Thermally conductive sheet

Abstract

The present invention is a thermally conductive sheet characterized by containing two or more fibrous thermally conductive fillers (B) having different average fiber diameters in a polymer matrix (A), and the ratio of the maximum diameter to the minimum diameter of each of the average fiber diameters being 2:1 to 45:1. Thus, the thermally conductive sheet having excellent thermal conductivity and low specific gravity is provided by being filled with two or more fibrous thermally conductive fillers having different average fiber diameters.

Description

Thermally conductive sheet

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

[0002] In recent years, the computerization of automobiles has become remarkable, increasing the number of electronic components installed and the demand for heat dissipation materials. Sheet-type heat dissipation materials are particularly popular due to their ease of mounting. Furthermore, with autonomous driving becoming a trend, heat dissipation materials are required to have even higher thermal conductivity. To achieve high thermal conductivity, it is generally necessary to increase the filling rate of the filler dispersed within the polymer, which tends to result in a high specific gravity exceeding 2.0. However, this poses a problem when promoting weight reduction to improve the fuel efficiency of gasoline-powered vehicles and the electric power efficiency of electric vehicles.

[0003] For example, Patent Document 1 proposes a carbon fiber-containing sheet in which two types of carbon fibers with different fiber diameters are blended and oriented in one direction to increase the contact area of ​​the carbon fibers and improve thermal conductivity.

[0004] However, in Patent Document 1, the ratio of the fiber diameters of the first fiber elements to the second fiber elements is 50:1 to 100:1, which means that the first fiber diameter is extremely large or the second fiber diameter is extremely small. This raises the issue of increasing the compound viscosity when blended with a synthetic resin, making it difficult to achieve a uniform compound, and also makes it difficult to increase the filling ratio of the second fiber elements, leaving room for improvement in increasing the contact area.

[0005] Furthermore, by filling multiple carbon fibers with different fiber diameters, the contact area could be increased further, and thermal conductivity could be improved.

[0006] Furthermore, Patent Document 2 proposes a heat dissipation sheet in which carbon fibers having an average effective fiber length of 5 to 120 μm, an average fiber diameter of 200 to 900 nm, and an aspect ratio of 30 to 1,000 are oriented in the plane direction in a polymer.

[0007] However, heat dissipation sheets that generally transfer heat generated by a heat generating element to a heat sink are often required to conduct heat in the thickness direction rather than in the surface direction of the heat dissipation sheet, leaving room for improvement in the direction of heat conduction.

[0008] Furthermore, Patent Document 3 proposes a thermally conductive sheet having a thermal conductivity of 10 W / m K or more, which contains a volume fraction of a thermally conductive filler including carbon fiber and an inorganic filler of 55 to 70% and a volume fraction of a two-component addition reaction type silicone of 30 to 45%.

[0009] However, when the thermally conductive filler is contained in the volume ratio of Patent Document 3, the specific gravity increases, and there is room for improvement in reducing the specific gravity.

[0010] JP 2019-111728 A JP 2021-008629 A JP 2022-127596 A

[0011] The present invention has been made to solve the above-mentioned problems, and aims to provide a thermally conductive sheet that has excellent thermal conductivity and a low specific gravity by filling it with two or more types of fibrous thermally conductive fillers with different average fiber diameters.

[0012] In order to solve the above problems, the present invention provides a thermally conductive sheet comprising a polymer matrix (A) containing two or more types of fibrous thermally conductive fillers (B) having different average fiber diameters, and the ratio of the maximum diameter to the minimum diameter of the average fiber diameters is 2:1 to 45:1.

[0013] Such a thermally conductive sheet has an appropriate fiber diameter ratio, which improves the packing efficiency of the fibrous thermally conductive filler and increases thermal conductivity.

[0014] The thermally conductive sheet preferably has a thermal conductivity in the thickness direction of 25 W / m·K or more and a specific gravity of less than 1.4.

[0015] Such a thermal conductive sheet can improve heat transfer efficiency by being placed between a heat generating body and a heat dissipating body. In addition, the inclusion of fibrous thermal conductive filler shortens the heat transfer path within the thermal conductive sheet, allowing for high thermal conductivity with a low filling amount, making it ideal for achieving low specific gravity.

[0016] The aspect ratio of the fibrous thermally conductive filler (B) is preferably 2 or more and less than 30.

[0017] Such a fibrous thermally conductive filler prevents the fillers from becoming entangled with each other, thereby improving the packing properties.

[0018] It is also preferred that at least one of the fibrous thermally conductive fillers (B) is a carbon fiber having an average fiber diameter of 8 μm or more.

[0019] Such fibrous thermally conductive fillers have high thermal conductivity in the longitudinal direction, so increasing the cross-sectional area can efficiently increase thermal conductivity. Furthermore, carbon has a lower specific gravity than common thermally conductive fillers such as alumina and aluminum nitride, making it suitable for reducing specific gravity.

[0020] At least one of the fibrous thermally conductive fillers (B) is preferably a carbon fiber having an average fiber diameter of 1 μm or more and less than 8 μm, or an average fiber diameter of less than 1 μm.

[0021] Such fibrous thermally conductive fillers are more likely to be disposed between fillers having an average fiber diameter of 8 μm or more, further increasing the thermal conductivity.

[0022] The proportion of the fibrous thermally conductive filler (B) contained in the thermally conductive sheet of the present invention is preferably 20 to 60 mass %.

[0023] Such a thermally conductive sheet is advantageous in achieving both high thermal conductivity and low specific gravity.

[0024] It is also preferred that the major axis direction of the fibrous thermally conductive filler (B) of the present invention is oriented in the thickness direction of the thermally conductive sheet.

[0025] Such a heat conductive sheet is preferable because it further shortens the heat transfer path in the thickness direction of the sheet.

