JP7725967B2 - Thermally conductive adhesive sheet, thermally conductive sheet-like substrate, and method for manufacturing a thermally conductive adhesive sheet - Google Patents

Description

The present invention relates to a thermally conductive adhesive sheet, a thermally conductive sheet-like substrate, and a method for producing a thermally conductive adhesive sheet.

In recent years, as electronic devices and semiconductors have become smaller, more dense, more powerful, and more powerful, the components that make them up have become increasingly highly integrated. Inside highly integrated devices, various components are tightly packed into a limited space, which can easily cause the inside of electronic devices to become very hot, which can lead to malfunctions.

Therefore, heat dissipation components such as heat sinks and heat sinks are used to dissipate heat from inside electronic devices and the like. Screws, adhesives, and adhesive sheets have been used to join heat dissipation components to components that generate heat. However, the use of adhesives and adhesive sheets is preferable in terms of miniaturization and high density of electronic devices and the like. Among adhesive sheets, pressure-sensitive adhesive sheets, i.e., pressure-sensitive adhesive sheets that do not require special heating or pressure application for application, have become increasingly popular. Furthermore, various heat-conductive adhesive sheets have been proposed that utilize heat-conductive adhesives containing thermally conductive particles with excellent thermal conductivity to efficiently transfer heat generated from components to heat dissipation components.
For example, Patent Documents 1 to 5 disclose so-called substrate-less thermally conductive adhesive sheets in which a thermally conductive adhesive layer containing thermally conductive particles is used as the thermally conductive adhesive sheet. Patent Documents 6 to 9 disclose thermally conductive adhesive sheets having a substrate. Patent Documents 10 and 11 disclose innovations regarding the substrate.

Japanese Patent Application Laid-Open No. 2002-294192 Japanese Patent Application Laid-Open No. 2002-285121 Japanese Patent Application Laid-Open No. 2004-027039 Japanese Patent Application Laid-Open No. 2014-167093 Japanese Patent Application Laid-Open No. 2019-189767 Japanese Patent Application Laid-Open No. 2014-034652 JP 2016-108438 A JP 2015-160937 A JP 2015-164996 A Japanese Patent Application Publication No. 10-292157 JP 2012-169599 A

The substrate-less thermally conductive adhesive sheets disclosed in Patent Documents 1 to 5 do not contain a substrate, and therefore, in theory, the thickness of the thermally conductive adhesive sheet can be made thin, and the heat generated from the heat-generating component can be efficiently transferred to the heat-dissipating component.
However, adhesive sheets consisting only of an adhesive layer without a substrate actually lack stiffness, and therefore have extremely poor punching processability. Many industrially used adhesive sheets are punched into small pieces of a size and shape that match the size and shape of the adherend, assuming the time of use, i.e., application. Adhesive sheets with extremely poor punching processability have the drawback of poor productivity of the small pieces. Furthermore, such substrate-less adhesive sheets have the drawback of extremely poor electrical insulation (breakdown voltage, etc.), making them unusable in areas where reliability as an electrical insulating material is highly required.

On the other hand, thermally conductive adhesive sheets with a substrate, such as those disclosed in Patent Documents 6 to 9, are superior to substrate-less ones in terms of punching processability and electrical insulation. However, because they have a substrate, it is difficult to efficiently transfer heat generated from the heat-generating component to the heat-dissipating component.

By using a substrate containing thermally conductive particles as disclosed in Patent Document 10, it is theoretically expected that the thermal conductivity of the entire thermally conductive pressure-sensitive adhesive sheet will be improved (thermal resistance will be reduced).
However, since the substrate containing thermally conductive particles could not be made sufficiently thin, even if the thermal conductivity of the entire thermally conductive adhesive sheet was improved using a substrate containing thermally conductive particles, the thermal conductivity of the entire thermally conductive adhesive sheet could not be sufficiently improved. The substrate containing thermally conductive particles is produced by mixing the thermally conductive particles into a molten resin using melt kneading or various mixing devices (single- or twin-screw extruders, rolls, Banbury mixers, various kneaders, etc.) so that the thermally conductive particles are uniformly dispersed, and then molding the mixture into a sheet using a molding machine such as a T-shaped molding machine or a calendar molding machine. This is because, with such a manufacturing method, the substrate containing thermally conductive particles could not be made sufficiently thin.

Patent Document 11 discloses a method for producing a thermally conductive film, in which a film-forming material containing a resin and a scaly filler is applied to a support to form a coating film, and then a magnetic field is used to orient the scaly filler in the thickness direction of the film, and the coating film is solidified (claim 7, [0044] to [0047], etc.).
When a composition containing scaly fillers is applied to a support and dried, the scaly fillers form a coating film while being oriented and settling to form layers in the plane direction, so the coating film and its solidified film have excellent thermal conductivity in the plane direction but poor thermal conductivity in the thickness direction.In contrast, by using a magnetic field to orient the scaly fillers so that they are parallel to the thickness direction of the film, thermal conductivity in the thickness direction can be improved.
However, when a scaly filler is used, the scaly filler is oriented in both the surface direction and the thickness direction, resulting in a coating film and its solidified film that are brittle and have poor extensibility. Pressure-sensitive adhesive sheets using a substrate with poor extensibility have poor punching processability despite having a substrate, and cannot withstand the tension during application. This tendency is particularly pronounced when the coating film and its solidified film are thin.

The object of the present invention is to provide a thermally conductive double-sided pressure-sensitive adhesive sheet that is thin, has excellent punching processability, is easy to apply, has excellent thermal conductivity in the thickness direction, and has excellent voltage resistance.

That is, the present invention provides a thermally conductive pressure-sensitive adhesive sheet having thermally conductive pressure-sensitive adhesive layers α1 and α2 on both sides of a sheet-like substrate β,
the sheet-like substrate β contains an insulating inorganic filler and has a thickness of 1 to 10 μm;
Among the insulating inorganic fillers observed in a cross section of the sheet-like base material β, a ratio of inorganic fillers having an aspect ratio: major axis a ^β / minor axis b ^β of 1 to 2 is 80% or more,
the thermally conductive adhesive layers α1 and α2 each independently contain an insulating inorganic filler, and the total thickness of the thermally conductive adhesive layers α1 and α2 is 1 to 45 μm;
The total thickness of the sheet-like substrate β and the thermally conductive adhesive layers α1 and α2 is 50 μm or less,
the ratio E'1/ _E'2 of the storage elastic modulus _E'1 at a temperature T1 that is 30°C higher than the temperature _T0 of the highest inflection point of the storage elastic modulus in the temperature range of 0°C to 150° _C to the storage elastic modulus E'2 at a temperature _{T2 that} is 50°C higher than the temperature _T0 of the inflection _point is 1 to 3;
The present invention relates to a thermally conductive adhesive sheet.

The present invention also relates to a sheet-like substrate β that contains an insulating inorganic filler, has a thickness of 1 to 10 μm, has an aspect ratio (long diameter a ^β /short diameter b ^β) of 1 to 2 for 80% or more of the insulating inorganic filler (100%) observed in a cross section, and has a storage modulus at 200°C of 10 ³ Pa or more.

Furthermore, the present invention provides a method for producing a thermally conductive adhesive sheet having thermally conductive adhesive layers α1 and α2 on both sides of a sheet-like substrate β, the method comprising the following steps [1] to [3]:
the ratio E'1/E'2 of the storage modulus E'1 at a temperature _T1 that is 30°C higher than the temperature _T0 of the highest inflection point of the storage modulus in the temperature range of 0 to 150° _C to the storage modulus _E'2 at a temperature _T2 that is 50° _C higher than the temperature _T0 of the inflection point is 1 to ₃ ;
The present invention relates to a method for producing a thermally conductive pressure-sensitive adhesive sheet.
[1] A process for producing a sheet-like substrate β using a dispersion for forming the sheet-like substrate β, the dispersion containing a solution or dispersion of a crosslinkable polymer, an insulating inorganic filler, and a crosslinking agent, the sheet-like substrate β having a storage modulus at 200°C of 10 ³ Pa or more, a thickness of 1 to 10 μm, and an aspect ratio (long diameter a ^β /short diameter b ^β) of 1 to 2 for 80% or more of the insulating inorganic filler of 100% observed in the cross section of the sheet-like substrate β.
[2] A process for producing two thermally conductive adhesive layers α1 and α2 using a dispersion for forming the thermally conductive adhesive layers α1 and α2, the dispersion containing a solution or dispersion of a crosslinkable polymer, an insulating inorganic filler, and a crosslinking agent.
[3] A step of laminating the thermally conductive adhesive layers α1 and α2 on both sides of the sheet-like substrate β so that the total thickness of the thermally conductive adhesive layers α1 and α2 is 1 to 45 μm and the total thickness of the sheet-like substrate β and the thermally conductive adhesive layers α1 and α2 is 50 μm or less.

The present invention makes it possible to provide a thermally conductive double-sided pressure-sensitive adhesive sheet that is thin, has excellent punching processability, is easy to apply, has excellent thermal conductivity in the thickness direction, and has excellent voltage resistance.

Simply put, the present invention relates to a thermally conductive adhesive sheet that contains an insulating inorganic filler and has thin thermally conductive adhesive layers α1 and α2 on both sides of a thin, heat-resistant sheet-like substrate β.

<<Sheet-like substrate β>>
The sheet-like substrate β constituting the thermally conductive pressure-sensitive adhesive sheet of the present invention will be described.
The sheet-like substrate β preferably contains an insulating inorganic filler, has a thickness of 1 to 10 μm, and of 100% of the insulating inorganic filler observed in a cross section, 80% or more of the inorganic filler have an aspect ratio (major axis a ^β /minor axis b ^β) of 1 to 2, and has a storage modulus at 200°C of 10 ³ MPa or more.
Generally, the storage modulus tends to decrease with increasing temperature, and those with a storage modulus of less than 10 ³ MPa soften and become almost liquid. By including a cured product of a crosslinkable polymer that functions as a film-forming component, the sheet-like substrate β has a storage modulus of 10 ³ MPa or more at 200°C, and functions as a support for the thermally conductive adhesive layers α1 and α2 in the thermally conductive adhesive sheet, even if the thickness is 1 to 10 μm.

<Insulating inorganic filler>
Examples of the insulating inorganic filler (hereinafter sometimes simply referred to as inorganic filler) contained in the sheet-like substrate β include metal hydroxides, metal oxides, ceramics, etc. Specific examples include aluminum hydroxide, magnesium hydroxide, aluminum oxide, silicon oxide, magnesium oxide, zinc oxide, titanium oxide, zirconium oxide, iron oxide, silicon carbide, boron nitride, aluminum nitride, titanium nitride, silicon nitride, and titanium boride, with aluminum nitride and aluminum oxide being preferred in terms of size and shape.
These inorganic fillers may be subjected to a surface treatment such as oxidation treatment, silane coupling treatment, or stearic acid treatment in order to improve moisture resistance and heat resistance, suppress surface activity due to ultraviolet rays, etc., or to improve dispersibility in a crosslinkable polymer described below, and reduce the interface with the resin (improving thermal conductivity).

