether; mono(meth)allyl ethers of halogen- and hydroxy-substituted (poly)alkylene glycols such as glycerin mono(meth)allyl ether, (meth)allyl-2-chloro-3-hydroxypropyl ether and (meth)allyl-2-hydroxy-3-chloropropyl ether; mono(meth)allyl ethers of polyhydric phenols such as eugenol and isoeugenol, and halogen-substituted products thereof; (meth)allyl thioethers of alkylene glycols such as (meth)allyl-2-hydroxyethyl thioether and (meth)allyl-2-hydroxypropyl thioether; and the like.

[0040] - Aromatic vinyl monomer unit - Examples of aromatic vinyl monomers that can form aromatic vinyl monomer units include styrene, α-methylstyrene, butoxystyrene, and vinylnaphthalene. Among these, styrene is preferred. These can be used alone or in combination of two or more.

[0041] -Nitrile Group-Containing Monomer Unit-Further, examples of nitrile group-containing monomers capable of forming nitrile group-containing monomer units include α,β-ethylenically unsaturated nitrile monomers. Specifically, the α,β-ethylenically unsaturated nitrile monomer is not particularly limited as long as it is an α,β-ethylenically unsaturated compound having a nitrile group, and examples include acrylonitrile; α-halogenoacrylonitriles such as α-chloroacrylonitrile and α-bromoacrylonitrile; and α-alkylacrylonitriles such as methacrylonitrile and α-ethylacrylonitrile. Among these, acrylonitrile and methacrylonitrile are preferred as nitrile group-containing monomers, with acrylonitrile being more preferred. These can be used alone or in combination of two or more.

[0042] Conjugated diene monomer unit Examples of conjugated diene monomers that can form conjugated diene monomer units include conjugated diene compounds having 4 or more carbon atoms, such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. Of these, 1,3-butadiene is preferred.

[0043] -Alkylene structural unit- The alkylene structural unit is represented by the general formula: -C ｎ H ２ｎ - [where n is an integer of 2 or greater]. The method for introducing alkylene structural units into low-molecular-weight acrylic resins is not particularly limited, and examples include the following methods (1) and (2): (1) a method in which a copolymer is prepared from a monomer composition containing a conjugated diene monomer (e.g., 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, etc.) and the copolymer is hydrogenated to convert the conjugated diene monomer units into alkylene structural units; and (2) a method in which a copolymer is prepared from a monomer composition containing a 1-olefin monomer (e.g., ethylene, propylene, 1-butene, 1-hexene, etc.). Among these, method (1) is preferred because it allows for easy production of low-molecular-weight acrylic resins.

[0044] <<Weight-Average Molecular Weight of Low-Molecular-Weight Acrylic Resin>> The weight-average molecular weight of an acrylic resin having a weight-average molecular weight of a predetermined value or less (low-molecular-weight acrylic resin) may be 20,000 or less, preferably 10,000 or less, more preferably 9,000 or less, even more preferably 8,000 or less, and even more preferably 7,000 or less. By using an acrylic resin with such a low weight-average molecular weight as the acrylic resin, adhesiveness is imparted to the thermal conductive sheet of the present invention, thereby achieving excellent adhesion to metals. Furthermore, in order to ensure the resin's properties (e.g., viscosity, binding ability to thermally conductive fillers), the weight-average molecular weight of the low-molecular-weight acrylic resin may be, for example, 200 or more, preferably 500 or more, more preferably 700 or more, even more preferably 1,000 or more, even more preferably 1,500 or more, even more preferably 2,000 or more, and even more preferably 2,500 or more.

[0045] <<Glass Transition Temperature of Low Molecular Weight Acrylic Resin>> The glass transition temperature (Tg) of the low molecular weight acrylic resin is not particularly limited, but is preferably the glass transition temperature at which the low molecular weight acrylic resin becomes liquid at room temperature and normal pressure. If the low molecular weight acrylic resin is liquid at room temperature and normal pressure, viscosity is imparted to the low molecular weight acrylic resin, which in turn imparts higher adhesiveness to the thermal conductive sheet and allows for better adhesion to metal. Such a glass transition temperature is preferably -80°C or higher, more preferably -70°C or higher, and even more preferably -60°C or higher, and preferably 0°C or lower, more preferably -10°C or lower, and even more preferably -30°C or lower.

[0046] <<Viscosity of Low Molecular Weight Acrylic Resin>> When the low molecular weight acrylic resin is liquid at room temperature and normal pressure, the viscosity of the low molecular weight acrylic resin is not particularly limited. However, from the viewpoint of imparting high adhesiveness to the thermal conductive sheet, thereby imparting better adhesion to metal, and further enabling the thermal conductive sheet to achieve both excellent tack and tensile strength and improving handleability, the viscosity of the low molecular weight acrylic resin at 25°C is preferably 3,000 mPa·s or more, more preferably 5,000 mPa·s or more, and even more preferably 10,000 mPa·s or more. Furthermore, from the viewpoint of ensuring shape retention during thermal conductive sheet molding, the viscosity of the low molecular weight acrylic resin is preferably 1,000,000 mPa·s or less, more preferably 800,000 mPa·s or less, and even more preferably 500,000 mPa·s or less. In the present invention, the "viscosity" of the low molecular weight acrylic resin can be measured at 25°C using an E-type viscometer.