[0026] The polymer matrix (A) is preferably a cured product of a curable polymer composition containing at least one selected from the group consisting of organopolysiloxane, polyurethane, polyacrylate, perfluoropolyether elastomer, unsaturated polyester resin, polyamide resin, epoxy resin, and phenolic resin.

[0027] If the polymer matrix (A) has such a structure, the filling property of the fibrous thermally conductive filler (B) will be good.

[0028] As described above, by filling two or more types of fibrous thermally conductive fillers with different average fiber diameters at an appropriate fiber diameter ratio, a thermally conductive sheet with excellent thermal conductivity and low specific gravity can be obtained.

[0029] As described above, there has been a demand for a thermally conductive sheet that has excellent thermal conductivity and a low specific gravity.

[0030] As a result of intensive research to solve the above problems, the inventors discovered that by filling fibrous thermally conductive fillers with different average fiber diameters, it is possible to obtain a thermally conductive sheet with excellent thermal conductivity and a low specific gravity, which led to the creation of the present invention.

[0031] That is, the present invention provides a thermally conductive sheet comprising a polymer matrix (A) and two or more types of fibrous thermally conductive fillers (B) having different average fiber diameters, and the ratio of the maximum diameter to the minimum diameter of the average fiber diameter is 2:1 to 45:1.

[0032] The present invention will be described in detail below, but the present invention is not limited thereto.

[0033] The thermally conductive sheet of the present invention is obtained by molding a filler-containing resin composition for a thermally conductive sheet, which contains two or more types of fibrous thermally conductive fillers (B) having different average fiber diameters in a polymer matrix (A), and in which the ratio of the maximum diameter to the minimum diameter of the average fiber diameter is 2:1 to 45:1, using a method described below.

[0034] [Filler-Containing Resin Composition] The filler-containing resin composition is a resin composition containing a polymer matrix (A) and two or more types of fibrous thermally conductive fillers (B) having different average fiber diameters.

[0035] [Polymer Matrix (A)] The polymer matrix, which is component (A) of the filler-containing resin composition, is not particularly limited, but in view of the filling ability of the fibrous thermally conductive filler, component (B), it is preferably one or more curable polymer compositions selected from the group consisting of organopolysiloxane, polyurethane, polyacrylate, perfluoropolyether elastomer, unsaturated polyester resin, polyamide resin, epoxy resin, and phenolic resin, more preferably an organopolysiloxane, perfluoropolyether elastomer, polyurethane, or polyacrylate, which have excellent flexibility, and even more preferably an organopolysiloxane or perfluoropolyether elastomer, which have excellent heat resistance and cold resistance.

[0036] [Curable Polymer Composition] Therefore, the component (A) is preferably a curable organopolysiloxane composition or a curable perfluoropolyether elastomer composition.

[0037] [Curable Organopolysiloxane Composition] The curable organopolysiloxane composition will now be described in detail.

[0038] Examples of curable organopolysiloxane compositions include thermosetting organopolysiloxane compositions, moisture-curing organopolysiloxane compositions, and electron beam-curing silicone resins. Examples of thermosetting silicone resins include addition-curing organopolysiloxane compositions and peroxide-curing organopolysiloxane compositions. Among these, addition-curing organopolysiloxane compositions are preferred, and addition-curing organopolysiloxane compositions containing the following components (A-1) to (A-3) are more preferred. (A-1) Alkenyl group-containing organopolysiloxane, (A-2) Organohydrogenpolysiloxane, (A-3) Platinum group metal-based curing catalyst

[0039] [(A-1) Alkenyl Group-Containing Organopolysiloxane] The alkenyl group-containing organopolysiloxane, component (A-1), is an organopolysiloxane having two or more silicon-bonded alkenyl groups per molecule, and serves as the main component of the filler-containing resin composition of the present invention (which provides a thermally conductive silicone cured product). Typically, the main chain portion is essentially composed of repeating diorganosiloxane units, but this may also include a branched structure as part of the molecular structure, or may be cyclic. However, from the perspective of physical properties such as the mechanical strength of the cured product, linear diorganopolysiloxanes are preferred.

[0040] Examples of the alkenyl group include vinyl, aryl, propenyl, isopropenyl, butenyl, hexenyl, and cyclohexenyl groups, each of which typically has about 2 to 8 carbon atoms. Of these, lower alkenyl groups such as vinyl and aryl groups are preferred, with vinyl being particularly preferred. It is preferred that two or more alkenyl groups are present in the molecule, but in order to improve the flexibility of the resulting cured product, it is preferred that the alkenyl groups be bonded only to silicon atoms at the molecular chain terminals.

[0041] The functional group other than the alkenyl group is a monovalent hydrocarbon group, and examples thereof include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, and dodecyl; cycloalkyl groups such as cyclopentyl, cyclohexyl, and cycloheptyl; aryl groups such as phenyl, tolyl, xylyl, naphthyl, and biphenylyl; and aralkyl groups such as benzyl, phenylethyl, phenylpropyl, and methylbenzyl. Among these, preferred are those having 1 to 10 carbon atoms, and particularly preferred are alkyl groups having 1 to 3 carbon atoms, such as methyl, ethyl, propyl, and 3,3,3-trifluoropropyl, and phenyl. Furthermore, the functional groups other than the alkenyl group bonded to the silicon atom do not necessarily have to be the same.

[0042] The kinematic viscosity of this organopolysiloxane at 25°C is usually preferably 10 to 100,000 mm ２ / s, particularly preferably 5,000 to 100,000 mm ２ / s. The viscosity is in the range of 10 mm ２ / s or more, the storage stability of the resulting resin composition is good, and ２ If the viscosity is 1 / s or less, the extensibility of the resulting resin composition will not be impaired.

[0043] In this specification, the kinematic viscosity is a value measured at 25°C using a Cannon-Fenske viscometer described in JIS Z 8803:2011.