The shape of the inorganic filler is preferably selected to be spherical, cubic, or polygonal so as to minimize the occurrence of orientation or deflection of the inorganic filler in the sheet-like substrate β. In other words, it is preferable to select a shape whose so-called aspect ratio (also called the "length/width" ratio or "long diameter/short diameter" ratio) is not extremely large and is as close to 1 as possible.
By selecting an inorganic filler so that it is contained as uniformly as possible in the sheet-like substrate β, it is possible to suppress embrittlement and reduced extensibility of the sheet-like substrate β due to the inclusion of the inorganic filler. The sheet-like substrate β, when thin, having a thickness of 1 to 10 μm, functions as a so-called core material of the double-sided PSA sheet and is responsible for stabilizing the shape of the entire double-sided PSA sheet. Therefore, it is preferable to select an inorganic filler used in the sheet-like substrate β that has an aspect ratio as close to 1 as possible.
Using a scanning electron microscope, the inorganic filler is observed at a magnification that allows observation of approximately 50 particles in one field of view, and 30 particles are selected in a spiral pattern from an arbitrary point approximately in the center of the field of view toward the outside, and the major axis ^aβ and minor axis ^bβ of each particle are determined. The major axis ^aβ and minor axis ^bβ of each particle are determined, and the average value of the aspect ratio of the 30 particles = major axis ^aβ / minor axis ^bβ is taken as the average aspect ratio of the inorganic filler. It is important to select inorganic fillers in which 80% or more of the 30 particles have an aspect ratio (major axis ^aβ / minor axis bβ ⁾ of 1 to 2, and it is preferable to select inorganic fillers in which 80% or more of the particles have an aspect ratio of 1 to 1.5, and it is more preferable to select inorganic fillers in which 80% or more of the particles have an aspect ratio of 1 to 1.3.
The inorganic filler is preferably selected to have an average particle size of 0.1 to 3 μm, more preferably 0.3 to 2 μm. The average particle size refers to the average value of particle sizes of 30 particles, and the particle size of each particle refers to half the sum of the major axis a ^β and the minor axis b ^β of each particle.

Furthermore, it is preferable to select an inorganic filler having a D90 particle size of 50 or less when the thickness of the sheet-like substrate β to be formed is taken as 100.
To improve thermal conductivity, it is effective to make the entire thermally conductive adhesive sheet as thin as possible. However, in order to achieve adhesive performance, the thermally conductive adhesive layers α1 and α2 described below must be made thick to a certain extent. Therefore, in order to make the entire thermally conductive adhesive sheet as thin as possible, it is important to make the sheet-like substrate β as thin as possible. On the other hand, a certain thickness is required for the production stability and ease of handling of the sheet-like substrate β. Therefore, the thickness of the sheet-like substrate β is 1 to 10 μm, preferably 2 to 9 μm, and more preferably 3 to 8 μm.

Since the thickness of the sheet-like substrate β is 1 to 10 μm, it is preferable to select an inorganic filler contained in the sheet-like substrate β having a D90 particle size of 50 or less when the thickness of the sheet-like substrate β to be formed is 100 so that the thickness falls within the above range.
The D90 particle size is the particle size when the cumulative volume percentage of the particle size distribution is 90%. The D90 particle size can be measured using a dynamic light scattering particle size distribution measuring device (such as the "Nanotrack UPA" manufactured by Nikkiso Co., Ltd. or the "LB-550" manufactured by Horiba, Ltd.). Methyl ethyl ketone, ethyl acetate, or the like can be used as a diluent during the measurement. Specifically, it is preferable to select an inorganic filler having a D90 particle size of 0.5 to 5 μm, and more preferably an inorganic filler having a D90 particle size of 1 to 3 μm.

The selected inorganic filler will also be observed in the cross section of the sheet-like substrate β, so by selecting such an inorganic filler, the aspect ratio (long diameter a ^β /short diameter b ^β ) of 80% or more of the insulating inorganic filler particles observed in the cross section of the sheet-like substrate β can be set to 1 to 2, preferably 1 to 1.5, and more preferably 1 to 1.3.
Furthermore, the average particle diameter of the insulating inorganic filler observed in the cross section of the sheet-like substrate β is preferably 60% or less of the thickness of the sheet-like substrate β, and the maximum diameter a ^β _max of the insulating inorganic filler is preferably less than the thickness of the sheet-like substrate β.
The aspect ratio, average particle diameter, and maximum diameter ^aβmax of the insulating inorganic filler observed in the cross section of the sheet-like substrate β _can be determined, as in the case of observing the insulating inorganic filler, by using a scanning electron microscope to observe the cross section of the sheet-like substrate β at a magnification that allows approximately 50 particles to be observed in one field of view, selecting 30 particles, and measuring the major axis ^aβ and minor axis ^bβ of each particle.

<Crosslinkable polymer>
As described above, the sheet-like substrate β preferably contains a cured product of a crosslinkable polymer that functions as a film-forming component. The crosslinkable polymer will now be described.
Examples of crosslinkable polymers include those that can react with a crosslinking agent (also called a curing agent) to form a cured product, as well as polymers that have self-crosslinking properties.
Examples of polymers that can react with a crosslinking agent to form a cured product include polyvinyl acetal resins, styrene-based elastomers, polyester resins, epoxy resins, urethane resins, urethane urea resins, phenolic resins, amino resins, and acrylic resins, with polyvinyl acetal resins and styrene-based elastomers being preferred.
Examples of functional groups that the crosslinkable polymer may have include alcoholic hydroxyl groups (hereinafter simply referred to as hydroxyl groups), phenolic hydroxyl groups, acid anhydride groups, carboxy groups, amino groups, cyanate groups, isocyano groups, cyanato groups, isocyanato groups, imidazole groups, pyrrole groups, acetal groups, acryloyl groups, methacryloyl groups, aldehyde groups, hydrazide groups, hydrazone groups, and phosphate groups, and the functional group is preferably at least one selected from the group consisting of alcoholic hydroxyl groups, carboxy groups, and acid anhydride groups.

<Polyvinyl acetal resin>
The polyvinyl acetal resin is a resin obtained by reacting polyvinyl alcohol with an aldehyde compound to form an acetal, and examples thereof include polyvinyl formal resin (also referred to as polyvinyl acetal resin in the narrow sense) derived from formaldehyde and polyvinyl butyral resin derived from butylaldehyde.

Examples of methods for producing polyvinyl acetal resins include a first step in which polyvinyl acetate is produced by radical polymerization of vinyl acetate, and the resulting polyvinyl acetate is hydrolyzed (saponified) in an alkaline solution, followed by separation, purification, and drying to obtain polyvinyl alcohol, and a second step in which the polyvinyl alcohol obtained in the first step is dissolved, to which an acid catalyst and formaldehyde or butylaldehyde are added, and the resulting mixture is condensed to produce a polyvinyl acetal resin. The resulting polyvinyl acetal resin has an acetyl group derived from vinyl acetate, a hydroxyl group derived from polyvinyl alcohol, and a six-membered ring structure derived from acetalization.
As the polyvinyl acetal resin, polyvinyl butyral resin is preferred because the resin itself has high extensibility and the extensibility is not significantly impaired even when a filler is blended.

From the viewpoint of film-forming properties, the weight-average molecular weight (Mw) of the polyvinyl butyral resin is preferably 10,000 or more, more preferably 20,000 or more, and preferably 30,000 or more. Furthermore, from the viewpoint of the suitability of the dispersion for forming the sheet-like substrate β to be applied to a release sheet, the weight-average molecular weight (Mw) of the polyvinyl butyral resin is preferably 1,000,000 or less, more preferably 500,000 or less, and even more preferably 250,000 or less.

The hydroxyl groups in the polyvinyl butyral resin react with a crosslinking agent described later to form a cured product. From the viewpoint of generating appropriate crosslinking and forming a sheet-like substrate β having a storage modulus of ¹⁰ MPa or more at 200°C, the content of units having hydroxyl groups in the polyvinyl butyral resin, i.e., units derived from unacetylated polyvinyl alcohol, is preferably 10 to 30 wt%, more preferably 15 to 25 wt%.
The hydroxyl value of the polyvinyl butyral resin is preferably 150 to 350 mgKOH/g, and more preferably 200 to 300 mgKOH/g. The hydroxyl value of the polyvinyl butyral resin can be determined as follows.

That is, the hydroxyl value is measured in accordance with JIS K 0070-1992 (acetylation method).
Approximately 25 g of acetic anhydride is taken, pyridine is added, and the total volume is brought to 100 mL. Stir thoroughly to prepare the acetylation reagent. Approximately 2 g of sample is accurately weighed into a flask, and 5 mL of acetylation reagent and 10 mL of pyridine are added. An air condenser is attached, and the mixture is heated at 100°C for 70 minutes. After cooling, 35 mL of toluene is added as a solvent from the top of the condenser, and the mixture is stirred. Then, 1 mL of water is added and stirred to decompose the acetic anhydride. To complete the decomposition, the mixture is heated again for 10 minutes and allowed to cool. After this, the condenser is washed with 5 mL of ethanol, and the washed ethanol is added to the sample solution. 50 mL of pyridine is then added as a solvent to the sample solution and stirred.
This sample solution is titrated potentiometrically with a 0.5 mol/L potassium hydroxide ethanol solution. The same procedure is repeated next time, but without the sample. Potentiometric titration is performed, and the hydroxyl value is calculated using the following formula:

(Formula) Hydroxyl value (mg KOH/g) = ((α - γ) × Δ × 28.5) / ε + η

α: Amount (mL) of 0.5 mol/L potassium hydroxide ethanol solution used in the blank test
γ: Amount (mL) of 0.5 mol/L potassium hydroxide ethanol solution used for the sample
Δ: Factor of 0.5 mol/L potassium hydroxide ethanol solution ε: Amount of sample collected (g)
η: Acid number

The acid value is measured in accordance with JIS K 0070-1992 (potentiometric titration method).
Phenolphthalein solution is added as an indicator to a solvent made by mixing toluene and ethanol in a volume ratio of 4:1, and the mixture is neutralized with 0.1 mol/L potassium hydroxide ethanol solution. Approximately 5 g of sample is accurately weighed into a beaker, 50 mL of solvent is added, and the mixture is stirred on a panel heater (80°C) for 6 hours. Potentiometric titration is then performed with 0.1 mol/L potassium hydroxide ethanol solution, and the acid value is calculated using the following formula.

(Formula) Acid value (mgKOH/g) = (γ×Δ×5.611)/ε

γ: Amount (mL) of 0.1 mol/L potassium hydroxide ethanol solution used for the sample
Δ: Factor of 0.1 mol/L potassium hydroxide ethanol solution ε: Sample (g)

The glass transition temperature (Tg) of the polyvinyl butyral resin is preferably 50 to 150°C, and more preferably 60 to 130°C. Using a polyvinyl butyral resin with a Tg within this range makes it easier to form a sheet-like substrate β with a higher storage modulus at 200°C.

Commercially available polyvinyl butyral resins include S-LEC "BL-1", "BL-2", "BL-2H", "BL-5", "BL-10", "BM-1", "BM-2", "BM-S", "BH-3", and "BH-S" manufactured by Sekisui Chemical Co., Ltd.
These polyvinyl acetal resins may be used singly or in combination of two or more.

<Styrene-based elastomer>
In this specification, "elastomer" refers to a polymer that has rubber elasticity at room temperature without undergoing vulcanization. In terms of chemical structure, elastomers generally have an ABA type block or an (A-B)n type multiblock structure. Furthermore, styrene-based elastomers refer to copolymers that have a block containing polystyrene (hereinafter also referred to as a polystyrene block).