[0047] <<Proportion of Low-Molecular-Weight Acrylic Resin in All Resins>> The proportion of low-molecular-weight acrylic resin in all resins contained in the thermal conductive sheet is not particularly limited, but is preferably 20% by mass or more, and more preferably 25% by mass or more, from the viewpoint of imparting excellent adhesion to metal to the thermal conductive sheet. Furthermore, the proportion of low-molecular-weight acrylic resin in all resins contained in the thermal conductive sheet is preferably 70% by mass or less, and more preferably 50% by mass or less, from the viewpoints of ensuring the fluororesin content, imparting excellent flame retardancy to the thermal conductive sheet, and ensuring shape retention during molding of the thermal conductive sheet.

[0048] When a thermally conductive sheet contains multiple low-molecular-weight acrylic resins, the content of the low-molecular-weight acrylic resins refers to the sum of the contents of each low-molecular-weight acrylic resin. For example, when multiple peaks corresponding to acrylic resins with a weight-average molecular weight below the predetermined value are detected by GPC, the peak area of ​​each peak can be used to represent the content of each low-molecular-weight acrylic resin, and the sum of these peak areas can be used to represent the content (total amount) of the low-molecular-weight acrylic resin. When a thermally conductive sheet contains both one or more low-molecular-weight acrylic resins and one or more acrylic resins with a weight-average molecular weight above the predetermined value, the sum of the peak areas of the peaks detected by GPC corresponding to acrylic resins with a weight-average molecular weight below the predetermined value can be used to represent the content of the low-molecular-weight acrylic resin. The peak areas of the peaks corresponding to acrylic resins with a weight-average molecular weight above the predetermined value can be excluded from the calculation of the content of the low-molecular-weight acrylic resin.

[0049] <<Method for Preparing Low-Molecular-Weight Acrylic Resin>> The method for preparing the low-molecular-weight acrylic resin described above is not particularly limited. For example, the low-molecular-weight acrylic resin can be prepared by polymerizing a monomer composition containing the above-described monomer to obtain a polymer, and then hydrogenating (hydrogenating) the resulting polymer as needed. Furthermore, when the low-molecular-weight acrylic resin contains the above-described functional group, it may be prepared by polymerizing a derivative substituted with the above-described functional group as a monomer, or by polymerizing an unsubstituted monomer and then substituting the polymer with the above-described functional group. The content of each monomer in the monomer composition used to prepare the low-molecular-weight acrylic resin can be determined based on the content of each repeating unit in the low-molecular-weight acrylic resin. The polymerization method is not particularly limited, and any method such as solution polymerization, suspension polymerization, bulk polymerization, or emulsion polymerization can be used. Furthermore, any reaction such as ionic polymerization, radical polymerization, or living radical polymerization can be used as the polymerization reaction. When a molecular weight modifier is used in preparing the low molecular weight acrylic resin, the amount of the molecular weight modifier is preferably less than 0.5 parts by mass per 100 parts by mass of the total of the monomers.

[0050] <Thermal Conductive Filler> The thermal conductive filler is an effective component for imparting excellent thermal conductivity to the thermal conductive sheet. The thermal conductive filler constituting the thermal conductive sheet is not particularly limited, and examples thereof include particulate materials such as alumina particles, zinc oxide particles, inorganic nitride particles, silicon carbide particles, magnesium oxide particles, and particulate carbon materials, as well as fibrous materials such as carbon nanotubes, vapor-grown carbon fibers, carbon fibers obtained by carbonizing organic fibers, and cut products thereof. Among these, it is preferable to use at least one type selected from the group consisting of inorganic nitride particles, particulate carbon materials, and fibrous carbon nanomaterials such as carbon nanotubes (CNTs). Note that the thermal conductive filler may be used alone or in combination of two or more types.

[0051] The amount of thermally conductive filler contained in the thermally conductive sheet is not particularly limited, and can be, for example, preferably 5 parts by mass or more, more preferably 50 parts by mass or more, even more preferably 70 parts by mass or more, and preferably 200 parts by mass or less, more preferably 160 parts by mass or less, and even more preferably 140 parts by mass or less per 100 parts by mass of the total resin.

[0052] <<Inorganic Nitride Particles>> Examples of inorganic nitride particles include boron nitride particles, aluminum nitride particles, and silicon nitride particles. These may be used alone, or two or more may be used in any ratio. Among these, boron nitride particles are preferred in terms of imparting electrical insulation and thermal conductivity to the thermal conductive sheet. Specific examples of commercially available boron nitride particles include the "PT" series (e.g., "PT-110") manufactured by Momentive Performance Materials Japan; the "Sho-BN UHP" series (e.g., "Sho-BN UHP-1") manufactured by Showa Denko K.K.; and "HSL" and "HS" manufactured by Dangdong Chemical Engineering Institute Co., Ltd.

[0053] <<Particulate Carbon Material>> The particulate carbon material is not particularly limited, and examples thereof include graphite such as artificial graphite, flake graphite, exfoliated graphite, natural graphite, acid-treated graphite, expandable graphite, and expanded graphite; carbon black; and the like. These may be used alone or in combination of two or more types in any ratio. Among these, expanded graphite is preferred. Use of expanded graphite can further improve the thermal conductivity of the thermal conductive sheet.