[0044] The organopolysiloxane of component (A-1) may be used either alone or in combination of two or more different viscosities.

[0045] [(A-2) Organohydrogenpolysiloxane] The organohydrogenpolysiloxane of component (A-2) is an organohydrogenpolysiloxane having an average of at least two, and preferably 2 to 100, hydrogen atoms directly bonded to silicon atoms (hydrosilyl groups) per molecule, and functions as a crosslinker for component (A-1). That is, the hydrosilyl groups in component (A-2) and the alkenyl groups in component (A-1) undergo addition via a hydrosilylation reaction promoted by the platinum group metal curing catalyst of component (A-3), described below, to form a three-dimensional network structure with a crosslinked structure. Note that if the number of hydrosilyl groups is less than two, the product will not cure.

[0046] The organohydrogenpolysiloxane used is one represented by the following average structural formula (1), but is not limited thereto. (In the formula, R is independently a hydrogen atom or a monovalent hydrocarbon group containing no aliphatic unsaturated bonds, and at least two, preferably 2 to 100, more preferably 2 to 10 hydrogen atoms per molecule are present, and e is an integer of 1 or greater, preferably an integer of 10 to 200.)

[0047] In the general formula (1), examples of the monovalent hydrocarbon group R containing no aliphatic unsaturated bonds other than hydrogen atoms include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, and dodecyl; cycloalkyl groups such as cyclopentyl, cyclohexyl, and cycloheptyl; aryl groups such as phenyl, tolyl, xylyl, naphthyl, and biphenylyl; and aralkyl groups such as benzyl, phenylethyl, phenylpropyl, and methylbenzyl. Among these, groups having 1 to 10 carbon atoms, and particularly 1 to 6 carbon atoms, are preferred, and alkyl groups having 1 to 3 carbon atoms, such as methyl, ethyl, propyl, and 3,3,3-trifluoropropyl, and phenyl are more preferred. Furthermore, it is not essential that all Rs be the same.

[0048] The amount of component (A-2) added is, for example, an amount such that the number of hydrosilyl groups derived from component (A-2) is 0.1 to 5.0 moles, preferably 0.3 to 2.0 moles, and more preferably 0.5 to 1.5 moles per mole of alkenyl groups derived from component (A-1). When the amount of hydrosilyl groups derived from component (A-2) is 0.1 mole or more per mole of alkenyl groups derived from component (A-1), sufficient curing occurs, and the strength of the cured product is sufficient to retain its shape as a molded product, so that it is not difficult to handle due to insufficient strength. Furthermore, when the amount is 5.0 moles or less, the flexibility of the cured product is good and it does not become brittle.

[0049] [(A-3) Platinum Group Metal-Based Curing Catalyst] The platinum group metal-based curing catalyst of component (A-3) is a catalyst for promoting the addition reaction between the alkenyl group derived from component (A-1) and the hydrosilyl group derived from component (A-2), and examples thereof include catalysts well known as catalysts used in hydrosilylation reactions. Specific examples thereof include platinum group metals such as platinum (including platinum black), rhodium, and palladium; ２ PtCl ４ ・nH ２ O, H ２ PtCl ６ ・nH ２O, NaHPtCl ６ ・nH ２ O, KHPtCl ６ ・nH ２ O, Na ２ PtCl ６ ・nH ２ O.K. ２ PtCl ４ ・nH ２ O, PtCl ４ ・nH ２ O, PtCl ２ , Na ２ HPtCl ４ ・nH ２ O (wherein n is an integer of 0 to 6, preferably 0 or 6), chloroplatinic acid and chloroplatinate salts, alcohol-modified chloroplatinic acid (see U.S. Pat. No. 3,220,972), complexes of chloroplatinic acid with olefins (see U.S. Pat. Nos. 3,159,601, 3,159,662 and 3,775,452), platinum black, platinum group metals such as palladium supported on a support such as alumina, silica or carbon, rhodium-olefin complexes, chlorotris(triphenylphosphine)rhodium (Wilkinson's catalyst), complexes of platinum chloride, chloroplatinic acid or chloroplatinate salts with vinyl group-containing siloxanes, particularly vinyl group-containing cyclic siloxanes.

[0050] The amount of component (A-3) used may be any effective amount, preferably 0.1 to 2,000 ppm, more preferably 50 to 1,000 ppm, calculated as the mass of platinum group metal element relative to component (A-1). Amounts of 0.1 ppm or more provide sufficient catalytic activity, while amounts of 2,000 ppm or less provide sufficient effect in promoting the addition reaction while also reducing costs.

[0051] [Curable Perfluoropolyether Elastomer Composition] The curable perfluoropolyether elastomer composition, which is another preferred embodiment, will now be described in detail.

[0052] Examples of curable perfluoropolyether elastomer compositions include thermosetting perfluoropolyether elastomer compositions, moisture-curing perfluoropolyether elastomer compositions, and electron beam-curing perfluoropolyether elastomers. Examples of thermosetting perfluoropolyether elastomers include addition-curing perfluoropolyether elastomer compositions and peroxide-curing perfluoropolyether elastomer compositions. Of these, addition-curing perfluoropolyether elastomer compositions are preferred, and addition-curing perfluoropolyether elastomer compositions containing the following components (A-4) to (A-6) are more preferred. (A-4) Alkenyl group-containing perfluoropolyether compound, (A-5) Fluorine-containing organohydrogenpolysiloxane, (A-6) Platinum group metal-based curing catalyst

[0053] [(A-4) Alkenyl Group-Containing Perfluoropolyether Compound] The alkenyl group-containing perfluoropolyether compound, component (A-4), is a perfluoropolyether compound having two or more alkenyl groups per molecule, and serves as the main component of the filler-containing resin composition of the present invention (which provides a thermally conductive perfluoropolyether cured product). Generally, the main chain portion is basically composed of repeating perfluoropolyether units (perfluorooxyalkylene units).

