WO2025220439A1 - Thermally conductive silicone potting composition and cured product thereof - Google Patents

Abstract

[Problem] To provide a thermally conductive silicone potting composition that has low viscosity and high flowability in spite of containing a large amount of a thermally conductive filler, is capable of flowing into a minute space, and exhibits a desired thermal conductivity after being cured. [Solution] The thermally conductive silicone potting composition comprises: (A) 100 parts by mass of a linear organopolysiloxane having, on average, 0.5 to 1.8 alkenyl groups bonded to a silicon atom at a molecular chain terminal per molecule; (B) 500-2,000 parts by mass of a thermally conductive filler; (C) 0.1 to 100 parts by mass of an organohydrogensiloxane having at least two SiH groups in one molecule; and (D) a hydrosilylation reaction catalyst.

Description

Thermally conductive silicone potting composition and its cured product

The present invention relates to a thermally conductive silicone potting composition and its cured product.

Due to growing awareness of global warming, the automotive industry is developing environmentally friendly vehicles such as hybrid vehicles, plug-in hybrid vehicles, and electric vehicles with the aim of reducing greenhouse gas emissions. In order to improve the fuel efficiency of these vehicles, the inverters installed in these vehicles are becoming more powerful and smaller.
Accordingly, components such as ICs and reactors within inverters have become smaller, resulting in increased heat generation. Conventionally, to protect such heat-generating components, the cooling efficiency of the components has been improved by interposing a thermally conductive silicone composition such as a thermally conductive silicone grease composition, a thermally conductive silicone gel composition, or a thermally conductive silicone potting composition between the heat-generating component and the cooler.

For example, Patent Document 1 proposes a method in which a cooler and a heat-generating component are assembled in advance, and then a highly fluid, thermally conductive silicone potting composition is poured into the assembly to thermally connect the heat-generating component and the cooler.
However, when practical fluidity is maintained using the method of Patent Document 1, the thermal conductivity is limited to approximately 1.0 W/m·K, which is insufficient to deal with the further increase in heat generation that has accompanied the recent trend toward miniaturization of devices and miniaturization of components.

As a solution to this problem, Patent Documents 2 to 5 propose silicone potting compositions that contain large amounts of thermally conductive fillers to achieve high thermal conductivity while also maintaining high fluidity.

However, although these compositions have high fluidity, they also have high viscosity, and with the recent trend toward smaller devices and finer components, it can be difficult to pot every corner of the components, and they have not been able to impart sufficient heat dissipation properties.
Therefore, in order to improve the performance of inverters, there is a strong demand for a thermally conductive silicone potting material that not only has high thermal conductivity and high fluidity, but also has a lower viscosity.

JP 2011-122000 A JP 2016-084378 A Japanese Patent Application Laid-Open No. 2019-077843 Japanese Patent Application Laid-Open No. 2019-077845 Japanese Patent Application Laid-Open No. 2021-113290

The present invention has been made in consideration of the above circumstances, and aims to provide a thermally conductive silicone potting composition that, despite containing a large amount of thermally conductive filler, has low viscosity and high flowability, can flow into minute spaces, and has the desired thermal conductivity after curing.

As a result of extensive research into achieving the above objectives, the inventors discovered that by using a base polymer with a specified amount of functional groups, it is possible to obtain a thermally conductive silicone potting composition that has low viscosity and high flowability, despite containing a large amount of thermally conductive filler, and thus completed the present invention.

That is, the present invention is
1. A thermally conductive silicone potting composition comprising: (A) 100 parts by weight of a linear organopolysiloxane having an average of 0.5 to 1.8 alkenyl groups bonded to silicon atoms at the molecular chain terminals per molecule; (B) 500 to 2,000 parts by weight of a thermally conductive filler; (C) 0.1 to 100 parts by weight of an organohydrogensiloxane having at least two SiH groups per molecule; and (D) a hydrosilylation reaction catalyst;
2. The thermally conductive silicone potting composition of 1, wherein the linear organopolysiloxane of component (A) has a viscosity of 10 to 500 mPa·s at 25°C as measured with a Brookfield type rotational viscometer.
3. (E) A thermally conductive silicone potting composition according to item 1, which comprises a silane compound represented by the following general formula (1) and an organopolysiloxane represented by the following general formula (2), in an amount of 0.1 to 30 parts by mass per 100 parts by mass of component (A):
(In the formula, ^R1 's each independently represent a monovalent hydrocarbon group, and n represents an integer of 0 to 30.)
4. The thermally conductive silicone potting composition of 3, wherein the thermally conductive filler (B) is surface-treated with component (E).
5. The thermally conductive silicone potting composition of 1, having a viscosity at 25°C of 30 Pa·s or less.
6. A cured product is provided by curing the thermally conductive silicone potting composition of any one of 1 to 5.