A styrene-based elastomer having a polystyrene structure in the molecule is preferred as the elastomer, since it can form a sheet-like substrate β having a larger storage modulus at 200° C. Specific examples include styrene-butadiene block copolymer, styrene-ethylene-propylene block copolymer, styrene-butadiene-styrene block copolymer (hereinafter also referred to as SBS), styrene-isoprene-styrene block copolymer (hereinafter also referred to as SIS), and styrene-ethylene-butylene-styrene block copolymer (hereinafter also referred to as SEBS).
In these styrene-based elastomers, the portions other than the polystyrene blocks are collectively considered to be one block. Furthermore, among the portions other than the polystyrene blocks, examples of blocks formed from units (residues) derived from two or more monomers include, in the above examples, copolymers of ethylene and propylene and copolymers of ethylene and butylene. Such blocks formed from units derived from two or more monomers other than the polystyrene blocks may be random copolymers or block copolymers.

The styrene-based elastomer preferably has a weight-average molecular weight of approximately 30,000 to 400,000, more preferably 50,000 to 300,000, and even more preferably 80,000 to 25,000. To prevent embrittlement caused by the addition of inorganic fillers, a highly reactive elastomer with a weight-average molecular weight of approximately 100,000 is used as the main component, and an elastomer with a weight-average molecular weight of approximately 200,000 can be used in combination.

It is important that the styrene-based elastomer has a functional group such as a carboxyl group or a carboxylic acid anhydride group that can react with the crosslinking agent described below, and a carboxylic acid anhydride group is preferred.

Examples of methods for introducing acid anhydride groups into styrene-based elastomers include polymerizing a monomer having an acid anhydride group as one of the raw materials for producing the styrene-based elastomer with other raw materials, introducing acid anhydride groups into side chains after polymer synthesis, and grafting. Examples include copolymerizing an appropriate amount of an ethylenically unsaturated carboxylic acid anhydride such as maleic anhydride, and synthesizing a styrene-based elastomer and then grafting the resulting polymer using an appropriate amount of an ethylenically unsaturated carboxylic acid anhydride such as maleic anhydride and a peroxide. After introducing the acid anhydride groups, carboxy groups can be introduced by ring-opening some or all of the acid anhydride groups using water, alcohol, amine, or the like.
Instead of acid anhydrides, an appropriate amount of ethylenically unsaturated carboxylic acid such as maleic acid can be used. In this case, acid anhydride groups can be introduced by converting some or all of the carboxy groups into anhydride groups after the introduction of carboxy groups.

The styrene-based elastomer having a carboxy group or an acid anhydride group preferably contains 5 to 60% by mass of polystyrene blocks, more preferably 10 to 50% by mass, and even more preferably 20 to 40% by mass, based on 100% by mass excluding the anhydride of the ethylenically unsaturated carboxylic acid.
When the polystyrene block content is 5% by mass or more, the solubility in solvents is improved and the solution stability of the composition for forming the sheet-like substrate β is excellent, and when the content is 60% by mass or less, the extensibility is also excellent, and a sheet-like substrate β with a larger storage modulus at 200°C can be formed.

The acid anhydride group value of the styrene-based elastomer having an acid anhydride group is preferably 0.1 to 40 mg _{CH 3} ONa/g, more preferably 1 to 30 mg _{CH 3} ONa/g, and even more preferably 5 to 20 mg _{CH 3} ONa/g. By using a styrene-based elastomer having an acid anhydride group with an acid anhydride group value of 0.1 mg CH 3 ONa/g or more, a cured product with a sufficiently high storage modulus at 200°C can be formed, and a thermally conductive adhesive sheet with excellent punching processability and voltage resistance can be formed. By using a styrene-based elastomer having an acid anhydride group with an acid anhydride group value of 40 mg CH ₃ ONa/g or less, the solution stability of the composition for forming the sheet-like substrate β can be excellent.

Other resins that can be used to form the sheet-like substrate β include polyester resin, epoxy resin, urethane resin, urethane urea resin, phenol resin, amino resin, acrylic resin, etc.

<Crosslinking agent>
The crosslinking agent can be appropriately selected from those capable of reacting with the functional groups of the crosslinkable polymer to form the sheet-like substrate β, which is a cured product.
When the crosslinkable polymer has an alcoholic hydroxyl group, examples of the crosslinking agent include a compound having an isocyanate group, a compound having an acid anhydride group, etc. When the crosslinkable polymer has a carboxy group or an acid anhydride group, examples of the crosslinking agent include a compound having an isocyanate group, a compound having an aziridinyl group, a compound having an epoxy group, a compound having an amino group, etc.

<Compounds Having Isocyanate Groups>
The compound having an isocyanate group is a compound having two or more isocyanates, and by reacting this component with the aforementioned polyvinyl acetal resin or styrene-based elastomer, a cured product can be formed having a storage modulus of 10 ³ MPa or more even at 200°C, making it possible to form a thermally conductive adhesive sheet that has excellent punching processability and voltage resistance.

Compounds having an isocyanate group include aromatic, aliphatic, araliphatic, and alicyclic compounds, and aromatic compounds are preferred from the viewpoints of the storage modulus of the cured product at 200°C, the punching processability of the thermally conductive adhesive sheet, and the withstand voltage.
Among aromatic compounds having an isocyanate group, specific examples of compounds having two isocyanate groups include 1,3-phenylene diisocyanate, 4,4'-diphenyl diisocyanate, 1,4-phenylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4'-toluidine diisocyanate, dianisidine diisocyanate, and 4,4'-diphenyl ether diisocyanate.
Among aromatic compounds having an isocyanate group, specific examples of compounds having three or more isocyanate groups include aromatic polyisocyanates such as 2,4,6-triisocyanate toluene and 1,3,5-triisocyanate benzene, and further examples include trimethylolpropane adducts of the diisocyanates described above, biuret products obtained by reaction with water, and trimers having an isocyanurate ring.

When using polyvinyl acetal resin as the crosslinkable polymer and a compound having an isocyanate group as the crosslinking agent, the compound having an isocyanate group is preferably blended in such a way that 0.01 to 1 mol of isocyanate groups are present per 1 mol of hydroxyl groups in the polyvinyl acetal resin, and a range of 0.05 to 0.5 mol is more preferable.

When a styrene-based elastomer resin is used as the crosslinkable polymer and a compound having an isocyanate group is used as the crosslinking agent, the compound having an isocyanate group is preferably blended in such a range that 0.5 to 20 mol of isocyanate groups are present per 1 mol of functional groups (i.e., carboxy groups or acid anhydride groups) in the styrene-based elastomer resin, and a range of 1 to 10 mol is more preferred.

By making the content of isocyanate groups 0.01 mol or more relative to the hydroxyl groups in the polyvinyl acetal resin, or by making the content of isocyanate groups 0.5 mol or more relative to the functional groups in the styrene-based elastomer resin, the crosslinking density can be increased, making it possible to form a cured product with a sufficiently large storage modulus even in an environment of 200°C, and to form a thermally conductive adhesive sheet that has excellent punching processability and voltage resistance.
By setting the isocyanate group content to 1 mol or less relative to the hydroxyl groups in the polyvinyl acetal resin, or by setting the isocyanate group content to 20 mol or less relative to the functional groups in the styrene-based elastomer resin, it is possible to maintain the inherent flexibility of the crosslinkable polymer and maintain tough physical properties that do not become embrittled even when inorganic fillers are blended.Furthermore, by setting the content within the preferred range, it is possible to produce a thermally conductive adhesive sheet that is excellent in processability without becoming embrittled.

<Compounds Having an Aziridinyl Group>
When a styrene-based elastomer resin is used as the crosslinkable polymer and a compound having an aziridinyl group is used as the crosslinking agent, the compound having an aziridinyl group is preferably blended in such a range that the amount of aziridinyl groups is 0.05 to 1 mol, more preferably 0.1 to 0.5 mol, per 1 mol of functional groups in the styrene-based elastomer resin. The reasons why the amount of aziridinyl groups is preferably 0.05 mol or more and 1 mol or less are the same as when a compound having an isocyanate group is used as the crosslinking agent.

<Compounds Having an Acid Anhydride Group, Compounds Having an Epoxy Group, and Compounds Having an Amino Group>
When a compound having an acid anhydride group, a compound having an epoxy group, or a compound having an amino group is used as a crosslinking agent, it is preferable to blend the acid anhydride group, etc., in a range of 0.05 to 1 mol per 1 mol of functional group in the selected crosslinkable polymer, and more preferably in a range of 0.1 to 0.5 mol. By blending within the preferred range, a cured product with a sufficiently large storage modulus can be formed even in a high-temperature environment, and it can be used as a substrate for a thermally conductive adhesive sheet that has excellent punching workability, excellent voltage resistance, and is not embrittled.

The sheet-like substrate β can be formed using a dispersion for forming the sheet-like substrate β, which contains a solution or dispersion of a crosslinkable polymer, an insulating inorganic filler, and a crosslinking agent.
That is, by applying the dispersion liquid for forming the sheet-like substrate β onto a release sheet and heating it, the liquid medium is removed, the crosslinkable polymer and the crosslinking agent are reacted, and a sheet-like substrate β containing an insulating inorganic filler in the cured product, which is the reaction product, can be formed on the release sheet.
The dispersion for forming the sheet-like substrate β preferably contains 100 to 500 parts by mass of insulating inorganic filler per 100 parts by mass of the crosslinkable polymer, more preferably 150 to 450 parts by mass, and even more preferably 200 to 400 parts by mass. From the viewpoint of the thermal conductivity of the sheet-like substrate β to be formed, the content of the insulating inorganic filler is preferably high. From the viewpoint of the coating suitability of the dispersion for forming the sheet-like substrate β, and from the viewpoint of suppressing the occurrence of voids, cracks, etc., forming a sheet-like substrate β as dense as possible, and forming a pressure-sensitive adhesive sheet with excellent voltage resistance, it is preferable that the content of the insulating inorganic filler is not too high.

The liquid medium contained in the dispersion for forming the sheet-like substrate β may be any medium capable of dissolving or dispersing the crosslinkable polymer and the crosslinking agent, and examples thereof include methyl ethyl ketone, ethyl acetate, and isopropyl alcohol.
The dispersion for forming the sheet-like substrate β can be obtained by adding an insulating inorganic filler to a solution or dispersion of a crosslinkable polymer, dispersing the filler, and then adding a crosslinking agent or a solution containing the crosslinking agent. Additives such as dispersants and leveling agents can also be added to the dispersion for forming the sheet-like substrate β. For dispersion, equipment such as a stirring motor, a grinding machine, a three-roll mill, a ball mill, a rotation/revolution mill, a planetary mill, or a bead mill can be used.

<<Thermal conductive adhesive layers α1, α2>>
The thermally conductive adhesive layers α1 and α2 constituting the thermally conductive adhesive sheet of the present invention will be described.
The adhesive layer preferably has a glass transition point of 0°C or lower to exhibit adhesiveness. Both of the thermally conductive adhesive layers α1 and α2 located on both sides of the sheet-like substrate β contain an insulating inorganic filler. The total thickness of the thermally conductive adhesive layers α1 and α2 is 1 to 45 μm, and the thicknesses of the thermally conductive adhesive layers α1 and α2 may be approximately the same or different.
In order to improve thermal conductivity, it is effective to make the entire thermally conductive adhesive sheet as thin as possible. Therefore, the thickness of each of the thermally conductive adhesive layers α1 and α2 is preferably 30 μm or less, and more preferably 25 μm or less.
On the other hand, in order to achieve adhesive performance, the thermally conductive adhesive layers α1 and α2 described below need to be thick to a certain extent. Therefore, the thickness of each of the thermally conductive adhesive layers α1 and α2 is preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 15 μm or more. In other words, it is preferable to set the thickness in the range of 15 to 25 μm.