[0054] Among them, the following general formula (2) CH ２ =CH-(X) ｚ -Rf １ -(X') ｚ -CH=CH ２ (2) wherein X is a group of the formula: -CH ２ -, -CH ２ O-, -CH ２ OCH ２ -or-Y-NR １ -CO- (wherein Y is a group of the formula: -CH ２ or the formula: is a divalent group represented by R １ is a hydrogen atom or a monovalent hydrocarbon group; X' is a divalent group represented by the formula: -CH ２ -, -CH２ O-, -CH ２ OCH ２ -or-CO-NR １ -Y'- (wherein Y' is a group of the formula: -CH ２ or the formula: is a divalent group represented by R １ is the same as above.) and Rf １ is represented by the following general formula (i): ｔ F ２ｔ [OCF ２ CF (CF ３ )] ｐ OCF ２ (CF ２ ) ｒ CF ２ O[CF(CF ３ )CF ２ O] ｑ C ｔ’ F ２ｔ’ - (i) (wherein p and q are integers of 1 to 150, and the average of the sum of p and q is 2 to 200, r is an integer of 0 to 6, and t and t' are 2 or 3), or the following general formula (ii): -C ｔ F ２ｔ [OCF ２ CF (CF ３ )] ｕ (OCF ２ ) ｖ O.C. ｔ’ F ２ｔ’ -(ii) (wherein u is an integer of 1 to 200, v is an integer of 1 to 50, and t and t' are the same as above), and z is independently 0 or 1.

[0055] Rf １ The lower limit of the degree of polymerization (p+q) or (u+v) of the divalent perfluoropolyether group represented by the formula (I) is not particularly limited as long as it is an integer of 2 or more, but in order to provide low contamination and low residual properties, it is preferably 30 or more, particularly preferably 80 or more.

[0056] R １When is a monovalent hydrocarbon group, examples thereof include monovalent hydrocarbon groups having 1 to 20 carbon atoms, and specific examples thereof include alkyl groups such as methyl, ethyl, propyl, butyl, hexyl, cyclohexyl, octyl, and decyl; alkenyl groups such as vinyl and allyl; aryl groups such as phenyl, tolyl, and naphthyl; and aralkyl groups such as benzyl and phenylethyl.

[0057] [(A-5) Fluorine-Containing Organohydrogenpolysiloxane] The fluorine-containing organohydrogenpolysiloxane of component (A-5) acts as a crosslinking agent or chain extender for component (A-4). For this purpose, the fluorine-containing organohydrogenpolysiloxane has two or more, preferably three or more, hydrogen atoms bonded to silicon atoms (i.e., hydrosilyl groups) per molecule. Component (A-5) can be used either alone or in combination of two or more.

[0058] From the viewpoint of compatibility with component (A-4), dispersibility, etc., the fluorine-containing organohydrogenpolysiloxane preferably has one or more groups selected from perfluoroalkyl groups, perfluoroalkylene groups, perfluorooxyalkyl groups, and perfluorooxyalkylene groups in each molecule. Examples of these perfluoro groups include those represented by the following general formula: Perfluoroalkyl group: C ｇ F ２ｇ＋１ (wherein g is an integer of 1 to 20, preferably 2 to 10) Perfluoroalkylene group: —C ｇ F ２ｇ (wherein g is the same as above) Perfluorooxyalkyl group: (wherein f is an integer of 2 to 200, preferably 2 to 100, and h is an integer of 1 to 3) Perfluorooxyalkylene group: (wherein i and j are integers of 1 or more, and the average of i+j is 2 to 200, preferably 2 to 100) -(CF ２ CF ２ O) ｋ (CF ２O) Ｌ CF ２ (wherein k and L each represent an integer of 1 or more, and the average of k+L is 2 to 200, preferably 2 to 100.)

[0059] The divalent linking group connecting these perfluoroalkyl groups, perfluorooxyalkyl groups, perfluoroalkylene groups, or perfluorooxyalkylene groups to the silicon atom may be an alkylene group, an arylene group, or a combination thereof, or these groups may be joined by an ether bond, an amide bond, a carbonyl bond, or the like, and examples thereof include -CH ２ CH ２ -, -CH ２ CH ２ CH ２ -, -CH ２ CH ２ CH ２ OCH ２ -, -CH ２ CH ２ CH ２ -NH-CO-, -CH ２ CH ２ CH ２ -N(Ph)-CO-, -CH ２ CH ２ CH ２ -N(CH ３ ) -CO-, -CH ２ CH ２ CH ２ -O-CO-, -Ph'-N(CH ３ )-CO- (where Ph represents a phenyl group and Ph' represents a phenylene group) and the like.

[0060] In the fluorine-containing organohydrogenpolysiloxane of component (A-5), examples of the monovalent fluorine-containing substituent, that is, the monovalent substituent bonded to a silicon atom other than an organic group containing a perfluoroalkyl group or a perfluorooxyalkyl group, include unsubstituted hydrocarbon groups such as hydrocarbon groups having 1 to 20 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl, butyl, hexyl, cyclohexyl, octyl, and decyl; alkenyl groups such as vinyl and allyl; aryl groups such as phenyl, tolyl, and naphthyl; and aralkyl groups such as benzyl and phenylethyl. Also included are substituted hydrocarbon groups in which at least a portion of the hydrogen atoms of these hydrocarbon groups have been substituted with chlorine atoms or the like, such as chloromethyl, chloropropyl, and cyanoethyl.

[0061] The fluorine-containing organohydrogenpolysiloxane of component (A-5) may be cyclic, linear, three-dimensional network, or a combination thereof. The number of silicon atoms in this fluorine-containing organohydrogenpolysiloxane is not particularly limited, but is usually about 3 to 60, and preferably about 4 to 30.