[0054] Expanded graphite can be obtained by, for example, chemically treating graphite such as flake graphite with sulfuric acid or the like to obtain expandable graphite, expanding the graphite by heat treatment, and then pulverizing the expanded graphite. Examples of expanded graphite include EC1500, EC1000, EC500, EC300, EC100, and EC50 (all trade names) manufactured by Ito Graphite Industries Co., Ltd., and CS-5, CS-30, CS-100, and CS-F400 manufactured by Marutoyo Foundry Manufacturing Co., Ltd.

[0055] Here, when the thermally conductive filler is a particulate carbon material, the content of the particulate carbon material in the thermally conductive filler is preferably 40% by mass or more, more preferably 50% by mass or more, and preferably 100% by mass or less. If the content of the particulate carbon material is equal to or greater than the above-mentioned lower limit, a heat transfer path can be formed well, thereby further improving the thermal conductivity of the thermally conductive sheet. Furthermore, if the content of the particulate carbon material is equal to or less than the above-mentioned upper limit, the blending of the particulate carbon material can suppress a decrease in the flexibility of the thermally conductive sheet, and can improve the adhesion between the thermally conductive sheet and the adherend (heat-generating body, heat-dissipating body), allowing the thermally conductive sheet to exhibit excellent thermal conductivity.

[0056] - Orientation of particulate carbon material - The orientation of the particulate carbon material in the thermally conductive sheet is preferably such that the angle between the major axis direction of the particulate carbon material and 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-mentioned predetermined range, the thermal resistance of the thermally conductive sheet can be reduced and thermal conductivity can be increased. In the present invention, the "orientation of the particulate carbon material" can be measured according to the method described in the examples of this specification.

[0057] -Average Particle Diameter of Particulate Carbon Material- The volume average particle diameter of the particulate carbon material is preferably 0.1 μm or more, more preferably 1 μm or more, and preferably 300 μm or less, and more preferably 250 μm or less. When the volume average particle diameter of the particulate carbon material is within the above range, the thermal conductivity of the thermal conductive sheet can be improved. The volume average particle diameter can be measured using a laser diffraction / scattering particle diameter measuring device (LA-960 series, manufactured by Horiba, Ltd.). Furthermore, the average particle diameter in the major axis direction of the particulate carbon material is preferably 30 μm or more, more preferably 50 μm or more, and preferably 300 μm or less, and more preferably 200 μm or less. Furthermore, the average particle diameter in the minor axis direction of the particulate carbon material is preferably 5 μm or more, more preferably 10 μm or more, and preferably 30 μm or less, and more preferably 20 μm or less. When the average particle diameter of the particulate carbon material is within the above range, the thermal conductivity of the thermal conductive sheet can be improved. In this specification, the "average particle size" can be determined by observing particulate carbon material with a scanning electron microscope (SEM), measuring the maximum diameter (major diameter) and minimum diameter (minor diameter) of any 50 particulate carbon material particles, and calculating the number-average value of the measured major diameter and minor diameter.

[0058] -Content of Particulate Carbon Material- The content of the particulate carbon material is preferably 20 parts by mass or more, more preferably 30 parts by mass or more, and even more preferably 50 parts by mass or more, and preferably 150 parts by mass or less, more preferably 130 parts by mass or less, and even more preferably 120 parts by mass or less, relative to 100 parts by mass of the total resin. When the content of the particulate carbon material is 20 parts by mass or more and 150 parts by mass or less, relative to 100 parts by mass of the total resin, the balance between hardness and adhesiveness of the thermal conductive sheet can be improved, and handleability can be improved. Furthermore, when the content of the particulate carbon material is 30 parts by mass or more, relative to 100 parts by mass of the total resin, the thermal conductivity of the thermal conductive sheet can be improved. Furthermore, when the content of the particulate carbon material is 100 parts by mass or less, relative to 100 parts by mass of the total resin, the adhesiveness of the thermal conductive sheet can be improved and powder falling off of the particulate carbon material can be sufficiently prevented.

[0059] <<Fibrous Material>> A fibrous carbon nanomaterial containing CNT that can be suitably used as a fibrous material may consist of CNT alone, or may be a mixture of CNT and a fibrous carbon nanomaterial other than CNT. Note that the CNT in the fibrous carbon nanomaterial is not particularly limited, and single-walled carbon nanotubes and / or multi-walled carbon nanotubes can be used, but the CNT is preferably a single-walled to five-walled carbon nanotube, and more preferably a single-walled carbon nanotube.

[0060] Furthermore, the above-mentioned fibrous carbon nanomaterial is not particularly limited, and it is preferable to use a fibrous carbon nanomaterial containing CNTs produced in accordance with a method (see International Publication No. 2006 / 011655) in which raw material compounds and a carrier gas are supplied to a substrate having a catalyst layer for CNT production on its surface to synthesize CNTs by chemical vapor deposition (CVD), and the catalytic activity of the catalyst layer is dramatically improved by the presence of a trace amount of oxidant (catalytic activator) in the system. Hereinafter, carbon nanotubes obtained by the super-growth method may be referred to as "SGCNTs." Herein, the fibrous carbon nanomaterial containing SGCNTs produced by the super-growth method may be composed solely of SGCNTs, or may contain, in addition to SGCNTs, other carbon nanostructures, such as non-cylindrical carbon nanostructures.