<Insulating inorganic filler>
The insulating inorganic filler (hereinafter sometimes abbreviated as inorganic filler) contained in the thermally conductive adhesive layers α1 and α2 may be, for example, a metal hydroxide, a metal oxide, a ceramic, etc. Specific examples include aluminum hydroxide, magnesium hydroxide, aluminum oxide, silicon oxide, magnesium oxide, zinc oxide, titanium oxide, zirconium oxide, iron oxide, silicon carbide, boron nitride, aluminum nitride, titanium nitride, silicon nitride, titanium boride, etc., with aluminum nitride and aluminum oxide being preferred in terms of size and shape.
These inorganic fillers may be subjected to a surface treatment such as oxidation treatment, silane coupling treatment or stearic acid treatment in order to improve the moist heat resistance of the filler or to improve dispersibility in the crosslinkable polymer described below.

The shape of the inorganic filler may be, for example, spherical, cubic, polygonal, elliptical, needle-like, scale-like, or a combination thereof.
From the viewpoint of reducing thermal resistance and improving thermal conductivity by making the thermally conductive adhesive layers α1 and α2 as thin as possible, spherical, cubic, or polygonal shapes are preferred, and shapes with an aspect ratio as close to 1 as possible are preferred.
On the other hand, from the viewpoint of improving thermal conductivity by increasing the number of contact points between inorganic fillers and creating a directionality or flow in heat propagation, scale-like or elliptical fillers or aggregates thereof can be used. As mentioned above, in the case of the sheet-like substrate β, it functions as a so-called core material of the double-sided pressure-sensitive adhesive sheet when it is thin, at 1 to 10 μm, and is responsible for stabilizing the entire double-sided pressure-sensitive adhesive sheet. Therefore, it is preferable that the inorganic filler contained in the sheet-like substrate β has an aspect ratio as close to 1 as possible. However, in the case of the thermally conductive pressure-sensitive adhesive layers α1 and α2, since they are originally adhesive layers and are soft, the impact of embrittlement is not as great as in the case of the sheet-like substrate β, so scale-like inorganic fillers such as boron nitride or aggregates thereof can also be used as the inorganic filler.

Since the thermally conductive adhesive layers α1 and α2 have a total thickness of 1 to 45 μm, it is preferable to select an inorganic filler contained in the thermally conductive adhesive layers α1 and α2 that is large enough to fit within this thickness. That is, when the thickness of the thermally conductive adhesive layer α1 to be formed is taken as 100, it is preferable to select an inorganic filler with a D90 particle size of 50 or less. The same applies to the thermally conductive adhesive layer α2.
The D90 particle size is the particle size when the cumulative volume percentage of the particle size distribution is 90%, and can be determined in the same manner as in the case of the inorganic filler in the sheet-like substrate β. In the case of the thermally conductive adhesive layers α1 and α2, specifically, it is preferable to select an inorganic filler having a D90 particle size of 1 to 10 μm, and more preferably an inorganic filler having a D90 particle size of 1 to 8 μm.
By selecting such an inorganic filler, it is possible to obtain a thermally conductive adhesive layer α1 in which the average particle diameter of the insulating inorganic filler observed in the cross section of the thermally conductive adhesive layer α1, i.e., the average value of the particle diameter which is ½ of the sum of the major axis ^aα1 and the minor axis ^bα1 , is 60% or less of the thickness of the thermally conductive adhesive layer α1, and _{the maximum diameter aα1max} ^of the insulating inorganic filler is less than the thickness of the thermally conductive adhesive layer α1. The same applies to the thermally conductive adhesive layer α2.

<Crosslinkable polymer>
As described above, the thermally conductive adhesive layers α1 and α2 contain a cured product of a crosslinkable polymer that functions as a film-forming component. The crosslinkable polymer will now be described. Note that, to distinguish it from the case of the crosslinkable polymer for forming the sheet-like substrate β, the crosslinkable polymer for forming the thermally conductive adhesive layers α1 and α2 is sometimes referred to as an adhesive resin.
Examples of crosslinkable polymers include those that can react with a crosslinking agent (also called a curing agent) to form a cured product, as well as polymers that have self-crosslinking properties.
Examples of polymers that can react with a crosslinking agent to form a cured product include acrylic resins, polyester resins, urethane resins, and silicone resins. Among these, acrylic resins are preferred in terms of weather resistance, heat resistance, functionality, cost, and ease of adhesive design depending on the intended use. Different types of crosslinkable polymers or the same type of crosslinkable polymers can be used to form the thermally conductive adhesive layers α1 and α2. Furthermore, when the same type of crosslinkable polymer is used, the same polymer or different polymers can be used.

Functional groups that crosslinkable polymers may have include alcoholic hydroxyl groups (hereinafter simply referred to as hydroxyl groups), phenolic hydroxyl groups, acid anhydride groups, carboxy groups, amino groups, cyanate groups, isocyano groups, cyanato groups, isocyanato groups, imidazole groups, pyrrole groups, acetal groups, acryloyl groups, methacryloyl groups, aldehyde groups, hydrazide groups, hydrazone groups, and phosphate groups, with alcoholic hydroxyl groups and carboxy groups being preferred.

Acrylic resins are polymers obtained by polymerizing acrylic monomers (monomer components having a (meth)acryloyl group in the molecule), and are preferably polymers obtained by polymerizing alkyl (meth)acrylate esters. Acrylic resins can be used alone or in combination of two or more types.

The above-mentioned (meth)acrylic acid alkyl esters are not particularly limited, but examples thereof include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, ( Examples of (meth)acrylic acid alkyl esters having an alkyl group containing 1 to 20 carbon atoms include nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, and eicosyl (meth)acrylate. Among these, (meth)acrylic acid alkyl esters having an alkyl group containing 1 to 12 carbon atoms (particularly 2 to 12 carbon atoms) are preferred, and (meth)acrylic acid alkyl esters having an alkyl group containing 4 to 9 carbon atoms are more preferred, from the perspective of easily balancing adhesive properties. The above (meth)acrylic acid alkyl esters can be used alone or in combination of two or more.

Of all the monomer components (100% by mass) that make up the acrylic resin, the proportion of the (meth)acrylic acid alkyl ester is not particularly limited, but is preferably 60% by mass or more (e.g., 60 to 99% by mass), more preferably 70% by mass or more (e.g., 70 to 98% by mass), and even more preferably 80% by mass or more (e.g., 80 to 98% by mass).

The acrylic resin may be a polymer containing only the above-mentioned (meth)acrylic acid alkyl ester as a constituent monomer component, but is preferably a copolymer containing a monomer having an alcoholic hydroxyl group or a carboxyl group that functions as a crosslinking point.

When a copolymerizable monomer contains a monomer having a hydroxyl group, the dispersibility of the inorganic filler is improved, and the wettability to the adherend is improved when the adhesive is applied, which is expected to result in improved thermal conductivity.
Examples of the monomer having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, and (4-hydroxymethylcyclohexyl)methyl methacrylate. Of these, 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate are preferred. The hydroxyl group-containing monomers can be used alone or in combination of two or more.
The amount of the monomer having a hydroxyl group is preferably 0.1 to 10% by mass, more preferably 1 to 5% by mass, of the total monomer components (100% by mass).

The carboxyl group-containing monomer is a monomer having one or more carboxyl groups in one molecule, and may be in the form of an anhydride. The carboxyl group-containing monomer is not particularly limited, but examples thereof include (meth)acrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, isocrotonic acid, maleic anhydride, and itaconic anhydride. The carboxyl group-containing monomer may be used alone or in combination of two or more.
The amount of the monomer having a carboxy group is preferably 0.1 to 15% by mass, and more preferably 1 to 10% by mass, of the total monomer components (100% by mass).

Other monomer components that make up acrylic resin can also include monomers with polar functional groups, such as nitrogen-containing monomers, sulfonic acid group-containing monomers, and phosphate group-containing monomers, as well as styrene.

From the viewpoint of film-forming properties, the weight-average molecular weight (Mw) of the acrylic resin is preferably 100,000 or more, more preferably 300,000 or more, and preferably 500,000 or more. Furthermore, from the viewpoint of the suitability of the dispersion for forming the sheet-like substrate β to be applied to a release sheet, the weight-average molecular weight (Mw) of the acrylic resin is preferably 2,000,000 or less, and more preferably 1,300,000 or less. A weight-average molecular weight (Mw) of 100,000 or more ensures sufficient durability, while a weight-average molecular weight of 2,000,000 or less provides fluidity and drying properties suitable for film production (coating).

The glass transition temperature (Tg) of the acrylic resin is preferably 0°C to -70°C, more preferably -20°C to -70°C, and even more preferably -40°C to -70°C. The glass transition temperature of the acrylic resin refers to the value calculated using the Fox equation from the monomer composition of the acrylic resin used, citing the glass transition temperatures of the homopolymers of each monomer listed in "Polymer Handbook" (edited by J. Brandrup and E.H. Immergut, Interscience Publishers).

<Crosslinking agent>
Examples of the crosslinking agent used in forming the thermally conductive adhesive layers α1 and α2 include the same crosslinking agents as those exemplified for forming the sheet-like substrate β.
When an acrylic resin having a hydroxyl group is used as the crosslinkable polymer and a compound having an isocyanate group is used as the crosslinking agent, the compound having an isocyanate group is preferably blended in an amount of 0.01 to 1 mol, more preferably 0.05 to 0.5 mol, of isocyanate groups per 1 mol of hydroxyl groups in the acrylic resin.
When an acrylic resin having a carboxy group is used as the crosslinkable polymer and a compound having an epoxy group is used as the crosslinking agent, the compound having an epoxy group is preferably blended in such a range that the epoxy group is present in an amount of 0.01 to 1 mol, more preferably 0.05 to 0.5 mol, per 1 mol of carboxy group in the acrylic resin.
By setting the content within the above range, sufficient crosslinking between polymers is formed, and the adhesive layer does not completely melt even in a heated environment, and the adhesive tape does not peel off. In particular, when an isocyanate curing agent is selected, good surface affinity is exhibited on a wide range of adherends, and stable adhesive properties can be achieved.
The compound having an epoxy group used as the crosslinking agent is preferably selected because it easily forms strong crosslinks between polymers and stabilizes adhesive properties under high temperature and high humidity conditions.

The thermally conductive adhesive layers α1 and α2 can be formed using a dispersion for forming the thermally conductive adhesive layers α1 and α2, which contains a solution or dispersion of a crosslinkable polymer, an insulating inorganic filler, and a crosslinking agent.
The dispersion for forming the thermally conductive adhesive layers α1 and α2 preferably contains 100 to 500 parts by mass, more preferably 150 to 450 parts by mass, and even more preferably 200 to 400 parts by mass of insulating inorganic filler per 100 parts by mass of crosslinkable polymer. From the viewpoint of the thermal conductivity of the thermally conductive adhesive layers α1 and α2 to be formed, the content of the insulating inorganic filler is preferably high. From the viewpoint of the coatability of the dispersion for forming the thermally conductive adhesive layers α1 and α2, it is preferable that the content of the insulating inorganic filler is not too high.