[0062] The molecular weight of the fluorine-containing organohydrogenpolysiloxane of component (A-5) is preferably in the range of 750 to 3,000, and more preferably 800 to 2,400.

[0063] The amount of component (A-5) added is an effective amount necessary to cure component (A-4) and obtain a cured product with the required rubber elasticity. In particular, the amount is such that 0.5 to 5.0 moles, and preferably 1.0 to 2.0 moles, of hydrosilyl groups in component (A-5) are supplied per mole of the total alkenyl groups in component (A-4) in the composition. If the amount added is sufficient, the degree of crosslinking will be sufficient; if it is not too high, chain extension will not be prioritized, resulting in sufficient curing, no foaming during curing, and no deterioration in the heat resistance, etc., of the resulting cured coating.

[0064] [(A-6) Platinum Group Metal Curing Catalyst] The platinum group metal curing catalyst of component (A-6) is a catalyst for promoting the addition reaction between the alkenyl group derived from component (A-4) and the hydrosilyl group derived from component (A-5), and examples thereof include catalysts well known as catalysts used in hydrosilylation reactions. Specific examples include the same curing catalysts as those exemplified for component (A-3) above.

[0065] The amount of component (A-6) used may be an effective amount as a catalyst, for example, 0.1 to 500 ppm (by mass) of platinum group metals relative to component (A-4) is preferred.

[0066] [Fibrous Thermally Conductive Filler (B)] The component (B) of the filler-containing resin composition of the present invention is two or more types of fibrous thermally conductive fillers having different average fiber diameters. The upper limit of the number of different average fiber diameters in the component (B) is not particularly limited, but can be, for example, four or less.

[0067] The shape of the fibrous thermally conductive filler is not particularly limited as long as it is fibrous, and examples thereof include fibers, needles, whiskers, and the like.

[0068] The size of the fibrous thermally conductive filler may be selected appropriately as needed and is not particularly limited, but it is preferable that the aspect ratio (average major axis length / average minor axis length) of the filler be 2 or more and less than 30, and more preferably 5 or more and less than 30.

[0069] If such a material is used, the fibrous thermally conductive filler particles will not become entangled with each other, improving the packing properties and the thermal conductivity.

[0070] The average major axis length and the average minor axis length of the fibrous thermally conductive filler can be measured using a scanning electron microscope (SEM), and the aspect ratio can be calculated from these values. For example, the aspect ratio of carbon fibers is the value of "average fiber length / average fiber diameter" of the carbon fibers.

[0071] The average fiber diameter of the fibrous thermally conductive filler is not particularly limited, but is preferably 0.1 to 20 μm, more preferably 0.3 to 15 μm. Preferably, one of the fibrous thermally conductive fillers is a carbon fiber having an average fiber diameter of 8 μm or more.

[0072] Such a fibrous thermally conductive filler has a high thermal conductivity in the major axis direction of the fibrous thermally conductive filler, and therefore the cross-sectional area in the horizontal axis direction is large, thereby efficiently increasing the thermal conductivity.

[0073] Furthermore, carbon has a lower specific gravity than common thermally conductive fillers such as alumina and aluminum nitride, and is therefore preferable for reducing the specific gravity.

[0074] At least one of the fibrous thermally conductive fillers is preferably a carbon fiber having an average fiber diameter of 1 μm or more and less than 8 μm, or an average fiber diameter of less than 1 μm.

[0075] Such fibrous thermally conductive fillers are more likely to be disposed between fillers having an average fiber diameter of 8 μm or more, further increasing the thermal conductivity.

[0076] The present invention is also characterized by containing two or more types of fibrous thermally conductive fillers with different average fiber diameters. The ratio of the maximum diameter to the minimum diameter of these average fiber diameters is 2:1 to 45:1, and preferably 3:1 to 40:1. A ratio of less than 2:1 is undesirable because interference between fibers is not sufficiently reduced, while a ratio of more than 45:1 is undesirable because the compound viscosity increases and it becomes difficult to increase the filling ratio of the minimum fiber diameter.

[0077] The content of the fibrous thermally conductive filler (component (B)) in the thermally conductive sheet of the present invention is preferably 20 to 60 mass %, more preferably 30 to 52 mass %, because this range provides a good balance between high thermal conductivity and low specific gravity.

[0078] It is also preferred that the major axis direction of the fibrous thermally conductive filler (component (B)) is oriented in the thickness direction of the thermally conductive sheet obtained by curing the composition.

[0079] Such a heat conductive sheet is preferable because it further shortens the heat transfer path in the thickness direction of the sheet.

[0080] [Other Components] The filler-containing resin composition may further contain, as necessary, components such as a solvent, a thixotropy-imparting agent, a dispersant, a curing agent, a curing accelerator, a retarder, a slight tackifier, a plasticizer, a flame retardant, an antioxidant, a stabilizer, and a colorant.

[0081] The filler-containing resin composition may also contain a filler other than the component (B). Specific examples include silica and titanium oxide. The amount of the filler to be added is not particularly limited, but is preferably 0 to 60 parts by mass, and more preferably 0 to 40 parts by mass, per 100 parts by mass of the component (A).

[0082] [Method for producing filler-containing resin composition] The filler-containing resin composition can be produced by mixing the polymer matrix, the fibrous thermally conductive filler, and, if necessary, the other components using a mixer or the like.

[0083] [Method for Producing Thermally Conductive Sheet] The method for producing the thermally conductive sheet of the present invention includes the steps of molding and curing the filler-containing resin composition, and examples thereof include the following methods.

[0084] There is a method for obtaining a thermally conductive sheet having excellent thermal conductivity in the thickness direction and a low specific gravity, comprising: (I-1) a preparation step of preparing a filler-containing resin composition containing the polymer matrix (A) and the thermally conductive filler (B); (I-2) a filling step of filling a mold with the filler-containing resin composition; (I-3) a molding step of curing the filler-containing composition filled in the mold by heating or the like to obtain a filler-containing molded body; and (I-4) a slicing step of slicing the filler-containing molded body to obtain a thermally conductive sheet.