[0061] When the thermally conductive filler contains a fibrous material, the content of the fibrous material in the thermally conductive filler is preferably 0.001% by mass or more, more preferably 0.005% by mass or more, and preferably 20% by mass or less, and more preferably 10% by mass or less. If the content of the fibrous material is equal to or greater than the above-mentioned lower limit, a heat transfer path can be formed well, thereby further improving the thermal conductivity of the thermally conductive sheet. Furthermore, if the content of the fibrous material is equal to or less than the above-mentioned upper limit, the flexibility of the thermally conductive sheet is prevented from decreasing due to the addition of the fibrous material, and the adhesion between the thermally conductive sheet and the adherend (heat-generating body, heat-dissipating body) is improved, allowing the thermally conductive sheet to exhibit excellent thermal conductivity.

[0062] <Other Components> In addition to the above-mentioned fluororesin, acrylic resin having a weight-average molecular weight of a predetermined value or less, and thermally conductive filler, the thermally conductive sheet of the present invention may further contain other components as desired. Examples of other components include resins other than fluororesin and acrylic resin (hereinafter referred to as "other resins"), acrylic resins having a weight-average molecular weight exceeding a predetermined value, additives, fillers, etc.

[0063] <<Other Resins>> As the other resins, known resins such as epoxy resins, silicone resins, etc. can be used. These may be used alone or in combination of two or more.

[0064] <<Acrylic Resin Having a Weight-Average Molecular Weight Above a Predetermined Value>> The thermally conductive sheet of the present invention may optionally further contain an acrylic resin having a weight-average molecular weight above the predetermined value, in addition to the acrylic resin having a weight-average molecular weight equal to or less than the predetermined value. The term "acrylic resin having a weight-average molecular weight above the predetermined value" refers to an acrylic resin having a weight-average molecular weight that exceeds the upper limit of the weight-average molecular weight defined above in the "acrylic resin having a weight-average molecular weight equal to or less than a predetermined value" (low molecular weight acrylic resin). For example, the term "acrylic resin" may be an acrylic resin having a weight-average molecular weight of more than 20,000. The term "acrylic resin" is as explained in the section "acrylic resin having a weight-average molecular weight equal to or less than a predetermined value."

[0065] <<Additives>> In addition, additives that can be blended into the thermally conductive sheet are not particularly limited, and examples thereof include flame retardants such as red phosphorus-based flame retardants and phosphate ester-based flame retardants; moisture absorbents; adhesion improvers such as silane coupling agents, titanium coupling agents, and acid anhydrides; wettability improvers such as nonionic surfactants and fluorine-based surfactants; and ion trapping agents such as inorganic ion exchangers. These may be used alone or in combination of two or more. In addition to the thermally conductive filler, resin particles, rubber particles, etc. may be blended in order to improve the strength and compressibility of the thermally conductive sheet.

[0066] <Method for forming thermally conductive sheet> The thermally conductive sheet of the present invention can be formed, without particular limitation, through a thermally conductive sheet pre-molding step, a laminate formation step, a slicing step, and the like, according to the method described in WO 2016 / 185688, for example.

[0067] <<Pre-thermal conductive sheet molding process>> In the pre-thermal conductive sheet molding process, a composition containing all of the components that form the thermal conductive sheet, i.e., the fluororesin, the acrylic resin having a weight-average molecular weight of a predetermined value or less, the thermal conductive filler, and optionally other components, is pressurized and molded into a sheet to obtain a pre-thermal conductive sheet.

[0068] [Composition] The composition can be prepared by mixing all of the components forming the thermally conductive sheet, i.e., the fluororesin, the acrylic resin having a weight-average molecular weight of a predetermined value or less, the thermally conductive filler, and optionally other components. The mixing of the above-mentioned components can be carried out using any known mixing device, such as a kneader, roll, or mixer, without any particular limitation. The mixing can also be carried out in the presence of a solvent, such as an organic solvent. The mixing time can be, for example, 5 minutes to 60 minutes. The mixing temperature can be, for example, 5°C to 150°C.

[0069] [Molding of Composition] The composition prepared as described above can be optionally degassed, optionally removed from the solvent, and crushed, and then pressed (primary pressing) to form into a sheet. The composition can be molded into a sheet using any known molding method, such as press molding, roll molding, or extrusion molding, as long as the molding method involves applying pressure. Among these, the composition is preferably formed into a sheet by roll molding, and more preferably formed into a sheet by passing the composition between rolls while sandwiched between protective films. The protective film is not particularly limited, and a sandblasted polyethylene terephthalate (PET) film or the like can be used. The roll temperature can be 5°C or higher and 150°C or lower.

[0070] [Pre-heat-conductive sheet] The thickness of the pre-heat-conductive sheet obtained by pressing the composition into a sheet is not particularly limited, and can be, for example, 0.05 mm or more and 2 mm or less.

[0071] <<Laminate Forming Step>> In the laminate forming step, a plurality of pre-thermally conductive sheets obtained in the pre-thermally conductive sheet molding step are stacked in the thickness direction, or the pre-thermally conductive sheets are folded or rolled up to obtain a laminate. Here, in order to further increase the adhesive strength between the surfaces of the pre-thermally conductive sheets in the laminate obtained in the laminate forming step and sufficiently suppress delamination of the laminate, the laminate forming step may be performed in a state where the surfaces of the pre-thermally conductive sheets are slightly dissolved with a solvent, or in a state where an adhesive is applied to the surfaces of the pre-thermally conductive sheets or an adhesive layer is provided on the surfaces of the pre-thermally conductive sheets, or the laminate formed by stacking the pre-thermally conductive sheets may be further heat-pressed (secondary pressurized) in the stacking direction.