Before curing, the thermally conductive silicone potting composition of the present invention has low viscosity and high flowability, allowing it to flow into minute spaces. After curing, it achieves the desired thermal conductivity and is able to protect heat-generating components without peeling off.
For this reason, the composition of the present invention is effective for potting when it is fixed to a cooler of a component having a fine structure, such as a transformer. In such components, after hardening, the composition has high thermal conductivity and can efficiently transfer heat from the component to the cooler, making it highly reliable even when used in high-temperature environments.

The present invention will be specifically described below.
The thermally conductive silicone potting composition of the present invention cures at room temperature or under heat and adheres to metals, organic resins, etc., and contains the following essential components (A) to (D):
(A) alkenyl group-containing organopolysiloxane (B) thermally conductive filler (C) organohydrogensiloxane (D) hydrosilylation reaction catalyst

[1] Component (A) Component (A) is a linear organopolysiloxane having an average of 0.5 to 1.8 alkenyl groups per molecule bonded to silicon atoms at the molecular chain terminals.
The average number of alkenyl groups per molecule of component (A) is preferably 0.8 to 1.8, and more preferably 1.0 to 1.6. If the number of alkenyl groups is less than 0.5, the strength of the cured product will be low or the composition will not cure, while if the number exceeds 1.8, the flowability of the composition will be reduced.

The viscosity of component (A) at 25°C is preferably 10 to 500 mPa·s, more preferably 10 to 200 mPa·s, and even more preferably 10 to 100 mPa·s. If the viscosity at 25°C is 10 mPa·s or higher, the composition will have excellent storage stability, and if it is 500 mPa·s or lower, the composition will have good flowability. The above viscosity is measured using a B-type rotational viscometer (the same applies hereinafter).

The alkenyl group bonded to the silicon atom is not particularly limited, but is preferably an alkenyl group having 2 to 10 carbon atoms, and more preferably an alkenyl group having 2 to 8 carbon atoms.
Specific examples thereof include vinyl, allyl, 1-butenyl, and 1-hexenyl groups, and among these, vinyl groups are preferred from the standpoint of ease of synthesis and cost.

The organic group bonded to the silicon atom other than the alkenyl group is not particularly limited, but is preferably a monovalent hydrocarbon group having 1 to 20 carbon atoms and excluding aliphatic unsaturated bonds, and more preferably a monovalent hydrocarbon group having 1 to 10 carbon atoms and excluding aliphatic unsaturated bonds.
Specific examples thereof include alkyl groups such as methyl, ethyl, n-propyl, n-butyl, n-hexyl, and n-dodecyl; aryl groups such as phenyl; and aralkyl groups such as 2-phenylethyl and 2-phenylpropyl.
In addition, some or all of the hydrogen atoms of these hydrocarbon groups may be substituted with halogen atoms such as chlorine, fluorine, or bromine, and specific examples thereof include halogen-substituted monovalent hydrocarbon groups such as fluoromethyl, 2-bromoethyl, chloromethyl, and 3,3,3-trifluoropropyl groups.
Among these, the organic groups bonded to silicon atoms other than alkenyl groups are preferably alkyl groups having 1 to 5 carbon atoms, and more preferably 90 mol % or more of which are methyl groups from the standpoint of ease of synthesis and cost.

Component (A) preferably does not contain an organoxysilyl group, and examples of organoxy groups include alkoxy groups, alkoxyalkoxy groups, alkenyloxy groups, and acyloxy groups.
The alkoxy group includes those having 1 to 10 carbon atoms, preferably 1 to 6 carbon atoms, and more preferably 1 to 3 carbon atoms, such as a methoxy group and an ethoxy group.
The alkoxyalkoxy group includes those in which each alkoxy group has 1 to 6 carbon atoms, preferably 1 to 3 carbon atoms, such as a methoxyethoxy group and a methoxypropoxy group.
The alkenyloxy group may have 2 to 6 carbon atoms, such as a vinyloxy group or an allyloxy group.
The acyloxy group may have 1 to 10 carbon atoms, such as an acetyloxy group or an octanoyloxy group.