[0085] Each of the above steps will be described in detail below.

[0086] Preparation Step (I-1) In this step, a filler-containing resin composition containing the polymer matrix (A) and the fibrous thermally conductive filler (B) is prepared according to the method for producing the filler-containing resin composition.

[0087] Filling Step (I-2) The prepared filler-containing resin composition is filled into a mold measuring 40 mm in the X-axis direction (width direction) × 40 mm in the Y-axis direction (length direction) × 80 mm in the Z-axis direction (height direction) using a dispenser. (Filling Method) (i) The composition is discharged in a uniform manner so that the discharge direction is parallel to the X-axis direction of the mold, and then stacked in the Z-axis direction and filled. (ii) The composition is discharged in a uniform manner so that the discharge direction is parallel to the Y-axis direction of the mold, and then stacked in the Z-axis direction and filled. In this way, a molded body in which the fibrous thermally conductive filler or the like is oriented can be obtained more preferably and efficiently. The number of times the composition is stacked in the Z-axis direction is preferably two or more, and more preferably 20 or more.

[0088] Molding Method (I-3) In the molding step, a filler-containing molded body can be obtained by heating the filler-containing resin composition filled in the mold. The curing conditions for the filler-containing resin composition are optimized depending on the type of resin in the filler-containing resin composition. For example, the following conditions are preferred for curing a filler-containing molded body obtained from a filler-containing resin composition using a silicone resin. The curing temperature is preferably 0 to 200°C, more preferably 10 to 150°C. The curing time is preferably 1 minute to 24 hours, more preferably 5 minutes to 1 hour.

[0089] Slicing Step (I-4) The slicing step is a step of slicing the filler-containing molded body obtained in the molding step to obtain a thermally conductive sheet of a desired thickness. Specifically, the slicing step involves the following steps (i) or (ii): (i) discharging the filler-containing molded body into a mold so that the discharging direction is parallel to the X-axis direction, and stacking it in the Z-axis direction. The filled filler-containing molded body is then parallel to the Y-Z plane; (ii) discharging the filler-containing molded body into a mold so that the discharging direction is parallel to the Y-axis direction, and stacking it in the Z-axis direction. The filled filler-containing molded body is then parallel to the X-Z plane. By slicing the molded body to a desired thickness, a thermally conductive sheet in which the major axis direction of the fibrous thermally conductive filler is oriented in the thickness direction can be obtained.

[0090] The present invention will be described in detail below, but the present invention is not limited thereto.

[0091] The kinematic viscosity is a value measured at 25°C using a Cannon-Fenske viscometer according to JIS Z 8803:2011, the average fiber length, average fiber diameter, and aspect ratio are determined by SEM measurement, and the average particle size is a cumulative average particle size (median diameter) on a volume basis measured using a Microtrac MT3300EXII particle size analyzer manufactured by Microtrac Bell Corporation.

[0092] <Addition Reaction Type Organopolysiloxane Composition> The components of the addition reaction type organopolysiloxane composition are the following (A-1) to (A-3).

[0093] Component (A-1) Organopolysiloxane represented by the following formula: 97.1% by mass In the above formula, n is the kinematic viscosity at 25°C, expressed as 30,000 mm ２ / s.

[0094] Component (A-2) Organohydrogenpolysiloxane represented by the following formula: 2.2% by mass In the above formula, o=27 and p=3, each of which is the average degree of polymerization.

[0095] Component (A-3) 5% chloroplatinic acid 2-ethylhexanol solution: 0.7% by mass

[0096] A filler-containing resin composition of the present invention was produced as follows.

[0097] <Filler-Containing Organopolysiloxane Composition A> A filler-containing organopolysiloxane composition A (specific gravity 1.32) was obtained by mixing 55.0 mass% of an addition reaction type organopolysiloxane composition (specific gravity 1.0), 30.4 mass% of carbon fiber A (average fiber length 250 μm, average fiber diameter 10 μm, aspect ratio 25, specific gravity 2.2), 3.6 mass% of carbon fiber B (average fiber length 40 μm, average fiber diameter 3 μm, aspect ratio 13, specific gravity 2.2), and 11.0 mass% of carbon fiber C (average fiber length 10 μm, average fiber diameter 0.5 μm, aspect ratio 20, specific gravity 2.2).

[0098] <Filler-Containing Organopolysiloxane Composition B> Filler-containing organopolysiloxane composition B (specific gravity 1.32) was obtained by mixing 55.0 mass% of an addition reaction type organopolysiloxane composition (specific gravity 1.0), 34.0 mass% of carbon fiber A (average fiber length 250 μm, average fiber diameter 10 μm, aspect ratio 25, specific gravity 2.2), and 11.0 mass% of carbon fiber B (average fiber length 40 μm, average fiber diameter 3 μm, aspect ratio 13, specific gravity 2.2).

[0099] <Filler-Containing Organopolysiloxane Composition C> A filler-containing organopolysiloxane composition C (specific gravity 1.32) was obtained by mixing 55.0 mass% of an addition reaction type organopolysiloxane composition (specific gravity 1.0), 34.0 mass% of carbon fiber A (average fiber length 250 μm, average fiber diameter 10 μm, aspect ratio 25, specific gravity 2.2), and 11.0 mass% of carbon fiber C (average fiber length 10 μm, average fiber diameter 0.5 μm, aspect ratio 20, specific gravity 2.2).

[0100] <Filler-Containing Organopolysiloxane Composition D> Filler-containing organopolysiloxane composition D (specific gravity 1.32) was obtained by mixing 55.0 mass% of an addition reaction type organopolysiloxane composition (specific gravity 1.0) and 45.0 mass% of carbon fiber D (average fiber length 300 μm, average fiber diameter 10 μm, aspect ratio 30, specific gravity 2.2).