[0072] From the viewpoint of efficiently suppressing delamination, it is preferable to apply secondary pressure to the obtained laminate in the stacking direction. The conditions for secondary pressure application are not particularly limited, and can be a pressure of 0.05 MPa or more and 0.5 MPa or less in the stacking direction at a temperature of 20°C or more and 170°C or less for 10 seconds to 30 minutes.

[0073] In a laminate obtained by stacking, folding, or rolling the primary sheet, the thermally conductive filler is presumably oriented in a direction substantially perpendicular to the stacking direction. For example, if the thermally conductive filler is a particulate carbon material and has a scale-like shape, the direction of the major axis of the main surface of the scale-like shape is presumably substantially perpendicular to the stacking direction.

[0074] <<Slicing Step>> In the slicing step, the laminate obtained in the laminate-forming step is sliced ​​at an angle of 45° or less relative to the stacking direction to obtain a thermally conductive sheet composed of slices of the laminate. 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 these, the knife processing method is preferred because it is easy to achieve a uniform thickness for the thermally conductive sheet. The cutting tool used to slice 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.

[0075] The thermally conductive sheet obtained through the slicing process is typically configured by joining strips (slices of the pre-thermal conductive sheet that constituted the laminate) in parallel, each strip containing a fluororesin, an acrylic resin having a weight-average molecular weight of a predetermined value or less, and a thermally conductive filler.

[0076] From the viewpoint of increasing the thermal conductivity of the thermal conduction sheet, the angle at which the laminate is sliced ​​is preferably 30° or less relative to the stacking direction, more preferably 15° or less relative to the stacking direction, and preferably approximately 0° relative to the stacking direction (i.e., in the direction along the stacking direction).

[0077] In the secondary sheet obtained in this manner, the thermally conductive filler is well oriented in the thickness direction, and the angle of the long axis direction of the thermally conductive filler with respect to the sheet surface is 60° to 90°. More specifically, when the thermally conductive filler is a particulate carbon material in a flake shape, the angle of the long axis direction of the main surface of the flake shape with respect to the sheet surface is 60° to 90°.

[0078] From the viewpoint of slicing the laminate easily, the temperature of the laminate during slicing is preferably −20° C. or higher and 30° C. or lower. Furthermore, for the same reason, the laminate to be sliced ​​is preferably sliced ​​while applying a pressure in a direction perpendicular to the stacking direction, and more preferably while applying a pressure of 0.1 MPa or higher and 0.5 MPa or lower in the direction perpendicular to the stacking direction.

[0079] In the thermally conductive sheet obtained through the various processes described above, the thermally conductive filler is well oriented in the thickness direction. For example, if the thermally conductive filler is a particulate carbon material in the form of flakes, the major axis of the main surface of the flakes is oriented in the thickness direction of the secondary sheet.

[0080] <Properties of Thermally Conductive Sheet> <Tack> The thermally conductive sheet of the present invention exhibits excellent measured values ​​for tack to metal. The measured value of tack to metal is an index of adhesion to metal (e.g., a metal heat sink), and the larger the measured value of tack to metal, the better the adhesion to metal. The measured value of tack to metal is, for example, 20 N / cm２ It is preferable that the strength is 25 N / cm or more. ２ More preferably, it is 200 N / cm or more. ２ Preferably, it is 100 N / cm or less. ２ It is more preferable that the "tack to metal" is not more than 100%. In the present invention, the "tack to metal" can be measured according to the method described in the examples of this specification.

[0081] <Tensile strength> The thermally conductive sheet of the present invention exhibits excellent measured values ​​for tensile strength. Tensile strength is an indicator of durability, with higher tensile strength indicating better durability. The tensile strength of the thermally conductive sheet is, for example, preferably 0.01 MPa or more, more preferably 0.02 MPa or more, and preferably 3.00 MPa or less, and more preferably 2 MPa or less. If the tensile strength is equal to or greater than the above lower limit, sufficient strength (durability) can be ensured. Furthermore, if the tensile strength is equal to or less than the above upper limit, the thermally conductive sheet can be easily manufactured. In the present invention, "tensile strength" can be measured according to the method described in the examples of this specification.

[0082] <Thermal Conductivity> The thermal conductive sheet of the present invention exhibits excellent measured values ​​for thermal conductivity. For example, the thermal conductivity of the thermal conductive sheet in the thickness direction at 25°C is preferably 1 W / m·K or more, more preferably 5 W / m·K or more, and even more preferably 10 W / m·K or more. Thermal conductivity is an index of thermal conductivity, and the higher the thermal conductivity, the better the thermal conductivity for efficiently transferring heat from a heat generating body to a heat dissipating body. In the present invention, "thermal conductivity" can be measured according to the method described in the examples of this specification.

[0083] <Flame Retardancy> The thermal conductive sheet of the present invention is evaluated as having excellent flame retardancy. In the present invention, the "flame retardancy" can be evaluated according to the method described in the examples of the present specification.

[0084] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" representing amounts are based on mass unless otherwise specified. In the examples and comparative examples, the viscosity of a liquid resin at room temperature and normal pressure; the Mooney viscosity of a solid resin at room temperature and normal pressure; the tack, tensile strength, thermal conductivity, and flame retardancy of a thermal conductive sheet to a metal substrate were measured or evaluated according to the following methods.