[2] Component (B) Component (B) is a thermally conductive filler that serves to impart thermal conductivity to the composition.
As the thermally conductive filler, conventionally known ones can be used, and specific examples thereof include aluminum powder, copper powder, silver powder, nickel powder, gold powder, alumina powder, zinc oxide powder, magnesium oxide powder, aluminum nitride powder, aluminum hydroxide powder, boron nitride powder, silicon nitride powder, diamond powder, carbon powder, indium, gallium, etc. Such thermally conductive fillers may be used alone or in combination of two or more.
The thermally conductive filler preferably has a thermal conductivity of 10 W/m·K or more in order to impart sufficient thermal conductivity to the composition.
The average particle size of the thermally conductive filler is preferably 0.1 to 100 μm, more preferably 0.5 to 90 μm. Within this range, aggregation of the thermally conductive filler particles is suppressed, resulting in excellent fluidity. Note that the average particle size in the present invention is the median diameter (D50) in the volume-based particle size distribution determined by laser diffraction method.
The thermally conductive filler may have any shape, such as an irregular shape or a spherical shape.

The amount of component (B) blended is 500 to 2,000 parts by mass, preferably 800 to 1,500 parts by mass, per 100 parts by mass of component (A). If it is less than 500 parts by mass, sufficient thermal conductivity may not be achieved, and if it exceeds 2,000 parts by mass, the composition may become highly viscous or its flowability may decrease.

[3] Component (C) Component (C) is an organohydrogensiloxane having at least two, preferably three or more, and more preferably 3 to 100 hydrogen atoms bonded to silicon atoms (SiH groups) per molecule.
The molecular structure of the organohydrogensiloxane of component (C) may be linear, branched, or network, and multiple organohydrogensiloxane chains may be linked by linking groups. Silicon-bonded hydrogen atoms may be present either at the molecular chain terminals (both terminals or one terminal) or non-terminal portions, or may be present in both locations.

The organic groups other than hydrogen atoms bonded to the silicon atoms in component (C) include monovalent hydrocarbon groups having 1 to 10 carbon atoms, excluding alkenyl groups. Specific examples include alkyl groups such as methyl, ethyl, propyl, and butyl; aryl groups such as phenyl and tolyl; aralkyl groups such as phenylethyl and phenylpropyl; and halogenated alkyl groups in which some or all of the hydrogen atoms in these groups have been replaced with halogen atoms such as chlorine, fluorine, or bromine, such as gamma-chloropropyl and 3,3,3-trifluoropropyl. Among these, alkyl groups having 1 to 6 carbon atoms are preferred, and alkyl groups having 1 to 3 carbon atoms are more preferred. In particular, from the standpoint of ease of synthesis and cost, it is more preferable that 90 mol % or more of the organic groups other than hydrogen atoms be methyl groups.

The kinematic viscosity of component (C) at 25°C is not particularly limited, but is preferably 1 to 10,000 ^mm2 /s, and more preferably 1 to 1,000 ^mm2 /s. The kinematic viscosity is measured at 25°C using a Cannon-Fenske viscometer (the same applies hereinafter). Several components with different viscosities may be used in combination as component (C).

Component (C) may be a compound containing a cyclic organohydrogensiloxane represented by the following general formula (3-1) and/or a cyclic organohydrogensiloxane represented by the following general formula (3-2). These compounds serve to crosslink with component (A) and to impart adhesive properties.

In each of the above formulas, R2 ^'s are each independently an alkyl group having 1 to 6 carbon atoms, preferably 1 to 3 carbon atoms, and specific examples thereof include methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, etc. Among these, from the standpoint of ease of synthesis and cost, it is preferable that 90 mol % or more of R2 ^'s are methyl groups.