The liquid medium contained in the dispersion liquid for forming the thermally conductive adhesive layers α1 and α2 may be any medium that can dissolve or disperse the crosslinkable polymer and crosslinking agent, and examples thereof include methyl ethyl ketone, ethyl acetate, and isopropyl alcohol.
The dispersion for forming the thermally conductive adhesive layers α1 and α2 can be obtained by adding an insulating inorganic filler to a solution or dispersion of a crosslinkable polymer, dispersing the filler, and then adding a crosslinking agent or a solution containing the crosslinking agent. Additives such as dispersants and leveling agents can also be added to the dispersion for forming the thermally conductive adhesive layers α1 and α2 as appropriate. Equipment such as a stirring motor, a grinding machine, a three-roll mill, a ball mill, a rotation/revolution mill, a planetary mill, or a bead mill can be used for dispersion.

The thermally conductive adhesive layers α1 and α2 may contain a tackifier (adhesive imparting agent) to enhance tack, adhesion to the adherend, and adhesive strength. Examples of tackifiers include polybutenes, rosin-based resins, terpene-based resins, petroleum-based resins (e.g., petroleum-based aliphatic hydrocarbon resins, petroleum-based aromatic hydrocarbon resins, petroleum-based aliphatic-aromatic copolymer hydrocarbon resins, petroleum-based alicyclic hydrocarbon resins (hydrogenated aromatic hydrocarbon resins), etc.), coumarone-based resins, etc. In terms of compatibility, petroleum-based resins and rosin-based resins are preferred. Tackifiers may be used singly or in combination of two or more types.

The content of tackifier in the thermally conductive adhesive layers α1 and α2 is preferably 1 part by mass or more per 100 parts by mass of the solids content of the main crosslinkable polymer, and more preferably 30 parts by mass or less. Incorporating 1 part by mass or more makes it easier to achieve properties such as tack, adhesion to the adherend, and enhanced adhesive strength, while using 30 parts by mass or less inhibits the crosslinking reaction that forms the main adhesive layer, suppresses bleeding of low molecular weight components and sagging at the edges when the product is heated, and reduces contamination of blades used in punching processes, etc.

<<Thermal conductive adhesive sheet>>
The thermally conductive adhesive sheet of the present invention is a thermally conductive adhesive sheet having thermally conductive adhesive layers α1 and α2 on both sides of a sheet-like substrate β, and the total thickness of the sheet-like substrate β and the thermally conductive adhesive layers α1 and α2 is 50 μm or less. Although a thinner total thickness of the sheet-like substrate β and the thermally conductive adhesive layers α1 and α2 is preferable, a thickness of 30 to 49 μm is preferred from the viewpoints of thickness stability, ease of handling, etc.
The thickness of the sheet-like substrate β and the thickness of the thermally conductive adhesive layers α1 and α2 are as described above.

In the thermally conductive pressure-sensitive adhesive sheet of the present invention, the ratio E'1/E'2 of the storage modulus _E'1 at a temperature T1 that is 30°C higher than the temperature _T0 of the highest inflection point of the storage modulus in the temperature range of ₀ ° _C to 150°C to the storage modulus _E'2 at a temperature _T2 that is 50°C higher than the temperature _T0 of the inflection _point is 1 to 3. If no clear inflection point is observed, the temperature _T0 is set to 150°C.
The storage modulus _E'1 at temperature _T1 is the temperature at which the polymers begin to separate and move, resulting in a semi-liquid state. Furthermore, the storage modulus _E'2 at temperature _T2 is a state in which liquefaction progresses and the modulus drops significantly, with _E'1 >>_E'2 . Therefore, in the present invention, it is important that the ratio of the storage modulus _E'1 at temperature _T1 to the storage modulus _E'2 at temperature _T2 , _E'1 / _E'2 , is 1 to 3. This indicates that liquefaction does not progress as the temperature rises from _T1 to _T2 , i.e., that the polymers are sufficiently crosslinked. Therefore, even when the thermally conductive pressure-sensitive adhesive sheet of the present invention is used in a high-temperature environment, the substrate can adequately support the entire tape and maintain its performance. It is even more preferable that _E'1 / _E'2 is in the range of 1 to 1.5.

In the thermally conductive pressure-sensitive adhesive sheet of the present invention, the thickness of the sheet-like substrate β containing an insulating inorganic filler is 1 to 10 μm, preferably 3 to 10 μm, and more preferably 3 to 8 μm.
In the sheet-like substrate β of the thermally conductive pressure-sensitive adhesive sheet of the present invention, 80% or more of the insulating inorganic filler particles observed in a cross section thereof have an aspect ratio (major axis a ^β /minor axis b ^β) of 1 to 2, preferably 1 to 1.5, and more preferably 1 to 1.3. By making the aspect ratio of the inorganic filler in the sheet-like substrate β 1 to 2, the extensibility of the thin sheet-like substrate β can be improved, and the application performance as a thermally conductive pressure-sensitive adhesive sheet can be improved.
The aspect ratio of the insulating inorganic filler observed in the cross section of the sheet-like substrate β is determined by observing the cross section of the sheet-like substrate β in the thermally conductive adhesive sheet using a scanning electron microscope at a magnification that allows approximately 50 particles to be observed in one field of view, selecting 30 particles in a spiral pattern from an arbitrary point approximately in the center of the field of view toward the outside, determining the major axis ^aβ and minor axis ^bβ of each particle, and calculating the aspect ratio of each particle: major axis ^aβ /minor axis ^bβ .

Furthermore, in terms of the thinness and extensibility of the sheet-like substrate β and the application performance as a thermally conductive pressure-sensitive adhesive sheet, it is preferable that the sheet-like substrate β in the thermally conductive pressure-sensitive adhesive sheet of the present invention has an average particle diameter of the insulating inorganic filler observed in the cross section of the sheet-like substrate ^β , i.e., an average value of the particle diameter which is 1/2 of the sum of the major axis a β and the minor axis b ^β , is 60% or less of the thickness of the sheet-like substrate β, and the maximum diameter a ^β _max of the insulating inorganic filler is less than the thickness of the sheet-like substrate β.

The thermally conductive pressure-sensitive adhesive sheet of the present invention can be formed, for example, as follows.
That is, the dispersions for forming the thermally conductive adhesive layers α1 and α2 are applied to the release sheet, respectively, and heated to remove the liquid medium, react the crosslinkable polymer with the crosslinking agent, and form the thermally conductive adhesive layers α1 and α2 on the release sheet, respectively, containing an insulating inorganic filler in the cured product, which is the reaction product. Separately, the thermally conductive adhesive layer α1 is superimposed on the sheet-like substrate β formed on the release sheet, to obtain an intermediate having a laminated structure of release sheet / sheet-like substrate β / thermally conductive adhesive layer α1 / release sheet, and then peel off the release sheet covering the sheet-like substrate β, and superimpose the thermally conductive adhesive layer α2 on the exposed sheet-like substrate β, to obtain a laminate having a laminated structure of release sheet / thermally conductive adhesive layer α2 / sheet-like substrate β / thermally conductive adhesive layer α1 / release sheet. In other words, a laminate can be obtained in which both sides of the thermally conductive adhesive sheet of the present invention are covered with a release sheet.

Alternatively, the thermally conductive pressure-sensitive adhesive sheet of the present invention can also be obtained by the following method.
That is, first, in the same manner as described above, a dispersion for forming the thermally conductive adhesive layer α2 is applied to a release sheet and heated to form a thermally conductive adhesive layer α2 on the release sheet. Separately, a dispersion for forming the thermally conductive adhesive layer α1 is applied to a sheet-like substrate β and heated to remove the liquid medium, react the crosslinkable polymer with the crosslinking agent, and form a thermally conductive adhesive layer α1 on the sheet-like substrate β. The surface of the formed thermally conductive adhesive layer α1 is covered with a release sheet to obtain an intermediate having a laminate structure of release sheet / sheet-like substrate β / thermally conductive adhesive layer α1 / release sheet. Next, the release sheet covering the sheet-like substrate β is peeled off, and the thermally conductive adhesive layer α2 is superimposed on the exposed sheet-like substrate β to obtain a laminate having a laminate structure of release sheet / thermally conductive adhesive layer α2 / sheet-like substrate β / thermally conductive adhesive layer α1 / release sheet. In other words, a laminate can be obtained in which both sides of the thermally conductive adhesive sheet of the present invention are covered with a release sheet.

By peeling off the release sheets covering both sides of the laminate and using the thermally conductive adhesive sheet of the present invention to attach a heat-generating component and a heat-dissipating component, the heat generated by the heat-generating component can be efficiently transferred to the heat-dissipating component.

The following examples and comparative examples will explain the present invention in detail. However, the present invention is not limited to these examples. Unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass." Furthermore, the blending amounts of each material shown in the tables are all values based on the solid content, excluding the solvent.

(Preparation of sheet-like substrate β)
<Preparation of Sheet-Like Substrate β-1>
In a 450 mL glass bottle, 10 g of pellets of maleic anhydride-modified styrene-based elastomer resin (FG1901 manufactured by Kraton Corporation, abbreviated as SEBS in Table 1) were dissolved in a solvent of toluene:methyl ethyl ketone (hereinafter referred to as MEK) = 9:1 (mass ratio) to prepare a solution with a solids content of approximately 10%. Next, 15.1 g of alumina powder AO-502 (average particle size: 0.3 μm) manufactured by Admatechs and 100 g of glass beads with a diameter of 5 mm were added to 100 g of the above solution, and the mixture was shaken for 60 minutes using a Scandex wet media disperser to disperse the alumina powder, and the glass beads were removed to prepare a dispersion.
To the dispersion, 0.06 g of an aziridine-based curing agent (Chemitaite PZ-33) and a solvent for dilution of toluene:MEK=9:1 (mass ratio) were added, and the mixture was mixed for 20 minutes with a mix rotor to prepare a coating liquid for forming a sheet-like substrate β-1 with a solid content of approximately 20%.
The coating liquid for forming the sheet-like substrate β-1 was applied to the release-treated surface of a release film manufactured by Toray Industries, Inc. (Cerapeel MF, thickness 50 μm) using a doctor blade, dried in an oven at 100°C for 2 minutes, and then aged at 40°C for 3 days to produce a sheet-like substrate β-1 with a thickness of approximately 5 μm on the release film.

<Preparation of Sheet-Like Substrates β-2 and β-3>
It was prepared in the same manner as in β-1, except that alumina powder AO-502 filler was added so as to obtain the filler blending amount shown in Table 1.

<Preparation of Sheet-Like Substrate β-4>
β-4 was produced in the same manner as β-1, except that the same coating liquid for forming the sheet-like substrate β-2 as β-2 was used, and the coating liquid was naturally filtered through a #500 mesh wire mesh to remove coarse particles, and the doctor blade clearance was adjusted to obtain the thickness shown in Table 1.

<Preparation of Sheet-Like Substrates β-5 to β-7>
β-5 to β-7 were prepared in the same manner as β-1, except that the same coating liquid for forming the sheet substrate β-2 as in β-2 was used and the doctor blade clearance was adjusted to obtain the thickness shown in Table 1.