[0101] <Filler-Containing Organopolysiloxane Composition E> A filler-containing organopolysiloxane composition E (specific gravity 1.32) was obtained by mixing 55.0 mass% of an addition reaction type organopolysiloxane composition (specific gravity 1.0), 34.0 mass% of carbon fiber A (average fiber length 250 μm, average fiber diameter 10 μm, aspect ratio 25, specific gravity 2.2), and 11.0 mass% of carbon fiber E (average fiber length 50 μm, average fiber diameter 10 μm, aspect ratio 5, specific gravity 2.2).

[0102] <Filler-Containing Organopolysiloxane Composition F> Filler-containing organopolysiloxane composition F (specific gravity 1.42) was obtained by mixing 45.6 mass% of an addition reaction type organopolysiloxane composition (specific gravity 1.0), 34.0 mass% of carbon fiber A (average fiber length 250 μm, average fiber diameter 10 μm, aspect ratio 25, specific gravity 2.2), and 20.4 mass% of carbon fiber E (average fiber length 50 μm, average fiber diameter 10 μm, aspect ratio 5, specific gravity 2.2).

[0103] <Filler-Containing Organopolysiloxane Composition G> A filler-containing organopolysiloxane composition G (specific gravity 1.3) was obtained by mixing 55.0 mass% of an addition reaction type organopolysiloxane composition (specific gravity 1.0) and 45.0 mass% of carbon fiber B (average fiber length 40 μm, average fiber diameter 3 μm, aspect ratio 13, specific gravity 2.2).

[0104] <Filler-Containing Organopolysiloxane Composition H> Filler-containing organopolysiloxane composition F (specific gravity 1.3) was obtained by mixing 55.0 mass% of an addition reaction type organopolysiloxane composition (specific gravity 1.0) and 45.0 mass% of carbon fiber C (average fiber length 10 μm, average fiber diameter 0.5 μm, aspect ratio 20, specific gravity 2.2).

[0105] Using the above filler-containing organopolysiloxane compositions A to H, thermally conductive sheets were produced as follows.

[0106] Example 1 The above filler-containing organopolysiloxane composition A was filled into a container using a dispenser (MOHNO MASTER V2) manufactured by Musashi Engineering Co., Ltd., with a nozzle having an inner diameter of 1.64 mm, so that the components were arranged in the Y-axis direction, forming 24 rows in the X-axis direction, and 24 columns in the Z-axis direction, and then heated at 80°C for 20 minutes to obtain a filler-containing molded body 1. The cured product, which was the filler-containing molded body, was sliced ​​parallel to the X-Z plane with a cutter knife to obtain a thermally conductive sheet 1 having a thickness of 2 mm.

[0107] Example 2 A thermally conductive sheet 2 was obtained in the same manufacturing method as in Example 1, except that the filler-containing organopolysiloxane composition was changed to the filler-containing organopolysiloxane composition B.

[0108] Example 3 A thermally conductive sheet 3 was obtained in the same manufacturing method as in Example 1, except that the filler-containing organopolysiloxane composition was changed to the filler-containing organopolysiloxane composition C.

[0109] Comparative Example 1 An attempt was made to produce a thermally conductive sheet using the same production method as in Example 1, except that the filler-containing organopolysiloxane composition was changed to the filler-containing organopolysiloxane composition D. However, the composition could not be ejected from the nozzle, and a thermally conductive sheet could not be obtained.

[0110] Comparative Example 2 A thermally conductive sheet 4 was obtained in the same manner as in Example 1, except that the filler-containing organopolysiloxane composition was changed to the filler-containing organopolysiloxane composition E.

[0111] Comparative Example 3 A thermally conductive sheet 5 was obtained in the same manner as in Example 1, except that the filler-containing organopolysiloxane composition was changed to the filler-containing organopolysiloxane composition F.

[0112] Comparative Example 4 A thermally conductive sheet 6 was obtained in the same manner as in Example 1, except that the filler-containing organopolysiloxane composition was changed to the filler-containing organopolysiloxane composition G.

[0113] Comparative Example 5 A thermally conductive sheet 7 was obtained in the same manner as in Example 1, except that the filler-containing organopolysiloxane composition was changed to the filler-containing organopolysiloxane composition H.

[0114] <Evaluation Items> The thermal conductivity and specific gravity of each of the thermally conductive sheets obtained in Examples 1 to 3 and Comparative Examples 2 to 5 were measured as follows.

[0115] <Thermal Conductivity> Each of the thermally conductive sheets obtained in Examples 1 to 3 and Comparative Examples 2 to 5 was punched out to form a circle with a diameter of 12.7 mm, and the thermal conductivity of each test piece was measured using a laser flash method (LFA 447 Nanoflash, manufactured by Netzsch GmbH) in accordance with JIS R 1611: 2010. The results are shown in Table 1 below.

[0116] <Specific Gravity> The specific gravity of each of the thermally conductive sheets obtained in Examples 1 to 3 and Comparative Examples 2 to 5 was measured at 25° C. in accordance with JIS K 6249: 2003. The results are shown in Table 1 below.

[0117]

[0118] Examples 1, 2, and 3 all had a carbon fiber content (total) of 45.0% and contained two or more types of carbon fibers with different average fiber diameters, with an average fiber diameter of 8 μm or more and 1 μm or more but less than 8 μm or less than 1 μm. The ratio of the maximum diameter to the minimum diameter of the average fiber diameter was within a range of 2:1 to 45:1. Example 1 had a thermal conductivity of 32.1 W / m K and a specific gravity of 1.30, Example 2 had a thermal conductivity of 26.0 W / m K and a specific gravity of 1.31, and Example 3 had a thermal conductivity of 28.1 W / m K and a specific gravity of 1.31.