^R3 's are each independently an epoxy group or a trialkoxysilyl group bonded to the silicon atom via a carbon atom or via a carbon atom and an oxygen atom.
Examples of epoxy groups bonded to a silicon atom via a carbon atom or via a carbon atom and an oxygen atom include 3-glycidoxypropyl, 3-glycidoxyethyl, and 3,4-epoxycyclohexylethyl groups.
Examples of trialkoxysilyl groups bonded to a silicon atom via a carbon atom or via a carbon atom and an oxygen atom include trimethoxysilylpropyl, trimethoxysilylpropylmethyl, trimethoxysilylethyl, triethoxysilylpropyl, triethoxysilylpropylmethyl, and triethoxysilylethyl groups.

In formula (3-1), i is an integer of 2 or more, j is an integer of 1 or more, and i+j is an integer of 4 to 12, preferably an integer of 4 to 8, more preferably an integer of 4 to 6, and even more preferably 4.
The siloxane units in formula (3-1) may be arranged in any order, and may be arranged randomly, in blocks, or alternately.
In formula (3-2), X is a divalent hydrocarbon group which may contain an ether bond, and is preferably a group containing a bisphenol A residue represented by the following formula (4).
k's are each independently an integer of 3 to 11, preferably an integer of 3 to 7, more preferably an integer of 3 to 5, and even more preferably 3.

(The dashed lines represent bonds to the silicon atom.)

Specific examples of component (C) include, but are not limited to, organohydrogensiloxanes represented by the following formula. Component (C) may be used alone or in combination of two or more types.

In terms of curability and the mechanical properties of the cured product, the blending amount of component (C) is 0.1 to 100 parts by mass, and more preferably 1 to 20 parts by mass, per 100 parts by mass of component (A). Less than 0.1 parts by mass will result in insufficient curing, while more than 100 parts by mass may result in reduced mechanical properties of the cured product.

[4] Component (D) Component (D) is a hydrosilylation catalyst. Any hydrosilylation catalyst that promotes the addition reaction between the alkenyl group of component (A) and the Si—H group of component (C) can be used, and any conventionally known catalyst can be used. Specifically, platinum group metal catalysts are preferred, and among these, catalysts selected from platinum and platinum compounds are preferred.

_Specific examples of catalysts include platinum (including platinum black); platinum group metals such as rhodium _{and palladium ; H2PtCl4.nH2O , H2PtCl6.H2O , NaHPtCl6.nH2O , KHPtCl6.nH2O , Na2PtCl6.nH2O , K2PtCl4.nH2O , PtCl4.nH2O , PtCl2 , Na2HPtCl4.nH2O ,} chloroplatinic acid; chloroplatinic acid salts; alcohol-modified chloroplatinic acid; complexes of chloroplatinic acid with olefins; platinum group metals such as platinum black or 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 chloroplatinic acid salts with vinyl group-containing siloxanes, and the like, may be used alone or in combination of two or more.

The amount of component (D) to be added should be an effective amount as a catalyst, an amount that can promote the reaction between components (A) and (C), and can be adjusted appropriately depending on the desired curing rate.
In particular, an amount equivalent to 0.1 to 7,000 ppm, and more preferably 1 to 6,000 ppm, by mass, calculated as platinum group metal atoms relative to the mass of component (A), is preferred. When the blending amount of component (E) is within the above range, more efficient catalytic action can be expected.

[5] Component (E) The thermally conductive silicone potting composition of the present invention preferably contains, as component (E), a silane compound represented by the following general formula (1) and an organopolysiloxane represented by the following general formula (2). This component acts on the surface of the thermally conductive filler and has the role of improving fluidity.

(In the formula, ^R1 's each independently represent a monovalent hydrocarbon group, and n represents an integer of 0 to 30.)