<Preparation of Sheet-Like Substrates β-8 to β-10>
The filler type of β-8 is spherical alumina DAM-03 (average particle size: 6.8 μm) manufactured by Denka Co., Ltd.
The filler type of β-9 is aluminum nitride AlN020AF (average particle size: 2.7 μm) manufactured by Thrutek Corporation.
The filler type for β-10 is magnesium oxide PyroXima 5301K (average particle size: 3 μm) manufactured by Kyowa Chemical Co., Ltd.
Each was changed and the thickness was adjusted to the values shown in Table 1, but the same method as for β-1 was used to prepare the film.

<Preparation of Sheet-Like Substrates β-11 to β-14>
In a 450 mL glass bottle, 10 g of polyvinyl butyral resin powder (S-LEC BHS manufactured by Sekisui Chemical Co., Ltd., abbreviated as PVB in Table 1) was dissolved in a solvent with a toluene:isopropyl alcohol (hereinafter referred to as IPA) ratio of 3:7 (by mass) to prepare a solution with a solids content of approximately 10%. Next, 25.3 g of alumina powder AO-502 (average particle size: 0.3 μm) manufactured by Admatechs and 100 g of glass beads with a diameter of 5 mm were added, and the mixture was shaken for 30 minutes using a Scandex wet media disperser to disperse the alumina powder. The glass beads were then removed to prepare a dispersion.
To the dispersion, 0.81 g of an isocyanate curing agent (Sumidur N3900, HDI nurate) and a dilution solvent of toluene:IPA=3:7 were added, and the mixture was mixed with a mix rotor for 20 minutes to prepare a coating liquid for forming a sheet-like substrate β-11 with a solid content of approximately 20%.
Using the obtained coating liquid for forming the sheet-like substrate β-11, β-11 to β-14 were produced in the same manner as β-1, except that the doctor blade clearance was adjusted so that the thickness of the formed sheet-like substrate would be the thickness shown in Table 1.

<Preparation of Sheet-Like Substrate β-15>
Instead of 10 g of polyvinyl butyral resin powder, 10 g of polyester resin (Vylon 200 manufactured by Toyobo Co., Ltd.) was used and dissolved in a solvent of toluene:IPA = 9:1 to obtain a solution with a solids content of approximately 10%, and a coating liquid for forming a sheet-like substrate β-15 with a solids content of approximately 20% was prepared in the same manner as in the case of sheet-like substrate β-11, except that 15.1 g of Admatechs alumina powder AO-502 (average particle size: 0.3 μm) was used, and a sheet-like substrate β-15 with a thickness of approximately 5 μm was produced.

<Preparation of Sheet-Like Substrate β-16>
A solution of a urethane prepolymer having an isocyanate group at its terminal was obtained by reacting 195 g of a polyester diol (P-2011, manufactured by Kuraray Co., Ltd.), 7 g of dimethylolbutanoic acid, and 40 g of isophorone diisocyanate in toluene. This was then reacted with 6 parts of isophorone diamine, 0.6 g of di-n-butylamine, and 113 g of 2-propanol, and toluene and 2-propanol were added to obtain a solution of a polyester polyurethane resin with a solids content of approximately 25%. The polyester polyurethane resin had an Mw of 100,000, a Tg of -5°C, and an acid value of 10 mgKOH/g.
A coating liquid for forming a sheet-like substrate β-16 having a solids content of about 20% was prepared in the same manner as in the case of the sheet-like substrate β-1, except that a solution of a polyester-based polyurethane resin having a solids content of about 10% was used instead of the styrene-based elastomer resin having a solids content of about 10%. A sheet-like substrate β-16 having a thickness of about 5 μm was prepared.

<Preparation of Comparative Sheet-Like Substrate β-101>
A comparative sheet-like substrate β-101 having a thickness of approximately 5 μm was prepared by preparing a coating liquid for forming a comparative sheet-like substrate β-101 having a solids content of approximately 20% in the same manner as in the case of the coating liquid for forming the sheet-like substrate β-11, except that an isocyanate curing agent (Sumidur N3900, HDI nurate) was not added.

<Preparation of Comparative Sheet-Like Substrate β-102>
A coating liquid for forming a comparative sheet-like substrate β-102 having a solids content of approximately 20% was prepared in the same manner as in the case of the coating liquid for forming the sheet-like substrate β-11, except that Admatechs alumina powder AO-502 (average particle size: 0.3 μm) was not added, and a comparative sheet-like substrate β-102 having a thickness of approximately 5 μm was prepared.

<Preparation of Comparative Sheet-Like Substrate β-103>
A comparative sheet-like substrate β-103 forming coating liquid with a solids content of about 20% was prepared in the same manner as in the case of the sheet-like substrate β-11 forming coating liquid, except that 11 g of Showa Denko K.K. boron nitride UHP-1K (average particle size: 4 μm) was used instead of 25.3 g of Admatechs' alumina powder AO-502 (average particle size: 0.3 μm), and a comparative sheet-like substrate β-103 with a thickness of about 15 μm was prepared.

[Cross-section observation of sheet-like substrate β-1 etc.]
The sheet-like substrate β-1 etc. together with the laminated release film was immersed in liquid nitrogen for 1 minute, and a razor was placed perpendicular to the sheet surface using tweezers in the liquid nitrogen, and the opposite side of the razor (the back side) was struck with a hammer to split the sheet-like substrate β-1 etc. in the thickness direction, thereby obtaining a cross section for observation. After returning to room temperature, the cross section was observed using a digital microscope VHX-7000 manufactured by Keyence Corporation.

[Measurement of storage modulus (E′) of sheet-like substrate β-1, etc.]
The sheet-like substrate β-1 etc. formed on the release film was punched out into test pieces 15 mm long and 5 mm wide, the release film was peeled off, and the sheet-like substrate β-1 etc. was measured for change in storage modulus (E') in the temperature range of -50°C to 250°C when tensile under conditions of load cell: 600 gf, heating rate: 1°C/min, chuck distance: 5 mm, and frequency: 5 Hz using a dynamic viscoelasticity measuring device DVA-200 (manufactured by IT Measurement Control Co., Ltd.). Of the temperatures at the inflection point of the storage modulus (E') in the range of 0 to 150°C, i.e., the maximum point of tan _{δ, the temperature T0 on the highest temperature side was determined, and E'1 at temperature T1} 30°C higher than T0 and _E'2 at temperature _T2 50°C higher than _T0 were determined, to derive _E'1 / _E'2 .

(Preparation of adhesive resin P)
<Adhesive resin P-1>
2-Ethylhexyl acrylate/2-hydroxyethyl acrylate/acrylic acid = 96.5/3/0.5 (mass ratio) was polymerized in a solvent consisting of acetone/ethyl acetate/toluene = 1/1/0.5 (mass ratio) to obtain a solution of acrylic resin P-1 (non-volatile content concentration: approximately 50%) having a weight average molecular weight of 900,000 and a glass transition temperature according to the FOX formula of -67°C.

<Adhesive resin P-2>
By changing the amount of polymerization initiator, a solution of acrylic resin P-2 (non-volatile content of about 50%) having a weight average molecular weight of 300,000 and the same monomer composition as adhesive resin P-1 was obtained.

<Adhesive resin P-3>
Butyl acrylate (BA)/acrylic acid/2-hydroxyethyl acrylate (HEA) = 96.5/3/0.5 (mass ratio) was polymerized in a solvent consisting of toluene/ethyl acetate = 1/1 (mass ratio) to obtain a solution of acrylic resin P-3 (non-volatile concentration: approximately 50%) having a weight average molecular weight of 300,000 and a glass transition temperature according to the FOX formula of -51°C.

[Weight average molecular weight (Mw) of adhesive resin P]
Measurement was carried out using a Shodex GPC-104/101 system manufactured by Showa Denko KK
Column Shodex KF-805L + KF-803L + KF-802
Detector: Differential Refractometer (RI)
Column temperature: 40°C
Eluent: tetrahydrofuran Flow rate: 1.0 mL/min Sample concentration: 0.2%
Standard sample for calibration curve: TSK standard polystyrene

<Dispersion liquid αS-1 for forming thermally conductive adhesive layer>
In a 450 mL glass bottle, 100 g of a solution containing 50 g of adhesive resin (P-1) in terms of solid content, 75 g of inorganic filler (T-1) described below, and MEK for dilution so that the solid content was approximately 60% were placed, and then 100 g of glass beads with a diameter of 5 mm were added, and the mixture was shaken for 60 minutes using a wet media disperser Scandex to disperse the inorganic filler (T-1), and the glass beads were removed to prepare a dispersion.
The dispersion was subjected to natural filtration through a #500 mesh wire netting to remove coarse particles, and then 0.1 g of the curing agent (S-1) described below and MEK for dilution were added and mixed with a mix rotor to prepare a dispersion αS-1 for forming a thermally conductive adhesive layer with a solid content of 50%.
The ratio of the isocyanate groups in the curing agent (S-1) to the hydroxyl groups in the adhesive resin (P-1), NCO/OH, in the dispersion αS-1 was 0.3.

<Dispersion liquid for forming thermally conductive adhesive layer αS-2, αS-3>
Dispersions αS-2 and αS-3 for forming a thermally conductive adhesive layer were prepared in the same manner as αS-1, except that inorganic filler (T-1) was used so that the inorganic filler content shown in Table 3 was obtained.

<Dispersion liquid for forming thermally conductive adhesive layer αS-4 to αS-5>
Dispersions αS-3 to αS-6 for forming a thermally conductive adhesive layer were prepared in the same manner as αS-2, except that inorganic fillers (T-2) to (T-4) were used instead of inorganic filler (T-1).

<Dispersion for forming thermally conductive adhesive layer αS-6>
Dispersion liquid αS-6 for forming a thermally conductive adhesive layer was prepared in the same manner as αS-2, except that inorganic filler (T-4) was used instead of inorganic filler (T-1) and the wire mesh used for natural filtration after dispersion was changed to #300 mesh.

<Dispersion liquid for forming thermally conductive adhesive layers αS-7 to αS-8>
Dispersions αS-7 to αS-8 for forming thermally conductive adhesive layers were prepared in the same manner as αS-1, except that solutions containing adhesive resins (P-2) to (P-3) were used instead of the solution containing adhesive resin (P-1).

<Dispersion for forming thermally conductive adhesive layer αS-9>
Dispersion liquid αS-9 for forming a thermally conductive adhesive layer was prepared in the same manner as αS-2, except that 0.6 parts of curing agent (S-2) was used instead of curing agent (S-1).
The ratio of the epoxy group in the curing agent (S-2) to the carboxy group in the adhesive resin (P-1) in the dispersion αS-9, ie, epoxy/COOH, was 0.3.

<Preparation of Comparative Example Dispersion αS-101 for Forming Thermally Conductive Adhesive Layer>
A dispersion liquid αS-101 for forming a comparative thermally conductive adhesive layer was prepared in the same manner as αS-2, except that the curing agent (S-1) was not added.

<Preparation of Comparative Example Dispersion αS-102 for Forming Thermally Conductive Adhesive Layer>
A dispersion liquid αS-102 for forming a comparative thermally conductive adhesive layer was prepared in the same manner as αS-2, except that the inorganic filler (T-1) was not blended.