[0085] (Method for measurement or evaluation) <Viscosity of liquid resin at room temperature and normal pressure> The viscosity (viscosity coefficient: P) of the liquid fluororesin at room temperature and normal pressure was measured at a temperature of 80°C using an E-type viscometer (manufactured by BROOKFIELD Corporation, device name "BROOKFIELD DIGITAL VISCOMETER MODEL DV-II Pro").

[0086] <Mooney viscosity of resin that is solid at room temperature and normal pressure> Mooney viscosity (ML １＋４ , 100°C) was measured at a temperature of 100°C using a Mooney viscometer (manufactured by Shimadzu Corporation, product name "MOONEY VISCOMETER SMV-202") in accordance with JIS-K6300. Generally, the lower the Mooney viscosity of a resin that is solid at room temperature and normal pressure, the higher the flexibility of the resin.

[0087] <Tack> The tack of the thermally conductive sheet to a metal substrate was measured using a probe tack tester (manufactured by Rhesca Corporation, product name "TAC1000") in an atmosphere at 25°C. A thermally conductive sheet punched to a diameter of 4 mm and attached with double-sided tape to the tip of a flat, circular probe with a diameter of 5 mm was pressed against a 3 cm square, 2 mm thick copper plate with a nickel-plated surface placed on a stage for 10 seconds with a load of 1.3 N (130 gf), and the force required to pull the probe with the thermally conductive sheet attached from the copper plate was measured. The measured value of tack to metal is an index of adhesion to metal, and a larger measured value of tack to metal indicates better adhesion to metal.

[0088] <Tensile Strength> A thermally conductive sheet was punched using a No. 2 dumbbell (dumbbell-shaped, width: 3 mm, length: 70 mm) conforming to JIS K7113 to produce five test pieces. Then, using a tensile tester (manufactured by Shimadzu Corporation, product name "AG-IS20kN"), the test pieces were pulled at a load cell of 50 N, a chuck distance of 35 mm, a speed of 25 mm / min, and a temperature of 23°C to measure the breaking strength (tensile strength). The test pieces were punched in such a way that the long axis of the dumbbell intersected the strips constituting the thermally conductive sheet at a 90-degree angle. The test pieces were punched at five locations: the center and four corners of the sheet (positions where part of the dumbbell was within 3 cm of the corner). The average of the measurements for the five test pieces was taken as the tensile strength of the thermally conductive sheet.

[0089] <Thermal conductivity> 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 method. [Thermal diffusivity α in the thickness direction] The thermal diffusivity in the thickness direction at 25°C was measured using a thermal property measuring device (manufactured by Bethel Co., Ltd., product name "Thermowave Analyzer TA35"). A thermally conductive sheet sliced ​​to a thickness of 0.3 mm was used for the measurement. [Constant-pressure specific heat Cp] The specific heat at 25°C 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 (density)] The specific heat was measured using an automatic hydrometer (manufactured by Toyo Seiki Seisakusho, product name "DENSIMETER-H"). Each measured value was then substituted into the following formula (I): λ = α × Cp × ρ (I) to determine the thermal conductivity λ (W / m K) in the thickness direction of the thermally conductive sheet at 25°C.

[0090] <Orientation angle of particulate carbon material> The orientation angle of the graphite (particulate carbon material) in the thermally conductive sheet was determined by observing a cross section of the thermally conductive sheet cut into a regular octagon using a scanning electron microscope (SEM, Hitachi High-Technologies Corporation's "SU-3500") at a magnification that fit the entire sheet from top to bottom. The magnification was 700x. Fifty lines were drawn along the major axis of the graphite (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 greater, a 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 graphite (particulate carbon material) in the thermally conductive sheet.

[0091] <Flame Retardancy> Ten test specimens were prepared by cutting the thermal conductive sheet into a size of 125 mm long x 13 mm wide. Five test specimens were then stored for 48 hours in an environment at 23°C and 50% relative humidity (I). Meanwhile, the remaining five test specimens were stored for 168 hours in an environment at 70°C (II) for aging treatment. In this way, two sets of five test specimens were prepared, each subjected to two treatments. Next, each test specimen in each set was lifted vertically and supported with a clamp, and absorbent cotton was placed approximately 300 mm below the supported test specimen. The air and gas flow rates of a Bunsen burner were adjusted to create a blue flame approximately 20 mm high. The Bunsen burner flame was applied to the lower end of the vertically supported test specimen (so that the flame intersected the test specimen by approximately 10 mm) for 10 seconds, after which the Bunsen burner flame was removed from the test specimen. Immediately after the flame on the test piece was extinguished, the Bunsen burner flame was applied to the test piece again and maintained for another 10 seconds, after which the test piece was removed from the Bunsen burner. The afterflame time (the time it burned with a flame) after the first flame contact, the afterflame time after the second flame contact, the second flameless combustion time (the time it burned without a flame after the flame was removed), and whether the test piece ignited the absorbent cotton or produced droplets while flaming were confirmed and evaluated. Specifically, for two sets of five test pieces, the following four conditions were determined to be met: (1) the afterflame time after the first and second flame contacts for each test piece was within 10 seconds, (2) the total afterflame time after the five flame contacts was within 50 seconds, (3) no droplets were produced that could ignite the absorbent cotton, and (4) the flameless combustion time after the second flame contact was within 30 seconds. A thermally conductive sheet in which all test pieces subjected to 48-hour storage (I) and aging treatment (II) satisfy all of the above conditions (1) to (4) can be said to have excellent flame retardancy. OK: All test pieces subjected to 48-hour storage (I) and aging treatment (II) satisfy all of the above conditions (1) to (4). NG: At least one of the test pieces subjected to 48-hour storage (I) and aging treatment (II) does not satisfy one or more of the above conditions (1) to (4) (out of specification). -: Unable to evaluate