The monovalent hydrocarbon group for ^R1 is not particularly limited, but is preferably a monovalent hydrocarbon group having 1 to 10 carbon atoms, more preferably a monovalent hydrocarbon group having 1 to 6 carbon atoms, and even more preferably a monovalent hydrocarbon group having 1 to 3 carbon atoms.
Specific examples of the monovalent hydrocarbon group include alkyl, alkenyl, aryl, and aralkyl groups, as well as halogenated monovalent hydrocarbon groups such as halogenated alkyl groups in which some or all of the hydrogen atoms of these monovalent hydrocarbon groups have been substituted with halogen atoms such as chlorine, fluorine, or bromine.
The alkyl group may be linear, branched, or cyclic, and specific examples thereof include linear alkyl groups such as methyl, ethyl, n-propyl, n-hexyl, and n-octyl groups; branched alkyl groups such as isopropyl, isobutyl, tert-butyl, and 2-ethylhexyl groups; and cyclic alkyl groups such as cyclopentyl and cyclohexyl groups.
Specific examples of the alkenyl group include vinyl, allyl, 1-butenyl, and 1-hexenyl groups.
Specific examples of the aryl group include phenyl and tolyl groups.
Specific examples of the aralkyl group include 2-phenylethyl and 2-methyl-2-phenylethyl groups.
Specific examples of halogenated alkyl groups include 3,3,3-trifluoropropyl, 2-(nonafluorobutyl)ethyl, and 2-(heptadecafluorooctyl)ethyl groups.
Among these, R ¹ is preferably a methyl group, a phenyl group, or a vinyl group, and more preferably a methyl group.

n is preferably an integer from 0 to 20, and more preferably an integer from 1 to 10.

When component (E) is used, the blending amount is 0.1 to 30 parts by mass, and more preferably 0.1 to 20 parts by mass, per 100 parts by mass of component (A).Within this range, a highly fluid composition is likely to be obtained.
The component (E) may be used alone or in combination of two or more types.

[7] Other Components In addition to the components (A) through (E) above, the thermally conductive silicone potting composition of the present invention may contain known additives, provided that the addition does not impair the object of the present invention.
Such additives include, for example, reaction inhibitors for the purpose of suppressing the curing reaction of the composition at room temperature and extending the shelf life and pot life.
The reaction inhibitor may be any known reaction inhibitor that can suppress the catalytic activity of component (D). Specific examples include acetylene alcohol compounds such as 1-ethynyl-1-cyclohexanol and 3-butyn-1-ol; nitrogen-containing compounds such as triallyl isocyanurate; organic phosphorus compounds; oxime compounds; and organic chloro compounds. These may be used alone or in combination of two or more. Of these, 1-ethynyl-1-cyclohexanol and triallyl isocyanurate are preferred.

When a reaction inhibitor is used, the amount added is preferably 0.01 to 5 parts by mass, and more preferably 0.05 to 1 part by mass, per 100 parts by mass of component (A), taking into consideration the shelf life and pot life of the composition, as well as the curability of the composition.
The reaction inhibitor may be diluted with an organic solvent such as toluene, xylene, or isopropyl alcohol to improve dispersibility in the composition.

In addition, the thermally conductive silicone potting composition of the present invention can also contain hindered phenol-based antioxidants, reinforcing and non-reinforcing fillers such as calcium carbonate, and colorants such as pigments and dyes.

The thermally conductive silicone potting composition of the present invention can be prepared by mixing the above-mentioned components (A) to (D), and optionally component (E) and other components, using a known method such as a gate mixer, kneader, or planetary mixer.

The thermally conductive silicone potting composition of the present invention may be a two-part composition in which a first part consisting of components (A), (B), and (D), and optionally component (E) and other components, and a second part consisting of components (A), (B), and (C), and optionally component (E) and other components, are prepared separately, and the first and second parts are mixed together before use. There may also be components that are shared by both the first and second parts. By forming the composition into such a two-part composition, storage stability can be further ensured.

The viscosity of the thermally conductive silicone potting composition of the present invention at 25°C is preferably 1 to 30 Pa·s, and more preferably 1 to 20 Pa·s, from the standpoints of dispersibility of the thermally conductive filler and workability. The viscosity is measured using a Brookfield type rotational viscometer at 20 rpm (approximate shear rate of 4.2 s ^-1 ).
Furthermore, the thermally conductive silicone potting composition preferably has a thixotropic index value of 1.3 or less after mixing and before curing. If the thixotropic index exceeds 1.3 after mixing and before curing, the composition may have poor fluidity, making it difficult for the composition to flow into heat-generating components with fine structures. Here, the thixotropic index is the viscosity measured at 10 rpm (approximate shear rate of 2.1 s ^-1 ) using a Brookfield viscometer divided by the viscosity measured at 20 rpm (approximate shear rate of 4.2 s ^-1 ).