[0119] Comparative Example 2 (carbon fibers A and E with an average fiber diameter of 10 μm), Comparative Example 4 (carbon fiber B with an average fiber diameter of 3 μm), and Comparative Example 5 (carbon fiber C with an average fiber diameter of 0.5 μm) had a carbon fiber content (total) of 45.0%, but all contained only one type of carbon fiber average fiber length. Comparative Example 2 had a thermal conductivity of 17.1 W / m·K and a specific gravity of 1.31, Comparative Example 4 had a thermal conductivity of 8.9 W / m·K and a specific gravity of 1.30, and Comparative Example 5 had a thermal conductivity of 4.7 W / m·K and a specific gravity of 1.31, which were lower than Examples 1 to 3. This result indicates that even if the loading amount of fibrous thermally conductive filler is the same, the thermal conductivity is increased by including two or more types of fibrous thermally conductive fillers with different average fiber diameters.

[0120] In Comparative Example 1, which had a carbon fiber content (total) of 45.0% but a single type of average fiber diameter (carbon fiber D with an average fiber diameter of 10 μm and an aspect ratio of 30), dispenser discharge was not possible in the thermal conductive sheet manufacturing process, and a sheet was not formed. This result indicates that if the average fiber diameter is 8 μm or more and the aspect ratio is 30 or more, the compound viscosity increases, making it difficult to achieve a high loading amount.

[0121] Comparative Example 3 (carbon fibers A and E with an average fiber diameter of 10 μm), which had a carbon fiber content (total) of 54.4 mass%, higher than those of Examples 1 to 3, had a single type of carbon fiber with an average fiber diameter of 10 μm, a thermal conductivity of 20.6 W / m·K, and a specific gravity of 1.40, which was lower than those of Examples 1 to 3. This result means that even if the loading amount of fibrous thermally conductive filler is low, the thermal conductivity can be increased by containing two or more types of fibrous thermally conductive fillers with different average fiber diameters and with a maximum / minimum average fiber diameter ratio in the range of 2:1 to 45:1.

[0122] Thus, the present invention can provide a thermally conductive sheet that has excellent thermal conductivity and a low specific gravity by filling the sheet with two or more types of fibrous conductive fillers that have different average fiber diameters and whose maximum / minimum average fiber diameter ratio is in the range of 2:1 to 45:1.

[0123] This specification includes the following aspects. [1] A thermally conductive sheet comprising a polymer matrix (A) and two or more types of fibrous thermally conductive fillers (B) having different average fiber diameters, wherein the ratio of the maximum diameter to the minimum diameter of the average fiber diameter is 2:1 to 45:1. [2] The thermally conductive sheet according to [1], wherein the thermal conductivity in the thickness direction of the thermally conductive sheet is 25 W / m·K or more and the specific gravity is less than 1.4. [3] The thermally conductive sheet according to [1] or [2], wherein the aspect ratio of the fibrous thermally conductive filler (B) is 2 or more and less than 30. [4] The thermally conductive sheet according to any one of [1] to [3], wherein at least one type of the fibrous thermally conductive filler (B) is a carbon fiber having an average fiber diameter of 8 μm or more. [5] The thermally conductive sheet according to any one of [1] to [4], wherein one or more of the fibrous thermally conductive fillers (B) are carbon fibers having an average fiber diameter of 1 μm or more but less than 8 μm or an average fiber diameter of less than 1 μm. [6] The thermally conductive sheet according to any one of [1] to [5], wherein the proportion of the fibrous thermally conductive filler (B) contained in the thermally conductive sheet is 20 to 60 mass%. [7] The thermally conductive sheet according to any one of [1] to [6], wherein the major axes of the fibrous thermally conductive fillers (B) are oriented in the thickness direction of the thermally conductive sheet. [8] The thermally conductive sheet according to any one of [1] to [7], wherein the polymer matrix (A) is a cured product of a curable polymer composition containing one or more selected from the group consisting of organopolysiloxane, polyurethane, polyacrylate, perfluoropolyether elastomer, unsaturated polyester resin, polyamide resin, epoxy resin, and phenolic resin.

[0124] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits similar effects is included within the technical scope of the present invention.

Claims

1. A thermally conductive sheet comprising a polymer matrix (A) and two or more types of fibrous thermally conductive fillers (B) having different average fiber diameters, the ratio of the maximum diameter to the minimum diameter of the average fiber diameter being 2:1 to 45:

1.

2. The thermally conductive sheet according to claim 1, characterized in that the thermal conductivity of the thermally conductive sheet in the thickness direction is 25 W / m·K or more and the specific gravity is less than 1.

4.

3. The thermally conductive sheet according to claim 1, characterized in that the aspect ratio of the fibrous thermally conductive filler (B) is 2 or more and less than 30.

4. The thermally conductive sheet according to claim 1, characterized in that at least one of the fibrous thermally conductive fillers (B) is a carbon fiber having an average fiber diameter of 8 μm or more.

5. The thermally conductive sheet according to claim 1, characterized in that one or more of the fibrous thermally conductive fillers (B) is a carbon fiber having an average fiber diameter of 1 μm or more and less than 8 μm, or an average fiber diameter of less than 1 μm.

6. The thermally conductive sheet according to claim 1, characterized in that the proportion of the fibrous thermally conductive filler (B) contained in the thermally conductive sheet is 20 to 60 mass %.

7. The thermally conductive sheet according to claim 1, characterized in that the long axis direction of the fibrous thermally conductive filler (B) is oriented in the thickness direction of the thermally conductive sheet.

8. The thermally conductive sheet according to claim 1, characterized in that the polymer matrix (A) is a cured product of a curable polymer composition containing one or more members selected from the group consisting of organopolysiloxane, polyurethane, polyacrylate, perfluoropolyether elastomer, unsaturated polyester resin, polyamide resin, epoxy resin, and phenolic resin.

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