(hardener S)
S-1: Desmodur N3900: HDI nurate manufactured by Sumika Kobestro Urethane Co., Ltd. S-2: JER630 (a compound having three epoxy groups manufactured by Mitsubishi Chemical Corporation)

(Inorganic filler T)
T-1: Aluminum oxide, manufactured by Denka Co., Ltd., DAW-03, average particle size 4 μm, average aspect ratio: 1.02, maximum particle size 8 μm
T-2: Aluminum oxide, manufactured by Showa Denko K.K., CBP-02, average particle size 2 μm, average aspect ratio: 1.01, maximum particle size 4 μm
T-3: Aluminum nitride, HF01 manufactured by Tokuyama Corporation, average particle size 3 μm, average aspect ratio: 1.2, maximum particle size 9 μm
T-4: Boron nitride, Denka Boronite, HGP, manufactured by Denka Co., Ltd., average particle size 6 μm, average aspect ratio > 10, maximum particle size 23 μm

(Example 1) Thermally conductive adhesive sheet W-1
The dispersion liquid αS-2 for forming the thermally conductive adhesive layer was applied to the release-treated surfaces of two release films (Cerapeel MF, thickness 50 μm) manufactured by Toray Industries, Inc. using a doctor blade, and then dried in an oven at 100°C for 2 minutes. Thereafter, precursor coating films, which were the precursors to the thermally conductive adhesive layers α1-2 and α2-2 and had a thickness of approximately 20 μm, were produced on each release film.
Separately, a sheet-like substrate β-1 of a styrene-based elastomer prepared on a release film was placed on top of the release film so that the sheet-like substrate surface and the precursor coating film of the thermally conductive adhesive layer α1-2 were in contact with each other, and a rubber roller was pressed against the release film and rotated to obtain an intermediate laminate.
The release film on the sheet substrate side of the intermediate laminate was peeled off, and the intermediate laminate was placed on top of the exposed sheet substrate surface so that the precursor coating film of the thermally conductive adhesive layer α2-2 was in contact with the surface. A rubber roller was pressed against the release film and rotated to produce a laminate of release film/precursor coating film of the thermally conductive adhesive layer α1-2/sheet substrate β-1/precursor coating film of the thermally conductive adhesive layer α2-2/release film. This was then aged at 40°C for 7 days to complete the reaction of any unreacted crosslinking agent present in each precursor coating film, yielding a laminate in which the thermally conductive adhesive sheet W-1 was sandwiched between release-treated films.

(Examples 2 and 3) Thermally conductive adhesive sheets W-2 and W-3
A laminate of release film/thermally conductive adhesive layer α1-2/sheet-like substrate β-2 (or β-3)/thermally conductive adhesive layer α2-2/release film was produced in the same manner as in Example 1, except that sheet-like substrate β-1 was replaced with sheet-like substrate β-2 or β-3 having a different inorganic filler content. In other words, a laminate in which thermally conductive adhesive sheets W-2 and W-3 were sandwiched between release-treated films was obtained.

(Examples 4 to 7) Thermally conductive adhesive sheets W-4 to W-7
Except for using sheet-like substrates β-4 to β-7 of different thicknesses instead of the sheet-like substrate β-1, laminates of release film/thermally conductive adhesive layer α1-2/sheet-like substrate β-4 to β-7/thermally conductive adhesive layer α2-2/release film were prepared in the same manner as in Example 1, that is, laminates in which thermally conductive adhesive sheets W-4 to W-7 were sandwiched between release-treated films were obtained.

(Examples 8 to 10) Thermally conductive adhesive sheets W-8 to W-10
Except for using sheet-like substrates β-8 to β-10 containing different types of inorganic filler instead of sheet-like substrate β-1, laminates of release film/thermally conductive adhesive layer α1-2/sheet-like substrates β-8 to β-10/thermally conductive adhesive layer α2-2/release film were prepared in the same manner as in Example 1, that is, laminates in which thermally conductive adhesive sheets W-4 to W-7 were sandwiched between release-treated films were obtained.

(Examples 11 to 14) Thermally conductive adhesive sheets W-11 to W-14
Except for using polyvinyl acetal-based sheet-like substrates β-11 to β-14 of different thicknesses instead of the sheet-like substrate β-1, laminates of release film/thermal conductive adhesive layer α1-2/sheet-like substrate β-11 to β-14/thermal conductive adhesive layer α2-2/release film were prepared in the same manner as in Example 1, that is, laminates in which thermally conductive adhesive sheets W-11 to W-14 were sandwiched between release-treated films were obtained.

(Examples 15 and 16) Thermally conductive adhesive sheets W-15 to W-16
Except for using sheet-like substrates β-15 to β-16, each of which was made of a polyester or polyester-based polyurethane as a crosslinkable polymer, instead of the sheet-like substrate β-1, in the same manner as in Example 1, a laminate of release film/thermally conductive adhesive layer α1-2/sheet-like substrate β-15 to β-16/thermally conductive adhesive layer α2-2/release film was produced, that is, a laminate in which thermally conductive adhesive sheets W-15 to W-16 were sandwiched between release-treated films was obtained.

(Example 17) Thermally conductive adhesive sheet W-17
As the thermally conductive adhesive layer, instead of α1-2 and α2-2 having a thickness of 20 μm, α1-4-1 and α2-4-1 having a thickness of 5 μm, which were formed using the dispersion liquid αS-4 for forming the thermally conductive adhesive layer, were used. A laminate in which the thermally conductive adhesive sheet W-17 was sandwiched between release-treated films was obtained in the same manner as in Example 1.

(Examples 18 to 19) Thermally conductive adhesive sheets W-18 to W-19
As the thermally conductive adhesive layer, instead of α1-2 and α2-2 having a thickness of 20 μm, α1-2-2 and α2-2-2 having a thickness of 10 μm, α1-2-3 having a thickness of 30 μm, and α2-2-3 having a thickness of 10 μm, which were formed using dispersion liquid αS-2 for forming thermally conductive adhesive layers, were used. Except for this, a laminate in which thermally conductive adhesive sheets W-18 to W-19 were sandwiched between release-treated films was obtained in the same manner as in Example 1.

(Examples 20 to 27) Thermally conductive adhesive sheets W-20 to W-27
Except for using dispersions αS-1, αS-3 to αS-9 as shown in Table 4 instead of dispersion αS-2 for forming the thermally conductive adhesive layer, laminates in which thermally conductive adhesive sheets 20 to W-27 were sandwiched between release-treated films were obtained in the same manner as in Example 1.

(Comparative Examples 1 and 2) Comparative thermally conductive adhesive sheets W-101 to W-102
Except for using dispersions αS-101 and αS-102 as shown in Table 4 instead of dispersion αS-2 for forming the thermally conductive adhesive layer, a laminate was obtained in the same manner as in Example 1, in which comparative thermally conductive adhesive sheets W-101 to W-102 were sandwiched between release-treated films.

(Comparative Example 3) Comparative thermally conductive adhesive sheet W-103
A laminate in which the comparative thermally conductive adhesive sheet W-103 was sandwiched between release-treated films was obtained in the same manner as in Example 1, except that 50 μm thick α1-2-103 and α2-2-103, which were formed using the dispersion αS-2 for forming the thermally conductive adhesive layer, were used instead of 20 μm thick α1-2 and α2-2.

(Comparative Examples 4 to 6) Comparative thermally conductive adhesive sheets W-104 to W-106
Except for using polyvinyl acetal-based sheet-like substrates β-101 to β-103 instead of the sheet-like substrate β-1, laminates of release film/thermally conductive adhesive layer α1-2/sheet-like substrates β-101 to β-103/thermally conductive adhesive layer α2-2/release film were prepared in the same manner as in Example 1, that is, laminates in which comparative thermally conductive adhesive sheets W-104 to W-106 were sandwiched between release-treated films were obtained.

[Measurement of average particle size, etc. of inorganic filler in each layer by cross-sectional observation of thermally conductive adhesive sheet W-1, etc.]
In the same manner as in the observation of the cross section of the sheet-like substrate β-1, etc., the cross section of the thermally conductive pressure-sensitive adhesive sheet W-1, etc. was observed, and the average particle size, maximum diameter, etc. of the inorganic filler contained in each layer was determined.

[Measurement of storage modulus (E′) of thermally conductive adhesive sheet W-1, etc.]
The laminate sandwiched between the release-treated films was punched out into test pieces 15 mm long and 5 mm wide, the release-treated films were peeled off, and the thermally conductive adhesive sheet W-1 etc. was measured for change in storage modulus (E') in the temperature range of -50°C to 250°C when tensile under conditions of load cell: 600 gf, heating rate: 5°C/min, chuck distance: 5 mm, frequency: 10 Hz using a dynamic viscoelasticity measuring device DVA- ₂₀₀ (manufactured by IT Measurement Control Co., Ltd.). Of the temperatures at which the storage modulus (E') inflection point in the range of 0 to 150°C, i.e., the maximum value of tan δ, the temperature T0 on the highest temperature side was determined, and _E'1 at temperature _{T1 30} °C higher than _T0 and E'2 at temperature _T2 50°C higher than _T0 were determined, to derive _E'1 / _E'2 .
In Comparative Example 1, the adhesive layer was not crosslinked and melted, so the storage modulus (E') of the thermally conductive adhesive sheet could not be measured.

<Evaluation of processability>
The laminate sandwiched between the release-treated films was cut into 100 rectangular pieces of 10 mm x 30 mm using a die-cutting machine, and the number of defective pieces was evaluated as follows.
Here, defective products are those in which the shape of the punched edge is distorted by the die-cutting process (chipping due to embrittlement of the sheet-like substrate β-1, etc.), or those in which the release film and the thermally conductive adhesive sheet W-1, etc. have peeled off.
S: 0 defective products (best)
A: 1 to 5 (excellent)
B: 6 to 10 pieces (standard)
C: 11 to 20 pieces (usable)
D: 21 to 100 pieces (not allowed)

<Thermal conductivity measurement>
One of the release films was peeled off from the laminate sandwiched between the release-treated films, and the exposed adhesive layer surface was subjected to gold vapor deposition plating using a small high-vacuum deposition apparatus VE-2013 manufactured by Vacuum Device Co., Ltd. Next, the release film was peeled off from the other surface as well, and gold vapor deposition plating was similarly performed.
A blackening agent (Black Guard Spray, manufactured by Fine Chemical Japan Co., Ltd.) was applied to one surface to blacken it, and then a laser was applied to the blackened surface using a Xe flash analyzer LFA 447 Nanoflash, manufactured by Netzsch Japan Co., Ltd., to measure the thermal diffusivity in the thickness direction at 23°C.
Separately, the specific heat of the sample (i.e., the thermally conductive adhesive sheet) was calculated by comparing it with single crystal sapphire, whose heat capacity is known, using a differential scanning calorimeter (DSC measuring device), and the density of the sample was measured using the water displacement method, and the thermal conductivity [W/(mK)] in the thickness direction was calculated using the following formula.
[Thermal conductivity] = [Thermal diffusivity] x [Density] x [Specific heat]

<Evaluation of thermal resistance value (before heating)>
The thermal resistance values were determined for the thermally conductive adhesive sheets of each Example and Comparative Example sandwiched between aluminum plates, and for the standard adhesive sheet described below sandwiched between aluminum plates.The thermal resistance values of the thermally conductive adhesive sheets of each Example and Comparative Example were evaluated based on how much smaller the thermal resistance value of the thermally conductive adhesive sheets of each Example and Comparative Example was than the thermal resistance value of the standard adhesive sheet.
Specifically, the release films were sequentially peeled off from the laminate sandwiched between the release films, and the conductive adhesive sheet of each Example and Comparative Example or the reference adhesive sheet was sandwiched between aluminum plates 10 mm long, 12 mm wide, and 2 mm thick and pressed together under a load of 1 kg for 1 minute. A MOSFET (heat source IC, 15 mm long, 10 mm wide) and thermistor A (thermometer) were attached side by side to one of the aluminum plates with epoxy adhesive (Cemedine Co., Ltd., Hi-Super 5), and thermistor B (thermometer) was attached to the center of the other aluminum plate with epoxy adhesive.
Using the Tokyo Devices IW7300-KIT thermal resistance measurement kit, heating was continued at a room temperature of 23°C with a MOSFET set to an output of 1W, and the temperature was profiled. When the temperatures of thermistor A and thermistor B became constant, the difference in the temperatures detected by the thermistors was read and the thermal resistance value (°C/W) before heating was calculated using the following formula.
[Thermal resistance value] = [Temperature difference] ÷ [MOSFET output]

<Evaluation of thermal resistance value (after heating)>
The thermally conductive adhesive sheet or reference adhesive sheet of each example and comparative example sandwiched between aluminum plates is heated at 100°C for 4 hours, then returned to 23°C, and the thermal resistance value (°C/W) is calculated at room temperature of 23°C in the same manner as above.