The thermally conductive silicone potting composition of the present invention preferably has a flowability of 100 mm or more at 23°C, the measurement method of which will be described in detail in the Examples below. When pouring the silicone potting composition into a cooler where a component with a fine structure, such as a transformer, is attached, the flowability is preferably 120 mm or more. The higher the flowability, the better, but because the measurement limit depends on the length of the aluminum plate, the upper measurement limit here is 400 mm.

The curing conditions for the thermally conductive silicone potting composition of the present invention are not particularly limited, and can be the same as those for conventionally known silicone gels.
After pouring, the thermally conductive silicone potting composition may be cured by the heat from the heat-generating component, or may be actively heated and cured. The heat-curing conditions are preferably 60 to 180°C, more preferably 80 to 150°C, and preferably 0.1 to 3 hours, more preferably 0.5 to 2 hours.

The thermal conductivity of the cured product of the thermally conductive silicone potting composition of the present invention at an ambient temperature of 25° C. is preferably 1.5 W/m K or more, and more preferably 2.0 W/m K or more. There is no particular upper limit, but it is usually 10.0 W/m K or less.
The hardness of the cured product is preferably 10 or more as measured by a type A durometer, and although there is no particular upper limit, it is usually 80 or less.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. In the following examples, the weight average molecular weight is a polystyrene-equivalent value measured by gel permeation chromatography (GPC).
The components used in the examples and comparative examples are shown below.

(A) Component A-1: A dimethylpolysiloxane in which 80% of the terminals are blocked with dimethylvinylsilyl groups and 20% are blocked with trimethylsilyl groups, and which has a viscosity of 50 mPa·s at 25°C. A-2: A dimethylpolysiloxane in which both terminals are blocked with dimethylvinylsilyl groups, and which has a viscosity of 60 mPa·s at 25°C.

(B) Component B-1: Alumina powder with an average particle size of 1.0 μm B-2: Alumina powder with an average particle size of 10 μm B-3: Alumina powder with an average particle size of 40 μm B-4: Alumina powder with an average particle size of 80 μm

(C) Component C-1: organohydrogensiloxane represented by the following formula:

C-2: Organohydrogensiloxane having an average structure represented by the following formula:
(In the formula, the order of the siloxane units in parentheses is not specified.)

C-3: Cyclic organohydrogensiloxane represented by the following formula:

C-4: Cyclic organohydrogensiloxane derivative represented by the following formula:

(D) Component D-1: Dimethylpolysiloxane solution of platinum-divinyltetramethyldisiloxane complex (both ends of which are capped with dimethylvinylsilyl groups, dissolved in dimethylpolysiloxane having a viscosity of 0.6 Pa·s at 25°C; platinum concentration: 1% by mass)

(E) Component E-1: Trimethylsilanol

E-2: Organopolysiloxane represented by the following formula

(F) Other Components F-1: 1-ethynyl-1-cyclohexanol F-2: triallyl isocyanurate

[Examples 1 to 3, Comparative Examples 1 to 3]
Components (A) through (E) and other components were mixed as follows to obtain a silicone potting composition.
Component (A), component (B), and component (E) were added to a 5L gate mixer (manufactured by Inoue Seisakusho Co., Ltd., product name: 5L Planetary Mixer) in the amounts shown in Table 1, and mixed at 25°C for 1 hour, followed by mixing under reduced pressure at 150°C for 2 hours to obtain a mixture (for Example 2, a mixture in which component (B) was surface-treated with component (E)). After cooling the mixture, component (D) was added and the mixture was mixed at 25°C for 30 minutes. Next, reaction inhibitors (F-1) and (F-2) were added and the mixture was mixed at 25°C for 30 minutes. Finally, component (C) was added and the mixture was mixed at room temperature for 30 minutes.