[0092] Example 1 Preparation of Composition 75 parts of a thermoplastic fluororesin that is liquid at room temperature and normal pressure (manufactured by Daikin Industries, Ltd., trade name "Dai-el G-101", viscosity (viscosity coefficient): 3300 P) as the fluororesin, 25 parts of a hydroxyl group-containing liquid acrylic polymer having a weight average molecular weight of 6,000 (manufactured by Toagosei Co., Ltd., trade name "ARUFON (registered trademark) UH-2190", viscosity 34,000 mPa·s @ 25°C, glass transition temperature (Tg) -47°C, hydroxyl value 33 mg·KOH / g resin) as the acrylic resin, and 80 parts of expanded graphite (manufactured by Ito Graphite Industries Co., Ltd., trade name "EC-300", volume average particle size: 50 μm) as the thermally conductive filler were mixed and stirred for 5 minutes using a Hobart mixer (manufactured by Kodaira Seisakusho Co., Ltd., trade name "ACM-5LVT type"). The resulting mixture was degassed under vacuum for 30 minutes to obtain a composition. The resulting composition was then placed in a crusher and crushed for 10 seconds.

[0093] <Formation of Pre-heat Conductive Sheet> Next, 1 kg of the crushed composition was sandwiched between sandblasted PET films (protective films) having a thickness of 50 μm, 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 pre-heat conductive sheet having a thickness of 0.5 mm.

[0094] <Formation of laminate> Next, the obtained pre-thermal conductive sheet was cut into a size of 150 mm length × 150 mm width × 0.5 mm thickness, and 120 sheets were stacked in the thickness direction of the pre-thermal conductive sheet. Further, the sheets were pressed (secondary pressing) in the stacking direction at a temperature of 25°C and a pressure of 0.1 MPa for 3 minutes to obtain a laminate with a height of approximately 60 mm.

[0095] <Formation of Thermally Conductive Sheet> The laminated side of the secondary-pressurized laminate was then pressed with a pressure of 0.3 MPa using a woodworking slicer (Marunaka Iron Works Co., Ltd., product name "Super Finishing Planer Super Mecha S") to slice at an angle of 0 degrees relative to the lamination direction (in other words, in the normal direction to the main surface of the laminated pre-thermally conductive sheet), thereby obtaining a thermally conductive sheet (configured by parallel-joined strips) measuring 150 mm long x 60 mm wide x 0.15 mm thick. The resulting thermally conductive sheet was then measured and evaluated for tack to the metal substrate, tensile strength, thermal conductivity, flame retardancy, and the orientation angle of the graphite (particulate carbon material) according to the methods described above. The results are shown in Table 1.

[0096] Example 2 A composition, a pre-heat conductive sheet, a laminate, and a heat conductive sheet were produced in the same manner as in Example 1, except that in preparing the composition, the amount of fluororesin used in Example 1 was changed to 50 parts and the amount of acrylic polymer used in Example 1 was changed to 50 parts. Measurements and evaluations were then carried out in the same manner as in Example 1. The results are shown in Table 1.

[0097] Example 3 A composition, a pre-heat conductive sheet, a laminate, and a heat conductive sheet were produced in the same manner as in Example 1, except that in preparing the composition, the amount of fluororesin used in Example 1 was changed to 90 parts and the amount of acrylic polymer used in Example 1 was changed to 10 parts. Measurements and evaluations were then carried out in the same manner as in Example 1. The results are shown in Table 1.

[0098] Example 4 A composition, a pre-heat conductive sheet, a laminate, and a heat conductive sheet were produced in the same manner as in Example 1, except that in preparing the composition, the amount of fluororesin used in Example 1 was changed to 25 parts and the amount of acrylic polymer used in Example 1 was changed to 75 parts. Measurements and evaluations were then carried out in the same manner as in Example 1. The results are shown in Table 1.

[0099] Example 5 In preparing the composition, a composition, a pre-heat conductive sheet, a laminate, and a heat conductive sheet were produced in the same manner as in Example 1, except that a hydroxyl-containing liquid acrylic polymer having a weight average molecular weight of 11,000 (manufactured by Toagosei Co., Ltd., trade name "ARUFON (registered trademark) UH-2000", viscosity 14,000 mPa·s @ 25°C, glass transition temperature (Tg) -55°C, hydroxyl value 20 mg·KOH / g resin) was used instead of the acrylic polymer used in Example 1. The results are shown in Table 1.

[0100] (Example 6) In preparing the composition, a thermoplastic fluororesin that is solid at room temperature and normal pressure (manufactured by 3M Japan Ltd., trade name "Dyneon FC2211", Mooney viscosity: 27 ml) was used. １＋４ A composition, a pre-heat conductive sheet, a laminate, and a heat conductive sheet were produced in the same manner as in Example 1, except that 20 parts of a thermoplastic fluororesin (100°C, 100°C) and 55 parts of the thermoplastic fluororesin that is liquid at room temperature and normal pressure used in Example 1 were stirred and mixed in the presence of 60 parts of ethyl acetate as a solvent, and the ethyl acetate was removed simultaneously with vacuum degassing. Measurements and evaluations were then carried out in the same manner as in Example 1. The results are shown in Table 1.