The reference adhesive sheet used was Neoflix 6571, a 50 μm thick adhesive tape (composition: acrylic adhesive/PET film/acrylic adhesive) manufactured by Nichiei Shinka Co., Ltd., which is a common double-sided adhesive tape that does not contain inorganic fillers. The thermal resistance of the reference adhesive sheet was 9.1 (°C/W) before heating and 10.5 (°C/W) after heating.

(Evaluation criteria)
The thermal resistance value of the thermally conductive adhesive sheet of each Example and Comparative Example was subtracted from the thermal resistance value of the reference adhesive sheet, and the difference was used to evaluate the sheet according to the following criteria:
S: Difference is 5°C/W or more (best)
A: The difference is 3°C/W or more and less than 5°C/W (excellent).
B: Difference is 2°C/W or more and less than 3°C/W (standard)
C: Difference is 1°C/W or more and less than 2°C/W (usable)
D: Difference is less than 1°C/W (unacceptable)

[Voltage resistance test]
The laminates of each Example and Comparative Example sandwiched between release-treated films were cut into 5 cm squares, the release-treated films were sequentially peeled off, 5 μm thick copper foil was attached to each side, and a load of 2 kgf was applied and the pieces were adhered with a rubber roll. For the adhered samples, a withstand voltage measuring instrument 8504 manufactured by Tsuruga Electric Co., Ltd. was used, with a charging terminal contacted with the adhesive layer α1 side and an energizing terminal contacted with the adhesive layer α2 side, and a direct current was applied in the atmosphere while increasing the voltage from 0.1 to 10 kV, and the applied voltage (breakdown voltage) was recorded.
S: No current flows when the applied voltage is 7 kV (best)
A: Conducts electricity at 5 kV or more and less than 7 kV (excellent)
B: Energized at 3 kV or more and less than 5 kV (standard)
C: Powered at 2 kV or more and less than 3 kV (usable)
D: Energize at less than 2 kV (not allowed)

<Heat resistance evaluation>
The laminate of each example and each comparative example sandwiched between release-treated films was cut to a size of 20 mm x 20 mm, the release film on the adhesive layer α1 side was peeled off, and the laminate was attached to a multi-heat sink for chips manufactured by Inex Co., Ltd. (made of aluminum, 20 mm x 20 mm x height 6 mm, approximately 100 g) so that the smooth surface of the heat sink and the square of the adhesive sheet were neatly overlapped.
Next, the release film on the adhesive layer α2 side is peeled off and attached to a 30 mm x 50 mm x 2 mm thick aluminum plate, and the aluminum plate is placed on a ceramic hot plate (manufactured by AS ONE Corporation) set to 150°C and tilted at 30 degrees so that the aluminum plate is in contact with it.
After 10 minutes, the aluminum plate was removed from the hot plate, and the heat sink and the thermally conductive adhesive tape were observed under a microscope at 10 to 20 times magnification to see if there was any misalignment in the tilted direction, and the results were evaluated according to the following criteria.
S: No deviation or deviation of less than 0.1 mm (best)
A: Deviation of 0.1 mm or more and less than 0.5 mm (excellent)
B: Deviation of 0.5 mm or more and less than 1 mm (standard)
C: There is a misalignment of 1 mm or more, and no defects in appearance such as foaming can be found on the exposed adhesive layer surface (usable)
D: There is a misalignment of 1 mm or more, and defects such as foaming can be seen on the exposed adhesive layer surface (unacceptable)

Claims (12)

A thermally conductive adhesive sheet having thermally conductive adhesive layers α1 and α2 on both sides of a sheet-like substrate β,
the sheet-like substrate β contains a cured product of a crosslinkable polymer and a crosslinking agent, and an insulating inorganic filler, has a storage modulus at 200°C of 10 ³ Pa or more, and has a thickness of 1 to 10 μm;
Among the insulating inorganic fillers observed in a cross section of the sheet-like base material β, a ratio of inorganic fillers having an aspect ratio: major axis a ^β / minor axis b ^β of 1 to 2 is 80% or more,
the thermally conductive adhesive layers α1 and α2 each independently contain a cured product of a crosslinkable polymer and a crosslinking agent, and an insulating inorganic filler, and the thermally conductive adhesive layers α1 and α2 have a total thickness of 1 to 45 μm;
The total thickness of the sheet-like substrate β and the thermally conductive adhesive layers α1 and α2 is 50 μm or less,
the ratio _E'1/E'2 of the storage elastic modulus _E'1 at a temperature T1 that is 30°C higher than the temperature _T0 of the highest inflection point of the storage elastic modulus in the temperature range of 0°C to 150° _C to the storage elastic modulus E'2 at a temperature _{T2 that} is 50°C higher than the temperature _T0 of the inflection _point is 1 to 3;
Thermally conductive adhesive sheet. an average particle diameter, which is half the sum of the major axis a ^β and the minor axis b ^β of the insulating inorganic filler observed in a cross section of the sheet-like base material β, is 60% or less of the thickness of the sheet-like base material β, and a maximum diameter a ^β _max of the insulating inorganic filler is less than the thickness of the sheet-like base material β;
The thermally conductive adhesive sheet according to claim 1. an average particle diameter, which is half the sum of a major axis ^aα1 and a minor axis ^bα1 of the insulating inorganic filler observed in a cross section of the thermally conductive adhesive layer α1, is 60% or less of the thickness of the thermally conductive adhesive layer α1, and a maximum diameter ^aα1max of the insulating inorganic filler is _less than the thickness of the thermally conductive adhesive layer α1;
an average particle diameter of the insulating inorganic filler observed in a cross section of the thermally conductive adhesive layer α2, which is half the sum of a major ^{axis aα2} and a minor axis ^bα2 , is 60% or less of the thickness of the thermally conductive adhesive layer α2, and _a maximum diameter ^aα2max of the insulating inorganic filler is less than the thickness of the thermally conductive adhesive layer α2;
The thermally conductive adhesive sheet according to claim 1 or 2. The thermally conductive adhesive sheet according to any one of claims 1 to 3 , wherein the crosslinkable polymer in the sheet-like substrate β is selected from the group consisting of polyvinyl acetal resin, styrene-based elastomer, polyester resin, and polyester polyurethane resin. The thermally conductive adhesive sheet according to any one of claims 1 to 4, wherein the crosslinkable polymer in the thermally conductive adhesive layers α1 and α2 is an acrylic resin. The thermally conductive adhesive sheet according to any one of claims 1 to 5, wherein the crosslinking agent in the sheet-like substrate β is selected from the group consisting of a compound having an isocyanate group, a compound having an acid anhydride group, a compound having an aziridinyl group, a compound having an epoxy group, and a compound having an amino group. The thermally conductive adhesive sheet according to any one of claims 1 to 6 , wherein the crosslinking agent in the thermally conductive adhesive layers α1 and α2 is selected from the group consisting of a compound having an isocyanate group, a compound having an acid anhydride group, a compound having an aziridinyl group, a compound having an epoxy group, and a compound having an amino group. A method for producing a thermally conductive adhesive sheet having thermally conductive adhesive layers α1 and α2 on both sides of a sheet-like substrate β, comprising the following steps [1] to [3]:
the ratio E'1/E'2 of the storage modulus E'1 at a temperature _T1 that is 30°C higher than the temperature _T0 of the highest inflection point of the storage modulus in the temperature range of 0 to 150° _C to the storage modulus _E'2 at a temperature _T2 that is 50° _C higher than the temperature _T0 of the inflection point is 1 to ₃ ;
A method for manufacturing a thermally conductive adhesive sheet.
[1] A process for producing a sheet-like substrate β using a dispersion for forming the sheet-like substrate β, the dispersion containing a solution or dispersion of a crosslinkable polymer, an insulating inorganic filler, and a crosslinking agent, the sheet-like substrate β having a storage modulus at 200°C of 10 ³ Pa or more, a thickness of 1 to 10 μm, and an aspect ratio (long diameter a ^β /short diameter b ^β) of 1 to 2 for 80% or more of the insulating inorganic filler of 100% observed in the cross section of the sheet-like substrate β.
[2] A process for producing two thermally conductive adhesive layers α1 and α2 using a dispersion for forming the thermally conductive adhesive layers α1 and α2, the dispersion containing a solution or dispersion of a crosslinkable polymer, an insulating inorganic filler, and a crosslinking agent.
[3] A step of laminating the thermally conductive adhesive layers α1 and α2 on both sides of the sheet-like substrate β so that the total thickness of the thermally conductive adhesive layers α1 and α2 is 1 to 45 μm and the total thickness of the sheet-like substrate β and the thermally conductive adhesive layers α1 and α2 is 50 μm or less. 9. The method for producing a thermally conductive adhesive sheet according to claim 8, wherein in the step [1], the dispersion for forming the sheet-like substrate β is produced using an insulating inorganic filler having a D90 particle diameter of 50 or less when the thickness of the sheet-like substrate β is 100. 10. The method for producing a thermally conductive adhesive sheet according to claim 8 or 9, wherein in the step [ 2 ], a dispersion for the thermally conductive adhesive layers α1 and α2 is produced using an insulating inorganic filler having a D90 particle size of 50 or less when the thickness of the thermally conductive adhesive layers α1 and α2 is 100. the crosslinkable polymer in the step [1] is selected from the group consisting of polyvinyl acetal resins, styrene-based elastomers, polyester resins, and polyester polyurethane resins;
the crosslinking agent in the step [1] is selected from the group consisting of a compound having an isocyanate group, a compound having an acid anhydride group, a compound having an aziridinyl group, a compound having an epoxy group, and a compound having an amino group;
A method for producing the thermally conductive pressure-sensitive adhesive sheet according to any one of claims 8 to 10 . the crosslinkable polymer in the step [2] is an acrylic resin,
the crosslinking agent in the step [2] is selected from the group consisting of a compound having an isocyanate group, a compound having an acid anhydride group, a compound having an aziridinyl group, a compound having an epoxy group, and a compound having an amino group;
A method for producing the thermally conductive pressure-sensitive adhesive sheet according to any one of claims 8 to 11 .