The resulting composition was measured for the following physical properties, and the results are shown in Table 2.
[1] Viscosity The viscosity of the thermally conductive silicone potting composition at 25°C was measured using a Brookfield viscometer (TVB-10H, manufactured by Toki Sangyo Co., Ltd.) with a No. 7 rotor at 20 rpm (approximate shear rate of 4.2 s ^-1 ).
[2] Flowability 0.60 mL of the thermally conductive silicone potting composition was weighed out and dropped onto an aluminum plate (JIS H 4000:2022, width 25 mm x length 400 mm x thickness 0.5 mm). Immediately after dropping, the aluminum plate was tilted at an angle of 28° and left to stand in an atmosphere of 23°C (±2°C) for 1 hour. After leaving the plate, the length of the thermally conductive silicone potting composition was measured from one end to the other of the flow.
[3] Thixotropic Index The viscosity of the thermally conductive silicone potting composition at 25°C was calculated by dividing the viscosity measured at 10 rpm (approximate shear rate of 2.1 ^s ) using a Brookfield viscometer (TVB-10H, manufactured by Toki Sangyo Co., Ltd.) with a No. 7 rotor by the viscosity measured at 20 rpm (approximate shear rate of 4.2 ^s ).
[4] Thermal Conductivity The thermally conductive silicone potting composition was press-cured to a thickness of 2.0 mm at 120°C for 10 minutes, and then heated in an oven at 120°C for 50 minutes. The thermal conductivity of the resulting cured product was measured in accordance with ISO 22007-2 using a hot disc thermophysical property analyzer TPA-501 (manufactured by Kyoto Electronics Manufacturing Co., Ltd.) at an ambient temperature of 25°C.
[5] Density The thermally conductive silicone potting composition was press-cured to a thickness of 2.0 mm at 120°C for 10 minutes, and then heated in an oven at 120°C for 50 minutes. The density of the resulting silicone sheet was measured in accordance with JIS K 6251:2017.
[6] Elongation at break and tensile strength A thermally conductive silicone potting composition was press-cured to a thickness of 2.0 mm at 120°C for 10 minutes, and then heated in an oven at 120°C for 50 minutes. The elongation at break and tensile strength of the resulting silicone sheet were measured in accordance with JIS K 6251:2017.
[7] Tensile shear adhesive strength A thermally conductive silicone potting composition was sandwiched between 1.0 mm thick aluminum plates (JIS H 4000:2022) to a thickness of 2.0 mm and an adhesive area of 25 mm x 10 mm. The composition was then heated at 120°C for 1 hour to cure the silicone potting composition, producing an adhesive test piece. The tensile shear adhesive strength of the resulting test piece was measured in accordance with JIS K 6850:1999.
[8] Hardness The thermally conductive silicone potting composition was press-cured to a thickness of 2.0 mm at 120°C for 10 minutes, and then heated in an oven at 120°C for 50 minutes. Three of the resulting silicone sheets were stacked, and the hardness was measured using a Type A durometer as specified in JIS K 6253:2012.

As shown in Table 2, the thermally conductive silicone potting compositions of Examples 1 and 2 exhibit low viscosity and high flowability, and are known to have excellent thermal conductivity after curing.
On the other hand, the thermally conductive silicone potting compositions of Comparative Examples 1 and 2, in which component (A) was changed to a linear organopolysiloxane terminated at both ends with dimethylvinylsilyl groups, were found to have high viscosity and poor flowability.

Claims (6)

A thermally conductive silicone potting composition comprising: (A) 100 parts by mass of a linear organopolysiloxane having an average of 0.5 to 1.8 alkenyl groups bonded to silicon atoms at the molecular chain terminals per molecule; (B) 500 to 2,000 parts by mass of a thermally conductive filler; (C) 0.1 to 100 parts by mass of an organohydrogensiloxane having at least two SiH groups per molecule; and (D) a hydrosilylation reaction catalyst. The thermally conductive silicone potting composition according to claim 1, wherein the viscosity of the linear organopolysiloxane of component (A) at 25°C measured with a Brookfield type rotational viscometer is 10 to 500 mPa·s. The thermally conductive silicone potting composition according to claim 1, which comprises (E) a silane compound represented by the following general formula (1) and an organopolysiloxane represented by the following general formula (2) in an amount of 0.1 to 30 parts by mass per 100 parts by mass of component (A):
(In the formula, ^R1 's each independently represent a monovalent hydrocarbon group, and n represents an integer of 0 to 30.) The thermally conductive silicone potting composition according to claim 3, wherein the thermally conductive filler (B) is surface-treated with component (E). The thermally conductive silicone potting composition according to claim 1, which has a viscosity of 30 Pa·s or less at 25°C. A cured product obtained by curing the thermally conductive silicone potting composition described in any one of claims 1 to 5.