[0101] Example 7 A composition, a pre-heat conductive sheet, a laminate, and a heat conductive sheet were produced in the same manner as in Example 1, except that in preparing the composition, 20 parts of the thermoplastic fluororesin that is solid at room temperature and normal pressure used in Example 6 and 40 parts of the thermoplastic fluororesin that is liquid at room temperature and normal pressure used in Example 1 were used as the fluororesin, and 40 parts of the acrylic polymer used in Example 1 were used as the acrylic resin. These were stirred and mixed in the presence of 60 parts of ethyl acetate as a solvent, and the ethyl acetate was removed simultaneously with vacuum degassing. Measurements and evaluations were then performed in the same manner as in Example 1. The results are shown in Table 1.

[0102] Example 8 In preparing the composition, a composition, a pre-heat conductive sheet, a laminate, and a heat conductive sheet were produced in the same manner as in Example 7, except that a non-functional liquid acrylic polymer having a weight average molecular weight of 2,300 (manufactured by Toagosei Co., Ltd., trade name "ARUFON (registered trademark) UP-1110", viscosity 3,500 mPa s @ 25°C, glass transition temperature (Tg) -64°C) was used instead of the acrylic polymer used in Example 7. Then, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0103] Example 9 In preparing the composition, a composition, a pre-heat conductive sheet, a laminate, and a heat conductive sheet were produced in the same manner as in Example 7, except that a non-functional liquid acrylic polymer having a weight average molecular weight of 1,700 (manufactured by Toagosei Co., Ltd., trade name "ARUFON (registered trademark) UP-1190", viscosity 6,000 mPa s @ 25°C, glass transition temperature (Tg) -57°C) was used instead of the acrylic polymer used in Example 7. Then, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0104] Comparative Example 1 A composition, a pre-heat conductive sheet, a laminate, and a heat conductive sheet were produced in the same manner as in Example 1, except that in preparing the composition, the amount of fluororesin used in Example 1 was changed to 100 parts and no acrylic resin was added. Measurements and evaluations were then carried out in the same manner as in Example 1. The results are shown in Table 1.

[0105] Comparative Example 2 A composition, a pre-heat conductive sheet, a laminate, and a heat conductive sheet were produced in the same manner as in Example 1, except that in preparing the composition, an acrylic polymer having a weight average molecular weight of 600,000 (trade name "Paraclon ME-2000", manufactured by Negami Chemical Industrial Co., Ltd., glass transition temperature (Tg) -35°C) was used as the acrylic resin instead of the acrylic polymer used in Example 1. Measurements and evaluations were then carried out in the same manner as in Example 1. The results are shown in Table 1.

[0106] (Comparative Example 3) A composition was prepared in the same manner as in Example 1, except that the amount of acrylic polymer used in Example 1 was changed to 100 parts and no fluororesin was added. However, the viscosity of this composition was so high that it could not be crushed, and therefore a pre-heat conductive sheet, a laminate, and a heat conductive sheet were not produced.

[0107] (Comparative Example 4) In preparing the composition, 50 parts of the same acrylic polymer (manufactured by Toagosei Co., Ltd., trade name "ARUFON (registered trademark) UH-2190") as used in Example 1 and 50 parts of the same acrylic polymer (manufactured by Negami Chemical Industrial Co., Ltd., trade name "PARACRON ME-2000") as used in Comparative Example 2 were used as the acrylic resin, and a composition was prepared in the same manner as in Example 1, except that no fluororesin was added. However, the viscosity of this composition was so high that it could not be disintegrated, and therefore a pre-heat conductive sheet, a laminate, and a heat conductive sheet were not produced.

[0108]

[0109] (Table notes) *The composition was too viscous to be crushed, so production was discontinued.

[0110] Table 1 shows that the thermally conductive sheet containing a fluororesin, an acrylic resin with a weight-average molecular weight of 20,000 or less, and a thermally conductive filler has good flame retardancy, thermal conductivity as measured by thermal conductivity, and durability as measured by tensile strength, and has excellent adhesion to metal as measured by tack to metal.

[0111] According to the present invention, it is possible to provide a thermally conductive sheet that has excellent adhesion to metals and flame retardancy.

Claims

1. A thermally conductive sheet comprising a fluororesin, an acrylic resin having a weight-average molecular weight of 20,000 or less, and a thermally conductive filler.

2. The thermally conductive sheet according to claim 1 , wherein the fluororesin comprises a fluororesin that is liquid at room temperature and normal pressure.

3. The thermally conductive sheet according to claim 2 , wherein the fluororesin includes both a fluororesin that is liquid at room temperature and normal pressure and a fluororesin that is solid at room temperature and normal pressure.

4. The thermal conductive sheet according to claim 1 , wherein the acrylic resin has a weight average molecular weight of 10,000 or less.

5. The thermally conductive sheet according to claim 1 , wherein the acrylic resin comprises an acrylic resin that is liquid at room temperature and normal pressure.

6. The thermally conductive sheet according to claim 1 , wherein the acrylic resin contains a hydroxyl group.

7. The thermal conductive sheet according to claim 1 , wherein the content of the acrylic resin is 20% by mass or more and 70% by mass or less of the total resin.

8. The thermal conductive sheet according to any one of claims 1 to 7, having a configuration in which strips made of a composition in which the fluororesin, the acrylic resin, and the thermally conductive filler are mixed are joined in parallel.